Rapid deletional peripheral CD8 T cell tolerance induced by allogeneic bone marrow: role of donor class II MHC and B cells

Donor programmed cell death 1 ligand 1 is required for organ transplant tolerance in major histocompatibility complex-mismatched mixed chimeras although programmed cell death 1 ligand 1 and major histocompatibility complex class II are not required for inducing chimerism

Induction of major histocompatibility complex (MHC) human leukocyte antigen (HLA)-mismatched mixed chimerism is a promising approach for organ transplantation tolerance; however, human leukocyte antigen-mismatched stable mixed chimerism has not been achieved in the clinic. Tolerogenic dendritic cell (DC) expression of MHC class II (MHC II) and programmed cell death 1 ligand 1 (PD-L1) is important for immune tolerance, but whether donor-MHC II or PD-L1 is required for the induction of stable MHC-mismatched mixed chimerism and transplant tolerance is unclear. Here, we show that a clinically applicable radiation-free regimen can establish stable MHC-mismatched mixed chimerism and organ transplant tolerance in murine models. Induction of MHC-mismatched mixed chimerism does not require donor cell expression of MHC II or PD-L1, but donor-type organ transplant tolerance in the mixed chimeras (MC) requires the donor hematopoietic cells and the organ transplants to express PD-L1. The PD-L1 expressed by donor hematopoietic cells and the programmed cell death 1 expressed by host cells augment host-type donor-reactive CD4+ and CD8+ T cell anergy/exhaustion and differentiation into peripheral regulatory T (pTreg) cells in association with the organ transplant tolerance in the MC. Conversely, host-type Treg cells augment the expansion of donor-type tolerogenic CD8+ DCs that express PD-L1. These results indicate that PD-L1 expressed by donor-type tolerogenic DCs and expansion of host-type pTreg cells in MHC-mismatched MCs play critical roles in mediating organ transplant tolerance.

Chimerism-Based Tolerance to Kidney Allografts in Humans: Novel Insights and Future Perspectives

Chronic rejection and immunosuppression-related toxicity severely affect long-term outcomes of kidney transplantation. The induction of transplantation tolerance – the lack of destructive immune responses to a transplanted organ in the absence of immunosuppression – could potentially overcome these limitations. Immune tolerance to kidney allografts from living donors has been successfully achieved in humans through clinical protocols based on chimerism induction with hematopoietic cell transplantation after non-myeloablative conditioning. Notably, two of these protocols have led to immune tolerance in a significant fraction of HLA-mismatched donor-recipient combinations, which represent the large majority of cases in clinical practice. Studies in mice and large animals have been critical in dissecting tolerance mechanisms and in selecting the most promising approaches for human translation. However, there are several key differences in tolerance induction between these models and humans, including the rate of success and stability of donor chimerism, as well as the relative contribution of different mechanisms in inducing donor-specific unresponsiveness. Kidney allograft tolerance achieved through durable full-donor chimerism may be due to central deletion of graft-reactive donor T cells, even though mechanistic data from patient series are lacking. On the other hand, immune tolerance attained with transient mixed chimerism-based protocols initially relies on Treg-mediated suppression, followed by peripheral deletion of donor-reactive recipient T-cell clones under antigenic pressure from the graft. These conclusions were supported by data deriving from novel high-throughput T-cell receptor sequencing approaches that allowed tracking of alloreactive repertoires over time. In this review, we summarize the most important mechanistic studies on tolerance induction with combined kidney-bone marrow transplantation in humans, discussing open issues that still need to be addressed and focusing on techniques developed in recent years to efficiently monitor the alloresponse in tolerance trials. These cutting-edge methods will be instrumental for the development of immune tolerance protocols with improved efficacy and to identify patients amenable to safe immunosuppression withdrawal.

