An essential contribution by IFN-gamma to CD8+ T cell-mediated rejection of pancreatic islet allografts

Recognizing Complexity of CD8 T Cells in Transplantation

The major role of CD8 + T cells in clinical and experimental transplantation is well documented and acknowledged. Nevertheless, the precise impact of CD8 + T cells on graft tissue injury is not completely understood, thus impeding the development of specific treatment strategies. The goal of this overview is to consider the biology and functions of CD8 + T cells in the context of experimental and clinical allotransplantation, with special emphasis on how this cell subset is affected by currently available and emerging therapies.

Cutting edge of immune response and immunosuppressants in allogeneic and xenogeneic islet transplantation

As an effective treatment for diabetes, islet transplantation has garnered significant attention and research in recent years. However, immune rejection and the toxicity of immunosuppressive drugs remain critical factors influencing the success of islet transplantation. While immunosuppressants are essential in reducing immune rejection reactions and can significantly improve the survival rate of islet transplants, improper use of these drugs can markedly increase mortality rates following transplantation. Additionally, the current availability of islet organ donations fails to meet the demand for organ transplants, making xenotransplantation a crucial method for addressing organ shortages. This review will cover the following three aspects: 1) the immune responses occurring during allogeneic islet transplantation, including three stages: inflammation and IBMIR, allogeneic immune response, and autoimmune recurrence; 2) commonly used immunosuppressants in allogeneic islet transplantation, including calcineurin inhibitors (Cyclosporine A, Tacrolimus), mycophenolate mofetil, glucocorticoids, and Bortezomib; and 3) early and late immune responses in xenogeneic islet transplantation and the immune effects of triple therapy (ECDI-fixed donor spleen cells (ECDI-SP) + anti-CD20 + Sirolimus) on xenotransplantation.

Lithium-induced apoptotic cell death is not accompanied by a noticeable inflammatory response in the kidney

Lithium (Li+) therapy is a valuable tool in psychiatric practice that remains underutilized due to safety concerns. Excessive plasma Li+ levels are nephrotoxic and can trigger a local immune response. Our understanding of the immunomodulatory effects of Li+ in the kidney is fragmentary. Here, we studied how immune mechanisms contribute to the development of Li+-induced adverse effects in the kidneys of C57BL/6NJ mice placed on a 0.3% lithium carbonate diet for 28 days. We combined histochemical techniques, immunoblotting, flow cytometry, qPCR and proteome profiler arrays to characterize renal tissue damage, infiltrating immune cells and cytokine markers, activation of pyroptotic and apoptotic cascades in the kidneys of mice receiving Li+-containing and regular diets. We found that biomarkers of tubular damage, kidney injury marker, KIM-1, and neutrophil gelatinase-associated lipocalin, NGAL, were elevated in the renal tissue of Li+-treated mice when compared to controls. This correlated with increased interstitial fibrosis in Li+-treated mice. Administration of Li+ did not activate the pro-inflammatory NLRP3 inflammasome cascade but promoted apoptosis in the renal tissue. The TUNEL-positive signal and levels of pro-apoptotic proteins, Bax, cleaved caspase-3, and caspase-8, were elevated in the kidneys of Li+-treated mice. We observed a significantly higher abundance of CD93, CCL21, and fractalkine, accumulation of F4.80+ macrophages with reduced M1/M2 polarization ratio and decreased CD4+ levels in the renal tissue of Li+-treated mice when compared to controls. Therefore, after 28 days of treatment, Li+-induced insult to the kidney manifests in facilitated apoptotic cell death without an evident pro-inflammatory response.

Single-Cell Landscape of Mouse Islet Allograft and Syngeneic Graft

Islet transplantation to treat the late stage of type 1 diabetic patient (T1DM) has recently made inspiring success in clinical trials. However, most patients experience a decline in islet graft function in one to three years due to immune rejection. Although the mechanisms of immune cells, including macrophages, dendritic cells (DCs), neutrophils, natural killer cells (NKs), B cells, and T cells, that mediate immune rejection have been investigated, the overall characteristics of immune infiltrates in islet allografts and syngeneic grafts remain unclear. Single-cell RNA sequencing (scRNA-seq) has provided us with new opportunities to study the complexity of the immune microenvironment in islet transplants. In the present study, we used scRNA-seq to comprehensively analyze the immune heterogeneity in the mouse model of islet transplantation. Our data revealed T lymphocytes and myeloid cells as the main immune components of grafts 7 days post-islet transplantation, especially in allografts. Moreover, our results indicated that allogeneic islet cells were transformed into antigen-presenting cell-like cells with highly expressed MHC class I molecules and genes involved in MHC class I-mediated antigen presentation. This transformation may dramatically facilitate the interaction with cytotoxic CD8⁺ T cells and promote the destruction of islet allografts. Our study provides insight into the transcriptomics and diverse microenvironment of islet grafts and their impacts on immune rejection.

