Transplantation: tolerance

The immunosuppressive regimens currently in use have been responsible for major improvements in transplanted organ acceptance, but long-term survival can be compromised by drug toxicity and/or chronic immune deficiency. The ultimate goal for transplantation is tolerance, defined as durable, donor-specific allograft acceptance in the absence of long-term immunosuppression. This review describes current experimental strategies for tolerance induction in primate models that are poised for clinical application.     

移植早期宿主成熟T细胞的灭活在诱导特异性免疫耐受中的作用

目的探讨参与早期急性排斥反应并导致非清髓性治疗诱导特异性免疫耐受失败的主要免疫细胞。方法通过动态观察受者三种不同预处理方案的骨髓移植后早期各阶段在外周血、脾脏、胸腺中的嵌合状态，再用T细胞敲除小鼠作为受者，以验证宿主成熟T细胞在急性排斥反应与诱导特并性免疫耐受中的重要作用。结果亚致死量全身放射治疗TBI（450cGy）4-BMT骨髓移植治疗组的BALB／C受者移植心脏均出现排斥反应，存活时间〈38d，骨髓移植后第7天在受者的外周血、脾脏中仍存活一定数量的宿主成熟T细胞（分别为0．23％与0．48％）。T细胞基因敲除c57BL／6小鼠作为受者，给予单次TBI（450cGy）＋BMT治疗，其移植心脏获长期存活，并获得完全嵌合状态。结论移植早期彻底清除或灭活宿主成熟T细胞是预防早期排斥反应的关键，也有利于供者骨髓T细胞植活，这对临床制定诱导特异性免疫耐受冶疗方案有着重要意义。     

Current pharmacotherapeutical options for the prevention of kidney transplant rejection

Introduction: Advances in immunosuppression and medical care over the past years resulted in better short- and long-term graft survival following kidney transplantation. Novel potent immunosuppressive agents, combinations of proven substances and the steadily expanding knowledge on the pathophysiology of kidney transplant rejection allows the extension of donor and recipient criteria, including the usage of organs from ABO-incompatible and crossmatch-positive donors, to overcome the increasing problem of organ shortage.

Areas covered: Immunosuppressive regimens for the prevention of kidney transplant rejections with a focus on regimens aiming at calcineurin inhibitor or steroid minimization, withdrawal or avoidance. Prevention of antibody-mediated rejections in standard-risk and sensitized recipients, as well as newly introduced immunosuppressive substances are covered.

Expert opinion: Currently applied immunosuppressive regimens are associated with excellent short-term graft survival. However, the long-term outcomes of different regimens substantially differ with regard to potential side effects, graft function, rejection and sensitization rates. The adverse effects of effective immunosuppression must carefully be balanced against the benefit, e.g., prevention of the development of donor-specific human leukocyte antigen antibodies and chronic (antibody-mediated) rejection. The choice of the appropriate immunosuppressive regimen requires clinical experience and careful consideration of recipient and transplant characteristics to achieve an optimal long-term graft survival.     

Induction of tolerance in organ recipients by hematopoietic stem cell transplantation

The use of hematopoietic stem cell transplantation (HSCT) for the establishment of mixed chimerism represents a viable and attractive approach for generating tolerance in transplantation biology, as it generally leads to durable immune tolerance, enabling the subsequent engraftment of organ transplants without the need for a deleterious continuous immunosuppressive therapy. However, in order to apply HSCT to patients in a manner that enables long term survival, transplant-related mortality must be minimized by eliminating the risk for graft-versus-host-disease (GVHD) and by reducing the toxicity of the conditioning protocol. T-cell depleted bone marrow transplants (TDBMT) have been shown to adequately eliminate GVHD. However, even in leukemia patients undergoing supralethal conditioning, mismatched TDBMT are vigorously rejected. This barrier can be overcome through the modulatory activity of CD34 cells, which are endowed with veto activity, by the use of megadose stem cell transplants. In mice, megadoses of Sca+lin-hematopoietic stem cells can induce mixed chimerism following sub-lethal conditioning. Nevertheless, the number of human CD34 cells that can be harvested is not likely to be sufficient to overcome rejection under reduced intensity conditioning (RIC), which might be acceptable in recipients of organ transplantation. To address this challenge, we investigated a novel source of veto cells, namely anti 3rd-party cytotoxic T cells (CTLs) which are depleted of GVH reactivity, combined with megadoses of purified stem cells and a RIC protocol. This approach might provide a safer modality for the induction of durable chimerism.     