Maternal microchimerism and cell-mediated immune-modulation enhance engraftment following semi-allogenic intrauterine transplantation

Successful intrauterine hematopoietic cell transplantation (IUT) for congenital hemoglobinopathies is hampered by maternal alloresponsiveness. We investigate these interactions in semi-allogenic murine IUT. E14 fetuses (B6 females × BALB/c males) were each treated with 5E+6 maternal (B6) or paternal (BALB/c) bone marrow cells and serially monitored for chimerism (>1% engraftment), trafficked maternal immune cells, and immune responsiveness to donor cells. A total of 41.0% of maternal IUT recipients (mIUT) were chimeras (mean donor chimerism 3.0 ± 1.3%) versus 75.0% of paternal IUT recipients (pIUT, 3.6 ± 1.1%). Chimeras showed higher maternal microchimerism of CD4, CD8, and CD19 than non-chimeras. These maternal cells showed minimal responsiveness to B6 or BALB/c stimulation. To interrogate tolerance, mIUT were injected postnatally with 5E+6 B6 cells/pup; pIUT received BALB/c cells. IUT-treated pups showed no changes in trafficked maternal or fetal immune cell levels compared to controls. Donor-specific IgM and IgG were expressed by 1%-3% of recipients. mIUT splenocytes showed greater proliferation of regulatory T cells (Treg) upon BALB/c stimulation, while B6 stimulation upregulated the pro-inflammatory cytokines more than BALB/c. pIUT splenocytes produced identical Treg and cytokine responses to BALB/c and B6 cells, with higher Treg activity and lower pro-inflammatory cytokine expression upon exposure to BALB/c. In contrast, naïve fetal splenocytes demonstrated greater alloresponsiveness to BALB/c compared to B6 cells. Thus pIUT, associated with increased maternal cell trafficking, modulates fetal Treg, and cytokine responsiveness to donor cells more efficiently than mIUT, resulting in improved engraftment. Paternal donor cells may be considered alternatively to maternal donor cells for intrauterine and postnatal transplantation to induce tolerance and maintain engraftment.

B cells as antigen-presenting cells in transplantation rejection and tolerance

Transplantation of fully allogeneic organs into immunocompetent recipients invariably elicits T cell and B cell responses that lead to the production of donor-specific antibodies (DSA). When immunosuppression is inadequate donor-specific T cell and B cell responses escape, leading to T cell-mediated rejection (TCMR), antibody mediated (ABMR) rejection, or mixed rejection (MR) exhibiting features of both TCMR and ABMR. Current literature suggests that ABMR is a major cause of late graft loss, and that new therapies to curtail the donor-specific humoral response are necessary. The majority of research into B cell responses elicited by allogeneic allografts in both preclinical models and clinical studies, has focused on the function of B cells as antibody-secreting cells and the pathogenic effects of DSA as mediators of ABMR. However, it has long been recognized that the DSA response to allografts is T cell-dependent, and that B cells engage in cognate interactions with T cells that provide "help" and promote B cell differentiation into antibody-secreting cells (ASCs). This review focusses the function of B cells as antigen-presenting cells (APCs) to T cells in lymphoid organs, how they may be critical APCs to T cell in the allograft, and the functional consequences of these interactions.

Vaccination with CD47 deficient tumor cells elicits an antitumor immune response in mice

Cancer cells are poorly immunogenic and have a wide range of mutations, which makes them unsuitable for use in vaccination treatment. Here, we show that elimination of CD47, a ligand for the myeloid cell inhibitory receptor SIRPα, from tumor cells by genetic deletion or antibody blocking, significantly improves the effectiveness of the immune response to tumour cells. In both solid and hematopoietic mouse tumor models, vaccination with tumor cells or tumor antigen-expressing cells, that lack CD47 or were pre-coated with anti-CD47 antibodies, achieved an antitumor immune response. The efficacy of this approach was synergistically enhanced when used in combination with anti-PD-1 antibodies. The induction of antitumor responses depends on SIRPα⁺CD11c⁺ DCs, which exhibit rapid expansion following introduction of CD47-deficient tumor cells. Our results indicate that CD47-deficient whole tumor cells can induce antitumor responses.

