Advances in transplantation immunology

Although there have been dramatic advances in clinical organ transplantation over the past 20 years, rejection, both acute and chronic, and the complications of immunosuppression remain major problems. Nevertheless as our understanding of the immune response to a vascularized organ allograft develops, so too will our ability to develop more specific immunosuppression. In any strategy for more specific immunosuppression compatibility for the major histocompatibility complex of antigens (HLA in man) is likely to be important. Monoclonal antibodies to T cell subpopulations, or even to T cells specifically activated by the graft, provide methods of suppressing the immune response at a more specific level. The recognition that stable grafts are maintained, at least in experimental rodent models, by T suppressor cells may allow development of precise methods of inducing the generation of such cells in clinical practice. The induction of tolerance in the adult animal can be achieved in a number of ways, the most promising of which for clinical application, is antigen pretreatment. If tolerance could be achieved in clinical practice within the not too distant future, then this would represent the attainment of the ultimate goal of transplantation. Members of the department who contributed to the work described in this lecture were M. Dallman, S. Fuggle, J. Madsen, G. Tellides, A. Ting and K. Wood This paper is based on a lecture given at the 87th Annual Congress of the Japanese Surgical Society, Tokyo, Japan, 1987.     

Natural Killer Cells and Liver Transplantation: Orchestrators of Rejection or Tolerance?

Natural killer (NK) cells are highly heterogeneous innate lymphocytes with a diverse repertoire of phenotypes and functions. Their role in organ transplantation has been poorly defined due to conflicting clinical and experimental data. There is evidence that NK cells can contribute to graft rejection and also to tolerance induction. In most solid organ transplantation settings, the role of NK cells is only considered from the perspective of the recipient immune system. In contrast to other organs, the liver contains major resident populations of immune cells, particularly enriched with innate lymphocytes such as NK cells, NKT cells, and gamma‐delta T cells. Liver transplantation therefore results in a unique meeting of donor and recipient immune systems. The unusual immune repertoire and tolerogenic environment of the liver may explain why this potentially inflammatory “meeting” often results in attenuated immune responses and reduced requirement for immunosuppression. Recent trials of immunosuppression withdrawal in liver transplant patients have identified NK cell features as possible predictors of tolerance. Here we propose that hepatic NK cells play a key role in the induction of tolerance post–liver transplant and examine potential mechanisms by which these cells influence liver transplant outcome.     

The TIM family of cosignaling receptors: emerging targets for the regulation of autoimmune disease and transplantation tolerance

Currently, lifelong immune suppression regimens are required for solid organ and cellular transplantation and carry significant increased risk of infection, malignancy, and toxicity. For non-life-saving procedures such as islet transplantation, the risk/benefit ratio of lifelong immunosuppression versus benefit from transplantation requires even more careful balance. The search for specific agents to modulate the immune system without chronic immunosuppression is important for the broad application of islet transplantation. The T-cell immunoglobulin mucin (TIM) family is a distinct group of coreceptors that are differentially expressed on T(H)1 and T(H)2 cells, and have the potential to regulate both cytotoxic and humoral immune responses. Completed murine studies demonstrate Tim pathways may be important in the regulation of tolerance to self (auto), harmless (allergic), and transplant (allo) antigen; however, the potential impact of targeting Tim coreceptors has yet to be fully explored in transplantation tolerance induction or autoimmune disease. The current review examines the impact of Tim coreceptor targeting as an emerging therapeutic option for regulating autoimmune diseases and prevention of allograft rejection.     

Overcoming memory T-cell responses for induction of delayed tolerance in nonhuman primates

The presence of alloreactive memory T cells is a major barrier for induction of tolerance in primates. In theory, delaying conditioning for tolerance induction until after organ transplantation could further decrease the efficacy of the regimen, since preexisting alloreactive memory T cells might be stimulated by the transplanted organ. Here, we show that such "delayed tolerance" can be induced in nonhuman primates through the mixed chimerism approach, if specific modifications to overcome/avoid donor-specific memory T-cell responses are provided. These modifications include adequate depletion of CD8+ memory T cells and timing of donor bone marrow administration to minimize levels of proinflammatory cytokines. Using this modified approach, mixed chimerism was induced successfully in 11 of 13 recipients of previously placed renal allografts and long-term survival without immunosuppression could be achieved in at least 6 of these 11 animals.     

