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The extracellular role of granzyme B in abdominal aortic aneurysm Ang, Lisa Shouning 2013

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     THE EXTRACELLULAR ROLE OF GRANZYME B IN ABDOMINAL AORTIC ANEURYSM by LISA SHOUNING ANG B.Sc. University of Waterloo, 2005   A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES (Pathology and Laboratory Medicine) THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver) December 2013   ? Lisa Shouning Ang 2013   ii  ABSTRACT Background: Abdominal aortic aneurysm (AAA) is an age-related disease characterized by progressive degradation of elastic lamellae, defective collagen architecture and medial smooth muscle cell loss. We previously demonstrated that knocking out the serine protease granzyme B (GZMB) reduces incidence and severity of AAA in mice; however, while GZMB is known for its role in apoptosis, it also accumulates extracellularly during inflammation and can cleave extracellular matrix (ECM) components such as decorin and fibrillin-1. We hypothesized that GZMB contributes to AAA development through the degradation of vascular ECM and that the inhibition of extracellular GZMB would reduce the incidence and severity of AAA progression.  Methods: Human aneurysmal samples were obtained and apolipoprotein E(apoE)-knockout (KO), GZMB/apoE-double knockout (GDKO) and perforin/apoE-DKO (PDKO) mice were implanted with osmotic minipumps releasing angiotensin II for 28 days to induce AAA formation. Additional apoE-KO mice were injected with the GZMB inhibitor, serpin A3N (SA3N, 4-120 ?g/kg) or anti-GZMB neutralizing antibody (1 mg/kg) prior to pump implantation. Tissues were assessed for aneurysm pathology, inflammation and ECM composition. Collagen content was analysed by second harmonic generation and transmission electron microscopy.  Results: Human aneurysmal tissues showed elevated levels of GZMB immunopositivity compared to controls. A significant reduction in AAA incidence and severity was observed in GDKO mice compared to apoE-KO, whereas perforin deficiency was not protective against AAA. A dose-dependent reduction in the frequency of aortic rupture was observed in mice that received SA3N or anti-GZMB antibody treatment. Pre-incubation with SA3N prevented decorin cleavage by GZMB in vitro. Reduced GZMB and a corresponding reduction in loss of adventitial decorin were observed in SA3N and anti-GZMB-treated mice while collagen density was increased. Adventitial collagen from SA3N-treated mice exhibited significantly higher fibre density and reduced fibril size irregularity.  Conclusions: GZMB promotes destruction of the elastic lamellae via degradation of fibrillin-1 and destabilization of elastic microfibrils while GZMB-mediated degradation of decorin contributes to loss of adventitial collagen organization and density. The extracellular inhibition of GZMB prevented decorin loss and enabled a beneficial remodelling of adventitial collagen in response to medial injury, leading to higher vessel tensile strength and increased resistance to aortic rupture. iii  PREFACE Chapter 1 contains text from a review entitled ?Inflammaging and proteases in abdominal aortic aneurysm? by Hendel A, Ang LS, Granville DJ (Curr Vasc Pharmacol. 2012 Jun 22). Figure 1 was taken from ?Granzyme B in injury, inflammation and repair? by Hiebert PR and Granville DJ (Trends Mol Med. 2012 Dec;18(12):732-41) and was drawn by Paul Hiebert. Chapter 4 contains text and images from ?Perforin-independent extracellular granzyme B activity contributes to abdominal aortic aneurysm? by Chamberlain CM, Ang LS, Boivin WA, Cooper DM, Williams SJ, Zhao H, Hendel A, Folkesson M, Swedenborg J, Allard MF, McManus BM, Granville DJ (Am J Pathol. 2010 Feb;176(2):1038-49).  Immunohistochemistry was performed by Hongyan Zhao. Chapter 5 contains text and images from ?Perforin-independent extracellular granzyme B activity contributes to abdominal aortic aneurysm? by Chamberlain CM, Ang LS, Boivin WA, Cooper DM, Williams SJ, Zhao H, Hendel A, Folkesson M, Swedenborg J, Allard MF, McManus BM, Granville DJ (Am J Pathol. 2010 Feb;176(2):1038-49). Movat?s Pentachrome staining was performed by Amrit Samra. Immunohistochemistry was performed with assistance from Tyler Varnals and Hongyan Zhao. Immunoprecipitation of mouse serum proteins was performed by Hongyan Zhao. Fibrillin-1 transcription levels were measured by Dawn Cooper. Confocal images of immune cells were taken by Alon Hendel. Chapter 6 contains text and images from ?Serpina3n attenuates granzyme B-mediated decorin cleavage and rupture in a murine model of aortic aneurysm? by Ang LS, Boivin WA, Williams SJ, Zhao H, Abraham T, Carmine-Simmen K, McManus BM, Bleackley RC, Granville DJ (Cell Death Dis. 2011 Sep 8;2:e209). Serpin A3N was kindly provided by Dr. R. Chris Bleackley. Movat?s Pentachrome staining was performed by Amrit Samra. Assessment of serpin inhibition was performed by Sarah Williams. Decorin cleavage assays were performed by Wendy Boivin. Second harmonic generation studies were performed iv  in collaboration with Dr. Thomas Abraham, and electron microscopy was performed in collaboration with Dr. David Walker and Fanny Chu.  All animal work was performed in accordance with the guidelines for animal experimentation approved by the Animal Experimentation Committee of the University of British Columbia. All research studies were conducted with the approval of the University of British Columbia Animal Care Committee and research was carried out under the certificates A10-0119, A06-0158 and A10-0084.    v  TABLE OF CONTENTS Abstract ......................................................................................................................................................................... ii Preface .......................................................................................................................................................................... iii Table of Contents .......................................................................................................................................................... v List of Tables ............................................................................................................................................................... viii List of Figures ................................................................................................................................................................ ix List of Abbreviations ...................................................................................................................................................... x Acknowledgements .................................................................................................................................................... xiv Chapter 1: Introduction ................................................................................................................................................. 1 1.1 Aorta: Structure and Function ............................................................................................................................. 1 1.1.1 Embryology ................................................................................................................................................... 1 1.1.2 Anatomy ....................................................................................................................................................... 2 1.1.3 Aortic Function ............................................................................................................................................. 4 1.2 The Pathophysiology of Abdominal Aortic Aneurysm ......................................................................................... 6 1.2.1 Genetics ........................................................................................................................................................ 7 1.2.2 Classification and Etiology ............................................................................................................................ 8 1.2.2 Pathophysiology ........................................................................................................................................... 9 1.3 The Extracellular Matrix ..................................................................................................................................... 11 1.3.1 Elastin ......................................................................................................................................................... 12 1.3.2 Collagen ...................................................................................................................................................... 13 1.4 The Role of Immune-Derived Proteases in AAA ................................................................................................ 14 1.4.1 Matrix Metalloproteinases ......................................................................................................................... 17 1.4.2 Elastase ....................................................................................................................................................... 24 1.4.3 Cathepsins .................................................................................................................................................. 27 1.4.4 Chymase and Tryptase ................................................................................................................................ 30 1.5 Granzyme B ........................................................................................................................................................ 34 1.5.1 GZMB-induced Cell Death ........................................................................................................................... 35 1.5.2 Extracellular GZMB Activity ........................................................................................................................ 37 1.5.3 Extracellular GZMB Substrates ................................................................................................................... 39 1.5.4 GZMB Inhibitors .......................................................................................................................................... 43 1.5.5 GZMB in Atherosclerosis ............................................................................................................................. 44 1.6 Murine Models of AAA ...................................................................................................................................... 46 1.6.1 Genetically Engineered Mouse Models of AAA .......................................................................................... 47 1.6.2 Chemically-Induced Mouse Models of AAA ............................................................................................... 49 vi  Chapter 2: Rationale, Hypothesis and Specific Aims ................................................................................................... 53 2.1 Rationale ............................................................................................................................................................ 53 2.2 Hypothesis ......................................................................................................................................................... 54 2.3 Specific Aims ...................................................................................................................................................... 54 Chapter 3: Methodology ............................................................................................................................................. 56 3.1 In Vitro Assays .................................................................................................................................................... 56 3.1.1 Fibrillin-1 Expression Assay ......................................................................................................................... 56 3.1.2 Murine GZMB Enzymatic Activity Assay ..................................................................................................... 56 3.1.3 Decorin Cleavage Assay .............................................................................................................................. 57 3.1.4 Statistics ...................................................................................................................................................... 58 3.2 In Vivo Assays ..................................................................................................................................................... 58 3.2.1 Human AAA and TAA .................................................................................................................................. 58 3.2.2 Mice ............................................................................................................................................................ 58 3.2.3 Murine Model of Angiotensin II-induced AAA ............................................................................................ 59 3.2.4 Serpin A3N Treatment ................................................................................................................................ 59 3.2.5 Anti-GZMB Antibody Treatment ................................................................................................................. 60 3.2.6 Tissue Collection and Gross Pathological Characterization ........................................................................ 60 3.2.7 Histology ..................................................................................................................................................... 61 3.2.8 Immunohistochemistry ............................................................................................................................... 61 3.2.9 Immunoprecipitation of Serum Fibrillin-1 Fragments ................................................................................ 61 3.2.10 Confocal Microscopy ................................................................................................................................ 62 3.2.11 Second Harmonic Generation Microscopy ............................................................................................... 63 3.2.12 Transmission Election Microscopy............................................................................................................ 63 3.2.13 Immuno-Electron Microscopy .................................................................................................................. 64 3.2.14 Statistics and Data Analysis ...................................................................................................................... 66 Chapter 4: GZMB in Human AAA and TAA ................................................................................................................... 67 4.1 Introduction ....................................................................................................................................................... 67 4.2 GZMB Is Elevated in Human AAA ....................................................................................................................... 67 4.3 GZMB Colocalizes to Immune Cells in Human TAA ............................................................................................ 68 4.5 Discussion .......................................................................................................................................................... 72 Chapter 5: Perforin Deficiency Is Not Protective Against Angiotensin II-induced Murine AAA .................................. 73 5.1 Introduction ....................................................................................................................................................... 73 5.2 GZMB Deficiency But Not Perforin Deficiency Improves Outcomes in AAA ...................................................... 73 5.2.1 Survival and Incidence of AAA .................................................................................................................... 73 vii  5.2.2 Medial Integrity .......................................................................................................................................... 74 5.3 Perforin Deficiency Does Not Reduce GZMB Levels in Ang II-induced AAA ...................................................... 78 5.4 Perforin Deficiency Does Not Prevent Medial Fibrillin-1 Loss ........................................................................... 80 5.5 GZMB Deficiency Reduces Fibrillin-1 Fragmentation in Mouse Serum ............................................................. 80 5.6 GZMB Treatment Does Not Alter Fibrillin-1 Expression In Vivo......................................................................... 80 5.7 Discussion .......................................................................................................................................................... 83 Chapter 6: Inhibition of GZMB Reduces Rate of Rupture in Angiotensin-II-induced AAA ........................................... 88 6.1 Introduction ....................................................................................................................................................... 88 6.2 SA3N Treatment ................................................................................................................................................ 88 6.2.1 SA3N Inhibits Murine GZMB Activity In Vitro ............................................................................................. 88 6.2.2 SA3N Dose-Dependently Improves Outcomes in AAA ............................................................................... 90 6.2.3 GZMB Immunopositivity Corresponds to Regions of Medial Disruption and Loss of Fibrillin-1 and Decorin ............................................................................................................................................................................. 90 6.2.4 SA3N Reduces GZMB Immunopositivity in Mouse Aorta ........................................................................... 93 6.2.5 SA3N Inhibits GZMB-mediated Cleavage of Decorin In Vitro ..................................................................... 93 6.2.6 SA3N Promotes Adventitial Thickening and Prevents Loss of Decorin ....................................................... 96 6.2.7 SA3N Reduces Loss of Collagen Density in AAA .......................................................................................... 96 6.2.8 SA3N Reduces Collagen Fibril Diameter Irregularity and Adventitial Decorin Loss at the Ultrastructural Level ................................................................................................................................................................... 100 6.3 Anti-GZMB Antibody Treatment ...................................................................................................................... 103 6.3.1 Anti-GZMB Antibody Treatment Improves Outcomes in AAA .................................................................. 103 6.3.2 Anti-GZMB Antibody Treatment Reduces GZMB Immunopositivity and Prevents Decorin Loss ............. 104 6.3.3 Anti-GZMB Antibody Treatment Reduces Loss of Collagen Density in AAA ............................................. 110 6.4 Decorin Loss and Collagen Density in Human TAA and Thoracic Dissection ................................................... 114 6.4.1 GZMB Immunopositivity Corresponds to Decorin Loss in TAA ................................................................. 114 6.4.2 Disorganized Collagen Architecture in Human TAA and Dissection ......................................................... 114 6.5 Discussion ........................................................................................................................................................ 119 Chapter 7: Conclusion ................................................................................................................................................ 126 References ................................................................................................................................................................. 130    viii  LIST OF TABLES Table 1: Immune  Proteases in AAA ............................................................................................................................ 15 Table 2: Extracellular GZMB Substrates and Potential Consequences in AAA ............................................................ 42 Table 3: Adventitial Collagen Fibril Diameters ........................................................................................................... 102   ix  LIST OF FIGURES Figure 1: GZMB-mediated apoptosis ........................................................................................................................... 37 Figure 2: Experimental overview and chapter outline ................................................................................................ 55 Figure 3: GZMB in non-atherosclerotic human aorta versus human AAA. .................................................................. 70 Figure 4: GZMB colocalization with macrophages and CD3+ cells in TAA .................................................................... 70 Figure 5: GZMB colocalization with mast cells in TAA ................................................................................................. 71 Figure 6: Kaplan-Meier survival curve for apoE-KO, PDKO and GDKO mice on Ang II ................................................ 75 Figure 7: Representative gross pathology, morphology and summary of outcomes .................................................. 76 Figure 8: GZMB deficiency reduces medial disruption ................................................................................................ 77 Figure 9: GZMB colocalizes to mast cells ..................................................................................................................... 78 Figure 10: GZMB immunostaining in apoE-KO and PDKO ........................................................................................... 79 Figure 11: Fibrillin-1 staining in apoE-KO and PDKO is reduced compared to GDKO .................................................. 81 Figure 12: GZMB cleaves fibrillin-1 in vitro and in vivo ................................................................................................ 82 Figure 13: SA3N inhibits murine GZMB enzymatic activity. ........................................................................................ 89 Figure 14: GZMB inhibition increases 28-day survival and reduces rate of aneurysm rupture. ................................. 91 Figure 15: GZMB is abundant in vessels exhibiting medial disruption. ....................................................................... 92 Figure 16: GZMB immunopositivity is reduced in SA3N-treated mice. ....................................................................... 94 Figure 17: GZMB cleavage of decorin is prevented by pre-incubation with SA3N in vitro. ......................................... 95 Figure 18: SA3N and GZMB deficiency promotes adventitial thickening and increased decorin content in AAA. ..... 98 Figure 19: SA3N treatment reduces the loss of collagen fibre density in Ang II-treated apoE-KO mice. .................... 99 Figure 20: SA3N reduces collagen fibril diameter irregularity. .................................................................................. 101 Figure 21: SA3N reduces decorin loss at the ultrastructural level ............................................................................. 103 Figure 22: Anti-GZMB antibody treatment improves outcome in Ang II-induced AAA ............................................. 105 Figure 23: Representative Movat?s pentachrome, GZMB and decorin staining in healthy aorta ............................. 106 Figure 24: Representative Movat?s pentachrome, GZMB and decorin staining in anti-gzmb treated non-ruptured aorta .......................................................................................................................................................................... 107 Figure 25: Representative Movat?s pentachrome, GZMB and decorin staining in IgG-treated non-ruptured aorta 108 Figure 26: Representative Movat?s pentachrome, GZMB and decorin staining in IgG-treated ruptured aorta ....... 109 Figure 27: Picrosirius Red: Anti-GZMB antibody treatment reduces loss of collagen density in AAA ....................... 111 Figure 28: SHG: Anti-GZMB antibody treatment reduces loss of collagen density in AAA........................................ 113 Figure 29: GZMB immunopositivity corresponds to decorin loss in TAA .................................................................. 116 Figure 30: Collagen density in human TAA and dissection ........................................................................................ 118 Figure 31: Current perspective on the extracellular role of GZMB in AAA ................................................................ 129   x  LIST OF ABBREVIATIONS ?1AT  Alpha-1-antitrypsin  AAA Abdominal aortic aneurysm ACT Alpha-1-antichymotrypsin ADAMTS-13 A disintegrin and metalloproteinase with thrombospondin motifs-13 AIF Apoptosis-inducing factor Ang I Angiotensin I Ang II Angiotensin II ANOVA Analysis of variance Anti-GZMB Anti-GZMB antibody AP ABC-alkaline phosphatase AP-1 Activator protein-1 APC Antigen presenting cell apoE Apolipoprotein E AT1 Angiotensin II receptor, type 1  AT2 Angiotensin II receptor, type 2 ATF Activating transcription factor AU Arbitrary units CBF Core binding factor CG Cathepsin G CMA-1 Mast cell chymase-1 COPD Chronic obstructive pulmonary disease CREB  Cyclic AMP-responsive element-binding protein crmA Cytokine response modifier-A  CTL Cytotoxic T lymphocyte CTLA-1 Cytotoxic T lymphocyte-associated serine esterase-1  DKO Double knockout  DPPI Dipeptidyl peptidase I  xi  ECM Extracellular matrix EDTA Ethylenediaminetetraacetic acid FDA Food and Drug Administration FGF-2 Fibroblast growth factor-2 GAG Glycosaminoglycan GDKO Granzyme B/apolipoprotein E-double knockout mice GZMA Granzyme A GZMB Granzyme B GZMH Granzyme H GZMM Granzyme M H&E Hematoxylin and eosin HCASMC Human coronary artery smooth muscle cell HDL High-density lipoprotein HIV-1 Human immunodeficiency virus-1 HMG CoA 3-Hydroxy-3-methylglutaryl coenzyme A  IFN-? Interferon-gamma IHC Immunohistochemistry IL-13 Interleukin-13 IL-1? Interleukin-1-alpha IL-1? Interleukin-1-beta IL-2 Interleukin-2 IL-6 Interleukin-6 IL-8 Interleukin-8 iNOS Inducible nitric oxide synthase  IS Immunological synapse KO Knockout  LDL Low-density lipoprotein 5-LO 5-lipoxygenase xii  LIX Lipopolysaccharide-induced CXC LLC Large latent complex LOX Lysyl oxidase LTC4 Leukotriene C4  MCL-1 Myeloid cell leukemia sequence-1 MCP-1 Monocyte attractant protein-1  MHC-II Major histocompatibility complex class II  MIP-2 Macrophage inflammatory protein-2 mMCP-4 Mouse mast cell protease- 4  mMCP-6 Mouse mast cell protease- 6 MMP-1 Matrix metalloproteinase-1 MMP-12 Matrix metalloproteinase-12 MMP-13 Matrix metalloproteinase-13 MMP-2 Matrix metalloproteinase-2 MMP-9 Matrix metalloproteinase-9 MPR Mannose-6-phosphate receptor MT1-MMP Membrane-type 1 matrix metalloproteinase NADPH Nicotinamide adenine dinucleotide phosphate (reduced) NK Natural killer NO Nitric oxide NR Non-ruptured NSAID Non-steroidal anti-inflammatory drug PA Phosphatidic acid PAI-1 Plasminogen activator-1 PAR-2 Protease activated receptor-2  PDGF Platelet-derived growth factor PDKO Perforin/apolipoprotein E-double knockout mice PEBP2 Polyomavirus enhancer binding protein 2 xiii  PGE2  Prostaglandin E2  PGG Pentagalloyl glucose PI-9 Protease inhibitor-9 PLD Phospholipase D RCL Reactive centre loop RGD Arg-Gly-Asp ROS Reactive oxygen species SA3N Serpin A3N SAAAVE Act Screening for Abdominal Aortic Aneurysm Very Efficiently Act Serpin Serine protease inhibitor SHG Second harmonic generation SMBM Smooth muscle cell basal medium SMC Smooth muscle cell SPI-6 Serine protease inhibitor-6 TAA Thoracic aortic aneurysm TEM Transmission electron microscopy TGF-? Transforming growth factor-beta TIMP-1 Tissue inhibitor of metalloproteinase-1 TNF-? Tumour necrosis factor-alpha TUNEL Terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling uPA Urokinase-type plasminogen activator  VCAM-1 Vascular cell adhesion molecule-1 VEGF Vascular endothelial growth factor  VEGFR-2 Vascular endothelial growth factor receptor-2 VSMC Vascular smooth muscle cell vWF von Willebrand factor WT Wild type ?1AT  Alpha-1-antitrypsin  xiv  ACKNOWLEDGEMENTS I would like to thank my family for their support and encouragement, and especially my parents, Richard and Sarah Ang, who brought me up to value knowledge and the pursuit of truth. I want to acknowledge my friends at the James Hogg Research Centre who have shared this seminal experience in graduate school with me, and helped to make it the most memorable time of my life thus far: Seti Boroomand, Hayley Hiebert, Paul Hiebert, Stephanie Warner, Wendy Boivin, Anna Meredith, Alon Hendel and Cleo Leung. I want to thank my supervisor, Dr. David Granville, who has been a supportive mentor since the very beginning, and who never gave up on me. I will be ever grateful to Hongyan Zhao, for her patience and willingness to help with every experiment. This project would not have been nearly as successful without her assistance and instruction. I would also like to thank all the members of the Granville laboratory, both past and present, for their support and solidarity. It has been an honour to work alongside you. I would like to thank the members of my advisory committee for their invaluable time and willingness to share their expertise, and for the constructive support and advice given to me over the years: Dr. David Walker, Dr. Ismail Laher, Dr. Bruce McManus and Dr. Michael Allard. I would like to acknowledge the members of the Genetically Engineered Models facility at the James Hogg Research Centre, with especial thanks to Lubos Bohunek and Tatjana Bozin, who selflessly gave of their time and effort and made all the animal work in this dissertation possible. I want to thank Fanny Chu for going the extra mile to help me obtain the electon microscopy images in this dissertation. The beautiful results speak for themselves.  