Immune monitoring of transplant patients in transient mixed chimerism tolerance trials

This review focuses on mechanistic studies performed in recipients of non-myeloablative bone marrow transplant regimens developed at Massachusetts General Hospital in HLA-identical and HLA-mismatched haploidentical combinations, initially as a platform for treatment of hematologic malignancies with immunotherapy in the form of donor leukocyte infusions, and later in combination with donor kidney transplantation for the induction of allograft tolerance. In patients with permanent mixed chimerism, central deletion may be a major mechanism of long-term tolerance. In patients in whom donor chimerism is only transient, the kidney itself plays a significant role in maintaining long-term tolerance. A high throughput sequencing approach to identifying and tracking a significant portion of the alloreactive T cell receptor repertoire has demonstrated biological significance in transplant patients and has been useful in pointing to clonal deletion as a long-term tolerance mechanism in recipients of HLA-mismatched combined kidney and bone marrow transplants with only transient chimerism.

Mechanisms of Mixed Chimerism-Based Transplant Tolerance

Immune responses to allografts represent a major barrier in organ transplantation. Immune tolerance to avoid chronic immunosuppression is a critical goal in the field, recently achieved in the clinic by combining bone marrow transplantation (BMT) with kidney transplantation following non-myeloablative conditioning. At high levels of chimerism such protocols can permit central deletional tolerance, but with a significant risk of graft-versus-host (GVH) disease (GVHD). By contrast, transient chimerism-based tolerance is devoid of GVHD risk and appears to initially depend on regulatory T cells (Tregs) followed by gradual, presumably peripheral, clonal deletion of donor-reactive T cells. Here we review recent mechanistic insights into tolerance and the development of more robust and safer protocols for tolerance induction that will be guided by innovative immune monitoring tools.

APC-targeted proinsulin expression inactivates insulin-specific memory CD8+ T cells in NOD mice

Type 1 diabetes (T1D) results from T-cell-mediated autoimmune destruction of pancreatic β cells. Effector T-cell responses emerge early in disease development and expand as disease progresses. Following β-cell destruction, a long-lived T-cell memory is generated that represents a barrier to islet transplantation and other cellular insulin-replacement therapies. Development of effective immunotherapies that control or ablate β-cell destructive effector and memory T-cell responses has the potential to prevent disease progression and recurrence. Targeting antigen expression to antigen-presenting cells inactivates cognate CD8⁺ effector and memory T-cell responses and has therapeutic potential. Here we investigated this in the context of insulin-specific responses in the non-obese diabetic mouse where genetic immune tolerance defects could impact on therapeutic tolerance induction. Insulin-specific CD8⁺ memory T cells transferred to mice expressing proinsulin in antigen-presenting cells proliferated in response to transgenically expressed proinsulin and the majority were rapidly deleted. A small proportion of transferred insulin-specific Tmem remained undeleted and these were antigen-unresponsive, exhibited reduced T cell receptor (TCR) expression and H-2K^d/insB_15-23 tetramer binding and expressed co-inhibitory molecules. Expression of proinsulin in antigen-presenting cells also abolished the diabetogenic capacity of CD8⁺ effector T cells. Therefore, destructive insulin-specific CD8⁺ T cells are effectively inactivated by enforced proinsulin expression despite tolerance defects that exist in diabetes-prone NOD mice. These findings have important implications in developing immunotherapeutic approaches to T1D and other T-cell-mediated autoimmune diseases.

Short-course rapamycin treatment enables engraftment of immunogenic gene-engineered bone marrow under low-dose irradiation to permit long-term immunological tolerance

Background

Application of genetically modified hematopoietic stem cells is increasingly mooted as a clinically relevant approach to protein replacement therapy, immune tolerance induction or conditions where both outcomes may be helpful. Hematopoietic stem and progenitor cell (HSPC)-mediated gene therapy often requires highly toxic pretransfer recipient conditioning to provide a ‘niche’ so that transferred HSPCs can engraft effectively and to prevent immune rejection of neoantigen-expressing engineered HSPCs. For widespread clinical application, reducing conditioning toxicity is an important requirement, but reduced conditioning can render neoantigen-expressing bone marrow (BM) and HSC susceptible to immune rejection if immunity is retained.

