A novel clinically relevant approach to tip the balance toward regulation in stringent transplant model

Metronomic capecitabine with rapamycin exerts an immunosuppressive effect by inducing ferroptosis of CD4+ T cells after liver transplantation in rat

Liver transplantation is one of the most effective treatments for hepatocellular carcinoma (HCC). The balance between inhibiting immune rejection and preventing tumor recurrence after liver transplantation is the key to determining the long-term prognosis of patients with HCC after liver transplantation. In our previous study, we found that capecitabine (CAP), an effective drug for the treatment of HCC, could exert an immunosuppressive effect after liver transplantation by inducing T cell ferroptosis. Recent studies have shown that ferroptosis is highly associated with autophagy. In this study, we confirmed that the autophagy inducer rapamycin (RAPA) combined with metronomic capecitabine (mCAP) inhibits glutathione peroxidase 4 (GPX4) and promotes ferroptosis in CD4+ T cells to exert immunosuppressive effects after rat liver transplantation. Compared with RAPA or mCAP alone, the combination of RAPA and mCAP could adequately reduce liver injury in rats with acute rejection after transplantation. The CD4+ T cell counts in peripheral blood, spleen, and transplanted liver of recipient rats significantly decreased, and the oxidative stress level and ferrous ion concentration of CD4+ T cells significantly increased in the combination group. In vitro, the combination of drugs significantly promoted autophagy, decreased GPX4 protein expression, and induced ferroptosis in CD4+ T cells. In conclusion, the autophagy inducer RAPA improved the mCAP-induced ferroptosis in CD4+ T cells. Our results support the concept of ferroptosis as an autophagy-dependent cell death and suggest that the combination of ferroptosis inducers and autophagy inducers is a new research direction for improving immunosuppressive regimens after liver transplantation.

Regulatory T cell homeostasis: Requisite signals and implications for clinical development of biologics

Novel biologics are currently being tested in clinical trials for the treatment of autoimmune diseases and the prevention of transplant allograft rejection. Their premise is to deliver highly efficient immunosuppression while minimizing side-effects, as they specifically target inflammatory mediators involved in the dysregulation of the immune system. However, the pleiotropism of soluble mediators and cell-to-cell interactions with potential to exert both proinflammatory and regulatory influences on the outcome of the immune response can lead to unpredictable results. Predicting responses to biologic drugs requires mechanistic understanding of the cell type-specific effect of immune mediators. Elucidation of the central role of regulatory T cells (Treg), a small subset of T cells dedicated to immune homeostasis, in preventing the development of auto- and allo-immunity has provided a deeper understanding of the signaling pathways that govern immune tolerance. This review focuses on the requisite signals that promote Treg homeostasis and discusses the anticipated outcomes of biologics targeting these signals. Our goal is to inform and facilitate the design of cell-specific biologics that thwart T effector cells (Teff) while promoting Treg function for the treatment of autoimmune diseases and the prevention of transplant rejection.

Autophagy-lysosome inhibitor chloroquine prevents CTLA-4 degradation of T cells and attenuates acute rejection in murine skin and heart transplantation

Background: The immune checkpoint cytotoxic T lymphocyte antigen-4 (CTLA-4), induced upon T cell activation but degraded quickly, has been targeted in the clinical therapy of advanced cancers and autoimmune diseases. However, whether inhibiting CTLA-4 degradation ameliorates transplant rejection remains unknown.

Methods: The CTLA-4 expression in activated murine T cells treated with the inhibitors mediating protein degradation was detected by flow cytometry (FCM). CD45.1 mice, which received TEa T cells and underwent heart transplantation, were administrated with the inhibitor. Subsequently, CTLA-4 expression of TEa T cells was analyzed. Murine skin and heart transplantation models were built, then the survival and histopathology of the allografts, and T cell subsets in the spleens of each group were compared.

Results: Chloroquine (CQ) was identified as an inhibitor of CTLA-4 degradation, which augmented both surface and total CTLA-4 expression in T cells. It considerably prolonged the skin and heart allograft survival time and reduced the infiltration of inflammatory cells in allografts. Besides decreasing the frequencies of the CD4⁺ and CD8⁺ effector T cells, especially IFN-γ producing T cells, CQ also increased the proportion of regulatory T cells in the spleen. The CTLA-4 blockade abrogated the benefits of CQ on the survival of heart allografts. Moreover, CQ enhanced CTLA-4 expression in activated human T cells and reduced the secretion of IFN-γ in human mixed lymphocyte reaction.

Conclusion: Targeting CTLA-4 degradation provides a novel means to prevent transplant rejection and induce transplant tolerance.

