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

T cell responsiveness to IL-10 defines the immunomodulatory effect of costimulation blockade via anti-CD154 and impacts transplant survival

Costimulation blockade (CoB)-based immunotherapy is a promising alternative to immunosuppression for transplant recipients; however, the current limited understanding of the factors that impact its efficacy restrains its clinical applicability. In this context, pro- and anti-inflammatory cytokines are being recognized as having an impact on T cell activation beyond effector differentiation. This study aims at elucidating the impact of direct IL-10 signaling in T cells on CoB outcomes. We used a full-mismatch skin transplantation model where recipients had a T cell-restricted expression of a dominant negative IL-10 receptor (10R-DN), alongside anti-CD154 as CoB therapy. Unlike wild-type recipients, 10R-DN mice failed to benefit from CoB. This accelerated graft rejection correlated with increased accumulation of T cells producing TNF-α, IFN-γ, and IL-17. In vitro experiments indicated that while lack of IL-10 signaling did not change the ability of anti-CD154 to modulate alloreactive T cell proliferation, the absence of this pathway heightened TH1 effector cell differentiation. Furthermore, deficiency of IL-10 signaling in T cells impaired Treg induction, a hallmark of anti-CD154 therapy. Overall, these findings unveil an important and novel role of IL-10 signaling in T cells that defines the success of CoB therapies and identifies a target pathway for obtaining robust immunoregulation.

Ruxolitinib attenuates acute rejection and can serve as an immune induction therapy in heart transplantation

The benefits of IL2RA antagonists in heart transplant patients are controversial. We aimed to elucidate the effects of IL2RA antagonists and identify targets that could be better than IL2RA antagonists. By using single-cell RNA sequencing of immune cells at different time points in patients receiving IL2RA antagonists, we identified nineteen types of cells. We revealed higher IL2RA expression in regulatory T cells (Tregs), suggesting that IL2RA antagonists attenuated IL-2-induced Treg activation. CD4_C04_IFNGR1 and CD8_C05_IFITM2 which had more cytotoxic effects, remained elevated at later time points. IFNGR1 was upregulated in these two subtypes, but was not expressed in Treg. Ruxolitinib targeted the pathways of IFNGR1 (JAK1/2) while not affecting the pathway of IL-2-induced Tregs activation (JAK3). Ruxolitinib showed prolonged survival compared to IL2RA mAb-treated mice. Our study provided dynamic changes of immune cells after IL2RA antagonists treatment at single-cell resolution. Ruxolitinib has potential as a new immunoinduction therapy without affecting Treg.

Costimulation Blockade in Vascularized Composite Allotransplantation

Vascular composite allotransplantation (VCA) is a field under research and has emerged as an alternative option for the repair of severe disfiguring defects that result from infections or traumatic amputation in a selected group of patients. VCA is performed in centers with appropriate expertise, experience and adequate resources to effectively manage the complexity and complications of this treatment. Lifelong immunosuppressive therapy, immunosuppression associated complications, and the effects of the host immune response in the graft are major concerns in VCA. VCA is considered a quality of life transplant and the risk-benefit ratio is dissimilar to life saving transplants. Belatacept seems a promising drug that prolongs patient and graft survival in kidney transplantation and it could also be an alternative approach to VCA immunosuppression. In this review, we are summarizing current literature about the role of costimulation blockade, with a focus on belatacept in VCA.

Multiphase Assembly of Small Molecule Microcrystalline Peptide Hydrogel Allows Immunomodulatory Combination Therapy for Long-Term Heart Transplant Survival

Combination therapies that target multiple pathways involved in immune rejection of transplants hold promise for patients in need of restorative surgery. Herein, a noninteracting multiphase molecular assembly approach is developed to crystallize tofacitinib, a potent JAK1/3 inhibitor, within a shear-thinning self-assembled fibrillar peptide hydrogel network. The resulting microcrystalline tofacitinib hydrogel (MTH) can be syringe-injected directly to the grafting site during surgery to locally deliver the small molecule. The rate of drug delivered from MTH is largely controlled by the dissolution of the encapsulated microcrystals. A single application of MTH, in combination with systemically delivered CTLA4-Ig, a co-stimulation inhibitor, affords significant graft survival in mice receiving heterotopic heart transplants. Locoregional studies indicate that the local delivery of tofacitinib at the graft site enabled by MTH is required for the observed enhanced graft survival.

