Costimulatory pathways in transplantation: challenges and new developments

Monocytes as an early risk factor for acute graft-versus-host disease after allogeneic hematopoietic stem cell transplantation

Acute graft-versus-host disease (aGVHD) is a major complication after allogeneic hematopoietic stem cell transplantation (allo-HSCT) and contributes to high morbidity and mortality. However, our current understanding of the development and progression of aGVHD after allo-HSCT remains limited. To identify the potential biomarkers for the prevention and treatment of aGVHD during the early hematopoietic reconstruction after transplantation, we meticulously performed a comparative analysis of single-cell RNA sequencing data from post-transplant patients with or without aGVHD. Prior to the onset of aGVHD, monocytes in the peripheral blood of patients with aGVHD experienced a dramatic rise and activation on day 21 post-transplantation. This phenomenon is closely aligned with clinical cohort results obtained from blood routine examinations. Furthermore, in vitro co-culture experiments showed that peripheral blood monocytes extracted from patients with aGVHD approximately 21 days post-transplantation induced a significantly higher proliferation rate of allogeneic T cells compared to those from patients without aGVHD. Our study indicates that monocytes could be a crucial early clinical risk factor for the development of aGVHD, and this insight could potentially guide the timing of monitoring efforts, recommending assessments at the pivotal juncture of approximately day 21 post-transplantation, shedding fresh light on the significance of early hematopoietic regeneration in relation to the onset of aGVHD.

Immunoprotection Strategies in β-Cell Replacement Therapy: A Closer Look at Porcine Islet Xenotransplantation

Type 1 diabetes mellitus (T1DM) is characterized by absolute insulin deficiency primarily due to autoimmune destruction of pancreatic β-cells. The prevailing treatment for T1DM involves daily subcutaneous insulin injections, but a substantial proportion of patients face challenges such as severe hypoglycemic episodes and poorly controlled hyperglycemia. For T1DM patients, a more effective therapeutic option involves the replacement of β-cells through allogeneic transplantation of either the entire pancreas or isolated pancreatic islets. Unfortunately, the scarcity of transplantable human organs has led to a growing list of patients waiting for an islet transplant. One potential alternative is xenotransplantation of porcine pancreatic islets. However, due to inter-species molecular incompatibilities, porcine tissues trigger a robust immune response in humans, leading to xenograft rejection. Several promising strategies aim to overcome this challenge and enhance the long-term survival and functionality of xenogeneic islet grafts. These strategies include the use of islets derived from genetically modified pigs, immunoisolation of islets by encapsulation in biocompatible materials, and the creation of an immunomodulatory microenvironment by co-transplanting islets with accessory cells or utilizing immunomodulatory biomaterials. This review concentrates on delineating the primary obstacles in islet xenotransplantation and elucidates the fundamental principles and recent breakthroughs aimed at addressing these challenges.

Immunoprotection of cellular transplants for autoimmune type 1 diabetes through local drug delivery

Type 1 diabetes mellitus (T1DM) is an autoimmune condition that results in the destruction of insulin-secreting β cells of the islets of Langerhans. Allogeneic islet transplantation could be a successful treatment for T1DM; however, it is limited by the need for effective, permanent immunosuppression to prevent graft rejection. Upon transplantation, islets are rejected through non-specific, alloantigen specific, and recurring autoimmune pathways. Immunosuppressive agents used for islet transplantation are generally successful in inhibiting alloantigen rejection, but they are suboptimal in hindering non-specific and autoimmune pathways. In this review, we summarize the challenges with cellular immunological rejection and therapeutics used for islet transplantation. We highlight agents that target these three immune rejection pathways and how to package them for controlled, local delivery via biomaterials. Exploring macro-, micro-, and nano-scale immunomodulatory biomaterial platforms, we summarize their advantages, challenges, and future directions. We hypothesize that understanding their key features will help identify effective platforms to prevent islet graft rejection. Outcomes can further be translated to other cellular therapies beyond T1DM.

Advances in Organ and Tissue Xenotransplantation

End-stage organ failure can result from various preexisting conditions and occurs in patients of all ages, and organ transplantation remains its only treatment. In recent years, extensive research has been done to explore the possibility of transplanting animal organs into humans, a process referred to as xenotransplantation. Due to their matching organ sizes and other anatomical and physiological similarities with humans, pigs are the preferred organ donor species. Organ rejection due to host immune response and possible interspecies infectious pathogen transmission have been the biggest hurdles to xenotransplantation's success. Use of genetically engineered pigs as tissue and organ donors for xenotransplantation has helped to address these hurdles. Although several preclinical trials have been conducted in nonhuman primates, some barriers still exist and demand further efforts. This review focuses on the recent advances and remaining challenges in organ and tissue xenotransplantation.

TNX-1500, a crystallizable fragment-modified anti-CD154 antibody, prolongs nonhuman primate cardiac allograft survival

Blockade of the CD40/CD154 T cell costimulation pathway is a promising approach to supplement or replace current clinical immunosuppression in solid organ transplantation. We evaluated the tolerability and activity of a novel humanized anti-CD154 monoclonal antibody, TNX-1500 (TNX), in a nonhuman primate heterotopic cardiac allogeneic (allo) transplant model. TNX-1500 contains a rupluzimab fragment antigen-binding region and an immunoglobin G4 crystallizable fragment region engineered to reduce binding to the crystallizable fragment gamma receptor IIa and associated risks of thrombosis. Recipients were treated for 6 months with standard-dose TNX (sTNX) monotherapy, low-dose TNX monotherapy (loTNX), or loTNX with mycophenolate mofetil (MMF) (loTNX + MMF). Results were compared with historical data using chimeric humanized 5c8 monotherapy dosed as for loTNX but discontinued at 3 months. Median survival time was similar for humanized 5c8 and both loTNX groups, but significantly longer with sTNX (>265 days) than with loTNX (99 days) or loTNX + MMF (88 days) (P < 0.05 for both comparisons against sTNX). Standard-dose TNX prevented antidonor alloantibody elaboration, inhibited chronic rejection, and was associated with a significantly reduced effector T cells/regulatory T cells ratio relative to loTNX with MMF. No thrombotic complications were observed. This study demonstrated that TNX was well tolerated, prolongs allograft survival, and prevents alloantibody production and cardiac allograft vasculopathy in a stringent preclinical nonhuman primate heart allotransplant model.

Immune-checkpoint expression in antigen-presenting cells (APCs) of cytomegaloviruses infection after transplantation: as a diagnostic biomarker

Cytomegalovirus (CMV), a member of the Herpesviridae family, mostly causes only slight feverish symptoms or can be asymptomatic in immunocompetent individuals. However, it is known to be particularly a significant cause of morbidity in immunocompromised patients, including transplant recipients, whose immune system has been weakened due to the consumption of immunosuppressor drugs. Therefore, the diagnosis of CMV infection after transplantation is crucial. New diagnostic methods for the quick detection of CMV have been developed as a result of understanding the clinical importance of invasive CMV. Antigen-presenting cells (APCs) and T cells are important components of the immune system and it may be possible to diagnose viral infections using immunological markers, such as lymphocytosis, cytotoxic T lymphocytes (CTL), and serum cytokine levels. Moreover, PD-1, CTLA 4, and TIGIT, which are expressed on certain T cells and antigen-presenting cells, are over-expressed during the infection. The assessment of CMV infection based on T cell and APC activity, and the expression of immunological checkpoints, can be helpful for the diagnosis of transplant patients at risk for CMV infection. In this review, we will investigate how immune checkpoints affect immune cells and how they impair organ transplantation after CMV infection.