Costimulation Blockade in Transplantation

T cells play a pivotal role in orchestrating immune responses directed against a foreign (allogeneic) graft. For T cells to become fully activated, the T-cell receptor (TCR) must interact with the major histocompatibility complex (MHC) plus peptide complex on antigen-presenting cells (APCs), followed by a second "positive" costimulatory signal. In the absence of this second signal, T cells become anergic or undergo deletion. By blocking positive costimulatory signaling, T-cell allo-responses can be aborted, thus preventing graft rejection and promoting long-term allograft survival and possibly tolerance (Alegre ML, Najafian N, Curr Mol Med 6:843-857, 2006; Li XC, Rothstein DM, Sayegh MH, Immunol Rev 229:271-293, 2009). In addition, costimulatory molecules can provide negative "coinhibitory" signals that inhibit T-cell activation and terminate immune responses; strategies to promote these pathways can also lead to graft tolerance (Boenisch O, Sayegh MH, Najafian N, Curr Opin Organ Transplant 13:373-378, 2008). However, T-cell costimulation involves an incredibly complex array of interactions that may act simultaneously or at different times in the immune response and whose relative importance varies depending on the different T-cell subsets and activation status. In transplantation, the presence of foreign alloantigen incites not only destructive T effector cells but also protective regulatory T cells, the balance of which ultimately determines the fate of the allograft (Lechler RI, Garden OA, Turka LA, Nat Rev Immunol 3:147-158, 2003). Since the processes of alloantigen-specific rejection and regulation both require activation of T cells, costimulatory interactions may have opposing or synergistic roles depending on the cell being targeted. Such complexities present both challenges and opportunities in targeting T-cell costimulatory pathways for therapeutic purposes. In this chapter, we summarize our current knowledge of the various costimulatory pathways in transplantation and review the current state and challenges of harnessing these pathways to promote graft tolerance (summarized in Table 10.1).

Total Recall: Can We Reshape T Cell Memory by Lymphoablation?

Despite recent advances in immunosuppression, donor-reactive memory T cells remain a serious threat to successful organ transplantation. To alleviate damaging effects of preexisting immunologic memory, lymphoablative induction therapies are used as part of standard care in sensitized recipients. However, accumulating evidence suggests that memory T cells have advantages over their naive counterparts in surviving depletion and expanding under lymphopenic conditions. This may at least partially explain the inability of existing lymphoablative strategies to improve long-term allograft outcome in sensitized recipients, despite the well-documented decrease in the frequency of early acute rejection episodes. This minireview summarizes the insights gained from both experimental and clinical transplantation as to the effects of existing lymphoablative strategies on memory T cells and discusses the latest research developments aimed at improving the efficacy and safety of lymphoablation.

Role of Memory T Cells in Allograft Rejection and Tolerance

Memory T cells are characterized by their low activation threshold, robust effector functions, and resistance to conventional immunosuppression and costimulation blockade. Unlike their naïve counterparts, memory T cells reside in and recirculate through peripheral non-lymphoid tissues. Alloreactive memory T cells are subdivided into different categories based on their origins, phenotypes, and functions. Recipients whose immune systems have been directly exposed to allogeneic major histocompatibility complex (MHC) molecules display high affinity alloreactive memory T cells. In the absence of any prior exposure to allogeneic MHC molecules, endogenous alloreactive memory T cells are regularly generated through microbial infections (heterologous immunity). Regardless of their origin, alloreactive memory T cells represent an essential element of the allograft rejection process and a major barrier to tolerance induction in clinical transplantation. This article describes the different subsets of alloreactive memory T cells involved in transplant rejection and examine their generation, functional properties, and mechanisms of action. In addition, we discuss strategies developed to target deleterious allospecific memory T cells in experimental animal models and clinical settings.

Impact of Immune-Modulatory Drugs on Regulatory T Cell

Immunosuppression strategies that selectively inhibit effector T cells while preserving and even enhancing CD4FOXP3 regulatory T cells (Treg) permit immune self-regulation and may allow minimization of immunosuppression and associated toxicities. Many immunosuppressive drugs were developed before the identity and function of Treg were appreciated. A good understanding of the interactions between Treg and immunosuppressive agents will be valuable to the effective design of more tolerable immunosuppression regimens. This review will discuss preclinical and clinical evidence regarding the influence of current and emerging immunosuppressive drugs on Treg homeostasis, stability, and function as a guideline for the selection and development of Treg-friendly immunosuppressive regimens.

CD4 T Cell Help via B Cells Is Required for Lymphopenia-Induced CD8 T Cell Proliferation

Ab-mediated lymphoablation is commonly used in solid organ and hematopoietic cell transplantation. However, these strategies fail to control pathogenic memory T cells efficiently and to improve long-term transplant outcomes significantly. Understanding the mechanisms of T cell reconstitution is critical for enhancing the efficacy of Ab-mediated depletion in sensitized recipients. Using a murine analog of anti-thymocyte globulin (mATG) in a mouse model of cardiac transplantation, we previously showed that peritransplant lymphocyte depletion induces rapid memory T cell proliferation and only modestly prolongs allograft survival. We now report that T cell repertoire following depletion is dominated by memory CD4 T cells. Additional depletion of these residual CD4 T cells severely impairs the recovery of memory CD8 T cells after mATG treatment. The CD4 T cell help during CD8 T cell recovery depends on the presence of B cells expressing CD40 and intact CD40/CD154 interactions. The requirement for CD4 T cell help is not limited to the use of mATG in heart allograft recipients, and it is observed in nontransplanted mice and after CD8 T cell depletion with mAb instead of mATG. Most importantly, limiting helper signals increases the efficacy of mATG in controlling memory T cell expansion and significantly extends heart allograft survival in sensitized recipients. Our findings uncover the novel role for helper memory CD4 T cells during homeostatic CD8 T cell proliferation and open new avenues for optimizing lymphoablative therapies in allosensitized patients.

