The future of organ transplantation: from the laboratory to the clinic

Progress in Liver Transplant Tolerance and Tolerance-Inducing Cellular Therapies

Liver transplantation is currently the most effective method for treating end-stage liver disease. However, recipients still need long-term immunosuppressive drug treatment to control allogeneic immune rejection, which may cause various complications and affect the long-term survival of the recipient. Many liver transplant researchers constantly pursue the induction of immune tolerance in liver transplant recipients, immunosuppression withdrawal, and the maintenance of good and stable graft function. Although allogeneic liver transplantation is more tolerated than transplantation of other solid organs, and it shows a certain incidence of spontaneous tolerance, there is still great risk for general recipients. With the gradual progress in our understanding of immune regulatory mechanisms, a variety of immune regulatory cells have been discovered, and good results have been obtained in rodent and non-human primate transplant models. As immune cell therapies can induce long-term stable tolerance, they provide a good prospect for the induction of tolerance in clinical liver transplantation. At present, many transplant centers have carried out tolerance-inducing clinical trials in liver transplant recipients, and some have achieved gratifying results. This article will review the current status of liver transplant tolerance and the research progress of different cellular immunotherapies to induce this tolerance, which can provide more support for future clinical applications.

Donor reactive regulatory T cells

PURPOSE OF REVIEW

Donor reactive regulatory T cells (Treg) play an important role in tolerance induction and maintenance in experimental transplant models. In this review we focus on the formation of the donor reactive Treg pool and explore the potential of these cells for therapeutic application in clinical transplantation.

RECENT FINDINGS

Donor reactive Treg can arise by both conversion of alloreactive nonregulatory cells and expansion of naturally occurring Treg (nTreg) cross-reactive with donor alloantigen but the quantitative contribution of each of these pathways is at present unclear. However, the fact that donor reactive Treg can be driven both in vivo and ex vivo by alloantigen challenge of nonregulatory precursors is encouraging as it demonstrates that the functional potential of these cells for use in clinical transplantation will not be limited by fortuitous cross-reactivity between nTreg and donor alloantigens. Treg can be generated in vivo by transplantation or alloantigen challenge in combination with Treg-permissive immunosuppression, or ex vivo by phenotypic selection or by polyclonal or antigen-specific stimulation. A number of ex-vivo protocols exist for the enrichment of Treg in the laboratory and in many cases these cells have demonstrable function both in vitro and in relevant graft-versus-host disease (GVHD) or organ transplant models. The challenge now is to understand the clinical opportunities and limitations that these populations present.

SUMMARY

Combined with appropriate immunosuppression, Treg generated/expanded in vivo or ex vivo may hold the final key to operational tolerance in clinical setting.

PD-1/PD-L1, PD-1/PD-L2, and other co-inhibitory signaling pathways in transplantation

Transplantation of cells, tissues and vascularized solid organs is a successful therapeutic intervention for many end-stage chronic diseases. The combination of co-stimulatory blockade with the delivery of negative signals to T cells through co-inhibitory receptors would provide a robust approach to modulating T-cell receptor signaling and improving alloantigen-specific control of transplant rejection. This approach based on fundamental knowledge of APC/T-cell interactions may complement conventional therapies in the near future to reinforce long-term allograft survival, and permit minimal immunosuppression. The focus of this review was primarily on two major co-inhibitory signaling pathways, namely PD-1/PD-L1/PD-L2 and BTLA/CD160/HVEM/LIGHT that have been thoroughly characterized in murine models of transplantation using genetically modified mice, specific monoclonal antibodies and fusion proteins.

