CD4+CD25+ regulatory T cells mediate acquired transplant tolerance.

The Holy Grail of clinical organ transplantation is the safe induction of allograft tolerance. Transplant tolerance has been successfully induced in animal models. Since T cells play a pivotal role in graft rejection, modulating T cell function has been the primary focus of studies aimed at inducing transplant tolerance. Rodent models of transplant tolerance induction include central deletion and peripheral mechanisms involving activation-induced cell death (AICD), anergy, immune deviation, and production of regulatory T cells. These mechanisms are not mutually exclusive. Although clonal deletion and anergy limit self-reactive T cells in the thymus, these mechanisms alone are not sufficient for controlling self-reactive T cells in the periphery. There is now evidence that the adult animal harbors two functionally distinct populations of CD4(+) T cells; one mediates autoimmune disease and the other dominantly inhibits it. The latter cells express CD4, CD25 and CTLA-4. These thymus-derived T cells have recently been shown to mediate the induction and maintenance of transplant tolerance. These CD4(+)CD25(+) T cells are similar in origin, phenotype, and function to those that maintain natural self-tolerance and T cell homeostasis in the periphery. Against this background, is it possible that alloantigen specific regulatory T cells might be generated and expanded ex vivo before organ transplantation and then infused to induce long-term tolerance, perhaps without the need for chronic immunosuppression?