Long-term control of diabetes in immunosuppressed nonhuman primates (NHP) by the transplantation of adult porcine islets

Pig islets are an alternative source for islet transplantation to treat type 1 diabetes (T1D), but reproducible curative potential in the pig-to-nonhuman primate (NHP) model has not been demonstrated. Here, we report that pig islet grafts survived and maintained normoglycemia for >6 months in four of five consecutive immunosuppressed NHPs. Pig islets were isolated from designated pathogen-free (DPF) miniature pigs and infused intraportally into streptozotocin-induced diabetic rhesus monkeys under pretreatment with cobra venom factor (CVF), anti-thymocyte globulin (ATG) induction and maintenance with anti-CD154 monoclonal antibody and low-dose sirolimus. Ex vivo expanded autologous regulatory T cells were adoptively transferred in three recipients. Blood glucose levels were promptly normalized in all five monkeys and normoglycemia (90-110 mg/dL) was maintained for >6 months in four cases, the longest currently up to 603 days. Intravenous glucose tolerance tests during the follow-up period showed excellent glucose disposal capacity and porcine C-peptide responses. Adoptive transfer of autologous regulatory T cells was likely to be associated with more stable and durable normoglycemia. Importantly, the recipients showed no serious adverse effects. Taken together, our results confirm the clinical feasibility of pig islet transplantation to treat T1D patients without the need for excessive immunosuppressive therapy.

Differential sensitivity of regulatory and effector T cells to cell death: a prerequisite for transplant tolerance

Despite significant progress achieved in transplantation, immunosuppressive therapies currently used to prevent graft rejection are still endowed with severe side effects impairing their efficiency over the long term. Thus, the development of graft-specific, non-toxic innovative therapeutic strategies has become a major challenge, the goal being to selectively target alloreactive effector T cells while sparing CD4⁺Foxp3⁺ regulatory T cells (Tregs) to promote operational tolerance. Various approaches, notably the one based on monoclonal antibodies or fusion proteins directed against the TCR/CD3 complex, TCR coreceptors, or costimulatory molecules, have been proposed to reduce the alloreactive T cell pool, which is an essential prerequisite to create a therapeutic window allowing Tregs to induce and maintain allograft tolerance. In this mini review, we focus on the differential sensitivity of Tregs and effector T cells to the depleting and inhibitory effect of these immunotherapies, with a particular emphasis on CD3-specific antibodies that beyond their immunosuppressive effect, also express potent tolerogenic capacities.

Belatacept treatment for two yr after liver transplantation is not associated with operational tolerance

Belatacept was recently evaluated in liver transplantation (LT) in a phase II multicenter trial, which was terminated prematurely. Patients were more than two yr post-LT at the time. As high rates of spontaneous tolerance after LT have been reported and as belatacept has marked immunomodulatory effects, we decided to maintain the belatacept patients enrolled at our center (n = 4) on MMF monotherapy. All belatacept patients on MMF monotherapy developed graft dysfunction consistent with acute rejection after a mean period of 10.3 (7-14) wk. Patients were therefore switched to triple therapy with CNI, MMF, and corticosteroids. Graft dysfunction resolved within 1-3 wk after switch. At the time of belatacept discontinuation, mean eGFR was 105.1 mL/min/1.73 m² (92.1-118.9) in belatacept patients compared to 58 mL/min/1.73 m² (36.1-98.2) in controls (p = 0.022). One yr after the switch to CNI therapy, eGFR had declined by 27.4 mL (19.2-39.3; p = 0.008). Thus, LT patients treated with belatacept show superior kidney function that declines upon institution of CNIs. MMF monotherapy following withdrawal of belatacept is associated with a high incidence of graft dysfunction. Belatacept has no obvious immunomodulatory effects in LT recipients that would be sufficient to allow drug withdrawal with a high rate of success.

Tregitope: Immunomodulation powerhouse

IVIG is frequently used in the 'pre-conditioning' regimens for higher risk transplants; its effects are attributed in part to induction of Tregs. We have identified regulatory T cell (Treg) epitopes, now known as Tregitopes, in IgG, the main component of intravenous immunoglobulin therapy (IVIg). Tregitopes provide one explanation for the expansion and activation of Treg cells following IVIg treatment. Tregitopes are peptides that exhibit high affinity binding to multiple human HLA Class II DR; they are conserved across IgG isotypes and mammalian species. In vitro and in vivo, for human PBMC and in animal models, Tregitopes activate Tregs. Studies to delineate the mechanism of action have shown that Tregitopes' effects are very similar to IVIg in vitro. Here we demonstrate that Tregitopes induce Tregs to produce IL-10, leading to modulation of dendritic cell phenotype (down-regulation of Class II, CD80 and CD86 and up-regulation of ILT3), and describe the effects of Tregitopes in the ABM-TCR-transgenic skin transplantation model. The discovery of Tregitopes in IgG and other autologous proteins may contribute to improved understanding of the mechanism of action of IVIg and lead to the application of these powerful immunomodulators to improve transplantation success and suppress autoimmune disease, in the future.

