Costimulation Blockade followed by a 12-Week Period of Cyclosporine A Facilitates Prolonged Drug-Free Survival of Rhesus Monkey Kidney Allografts

CD40-CD40L Blockade: Update on Novel Investigational Therapeutics for Transplantation

Effective immune responses require antigen presentation by major histocompatibility complexes with cognate T-cell receptor and antigen-independent costimulatory signaling for T-cell activation, proliferation, and differentiation. Among several costimulatory signals, CD40-CD40L is of special interest to the transplantation community because it plays a vital role in controlling or regulating humoral and cellular immunity. Blockade of this pathway has demonstrated inhibition of donor-reactive T-cell responses and prolonged the survival of transplanted organs. Several anti-CD154 and anti-CD40 antibodies have been used in the transplantation model and demonstrated the potential of extending allograft and xenograft rejection-free survival. The wide use of anti-CD154 antibodies was hampered because of thromboembolic complications in transplant recipients. These antibodies have been modified to overcome the thromboembolic complications by altering the antibody binding fragment (Fab) and Fc (fragment, crystallizable) receptor region for therapeutic purposes. Here, we review recent preclinical advances to target the CD40-CD40L pair in transplantation.

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.

An Evaluation of 20 Years of EU Framework Programme-Funded Immune-Mediated Inflammatory Translational Research in Non-Human Primates

Aging western societies are facing an increasing prevalence of chronic inflammatory and degenerative diseases for which often no effective treatments exist, resulting in increasing health-care expenditure. Despite high investments in drug development, the number of promising new drug candidates decreases. We propose that preclinical research in non-human primates can help to bridge the gap between drug discovery and drug prescription. Translational research covers various stages of drug development of which preclinical efficacy tests in valid animal models is usually the last stage. Preclinical research in non-human primates may be essential in the evaluation of new drugs or therapies when a relevant rodent model is not available. Non-human primate models for life-threatening or severely debilitating diseases in humans are available at the Biomedical Primate Research Centre (BPRC). These have been instrumental in translational research for several decades. In order to stimulate European health research and innovation from bench to bedside, the European Commission has invested heavily in access to non-human primate research for more than 20 years. BPRC has hosted European users in a series of transnational access programs covering a wide range of research areas with the common theme being immune-mediated inflammatory disorders. We present an overview of the results and give an account of the studies performed as part of European Union Framework Programme (EU FP)-funded translational non-human primate research performed at the BPRC. These data illustrate the value of translational non-human primate research for the development of new therapies and emphasize the importance of EU FP funding in drug development.

Lymphoid-Like Structures with Distinct B Cell Areas in Kidney Allografts are not Predictive for Graft Rejection. A Non-human Primate Study

Kidney allograft biopsies were analyzed for the presence of B cell clusters/aggregates using CD20 staining. Few B cells were found in the diffuse interstitial infiltrates, but clusters of B cells were found in nodular infiltrates. These nodular infiltrates were smaller shortly after transplantation, and their size increased over time. At the time of clinical rejection, the nodules often presented as tertiary lymphoid structures (TLS) with lymphoid-like follicles. The presence of small B cell clusters during the first 2 months after transplantation was not associated with early rejection. Even in animals that did not reject their allograft, TLS-like structures were present and could disappear over time. Although TLS were more often found in samples with interstitial fibrosis and tubular atrophy (IFTA), TLS were also present in samples without IFTA. The presence and density of clusters resembling tertiary lymphoid structures most likely reflect an ongoing immune response inside the graft and do not necessarily signify a poor graft outcome or IFTA.

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.

In vitro testing of an anti-CD40 monoclonal antibody, clone 2C10, in primates and pigs

BACKGROUND

The CD40/CD154 and CD28/B7 pathways are important in allo- and xeno-transplantation. Owing to the thrombotic complications of anti-CD154mAb, anti-CD40mAb has emerged as a promising inhibitor of costimulation. Various clones of anti-CD40mAb have been developed against primate species, e.g., clone 2C10 against rhesus monkeys. We have compared the in vitro efficacy of 2C10 to prevent a T cell response in primates and pigs.

METHODS

The binding of 2C10 to antigen-presenting cells (PBMCs [B cells]) of humans, rhesus and cynomolgus monkeys, baboons, and pigs was measured by flow cytometry, and was also tested indirectly by a blocking assay. The functional capacity of 2C10 was tested by mixed lymphocyte reaction (MLR) with polyclonal stimulation by phytohemagglutinin (PHA) and also with wild-type pig aortic endothelial cells (pAECs) as stimulators.

RESULTS

There was a significant reduction in binding of 2C10 to baboon PBMCs compared to rhesus, cynomolgus, and human PBMCs, and minimal binding to pig PBMCs. The blocking assay confirmed that the binding of 2C10 was significantly lower to baboon PBMCs when compared to the other primate species tested. The functional assay with PHA showed significantly reduced inhibition of PBMC proliferation in humans, cynomolgus monkeys, and baboons compared to rhesus monkeys, which was confirmed on MLR with pAECs.

