FN18-CRM9 immunotoxin promotes tolerance in primate renal allografts

Challenges and opportunities in the islet transplantation microenvironment: a comprehensive summary of inflammatory cytokine, immune cells, and vascular endothelial cells

It is now understood that islet transplantation serves as a β-cell replacement therapy for type 1 diabetes. Many factors impact the survival of transplanted islets, especially those related to the microenvironment. This review explored microenvironmental components, including vascular endothelial cells, inflammatory cytokines, and immune cells, and their profound effects on post-islet transplantation survival rates. Furthermore, it revealed therapeutic strategies aimed at targeting these elements. Current evidence suggests that vascular endothelial cells are pivotal in facilitating vascularization and nutrient supply and establishing a new microcirculation network for transplanted islets. Consequently, preserving the functionality of vascular endothelial cells emerges as a crucial strategy to enhance the survival of islet transplantation. Release of cytokines will lead to activation of immune cells and production and release of further cytokines. While immune cells hold undeniable significance in regulating immune responses, their activation can result in rejection reactions. Thus, establishing immunological tolerance within the recipient's body is essential for sustaining graft functionality. Indeed, future research endeavors should be directed toward developing precise strategies for modulating the microenvironment to achieve higher survival rates and more sustained transplantation outcomes. While acknowledging certain limitations inherent to this review, it provides valuable insights that can guide further exploration in the field of islet transplantation. In conclusion, the microenvironment plays a paramount role in islet transplantation. Importantly, we discuss novel perspectives that could lead to broader clinical applications and improved patient outcomes in islet transplantation.

CD3e-immunotoxin spares CD62Llo Tregs and reshapes organ-specific T-cell composition by preferentially depleting CD3ehi T cells

CD3-epsilon(CD3e) immunotoxins (IT), a promising precision reagent for various clinical conditions requiring effective depletion of T cells, often shows limited treatment efficacy for largely unknown reasons. Tissue-resident T cells that persist in peripheral tissues have been shown to play pivotal roles in local and systemic immunity, as well as transplant rejection, autoimmunity and cancers. The impact of CD3e-IT treatment on these local cells, however, remains poorly understood. Here, using a new murine testing model, we demonstrate a substantial enrichment of tissue-resident Foxp3+ Tregs following CD3e-IT treatment. Differential surface expression of CD3e among T-cell subsets appears to be a main driver of Treg enrichment in CD3e-IT treatment. The surviving Tregs in CD3e-IT-treated mice were mostly the CD3e^dimCD62L^lo effector phenotype, but the levels of this phenotype markedly varied among different lymphoid and nonlymphoid organs. We also found notable variations in surface CD3e levels among tissue-resident T cells of different organs, and these variations drive CD3e-IT to uniquely reshape T-cell compositions in local organs. The functions of organs and anatomic locations (lymph nodes) also affected the efficacy of CD3e-IT. The multi-organ pharmacodynamics of CD3e-IT and potential treatment resistance mechanisms identified in this study may generate new opportunities to further improve this promising treatment.

Introducing thymus for promoting transplantation tolerance

Establishing tolerance remains a central, if elusive, goal of transplantation. In solid-organ transplantation, one strategy for inducing tolerance has been cotransplantation of various forms of thymic tissue along with another organ. As one of the biological foundations of central tolerance, thymic tissue carries with it the ability to induce tolerance to any other organ or tissue from the same donor (or another donor tissue-matched to the thymic tissue) if successfully transplanted. In this review, we outline the history of this approach as well as work to date on its application in organ transplantation, concluding with future directions. We also review our experience with allogeneic processed thymus tissue for the treatment of congenital athymia, encompassing complete DiGeorge syndrome and other rare genetic disorders, and consider whether allogeneic processed thymic tissue implantation may offer a novel method for future experimentation with tolerance induction in organ transplantation.

Mycophenolate mofetil versus azathioprine in kidney transplant recipients on steroid-free, low-dose cyclosporine immunosuppression (ATHENA): A pragmatic randomized trial

BACKGROUND

We compared protection of mycophenolate mofetil (MMF) and azathioprine (AZA) against acute cellular rejection (ACR) and chronic allograft nephropathy (CAN) in kidney transplant recipients on steroid-free, low-dose cyclosporine (CsA) microemulsion maintenance immunosuppression.

METHODS AND FINDINGS

ATHENA, a pragmatic, prospective, multicenter trial conducted by 6 Italian transplant centers, compared the outcomes of 233 consenting recipients of a first deceased donor kidney transplant induced with low-dose thymoglobulin and basiliximab and randomized to MMF (750 mg twice/day, n = 119) or AZA (75 to 125 mg/day, n = 114) added-on maintenance low-dose CsA microemulsion and 1-week steroid. In patients without acute clinical or subclinical rejections, CsA dose was progressively halved. Primary endpoint was biopsy-proven CAN. Analysis was by intention to treat. Participants were included between June 2007 and July 2012 and followed up to August 2016. Between-group donor and recipient characteristics, donor/recipient mismatches, and follow-up CsA blood levels were similar. During a median (interquartile range (IQR)) follow-up of 47.7 (44.2 to 48.9) months, 29 of 87 biopsied patients on MMF (33.3%) versus 31 of 88 on AZA (35.2%) developed CAN (hazard ratio (HR) [95% confidence interval (CI)]: 1.147 (0.691 to 1.904, p = 0.595). Twenty and 21 patients on MMF versus 34 and 14 on AZA had clinical [HR (95% CI): 0.58 (0.34 to 1.02); p = 0.057) or biopsy-proven subclinical [HR (95% CI): 1.49 (0.76 to 2.92); p = 0.249] ACR, respectively. Combined events [HR (95% CI): 0.85 (0.56 to 1.29); p = 0.438], patient and graft survival, delayed graft function (DGF), 3-year glomerular filtration rate (GFR) [53.8 (40.6;65.7) versus 49.8 (36.8;62.5) mL/min/1.73 m2, p = 0.50], and adverse events (AEs) were not significantly different between groups. Chronicity scores other than CAN predict long-term graft outcome. Study limitations include small sample size and unblinded design.