Tolerance-inducing immunosuppressive strategies in clinical transplantation: an overview

The significant development of immunosuppressive drug therapies within the past 20 years has had a major impact on the outcome of clinical solid organ transplantation, mainly by decreasing the incidence of acute rejection episodes and improving short-term patient and graft survival. However, long-term results remain relatively disappointing because of chronic allograft dysfunction and patient morbidity or mortality, which is often related to the adverse effects of immunosuppressive treatment. Thus, the induction of specific immunological tolerance of the recipient towards the allograft remains an important objective in transplantation. In this article, we first briefly describe the mechanisms of allograft rejection and immune tolerance. We then review in detail current tolerogenic strategies that could promote central or peripheral tolerance, highlighting the promises as well as the remaining challenges in clinical transplantation. The induction of haematopoietic mixed chimerism could be an approach to induce robust central tolerance, and we describe recent encouraging reports of end-stage kidney disease patients, without concomitant malignancy, who have undergone combined bone marrow and kidney transplantation. We discuss current studies suggesting that, while promoting peripheral transplantation tolerance in preclinical models, induction protocols based on lymphocyte depletion (polyclonal antithymocyte globulins, alemtuzumab) or co-stimulatory blockade (belatacept) should, at the current stage, be considered more as drug-minimization rather than tolerance-inducing strategies. Thus, a better understanding of the mechanisms that promote peripheral tolerance has led to newer approaches and the investigation of individualized donor-specific cellular therapies based on manipulated recipient regulatory T cells.     

异基因骨髓移植和非清髓性干细胞移植嵌合形成过程的比较

目的 研究异基因骨髓移植（allo-BMT）和非清髓性干细胞移植（NST）两种移植方式在供体细胞嵌合状态的形成及转归上的差异,探讨早期供体细胞植入的关键因素。方法 对20例接受allo-BMT和18例NST的患者进行回顾性比较,研究两组患者疾病类型、干细胞来源、预处理方案和移植物抗宿主病预防方案。用复合扩增荧光标记STR-PCR结合毛细管电泳方法对移植后＋7、＋14、＋21d,＋1、＋3、＋6、＋9、＋12个月的嵌合体进行动态检测。结果 （1）NST组在受体年龄、单个核细胞（MNC）、CD34^＋及T细胞数量上均明显高于BMT组,造血重建方面,中性粒细胞绝对值恢复时间与BMT组无差别,但血小板恢复明显早于BMT组。（2）NST组患者供体细胞完全嵌合状态（FDC）的建立比BMT组早（1个月vs 3个月）,移植后早期（＋1个月）FDC比例亦明显高于BMT组（38．9％vs 20%）,而混合嵌合状态（MC）的发生率明显低于BMT组（61．2％ vs 80％）,移植1个月后各时间段两组在嵌合体形成上均无显著性差别。（3）氟达拉滨为基础的NST预处理方案与标准预处理方案相比并未延迟供体细胞的植入。（4）NST组慢性移植物抗宿主病的发生率明显高于BMT组（80％vs 50%,P〈0．01）,与NST组输入高剂量的CD34^＋细胞相关。结论 在供体细胞早期植入和嵌合体形成的过程中,移植物中造血干细胞和T细胞数量至关重要,并可能起决定性作用。     

Immunopharmacologic Therapy in Renal Transplantation

The long-term kidney allograft survival rate is still far from optimum. Conventional immunosuppressive drugs used to prevent allograft rejection are associated with significant side effects. Moreover, withdrawal of these agents is often associated with graft loss due to rejection. No treatment is available for chronic rejection. Graft tolerance is difficult to achieve in humans, and therefore a continued goal in organ transplantation is to develop immunosuppressive regimens that are associated with fewer side effects and decreased rates of rejection, and that promote graft tolerance. The advent of newer pharmacologic agents and bioreagents is expected to improve patient and graft survival rates.     