Immune checkpoint inhibitors in kidney transplantation

PURPOSE OF REVIEW

The development of immune checkpoint inhibitor (ICI) immunotherapy has revolutionized the treatment of several cancers. Malignancies are one of the leading causes of death in solid organ transplant recipients (SOTRs). Although ICI treatment may be an effective option in treating malignancies in SOTRs, concerns about triggering allograft rejection have been raised in this population. Herein, we will review currently available data regarding patients, allograft and malignancy outcomes in SOTRs who received ICI therapy.

RECENT FINDINGS

Cancer incidence is three to five-fold higher among SOTRs, compared with the general population. Skin cancer is the most prevalent cancer after transplant, followed by kidney cancer, lymphoma and Kaposi sarcoma. There are no large prospective studies evaluating ICI therapy's use for treating cancers in SOTRs. However, retrospective studies have shown that ICI treatment may be associated with improved malignancy outcomes and overall survival (OS). However, the risk of allograft rejection is high (around 40%) of whom about half lose their allograft. Maintaining higher levels of immunosuppression may be associated with a lower risk of allograft rejection, but potentially worse malignancy outcomes.

SUMMARY

Although ICI treatment may be associated with improved patient and malignancy outcomes, the risk of allograft rejection and loss are high. Prospective studies are needed to confirm the benefits of ICI therapy in SOTRs and to evaluate the optimal immunosuppression regimen modifications, if any, to improve patient, malignancy and allograft outcomes in transplant recipients.

Delivery of costimulatory blockade to lymph nodes promotes transplant acceptance in mice

The lymph node (LN) is the primary site of alloimmunity activation and regulation during transplantation. Here, we investigated how fibroblastic reticular cells (FRCs) facilitate the tolerance induced by anti-CD40L in a murine model of heart transplantation. We found that both the absence of LNs and FRC depletion abrogated the effect of anti-CD40L in prolonging murine heart allograft survival. Depletion of FRCs impaired homing of T cells across the high endothelial venules (HEVs) and promoted formation of alloreactive T cells in the LNs in heart-transplanted mice treated with anti-CD40L. Single-cell RNA sequencing of the LNs showed that anti-CD40L promotes a Madcam1+ FRC subset. FRCs also promoted the formation of regulatory T cells (Tregs) in vitro. Nanoparticles (NPs) containing anti-CD40L were selectively delivered to the LNs by coating them with MECA-79, which binds to peripheral node addressin (PNAd) glycoproteins expressed exclusively by HEVs. Treatment with these MECA-79-anti-CD40L-NPs markedly delayed the onset of heart allograft rejection and increased the presence of Tregs. Finally, combined MECA-79-anti-CD40L-NPs and rapamycin treatment resulted in markedly longer allograft survival than soluble anti-CD40L and rapamycin. These data demonstrate that FRCs are critical to facilitating costimulatory blockade. LN-targeted nanodelivery of anti-CD40L could effectively promote heart allograft acceptance.

An efficient combination immunotherapy for antitumor immunity without accelerating cardiac allograft rejection

Immunotherapy with immune checkpoint inhibitors (ICIs), including antibodies against programmed cell death protein-1 (PD-1) and its receptor programmed cell death ligand-1 (PD-L1), represents a promising systematic treatment for advanced human malignancies. Transplantation remains the ultimate therapy for end-stage organ diseases. However, the efficacy of ICI treatment in solid organ transplant (SOT) recipients remains controversial. We established a transgenic primary liver cancer mouse model and performed allogeneic heterotopic heart transplantation. Different treatments were performed and survival curves were calculated. Graft samples were collected, and immune cells and the cell surface expression of PD-L1 were analysed by flow cytometry. Inflammatory cytokine levels in the serum were measured by an inflammatory array. The specificity of the histochemical techniques was tested by staining sections. A combination immunotherapy comprising a BET protein inhibitor (JQ1) and an immune checkpoint inhibitor (anti-PD-L1 antibody) was administered to primary liver cancer model mice bearing cardiac allografts. Interestingly, the combination immunotherapy effectively suppressed the progression of primary liver cancer but did not accelerate allograft rejection. In accordance with our previous findings, BET protein inhibition enhances the expression of a putative membrane transporter (Rab8A), which upregulates the expression of PD-L1 on the plasma membrane in a transgenic primary liver cancer mouse model. This may be a crucial mechanism of tumour progression arrest. Our data showed that heart transplantation upregulated the expression of the proinflammatory factor IFN-γ and suggested that BET protein inhibition (with JQ1) decreased PD-L1 expression in heart tissues after cardiac transplantation. This phenomenon was accompanied by enhanced infiltration of inflammatory IFN-γ. Our study provides a novel and efficient therapeutic strategy for SOT recipients.

Ox40-Cre-mediated deletion of BRD4 reveals an unexpected phenotype of hair follicle stem cells in alopecia

BRD4 is a bromodomain extraterminal domain family member and functions primarily as a chromatin reader regulating genes involved in cell-fate decisions. Here, we bred Brd4fl/fl Ox40-Cre mice in which Brd4 was conditionally deleted in OX40-expressing cells to examine the role of BRD4 in regulating immune responses. We found that the Brd4fl/fl Ox40-Cre mice developed profound alopecia and dermatitis, while other organs and tissues were not affected. Surprisingly, lineage-tracing experiments using the Rosa26fl/fl-Yfp mice identified a subset of hair follicle stem cells (HFSCs) that constitutively express OX40, and deletion of Brd4 specifically in such HFSCs resulted in cell death and a complete loss of skin hair growth. We also found that death of HFSCs triggered massive activation of the intradermal γδ T cells, which induced epidermal hyperplasia and dermatitis by producing the inflammatory cytokine IL-17. Interestingly, deletion of Brd4 in Foxp3+ Tregs, which also constitutively express OX40, compromised their suppressive functions, and this, in turn, contributed to the enhanced activation of γδ T cells, as well as the severity of dermatitis and hair follicle destruction. Thus, our data demonstrate an unexpected role of BRD4 in regulating skin follicle stem cells and skin inflammation.

Genetic engineering of pigs for xenotransplantation to overcome immune rejection and physiological incompatibilities: The first clinical steps

Xenotransplantation has the potential to solve the shortfall of human organ donors. Genetically modified pigs have been considered as potential animal donors for human xenotransplantation and have been widely used in preclinical research. The genetic modifications aim to prevent the major species-specific barriers, which include humoral and cellular immune responses, and physiological incompatibilities such as complement and coagulation dysfunctions. Genetically modified pigs can be created by deleting several pig genes related to the synthesis of various pig specific antigens or by inserting human complement- and coagulation-regulatory transgenes. Finally, in order to reduce the risk of infection, genes related to porcine endogenous retroviruses can be knocked down. In this review, we focus on genetically modified pigs and comprehensively summarize the immunological mechanism of xenograft rejection and recent progress in preclinical and clinical studies. Overall, both genetically engineered pig-based xenografts and technological breakthroughs in the biomedical field provide a promising foundation for pig-to-human xenotransplantation in the future.

