Regulatory T cells and dendritic cells in transplantation medicine

Transplant Immunology 11 (2003) 231–233

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Regulatory T cells and dendritic cells in transplantation medicine

Transplantation medicine has made immense progress over the last 50 years since the concept of immunological tolerance first emerged. This is largely tributary to the development of new immunosuppressive drugs and treatment strategies. However, major problems associated with the collateral effects of the immunosuppressive treatment still remain unresolved. Chronic rejection on one side, malignancies and infections on the other, threaten the long-term graft and patient’s survival. Certain drugs, such as calcineurin inhibitors, suppress the generation of effector T helper and cytotoxic cells, yet interfere with the induction of tolerance, probably by blocking the development of regulatory T cells. The clinicians are faced with the dilemma of using the maximum tolerated dose to prevent rejection or a minimally efficient immunosuppressive dose, which may avoid the risk of collateral effects. After the discovery of cyclosporine, triple therapy with steroids, cyclosporine and azathioprine has been used most extensively. Mycophenolate has replaced azathioprine as a component of triple therapy and finally, Tacrolimus has replaced cyclosporine. Polyclonal or monoclonal (anti-CD3) anti-lymphocytic antibodies have been added to treatment during the early induction phase or for reversal of the acute rejection episodes. More recently, Daclizumab a humanized monoclonal antibody, which blocks the alpha chain of the IL-2 receptor and Sirolimus have been added to the conventional treatment. The frequency of acute rejection episodes has been reduced from approximately 40% to 10–15% and the 1-year graft survival has reached 90% for most of the organ transplants. However, the long-term results are still unknown and remain to be evaluated. While the generation of new immunosuppressive agents is largely the merit of research performed by pharmaceutical companies, the understanding of basic mechanisms of immune recognition and regulatory networks is the first basic research. Crucial to our understanding of the phenomenon of allograft rejection has been the discovery that recipient T cells can recognize and react against the graft either directly (the direct recognition pathway) or indirectly

(the indirect recognition pathway). It has been shown that both pathways mediate the growth and the differentiation of alloantibody producing B cells, and contribute to acute and chronic rejection. However, both pathways can be inhibited by particular subsets of specialized T cells, which display regulatory and suppressor function. Regulatory T cells, (TR) display a characteristic CD4qCD25q phenotype and act as natural inhibitors of T cell reactivity to autoantigens. ‘Natural’ TR arises in the thymus, is present at birth and persists through adult life. Upon triggering of their TCR, they interact with other T cells inhibiting their response to any kind of stimulation. This inhibition is not antigen specific and thus, it is unlikely that expansion of naturally occurring CD4qCD25q T cells may find their clinical application in strategies aimed to induce antigen– specific tolerance in transplantation. Another category of T cells which share the CD4qCD25q phenotype with natural TR are the IL-10 producing TR1 cells. These cells can be induced both in vivo and in vitro by multiple antigenic stimulations in the presence of IL-10. There is still a controversy in the literature about their antigen specificity and mechanism of action. In certain experimental systems, it has been shown that the inhibitory effect of TR1 cells is antigen– nonspecific and mediated by IL-10. In other systems, both antigen–specificity and cell-to-cell interaction with other T cells or APCs have been shown to occur. ‘Induced’ TR1 cells may be of interest in clinical medicine if it becomes feasible to generate and expand alloantigen–specific populations. Regulatory T cells, which are derived from the CD8 subset of lymphocytes have also been described. They resemble TR1 cells producing IL-10 and acting in an antigen–nonspecific manner, although in some systems they appear to be antigen specific. Mature or immature plasmacytoid dendritic cells as well as immature myeloid dendritic cells seem to be able to induce IL-10 producing CD8q TR cells. Again, antigen specificity is a pre-requisite for contemplating the use of CD8q TR cells in transplantation. Finally, a distinct category of inhibitory T cells are the CD8qCD28y T suppressor (TS) cells. These cells have