Regulatory T cells as immunotherapy

Regulatory T cells (Tregs) suppress exuberant immune system activation and promote immunologic tolerance. Because Tregs modulate both innate and adaptive immunity, the biomedical community has developed an intense interest in using Tregs for immunotherapy. Conditions that require clinical tolerance to improve outcomes – autoimmune disease, solid organ transplantation, and hematopoietic stem cell transplantation – may benefit from Treg immunotherapy. Investigators have designed ex vivo strategies to isolate, preserve, expand, and infuse Tregs. Protocols to manipulate Treg populations in vivo have also been considered. Barriers to clinically feasible Treg immunotherapy include Treg stability, off-cell effects, and demonstration of cell preparation purity and potency. Clinical trials involving Treg adoptive transfer to treat graft versus host disease preliminarily demonstrated the safety and efficacy of Treg immunotherapy in humans. Future work will need to confirm the safety of Treg immunotherapy and establish the efficacy of specific Treg subsets for the treatment of immune-mediated disease.

Attenuation of donor-reactive T cells allows effective control of allograft rejection using regulatory T cell therapy

Regulatory T cells (Tregs) are essential for the establishment and maintenance of immune tolerance, suggesting a potential therapeutic role for Tregs in transplantation. However, Treg administration alone is insufficient in inducing long-term allograft survival in normal hosts, likely due to the high frequency of alloreactive T cells. We hypothesized that a targeted reduction of alloreactive T effector cells would allow a therapeutic window for Treg efficacy. Here we show that preconditioning recipient mice with donor-specific transfusion followed by cyclophosphamide treatment deleted 70-80% donor-reactive T cells, but failed to prolong islet allograft survival. However, infusion of either 5 × 10(6) Tregs with direct donor reactivity or 25 × 10(6) polyclonal Tregs led to indefinite survival of BALB/c islets in more than 70% of preconditioned C57BL/6 recipients. Notably, protection of C3H islets in autoimmune nonobese diabetic mice required islet autoantigen-specific Tregs together with polyclonal Tregs. Treg therapy led to significant reduction of CD8(+) T cells and concomitant increase in endogenous Tregs among graft-infiltrating cells early after transplantation. Together, these results demonstrate that reduction of the donor-reactive T cells will be an important component of Treg-based therapies in transplantation.

Targeting CD28 to prevent transplant rejection

INTRODUCTION

The pivotal role of costimulatory pathways in regulating T-cell activation versus tolerance has stimulated tremendous interest in their manipulation for therapeutic purposes. Of these, the CD28-B7 pathway is arguably the most important and best studied. Therapeutic targets of CD28 are currently used in the treatment of melanoma, autoimmune diseases and in transplantation.

AREAS COVERED

In this review, we summarize our current knowledge of CD28 and cytotoxic T-lymphocyte antigen-4 (CTLA-4) signaling, and review the current state and challenges of harnessing them to promote transplant tolerance.

EXPERT OPINION

Despite the success of belatacept, a first-in-class CTLA-4 fusion protein now clinically used in transplantation, it is apparent that we have only scratched the surface in understanding the complexities of how costimulatory pathways modulate the immune system. Our initial assumption that positive costimulators activate effector T cells and prevent tolerance, while negative costimulators inhibit effector T cells and promote tolerance, is clearly an oversimplified view. Indeed, belatacept is not only capable of blocking deleterious CD28-B7 interactions that promote effector T-cell responses but can also have undesired effects on tolerogenic regulatory T-cell populations.

Rejection and regulation: a tight balance

PURPOSE OF REVIEW

Achieving allograft tolerance is the holy grail of transplantation. However, tolerance and rejection are two extreme ends of a scale that can be tipped in either direction. We review the novel effector and regulatory mechanisms involved and factors that tip the balance in favor of rejection or regulation.

RECENT FINDINGS

It is increasingly recognized that established T-cell phenotypes could change their commitments. New data point to the plasticity of Th17 cells in vivo with a reciprocal balance of Th17 cells and regulatory T cells (Tregs) driven by the local cytokine environment. Treg-cell profiles have been linked to acute and chronic allograft outcomes, and emerging data also indicate a novel role of a regulatory B-cell population. Current research efforts are looking into factors that tip the balance toward allograft tolerance by targeting cytokines, novel costimulatory pathways such as T-cell immunoglobulin mucin molecules, and components of innate immunity, particularly dendritic cells.

SUMMARY

The balance of effector and regulatory mechanisms contributing to allograft outcome is very complex. It is likely that targeting multiple pathways will be required to achieve tolerance. Further studies are warranted to define this balance and identify optimal combination of therapeutic interventions.

