THE replacement of malfunctioning tissues and organs has long been a clinical objective in medicine. Modern transplantation has been made possible by improved clinical care, a better understanding of immunogenetics, cross-matching of antibodies, and the development of several nonspecific immunosuppressive agents, including corticosteroids, azathioprine, antilymphocyte serum, cyclosporine, and monoclonal antibodies. The combination of HLA matching and immunosuppression has greatly improved both graft and patient survival, but several complications persist: infection, hypertension, drug toxicity, recurrence of disease, and psychosocial problems. Identifying more specific modes of immunosuppression therefore remains a challenge for clinical medicine.

A thorough understanding of the cellular, biochemical, and . . .