Ablation of BATF Alleviates Transplant Rejection via Abrogating the Effector Differentiation and Memory Responses of CD8+ T Cells

Allogeneic CD8⁺ T cells are prominently involved in allograft rejection, but how their effector differentiation and function are regulated at a transcriptional level is not fully understood. Herein, we identified the basic leucine zipper ATF-like transcription factor (BATF) as a key transcription factor that drives the effector program of allogeneic CD8⁺ T cells. We found that BATF is highly expressed in graft-infiltrating CD8⁺ T cells, and its ablation in CD8⁺ T cells significantly prolonged skin allograft survival in a fully MHC-mismatched transplantation model. To investigate how BATF dictates allogeneic CD8⁺ T cell response, BATF^-/- and wild-type (WT) CD8⁺ T cells were mixed in a 1:1 ratio and adoptively transferred into B6.Rag1^-/- mice 1 day prior to skin transplantation. Compared with WT CD8⁺ T cells at the peak of rejection response, BATF^-/- CD8⁺ T cells displayed a dysfunctional phenotype, evident by their failure to differentiate into CD127^-KLRG1⁺ terminal effectors, impaired proliferative capacity and production of pro-inflammatory cytokines/cytotoxic molecules, and diminished capacity to infiltrate allografts. In association with the failure of effector differentiation, BATF^-/- CD8⁺ T cells largely retained TCF1 expression and expressed significantly low levels of T-bet, TOX, and Ki67. At the memory phase, BATF-deficient CD8⁺ T cells displayed impaired effector differentiation upon allogeneic antigen re-stimulation. Therefore, BATF is a critical transcriptional determinant that governs the terminal differentiation and memory responses of allogeneic CD8⁺ T cells in the transplantation setting. Targeting BATF in CD8⁺ T cells may be an attractive therapeutic approach to promote transplant acceptance.

Leflunomide Inhibits rat-to-Mouse Cardiac Xenograft Rejection by Suppressing Adaptive Immune Cell Response and NF-κB Signaling Activation

Xenotransplantation is a potential solution for the severe shortage of human donor organs and tissues. The generation of humanized animal models attenuates strong innate immune responses, such as complement-mediated hyperacute rejection. However, acute vascular rejection and cell mediated rejection remain primary barriers to xenotransplantation, which limits its clinical application. In this study, we systematically investigated the immunosuppressive effect of LEF using a rat-to-mouse heart xenotransplantation model. SD rat xenogeneic hearts were transplanted into C57BL/6 mice, and survived 34.5 days after LEF treatment. In contrast, BALB/c allogeneic hearts were transplanted into C57BL/6 mice, and survived 31 days after LEF treatment. Compared to normal saline treatment, LEF treatment decreased xenoreactive T cells and CD19⁺ B cells in recipient splenocytes. Most importantly, LEF treatment protected myocardial cells by decreasing xenoreactive T and B cell infiltration, inflammatory gene expression, and IgM deposition in grafts. In vivo assays revealed that LEF treatment eliminated xenoreactive and alloreactive T and B lymphocytes by suppressing the activation of the NF-κB signaling pathway. Taken together, these observations complement the evidence supporting the potential use of LEF in xenotransplantation.