Last, but not least, I would like to thank my husband, Sorin Alexander, for his unconditional love, support and endless patience with me. You were my inspiration all along.1  CHAPTER 1: INTRODUCTION 1.1 AORTA: STRUCTURE AND FUNCTION The aorta is the largest artery in the body and extends from the aortic valve in the heart to the iliac bifurcation. It is the main vessel through which oxygenated blood is distributed to all parts of the body and is thus crucial to the circulatory system.  1.1.1 EMBRYOLOGY The development of the aorta is an integral part of the development of the conotruncus, or the cardiac outflow tract. Malformations in the conotruncus account for a large percentage of clinically relevant congenital heart defects such as persistent truncus arteriosus, transposition of the great arteries, double outlet of the right ventricle, and tetralogy of Fallot.1 The cardiac outflow tract begins as a single tube that extends from the distal end of the primitive right ventricle to the end of the pericardial cavity. It is composed of three layers: an outer layer of myocytes, a middle layer of cardiac jelly and a single layer of endothelial cells lining the lumen.2  The outflow tract develops a characteristic bend with proximal and distal portions,3 with the distal portion connecting to the bilateral dorsal aortae through the first pair of pharyngeal arch arteries at the aortic sac.4 In humans, the pharyngeal arches develop during the fourth week of gestation as a series of mesodermal outpouchings on either side of the pharynx. The second, third, fourth and sixth pairs of pharyngeal arches appear in sequence and arise from the ventral side of the aortic sac, from where they develop to meet the dorsal aortae.4  The first and second pairs of arches degenerate early, with a remnant of the first arch developing into the maxillary artery. The third arches form the origin for the left and right common carotid arteries, while the left fourth arch is retained as a part of the mature left aortic arch and the 2  right fourth arch forms the most proximal part of the right subclavian artery. The aortic sac is partitioned by the aortopulmonary septum resulting in the separation of the intrapericardial aorta and the pulmonary trunk, as well as the origins of the fourth and sixth arches. The proximal part of the sixth arches form the right and left pulmonary arteries. While the distal section of the right arch degenerates, the distal part of the left arch forms the ductus arteriosus.4 In the developing foetus, this duct remains open and allows for blood to bypass the immature pulmonary system, but should close within a few days of birth and develop into the ligamentum arteriosum, a fibrous band connecting the left pulmonary artery to the inferior surface of the aortic arch. The seventh intersegmental arteries enlarge and migrate towards the head along the dorsal aortae. The right dorsal aorta and first through sixth intersegmental arteries degenerate, leaving the seventh right intersegmental artery to become the right subclavian artery. The seventh left intersegmental artery arrives at the fourth arch, in between the ductus arteriosus and the left common carotid artery (third arch) and becomes the left subclavian artery. This leaves the mature aorta to derive from the persistence of the intrapericardial aorta, the left fourth arch and the left dorsal aorta.4  1.1.2 ANATOMY  The aorta can be divided into four main sections: the aortic root and ascending aorta, the aortic arch, the thoracic descending aorta and the abdominal aorta. At the aortic root, the aortic valve sits at the origin of the aorta and guards the left ventricular outflow tract. The three semilunar leaflets normally bridge the anatomic ventriculo-arterial junction. Immediately above the aortic valve are the three aortic sinuses, from which the left posterior aortic sinus gives rise to the left coronary artery and the anterior aortic sinus gives rise to the right coronary artery.4 The valve leaflets are typically nearly equal in size, and are thin and pliable in young, healthy individuals but often become thick and stiff with age.5 Like the sinuses, the ascending aorta is enclosed 3  within the pericardium and in adults, it measures approximately 3 cm across in diameter and 5 cm in length.4  The aortic arch begins at the level of the superior attachment to the fibrous pericardium and is entirely external to the pericardial sac. The arch curves cephalad and then posterior and to the left to cross in front of the trachea before descending into the thorax to become the thoracic aorta. The three major arteries that arise from the arch and supply the head, neck and arms are the brachiocephalic, left common carotid and left subclavian arteries.4 The thoracic aorta continues from the arch and begins descent approximately level with the fourth thoracic vertebra, beginning on the left and shifting to the midline at the twelfth vertebra. There, it passes through the aortic hiatus in the diaphragm and becomes the abdominal aorta. The thoracic aorta gives rise to the visceral, intercostal, subcostal and superior phrenic arteries.4  The abdominal aorta begins at the aortic hiatus and ends at the iliac bifurcation. It gives rise to major arteries such as the iliac, renal and mesenteric arteries, and narrows to approximately two-thirds the width of the thoracic aorta.4 Throughout its length, the aorta is composed of three layers: a thin tunica intima made up of a single layer of endothelial cells sitting on a basement membrane, a thick tunica media comprised of smooth muscle cells (SMC) embedded in a dense matrix of fibrillar structural proteins such as elastin, and lastly, the tunica adventitia, which contains fibroblasts and collagen fibres.4 It is the media that provides the aorta with its viscoelastic properties via concentric bands of elastin filaments that associate with collagen fibres and SMC into lamellar units or lamellae.6 In mammals, the first 28 to 30 lamellar units from luminal surface are an avascular zone that does not have a dedicated blood supply, and instead must rely on diffusion of oxygen and nutrients across the intima from the lumen.7 If additional lamellar units are present, they must be supplied by a network of vasa vasorum which penetrates the media from the adventitia. As such, the abdominal aorta with 28 to 32 lamellar units, is typically 4  completely avascular and lacking a vasa vasorum, whereas the thoracic aorta contains 55 to 60 lamellar units and contains both vascular and avascular zones. 7 Aortic medial thickness increases from birth to adulthood; however, this is achieved by synthesizing lamellar units in the thoracic aorta,8 whereas expansion occurs by widening existing lamellar units in the abdominal aorta.9 While both mechanisms maintain the same ratio of aortic diameter to medial thickness, the difference in assembly may account for the reduced elastin content in the abdominal aorta compared to the thoracic segment.  1.1.3 AORTIC FUNCTION The arterial system can be seen as passive structure that accepts blood in waves from the left ventricle and distributes it to the organs and tissues of the body. As such, it can be said to have two major roles:  Conduit: to distribute blood to the organs and tissues Cushion: to moderate flow pulsations arriving via left ventricular discontinuous ejection Both positions are essential in a functional arterial system and when healthy, the human arterial system is relatively efficient conduit and cushion. Mean blood pressure loss is less than 1 mmHg between the aorta and the radial artery, and further decreases by approximately 2 mmHg for arteries that are less than 0.5 mm in diameter,10 while pulsatile energy loss in the circulation has been shown to be only 10% of steady energy loss.11 One of the earliest models of the arterial system was described in 1733 by Stephen Hales in his treatise on blood pressure, entitled Haemastaticks.12 Interestingly, he was also the first person to measure arterial blood pressure in an animal.10 Hales compared the arterial system to a contemporary fire engine of his day, where an air-filled dome, or Windkessel, would function as a cushion, with an attached hose as a conduit.13In this system, the Windkessel converted pulsatile flow into constant flow, 5  and Hales likened arterial distensibility to the Windkessel, the distributing arteries to the hose, and peripheral resistance to the hose nozzle. This simple system is essentially correct but now viewed as oversimplified because cushion and conduit functions cannot be separated into specific arteries and are performed by the entire system.  A more modern model was proposed by McDonald, Taylor and Womersley in the 1950s,14, 15 where the arterial system was described as a simple distensible tube which connects the heart to peripheral resistance. The analogy used was that of an electrical transmission line with high resistance at the end (peripheral resistance) that creates a mean voltage (pressure). This model can account for both differences in pressure at different points at the same time, and for amplification and reflection of the pressure wave; however, peripheral resistance is restricted to a single reflection point and it is assumed that the distance between the heart and the peripheral resistance is always constant, which is not true for different organs and tissue systems.10   With regards to the aorta, it has been suggested that its main function is to act as the chief cushion in the arterial tree, providing up to 70% of the entire cushioning capacity of the system. Compared to the peripheral arteries, the aorta is passive and does not constrict or dilate.10  Distensibility of the aorta is largely dependent on its elastic components, and the ratio of elastin to collagen in the vessel wall. In the thoracic aorta, this ratio is 6:4 compared to 3:7 in the rest of the arterial tree.16 As the elastic modulus of collagen is greater than 300 times that of elastin,17, 18 the proximal thoracic aorta is typically much more distensible than the distal abdominal aorta. When measured, abdominal aortic stiffness was documented at 569 ? 138 N/cm2, compared to 261 ? 26 N/cm2 in thoracic aorta,19, 20 and this can readily account for the increase in pulse wave velocity from the proximal aorta to the distal abdominal aorta.21-23 6  Additionally, the elastic properties, or the relationship between arterial pressure and diameter in the aorta is not linear and is attributed to different loading on different components in the aorta.18, 24 At low levels of pressure, wall stress is typically supported by the more distensible elastin fibres, whereas at high pressure, support of wall stress is taken on by the stiffer collagen fibres, and these properties help maintain structural integrity of the aortic wall.10   1.2 THE PATHOPHYSIOLOGY OF ABDOMINAL AORTIC ANEURYSM Aneurysm is derived from the Greek word ?????????, meaning widening,25 and is clinically defined as a permanent focal dilation of an artery over 50% from its original diameter and is  characterized by chronic inflammation, destructive remodeling of the ECM and progressive weakening of the vessel wall.26 Approximately 80% of all aortic aneurysms occur in the abdominal region.27  Abdominal aortic aneurysm (AAA) is an increasingly common and frequently fatal clinical condition, responsible for over 15 000 deaths per year in the United States.27    The incidence of AAA measuring 2.9-4.9 cm in diameter ranges from 1.3% in men 45-54 years of age to 12.5% in men 75-84 years of age. In women, prevalence ranges from 0% to 5.2% in the same age groups.28  Rupture occurs in approximately 20% of AAA exceeding 5 cm in diameter but is expected in over 50% of AAA greater than 7 cm.28  Currently, the only effective intervention is open surgical or endovascular repair, however, operative mortality for elective open surgical repair is 5.6%.29  Endovascular repair is associated with lower operative mortality, but carries an increased risk of rupture after four years post-surgery and a greater probability of reintervention.30  Total mortality for rupture has been reported to be as high as 90%;28 many such patients do not reach emergency treatment in time and those that do have high surgical mortality.31  Due to the risk of rupture in combination with the asymptomatic nature of AAA, in 2006 the US Congress passed the Screening for Abdominal Aortic Aneurysm Very Efficiently (SAAAVE) Act for men over the age of 65 as they enrol in Medicare, men between the ages of 65 and 75 who have smoked over 100 cigarettes in their lifetime, and both men and women with a family history of AAA. In 7  September 2008, the Canadian Society for Vascular Surgery recommended a similar screening program. As such, as patient and provider awareness and education increases, it is anticipated that early stage, pharmaceutical options geared towards slowing or preventing aneurysm progression and rupture will be in great demand.27 1.2.1 GENETICS In 1977, a report by Clifton32 about a family of three brothers who all suffered ruptured AAA sparked off an period of intensive research into the genetic basis for the disease. It is now understood that having a first-degree relative with AAA increases lifetime risk approximately 10 fold, with estimates ranging from 18-22%33-35 but no specific gene or mutation has been definitively identified as the cause of familial AAA. Various inheritance patterns have been noted, including male expression bias,36 autosomal recessive,37 and autosomal dominant with low penetrance,38 but given the state of knowledge regarding AAA today, it is far more likely that AAA is a complex, multifactoral disorder with multiple genetic and environmental risk factors39 and possibly multiple pathological mechanisms.  Suggested targets include genes involved in extracellular matrix (ECM) production and degradation, inflammation, cell adhesion, atherosclerosis, SMC contraction, intracellular protein turnover and cell signalling that have been observed to be either up- or downregulated in AAA.40 Additionally, specific polymorphisms of heme oxygenase 1, angiotensin converting enzyme, apolipoprotein E (apoE), plasminogen activator-1 (PAI-1), tissue inhibitor of metalloproteinase-1 (TIMP-1) and many others have been associated with AAA development and pathogenesis.40 Of note, a genome wide linkage analysis mapped a locus for familial AAA to a specific location on chromosome 19q13.3,41 further supporting the hypothesis that development of AAA may have a significant genetic aspect. 8  More recently, several groups have made a concerted effort to further identify circulating biomarkers for AAA.42-44 While not directly looking for genetic risk factors, the identification of novel biomarkers may shed further light on the actual culprits involved in AAA pathogenesis, and may aid with the development of genetic screening assays to identify patients at greatest risk for developing AAA. 1.2.2 CLASSIFICATION AND ETIOLOGY AAA may be classified according to various categorization schemes. These include classification by etiology, location or shape.  Typical etiological categories include:45 - Congenital or developmental e.g. Ehlers-Danlos, Marfan?s syndrome - Mechanical or hemodynamic e.g. Arteriovenus fistula-associated, postenotic - Traumatic e.g. blunt trauma - Inflammatory, non-infectious e.g. Kawasaki?s disease, polyarteritis, Takayasu?s disease  - Infectious e.g. Bacterial, fungal, spirochete infection - Degenerative, non specific e.g. Arteriosclerotic - Anastomotic or postarteriostomy Typical location categories include:25 - Infrarenal (entirely below the renal arteries) - Juxtarenal (infrarenal aorta adjacent to or including the lower margin of renal artery origins) - Pararenal (including the juxtarenal aorta and entire renal artery origins) - Suprarenal (involves the aorta above the renal arteries) Typical shape categories include: 25 - Fusiform - Saccular - Pseudoaneurysm, external diameter dilation without internal dilation Etiologically, the vast majority of AAA is degenerative, 25 and has traditionally been described as a complication of advanced atherosclerosis; however, it is more recently being viewed as a separate entity with similar risk factors. That said, the most significant risk factors for AAA are male gender, cigarette smoking, age, family history, atherosclerotic disease, low high-density lipoprotein (HDL) and high low-9  density lipoprotein (LDL).28, 46, 47 Hypertension is only weakly associated with AAA25 while diabetes may even be protective against development of AAA.48 1.2.2 PATHOPHYSIOLOGY The hallmark of degenerative AAA is a persistent proteolytic imbalance that results in excessive matrix destruction and progressive loss of vessel wall structural integrity.49 AAA are histologically characterized by fragmentation of elastic lamellae and loss of elastin. The medial layer grows thinner with progressive elastin and SMC loss and is eventually remodelled with collagen. Atherosclerosis is often present, but the thin media maybe used to differentiate AAA from atherosclerotic occlusive disease where intimal-medial thickening is prominent.25 A concentric laminated thrombus frequently lines the intimal surface and may preserve normal blood flow but has also been suggested to exacerbate proteolytic degradation of the vascular wall beneath it.50 Another differentiating factor between AAA and atherosclerosis is the location of the inflammatory infiltrate. In AAA, chronic transmural inflammation is a principal characteristic, with mononuclear cells, lymphocytes, blood plasma cells and mast cells infiltrating the media and adventitia, while in atherosclerosis, infiltrating inflammatory cells are largely restricted to the intima. 25 Hemodynamic factors within the abdominal aorta are critical in the development of AAA.51 Not only does the reflection of pulse waves at the iliac bifurcation increase wall stress on the infrarenal aorta,52 but the abdominal aorta is already much less compliant than the thoracic aorta with far fewer elastic lamellar units and a greatly reduced elastin:collagen ratio to begin with.6 Following initial aneurysm formation, further dilation of the aorta is promoted by the increase in tangential stress.25 As aneurysmal disease involves elastin fragmentation and SMC loss, an aortic region that experiences higher wall stress, has fewer lamellar units, decreased elastin content and poorer nutrient delivery to the endogenous cell population is at greater risk of developing AAA.  10  Chronic transmural inflammation is one of the hallmarks of degenerative AAA and while the exact mechanisms involved are unclear, the inflammatory milieu is very likely responsible for the destruction of vascular wall components, directly leading to loss of structural integrity. Many inflammatory molecules have been implicated, such as the chemokines monocyte attractant protein-1 (MCP-1) and interleukin-8 (IL-8).53 It has not yet been determined whether these are the cause of, or the result of inflammation in the aneurysmal aorta. With the destruction of the elastic lamellae, it has been hypothesised that the production of ECM peptide fragments may present chemoattractant activity for macrophages and other inflammatory cells. 25 It is also well documented that the inflammatory milieu surrounding degenerative AAAs produce significant amounts of proinflammatory cytokines such as tumour necrosis factor-? (TNF-?), interleukin-1? (IL-1?), and interferon-? (IFN-?).54 While the lack of a vasa vasorum in the abdominal aorta initially results in poor nutrient delivery to medial SMC, chronic inflammation often results in medial neovascularisation of the aneurysmal wall. This further contributes to AAA development by recruiting more inflammatory cells and providing a direct microvascular route to the tissue for the newly summoned infiltrate.51 Medial neoangiogenesis has been shown to be spatially associated with areas of greatest elastin degradation and inflammatory cell infiltrate.25  SMCs play a fundamental, protective role in maintaining aortic architecture and regulating physiological remodelling processes, however,  aneurysmal media can lose up to 74% of SMC numbers.55 Previous studies have determined that this loss is mostly due to SMC apoptosis. Indeed, the expression of various mediators of programmed cell death such as p53 and p21 have been shown to be upregulated in AAA tissues.56 Infiltrating T cells are also sources of cytotoxic mediators such as perforin and Fas, further promoting SMC loss.57 It has been suggested that ischemia may further damage SMC due to the lack of a vasa vasorum in the abdominal aorta combined with the presence of intramural thrombus and 11  intimal plaque development. As advanced age is a factor in AAA development, SMC senescence may also play a role in SMC loss. Similarly, medial SMC isolated from AAA display a distinct morphological appearance when cultured in vitro, and furthermore have a diminished proliferative ability compared to SMC from controls.58  The effect of the intraluminal thrombus on wall stress is the subject of much controversy. While some believe it may transmit pulsatile stress to the underlying wall, there is also evidence that the presence of thrombus may diminish the tensile stress present because it normalizes blood flow.59, 60 Presence of thrombus results in hypoxia and diminish oxygen and nutrient diffusion to the media,61 and has been associated with rapid dilation62  and increased inflammation, revascularization and wall weakening.63 It has been proposed that the intraluminal thrombus may act as a trap for proteases such as matrix metalloproteinase-9 (MMP-9) and result in increased proteolytic degradation.50, 64  The presence of plasminogen and its activator, urokinase-type plasminogen activator (uPA) has been observed and may result in further activation of MMPs which are known elastolytic and collagenolytic enzymes.50     1.3 THE EXTRACELLULAR MATRIX The extracellular matrix (ECM) is the intricate network of diverse macromolecules that surround the cells in all tissues. It was commonly believed that the matrix served in a purely structural capacity but it is now understood that the ECM takes a very active role in regulating the cells that are embedded within it and is capable of influencing cell development, survival, migration, proliferation, phenotype and function. 65 The ECM is by and large composed of two main groups of macromolecules: 1. fibrous proteins such as elastin, collagen, fibronectin and laminin, and 2. glycosaminoglycans (GAGs). 12  GAGs are long, unbranched polysaccharide chains with a repeating disaccharide unit. Based on dissacharide structure and sulfation, there are four types of GAGs: 1. hyaluronan, 2. chondroitin/dermatan sulfate, 3. heparan sulfate, 4. keratan sulfate. 65 Because of the stiffness and hydrophilicity of the polysaccharide chains, GAGs tend to occupy a very large volume relative to their mass. Their highly negatively charge attracts cations such as Na+, that cause the diffusion of water into the ECM providing the turgor that enables the ECM to withstand compressive forces. 65 GAGs are typically found covalently bound to protein cores except for hyaluronan, forming a class of proteins known as the proteoglycans. Proteoglycans are believed to function as a reservoir for growth factors and cytokines in the ECM and may mediate the growth factor gradients that are critical for guidance in many developmental processes.66 Extracellular proteases, such as the MMPs,  may then regulate growth factor bioavailability by degrading ECM components and releasing sequestered cytokines and activating latent growth factors. 67, 68 However, some growth factors require heparin sulfate as a cofactor in binding their receptors69 and numerous ECM proteins can bind growth factors directly without the involvement of GAGs, 66 while many ECM receptors such as the integrins function as signal transduction receptors, 66 further supporting the active role of the ECM in mediating cell processes eg. vascular endothelial growth factor (VEGF) binds to fibronectin type III domains in fibronectin and tenascin-C, which promotes cell proliferation.70, 71  1.3.1 ELASTIN Elastic fibres in the ECM are found in almost all organ systems and are of critical importance and great abundance in tissues that must withstand periodic stress.72 These fibres are responsible for the pliancy and distensibility in connective tissues and arterial walls, and influence both structure and hemodynamic function in these tissues.73 They consist of an internal core of cross-linked elastin 13  surrounded by a sheath of 10-12nm microfibrils that are largely comprised of two large glycoproteins called fibrillin-1 and -2.72 Although fibrillin-1 and -2 share partially overlapping expression patterns, fibrillin-2 tends to be strongly expressed in developing tissues, with fibrillin-1 being the most abundant isoform throughout life.74  Fibrillin-1 in particular is believed to serve as a scaffold for the deposition of tropoelastin during elastic fibrogenesis,75 may mediate cell binding via ?v?3- and ?5?1-integrins76, 77 and has been shown to associate with focal adhesions.78 Fibrillin-1 regulates the bioavailability of transforming growth factor-? (TGF-?) and mutations in fibrillin-1 result in Marfan syndrome, a connective tissue disorder which is associated with the development of arterial aneurysms, elevated TGF-? signalling and other severe complications in the skeletal, ocular, pulmonary and central nervous systems.79, 80 1.3.2 COLLAGEN Collagens are one of the most important structural proteins in vertebrates and typically the most common, constituting approximately 25% of total protein mass in mammals.65  Within vascular ECM, fibrillar collagens (type I and III) are responsible for the provision of tensile strength and also contribute to the extensile properties of the vessel wall.81 Collagen proteins are characterized by the presence of a triple-helical domain consisting of polypeptide chains with a repeating Gly-X-Y sequence and the ability to form supramolecule aggregates.82 Collagens are comprised of three polypeptide ? chains that each coil into a left-handed helix and associate together to form a right-handed super helix.83 After transcription of procollagen genes, the ? chains are synthesized by endoplasmic ribosomes and the pre-pro-collagen peptide undergoes N-terminal processing to remove signal peptides after translocation across the membrane of the rough endoplasmic reticulum.84 After the hydroxylation and glycosylation of lysines and prolines, 3 pro-peptides associate and twist together to form the procollagen triple helix with the assistance of molecular chaperones such as HSP47.85 The procollagen 14  triple helix is then transported to the Golgi complex, where phosphorylation of serine and sulfation of tyrosine occurs, after which it is secreted outside of the cell. Collagen peptidases cleave off the propeptide to produce tropocollagen which undergo polymerization by lysyl oxidase to form collagen fibrils. Collagen fibrils then group together to form larger collagen fibre bundles. 