Alternatives to calcineurin inhibition in renal transplantation: belatacept, the first co-stimulation blocker

PD-L1 overexpression conveys tolerance of mesenchymal stem cell-derived cardiomyocyte-like cells in an allogeneic mouse model

Although the autologously transplanted cells are immunologically durable, allogeneic cell transplantation is inevitable in a series of cases. Mesenchymal stem cells (MSCs) are one of the suitable candidates for cardiac tissue regeneration that have been shown to acquire immunogenicity concurrent with cardiomyogenic differentiation. The present study aimed to exploit PD-L1, as a key immunomodulatory checkpoint ligand to protect the MSCs-derived cardiomyocyte-like cells (CLCs) against the detrimental alloimmunity. Mouse bone marrow-derived MSCs were stably transduced to overexpress PD-L1. MSCs were in vitro differentiated into CLCs and the expressions of immunologic molecules were compared between MSCs and CLCs. The in vitro and in vivo allogeneic immune responses were also examined. The differentiated CLCs had higher expressions of MHC-I and CD80. Upon in vitro coculture with allogeneic splenocytes, CLCs caused more CD4+ and CD8+ T cell activation, lymphocyte proliferation, and interferon-γ (IFN-γ) release in comparison to MSCs. PD-L1 overexpression on CLCs decreased the activation of CD8+ T cells, proliferation of lymphocytes, and release of IFN-γ. The PD-L1-overexpressing CLCs elicited lower in vivo CD4+ and CD8+ T cell activation and reduced the anti-donor antibody response accompanied by increased durability and reduced T cell infiltration. The present study verified the potential of PD-L1 overexpression as a preparative strategy for the protection of allogeneic MSCs-derived CLCs against the detrimental alloreaction.

An update on potentials and promises of T cell co-signaling molecules in transplantation

The promising outcomes of immune-checkpoint based immunotherapies in cancer have provided a proportional perspective ahead of exploiting similar approaches in allotransplantation. Belatacept (CTLA-4-Ig) is an example of costimulation blockers successfully exploited in renal transplantation. Due to the wide range of regulatory molecules characterized in the past decades, some of these molecules might be candidates as immunomodulators in the case of tolerance induction in transplantation. Although there are numerous attempts on the apprehension of the effects of co-signaling molecules on immune response, the necessity for a better understanding is evident. By increasing the knowledge on the biology of co-signaling pathways, some pitfalls are recognized and improved approaches are proposed. The blockage of CD80/CD28 axis is an instance of evolution toward more efficacy. It is now evident that anti-CD28 antibodies are more effective than CD80 blockers in animal models of transplantation. Other co-signaling axes such as PD-1/PD-L1, CD40/CD154, 2B4/CD48, and others discussed in the present review are examples of critical immunomodulatory molecules in allogeneic transplantation. We review here the outcomes of recent experiences with co-signaling molecules in preclinical studies of solid organ transplantation.

B cells in operational tolerance

Transplantation is currently the therapy of choice for endstage organ failure even though it requires long-term immunosuppresive therapy, with its numerous side effects, for acceptance of the transplanted organ. In rare cases however, patients develop operational tolerance, that is, graft survival without immunosuppression. Studies conducted on these patients reveal genetic, phenotypic, and functional signatures. They provide a better understanding of the immunological mechanisms involved in operational tolerance and define biomarkers that could be used to adapt immunosuppressive treatment to the individual, safely reduce immunosuppression doses, and ideally and safely guide immunosuppression withdrawal. This review summarizes studies that suggest a role for B cells as biomarkers of operational tolerance and discusses the use of B cells as a predictive tool for immunologic risk.

Maintenance immunosuppressants in the management of antibody-mediated renal allograft rejection: which regimen is best?