The Presence of Activated T Cell Subsets prior to Transplantation Is Associated with Increased Rejection Risk in Pancreas Transplant Recipients

Pancreas and islet transplantation (PTx) are currently the only curative treatment options for type 1 diabetes. CD4⁺ and CD8⁺ T cells play a pivotal role in graft function, rejection, and survival. However, characterization of immune cell status from patients with and without rejection of the pancreas graft is lacking. We performed multiparameter immune phenotyping of T cells from PTx patients prior to and 1 y post-PTx in nonrejectors and histologically confirmed rejectors. Our results suggest that rejection is associated with presence of elevated levels of activated CD4⁺ and CD8⁺ T cells with a gut-homing phenotype both prior to and 1 y post-PTx. The CD4⁺ and CD8⁺ T cells were highly differentiated, with elevated levels of type 1 inflammatory markers (T-bet and INF-γ) and cytotoxic components (granzyme B and perforin). Furthermore, we observed increased levels of activated FOXP3⁺ regulatory T cells in rejectors, which was associated with a hyporesponsive phenotype of activated effector T cells. Finally, activated T and B cell status was correlated in PTx patients, indicating a potential interplay between these cell types. In vitro treatment of healthy CD4⁺ and CD8⁺ T cells with tacrolimus abrogated the proliferation and cytokine (INF-γ, IL-2, and TNF-α) secretion associated with the type 1 inflammatory phenotype observed in pre- and post-PTx rejectors. Together, our results suggest the presence of activated CD4⁺ and CD8⁺ T cells prior to PTx confer increased risk for rejection. These findings may be used to identify patients that may benefit from more intense immunosuppressive treatment that should be monitored more closely after transplantation.

Interferon regulatory factor 4 deficiency in CD8+ T cells abrogates terminal effector differentiation and promotes transplant acceptance

Allogeneic CD8⁺ cytotoxic T cells play an essential role in rejecting transplanted allografts, but how their effector function is regulated on a transcriptional level remains unclear. Herein, we investigate the role of interferon regulatory factor 4 (IRF4) in controlling CD8⁺ T-cell function in response to transplant. B6.Rag1^-/- mice were adoptively transferred with CD8⁺ T cells isolated from either Irf4^fl/fl Cd4-Cre (T-cell-specific Irf4-deficient) or Irf4^fl/fl control mice, followed by BALB/c skin transplantation. Recipients that received Irf4-deficient CD8⁺ T cells permanently accepted the skin allografts, whereas recipients that received control CD8⁺ T cells acutely rejected the transplanted skins. Mechanistically, compared with the transferred control CD8⁺ T cells in B6.Rag1^-/- recipients, the transferred Irf4-deficient CD8⁺ T cells lost the capacity to differentiate into CD127^- KLRG1⁺ terminal effector cells, barely produced effector cytokines and cytotoxic molecules (e.g. IL-2, IFN-γ, TNF-α, granzyme A and granzyme B), and displayed defect in proliferative capacity, evident by their decreased Ki67 expression and lower frequencies. Moreover, the transferred Irf4-deficient CD8⁺ T cells displayed low expression of transcription factors ID2 and T-bet that govern the terminal effector T-cell programmes, and high expression of transcription factor TCF1 that maintains the naïve-memory T-cell programmes. Hence, IRF4 deficiency in CD8⁺ T cells abrogates their terminal effector differentiation and promotes transplant acceptance. These findings suggest that targeting IRF4 expression represents an attractive and promising therapeutic approach for inducing transplant acceptance.

Rejection of xenogeneic porcine islets in humanized mice is characterized by graft-infiltrating Th17 cells and activated B cells

Xenogeneic porcine islet transplantation is a promising potential therapy for type 1 diabetes (T1D). Understanding human immune responses against porcine islets is crucial for the design of optimal immunomodulatory regimens for effective control of xenogeneic rejection of porcine islets in humans. Humanized mice are a valuable tool for studying human immune responses and therefore present an attractive alternative to human subject research. Here, by using a pig-to-humanized mouse model of xenogeneic islet transplantation, we described the human immune response to transplanted porcine islets, a process characterized by dense islet xenograft infiltration of human CD45⁺ cells comprising activated human B cells, CD4⁺ CD44⁺ IL-17⁺ Th17 cells, and CD68⁺ macrophages. In addition, we tested an experimental immunomodulatory regimen in promoting long-term islet xenograft survival, a triple therapy consisting of donor splenocytes treated with ethylcarbodiimide (ECDI-SP), and peri-transplant rituximab and rapamycin. We observed that the triple therapy effectively inhibited graft infiltration of T and B cells as well as macrophages, promoted transitional B cells both in the periphery and in the islet xenografts, and provided a superior islet xenograft protection. Our study therefore indicates an advantage of donor ECDI-SP treatment in controlling human immune cells in promoting long-term islet xenograft survival.

Linking innate immunity and chronic antibody-mediated allograft rejection

PURPOSE OF REVIEW

To summarize recent findings linking donor-specific antibodies with innate immunity resulting in chronic allograft rejection.