Vascularized composite allotransplantation combined with costimulation blockade induces mixed chimerism and reveals intrinsic tolerogenic potential

Vascularized composite allotransplantation (VCA) has become a valid therapeutic option to restore form and function after devastating tissue loss. However, the need for high-dose multidrug immunosuppression to maintain allograft survival is still hampering more widespread application of VCA. In this study, we investigated the immunoregulatory potential of costimulation blockade (CoB; CTLA4-Ig and anti-CD154 mAb) combined with nonmyeoablative total body irradiation (TBI) to promote allograft survival of VCA in a fully MHC-mismatched mouse model of orthotopic hind limb transplantation. Compared with untreated controls (median survival time [MST] 8 days) and CTLA4-Ig treatment alone (MST 17 days), CoB treatment increased graft survival (MST 82 days), and the addition of nonmyeloablative TBI led to indefinite graft survival (MST > 210 days). Our analysis suggests that VCA-derived BM induced mixed chimerism in animals treated with CoB and TBI + CoB, promoting gradual deletion of alloreactive T cells as the underlying mechanism of long-term allograft survival. Acceptance of donor-matched secondary skin grafts, decreased ex vivo T cell responsiveness, and increased graft-infiltrating Tregs further indicated donor-specific tolerance induced by TBI + CoB. In summary, our data suggest that vascularized BM-containing VCAs are immunologically favorable grafts promoting chimerism induction and long-term allograft survival in the context of CoB.

Schistosomal extracellular vesicle-enclosed miRNAs modulate host T helper cell differentiation

During the chronic stage of Schistosoma infection, the female lays fertile eggs, triggering a strong anti-parasitic type 2 helper T-cell (Th2) immune response. It is unclear how this Th2 response gradually declines even though the worms live for years and continue to produce eggs. Here, we show that Schistosoma mansoni downregulates Th2 differentiation in an antigen-presenting cell-independent manner, by modulating the Th2-specific transcriptional program. Adult schistosomes secrete miRNA-harboring extracellular vesicles that are internalized by Th cells in vitro. Schistosomal miRNAs are found also in T helper cells isolated from Peyer's patches and mesenteric lymph nodes of infected mice. In T helper cells, the schistosomal miR-10 targets MAP3K7 and consequently downmodulates NF-κB activity, a critical transcription factor for Th2 differentiation and function. Our results explain, at least partially, how schistosomes tune down the Th2 response, and provide further insight into the reciprocal geographic distribution between high prevalence of parasitic infections and immune disorders such as allergy. Furthermore, this worm-host crosstalk mechanism can be harnessed to develop diagnostic and therapeutic approaches for human schistosomiasis and Th2-associated diseases.

CTLA4-Ig in combination with FTY720 promotes allograft survival in sensitized recipients