Pathological features of antibody-mediated rejection

Although cell-mediated rejection has remained the most common form of graft rejection after organ transplantation, antibody-mediated rejection has recently gained much significance in clinical transplantation. New evidence points to an antibody-mediated rejection contributing not only to hyperacute and acute but also to chronic allograft rejection. In addition, in discordant xenotransplantation, severe forms of antibody-mediated rejection, including hyperacute rejection and acute humoral xenograft rejection, represent major immunological barriers to successful xenotransplantation. Antibody-mediated rejection in both allotransplantation and xenotransplantation typically does not respond to conventional anti-rejection therapy, so it has recently been recognized as a major cause of graft loss. Histopathology remains the most definitive and reliable tool for the diagnosis of graft rejection in both allografts and xenografts. In this review, we discuss the concept that microvascular injury is a characteristic feature of antibody-mediated rejection that develops in hyperacute, acute and chronic antibody-mediated rejection in both allografts and discordant xenografts as well as in kidney and heart grafts. We also review work indicating that endothelial cell activation and endothelial cell death in the microvasculature can contribute to ultimate graft loss by triggering capillary destruction, interstitial hemorrhage, and platelet-rich microthrombi in hyperacute and acute antibody-mediated rejection as well as with the formation and progression of fibrotic scars in chronic antibody-mediated rejection.     

Pharmacotherapeutic agents in xenotransplantation

The ability to transplant pig organs into humans would resolve the current crisis in the supply of cadaveric human organs for the treatment of end stage disease. Several immunologic barriers need to be overcome if pig-to-primate transplantation is to be successful. The presence of preformed antibodies in humans, apes and Old World monkeys directed against galactose epitopes on pig vascular endothelium provides the major barrier, as binding of antibody to antigen leads to graft destruction by complement activation and other mechanisms. Hyperacute rejection can result from the action of complement. If this is prevented, delayed antibody-mediated rejection develops, which can be associated with a state of consumptive coagulopathy (disseminated intravascular coagulation, DIC). Efforts being made to overcome antibody-mediated rejection include depletion of antibody by extracorporeal immunoadsorption, prevention of an induced antibody response by co-stimulatory blockade, B-cell and/or plasma cell depletion, depletion or inhibition of complement, or the use of organs from pigs transgenic for a human complement regulatory protein, such as hDAF. The ultimate solution would be the induction of both B- and T-cell tolerance to the transplanted pig organ, which is being explored by attempting to induce haematopoietic cell chimerism. One complication of this is a thrombotic microangiopathy, similar to thrombotic thrombocytopenic purpura. The many and diverse roles in which pharmacotherapy is involved in attempts to overcome the barriers of xenotransplantation are reviewed and current progress, particularly in our own laboratory, is discussed.     

Pharmacotherapeutic agents in xenotransplantation

The ability to transplant pig organs into humans would resolve the current crisis in the supply of cadaveric human organs for the treatment of end stage disease. Several immunologic barriers need to be overcome if pig-to-primate transplantation is to be successful. The presence of preformed antibodies in humans, apes and Old World monkeys directed against galactose epitopes on pig vascular endothelium provides the major barrier, as binding of antibody to antigen leads to graft destruction by complement activation and other mechanisms. Hyperacute rejection can result from the action of complement. If this is prevented, delayed antibody-mediated rejection develops, which can be associated with a state of consumptive coagulopathy (disseminated intravascular coagulation, DIC). Efforts being made to overcome antibody-mediated rejection include depletion of antibody by extracorporeal immunoadsorption, prevention of an induced antibody response by co-stimulatory blockade, B-cell and/or plasma cell depletion, depletion or inhibition of complement, or the use of organs from pigs transgenic for a human complement regulatory protein, such as hDAF. The ultimate solution would be the induction of both B- and T-cell tolerance to the transplanted pig organ, which is being explored by attempting to induce haematopoietic cell chimerism. One complication of this is a thrombotic microangiopathy, similar to thrombotic thrombocytopenic purpura. The many and diverse roles in which pharmacotherapy is involved in attempts to overcome the barriers of xenotransplantation are reviewed and current progress, particularly in our own laboratory, is discussed.     