82, 84 Decorin is a small leucine-rich proteoglycan expressed by fibroblasts, myocytes and SMC 86, 87 and is believed to regulate numerous functions in the ECM, including the regulation of collagen fibrillogenesis,88 collagen degradation,89 cell growth90-92 and cell signalling.93, 94 Decorin is composed of a 40kDa core protein called decoron, which is attached to a single chondroitin/dermatan sulfate GAG chain.95 Decoron functions as an anchor between decorin molecules and collagen fibrils, providing elasticity and improving the tensile properties of collagen.96, 97 During collagen fibrillogenesis, decorin is believed to regulate fibril formation, fusion and organization.88 In addition to these structural roles, decorin binds and sequesters all three isoforms of TGF-?. When bound to collagen, this interaction with TGF-? occurs directly to decoron and not via the GAG chain. The degradation of decorin during inflammation and remodelling increases the bioavailability of TGF-?68, 98, 99 and has implications for the mediation of inflammation, and the inhibition of cell proliferation.    1.4 THE ROLE OF IMMUNE-DERIVED PROTEASES IN AAA AAA is characterized by a persistent proteolytic imbalance that results in the destruction of the vascular wall.49 In section 1.4 and 1.5, the role of key immune-derived proteases involved in the pathogenesis of AAA will be described. They are summarized in Table 1. .   15  TABLE 1: IMMUNE  PROTEASES IN AAA Enzyme Source within the aneurysmal aorta Pathological contribution  Extracellular Endogenous inhibitor  Elastase ? Macrophages ? Neutrophils ? SMC ? Cleavage of elastin and destruction of elastic lamellae ? Generates proinflammatory elastin fragments (elastokines) ? ?-1-antitrypsin  Chymase ? Mast cells ? Activating MMPs and cathepsins ? resulting in elastic lamellar fragmentation ? Promote immune infiltration  ? SMC apoptosis ? Neovascularization ? Convert Ang I to Ang II ? ?1-antichymotrypsin ? ?2?macroglobulin ? ?1-protease inhibitor Tryptase ? Mast cells ? Activating MMPs and cathepsins ? resulting in elastic lamellae fragmentation ? Promote immune infiltration  ? SMC apoptosis Unknown Cathepsins ? Macrophages ? Neutrophils ? SMC ? Mast cells ? Cleavage of elastin and destruction of elastic lamellae ? Generates proinflammatory elastin fragments (elastokines) ? Promote immune infiltration ? Promoting angiogenesis ? Activating MMPs ? Cystatin C   MMP-9 ? Macrophages ? Neutrophils  ? Cleavage of collagens (I, IV, V, VII, X, XI, XIV), elastin, fibronectin and plasminogen ? Promote rupture ? Activate VEGF, angiogenesis ? Activate, liberate TGF-?  ? Macrophage migration ? TIMP-1 ? TIMP-2 ? TIMP-3 ? TIMP-4   16  TABLE 1: IMMUNE PROTEASES IN AAA (cont?d) Enzyme Source within the aneurysmal aorta Pathological contribution  Extracellular Endogenous inhibitor  MMP-2 ? SMC ? Fibroblasts ? Macrophages ? Cleavage of collagens (I, IV, V, VII, X, XI, XIV) elastin, fibronectin, laminin, MMP-1, MMP-9 and MMP-13 ? Activate, liberate TGF-?  ? Promotes invasive migration of VSMCs ? TIMP-1 ? TIMP-2 ? TIMP-3 ? TIMP-4 MMP-1 ? Fibroblasts ? SMC ? Endothelial cells ? Monocytes ? Cleavage of collagen (I, II, III, VII, VIII, X), aggrecan, gelatine, MMP-2 and MMP-9 ? TIMP-1 ? TIMP-2 ? TIMP-3 ? TIMP-4 MMP-12 ? Macrophage ? Foam cell ? Cleavage of collagen IV, gelatine, elastin, fibronectin, casein, fibrinogen, plasminogen, MMP-2 ? Highest affinity for elastin and elastolytic cleavage ? TIMP-1 MMP-13 ? SMC ? Cleavage of Collagens (I, II, III, IV, IX, X, XIV) gelatin, MMP-9 ? TIMP-1 ? TIMP-2 ? TIMP-3  MT1-MMP ? SMC ? Macrophage ? Activate pro-MMP2 ? Promote macrophage-dependent elastolysis ? TIMP-1 Granzyme B ? CTL ? NK Cell ? Treg ? Basophils ? Type II pneumocytes ? Macrophage ? Mast cell ? Keratinocytes ? Chondrocytes ? Cleavage of fibrillin-1, decorin ? Disruption of elastic lamellae ? Dysregulation of adventitial remodelling ? Potentiates pro-inflammatory activity of IL-1? ? Serpin A3N (only found in mice)  (Adapted from Hendel et al. Curr Vasc Pharmacol, (2012).100   17  1.4.1 MATRIX METALLOPROTEINASES Matrix metalloproteinases (MMPs) are a large family of endopeptidases that are required for physiological homeostasis and remodelling processes in the ECM but have also been implicated in pathological disease conditions involving tissue degradation and degeneration, such as rheumatoid and osteo-arthritis,101, 102 osteoporosis,103 and metastasis.104 Given that dysregulation of ECM turnover is a hallmark of AAA pathogenesis, many studies have focused on the role of MMPs in AAA development and progression. The link between MMPs with aortic degeneration was first noted in 1991, when Senior et al. demonstrated that both MMP-2 and MMP-9 were capable of cleaving elastin.105 Since then, the investigation of MMPs in aneurysmal disease has largely focused on these two proteases and their corresponding endogenous inhibitors. Of the two, MMP-9 is arguably the most prominent in the pathogenesis of AAA.106  MMP-9 expression and activity is upregulated in human AAA,107-109 and is significantly increased at the site of aneurysmal rupture.110, 111 While absent in normal tissues,112 it is the predominant source of gelatinolytic activity in the luminal and parietal thrombus.64 Furthermore, plasma levels of MMP-9 are elevated in AAA patients113 and Type A and B aortic dissection within an hour of onset of symptoms compared to controls.114  Given the nature of MMP involvement in aneurysm pathology, numerous studies have attempted to demonstrate an association between MMP levels, their proteolytic activity in the aorta and the size and frequency of rupture. Freestone et al. identified MMP-2 as the principal source of gelatinase activity in small aneurysms (4.0-5.5 cm), whereas zymography data indicated a size-associated increase in MMP-9 activity in larger aneurysms (5.5-10 cm).115 A subsequent study by McMillan et al. reported a four-fold elevation in MMP-9 mRNA transcript levels as measured by 18  competitive PCR in medium-sized AAA (5.0-6.9 cm) compared to small AAA (3.0-4.9 cm), large AAA(>7 cm) and normal aortas.116 In comparison, Petersen et al. measured MMP-2 and MMP-9 levels by ELISA in medium-sized ruptured AAA (5-7 cm) and large asymptomatic unruptured AAA (>7 cm).111 They determined that MMP-9 levels were significantly higher in medium ruptured AAA compared to large asymptomatic AAA, whereas MMP-2 levels were higher in asymptomatic AAA and demonstrated a positive correlation with diameter of asymptomatic AAA. Interestingly, they did not find a correlation between MMP-9 levels and aortic diameter, suggesting that increased MMP-9 levels may influence aortic rupture independently of dilation.  MMP-9 colocalizes to infiltrating macrophages in AAA in the media and media-adventitia junction.107, 117 The preferential localization to discrete areas adjacent to the adventitial vasa vasorum suggests that MMP-9 is involved in matrix remodelling processes associated with neovascularisation.116 While macrophages are the primary source of MMP-9 in the aortic wall, polymorphonuclear leukocytes are responsible for the majority of MMP-9 release in the mural thrombus.50 MMP-9 synthesis by vascular smooth muscle cells (VSMCs) is also elevated when compared to normal and atherosclerotic controls.118  Several animal models have been employed to investigate the role of MMP-9 in AAA. MMP-9 deficiency has proven protective against aortic dilation and progressive destruction of elastin induced by elastase infusion119 and calcium chloride application.120 Similar to human AAA, histological analysis confirmed that macrophages are the primary source for MMP-9 in the animal models.  In the latter study, it was found that the reinfusion of competent macrophages from wild-type mice into knockout (KO) animals was sufficient to reconstitute AAA in MMP-9-KO but not MMP-2-KO mice, suggesting that these two MMPs may work in concert in the pathogenesis of AAA, with MMP-2 enhancing the degenerative abilities of MMP-9.  19  Much attention has been given to the gelatinolytic and elastinolytic activity of the MMPs; however, there is growing interest in other functions that MMPs may serve in AAA pathogenesis. VEGF bioavailability is regulated extracellularly by numerous MMPs through intramolecular processing. Lee et al. have demonstrated that MMP-3, MMP-7, MMP- 9 and MMP-19 are able to cleave VEGF in an efficient manner, releasing a soluble, biologically active 16 kDa fragment that phosphorylates VEGF receptor 2 (VEGFR2) and induces angiogenesis121 VEGF has also been shown to be upregulated in human AAA 122, 123 and exacerbates the development of angiotensin-II-induced AAA in mice.124 Similarly, MMP-2 and MMP-9 are capable of liberating and activating TGF-? from the inactive Large Latent Complex (LLC).125 The exact role of TGF-? in the pathogenesis of AAA is unclear given that it has both angiogenic and anti-angiogenic properties,126 and has been shown to promote both the synthesis and degradation of the ECM.127 However, dysregulation of TGF-? signalling is recognised as a characteristic of Marfan syndrome,128 where patients frequently suffer from vascular defects including dissection and aneurysm.  Compared to MMP-2 and MMP-9, little is known about the involvement of other MMPs in aneurysm pathogenesis. MMP-12 is selectively expressed in macrophages and foam cells in AAA and atherosclerotic lesions129 and it has been suggested that MMP-12 is responsible for up to 95% of macrophage-mediated elastin degradation in AAA.129 Like all other MMPs, MMP-12 is secreted in inactive pro-enzyme form and requires extracellular activation. Interestingly, mice deficient in both apoE and uPA exhibit reduced pro-MMP-12 activation and are protected against medial destruction and aneurysm formation.130 IL-13 induction of MMP-2, MMP-9, MMP-13 and MMP-14 is decreased in the absence of MMP-12,131 suggesting that IL-13-induced inflammatory infiltration is, at least in part, mediated by an MMP-12-dependent pathway. Taken together, there is sufficient evidence to support a role for MMP-12 in AAA pathogenesis; however, results from animal studies have been conflicting. Pyo et al. reported that MMP-12-KO mice were not protected against elastase-induced aneurysmal degeneration and dilation, with similar incidence rates and elastin loss compared to wild-type 20  controls;119 whereas Longo et al. showed that MMP-12-KO mice exhibit a reduction in aortic dilation, loss of elastic lamellae and macrophage infiltration following calcium chloride treatment.129  It is probable that these differences are model-specific and it is still unclear which model is a more accurate representation of human AAA.  Membrane-type-1 MMP (MT1-MMP) is 66 kDa132 integral plasma membrane protein that functions at the cell surface and is the dominant activator of MMP-2. Nollendorfs et al. were able to localize MT1-MMP to both VSMCs and macrophages and show elevated levels of MT1-MMP mRNA and protein in human AAA.133 In a rat model of elastase-infusion, early MT1-MMP expression was associated with increased activation of MMP-2134 whereas MT1-MMP protein levels were shown to increase progressively in a murine calcium chloride-induced thoracic aneurysm model for 16 weeks post- induction.135 While known substrates for MT1-MMP include collagen (I, II, III), gelatin, casein, elastin, fibronectin, laminin, MMP-2 and MMP-13,106 MT1-MMP is of interest in the context of AAA pathogenesis primarily because of its ability to activate pro-MMP-2. However, a recent study by Xiong et al. has demonstrated that MT1-MMP can regulate macrophage-dependent elastolytic activity in AAA development independently of MMP-2 and MMP-9.136   Additional roles for MMP-1 and MMP-13 have also been implicated in AAA pathogenesis. Elevated levels of MMP-1 protein and mRNA have been observed in aneurysmal adventitia 137, 138 and elevated plasma levels of MMP-1 are associated with AAA rupture 139. Lizarbe et al. found that inhibition of inducible NO synthase (iNOS) protects against elastase-induced AAA by inhibiting production of MMP-13140 and that MMP-13-KO mice exhibit increased extracellular MMP inducer (CD147) and reduced aortic dilation following elastase infusion,140 suggesting that NO may contribute to AAA pathogenesis by modulating the activity of MMP-13.  21  Due to the significant roles of MMPs in AAA pathogenesis it is essential to explore the role of their counterpart inhibitors during disease progression. Tissue inhibitors of matrix metalloproteinases (TIMPs) are the endogenous inhibitors of the MMPs and represent a fundamental level of MMP regulation. TIMPs comprise 4 known glycoproteins: TIMP-1, TIMP-2, TIMP-3 and TIMP-4, of which TIMP-1 has received the most scrutiny.  TIMP-1 inhibits MMP-1, MMP-2, MMP-3, MMP-9 and MT1-MMP 106 and its expression is localized to VSMCs116, 141 but can be induced in vitro by numerous other cell types such as endothelial cells (EC) and fibroblasts in response to external stimuli.142 By blocking MT1-MMP and MMP-3, TIMP-1 effectively downregulates the activation of MMP-2 and MMP-9 respectively.143 Plasma levels of TIMP-1 are significantly elevated in AAA patients compared to atherosclerotic or healthy controls144 but curiously, TIMP-1 mRNA levels are 250 times higher than MMP-1 and 11 times higher than MMP-9 levels but protein ratios of MMP-1:TIMP-1 and MMP-9:TIMP-1 are significantly higher in aneurysmal aortas.138  Some of the strongest experimental data supporting a protective role for TIMP-1 in aneurysm development comes from studies in mice where the effect of TIMP-1 deficiency and overexpression was assessed. Silence et al. were able to show that TIMP-1/apoE-double KO (DKO) mice on cholesterol rich diets had increased incidence of aneurysm compared to TIMP-1 +/+ mice.145 TIMP-1 deficiency resulted in enhanced MMP activity and macrophage infiltration.145 These observations were confirmed by Lemaitre et al. who additionally reported increased medial degradation and incidence of pseudoaneurysm in TIMP-1/apoE-DKO mice.146 TIMP-1 deficient mice also show increased susceptibility to aneurysm induction by elastase infusion, presenting with higher incidence and greater severity.147 Finally, overexpression of TIMP-1 was able to prevent aortic degeneration, dilation and rupture in a rat xenograft model of AAA.143  22  Cultured VSMCs from both AAA and healthy patients constitutively express both MMP-2 and TIMP-2, its endogenous inhibitor 106 but increased expression of TIMP-2 and MMP-2 in AAA colocalizes to both VSMCs and macrophages.106, 141 Because the activation of MMP-2 requires the formation of the trimolecular complex of MT1-MMP, TIMP-2 and pro-MMP-2, TIMP-2 can theoretically both promote and inhibit matrix degradation in AAA. Xiong et al. probed this question by inducing aneurysm in TIMP-2-/- mice by application of calcium chloride; however, they concluded that the targeted deletion of TIMP-2 attenuates aneurysmal development, in spite of its supposed role as an MMP inhibitor 148 implying that the relationships between the MMPs and TIMPs may be more complex than initially assumed, and warrants further investigation.  Of all the MMPs involved in AAA pathogenesis, MMP-2 and MMP-9 are the most well characterized and evidence from both experimental and observational studies may imply that inhibition of these proteases will be a beneficial treatment approach for AAA. In that respect, doxycycline has been examined as potential drug that can inhibit MMPs activity. Doxycycline is a synthetic tetracycline derivative that functions both as an antibiotic and a broad-spectrum MMP inhibitor. It binds to the MMP active site and calcium ion site and induces a conformational change that results in the loss of enzymatic activity.149, 150 Doxycycline also suppresses activation of pro-MMP-2 and inhibits transcription of MMP-9 in macrophages.151, 152 In vitro studies show that doxycycline reduces MMP-9 activity and prevents elastin degradation153 while data from animal studies show that doxycycline treatment can inhibit MMP activity in vivo, preventing elastin degradation and aortic dilation in both an elastase-infusion rat model154 and dose-dependently in a murine calcium chloride model.155 A preliminary study in AAA patients (100 mg doxycycline twice daily for 3-6 months) achieved similar plasma levels to murine models where aortic dilation was reduced by 33-66%.155 Curci et al. reported that patients who were given doxycycline for 3 weeks prior to conventional aneurysm repair presented with significantly lower MMP-9 expression and activity in the aortic wall compared to controls152 while Lindeman et al. showed 23  that a 2-week regimen of doxycycline treatment prior to repair profoundly reduced aortic wall neutrophil and cytotoxic T cell infiltration.156 A small study by Baxter et al. showed significant reduction in MMP-9 plasma levels after 6 months of doxycycline treatment157 but in a follow-up study on aneurysmal growth rate, the difference between treated and control groups did not achieve statistical significance.158  While direct inhibition of MMP activity might appear to be an attractive target for therapeutic intervention in AAA, it would be unwise to ignore the results from the use of broad spectrum MMP inhibitors in anti-cancer drug development programs where immensely promising preclinical results were subsequently followed by complete failure in clinical trials.159 Firstly, many MMPs play important physiological roles in maintaining cellular homeostasis in normal tissues and in regulation of extracellular signalling networks;160-162 thus, the use of broad spectrum inhibitors to block the pathological aspects of MMPs may have deleterious effects on overall patient outcome that may be masked in small studies when these ?anti-target? functions of MMPs are affected.161 Significant proportions of patients in these clinical trials developed severe musculoskeletal side effects,163-165 necessitating the lowering of drug dosage below minimum inhibitory levels, confounding the results and indicating a very low therapeutic index.166 These clinical trials were also performed with patients with advanced progression of disease while preclinical data indicated that MMP inhibition would be preferentially effective in earlier stages of disease.161, 167 Similarly in AAA, differential effects of MMPs have been observed at different stages of aneurysm growth,106 suggesting that targeted inhibition of individual MMPs could prove more beneficial than broad spectrum inhibition with less severe side effects. Besides blocking MMP activity directly, another option that has been considered is to target the inflammatory response which activates and induces MMP expression. Non-steroidal anti-inflammatory 24  drugs (NSAIDs), such as indomethacin, reduce IL-1? and IL-6 release in aortic explants and have been shown to reduce MMP-9 activity and attenuate elastase-induced aneurysm in rats.168 Indomethacin also downregulates synthesis of prostaglandin E2 (PGE2), which in turn regulates MMP-9 expression, reducing the inflammatory response and aortic dilation.169 There has been much interest in the use of statins, or 3-Hydroxy-3-methylglutaryl coenzyme A  (HMG CoA) reductase inhibitors as part of AAA therapeutic strategy because not only do they reduce MMP-9 secretion by macrophages,170 but also because of its beneficial effects on endothelial function, prevention of lipoprotein cholesterol oxidation, inhibition of macrophage and VSMC migration stabilization of atherosclerotic plaque and anti-inflammatory effects.171 Initial studies were performed with cerivastatin, which has since been withdrawn from the market. However, at maximum clinical doses, cerivastatin downregulated the production of MMP-9 in explanted AAA organ cultures.172 Patients given a 3-week pre-operative regimen of simvastatin also demonstrated reduced MMP-9 activity in the aortic wall.173 Observational studies following the growth of untreated AAA in patients have reported that statin use attenuates or even halts expansion completely174, 175 independently of other cardiovascular risk factors.   1.4.2 ELASTASE Destruction of the elastic lamellae is one of the major hallmarks of aneurysmal disease, and this can lead to weakening of the aortic wall and subsequent dilation.176 Quantification of protein content indicates up to 90% loss of elastin in human AAA specimens compared to healthy controls.177, 178 The degradation of elastin by elastase may contribute to AAA development not only through reduction in structural stability but also through the generation of elastin-derived peptides that may serve as chemoattractant for inflammatory cells, further exacerbating inflammation in the aneurysmal aorta.179 25  As such, these observations formed the basis for the development of the widely used rodent elastase infusion model of AAA.180  Although there are known inconsistencies in outcome with this model, primarily due to variation in the preparation of elastase,181 as well as the relatively complicated and invasive experimental procedure required, many recent studies have relied heavily on this model to investigate AAA pathogenesis. Although the degradation of elastin may serve as a primary event in AAA pathogenesis, 182 the use of elastase infusion may overshadow other initiating factors leading to this disorder and hinder the study of endogenous elastases involved in the progression of disease.  Evidence for the role of elastase in AAA development originated from earlier studies examining both local and systemic levels of elastase in AAA patients. 183-185 Sources for elastase include neutrophils, macrophages (MMP-12, discussed previously) as well as pancreatic and smooth muscle cells 186 Neutrophil elastase is highly upregulated throughout the adventitia 187 and within the intraluminal thrombi of aneurysmal aorta,188 providing two sources for this enzyme from opposite sides of the aortic wall. Studies performed with neutrophil-depleted mice have shown a significant reduction in aneurysm formation,189 however the role of neutrophil elastase per se was not directly evaluated in the this study due to the use of the elastase infusion model. Interestingly, blood samples of AAA patients show increased neutrophil elastase levels in both pre-operative183 as well as post-operative patients with an increase in neutrophil elastase secretion in the latter group when compared to patients with occlusive aortic disease.190 These findings suggest that neutrophil elastase release may serve as a primary event in aneurysm development that is independent of the presence of the aneurysm itself as levels of this enzyme remain high six months after aortic repair surgery. 190 Most importantly, this may also highlight the need for continued anti-elastase therapy after aortic repair as elevated elastase levels will likely continue to exert its harmful effects, hastening the need for reintervention in the future. Interestingly, it 26  has also been found that the elastase activity in aortic tissue of aneurysm patients without concomitant occlusive disease was significantly higher than in aneurysm patients that were diagnosed with occlusive disease, or occlusive disease alone,183 suggesting that elastase-dependent mechanisms may play a more significant role in non-atherosclerotic AAA, and may have ramifications for the development of treatment strategies in these patients. The observation of elevated elastase levels in AAA patients led researchers to examine the relationship between this protease and its endogenous inhibitor, alpha-1-antitrypsin (?1AT).  ?1AT deficiency is known to play a major role in the development of lung emphysema. Reduced inhibition of neutrophil elastase leads to increased proteolytic activity resulting in degradation of the alveoli wall.191 Thus, it was postulated that an imbalance between elastase and ?1AT may also play a role in AAA pathogenesis. This imbalance was indirectly demonstrated by comparing the inhibition of exogenous elastase by plasma derived from either healthy controls or AAA patients. Plasma from healthy controls demonstrated increased elastase inhibition compared to plasma from AAA patients, indicating a reduction of anti-protease activity in AAA plasma; however, a direct measurement of ?1AT in these samples was not performed.182 On the other hand, there is no increase in the frequency of ?1AT gene deficiency in AAA patients compared to the general population, suggesting that changes in ?1AT activity in AAA patients are independent of gene expression.192 A recent study demonstrated that ?1AT protein levels are reduced in human ascending aortic aneurysm and aortic dissection compared to control aortas, without significant changes in ?1AT mRNA levels between aneurysm tissues and control.193 While augmentation therapy using exogenous purified ?1AT administration is approved by the US Food and Drug Administration (FDA) for ?1AT deficient patients with evidence of respiratory disease,194 this treatment approach remains controversial with a major concern being the excessive cost for treatment. Interestingly, peri-adventitial treatment with an elastin-stabilizing agent, pentagalloyl glucose (PGG), protected rat aorta from developing AAA after peri-aortic application of calcium chloride (CaCl2) 27  solution.195 Furthermore, administration of PGG on established aneurysm, 28 days post-CaCl2 application, resulted in attenuation of AAA progression and reduction in aortic diameter was observed. It was suggested that PGG binds to the elastic lamellae and prevent elastin degradation when faced with the mounting immune response that develops in the aorta of these rats.195 While in its current state, PGG treatment must be administered peri-adventitially and thus rendered less relevant as a pharmacological agent,  it may still be useful as an adjunct treatment during aortic surgical repair to minimize post-surgery aneurysm recurrence.195  1.4.3 CATHEPSINS Cysteine cathepsins are a family of lysosomal proteases that are potent elastolytic and collagenolytic enzymes.196 Due to their involvement in numerous chronic inflammatory disease such as atherosclerosis,197 arthritis 198 and emphysema,199 their expression and release by macrophages, SMC,200 neutrophils and mast cells, and their potent elastolytic abilities,201 cathepsins are strong candidates for key roles in AAA onset and/or progression.202  Of the 11 known cysteine cathepsins, cathepsin K has the most potent elastolytic ability,196while cathepsin K, L and S have demonstrated collagenolytic enzyme activity at physiologically relevant temperatures, a function that was previously only attributed to the MMPs.203 Several studies have demonstrated a significant increase in both protein expression and activity of cathepsins B,H, S, K, and L in human AAA samples compared to control aortas.49, 200, 202, 204, 205 Increased activity of cathepsins B, H, L, and S appear to be atheroma-independent as the activity of these proteases have been shown to be higher in AAA compared to atherosclerotic aortas.205 Furthermore, elevated activity of cathepsin S was observed in both growing aneurysm and ruptured human AAA samples, indicating a potential causative role for this particular cathepsin in disease progression.49  28  The original evidence for cathepsin involvement in aneurysm pathogenesis came from experiments showing dysregulated cathepsin activity during disease progression. A number of studies have shown a significant reduction in cystatin C, the endogenous extracellular inhibitor of cathepsins, in human AAA specimens compared to controls and is associated with aneurysm progression and increased expansion rate.49, 202, 205-208 Cystatin C deficiency was observed both at the tissue level and in the serum, indicating a potential imbalance between cathepsins and their inhibitors resulting in a net increase in cathepsin activity during AAA development.202, 207, 208 Pro-inflammatory cytokines,209 reduced TGF-? signalling207 and degradation of cystatin C by neutrophil-derived proteases such as neutrophil elastase and MMP-8,49 are all mechanisms that may alter the balance  between cystatin C and cathepsins in aneurysmal aortas. Furthermore, it has been shown that cystatin C/apoE-DKO mice exhibit increased cathepsin activity compared to wild type (WT) control mice210, 211 and experience greater aortic dilation, increased elastic lamina degradation and elevated immune infiltration. As chymase and tryptase can regulate the activity and expression of cathepsins, reduced cathepsin activity has also been attributed to chymase and tryptase regulation as deficiencies of the latter proteases are protective against AAA development in mice.212, 213  Cathepsin C (also referred as dipeptidyl peptidase I (DPPI), a protease responsible for the activation of zymogen granule enzymes, may also contribute to AAA pathogenesis through increased activation and release of granule enzymes and promoting neutrophil recruitment to the affected aorta.214 Cathepsin C expression is associated with AAA development while deficiency of this protease results in reduced aneurysm formation, greater preservation of elastic lamellae and reduced inflammation.214  Various cathepsin-KO mice models have been developed and it has been demonstrated that cathepsin L deficiency leads to reduced AAA development, reduced elastin fragmentation and reduced 29  inflammation in elastase infused aortas and in periaortic CaCl2 treated mice,215 while cathepsin S deficiency reduced aortic lesion expression and activity of matrix metalloproteinase MMP-2, MMP-9, and cathepsin K as well as significantly reducing AAA lesion media SMC apoptosis and inflammatory cell accumulation and proliferation.216 Cathepsin S was also shown to promote angiogenesis, monocyte and T-cell transmigration, and T-cell proliferation in vitro.216 Contradictory observations have been published regarding the effect of cathepsin K deficiency in animal models of AAA. Bai et al, reported that cathepsin K deficiency did not result in any difference in aneurysm development or changes to the elastic lamellae in Angiotensin II (Ang II)-infused apoE-KO mice,217 while Sun et al. reported that cathepsin K deficiency did result in reduced aneurysm formation, reduced elastin fragmentation, reduced SMC apoptosis and lower inflammatory infiltrate using the elastase infusion model of AAA.218 These contradicting results may be attributed to the difference in experimental models used to induce AAA in these mice. In elastase-infused mice, inflammation is stimulated  locally, while in the Ang II model a more systemic induction occurs, which may promote peripheral inflammatory cell recruitment to the aorta.218 Although not directly assessed, it was hypothesized in both manuscripts that changes in inflammatory status due to Ang II infusion might support further activation of immune cells at the site of aneurysm development leading to overexpression and activation of other proteases and cathepsins, such as cathepsin S and cathepsin C, leading to increased proteolytic activity exerted by these enzymes, potentially obscuring the effect of cathepsin K in AAA development.217, 218   Although the physiological inducers of cathepsin activation and release in human AAA are yet to be determined, the observations from both human AAA tissue and animal models of AAA demonstrate the importance of maintaining the balance between cysteine cathepsins and their inhibitors, and that 30  dysregulation of cathepsin activity may contribute to loss of arterial integrity, vascular remodelling and aortic dilation in AAA.  1.4.4 CHYMASE AND TRYPTASE Chymases and tryptases are serine proteases that are largely restricted to mast cells where they constitute the majority of endopeptidases stored within the secretory granules.219 A potential role for chymases and tryptases in the development of AAA was suggested when an increase in mast cells was observed in the media and adventitia of AAA lesions.220, 221 The number of mast cells in the outer media and adventitia of human AAA specimens was significantly higher than the numbers typically observed in both early and advanced atherosclerotic lesions, as well as healthy aorta. Mast cell count increased up to 4-fold in aneurysmal non-ruptured aorta compared with non-dilated aortas in autopsy cases, indicating the specific localization of mast cells to the aneurysmal tissue.221 Furthermore, mast cell numbers in human AAA correlated with the degree of aortic dilation observed.221 While the exact mechanism involved in mast cell recruitment is yet unclear, elevated mRNA and protein levels of leukotriene C4 (LTC4) and its three key enzymes (5-lipoxygenase (5-LO), 5-LO-activating protein, LTC4 synthase) were localized to regions of increased inflammatory cell infiltrate, suggesting that LTC4 participates in mast cell recruitment in aneurysmal development. Direct involvement of mast cells in AAA has been shown in various experimental animal models. In both the periaortic CaCl2 model and the elastase-infusion model, the lack of functional mast cells in KitW-sh/W-sh mice protected the animals from AAA development.212, 222 Mast cell deficiency reduced medial elastin degradation, SMC loss, adventitial angiogenesis and lesion activity of various cysteine cathepsins, MMP-2 and MMP-9. Reduced AAA formation, levels of inflammatory cytokines and protease activity was restored in KitW-sh/W-sh  mice that received exogenous WT mast cells.222 Mice treated with mast cell activator, compound 48/80, exhibited increased AAA dilation, upregulated protease expression and 31  greater elastin loss compared to controls, while mice treated with mast cell stabilizer, Cromolyn, saw a suppression of similar phenotypes.222 Similar results were observed in rats, where an increase in mast cells was observed at 3,7 and 14 days after periaortic CaCl2 application.221 The absence of mast cells in Ws/Ws rats or treatment with mast cell stabilizer, tranilast, preserved elastic lamellae, reduced MMP-2 and -9 activity and prevented AAA development.221  These findings support a regulatory role for mast cells in AAA development and preventing mast cell degranulation may prove to be an attract therapeutic target, and justify further study of mast cell recruitment and activation mechanisms in the aneurysmal aorta. The mast cell-specific proteases include chymase, tryptase and carboxypeptidase A and together they constitute 25% of total cellular protein, of which tryptase accounts for nearly 20%.223 Mast cell accumulation in AAA lesions are typically confirmed by the use of chymase or tryptase antibody-mediated immunohistological analysis.224 In a prospective clinical follow-up study, it was determined that plasma levels of chymase in Danish men correlated with annual AAA growth rate but was not associated with initial AAA size.225 In a separate study, plasma tryptase levels were significantly elevated in AAA patients compared to controls, although many in the AAA group were smokers and had other cardiovascular complications; however, tryptase levels also correlated with annual growth rates and regression analysis confirmed that higher tryptase levels indicated a higher likelihood of requiring later surgical repair even after adjustment for more than 10 potential AAA risk factors.213   The substrate specificities of both tryptase and chymase makes them pertinent to AAA pathogenesis. These include several ECM proteins including procollagen, VE-cadherin, fibronectin and vitronectin (reviewed in 224, 226). Cleavage of fibronectin and vitronectin may lead to the disruption of the SMC focal adhesion complex and subsequent apoptosis.224 Chymase also inhibits SMC collagen synthesis227 while tryptase induces the endothelial cell production of chemokines IL-8 and MCP-1228 and 32  both have been demonstrated to activate endothelial protease activated receptor-2 (PAR-2)224, 229 which may affect regulation of vascular tone and permeability and contribute to pro-inflammatory response as observed in atherosclerosis.230 While the ability of these enzymes to directly degrade the elastic lamellae is unclear, the potential role of chymase and tryptase in aortic wall deterioration may also rely on their ability to regulate MMP and cathepsin activity.231, 232 Both proteases activate MMP-1 and MMP-3, which subsequently activate MMP-9.233, 234 Furthermore, chymase converts angiotensin I (Ang I) to Ang II .235 Ang II may play a significant role in AAA development as it promotes inflammation within the aorta, while Ang II infusion is a widely used mouse model of AAA and is discussed in section 1.6.236 In this model, oral administration of chymase inhibitor, NK3201 significantly reduced AAA development, monocyte recruitment, and MMP-9 expression and activity.237 Similar results were obtained using this inhibitor in elastase-perfused dogs238 and hamsters.239  In experiments using elastase-infused mice deficient in mouse mast cell protease- 4 (mMCP-4), the most relevant human chymase ortholog, it was shown that these mice demonstrate significant attenuation of aneurysm formation, reduced immune infiltration, higher medial SMC content and less neovascularisation within the adventitia.212 Reduced  expression and activity of cysteine cathepsins, chymase-mediated SMC apoptosis and chymase-mediated angiogenesis in these mice were also noted when compared to WT controls.212 Mice deficient in mouse mast cell protease-6 (mMCP-6) or murine tryptase, did not develop aortic dilation at 7, 14 or 56 days post elastase-perfusion. Lesions macrophage and T cell infiltration and major histocompatibility complex class II (MHC-II) molecules were also greatly reduced,213 while endothelial cells from Mcpt6-/- mice had significant reduction in the production of  TNF-?, IL-6 and of chemokine KC (IL-8 homolog), macrophage inflammatory protein-2 (MIP-2) and lipopolysaccharide-induced CXC (LIX); all of which may contribute to reduced leukocyte infiltration in the vascular tissues.213 33  Deficiency in mMCP6 has also resulted in reduced SMC apoptosis, reduced cysteinyl cathepsin activity and decreased immune infiltration. However, as opposed to mMCP4, mMCP6 mice exhibited no effect on neovascularization 213. A discussion of the potential roles for chymase and tryptase in AAA would not be complete without an investigation of their endogenous inhibitors within the context of this disease; however, knowledge in this area is lacking due to scarcity of known specific endogenous inhibitors for these enzymes. Earlier studies have identified ?1-antichymotrypsin (ACT), ?2?macroglobulin and ?1-protease inhibitor as probable inhibitors of chymase, however their inhibitory effect in vivo still requires clarification as chymase in its physiological role is conjugated with heparin proteoglycan and is resistant to inhibition by these inhibitors.240  Endogenous inhibitors for tetrameric tryptase have yet to be identified.226 For this reason, most work in this area has been devoted to the development of small molecule inhibitors against these enzymes. Although most of these synthetic inhibitors were developed to inhibit human chymase and tryptase, to date, they have only been tested in animal models of disease, thus the efficacy of these inhibitors in humans remains to be determined.226 This issue may become more complicated when interpreting data from studies in mice, considering that there are  multiple gene orthologs of chymases (mMCP-1, -2, -4, -5, and -9) that are present in mice but absent in humans.226 Further, chymase and tryptase are involved in a complex regulatory network that controls of the activity and expression of other proteases (MMPs, cathepsins) both in mast cells and in other cell types (SMC, monocytes),  regulates leukocyte migration and the modulation of the cytokine milieu during inflammation. Moreover, chymase and tryptase may also have anti-inflammatory roles as they can degrade key inflammatory cytokines and this is highly suggestive of a role for them in dampening the immune response.219 Extreme caution will be required in pursuing chymase and tryptase inhibition as a therapeutic strategy to treat AAA since interruption of their physiological activity may have immense impact on several pathways which are currently either unknown or ill defined. Nevertheless, 34  the relative clinical success of mast cell stabilizers in asthma, as well as the promising results from studies using specific chymase inhibitors in several animal models of AAA,237, 238, 241 do indicate significant potential for this treatment approach.  1.5 GRANZYME B In 1978, Hatcher et al. isolated a cytotoxic protease from lymphocytes that was capable of inducing apoptosis in target cells.242 Shortly after, the granzymes, or granule enzymes,  were identified as a family of conserved serine proteases found in the granules of cytotoxic T lymphocytes (CTL) and natural killer (NK) cells.243-248 There are 5 known granzymes expressed in humans: A, B, H, K, and M; and 11 expressed in mice: A, B, C, D, E, F, G, K, L, M, and N.249 Of these, granzymes A and B (GZMA and GZMB) are the most abundant and subsequently, also the most well-characterized. GZMB, in particular, has been studied extensively for its central role in mediating programmed cell death. GZMB is a 32 kDa chymotrypsin-like serine protease and is characterized by a His-Asp-Ser catalytic triad in its active site.250 It is variably known as cytotoxic T lymphocyte-associated serine esterase-1 (CTLA-1) or granzyme 2 and possesses the unusual ability to cleave substrates at aspartic acid residues. This specificity is reliant on the presence of an arginine residue at position 226 within the binding pocket.251  The GZMB cluster is located on chromosome 14 in both mice and humans, but where the cluster in humans comprises GZMB, granzyme H (GZMH), cathepsin G (CG), and mast cell chymase-1 (CMA-1), the cluster in mice consists of GZMB, granzymes C, D, E, F, G and N, CG and CMA-1. Within CTLs, GZMB transcription is induced by T cell receptor activation and co-stimulation with cytokines.252 The upstream promoter region for GZMB contains binding sites for two general transcription factors: activating 35  transcription factor/cyclic AMP-responsive element-binding protein (ATF/CREB) and activator protein-1 (AP-1); and two lymphoid-specific transcription factors: Ikaros and core-binding factor (CBF/PEBP2).253-256 Transcriptional activation requires all 4 factors to act in concert and a single mutation to any of the transcription factor binding sites will abrogate all expression of GZMB.253, 255  1.5.1 GZMB-INDUCED CELL DEATH The granule-mediated cell death pathway is the primary means used for the clearance of pathogen-infected cells and the removal of tumours (reviewed in Russell and Ley257) and the primary effectors of this pathway are the granzymes. Most lymphocytes express GZMB constitutively and further upregulate transcription when activated. GZMB expression in CTLs and NK cells is regulated by many of the same factors that stimulate immune cell activation, such as the composition of the cytokine milieu, receptor engagement and presence of CD4+ T cell populations.252 GZMB is synthesized as a pro-peptide and is tagged with a mannose-6-phosphate receptor (MPR) to target it to lytic granules.258 Upon entering the granule, GZMB is activated by cleavage of a 2 amino acid propeptide domain by cathepsin C and then stored in complex with the chondroitin sulfate proteoglycan, serglycin.259-262 The acidic pH inside the lytic granules and storage with serglycin serve to minimize proteolytic activity until engagement.252 When the cytotoxic cell engages with its target, a rapid polarization of lytic granules occurs towards the immunological synapse (IS).263 The movement of lytic granules and other soluble factors across the IS is directional and enabled by the microtubule-organizing centre.264, 265 Upon delivery to the target site, the lytic granule fuses with the target cell plasma membrane and its contents are endocytosed into the target cytoplasm. Because GZMB must access intracellular substrates to initiate the apoptotic cascade, a key step in GZMB function is necessarily its entry into the targets cell 36  cytoplasm. By convention, it was believed that this was mediated by perforin; however the exact mechanism by which this occurs is still under investigation. Perforin is a calcium-dependent cytolytic protein that multimerizes in the target cell plasma membrane, forming a pore 5-20 nm across.266 While traditional models suggest that perforin grants GZMB entry into the target cell cytoplasm through the formation of these pores, it has also been shown that GZMB can be endocytosed via binding to the mannose-6-phosphate receptor in the absence of perforin.267 Although its exact role in facilitating GZMB entry into the target cell is still unclear, perforin is definitely required for GZMB-mediated apoptosis as perforin deficiency is associated with impaired lymphocyte-mediated cytotoxicity, while perforin-deficient mice are highly susceptible to viral infection268, 269 and cancer.270, 271   Upon entry into the target cell cytoplasm, (see Figure 1) GZMB exhibits broad substrate specificity and has been shown to cleave and activate caspase-3, -7 and -8 in vivo.272-274 Activation of executioner caspase-3 is integral to the apoptotic cascade as it processes the inhibitor of caspase-activated deoxyribonuclease, DNA damage sensor, nuclear lamins and others (reviewed in Hengartner275) GZMB also cleaves Bid to gtBid, which then translocates to the mitochondria where it interacts with Bax and Bak to disrupt mitochondrial membrane integrity,276-278 leading to increased membrane permeability and the release of cytochrome c and apoptosis-inducing factor (AIF)279, 280 which stimulates the formation of the apoptosome. GZMB has also been shown to cleave anti-apoptotic protein myeloid cell leukemia sequence-1 (MCL-1).281 This leads to the release of pro-apoptotic Bcl-2 family member Bim which can also induce mitochondrial outer membrane permeabilization and cytochrome c release.     37  FIGURE 1: GZMB-MEDIATED APOPTOSIS GZMB and perforin are released into the immunological synapse from the granules of the cytotoxic cell towards the target. They are taken into the target cell via endocytosis and GZMB gains access to the target cell cytoplasm via a perforin-mediated mechanism.GZMB cleaves various intracellular substrates initiating caspase-independent and caspase-dependent apoptosis. Reproduced with permission from Hiebert and Granville, Trends Mol Med, (2012).282    1.5.2 EXTRACELLULAR GZMB ACTIVITY Although the granzymes were originally discovered as both intracellular and extracellular proteases, the vast majority of research has focused on their intracellular functions; however, there has been a recent resurgence in interest regarding the extracellular abilities of GZMB in particular. While it was once believed that GZMB was expressed exclusively by CTLs and NK cells, it has been observed that under pro-inflammatory conditions, GZMB can be expressed by numerous immune (CD4+ cells, mast cells, macrophages, neutrophils, basophils, dendritic cells, T regulatory cells) and non-immune cell types (SMC, chondrocytes, keratinocytes, pneumocytes, Sertoli cells, spermatocytes, granulosa cells and syncytial trophoblasts).283-293  Although certain cell types such as keratinocytes, chondrocytes and neutrophils can express both GZMB and perforin together, other types such as mast cells express GZMB in the absence of perforin, which would be indicative of GZMB having a purely extracellular function in these cells.284, 293-295 Upon examining the composition of the GZMB gene cluster on chromosome 14, it becomes apparent 38  that GZMB is clustered together with numerous mast cell proteases apart from perforin and GZMA and as a result, GZMB can be expressed by myeloid cells independently of perforin.288, 290, 296  25 years ago, Kramer and Simon suggested that a proportion of granule proteases may leak from the IS during CTL degranulation.297 It was now known that up to a third of GZMA and GZMB maybe be constitutively secreted by T cells,298 with NK cells primarily releasing active proteases via the granule pathway and CTLs secreting inactive zymogen in a granule-independent manner, suggesting the possibility for the existence of extracellular GZMB activator(s).299 GZMB is present at low levels in the plasma of healthy individuals with the median reported levels ranging from 20-40pg/ml.300, 301 Serum levels of GZMB are known to be elevated in numerous inflammatory diseases such as Epstein-Barr virus infection, arthritis, human immunodeficiency virus-1 (HIV-1) infection amongst others.300-302  GZMB levels have also been reported to be elevated in the synovial fluid of rheumatoid arthritis patients,303 cerebrospinal fluid of multiple sclerosis304 and Rasmussen encephalitis patients,305 and bronchoalveolar lavage in chronic obstructive pulmonary disease (COPD)306 and lung inflammation.307  Furthermore, although the endogenous inhibitor of GZMB, protease inhibitor-9 (PI-9), is present in the plasma, GZMB retains up to 70% of its activity, suggesting that PI-9 is unable to inhibit GZMB effectively in the circulation,308, 309 and in the absence of other extracellular inhibitors, it is possible that GZMB may be largely unregulated in comparison to other ECM proteases like the MMPs, which are tightly regulated by their inhibitors.    39  1.5.3 EXTRACELLULAR GZMB SUBSTRATES The earliest extracellular substrates for GZMB were identified in the 1990?s, and it is now accepted that GZMB cleaves a variety of cell surface receptors, cytokines and has numerous targets in the ECM and the blood clotting cascade. GZMB has been shown to cleave cell surface receptors such the nicotinic acetylcholine receptor,310 the neuronal glutamate receptor,311 Notch-1,312 and fibroblast growth factor receptor-1.312 Cleavage of the latter 2 receptors may affect cell signalling pathways involved in tumour survival and antiviral activity, while cleavage of the former two uncover cryptic epitopes that generate autoantigenic fragments. This may trigger an autoimmune response, further exacerbating or possibly inducing a state of chronic inflammation. GZMB cleaves a number of targets involved in blood clotting, amongst them are plasmin,313 plasminogen,313 von Willebrand factor (vWF)314 and matrix fibrinogen.313, 314  Plasminogen is the precursor of plasmin, which mediates the dissolution of fibrin clots,315 amongst other proteolytic functions. While plasmin is considered proangiogenic, cleavage of both plasminogen and plasmin by GZMB results in the production of a 38 kDa fragment called angiostatin, which is antiangiogenic313 and has been shown to induce endothelial cell apoptosis.316 GZMB cleaves vWF in the A1-3 domains with comparable efficiency to ADAMTS-13314 As this region is necessary for interaction with platelets, GZMB cleavage can result in impaired platelet aggregation.314  Interleukin-1? (IL-1?) is a pro-inflammatory cytokine that is known to be elevated in AAA.317, 318 It signals through the type I IL-1 receptor319 and IL-1 signalling activates prostaglandin synthesis,320 induces nitric oxide synthase in smooth muscle,321 and promotes the release of other pro-inflammatory mediators, including IL-6,322 a prominent cytokine observed consistently in aneurysmal tissue.323 A recent study by Afonina et al. identified IL-1? as a substrate for GZMB, where cleavage by GZMB 40  potentiates the biological activity of this cytokine both in vitro and in vivo.324 GZMB -processed IL-1? also exhibited greater potency as an immuno-adjuvant when injected into mice with ovalbumin.324  Numerous targets for GZMB cleavage have been identified in the ECM, namely aggrecan,325 fibronectin,326, 327 vitronectin, 76 decorin,98 fibrillin-1,328 biglycan,98 betaglycan,98 laminin ?576 and others. The  Arg?Gly?Asp (RGD) domain is the recognition sequence for integrin binding329 in many ECM proteins and GZMB is known to or purported to cleave it in vitronectin and fibronectin,326 indicating that GZMB-mediated proteolysis of these glycoproteins may alter cell?matrix interactions and affect cell migration and adhesion.326  Many ECM components serve in a structural capacity. For instance, decorin88 and biglycan330 regulate collagen fibrillogenesis while fibrillin-1 associates with elastin to form elastic microfibrils331 in the vasculature. Degradation of these components may destabilize the vessel, weakening it and rendering it unable to withstand pulsatile blood flow. However, GZMB cleavage of ECM components does not only result in mechanical damage and direct loss of structural integrity in the target tissues. Of the known substrates, fibronectin, laminin and vitronectin have major roles in cell adhesion326 and cleavage of these proteins can result in cell detachment and death by anoikis.327 Choy et al. have shown that up to 30% of CTL-induced cell death was GZMB-dependent but perforin-independent after describing anoikis in cultured SMC following fibronectin cleavage.327 Buzza et al. took this further by culturing endothelial cells on pure fibronectin, laminin and vitronectin matrices and also observed cell death by anoikis following incubation with GZMB in the absence of perforin.326  It has been suggested that ECM fragments generated by GZMB cleavage may possess chemotactic properties, recruiting inflammatory cells to the site of injury and further exacerbating the inflammatory state.332 Fibronectin fragments have been shown to have chemotactic properties, attracting neutrophils and monocytes,332-334 and additionally, have the ability to induce MMP expression 41  in chondrocytes.335 ECM fragments may further serve as signalling molecules to neighbouring cells,312 propagating the inflammatory phenotype in the vicinity of injury. The ECM also acts a reservoir for growth factors and cytokines, and the interaction between various ECM components, attendant proteases and these molecules can influence their storage, concentration, activation status, synthesis and degradation.67, 68 For example, the TGF-? family can be sequestered in both active and latent forms by various proteoglycans in the ECM,336 and degradation by GZMB could induce unregulated release of these molecules, elevating circulating levels as well as influencing and unbalancing various signalling pathways in the surrounding tissue. A selection of AAA-relevant extracellular GZMB substrates are summarized in Table 2.   42  TABLE 2: EXTRACELLULAR GZMB SUBSTRATES AND POTENTIAL CONSEQUENCES IN AAA Substrate Role in vasculature and circulation Potential consequences of cleavage Status in AAA References Decorin Regulates collagen fibril organization, sequesters growth factors, regulates cell proliferation and endothelial cell-matrix interactions during angiogenesis. Impaired collagen fibrillogenesis, loss of vessel tensile strength and distensibility. Release of TGF-?. Differential localization and altered ratio of decorin to other proteoglycans has been observed. 88, 98, 337-340 Biglycan Regulates collagen assembly and cell proliferation, sequesters cytokines and growth factors. Impaired collagen fibrillogenesis, loss of vessel tensile strength and distensibility. Release of TGF-?.  60% reduction in biglycan mRNA expression was observed in human AAA. Biglycan deficiency resulted in spontaneous aortic dissection and rupture in mice. 98, 330, 340-342 Fibrillin-1 Associates with elastin as a major scaffolding component of elastic microfibrils which provide load bearing support for the entire vessel.  Destruction of the elastic lamellar unit, loss of vessel elasticity and structural integrity. Mutations in FBN-1 lead to inability of vessel wall to sustain hemodynamic stress due to mechanical collapse of the elastic lamellae resulting in dissecting aortic aneurysms in Marfan?s syndrome.  