CONCLUSIONS

Since both the binding and functional activity of 2C10 in the baboon is lower than in rhesus monkeys, in vivo treatment using 2C10 in the baboon might require a higher dose or more frequent administration in comparison to rhesus monkeys. It may also be beneficial to develop species-specific clones of anti-CD40mAb.

Initial in vivo experience of pig artery patch transplantation in baboons using mutant MHC (CIITA-DN) pigs

BACKGROUND

In the pig-to-nonimmunosuppressed baboon artery patch model, a graft from an α1,3-galactosyltransferase gene-knockout pig transgenic for human CD46 (GTKO/CD46) induces a significant adaptive immune response (elicited anti-pig antibody response, increase in T cell proliferation on MLR, cellular infiltration of the graft), which is effectively prevented by anti-CD154mAb-based therapy.

METHODS

As anti-CD154mAb is currently not clinically applicable, we evaluated whether it could be replaced by CD28/B7 pathway blockade or by blockade of both pathways (using belatacept + anti-CD40mAb [2C10R4]). We further investigated whether a patch from a GTKO/CD46 pig with a mutant human MHC class II transactivator (CIITA-DN) gene would allow reduction in the immunosuppressive therapy administered.

RESULTS

When grafts from GTKO/CD46 pigs were transplanted with blockade of both pathways, a minimal or insignificant adaptive response was documented. When a GTKO/CD46/CIITA-DN graft was transplanted, but no immunosuppressive therapy was administered, a marked adaptive response was documented. In the presence of CD28/B7 pathway blockade (abatacept or belatacept), there was a weak adaptive response that was diminished when compared with that to a GTKO/CD46 graft. Blockade of both pathways prevented an adaptive response.

CONCLUSION

Although expression of the mutant MHC CIITA-DN gene was associated with a reduced adaptive immune response when immunosuppressive therapy was inadequate, when blockade of both the CD40/CD154 and CD28/B7 pathways was present, the response even to a GTKO/CD46 graft was suppressed. This was confirmed after GTKO/CD46 heart transplantation in baboons.

Lung transplantation: chronic allograft dysfunction and establishing immune tolerance

Despite significant medical advances since the advent of lung transplantation, improvements in long-term survival have been largely unrealized. Chronic lung allograft dysfunction, in particular obliterative bronchiolitis, is the primary limiting factor. The predominant etiology of obliterative bronchiolitis involves the recipient's innate and adaptive immune response to the transplanted allograft. Current therapeutic strategies have failed to provide a definitive treatment paradigm to improve long-term outcomes. Inducing immune tolerance is an emerging therapeutic strategy that abrogates allograft rejection, avoids immunosuppression, and improves long-term graft function. The aim of this review is to discuss the key immunologic components of obliterative bronchiolitis, describe the state of establishing immune tolerance in transplantation, and highlight those strategies being evaluated in lung transplantation.

T-cell co-stimulatory blockade in transplantation: two steps forward one step back!

INTRODUCTION

The concern about nephrotoxicity with calcineurin inhibitors led to the search of novel agents for immunosuppression. Based on the requirement of T-cell co-stimulatory signals to fully activated naïve T cells, it became clear that blocking these pathways could be an appealing therapeutic target. However, some unexpected findings were noticed in the recent clinical trials of belatacept, including a higher rate of rejection, which warranted further investigation with some interesting concepts emerging from the bench.

AREAS COVERED

This article aims to review the literature of the B7:CD28 co-stimulatory blockade in transplantation, including the basic immunology behind its development, clinical application and potential limitations.

EXPERT OPINION

Targeting co-stimulatory pathways were found to be much more complex than initially anticipated due to the interplay between not only various co-stimulatory pathways but also various co-inhibitory ones. In addition, co-stimulatory signals have different roles in diverse immune cell types. Therefore, targeting CD28 ligands with cytotoxic T lymphocyte antigen-4 (CTLA4)-Ig may have some deleterious effects, including the inhibition of regulatory T cells, blockade of co-inhibitory signals (CTLA4) and promotion of Th17 cells. Co-stimulatory independence of memory T cells was another unforeseen limitation. Learning how to better integrate co-stimulatory targeting with other immunosuppressive agents will be critical for the improvement of long-term graft survival.

Comparative analysis of inflammatory infiltrates in collagen-induced arthritis, kidney graft rejection and delayed-type hypersensitivity in non-human primates

OBJECTIVES

Non-human primates are immunologically closely related to humans providing relevant models of inflammatory disorders often used to evaluate new immunomodulating therapies. The aim of the study was to compare inflammatory infiltrates of acute graft rejection (AR) and collagen-induced arthritis (CIA) to delayed-type hypersensitivity (DTH) reactions as the latter model may serve as a less invasive animal model.