Methods

BM or HSPC-expressing OVA ubiquitously (actin.OVA) or targeted to MHC II+ cells was transferred using low-dose (300 cGy) total body irradiation. Recipients were administered rapamycin, cyclosporine or vehicle for 3 weeks commencing at BM transfer. Engraftment was determined using CD45 congenic donors and recipients. Induction of T-cell tolerance was tested by immunising recipients and analysing in-vivo cytotoxic T-lymphocyte (CTL) activity. The effect of rapamycin on transient effector function during tolerance induction was tested using an established model of tolerance induction where antigen is targeted to dendritic cells.

Results

Immune rejection of neoantigen-expressing BM and HSPCs after low-dose irradiation was prevented by a short course of rapamycin, but not cyclosporine, treatment. Whereas transient T-cell tolerance developed in recipients of OVA-expressing BM administered vehicle, only when engraftment of neoantigen-expressing BM was facilitated with rapamycin treatment did stable, long-lasting T-cell tolerance develop. Rapamycin inhibited transient effector function development during tolerance induction and inhibited development of CTL activity in recipients of OVA-expressing BM.

Conclusions

Rapamycin acts to suppress acquisition of transient T-cell effector function during peripheral tolerance induction elicited by HSPC-encoded antigen. By facilitating engraftment, short-course rapamycin permits development of long-term stable T-cell tolerance.

Electronic supplementary material

The online version of this article (doi:10.1186/s13287-017-0508-3) contains supplementary material, which is available to authorized users.

Immunomodulatory Effects of Different Cellular Therapies of Bone Marrow Origin on Chimerism Induction and Maintenance Across MHC Barriers in a Face Allotransplantation Model

Many more patients would benefit from vascularized composite allotransplantation if less toxic and safer immunosuppressive protocols will become available. Tolerance induction protocols with donor cells co-transplantation are one of the promising pathways to reduce maintenance immunosupressive regimens. We investigated the role of donor bone marrow cells (BMC), mesenchymal stromal cells (MSC) and in vivo created chimeric cells (CC) used as supportive therapies in a fully MHC-mismatched rat face transplantation model. Twenty-four fully MHC-mismatched hemiface transplantations were performed between ACI (RT1(a)) donors and Lewis (RT1(l)) recipients under combined seven-day immunosuppressive regimen of anti-αβ-T-cell receptor (TCR) monoclonal antibody and cyclosporin A. We studied four experimental groups-group 1: no cellular therapy; group 2: supportive therapy with BMC; group 3: supportive therapy with MSC; group 4: supportive therapy with CC generated in a primary chimera. We evaluated clinical and histological rejection grades, transplanted cells migration, donor-specific chimerism in the peripheral blood and bone marrow compartments, and CD4(+)/CD25(+) T-cell levels. Face allograft rejection was observed at 26.8 ± 0.6 days post-transplant (PT) in the absence of cellular therapy, at 34.5 ± 1.1 days for group 2, 29.3 ± 0.8 days for group 3, and 30.3 ± 1.38 PT for group 4. The longest survival was observed in allografts supported by co-transplantation of BMC. All support in cellular therapies delayed face allograft rejection by chimerism induction and/or immunomodulatory properties of co-transplanted cells. Survival time was comparable between groups, however, further studies, with different cell dosages, delivery routes and delivery times are required.

Longitudinal studies of a B cell-derived signature of tolerance in renal transplant recipients

Biomarkers of transplant tolerance would enhance the safety and feasibility of clinical tolerance trials and potentially facilitate management of patients receiving immunosuppression. To this end, we examined blood from spontaneously tolerant renal transplant recipients and patients enrolled in two interventional tolerance trials using flow cytometry and gene expression profiling. Using a previously reported tolerant cohort as well as newly identified tolerant patients, we confirmed our previous finding that tolerance was associated with increased expression of B cell-associated genes relative to immunosuppressed patients. This was not accounted for merely by an increase in total B cell numbers, but was associated with the increased frequencies of transitional and naïve B cells. Moreover, serial measurements of gene expression demonstrated that this pattern persisted over several years, although patients receiving immunosuppression also displayed an increase in the two most dominant tolerance-related B cell genes, IGKV1D-13 and IGLL-1, over time. Importantly, patients rendered tolerant via induction of transient mixed chimerism, and those weaned to minimal immunosuppression, showed similar increases in IGKV1D-13 as did spontaneously tolerant individuals. Collectively, these findings support the notion that alterations in B cells may be a common theme for tolerant kidney transplant recipients, and that it is a useful monitoring tool in prospective trials.

