Regulatory T cells and transplantation tolerance.

Rodent models of transplantation have demonstrated that it is possible to induce specific immunological tolerance of donor antigens and indefinite graft survival in the absence of any continued immunosuppression. If this situation could be achieved clinically it would avoid many of the longer term complications of organ grafting, such as the increased risk of infection and cancer and the nephrotoxicity of many immunosuppressive agents. In this review we shall consider the interplay between regulatory T cells, dendritic cells and the graft itself and the resulting local protective mechanisms that are coordinated to maintain the tolerant state. We will discuss how both anti-inflammatory cytokines and negative costimulatory interactions can elicit a number of interrelated mechanisms to regulate both T cell and antigen-presenting cell activity. The induction and maintenance of tolerance via acquired local immune privilege has implications for the design of therapeutic regimens and the monitoring of the tolerant status of patients being weaned off immunosuppression.