The thymus is required for the ability of FTY720 to prolong skin allograft survival across different histocompatibility MHC barriers

The immunosuppressive effect of FTY720 is associated with the reversible sequestration of lymphocytes from the blood and the spleen into secondary lymphoid organs and reduced egress of mature thymocytes from the thymus. This work was designed to dissect the differential effect of FTY720 on CD4 and CD8 T cell-mediated mechanisms of skin graft rejection in the presence (euthymic) or absence (thymectomized) of thymic output. To that end, untreated and FTY720-treated euthymic (Euthy) and thymectomized (ATX) mice received skin allografts across a full, class II or class I major histocompatibility complex (MHC) mismatched (MM) barriers and graft survival was monitored. We demonstrate that a short course of FTY720 treatment significantly augments the survival of full, class I and class II MHC MM skin grafts compared to the nontreated controls. Interestingly, FTY720-treated Euthy recipients showed a significantly prolonged skin allograft survival compared to FTY720-treated ATX mice. These results together show that FTY720 impairs both CD4 and CD8 T cell-mediated mechanisms of rejection and, more importantly, the presence of the thymus is necessary for the ability of FTY720 to modulate skin allograft rejection across different histocompatibility MHC barriers.

Novel therapies in transplantation

Immunosuppressant drugs are crucial in order to protect transplanted kidney, heart or liver against the body's natural defence mechanisms. These drugs are used to prevent both acute rejection of the organ as well as chronic deterioration of the graft over longer periods of time. Currently used immunosuppressant drugs are calcineurin inhibitors (cyclosporin and tacrolimus), steroids (prednisolone and methylprednisolone), antimetabolites (azathioprine and mycophenolate mofetil), antiproliferatives (sirolimus) and monoclonal antibodies that are used as an induction therapy (basilximab, daclizumab and muromonab). Over time, we have learnt more about existing immunosuppressive choices and the ways to monitor these drugs, and the current trend in immunosuppression therapy is towards tailoring the therapy according to an individual patient. The major issue that is now emerging is not how to prevent acute rejection, as current drugs are all efficacious, but rather how to avoid the long-term side effects that can harm both the graft and host, and negatively influence compliance. The market in transplantation is considerable and still growing; the whole market including the immuno-suppressants used in autoimmune diseases had sales of approximately pound 1.5 billion in 2001. There are currently 23 new drugs in advanced clinical development intended to be used in either organ transplantation or as treatment of autoimmune diseases. The new drugs that are intended to be used in transplantation are mostly analogues of currently used drugs with improved safety and pharmaceutical profiles.

Tolerance induction in clinical transplantation

Introduction of modern immunosuppressive agents has led to great success of allotransplantation in humans, and survival rates for all solid organs have been dramatically improved. However, a constant proportion of organs is lost every year due to chronic allograft rejection and immunosuppressive drug toxicity. This has led to a situation where, despite the of donor organ shortage, about one third of the patients on the kidney transplant waiting list are listed for a retransplant. The induction of donor-specific tolerance has the potential of at least partially resolving this problem, since it might prevent chronic rejection and drug toxicity at the same time. For a variety of protocols, successful tolerance induction has been demonstrated in rodent models. However, translation of such protocols to large animal models and on clinical trials has turned out to be very difficult. This review briefly describes mechanisms and barriers to transplantation tolerance, and then focuses on pre-clinical and clinical studies in non-human primates and humans. We have divided the strategies into two groups, based on the principle mechanisms of tolerance induction: the first group are protocols not using hematopoietic stem cell transplantation (HCT) as part of there regimen. They rely mainly on intensive T cell depletion (either by total body irradiation, total lymphoid irradiation or treatment with T cell-depleting agents such as anti-thymocyte globulin, anti-CD52 antibody or CD3 immunotoxin), which have been combined with costimulatory blockade, signaling blockade or donor antigen infusion. The second group are HCT-based protocols combining HCT with T cell-depleting agents and cytoreductive treatment. So far, only two protocols (one with total lymphoid irradiation and anti-thymocyte globulin, but no HCT; one with HCT, cyclophosphamide, anti-thymocyte globulin and thymic irradiation) have been translated into successful human studies. We summarize and discuss the results of these trials and suggest goals for further studies for the development tolerance protocols applicable for a broad population of allograft recipients.