Despite recent evidence of improved graft outcomes and safety, the high incidence of early acute cellular rejection with belatacept, a high-affinity CTLA4-Ig, has limited its use in clinical transplantation. Here we define how the incomplete control of endogenous donor-reactive memory T cells results in belatacept-resistant rejection in an experimental model of BALB/c.2W-OVA donor heart transplantation into C57BL/6 recipients presensitized to donor splenocytes. These sensitized mice harbored modestly elevated numbers of endogenous donor-specific memory T cells and alloantibodies compared with naive recipients. Continuous CTLA4-Ig treatment was unexpectedly efficacious at inhibiting endogenous graft-reactive T cell expansion but was unable to inhibit late CD4+ and CD8+ T cell infiltration into the allografts, and rejection was observed in 50% of recipients by day 35 after transplantation. When CTLA4-Ig was combined with the sphingosine 1-phosphate receptor-1 (S1PR1) functional antagonist FTY720, alloantibody production was inhibited and donor-specific IFN-γ-producing T cells were reduced to levels approaching nonsensitized tolerant recipients. Late T cell recruitment into the graft was also restrained, and graft survival improved with this combination therapy. These observations suggest that a rational strategy consisting of inhibiting memory T cell expansion and trafficking into the allograft with CTLA4-Ig and FTY720 can promote allograft survival in allosensitized recipients.

Combining Theoretical and Experimental Techniques to Study Murine Heart Transplant Rejection

The quality of life of organ transplant recipients is compromised by complications associated with life-long immunosuppression, such as hypertension, diabetes, opportunistic infections, and cancer. Moreover, the absence of established tolerance to the transplanted tissues causes limited long-term graft survival rates. Thus, there is a great medical need to understand the complex immune system interactions that lead to transplant rejection so that novel and effective strategies of intervention that redirect the system toward transplant acceptance (while preserving overall immune competence) can be identified. This study implements a systems biology approach in which an experimentally based mathematical model is used to predict how alterations in the immune response influence the rejection of mouse heart transplants. Five stages of conventional mouse heart transplantation are modeled using a system of 13 ordinary differential equations that tracks populations of both innate and adaptive immunity as well as proxies for pro- and anti-inflammatory factors within the graft and a representative draining lymph node. The model correctly reproduces known experimental outcomes, such as indefinite survival of the graft in the absence of CD4⁺ T cells and quick rejection in the absence of CD8⁺ T cells. The model predicts that decreasing the translocation rate of effector cells from the lymph node to the graft delays transplant rejection. Increasing the starting number of quiescent regulatory T cells in the model yields a significant but somewhat limited protective effect on graft survival. Surprisingly, the model shows that a delayed appearance of alloreactive T cells has an impact on graft survival that does not correlate linearly with the time delay. This computational model represents one of the first comprehensive approaches toward simulating the many interacting components of the immune system. Despite some limitations, the model provides important suggestions of experimental investigations that could improve the understanding of rejection. Overall, the systems biology approach used here is a first step in predicting treatments and interventions that can induce transplant tolerance while preserving the capacity of the immune system to protect against legitimate pathogens.

Delayed Cytotoxic T Lymphocyte-Associated Protein 4-Immunoglobulin Treatment Reverses Ongoing Alloantibody Responses and Rescues Allografts From Acute Rejection

Antibody-mediated rejection has emerged as the leading cause of late graft loss in kidney transplant recipients, and inhibition of donor-specific antibody production should lead to improved transplant outcomes. The fusion protein cytotoxic T lymphocyte-associated protein 4-immunoglobulin (CTLA4-Ig) blocks T cell activation and consequently inhibits T-dependent B cell antibody production, and the current paradigm is that CTLA4-Ig is effective with naïve T cells and less so with activated or memory T cells. In this study, we used a mouse model of allosensitization to investigate the efficacy of continuous CTLA4-Ig treatment, initiated 7 or 14 days after sensitization, for inhibiting ongoing allospecific B cell responses. Delayed treatment with CTLA4-Ig collapsed the allospecific germinal center B cell response and inhibited alloantibody production. Using adoptively transferred T cell receptor transgenic T cells and a novel approach to track endogenous graft-specific T cells, we demonstrate that delayed CTLA4-Ig minimally inhibited graft-specific CD4(+) and T follicular helper responses. Remarkably, delaying CTLA4-Ig until day 6 after transplantation in a fully mismatched heart transplant model inhibited alloantibody production and prevented acute rejection, whereas transferred hyperimmune sera reversed the effects of delayed CTLA4-Ig. Collectively, our studies revealed the unexpected efficacy of CTLA4-Ig for inhibiting ongoing B cell responses even when the graft-specific T cell response was robustly established.