331, 343, 344 IL-1? Activates prostaglandin synthesis, induces iNOS in SMC, initiation of ischemia induced inflammation. Cleavage augments proinflammatory biological activity, increases potency as immune-adjuvant.  IL-1? is elevated in AAA and promotes release of proinflammatory mediators eg. IL-6. 320, 322, 345, 346 Fibronectin Cell adhesion, growth, differentiation and migration. Mediates SMC phenotype.    Impaired integrin binding. Impaired tissue repair, blood clotting, and cell migration and adhesion, SMC apoptosis and anoikis. Loss of fibronectin in aneurysmal basement membrane observed, but is densely expressed in ruptured aneurysms and atherosclerotic lesions 326, 327, 347, 348 von Willebrand factor Formation of platelet aggregates and clotting at sites of vascular endothelial cell injury. Cleavage site within the domain responsible for platelet interaction, delaying thrombosis. Peri-operative thrombocytopenia is associated with poor outcome in ruptured AAA. 349 Plasminogen  Precursor of plasmin, which is proangiogenic and has both proteolytic and fibrinolytic effects on the ECM, enabling cell migration.  Production of antiangiogenic angiostatin, defective clot clearance, impaired cell migration and angiogenesis, induces endothelial cell apoptosis. PAI-1 reduces AAA incidence and rupture. tPA activation associated with greater expansion of AAA.  313, 316, 350, 351 Vitronectin Cell adhesion, hemostasis, SMC migration and chemotaxis via Ras, ERK1/2, and p38.  Binds PAI-1 and regulates plasminogen-mediated proteolysis of ECM.  Impaired SMC migration and proliferation. Disruption of growth factor binding.  Identified as a biomarker for AAA and correlated for AAA expansion rate. 326, 352-355 43  1.5.4 GZMB INHIBITORS Because of the role GZMB plays in mediating cytotoxicity, cells expressing GZMB need a means of controlling extraneous GZMB in order to forestall their own autolysis. Currently, the only known endogenous inhibitor of GZMB in humans is the intracellular PI-9, which is a member of the large family of serine protease inhibitors (serpins) and is also known as serpin B9.356 Serpins inhibit serine protease activity by acting as a pseudo-substrate for their target protease. They contain a unique reactive centre loop (RCL) near the carboxy terminus that undergoes an irreversible conformational change when the serpin complexes with its protease, which cleaves at the RCL between two amino acid residues termed P1 and P1?. Serpin-protease specificity is typically dictated by the identity of P1.357 Although GZMB, like the caspases, preferentially cleave substrates after aspartic acid, it is also able to cleave after various other acidic residues. So while GZMB is able to cleave after the P1 glutamic acid in PI-9, caspases typically do not,358 and this would appear to confer very high specificity of interaction between PI-9 and GZMB.359 As expected, PI-9 is expressed by various immune cell types (T-cells, B-cells) and antigen presenting cells (APCs) 356, 359, 360 to protect themselves against misdirected GZMB, but it has also been observed that high levels of PI-9 expression may be found in VSMCs (VSMCs),361 hepatocytes362, 363 and endothelial cells,364 as well as various cells types in immune-privileged tissues such as the eyes, testes, ovaries and placenta.360 In mice, the murine ortholog to PI-9 is known as serine protease inhibitor-6 (SPI-6) and is similarly capable of regulating intracellular murine GZMB activity.365 Interestingly, human PI-9 is not capable of inhibiting murine GZMB, and neither is murine GZMB very effective at inducing apoptosis in human cells,365 which would be indicative of a significant difference in functional specificities between the human and murine granzymes. 44  While no extracellular inhibitors of GZMB have been identified in humans, Sipione et al.366 discovered an inhibitor of both murine and human GZMB, called serpin A3N (SA3N), that is expressed and secreted by mouse Sertoli cells in the seminiferous tubules, where it is believed to act in concert with SPI-6 to modulate activity of GZMB and protect islet grafts from allo-, auto-, and xenoimmune mechanisms of destruction. SA3N is now known to be expressed in the brain, testes, spleen, lung and thymus and has a high degree of homology with human ACT or serpin A3.367 While in humans there is a single gene coding for ACT, repeated duplications have resulted in a cluster of 13 closely related genes at the same locus in mice.367 Interestingly, while ACT shares homology with SA3N, it does not appear to have an inhibitory effect on GZMB.368 GZMB activity may also be indirectly affected by other granzymes. Granzyme M (GZMM) has been shown to inactivate PI-9,369 and could therefore deregulate GZMB activity, while GZMH may potentiate GZMB antiviral capabilities by degrading adenoviral protein Ad5-100K, which is required for viral replication and also inhibits GZMB activity.370 Several viral serpins have been found to inhibit GZMB, of which cytokine response modifier-A (crmA)371-373 and Serp-2 are the most well-known. CrmA contains an aspartic acid at P1356 and is known to inhibit various intracellular cysteine proteases involved in apoptosis and to prevent the maturation of IL-1?. Myxoma virus serp2 has been shown to be a weak inhibitor of GZMB, but unlike crmA, is unable to prevent apoptosis in cowpox virus-infected cells.374  1.5.5 GZMB IN ATHEROSCLEROSIS With the discovery that GZMB can be expressed and secreted by numerous immune and non-immune cell types, and the identification of novel extracellular substrates, the role of GZMB in disease is now being re-evaluated. During conditions of chronic inflammation, the infiltration of immune cells can result in the elevation of GZMB levels both at the site of injury and in the circulation.  45  Atherosclerosis is responsible for the majority of cerebrovascular disease, coronary artery disease, myocardial infarction and stroke375 376 and is currently the leading cause of death worldwide.28 It can be described as a chronic inflammatory disease of the arterial wall that results in lipid-lesion formation and luminal narrowing of the vessel, typically at locations where laminar flow is disrupted. T lymphocytes and macrophages are the key immune mediators in atherosclerotic lesions and also the most abundant,377-379 although smaller populations of neutrophils, mast cells, dendritic cells and NK cells are also involved. 380-382  Atherosclerosis begins with endothelial dysfunction, which is characterized by reduced vasodilation,  release of proinflammatory cytokines, enhanced pro-thrombotic functions and increased cell adhesion molecule expression.383 It may be caused by hemodynamic stress, chronic hyperlipidemia or exposure to oxidative stress resulting from smoking, hypertension or diabetes mellitus.384 The release of cytokines recruits immune cells to the region of endothelial dysfunction, where oxidative stress has lowered nitric oxide (NO) bioavailability and upregulated the expression of cell adhesion molecules such as vascular cell adhesion molecule-1 (VCAM-1) which binds  monocytes and T lymphocytes,384 and induced MCP-1 expression which recruits mononuclear phagocytes.385 As immune cells migrate into the intima, macrophages engulf oxidised lipoproteins and mature into lipid-laden foam cells,283, 377, 386 which then secrete growth factors stimulating VSMC migration, proliferation and the synthesis of collagen and proteoglycans, forming a fibrous matrix that develops into a plaque with a fibrous cap. 384  The level of GZMB expression has been shown to correlate with disease severity and has been observed in both the lipid rich and cellular areas as well as in the shoulder regions of atherosclerotic plaques.283 Within the intima and media, GZMB has been found to colocalize to SMC, macrophages and T cells, but has also been observed extracellularly.283 Additionally, elevated levels of circulating GZMB correspond to carotid plaque instability and incidence of cerebrovascular events,387 while Tsuru et al.388 46  found that GZMB levels in peripheral blood mononuclear cells were elevated in patients with unstable angina pectoris compared to patients with stable angina pectoris, making it highly indicative that GZMB affects plaque stability.  GZMB colocalization to TUNEL-positive foam cells suggests that GZMB may be responsible for cell death in the lipid-rich necrotic core;283 however, GZMB can cleave numerous ECM proteins found in the atherosclerotic plaque such as fibronectin, vitronectin and laminin and thereby contribute to ECM fragmentation 326, 327 and induce perforin-independent SMC apoptosis via anoikis.327  The effect of elevated GZMB activity in atherosclerosis may vary with the stage of lesion development, as in the early stages of atherosclerosis, GZMB-induced SMC apoptosis may act to reduce intimal hyperplasia,389 while in more advanced lesions with fibrous caps, increased apoptosis of SMC and ECM degradation may undermine cap integrity and contribute to plaque rupture.   1.6 MURINE MODELS OF AAA While the critical hallmarks of aneurysmal disease have been well characterized, there is still a general paucity of information on the actual sequence of events that result in the initiation, progression and rupture of the aneurysm. Given the nature of the disease, its location, and typical asymptomatic presentation until advanced dilation or rupture, acquisition of tissue in the formative stages of disease is not a viable option, even after the implementation of early screening programs. Researchers are thus restricted to observational studies of samples obtained during surgical repair for severe AAA over 5 cm,180  where results may not necessarily reflect the processes involved in the formative stages of disease. 47  Various in vitro models have been developed to study the pathogenesis of AAA and to test novel treatment and diagnostic techniques. Thompson et al.390, 391 published a model of aortic organ culture that was subjected to degradation by pancreatic elastase to determine the effect of leukocyte infiltration and shear stress on MMP production within the wall. Other in vitro methods have been described for the measurement of endoleakage392, 393 and direct intra-aneurysm pressure.394 These models have their uses, but in general, can only be maintained in culture for a limited time, and any in vitro system, no matter how controlled, will never fully be able to recapitulate the true state of the in vivo environment and will still require further validation prior to clinical use.  Animal models of disease ideally mimic the cellular and biochemical characteristics of the human form of the disease; however, in the case of AAA, there are numerous challenges to gauging the fidelity with which animal models replicate the human condition.180 Even so, given the limitations of using human specimens and in vitro systems, various animal models of AAA have grown increasingly important in providing an experimental framework to test hypotheses relating to aneurysm initiation, progression and potential therapeutic strategies. While models in larger animals have been described, rodents and in particular, mice, have been the predominant model organism used to develop experimental models of AAA for various reasons. They are the most well-characterized laboratory research animal, with relatively short life cycle, documented genetic backgrounds and are easily manipulated for the regulation of specific genes.  Murine models of AAA can be classified into two general categories: Genetically engineered models, and chemically-induced models. 1.6.1 GENETICALLY ENGINEERED MOUSE MODELS OF AAA The blotchy mouse has an X chromosome mutation that leads to abnormal copper absorption in the intestinal tract.395 Copper is a required cofactor for lysyl oxidase (LOX) and thus, these mice exhibit 48  defective collagen and elastin crosslinking and which result in abnormalities in connective tissue architecture. 396This predisposes them to develop aneurysms throught the aorta, but primarily in the thoracic region.395 Based on these findings, it was suggested that altered copper metabolism may be a contributing factor to aneurysm pathogenesis in humans but no evidence of reduced copper levels has been observed in human AAA.397, 398 While this model recapitulates some traits of the human disease, such as male gender bias for rupture,399 other abnormalities such as emphysema and neurological dysfunction ultimately limit the usefulness of this model.400 Similarly, mice with a genetically engineered deficiency in Lox are not viable models of AAA as they typically die in the perinatal period from rupture of thoracic aortic aneurysms (TAA).401 Various MMPs have been implicated in the pathogenesis of AAA, and mice deficient in MMP-3402 and TIMP-1145, 146 have been shown to develop medial dissections and small aneurysms in both the thoracic and the abdominal aorta. The lack of specificity may indicate a generalized destruction of aortic ECM and may obscure the true mechanism behind AAA development.180 ApoE and LDL receptor knockout (LDLR-KO) mice fed on high fat diets have both been shown to develop suprarenal AAA.403 AAA development was reduced in apoE-KO mice that were also deficient for urokinase and this was determined to be due to an inability to activate MMP-12 via a plasmin-dependent pathway.130 Elastin degradation and vessel dilation in LDLR-KO mice were reduced by treatment with a broad-spectrum MMP inhibitor, CGS 27023A, without affecting concurrent atherosclerotic lesions.404 The Tsukuba hypertensive mouse is produced by crossing transgenic mice that express human renin and angiotensinogen.405 These mice developed chronic hypertension and died of ruptured AAA after 10 days of being fed drinking water containing 1% sodium chloride. Rupture was not associated 49  with hypertension, but warrants further study of the effects of high sodium diets on the renin-angiotensin system and association with AAA in humans.180 1.6.2 CHEMICALLY-INDUCED MOUSE MODELS OF AAA Chemically-induced models of AAA have proven to be useful in elucidating the molecular mechanisms involved in aneurysm formation and for testing of potential pharmacotherapeutics. The three models described here are the most commonly employed animal models of AAA. The elastase-induced model was first described by Anidjar et al. in rats406 and is still a frequently used model of AAA today. It was developed based on the observation of disrupted elastin in AAA and knowledge of the critical role that elastin plays in providing vessel distensibility and maintaining structural integrity.180 The model was adapted for use in mice,119 whereby the abdominal aorta is isolated by a distal suture and a catheter is surgically inserted into the aorta at the iliac bifurcation. Porcine pancreatic elastase is infused into the aorta and incubated for 5 minutes before normal flow is restored. Elastase infusion via this method induces immediate dilation, presumed to be caused by the mechanical effects of the procedure, but by day 14, significantly greater dilation is observed in elastase-infused mice compared to heat-inactivated controls, accompanied by extensive destruction of elastic lamellae and adventitial nflammation mediated predominantly by macrophages. Aneurysm in this model is defined as a 100% increase in diameter to account for the initial mechanical dilation.180 It should be noted that the source and method of preparation of the elastase are critical to the outcome in this method, and it has been suggested that the resulting inflammatory response is attributable to a contaminant in certain preparations of elastase, and not to the elastase itself.181, 407 Decreased expression of both endothelial and neural nitric oxide synthase, but significantly increased iNOS has been observed in elastase-induced AAA. Interestingly, although iNOS has been implicated in inflammatory vascular diseases, male iNOS-deficient mice did not receive protection from development 50  of elastase-induced AAA, while female mice experienced greater incidence and developed larger AAA in the absence of iNOS. This effect was reversed by oophrectomy, suggesting an interaction between iNOS and gonadal hormones.408 The peri-aortic application of CaCl2 to induce aneurysm formation was first described in rabbits409 and has been adapted for induction of AAA in mice.410 Using a cotton wool tip, 0.25M CaCl2 is applied directly to the abdominal aorta for 15 minutes and aortic diameter was found to increase by 64% after 2 weeks, and 110% after 3 weeks.410 Similar to human AAA, structural disruption of the medial elastic lamellae and activation of the inflammatory response in the media and on the luminal surface was observed, indicating that this might be a clinically relevant model of disease. The CaCl2 model was also used to define the role of MMPs in AAA development. While MMP-2-KO and MMP-9-KO mice do not develop aneurysms,120 aortic dilation is ablated in MMP-12-KO mice compared to WT controls after CaCl2 application. The systemic infusion of Ang II has been shown to induce a localized inflammatory response resulting in suprarenal aneurysm formation in hyperlipidemic LDLR-KO or apoE-KO mice.411, 412 Ang II is typically administered at 500-1000ng/kg per minute via subcutaneously implanted osmotic pumps for 28 days.180 This model also exhibits male gender bias, with males more susceptible than females. Androgen interaction may play a role as orchidectomy reduces incidence and severity in male mice to levels similar to that observed in females.413   Macrophage infiltration into the suprarenal aorta and the subsequent disruption of elastin fibres have been suggested as the precipitating event and may occur within a few days of pump implantation.414 After 3-10 days, medial dissection occurs with the formation of a prominent vascular hematoma.414 This thrombus is distinct from the laminated thrombus observed in human AAA as it results in only external diameter dilation, but may also exacerbate proteolytic activity in the dissected 51  aortic wall.180 ECM deposition and leukocyte infiltration follow thrombus formation, and if rupture does not occur, the dilated aorta slowly regains circumferential elastin fibres and the lumen is re-endothelialized. Extensive neovascularization occurs throughout the remodelled tissue. If Ang II infusion is continued past 28 days, large atherosclerotic lesions characterized by lipid-laden foam cells, develop throughout the intima of the abdominal aorta.414   Co-infusion of angiotensin II receptor, type 1 (AT1) antagonist, Losartan, with Ang II is able to completely inhibit aneurysm formation in this model, whereas co-infusion with angiotensin II receptor, type 2 (AT2) receptor antagonist, PD123319 was shown to increase severity and incidence of AAA,415 indicating that Ang II initiates aneurysm formation via signaling through the AT1 receptor. AT1 signaling in endothelial cells upregulates expression of E-selectin and VCAM-1, promoting leukocyte adhesion.416, 417 In VSMC, Ang II induces the production of phosphatidic acid (PA) via phospholipase D (PLD)-mediated phosphatidylcholine hydrolysis. PA then activates nicotinamide adenine dinucleotide phosphate (NADPH) oxidase wich results in the production of reactive oxygen species (ROS).418, 419 Elevated levels of ROS cause VSMC hypertrophy and hyperplasia, lipid peroxidation and the production of MMP-2 and -9.418-420 This initiates a localized inflammatory response and recruits monocytes and lymphocytes to the vessel wall. AT1 receptor activation in macrophages leads to increased 12/15 lipoxygenase and ROS production, LDL oxidation, cholesterol synthesis and decreased cholesterol efflux, further promoting the inflammatory state. Therefore, aneurysm formation in this model is inflammatory, and has also been shown to be independent of blood pressure elevation, as treatment with hydralazine, which lowers systemic blood pressure, had  no effect on AAA incidence.421  In conclusion, while in vivo animal models have been crucial to our current understanding of the pathogenesis of AAA and continue to be valuable tools for the development of new diagnostic tools, pharmacotherapy and repair techniques, they must be used with the understanding that each model 52  has its limitations and the purpose of the study is an important factor to consider when making an appropriate choice of model. These include but are not limited to interspecies variation in anatomy, genetics, physiology and pharmacodynamics.422 However, the consistency of outcomes across various models in comparison to human AAA is strongly indicative that aneurysm formation involves a common mechanism of ECM destruction and loss of structural integrity.   53  CHAPTER 2: RATIONALE, HYPOTHESIS AND SPECIFIC AIMS 2.1 RATIONALE The exact etiology of AAA pathogenesis is yet unclear but it is currently believed that a key event in the development of aneurysms involves the proteolytic cleavage of ECM components such as collagen and elastin that results in a mechanical weakening of the vessel wall.26  Analysis of human AAA tissue shows marked degradation of medial elastin fibres, thinning of the media, adventitial hypertrophy and an accumulation of T and B lymphocytes.26  Atherosclerosis and the presence of thrombi are also frequently noted. Various proteases such as matrix metalloproteinases (MMP-2, 7, 9, 12),106 cathepsins202 and elastases183 have all been put forward as possible contributors towards the degradation of vessel wall ECM.  It has also been suggested that medial SMC apoptosis plays a significant role in the pathogenesis of AAA.  Substantial p53 mRNA expression has been observed in AAA tissue as well as a considerable decrease in SMC density.423 Loss of SMCs would result in instability of the medial architecture and affect matrix remodelling processes, resulting in medial degradation that is a key characteristic of aneurysmal disease. GZMB is a 32 kDa serine protease that plays an important role in immune cell-mediated apoptosis. In its traditional role as a mediator of cellular apoptosis, GZMB is released from the granules of an effector cell together with perforin, a pore-forming enzyme that is required for the entry of GZMB into the cytoplasm of the target cell where it can trigger the apoptotic cascade, resulting in cell death.252  In addition to this intracellular pathway, it has been shown that GZMB is also released into the extracellular space during chronic inflammatory states and viral infections.368 Outside of the cell, GZMB has been found to exhibit extracellular proteolytic abilities and is capable of cleaving numerous ECM 54  proteins such as fibronectin, vitronectin, laminin, aggrecan326, 327 and the elastin scaffolding protein fibrillin-1344 which would result in a deterioration of vessel wall integrity. As further support for a pathogenic extracellular role, it has been observed that GZMB levels are elevated in advanced human atherosclerotic and allograft vasculopathy lesions compared to healthy arteries.283 Additionally, high plasma GZMB levels correspond to carotid plaque instability and increased cerebrovascular events in humans,387 while increased GZMB production in peripheral blood mononuclear cells was observed in patients with unstable versus stable angina.388 Given that GZMB possesses both intracellular and extracellular capabilities, it is possible that GZMB contributes to the development of AAA and dissection in our AngII-induced murine model via one of, or both, the perforin-mediated apoptotic pathway and its extracellular degradative ability. An experimental  overview and chapter outline is described in Figure 2. 2.2 HYPOTHESIS I hypothesize that GZMB contributes to the pathogenesis of AAA through the proteolyses of extracellular matrix proteins leading to vessel wall instability and rupture. 2.3 SPECIFIC AIMS Aim 1: To assess the presence and role of GZMB in murine and human abdominal aortic aneurysm. Aim 2: To determine if GZMB contributes to aneurysm formation through an intracellular, perforin-dependent and/or extracellular perforin-independent  mechanism. Aim 3: To determine if the administration of extracellular GZMB inhibitors can reduce the incidence and severity of disease in a mouse model of aortic aneurysm. 55   FIGURE 2: EXPERIMENTAL OVERVIEW AND CHAPTER OUTLINE    56  CHAPTER 3: METHODOLOGY 3.1 IN VITRO ASSAYS 3.1.1 FIBRILLIN-1 EXPRESSION ASSAY Human coronary artery smooth muscle cells (HCASMC) (Cambrex, Walkersville, MD) were grown in Smooth muscle cell basal medium (SMBM) containing supplements and 5% FBS, according to the supplier's instructions (LONZA, Walkersville, MD), and were used after reaching 80% confluence. Prior to treatment, cells were starved for 24 hours in serum-starved media (supplemented SMBM (LONZA) + 0.2% FBS). Following starvation, cells were treated with 10 nM or 25 nM of GZMB (Alexis Biochemicals, Farmingdale, NY) for 24 hours. Controls were treated with PBS. At 24 hours, cells were lysed with TRIzol (Invitrogen, Carlsbad, CA), RNA was extracted according to manufacturer?s instructions, and total RNA quantified by absorbance at 260 nm. To eliminate genomic DNA contamination, total RNA (1.5 ?g) was DNAse I-treated (Invitrogen) according to manufacturer?s instructions. cDNA was prepared from 1 ?g of total RNA using 50 ?M of oligo (dT)20 primer and Superscript III reverse transcriptase (Invitrogen), according to manufacturer?s instructions. All PCR reactions were carried out in 25 ?l volumes using Platinum Taq DNA polymerase (Invitrogen) and previously published primers for human fibrillin-1 (Forward, 5?-GTGAGATCAACATCAATGGAGC-3?; Reverse, 5?-TTACACACTCCTGGGAACACTTC-3?) and the universal GAPDH primers (Forward, 5?-CATGTTCGTCATGGGTGTGA-3?; Reverse, 5?-GACTGTGGTCATGAGTCCTT-3?).424 Equal aliquots of the PCR products were electrophoresed on a 2% agarose gel, stained with SafeView and photographed.  3.1.2 MURINE GZMB ENZYMATIC ACTIVITY ASSAY Increasing concentrations of SA3N (0.625 nM ? 80 nM) or anti-GZMB neutralizing antibody (0.113 - 14.15?g/ml, R&D Systems, Minneapolis, MN) were pre-incubated with active recombinant mGZMB (20nM, Sigma Aldrich, St Louis, MO) for 25 min in reaction buffer (50 mM HEPES, 10% sucrose, 0.1% 57  CHAPS, 5 mM DTT). Cleavage of the colorimetric GZMB substrate Ac-IEPD-pNA (1 mM, Calbiochem, EMD Chemicals, Gibbstown, NJ) in duplicate reactions was monitored by measuring absorbance at 405 nm using a plate reader (Magellan Tecan SaFire 2, Tecan Group Ltd., M?nnedorf, Switzerland) in kinetic mode. The initial rate of each reaction was calculated, and percent mGZMB activity for each concentration was determined relative to mGZMB activity in the absence of inhibitor. Inhibition was determined in 3 separate experiments. 3.1.3 DECORIN CLEAVAGE ASSAY Western blot: Purified human GZMB (100  nM; Axxora, San Diego CA) was pre-incubated with or without recombinant SA3N (240 nM) in 50 mM Tris buffer, pH 7.4, for 25 min at room temperature (RT) prior to incubation with recombinant human decorin (0.16?g; Abnova, Walnut, CA) for 24 hours at RT. After incubation, proteins were denatured, separated on a 10% SDS-polyacrylamide gel, transferred to a nitrocellulose membrane and blocked with 10% skim milk. The membrane was probed using a mouse anti-human decorin antibody (1:200, R&D Systems) and IRDye? 800 conjugated affinity purified anti-mouse IgG (1:3000, Rockland Inc., Gilbertsville, PA). Bands were imaged using the Odyssey Infrared Imaging System (LI-COR Biotechnology, Lincoln, NE).  Ponceau stain: Purified human GZMB (80 nM; Axxora) was pre-incubated with or without recombinant SA3N (240 nM) in 50 mM Tris buffer, pH 7.4, for 25 min at RT prior to incubation with recombinant human decorin (0.3 ?g; Abnova) for 24 hours at RT. After incubation, proteins were denatured, separated on a 10% SDS-polyacrylamide gel and transferred to a nitrocellulose membrane. Ponceau stain was used to identify cleavage fragments.   58  3.1.4 STATISTICS Statistical analysis was compiled in Graphpad Prism 5 (Graphpad Software, La Jolla, CA). SA3N IC50 concentrations were determined using nonlinear regression analysis. For all tests, significant difference was set at p < 0.05. 3.2 IN VIVO ASSAYS 3.2.1 HUMAN AAA AND TAA  Formalin-fixed, paraffin-embedded human AAA samples were obtained in accordance with the ethical protocols at the Karolinska Institute, Sweden. Formalin-fixed, paraffin-embedded human TAA samples were obtained in accordance with the ethical protocols at the Biobank Cardiovascular Registry, St. Paul's Hospital, Vancouver, BC.  3.2.2 MICE All procedures were done in accordance with the guidelines for animal experimentation approved by the Animal Experimentation Committee of the University of British Columbia.  