PD-1-dependent mechanisms maintain peripheral tolerance of donor-reactive CD8+ T cells to transplanted tissue

Targeted depletion of PD-1-expressing cells induces immune tolerance through peripheral clonal deletion

Thymic negative selection of the T cell receptor (TCR) repertoire is essential for establishing self-tolerance and acquired allograft tolerance following organ transplantation. However, it is unclear whether and how peripheral clonal deletion of alloreactive T cells induces transplantation tolerance. Here, we establish that programmed cell death protein 1 (PD-1) is a hallmark of alloreactive T cells and is associated with clonal expansion after alloantigen encounter. Moreover, we found that diphtheria toxin receptor (DTR)-mediated ablation of PD-1+ cells reshaped the TCR repertoire through peripheral clonal deletion of alloreactive T cells and promoted tolerance in mouse transplantation models. In addition, by using PD-1-specific depleting antibodies, we found that antibody-mediated depletion of PD-1+ cells prevented heart transplant rejection and the development of experimental autoimmune encephalomyelitis (EAE) in humanized PD-1 mice. Thus, these data suggest that PD-1 is an attractive target for peripheral clonal deletion and induction of immune tolerance.

Fine-tuning tumor- and allo-immunity: advances in the use of immune checkpoint inhibitors in kidney transplant recipients

Cancer is a common complication after kidney transplantation. Kidney transplant recipients (KTR) have a 2- to 4-fold higher risk of developing cancer compared to the general population and post-transplant malignancy is the third most common cause of death in KTR. Moreover, it is well known that certain cancer types are overrepresented after transplantation, especially non-melanoma skin cancer. Immune checkpoint inhibitors (ICI) have revolutionized the treatment of cancer, with remarkable survival benefit in a subgroup of patients. ICI are monoclonal antibodies that block the binding of specific co-inhibitory signaling molecules. Cytotoxic T lymphocyte-associated antigen-4 (CTLA-4), programmed cell death protein 1 (PD-1), and its ligand programmed cell death ligand 1 (PD-L1) are the main targets of ICI. Solid organ transplant recipients (SOTR) have been excluded from clinical trials owing to concerns about tumor response, allo-immunity, and risk of transplant rejection. Indeed, graft rejection has been estimated as high as 48% and represents an emerging problem. The underlying mechanisms of organ rejection in the context of treatment with ICI are poorly understood. The search for restricted antitumoral responses without graft rejection is of paramount importance. This review summarizes the current knowledge of the use of ICI in KTR, the potential mechanisms involved in kidney graft rejection during ICI treatment, potential biomarkers of rejection, and how to deal with rejection in clinical practice.

Research progress on CD8+ T cell immune regulation in allogenic transplantation

With advances in tissue typing, organ preservation techniques, and clinical surgery, organ transplantation has gained popularity as a treatment option for various end-stage diseases. Allogeneic transplantation has been widely adopted and extensively researched in clinical practice. Despite significant breakthroughs and progress in immunosuppression, this procedure is still associated with several adverse reactions and complications. Therefore, there is a continuing need to explore new immunological approaches to provide fresh insights and guidance for clinical transplantation. CD8+ T cells, traditionally known for their cytotoxic function and their ability to recognize transplanted organs as "non-self" entities, display cytotoxicity. However, recent studies have unveiled that CD8+ T cells have various subtypes and functions that extend beyond conventional cytotoxicity. These CD8+ T cell subtypes include Effector CD8+ T cells, Memory CD8+ T cells, and CD8Treg cells. This review examines the immune regulatory mechanisms of CD8+ T cells in allogeneic transplantation and discusses the potential applications of CD8+ T cells in treating tumors in transplant recipients who are receiving immunosuppressive therapy. These findings offer theoretical guidance for reducing post-transplant rejection reactions and improving adverse prognoses, offering new hope for improved clinical survival rate.

Immune Checkpoints in Solid Organ Transplantation

Allogenic graft acceptance is only achieved by life-long immunosuppression, which comes at the cost of significant toxicity. Clinicians face the challenge of adapting the patients' treatments over long periods to lower the risks associated with these toxicities, permanently leveraging the risk of excessive versus insufficient immunosuppression. A major goal and challenge in the field of solid organ transplantation (SOT) is to attain a state of stable immune tolerance specifically towards the grafted organ. The immune system is equipped with a set of inhibitory co-receptors known as immune checkpoints (ICs), which physiologically regulate numerous effector functions. Insufficient regulation through these ICs can lead to autoimmunity and/or immune-mediated toxicity, while excessive expression of ICs induces stable hypo-responsiveness, especially in T cells, a state sometimes referred to as exhaustion. IC blockade has emerged in the last decade as a powerful therapeutic tool against cancer. The opposite action, i.e., subverting IC for the benefit of establishing a state of specific hypo-responsiveness against auto- or allo-antigens, is still in its infancy. In this review, we will summarize the available literature on the role of ICs in SOT and the relevance of ICs with graft acceptance. We will also discuss the possible influence of current immunosuppressive medications on IC functions.

Antithymocyte Globulin Inhibits CD8+ T Cell Effector Functions via the Paracrine Induction of PDL-1 on Monocytes

BACKGROUND

Antithymocyte globulins (ATG) are T cell-depleting antibodies used in solid organ transplantation for induction therapy in sensitized patients with a high risk of graft rejection. Previously described effects besides the depletion of T cells have suggested additional modes of action and identified further cellular targets.

METHODS

We examined the transcriptional changes arising in immune cells from human blood after ex vivo stimulation with ATG at the single-cell level to uncover additional mechanisms by which ATG regulates T cell activity and effector functions.

FINDINGS

Analysis of the paracrine factors present in the plasma of ATG-treated whole blood revealed high levels of chemokines and cytokines, including interferon-γ (IFN-γ). Furthermore, we identified an increase in the surface expression of the programmed death ligand 1 (PDL-1) on monocytes mediated by the released paracrine factors. In addition, we showed that this induction is dependent on the activation of JAK/STAT signaling via the binding of IFN-γ to interferon-γ receptor 1 (IFN-γR1). Lastly, we demonstrated that the modulation of the immune regulatory axis of programmed cell death protein 1 (PD1) on activated CD8⁺ T cells with PDL-1 found on monocytes mediated by ATG potently inhibits effector functions including the proliferation and granzyme B release of activated T cells.

INTERPRETATION

Together, our findings represent a novel mode of action by which ATG exerts its immunosuppressive effects.

T-cell Exhaustion in Organ Transplantation

Exhaustion of T cells occurs in response to long-term exposure to self and foreign antigens. It limits T cell capacity to proliferate and produce cytokines, leading to an impaired ability to clear chronic infections or eradicate tumors. T-cell exhaustion is associated with a specific transcriptional, epigenetic, and metabolic program and characteristic cell surface markers' expression. Recent studies have begun to elucidate the role of T-cell exhaustion in transplant. Higher levels of exhausted T cells have been associated with better graft function in kidney transplant recipients. In contrast, reinvigorating exhausted T cells by immune checkpoint blockade therapies, while promoting tumor clearance, increases the risk of acute rejection. Lymphocyte depletion and high alloantigen load have been identified as major drivers of T-cell exhaustion. This could account, at least in part, for the reduced rates of acute rejection in organ transplant recipients induced with thymoglobulin and for the pro-tolerogenic effects of a large organ such as the liver. Among the drugs that are widely used for maintenance immunosuppression, calcineurin inhibitors have a contrasting inhibitory effect on exhaustion of T cells, while the influence of mTOR inhibitors is still unclear. Harnessing or encouraging the natural processes of exhaustion may provide a novel strategy to promote graft survival and transplantation tolerance.