Substratum interactions determine immune response to allogeneic transplants of endothelial cells

Endothelial cells (ECs) are central to vascular health but also interact with and regulate the immune system. Changes in endothelial state enable immune cells to migrate into the tissue to facilitate repair and fight infection. ECs modulate the function of immune cells through the expression of adhesion molecules, chemokines, major histocompatibility complex (MHC), and an array of co-stimulatory and inhibitor molecules. These interactions allow ECs to act as antigen presenting cells (APCs) and influence the outcome of immune recognition. This study elucidates how EC microenvironment, vascular cell biology, and immune response are not only connected but interdependent. More specifically, we explored how cell-substratum interactions influence EC antigen presentation and co-stimulation, and how these differences affect allorecognition in animal models of cell transplantation. Investigation of EC state was carried out using RNA sequencing while assessment of the allogeneic response includes measurements of immune cell cytotoxic ability, T cell proliferation, cytokine release, serum antibodies, and histological staining. Differences in substratum led to divergent EC phenotypes which in turn influenced immune response to transplanted cells, both due to the physical barrier of matrix-adhesion and differences in expression of surface markers. ECs grown in 2D on tissue culture plastic or in 3D on collagen scaffolds had significantly different basal levels of MHC expression, co-stimulatory and adhesion molecules. When treated with cytokines to mimic an inflammatory state, ECs did not converge to a single phenotype but rather responded differently based on their substratum. Generally, 3D ECs were more responsive to inflammatory stimuli than 2D ECs. These unique expression patterns measured in vitro also influence immune recognition in vivo. ECs grown in 2D were more likely to provoke a cytotoxic response while 3D ECs induced T cell proliferation. ECs are uniquely configured to sense not only local flow and mechanical forces but a range of markers related to systemic state, including immune function. ECs interact with immune cells with differing results depending on the environment in which the EC-lymphocyte interaction occurs. Therefore, understanding this relationship is essential to predicting and modifying the outcome of EC-immune interacts. We specifically examined the relationship between EC substratum and allorecognition.

Balancing B cell responses to the allograft: implications for vaccination

Balancing enough immunosuppression to prevent allograft rejection and yet maintaining an intact immune system to respond to vaccinations, eliminate invading pathogens or cancer cells is an ongoing challenge to transplant physicians. Antibody mediated allograft rejection remains problematic in kidney transplantation and is the most common cause of graft loss despite current immunosuppressive therapies. The goal of immunosuppressive therapies is to prevent graft rejection; however, they prevent optimal vaccine responses as well. At the center of acute and chronic antibody mediated rejection and vaccine responses is the B lymphocyte. This review will highlight the role of B cells in alloimmune responses including the dependency on T cells for antibody production. We will discuss the need to improve vaccination rates in transplant recipients and present data on B cell populations and SARS-CoV-2 vaccine response rates in pediatric kidney transplant recipients.

Emerging local immunomodulatory strategies to circumvent systemic immunosuppression in cell transplantation

INTRODUCTION

Cell transplantation is a promising curative therapeutic strategy whereby impaired organ functions can be restored without the need for whole organ transplantation. A key challenge in allotransplantation is the requirement for life-long systemic immunosuppression to prevent rejection, which is associated with serious adverse effects such as increased risk of opportunistic infections and the development of neoplasms. This challenge underscores the urgent need for novel strategies to prevent graft rejection while abrogating toxicity-associated adverse events.

AREAS COVERED

We review recent advances in immunoengineering strategies for localized immunomodulation that aim to support allograft function and provide immune tolerance in a safe and effective manner.

EXPERT OPINION

Immunoengineering strategies are tailored approaches for achieving immunomodulation of the transplant microenvironment. Biomaterials can be adapted for localized and controlled release of immunomodulatory agents, decreasing the effective dose threshold and frequency of administration. The future of transplant rejection management lies in the shift from systemic to local immunomodulation with suppression of effector and activation of regulatory T cells, to promote immune tolerance.

Posttransplant VEGFR1R2 Trap Eye Drops Inhibit Corneal (Lymph)angiogenesis and Improve Corneal Allograft Survival in Eyes at High Risk of Rejection

Purpose

To assess whether topical application of VEGFR1R2 Trap after corneal transplantation can impair corneal (lymph)angiogenesis and promote murine corneal allograft survival in eyes at high risk of rejection.

Methods

We used the murine model of suture-induced neovascularization and subsequent keratoplasty in eyes at high risk of rejection, which is an established model for local drug application. After transplantation, the mice were treated with either VEGFR1R2 Trap (aflibercept) or human IgG Fc as eye drops for 2 weeks (three times/d). Deposition of VEGFR1R2 Trap in corneal tissue was detected by immunohistochemistry. Two and 8 weeks after transplantation, corneal (lymph)angiogenesis was assessed morphometrically. Dendritic cells (DCs) and regulatory T cells (Tregs) in the draining lymph nodes (dLNs) were examined by flow cytometry. Allograft survival was determined by corneal graft opacity scores.

Results

Topically applied VEGFR1R2 Trap penetrated into corneal host and graft stroma after keratoplasty in eyes at high risk of rejection. Additional postsurgical corneal hemangiogenesis (P < 0.0001) and lymphangiogenesis (P < 0.01) as well as infiltrating CD45⁺ leukocytes (P < 0.001) and macrophages (P < 0.01) were significantly reduced in the VEGFR1R2 Trap group compared to controls. VEGFR1R2 Trap eye drops significantly decreased the frequency of total CD11c⁺ DCs (P < 0.01), as well as activated CD11c⁺MHC II⁺ DCs (P < 0.01) and CD11c⁺CD40⁺ DCs (P < 0.05). In contrast, the frequency of CD200R⁺ regulatory DCs (P < 0.05) and Tregs in dLNs (P < 0.01) was enhanced. Moreover, long-term allograft survival was also improved (P < 0.05).

Conclusions

Temporary, topical application of VEGFR1R2 Trap after corneal transplantation can achieve sufficient anti-VEGF activity, inhibit additional (lymph)angiogenesis, and significantly improve corneal allograft survival in eyes at high risk of rejection.

Translational Relevance

VEGFR1R2 Trap eye drops after transplantation present a new therapeutic option for patients undergoing corneal transplantation and are at high risk of graft rejection.

Preincubation of donor tissue with a VEGF cytokine trap promotes subsequent high-risk corneal transplant survival

AIMS

Pathological neovascularisation of the host bed and the transplant itself is the main risk factor for graft rejection after corneal transplantation. This study aims to prevent this process by preincubation of the corneal donor tissue ex vivo with an antivascular endothelial growth factor (VEGF) cytokine trap blocking additional postsurgical hemangiogenesis and lymphangiogenesis to promote high-risk graft survival.

METHODS

The donor tissue was preincubated with a VEGFR1R2 cytokine trap for 24 hours prior to murine high-risk corneal transplantation (human IgG Fc was used as the control). The distribution of VEGFR1R2 Trap in the cornea was investigated by immunohistochemistry. Corneas were excised to quantify the blood vessels (BVs) and lymphatic vessels (LVs) and draining lymph nodes (dLNs) were harvested to analyse the phenotype of dendritic cells (DCs) and T cells at week 1, 2 and 8 post-transplantation. Graft survival was compared between preincubation with VEGFR1R2 Trap and human IgG Fc in high-risk recipients.

RESULTS

VEGFR1R2 Trap was present in the graft for at least 2 weeks after surgery and additionally diffused into the corneal recipient. BVs, LVs and macrophages in the whole cornea were significantly decreased 1-week and 2-week post-transplantation (p<0.05). In dLNs the frequency of CD11c+DCs was significantly reduced, whereas CD200R+ regulatory DCs were significantly increased after keratoplasty (p<0.05). Furthermore, long-term high-risk graft survival was significantly improved (p<0.01).