Short-term MyD88 inhibition ameliorates cardiac graft rejection and promotes donor-specific hyporesponsiveness of skin grafts in mice

Recognition of evolutionarily conserved ligands by Toll-like receptors (TLRs) triggers signaling cascades in innate immune cells to amplify adaptive immune responses. Nearly all TLRs require MyD88 to transduce downstream signaling. MyD88 deficiency has been shown to promote the allograft acceptance in mice. However, direct evidence for therapeutic potential of MyD88 inhibitors remains lacking. Herein, we used a MyD88 inhibitor, namely ST2825, to explore its therapeutic potential and mechanisms in fully allogeneic skin and heart transplant models. Phenotypic maturation of dendritic cells stimulated by TLR ligands was alleviated by ST2825 in parallel with reduced T-cell proliferation in vitro. A short-course treatment with ST2825 significantly prolonged cardiac graft survival (mean survival time = 18.5 ± 0.92 days vs. 7.25 ± 0.46 days). ST2825-treated group had significantly reduced proinflammatory cytokines in allografts compared with control group. ST2825 combined with anti-CD154 induced long-term skin allograft acceptance in about one-third of recipients (>100 days). 'Skin-tolerant' recipients showed attenuated donor-specific IFN-γ responses, intact IL-4 responses, and compromised alloantibody responses. We conclude that MyD88 inhibitor ST2825 attenuates acute cardiac rejection and promotes donor-specific hyporesponsiveness in stringent skin transplant models. The direct evidence suggests that pharmacological inhibition of MyD88 hold promising potential for transplant rejection.

Ubiquitous LEA29Y Expression Blocks T Cell Co-Stimulation but Permits Sexual Reproduction in Genetically Modified Pigs

We have successfully established and characterized a genetically modified pig line with ubiquitous expression of LEA29Y, a human CTLA4-Ig derivate. LEA29Y binds human B7.1/CD80 and B7.2/CD86 with high affinity and is thus a potent inhibitor of T cell co-stimulation via this pathway. We have characterized the expression pattern and the biological function of the transgene as well as its impact on the porcine immune system and have evaluated the potential of these transgenic pigs to propagate via assisted breeding methods. The analysis of LEA29Y expression in serum and multiple organs of CAG-LEA transgenic pigs revealed that these animals produce a biologically active transgenic product at a considerable level. They present with an immune system affected by transgene expression, but can be maintained until sexual maturity and propagated by assisted reproduction techniques. Based on previous experience with pancreatic islets expressing LEA29Y, tissues from CAG-LEA29Y transgenic pigs should be protected against rejection by human T cells. Furthermore, their immune-compromised phenotype makes CAG-LEA29Y transgenic pigs an interesting large animal model for testing human cell therapies and will provide an important tool for further clarifying the LEA29Y mode of action.

The podocyte as a direct target for treatment of glomerular disease?

The Centers for Disease Control and Prevention estimates more than 10% of adults in the United States, over 20 million Americans, have chronic kidney disease (CKD). A failure to maintain the glomerular filtration barrier directly contributes to the onset of CKD. The visceral epithelial cells, podocytes, are integral to the maintenance of this renal filtration barrier. Direct podocyte injury contributes to the onset and progression of glomerular diseases such as minimal change disease (MCD), focal segmental glomerular sclerosis (FSGS), diabetic nephropathy, and HIV-associated nephropathy (HIVAN). Since podocytes are terminally differentiated with minimal capacity to self-replicate, they are extremely sensitive to cellular injury. In the past two decades, our understanding of the mechanism(s) by which podocyte injury occurs has greatly expanded. With this newfound knowledge, therapeutic strategies have shifted to identifying targets directed specifically at the podocyte. Although the systemic effects of these agents are important, their direct effect on the podocyte proves to be essential in ameliorating glomerular disease. In this review, we highlight the mechanisms by which these agents directly target the podocyte independent of its systemic effects.