Induction of transplantation tolerance via regulatory T cells

In the last two decades, graft survival has been greatly improved by the introduction of efficient immunosuppressive drugs. On the other hand, late graft loss caused by chronic rejection together with the side effects of long-term immunosuppression, remain major obstacles for successful transplantation. Operational tolerance, which is defined by the lack of acute and chronic rejection and indefinite graft survival with normal graft function in the absence of chronic immunosuppression, represents an attractive alternative. Several approaches have been explored to achieve transplantational tolerance, which is considered the "Holy Grail" of transplantation, including induction of central tolerance by establishing mixed chimerism through hematopoietic stem cell transplantation or induction of peripheral tolerance through modulation of allogeneic immune responses. Graft-specific alloreactive T cells, which largely mediate graft rejection, can be silenced through different mechanisms, including deletion, which may occur within the thymus or in the lymphoid organs; anergy, in which alloreactive T cells cannot adequately respond following restimulation with the specific antigen; and suppression, which may be mediated by direct interactions with regulatory T cells (Tregs) or by soluble factors produced by Tregs. This review attempts to summarize the most novel and successful strategies to achieve operational tolerance via induction of Tregs.     

Tacrolimus: a further update of its pharmacology and therapeutic use in the management of organ transplantation

Tacrolimus (FK-506) is an immunosuppressant agent that acts by a variety of different mechanisms which include inhibition of calcineurin. It is used as a therapeutic alternative to cyclosporin, and therefore represents a cornerstone of immunosuppressive therapy in organ transplant recipients. Tacrolimus is now well established for primary immunosuppression in liver and kidney transplantation, and experience with its use in other types of solid organ transplantation, including heart, lung, pancreas and intestinal, as well as its use for the prevention of graft-versus-host disease in allogeneic bone marrow transplantation (BMT), is rapidly accumulating. Large randomised nonblind multicentre studies conducted in the US and Europe in both liver and kidney transplantation showed similar patient and graft survival rates between treatment groups (although rates were numerically higher with tacrolimus- versus cyclosporin-based immunosuppression in adults with liver transplants), and a consistent statistically significant advantage for tacrolimus with respect to acute rejection rate. Chronic rejection rates were also significantly lower with tacrolimus in a large randomised liver transplantation trial, and a trend towards a lower rate of chronic rejection was noted with tacrolimus in a large multicentre renal transplantation study. In general, a similar trend in overall efficacy has been demonstrated in a number of additional clinical trials comparing tacrolimus- with cyclosporin-based immunosuppression in various types of transplantation. One notable exception is in BMT, where a large randomised trial showed significantly better 2-year patient survival with cyclosporin over tacrolimus, which was primarily attributed to patients with advanced haematological malignancies at the time of (matched sibling donor) BMT. These survival results in BMT require further elucidation. Tacrolimus has also demonstrated efficacy in various types of transplantation as rescue therapy in patients who experience persistent acute rejection (or significant adverse effect's) with cyclosporin-based therapy, whereas cyclosporin has not demonstrated a similar capacity to reverse refractory acute rejection. A corticosteroid-sparing effect has been demonstrated in several studies with tacrolimus, which may be a particularly useful consideration in children receiving transplants. The differences in the tolerability profiles of tacrolimus and cyclosporin may well be an influential factor in selecting the optimal treatment for patients undergoing organ transplantation. Although both drugs have a similar degree of nephrotoxicity, cyclosporin has a higher incidence of significant hypertension, hypercholesterolaemia, hirsutism and gingival hyperplasia, while tacrolimus has a higher incidence of diabetes mellitus, some types of neurotoxicity (e.g. tremor, paraesthesia), diarrhoea and alopecia. Conclusion: Tacrolimus is an important therapeutic option for the optimal individualisation of immunosuppressive therapy in transplant recipients.     