CONCLUSIONS

Preincubation of corneal donor tissue with a VEGFR1R2 cytokine trap can significantly promote subsequent high-risk corneal transplant survival and thereby opens new treatment avenues for high-risk corneal transplantation.

A Hypothetical Model Suggesting Some Possible Ways That the Progesterone Receptor May Be Involved in Cancer Proliferation

Cancer and the fetal-placental semi-allograft share certain characteristics, e.g., rapid proliferation, the capacity to invade normal tissue, and, related to the presence of antigens foreign to the host, the need to evade immune surveillance. Many present-day methods to treat cancer use drugs that can block a key molecule that is important for one or more of these characteristics and thus reduce side effects. The ideal molecule would be one that is essential for both the survival of the fetus and malignant tumor, but not needed for normal cells. There is a potential suitable candidate, the progesterone induced blocking factor (PIBF). The parent 90 kilodalton (kDa) form seems to be required for cell-cycle regulation, required by both the fetal-placental unit and malignant tumors. The parent form may be converted to splice variants that help both the fetus and tumors escape immune surveillance, especially in the fetal and tumor microenvironment. Evidence suggests that membrane progesterone receptors are involved in PIBF production, and indeed there has been anecdotal evidence that progesterone receptor antagonists, e.g., mifepristone, can significantly improve longevity and quality of life, with few side effects.

Gut microbes enlarged the protective effect of transplanted regulatory B cells on rejection of cardiac allografts

BACKGROUND

Regulatory B cells (Bregs) play an important role in maintaining immune homeostasis and have the potential to induce tolerance. Previous work has found that Breg cells are involved in heart transplantation tolerance. However, the effect of Breg on the transplantation tolerance and the underlying mechanisms remain to be clarified.

METHODS

Using a within-species heart transplantation model, we aimed to investigate the role of CD19⁺CD5⁺CD1d^high Bregs isolated from transplanted mice in preventing transplant rejection in vivo. We also explored the effects of CD40 and tumor necrosis factor receptor-associated factor 6 (TRAF6) ubiquitin ligase on Breg-mediated prolongation of survival in heart transplant (HT) mice, and the regulatory effects of downstream Cdk4 and Cdk6 proteins on dendritic cells (DCs), which clarified the function and molecular mechanism of Breg cells in HT mice.

RESULTS

Our data suggest that adoptive transfer of the transplanted Bregs served as an effective tolerance-inducing mechanism in HT mice and was involved in the CD40-TRAF6 signaling pathway in DCs. Moreover, DCs collected from the Breg treated HT mice also prolonged the survival of HT mice. Furthermore, DC-specific knockout of TRAF6 diminished Breg-mediated prolongation of survival in HT mice. Interestingly, gut microbes from donors increased the survival of cardiac allografts both in both the absence and presence of Bregs but were not implicated in CD40-TRAF6 signaling.

CONCLUSIONS

These findings reveal a role of Breg cells in the induction of transplantation tolerance through the blockade of the CD40-TRAF6 signaling pathway, which might be used in the treatment of HT in the clinic.

Mesenchymal Stem Cells from Human Exfoliated Deciduous Teeth and the Orbicularis Oris Muscle: How Do They Behave When Exposed to a Proinflammatory Stimulus?

Mesenchymal stem cells (MSCs) have been studied as a promising type of stem cell for use in cell therapies because of their ability to regulate the immune response. Although they are classically isolated from the bone marrow, many studies have sought to isolate MSCs from noninvasive sources. The objective of this study was to evaluate how MSCs isolated from the dental pulp of human exfoliated deciduous teeth (SHED) and fragments of the orbicularis oris muscle (OOMDSCs) behave when treated with an inflammatory IFN-γ stimulus, specifically regarding their proliferative, osteogenic, and immunomodulatory potentials. The results demonstrated that the proliferation of SHED and OOMDSCs was inhibited by the addition of IFN-γ to their culture medium and that treatment with IFN-γ at higher concentrations resulted in a greater inhibition of the proliferation of these cells than treatment with IFN-γ at lower concentrations. SHED and OOMDSCs maintained their osteogenic differentiation potential after stimulation with IFN-γ. Additionally, SHED and OOMDSCs have been shown to have low immunogenicity because they lack expression of HLA-DR and costimulatory molecules such as CD40, CD80, and CD86 before and after IFN-γ treatment. Last, SHED and OOMDSCs expressed the immunoregulatory molecule HLA-G, and the expression of this antigen increased after IFN-γ treatment. In particular, an increase in intracellular HLA-G expression was observed. The results obtained suggest that SHED and OOMDSCs lack immunogenicity and have immunomodulatory properties that are enhanced when they undergo inflammatory stimulation with IFN-γ, which opens new perspectives for the therapeutic use of these cells.

The many shades of macrophages in regulating transplant outcome

The shift of emphasis from short-term to long-term graft outcomes has led to renewed interests in how the innate immune cells regulate transplant survival, an area that is traditionally dominated by T cells in the adaptive system. This shift is driven largely by the limited efficacy of current immunosuppression protocols which primarily target T cells in preventing chronic graft loss, as well as by the rapid advance of basic sciences in the realm of innate immunity. In fact, the innate immune cells have emerged as key players in the allograft response in various models, contributing to both graft rejection and graft acceptance. Here, we focus on the macrophages, highlighting their diversity, plasticity and emerging features in transplant models, as well as recent developments in our studies of diverse subsets of macrophages. We also discuss challenges, unsolved questions, and emerging approaches in therapeutically modulating macrophages in further improvement of transplant outcomes.

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).

C-kit-derived CD11b+ cells are critical for cardiac allograft prolongation by autologous C-kit+ progenitor cells

Autologous C-kit⁺ cells robustly prolong cardiac allografts. As C-kit⁺ cells can transdifferentiate to hematopoietic cells as well as non-hematopoietic cells, we aimed to clarify the class(es) of C-kit-derived cell(s) required for cardiac allograft prolongation. Autologous C-kit⁺ cells were administered post-cardiac transplantation and allografts were evaluated for C-kit⁺ inoculum-derived cells. Results suggested that alloimmunity was a major signal for trafficking of C-kit-derived cells to the allograft and demonstrated that C-kit⁺ inoculum-derived cells expressed CD11b early after transfer. Allograft survival studies with CD11b-DTR C-kit⁺ cells demonstrated a requirement for C-kit⁺-derived CD11b⁺ cells. Co-therapy studies demonstrated near complete abrogation of acute rejection with concomitant CTLA4-Ig therapy and no loss of prolongation in combination with Cyclosporine A. These results strongly implicate a C-kit-derived myeloid population as critical for allograft preservation and demonstrate the potential therapeutic application of autologous C-kit⁺ progenitor cells as calcineurin inhibitor-sparing agents and possibly as co-therapeutics for durable graft survival.