MATERIALS AND METHODS

Tissue samples of AR, CIA and DTH were obtained from rhesus monkeys used in several pre-clinical studies. The infiltrate composition was determined by immunohistochemical analysis.

RESULTS

The infiltrates in AR consisted of T cells, macrophages and B cells. The presence of lymphoid structures in AR suggested ongoing intragraft immune activation. The synovia of CIA contained predominantly macrophages and few T cells. The DTH infiltrates were dominated by T cells when the challenged was ovalbumin (OVA) and by macrophages when the challenge was tetanus toxoid (TT).

CONCLUSIONS

The histology of AR resembles aspects of DTH to OVA while that of CIA showed similarities of the DTH to TT. The DTH reaction could serve as a model to study immunomodulating drugs for acute rejection and the acute inflammatory phase of autoimmunity.

Enhanced de novo alloantibody and antibody-mediated injury in rhesus macaques

Chronic allograft rejection is a major impediment to long-term transplant success. Humoral immune responses to alloantigens are a growing clinical problem in transplantation, with mounting evidence associating alloantibodies with the development of chronic rejection. Nearly a third of transplant recipients develop de novo antibodies, for which no established therapies are effective at preventing or eliminating, highlighting the need for a nonhuman primate model of antibody-mediated rejection. In this report, we demonstrate that depletion using anti-CD3 immunotoxin (IT) combined with maintenance immunosuppression that included tacrolimus with or without alefacept reliably prolonged renal allograft survival in rhesus monkeys. In these animals, a preferential skewing toward CD4 repopulation and proliferation was observed, particularly with the addition of alefacept. Furthermore, alefacept-treated animals demonstrated increased alloantibody production (100%) and morphologic features of antibody-mediated injury. In vitro, alefacept was found to enhance CD4 effector memory T cell proliferation. In conclusion, alefacept administration after depletion and with tacrolimus promotes a CD4+memory T cell and alloantibody response, with morphologic changes reflecting antibody-mediated allograft injury. Early and consistent de novo alloantibody production with associated histological changes makes this nonhuman primate model an attractive candidate for evaluating targeted therapeutics.

Bidirectional alloreactivity: A proposed microchimerism-based solution to the NIMA paradox

The NIMA paradox is the observation that in transplants of allogeneic kidneys or hematopoietic stem cells, siblings benefit from re-exposure to non-inherited maternal antigens (NIMA), whereas re-exposure to a transplant from mother herself, theoretically the ideal "NIMA" donor, does not yield clinical results superior to a father-donated allograft. Recent observations of bidirectional alloreactivity in kidney and cord blood transplantation offer a possible solution to this paradox. If correct, the proposed solution points the way to clinical applications of microchimerism in solid organ and hematopoetic transplants.

Nondepleting anti-CD40-based therapy prolongs allograft survival in nonhuman primates

Costimulation blockade of the CD40/CD154 pathway has been effective at preventing allograft rejection in numerous transplantation models. This strategy has largely depended on mAbs directed against CD154, limiting the potential for translation due to its association with thromboembolic events. Though targeting CD40 as an alternative to CD154 has been successful at preventing allograft rejection in preclinical models, there have been no reports on the effects of CD40-specific agents in human transplant recipients. This delay in clinical translation may in part be explained by the presence of cellular depletion with many CD40-specific mAbs. As such, the optimal biologic properties of CD40-directed immunotherapy remain to be determined. In this report, we have characterized 3A8, a human CD40-specific mAb and evaluated its efficacy in a rhesus macaque model of islet cell transplantation. Despite partially agonistic properties and the inability to block CD40 binding of soluble CD154 (sCD154) in vitro, 3A8-based therapy markedly prolonged islet allograft survival without depleting B cells. Our results indicate that the allograft-protective effects of CD40-directed costimulation blockade do not require sCD154 blockade, complete antagonism or cellular depletion, and serve to support and guide the continued development of CD40-specific agents for clinical translation.

Antigen-specific induced Foxp3+ regulatory T cells are generated following CD40/CD154 blockade

Blockade of the CD40/CD154 pathway potently attenuates T-cell responses in models of autoimmunity, inflammation, and transplantation. Indeed, CD40 pathway blockade remains one of the most powerful methods of prolonging graft survival in models of transplantation. But despite this effectiveness, the cellular and molecular mechanisms underlying the protective effects of CD40 pathway blockade are incompletely understood. Furthermore, the relative contributions of deletion, anergy, and regulation have not been measured in a model in which donor-reactive CD4(+) and CD8(+) T-cell responses can be assessed simultaneously. To investigate the impact of CD40/CD154 pathway blockade on graft-specific T-cell responses, a transgenic mouse model was used in which recipients containing ovalbumin-specific CD4(+) and CD8(+) TCR transgenic T cells were grafted with skin expressing ovalbumin in the presence or absence of anti-CD154 and donor-specific transfusion. The results indicated that CD154 blockade altered the kinetics of donor-reactive CD8(+) T-cell expansion, delaying differentiation into IFN-γ(+) TNF(+) multifunctional cytokine producers. The eventual differentiation of cytokine-producing effectors in tolerant animals coincided with the emergence of an antigen-specific CD4(+) CD25(hi) Foxp3(+) T-cell population, which did not arise from endogenous natural T(reg) but rather were peripherally generated from naïve Foxp3(-) precursors.