Regulatory T cells and transplantation tolerance

Rodent models of transplantation have demonstrated that it is possible to induce specific immunological tolerance of donor antigens and indefinite graft survival in the absence of any continued immunosuppression. If this situation could be achieved clinically it would avoid many of the longer term complications of organ grafting, such as the increased risk of infection and cancer and the nephrotoxicity of many immunosuppressive agents. In this review we shall consider the interplay between regulatory T cells, dendritic cells and the graft itself and the resulting local protective mechanisms that are coordinated to maintain the tolerant state. We will discuss how both anti-inflammatory cytokines and negative costimulatory interactions can elicit a number of interrelated mechanisms to regulate both T cell and antigen-presenting cell activity. The induction and maintenance of tolerance via acquired local immune privilege has implications for the design of therapeutic regimens and the monitoring of the tolerant status of patients being weaned off immunosuppression.     

Transplantation tolerance: mechanisms and strategies?

The goal of transplantation is to induce tolerance to the transplanted tissue. We believe that this will only be possible by exploring strategies normally used to endure and maintain self-tolerance. Tolerance can be defined as a state where the immune system does not respond to a specific antigen. This is in sharp contrast with immunosuppression, which decreases the immune response to a myriad of antigens and requires continued medication. T cells play an essential role in the immune response to alloantigens, because animals devoid of T cells do not reject transplanted organs. In this article we will focus on the mechanism of T cell tolerance and how the immune system may be manipulated to achieve tolerance to alloantigens.     

Approaching the promise of operational tolerance in clinical transplantation

Long-term acceptance of transplanted organs without requirement for indefinite immunosuppression remains the ultimate goal of transplant clinicians and scientists. This clinical state of allograft acceptance termed "operational tolerance" has been elusive in routine practice. However, there are published reports of recipients where immunosuppression has been discontinued, by intention or patient noncompliance, in which the outcome is a nondestructive immune response and normal function. The question now arises how clinical operational tolerance might be achieved in the majority of recipients. This review provides an overview of current approaches to achieve operational tolerance, including the use of donor bone marrow and depletion of recipient T cells and the resistance of liver transplants to rejection. It also describes the key role of clinical immune monitoring and future approaches to tolerance induction including inhibition of T-cell signaling, manipulation of costimulatory pathways, and expansion of regulatory T cells. The principles of these experimental approaches may ultimately be extended to provide safe and effective control of transplant rejection and induction of clinical operational tolerance.     

CD4+CD25+ regulatory T cells mediate acquired transplant tolerance

The Holy Grail of clinical organ transplantation is the safe induction of allograft tolerance. Transplant tolerance has been successfully induced in animal models. Since T cells play a pivotal role in graft rejection, modulating T cell function has been the primary focus of studies aimed at inducing transplant tolerance. Rodent models of transplant tolerance induction include central deletion and peripheral mechanisms involving activation-induced cell death (AICD), anergy, immune deviation, and production of regulatory T cells. These mechanisms are not mutually exclusive. Although clonal deletion and anergy limit self-reactive T cells in the thymus, these mechanisms alone are not sufficient for controlling self-reactive T cells in the periphery. There is now evidence that the adult animal harbors two functionally distinct populations of CD4(+) T cells; one mediates autoimmune disease and the other dominantly inhibits it. The latter cells express CD4, CD25 and CTLA-4. These thymus-derived T cells have recently been shown to mediate the induction and maintenance of transplant tolerance. These CD4(+)CD25(+) T cells are similar in origin, phenotype, and function to those that maintain natural self-tolerance and T cell homeostasis in the periphery. Against this background, is it possible that alloantigen specific regulatory T cells might be generated and expanded ex vivo before organ transplantation and then infused to induce long-term tolerance, perhaps without the need for chronic immunosuppression?     