Resident memory T cells form during persistent antigen exposure leading to allograft rejection

Tissue-resident memory T cells (T_RM) contained at sites of previous infection provide local protection against reinfection. Whether they form and function in organ transplants where cognate antigen persists is unclear. This is a key question in transplantation as T cells are detected long term in allografts, but it is not known whether they are exhausted or are functional memory T cells. Using a mouse model of kidney transplantation, we showed that antigen-specific and polyclonal effector T cells differentiated in the graft into T_RM and subsequently caused allograft rejection. T_RM identity was established by surface phenotype, transcriptional profile, and inability to recirculate in parabiosis and retransplantation experiments. Graft T_RM proliferated locally, produced interferon-γ upon restimulation, and their in vivo depletion attenuated rejection. The vast majority of antigen-specific and polyclonal T_RM lacked phenotypic and transcriptional exhaustion markers. Single-cell analysis of graft T cells early and late after transplantation identified a transcriptional program associated with transition to the tissue-resident state that could serve as a platform for the discovery of therapeutic targets. Thus, recipient effector T cells differentiate into functional graft T_RM that maintain rejection locally. Targeting these T_RM could improve renal transplant outcomes.

MCMV Dissemination from Latently-Infected Allografts Following Transplantation into Pre-Tolerized Recipients

Transplantation tolerance is achieved when recipients are unresponsive to donor alloantigen yet mobilize against third-party antigens, including virus. After transplantation, cytomegalovirus (CMV) reactivation in latently-infected transplants reduces allograft viability. To determine if pre-tolerized recipients are resistant to viral dissemination in this setting, we transfused chemically-fixed donor splenocytes (1-ethyl-3- (3′-dimethyl-aminopropyl)-carbo-diimide (ECDI)-treated splenocytes (ECDIsp)) to induce donor antigen tolerance without immunosuppression. In parallel, we implanted donor islet cells to validate operational tolerance. These pre-tolerized recipients were implanted with murine CMV (MCMV) latently-infected donor kidneys (a validated model of CMV latency) to monitor graft inflammation and viral dissemination. Our results indicate that tolerance to donor islets was sustained in recipients after implantation of donor kidneys. In addition, kidney allografts implanted after ECDIsp and islet implantation exhibited low levels of fibrosis and tubulitis. In contrast, kidney cellular and innate immune infiltrates trended higher in the CMV group and exhibited increased markers of CD8⁺ T cell activation. Tolerance induction was unable to prevent increases in MCMV-specific CD8⁺ T cells or dissemination of viral IE-1 DNA. Our data suggest that latently-infected allografts are inherently more susceptible to inflammation that is associated with viral dissemination in pre-tolerized recipients. Thus, CMV latently-infected allografts require enhanced strategies to protect allograft integrity and viral spread.

PD-1 and PD-L1 expression on T-cell subsets in women with unexplained recurrent pregnancy losses

PROBLEM

Does programmed death-1 (PD-1)/programmed death ligand-1 (PD-L1) expression on the T-cell subsets such as T helper (Th) 1, Th17, and Treg cells differentiate women with recurrent pregnancy losses (RPL) from normal fertile women?

METHOD OF STUDY

The study was designed as a prospective cohort study. Forty-five women with two or more RPL of unknown etiology and twenty fertile women who had at least one or more live-born infants were enrolled prospectively from Jan 2017 to Jul 2019. PD-1 and PD-L1 expression on T-cell subsets were measured by flow cytometric analysis.

RESULTS

The proportions of PD-1⁺ Th1 (CD4⁺ /IFN-γ⁺ /CD279⁺ and CD4⁺ /TNF-α⁺ /CD279⁺ ) and PD-1⁺ Th17 cells (CD4⁺ /IL17⁺ /CD279⁺ ) were significantly lower in RPL group than those of controls (P < .05, respectively). The proportion of PD-1⁺ Tregs (CD4⁺ /CD25⁺ /CD127^dim/- /CD279⁺ ) in RPL group was not different from that of controls. The proportion of PD-L1⁺ Th17 cells (CD4⁺ IL17⁺ CD274⁺ ) was significantly lower as compared with that of /controls (P < .05). However, the proportions of PD-L1⁺ Th1 (CD4⁺ /IFN-γ⁺ /CD274⁺ and CD4⁺ /TNF-α⁺ /CD274⁺ ) and PD-L1⁺ Treg (CD4⁺ /CD25⁺ /CD127^dim/- /CD274⁺ ) cells were not different between the RPL group and controls (P > .05, respectively). In Th1, Th17 and Treg cells, the proportions of PD-L1⁺ (CD274⁺ ) cells were significantly higher than those of PD-1⁺ (CD279⁺ ) cells in both RPL group and controls (P < .05, respectively).

CONCLUSION

PD-1 and PD-L1 expressions on Th17 cells as well as PD-1 expression on Th1 cells were significantly downregulated in women with RPL, which may lead to increased Th1 and Th17 immunity, and imbalance between Th17, Th1, and Treg cells in women with RPL.

CD8+CD122+PD-1+ Tregs Synergize With Costimulatory Blockade of CD40/CD154, but Not B7/CD28, to Prolong Murine Allograft Survival

A transplanted organ is always rejected in the absence of any immunosuppressive treatment due to vigorous alloimmunity. However, continuously global immunosuppression with a conventional immunosuppressant may result in severe side effects, including nephrotoxicity, tumors and infections. Tregs have been widely used to inhibit allograft rejection, especially in animal models. However, it's well accepted that administration of Tregs alone is not satisfactory in immune-competent wild-type animals. Therefore, it's imperative to promote Treg therapies under the cover of other approaches, including costimulatory blockade. In the present study, we demonstrated that administration of in vitro-expanded CD8⁺CD122⁺PD-1⁺ Tregs synergized with costimulatory blockade of CD40/CD154, but not B7/CD28, to prolong skin allograft survival in wild-type mice and to reduce cellular infiltration in skin allografts as well. Treg treatment and blockade of CD40/CD154, but not B7/CD28, also exhibited an additive effect on suppression of T cell proliferation in vitro and pro-inflammatory cytokine expression in skin allografts. Importantly, blocking B7/CD28, but not CD40/CD154, costimulation decreased the number of transferred CD8⁺CD122⁺PD-1⁺ Tregs and their expression of IL-10 in recipient mice. Furthermore, it's B7/CD28, but not CD40/CD154, costimulatory blockade that dramatically reduced IL-10 production by CD8⁺CD122⁺PD-1⁺ Tregs in vitro, suggesting that B7/CD28, but not CD40/CD154, costimulation is critical for their production of IL-10. Indeed, infusion of IL-10-deficient CD8⁺CD122⁺PD-1⁺ Tregs failed to synergize with anti-CD154 Ab treatment to further prolong allograft survival. Our data may explain why blocking B7/CD28 costimulatory pathway does not boost IL-10-dependent Treg suppression of alloimmunity. Thus, these findings could be implicated in clinical organ transplantation.