The role of B cells in solid organ transplantation

The role of antibodies in chronic injury to organ transplants has been suggested for many years, but recently emphasized by new data. We have observed that when immunosuppressive potency decreases either by intentional weaning of maintenance agents or due to homeostatic repopulation after immune cell depletion, the threshold of B cell activation may be lowered. In human transplant recipients the result may be donor-specific antibody, C4d+ injury, and chronic rejection. This scenario has precise parallels in a rhesus monkey renal allograft model in which T cells are depleted with CD3 immunotoxin, or in a CD52-T cell transgenic mouse model using alemtuzumab to deplete T cells. Such animal models may be useful for the testing of therapeutic strategies to prevent DSA. We agree with others who suggest that weaning of immunosuppression may place transplant recipients at risk of chronic antibody-mediated rejection, and that strategies to prevent this scenario are needed if we are to improve long-term graft and patient outcomes in transplantation. We believe that animal models will play a crucial role in defining the pathophysiology of antibody-mediated rejection and in developing effective therapies to prevent graft injury. Two such animal models are described herein.

CD40-specific costimulation blockade enhances neonatal porcine islet survival in nonhuman primates

The widespread clinical implementation of alloislet transplantation as therapy for type 1 diabetes has been hindered by the lack of suitable islet donors. Pig-to-human islet xenotransplantation is one strategy with potential to alleviate this shortage. Long-term survival of porcine islets has been achieved using CD154-specific antibodies to interrupt the CD40/CD154 costimulation pathway; however, CD154-specific antibodies seem unlikely candidates for clinical translation. An alternative strategy for CD40/CD154 pathway interruption is use of CD40-specific antibodies. Herein, we evaluate the ability of a chimeric CD40-specific monoclonal antibody (Chi220) to protect islet xenografts. Neonatal porcine islets (~50,000 IEQ/kg) were transplanted intraportally into pancreatectomized diabetic macaques. Immunosuppression consisted of induction therapy with Chi220 and the IL-2 receptor-specific antibody basiliximab, and maintenance therapy with sirolimus and the B7-specific fusion protein belatacept. Chi220 effectively promoted xenoislet engraftment and survival, with five of six treated recipients achieving insulin-independent normoglycemia (median rejection-free survival 59 days; mean 90.8 days, maximum 203 days). No thromboembolic phenomena were observed. CD40 represents a promising alternative to CD154 as a therapeutic target, and the efficacy of CD40-specific antibodies in islet xenotransplantation warrants further investigation.

Co-stimulation blockade as a new strategy in kidney transplantation: benefits and limits

New immunosuppressive drugs have greatly decreased the frequency of graft failure due to acute rejection but have had little impact on long-term graft survival. This is due, at least in part, to the broad non-immune effects of the current immunosuppressive drugs, which are involved in the death of patients and in chronic allograft dysfunction, particularly due to their nephrotoxicity. Recent progress in the development of biologicals, i.e. antibodies and fusion proteins, allows precise targeting of the immune system, preventing the non-immune side effects encountered with current protocols. In particular, targeting of the two most important co-stimulation pathways critical for T-cell activation, i.e. B7/CD28 and CD40/CD40L, has provided excellent results in many experimental models of organ transplantation. This has led to the clinical development of belatacept, a cytotoxic T-lymphocyte-associated antigen 4 immunoglobulin (CTLA4-Ig) fusion protein, which has proved to be efficient in preventing acute rejection in kidney transplant recipients. Its use is associated with improved renal function and a better metabolic profile than calcineurin inhibitors. However, because belatacept does not selectively target alloreactive T lymphocytes and must be combined with classical immunosuppressive drugs, infectious and neoplastic complications may occur, particularly post-transplantation lymphoproliferative disorders. We also address the current development of molecules targeting other co-stimulatory pathways (CD40/CD40L, leukocyte function-associated antigen [LFA]-1/intercellular adhesion molecule [ICAM], CD2/LFA-3). Many unresolved issues regarding the use of co-stimulation blocking agents are also discussed, e.g. their long half-life, which can be problematic in cases of serious adverse events, their long-term safety and efficacy, and the lack of monitoring tools to allow modulation of their use over time.

Memory T-cell-specific therapeutics in organ transplantation

PURPOSE OF REVIEW

This review details the role of memory T cells in physiologic and allospecific immunity, and summarizes the effects of immunosuppressive agents used to manipulate their function in the context of organ transplantation.

