Chronic rejection (CR) is the leading cause of late allograft failure. Manifestations of chronic rejection (CR) include fibrosis of the graft and vascular neointimal development, referred to as transplant associated vasculopathy (TAV). Despite continued investigation, the underlying mechanisms responsible for these disease manifestations remain poorly defined. TGFB has been implicated in fibrosis of the graft, and smooth muscle (SM) cell migration and proliferation are thought to be critical in TAV. This proposal will employ gene transfer strategies and mAb as targeted therapies of allograft fibrosis and TAV in the mouse vascularized cardiac allograft model, where transient depletion of recipient CDA4+ T cells results in prolonged graft survival and the development of CS. Results will be compared to recipients treated with multiple doses of anti-CD40L mAb, which do not develop CR. In Specific Aim 1, cellular and cytokine mediators of CR will be identified by adoptive transfer of selected cell populations into SCID allograft recipients, and modulating responses with neutralizing mAb. The role of regulatory T cells in CR will be explored to test the hypothesis that Treg function to prevent Th1 and Th2 responses associated with acute rejection, but allow CR inducing factors to be expressed. In Specific Aim 2, the direct participation of TGFB in graft fibrosis will be assessed by inhibiting local TGFB activity by transfection with decorin, an inhibitor of TGFB. TGFB transfected grafts will be placed in recipients treated with multiple doses of anti-CD40L that do not develop CR. Since connective tissue growth factor (CTGF) is the down stream mediator of TGFB induced fibrosis, targeting CTGF will be explored to test the hypothesis that TGFB induced fibrosis may be segregated from the beneficial anti-inflammatory activities of this cytokine. In Specific Aim 3, gene transfer of the cyclin dependent kinase (cdk) inhibitors p21 and p27, will be employed to test the hypothesis that limiting vascular SM cell proliferation will inhibit the progression of TAV. Heme oxygenase-1 (HO-1) gene transfer into p21 deficient allografts will be performed to test the hypothesis that the reported protective effects of HO-1 over expression in TAV are mediated in part by regulation of upregulation of p21. Hence, the proposed studies will identify underlying mechanisms responsible for fibrosis and TAV. Further, these studies will shed light on the interactions between TGFB, decorin, CTGF, p21/p27, and HO-1 as they relate to CR. Studies of this nature have not performed previously, and will rigorously explore the use of these novel therapeutic targets in preventing the progression of CR.