Antibody-mediated rejection (AMR) is a pivotal cause of long-term graft failure following renal transplantation. De novo donor-specific antibody reduction is essential to prevent AMR and improve long-term graft survival in renal transplant recipients. Although the number of early AMR episodes can be successfully controlled by attenuating de novo donor-specific antibodies, the long-term outcomes are unsatisfactory. Numerous studies have focused on new strategies to reverse AMR, but the available evidence suggests that maintenance immunosuppressive agents play important roles. This article reviews data on the use of various maintenance immunosuppressive strategies in the management of AMR, with a focus on antibody-mediated kidney transplant rejection. Its aim is to help provide options benefitting long-term graft survival in renal transplant recipients.

Selective CD28 Antagonist Blunts Memory Immune Responses and Promotes Long-Term Control of Skin Inflammation in Nonhuman Primates

Novel therapies that specifically target activation and expansion of pathogenic immune cell subsets responsible for autoimmune attacks are needed to confer long-term remission. Pathogenic cells in autoimmunity include memory T lymphocytes that are long-lived and present rapid recall effector functions with reduced activation requirements. Whereas the CD28 costimulation pathway predominantly controls priming of naive T cells and hence generation of adaptive memory cells, the roles of CD28 costimulation on established memory T lymphocytes and the recall of memory responses remain controversial. In contrast to CD80/86 antagonists (CTLA4-Ig), selective CD28 antagonists blunt T cell costimulation while sparing CTLA-4 and PD-L1-dependent coinhibitory signals. Using a new selective CD28 antagonist, we showed that Ag-specific reactivation of human memory T lymphocytes was prevented. Selective CD28 blockade controlled both cellular and humoral memory recall in nonhuman primates and induced long-term Ag-specific unresponsiveness in a memory T cell-mediated inflammatory skin model. No modification of memory T lymphocytes subsets or numbers was observed in the periphery, and importantly no significant reactivation of quiescent viruses was noticed. These findings indicate that pathogenic memory T cell responses are controlled by both CD28 and CTLA-4/PD-L1 cosignals in vivo and that selectively targeting CD28 would help to promote remission of autoimmune diseases and control chronic inflammation.

Tolerant Kidney Transplant Patients Produce B Cells with Regulatory Properties

Whereas a B cell-transcriptional profile has been recorded for operationally tolerant kidney graft patients, the role that B cells have in this tolerance has not been reported. In this study, we analyzed the role of B cells from operationally tolerant patients, healthy volunteers, and kidney transplant recipients with stable graft function on T cell suppression. Proliferation, apoptosis, and type I proinflammatory cytokine production by effector CD4(+)CD25(-) T cells were measured after anti-CD3/anti-CD28 stimulation with or without autologous B cells. We report that B cells inhibit CD4(+)CD25(-) effector T cell response in a dose-dependent manner. This effect required B cells to interact with T-cell targets and was achieved through a granzyme B (GzmB)-dependent pathway. Tolerant recipients harbored a higher number of B cells expressing GzmB and displaying a plasma cell phenotype. Finally, GzmB(+) B-cell number was dependent on IL-21 production, and B cells from tolerant recipients but not from other patients positively regulated both the number of IL-21(+) T cells and IL-21 production, suggesting a feedback loop in tolerant recipients that increases excessive B cell activation and allows regulation to take place. These data provide insights into the characterization of B cell-mediated immunoregulation in clinical tolerance and show a potential regulatory effect of B cells on effector T cells in blood from patients with operationally tolerant kidney grafts.