RECENT FINDINGS

Memory T cells are lymphocytes with characteristics that are thought to promote anamnestic immune responses. They have a unique capacity to generate rapid effector functions upon secondary exposure to a pathogen, and this capacity is achieved through truncated requirements for antigen presentation, reduced activation thresholds, and enhanced trafficking and adhesion mechanisms. In general, these same mechanisms also appear to evoke improved efficiency in mediating allograft rejection. The phenotype of these cells has been increasingly well defined and associated with a characteristic pattern of susceptibility to immunosuppressive agents. This knowledge is now being exploited in the development of immune therapeutic regimens to selectively mollify T memory cell effects.

SUMMARY

A specific targeting of memory T cells has potential to prevent allograft rejection in a more precise manner than current means of immunosuppression. However, these benefits will be balanced by the reciprocal risk of susceptibility to recurrent infection.

Anti-CD40 monoclonal antibody synergizes with CTLA4-Ig in promoting long-term graft survival in murine models of transplantation

Blockade of the CD40/CD154 signaling pathway using anti-CD154 Abs has shown promise in attenuating the alloimmune response and promoting long-term graft survival in murine model systems, although side effects observed in humans have hampered its progression through clinical trials. Appropriately designed anti-CD40 Abs may provide a suitable alternative. We investigated two isoforms of a novel monoclonal rat anti-mouse CD40 Ab (7E1) for characteristics and effects mirroring those of anti-CD154: 7E1-G1 (an IgG1 isotype); and 7E1-G2b (an IgG2b isotype). In vitro proliferation assays to measure the agonist properties of the two anti-CD40 Abs revealed similar responses when plate bound. However, when present as a soluble stimulus, 7E1-G1 but not 7E1-G2b led to proliferation. 7E1-G2b was as effective as anti-CD154 when administered in vivo in concert with CTLA4-Ig in promoting both allogeneic bone marrow chimerism and skin graft survival, whereas 7E1-G1 was not. The protection observed with 7E1-G2b was not due to depletion of CD40-bearing APCs. These data suggest that an appropriately designed anti-CD40 Ab can promote graft survival as well as anti-CD154, making 7E1-G2b an attractive substitute in mouse models of costimulation blockade-based tolerance regimens.

A human anti-CD40 monoclonal antibody, 4D11, for kidney transplantation in cynomolgus monkeys: induction and maintenance therapy

Blockade of CD40-CD154 signaling pathway is an attractive strategy to induce potent immunosuppression and tolerance in organ transplantation. Due to its strong immunosuppressive effect shown in nonhuman primate experiments, anti-CD154 monoclonal antibodies (mAbs) have been tried in clinical settings, but it was interrupted by unexpected thromboembolic complications. Thus, inhibition of the counter molecule, CD40, has remained an alternative approach. In the previous preliminary study, we have shown that 4D11, a novel fully human anti-CD40 mAb, has a fairly potent immunosuppressive effect on kidney allograft in nonhuman primates. In this study, we aimed to confirm the efficacy and untoward events of the 2-week induction and 180-day maintenance 4D11 treatments. In both, 4D11 significantly suppressed T-cell-mediated alloimmune responses and prolonged allograft survival. Addition of weekly 4D11 administration after the induction treatment further enhanced graft survival. Complete inhibition of both donor-specific Ab and anti-4D11 Ab productions was obtained only with higher-dose maintenance therapy. No serious side effect including thromboembolic complications was noted except for a transient reduction of hematocrit in one animal, and decrease of peripheral B-cell counts in all. These results indicate that the 4D11 appears to be a promising candidate for immunosuppression in clinical organ transplantation.

Translating costimulation blockade to the clinic: lessons learned from three pathways

SUMMARY

As the recognition that costimulatory signals are critical for optimal T-cell activation, proliferation, and differentiation, there has been an explosion in the study of costimulatory molecules and their roles in enhancing anti-donor T-cell responses following transplantation. Here, we focus on the bench-to-beside translation of blocking agents designed to target three critical costimulatory pathways: the CD28/CD80/CD86 pathway, the CD154/CD40 pathway, and the lymphocyte function associated antigen-1/intercellular adhesion molecule pathway. While blockade of each of these pathways proved promising in inhibiting donor-reactive T-cell responses and promoting long-term graft survival in murine models of transplantation, the progression of development of therapeutic agents to block these pathways has each taken a slightly different course. Both logistical and biological pitfalls have accompanied the translation of blockers of all three pathways into clinically applicable therapies, and the development of costimulatory blockade as a substitute for current standard-of-care calcineurin inhibitors has by no means reached completion. Collaboration between both the basic and clinical arenas will further propel the development of costimulation blockers currently in the pipeline, as well as of novel methods to target these critical pathways during transplantation.