CONCLUSIONS

In this study, we found that in deceased donor kidney transplant recipients on low-dose CsA and no steroids, MMF had no significant benefits over AZA. This finding suggests that AZA, due to its lower costs, could safely replace MMF in combination with minimized immunosuppression.

TRIAL REGISTRATION

ClinicalTrials.gov NCT00494741; EUDRACT 2006-005604-14.

Experimental modeling of desensitization: What have we learned about preventing AMR?

During the past 5 decades, short-term outcomes in kidney transplant have significantly improved, in large part due to reduced rates and severity of acute rejection. Development of better immunosuppressive maintenance agents, as well as new induction therapies, helped make these advances. Nonhuman primate models provided a rigorous testing platform to evaluate candidate biologics during this process. However, antibody-mediated rejection remains a major cause of late failure of kidney allografts despite advances made in pharmacologic immunosuppression and strategies developed to facilitate improved donor-recipient matching. Our laboratory has been actively working to develop strategies to prevent and treat antibody-mediated rejection and immunologic sensitization in organ transplant, relying largely on a nonhuman primate model of kidney transplant. In this review, we will cover outcomes achieved by managing antibody-mediated rejection or sensitization in nonhuman primate models and discuss promises, limitations, and future directions for this model.

Targeted Diphtheria Toxin-Based Therapy: A Review Article

Cancer remains one of the leading causes of death worldwide. Conventional therapeutic strategies usually offer limited specificity, resulting in severe side effects and toxicity to normal tissues. Targeted cancer therapy, on the other hand, can improve the therapeutic potential of anti-cancer agents and decrease unwanted side effects. Targeted applications of cytolethal bacterial toxins have been found to be especially useful for the specific eradication of cancer cells. Targeting is either mediated by peptides or by protein-targeting moieties, such as antibodies, antibody fragments, cell-penetrating peptides (CPPs), growth factors, or cytokines. Together with a toxin domain, these molecules are more commonly referred to as immunotoxins. Targeting can also be achieved through gene delivery and cell-specific expression of a toxin. Of the available cytolethal toxins, diphtheria toxin (DT) is one of the most frequently used for these strategies. Of the many DT-based therapeutic strategies investigated to date, two immunotoxins, Ontak^TM and Tagraxofusp^TM, have gained FDA approval for clinical application. Despite some success with immunotoxins, suicide-gene therapy strategies, whereby controlled tumor-specific expression of DT is used for the eradication of malignant cells, are gaining prominence. The first part of this review focuses on DT-based immunotoxins, and it then discusses recent developments in tumor-specific expression of DT.

Translational impact of NIH-funded nonhuman primate research in transplantation

The National Institutes of Health (NIH) has long supported using nonhuman primate (NHP) models for research on kidney, pancreatic islet, heart, and lung transplantation. The primary purpose of this research has been to develop new treatments for down-modulating or preventing deleterious immune responses after transplantation in human patients. Here, we discuss NIH-funded NHP studies of immune cell depletion, costimulation blockade, regulatory cell therapy, desensitization, and mixed hematopoietic chimerism that either preceded clinical trials or prevented the human application of therapies that were toxic or ineffective.

Transplant research in nonhuman primates to evaluate clinically relevant immune strategies in organ transplantation

Research in transplant immunology using non-human primate (NHP) species to evaluate immunologic strategies to prevent rejection and prolong allograft survival has yielded results that have translated successfully into human organ transplant patient management. Other therapies have not proceeded to human translation due to failure in NHP testing, arguably sparing humans the futility and risk of such testing. The NHP transplant models are ethically necessary for drug development in this field and provide the closest analogue to human transplant patients available. The refinement of this resource with respect to colony MHC typing, reagent and assay development, and availability to the research community has greatly enhanced knowledge about transplant immunology and drug development.

Nonhuman primate models of transplant tolerance: closer to the holy grail

PURPOSE OF REVIEW

Transplantation tolerance, successful acceptance of an organ without the perils of immunosuppression, has been a central goal of transplant research. Many strategies to achieve this tolerance have been examined over the past three decades, culminating in several human trials of transplant tolerance. This progression from the 'benchtop to the clinic' has depended on the successful implementation of these tolerance strategies in nonhuman primates. This review will examine the described methods of transplant tolerance induction in nonhuman primates.

RECENT FINDINGS

Although costimulatory blockade and mixed chimerism have an established record of achieving transplant tolerance in nonhuman primates, some of the most innovative recent techniques of tolerance induction have relied on cellular transfer. This review will fully examine the role of regulatory T-cell transfer and the use of mesenchymal stem/stromal cells to promote tolerance of organ allografts in nonhuman primates.

SUMMARY

Use of translational nonhuman primate transplant models is a vital intermediate step to advance new approaches of transplant tolerance induction from the lab to the clinic. This review will explore numerous techniques of tolerance induction that have been piloted in primates, including depletional techniques, induction of mixed hematopoietic chimerism, costimulation blockade, and adoptive transfer of tolerogenic cell populations.

Alemtuzumab induction therapy in solid organ transplantation

Alemtuzumab (Campath) is a monoclonal antibody that has a profound lymphocyte-depleting effect, targeting the CD52 antigen that is present on all lymphocytes. Alemtuzumab has been used for the treatment of chronic lymphocytic leukaemia and various autoimmune disorders, and has also shown potential as an induction agent in the prevention of rejection following solid organ transplantation. Alemtuzumab has been studied in randomised controlled trials and has demonstrated low levels of rejection in renal transplant recipients compared with other induction agents, albeit mainly in the early months following transplantation. Studies have shown that alemtuzumab enables the use of lower calcineurin inhibitor (CNI) maintenance drugs; however, this reduction in nephrotoxic immunosuppression has not consistently been matched by an improvement in renal function. The hypothesis has been suggested that alemtuzumab might allow the development of immunosuppressive regimens that avoid CNIs completely; studies have investigated the combination of alemtuzumab with mammalian target of rapamycin-inhibitor maintenance therapy, and, in particular, sirolimus. Initial studies with this combination showed that regimens of sirolimus alone and of sirolimus with mycophenolate mofetil were unsuccessful, with a high rate of rejection and complications. Subsequent studies have targeted the combination of alemtuzumab induction with a short course of a CNI, before switching to maintenance therapy with sirolimus. This regimen might combine good protection from acute cellular rejection and chronic nephrotoxicity. A randomised controlled trial has been established to study this regimen, with results pending.