Rapamycin and CTLA4Ig synergize to induce stable mixed chimerism without the need for CD40 blockade

The mixed chimerism approach achieves donor-specific tolerance in organ transplantation, but clinical use is inhibited by the toxicities of current bone marrow (BM) transplantation (BMT) protocols. Blocking the CD40:CD154 pathway with anti-CD154 monoclonal antibodies (mAbs) is exceptionally potent in inducing mixed chimerism, but these mAbs are clinically not available. Defining the roles of donor and recipient CD40 in a murine allogeneic BMT model, we show that CD4 or CD8 activation through an intact direct or CD4 T cell activation through the indirect pathway is sufficient to trigger BM rejection despite CTLA4Ig treatment. In the absence of CD4 T cells, CD8 T cell activation via the direct pathway, in contrast, leads to a state of split tolerance. Interruption of the CD40 signals in both the direct and indirect pathway of allorecognition or lack of recipient CD154 is required for the induction of chimerism and tolerance. We developed a novel BMT protocol that induces mixed chimerism and donor-specific tolerance to fully mismatched cardiac allografts relying on CD28 costimulation blockade and mTOR inhibition without targeting the CD40 pathway. Notably, MHC-mismatched/minor antigen-matched skin grafts survive indefinitely whereas fully mismatched grafts are rejected, suggesting that non-MHC antigens cause graft rejection and split tolerance.

Hematopoietic stem cell infusion/transplantation for induction of allograft tolerance

PURPOSE OF REVIEW

The present review updates the current status of basic, preclinical, and clinical research on donor hematopoietic stem cell infusion for allograft tolerance induction.

RECENT FINDINGS

Recent basic studies in mice provide evidence of significant involvement of both central deletional and peripheral regulatory mechanisms in induction and maintenance of allograft tolerance effected through a mixed chimerism approach with donor hematopoietic stem cell infusion. The presence of heterologous memory T cells in primates hampers the induction of persistent chimerism. Durable mixed chimerism, however, now has been recently induced in inbred major histocompatibility complex-mismatched swine, resulting in tolerance of vascularized composite tissue allografts. In clinical transplantation, allograft tolerance has been achieved in human leukocyte antigen-mismatched kidney transplantation after the induction of transient mixed chimerism or persistent full donor chimerism.

SUMMARY

Tolerance induction in clinical kidney transplantation has been achieved by donor hematopoietic stem cell infusion. Improving the consistency and safety of tolerance induction and extending successful protocols to other organs, and to organs from deceased donors, are critical next steps to bringing tolerance to a wider range of clinical applications.

Induction of transplantation tolerance through regulatory cells: from mice to men

Organ transplantation results in the activation of both innate and adaptive immune responses to the foreign antigens. While these responses can be limited with the use of systemic immunosuppressants, the induction of regulatory cell populations may be a novel strategy for the maintenance of specific immunological unresponsiveness that can reduce the severity of the detrimental side effects of current therapies. Our group has extensively researched different regulatory T-cell induction protocols for use as cellular therapy in transplantation. In this review, we address the cellular and molecular mechanisms behind regulatory T-cell suppression and their stability following induction protocols. We further discuss the use of different hematopoietically derived regulatory cell populations, including regulatory B cells, regulatory macrophages, tolerogenic dendritic cells, and myeloid-derived suppressor cells, for the induction of transplantation tolerance in light of new clinical trials developing therapies with some of these populations.

B cells with immune-regulating function in transplantation

In transplantation, the contribution of B cells to the rejection or acceptance of the allograft is a topic of major interest. The presence of donor-specific antibodies in transplant recipients is often associated with decreased graft function and rejection, clearly indicating a pathogenetic role of B cells in transplantation. However, data from studies in humans and rodents suggest that under certain conditions, B cells have the capacity to control or regulate the immune response to a transplanted organ. Although a great deal of attention has been focused on B cells in human and murine models of autoimmunity, our understanding of the role of these cells in transplantation is limited at present. Indeed, results in this setting are controversial and seem to depend on the model system used or the clinical situation studied. Here, we review the current understanding of the various phenotypes and roles that have been associated with immune-regulating B cells. We also discuss the mechanisms employed by subsets of these regulatory B cells to control the immune response in transplant recipients and in animal models of transplantation.