Fc-disabled anti-mouse CD40L antibodies retain efficacy in promoting transplantation tolerance

CD40L antibodies have proven to be powerful immunosuppressive agents in nonhuman primates but unfortunately perturb blood coagulation. Neither the therapeutic nor the prothrombotic mechanism of anti-CD40L is defined sufficiently to determine whether these effects can be uncoupled. Recent evidence suggests that the Fc region of anti-CD40L antibodies interacting with Fc receptors plays an important role in stabilizing platelet aggregates. An Fc-disabled, aglycosylated anti-CD40L heavy chain variant was therefore created to determine whether it might still be useful in promoting transplantation tolerance. In a number of mouse models an engineered aglycosyl anti-CD40L recapitulated the effects of the intact anti-CD40L antibody in tolerance protocols involving transplantation of allogeneic bone marrow and skin. In contrast, another anti-CD40L variant with a conventional rat gamma2b heavy chain was less effective in ensuring long-term skin graft survival, possibly associated with its faster clearance from the circulation. These results show that short pulses of anti-CD40L antibody therapy may still be useful in tolerance protocols even when the Fc region is disabled.

PD-1-dependent mechanisms maintain peripheral tolerance of donor-reactive CD8+ T cells to transplanted tissue

Peripheral mechanisms of self-tolerance often depend on the quiescent state of the immune system. To what degree such mechanisms can be engaged in the enhancement of allograft survival is unclear. To examine the role of the PD-1 pathway in the maintenance of graft survival following blockade of costimulatory pathways, we used a single-Ag mismatch model of graft rejection where we could track the donor-specific cells as they developed endogenously and emerged from the thymus. We found that graft-specific T cells arising under physiologic developmental conditions at low frequency were actively deleted at the time of transplantation under combined CD28/CD40L blockade. However, this deletion was incomplete, and donor-specific cells that failed to undergo deletion up-regulated expression of PD-1. Furthermore, blockade of PD-1 signaling on these cells via in vivo treatment with anti-PD-1 mAb resulted in rapid expansion of donor-specific T cells and graft loss. These results suggest that the PD-1 pathway was engaged in the continued regulation of the low-frequency graft-specific immune response and thus in maintenance of graft survival.

Characterization of naturally occurring CD4+CD25+ regulatory T cells in rhesus monkeys

BACKGROUND

Translational research in a relevant preclinical model is recommended before Treg-inducing protocols can be implemented in humans. We have characterized rhesus monkey CD25 cells phenotypically and functionally.

METHODS

The phenotype of CD4(+)CD25(high) cells was determined by FACS, focusing on established markers of mouse and human Treg cells. Percentages of cells positive for CD45RA, CD62L, and intracellular CTLA-4 and FOXP3 were compared between CD4(+)CD25(high) and CD4(+)CD25(-) cells. CD25 cells stimulated with anti-CD3, ConA, and/or allogeneic peripheral blood mononuclear cells were mixed with freshly isolated CD25 cells. The suppressive activity of the CD25 cells in vitro was assessed using several experimental conditions.

RESULTS

Rhesus monkey CD4(+)CD25(high) cells expressed high intracellular levels of CTLA-4 and FOXP3, whereas expression was negligible in CD4(+)CD25(-) cells. The CD25(high) population was mostly CD45RA(-), indicative of a memory phenotype. The CD25(+) cells were anergic, because they showed low proliferative responses, no interleukin-2 production, and some interferon-gamma and interleukin-10 production. Proliferation of CD4(+)CD25(-) cells stimulated by anti-CD3 or allogeneic cells was decreased when CD4(+)CD25(-) cells were added at a 1:1 ratio. In addition, we found that CD25 cells inhibited the interleukin-2 and interferon-gamma production by anti-CD3-stimulated CD25 cells in a dose-dependent fashion, through a cell-cell contact-dependent mechanism.

CONCLUSIONS

Rhesus monkey CD4(+)CD25(+) cells have similar phenotypic and functional characteristics as natural Tregs in humans. These findings allow testing of Treg expansion and induction protocols in a relevant preclinical model, the rhesus monkey.

Preclinical assessment of therapeutic antibodies against human CD40 and human interleukin-12/23p40 in a nonhuman primate model of multiple sclerosis

BACKGROUND

Proinflammatory cytokines, such as interleukin (IL)-12 and IL-23, and costimulatory molecules on antigen-presenting cells (APC), such as CD40, are critical to autoreactive T cell activation by APC, and hence, are considered relevant targets of therapy for immune-mediated inflammatory diseases (IMID).

OBJECTIVE

The current review discusses the preclinical evaluation of two novel immunotherapeutic monoclonal antibodies (mAbs), one directed against human IL-12/23p40 and the other against CD40. As the antibodies only recognize their target molecule in primates, the efficacy could not be tested in rodent models.