Interleukin 2 receptor-targeted therapy--rationale and applications in organ transplantation

Despite major advances in immunopharmacology for use in clinical organ transplantation, graft rejection and drug-induced side effects remain the major problems with currently available immunosuppressive modalities. Recent advances in hybridoma technology have produced relatively effective and reproducible biological immunosuppression with monoclonal antibodies; indeed, OKT3 and anti-T12 mAbs have been employed with considerable success as adjuncts to chemical suppression in treating rejection. Nonetheless, the use of such antibodies broadly reactive to differentiation antigens on T lymphocytes does not solve the problems of side effects caused by general immunosuppression. An ideal therapeutic agent should target only lymphocytes that participate in rejection of foreign tissue without affecting physiological host immune surveillance and normal defense mechanisms. Theoretically, this goal could be achieved by "antigenic suicide," or by using the appropriate antiidiotypic antibodies or mAbs against the antigen combining site of T cell receptors. However, because of the intense polymorphism of transplantation antigens and the vast genetic repertoire encoding for the T cell antigen receptor, success of such forms of specific immunosuppression, at least at this time, is highly improbable.     

Induction of Tolerance in Solid Organ Transplantation: The Rationale to Develop Clinical Protocols in Liver Transplantation

Minimization or withdrawal of immunosuppressive treatments after organ transplantation represents a major objective for improving quality of life and long-term survival of grafted patients. Such a goal may be reached under some clinical conditions, particularly in liver transplantation, making these patients good candidates for tolerance trials. In this context in liver transplantation, the central questions are (1) how to promote the natural propensity of the liver graft to be accepted, (2) which type of immunosuppressive drug should be used for induction and maintenance, and (3) which biomarkers could be used to discriminate tolerant patients from those requiring long-term immunosuppression. Induction therapies using aggressive T-cell-depleting agents may favor graft acceptance. However, persistent and/or rapidly reemerging cell lines, such as memory-type cells or CD8⁺ T cells, could represent a significant barrier for induction of tolerance. The type of maintenance drugs also remains questionable. Calcineurin inhibitors may be eventually deleterious in the context of tolerance protocols, through inhibitory effects on regulatory T cells, that are not observed with rapamycin. In conclusion, significant efforts must be made to achieve reliable strategies for immunosuppression minimization or withdrawal after organ transplantation into the clinics.     

Impact of immunosenescence on transplant outcome

Aging affects all compartments of the immune response and has a major impact on transplant outcome and organ quality. Although clinical trials in the aging transplant population remain rare, our current understanding of immunosenescence provides a basis for an age‐adapted immunosuppression and organ allocation with the goal to optimize utilization and to improve outcomes in older recipients. From a more general perspective, understanding the mechanisms and consequences of immunosenescence will have a broad impact on immune therapies in and beyond transplantation.     

Evolving Paradigms That Determine the Fate of an Allograft

Despite the many advances in both immunological knowledge and the practical application of clinical immunosuppression, the holy grail of indefinite graft survival with immune tolerance in clinical solid organ transplantation remains a distant dream. The tremendous progress made in understanding the molecular and cellular basis of allograft rejection has not been translated into durable modalities that have advanced clinical care and outcomes. Indeed, currently used drugs and treatment protocols, largely directed at inhibiting alloreactive T cells, have not optimally improved allograft survival or function. A shift in emphasis, focusing on under appreciated immune pathways must now be considered to make further improvement. We highlight three areas of recent interest, complement, NK cells and lymphatics, which reinforce the concept that the transplant community must direct attention on how the immune system as a whole responds to a transplant. The current challenge is to integrate molecular, cellular and anatomic concepts to achieve the equivalent of a unified field theory of the immune response to organ transplants.     

Early immunosuppression withdrawal after living donor liver transplantation and donor stem cell infusion.