RECENT FINDINGS

Studies in recent years highlight the significance of donor-specific antibodies (DSA) in both acute and chronic allograft rejection. Since chronic rejection is the leading cause of graft failure, this review centers on the contribution of three areas of innate immunity of particular recent focus: complement, NK cells, and macrophages. Recent advances indicate the diverse roles that complement components play both in directly initiating allograft injury and indirectly by contributing to enhanced alloreactivity. NK cells also have emerged as an additional innate response that directly links DSA with chronic graft injury. Finally, recent studies identify alternatively activated macrophages as an additional arm of innate immunity contributing to chronic allograft rejection.

SUMMARY

Chronic allograft rejection involves a significant contribution of DSA and differing pathways of the innate immune system. However, key issues remain unresolved. First, it is not always clear which of these varied sources of innate immunity contributing to chronic rejection may be antibody dependent. Moreover, it is not yet clear if these innate pathways represent independent routes that contribute to chronic rejection or rather act in concert to mediate allograft injury.

IFN-γ Control of an Effector/Target Combination for Skin Allograft Rejection: Macrophage/Skin Components in Normal Mice or T Cell/Endothelial Cells in IFN-γ-Deficient Mice

Organ, skin, or cell allografts are acutely rejected from normal mice, whereas vascularized organ allografts, but not allografted Meth A cells, are rejected from interferon-γ (IFN-γ)-deficient mice. Here we explored effector/target combinations for i.p. allografted Meth A (cytotoxic T lymphocyte [CTL]-resistant) or RLmale1 (CTL-susceptible) cells into or for BALB/c skin (skin components: CTL resistant) onto normal or IFN-γ-deficient C57BL/6 mice. After allografting, normal mice showed more infiltration but only a little thrombosis/hemorrhage. Monocyte/macrophage MHC receptor (MMR)⁺ macrophages (on days 5-10) and T cell receptor (TCR)⁺ CTLs (on days 7-9) were cytotoxic against Meth A cells or skin components and RLmale1 cells, respectively, and the allografts were rejected. After allografting into IFN-γ-deficient mice, MMR^- macrophages and highly activated TCR⁺ CTLs were induced, and the mice died of hemorrhagic ascites with Meth A cells and more acutely rejected RLmale1 cells. The CTLs on days 4-6 were inactive toward skin components at an in vivo effector/target ratio but injured endothelial cells to cause severe thrombosis/hemorrhage and more acute rejection of skin allografts. These results indicate that IFN-γ-dependent MMR expression was essential for macrophage-mediated cytolysis of allogeneic skin components and that IFN-γ-deficient mice more acutely rejected skin allograft by causing CTL-induced injury to endothelial cells.

A combination regimen of low-dose bortezomib and rapamycin prolonged the graft survival in a murine allogeneic islet transplantation model

As the first FDA-approved proteasome inhibitor drug, bortezomib has been used for the treatment of multiple myeloma and lymphoma. However, its effects alone or in combination with other immunosuppressants on allogeneic islet transplantation have not been reported so far. In this study, we showed that the short-term combination treatment of low-dose bortezomib and rapamycin significantly prolonged the survival of islet allografts. Short-term treatment of low-dose (0.05 mg/kg or 0.1 mg/kg) bortezomib reduced the MHC class II expression in dendritic cells (DCs) of alloantigen-sensitized mice, and prolonged the islet allograft survival for up to 50 days in diabetic mice. Notably, when bortezomib was combined with rapamycin, it induced islet-specific immunological tolerance which allowed the acceptance of a second graft without additional immunosuppression. This regimen dramatically reduced the alloantigen-specific IFN-γ-producing T cells in the spleen, and increased regulatory T cells both at the graft site and in the spleen. Therefore, we propose that short-term treatment of low-dose bortezomib and rapamycin could be a new tolerance-promoting immunosuppressive regimen for allogeneic islet transplantation.

Differential Impact of T-bet and IFNγ on Pancreatic Islet Allograft Rejection

BACKGROUND

T cell-mediated graft rejection is mostly correlated with potent Th1 responses. However, because IFNγ mice reject their graft as efficiently as wild-type (WT) mice, the exact contribution of IFNγ and its transcription factor T-bet remains a matter of debate. Here, we address this question in the context of pancreatic islet allograft to better inform the molecular pathways that hampers islet survival in vivo.