The role of Thymoglobulin induction in kidney transplantation: an update

The rabbit antithymocyte globulin Thymoglobulin first became available over 25 yr ago and is the most widely used lymphocyte-depleting preparation in solid organ transplantation. Thymoglobulin targets a wide range of T-cell surface antigens as well as natural killer-cell antigens, B-cell antigens, plasma cell antigens, adhesion molecules and chemokine receptors, resulting in profound, long-lasting T-cell depletion. Randomized studies have established the anti-rejection efficacy of Thymoglobulin in kidney transplantation. Experimental and clinical data suggest that Thymoglobulin administration may ameliorate ischemia reperfusion injury, thus reducing the incidence of delayed graft function (DGF). Studies have demonstrated the benefit of using Thymoglobulin to facilitate immunosuppression minimization, both for corticosteroid and calcineurin inhibitor (CNI) withdrawal or avoidance, with potential improvement in cardiovascular and renal outcomes. The optimal cumulative dose for Thymoglobulin induction is 6-7.5 mg/kg, with vigilant short- and long-term monitoring of hematological status. Induction with Thymoglobulin is now indicated in immunologically high-risk patients, in those at increased risk of DGF and to maintain efficacy in low-risk transplant recipients receiving steroid or CNI minimization or avoidance regimens. We suggest that in future trials Thymoglobulin be tested with costimulation signal blockers and other immunosuppressants with the objective of establishing operational tolerance.

Influence of pharmacological immunomodulatory agents on CD4(+)CD25(high)FoxP3(+) T regulatory cells in humans

T regulatory cells (Tregs) play a critical role in the immunologic tolerance to the graft in transplantation. Thus, due to their immunosuppressive capability, ex vivo expanded Tregs may be used as a cellular therapy and an attractive novel strategy to control chronic rejection and eliminate need for lifelong pharmacological immunosuppression. Since Treg therapy is still in its infancy, initially Tregs still need to be applied in combination with pharmacological agents to prevent rejection. Fortunately, some of the medications have been shown to enhance the function and number of Tregs. In the clinic, different immunosuppressive regimens are used for individual patients for different types of organ transplantation. In this review, we present the most commonly used pharmacological agents for immunosuppression and discuss how they affect the Treg population. It is extremely difficult to dissect the effect of single agent on Tregs population in clinical settings since usually the combination of several medications is applied at the same time for graft protection. Nevertheless, experimental and clinical data indicate that thymoglobulin as immunosuppressive induction and mTOR inhibitors as immunosuppressive maintenance agents have the most beneficial effect on Treg population in the blood. Among supplemental agents promoting Tregs, anti-TNFα preparations have been in clinical use (in autoimmune diseases) for many years, so they are optimal candidates for testing in transplant settings in combination with Treg based cellular therapy.

Pretransplant antithymocyte globulin has increased efficacy in controlling donor-reactive memory T cells in mice

Antibody-mediated lymphocyte depletion is frequently used as induction therapy in sensitized transplant patients. Although T cells with an effector/memory phenotype remain detectable after lymphoablative therapies in human transplant recipients, the role of preexisting donor-reactive memory in reconstitution of the T cell repertoire and induction of alloimmune responses following lymphoablation is poorly understood. We show in a mouse cardiac transplantation model that antidonor immune responses following treatment with rabbit antimouse thymocyte globulin (mATG) were dominated by T cells derived from the preexisting memory compartment. Administration of mATG 1 week prior to transplantation (pre-TP) was more efficient in targeting preexisting donor-reactive memory T cells, inhibiting overall antidonor T cell responses, and prolonging heart allograft survival than the commonly used treatment at the time of transplantation (peri-TP). The failure of peri-TP mATG to control antidonor memory responses was due to faster recovery of preexisting memory T cells rather than their inefficient depletion. This rapid recovery did not depend on T cell specificity for donor alloantigens suggesting an important role for posttransplant inflammation in this process. Our findings provide insights into the components of the alloimmune response remaining after lymphoablation and may help guide the future use of ATG in sensitized transplant recipients.

Prolonged, low-dose anti-thymocyte globulin, combined with CTLA4-Ig, promotes engraftment in a stringent transplant model

BACKGROUND

Despite significant nephrotoxicity, calcineurin inhibitors (CNIs) remain the cornerstone of immunosuppression in solid organ transplantation. We, along with others, have reported tolerogenic properties of anti-thymocyte globulin (ATG, Thymoglobulin®), evinced by its ability both to spare Tregs from depletion in vivo and, when administered at low, non-depleting doses, to expand Tregs ex vivo. Clinical trials investigating B7/CD28 blockade (LEA29Y, Belatacept) in kidney transplant recipients have proven that the replacement of toxic CNI use is feasible in selected populations.

METHODS

Rabbit polyclonal anti-murine thymocyte globulin (mATG) was administered as induction and/or prolonged, low-dose therapy, in combination with CTLA4-Ig, in a stringent, fully MHC-mismatched murine skin transplant model to assess graft survival and mechanisms of action.