Male apoE-KO mice (C57Bl/6 background), GZMB-KO mice (C57Bl/6 background) and perforin-KO mice (C57Bl/6 background) were obtained from Jackson Laboratories, Bar Harbor, ME (Stock Numbers 002052, 002248, 002407).  The GZMB/apoE-DKO (GDKO) mice and perforin/apoE-DKO (PDKO) mice were generated by crossing the apoE-KO and GZMB-KO or apoE-KO and perforin-KO mouse strains, respectively.  Genotyping of the mice was performed using primers and polymerase chain reaction (PCR) protocols designed from Jackson Laboratories (GZMB primers:  5'-CTG CTA CTG CTG ACC TTG TCT-3', 5'-TGA GGA CAG CAA TTC CAT CTA-3' and 5'-TTC CTC GTG CTT TAC GGT ATC-3';  apoE primers:5'-   GCC TAG CCG AGG GAG AGC CG   -3', 5'- TGT GAC TTG GGA GCT CTG CAG C   -3' and 5'-   GCC GCC CCG ACT GCA TCT   -3'  and perforin primers: 5'- GCT ATC AGG ACA TAG CGT TGG   -3', 5'-   GGA GGC TCT GAG ACA GGC TA   -3' and 5'-   TAC CAC CAA ATG GGC CAA G -3') (Sigma Genosys, Oakville, ON).  All mice were 59  housed at The Genetic Engineered Models (GEM) facility (UBC James Hogg Research Centre, St. Paul?s Hospital, Vancouver, BC).  3.2.3 MURINE MODEL OF ANGIOTENSIN II-INDUCED AAA Mice aged 11-13 weeks received either 28 days of Ang II (Sigma Aldrich, St. Louis, MO) infusion at 1000 ng/min/kg or saline infusion from a subcutaneous 1004 model ALZET? mini osmotic pump (DURECT Corporation, Cupertino, CA) as previously described.411  Briefly, an osmotic pump was filled with Ang II or saline solution, primed at 37?C for 24 hours in saline, and surgically implanted subcutaneously posterior to the scapula of the mouse.  During the implantation procedure, mice were anesthetized with gaseous anesthetic at a flow rate of 1.5 litres/min of oxygen with 2.5% isoflurane delivered via a Baines system using a calibrated tabletop anesthetic machine, administered from a rodent nose cone.  Depth of anesthesia was monitored by toe pinch response and respiration.  Eyes were protected using ocular lubricant.  Post-surgical pain control consisted of a subcutaneous injection of buprenorphine.  Animals were monitored daily for the duration of the experiment. 3.2.4 SERPIN A3N TREATMENT ApoE-KO mice were given a tail vein injection of sterile injectable saline (sham treatment) (n = 12) or recombinant SA3N diluted in sterile injectable saline at one of four doses: 120?g/kg per mouse (n = 11), 40 ?g/kg per mouse (n = 18), 20 ?g/kg per mouse (n = 10) or 4 ?g/kg per mouse (n = 11) prior to Ang II pump implantation on day 0. An additional group of apoE-KO mice (n = 28) were given tail vein injections of recombinant SA3N diluted in sterile injectable saline at 40 ?g/kg per mouse on day 0 prior to pump implantation and on day 4, 7, 14 and 21 post- pump implantation. SA3N was kindly provided by Dr. R. Chris Bleackley, University of Alberta.   60  3.2.5 ANTI-GZMB ANTIBODY TREATMENT ApoE-KO mice were given a tail vein injection of control IgG or anti-GZMB neutralizing antibody diluted in sterile injectable saline at 1 mg/kg (n = 5 per group) on day 0 prior to pump implantation and on day 4, 7, 14 and 21 post-pump implantation. 3.2.6 TISSUE COLLECTION AND GROSS PATHOLOGICAL CHARACTERIZATION At 28 days post-osmotic pump implantation, tissues from all surviving mice were collected. Blood was collected by cardiac puncture following CO2 euthanasia in ethylenediaminetetraacetic acid (EDTA) (Sarstedt Monovette, Sarstedt, N?mbrecht, Germany) and RBC removed by centrifugation.  The ventral cavity was opened, the right atrium was cut, and a catheter was inserted into the left ventricle. Sterile saline, and then 4% formalin (Fisher Scientific, Fairlawn, NJ), were perfused at a constant pressure of 100 mmHg using a pressurized tubing system until no blood was observed exiting the incision in the right atria. The heart, aorta to the iliac bifurcation, and kidneys were dissected from the mouse and photographed.  At this point, a gross description of the aorta was made and grouped under the following categories: No visible pathology; Small localized saccular AAA below diaphragm with or without visible hematoma (28 day survival); Large dissecting AAA beginning in the suprarenal aorta and extending beyond the diaphragm, into the mid-thoracic aorta (28 day survival); Large ruptured AAA with death by exsanguination into abdominal cavity (<28 day survival).  Observations were subsequently confirmed by a clinical and experimental pathologist blinded to treatment type. Necropsy was performed for all mice that died prior to 28 days to determine cause of death. Tissues were stored in fresh 10% formalin overnight before being embedded and serial sectioned into 5 or 10 ?m sections.   61  3.2.7 HISTOLOGY Formalin-fixed, paraffin-embedded murine abdominal aortic sections were sections were de-paraffinized and rehydrated in xylene and ethanol, then stained for hematoxylin and eosin (H&E), Movat?s pentachrome, and picrosirius red as previously described. 283  3.2.8 IMMUNOHISTOCHEMISTRY Formalin-fixed, paraffin-embedded murine abdominal aortic sections were sections were de-paraffinized and rehydrated in xylene and ethanol, then antigen retrieval was performed by boiling slides in citrate buffer (pH 6.0) to expose antigens masked in the fixation process as per recommended protocol from antibody supplier. Immunohistochemistry (IHC) was performed using goat anti-human fibrillin-1 Abs (N-19 and C-19, Santa Cruz Biotechnology, Inc., Santa Cruz, CA), rabbit anti-mouse GZMB (Abcam, Cambridge, MA), goat anti-mouse decorin (R&D Systems, Minneapolis, MN) as previously described.425  Negative control staining was performed in the same manner for all tested antibodies, with absence of primary antibody. For the GrB/Mast cell double stains, slides were prepared by removing paraffin, hydrating, and treated with boiling citrate buffer. Blocking was done by adding 10% goat serum to all sections and incubating for 30 min. The primary antibody used was a rabbit anti-mouse GrB antibody (1:100 in 10% goat serum). After overnight incubation, a goat anti-rabbit secondary antibody was applied and incubated for 1 hour. An ABC-alkaline phosphatase (AP) reagent was then prepared and applied. After 30 minutes incubation, Vector Red substrate was prepared and applied to the sections to produce the red GrB stain. The sections were counterstained with alcian blue (in 0.7N HCL) for 20 minutes. 3.2.9 IMMUNOPRECIPITATION OF SERUM FIBRILLIN-1 FRAGMENTS Fibrillin-1 fragments in mouse blood serum were isolated by immunoprecipitation then analyzed by western blotting. Briefly, proteins were denatured, separated on a 10% SDS-polyacrylamide gel, 62  transferred to a nitrocellulose membrane and blocked with 10% skim milk. The membrane was then probed using goat anti-human fibrillin-1 antibodies (N-19 and C-19, Santa Cruz Biotechnology, Inc., Santa Cruz, CA) and bands were imaged using the Odyssey Infrared Imaging System (LI-COR Biotechnology, Lincoln, NE). 3.2.10 CONFOCAL MICROSCOPY Formalin-fixed, paraffin-embedded human TAA sections were de-paraffinized and rehydrated in xylene and ethanol. Antigen retrieval was performed by boiling slides in citrate buffer (pH 6.0) to expose antigens masked in the fixation process as per recommended protocol from antibody supplier. To demonstrate colocalization of GZMB in macrophages, background staining was blocked by incubation of sections in 10% goat serum. Sections were incubated in a 1:100 dilution of rabbit anti-GZMB (Abcam) overnight, followed by incubation in biotinylated goat anti-rabbit secondary antibody (Vector laboratories) and ABC reagent (Vector laboratories). GZMB staining was visualized with TSA(TM) Plus Fluorescence Systems cyanine 3 tyramide (PerkinElmer Life Sciences, Inc., Boston, MA). To assess for macrophages, slides were incubated with avidin and biotin to prevent interaction of labelling reagents and blocked with 10% horse serum. Sections were incubated in a 1:100 mouse anti-MAC387 (AbD Serotec, UK) overnight followed by incubation in biotinylated horse anti-mouse secondary antibody (Vector laboratories) and ABC reagent (Vector laboratories). MAC387 staining was visualized with TSA(TM) Plus Fluorescence Systems fluorescein tyramide (PerkinElmer Life Sciences, Inc.). Similarly, for co-localization of GZMB in lymphocytes, slides were first stained for GZMB with mouse anti-GZMB (Dr. Joseph Trapani) then incubated with avidin and biotin to prevent interaction of labelling reagents and blocked with 10% goat serum. Sections were incubated with 1:100 mouse anti-CD3 (Abcam) overnight followed by incubation in biotinylated goat anti-rabbit secondary antibody (Vector laboratories) and ABC reagent (Vector laboratories). CD3 staining was visualized with TSA(TM) Plus Fluorescence Systems fluorescein tyramide (PerkinElmer Life Sciences, Inc.). Confocal images of fluorescently labelled tissue 63  sections were acquired with a Leica AOBS SP2 laser scanning inverted confocal microscope (Leica, Heidelberg, Germany). Excitation beams were produced by Ar (488 nm for fluorescein) and HeNe (543 nm for cyanine 3) lasers (Leica AOBS SP2 module) respectively. Images from these dual stained samples were acquired sequentially to eliminate cross-talk between the emission signals and acquired images were overlaid using Volocity software (Improvisions, UK).  3.2.11 SECOND HARMONIC GENERATION MICROSCOPY Collagen structures in formalin-fixed tissue specimens were visualized using second harmonic generation (SHG) microscopy as previously described.426, 427 Ultra-short laser pulse was focused on the specimen through a Leica 63X/1.2 NA Plan-Apochromat water immersion objective. SHG signal in the forward direction originating from the histological specimens was captured using a non-descanned detector in the transmission geometry. In this non-descanned PMT detector (R6357, Hamamatsu, Shizuoka, Japan), a 440/20 nm band pass filter (MP 440/20, Chroma Technology, USA) was used to collect spectrally clean SHG signal. The gain and offset of the PMTs were adjusted for optimized detection using the color gradient to avoid pixel intensity saturation and background and these settings kept constant for all measurements. The 3D image restoration from the collected z-section images was performed using Volocity software (Improvisions, UK). For collagen signal density calculations, a noise removal filter with kernel size of 3X3 was used to define the boundary between foreground and background, and the lower threshold in the histogram was set to mean voxel intensity value. The total SHG signal intensity values thus generated were normalized by the total collagen volume (?m3) and expressed in arbitrary units (AU). 3.2.12 TRANSMISSION ELECTION MICROSCOPY All transmission electron microscopy (TEM) procedures were performed at the James Hogg Research Centre/Institute for Heart + Lung Health (JHRC/HLI). For sample processing, each mouse was perfused 64  with Krebs? buffer through the left ventricle until solution ran clear. Tissues were harvested and fixed overnight in 2.5% glutaraldehyde in 0.1 M sodium cacodylate buffer. After rinsing 3 times in  0.1M sodium cacodylate buffer, samples were post-fixed in a 1:1 mixture of 2% osmium tetroxide and 0.2M sodium cacodylate buffer then rinsed in distilled water and dehydrated in a graded series of acetone washes, then incubated with 1:1 solution of Epon and acetone for 2 hours. Samples were incubated overnight in a 2:1 solution of Epon and acetone then changed to 100% Epon for up to 8 hours, before embedded in pure Epon and polymerized at 60-65?C for 24 hours. Sections with a thickness of 60 nm were cut using a Leica EM UC6 microtome (Leica) and collected on nickel grids. Samples were analyzed on a Tecnai 12 Transmission Electron Microscope (FEI, Hillsboro, OR) TEM at the JHRC/HLI. Aortas were divided into healthy and dilated segments. 20 images of collagen fibrils in cross section from each segment were taken and analyzed for morphology and diameter.  3.2.13 IMMUNO-ELECTRON MICROSCOPY All immunogold electron microscopy procedures were performed at the James Hogg Research Centre/Institute for Heart + Lung Health (JHRC/HLI). For sample processing, each mouse was perfused with Krebs? buffer through the left ventricle until solution ran clear. Aortic rings of approximately 4 to 6 mm (length) x 2 to 3 mm (diameter) were excised and immediately fixed in 4% paraformaldehyde (Cat. 15710, Ted Pella Inc., Redding, CA) + 0.05% glutaraldehyde (Cat. 18427, Ted Pella Inc.) in PBS  at pH 7 for 2 hours at 4?C. Then rings were further cross sectioned into several pieces, each having a length of 2 mm. Care was taken to keep tissue including thrombus intact. Samples were further fixed overnight at 4?C. The next day, samples were processed using a Pelco Biowave Microwave. Briefly, samples were dehydrated in a graded series of ethanol (50%, 70%, 90% and twice at 100%) for 40 seconds with temperature restriction to 37?C. Infiltration was carried out with 50% LR White resin (Cat. 14381, Electron Microscopy Sciences) in ethanol, then 100% LR White resin, both steps for 15 min with 65  temperature restriction of 45?C. Finally these steps were carried out without the microwave: Infiltration with a second change of 100% LR White resin at room temperature overnight and then polymerization at 50?C the next day. Sections with a thickness of 60 nm were cut using a Leica EM UC6 microtome (Leica) and collected on nickel grids. For immunogold labeling of decorin, primary affinity-purified polyclonal goat anti-mouse decorin IgG antibody (Cat.AF1060, R&D Systems) was diluted at 1:20. Secondary antibody, F(ab?) 2 Fragment of ultra-small rabbit-anti-goat IgG, H&L (Cat. 25220, Electron Microscopy Sciences, Hatfield, PA) was diluted at 1:50. The following procedure was carried out using the Pelco Biowave Microwave with temperature restriction to 37?C. First, free aldehydes in sections were blocked using 50 mM glycine (BP381-500, Fisher Scientific, Waltham, MA) in PBS at pH 7.4 for 5 min. Sections were then blocked in 5% rabbit serum blocking solution containing 0.18% Cold Water Fish Skin Gelatin (Cat. 15717, Ted Pella, Inc.) for 3 x 5 min with 2 min rest, washed 3 x 5 min in acetylated-BSA containing (BSA-c, Cat. 25557, Electron Microscopy Sciences) PBS buffer (0.15% BSA-c, pH 7.4) and incubated in primary antibody made in the same BSA-c buffer for 6 x 5 min with 2 min rest. Control sections were incubated in normal rabbit serum diluted at 1:20 or PBS buffer alone. Subsequently, sections were washed in BSA-c buffer for 3 x 5 min, and incubated in secondary antibody for 5 x 5 min with 2 min rest. The following was carried out at room temperature without microwave: BSA-c buffer for 6 x 5 min, PBS for 3 x 5 min, 2% glutaraldehyde in PBS for 5 min, distilled water for 5 x 2 min, and silver enhancement for gold labeling with Silver R-Gent SE-EM (Cat. 5000.033, Aurion, Wageningen, The Netherlands) for 40 min. Finally, sections were then washed 5 x 2 min in distilled water, stained in 2% uranyl acetate and then lead citrate, washed, air dried and finally analyzed on a Tecnai 12 Transmission Electron Microscope (FEI) TEM at the JHRC/HLI.    66   3.2.14 STATISTICS AND DATA ANALYSIS  Statistical analysis was compiled in Graphpad Prism 5 (Graphpad Software). Survival curves were assessed using Log rank test for trend and Log rank/Mantel-Cox analysis. AAA incidence was assessed by Chi Square test for trend across all groups. SHG collagen signal density was assessed by one-way analysis of variance (ANOVA) and unpaired Student?s t-test. Collagen fibril diameter was measured with Image ProPlus (Media Cybernetics, Inc., Rockville, MD) and standard deviation, standard error, range, mean and median were assessed with Graphpad Prism 5 (Graphpad Software). For all tests, significant difference was set at p < 0.05.   67  CHAPTER 4: GZMB IN HUMAN AAA AND TAA 4.1 INTRODUCTION Elevated GZMB levels have been reported in advanced human atherosclerotic and allograft vasculopathy lesions but not in healthy coronary arteries.283  In the latter study, GZMB was present in lymphocytes, macrophage foam cells, medial and intimal SMC and it was found that extracellular staining increased with disease severity.283  High GZMB levels in the plasma correspond to increased carotid artery plaque instability and increased cerebrovascular events in humans.428  Furthermore, elevated GZMB production is observed in peripheral blood mononuclear cells isolated from patients with unstable angina pectoris compared to cells from patients with stable angina pectoris.388 As aneurysmal disease shares many risk factors with these conditions, GZMB is therefore a logical target for investigation as an effector agent in the pathogenesis of AAA.  In the current chapter, human AAA and TAA samples were assessed for the presence of GZMB in the vessel wall and localization within various immune cell types. 4.2 GZMB IS ELEVATED IN HUMAN AAA AAA tissue and healthy, non-atherosclerotic aorta tissue were stained for the presence of GZMB. No GZMB immunopositivity was observed in control healthy aorta (Figure 3A) while GZMB was abundantly present in AAA tissue (Figure 3B-F). GZMB positivity was noted in phagocytic cells trapped in the intima and fibrin platelet-red cell thrombus, and also in thrombic material attached to the deep atheroma. GZMB was also noted in cells in proximity to the medial neovasculature, in adventitial collagen layers, in remote adventitia containing nerves, and particularly in large collections of lymphocytes in the adventitia. Moderate staining was observed extracellularly and in the occasional SMC. GZMB positivity was also found in thrombic material with red cells, suggestive of GZMB in the 68  plasma. Intense granular cytoplasmic GZMB was also identified in neuroganglion cells of the remote adventitia.  4.3 GZMB COLOCALIZES TO IMMUNE CELLS IN HUMAN TAA Human TAA tissue was assessed for the presence of GZMB in various immune cell types (Figure 4 and Figure 5). GZMB was found to co-localize to mast cells in the adventitia when visualized by immunohistochemistry. Macrophages in the intima, adventitia and intraluminal thrombus, and CD3+ lymphocytes in the intima, media, adventitia and intraluminal thrombus were found to contain GZMB when visualized by confocal microscopy.        69   70   No GZMB immunopositivity was observed in control healthy aorta (A) while GZMB was abundantly present in AAA tissue (B-F). GZMB positivity was observed in the media and adventitia but particularly in the adventitial lymphocytes and extracellular connective tissue layers of AAA tissue (B).  Staining in the medial thrombus (C) was largely extracellular, however, within the intraluminal thrombus (D-E), GZMB positivity was noted in cellular elements trapped and scattered throughout the fibrin platelet red cell thrombus. GZMB was also noted in remote adventitia containing nerves, and in large collections of lymphocytes in the adventitia. Intense granular cytoplasmic GZMB was identified in nerve ganglion cells of the remote adventitia (F).  Negative control, no GZMB primary antibody, shows lack of non-specific staining (G). Scale bar at 40X = 50 ?m.   FIGURE 4: GZMB COLOCALIZATION WITH MACROPHAGES AND CD3+ CELLS IN TAA GZMB co-localization is observed in macrophages of the intraluminal thrombus when visualized by confocal microscopy. GZMB positivity is seen in red (B), macrophage marker MAC387 in green (A) and combined image in (C). Scale bar = 25 ?m. GZMB also co-localizes to lymphocytes in the intraluminal thrombus. GZMB positivity is seen in red (E), CD3 in green (D) and combined image in (F). Scale bar = 100 ?m. FIGURE 3: GZMB IN NON-ATHEROSCLEROTIC HUMAN AORTA VERSUS HUMAN AAA. 71    FIGURE 5: GZMB COLOCALIZATION WITH MAST CELLS IN TAA GZMB expression was observed in mast cells (arrows in A, B) of the adventitia in sections of human TAA stained for GZMB (red) by IHC and counterstained with Alcian blue. Scale bar: 40X = 50 ?m.     72  4.5 DISCUSSION Although GZMB expression was initially thought to be restricted to CTL and NK cells, it is now clear that, under conditions of cellular stress, aging and disease, GZMB expression can be induced in, and secreted by other types of immune cells (macrophages, mast cells, neutrophils) and non-immune cells (keratinocytes, chondrocytes, VSMC).283, 286, 429-432  As such, many cell types could potentially act as a source of GZMB in age-related degenerative diseases such as AAA. We observed substantial GZMB immunopositivity in human AAA whereas GZMB levels are minimal in corresponding healthy control aortas. Elevated GZMB was observed in lymphocytes trapped in the intraluminal thrombus of AAA. In addition, GZMB co-localized to macrophages (intima, adventitia and thrombus) and lymphocytes (intima, media, adventitia and thrombus) and mast cells (adventitia) in human TAA (Figure 4 and Figure 5), readily demonstrating a source of GZMB in situ. A role for GZMB in AAA rupture would be consistent with previous studies demonstrating that the aneurysm wall covered with thrombus exhibits increased inflammation, mast cell activation221 and association with neovessels in the media and adventitia220, macrophage infiltration,414, 433-435 SMC apoptosis and ECM degradation, thus subjecting this region to greater risk of rupture.436 Based on this immunohistochemical assessment of human aneurysmal disease, and given that GZMB is well characterized for its role in perforin-mediated apoptosis and is now known to degrade various extracellular targets in the ECM,326, 327 we hypothesize that GZMB could contribute to AAA pathogenesis through medial and adventitial weakening in addition to the destabilizing effects initiated at the intraluminal thrombus and this warrants experimental testing in a suitable animal model for further elucidation.     73  CHAPTER 5: PERFORIN DEFICIENCY IS NOT PROTECTIVE AGAINST ANGIOTENSIN II-INDUCED MURINE AAA 5.1 INTRODUCTION The proteolytic mechanisms that are associated with ECM degradation in AAA are areas of active investigation. Several types of proteases, including matrix metalloproteinases (MMP)-2, -7, -9, -12, cathepsins, plasminogen activators, and elastases, have been proposed to contribute to the degradation of fibrillar ECM proteins.437  In the aorta, microfibrils associate with elastin in the tunica media to form the concentric lamellae that separate individual SMC layers and confer elasticity to the aortic wall. Microfibrils also act to stabilize the vessel wall by connecting lamellar rings to one another, to SMC, and to the sub-endothelial basement membrane.343 Fibrillin-1 is the major scaffolding component of microfibrils and thus plays a key role in maintaining vessel wall stability. It has previously been shown to be a cleavage target for the serine protease GZMB344 and in the current chapter, we shall examine possible roles for GZMB in aneurysmal pathogenesis via a well-established mouse model of Ang II-induced AAA.438  In this model, macrophage accumulation in the media of the suprarenal aorta and dissection precede the formation of aneurysm and atherosclerosis in Ang II-infused apoE-KO mice. While thrombi are usually constrained by adventitial tissue, rupture of the abdominal aorta and subsequent death due to abdominal bleeding is often observed.414  We hypothesize that GZMB is elevated in AAA and contributes to vessel wall instability and aneurysm formation. To test this, GDKO and PDKO mice were generated and infused with Ang II to induce AAA formation.  5.2 GZMB DEFICIENCY BUT NOT PERFORIN DEFICIENCY IMPROVES OUTCOMES IN AAA 5.2.1 SURVIVAL AND INCIDENCE OF AAA Ang II-treated GDKO mice but not PDKO mice had a significant increase in 28-day survival (92.86%; n = 14, 56.25%, n = 16) when compared to apoE-KO mice (53.33%; n = 15). Survival curves 74  (Figure 6) were analysed by log-rank test and a statistically significant difference between survival curves was found (P = 0.002). Upon necropsy, all mice that died prematurely (<28 days) were found to have large aortic dissections and extensive blood clots in the abdominal cavity suggesting that the cause of death was rupture and exsanguination. When surviving mice from Ang II-treated groups were assessed, a further 40.0% of apoE-KO mice and 43.75% of PDKO mice were found to have AAA compared with only 28.57% of GDKO mice, reducing total incidence of aneurysm-related pathology from 86.67% in apoE-KO mice and 87.50% in PDKO mice to 35.71% in GDKO mice (Figure 7). Rupture was observed in only 1/14 GDKO mice and aneurysms that were observed in this group were all small and localized, suggesting that GZMB contributes to the onset and progression of aneurysm. No dissection, AAA or mortality was observed in any of the saline-infused groups. Representative images of aorta gross morphology and cross sections are shown in Figure 7. 5.2.2 MEDIAL INTEGRITY As previously observed,414 medial disruption characterised by the loss of elastin is a predominant pathological feature of AAA; both in human aneurysms and in Ang II-induced murine AAA. While incidence of AAA and dissection was much reduced in GZMB-deficient mice, severe elastin breakage was frequently observed in conjunction with aneurysm development in both apoE-KO and PDKO mice. Severe medial disruption, as seen in apoE-KO and PDKO mice, was not observed in GDKO mice (Figure 8). 75     FIGURE 6: KAPLAN-MEIER SURVIVAL CURVE FOR APOE-KO, PDKO AND GDKO MICE ON ANG II Mice were implanted with mini-osmotic pumps containing saline or Ang II as described to induce aneurysm formation. Lines represent percentage of mice alive on each day post-implantation. No death was observed in saline control groups (n = 8 for apoE-KO, n= 11 for GDKO, n = 5 for PDKO).  In contrast, 92.86% of GDKO (n = 14), 53.33% of apoE-KO (n = 15) and 56.25% of PDKO (n = 16) infused with Ang II survived to 28 days. Survival curves were significantly different when analysed by log-rank test (p = 0.002).  76    FIGURE 7: REPRESENTATIVE GROSS PATHOLOGY, MORPHOLOGY AND SUMMARY OF OUTCOMES Upon euthanasia or necropsy, hearts and aortas were harvested and graded for severity of phenotype. Aortas were cross sectioned and stained with H&E to assess morphology. (A, E) Heart and aorta with no visible pathology (all saline groups, apoE 13.33%, GDKO 64.29%, PDKO 12.50%). (B, F) Small localized AAA with or without visible hematoma (apoE 13.3%, GDKO 28.57%, PDKO 37.50%). (C, G) Large dissecting AAA with visible hematoma in false lumen surrounding the abdominal aorta and extending above the diaphragm (apoE 26.67%, GDKO 0%, PDKO 6.25%). (D, H) Fatal ruptured dissecting AAA with abdominal bleeding (apoE 46.7%, GDKO 7.14%, PDKO 43.75%) was also observed. Outcomes are summarised in (I). Scale bar: 4X = 1mm. 77   FIGURE 8: GZMB DEFICIENCY REDUCES MEDIAL DISRUPTION Formalin-fixed tissues collected from mice that survived to 28 days were assessed using Movat?s Pentachrome as described. (A, B) Representative Aorta from GDKO mouse with no medial disruption, minimal adventitial thickening. (C, D) Aorta from apoE-KO with elastin breakage, pronounced adventitial thickening, but no visible hematoma. (E, F) Aorta from PDKO mouse with small AAA, elastin breakage, remodelled medial hematoma. (G, H) Aorta from PDKO mouse with dissection, elastin breakage and pronounced dilation. Scale bars: 4X = 1mm, 40X = 50 ?m      78  5.3 PERFORIN DEFICIENCY DOES NOT REDUCE GZMB LEVELS IN ANG II-INDUCED AAA When assessed by IHC staining, GZMB was elevated in the aortas of apoE-KO and PDKO mice that developed AAA following Ang II infusion (Figure 10). No GZMB immunopositivity was observed in healthy, age-matched controls that received saline infusion. GZMB was particularly abundant in the thrombus, both in trapped lymphocytes and in the extracellular space, as well as in the adventitia of large, dissecting AAA. Furthermore, GZMB was shown to colocalize to mast cells in the adventitia of apoE-KO mice that developed AAA (Figure 9).          