METHODS

Pancreatic islets from BALB/c mice were transplanted in WT, IFNγ, or T-bet C57BL/6 mice. Graft survival and the induction of effector and cytotoxic T-cell responses were monitored.

RESULTS

Rejection of fully mismatched islet allografts correlated with high expression of both IFNγ and T-bet in WT recipients. However, allogeneic islets were permanently accepted in T-bet mice, in contrast to IFNγ hosts. Long-term survival correlated with decreased CD4 and CD8 T-cell infiltrates, drastically reduced donor-specific IFNγ and tumor necrosis factor tumor necrosis factor α responses and very low expression of the cytotoxic markers granzyme B, perforin, and FasLigand. In addition, in vitro and in vivo data pointed to an increased susceptibility of T-bet CD8 T cell to apoptosis. These observations were not reported in IFNγ mice, which have set up compensatory effector mechanisms comprising an increased expression of the transcription factor Eomes and cytolytic molecules as well as tumor necrosis factor α-mediated but not IL-4 nor IL-17-mediated allogeneic responses.

CONCLUSIONS

Anti-islet T-cell responses require T-bet but not IFNγ-dependent programs. Our results provide new clues on the mechanisms dictating islet rejection and may help refine the therapeutic/immunosuppressive regimens applied in diabetic patients receiving islets or pancreas allografts.

Critical Role of Macrophage FcγR Signaling and Reactive Oxygen Species in Alloantibody-Mediated Hepatocyte Rejection

Humoral alloimmunity negatively impacts both short- and long-term cell and solid organ transplant survival. We previously reported that alloantibody-mediated rejection of transplanted hepatocytes is critically dependent on host macrophages. However, the effector mechanism(s) of macrophage-mediated injury to allogeneic liver parenchymal cells is not known. We hypothesized that macrophage-mediated destruction of allogeneic hepatocytes occurs by cell-cell interactions requiring FcγRs. To examine this, alloantibody-dependent hepatocyte rejection in CD8-depleted wild-type (WT) and Fcγ-chain knockout (KO; lacking all functional FcγR) transplant recipients was evaluated. Alloantibody-mediated hepatocellular allograft rejection was abrogated in recipients lacking FcγR compared with WT recipients. We also investigated anti-FcγRI mAb, anti-FcγRIII mAb, and inhibitors of intracellular signaling (to block phagocytosis, cytokines, and reactive oxygen species [ROS]) in an in vitro alloantibody-dependent, macrophage-mediated hepatocytoxicity assay. Results showed that in vitro alloantibody-dependent, macrophage-mediated hepatocytotoxicity was critically dependent on FcγRs and ROS. The adoptive transfer of WT macrophages into CD8-depleted FcγR-deficient recipients was sufficient to induce alloantibody-mediated rejection, whereas adoptive transfer of macrophages from Fcγ-chain KO mice or ROS-deficient (p47 KO) macrophages was not. These results provide the first evidence, to our knowledge, that alloantibody-dependent hepatocellular allograft rejection is mediated by host macrophages through FcγR signaling and ROS cytotoxic effector mechanisms. These results support the investigation of novel immunotherapeutic strategies targeting macrophages, FcγRs, and/or downstream molecules, including ROS, to inhibit humoral immune damage of transplanted hepatocytes and perhaps other cell and solid organ transplants.

Localized immune tolerance from FasL-functionalized PLG scaffolds

Intraportal allogeneic islet transplantation has been demonstrated as a potential therapy for type 1 diabetes (T1D). The placement of islets into the liver and chronic immunosuppression to control rejection are two major limitations of islet transplantation. We hypothesize that localized immunomodulation with a novel form of FasL chimeric with streptavidin, SA-FasL, can provide protection and long-term function of islets at an extrahepatic site in the absence of chronic immunosuppression. Allogeneic islets modified with biotin and engineered to transiently display SA-FasL on their surface showed sustained survival following transplantation on microporous scaffolds into the peritoneal fat in combination with a short course (15 days) of rapamycin treatment. The challenges with modifying islets for clinical translation motivated the modification of scaffolds with SA-FasL as an off-the-shelf product. Poly (lactide-co-glycolide) (PLG) was conjugated with biotin and fabricated into particles and subsequently formed into microporous scaffolds to allow for rapid and efficient conjugation with SA-FasL. Biotinylated particles and scaffolds efficiently bound SA-FasL and induced apoptosis in cells expressing Fas receptor (FasR). Scaffolds functionalized with SA-FasL were subsequently seeded with allogeneic islets and transplanted into the peritoneal fat under the short-course of rapamycin treatment. Scaffolds modified with SA-FasL had robust engraftment of the transplanted islets that restored normoglycemia for 200 days. Transplantation without rapamycin or without SA-FasL did not support long-term survival and function. This work demonstrates that scaffolds functionalized with SA-FasL support allogeneic islet engraftment and long-term survival and function in an extrahepatic site in the absence of chronic immunosuppression with significant potential for clinical translation.