The importance of the PD-1/PD-L1 pathway at the maternal-fetal interface

Background

Our goal with this study was to investigate the contribution of PD-1/PD-L1 immune-checkpoint pathway to maternal immunotolerance mechanisms.

Methods

Thirteen healthy pregnant women and 10 non-pregnant controls were involved in this project. PBMCs and DICs were isolated from peripheral blood and from decidual tissues. After the characterization of different immune cell subsets, we used fluorochrome-conjugated monoclonal antibodies to measure the expression level of PD-1, PD-L1, NKG2D, and CD107a molecules by flow cytometry.

Results

We measured significant alternations in the proportion of decidual immune cell subsets compared to the periphery. Elevated PD-1 expression by decidual CD8+ T, CD4+ T, and NKT-like cells were also detected accompanied by the increased PD-L1 expression by decidual CD4+ T, Treg, NKT-like and CD56 + NK cell subsets compared to peripheral blood. The cytotoxic potential was significantly higher in PD-1- decidual immune cells compared to the periphery, however we measured a significantly lower cytotoxicity in the decidual PD-1+ CD8+ T cells compared with the peripheral subsets. An activation receptor NKG2D expression was decreased by the PD-1+ CD8+ T subsets in the first trimester compared to non-pregnant condition but the expression level of the decidual counterparts was significantly elevated compared to the periphery. The cytotoxic potential of decidual PD1/NKG2D double positive CD8+ T cells was significantly decreased compared to the peripheral subsets.

Conclusions

Based on our results we assume that PD-1/PD-L1 pathway might have a novel role in the maintaining of the local immunological environment. Accompanied by NKG2D activating receptor this checkpoint interaction could regulate decidual CD8 Tc cell subsets and may contribute maternal immunotolerance.

T cell exhaustion implications during transplantation

Exhaustion of lymphocyte function, particularly T cell exhaustion, due to prolonged exposure to a high load of foreign antigen is commonly seen during chronic viral infection as well as antitumor immune responses. This phenomenon has been associated with a determined molecular mechanism and phenotypic manifestations on the cell surface. In spite of investigation of exhaustion, mostly about CD8 responses toward viral infections, recent studies have reported that chronic exposure to antigen may develop exhaustion in CD4 + T cells, B cells, and NK cells. Little is known with respect to lymphocyte exhaustion during transplantation and its effect on aberrant anti-graft responses. Through a same mechanobiology observed during chronic exposure of foreign viral antigens, alloantigen persistence mediated by allograft could develop a favorable circumstance for exhaustion of T cells responding to allograft. However, to achieve better manipulation approaches of this event to reduce the complications during transplantation, we need to be armed with a bulk of knowledge with regard to quality and quantity of T cell exhaustion occurring in various allografts, the kinetics of exhaustion development, the impression of immunosuppressive agents on the exhaustion, and the influence of exhaustion on graft survival and immune tolerance.

CD28 blockade controls T cell activation to prevent graft-versus-host disease in primates

Controlling graft-versus-host disease (GVHD) remains a major unmet need in stem cell transplantation, and new, targeted therapies are being actively developed. CD28-CD80/86 costimulation blockade represents a promising strategy, but targeting CD80/CD86 with CTLA4-Ig may be associated with undesired blockade of coinhibitory pathways. In contrast, targeted blockade of CD28 exclusively inhibits T cell costimulation and may more potently prevent GVHD. Here, we investigated FR104, an antagonistic CD28-specific pegylated-Fab', in the nonhuman primate (NHP) GVHD model and completed a multiparameter interrogation comparing it with CTLA4-Ig, with and without sirolimus, including clinical, histopathologic, flow cytometric, and transcriptomic analyses. We document that FR104 monoprophylaxis and combined prophylaxis with FR104/sirolimus led to enhanced control of effector T cell proliferation and activation compared with the use of CTLA4-Ig or CTLA4-Ig/sirolimus. Importantly, FR104/sirolimus did not lead to a beneficial impact on Treg reconstitution or homeostasis, consistent with control of conventional T cell activation and IL-2 production needed to support Tregs. While FR104/sirolimus had a salutary effect on GVHD-free survival, overall survival was not improved, due to death in the absence of GVHD in several FR104/sirolimus recipients in the setting of sepsis and a paralyzed INF-γ response. These results therefore suggest that effectively deploying CD28 in the clinic will require close scrutiny of both the benefits and risks of extensively abrogating conventional T cell activation after transplant.

Anti-LFA-1 induces CD8 T-cell dependent allograft tolerance and augments suppressor phenotype CD8 cells

The induction of tolerance to transplanted organs is a major objective in transplantation immunology research. Lymphocyte function-associated antigen-1 (LFA-1) interactions have been identified as a key component of the T-cell activation process that may be interrupted to lead to allograft tolerance. In mice, αLFA-1 mAb is a potent monotherapy that leads to the induction of donor-specific transferable tolerance. By interrogating important adaptive and innate immunity pathways, we demonstrate that the induction of tolerance relies on CD8⁺T-cells. We further demonstrate that αLFA-1 induced tolerance is associated with CD8⁺CD28^-T-cells with a suppressor phenotype, and that while CD8 cells are present, the effector T-cell response is abrogated. A recent publication has shown that CD8⁺CD28^- cells are not diminished by cyclosporine or rapamycin, therefore CD8⁺CD28^- cells represent a clinically relevant population. To our knowledge, this is the first time that a mechanism for αLFA-1 induced tolerance has been described.

Identification of IL-10-secreting CD8+CD28-PD-1+ regulatory T cells associated with chronic hepatitis C virus infection

CD8⁺CD28^- regulatory T cells (Tregs) play important roles in chronic viral infections. Programmed death 1 (PD-1) is highly expressed on hepatitis C virus (HCV)-specific CTLs. However, little is known regarding the role of CD8⁺CD28-PD1⁺ T cells in hepatitis C. Herein, we found that the frequency of CD8⁺CD28^-PD1⁺, but not CD8⁺CD28^-PD1^- T cells, correlated with markers of chronic hepatitis C virus (HCV) infection and the response to treatment. Our results showed that CD8⁺CD28-PD1⁺ T cells were significantly elevated in chronic HCV-infected patients and there was a distinct correlation between the frequency of CD8⁺CD28-PD1⁺ T cells and serum levels of HCV RNA. During a 48-week course of treatment with peg-IFN-a2a plus ribavirin, dynamic changes in the frequencies of CD8⁺CD28-PD1⁺ T cells were observed, associated with the virologic response. IL-10 secretion may explain the suppressive function of CD8⁺CD28-PD1⁺ T cells in chronic HCV-infected patients. Overall, our study demonstrates that PD-1 is an important marker of CD8⁺CD28^- Tregs in chronic HCV infection. Thus, the frequency and regulatory function of CD8⁺CD28-PD1⁺ T cells play vital roles in HCV infection and the response to treatment.

