UBC Theses and Dissertations
Utilizing the transporter associated with antigen processing (tap) to enhance immune responsiveness to cancers and viruses Alimonti, Judie Barbara
For cytotoxic T lymphocytes (CTL) to recognize infected, mutated, or cancerous cells, an antigenic peptide must be presented on the cell surface by the major histocompatibility complex I (MHC I). The majority of peptides bound to MHC I arise from the cytosolic degradation of proteins. These peptides are translocated by transporters associated with antigen processing (TAP), into the endoplasmic reticulum (ER) where they bind with MHC class I before proceeding to the cell surface for recognition by CD8+ T lymphocytes. In TAP deficient cell lines the MHC class I surface expression is low resulting in poor recognition by T cells. Therefore TAP is important in the loading of MHC class I with peptide, and this thesis examines TAP's role in antigen processing and presentation and its effect on CTL mediated killing. Three aspects are considered: 1) the use of TAP as an adjuvant in vaccines, 2) the involvement of TAP in secreting antigen which sensitizes cells to CTL killing, and 3) restoring TAP in TAP deficient cancer cells to improve immune recognition and destruction of the tumour. CTL play an essential role in eliminating viral pathogens, and adjuvants can promote the induction of CD8+ CTL when injected along with antigenic peptide. The first study demonstrated a > 4.8 X increase in the frequency of specific CTL when TAP was included in a vaccine containing a cytotoxic peptide. Thus TAP can act as an adjuvant to enhance an immune response to a viral cytotoxic epitope. TAP is the major transporter of peptides for MHC class I assembly in the ER lumen. The second section in this study shows that there is an alternative fate for ER lumenal peptides other than binding to MHC class I. The peptides can be actively secreted from the cell through the constitutive secretory pathway in a TAP dependent manner. Furthermore, the secreted peptides can sensitize surrounding cells to be recognized by CTL. This demonstrates that CTL recognition of neighbouring uninfected cells presenting viral antigen involves TAP. Some cancer cells, like CMT.64, escape CTL mediated destruction due to low surface concentration of MHC class I. CMT.64 cells transfected with TAP restores viral antigen presentation to CTL in vitro. The final section of this thesis explores the role of TAP in restoring antigen presentation and immune recognition of cancer cells in vivo. Although TAP functions as a heterodimer (TAP1 and 2), re-expression of TAP1 in CMT.64 improved survival of tumour bearing animals dramatically. Furthermore, a Vaccinia Virus containing TAP1 could be used as a form of cancer therapy in CMT.64 burdened mice. These findings argue that T cell recognition of the tumours increases with the addition of TAP. Collectively, the addition of TAP to either wild type cells, and TAP deficient cells increased immunogenicity. This supports a role for TAP in the CTL mediated recognition of both viruses and cancers, and the use of TAP in the development of vaccines, or cancer therapeutics.
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