Immunosuppressive effect of arsenic trioxide on islet xenotransplantation prolongs xenograft survival in mice

The role of arsenic trioxide (As₂O₃) in inhibiting immune rejection and prolonging islet allograft survival has been identified in islet allotransplantation. This study aims to explore the role of As₂O₃ in islet xenotransplantation and the action mechanism. The streptozotocin (STZ) was used in C57BL/6 mice to induce the type 1 diabetes mellitus (T1DM) for xenotransplantation models establishment. Donor islets were isolated by digesting. The flow cytometry (FCM) was used to analyze lymphocyte types. The blood sugar level was detected by using intraperitoneal glucose tolerance test (IPGTT). The serum level of cytokines was determined by the enzyme-linked immunosorbent assay (ELIZA). The cell proliferation was measured by MTT assay. The mRNA levels were quantified with qRT-PCR. As₂O₃ prolonged the survival of the recipient mice but had no influence on body weight. As₂O₃ protected the function of xenograft in insulin secretion and suppressed immune rejection of recipient. As₂O₃ inhibited proliferation of T lymphocyte and increased the proportion of Foxp3⁺ regulatory T cells in recipient mice. As₂O₃ inhibited activation and promoted clonal anergy of T lymphocyte. As₂O₃ decreased total number of B cells and reduced partial antibody levels in recipient mice. As₂O₃ and leflunomide showed a synergistic effect in suppressing islet xenotransplant rejection. As₂O₃ prolongs islet xenograft survival by inhibiting cellular immune response, and increasing Foxp3⁺ regulatory T cells, while decreasing partial antibody levels in serum.

Evaluation of biomaterial scaffold delivery of IL-33 as a localized immunomodulatory agent to support cell transplantation in adipose tissue

Introduction

The development of novel immunomodulatory strategies that might decrease the need for systemic immune suppression would greatly enable the utility of cell-based therapies. Cell transplantation on biomaterial scaffolds offers a unique opportunity to engineer a site to locally polarize immunogenic antigen generation. Herein, we investigated the localized delivery of IL-33, which is a novel cytokine that has been shown to have beneficial immunomodulatory effects in certain transplant models as mediating anti-inflammatory properties in the adipose tissue, to determine its feasibility for use as an immunomodulatory agent.

Results

Localized IL-33 delivery from poly(lactide-co-glycolide) (PLG) scaffolds implanted into the epididymal fat specifically increased the Foxp3+ population of CD4+ T cells in both blank scaffold implants and scaffolds seeded with allogeneic islets. In allogeneic islet transplantation, we found IL-33 delivery results in a local upregulation of graft-protective T cells where 80% of the local CD4+ population is Foxp3+ and overall numbers of graft destructive CD8+ T cells are decreased, resulting in a prolonged graft survival. Interestingly, local IL-33 also delayed islet engraftment by primarily inducing a local upregulation of Th2 cytokines, including IL-4 and IL-5, leading to increased populations of ST2+ Type 2 innate lymphoid cells (ILC2s) and Siglec F+ eosinophils.

Conclusions

These results suggest that local IL-33 delivery from biomaterial scaffolds can be used to increase Tregs enriched in adipose tissue and reduce graft-destructive T cell populations but may also promote innate cell populations that can delay cell engraftment.