PD-1 expression on CD8+ T cells regulates their differentiation within lung allografts and is critical for tolerance induction

Immunological requirements for rejection and tolerance induction differ between various organs. While memory CD8⁺ T cells are considered a barrier to immunosuppression-mediated acceptance of most tissues and organs, tolerance induction after lung transplantation is critically dependent on central memory CD8⁺ T lymphocytes. Here we demonstrate that costimulation blockade-mediated tolerance after lung transplantation is dependent on programmed cell death 1 (PD-1) expression on CD8⁺ T cells. In the absence of PD-1 expression, CD8⁺ T cells form prolonged interactions with graft-infiltrating CD11c⁺ cells; their differentiation is skewed towards an effector memory phenotype and grafts are rejected acutely. These findings extend the notion that requirements for tolerance induction after lung transplantation differ from other organs. Thus, immunosuppressive strategies for lung transplant recipients need to be tailored based on the unique immunological properties of this organ.

Differential Inhibitory Receptor Expression on T Cells Delineates Functional Capacities in Chronic Viral Infection

Inhibitory receptors have been extensively described for their importance in regulating immune responses in chronic infections and cancers. Blocking the function of inhibitory receptors such as PD-1, CTLA-4, 2B4, Tim-3, and LAG-3 has shown promise for augmenting CD8 T cell activity and boosting pathogen-specific immunity. However, the prevalence of inhibitory receptors on CD4 T cells and their relative influence on CD4 T cell functionality in chronic HIV infection remains poorly described. We therefore determined and compared inhibitory receptor expression patterns of 2B4, CTLA-4, LAG-3, PD-1, and Tim-3 on virus-specific CD4 and CD8 T cells in relation to their functional T cell profile. In chronic HIV infection, inhibitory receptor distribution differed markedly between cytokine-producing T cell subsets with, gamma interferon (IFN-γ)- and tumor necrosis factor alpha (TNF-α)-producing cells displaying the highest and lowest prevalence of inhibitory receptors, respectively. Blockade of inhibitory receptors differentially affected cytokine production by cells in response to staphylococcal enterotoxin B stimulation. CTLA-4 blockade increased IFN-γ and CD40L production, while PD-1 blockade strongly augmented IFN-γ, interleukin-2 (IL-2), and TNF-α production. In a Friend retrovirus infection model, CTLA-4 blockade in particular was able to improve control of viral replication. Together, these results show that inhibitory receptor distribution on HIV-specific CD4 T cells varies markedly with respect to the functional subset of CD4 T cells being analyzed. Furthermore, the differential effects of receptor blockade suggest novel methods of immune response modulation, which could be important in the context of HIV vaccination or therapeutic strategies.IMPORTANCE Inhibitory receptors are important for limiting damage by the immune system during acute infections. In chronic infections, however, their expression limits immune system responsiveness. Studies have shown that blocking inhibitory receptors augments CD8 T cell functionality in HIV infection, but their influence on CD4 T cells remains unclear. We assessed the expression of inhibitory receptors on HIV-specific CD4 T cells and their relationship with T cell functionality. We uncovered differences in inhibitory receptor expression depending on the CD4 T cell function. We also found differences in functionality of CD4 T cells following blocking of different inhibitory receptors, and we confirmed our results in a Friend virus retroviral model of infection in mice. Our results show that inhibitory receptor expression on CD4 T cells is linked to CD4 T cell functionality and could be sculpted by blockade of specific inhibitory receptors. These data reveal exciting possibilities for the development of novel treatments and immunotherapeutics.

Fifty Shades of Tolerance: Beyond a Binary Tolerant/Non-Tolerant Paradigm

Purpose of review

It has long been considered that tolerance in a transplant recipient is a binary all-or-none state: either the graft is accepted without immunosuppression identifying the recipient as tolerant, or the recipient rejects the graft and is not tolerant. This tolerance paradigm, however, does not accurately reflect data emerging from animal models and patients and requires revision.

Recent Findings

It is becoming appreciated that there may be different gradations in the quality of tolerance based on underlying cellular mechanisms of immunological tolerance, and that individuals may enhance their tolerance by strengthening or combining different cellular mechanisms. Furthermore, evidence suggests that even if tolerance is lost, the loss may be only temporary, and in some circumstances tolerance can be restored.

Summary

Shifting our focus from an all-or-nothing tolerance paradigm to one with many shades may help us better understand how tolerance operates, and how this state may be tracked and enhanced for better patient outcomes.

Advances in targeting co-inhibitory and co-stimulatory pathways in transplantation settings: the Yin to the Yang of cancer immunotherapy

In the past decade, the power of harnessing T-cell co-signaling pathways has become increasingly understood to have significant clinical importance. In cancer immunotherapy, the field has concentrated on two related modalities: First, targeting cancer antigens through highly activated chimeric antigen T cells (CAR-Ts) and second, re-animating endogenous quiescent T cells through checkpoint blockade. In each of these strategies, the therapeutic goal is to re-ignite T-cell immunity, in order to eradicate tumors. In transplantation, there is also great interest in targeting T-cell co-signaling, but with the opposite goal: in this field, we seek the Yin to cancer immunotherapy's Yang, and focus on manipulating T-cell co-signaling to induce tolerance rather than activation. In this review, we discuss the major T-cell signaling pathways that are being investigated for tolerance induction, detailing preclinical studies and the path to the clinic for many of these molecules. These include blockade of co-stimulation pathways and agonism of coinhibitory pathways, in order to achieve the delicate state of balance that is transplant tolerance: a state which guarantees lifelong transplant acceptance without ongoing immunosuppression, and with preservation of protective immune responses. In the context of the clinical translation of immune tolerance strategies, we discuss the significant challenge that is embodied by the fact that targeted pathway modulators may have opposing effects on tolerance based on their impact on effector vs regulatory T-cell biology. Achieving this delicate balance holds the key to the major challenge of transplantation: lifelong control of alloreactivity while maintaining an otherwise intact immune system.

PD-L1 Is Not Constitutively Expressed on Tasmanian Devil Facial Tumor Cells but Is Strongly Upregulated in Response to IFN-γ and Can Be Expressed in the Tumor Microenvironment

The devil facial tumor disease (DFTD) is caused by clonal transmissible cancers that have led to a catastrophic decline in the wild Tasmanian devil (Sarcophilus harrisii) population. The first transmissible tumor, now termed devil facial tumor 1 (DFT1), was first discovered in 1996 and has been continually transmitted to new hosts for at least 20 years. In 2015, a second transmissible cancer [devil facial tumor 2 (DFT2)] was discovered in wild devils, and the DFT2 is genetically distinct and independent from the DFT1. Despite the estimated 136,559 base pair substitutions and 14,647 insertions/deletions in the DFT1 genome as compared to two normal devil reference genomes, the allograft tumors are not rejected by the host immune system. Additionally, genome sequencing of two sub-strains of DFT1 detected greater than 15,000 single-base substitutions that were found in only one of the DFT1 sub-strains, demonstrating the transmissible tumors are evolving and that generation of neoantigens is likely ongoing. Recent evidence in human clinical trials suggests that blocking PD-1:PD-L1 interactions promotes antitumor immune responses and is most effective in cancers with a high number of mutations. We hypothesized that DFTD cells could exploit the PD-1:PD-L1 inhibitory pathway to evade antitumor immune responses. We developed recombinant proteins and monoclonal antibodies (mAbs) to provide the first demonstration that PD-1 binds to both PD-L1 and PD-L2 in a non-placental mammal and show that PD-L1 is upregulated in DFTD cells in response to IFN-γ. Immunohistochemistry showed that PD-L1 is rarely expressed in primary tumor masses, but low numbers of PD-L1+ non-tumor cells were detected in the microenvironment of several metastatic tumors. Importantly, in vitro testing suggests that PD-1 binding to PD-L1 and PD-L2 can be blocked by mAbs, which could be critical to understanding how the DFT allografts evade the immune system.