Tolerance induction: hematopoietic chimerism

PURPOSE OF REVIEW

Although numerous experimental models to induce allograft tolerance have been reported, it has been difficult to translate these basic studies to clinical transplantation. However, successful induction of tolerance in HLA-mismatched kidney transplantation has recently been reported. In this review, recent progress in tolerance induction in preclinical (nonhuman primates) and clinical transplantation is summarized.

RECENT FINDINGS

Among many clinical trials to induce renal allograft tolerance, success has so far been achieved only by combining donor bone marrow with organ transplantation. Induction of renal allograft tolerance by transient or durable mixed chimerism has been reported in HLA-matched or mismatched kidney transplant recipients. More recently, renal allograft tolerance by induction of full donor chimerism has also been reported using a more intensified preparative conditioning regimen.

SUMMARY

Durable allograft tolerance has been achieved by induction of hematopoietic chimerism in clinical kidney transplantation, with outstanding long-term results in successful cases. However, these approaches have been associated with higher early complications than are seen following transplantation with conventional immunosuppression. Improvements in the consistency and safety of tolerance induction and extension of successful protocols to other organs will be the next steps in bringing tolerance to a wider range of clinical applications.

Primate models in organ transplantation

Large animal models have long served as the proving grounds for advances in transplantation, bridging the gap between inbred mouse experimentation and human clinical trials. Although a variety of species have been and continue to be used, the emergence of highly targeted biologic- and antibody-based therapies has required models to have a high degree of homology with humans. Thus, the nonhuman primate has become the model of choice in many settings. This article will provide an overview of nonhuman primate models of transplantation. Issues of primate genetics and care will be introduced, and a brief overview of technical aspects for various transplant models will be discussed. Finally, several prominent immunosuppressive and tolerance strategies used in primates will be reviewed.

Lymphodepletional strategies in transplantation

Because lymphocytes were shown to mediate transplant rejection, their depletion has been studied as a mechanism of preventing rejection and perhaps inducing immunologic tolerance. Agents that profoundly deplete lymphocytes have included monoclonal antibodies, cytotoxic drugs, and radiation. We have studied several such agents but focused on antibodies that deplete not only peripheral blood lymphocytes, but also lymph node lymphocytes. Depletion of lymph node T lymphocytes appears to permit peripheral tolerance at least for T cells in animal models. Nevertheless, B-cell responses may be resistant to such approaches, and T memory cells are likewise relatively resistant to depleting antibodies. We review the experimental and clinical approaches to depletion strategies and outline some of the pitfalls of depletion, such as limitations of currently available agents, duration of tolerance, infection, and malignancy. It is notable that most tolerogenic strategies that have been attempted experimentally and clinically include depleting agents even when they are not named as the underlying strategy. Thus, there is an implicitly acknowledged role for reducing the precursor frequency of donor antigen-specific lymphocytes when approaching the daunting goal of transplant tolerance.

Delayed anti-CD3 therapy results in depletion of alloreactive T cells and the dominance of Foxp3+ CD4+ graft infiltrating cells

The engineered Fc-nonbinding (crystallizable fragment-nonbinding) CD3 antibody has lower mitogenicity and a precise therapeutic window for disease remission in patients with type 1 diabetes. Before anti-CD3 can be considered for use in transplantation, the most effective timing of treatment relative to transplantation needs to be elucidated. In this study anti-CD3F(ab')2 fragments or saline were administered intravenously for 5 consecutive days (early: d1-3 or delayed: d3-7) to mice transplanted with a cardiac allograft (H2(b)-to-H2(k); d0). Survival of allografts was prolonged in mice treated with the early protocol (MST = 48 days), but most were rejected by d100. In contrast, in mice treated with the delayed protocol allografts continued to survive long term. The delayed protocol significantly inhibited donor alloreactivity at d30 as compared to the early protocol. A marked increase in Foxp3(+) T cells (50.3 ± 1.6%) infiltrating the allografts in mice treated with the delayed protocol was observed (p < 0.0001 vs. early (24.9 ± 2.1%)) at d10; a finding that was maintained in the accepted cardiac allografts at d100. We conclude that the timing of treatment with anti-CD3 therapy is critical for inducing long-term graft survival. Delaying administration effectively inhibits the alloreactivity and promotes the dominance of intragraft Foxp3(+) T cells allowing long-term graft acceptance.

Tolerogenic therapies in transplantation

Since the concept of immunologic tolerance was discovered in the 1940s, the pursuit of tolerance induction in human transplantation has led to a rapid development of pharmacologic and biologic agents. Short-term graft survival remains an all-time high, but successful withdrawal of immunosuppression to achieve operational tolerance rarely occurs outside of liver transplantation. Collaborative efforts through the NIH sponsored Immune Tolerance Network and the European Commission sponsored Reprogramming the Immune System for Establishment of Tolerance consortia have afforded researchers opportunity to evaluate the safety and efficacy of tolerogenic strategies, investigate mechanisms of tolerance, and identify molecular and genetic markers that distinguish the tolerance phenotype. In this article, we review traditional and novel approaches to inducing tolerance for organ transplantation, with an emphasis on their translation into clinical trials.