Association of CTLA4 Gene Polymorphism with Transfusion Reaction after Infusion of Leukoreduced Blood Component

Leukocytes and cytokines in blood units have been known to be involved in febrile non-hemolytic transfusion reaction (FNHTR), and these adverse reactions still occur while using pre-storage leukoreduced blood products. Blood transfusion is similar to transplantation because both implant allogeneic cells or organs into the recipient. CTLA4 gene polymorphism was found to be associated with graft-versus-host disease in hematopoietic stem cell transplantation. We performed a prospective cohort study at a major tertiary care center to investigate the correlation of CTLA4 gene polymorphism and transfusion reactions. Selected CTLA4 gene SNPs were genotyped and compared between patients with transfusion-associated adverse reactions (TAARs) and healthy controls. Nineteen patients and 20 healthy subjects were enrolled. There were 4 SNPs showing differences in allele frequency between patients and controls, and the frequency of “A” allele of rs4553808, “G” allele of rs62182595, “G” allele of rs16840252, and “C” allele of rs5742909 were significantly higher in patients than in controls. Moreover, these alleles also showed significantly higher risk of TAARs (OR = 2.357, 95%CI: 1.584–3.508, p = 0.02; OR = 2.357, 95%CI: 1.584–3.508, p = 0.02; OR = 2.462, 95%CI: 1.619–3.742, p = 0.008; OR = 2.357, 95%CI: 1.584–3.508, p = 0.02; OR = 2.357, 95%CI: 1.584–3.508, p = 0.02, respectively). The present study demonstrated the correlation of CTLA4 gene polymorphism and transfusion reaction, and alleles of 4 CTLA4 SNPs with an increased risk of TAARs were found. It is important to explore the potential immune regulatory mechanism affected by SNPs of costimulatory molecules, and it could predict transfusion reaction occurrence and guide preventive actions.

Impact of infection on transplantation tolerance

Allograft tolerance is the ultimate goal of organ transplantation. Current strategies for tolerance induction mainly focus on inhibiting alloreactive T cells while promoting regulatory immune cells. Pathogenic infections may have direct impact on both effector and regulatory cell populations, therefore can alter host susceptibility to transplantation tolerance induction as well as impair the quality and stability of tolerance once induced. In this review, we will discuss existing data demonstrating the effect of infections on transplantation tolerance, with particular emphasis on the role of the stage of infection (acute, chronic, or latent) and the stage of tolerance (induction or maintenance) in this infection-tolerance interaction. While the deleterious effect of acute infection on tolerance is mainly driven by proinflammatory cytokines induced shortly after the infection, chronic infection may generate exhausted T cells that could in fact facilitate transplantation tolerance. In addition to pathogenic infections, commensal intestinal microbiota also has numerous significant immunomodulatory effects that can shape the host alloimmunity following transplantation. A comprehensive understanding of these mechanisms is crucial for the development of therapeutic strategies for robustly inducing and stably maintaining transplantation tolerance while preserving host anti-pathogen immunity in clinically relevant scenarios.

The Immunomodulatory Properties of Amniotic Cells: The Two Sides of the Coin

Among the many cell types useful in developing therapeutic treatments, human amniotic cells from placenta have been proposed as valid candidates. Both human amniotic epithelial and mesenchymal stromal cells, and the conditioned medium generated from their culture, exert multiple immunosuppressive activities. Indeed, they inhibit T and B cell proliferation, suppress inflammatory properties of monocytes, macrophages, dendritic cells, neutrophils, and natural killer cells, while promoting induction of cells with regulatory functions such as regulatory T cells and anti-inflammatory M2 macrophages. These properties have laid the foundation for their use for the treatment of inflammatory-based diseases, and encouraging results have been obtained in different preclinical disease models where exacerbated inflammation is present. Moreover, an immune-privileged status of amniotic cells has been often highlighted. However, even if long-term engraftment of amniotic cells has been reported into immunocompetent animals, only few cells survive after infusion. Furthermore, amniotic cells have been shown to be able to induce immune responses in vivo and, under specific culture conditions, they can stimulate T cell proliferation in vitro. Although immunosuppressive properties are a widely recognized characteristic of amniotic cells, immunogenic and stimulatory activities appear to be less reported, sporadic events. In order to improve therapeutic outcome, the mechanisms responsible for the suppressive versus stimulatory activity need to be carefully addressed. In this review, both the immunosuppressive and immunostimulatory activity of amniotic cells will be discussed.

Ablation of interferon regulatory factor 4 in T cells induces "memory" of transplant tolerance that is irreversible by immune checkpoint blockade

Achieving transplant tolerance remains the ultimate goal in the field of organ transplantation. We demonstrated previously that ablation of the transcription factor interferon regulatory factor 4 (IRF4) in T cells induced heart transplant acceptance by driving allogeneic CD4⁺ T cell dysfunction. Herein, we showed that heart-transplanted mice with T cell-specific IRF4 deletion were tolerant to donor-specific antigens and accepted the subsequently transplanted donor-type but not third-party skin allografts. Moreover, despite the rejection of the primary heart grafts in T cell-specific Irf4 knockout mice under immune checkpoint blockade, the establishment of donor-specific tolerance in these mice was unhindered. By tracking alloantigen-specific CD4⁺ T cells in vivo, we revealed that checkpoint blockade restored the expression levels of the majority of wild-type T cell-expressed genes in Irf4-deficient T cells on day 6 post-heart grafting, indicating the initial reinvigoration of Irf4-deficient T cells. Nevertheless, checkpoint blockade did not restore cell frequency, effector memory cell generation, and IFN-γ/TNF-α production of Irf4^-/- alloreactive T cells at day 30 post-heart grafting. Hence, targeting IRF4 represents a potential therapeutic strategy for driving intrinsic T cell dysfunction and achieving alloantigen-specific transplant tolerance.

Design, Synthesis, and Evaluation of Novel Immunomodulatory Small Molecules Targeting the CD40⁻CD154 Costimulatory Protein-Protein Interaction

We report the design, synthesis, and testing of novel small-molecule compounds targeting the CD40⁻CD154 (CD40L) costimulatory interaction for immunomodulatory purposes. This protein-protein interaction (PPI) is a TNF-superfamily (TNFSF) costimulatory interaction that is an important therapeutic target since it plays crucial roles in the activation of T cell responses, and there is resurgent interest in its modulation with several biologics in development. However, this interaction, just as all other PPIs, is difficult to target by small molecules. Following up on our previous work, we have now identified novel compounds such as DRI-C21091 or DRI-C21095 that show activity (IC50) in the high nanomolar to low micromolar range in the binding inhibition assay and more than thirty-fold selectivity versus other TNFSF PPIs including OX40⁻OX40L, BAFFR-BAFF, and TNF-R1-TNFα. Protein thermal shift (differential scanning fluorimetry) assays indicate CD154 and not CD40 as the binding partner. Activity has also been confirmed in cell assays and in a mouse model (alloantigen-induced T cell expansion in a draining lymph node). Our results expand the chemical space of identified small-molecule CD40⁻CD154 costimulatory inhibitors and provide lead structures that have the potential to be developed as orally bioavailable immunomodulatory therapeutics that are safer and less immunogenic than corresponding biologics.

The Costimulatory Pathways and T Regulatory Cells in Ischemia-Reperfusion Injury: A Strong Arm in the Inflammatory Response?

Costimulatory molecules have been identified as crucial regulators in the inflammatory response in various immunologic disease models. These molecules are classified into four different families depending on their structure. Here, we will focus on various ischemia studies that use costimulatory molecules as a target to reduce the inherent inflammatory status. Furthermore, we will discuss the relevant role of T regulatory cells in these inflammatory mechanisms and the costimulatory pathways in which they are involved.

Could OX40 agonist antibody promote activation of the anti-tumor immune response in gastric cancer?

BACKGROUND AND OBJECTIVES

OX40, a membrane-bound molecule of the tumor-necrosis-factor-receptor superfamily, is a critical costimulatory receptor during the immune response, especially to T cells, but studies described their presence of OX-40 on neutrophils and monocytes, suggesting a potential role in the activation of immune response. Our aim was to characterize costimulatory receptors OX40 expression on circulating leukocytes in gastric cancer to identify novel targets for immunotherapy.