Redox-Dependent Inflammation in Islet Transplantation Rejection

Type 1 diabetes is an autoimmune disease that results in the progressive destruction of insulin-producing pancreatic β-cells inside the islets of Langerhans. The loss of this vital population leaves patients with a lifelong dependency on exogenous insulin and puts them at risk for life-threatening complications. One method being investigated to help restore insulin independence in these patients is islet cell transplantation. However, challenges associated with transplant rejection and islet viability have prevented long-term β-cell function. Redox signaling and the production of reactive oxygen species (ROS) by recipient immune cells and transplanted islets themselves are key players in graft rejection. Therefore, dissipation of ROS generation is a viable intervention that can protect transplanted islets from immune-mediated destruction. Here, we will discuss the newly appreciated role of redox signaling and ROS synthesis during graft rejection as well as new strategies being tested for their efficacy in redox modulation during islet cell transplantation.

Evolving Approaches in the Identification of Allograft-Reactive T and B Cells in Mice and Humans

Whether a transplanted allograft is stably accepted, rejected, or achieves immunological tolerance is dependent on the frequency and function of alloreactive lymphocytes, making the identification and analysis of alloreactive T and B cells in transplant recipients critical for understanding mechanisms, and the prediction of allograft outcome. In animal models, tracking the fate of graft-reactive T and B cells allows investigators to uncover their biology and develop new therapeutic strategies to protect the graft. In the clinic, identification and quantification of graft-reactive T and B cells allows for the early diagnosis of immune reactivity and therapeutic intervention to prevent graft loss. In addition to rejection, probing of T and B cell fate in vivo provides insights into the underlying mechanisms of alloimmunity or tolerance that may lead to biomarkers predicting graft fate. In this review, we discuss existing and developing approaches to track and analyze alloreactive T and B cells in mice and humans and provide examples of discoveries made utilizing these techniques. These approaches include mixed lymphocyte reactions, trans-vivo delayed-type hypersensitivity, enzyme-linked immunospot assays, the use of antigen receptor transgenic lymphocytes, and utilization of peptide-major histocompatibility multimers, along with imaging techniques for static multiparameter analysis or dynamic in vivo tracking. Such approaches have already refined our understanding of the alloimmune response and are pointing to new ways to improve allograft outcomes in the clinic.

Successful Treatment of T Cell-Mediated Acute Rejection with Delayed CTLA4-Ig in Mice

Clinical observations that kidney transplant recipients receiving belatacept who experienced T cell-mediated acute rejection can be successfully treated and subsequently maintained on belatacept-based immunosuppression suggest that belatacept is able to control memory T cells. We recently reported that treatment with CTLA4-Ig from day 6 posttransplantation successfully rescues allografts from acute rejection in a BALB/c to C57BL/6 heart transplant model, in part, by abolishing B cell germinal centers and reducing alloantibody titers. Here, we show that CTLA4-Ig is additionally able to inhibit established T cell responses independently of B cells. CTLA4-Ig inhibited the in vivo cytolytic activity of donor-specific CD8⁺ T cells, and the production of IFNγ by graft-infiltrating T cells. Delayed CTLA4-Ig treatment did not reduce the numbers of graft-infiltrating T cells nor prevented the accumulation of antigen-experienced donor-specific memory T cells in the spleen. Nevertheless, delayed CTLA4-Ig treatment successfully maintained long-term graft acceptance in the majority of recipients that had experienced a rejection crisis, and enabled the acceptance of secondary BALB/c heart grafts transplanted 30 days after the first transplantation. In summary, we conclude that delayed CTLA4-Ig treatment is able to partially halt ongoing T cell-mediated acute rejection. These findings extend the functional efficacy of CTLA4-Ig therapy to effector T cells and provide an explanation for why CTLA4-Ig-based immunosuppression in the clinic successfully maintains long-term graft survival after T cell-mediated rejection.

Host Expression of the CD8 Treg/NK Cell Restriction Element Qa-1 is Dispensable for Transplant Tolerance

Disruption of the non-classical Major Histocompatibility Complex (MHC) Ib molecule Qa-1 impairs CD8 Treg and natural killer (NK) cell function and promotes a lupus-like autoimmune disease. This immune perturbation would be expected to enhance anti-transplant responses and impair tolerance induction, but the effect of Qa-1 deficiency on the transplant response has not been previously reported. Qa-1 deficiency enhanced CD4 TFH and germinal center (GC) B cell numbers in naïve mice and hastened islet allograft rejection. Despite enhanced immunity in B6.Qa-1^-/- mice, these mice did not generate an excessive primary CD4 TFH cell response nor an enhanced alloantibody reaction. Both CD8 Tregs and NK cells, which often regulate other cells through host Qa-1 expression, were targets of anti-CD45RB therapy that had not been previously recognized. However, B6.Qa-1^-/- mice remained susceptible to anti-CD45RB mediated suppression of the alloantibody response and transplant tolerance induction to mismatched islet allografts. Overall, despite enhanced immunity as demonstrated by augmented CD4 TFH/GC B cell numbers and hastened islet allograft rejection in naïve 12-week old Qa-1 deficient mice, the CD8 Treg/NK cell restriction element Qa-1 does not regulate the primary cellular or humoral alloresponse and is not required for long-term transplant tolerance.