FIGURE 9: GZMB COLOCALIZES TO MAST CELLS Aortas from apoE-KO mice that developed AAA were double-stained for GZMB and mast cells, GZMB was visualized by Vector Red and mast cells were counterstained with Alcian Blue. Mast cells are indicated by arrows. Scale bar 10X = 200?m, 40X = 50?m. 79    FIGURE 10: GZMB IMMUNOSTAINING IN APOE-KO AND PDKO GZMB-immunopositivity is observed in the medial thrombus of an apoE-KO mouse that suffered a small, saccular AAA (A, B) and the adventitia of a PDKO mouse that suffered a large dissecting AAA (C, D). Trapped lymphocytes in the thrombus are indicated by arrows. GZMB staining is not observed in apoE-KO mice that received saline infusion (E, F). Scale bars: 4X = 1mm, 40X = 50 ?m. 80  5.4 PERFORIN DEFICIENCY DOES NOT PREVENT MEDIAL FIBRILLIN-1 LOSS The levels of fibrillin-1 immunopositivity in the tunica media of apoE-KO and PDKO mice that received Ang II infusion was frequently greatly reduced, especially in the tunica media of dilated, aneurysmal aorta in the regions adjacent to the thrombus compared to GDKO mice. (Figure 11) 5.5 GZMB DEFICIENCY REDUCES FIBRILLIN-1 FRAGMENTATION IN MOUSE SERUM ApoE-KO and GDKO mice blood serum levels of fibrillin-1 were assessed by immunoprecipitation and western blotting. All samples tested have a band at approximately 50kD representing immunoglobulin heavy chain; however, it was observed that samples from apoE-KO mice with aneurysms following infusion with Ang II have a distinct second band at approximately 45 kD that is not seen in samples from GDKO mice that received similar treatment or in apoE-KO mice that received the saline control, suggesting that significant fibrillin-1 fragmentation is prevented in GZMB-deficient mice  compared to apoE-KO controls following Ang II infusion (Figure 12). 5.6 GZMB TREATMENT DOES NOT ALTER FIBRILLIN-1 EXPRESSION IN VIVO Fibrillin-1 transcription levels in HCASMC following treatment with GZMB were assessed by RT-PCR (Figure 12C). It was found that there was no difference in fibrillin-1 expression in HCASMC treated with GZMB compared to controls, suggesting that GZMB does not have any direct effect on fibrillin-1 transcription levels.  81   FIGURE 11: FIBRILLIN-1 STAINING IN APOE-KO AND PDKO IS REDUCED COMPARED TO GDKO Decreased fibrillin-1 staining, as indicated by red colour, was observed in apoE-KO and PDKO mice (A and B respectively) compared to GDKO mice (C) that received Ang II. Scale bar = 50?m.   82   FIGURE 12: GZMB CLEAVES FIBRILLIN-1 IN VITRO AND IN VIVO (A) GZMB cleaves fibrillin-1 from HCASMC-generated ECM in vitro.  HCASMC were cultured to confluency, lysed, and ECM was biotinylated as described. Human GZMB, isolated from IL-2 stimulated lymphocytes was then added to biotinylated ECM and supernatants were collected to assess ECM fragments that were released from the plate. GZMB-mediated fibrillin-1 cleavage was attenuated by the GZMB inhibitor DCI but not by the inhibitor solvent control DMSO. Fibrillin-1 fragments from the supernatant are indicated by an arrow. Gel is representative of 6 independent experiments. (B) Fibrillin-1 fragmentation is prevented in GDKO mouse serum compared to apoE-KO following Ang II infusion. Fibrillin-1 in mouse serum was isolated by immunoprecipitation and analyzed by western blotting. ApoE-KO mice with aneurysm show a distinct band at approximately 45 kD that is not observed in GDKO on Ang II and apoE-KO on saline. (C) In vitro fibrillin-1 expression is not directly affected by GZMB treatment. HCASMC were treated with GZMB and assessed for fibrillin-1 transcription by RT-PCR. No difference was observed in cells that received treatment compared to controls.    83  5.7 DISCUSSION While GZMB was absent in healthy, non-atherosclerotic aorta, intense immunopositivity was observed in AAA tissue from both humans (Figure 3) and mice (Figure 10). This complements previous studies where GZMB expression has been shown to increase in both the lesion and plasma as atherosclerotic disease severity increases283, 428 however, the mechanism by which GZMB exerts its pathogenic effects are unclear. As it is difficult to demonstrate a causative relationship between GZMB and AAA in humans, we have utilized the well-established murine model of Ang II-induced AAA. In this model, macrophage accumulation in the media, medial disruption and dissection precede AAA and atherosclerosis.414  GZMB deficiency resulted in a significant decrease in the total incidence of AAA from 86.67% in apoE-KO and 87.5% in PDKO mice to 35.71% in GDKO. Incidence of rupture was also reduced from 46.67% in apoE-KO and 43.75% in PDKO to 7.14% in GDKO resulting in a much improved survival rate for GZMB-deficient mice (Figure 6 and Figure 7).  It is important to note that the GZMB-KO mouse439 is a cluster knockdown in which some of the lesser abundant ?orphan? granzymes, unique to mice located close to GZMB on chromosome 14 (C, F, D, and G) have been reported to exhibit reduced expression.440 However, it is unlikely that this would affect the outcome of this study as these granzymes are not present in humans and GZMB is highly expressed in the area of injury in both mice and humans while these other granzymes are not detectable in the vasculature (unpublished observations) and their physiological role in general  has not been determined. The results from the PDKO group are of particular note because perforin is essential for GZMB-mediated internalization and induction of apoptosis in target cells.441  As GZMB is only one of multiple granzymes released towards target cells, many studies previously utilized perforin-KO mice to evaluate the role of the granule pathway in disease with the assumption that perforin is necessary for granzyme internalization and apoptosis. In this scenario, if perforin deficiency did not affect outcome, it was often indirectly concluded that granzymes were not involved and/or did not contribute to disease outcome. 84  One shortcoming of this approach, as demonstrated in the present study, was that it ignored the possibility that granzymes could exhibit perforin-independent, extracellular activity that might contribute to disease. As perforin deficiency in this model does not provide any protective effect, it is unlikely that the classical GZMB/perforin-mediated apoptosis pathway is involved in AAA and this would suggest that the pathological effects exerted by GZMB are largely extracellular. In support of this concept, GZMB is capable of cleaving numerous extracellular proteins (reviewed in Boivin et al.442) This does not rule out a role for GZMB in cell death as extracellular GZMB can induce perforin-independent detachment-mediated apoptosis, or anoikis, of VSMC in vitro through the cleavage of ECM.327  Although we did not observe a significant difference in SMC apoptosis at 28 days post-implant (data not shown), it is often difficult to capture apoptotic cells in vivo as apoptotic cell debris is rapidly removed by neighboring cells and it is possible that measurable levels of apoptosis may be observed at an earlier time-point. Nonetheless, the protective effect of GZMB deficiency on AAA is clearly evident. The ability for GZMB to cleave ECM components such as fibrillin-1, decorin, fibronectin, vitronectin, laminin and aggrecan98, 325-327 has been documented previously.  Of note, fibrillin-1 is the major scaffolding component of microfibrils and plays a key role in maintaining vessel wall stability.  In the aorta, fibrillin-1 associates with elastin to form the concentric elastic lamellae of the tunica media that confer elasticity to the vessel.  In addition, microfibrils not associated with elastin act to stabilize the vessel wall by connecting lamellar rings to one another, to SMC, and to the sub-endothelial basement membrane.331, 343  Together with collagen, fibrillin-1 microfibrils in the adventitia provide load-bearing support for the entire vessel.443  Increased GZMB staining and correspondingly reduced fibrillin-1 staining were observed in the aneurysmal aortas of apoE-KO and PDKO mice when compared to GDKO mice, suggesting that GZMB 85  degradation could contribute to the loss of fibrillin-1 (Figure 11).  Furthermore, western blot analysis of mouse blood serum showed the presence of an extra fibrillin-1 fragment in apoE-KO samples that was not observed in GDKO mice following infusion of Ang II (Figure 12B) suggesting that GZMB deficiency prevents the degradation of fibrillin-1. In addition to this, in vitro GZMB treatment of HCASMC did not have any direct effect on fibrillin-1 transcription levels (Figure 12C) supporting the assertion that any decrease in fibrillin-1 observed in the aortas of apoE-KO mice was most likely attributed to proteolytic cleavage by GZMB. It should be noted that the fibrillin-1 fragment size detected in mouse serum differs from the fragments observed in human SMC-generated ECM supernatant. There could be a number of reasons for this discrepancy. Firstly, it is possible that human GZMB cleaves human fibrillin-1 at a different site on the protein compared to its murine counterpart. Indeed, previous studies have shown that mouse and human granzymes can differ in substrate specificities.444 Secondly, different antibodies were required to detect mouse versus human fibrillin-1, as such, while we were unable to detect the larger fragment previously seen in the in vitro study, it is possible that fibrillin-1 might have multiple cleavage sites or that the murine antibody does not detect this fragment. These results may be of relevance to the fibrillin-1 deficiencies associated with Marfan syndrome, but whether structural changes in fibrillin-1 due to mutations associated with Marfan syndrome predispose it to GZMB cleavage is unknown.  Fibrillin-1 null mice die perinatally from ruptured aortic aneurysm and impaired lung function.445 They have abnormally smooth elastic lamellae and exhibit a loss in VSMC attachments normally mediated by fibrillin-1.446 Hence, the degradation of fibrillin-1 by GZMB could contribute to medial disruption and subsequent fragmentation of elastic lamellae that is commonly observed in aneurysms as shown in Figure 3, but also affect VSMC attachment and phenotype, ultimately resulting in a decrease in VSMC, loss of structural integrity and a predisposition towards dilation, dissection and the subsequent formation of aneurysms or rupture.447  86  One of the major risk factors for aneurysm formation is advanced age. During aging, the elastin to collagen ratio is reduced thereby leading to arterial stiffness and reduced compliance during contraction.448-450  Chronic inflammation during atherosclerosis is associated with vessel wall remodelling and a loss of integrity. GZMB levels increase in the intima, media and adventitia with the severity of atherosclerotic disease,283 a condition that is associated with increased risk for developing AAA.  Elevated plasma levels of GZMB are found in patients with unstable versus stable carotid plaques and are associated with an increased occurrence of cerebral vascular events suggesting that GZMB contributes to plaque instability.428  Furthermore, a recent study has suggested a link between GZMB and unstable angina pectoris in which mononuclear cells from unstable angina pectoris patients exhibited greater GZMB production compared to cells from stable angina pectoris or healthy controls.388  As GZMB has been previously found to retain its activity in plasma,451 it is not unreasonable to propose a mechanism whereby chronic inflammation with macrophage infiltration and mast cell activation could lead to increased extracellular GZMB levels in and around the vessel wall. GZMB then cleaves ECM components such as fibrillin-1, contributing to the loss of elastic lamellae, medial degeneration, vessel wall instability and subsequent aneurysm formation, after which further assault by GZMB on the adventitial layer that maintains the structural integrity of the vessel could lead to rupture of the aorta.  As such, the greatest concern upon diagnosis of an aneurysm is the significantly increased risk of fatal aortic rupture. GZMB-deficiency appears not only to reduce incidence of AAA formation but also considerably reduces the incidence of aortic rupture in our model. While premature fatal rupture occurred in over half of apoE-KO and PDKO mice, only 1 in 14 GDKO mice was found to have died from rupture and exsanguination.  In this model, the preliminary dissection that disrupts the media and results in a visible hematoma is usually constrained initially by the tunica adventitia, allowing for remodeling of the 87  thrombus and reforming of the endothelium.414  Combining the observation of greatly reduced rupture in GDKO mice with the extensive GZMB immunopositivity observed in the adventitia, it is possible that increased GZMB activity contributes to adventitial degeneration and weakening thereby facilitating expansion and susceptibility to rupture. This could account for the fact that AAA observed in apoE-KO and PDKO were more likely to have expanded with loss of adventitia and progressed to premature rupture of the vessel wall. When the apoE-KO and PDKO mice that died prematurely were examined, large blood clots were always found in the abdominal cavity and thrombus material often spanned the entire length of the aorta from the kidneys to the heart. It is exciting to speculate that GZMB inhibition could limit aneurysm expansion by maintaining adventitial structural integrity; and shall be explored in the subsequent chapter.     88  CHAPTER 6: INHIBITION OF GZMB REDUCES RATE OF RUPTURE IN ANGIOTENSIN-II-INDUCED AAA 6.1 INTRODUCTION In Chapter 5, it was shown, using GDKO and PDKO mice, that the serine protease GZMB contributes to AAA through an extracellular, perforin-independent mechanism and is abundantly expressed in the thrombus and adventitia of aneurysmal aortas,328 making GZMB an attractive therapeutic target for the treatment of AAA.  GZMB has previously been shown to cleave the ECM proteoglycan decorin.98 Decorin is a small chondroitin/dermatan sulphate proteoglycan belonging to the family of small leucine-rich proteoglycans. In healthy aorta, decorin localizes primarily to the adventitia, where it interacts extensively with other ECM proteins and plays an important role in ECM assembly. Decorin plays numerous roles in collagen organization, formation of tight collagen bundles and is thus considered a mediator of adventitial tensile strength. In the subsequent chapter, we explore the hypothesis that the inhibition of GZMB would reduce the incidence and severity of aneurysm development in the Ang II-induced murine model of AAA and examine the effect of GZMB inhibition on AAA rupture with two different inhibitors: Serpin A3N (SA3N) and a neutralizing anti-GZMB antibody (Anti-GZMB).   6.2 SA3N TREATMENT 6.2.1 SA3N INHIBITS MURINE GZMB ACTIVITY IN VITRO Pre-incubation with SA3N dose-dependently inhibited cleavage of the GZMB tetrapeptide substrate IEPD-pNA by murine GZMB (Figure 13, IC50 = 11.83 nM, range 7.925 - 17.66 nM).  These results are consistent with previous results demonstrating that SA3N inhibits both recombinant human GZMB and mouse cytotoxic T lymphocyte degranulate GZMB. 366 89   FIGURE 13: SA3N INHIBITS MURINE GZMB ENZYMATIC ACTIVITY. Mouse GZMB (20 nM) mediated cleavage of the colorimetric substrate Ac-IEPD-pNA (1 mM) was inhibited by increasing concentrations of SA3N. Percent activity was determined as the change in initial rate relative to the initial rate in the absence of SA3N. IC50 = 11.83 nM (range 7.925 - 17.66 nM). Data represent the mean + SEM of 3 experiments.     90  6.2.2 SA3N DOSE-DEPENDENTLY IMPROVES OUTCOMES IN AAA Necropsy was performed on all mice that died prematurely before the 28-day time point and in all cases confirmed the presence of large ruptured aortic dissections and extensive blood clots in the abdominal cavity suggesting that death was caused by exsanguination following AAA rupture. The survival rate of mice that received a saline sham treatment was 50.0% while SA3N treatment improved survival of mice in a dose-dependent manner (Figure 14A, SA3N 120 ?g/kg, n = 9/11, 81.8%; SA3N 40 ?g/kg, n = 15/18, 83.3%; SA3N 20 ?g/kg, n = 7/10, 70.0%; SA3N 4 ?g/kg, n = 6/11, 54.5%; saline sham, n = 6/12, 50.0%). Log rank test for trend shows a significant increase in survival with dose of SA3N received (p = 0.0207). Mice that received 40 ?g/kg SA3N demonstrated a significant increase in survival when compared to mice that received sham treatment (Log rank/Mantel-Cox test, p = 0.0370). Incidence of aneurysm rupture was reduced from 50.0% in saline controls to 18.2% in mice that received 120 ?g/kg SA3N; 16.7% in those that received 40 ?g/kg (16.7%); 30.0% in those that received 20 ?g/kg and 45.5% in the group that received 4 ?g/kg (Figure 14B).   6.2.3 GZMB IMMUNOPOSITIVITY CORRESPONDS TO REGIONS OF MEDIAL DISRUPTION AND LOSS OF FIBRILLIN-1 AND DECORIN Serial sections of abdominal aorta from a sham-treated mouse following aortic rupture were stained for Movat?s Pentachrome (Figure 15A-C), GZMB (Figure 15D-F), and decorin (Figure 15G-I). Medial GZMB staining (Figure 15F) corresponded to the region of vessel dilation where profound loss of elastic lamellae was observed (Figure 15C). The same region exhibits a marked reduction in decorin (Figure 15I) proximal to the site of aneurysm. Minimal GZMB staining was noted on the non-dilated, healthy side of the vessel wall (Figure 15E) which displayed intact elastic lamellae (Figure 15B) and where robust decorin (Figure 15H) was observed.  91   FIGURE 14: GZMB INHIBITION INCREASES 28-DAY SURVIVAL AND REDUCES RATE OF ANEURYSM RUPTURE. (A) Survival. SA3N 120 ?g/kg (81.8%, n = 11); SA3N 40 ?g/kg (83.3%, n=18); SA3N 20 ?g/kg (70.0%, n = 10); SA3N 4 ?g/kg (54.5%, n = 11); Saline sham (50.0%, n = 12). Log rank test for trend shows a significant increase in survival with dose of SA3N received (p = 0.0207). (B) Total aneurysm incidence and rupture. SA3N 120 ?g/kg (Incidence 63.3%; Rupture 18.2%); SA3N 40 ?g/kg (77.8%; 16.7%); SA3N 20 ?g/kg (90.0%; 30.0%); SA3N 4 ?g/kg (81.8%; 45.5%); Saline sham (91.7%; 50.0%).   92      FIGURE 15: GZMB IS ABUNDANT IN VESSELS EXHIBITING MEDIAL DISRUPTION. Serial sections of abdominal aorta were taken from a sham-treated mouse following aortic rupture and stained for Movat?s Pentachrome (4X: A, 40X: B, C), GZMB (4X: D, 40X: E, F), and decorin (4X: G, 40X: H, I).   GZMB staining by immunohistochemistry (D, F) corresponds to regions of medial disruption and elastin fragmentation (A, C) and loss of decorin in the adventitia (I). The non-dilated side of the aorta has reduced GZMB staining in the media and adventitia (D, E) and corresponds to intact elastic lamellae (A, B) and decorin (G,H). Scale bars: 4X = 500?m; 40X = 50 ?m. 93  6.2.4 SA3N REDUCES GZMB IMMUNOPOSITIVITY IN MOUSE AORTA GZMB was detected in the aortas of apoE-KO sham-treated mice (Figure 15 and Figure 16) that developed AAA and was particularly elevated in aneurysms that progressed to rupture. Conversely, immunodetection of GZMB was reduced in animals receiving SA3N treatment particularly in the adventitia even when small aneurysms were present (Figure 16). GZMB was found to colocalize to mast cells in murine aneurysmal tissue (Figure 9). These mast cells were often associated with areas of high density GZMB staining in the adventitia and thrombi. No clear dose-dependent trend was observed regarding the concentration of mast cells and co-localization with GZMB (data not shown). Non-ruptured aneurysms with similar pathology from both SA3N- and sham-treated groups were assessed for immune cell infiltration (CD3-positive T lymphocytes and activated macrophages); however no significant difference in the levels of T lymphocytes or macrophages was observed (data not shown). 6.2.5 SA3N INHIBITS GZMB-MEDIATED CLEAVAGE OF DECORIN IN VITRO Active, purified human GZMB was incubated in vitro with recombinant human decorin protein. Decorin loss was observed by western blot (Figure 17A) and cleavage products were detected by Ponceau staining (Figure 17B). GZMB incubation resulted in production of numerous cleavage fragments ranging in size from approximately 15-55 kDa. GZMB-mediated decorin cleavage was abolished in the presence of SA3N.     94   FIGURE 16: GZMB IMMUNOPOSITIVITY IS REDUCED IN SA3N-TREATED MICE. GZMB staining is minimal in healthy aorta (A) in both the media (represented by ?M?) and the adventitia (represented by ?Ad?). GZMB expression is increased particularly in the adventitia of sham-treated mice that experienced rupture (B). Reduced GZMB was observed in SA3N-treated mice with no AAA (C) and small, remodelled AAA (D). Scale bars: 4X = 500 ?m; 40X = 50 ?m.      95   FIGURE 17: GZMB CLEAVAGE OF DECORIN IS PREVENTED BY PRE-INCUBATION WITH SA3N IN VITRO. Purified GZMB was pre-incubated with or without SA3N for 25 minutes at RT prior to incubation with recombinant human decorin for 24 hr at RT. Proteins were separated by SDS-PAGE, transferred to a nitrocellulose membrane and visualized by western blotting for decorin (A) and Ponceau stain (B). In Figure 5A, full length decorin is indicated by an asterisk (*) and is visible at approximately 65 kDa in lane 1. Decorin is severely reduced following incubation with GZMB in lane 2. In lane 3, Decorin loss is not observed following pre-incubation of GZMB with SA3N. In Figure 5B, SA3N and SA3N/GZMB complex at approximately 47 KDa and 70 KDa, respectively (indicated by ^) are visible in lanes 2, 3 and 6. Full length decorin is indicated by the asterisk (*) at approximately 65 KDa in lanes 4, 5 and 6. Decorin cleavage fragments in lane 5 are marked by arrows on the right side of the membrane.  96  6.2.6 SA3N PROMOTES ADVENTITIAL THICKENING AND PREVENTS LOSS OF DECORIN  Decorin was markedly reduced in the adventitial regions of aortas from saline sham-treated mice that developed aneurysm (Figure 18I) and exhibited rupture (Figure 18F) compared to aortas of healthy animals (Figure 18C). The adventitia surrounding the thrombus was thin and collagen content was also greatly reduced (Figure 18D, G). With picrosirius red staining under polarized light, adventitial collagen fibres from sham-treated ruptured and non-ruptured aneurysmal aorta (Figure 18D, G) appear green and yellow whereas adventitial collagen fibres from healthy aorta (Figure 18A) have a higher proportion of orange and red fibres, indicating higher fibre density. In comparison, the majority of aneurysms observed in the GDKO and SA3N-treated groups that received the two highest doses (120 and 40 ?g/kg) were small aneurysms with minimal thrombus that did not progress to rupture and displayed thickened adventitia, increased decorin staining (Figure 18L, R) and collagen fibres of higher density (Figure 18J, P) compared to ruptured, sham-treated aortas.  6.2.7 SA3N REDUCES LOSS OF COLLAGEN DENSITY IN AAA Collagen fibre imaging by SHG confirmed a reduction in collagen density loss in SA3N-treated mice compared to the saline sham-treated groups (Figure 19). Analysis by one-way ANOVA demonstrated a significant difference across all groups (p = 0.0226). SHG signal density representing fibre density was reduced in ruptured and non-ruptured (NR) sham-treated groups compared to normal and SA3N-treated mice, however only the difference between normal and sham-treated groups achieved statistical significance (sham NR, p = 0.0268; sham ruptured, p = 0.0218). Notable differences in collagen architecture and overall appearance were also observed in sham-treated aneurysmal aortas compared to healthy and SA3N-treated groups, with profound loss of collagen organization and thick bundle formation. 97   98  FIGURE 18: SA3N AND GZMB DEFICIENCY PROMOTES ADVENTITIAL THICKENING AND INCREASED DECORIN CONTENT IN AAA. Sections stained with picrosirius red are shown in the first column (A, D, G, J, M , P). Tissues immunostained for decorin are shown in the second column (B, E, H, K, N, Q) and enlarged for emphasis at 40X in the third column (C, F, I, L O, R). When stained with picrosirius red, thick collagen fibres evidenced by strong birefringence (red) under polarized light (A) are visible in the adventitia of healthy normal aorta, as well as robust decorin staining (B, C). Non-ruptured aneurysms from sham-treated mice exhibit bulbous thrombus, reduced decorin staining (H, I) and thinner collagen fibres of lower density (G) as evidenced by predominance of yellow colour when stained with picrosirius red and seen under polarized light. Ruptured aneurysms from sham-treated mice demonstrate even greater loss of collagen content (D) and a similarly reduced amount of decorin in the adventitia (E, F) compared to healthy aortas. In comparison, aortas from mice that received SA3N (120 ?g/kg) prior to Ang II pump implantation demonstrate a significant increase in collagen and a thickened adventitial layer, with a greater proportion of red and orange fibres under polarized light (J). Decorin content in the adventitia is also greatly increased (K, L). Morphology and levels of collagen and decorin in normal, non-diseased aorta from GDKO mice resemble healthy controls. (M, N, O) while GDKO that develop small, localized aneurysm display thickened adventitia (P) and increased decorin content (Q, R) similar to SA3N-treated mice. Scale bars: 4X = 500 ?m; 40X = 50 ?m.  99   FIGURE 19: SA3N TREATMENT REDUCES THE LOSS OF COLLAGEN FIBRE DENSITY IN ANG II-TREATED APOE-KO MICE. Adventitial collagen from healthy mouse (A), sham-treated mouse with ruptured aorta (B), SA3N-treated mouse (C), and sham-treated mouse, non-ruptured (NR) aorta (D) were assessed by SHG. (Scale bar: 25 ?m). SHG signal densities are summarized in (E) (n = 3 per group, * p < 0.05). Error bars indicate SEM. 100  6.2.8 SA3N REDUCES COLLAGEN FIBRIL DIAMETER IRREGULARITY AND ADVENTITIAL DECORIN LOSS AT THE ULTRASTRUCTURAL LEVEL Adventitial collagen fibrils from both the healthy and dilated segments of aneurysmal aortas were visualized by TEM and analyzed for appearance and diameter. (Figure 20A) Collagen fibril diameter from the healthy segments of both SA3N and saline sham-treated were comparable (SA3N range = 24.80-67.98 nm, mean = 42.03 nm, median = 41.10, n = 4867; Saline sham range: 24.24-68.82 nm, mean: 42.05 nm, median = 40.93nm, n = 5099), however, collagen  fibril diameter from the dilated segment of sham-treated aortas showed much greater range in size and appearance when compared with dilated segments of SA3N-treated aortas  (SA3N range = 26.20-68.87 nm, mean = 47.87 nm, median = 47.82,n  = 4179; Saline sham range: 8.32-106.1 nm, mean: 44.73 nm, median = 42.05, n = 5568) (Table 3) suggesting that SA3N treatment was able to reduce fibril diameter irregularity in aneurysmal aortas. In a preliminary pilot study, decorin at the ultrastructural level was visualized by immunogold labeling and imaged by TEM. Decorin immunopositivity was robust in the healthy segments of both SA3N and sham-treated aortas. Decorin loss in the dilated segments was profoundly reduced in the SA3N-treated aorta in comparison to the sham-treated aorta.    101     FIGURE 20: SA3N REDUCES COLLAGEN FIBRIL DIAMETER IRREGULARITY. Adventitial collagen fibrils from the healthy and dilated segments of aneurysmal aortas from both SA3N- and saline-treated mice were visualized by TEM (A) and analyzed for appearance and diameter (B). 20 images per segment were obtained of collagen fibrils in cross section. Fibrils from the dilated segments of saline-treated mice exhibited much greater range in fibril size within the same field of view compared to fibrils from SA3N-treated mice.   B A 102  TABLE 3: ADVENTITIAL COLLAGEN FIBRIL DIAMETERS  SA3N Healthy Saline Healthy SA3N Dilated Saline Dilated Fibril Count  4867 5099 4179 5568      Minimum (nm) 24.8 24.24 26.2 8.32 25% Percentile (nm) 37.44 37.28 42.44 37.21 Median (nm) 41.1 40.93 47.82 42.05 75% Percentile (nm) 45.76 46.08 52.95 49.24 Maximum (nm) 67.98 68.82 68.87 106.1      Mean (nm) 42.03 42.05 47.87 44.73 Std. Deviation (nm) 6.407 6.408 7.145 11.62 Std. Error (nm) 0.09183 0.08974 0.1105 0.1557      Lower 95% CI of mean (nm) 41.85 41.88 47.65 44.43 Upper 95% CI of mean (nm) 42.21 42.23 48.08 45.04      Coefficient of variation 15.24% 15.24% 14.93% 25.97%   103   FIGURE 21: SA3N REDUCES DECORIN LOSS AT THE ULTRASTRUCTURAL LEVEL Decorin at the ultrastructural level was visualized by immunogold staining. Decorin staining indicated by black spots is robust in the healthy segments of both saline sham- and SA3N-treated aortas, but is very much reduced in dilated segments of saline sham aortas. Decorin loss in the dilated segment is rescued by SA3N treatment.     