Acquired transplant tolerance.

Increasing the acceptance rate of organs is the central goal of transplantation research. Long-term survival of vascularized organs without chronic immunosuppressive therapy has been achieved in experimental animals. In humans, the possibility of achieving immunological tolerance and a drug-free state has been reported occasionally in patients who after withdrawal of immunosuppressants because of major toxicity still carry a functioning graft. It has been proposed that organ transplant implies a migratory flux of donor 'passenger' leukocytes out of the graft into the recipient tissue or organs, to establish a persistent condition of 'microchimerism'. Although there is evidence that the same migratory mechanisms apply to all organ grafts, migration of 'passenger' leukocytes is less in kidney and heart than in liver. To enhance the acceptance of organs less tolerogenic than liver, perioperative infusion of donor bone marrow has been attempted to increase the donor 'passenger' leukocyte load. It has been suggested that the established microchimerism is not only associated with long-term acceptance of the graft, but it also plays an active role in induction and maintenance of donor-specific unresponsiveness. However, the intimate mechanism(s) responsible for prolonged graft survival in this setting remain speculative. Experimental evidence is also available that the thymus plays a major role in the development of self-tolerance and is critical in the induction of acquired tolerance to exogenous antigens. It has been reported that after intrathymic injection of donor cells clonal deletion of maturing thymocytes occurs and is the major mechanism in the induction of donor-specific tolerance, since peripheral T-cell component would be devoid of alloreactive population. Studies are warranted in the near future to explore whether the thymus technique can be employed to prolong survival or induce tolerance to allograft in humans. An interesting novel strategy for transplant tolerance is also the oral administration of alloantigens, which has been recently applied to the cardiac transplant model in rat. All these approaches will have a major impact in the near future on transplant medicine, opening new perspectives to obtain indefinite graft survival.