Mixed chimerism through donor bone marrow transplantation: a tolerogenic cell therapy for application in organ transplantation

PURPOSE OF REVIEW

Organ transplantation is the state-of-the-art treatment for end-stage organ failure; however, long-term graft survival is still unsatisfactory. Despite improved immunosuppressive drug therapy, patients are faced with substantial side effects and the risk of chronic rejection with subsequent graft loss. The transplantation of donor bone marrow for the induction of mixed chimerism has been recognized to induce donor-specific tolerance a long time ago, but safety concerns regarding toxicities of current bone marrow transplantation (BMT) protocols impede widespread application.

RECENT FINDINGS

Recent studies in nonhuman primates and kidney transplant patients have demonstrated successful induction of allograft tolerance even though--in contrast to murine models--only transient chimerism was achieved. Progress toward the development of nontoxic murine BMT protocols revealed that Treg therapy is a potent therapeutic adjunct eliminating the need for cytotoxic recipient conditioning. Furthermore, new insight into the mechanisms underlying tolerization of CD4 and CD8 T cells in mixed chimeras has been gained and has identified possible difficulties impeding clinical translation.

SUMMARY

This review will address the recent advances in murine models as well as findings from the first clinical trials for the induction of tolerance through mixed chimerism. Both the potential for more widespread clinical application and the remaining hurdles and challenges of this tolerance approach will be discussed.

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

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

IL-10 deficiency blocks the ability of LPS to regulate expression of tolerance-related molecules on dendritic cells

Interleukin-10 (IL-10) is an anti-inflammatory cytokine that plays an important role in regulating the local inflammatory immune response, but regulatory mechanisms of this cytokine have not been fully elucidated. Here, we demonstrate that IL-10 deficiency renders LPS treatment ineffective in regulating the expression of CD40, CD80, CD86, B7-H2, and B7-DC on dendritic cells (DCs) and blocks upregulation of IL-27. This inability to respond to LPS was found in both IL-10(-/-) bone marrow derived and splenic DCs. Compared with wild-type DCs, IL-10(-/-) DCs expressed similar levels of TLR4 and CD14, but produced less LPS-binding protein. The deficiency in LPS-binding protein production may explain the failure of IL-10(-/-) DCs to respond normally to LPS. Moreover, lack of IL-10 modulated the proportions of CD11c(+) CD8(+) and CD11c(+) B220(+) DCs, which play an important role in local inflammatory responses and tolerance. IL-10 deficiency also blocked expression of galectin-1, CD205, and CD103, which are necessary for central and peripheral tolerance. While they did not respond to LPS, IL-10(-/-) DCs produced increased levels of IL-6 and CCL4 after TNF-α treatment. Together, our results demonstrate that IL-10 deficiency affects the immune functions of DCs, which may contribute to the increased severity of autoimmune diseases seen in IL-10(-/-) mice.

Hematopoietic chimerism and transplantation tolerance: a role for regulatory T cells

The immunosuppressive regimens currently used in transplantation to prevent allograft destruction by the host's immune system have deleterious side effects and fail to control chronic rejection processes. Induction of donor-specific non-responsiveness (i.e., immunological tolerance) to transplants would solve these problems and would substantially ameliorate patients' quality of life. It has been proposed that bone marrow or hematopoietic stem-cell transplantation, and resulting (mixed) hematopoietic chimerism, lead to immunological tolerance to organs of the same donor. However, a careful analysis of the literature, performed here, clearly establishes that whereas hematopoietic chimerism substantially prolongs allograft survival, it does not systematically prevent chronic rejection. Moreover, the cytotoxic conditioning regimens used to achieve long-term persistence of chimerism are associated with severe side effects that appear incompatible with a routine use in the clinic. Several laboratories recently embarked on different studies to develop alternative strategies to overcome these issues. We discuss here recent advances obtained by combining regulatory T cell infusion with bone-marrow transplantation. In experimental settings, this attractive approach allows development of genuine immunological tolerance to donor tissues using clinically relevant conditioning regimens.