Role of regulatory T cells in the promotion of transplant tolerance

Liver transplantation is now recognized as the most effective therapy for patients with end-stage acute and chronic liver failure. Despite outstanding short-term graft and patient survival, liver transplantation continues to face several major challenges, including poor long-term graft survival due to chronic rejection and major side effects of long-term immunosuppressive therapy (which is required for the prevention of rejection). The ability to produce a state of tolerance after transplantation would potentially obviate long-term immunosuppression. Self-tolerance and immune homeostasis involve both central and peripheral immunoregulatory mechanisms. To date, studies have shown that many subsets of regulatory T cells (Tregs) control immune responses to foreign and alloantigens. The identification of Tregs that are positive for CD4, CD25, and the transcription factor forkhead box (Foxp3) has resulted in major advances in our understanding of the immunology of rejection and the development of transplant tolerance. In this article, we focus on the importance of Tregs in tolerance induction in experimental models of liver transplantation. Furthermore, we discuss the therapeutic potential of Tregs for the promotion of tolerance in transplant patients and highlight recent clinical trials of Treg-based therapies.

Effect of pre-existing anti-diphtheria toxin antibodies on T cell depletion levels following diphtheria toxin-based recombinant anti-monkey CD3 immunotoxin treatment

Diphtheria toxin (DT)-based anti-CD3 immunotoxins have clinical relevance in numerous applications including autoimmune disease therapies and organ transplantation tolerance protocols. Pre-existing anti-DT antibodies acquired either by vaccination against diphtheria toxin or infections with C. diphtheriae may interfere or inhibit the function of these anti-CD3 immunotoxins. Previously, a full-length anti-rhesus monkey CD3 immunotoxin, FN18-CRM9, was shown to be less effective at depleting circulating T cells in animals with pre-existing anti-DT antibody titers than in animals without antibodies, and subsequent doses were ineffective. In this study, the T cell depletion function of a truncated DT based recombinant anti-monkey CD3 immunotoxin, A-dmDT390-scfbDb (C207), as part of a reduced intensity conditioning regimen prior to hematopoietic cell transplantation, was compared between two groups of monkeys: those with and without pre-existing anti-diphtheria titers. T cell depletion was comparable in both groups of monkeys, and therefore appeared to be unaffected by the presence of moderate levels of pre-existing anti-diphtheria antibodies.

Influence of induction modality on the outcome of deceased donor kidney transplant recipients discharged on steroid-free maintenance immunosuppression

BACKGROUND

Over last several years, alemtuzumab induction has been increasingly used in kidney transplantation especially in patients maintained on steroid-free immunosuppression. It is unclear which induction agent is associated with better graft and patient outcomes in these patients.

METHODS

Using Organ Procurement and Transplant Network/United Network of Organ Sharing database, graft and patient survivals were compared with multivariate analysis for deceased donor kidney transplant recipients from January 2000 to December 2008 who received induction with rabbit-antithymocyte globulin (r-ATG), alemtuzumab, or an interleukin-2 (IL-2) receptor blocker and were discharged on a calcineurin inhibitor/mycophenolate mofetil/steroid-free immunosuppression.

RESULTS

When compared with r-ATG (n=5348), adjusted graft survival was inferior with alemtuzumab (n=2428, hazards ratio [HR] 1.26, 95% confidence interval [CI] 1.10-1.43, P=0.001) and IL-2 receptor blocker (n=1396, HR 1.19, 95% CI 1.01-1.39, P=0.04) inductions and patient survival was inferior with alemtuzumab (HR 1.29, 95% CI 1.08-1.55, P=0.006). Alemtuzumab induction was associated with higher adjusted graft failure risks in patients with panel reactive antibody more than 20% (HR 1.30, 95% CI 1.01-1.68, P=0.04), recipients of expanded criteria donor kidneys (HR 1.58, 95% CI 1.23-2.02, P<0.001), and kidneys with cold ischemia time more than 24 hr (HR 1.31, 95% CI 1.04-1.65, P=0.02) and higher patient death risks in recipients of expanded criteria donor kidney (HR 1.66, 95% CI 1.20-1.30, P=0.002) and kidneys with cold ischemia time more than 24 hr (HR 1.44, 95% CI 1.04-2.00, P=0.03). Adjusted graft survival rates were similar for different induction agents in the low-immune risk group.

CONCLUSIONS

When compared with alemtuzumab and IL-2 receptor blocker, r-ATG induction seems to be associated with superior outcomes in deceased donor kidney transplant recipients maintained on calcineurin inhibitor/mycophenolate mofetil/steroid-free regimen.

Perioperative Minimal Induction Therapy: A Further Step toward More Effective Immunosuppression in Transplantation

Dual induction with low doses of rabbit anti-human thymoglobulin (RATG) and basiliximab effectively and safely prevented allograft rejection in high-risk renal transplant recipients. To assess whether treatment timing affects efficacy and tolerability, in this single-center, matched-cohort study, we compared posttransplant outcomes in 25 patients and 50 gender-, age-, and treatment-matched reference patients induced with the same course of 7 daily RATG infusions (0.5 mg/kg/day) started before or after engraftment, respectively. All subjects received basiliximab (20 mg) before and 4 days after transplantation, withdrew steroids within 6 days after surgery, and were maintained on steroid-free immunosuppression with cyclosporine and mycophenolate mofetil or azathioprine. Over 12 months after transplant, 1 patient (4%) and 13 reference patients (26%) had acute rejection episodes. One patient and 5 reference-patients required dialysis therapy because of delayed graft function. In all patients circulating CD4+ and CD8+ T lymphocytes were fully depleted before engraftment. Both treatments were well tolerated. In kidney transplantation, perioperative RATG infusion enhances the protective effect of low-dose RATG and basiliximab induction against graft rejection and delayed function, possibly because of more effective inhibition of early interactions between circulating T cells and graft antigens.

