The beginning of clinical tolerance in solid organ allografts

Development of effective multidrug immunosuppressive regimens and improvements in the management of chronically immunosuppressed patients have produced extraordinary patient and allograft survival in clinical organ transplantation. Unfortunately, significant problems of morbidity and mortality related to chronic immunosuppression remain. Thus, there is an enormous motivation and interest in inducing specific unresponsiveness (tolerance) to clinical solid organ allografts. Operational clinical tolerance may be defined as stable, normal graft function in the total absence of a requirement for maintenance immunosuppression. Alternatively, the concept of employing tolerogenic strategies to permit graft acceptance with dramatically reduced immunosuppression requirements is referred to as prope' or minimal immunosuppression tolerance. There have been isolated examples of clinical tolerance, usually in the context of spontaneous or induced donor chimerism, excellent HLA matching, and/or drug weaning or patient noncompliance. The various attempts that are currently being employed to induce some type of clinical tolerance are reviewed in this manuscript. Strategies in which all immunosuppression was to be withdrawn from the recipient (donor-specific unresponsiveness) are first discussed. These include strategies that utilize initial immunoablation with varying doses of irradiation and/or lymphocytic antibodies with or without donor-specific bone marrow infusion and short-term standard immunosuppressive therapy. Strategies to induce prope' or minimal immunosuppression tolerance that utilize induction lymphoablation with polyclonal or monoclonal antilymphocyte antibodies, with or without donor bone marrow infusion, followed by limited low-dose immunosuppressive therapy are also discussed. The ethical considerations in testing clinical tolerance strategies and protocols are discussed in detail. The limited number of clinical tolerance studies already available affirms that carefully supervised weaning of immunosuppressive drugs in controlled tolerance trials is not unreasonable, especially when monitored by protocol allograft biopsies. Initial results suggest that aggressively treated low-grade steroid-responsive rejection reactions in the absence of immune-mediated tissue destruction does not necessarily require resumption of high-dose immunosuppression. Finally, the role of donor bone marrow infusions in facilitating tolerance/hyporesponsiveness induction needs to be studied and expanded.