RESULTS

As a preclinical IMID model for the in vivo evaluation of both mAbs, we have used the experimental autoimmune/allergic encephalomyelitis (EAE) model in common marmoset monkeys (Callithrix jacchus). Both mAbs show beneficial activities in the EAE model when administered early in disease development as well as after the onset of brain inflammation. The treatment effects were evaluated using a combination of quantitative magnetic resonance imaging and a series of ex vivo and immunopathological evaluations.

CONCLUSION

The promising effects during ongoing disease in a relevant preclinical IMID model illustrate the potential of these two antibodies as treatment of IMID, in particular for multiple sclerosis on which disease EAE has been modeled.

A novel fully human anti-CD40 monoclonal antibody, 4D11, for kidney transplantation in cynomolgus monkeys

BACKGROUND

CD40-CD154 pathway blockade by anti-CD154 monoclonal antibodies (mAbs) significantly prolongs allograft survival in nonhuman primates. However, thromboembolic complications have prevented clinical application. Thus, blockade of the counter molecule by a novel fully human anti-CD40 mAb, 4D11, is an attractive alternative.

METHODS

Kidney transplantations were performed between outbred cynomolgus monkeys (stimulation index >3 in a mixed lymphocyte reaction). The animals were divided into five groups: nontreatment control (Group 1, n=3), 10-week treatment with either 10 mg/kg (Group 2, n=3), 20 mg/kg (Group 3, n=3), or 40 mg/kg (Group 4, n=1), and 4-week treatment (Group 5, n=1 each) with 10 mg/kg, 20 mg/kg, or 40 mg/kg followed by monthly administration. Graft survival, biochemistry, complete blood counts, lymphocyte phenotypes, blood drug levels, antidonor and antidrug antibodies, and renal histology were examined.

RESULTS

Survival (days) was as follows: Group 1 (5, 6, 7), Group 2 (150, 108, 108), Group 3 (84, 108, 379), Group 4 (147), and Group 5 (147, 102, 112). Two animals in Group 3 with normal graft function were killed upon development of hydronephrosis and cerebral infarction. B lymphocytes fell to one-third of the preoperative value at 4 weeks after transplantation in all animals. Antidonor antibodies developed in most of the animals after stopping drug treatment or at the time of death. No animals except for one formed anti-4D11 antibody.

CONCLUSION

4D11 appears to be a promising agent for antirejection treatment in clinical organ transplantation.

Expression patterns of regulatory T-cell markers in accepted and rejected nonhuman primate kidney allografts

The identification of FOXP3 expressing cells in recipients of an allograft, in particular inside the graft itself, may help to define criteria for immunosuppressive drug withdrawal. We therefore examined expression patterns of several regulatory T-cell (Treg) markers in kidney biopsies and kidney tissues taken at the time of graft rejection from monkeys treated with alpha CD40, alpha CD86, CsA, a combination of these or after drug withdrawal. In advanced stages of rejection, organized multifocal nodular infiltrates, with mature dendritic cells, T cells and B cells could be found. In contrast, interstitial infiltrates contain more macrophages, less T cells and few B cells. Cells expressing FOXP3, CD25 and CTLA-4 were mainly found in nodular infiltrates of rejected tissue samples. A significant correlation was found between the percentage FOXP3(+) cells and markers for rejection, i.e. creatinine levels and Banff interstitial and tubular infiltrate scores. The type of immunosuppression did not influence the percentage of cells expressing Treg markers. Three animals with prolonged drug-free survival showed low numbers of FOXP3(+) cells. We conclude that the presence of intragraft FOXP3(+) cells is not confined to tolerated grafts, but should be considered as part of the normal immune response during rejection.

T cell costimulatory pathways in allograft rejection and tolerance: what's new?

PURPOSE OF REVIEW

The induction or maintenance of allograft tolerance remains an ongoing challenge. One approach to the development of tolerogenic strategies involves targeting T-cell costimulatory signals. The two most widely studied costimulatory pathways are the CD28/B7 and CD40/CD154 pathways, and blocking of both, either alone or in combination, has been shown to prolong allograft survival in rodents and primates. Recent work revealed that CD28-independent 'novel costimulatory' pathways exist, which can mediate allograft rejection. This review highlights new studies on the role of these pathways in allograft rejection and tolerance.

RECENT FINDINGS

NK cells, CD8 T cells, and memory-effector responses appear to be less dependent on CD28 and/or CD154 costimulation, and utilize these novel costimulatory pathways for activation. The novel signals differ in their ability to enhance or inhibit T-cell activation, in their temporal and spatial expression patterns, and in their relative importance within the hierarchy of costimulatory signals. Emerging data suggest that costimulatory molecules are expressed on parenchymal cells.

SUMMARY

A strategy to induce tolerance might involve targeting novel costimulatory signals particularly at the time point of maximal expression, and delivering negative signals, while inhibiting the positive signals that drive T-cell alloresponses.