RESULTS

Prolonged, low-dose mATG, combined with CTLA4-Ig, effectively promotes engraftment in a stringent transplant model. Our data demonstrate that mATG achieves graft acceptance primarily by promoting Tregs, while CTLA4-Ig enhances mATG function by limiting activation of the effector T cell pool in the early stages of treatment, and by inhibiting production of anti-rabbit antibodies in the maintenance phase, thereby promoting regulation of alloreactivity.

CONCLUSION

These data provide the rationale for development of novel, CNI-free clinical protocols in human transplant recipients.

Context and location dependence of adaptive Foxp3(+) regulatory T cell formation during immunopathological conditions

Circulating Foxp3(+) regulatory T cells (Treg) may arise in the thymus (natural Treg, nTreg) or through the adaptive upregulation of Foxp3 after T cell activation (induced Treg, iTreg). In this brief review, we explore evidence for the formation and function of iTreg during pathologic conditions. Determining the ontogeny and function of Treg populations has relied on the use of manipulated systems in which either iTreg or nTreg are absent, or lineage tracing of T cell clones through repertoire analyses. iTreg appear particularly important at mucosal interfaces. iTreg can also ameliorate tissue-specific autoimmunity and are a prominent source of tumor-infiltrating Treg in some models. However, under many conditions, including in CNS autoimmunity, diabetes, and some tumor systems, iTreg formation appears limited. The immunological contribution of iTreg is thus highly context dependent. Deciphering immune parameters responsible for iTreg formation and their role in modulating pathologic immune responses will be important.

Translating tolerogenic therapies to the clinic - where do we stand?

Manipulation of the immune system to prevent the development of a specific immune response is an ideal strategy to improve outcomes after transplantation. A number of experimental techniques exploiting central and peripheral tolerance mechanisms have demonstrated success, leading to the first early phase clinical trials for tolerance induction. The first major strategy centers on the facilitation of donor-cell mixed chimerism in the transplant recipient with the use of bone marrow or hematopoietic stem cell transplantation. The second strategy, utilizing peripheral regulatory mechanisms, focuses on cellular therapy with regulatory T cells. This review examines the key studies and novel research directions in the field of immunological tolerance.

Review article: use of induction therapy in liver transplantation

Induction therapy is used relatively infrequently in liver transplantation, but developments in induction regimens and strategies for their use are prompting a re-examination of its benefits. Rabbit antithymocyte globulin (rATG) induces protracted, dose-dependent lymphocytopenia with preferential reconstitution of regulatory T-lymphocytes. Non-depleting interleukin-2 receptor antagonists (IL-2RA) act selectively on activated T-lymphocytes with a shorter duration of effect. IL-2RA induction with delayed and reduced calcineurin inhibitor (CNI) exposure appears to preserve efficacy, while more aggressive CNI minimisation has been attempted successfully using rATG. Steroid-free tacrolimus monotherapy with rATG or IL-2RA induction is effective if adequate tacrolimus exposure is maintained. Early concerns that addition of induction to a conventional maintenance regimen could lead to accelerated progression of hepatitis C disease, or to an increased risk of hepatocellular cancer recurrence, now appear unfounded using modern regimens. Similarly, with routine use of systemic prophylaxis, recent prospective and retrospective data have not shown a higher rate of infections overall, or cytomegalovirus infection specifically, using rATG or IL-2RA induction. Historical evidence that lymphocyte-depleting agents increased the risk of non-Hodgkin lymphoma has not been confirmed for rATG. Wider use of induction in liver transplantation is now merited, using individualized strategies to support reduced CNI exposure or steroid-free immunosuppression.

BIOMARKERS OF OPERATIONAL TOLERANCE IN SOLID ORGAN TRANSPLANTATION

INTRODUCTION: Long-term immunosuppressive therapy represents a huge burden on transplant recipients, but currently cannot be omitted. Improving long-term transplant outcome by immunosuppressive drug withdrawal may be achieved in patients who have developed (partial) immunological unresponsiveness towards their graft, either spontaneously or through tolerance induction. Reliable biomarkers are essential to define such immunological unresponsiveness and will facilitate controlled immunosuppressive drug weaning as well as provide surrogate end-points for tolerance induction trials. AREAS COVERED: Tolerance biomarkers have been defined for both liver and kidney transplantation and can accurately identify operationally tolerant transplant recipients retrospectively. These two tolerance fingerprints are remarkably different, indicating the involvement of distinct mechanisms. Limited data suggest that tolerance biomarkers can be detected in immunosuppressed transplant recipients. Whether these patients can safely have their immunosuppressive drugs withdrawn needs to be established. EXPERT OPINION: Mechanistic interpretation of the kidney transplant tolerance biomarker profile dominated by B cell markers remains a challenge in light of experimental evidence suggesting the pivotal involvement of regulatory T cells. Therefore, defining animal models that resemble human transplant tolerance is crucial in understanding the underlying mechanisms. Additionally, to ensure patient safety while monitoring for tolerance, it is essential to develop biomarkers to non-invasively detect early signs of rejection as well.