Alemtuzumab as compared to alternative contemporary induction regimens

Between 1 January 2002 and 31 December 2007, our center performed 1687 adult renal transplants. A retrospective analysis was performed to compare outcomes between patients receiving alemtuzumab (n = 632) and those receiving either basiliximab (n = 690) or thymoglobulin (n = 125). Patients receiving alemtuzumab were younger (49 vs. 51 years, P = 0.02), had fewer HLA matches (1.7 vs. 2.0, P < 0.0001), were more likely to have a cytomegalovirus (CMV) donor(+)/recipient(-) transplant (22% vs. 17%, P = 0.03) and were less likely to receive a living donor allograft (32% vs. 37%, P = 0.04). Alemtuzumab recipients were less likely to receive tacrolimus (35% vs. 47%, P < 0.0001). The 1-, 3-, and 5-year cumulative incidence of antibody-mediated rejection (AMR) in alemtuzumab-treated patients was 19%, 24%, and 27%, vs. 11%, 15%, and 18% for the other group (P < 0.0001). The 1-, 3-, and 5-year allograft survival in the alemtuzumab group was 88%, 75%, and 67%, vs. 91%, 82%, and 74% for the other group (P < 0.0001). Patient survival was equivalent. Alemtuzumab was an independent risk factor for living donor allograft loss (HR 2.0, P = 0.004), opportunistic infections (HR 1.3, P = 0.01), CMV infections (HR 1.6, P = 0.001), and AMR (HR 1.5, P = 0.002). The significantly worse graft survival in the alemtuzumab cohort may be due to the increased rates of AMR and infectious complications.

Moving to tolerance: clinical application of T regulatory cells

Decreasing the incidence of chronic rejection and reducing the need for life-long immunosuppression remain important goals in clinical transplantation. In this article, we will review how regulatory T cells (Treg) came to be recognized as an attractive way to prevent or treat allograft rejection, the ways in which Treg can be manipulated or expanded in vivo, and the potential of in vitro expanded/generated Treg for cellular therapy. We will describe the first regulatory T cell therapies that have been or are in the process of being conducted in the clinic as well as the safety concerns of such therapies and how outcomes may be measured.

Induction immunosuppression improves long-term graft and patient outcome in organ transplantation: an analysis of United Network for Organ Sharing registry data

BACKGROUND

Induction agents have been shown to reduce the rate of acute rejection. They have not been clearly shown to improve graft and patient survival.

METHODS

United Network for Organ Sharing registry data were analyzed to show the status of induction therapy in the United States and to determine the effect of induction therapy on long-term graft and patient survival.

RESULTS

Since establishment of the United Network for Organ Sharing renal transplant registry, there have been three distinct eras of induction regimen: (1) the low-induction, old antibody era, 1987 to 1993, when antilymphocyte globulin and muromonab-CD3 were the major agents; (2) a high-induction, transitional era, 1994 to 2002, when basiliximab (1998), daclizumab (1998), and rabbit antithymocyte globulin (rATG; 1999) replaced antilymphocyte globulin and muromonab-CD3, with maintenance agents also used; (3) the high-induction, modern antibody era, 2003 to present, with most patients receiving rATG, basiliximab, daclizumab, or alemtuzumab (2003). Induction recipients had higher graft and patient survival rates than nonrecipients in all categories of organ transplant. The improvement was statistically significant in kidney, liver, and lung transplants, although liver and lung recipients had a lower percentage of patients receiving induction than did kidney patients. Kidney transplant recipients on alemtuzumab with steroids had the lowest risk of graft failure, followed by those on alemtuzumab alone, rATG with steroids, rATG alone, and then basiliximab with steroids. Improvement was not statistically significant with daclizumab (alone or with steroids), basiliximab alone, or steroids alone.

CONCLUSION

Induction immunosuppression improved graft and patient outcome for most organ transplants. Depleting agents (alemtuzumab and rATG)--especially in combination with steroids--seem to be more efficient in preventing renal graft failure than nondepleting agents (basiliximab and daclizumab).

Immunologic basis of graft rejection and tolerance following transplantation of liver or other solid organs

Transplantation of organs between genetically different individuals of the same species causes a T cell-mediated immune response that, if left unchecked, results in rejection and graft destruction. The potency of the alloimmune response is determined by the antigenic disparity that usually exists between donors and recipients and by intragraft expression of proinflammatory cytokines in the early period after transplantation. Studies in animal models have identified many molecules that, when targeted, inhibit T-cell activation. In addition, some of these studies have shown that certain immunologic interventions induce transplantation tolerance, a state in which the allograft is specifically accepted without the need for chronic immunosuppression. Tolerance is an important aspect of liver transplantation, because livers have a unique microenvironment that promotes tolerance rather than immunity. In contrast to the progress achieved in inducing tolerance in animal models, patients who receive transplanted organs still require nonspecific immunosuppressant drugs. The development of calcineurin inhibitors has reduced the acute rejection rate and improved short-term, but not long-term, graft survival. However, long-term use of immunosuppressive drugs leads to nephrotoxicity and metabolic disorders, as well as manifestations of overimmunosuppression such as opportunistic infections and cancers. The status of pharmacologic immunosuppression in the clinic is therefore not ideal. We review recently developed therapeutic strategies to promote tolerance to transplanted livers and other organs and diagnostic tools that might be used to identify patients most likely to accept or reject allografts.

Bringing transplantation tolerance into the clinic: lessons from the ITN and RISET for the Establishment of Tolerance consortia

PURPOSE OF REVIEW

Fundamental discoveries during the 1990s revolutionized our understanding of transplantation tolerance and our ability to create it in animal models. The Immune Tolerance Network (ITN) and Reprogramming the Immune System for the Establishment of Tolerance (RISET) consortia were created to leverage these advances and work towards the goal of achieving clinical tolerance in transplantation. This article highlights their accomplishments and challenges during the past decade.

RECENT FINDINGS

In interventional trials, renal allograft tolerance has been achieved using bone marrow transplantation with nonmyeloablative protocols to induce transient hematopoietic chimerism. Drug minimization in renal transplantation was achieved with Campath-1H induction therapy and also with cellular therapy using 'transplant acceptance inducing cells'. Successful drug withdrawal was accomplished in long-term stable pediatric liver transplant recipients. Finally, 'registry' trials of tolerant kidney recipients revealed a B-cell signature of tolerance, which will form the basis for future investigations of its use as a biomarker for drug minimization or withdrawal in selected patients.