Long-term results of organ transplantation are still limited by serious side effects of immunosuppressive drugs. A major issue, therefore, is to elaborate novel therapeutic protocols allowing withdrawal or minimization of immunosuppressive therapy after transplantation. We report on 3 patients prospectively enrolled in an original protocol designed to promote graft acceptance in living donor liver transplantation, using posttransplant conditioning with high doses of antithymocyte globulin followed by injection of donor-derived stem cells. In 2 patients, early immunosuppression withdrawal was possible, without subsequent graft deterioration. In these 2 cases, in vitro studies showed indices of immunological tolerance as assessed by specific hyporesponsiveness to donor alloantigens in mixed lymphocytes culture. In the third patient, acute rejection rapidly occurred after discontinuation of immunosuppression, and minimal immunosuppression has to be maintained during long-term follow-up. In this case, a clearly distinct immunoreactive profile was observed as compared to tolerant patients, as no specific modulation of the antidonor response was observed in vitro. Of note, no macrochimerism could be detected in any of the 3 patients during the follow-up. In conclusion, these clinical observations demonstrated that, despite the absence of macrochimerism, donor stem cells infusion combined with recipient conditioning may allow early immunosuppression withdrawal or minimization after liver transplantation.     

Systemic inflammation in xenograft recipients (SIXR)

Dysregulation of the coagulation system commonly develops in pig xenograft recipients, and remains an obstacle to successful pig organ xenotransplantation. Uncontrolled activation of coagulation leads to consumptive coagulopathy (CC) in the recipients, and thrombotic microangiopathy (TM) in the grafts. T cell‐directed immunosuppression successfully prevents the adaptive immune response to pig antigens after xenotransplantation and prolongs survival of organ xenografts. In some reports, T cell‐directed immunosuppression was able to delay (or prevent) the development of CC and/or TM. Recent reports have confirmed that inflammation can lead to activation of the coagulation system. Additionally, pro‐coagulant proteins, e.g. thrombin, are considered as pro‐inflammatory factors. In fact, an amplification loop is suggested to exist between inflammation and coagulation, leading to escalation of each other. Our in vitro data indicate that thrombin activation of pig endothelial cells is associated with upregulation of human T cell responses, suggesting that control of activation of coagulation and prevention of thrombin activation may facilitate the regulation of immune responses to xenografts in vivo. We hypothesize that a state of systemic inflammation develops after pig organ xenotransplantation, which is generated by both adaptive and innate immune responses. Even if T cell‐directed immunosuppression can control activation of coagulation induced by adaptive immune responses, pro‐inflammatory signals induced by the innate immune system can still promote activation of coagulation. We studied two models of xenotransplantation of different antigen loads; (i) pig aortic patch xenotransplantation, i.e. low antigen load, and (ii) pig organ (heart and kidney) xenotransplantation, i.e. high antigen load. We evaluated activation of coagulation, development of a T cell‐dependent immune response, and production of innate and adaptive pro‐inflammatory factors. In recipients of a low antigen load xenograft, effective prevention of the adaptive immune response by T cell‐directed immunosuppression (i.e. suppression of T cell proliferation in response to pig antigens and prevention of elicited antibody production) was associated with reduced thrombin activation. However, there was (i) upregulation of C‐reactive protein (CRP) and (ii) fibrinogen levels, (iii) increased IL‐6 production in the circulation, and (iv) an increase in the absolute number of innate immune cells (monocytes and neutrophils). Furthermore, (v) monocytes and dendritic cells showed significant upregulation of tissue factor expression, and aggregation with platelets after transplantation. In recipients with a high antigen load xenograft, short‐term organ survival was associated with high levels of CRP and IL‐6 early after transplantation. In long‐term organ survival, high levels of CRP and IL‐6 preceded the development of CC. There was intense CRP deposition in kidney xenografts (more than in heart xenografts) suggesting a stronger innate immune response. Additionally, CRP‐positive cells were detected in native lungs, suggesting an innate systemic inflammatory response. In conclusion, efficient blockade of the T cell‐dependent adaptive immune response in xenograft recipients is associated with systemic upregulation of inflammatory markers. Systemic inflammation in xenograft recipients (SIXR) is associated with upregulation of tissue factor expression on innate immune cells and their aggregation with platelets. As inflammation is known to break tolerance after transplantation, understanding the underlying mechanisms and regulation of SIXR may be necessary to achieve long‐term survival of organ xenografts (and T cell tolerance to pig antigens). Also, further genetic modifications of donor pigs to express anti‐inflammatory proteins may be essential.     