Targeting the CD80/CD86 costimulatory pathway with CTLA4-Ig directs microglia toward a repair phenotype and promotes axonal outgrowth

Among the costimulatory factors widely studied in the immune system is the CD28/cytotoxic T-lymphocyte antigen-4 (CTLA4)-CD80/CD86 pathway, which critically controls the nature and duration of the T-cell response. In the brain, up-regulated expression of CD80/CD86 during inflammation has consistently been reported in microglia. However, the role of CD80/CD86 molecules has mainly been studied in a context of microglia-T cell interactions in pathological conditions, while the function of CD80/CD86 in the regulation of intrinsic brain cells remains largely unknown. In this study, we used a transgenic pig line in which neurons express releasable CTLA4-Ig, a synthetic molecule mimicking CTLA4 and binding to CD80/CD86. The effects of CTLA4-Ig on brain cells were analyzed after intracerebral transplantation of CTLA4-Ig-expressing neurons or wild-type neurons as control. This model provided in vivo evidence that CTLA4-Ig stimulated axonal outgrowth, in correlation with a shift of the nearby microglia from a compact to a ramified morphology. In a culture system, we found that the CTLA4-Ig-induced morphological change of microglia was mediated through CD86, but not CD80. This was accompanied by microglial up-regulated expression of the anti-inflammatory molecule Arginase 1 and the neurotrophic factor BDNF, in an astrocyte-dependent manner through the purinergic P2Y1 receptor pathway. Our study identifies for the first time CD86 as a key player in the modulation of microglia phenotype and suggests that CTLA4-Ig-derived compounds might represent new tools to manipulate CNS microglia.

Anti-CD28 Antibody-Initiated Cytokine Storm in Canines

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CD28 signal blockade after T cell receptor activation is under intense investigation as a tolerance-inducing therapy for transplantation. Our goal is to produce a CD28-specific reagent as a therapy for the prevention of graft rejection and graft-versus-host disease in the canine model of allogeneic hematopoietic cell transplantation.

FR104, an antagonist anti-CD28 monovalent fab' antibody, prevents alloimmunization and allows calcineurin inhibitor minimization in nonhuman primate renal allograft

Selective targeting of CD28 might represent an effective immunomodulation strategy by preventing T cell costimulation, while favoring coinhibition since inhibitory signals transmitted through CTLA-4; PD-L1 and B7 would not be affected. We previously showed in vitro and in vivo that anti-CD28 antagonists suppress effector T cells while enhancing regulatory T cell (Treg) suppression and immune tolerance. Here, we evaluate FR104, a novel antagonist pegylated anti-CD28 Fab' antibody fragment, in nonhuman primate renal allotransplantation. FR104, in association with low doses of tacrolimus or with rapamycin in a steroid-free therapy, prevents acute rejection and alloantibody development and prolongs allograft survival. However, when FR104 was associated with mycophenolate mofetil and steroids, half of the recipients rejected their grafts prematurely. Finally, we observed an accumulation of Helios-negative Tregs in the blood and within the graft after FR104 therapy, confirmed by Treg-specific demethylated region DNA analysis. In conclusion, FR104 reinforces immunosuppression in calcineurin inhibitor (CNI)-low or CNI-free protocols, without the need of steroids. Accumulation of intragraft Tregs suggested the promotion of immunoregulatory mechanisms. Selective CD28 antagonists might become an alternative CNI-sparing strategy to B7 antagonists for kidney transplant recipients.

Use of CTLA4Ig for induction of mixed chimerism and renal allograft tolerance in nonhuman primates

We have previously reported successful induction of renal allograft tolerance via a mixed chimerism approach in nonhuman primates. In those studies, we found that costimulatory blockade with anti-CD154 mAb was an effective adjunctive therapy for induction of renal allograft tolerance. However, since anti-CD154 mAb is not clinically available, we have evaluated CTLA4Ig as an alternative agent for effecting costimulation blockade in this treatment protocol. Two CTLA4Igs, abatacept and belatacept, were substituted for anti-CD154 mAb in the conditioning regimen (low dose total body irradiation, thymic irradiation, anti-thymocyte globulin and a 1-month posttransplant course of cyclosporine [CyA]). Three recipients treated with the abatacept regimen failed to develop comparable lymphoid chimerism to that achieved with anti-CD154 mAb treatment and these recipients rejected their kidney allografts early. With the belatacept regimen, four of five recipients developed chimerism and three of these achieved long-term renal allograft survival (>861, >796 and >378 days) without maintenance immunosuppression. Neither chimerism nor long-term allograft survival were achieved in two recipients treated with the belatacept regimen but with a lower, subtherapeutic dose of CyA. This study indicates that CD28/B7 blockade with belatacept can provide a clinically applicable alternative to anti-CD154 mAb for promoting chimerism and renal allograft tolerance.