Experimental models of B cell tolerance in transplantation

The use of conventional immunosuppression has successfully improved short-term allograft survival, however, long-term allograft survival has remained static and is complicated by serious side effects secondary to the long-term use of immunosuppressive agents. Immunological tolerance is the ultimate goal of organ transplantation, however it is an infrequent event in humans. Accordingly, over the past several decades, there has been a push to fully understand both the cellular and molecular mechanisms that play a role in the induction and maintenance of tolerance, with recent data implicating B cells and donor specific alloantibody as a barrier to and potential mediator of allograft tolerance. The study of B cells and alloantibody in transplant tolerance has evolved over recent years from using rodent models to non-human primate models. This review will discuss the role of B cells and alloantibody as antagonists and facilitators of transplantation tolerance, and highlight the experimental models developed for elucidating the mechanisms of B cell tolerance to alloantigen.

Dendritic cells transfected with PD-L1 recombinant adenovirus induces T cell suppression and long-term acceptance of allograft transplantation

The purpose of this study is to assess the potential of dendritic cells transfected with PD-L1 recombinant adenovirus induces CD8+ T cell suppression and kidney allograft tolerance. To prove it, DCs transfected with PD-L1 recombinant adenovirus (DC/Ad-PD-L1) were transferred into the MHC-mismatched rat kidney transplants. After kidney transplantation, the mixed lymphocyte reaction (MLR) assay and kidney function were analyzed. The results demonstrated that after administration of DC/Ad-PD-L1, the proliferation, cytokines secretion and activation marker expression of CD8+ T cells were suppressed. In addition, DC/Ad-PD-L1 could improve kidney function and survival of transplants. The findings suggested that DC/Ad-PD-L1 could induce CD8+ T cell tolerance and lead to kidney allograft tolerance, which provided a promising finding for clinical application.

LAG-3, TGF-β, and cell-intrinsic PD-1 inhibitory pathways contribute to CD8 but not CD4 T-cell tolerance induced by allogeneic BMT with anti-CD40L

Administration of a single dose of anti-CD40L mAb at the time of allogeneic BM transplantation tolerizes peripheral alloreactive T cells and permits establishment of mixed hematopoietic chimerism in mice. Once engrafted, mixed chimeras are systemically tolerant to donor Ags through a central deletion mechanism and will accept any donor organ indefinitely. We previously found that the PD-1/PD-L1 pathway is required for CD8 T-cell tolerance in this model. However, the cell population that must express PD-1 and the role of other inhibitory molecules were unknown. Here, we report that LAG-3 is required for long-term peripheral CD8 but not CD4 T-cell tolerance and that this requirement is CD8 cell-extrinsic. In contrast, adoptive transfer studies revealed a CD8 T cell-intrinsic requirement for CTLA4/B7.1/B7.2 and for PD-1 for CD8 T-cell tolerance induction. We also observed that both PD-L1 and PD-L2 are independently required on donor cells to achieve T-cell tolerance. Finally, we uncovered a requirement for TGF-β signaling into T cells to achieve peripheral CD8 but not CD4 T-cell tolerance in this in vivo system.

The power and the promise of restimulation-induced cell death in human immune diseases

Controlled expansion and contraction of lymphocytes both during and after an adaptive immune response are imperative to sustain a healthy immune system. Both extrinsic and intrinsic pathways of lymphocyte apoptosis are programmed to eliminate cells at the proper time to ensure immune homeostasis. Genetic disorders of apoptosis described in mice and humans have established Fas and Bim as critical pro-apoptotic molecules responsible for T-cell death in response to T-cell receptor restimulation and cytokine withdrawal, respectively. Emerging evidence prompts revision of this classic paradigm, especially for our understanding of restimulation-induced cell death (RICD) and its physiological purpose. Recent work indicates that RICD employs both Fas and Bim for T-cell deletion, dispelling the notion that these molecules are assigned to mutually exclusive apoptotic pathways. Furthermore, new mouse model data combined with our discovery of defective RICD in X-linked lymphoproliferative disease (XLP) patient T cells suggest that RICD is essential for precluding excess T-cell accumulation and associated immunopathology during the course of certain infections. Here, we review how these advances offer a refreshing new perspective on the phenomenon of T-cell apoptosis induced through antigen restimulation, including its relevance to immune homeostasis and potential for therapeutic interventions.

