Inactivating the Innate Immune Receptor CD11b With a First-in-Class Monoclonal Antibody Prolongs the Survival of Kidney Allografts in Nonhuman Primates.

Backgournd: Peritransplant ischemia/reperfusion injury (IRI) plays a central pathogenic role in nondelayed or delayed kidney allograft function immediately after transplantation and increases the risk of subsequent rejection. Potential therapies targeting specific cytokines or complement proteins to limit IRI have failed in clinical trials. Monoclonal antibody 107 (mAb107), a "pure" (nonactivating) inhibitor of the archetypal innate immune receptor integrin CD11b, has been shown to extend the survival of IRI nonhuman primate native kidneys in an in situ model.

Cardiac allograft tolerance can be achieved in nonhuman primates by donor bone marrow and kidney cotransplantation.

Long-term, immunosuppression-free allograft survival has been induced in human and nonhuman primate (NHP) kidney recipients after nonmyeloablative conditioning and donor bone marrow transplantation (DBMT), resulting in transient mixed hematopoietic chimerism. However, the same strategy has consistently failed in NHP heart transplant recipients. Here, we investigated whether long-term heart allograft survival could be achieved by cotransplanting kidneys from the same donor.

Adenosine triphosphate-mediated signaling of P2X7 receptors controls donor extracellular vesicle release and major histocompatibility complex cross-decoration after allotransplantation.

After skin allotransplantation, intercellular transfer of donor major histocompatibility complex molecules mediated primarily by extracellular vesicles (EVs) released by the allograft is known to contribute to semidirect and indirect activation of alloreactive T cells involved in graft rejection. At the same time, there is ample evidence showing that initiation of adaptive alloimmunity depends on early innate inflammation caused by tissue injury and subsequent activation of myeloid cells (macrophages and dendritic cells) recognizing danger-associated molecular patterns. Among these danger-associated molecular patterns, extracellular adenosine triphosphate plays a key role in innate inflammation by binding to P2X7 receptors (P2X7Rs).

Intercellular transfer of MHC molecules in T cell alloimmunity and allotransplantation.

After transplantation of allogeneic tissues and organs, recognition by recipient T cells of donor MHC molecules initiates the pro-inflammatory adaptive immune response leading to allograft rejection. T cell allorecognition has long been known to be mediated via two distinct pathways: the direct pathway in which T cells recognize intact allogeneic MHC molecules displayed on donor cells and the indirect pathway whereby T cells recognize donor MHC peptides processed and presented by recipient antigen-presenting cells (APCs). It is believed that direct allorecognition is the driving force behind early acute allograft rejection while indirect allorecognition is involved in chronic allograft rejection, a progressive condition characterized by graft vasculopathy and tissue fibrosis.

Pathways of Antigen Recognition by T Cells in Allograft Rejection.

The adaptive immune response leading to the rejection of allogeneic transplants is initiated and orchestrated by recipient T cells recognizing donor antigens. T-cell allorecognition is mediated via 3 distinct mechanisms: the direct pathway in which T cells recognize allogeneic major histocompatibility complex (MHC) molecules on donor cells, the indirect pathway through which T cells interact with donor peptides bound with self-MHC molecules on recipient antigen-presenting cells, and the recently described semidirect pathway whereby T cells recognize donor MHC proteins on recipient antigen-presenting cells. In this article, we present a description of each of these allorecognition pathways and discuss their role in acute and chronic rejection of allogeneic transplants.

Tolerance of a Vascularized Composite Allograft Achieved in MHC Class-I-mismatch Swine Mixed Chimerism.

Background: Vascularized composite allografts (VCAs) allow reconstruction of devastating injuries and amputations, yet require lifelong immunosuppression that is associated with significant morbidity. Induction of immune tolerance of VCAs would permit widespread use of these procedures. VCAs are acquired from deceased donors most likely to be -MHC-mismatched (in contrast to living-related renal transplant donor-recipient pairs matched at one MHC haplotype).

TRACE-seq: A transgenic system for unbiased and non-invasive transcriptome profiling of living cells.

Dynamic profiling of changes in gene expression in response to stressors in specific microenvironments without requiring cellular destruction remains challenging. Current methodologies that seek to interrogate gene expression at a molecular level require sampling of cellular transcriptome and therefore lysis of the cell, preventing serial analysis of cellular transcriptome. To address this area of unmet need, we have recently developed a technology allowing transcriptomic analysis over time without cellular destruction.