METHODS

Peripheral blood mononuclear cells were isolated from 24 gastric cancer patients and 34 healthy controls and the expression of costimulatory (OX40) receptors were analyzed on T cells, neutrophil and monocyte using monoclonal antibodies by flow cytometry.

RESULTS

We found that the higher levels of OX40 + T cells, monocytes/OX40+ and neutrophils/OX40+ from gastric cancer patients when compared to controls (P < 0.0001), and also higher levels of OX40+ T cells when compared to stages III and IV (P  = 0.02). Percentage levels of total T cells were similar between patients and controls.

CONCLUSIONS

OX40 as a therapeutic agent has been investigated in many preclinical tumor models. Our findings suggest that of levels of costimulatory in T cells in GC will direct future studies on the role that costimulatory receptors play in the failure of T cell-mediated immunity.

Current status of costimulatory blockade in renal transplantation

PURPOSE OF REVIEW

The review will focus on the impact and current status of costimulatory blockade in renal transplantation.

RECENT FINDINGS

The mainstay of immunosuppression in kidney transplantation is calcineurin inhibitors (CNIs) which have reduced acute rejection rates but failed to improve long-term allograft survival. Their cardiometabolic side-effects and nephrotoxicity have shifted the focus of investigation to CNI-free regimens. Costimulation blockade with belatacept, a second generation, higher avidity variant of cytotoxic T-lymphocyte associated protein 4 has emerged as part of a CNI-free regimen. Belatacept has demonstrated superior glomerular filtration rate compared with CNIs, albeit with an increased risk of early and histologically severe rejection. Focus on optimizing the belatacept regimen is underway. ASKP1240, which blocks the cluster of differentiation 40 (CD40)/CD154 costimulatory pathway, has just completed a phase 2 trial with a CNI-free regimen. CFZ533, an anti-CD40, is also poised to be tested in a phase 2 trial in renal transplantation. Nonagonistic CD28 antibodies have re-emerged with two anti-CD28 candidates in preclinical development.

SUMMARY

A reliable, CNI-free regimen that maintains low acute rejection rates and improves long-term renal allograft survival has become an achievable goal with costimulation blockade. The task of clinicians and researchers is to find the optimal combinations to maintain safety and improve efficacy.

Toward Small-Molecule Inhibition of Protein-Protein Interactions: General Aspects and Recent Progress in Targeting Costimulatory and Coinhibitory (Immune Checkpoint) Interactions

Protein-Protein Interactions (PPIs) that are part of the costimulatory and coinhibitory (immune checkpoint) signaling are critical for adequate T cell response and are important therapeutic targets for immunomodulation. Biologics targeting them have already achieved considerable clinical success in the treatment of autoimmune diseases or transplant recipients (e.g., abatacept, belatacept, and belimumab) as well as cancer (e.g., ipilimumab, nivolumab, pembrolizumab, atezolizumab, durvalumab, and avelumab). In view of such progress, there have been only relatively limited efforts toward developing small-molecule PPI inhibitors (SMPPIIs) targeting these cosignaling interactions, possibly because they, as all other PPIs, are difficult to target by small molecules and were not considered druggable. Nevertheless, substantial progress has been achieved during the last decade. SMPPIIs proving the feasibility of such approaches have been identified through various strategies for a number of cosignaling interactions including CD40-CD40L, OX40-OX40L, BAFFR-BAFF, CD80-CD28, and PD-1-PD-L1s. Here, after an overview of the general aspects and challenges of SMPPII-focused drug discovery, we review them briefly together with relevant structural, immune-signaling, physicochemical, and medicinal chemistry aspects. While so far only a few of these SMPPIIs have shown activity in animal models (DRI-C21045 for CD40-D40L, KR33426 for BAFFR-BAFF) or reached clinical development (RhuDex for CD80-CD28, CA-170 for PD-1-PD-L1), there is proof-of-principle evidence for the feasibility of such approaches in immunomodulation. They can result in products that are easier to develop/ manufacture and are less likely to be immunogenic or encounter postmarket safety events than corresponding biologics, and, contrary to them, can even become orally bioavailable.

Small-Molecule Inhibitors of the CD40-CD40L Costimulatory Protein-Protein Interaction

Costimulatory interactions are required for T cell activation and development of an effective immune response; hence, they are valuable therapeutic targets for immunomodulation. However, they, as all other protein-protein interactions, are difficult to target by small molecules. Here, we report the identification of novel small-molecule inhibitors of the CD40-CD40L interaction designed starting from the chemical space of organic dyes. For the most promising compounds such as DRI-C21045, activity (IC50) in the low micromolar range has been confirmed in cell assays including inhibition of CD40L-induced activation in NF-κB sensor cells, THP-1 myeloid cells, and primary human B cells as well as in murine allogeneic skin transplant and alloantigen-induced T cell expansion in draining lymph node experiments. Specificity versus other TNF-superfamily interactions (TNF-R1-TNF-α) and lack of cytotoxicity have also been confirmed at these concentrations. These novel compounds provide proof-of-principle evidence for the possibility of small-molecule inhibition of costimulatory protein-protein interactions, establish the structural requirements needed for efficient CD40-CD40L inhibition, and serve to guide the search for such immune therapeutics.

Associations between HVEM/LIGHT/BTLA/CD160 polymorphisms and the occurrence of antibody-mediate rejection in renal transplant recipients

Antibody-mediated rejection (ABMR) is a serious complications that can occur following renal transplantation. The production of donor-specific antibodies by the humoral immune response can trigger costimulatory signals, which are crucial in activating immune cells, and therefore, playing a potential role in ABMR. To investigate the role of HVEM/LIGHT/BTLA/CD160 polymorphisms in ABMR, we retrospectively analyzed 200 renal transplant recipients. We adopted next-generation sequencing (NGS) to identify HVEM/LIGHT/BTLA/CD160 single-nucleotide polymorphisms (SNPs) in the genotypes of these patients. We divided the patients into two groups: those with ABMR and those who were stable. We adopted multiple models and performed regression analysis after adjusting for multiple confounding variables, to determine the correlation between the SNPs and ABMR. We obtained 41 high-quality SNPs readouts. However, we did not observe any significant association between these polymorphisms and the pathogenesis of ABMR in any of the models.Nevertheless, since there is evidence suggesting the involvement of costimulatory signals in graft rejection, further research should be conducted to better understand how genetic polymorphisms may be involved in ABMR.

Disruption of Transplant Tolerance by an "Incognito" Form of CD8 T Cell-Dependent Memory

Several approaches successfully achieve allograft tolerance in preclinical models but are challenging to translate into clinical practice. Many clinically relevant factors can attenuate allograft tolerance induction, including intrinsic genetic resistance, peritransplant infection, inflammation, and preexisting antidonor immunity. The prevailing view for immune memory as a tolerance barrier is that the host harbors memory cells that spontaneously cross-react to donor MHC antigens. Such preexisting "heterologous" memory cells have direct reactivity to donor cells and resist most tolerance regimens. In this study, we developed a model system to determine if an alternative form of immune memory could also block tolerance. We posited that host memory T cells could potentially respond to donor-derived non-MHC antigens, such as latent viral antigens or autoantigens, to which the host is immune. Results show that immunity to a model nonself antigen, ovalbumin (OVA), can dramatically disrupt tolerance despite undetectable initial reactivity to donor MHC antigens. Importantly, this blockade of tolerance was CD8⁺ T cell-dependent and required linked antigen presentation of alloantigens with the test OVA antigen. As such, this pathway represents an unapparent, or "incognito," form of immunity that is sufficient to prevent tolerance and that can be an unforeseen additional immune barrier to clinical transplant tolerance.