Tracking of TCR-Transgenic T Cells Reveals That Multiple Mechanisms Maintain Cardiac Transplant Tolerance in Mice

Solid organ transplantation tolerance can be achieved following select transient immunosuppressive regimens that result in long-lasting restraint of alloimmunity without affecting responses to other antigens. Transplantation tolerance has been observed in animal models following costimulation or coreceptor blockade therapies, and in a subset of patients through induction protocols that include donor bone marrow transplantation, or following withdrawal of immunosuppression. Previous data from our lab and others have shown that proinflammatory interventions that successfully prevent the induction of transplantation tolerance in mice often fail to break tolerance once it has been stably established. This suggests that established tolerance acquires resilience to proinflammatory insults, and prompted us to investigate the mechanisms that maintain a stable state of robust tolerance. Our results demonstrate that only a triple intervention of depleting CD25⁺ regulatory T cells (Tregs), blocking programmed death ligand-1 (PD-L1) signals, and transferring low numbers of alloreactive T cells was sufficient to break established tolerance. We infer from these observations that Tregs and PD-1/PD-L1 signals cooperate to preserve a low alloreactive T cell frequency to maintain tolerance. Thus, therapeutic protocols designed to induce multiple parallel mechanisms of peripheral tolerance may be necessary to achieve robust transplantation tolerance capable of maintaining one allograft for life in the clinic.

T Cell Cosignaling Molecules in Transplantation

The ultimate outcome of alloreactivity versus tolerance following transplantation is potently influenced by the constellation of cosignaling molecules expressed by immune cells during priming with alloantigen, and the net sum of costimulatory and coinhibitory signals transmitted via ligation of these molecules. Intense investigation over the last two decades has yielded a detailed understanding of the kinetics, cellular distribution, and intracellular signaling networks of cosignaling molecules such as the CD28, TNF, and TIM families of receptors in alloimmunity. More recent work has better defined the cellular and molecular mechanisms by which engagement of cosignaling networks serve to either dampen or augment alloimmunity. These findings will likely aid in the rational development of novel immunomodulatory strategies to prolong graft survival and improve outcomes following transplantation.

Tolerance induction using nanoparticles bearing HY peptides in bone marrow transplantation

Allogeneic cell therapies have either proven effective or have great potential in numerous applications, though the required systemic, life-long immunosuppression presents significant health risks. Inducing tolerance to allogeneic cells offers the potential to reduce or eliminate chronic immunosuppression. Herein, we investigated antigen-loaded nanoparticles for their ability to promote transplant tolerance in the minor histocompatibility antigen sex-mismatched C57BL/6 model of bone marrow transplantation. In this model, the peptide antigens Dby and Uty mediate rejection of male bone marrow transplants by female CD4+ and CD8+ T cells, respectively, and we investigated the action of nanoparticles on these T cell subsets. Antigens were coupled to or encapsulated within poly(lactide-co-glycolide) (PLG) nanoparticles with an approximate diameter of 500 nm. Delivery of the CD4-encoded Dby epitope either coupled to or encapsulated within PLG particles prevented transplant rejection, promoted donor-host chimerism, and suppressed proliferative and IFN-γ responses in tolerized recipients. Nanoparticles modified with the Uty peptide did not induce tolerance. The dosing regimen was investigated with Dby coupled particles, and a single dose delivered the day after bone marrow transplant was sufficient for tolerance induction. The engraftment of cells was significantly affected by PD-1/PDL-1 costimluation, as blockade of PD-1 reduced engraftment by ∼50%. In contrast, blockade of regulatory T cells did not impact the level of chimerism. The delivery of antigen on PLG nanoparticles promoted long-term engraftment of bone marrow in a model with a minor antigen mismatch in the absence of immunosuppression, and this represents a promising platform for developing a translatable, donor-specific tolerance strategy.

T-cell exhaustion in allograft rejection and tolerance

PURPOSE OF REVIEW

The role of T-cell exhaustion in the failure of clearance of viral infections and tumors is well established. There are several ongoing trials to reverse T-cell exhaustion for treatment of chronic viral infections and tumors. The mechanisms leading to T-cell exhaustion and its role in transplantation, however, are only beginning to be appreciated and are the focus of the present review.

RECENT FINDINGS

Exhausted T cells exhibit a distinct molecular profile reflecting combinatorial mechanisms involving the interaction of multiple transcription factors important in control of cell metabolism, acquisition of effector function and memory capacity. Change of microenvironmental cues and limiting leukocyte recruitment can modulate T-cell exhaustion. Impaired leukocyte recruitment induces T-cell exhaustion and prevents allograft rejection.

SUMMARY

Preventing or reversing T-cell exhaustion may lead to prevention of transplant tolerance or triggering of rejection; therefore, caution should be exercised in the use of agents blocking inhibitory receptors for the treatment of chronic viral infections or tumors in transplant recipients. Further definition of the role of T-cell exhaustion in clinical transplantation and an understanding of the mechanisms of induction of T-cell exhaustion are needed to develop strategies for preventing allograft rejection and induction of tolerance.

IL-7 receptor blockade following T cell depletion promotes long-term allograft survival

T cell depletion is commonly used in organ transplantation for immunosuppression; however, a restoration of T cell homeostasis following depletion leads to increased memory T cells, which may promote transplant rejection. The cytokine IL-7 is important for controlling lymphopoiesis under both normal and lymphopenic conditions. Here, we investigated whether blocking IL-7 signaling with a mAb that targets IL-7 receptor α (IL-7Rα) alone or following T cell depletion confers an advantage for allograft survival in murine transplant models. We found that IL-7R blockade alone induced indefinite pancreatic islet allograft survival if anti-IL-7R treatment was started 3 weeks before graft. IL-7R blockade following anti-CD4- and anti-CD8-mediated T cell depletion markedly prolonged skin allograft survival. Furthermore, IL-7 inhibition in combination with T cell depletion synergized with either CTLA-4Ig administration or suboptimal doses of tacrolimus to induce long-term skin graft acceptance in this stringent transplant model. Together, these therapies inhibited T cell reconstitution, decreased memory T cell numbers, increased the relative frequency of Tregs, and abrogated both cellular and humoral alloimmune responses. Our data suggest that IL-7R blockade following T cell depletion has potential as a robust, immunosuppressive therapy in transplantation.

