Calcineurin inhibitors are not antagonistic to tolerance induction

The concept of “partial” clinical tolerance

The status of "partial" tolerance to organ allografts versus the status of complete tolerance is the main topic of this paper. Progress made in immunosuppression, particularly by use of various lymphocyte depleting agents for "induction therapy", seems to favor the subsequent development of T cells with suppressor/regulatory properties. The effective deletion of alloreactive T helper and cytotoxic cells in conjunction with the expansion of antigen-specific suppressor (CD8 + CD28 - FOXP3+) and regulatory (CD4 + CD25+ FOXP3+) T cells creates a milieu in which the graft is well tolerated under an "umbrella" of low dosage immunosuppression. The most effective induction treatment is Campath-1H, although ATGAM at high dosage is also widely used. Total lymphoid irradiation (TLI) is another very effective pretreatment strategy in spite of the risks which are associated with it. The induction of "partial" tolerance is a step in the right direction for exploring strategies that may lead to the induction of complete tolerance. It is safe for the patients and can prolong significantly the function of the graft, preventing the onset of chronic rejection.

Clinical trials: where are we now?

Clinical trials in transplantation have focused on improving outcomes and minimizing side effects associated with renal transplantation. Although immunologic tolerance, which means complete freedom from immunosuppressive drugs and maintenance of excellent long-term graft function, has seldom been achieved, in rare cases, this has been accomplished. Most current clinical trials focus on minimization of steroid use and calcineurin inhibitor use as a step toward tolerance, sometimes termed prope tolerance. Alternatively, new immunosuppressive agents are studied to assess their efficacy in preventing graft rejection with the anticipation of lesser toxicity. This review is organized in a case presentation style with actual cases from the University of Wisconsin kidney transplant experience presented as illustrations of actual scenarios in clinical trials. Lessons learned from these particular patients are then summarized with reference to the literature associated with the case. Using this format, some of the important lessons learned from clinical trials are outlined and directions for future study are noted. Clinical trials have permitted a dramatic improvement in graft survival and lowering of infectious and malignant side effects over the past 30-40 years. Nevertheless, we remain far from achieving true tolerance in patients for a variety of reasons.

Clinical trials of transplant tolerance: slow but steady progress

The search for tolerance therapies that would thwart the alloimmune response following organ transplantation while preserving a patient's protective immune response has been a formidable goal for clinical immunologists. Over the past few decades, a more detailed understanding of the molecular events associated with T-cell recognition and activation has demonstrated the feasibility of various tolerance approaches, such as costimulation blockade, in numerous animal models of both autoimmunity and transplantation. Yet, only a few promising new therapies have reached the early stages of human clinical development. In contrast, the use of T-cell depleting induction therapy has become widespread, and new trials have been designed with immunosuppressive drug withdrawal in mind. Furthermore, nonmyeloablative mixed chimeric approaches have allowed complete immunosuppressive withdrawal in some limited cases. In the course of these investigations, however, what has become increasingly clear is that the distinctions between immunosuppression and tolerance have been blurred as the success and durability of the therapies rely as much on the state of the organ and organism as they do the mechanism of action of the drug. In this review, we provide a summary of the progress and lessons in promoting clinical transplant tolerance and an overview of promising agents.