hCTLA4-Ig transgene expression in keratocytes modulates rejection of corneal xenografts in a pig to non-human primate anterior lamellar keratoplasty model

BACKGROUND

Human corneal allografting is an established procedure to cure corneal blindness. However, a shortage of human donor corneas as well as compounding economic, cultural, and organizational reasons in many countries limit its widespread use. Artificial corneas as well as porcine corneal xenografts have been considered as possible alternatives. To date, all preclinical studies using de-cellularized pig corneas have shown encouraging graft survival results; however, relatively few studies have been conducted in pig to non-human primate (NHP) models, and particularly using genetically engineered donors.

METHODS

In this study, we assessed the potential benefit of using either hCTLA4-Ig transgenic or α1,3-Galactosyl Transferase (GT) Knock-Out (KO) plus transgenic hCD39/hCD55/hCD59/fucosyl-transferase pig lines in an anterior lamellar keratoplasty pig to NHP model.

RESULTS

Corneas from transgenic animals expressing hCTLA4-Ig under the transcriptional control of a neuron-specific enolase promoter showed transgene expression in corneal keratocytes of the stroma and expression was maintained after transplantation. Although a first acute rejection episode occurred in all animals during the second week post-keratoplasty, the median final rejection time was 70 days in the hCTLA4-Ig group vs. 21 days in the wild-type (WT) control group. In contrast, no benefit for corneal xenograft survival from the GTKO/transgenic pig line was found. At rejection, cell infiltration in hCTLA4Ig transgenic grafts was mainly composed of macrophages with fewer CD3+ CD4+ and CD79+ cells than in other types of grafts. Anti-donor xenoantibodies increased dramatically between days 9 and 14 post-surgery in all animals.

CONCLUSIONS

Local expression of the hCTLA4-Ig transgene dampens rejection of xenogeneic corneal grafts in this pig-to-NHP lamellar keratoplasty model. The hCTLA4-Ig transgene seems to target T-cell responses without impacting humoral responses, the control of which would presumably require additional peripheral immunosuppression.

Unique B cell differentiation profile in tolerant kidney transplant patients

Operationally tolerant patients (TOL) display a higher number of blood B cells and transcriptional B cell signature. As they rarely develop an allo-immune response, they could display an abnormal B cell differentiation. We used an in vitro culture system to explore T-dependent differentiation of B cells into plasma cells. B cell phenotype, apoptosis, proliferation, cytokine, immunoglobulin production and markers of differentiation were followed in blood of these patients. Tolerant recipients show a higher frequency of CD20(+) CD24(hi) CD38(hi) transitional and CD20(+) CD38(lo) CD24(lo) naïve B cells compared to patients with stable graft function, correlating with a decreased frequency of CD20(-) CD38(+) CD138(+) differentiated plasma cells, suggestive of abnormal B cell differentiation. B cells from TOL proliferate normally but produce more IL-10. In addition, B cells from tolerant recipients exhibit a defective expression of factors of the end step of differentiation into plasma cells and show a higher propensity for cell death apoptosis compared to patients with stable graft function. This in vitro profile is consistent with down-regulation of B cell differentiation genes and anti-apoptotic B cell genes in these patients in vivo. These data suggest that a balance between B cells producing IL-10 and a deficiency in plasma cells may encourage an environment favorable to the tolerance maintenance.