2B4 (CD244) induced by selective CD28 blockade functionally regulates allograft-specific CD8+ T cell responses

Blockade of CD28 signals results in the up-regulation of 2B4 on primary CD8⁺ effectors and plays a critical role in controlling antigen-specific CD8⁺ T cell responses.

Mounting evidence in models of both autoimmunity and chronic viral infection suggests that the outcome of T cell activation is critically impacted by the constellation of co-stimulatory and co-inhibitory receptors expressed on the cell surface. Here, we identified a critical role for the co-inhibitory SLAM family member 2B4 (CD244) in attenuating primary antigen-specific CD8⁺ T cell responses in the presence of immune modulation with selective CD28 blockade. Our results reveal a specific up-regulation of 2B4 on antigen-specific CD8⁺ T cells in animals in which CD28 signaling was blocked. However, 2B4 up-regulation was not observed in animals treated with CTLA-4 Ig (abatacept) or CD28 blockade in the presence of anti–CTLA-4 mAb. 2B4 up-regulation after CD28 blockade was functionally significant, as the inhibitory impact of CD28 blockade was diminished when antigen-specific CD8⁺ T cells were deficient in 2B4. In contrast, 2B4 deficiency had no effect on CD8⁺ T cell responses during unmodified rejection or in the presence of CTLA-4 Ig. We conclude that blockade of CD28 signals in the presence of preserved CTLA-4 signals results in the unique up-regulation of 2B4 on primary CD8⁺ effectors, and that this 2B4 expression plays a critical functional role in controlling antigen-specific CD8⁺ T cell responses.

PD-1 regulates T cell proliferation in a tissue and subset-specific manner during normal mouse pregnancy

The regulation of T cell homeostasis during pregnancy has important implications for maternal tolerance and immunity. Evidence suggests that Programmed Death-1 (PD-1) participates in regulation of T cell homeostasis and peripheral tolerance. To examine the contribution of PD-1 signaling on T cell homeostasis during normal mouse pregnancy, we examined T cell number or proportion, PD-1 expression, proliferation, and apoptosis by flow cytometry, BrdU incorporation, and TUNEL assay in pregnant mice given anti-PD-1 blocking antibody or control on days 10, 12, and 14 of gestation. We observed tissue, treatment, and T cell-specific differences in PD-1 expression. Both pregnancy and PD-1 blockade increased T cell proliferation in the spleen, yet this effect was limited to CD4 T cells in the uterine- draining nodes. In the uterus, PD-1 blockade markedly altered the composition of the T cell pool. These studies support the idea that pregnancy is a state of dynamic T cell homeostasis and suggest that this state is partially supported by PD-1 signaling.

Role of the PD-1 pathway in the immune response

Understanding immunoregulatory mechanisms is essential for the development of novel interventions to improve long-term allograft survival. Programmed death 1 (PD-1) and its ligands, PD-L1 and PD-L2, have emerged as critical inhibitory signaling pathways that regulate T cell response and maintain peripheral tolerance. PD-1 signaling inhibits alloreactive T cell activation, and can promote induced regulatory T cell development. Furthermore, the upregulation of PD-L1 on nonhematopoietic cells of the allograft may actively participate in the inhibition of immune responses and provide tissue-specific protection. In murine transplant models, this pathway has been shown to be critical for the induction and maintenance of graft tolerance. In this review, we discuss the current knowledge of the immunoregulatory functions of PD-1 and its ligands and their therapeutic potential in transplantation.

Minor antigens on transfused RBCs crossprime CD8 T cells but do not induce full effector function

HLA-matched bone marrow transplantation (BMT) is a cure for nonmalignant hematological disorders; however, rejection rates are high and correlate with the number of antecedent transfusions. Recently, using murine models, we reported that minor antigens (mHAs) in transfused leukoreduced red blood cell (RBC) or platelet units induce rejection of subsequent BMT. To study RBCs as an immunogen, we utilized transgenic donors that express a model mHA selectively on RBCs (HOD mouse). Transfusion of HOD blood did not induce BMT rejection of marrow that shared mHAs with the HOD RBCs. Similarly, no endogenous anti-HOD CD8(+) T-cell response was detected with antigen-specific tetramer reagents. Adoptively transferred OT-I T cells rapidly expanded after HOD blood transfusion; however, only a semi-effector phenotype was observed (tumor necrosis factor-α and interferon-γ secretion, but essentially no Granzyme B). After initial expansion, OT-I T cells contracted rapidly to very low levels. A similar trend was observed by in vivo CTL assay, with only transient lytic activity. Together, these data indicate that RBCs may not be the component of RBC units that induces BMT rejection, and suggest that contaminating platelets or leukocytes may be responsible.

Lacto-N-fucopentaose III, a pentasaccharide, prolongs heart transplant survival

BACKGROUND

Lacto-N-fucopentaose III (LNFPIII) is a pentasaccharide containing the Lewis(x) trisaccharide that is found on schistosome eggs and in breast milk. LNFPIII conjugates suppress host immune responses and have therapeutic efficacy in mouse models of psoriasis and type 1 diabetes.

METHODS

We used nonvascularized neonatal ear-heart transplantation and heterotopic vascularized heart transplantation models to evaluate immunosuppressive effects of LNFPIII and subsequently analyzed the mechanism.

RESULTS

We found that administration of LNFPIII conjugates prolonged median graft survival by 80% when 1-day-old DBA/2 hearts were transplanted into ears of B6 mice. A similar graft prolongation was observed in a fully vascularized heterotopic heart transplantation model (DBA/2 into B6), No prolongation was observed with carrier protein (human serum albumin [HSA] or dextran) alone. We found increased programmed death ligand 1 (PD-L1) expression on F4/80 macrophages, CD4+ T cells, and CD11b+ CD11c+ (myeloid) dendritic cells, and increased arginase1 and Ym1 expression, typical of alternatively activated macrophages, in the draining (cervical) lymph node cells. We found accumulation of Foxp3+ regulatory T cells (Tregs) in the lymph nodes draining donor hearts, suggesting a possible role of Treg induction in graft prolongation. Anti-PD-L1 antibody treatment abrogated LNFPIII-mediated the graft survival benefit and Treg accumulation. LNFPIII-treated macrophages had increased PD-L1 expression and significantly prolonged DBA/2 allograft survival when injected intraperitoneally into B6 recipient mice.

CONCLUSIONS

LNFPIII prolongs fully allogeneic graft survival in both vascularized and nonvascularized allograft transplantation models. The mechanism of graft prolongation seems to involve both alternatively activated PDL-1 macrophages and recruitment of Foxp3+ Treg cells.