SUMMARY

Although transplantation tolerance is not yet reliably achieved in a clinical setting, collaborative efforts, such as those of the ITN and RISET networks, are an effective means to synergize intellectual and financial resources to bring this goal closer to reality.

Induction of tolerance in clinical kidney transplantation

Induction of donor-specific tolerance has been an ultimate goal in organ transplantation. Although numerous regimens for the induction of allograft tolerance have been developed in rodents, their application to primates has been limited. The approaches that have been successfully applied in primates can be divided into (i) use of total lymphoid irradiation, (ii) costimulatory blockade, (iii) profound depletion of recipient T cells, (iv) infusion of regulatory cells and (v) donor bone marrow (DBM) infusion/transplantation. Among these approaches, successful allograft tolerance has been achieved in clinical kidney transplantation using DBM transplantation.

Early and limited use of tacrolimus to avoid rejection in an alemtuzumab and sirolimus regimen for kidney transplantation: clinical results and immune monitoring

Alemtuzumab induction with 60 days of tacrolimus treatment and continuous sirolimus treatment prevented acute rejection in nine of 10 consecutive renal allograft recipients. All patients are alive with a functioning kidney graft at 27-39 months of follow-up. Extensive immune monitoring was performed in all patients. Alloantibody detection, cytokine kinetics assay (CKA), and trans vivo delayed-type hypersensitivity (DTH) assay were performed every 6 months showing correlation with clinical evolution. Despite alloantibody presence in five patients, eight patients remain without the need for specific treatment and only sirolimus monotherapy in decreasing dosage. Four patients take only 1 mg sirolimus daily with levels of 3-4 ng/mL. One patient showed clinical signs of rejection at month 9 post-transplant, with slow increase in serum creatinine and histological signs of mixed cellular (endarteritis) and humoral rejection (C4d positivity in peritubular capillaries and donor-specific antibody (DSA)). In summary, the addition of tacrolimus therapy for 2 months to a steroid-free, alemtuzumab induction and sirolimus maintenance protocol limited the previously shown acute rejection development. Nevertheless, alloantibody was present in serum and/or C4d present on 1-year biopsy in half the patients. The combination of CKA and DSA monitoring or the performance of transvivo DTH correlated with immune status of the patients.

Portal venous donor-specific transfusion in conjunction with sirolimus prolongs renal allograft survival in nonhuman primates

Pretransplant exposure to donor antigen is known to modulate recipient alloimmunity, and frequently results in sensitization. However, donor-specific transfusion (DST) can have a protolerant effect that is dependent on route, dose and coadministered immunosuppression. Rodent studies have shown in some strain combinations that portal venous (PV) DST alone can induce tolerance, and uncontrolled clinical use of PVDST has been reported. In order to determine if pretransplant PVDST has a clinically relevant salutary effect, we studied it and the influence of concomitant immunosuppression in rhesus monkeys undergoing renal allotransplantation. Animals received PVDST with unfractionated bone marrow and/or tacrolimus or sirolimus 1 week prior to transplantation. Graft survival was assessed without any posttransplant immunosuppression. PVDST alone or in combination with tacrolimus was ineffective. However, PVDST in combination with sirolimus significantly prolonged renal allograft survival to a mean of 24 days. Preoperative sirolimus alone had no effect, and peripheral DST with sirolimus prolonged graft survival in 2/4 animals, but resulted in accelerated rejection in 2/4 animals. These data demonstrate that PVDST in combination with sirolimus delays rejection in a modest but measurable way in a rigorous model. It may thus be a preferable method for donor antigen administration.

Alemtuzumab induction in deceased donor kidney transplantation

BACKGROUND

The use of alemtuzumab for induction therapy in kidney transplantation has been increasing. Herein is a report of graft outcomes associated with alemtuzumab induction from the Organ Procurement and Transplantation Network/United Network for Organ Sharing database.

METHODS

A total of 14,362 deceased donor kidney transplants from 2003 to 2004 received no induction (n=4,364), antithymocyte globulin (ATG; n=4,930), interleukin-2 receptor antagonists (IL-2RA; n=4,378), or alemtuzumab (n=690). Acute rejection within the initial hospitalization, 6 months, and 1 year; graft survival; and rejection-free survival were examined. Graft and rejection-free survival of alemtuzumab recipients maintained with tacrolimus (FK) or cyclosporine (CSA), mycophenolate mofetil (MMF), and steroids versus no calcineurin inhibitors (CNI), MMF, and steroids were compared.

RESULTS

Alemtuzumab recipients had less acute rejection during the initial hospitalization (2.3%) than no induction, ATG, and IL-2RA (7.6%, 3.4%, and 4.8%, respectively; P<0.001). There was increased acute rejection at 6 months and 1 year with alemtuzumab (14.5% and 19.2%) compared to no induction (12.7% and 14.8%, P<0.001), ATG (8.2% and 10.2%, P<0.001), and IL-2RA (11.1% and 13.0%, P<0.001) with no difference in adjusted relative risk for graft loss. Alemtuzumab recipients receiving FK or CSA, MMF, and steroids had increased graft (FK/MMF/steroids, P<0.001, CSA/MMF/steroids, P=0.007) and rejection-free survival (FK/MMF/steroids, P<0.001, CSA/MMF/steroids, P=0.006) over 24 months compared to no CNI, MMF, and steroids.

CONCLUSIONS

Despite reduced early rejection, acute rejection rates at 6 months and 1 year with alemtuzumab induction exceeded other forms of induction therapy. Maintenance with CNI-based immunosuppression may improve graft and rejection-free survival compared to CNI-free regimens among alemtuzumab recipients.

Transplantation Tolerance

Tolerance following organ transplantation was first described in experimental models over 50 years ago. Reports of tolerance in clinical transplantation have appeared in the literature sporadically for decades. Despite this long-standing fascination with transplantation tolerance, the ability to reproducibly induce tolerance in humans undergoing organ transplantation has remained elusive. Recent advances in our knowledge of the mechanisms that contribute to the induction and maintenance of tolerance as well as those factors that oppose tolerance may allow the design of clinical trials aimed at introducing tolerance-inducing strategies into clinical transplantation.