Upregulation of miR-142-3p in peripheral blood mononuclear cells of operationally tolerant patients with a renal transplant

Achieving drug-free tolerance or successfully using only small doses of immunosuppression is a major goal in organ transplantation. To investigate the potential mechanisms by which some kidney transplant recipients can achieve operational tolerance, we compared the expression profiles of microRNA in peripheral blood mononuclear cells of operationally tolerant patients with those of stable patients treated with conventional immunosuppression. B cells from operationally tolerant patients overexpressed miR-142-3p. The expression of miR-142-3p was stable over time and was not modulated by immunosuppression. In Raji B cells, overexpression of miR-142-3p modulated nearly 1000 genes related to the immune response of B cells, including potential miR-142-3p targets and molecules previously identified in the blood of operationally tolerant patients. Furthermore, our results suggested that a negative feedback loop involving TGF-β signaling and miR-142-3p expression in B cells may contribute to the maintenance of tolerance. In summary, miR-142-3p expression in peripheral blood mononuclear cells correlates with operational tolerance. Whether upregulation of miR-142-3p modulates inflammatory responses to promote tolerance or is a result of this tolerance state requires further study.     

The impact of donor age and recipient age on clinical course and immune response after organ transplantation

Organ shortage and increasing numbers of older patients on the waiting list represent major challenges in organ transplantation. As a consequence, older donor organs are increasingly used and preferentially allocated to elderly recipients. However, knowledge on donor age– and recipient age–dependent immune responses is limited. We reviewed the impact of donor age and recipient age on clinical outcomes. An increased immunogenicity of older grafts and an altered immune response of elderly recipients may require age-specific immunosuppression. Age-related immune responses may also impact current preclinical tolerance protocols.     

Specific alloantigen self-control by regulatory T cells in organ transplantation: a review

Multidrug immunosuppressive protocols have increased short-term patient and graft survival rates from 50% to 90% in the past two decades. Unfortunately, chronic graft rejection still remains the main cause of long-term failure and patients must undergo lifelong immunosuppression. The severe side effects such as life-threatening infections, secondary malignancies, and cardiovascular dysfunction all together include roughly 50% of deaths among kidney transplant patients with functioning grafts. Therefore, it should be of crucial importance to reduce immunosuppression and seek induction of specific tolerance to donor alloantigens. Several investigations have suggested that the acquisition of tolerance to self and/or foreign antigens is dependent on the number and function of naturally occurring and acquired regulatory T cells, which can control all aggressive T cells. The regulatory T cells together with their receptors, costimulatory molecules, cytokines, chemokines, and growth factors all contribute to maintain an equilibrium between aggressive and suppressive effector immune responses. As a consequence of increased knowledge, new immunosuppressive approaches based on either alloantigen-specific regulatory T-cell expansion in vivo or in vitro have been proposed to achieve donor-specific transplantation tolerance in kidney allograft recipients. This contribution attempted to summarize knowledge about regulatory T cells and developing methods to induce specific tolerance in kidney transplantation.     

T-cell activation and transplantation tolerance

Transplantation of allogeneic or "nonself" tissues stimulates a robust immune response leading to graft rejection, and therefore, most recipients of allogeneic organ transplants require the lifelong use of immune suppressive agents. Excellent outcomes notwithstanding, contemporary immunosuppressive medications are toxic, are often not taken by patients, and pose long-term risks of infection and malignancy. The ultimate goal in transplantation is to develop new treatments that will supplant the need for general immunosuppression. Here, we will describe the development and application of costimulation blockade to induce transplantation tolerance and discuss how the diverse array of signals that act on T cells will determine the balance between graft survival and rejection.