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been generated after chronic (multiple) stimulation of T cells with allogeneic, xenogeneic or antigen-pulsed autologous APC. TS are HLA class Iypeptide specific have a restricted TCR repertoire, produce no cytokines and are negative for CD28, CD40L and perforin. They inhibit the proliferation of CD4q TH cells with cognate antigen specificity and render bridging APC tolerogenic by a contact dependent mechanism. Upon interaction with CD8qCD28y TS, APC upregulate two inhibitory receptors named ILT3 and ILT4, which inhibit the transcription of certain costimulatory molecules (CD40, CD80 and CD86) in the APC and transmit a negative signal to ¨ or primed CD4q T cells, which CD4q T cells. Naıve recognize HLA class IIypeptide complexes on the tolerogenic APC become anergic and acquire regulatory function. These anergic CD4qCD25q T cells inhibit the activation and differentiation of the TH1 cells by inducing the upregulation of ILT3 and ILT4 on APC, which become tolerogenic. Since upregulation of ILT3 and ILT4 can be induced in immature myeloid DC not only by exposing them to CD8qCD28y TS or CD4qCD25q TR, but also by treating the APC with vit.D3, IL-10 andyor, IFN-a it appears that the induction of these molecules is the hallmark of tolerogenic DC. It is essential to understand whether regulatory and suppressor T cells develop after transplantation, and whether such knowledge permits the identification of patients who may be benefited from minimal immunosuppression. We can imagine the following scenario. Early after transplantation, in the inflammatory milieu created by the operative trauma, recipient CD4q T cells may become activated through direct recognition of alloantigens expressed on the membrane of ‘passenger’ donor APC. TH may proliferate, produce and induce a cascade of cytokines and elicit the generation of CD8q cytotoxic T lymphocytes, which will damage the graft causing an acute rejection episode. Successful immunosuppressive therapy will reverse rejection inhibiting the proliferation of alloreactive CD4q T helper cells. Donor APC migrates out of the graft, which thus becomes less immunogenic. Host DC ingest apoptotic and necrotic cells of the graft, process them into peptides and present them to T cells from the regional lymphnodes. However, under the coverage of immunosuppression CD4q T cells that recognize donor allopeptides may be unable to secrete IL2 and to proliferate. In contrast, CD8qCD28y T cells, which do not require costimulation via the B7 or CD40 molecules may still receive the activation signal (signal 1) interacting with non-professional APC of the graft, i.e. with endothelial cells (EC) or with autologous DC which have processed donor alloantigens. The prevalence of this population may prevent the cytotoxic activ-

ity of CD8qCD28q CTL since EC do not express the costimulatory B7 molecules. CD8qCD28y TS may also inhibit the indirect pathway tolerizing the host APC, which present allopeptides bound to MHC class II antigens to CD4q TH cells. Hence, the early quiescence may be maintained predominantly by CD8qCD28y TS. The effect of TS on EC may be similar to their effect on allogeneic donor APC, rendering them tolerogenic to recipient CD4q TH endowed with direct recognition ability. These alloreactive CD4q TH cells may become anergicyregulatory T cells. The state of quiescence, however, may be disturbed by changes in immunosuppressive therapy andyor inflammatory events. This may activate both direct and indirect alloreactivity resulting in secondary acute andy or chronic rejection. It is possible that indirect alloreactivity is also subjected to regulation via CD8qCD28y and CD4qCD25q T cells. Host DC exposed to cytokines, such as IL-10 andyor IFN-a may induce the preferential differentiation of ILT3high ILT4high DC, which may polarize the responding T cells to an inhibitory phenotype. However, some of these cytokines such as IL-10 may promote the differentiation of B cells, which recognize specifically donor MHC molecules. It is conceivable that antibody mediated chronic rejection, which occurs late following transplantation is the end result of a seemingly successful induction of immunoregulatory cytokines. Unfortunately, no efficient treatment for chronic rejection is currently available. Although immunologic tolerance has been induced successfully in rodents by numerous protocols, including treatment with anti CD4, anti-CD25, anti CTLA4, antiCD40L antibodies, no equivalent success has been obtained in humans. Certain experimental protocols, however, yielded encouraging results yet were abandoned for various reasons. Examples include the protocol developed by S. Strober, which was based on the use of total lymphoid irradiation followed by low dose immunosuppression. Although complete and stable tolerance has been shown to be inducible by this method, the risk involved overweighs the potential benefits. Similarly, the use of donor-specific transfusions or transplantation of donor bone marrow cells yielded promising results yet stable tolerance could not be achieved. Transfusions of blood or bone marrow cells may induce the generation of regulatory T cells andyor microchimerism yet none of these phenomena appeared to bypass the need of iatrogenically induced immunosuppression. This issue presents the current state of the art in the study of regulatory networks. It discusses the central role of tolerogenic dendritic cells, the characteristics of naturally occurring and induced CD4qCD25q regulatory T

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cells, the role of CD8qCD28y T suppressor cells, and the experimental and clinical evidence that these cells play an important role in transplantation. We hope that this information will stimulate further studies on the mechanism of allogeneic tolerance induction, so that the

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use of non-specific immunosuppressive therapy can be minimized. Prof. Raffaello Cortesini Editor-in-Chief