1,25(OH)2D3 Promotes the Efficacy of CD28 Costimulation Blockade by Abatacept

Inhibition of the CD28:CD80/CD86 T cell costimulatory pathway has emerged as an effective strategy for the treatment of T cell–mediated inflammatory diseases. However, patient responses to CD28-ligand blockade by abatacept (CTLA-4-Ig) in conditions such as rheumatoid arthritis are variable and often suboptimal. In this study, we show that the extent to which abatacept suppresses T cell activation is influenced by the strength of TCR stimulation, with high-strength TCR stimulation being associated with relative abatacept insensitivity. Accordingly, cyclosporin A, an inhibitor of T cell stimulation via the TCR, synergized with abatacept to inhibit T cell activation. We also observed that 1,25-dihydroxyvitamin D3 enhanced the inhibition of T cell activation by abatacept, strongly inhibiting T cell activation driven by cross-linked anti-CD3, but with no effect upon anti-CD28 driven stimulation. Thus, like cyclosporin A, 1,25-dihydroxyvitamin D3 inhibits TCR-driven activation, thereby promoting abatacept sensitivity. Vitamin D3 supplementation may therefore be a useful adjunct for the treatment of conditions such as rheumatoid arthritis in combination with abatacept to promote the efficacy of treatment.

Nonimmunologic targets of immunosuppressive agents in podocytes

Proteinuria is a characteristic finding in glomerular diseases and is closely associated with renal outcomes. In addition, therapeutic interventions that reduce proteinuria improve renal prognosis. Accumulating evidence has demonstrated that podocytes act as key modulators of glomerular injury and proteinuria. The podocyte, or glomerular visceral epithelial cell, is a highly specialized and differentiated cell that forms interdigitated foot processes with neighboring podocytes, which are bridged together by an extracellular structure known as the "slit diaphragm" (SD). The SD acts as a size- and charge-selective barrier to plasma protein. Derangement of SD structure or loss of SD-associated protein results in podocyte injury and proteinuria. During the past decades, several immune-modulating agents have been used for the treatment of glomerular diseases and for the reduction of proteinuria. Interestingly, recent studies have demonstrated that immunosuppressive agents can have a direct effect on the SD-associated proteins and stabilize actin cytoskeleton in podocyte and have therefore introduced the concept of nonimmunologic mechanism of renoprotection by immunomodulators. This review focuses on the evidence that immuno-modulating agents directly target podocytes.

Belatacept in kidney transplantation

PURPOSE OF REVIEW

Calcineurin inhibitors (CNIs) play a major role in long-term renal allograft dysfunction because of their nephrotoxic side-effects. Belatacept, a selective costimulation blockade agent, is the first biological agent approved for maintenance immunosuppression in renal transplantation.

RECENT FINDINGS

Studies have shown better preservation of glomerular filtration rate and improved metabolic end points with belatacept when compared with CNIs. More recent studies have shown that belatacept can be an effective first-line immunosuppressive agent with complete avoidance of CNIs and corticosteroids.

SUMMARY

Newer biological agents like belatacept may replace CNIs/corticosteroids in renal transplant recipients, with a benefit of better short-term and long-term renal function, better compliance, and ultimately a possible improvement in long-term renal allograft survival.