CD47 is required for suppression of allograft rejection by donor-specific transfusion

CD47 is a ligand of the inhibitory receptor, signal regulatory protein (SIRP)alpha, and its interaction with SIRPalpha on macrophages prevents phagocytosis of autologous hematopoietic cells. CD47-SIRPalpha signaling also regulates dendritic cell (DC) endocytosis, activation, and maturation. In this study, we show that CD47 expression on donor cells plays an important role in suppression of allograft rejection by donor-specific transfusion (DST). DST was performed by i.v. injection of splenocytes from C57BL/6 donors into MHC class I-disparate bm1 mice 7 d prior to donor skin grafting. Administration of wild-type (WT) C57BL/6 donor splenocytes markedly prolonged donor skin survival in bm1 mouse recipients. In contrast, bm1 mice receiving DST from CD47 knockout (KO) donors showed no inhibition or even acceleration of donor skin graft rejection compared with non-DST control (naive) bm1 mice. T cells from bm1 mice receiving CD47 KO, but not WT, DST exhibited strong anti-donor responses. The ability of DST to suppress alloresponses was positively correlated with the density of CD47 molecules on donor cells, as CD47(+/-) DST was able to prolonged donor skin survival, but to a significantly less extent than WT DST. Furthermore, DCs from CD47 KO, but not WT, DST recipients showed rapid activation and contributed to donor skin rejection. These results show for the first time that CD47 on donor cells is required to repress recipient DC activation and suppress allograft rejection after DST, and suggest CD47 as a potential target for facilitating the induction of transplant tolerance.

Mechanisms of transplantation tolerance in animals and humans

Donor-specific immune tolerance would avoid the toxicities of chronic immunosuppressive therapies while preventing graft rejection. Hematopoietic cell transplantation has shown preliminary success for intentional tolerance induction in pilot clinical trials. The mechanisms of tolerance in these trials and the animal studies leading up to them are discussed.

Hematopoietic cell transplantation for tolerance induction: animal models to clinical trials

The induction of donor-specific immune tolerance is the "holy grail" of transplantation, as it would avoid the toxicities of chronic immunosuppressive therapies while preventing acute and chronic graft rejection. A large number of approaches to tolerance induction have been described in the experimental literature, but only hematopoietic cell transplantation has shown preliminary success for intentional tolerance induction in pilot clinical trials. This review summarizes the conditions that allow progress to be made in moving strategies for tolerance induction from the bench to the bedside and discuss the mechanisms by which tolerance may be achieved through hematopoietic cell transplantation.

Influence of bone marrow graft B lymphocyte subsets on outcome after HLA-identical sibling transplants

The potential role of the infused B cell subset after Hematopoietic Stem Cell Transplantation has not been yet studied. The present study analyzed the impact of B cells on transplant outcome in 254 patients who received a bone marrow graft from a human leucocyte antigen-identical sibling donor. The influence of B lineage-specific hematopoietic progenitor cells (CD34(+) CD19(+)) and B cells (immature and mature B cells, CD34(-) CD19(+)) was also analyzed. All included patients received a myeloablative regimen. The cumulative incidence function of acute graft-versus-host (GvHD) grade II to IV was 48% and was inversely associated with the number of CD34(+) CD19(+). There were no statistically significant associations between B cell subsets and chronic GvHD or survival. The CD34(+) CD19(+) B cell subset remained significantly associated with acute GvHD in multivariate analysis (Relative risk = 0.32, 95% confidence interval: 0.11-0.92, P = 0.035). In conclusion, a higher B lineage-specific hematopoietic progenitor cells (CD34(+) CD19(+)) cell dose is associated with a significant decrease incidence of acute GvHD.