Chronic rejection, the major limitation of cardiac transplantation, is characterized by pathological remodeling and dysfunction of coronary arteries, termed graft arteriosclerosis (GA). The pathogenesis of GA is poorly understood, but is likely immune-mediated and may result from chronic delayed-type hypersensitivity responses by recipient T cells to donor vascular antigens through the secretion of cytokines, such as interferon-gamma (IFN-gamma). Paradoxically, IFN-gamma is generally thought to have an antiproliferative effect on vascular smooth muscle cells (VSMCs) and was considered to function as a proarteriosclerotic agent solely because of its immunomodulatory effects on endothelial cells and infiltrating leukocytes. However, we have found that IFN-gamma elicits arteriosclerosis in the absence of leukocytes. Our prior observations and current preliminary studies have led us to hypothesize that IFN-gamma induces VSMC proliferation that depends on a mTOR/p70S6K pathway, sensitizes VSMCs to apoptosis through upregulation of XAF1 and Noxa, and primes VSMCs for innate immune responses to fragmented nucleic acids by induction of RIG-I and MDA5. These disparate effects of IFN-gamma on VSMC survival and inflammation interact and cause intimal expansion, outward vascular remodeling, and vasodysfunction of conduit coronary arteries which ultimately determine lumen size and blood flow. We further hypothesize that these direct actions of IFN-gamma on VSMCs will be inhibited by peroxisome proliferator-activated receptor (PPAR)gamma ligands. To test our hypotheses with the experiments planned in this project, we have formed productive collaborations with other investigators of the program application and together we have developed novel models of GA in which human coronary arteries are interposed in severely immunodeficient mouse hosts that produce human IFN-gamma by adenoviral vector infection. Our methods are supplemented by mouse artery transplantation models and by cellular and molecular studies of human VSMCs. We will use these approaches to elucidate the effects of IFN-gamma on arterial tissue in vivo and in vitro. The outcomes of these studies will provide considerable new information about the role of IFN-gamma in GA, may identify novel therapeutic targets to treat and image GA, and investigate mechanisms of inhibiting GA by existing pharmacological agents.
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