Combating chronic renal allograft dysfunction : optimal immunosuppressive regimens

Kidney transplantation is the best treatment for patients with end-stage renal disease, both in terms of survival benefit and quality of life. The major limitation is the continuing shortage of kidneys suitable for transplantation, reinforcing the need to maximise graft survival. After the first year of transplantation, chronic renal allograft dysfunction (CRAD) is the first cause of late graft deterioration and failure. CRAD has been defined as a progressive renal dysfunction, independent of acute rejection, drug toxicity and recurrent or de novo nephropathy, with features on biopsy of chronic allograft nephropathy (CAN) characterised by vascular intimal hyperplasia, tubular atrophy, interstitial fibrosis and chronic transplant glomerulopathy. Protocol biopsy-based studies have demonstrated a high and early prevalence of CAN lesions during the first year in patients with normal and stable renal function. Beyond 1 year, the injuries associated with calcineurin inhibitors (CNIs) appear to be very common. The physiopathology of CRAD is complex and multifactorial. Both alloantigen-dependent factors (acute rejection, HLA matching, donor-specific antibodies, inadequate immunosuppression) and alloantigen-independent factors (donor age, brain death, ischaemia/reperfusion injuries, hypertension, hyperlipidaemia, cytomegalovirus, CNI-related nephrotoxicity) are involved. Consequently, CRAD appears as a dynamic process, evolving with time, and immunosuppressive regimens need to be modulated in order to provide the most suitable treatment at the different phases of its natural history. On the basis of this scheme, the new paradigm would be the use of a CNI-based regimen during the period of maximal risk of (subclinical) acute rejection, followed by a conversion to a CNI-free regimen in order to avoid the long-term consequences of nephrotoxicity. Fortunately, new agents are being introduced in clinical practice providing a large range of combinations and allowing individualisation of immunosuppressive regimens. Large, prospective, multicentre trials are warranted, and the challenge is to define new endpoints of CRAD and to determine the best therapeutic strategy.     

Immunosuppressive strategies and management

<正>Advances in immunosuppressive therapy have significantly improved short-term allograft and patient survival.However,chronic allograft failure,antibody mediated rejection,recurrent diseases and immunosuppressive drug associated adverse effects remain serious barriers to long-term survival and quality of life.New immunosuppressive agents and protocols are being evaluated to combat these problems.Importantly, clinicians must work to manage post-transplant complications and avoid complex medication regimens,which will potentiate drug interactions and non-compliance.     

The fourth barrier

At the entrance of a new era, clinical xenotransplantation is a valued and auspicious option in tackling the problem of donor shortage. Because of ethical and anatomical issues, domestic farm animals are considered the most favourable species for organ donation, but transplantation of their organs leads to a complex process of rejection. Mechanistically, three immunological barriers, namely hyperacute rejection, delayed xenograft rejection and a subsequent cellular rejection, are distinguished. A fifth (microbiological) barrier is also being recognised. This review focuses on problems regarding the fourth barrier, i.e. physiology, in possible clinical settings and their corresponding animal models. Besides anatomical differences and posture, biochemical differences may have a severe impact on recipient survival. Differences in blood components and electrolyte and other biochemical concentrations are easily detected throughout the species considered for xenotransplantation. Enzymes and hormones have complex routes of action, activation and inhibition, and their molecular differences can impede function. As infusion or medicine may correct certain imbalances in electrolytes and proteins, problems with complex interactions might be difficult to retrieve and solve. Experimentally, survival of discordant xenografts show promising results, but the first physiological problems have already been detected. So, based upon the few experimental data available and the comparison of veterinary physiology, one might expect differences between the organs grafted, regarding the possible occurrence of physiological problems. Moreover, precautions must be taken to extrapolate long-term survival, because of species specificity.     

96例活体肾移植患者免疫抑制剂应用情况分析

目的:了解我院活体肾移植患者住院期间免疫抑制剂的应用情况。方法:回顾性调查我院2005～2007年活体肾移植患者病历96份,分别对免疫抑制剂品种、相关费用及不良反应情况进行统计、分析。结果:活体肾移植患者平均住院费用为48385.85元,免疫抑制剂平均费用为17364.46元;术后急性排斥反应发生率为5.74%,免疫抑制剂相关不良反应发生率为54.17%。结论:活体肾移植具有排斥反应小、花费少的特点。今后应就活体肾移植患者如何降低免疫抑制剂用量,减少诱导免疫耐受治疗,施行环孢素撤除治疗等展开深入研究。     