Divergent Function of Programmed Death-Ligand 1 in Donor Tissue versus Recipient Immune System in a Murine Model of Bronchiolitis Obliterans

Costimulatory molecules, such as the programmed death ligand (PD-L1), might exert differential effects on T-cell function, depending on the clinical setting and/or immunological environment. Given the impact of T cells on bronchiolitis obliterans (BO) in lung transplantation, we used an established tracheal transplant model inducing BO-like lesions to investigate the impact of PD-L1 on alloimmune responses and histopathological outcome in BO. In contrast to other transplant models in which PD-L1 generally shows protective functions, we demonstrated that PD-L1 has divergent effects depending on its location in donor versus recipient tissue. Although PD-L1 deficiency in donor tissue worsened histopathological outcome, and increased systemic inflammatory response, recipient PD-L1 deficiency induced opposite effects. Mechanistic studies revealed PD-L1-deficient recipients were hyporesponsive toward alloantigen, despite increased numbers of CD8+ effector T cells. The function of PD-L1 on T cells after unspecific stimulation was dependent on both cell type and strength of stimulation. This novel function of recipient PD-L1 may result from the high degree of T-cell activation within the highly immunogenic milieu of the transplanted tissue. In this model, both decreased T-cell alloimmune responses and the reduction of BO in PD-L1-deficient recipients suggest a potential therapeutic role of selectively blocking PD-L1 in the recipient. Further investigation is warranted to determine the impact of this finding embedded in the complex pathophysiological context of BO.

TNF superfamily receptor OX40 triggers invariant NKT cell pyroptosis and liver injury

Tissue-resident immune cells play a key role in local and systemic immune responses. The liver, in particular, hosts a large number of invariant natural killer T (iNKT) cells, which are involved in diverse immune responses. However, the mechanisms that regulate survival and homeostasis of liver iNKT cells are poorly defined. Here we have found that liver iNKT cells constitutively express the costimulatory TNF superfamily receptor OX40 and that OX40 stimulation results in massive pyroptotic death of iNKT cells, characterized by the release of potent proinflammatory cytokines that induce liver injury. This OX40/NKT pyroptosis pathway also plays a key role in concanavalin A-induced murine hepatitis. Mechanistically, we demonstrated that liver iNKT cells express high levels of caspase 1 and that OX40 stimulation activates caspase 1 via TNF receptor-associated factor 6-mediated recruitment of the paracaspase MALT1. We also found that activation of caspase 1 in iNKT cells results in processing of pro-IL-1β to mature IL-1β as well as cleavage of the pyroptotic protein gasdermin D, which generates a membrane pore-forming fragment to produce pyroptotic cell death. Thus, our study has identified OX40 as a death receptor for iNKT cells and uncovered a molecular mechanism of pyroptotic cell death. These findings may have important clinical implications in the development of OX40-directed therapies.

Tim-3 and PD-1 regulate CD8+ T cell function to maintain early pregnancy in mice

During pregnancy, CD8⁺ T cells are important regulators in the balance of fetal tolerance and antiviral immunity. T-cell immunoglobulin mucin-3 (Tim-3) and programmed cell death-1 (PD-1) are well-recognized negative co-stimulatory molecules involved in viral persistence and tumor metastasis. Here, we demonstrate that CD8⁺ T cells co-expressing Tim-3 and PD-1 were down-regulated in the deciduae of female mice in abortion-prone matings compared with normal pregnant mice. In addition to their reduced numbers, the Tim-3⁺PD-1⁺CD8⁺ T cells produced lower levels of the anti-inflammatory cytokines interleukin (IL)-4 and IL-10, as well as a higher level of the pro-inflammatory cytokine interferon (IFN)-γ, relative to those from normal pregnancy. Furthermore, normal pregnant CBA/J females challenged with Tim-3- and/or PD-1-blocking antibodies were more susceptible to fetal resorption. These findings indicate that Tim-3 and PD-1 pathways play critical roles in regulating CD8⁺ T cell function and maintaining normal pregnancy.

The Induction and Maintenance of Transplant Tolerance Engages Both Regulatory and Anergic CD4+ T cells

Therapeutic tolerance to self-antigens or foreign antigens is thought to depend on constant vigilance by Foxp3⁺ regulatory T cells (Tregs). Previous work using a pancreatic islet allograft model and a short pulse of CD3 antibody therapy has shown that CD8⁺ T cells become anergic and use TGFβ and coinhibitory signaling as their contribution to the tolerance process. Here, we examine the role of CD4⁺ T cells in tolerization by CD3 antibodies. We show that both Foxp3⁺ Tregs and CD4⁺ T cell anergy play a role in the induction of tolerance and its maintenance. Foxp3⁺ Tregs resisted CD3 antibody-mediated depletion, unlike intragraft Th1 CD4⁺ lymphocytes coexpressing granzyme B and Tbx21, which were selectively eliminated. Tregs were mandatory for induction of tolerance as their depletion at the time of CD3 antibody therapy or for a short time thereafter, by an antibody to CD25 (PC61), led to graft rejection. Early treatment with CTLA-4 antibody gave the same outcome. In contrast, neither PC61 nor anti-CTLA-4 given late, at day 100 posttransplant, reversed tolerance once established. Ablation of Foxp3 T cells after diphtheria toxin injection in tolerant Foxp3^DTR recipient mice provided the same outcome. Alloreactive T cells had been rendered intrinsically unresponsive as total CD4⁺ or Treg-deprived CD4⁺ T cells from tolerant recipients were unable to mount donor-specific IFN-γ responses. In addition, intragraft Treg-deprived CD4⁺ T cells lacked proliferative capacities, expressed high levels of the inhibitory receptor PD-1, and exhibited a CD73^hiFR4^hi phenotype, thus reflecting a state of T cell anergy. We conclude that Tregs play a substantive and critical role in guiding the immune system toward tolerance of the allograft, when induced by CD3 antibody, but are less important for maintenance of the tolerant state, where T cell anergy appears sufficient.

Belatacept after kidney transplantation in adolescents: a retrospective study

Regardless of recipient age at kidney transplantation (KTx), patients are at greatest risk for graft loss in adolescence, partly due to nonadherence to an oral immunosuppressive regimen. Belatacept, a non-nephrotoxic, first-in-class immunosuppressant that inhibits costimulation of T cells requires intravenous application only every 4 weeks, potentially leading to better adherence. However, it is only approved for use in adults. We report here the findings of the first study of belatacept in adolescents, comprising all patients in our department switched to belatacept post-KTx. Six patients (median age 15.5 years) were switched after a median of 7.5 months (range 23 days to 12 years), treatment range 3-28 months (cumulative 83 months): Three patients switched early (<3 months after KTx) had increased estimated glomerular filtration rate (GFR); one patient switched 12 years post-KTx has stable GFR; two patients were switched following rapid decline of and with markedly impaired GFR, changing slope in one patient. One patient had one acute rejection. In addition of two patients who received belatacept for other conditions, the only relevant adverse event was neutropenia (after a cumulative 109 months). Belatacept as primary immunosuppression is an option in Epstein-Barr virus-seropositive nonadherent adolescents if administered sufficiently early before deterioration of graft function.