The role of co-inhibitory signals in spontaneous tolerance of weakly mismatched transplants

The immune system of female H-2(b) (C57BL/6) mice is a strong responder against the male minor-H antigen. However rejection or acceptance of such weakly mismatched grafts depends on the type of tissue transplanted. The mechanism responsible for such spontaneous graft acceptance, and its relationship to the natural mechanisms of tolerance of self antigens is unknown. Co-inhibitory molecules negatively regulate immune responses, and are important for self tolerance. We examined whether co-inhibitory molecules play a critical role in "spontaneous" allograft tolerance. Naïve or donor sensitized diabetic female C57BL/6 (B6) wild type (WT), PD-1(-/-), and BTLA(-/-) mice were transplanted with freshly isolated syngeneic male islet grafts. The role of co-inhibitors during priming of anti-donor responses and graft challenge was also assessed using monoclonal antibodies targeting co-inhibitory receptors. Among the co-inhibitor (CTLA-4, PD-1) specific antibodies tested, only anti-PD-1 showed some potential to prevent spontaneous acceptance of male islet grafts. All BTLA(-/-) and almost all PD-1(-/-) recipients maintained the ability to spontaneously accept male islet grafts. While spontaneous graft acceptance in naïve recipients was only weakly PD-1 dependent, tolerance induced by the accepted islets was found to be highly PD-1 dependent. Furthermore, spontaneous graft acceptance in pre-sensitized recipients showed an absolute requirement for recipient PD-1 but not BTLA. Thus, the PD-1 pathway, involved in self tolerance, plays a critical role in spontaneous tolerance induced by weakly mismatched grafts in naïve recipients and spontaneous graft acceptance in pre-sensitized recipients.

CD28 blockade induces division-dependent downregulation of interleukin-2 receptor alpha

BACKGROUND

Blockade of T cell costimulatory molecules represents a promising new method of attenuating donor-reactive T cell responses to promote graft survival following transplantation. However, recent studies in murine models have shown the presence of an initial high frequency of naïve donor-reactive T cells may render this strategy ineffective.

METHODS

In this report, we examined the phenotypic changes associated with CD28 blockade on T cells stimulated at increasing precursor frequencies in vitro.

RESULTS

We found that treatment with the CD28 blocker CTLA-4 Ig resulted in downregulation of the alpha chain of the IL-2 receptor (CD25) in a division-dependent manner. Significantly, blockade of the CD28 pathway was more effective in down-regulating CD25 when the donor-reactive T cell population was present at low as compared to high precursor frequency.

CONCLUSIONS

These results imply that treatment with CD28 blockers and anti-CD25 mAbs may cooperate in promoting graft survival under conditions of low MHC matching where the donor-reactive T cell precursor frequency is high.

Overcoming the memory barrier in tolerance induction: molecular mimicry and functional heterogeneity among pathogen-specific T-cell populations

PURPOSE OF REVIEW

This review highlights recent advances in our understanding of the frequency and nature of alloreactivity among memory T-cell populations, and discusses recent successes in experimentally targeting these populations in order to prolong graft survival.

RECENT FINDINGS

Recent studies suggest that not only is alloreactivity present within peripheral T-cell compartments of normal healthy individuals, but cross-reactivity between viral-specific T cells and allotropes may in fact be a very common occurrence. Furthermore, this cross-reactivity functions at the level of molecular mimicry of T-cell receptor recognition. Therapeutics that specifically target cell surface molecules or effector pathways used by memory T cells to mediate graft rejection will likely be required in order to attenuate the donor-reactive memory T-cell response during transplantation.

SUMMARY

A major challenge facing the field over the next decade is to define the heterogeneity that exists within memory T-cell populations that impacts graft survival. Understanding the functional and phenotypic differences that modify the memory T-cell barrier to tolerance induction might allow a strategy in which strength of immunosuppression could be tailored to fit the immunological history of a given transplant recipient in order to minimize nonimmune toxicities, maximize protective immunity, and prolong graft survival.

High-frequency alloreactive T cells augment effector function of low-frequency CD8+ T-cell responses under CD28/CD154 blockade

BACKGROUND

Blockade of costimulatory molecules is a potent method of inducing long-term graft survival. We have previously addressed the issue of donor-reactive T-cell precursor frequency on relative costimulation dependence and found that the presence of a high precursor frequency of donor-reactive CD8 T cells resulted in costimulation blockade-resistant graft rejection, whereas the presence of a low-frequency donor-reactive population did not. To address the mechanisms by which high-frequency T cells obviated the requirement for costimulation, we asked whether a low-frequency population responding concomitantly with a high-frequency response also demonstrated costimulation independence.

METHODS

A model system was established in which B6 mice containing a low frequency of anti-membrane bound chicken ovalbumin (mOVA) responders and a high frequency of anti-BALB/c responders received a skin graft from B6.mOVAxBALB/c F1 donors in the presence or absence of cytotoxic T-lymphocyte antigen-4 Ig/anti-CD154 costimulatory blockade.

RESULTS

The results revealed that in the presence of costimulation blockade, high-frequency anti-BALB/c T cells augmented the effector activity of low-frequency anti-mOVA T cells, but it did not enhance the accumulation of anti-mOVA T cells capable of mediating graft rejection.

CONCLUSIONS

These results demonstrate that both antigen-specific and antigen-independent factors contribute to the relative costimulation independence of high-frequency T-cell responses.

Cutting edge: programmed death-1 defines CD8+CD122+ T cells as regulatory versus memory T cells

Recent convincing data have shown that naturally occurring CD8(+)CD122(+) T cells are also regulatory T cells. Paradoxically, CD8(+)CD122(+) T cells have been well described as memory T cells. Given their critical role in tolerance versus long-term immunity, it is important to reconcile this profound dichotomy. In this study, we reported that CD8(+)CD122(+) T cells contain both programmed death-1 (PD-1)(-) and PD-1(+) populations. It was CD8(+)CD122(+)PD-1(+) T cells, but not their PD-1(-) counterparts, that suppressed T cell responses in vitro and in vivo. This suppression was largely dependent on their production of IL-10. Moreover, the costimulatory signaling of both CD28 and PD-1 is required for their optimal IL-10 production. In contrast, Ag-specific CD8(+)CD122(+)PD-1(-) T cells were bona fide memory T cells. Thus, CD8(+)CD122(+) T cells can be either regulatory T or memory T cells, depending on their PD-1 expression and Ag specificity. This study reconciles previously contradictory findings and has important implications for tolerance induction.

Co-inhibitory molecules: Controlling the effectors or controlling the controllers?

Nearly forty years ago the concept was proposed that lymphocytes are negatively regulated by what are now called co-inhibitory signals. Nevertheless, it is only the more recent identification of numerous co-inhibitors and their critical functions that has brought co-inhibition to the forefront of immunologic research. Although co-inhibitory signals have been considered to directly regulate conventional T cells, more recent data has indicated a convergence between co-inhibitory signals and the other major negative control mechanism in the periphery that is mediated by regulatory T cells. Furthermore, it is now clear that lymphocytes are not the sole domain of co-inhibitory signals, as cells of the innate immune system, themselves controllers of immunity, are regulated by co-inhibitors they express. Thus, in order to better understand negative regulation in the periphery and apply this knowledge to the treatment of disease, a major focus for the future should be the definition of the conditions where co-inhibition controls effector cells intrinsically versus extrinsically (via regulatory or innate cells).