Lymphodepletion and homeostatic proliferation: implications for transplantation

Control of the alloimmune response requires elimination and/or suppression of alloreactive immune cells. Lymphodepleting induction therapies are increasingly used to accomplish this goal, both as part of tolerance induction protocols or to reduce the requirements for maintenance immunosuppression in the peritransplant setting. However, it is well recognized that lymphopenia induces compensatory proliferation of immune cells, generally termed ``homeostatic proliferation,'' which favors the emergence of memory T cells. Paradoxically therefore, the result may be a situation that favors graft rejection and/or makes tolerance difficult to achieve or sustain. Yet all depletion is not alike, particularly with respect to the timing of reconstitution and the types of cells that repopulate the host. Thus, to design more effective induction strategies it is important to understand the homeostatic mechanisms, which exist to maintain a balanced repertoire of naïve and memory T and B cells in the periphery and how they respond to lymphodepletion. Here we will review the biology of homeostatic proliferation stimulated by lymphopenia, the effects of specific depleting agents on reconstitution of the T- and B-cell immune repertoire, drawing from both from animal models and human experience, and potential strategies to enhance allodepletion while minimizing the adverse effects of homeostatic proliferation.

Ex vivo expansion of human Tregs by rabbit ATG is dependent on intact STAT3-signaling in CD4⁺ T cells and requires the presence of monocytes

The addition of low, nondepleting doses of rabbit antithymocyte globulin (ATG) to human peripheral blood mononuclear cells has been shown to expand functional CD4(+) CD25(+) FoxP3(+) regulatory T cells (Tregs) in vitro. This report is the first to elucidate the exact cellular mechanisms of ATG-mediated Treg expansion. CD4(+) T cells require monocytes, but not other antigen presenting cell subsets, to be present in coculture to expand Tregs. However, T cells do not require direct cell-cell contact with monocytes, suggesting the importance of soluble factors. Moreover, ATG initially "reprograms" CD4(+) T cells, but not monocytes, and induces STAT3 and STAT5 signaling in CD4(+) cells. These reprogrammed CD4(+) T cells subsequently secrete GM-CSF and IL-10 only in case of intact STAT3 signaling, which in turn promote the generation of tolerogenic CD14(+) CD11c(+) dendritic cells characterized by enhanced IL-10 and decreased IL-12 production. Treg expansion following ATG treatment is accompanied by enhanced gene expression of both GM-CSF and Bcl-2, but not TGF-β, in peripheral blood mononuclear cells. These results demonstrate that ex vivo expansion of human Tregs by ATG is due to its ability to reprogram CD4(+) T cells in a STAT3-dependent but TGF-β-independent manner, leading to the generation of monocyte-derived dendritic cells with a tolerogenic cytokine profile.

CD4(+)Foxp3(+) regulatory T cell therapy in transplantation

Regulatory T cells (Tregs) are long-lived cells that suppress immune responses in vivo in a dominant and antigen-specific manner. Therefore, therapeutic application of Tregs to control unwanted immune responses is an active area of investigation. Tregs can confer long-term protection against auto-inflammatory diseases in mouse models. They have also been shown to be effective in suppressing alloimmunity in models of graft-versus-host disease and organ transplantation. Building on extensive research in Treg biology and preclinical testing of therapeutic efficacy over the past decade, we are now at the point of evaluating the safety and efficacy of Treg therapy in humans. This review focuses on developing therapy for transplantation using CD4(+)Foxp3(+) Tregs, with an emphasis on the studies that have informed clinical approaches that aim to maximize the benefits while overcoming the challenges and risks of Treg cell therapy.

Belatacept: from rational design to clinical application

Gradually improved immunosuppression has contributed significantly to the progress achieved in transplantation medicine so far. Nevertheless, current drug regimens are associated with late graft loss--in particular as a result of immunologic damage or drug toxicity--and substantial morbidity. Recently, the costimulation blocker belatacept (marketed under the name Nulojix®) has been approved for immunosuppression in renal transplantation. Belatacept (a mutated version of CTLA4Ig) is a fusion protein rationally designed to block CD28, a critical activating receptor on T cells, by binding and saturating its ligands B7-1 and B7-2. In phase II and III trials, belatacept was compared with cyclosporine (in combination with basiliximab, MMF, and steroids). Advantages observed with belatacept include superior graft function, preservation of renal structure and improved cardiovascular risk profile. Concerns associated with belatacept are a higher frequency of cellular rejection episodes and more post-transplant lymphoproliferative disorder (PTLD) cases especially in EBV seronegative patients, who should be excluded from belatacept-based regimens. Thus, after almost three decades of calcineurin inhibitors as mainstay of immunosuppression, belatacept offers a potential alternative. In this article, we will provide an overview of belatacept's preclinical development and will discuss the available evidence from clinical trials.

