Acute and chronic rejection remains a major obstacle to the success of transplantation. The main strategy to prevent the recipient’s immune system from rejecting the graft is the administration of immunosuppressive drugs. As these drugs weaken the recipient’s immune system, they can be predisposed to infections and certain forms of cancer. Therefore, the goal in transplantation remains the development of strategies to induce specific immune tolerance, so that the immune system does not reject the graft but otherwise maintains normal function. There is increasing evidence that regulatory T cells can control alloimmune responses and could therefore play an important role in the induction and maintenance of such specific immune tolerance to allografts. Vascular endothelial cells, which line the blood vessels of grafted organs, have been historically thought to have no immunological function. However, recent studies have suggested that they do play an active role in regulating immune responses in allograft rejection. We have previously demonstrated that allograft vascular endothelial cells can activate a subset of the recipient’s T lymphocytes (CD8⁺), induce their expansion and trigger allograft rejection. Vascular endothelial cells don’t induce an expansion in the other subset of T lymphocytes (CD4⁺). Interestingly, recent studies in our laboratory have shown that vascular endothelium can induce a population of regulatory CD4⁺ T cells, which can suppress alloimmune responses. In this project we will examine the mechanism how vascular endothelium induces regulatory CD4⁺ T cells. We will analyze which costimulatory pathways are important in this induction. We will also test whether these vascular endotheliuminduced regulatory T cells can suppress alloimmune responses in vivo and prevent the rejection of heart allografts. The proposed experiments will further dissect our novel observation, provide insight into how the allograft itself can regulate alloimmune responses and could lead to the development of novel strategies to induce tolerance to allografts.