Programmed death-1 signaling is essential for the skin allograft protection by alternatively activated dendritic cell infusion in mice

BACKGROUND

Alternatively activated dendritic cell (aaDC) can prolong allograft survival in the mouse model. However, the molecular mechanism(s) by which these DCs function to regulate alloreactive T-cell responses remains to be clearly defined.

METHODS

Bone marrow-derived DCs were incubated in the presence of interleukin (IL)-10 (immature DC), stimulated with lipopolysaccharide only (mature DC), or pretreated with IL-10 and then activated with lipopolysaccharide (aaDC). These cells were compared for their phenotypes and regulatory capacities both in vitro and in vivo. In addition, programmed death-1 (PD-1)/PD-L pathway was blocked to test its contribution to the regulatory function of aaDC.

RESULTS

The expression of surface major histocompatibility complex class II, CD80, and CD86 on aaDC was lower than that on mDC, whereas aaDC had a higher expression of PD-L1 and PD-L2 compared with immature DC or untreated DC. In vitro co-culture of aaDC with allogeneic T cells led to a significant decrease in the T-cell response as well as a reduction of interferon-gamma secretion and an enhanced IL-10 production while CD4 CD25 Foxp3 T cells were expanded. Interestingly, these regulatory effects of aaDC were partially abolished when PD-1/PD-L pathway was blocked using anti-PD-1 blocking antibody. Infusion of BALB/c donor-derived aaDC into naive C57BL/6 recipients resulted in a significantly prolonged skin allograft survival, which was, at least in part, PD-1/PD-L pathway dependent.

CONCLUSION

Our data indicate that the PD-1/PD-L pathway plays an important role in aaDC-mediated prolongation of skin allograft survival.

Expression of a CD200 transgene is necessary for induction but not maintenance of tolerance to cardiac and skin allografts

CD200, a type 2 transmembrane molecule of the Ig supergene family, can induce immunosuppression in a number of biological systems, as well as promote increased graft acceptance, following binding to its receptors (CD200Rs). Skin and cardiac allograft acceptance are readily induced in transgenic mice overexpressing CD200 under control of a doxycycline-inducible promoter, both of which are associated with increased intragraft expression of mRNAs for a number of genes associated with altered T cell subset differentiation, including GATA-3, type 2 cytokines (IL-4, IL-13), GITR, and Foxp3. Interestingly, some 12-15 days after grafting, induction of transgenic CD200 expression can be stopped (by doxycycline withdrawal), without obvious significant effect on graft survival. However, neutralization of all CD200 expression (including endogenous CD200 expression) by anti-CD200 mAb caused graft loss, as did introduction of an acute inflammatory stimulus (LPS, 10 microg/mouse, delivered by i.p. injection). We conclude that even with apparently stably accepted tissue allografts, disruption of the immunoregulatory balance by an intense inflammatory stimulus can cause graft loss.

Interaction of programmed death-1 and programmed death-1 ligand-1 contributes to testicular immune privilege

BACKGROUND

Immune responses are tempered in immunologically privileged sites including the testis. Previous studies have shown that islet transplantation in the testis significantly prolongs islet allograft survival. However, mechanisms underlying testicular immune privilege and intratesticular allograft survival remain unclear.

METHODS

Allogeneic murine islets were transplanted in the testis. Programmed death-1 ligand-1 (PD-L1) expression was detected by immunohistochemstry and real-time polymerase chain reaction. Infiltrating T-cell proliferation was measured by bromodeoxyuridine uptakes, whereas their apoptosis was quantified by terminal deoxynucleotide transferase-mediated dUTP nick-end labeling methods. Transgenic T cells were used to track allospecific memory T-cell generation.

RESULTS

We found that programmed death-1 (PD-1):PD-L1 negative costimulation is essential for prolonged survival of intratesticular islet allografts, as blocking PD-L1 or PD-1, but not PD-L2 and cytotoxic T-lymphocyte antigen 4, abrogated long-term survival of intratesticular islet allografts. As controls, blocking PD-1 or PD-L1 did not significantly accelerate the acute rejection of islet allografts transplanted under the renal capsule, a conventional islet-grafting site. We also found for the first time that PD-L1 is constitutively expressed mainly by spermatocytes and spermatids in seminiferous tubules of the testis. Moreover, infiltrating T cells underwent less vigorous proliferation but faster apoptosis in the testis than in the kidney. Blocking PD-1:PD-L1 costimulation largely abolished the suppression of T-cell proliferation and acceleration of T-cell apoptosis. Importantly, testicular immune privilege significantly suppressed the generation and proliferation of donor-specific memory CD8 T cells.

CONCLUSIONS

The constitutive expression of PD-L1 in the testis is an important mechanism underlying testicular immune privilege and long-term survival of intratesticular islet allografts.

Translating costimulation blockade to the clinic: lessons learned from three pathways

SUMMARY

As the recognition that costimulatory signals are critical for optimal T-cell activation, proliferation, and differentiation, there has been an explosion in the study of costimulatory molecules and their roles in enhancing anti-donor T-cell responses following transplantation. Here, we focus on the bench-to-beside translation of blocking agents designed to target three critical costimulatory pathways: the CD28/CD80/CD86 pathway, the CD154/CD40 pathway, and the lymphocyte function associated antigen-1/intercellular adhesion molecule pathway. While blockade of each of these pathways proved promising in inhibiting donor-reactive T-cell responses and promoting long-term graft survival in murine models of transplantation, the progression of development of therapeutic agents to block these pathways has each taken a slightly different course. Both logistical and biological pitfalls have accompanied the translation of blockers of all three pathways into clinically applicable therapies, and the development of costimulatory blockade as a substitute for current standard-of-care calcineurin inhibitors has by no means reached completion. Collaboration between both the basic and clinical arenas will further propel the development of costimulation blockers currently in the pipeline, as well as of novel methods to target these critical pathways during transplantation.

Intestinal tolerance is converted to autoimmune enteritis upon PD-1 ligand blockade

The B7 family member programmed death-1 ligand (PD-L1) has been shown to play an inhibitory role in the regulation of T cell responses in several organs. However, the role of PD-L1 in regulating tolerance to self-Ags of the small intestine has not been previously addressed. In this study, we investigated the role of PD-L1 in CD8(+) T cell tolerance to an intestinal epithelium-specific Ag using the iFABP-tOVA transgenic mouse model, in which OVA is expressed as a self-Ag throughout the small intestine. Using adoptive transfer of naive OVA-specific CD8(+) T cells, we show that loss of PD-1:PD-L1 signaling, by either Ab-mediated PD-L1 blockade or transfer of PD-1(-/-) T cells, leads to considerable expansion of OVA-specific CD8(+) T cells and their differentiation into effector cells capable of producing proinflammatory cytokines. A fatal CD8(+) T cell-mediated inflammatory response develops rapidly against the small bowel causing destruction of the epithelial barrier, severe blunting of intestinal villi, and recruitment and activation of myeloid cells. This response is highly specific because immune destruction selectively targets the small intestine but not other organs. Collectively, these results indicate that loss of the PD-1:PD-L1 inhibitory pathway breaks CD8(+) T cell tolerance to intestinal self-Ag, thus leading to severe enteric autoimmunity.