A Mixed-chimerism Protocol Utilizing Thymoglobulin and Belatacept Did Not Induce Lung Allograft Tolerance, Despite Previous Success in Renal Allotransplantation.

Background: In kidney transplantation, long-term allograft acceptance in cynomolgus macaques was achieved using a mixed-chimerism protocol based on the clinically available reagents, rabbit anti-thymocyte globulin (ATG), and belatacept. Here, we have tested the same protocol in cynomolgus macaques transplanted with fully allogeneic lung grafts.

Methods: Five cynomolgus macaques underwent left orthotopic lung transplantation.

T cell antigenicity and immunogenicity of allogeneic exosomes.

Extracellular vesicles, including exosomes, are regularly released by allogeneic cells after transplantation. Recipient antigen-presenting cells (APCs) capture these vesicles and subsequently display donor MHC molecules on their surface. Recent evidence suggests that activation of alloreactive T cells by the so-called cross-dressed APCs plays an important role in initiating the alloresponse associated with allograft rejection.

Contrasting effects of B cell depletion on CD4 and CD8 memory T cell responses generated after transplantation.

Alloreactive memory T cells play a key role in transplantation by accelerating allograft rejection and preventing tolerance induction. Some studies using µMT mice, which are constitutionally devoid of B cells, showed that B cells were required for the generation of memory T cells after allotransplantation. However, whether B cell depletion in normal adult mice has the same effect on memory responses by CD4 and CD8 T cells activated after transplantation has not been thoroughly investigated.

Extracellular vesicles in allograft rejection and tolerance.

Extracellular vesicles (EVs), including exosomes, ectosomes and apoptotic vesicles, play an essential role in communication between cells of the innate and adaptive immune systems. Recent studies showed that EVs released after transplantation of allogeneic tissues and organs are involved in the immune recognition and response leading to rejection or tolerance in mice. After skin, pancreatic islet, and solid organ transplantation, donor-derived EVs were shown to initiate direct inflammatory alloresponses by T cells leading to acute rejection.

Why some organ allografts are tolerated better than others: new insights for an old question.

Purpose Of Review: There is great variability in how different organ allografts respond to the same tolerance induction protocol. Well known examples of this phenomenon include the protolerogenic nature of kidney and liver allografts as opposed to the tolerance-resistance of heart and lung allografts. This suggests there are organ-specific factors which differentially drive the immune response following transplantation.

Maintaining T cell tolerance of alloantigens: Lessons from animal studies.

Achieving host immune tolerance of allogeneic transplants represents the ultimate challenge in clinical transplantation. It has become clear that different cells and mechanisms participate in acquisition versus maintenance of allograft tolerance. Indeed, manipulations which prevent tolerance induction often fail to abrogate tolerance once it has been established.

Emerging role of exosomes in allorecognition and allograft rejection.

Purpose Of Review: This article reviews recent literature on the nature of extracellular vesicles released by allogeneic transplants and examine their role in T-cell alloimmunity involved in rejection and tolerance of these grafts.

Recent Findings: Donor cells release extracellular vesicles, including exosomes, after transplantation of allogeneic organs and tissues. Consequently, recipient APCs take up these exosomes and present donor MHC antigens on their surface (allo-MHC cross-dressing) thus, activating some alloreactive T cells via a mechanism called semi-direct pathway of allorecognition.

Role of Memory T Cells in Allograft Rejection and Tolerance.

Memory T cells are characterized by their low activation threshold, robust effector functions, and resistance to conventional immunosuppression and costimulation blockade. Unlike their naïve counterparts, memory T cells reside in and recirculate through peripheral non-lymphoid tissues. Alloreactive memory T cells are subdivided into different categories based on their origins, phenotypes, and functions.

A Paradigm Shift on the Question of B Cells in Transplantation? Recent Insights on Regulating the Alloresponse.

B lymphocytes contribute to acute and chronic allograft rejection through their production of donor-specific antibodies (DSAs). In addition, B cells present allopeptides bound to self-MHC class II molecules and provide costimulation signals to T cells, which are essential to their activation and differentiation into memory T cells. On the other hand, both in laboratory rodents and patients, the concept of effector T cell regulation by B cells is gaining traction in the field of transplantation.