T-cell co-stimulatory blockade in kidney transplantation: back to the bench

It is believed that blocking positive T-cell co-stimulatory pathways should lead to long-term graft acceptance. Despite the exciting initial achievements in experimental animal models, targeting co-stimulatory pathways has shown to be much more complex in the clinic. In addition to multiple binding partners, some co-stimulatory interactions have been found to be inhibitory in nature, whereas others were demonstrated to be important in the development of regulatory T cells. Moreover, memory T cells have been shown to be resistant to co-stimulation blockade. Herein we focus on the B7:CD28 pathway and describe the evolution of targeting this pathway with cytotoxic T-lymphocyte antigen-4-Ig from bench to clinic. We also attempt to address possible causes for the unexpected high rejection rate observed in the phase III clinical trials with belatacept, using experimental data obtained from basic science research.

Paradoxical aspects of rapamycin immunobiology in transplantation

Rapamycin has long been considered an immunosuppressive agent due to its antiproliferative effects on immune cells, and is currently used as a component of antirejection regimens in transplantation. Despite the large number of mechanistic and clinical studies investigating the impact of rapamycin on cell-mediated immunity, several paradoxes concerning rapamycin immunobiology remain. In particular, emerging evidence suggests that under certain circumstances rapamycin can exert immunostimulatory effects, boosting T cell responses in the face of pathogen infections and vaccines. Here, we review recent findings concerning the contradictory outcomes of rapamycin induced mTOR inhibition on CD4(+) and CD8(+) T cell responses in transplantation and protective immunity. These studies suggest that the conditions under which T cells are stimulated can profoundly modify the impact of rapamycin on antigen-specific T cell responses. Thus, further investigation into the cellular and molecular pathways underlying the dichotomous effects of rapamycin in transplantation is required to harness the full potential of this immunomodulatory agent to promote graft survival and maximize protective immunity.

Operational tolerance: past lessons and future prospects

Every liver transplant (LT) center has had patients who either self-discontinue immunosuppressive (IS) therapy or are deliberately withdrawn due to a research protocol or clinical concern (ie, lymphoproliferative disorder [LPD], overwhelming infection). This is understandable because maintenance IS therapy, particularly calcineurin inhibitors (CNI), is associated with significant cost, side effects, and considerable long-term morbidity and mortality. Detrimental effects of IS therapy include increased risk of cardiovascular disease, metabolic syndrome, bone loss, opportunistic and community-acquired infections, and malignancy. In fact, LT recipients have among the highest rates of chronic kidney disease and associated mortality among all nonkidney solid organ recipients. This mortality is only ameliorated by undergoing a curative kidney transplant, usurping costs and valuable organ resources. The search for improved treatment algorithms includes trial and error CNI dose minimization, the use of alternative IS agents (antimetabolites, mammalian target of rapamycin [mTOR] inhibitors), or even complete CNI withdrawal. Yet those who are successful in achieving such operational tolerance (no immunosuppression and normal allograft function) are considered lucky. The vast majority of recipients will fail this approach, develop acute rejection or immune-mediated hepatitis, and require resumption of IS therapy. As such, withdrawal of IS following LT is not standard-of-care, leaving clinicians to currently maintain transplant patients on IS therapy for life. Nonetheless, the long-term complications of all IS therapies highlight the need for strategies to promote immunologic or operational tolerance. Clinically applicable biomarker assays signifying the potential for tolerance as well as tolerogenic IS conditioning are invariably needed if systematic, controlled rather than "hit or miss" approaches to withdrawal are considered. This review will provide an overview of the basic mechanisms of tolerance, particularly in relation to LT, data from previous IS withdrawal protocols and biomarker studies in tolerant recipients, and a discussion on the prospect of increasing the clinical feasibility and success of withdrawal.

TIM-3: a novel regulatory molecule of alloimmune activation

T cell Ig domain and mucin domain (TIM)-3 has previously been established as a central regulator of Th1 responses and immune tolerance. In this study, we examined its functions in allograft rejection in a murine model of vascularized cardiac transplantation. TIM-3 was constitutively expressed on dendritic cells and natural regulatory T cells (Tregs) but only detected on CD4(+)FoxP3(-) and CD8(+) T cells in acutely rejecting graft recipients. A blocking anti-TIM-3 mAb accelerated allograft rejection only in the presence of host CD4(+) T cells. Accelerated rejection was accompanied by increased frequencies of alloreactive IFN-γ-, IL-6-, and IL-17-producing splenocytes, enhanced CD8(+) cytotoxicity against alloantigen, increased alloantibody production, and a decline in peripheral and intragraft Treg/effector T cell ratio. Enhanced IL-6 production by CD4(+) T cells after TIM-3 blockade plays a central role in acceleration of rejection. Using an established alloreactivity TCR transgenic model, blockade of TIM-3 increased allospecific effector T cells, enhanced Th1 and Th17 polarization, and resulted in a decreased frequency of overall number of allospecific Tregs. The latter is due to inhibition in induction of adaptive Tregs rather than prevention of expansion of allospecific natural Tregs. In vitro, targeting TIM-3 did not inhibit nTreg-mediated suppression of Th1 alloreactive cells but increased IL-17 production by effector T cells. In summary, TIM-3 is a key regulatory molecule of alloimmunity through its ability to broadly modulate CD4(+) T cell differentiation, thus recalibrating the effector and regulatory arms of the alloimmune response.