Targeting CD28, CTLA-4 and PD-L1 costimulation differentially controls immune synapses and function of human regulatory and conventional T-cells

CD28, CTLA-4 and PD-L1, the three identified ligands for CD80/86, are pivotal positive and negative costimulatory molecules that, among other functions, control T cell motility and formation of immune synapse between T cells and antigen-presenting cells (APCs). What remains incompletely understood is how CD28 leads to the activation of effector T cells (Teff) but inhibition of suppression by regulatory T cells (Tregs), while CTLA-4 and PD-L1 inhibit Teff function but are crucial for the suppressive function of Tregs. Using alloreactive human T cells and blocking antibodies, we show here by live cell dynamic microscopy that CD28, CTLA-4, and PD-L1 differentially control velocity, motility and immune synapse formation in activated Teff versus Tregs. Selectively antagonizing CD28 costimulation increased Treg dwell time with APCs and induced calcium mobilization which translated in increased Treg suppressive activity, in contrast with the dampening effect on Teff responses. The increase in Treg suppressive activity after CD28 blockade was also confirmed with polyclonal Tregs. Whereas CTLA-4 played a critical role in Teff by reversing TCR-induced STOP signals, it failed to affect motility in Tregs but was essential for formation of the Treg immune synapse. Furthermore, we identified a novel role for PD-L1-CD80 interactions in suppressing motility specifically in Tregs. Thus, our findings reveal that the three identified ligands of CD80/86, CD28, CTLA-4 and PD-L1, differentially control immune synapse formation and function of the human Teff and Treg cells analyzed here. Individually targeting CD28, CTLA-4 and PD-L1 might therefore represent a valuable therapeutic strategy to treat immune disorders where effector and regulatory T cell functions need to be differentially targeted.

Antagonist properties of monoclonal antibodies targeting human CD28: role of valency and the heavy-chain constant domain

Antagonist antibodies targeting CD28 have been proposed as an alternative to the use of CD80/86 antagonists to modulate T cell responses in autoimmunity and transplantation. Advantages would be the blockade of CD28-mediated co-stimulatory signals without impeding the co-inhibitory signals dependent on CD80 interactions with CTLA-4 and PD-L1 that are important for the control of immune responses and for the function of regulatory T cells. Anti-CD28 antibodies are candidate antagonists only if they prevent access to the CD80/86 ligands without simultaneously stimulating CD28 itself, a process that is believed to depend on receptor multimerization. In this study, we evaluated the impact of different formats of a potentially antagonist anti-human CD28 antibody on T cell activation. In particular, we examined the role of valency and of the presence of an Fc domain, two components that might affect receptor multimerization either directly or in the presence of accessory cells expressing Fc receptors. Among monovalent (Fab', scFv), divalent (Fab'2), monovalent-Fc (Fv-Fc) and divalent-Fc (IgG) formats, only the monovalent formats showed consistent absence of induced CD28 multimerization and absence of associated activation of phosphoinositol-3-kinase, and clear antagonist properties in T cell stimulation assays. In contrast, divalent antibodies showed agonist properties that resulted in cell proliferation and cytokine release in an Fc-independent manner. Conjugation of monovalent antibodies with polyethylene glycol, α-1-antitrypsin or an Fc domain significantly extended their in vivo half-life without modifying their antagonist properties. In conclusion, these data indicate that monovalency is mandatory for maintaining the antagonistic activity of anti-CD28 monoclonal antibodies.

Belatacept utilization recommendations: an expert position

INTRODUCTION

There is a continuing need for an immunosuppressive therapy that offers a high benefit-risk profile for renal transplant recipients, supporting long-term patient and graft survival while minimizing cumulative nephrotoxicity and other side effects. Belatacept , the first biological agent developed for primary maintenance immunosuppression, was recently approved for use in Europe. Belatacept combined with corticosteroids and a mycophenolic acid is indicated for prophylaxis of graft rejection in adults receiving renal transplant. Its use is contraindicated in Epstein-Barr virus seronegative or serostatus unknown patients due to increased risk of developing posttransplant lymphoproliferative disorder.