Depletion of CD8 memory T cells for induction of tolerance of a previously transplanted kidney allograft

Heterologous immunologic memory has been considered a potent barrier to tolerance induction in primates. Induction of such tolerance for a previously transplanted organ may be more difficult, because specific memory cells can be induced and activated by a transplanted organ. In the current study, we attempted to induce tolerance to a previously transplanted kidney allograft in nonhuman primates. The conditioning regimen consisted of low dose total body irradiation, thymic irradiation, antithymocyte globulin, and anti-CD154 antibody followed by a brief course of a calcineurin inhibitor. This regimen had been shown to induce mixed chimerism and allograft tolerance when kidney transplantation (KTx) and donor bone marrow transplantation (DBMT) were simultaneously performed. However, the same regimen failed to induce mixed chimerism when delayed DBMT was performed after KTx. We found that significant levels of memory T cells remained after conditioning, despite effective depletion of naïve T cells. By adding humanized anti-CD8 monoclonal antibody (cM-T807), CD8 memory T cells were effectively depleted and these recipients successfully achieved mixed chimerism and tolerance. The current studies provide 'proof of principle' that the mixed chimerism approach can induce renal allograft tolerance, even late after organ transplantation if memory T-cell function is adequately controlled.

From current immunosuppressive strategies to clinical tolerance of allografts

In order to prevent allograft rejection, most current immunosuppressive drugs nonspecifically target T-cell activation, clonal expansion or differentiation into effector cells. Experimental models have shown that it is possible to exploit the central and peripheral mechanisms that normally maintain immune homeostasis and tolerance to self-antigens, in order to induce tolerance to alloantigens. Central tolerance results from intrathymic deletion of T cells with high avidity for thymically expressed antigens. Peripheral tolerance to nonself-molecules can be achieved by various mechanisms including deletion of activated/effector T cells, anergy induction and active regulation of effector T cells. In this article, we briefly discuss the pathways of allorecognition and their relevance to current immunosuppressive strategies and to the induction of transplantation tolerance (through haematopoietic mixed chimerism, depleting protocols, costimulatory blockade and regulatory T cells). We then review the prospect of clinical applicability of these protocols in solid organ transplantation.

The evolving role of alemtuzumab (Campath-1H) for immunosuppressive therapy in organ transplantation

Alemtuzumab is a monoclonal anti-CD52 antibody, which has been used extensively off label in solid organ transplantation. Its primary use has been as an induction agent at the time of organ transplantation, although there is limited experience using it to treat steroid-resistant rejection. Prolonged lymphocyte depletion can be expected following alemtuzumab treatment even with one dose of 30 mg intravenously. The nature and kinetics of lymphocyte repopulation depend on the maintenance immunosuppression being administered. In comparison with Thymoglobulin, a polyclonal depleting antibody preparation, alemtuzumab offers significant practical benefits with lower cost, fewer side effects in administration, and no specific issues with i.v. access. The risks and benefits of depleting induction agents, such as alemtuzumab, are compared with nondepleting agents, such as anti-CD25 induction therapy. While the majority of experience in solid organ transplantation has been in kidney transplantation, there is more limited experience in liver, pancreas, islet, small bowel, and lung transplantation. We herein review some of the lessons learned from clinical experience to date in solid organ transplantation using alemtuzumab as an immunosuppressant.

Is clinical tolerance realistic in the next decade?

Tolerance to allografts would mean a better quality of life and prognosis for transplant patients. Despite the first descriptions of tolerance to alloantigens over 50 years ago, deliberately induced tolerance in the clinic on a wide scale remains a goal that is not quite in reach. However, much progress has been made in understanding tolerance in rodent models and in the few reports of induced or spontaneously occurring tolerance in humans. Here, we review this progress made in the quest to achieve clinical tolerance.

Transplant tolerance in non-human primates: progress, current challenges and unmet needs

Given the significant morbidity associated with current post-transplant immunosuppressive regimens, induction of immune tolerance continues to be an important goal of clinical organ transplantation. While many strategies for inducing tolerance have been successfully applied in murine models, significant barriers are faced when translating these approaches to the clinic. This has necessitated pre-clinical studies in the more closely related model system, the non-human primates (NHP). In this review, we will discuss the four most prominent strategies for inducing transplantation tolerance and highlight their relative success and shortcomings in NHP. These strategies are: (1) T-cell costimulation blockade (2) mixed chimerism induction (3) T-cell depletion and (4) tolerance induction through regulatory T-cells. After discussing the progress that has been made with each of these strategies, we will identify this field's most pressing unmet needs and discuss how we may best overcome the resulting barriers to tolerance induction.

CAMPATH: from concept to clinic

Lymphocyte depletion has a long history in the area of therapeutic immunosuppression. CAMPATH-1H (alemtuzumab) was generated in an attempt to replace anti-lymphocyte globulins in the transplant arena. Its efficacy in killing lymphocytes has established it as a licensed drug for the management of chronic lymphocyte leukaemia. Short-term therapy with alemtuzumab has demonstrated long-term benefit in a number of autoimmune conditions. This drug has the potential to facilitate recruitment of tolerance processes so enabling drug minimization in transplantation, autoimmune and hypersensitivity diseases.

