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Matrix metalloproteinases, inflammation, and matrix remodeling in coxsackievirus-induced myocarditis Cheung, Caroline Tsui Yee


Myocarditis, or inflammation and injury of the heart muscle, induced by infection with coxsackievirus B3 (CVB3) is believed to lead to longterm heart disorders. Under pathological conditions, dysregulation of cardiac extracellular matrix (ECM) turnover results in maladaptive remodeling, progression of disease, and depressed cardiac function. The mechanisms by which this ECM alteration occurs are unclear, but inflammatory cells may be involved as secretors of matrix metalloproteinases (MMPs), cytokines, and growth factors that regulate ECM homeostasis. I hypothesize that MMPs play important roles in viral myocarditis. These enzymes degrade interstitial molecules, but they also regulate the immune system via modulation of cytokines and growth factors. To investigate the proposed hypothesis, I first examined the expression profile of MMP-2, MMP-8, MMP-9, MMP-12, and the tissue inhibitor of metalloproteinases (TIMPs), at key milestones in the evolution of myocarditis using a mouse model. To further explore the mechanisms, I infected MMP-8, MMP-9, and MMP-12 knockout mice, and examined how these deficiencies affected the immune response and matrix remodeling. Lastly, I inhibited the 4-1BB pathway, a major T-lymphocyte co-stimulatory molecule, to examine the synergy between the immune response and matrix remodeling. Following CVB3 infection, increased MMP-2, MMP-9, and MMP-12 expression was detected with corresponding decreases in TIMP expression, suggesting that an imbalance in MMPs and their inhibitors may result in increased protease activity. To examine the roles of the individual MMPs during CVB3 infection, I infected mice lacking MMP-8, MMP-9, or MMP-12. Following viral infection, mice deficient in MMP-12 and MMP-9 experienced increased viral titers and tissue injury while MMP-8 knockout mice showed minimal differences as compared to controls. The mechanisms by which the MMPs operate may be in ECM remodeling, but also modulation of the immune response, notably T-cells. In order to determine the effect of the immune response on matrix remodeling, I inhibited the 4-1BB receptor, a major T-cell stimulatory pathway, and observed a decrease in inflammation and matrix remodeling, while MMP-12 was decreased, in association with an improvement in cardiac function. My results suggest that MMPs may play crucial roles in CVB3 infection through their immune modulation and matrix remodeling activities.

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