The Fourth Barrier

Summary

At the entrance of a new era, clinical xenotransplantation is a valued and auspicious option in tackling the problem of donor shortage. Because of ethical and anatomical issues, domestic farm animals are considered the most favourable species for organ donation, but transplantation of their organs leads to a complex process of rejection. Mechanistically, three immunological barriers, namely hyperacute rejection, delayed xenograft rejection and a subsequent cellular rejection, are distinguished. A fifth (microbiological) barrier is also being recognised. This review focuses on problems regarding the fourth barrier, i.e. physiology, in possible clinical settings and their corresponding animal models. Besides anatomical differences and posture, biochemical differences may have a severe impact on recipient survival. Differences in blood components and electrolyte and other biochemical concentrations are easily detected throughout the species considered for xenotransplantation. Enzymes and hormones have complex routes of action, activation and inhibition, and their molecular differences can impede function. As infusion or medicine may correct certain imbalances in electrolytes and proteins, problems with complex interactions might be diffiult to retrieve and solve. Experimentally, survival of discordant xenografts show promising results, but the first physiological problems have already been detected. So, based upon the few experimental data available and the comparison of veterinay physiology, one might expect differences between the organs grafted, regarding the possible occurrence of physiological problems. Moreover, precautions must be taken to extrapolate long-term survival, because of species specificity.     

The search for immunosuppressive therapies to induce tolerance in organ transplantation

Organ transplantation has become a major therapeutic option for patients with irreversible organ diseases. Immunosuppressive agents are usually required to prevent allograft rejection in patients who undergo an organ transplantation. Such drugs suppress both specific and nonspecific immunity, and render the recipient more susceptible to both infection and malignancy. Therefore, the development of more effective and less toxic immunosuppressive agents could improve the clinical outcomes of organ transplant recipients. Since the early days of clinical and experimental liver transplantation, it has been known that the liver is less likely to be rejected in comparison to other organs and may be tolerogenic even across a fully allogeneic MHC barrier in some specific cases. Spontaneous acceptance of liver allografts has been observed in several species. Orthotopic liver transplantation (OLT) in certain rat strains is accepted without immunosuppressive agents and serum from post-OLT recipients displays immunosuppressive activity. Attempts have been made to identify the immunosuppressive factors that are present in post-OLT serum to elucidate the mechanism of immunological tolerance and to discover novel immunosuppressive agents for potential use in organ transplantation. In this review we will focus on established and recent findings in the identification of immunosuppressive factors in a rat tolerogenic OLT model. The most recent therapeutic methods in organ transplantation and future prospects will be discussed.     

Immunosuppressive preconditioning or induction regimens : evidence to date

The success of solid organ transplantation has been directly related to the development of immunosuppressive drug therapies. Preconditioning or induction therapy was developed to reduce early immunological and nonimmunological renal injury, with the goal of increasing long-term graft survival. However, the routine induction of immunological tolerance to solid organ allograft is currently not achievable because of the morbidity and mortality related to the immunosuppressive regimens themselves. The different therapeutic preconditioning or induction agents and their associated effects on cellular rejection, graft survival outcomes and the need for multiagent post-transplant maintenance therapy are reviewed.     

Potential use of genetically modified pigs as organ donors for transplantation into humans

1. Transplantation is currently viewed as a successful treatment for end-stage organ failure. Its more widespread use is severely limited by a shortage of human organ donors. This has stimulated the scientific and medical communities to look at the potential use of animals to solve this problem. For a number of reasons, the pig appears to be the preferred species. 2. The initial immunological problem encountered in pig-to-primate transplantation is a process of hyperacute rejection, which is mediated by the binding of antibodies to the graft endothelium followed by the activation of the complement cascade. We have sought to overcome these initial immunological challenges by focusing our attention not only on the recipient of the graft but also on the donor. Therefore, we have generated transgenic animals with specific genetic modifications designed to inhibit the activation of the complement cascade. Upon transplantation to baboons of organs derived from these transgenic pigs, we have been able to demonstrate that hyperacute rejection can be prevented. We will discuss the generation of these and other transgenic animals and their potential role in clinical xenotransplantation.