6.3 ANTI-GZMB ANTIBODY TREATMENT 6.3.1 ANTI-GZMB ANTIBODY TREATMENT IMPROVES OUTCOMES IN AAA Mice were given a tail vein injection of anti-GZMB (n = 9) or IgG control (n = 8) at 1 mg/kg on day 0, 4, 7, 14 and 21. Necropsy was performed on all mice that died prematurely before the 28-day time point and in all cases confirmed the presence of large ruptured aortic dissections and extensive blood clots in the abdominal cavity suggesting that death was caused by exsanguination following AAA rupture. Survival at 28 days was increased from 50.0% in the IgG control group to 77.8% in mice that received anti-GZMB, however, analysis of survival curves by Log-rank (Mantel Cox) test showed no 104  significant difference (p = 0.247). The incidence of aneurysmal pathology was reduced from 100% to 77.8% in mice that received anti-GZMB compared to IgG control. Incidence of aneurysm rupture and premature death was reduced from 50.0% in the IgG control group controls to 22.2% in mice that received anti-GZMB. (Figure 22). 6.3.2 ANTI-GZMB ANTIBODY TREATMENT REDUCES GZMB IMMUNOPOSITIVITY AND PREVENTS DECORIN LOSS  Abdominal aorta from both IgG and anti-GZMB-treated mice were obtained and visualized with Movat?s Pentachrome, and stained for GZMB and decorin content. Healthy aorta from the anti-GZMB-treated group is shown in Figure 23, non-ruptured aneurysmal aortas from IgG and anti-GZMB groups in Figure 25 and Figure 24 respectively, and ruptured aorta from the IgG-treated group in Figure 26. GZMB immunopositivity is minimal in healthy aorta and the healthy, non-dilated segments of aneurysmal aorta in both IgG and anti-GZMB-treated aortas. Similarly, decorin staining is robust in these areas. However, GZMB immunopositivity is greatly elevated in the dilated adventitia surrounding the thrombus in IgG control aortas compared to mice that received anti-GZMB treatment, and correspondingly, decorin content is much reduced in these areas. As previously observed in SA3N-treated mice, non-ruptured AAA that received anti-GZMB treatment developed a greatly thickened adventitia and did not display large hematoma or progress towards severe dissection extending past the diaphragm. 105    FIGURE 22: ANTI-GZMB ANTIBODY TREATMENT IMPROVES OUTCOME IN ANG II-INDUCED AAA (A) Anti-GZMB antibody treatment increased survival from 50.0% to 77.8% and (B) reduced incidence of AAA from 100% to 77.8% and reduced rate of rupture from 50.0% to 22.2% compared to mice that received control IgG injections. 0 20 40 60 80 100 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 Percent Survival (%) Day IgG Anti-GZMB 0 20 40 60 80 100 IgG Anti-GZMB Percent Incidence (%) Small AAA Rupture A B 106    FIGURE 23: REPRESENTATIVE MOVAT?S PENTACHROME, GZMB AND DECORIN STAINING IN HEALTHY AORTA Minimal GZMB staining and robust decorin staining is observed in the adventitia. Scale bars: 5X = 200 ?m, 40X = 25 ?m.      107   FIGURE 24: REPRESENTATIVE MOVAT?S PENTACHROME, GZMB AND DECORIN STAINING IN ANTI-GZMB TREATED NON-RUPTURED AORTA Aortas from mice that received anti-GZMB antibody treatment and developed small non-ruptured AAA exhibited thickened adventitia surrounding regions of elastin degradation. GZMB staining is minimal in both the healthy and dilated segment, while decorin staining is robust. Scale bars: 5X = 200 ?m, 40X = 25 ?m.       108   FIGURE 25: REPRESENTATIVE MOVAT?S PENTACHROME, GZMB AND DECORIN STAINING IN IGG-TREATED NON-RUPTURED AORTA Aortas from mice that received IgG control antibody treatment and developed small non-ruptured AAA exhibited large hematoma and extensive elastin degradation. GZMB staining is pronounced in the medial thrombus while decorin staining is reduced in the thin adventitia surrounding the thrombus. Scale bars: 5X = 200 ?m, 40X = 25 ?m.      109   FIGURE 26: REPRESENTATIVE MOVAT?S PENTACHROME, GZMB AND DECORIN STAINING IN IGG-TREATED RUPTURED AORTA Aortas from mice that received IgG control antibody treatment and developed ruptured AAA exhibited large hematoma and extensive elastin degradation. GZMB staining is pronounced both in the medial thrombus and adventitia while decorin staining is severely depleted in the thin adventitia surrounding the thrombus. Scale bars: 5X = 200 ?m, 40X = 25 ?m.     110  6.3.3 ANTI-GZMB ANTIBODY TREATMENT REDUCES LOSS OF COLLAGEN DENSITY IN AAA Collagen fibre morphology and density were measured by picrosirius red (Figure 27) and SHG (Figure 28). With picrosirius red staining under polarized light, adventitial collagen fibres from IgG-treated ruptured and non-ruptured aneurysmal aorta appear green and yellow whereas adventitial collagen fibres from healthy aorta have a higher proportion of orange and red fibres, indicating higher fibre density. SHG signal density representing fibre density was significantly reduced in ruptured and non-ruptured IgG-treated mice compared to anti-GZMB-treated mice (n = 4, p = 0.000927). Notable differences in collagen architecture and overall appearance were also observed in IgG-treated aneurysmal aortas compared to anti-GZMB-treated mice, with profound loss of collagen organization and thick bundle formation, similar to that observed in SA3N vs. saline-treated mice.   111   FIGURE 27: PICROSIRIUS RED: ANTI-GZMB ANTIBODY TREATMENT REDUCES LOSS OF COLLAGEN DENSITY IN AAA When stained with picrosirius red, thick collagen fibres evidenced by strong birefringence (red) under polarized light are visible in the adventitia of normal aorta. Non-ruptured and ruptured AAA  from IgG-treated mice display thinner collagen fibres of lower density as evidenced by predominance of yellow colour when stained with picrosirius red and seen under polarized light. In comparison, non-ruptured AAA from mice that received anti-GZMB antibody display collagen that more greatly resembles healthy controls with a greater proportion of red and orange fibres under polarized light. Scale bars: 40X = 50 ?m. 112  A 113   FIGURE 28: SHG: ANTI-GZMB ANTIBODY TREATMENT REDUCES LOSS OF COLLAGEN DENSITY IN AAA Adventitial collagen from normal aorta, anti-GZMB-treated non-ruptured AAA, IgG-treated non-ruptured aorta and IgG-treated ruptured aorta were assessed by SHG (A). (Scale bar: 37 ?m). SHG signal densities are summarized in (B) where anti-GZMB-treated mice exhibited significantly higher signal density compared to IgG-treated mice (n = 4 per group, * p < 0.05). Error bars indicate SD.      B 114  6.4 DECORIN LOSS AND COLLAGEN DENSITY IN HUMAN TAA AND THORACIC DISSECTION 6.4.1 GZMB IMMUNOPOSITIVITY CORRESPONDS TO DECORIN LOSS IN TAA Tissues obtained from TAA and thoracic aortic dissection were assessed for GZMB immunopositivity relative to non-atherosclerotic, healthy thoracic aorta (Figure 29). Increased GZMB levels were observed both intracellularly and extracellularly in the thrombic material and infiltrating cells in the vessel wall of the false lumen in acute aortic dissections and corresponded with reduced staining for decorin. GZMB levels appeared to increase with severity of disease in TAA samples tested, with GZMB immunopositivity observed in infiltrating immune cells in the adventitia and adventitial-medial junction in mild TAA and significantly stronger staining both intracellularly and extracellularly in severe TAA. Decorin staining was reduced in intensity in both mild and severe TAA when compared to healthy aorta. 6.4.2 DISORGANIZED COLLAGEN ARCHITECTURE IN HUMAN TAA AND DISSECTION Collagen fibres from healthy, non-atherosclerotic thoracic aorta (n = 4), TAA (n = 6) and thoracic dissection (n = 5) were visualized by Second Harmonic Generation (SHG) and assessed for signal density (Figure 30). Both TAA and dissection samples displayed profound differences in collagen architecture, with a marked realignment of the fibre bundles and a loss of the characteristic interweaving of adventitial aortic collagen as observed in healthy control samples. Signal density was lower in both TAA and thoracic dissections, and this reached significance in aortic dissections compared to healthy controls (p = 4.46E-5). 115     116  FIGURE 29: GZMB IMMUNOPOSITIVITY CORRESPONDS TO DECORIN LOSS IN TAA Increased GZMB levels were observed both intracellularly and extracellularly in the thrombic material and infiltrating cells in acute aortic dissections and corresponded with reduced staining for decorin. GZMB levels appeared to increase with severity of disease in TAA samples. Scale bar: 20X = 50 ?m  117    A 118   FIGURE 30: COLLAGEN DENSITY IN HUMAN TAA AND DISSECTION Adventitial collagen from healthy control aortas, thoracic dissection, and TAA were assessed by SHG (A). (Scale bar: 37 ?m). SHG signal densities are summarized in (B). Only the signal density from the dissections achieved statistical significance when compared to controls. (healthy control n = 4, dissection n = 5, TAA n = 6; * p < 0.05).      B 119  6.5 DISCUSSION While once thought to function exclusively as a pro-apoptotic, immune-secreted serine protease, GZMB can exert other perforin- and/or apoptosis-independent roles in pathogenesis.452 Indeed, it is now well-established that GZMB accumulates extracellularly in bodily fluids such as plasma, synovial fluid, cerebrospinal fluid, and bronchoalveolar lavage  during conditions of aging and chronic inflammation (Reviewed in 453). Furthermore, in addition to cytotoxic lymphocytes, other cell types, both immune and non-immune, are capable of expressing and secreting this protease alone, or in combination with perforin, into the extracellular milieu (reviewed in 453). GZMB also retains its activity in plasma 308 and is capable of cleaving ECM in vitro (reviewed in 453). As no extracellular inhibitors of GZMB have been identified in humans, in addition to being a marker of inflammation, extracellular GZMB activity may contribute to vascular pathogenesis. Indeed, in human atherosclerosis, elevated levels of GZMB are associated with increased disease severity and unstable plaque formation.387 In further support of an extracellular role for GZMB in vascular disease, we demonstrate in Chapter 5 that GZMB contributes to Ang II-induced aortic aneurysm in a perforin-independent manner. While GDKO mice were afforded significant protection against both progression and rupture compared to apoE-KO controls, no protection was observed in PDKO mice. As this study suggested that extracellular GZMB was responsible for AAA in part through the cleavage of ECM, it was logical to determine if extracellular inhibitors of GZMB could elicit a similar effect. Both SA3N and anti-GZMB significantly improved survival by reducing adventitial decorin degradation and preventing aneurysm rupture. These treatments also resulted in a decrease in overall incidence of aneurysm development that did not reach significance; however, the small unruptured aneurysms in the anti-GZMB and SA3N-treated groups were morphologically different to those observed in corresponding control animals. Aneurysms in the control groups consistently exhibited medial hematomas consistent to those previously observed by us and others 328, 411, 414 with a disordered, 120  fibrous adventitia displaying mostly yellow-green fibres when stained with picrosirius red, indicating less tightly packed collagen fibres of smaller diameter. In contrast, aneurysms in the GDKO, anti-GZMB and SA3N-treated groups, especially at the higher doses, demonstrated a remarkable thickening of the adventitial layer with red and orange collagen fibres when stained with picrosirius red. This suggests that the collagen fibres in the adventitia of these mice exhibited increased thickness, density and alignment454 compared to their saline and IgG-treated counterparts. Indeed, when examined by SHG, collagen from saline and IgG-treated mice demonstrated a significant reduction in collagen fibre density and exhibited unidirectional realignment of fibres, whereas collagen morphology from SA3N and anti-GZMB-treated aortas more closely resembled the interwoven network as seen in healthy controls. Similar observations were made of the collagen architecture in the dilated portions of human TAA and dissected aortas, suggesting that this is not a feature that is unique to the murine model, and encouragingly, may also be remedied by the administration of GZMB inhibitors as a possible therapeutic agent in the human forms of disease.  The Law of Laplace states that for any given internal fluid pressure, vascular wall tension is proportional to the radius of the vessel, and inversely proportional to vessel wall thickness.455 This implies that larger vessels require stronger, thicker walls than smaller vessels as a vessel of larger radius must be able to withstand greater tension. Similarly, dilated aneurysmal arteries experience increased wall tension and it follows that increasing wall thickness (as observed in the adventitia of GDKO, anti-GZMB and SA3N-treated animals that developed aneurysms), without overly compromising distensibility, will effectively reduce the tension experienced by the vessel.  The adventitia is responsible for maintaining the circumferential structural integrity of the vessel456 and its tensile strength is largely reliant on the organization and morphology of its major constituent, type I collagen.457 While medial elastin and fibrillin-1 loss is a critical factor in aneurysm 121  formation, aneurysmal growth and rupture is dependent on defective collagen homeostasis.51 Consistent differences in the optical properties of aneurysmal collagen have been noted when compared to normal vessels, suggesting a difference in fibril organization.458 To corroborate this, we have observed an increase in fibril diameter irregularity (Figure 20) in aneurysmal adventitia that is greatly reduced when treated with SA3N. Collagen content is both disorganized and reduced in human aneurysms and dissections of the ascending aorta, while in human AAA, turnover of collagen III is increased as evidenced by increased levels of aminoterminal propeptide of type III collagen and associated with impaired collagen fibrillogenesis.459, 460 In a study on collagen cross-linking, Carmo et al. observed an overall decrease in collagen content in human AAA specimens and concluded that new collagen synthesis in aneurysmal tissues was defective and lacking proper cross-linking,461 rendering it more susceptible to proteolytic attack.  Our findings in both murine and human aneurysmal disease related to the loss of collagen organization and packing are consistent with the recent findings of Lindeman et al 462 in which a loss of collagen microarchitecture was observed in the adventitia of AAA and Marfan syndrome patients. In the latter, it was proposed that the loose, ribbon-like appearance of collagen that is observed in a normal adventitia, but not aneurysmal adventitia, acts as a coherent network to maintain mechanical strength and allows for vessel dilation and that this is lost in growing aneurysms.462 Given that we had also observed a loss of adventitial collagen organization in the mouse AAA model, we were interested in how GZMB might affect this process. In saline sham and IgG treated mice that experienced rupture, strong intracellular and extracellular GZMB staining was apparent in the adventitia (Figure 16, Figure 25, Figure 26). In contrast, minimal staining was evident in the SA3N and anti-GZMB-treated groups. While these results suggest that SA3N administration could lower GZMB levels or that the non-functional GZMB-SA3N complex may 122  have been cleared from the vessel, it is also very possible that the non-competitive binding and/or conformational change of GZMB within the GZMB-SA3N complex results in reduced recognition and binding of the detection antibody to GZMB during IHC staining. It is, however, expected that these effects if present, should be largely alleviated in the anti-GZMB groups as the neutralizing antibody is polyclonal. In analysis of non-ruptured AAA, normal levels of adventitial decorin staining were observed on the non-dilated side of the vessel where the elastic lamellae are still intact. Conversely, on the dilated side where aortic rupture occurred and GZMB staining was most prominent, decorin staining was nearly absent. It has been previously shown that GZMB degrades decorin in vitro425 and here we demonstrate that pre-incubation with SA3N is capable of preventing cleavage by GZMB (Figure 17). Decorin staining was markedly elevated in GDKO, further implying that GZMB is responsible for decorin degradation.  Decorin immunopositivity was strong in SA3N and anti-GZMB-treated mice (Figure 18 and Figure 24), which also displayed reduced levels of GZMB immunopositivity in the thickened adventitia compared to controls. GZMB staining was elevated in both saline and IgG controls (Figure 16, Figure 25, Figure 26), particularly when rupture was observed while decorin staining was nearly depleted in the adventitia surrounding the medial thrombus (Figure 18, Figure 25, Figure 26) and at zones of dilation. As further evidence, GZMB immunopositivity also corresponds to loss of decorin in human thoracic dissection and severity of disease in human TAA. Because collagen is the main structural element of the adventitia, providing the mechanical and tensile strength, as well as forming the scaffold for the ECM, the organization and orientation of the collagen fibrils is critical and largely determined by interactions with proteoglycans such as decorin through interfibrillar bridges.463 Decoron, the core protein of the decorin molecule, functions as a strong anchor for the elastic glycan bridge between each decorin molecule and collagen fibril, conveying 123  elasticity and allowing for reversible deformation during movement of the vessel wall.97  Decorin is a key mediator of collagen fibrillogenesis 88, 337 and its incorporation into collagen fibrils improves tensile properties.96  Decorin-deficient mice have fragile skin with markedly reduced tensile strength.88, 464 Dermal collagen from these mice exhibit highly irregular fibril diameter and abnormal fibrillar organization,88 suggesting that decorin plays a crucial role in fibril formation, fusion and maintaining stability. Furthermore, Iwasaki et al 338 have found that in vitro collagen gels cultured in the absence of, or with low concentrations of decorin produced highly porous fibre networks with poor elasticity, whereas gels cultured with higher concentrations of decorin produced fibre networks of significantly higher density and elastic properties. Reese et al.465 have further demonstrated that the addition of decorin during the fibrillogenesis of Type I collagen increases the linear elastic modulus and tensile strength of collagen gels in vitro. This corroborates our own observation of higher adventitial collagen fibre density in SA3N and anti-GZMB-treated animals that also present with correspondingly higher decorin immunopositivity compared to controls.  Along with other proteoglycans, decorin can sequester and regulate the activity of growth factors such as TGF-?,466 fibroblast growth factor-2 (FGF-2),467 TNF-?,468 and platelet-derived growth factor (PDGF)469 making decorin a critical regulator of numerous receptor-specific signalling cascades and giving decorin the ability to influence repair and remodelling processes in damaged tissues.  Under specific conditions, decorin binds to the forming collagen fibrils, functioning as a spacer and slowing down collagen fibril association and fusion. This allows sufficient time for optimal interactions and cross-linking to occur, and thus forces the formation of well-organized fibrils of uniform diameter optimized for local conditions.88 In further support of this, we have observed that fibril-associated decorin is greatly reduced in the dilated regions of aneurysmal aorta which are more susceptible to rupture.  124  We have demonstrated that both human and murine aneurysmal disease exhibit a similar loss of collagen architecture and that GZMB is elevated in both conditions. We report that GZMB inhibition by SA3N was able to reduce decorin cleavage, subsequent collagen disorganization and aneurysmal rupture in murine AAA. These results were confirmed by the obtainment of comparable results with a more specific, neutralizing anti-GZMB antibody. Similar observations of adventitial thickening and reduced loss of decorin were also observed in GZMB-deficient mice following AAA induction, suggesting that the cleavage of decorin is most likely due to GZMB. As further support, SA3N treatment prevented GZMB cleavage of decorin in vitro. Based on these findings, we propose that reducing GZMB in the inflammatory milieu surrounding aneurysmal induction promotes beneficial adventitial collagen remodelling by preventing the degradation of decorin, thereby facilitating decorin-mediated fibrillogenesis and reinforcement of the adventitia, reducing wall tension and thus satisfying the Law of Laplace. The vessel consequently retains sufficient tensile strength and flexibility to maintain structural integrity, constrain dilation and prevent rupture.  Although we did not detect a change in CD3+ lymphocytes or macrophages following SA3N treatment compared to non-ruptured sham-treated controls at 28 days post-induction, immune cell infiltration is difficult to accurately assess and quantify in ruptured aortas in this model and will require additional study at earlier time points. In Chapter 5, it was determined that GZMB deficiency, but not perforin deficiency protects against AAA development in this model.328 While this rules out a role for GZMB/perforin-mediated apoptosis, there is indeed the possibility that GZMB could be contributing to aneurysm development through GZMB-mediated anoikis through ECM cleavage. Indeed, we and others have shown that GZMB cleavage of fibronectin can result in detachment-induced death of fibroblasts, 429 endothelial cells326 and smooth muscle cells;327 the loss of which may significantly contribute to aneurysm pathogenesis. 125  Although SA3N is a potent inhibitor of GZMB, SA3N is also known to bind and inhibit human leukocyte elastase (neutrophil elastase), 367 another protease implicated in AAA pathogenesis. Leukocyte elastase, however, does not cleave decorin.340 As elastase-digested aortas are utilized to assess decorin and biglycan expression in AAA,340 it is highly unlikely that inhibition of leukocyte elastase was responsible for prevention of decorin degradation in this model. This is further supported by the observation that decorin cleavage is also prevented in GZMB-deficient mice and similar results were achieved with a neutralizing anti-GZMB antibody. While the generation of small molecule GZMB inhibitors is in progress, at the present time, no other extracellular inhibitors of mouse GZMB are available.  In conclusion, because we have shown that small aneurysms in GDKO and GZMB-deficient mice develop similar morphology and show reduced propensity to decorin cleavage, adventitial collagen disorganization and rupture, we believe a strong argument can be made for pursuing further investigations into GZMB as a therapeutic target for reducing the progression and rupture of human aneurysms and dissections.      126  CHAPTER 7: CONCLUSION AAA is a complex, multifactorial disease that is characterized by elastin degradation and impaired collagen turnover in the aortic wall, and is likely associated with the effects of multiple genes in combination with various lifestyle and environmental factors. In the present study, GZMB levels were found to be elevated in human AAA and that GZMB colocalizes to macrophages, lymphocytes and mast cells in aneurysmal lesions. Furthermore, we confirm that GZMB has the ability to cleave numerous ECM proteins such as fibrillin-1 and decorin. Knocking out the serine protease GZMB reduced the incidence of aneurysmal disease in an Ang II-induced murine model of AAA, while perforin deficiency was not protective, indicating that GZMB contributes to the pathogenesis of AAA via its extracellular abilities. In the same model, GZMB deficiency prevented the loss of medial fibrillin-1 while a reduced level of fibrillin-1 fragments was observed in mouse serum. Because fibrillin-1 serves as a scaffold for elastin microfibrils, this would suggest that GZMB may play a role in the destruction of the elastic lamellae, a defining characteristic of aneurysmal disease.  Subsequently, we were able to show that inhibition of GZMB by both murine serpin A3N and a more specific neutralizing anti-GZMB antibody was able to reduce incidence of medial hematoma and decrease the rate of aneurysmal rupture in the same Ang II-induced model of AAA. In apoE-KO mice that received saline sham or IgG control treatment, GZMB expression corresponded to regions of decorin loss, and as decorin is a critical mediator of collagen fibrillogenesis, GZMB-mediated decorin degradation may be responsible for the irregularity of collagen fibril diameter and for the loss of collagen architecture and density in the adventitia seen in these mice. Correspondingly, GZMB deficiency prevented decorin loss, reduced collagen fibril irregularity, and preserved collagen architecture and 127  density. It is likely that this collagen remodelling constrained disease progression and accounted for the reduction in rupture rates seen in mice that received inhibitor treatment. In this model, Ang II signals through the AT1 receptor and initiates a localized inflammatory response in the suprarenal aorta that is largely macrophage and T-cell-driven. It is still unclear whether the elevation in GZMB immunopositivity is a result or part of the cause of this inflammation; nonetheless, we propose that GZMB?s role in AAA pathogenesis is two-fold: 1. GZMB-mediated cleavage of fibrillin-1 contributes to destruction of elastic lamellae, and 2. GZMB-mediated cleavage of decorin impairs the ability of the adventitia to respond to and compensate for medial injury and that collagen fibrillogenesis in the absence of decorin results in disorganized, fibrotic adventitial remodelling that is unable to support hemodynamic force or constrain further dilation of the aorta (Figure 31, part 1-4a). In summary, the inhibition of GZMB in the aneurysmal wall as a potential treatment strategy could preserve fibrillin-1 and decorin content. Collagen fibrillogenesis following medial injury in the presence of sufficient decorin will promote healthier adventitial remodelling that retains more of its original tensile strength and thus, be better able to constrain dilation and prevent rupture of the aorta (Figure 31, part 1-4b).   128  129  FIGURE 31: CURRENT PERSPECTIVE ON THE EXTRACELLULAR ROLE OF GZMB IN AAA 1) Inflammation and the presence of endogenous and infiltrating macrophages and leukocytes results in elevated protease activity in the aortic wall. 2a) Protease (eg. MMPs, cathepsins, GZMB etc.) activity leads to destruction of elastic lamellae either via direct degradation of elastin (eg. MMPs) or degradation of microfibril components (eg. GZMB cleavage of fibrillin-1). 3a) Inflammation advances, mast cells infiltrate the aneurysmal adventitia; GZMB levels remain elevated and continue degradation of vascular wall components (eg. decorin). Elastin loss confers larger percentage of tensile load on adventitial collagen and subsequent decrease in tensile strength causes dilation of aorta while release of TGF-? and other growth factors triggers collagen remodelling. 4a) Localized dilation of the aorta and abnormal blood flow allow for the formation of laminated thrombus. Collagen remodelling in the absence of sufficient decorin results in disorganized adventitial collagen with poor density and abnormal architecture that has insufficient tensile strength to restrain further dilation. Eventual loss of structural integrity leads to aortic rupture. 2b) Inhibition of GZMB in the inflammatory milieu lessens the degradation of the elastic microfibrils and subsequent loss of tensility. 3b) GZMB inhibition prevents degradation of decorin and collagen remodelling in the presence of decorin promotes healthier, organized collagen architecture with regular fibril diameter and spacing that may provide sufficient tensile strength to constrain dilation. 4b) Maintenance of adventitial integrity allows for re-endothelialization of the aorta and the re-formation of elastin fibres in the tunica media.    130  REFERENCES 1. Edmonds LD, James LM: Temporal trends in the birth prevalence of selected congenital malformations in the Birth Defects Monitoring Program/Commission on Professional and Hospital Activities, 1979-1989, Teratology 1993, 48:647-649 2. Manasek FJ: Embryonic development of the heart. I. 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