Graft-Infiltrating Macrophages Adopt an M2 Phenotype and Are Inhibited by Purinergic Receptor P2X7 Antagonist in Chronic Rejection

Macrophages exhibit diverse phenotypes and functions; they are also a major cell type infiltrating chronically rejected allografts. The exact phenotypes and roles of macrophages in chronic graft loss remain poorly defined. In the present study, we used a mouse heart transplant model to examine macrophages in chronic allograft rejection. We found that treatment of C57BL/6 mice with CTLA4 immunoglobulin fusion protein (CTLA4-Ig) prevented acute rejection of a Balb/c heart allograft but allowed chronic rejection to develop over time, characterized by prominent neointima formation in the graft. There was extensive macrophage infiltration in the chronically rejected allografts, and the graft-infiltrating macrophages expressed markers associated with M2 cells but not M1 cells. In an in vitro system in which macrophages were polarized into either M1 or M2 cells, we screened phenotypic differences between M1 and M2 cells and identified purinergic receptor P2X7 (P2x7r), an adenosine triphosphate (ATP)-gated ion channel protein that was preferentially expressed by M2 cells. We further showed that blocking the P2x7r using oxidized ATP (oATP) inhibited M2 induction in a dose-dependent fashion in vitro. Moreover, treatment of C57BL/6 recipients with the P2x7r antagonist oATP, in addition to CTLA4-Ig treatment, inhibited graft-infiltrating M2 cells, prevented transplant vasculopathy, and induced long-term heart allografts survival. These findings highlight the importance of the P2x7r-M2 axis in chronic rejection and establish P2x7r as a potential therapeutic target in suppression of chronic rejection.

T Cells Going Innate

Natural killer (NK) cell receptors (NKRs) play a crucial role in the homeostasis of antigen-experienced T cells. Indeed, prolonged antigen stimulation may induce changes in the receptor repertoire of T cells to a profile that features NKRs. Chronic antigen exposure, at the same time, has been shown to trigger the loss of costimulatory CD28 molecules with recently reported intensified antigen thresholds of antigen-experienced CD8(+) T cells. In transplantation, NKRs have been shown to assist allograft rejection in a CD28-independent fashion. We discuss here a role for CD28-negative T cells that have acquired the competency of the NKR machinery, potentially promoting allorecognition either through T cell receptor (TCR) crossreactivity or independently from TCR recognition. Collectively, NKRs can bring about innate-like T cells by providing alternative costimulatory pathways that gain relevance in chronic inflammation, potentially leading to resistance to CD28-targeting immunosuppressants.

Notch and its ligands in alloimmunity and rejection

PURPOSE OF REVIEW

The Notch signaling pathway is known to play a pivotal role in T- and B-cell development and fate, presenting it as an attractive therapeutic target in alloimmunity. This review provides an overview of the mechanisms of Notch signaling, focusing on new insights into its diverse functions in T-cell activation, differentiation and memory subset formation, and the consequences thereof in transplantation.

RECENT FINDINGS

Recent evidence has shown that while not critical for early antigen-specific CD4 T-cell activation, Notch signaling regulates the survival of memory CD4 T cells via control of glycolytic metabolism; in contrast, Notch signaling is critical for the generation of short-lived CD8 effector T cells, but not memory CD8 cells. Transient, selective inhibition of various Notch receptors and ligands in models of solid organ transplantation has been shown to successfully modulate the alloimmune response, affecting the balance between effector and regulatory cells, with particular influence on the natural regulatory T-cell population.

SUMMARY

These studies reveal diverse roles for individual Notch receptors and ligands in peripheral immunity and indicate that selective targeting of the Notch pathway is a promising, novel approach for immune modulation in transplantation; the advent of therapeutic human antibodies to neutralize both the Notch ligands and the individual Notch receptors suggests that this approach could be efficiently developed.

Characterisation of synovial fluid and infrapatellar fat pad derived mesenchymal stromal cells: The influence of tissue source and inflammatory stimulus

The infrapatellar fat pad (FP) and synovial fluid (SF) in the knee serve as reservoirs of mesenchymal stromal cells (MSCs) with potential therapeutic benefit. We determined the influence of the donor on the phenotype of donor matched FP and SF derived MSCs and examined their immunogenic and immunomodulatory properties before and after stimulation with the pro-inflammatory cytokine interferon-gamma (IFN-γ). Both cell populations were positive for MSC markers CD73, CD90 and CD105, and displayed multipotency. FP-MSCs had a significantly faster proliferation rate than SF-MSCs. CD14 positivity was seen in both FP-MSCs and SF-MSCs, and was positively correlated to donor age but only for SF-MSCs. Neither cell population was positive for the co-stimulatory markers CD40, CD80 and CD86, but both demonstrated increased levels of human leukocyte antigen-DR (HLA-DR) following IFN-γ stimulation. HLA-DR production was positively correlated with donor age for FP-MSCs but not SF-MSCs. The immunomodulatory molecule, HLA-G, was constitutively produced by both cell populations, unlike indoleamine 2, 3-dioxygenase which was only produced following IFN-γ stimulation. FP and SF are accessible cell sources which could be utilised in the treatment of cartilage injuries, either by transplantation following ex-vivo expansion or endogenous targeting and mobilisation of cells close to the site of injury.

Therapeutics for Graft-versus-Host Disease: From Conventional Therapies to Novel Virotherapeutic Strategies

Allogeneic hematopoietic stem cell transplantation (allo-HSCT) has a curative potential for many hematologic malignancies and blood diseases. However, the success of allo-HSCT is limited by graft-versus-host disease (GVHD), an immunological syndrome that involves inflammation and tissue damage mediated by donor lymphocytes. Despite immune suppression, GVHD is highly incident even after allo-HSCT using human leukocyte antigen (HLA)-matched donors. Therefore, alternative and more effective therapies are needed to prevent or control GVHD while preserving the beneficial graft-versus-cancer (GVC) effects against residual disease. Among novel therapeutics for GVHD, oncolytic viruses such as myxoma virus (MYXV) are receiving increased attention due to their dual role in controlling GVHD while preserving or augmenting GVC. This review focuses on the molecular basis of GVHD, as well as state-of-the-art advances in developing novel therapies to prevent or control GVHD while minimizing impact on GVC. Recent literature regarding conventional and the emerging therapies are summarized, with special emphasis on virotherapy to prevent GVHD. Recent advances using preclinical models with oncolytic viruses such as MYXV to ameliorate the deleterious consequences of GVHD, while maintaining or improving the anti-cancer benefits of GVC will be reviewed.

Memory T cells in organ transplantation: progress and challenges

Antigen-experienced T cells, also known as memory T cells, are functionally and phenotypically distinct from naive T cells. Their enhanced expression of adhesion molecules and reduced requirement for co-stimulation enables them to mount potent and rapid recall responses to subsequent antigen encounters. Memory T cells generated in response to prior antigen exposures can cross-react with other nonidentical, but similar, antigens. This heterologous cross-reactivity not only enhances protective immune responses, but also engenders de novo alloimmunity. This latter characteristic is increasingly recognized as a potential barrier to allograft acceptance that is worthy of immunotherapeutic intervention, and several approaches have been investigated. Calcineurin inhibition effectively controls memory T-cell responses to allografts, but this benefit comes at the expense of increased infectious morbidity. Lymphocyte depletion eliminates allospecific T cells but spares memory T cells to some extent, such that patients do not completely lose protective immunity. Co-stimulation blockade is associated with reduced adverse-effect profiles and improved graft function relative to calcineurin inhibition, but lacks efficacy in controlling memory T-cell responses. Targeting the adhesion molecules that are upregulated on memory T cells might offer additional means to control co-stimulation-blockade-resistant memory T-cell responses.