AREAS COVERED

This review provides practical recommendations for the use of belatacept, based on safety and efficacy data from Phase II and Phase III clinical trials in de novo kidney transplant recipients.

EXPERT OPINION

Treatment with belatacept is associated with improved long-term graft function, making belatacept an important option for prevention of kidney allograft rejection. Furthermore, efficacy and safety data over several years of therapy suggest that belatacept is particularly suitable for long-term immunosuppression, and the selective targeting offered by belatacept may help avoid some of the non-specific chronic safety risks associated with calcineurin inhibitors and steroids. Future studies will clarify the optimal regimen for belatacept usage.

Preclinical efficacy and immunological safety of FR104, an antagonist anti-CD28 monovalent Fab' antibody

Antagonist anti-CD28 antibodies prevent T cell costimulation and differentiate from CTLA4Ig since they cannot block CTLA-4 and PDL-1 coinhibitory signals. They demonstrated efficacy in suppressing effector T cells while enhancing regulatory T cells function and immune tolerance. However, anti-CD28 antibodies devoid of immunotoxicity and with a good pharmacokinetic profile have not yet been developed. Here, we describe FR104, a novel humanized pegylated anti-CD28 Fab' antibody fragment presenting a long elimination half-life in monkeys. In vitro, FR104 failed to induce human T cell proliferation and cytokines secretion, even in the presence of anti-CD3 antibodies or when cross-linked with secondary antibodies. Furthermore, in humanized NOD/SCID mice adoptively transferred with human PBMC, whereas superagonist and divalent antibodies elicited rapid cytokines secretion and human T cell activation, FR104 did not. These humanized mice developed a florid graft-versus-host disease, which was prevented by administration of FR104 in a CTLA4-dependent manner. Interestingly, administration of high doses of CTLA4-Ig was ineffective to prevent GVHD, whereas administration of low doses was partially effective. In conclusion, we demonstrated that FR104 is devoid of agonist activity on human T cells and thus compatible with a clinical development that might lead to higher therapeutic indexes, by sparing CTLA-4, as compared to CD80/CD86 antagonists.

OX40 and CD30 signals in CD4(+) T-cell effector and memory function: a distinct role for lymphoid tissue inducer cells in maintaining CD4(+) T-cell memory but not effector function

CD4(+) effector and memory T cells play a pivotal role in the development of both normal and pathogenic immune responses. This review focuses on the molecular and cellular mechanisms that regulate their development, with particular focus on the tumor necrosis factor superfamily members OX40 (TNFRSF4) and CD30 (TNFRSF8). We discuss the evidence that in mice, these molecular signaling pathways act synergistically to regulate the development of both effector and memory CD4(+) T cells but that the cells that regulate memory versus effector function are distinct, effectively allowing the independent regulation of the memory and effector CD4(+) T-cell pools.

Lymphoid tissue inducer cells: innate cells critical for CD4+ T cell memory responses?

Lymphoid tissue inducer cells (LTi) are a relatively new arrival on the immunological cellular landscape, having first been characterized properly only 15 years ago. They are members of an emerging family of innate lymphoid cells (ILCs). Elucidation of their function reveals links not only with the ancient innate immune system, but also with adaptive immune responses, in particular the development of lymph nodes and CD4(+) T cell memory immune responses, which on one hand underpin the success of vaccination strategies, and on the other hand drive many human immunologically mediated diseases. This perspective article is not an exhaustive account of the role of LTi in the development of lymphoid tissues, as there have been many excellent reviews published already. Instead, we combine current knowledge of genetic phylogeny and comparative immunology, together with classical mouse genetics, to suggest how LTi might have evolved from a primitive lymphocytic innate cell in the ancestral 500-million-year-old vertebrate immune system into a cell critical for adaptive CD4(+) T cell immune responses in mammals.