Allorecognition by T Lymphocytes and Allograft Rejection.

Recognition of donor antigens by recipient T cells in secondary lymphoid organs initiates the adaptive inflammatory immune response leading to the rejection of allogeneic transplants. Allospecific T cells become activated through interaction of their T cell receptors with intact allogeneic major histocompatibility complex (MHC) molecules on donor cells (direct pathway) and/or donor peptides presented by self-MHC molecules on recipient antigen-presenting cells (APCs) (indirect pathway). In addition, recent studies show that alloreactive T cells can also be stimulated through recognition of allogeneic MHC molecules displayed on recipient APCs (MHC cross-dressing) after their transfer cell-cell contact or through extracellular vesicles (semi-direct pathway).

Donor exosomes rather than passenger leukocytes initiate alloreactive T cell responses after transplantation.

Transplantation of allogeneic organs and tissues represents a lifesaving procedure for a variety of patients affected with end-stage diseases. Although current immunosuppressive therapy prevents early acute rejection, it is associated with nephrotoxicity and increased risks for infection and neoplasia. This stresses the need for selective immune-based therapies relying on manipulation of lymphocyte recognition of donor antigens.

Induced regulatory T cells in allograft tolerance via transient mixed chimerism.

Successful induction of allograft tolerance has been achieved in nonhuman primates (NHPs) and humans via induction of transient hematopoietic chimerism. Since allograft tolerance was achieved in these recipients without durable chimerism, peripheral mechanisms are postulated to play a major role. Here, we report our studies of T cell immunity in NHP recipients that achieved long-term tolerance versus those that rejected the allograft (AR).

Persistence of Indirect but Not Direct T Cell Xenoresponses in Baboon Recipients of Pig Cell and Organ Transplants.

We investigated the contributions of direct and indirect T cell antigen recognition pathways to the immune response to porcine antigens in naïve baboons and baboon recipients of pig xenografts. In naïve baboons, in vitro culture of peripheral blood T cells with intact pig cells (direct xenorecognition pathway) or pig cell sonicates and baboon antigen-presenting cells (indirect xenorecognition pathway) induced the activation and expansion of xenoreactive T cells producing proinflammatory cytokines, interleukin-2 and interferon-γ. Primary indirect xenoresponses were mediated by preexisting memory T cells, whose presence is not typically observed in primary alloresponses.

B Cell Depletion With an Anti-CD20 Antibody Enhances Alloreactive Memory T Cell Responses After Transplantation.

Alloreactive memory T cells mediate accelerated allograft rejection and transplant tolerance resistance. Recent studies have shown that B cell deficient-μMT mice fail to mount donor-specific memory T cell responses after transplantation. At the same time, other studies showed that pretransplant B cell depletion using rituximab (IgG1 anti-CD20 mAb) combined with cyclosporine A promoted the survival of islet allografts in monkeys.

Repeated Injections of IL-2 Break Renal Allograft Tolerance Induced via Mixed Hematopoietic Chimerism in Monkeys.

Tolerance of allografts achieved in mice via stable mixed hematopoietic chimerism relies essentially on continuous elimination of developing alloreactive T cells in the thymus (central deletion). Conversely, while only transient mixed chimerism is observed in nonhuman primates and patients, it is sufficient to ensure tolerance of kidney allografts. In this setting, it is likely that tolerance depends on peripheral regulatory mechanisms rather than thymic deletion.

Pilot Study Evaluating Regulatory T Cell-Promoting Immunosuppression and Nonimmunogenic Donor Antigen Delivery in a Nonhuman Primate Islet Allotransplantation Model.

The full potential of islet transplantation will only be realized through the development of tolerogenic regimens that obviate the need for maintenance immunosuppression. Here, we report an immunotherapy regimen that combines 1-ethyl-3-(3'-dimethylaminopropyl)-carbodiimide (ECDI)-treated donor lymphoid cell infusion (ECDI-DLI) with thymoglobulin, anti-interleukin-6 receptor antibody and rapamycin to achieve prolonged allogeneic islet graft survival in a nonhuman primate (NHP) model. Prolonged graft survival is associated with Treg expansion, donor-specific T cell hyporesponsiveness and a transient absence of donor-specific alloantibody production during the period of graft survival.