Induction of transplantation tolerance in non-human primate preclinical models

Short-term outcomes following organ transplantation have improved considerably since the availability of cyclosporine ushered in the modern era of immunosuppression. In spite of this, many of the current limitations to progress in the field are directly related to the existing practice of relatively non-specific immunosuppression. These include increased risks of opportunistic infection and cancer, and toxicity associated with long-term immunosuppressive drug exposure. In addition, long-term graft loss continues to result in part from a failure to adequately control the anti-donor immune response. The development of a safe and reliable means of inducing tolerance would ameliorate these issues and improve the lives of transplant recipients, yet given the improving clinical standard of care, the translation of new therapies has become appropriately more cautious and dependent on increasingly predictive preclinical models. While convenient and easy to use, rodent tolerance models have not to date been reliably capable of predicting a therapy's potential efficacy in humans. Non-human primates possess an immune system that more closely approximates that found in humans, and have served as a more rigorous preclinical testing ground for novel therapies. Prior to clinical adaptation therefore, tolerance regimens should be vetted in non-human primates to ensure that there is sufficient potential for efficacy to justify the risk of its application.

Development of tolerogenic strategies in the clinic

The study of tolerance in the clinic can be divided into three areas: (i) focused evaluation of existing tolerant transplant recipients as to their mechanism of tolerance; (ii) prospective tolerance trials, such as combined bone marrow and kidney transplantation as well as T cell depletion followed by subsequent weaning of immunosuppression; and (iii) immunologic assays to assess the likelihood of rejection or tolerance. Frankly, a very small number of patients have been transplanted with the intention of removing all immunosuppressive therapy, but several clinical trials with this aim are currently in progress, largely sponsored by the Immune Tolerance Network, a joint venture between the National Institutes of Health and the Juvenile Diabetes Research Foundation. Similarly, a reliable assay to assess tolerance has not yet been developed but a variety of approaches towards assessing rejection, and in some cases tolerance, are being developed. It would be accurate to state that many of the experimental and preclinical approaches to the induction of tolerance have resulted in better immunosuppression for human transplantation, but reliable tolerance strategies in humans have not yet been achieved. Combined bone marrow and kidney transplantation may be considered as one exception to this, but such a strategy is not generally applicable to the vast majority of solid organ transplant recipients. This review will summarize efforts to date, particularly focusing on kidney transplantation.

Randomized trial of Alemtuzumab for prevention of graft rejection and preservation of renal function after kidney transplantation

BACKGROUND

A randomized, multicenter, controlled trial was undertaken to evaluate the safety and efficacy of Alemtuzumab, a powerful lytic agent for both T and B lymphocytes, in the prophylaxis of rejection in renal transplantation (RTx).

METHODS

Thirty patients were randomized to receive Alemtuzumab together with low-dose cyclosporine (CsA) monotherapy (CAMPATH, n = 20) or to full doses of CsA with azathioprine and corticosteroids (Standard, n = 10). CsA was administered at doses to achieve whole-blood trough CsA levels of 90 to 110 ng/mL and 180 to 225 ng/mL in CAMPATH and Standard groups, respectively.

RESULTS

Per protocol, CsA trough levels were lower in patients assigned to CAMPATH post-RTx (median trough level of 119 vs. 166 ng/mL at 6 months, CAMPATH vs. Standard; 95% confidence interval, -92 to -34). At 6 months post-RTx, serum creatinine, graft and patient survivals, incidence of biopsy proven acute rejection (25% vs. 20%, CAMPATH vs. Standard), overall treatment failure, and severe and moderate infections were comparable. Whereas all patients receiving Standard therapy required maintenance corticosteroids at 6 months, of the 17 of 20 patients with functioning grafts in CAMPATH, 15 (88%, 95% confidence interval, 53%-97%) were steroid free.

CONCLUSION

These results suggest that Alemtuzumab is an effective induction agent that permits low-dose steroid-free immunosuppression in RTx.

Results from a human renal allograft tolerance trial evaluating T-cell depletion with alemtuzumab combined with deoxyspergualin

BACKGROUND

Perioperative lymphocyte depletion induces allograft tolerance in some animal models, but in humans has only been shown to reduce immunosuppressive requirements. Without maintenance immunosuppression, depleted human renal allograft recipients experience rejection characterized by infiltration of the allograft with monocytes and macrophages. T-cell depletion combined with a brief course of deoxyspergualin (DSG), a drug with inhibitory effects on monocytes and macrophages, induces tolerance in nonhuman primates. We therefore performed a trial to determine if lymphocyte depletion with alemtuzumab combined with DSG would induce tolerance in humans.

METHODS

Five recipients of live donor kidneys were treated perioperatively with alemtuzumab and DSG and followed postoperatively without maintenance immunosuppression. Patients were evaluated clinically, by flow cytometry, and by protocol biopsies analyzed immunohistochemically and with real-time polymerase chain reaction. Results were compared to previously studied patients receiving alemtuzumab alone or standard immunosuppression.

RESULTS

Despite profound T-cell depletion and therapeutic DSG dosing, all alemtuzumab/DSG patients developed reversible rejection that was similar in timing, histology, and transcriptional profile to that seen in patients treated with alemtuzumab alone. Chemokine expression was marked prior to and during rejections.

CONCLUSIONS

We conclude that treatment with alemtuzumab and DSG does not induce tolerance in humans. Chemokine production may not be adequately suppressed using this approach.

Challenges in therapeutic strategies for transplantation: Where now from here?

The current standard of care in transplantation reliably achieves acceptable graft and patient survival but still depends on life long immunosuppression in most patients. Current strategies employ medications that, in general, inhibit distal events mediating rejection, namely T cell activation and cytotoxicity. They do not typically interfere with initial allorecognition or the factors that influence the direction of an immune response (towards cytotoxicity as opposed to anergy or regulation). Given the exponential amplification of immune responses, these proximal targets may be more efficient in preventing rejection. Recent laboratory investigations have identified several approaches, e.g., costimulation blockade, depletion, and hematopoietic chimerism, that influence the initial stages of the alloimmune response, or establish self-perpetuating means of eliminating rejection without chronic immunosuppression. This manuscript reviews methods of immune manipulation that the authors view as promising for future exploitation and transfer to the clinic. These therapies are similar in that they are viewed as attempts to influence the ability of the body to mount an immune response and its subsequent direction, as opposed to supplying late effector phase inhibition. While it is recognized as unlikely that any one therapy will universally lead to tolerance, the authors propose that these concepts will make immunosuppressive drug minimization more readily successful.