No synergy between ATG induction and costimulation blockade induced kidney allograft survival in rhesus monkeys

BACKGROUND

Costimulation blockade with antibodies directed against human CD40 and CD86 leads to prolonged kidney allograft survival in rhesus monkeys, but fails to induce permanent graft acceptance. We have tested whether costimulation blockade is more effective after peripheral T-cell ablation with antithymocyte globulin (ATG), with the aim to remove already primed autoreactive cells present in the normal repertoire.

METHODS

Rhesus monkeys were transplanted with a mismatched kidney allograft. ATG was given around the time of transplantation (day -1 and 0). Costimulation blockade with anti-CD40+anti-CD86 was given at tapering dosages from day -1 to 56. Cyclosporin A (CsA) was given from day 42 onwards and first rejections occurring after day 42 were treated with prednisone.

RESULTS

We observed accelerated rejection in ATG-treated monkeys, compared to animals receiving only costimulation blockade. The accelerated rejection of the kidney allograft occurred despite the application of rejection therapy with steroids and CsA. Three of the five ATG-treated animals were found seropositive for donor-specific alloantibodies. Early biopsies (day 21) from animals treated with ATG and anti-CD40+anti-CD86 show substantially reduced expression of cytotoxic T lymphocyte-associated antigen-4 (CTLA-4) and forkhead box P3 (FOXP3) in focal infiltrates as compared to animals treated with only costimulation blockade. Furthermore, we observed the rapid reappearance of CD8 T-cells with a memory phenotype (disappearance of naive CD95/CD11a T-cells) in peripheral blood.

CONCLUSION

We conclude that (subtotal) T-cell depletion using ATG does not add to costimulation blockade induced kidney allograft survival.

Costimulation blockade and its possible future use in clinical transplantation

The nonimmune effects of currently used immunosuppressive drugs result in a high incidence of late graft loss due to nephrotoxicity and death of patients. As an immune-specific alternative to conventional immunosuppressants, new biotechnology tools can be used to block the costimulation signals of T-cell activation. Many experimental studies--particularly preclinical studies in nonhuman primates--have focused on blocking the 'classical' B7/CD28 and CD40/CD40L pathways, which are critical in primary T-cell activation. Here, we review the limitations, the recent advances and the first large-scale clinical application of the CTLA4-Ig fusion protein to block the B7/CD28 costimulation pathway. We also focus on new B7/CD28 and tumor necrosis factor (TNF)/TNF-R family costimulatory molecules that can deliver positive or negative costimulation signals regulating the alloimmune response. Strategies that use single agents to block costimulation have often proved to be insufficient. Given the diversity of the different costimulation molecules, future strategies for human transplantation may involve the simultaneous blockade of several selected pathways or the simultaneous use of conventional immunosuppressants.

Reshaping the past: Strategies for modulating T-cell memory immune responses

Memory T cells are generated following an initial encounter with antigen, persist over the lifetime of an individual, and mediate rapid and robust functional responses upon antigenic recall. While immune memory is generally associated with protective immune response to pathogens, memory T cells can be generated to diverse types of antigens including autoantigens and alloantigens through homologous or crossreactive priming and comprise the majority of circulating T cells during adulthood. Memory T cells can therefore play critical roles in propagating and perpetuating autoimmune disease and in mediating allograft rejection, although the precise pathways for regulation of memory immune responses remain largely undefined. Moreover, evaluating and designing strategies to modulate memory T-cell responses are challenging given the remarkable heterogeneity of memory T cells, with different subsets predominating in lymphoid versus non-lymphoid tissue sites. In this review, we discuss what is presently known regarding the effect of current immunomodulation strategies on the memory T-cell compartment and potential strategies for controlling immunological recall.

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.

A new look at blockade of T-cell costimulation: a therapeutic strategy for long-term maintenance immunosuppression

Activated T cells orchestrate the immune response that results in graft rejection; therefore, a common goal among current immunosuppressive therapies is to block T-cell activation, proliferation and function. Current immunosuppressive regimens that inhibit T cells and immune cells have greatly reduced the incidence of acute rejection following solid-organ transplant. However, the expected improvements in long-term outcomes have not been realized. This may be related to the non-immune side effects of current maintenance immunosuppressants, which target ubiquitously expressed molecules. The focus in transplantation research is shifting in search of maintenance immunosuppressive regimens that might offer improved long-term outcomes by providing efficacy in prevention of acute rejection combined with reduced toxicities. An emerging therapeutic strategy involves an immunoselective maintenance immunosuppressant that inhibits full T-cell activation by blocking the interaction between costimulatory receptor-ligand pairs. This review describes costimulatory pathways and the development of molecules, which inhibit them in the context of transplantation research. Recent clinical data using the selective costimulation blocker, belatacept (LEA29Y), as a part of a CNI-free maintenance immunosuppressive regimen in renal transplantation is highlighted.