Braking bad: novel mechanisms of CTLA-4 inhibition of T cell responses

The coinhibitory receptor cytotoxic T-lymphocyte antigen 4 (CTLA-4) is a master regulator of T cell responses and its function is critical in models of transplant tolerance. The CD28/CTLA-4 pathway is also an important therapeutic target, as the costimulation blocker belatacept was recently approved for use following renal transplantation. While the traditional model of CTLA-4 coinhibition focuses on its ability to directly counteract CD28 costimulation, recently this paradigm has significantly broadened. Recent work has uncovered the ability of CTLA-4 to act as a cell-extrinsic coinhibitory molecule on CD4(+) T cell effectors. While it has been appreciated that CTLA-4 is required for FoxP3(+) regulatory T cell (Treg) suppression, current studies have elucidated important differences in the function of CTLA-4 on Tregs compared to effectors. CTLA-4 expression patterns also differ by T cell subset, with Th17 cells expressing significantly higher levels of CTLA-4. Thus, in contrast to the traditional model of CTLA-4 as a negative receptor to counter CD28 costimulation, recent work has begun to define CTLA-4 as a global regulator of T cell responses with subset-specific functions. Future studies must continue to uncover the molecular mechanisms that govern CTLA-4 function. These novel findings have implications for novel strategies to maximize the regulatory potential of CTLA-4 during allogeneic T cell responses.

Understanding the CD28/CTLA-4 (CD152) pathway and its implications for costimulatory blockade

T cell activation is a key event in the adaptive immune system and vital in the generation of protective cellular and humoral immunity. Activation is required to generate CD4 effector T cell responses and provide help for B cell and cytotoxic T cell responses. While defective T responses to foreign antigen result in infectious pathology, over-reactive T cell responses against self-antigens result in autoimmunity and, in a transplantation setting, tissue rejection. Understanding how T cell activation is normally regulated is critical to therapeutic intervention and the CD28/CTLA-4 (CD152) pathway represents the initial activation checkpoint in molecular terms. In particular, while the CTLA-4 pathway is well established as an essential regulator of self-reactivity, its mechanism of action is still uncertain. Such mechanistic issues are important given its central position in T cell activation and the increasing number of therapeutic modalities aimed at manipulating the CD28/CTLA-4 pathway. Here, we provide an updated view of CTLA-4 biology, reviewing the established features of the system and highlighting its interplay with CD28. We then discuss how recent progress in our understanding of this pathway affects our interpretations following intervention.

Defining the acute kidney injury and repair transcriptome

The mammalian kidney has an intrinsic ability to repair after significant injury. However, this process is inefficient: patients are at high risk for the loss of kidney function in later life. No therapy exists to treat established acute kidney injury (AKI) per se: strategies to promote endogenous repair processes and retard associated fibrosis are a high priority. Whole-organ gene expression profiling has been used to identify repair responses initiated with AKI, and factors that may promote the transition from AKI to chronic kidney disease. Transcriptional profiling has shown molecular markers and potential regulatory pathways of renal repair. Activation of a few key developmental pathways has been reported during repair. Whether these are comparable networks with similar target genes with those in earlier nephrogenesis remains unclear. Altered microRNA profiles, persistent tubular injury responses, and distinct late inflammatory responses highlight continuing kidney pathology. Additional insights into injury and repair processes will be gained by study of the repair transcriptome and cell-specific translatome using high-resolution technologies such as RNA sequencing and translational profiling tailored to specific cellular compartments within the kidney. An enhanced understanding holds promise for both the identification of novel therapeutic targets and biomarker-based evaluation of the damage-repair process.

Co-stimulatory blockade with belatacept in kidney transplantation

Calcineurin inhibitors (CNIs) are the cornerstone of immunosuppression after transplantation but can exert negative effects on chronic allograft function and metabolic profile. Belatacept, a selective co-stimulation blocker of T cells, is the first biologic agent approved for maintenance immunosuppression in kidney transplantation. Studies reveal better preservation of glomerular filtration rate and improved metabolic end points with belatacept when compared to CNIs. Increased incidence of acute rejection is noted with belatacept but overall graft survival looked similar at 5 years. Risk for posttransplant lymphoproliferative disorder is higher in Epstein-Barr virus-seronegative recipients of belatacept. Belatacept seems to be a promising drug, and further experience with its use will define its future role.