Survival of an Islet Allograft Deficient in iNOS after Implantation into Diabetic NOD Mice

Proinflammatory cytokines play a major role in rejection of pancreatic islet allografts and in type 1 diabetes (T1D). In rodent islets, exposure to IL-1β alone or combined with IFN-γ induces expression of inducible nitric oxide synthase (iNOS). Inhibition of iNOS or a deletion of the iNOS gene has been shown to be protective in animal models of T1D. In the present study we tested the hypothesis that transplantation of pancreatic islets deficient in iNOS (iNOS–/–) would permit increased graft survival. Pancreatic islets isolated from wild-type (wt) mice and iNOS–/– mice were allogeneically transplanted beneath the kidney capsule of spontaneously diabetic NOD mice. When blood glucose increased above 12.0 mM after preceding normalization of hyperglycemia, animals were sacrificed. Histological examinations of grafts were performed and graft gene expression was analyzed by real-time PCR. Transplantations of the two types of islets could reverse hyperglycemia and the grafts functioned for on average 1 week posttransplantation. Morphological examination of both types of islet grafts showed immune cell infiltration around and within the grafts. Remaining endocrine cells could be observed in wt and iNOS–/– islet grafts. In the removed grafts iNOS-/islet tissue contained higher mRNA levels of insulin, proinsulin convertases (PC-1 and PC-2), and IL-1β compared to transplanted wt islets. The assessments of insulin, PC-1 and PC-2 mRNAs of the grafts suggest that the iNOS–/– islets may be more resistant to destruction in the transplantation model used; however, this was not sufficient to prolong the period of normoglycemia posttransplantation.

Comparison of Tolerated and Rejected Islet Grafts: A Gene Expression Study

Recently we showed that donor-specific tolerance to MHC-matched islet allografts in diabetic NOD mice could be induced by simultaneous islet and bone marrow transplantation. Mononuclear cell infiltration surrounding the islets was also found in tolerated grafts. In this study, we compared gene expression in the tolerated and rejected islet grafts by using Affymetrix Murine U74A oligonucleotide arrays. To confirm the results of microarray analysis, we performed real-time PCR and RNase protection assay on selected genes. Of over 12,000 genes studied, 57 genes were expressed at consistently higher levels in tolerated islet grafts, and 524 genes in rejected islet grafts. Genes from a variety of functional clusters were found to be different between rejected and tolerated grafts. In the rejected islet grafts, a number of T-cell surface markers and of cytotoxicity-related genes were highly expressed. Also in the rejected grafts, a number of cytokines and chemokines and their receptors were highly expressed. The differential expression of selected genes found by microarray analysis was also confirmed by real-time PCR and RNase protection assay. Our results indicated that gene microarray analysis can help us to detect gene expression differences representative of the biologic mechanisms of tolerance and rejection.

Interferon Gamma and Contact-dependent Cytotoxicity Are Each Rate Limiting for Natural Killer Cell-Mediated Antibody-dependent Chronic Rejection

Natural killer (NK) cells are key components of the innate immune system. In murine cardiac transplant models, donor-specific antibodies (DSA), in concert with NK cells, are sufficient to inflict chronic allograft vasculopathy independently of T and B cells. In this study, we aimed to determine the effector mechanism(s) required by NK cells to trigger chronic allograft vasculopathy during antibody-mediated rejection. Specifically, we tested the relative contribution of the proinflammatory cytokine interferon gamma (IFN-γ) versus the contact-dependent cytotoxic mediators of perforin and the CD95/CD95L (Fas/Fas ligand [FasL]) pathway for triggering these lesions. C3H/HeJ cardiac allografts were transplanted into immune-deficient C57BL/6 rag-/- γc-/- recipients, who also received monoclonal anti-major histocompatibility complex (MHC) class I DSA. The combination of DSA and wild-type NK cell transfer triggered aggressive chronic allograft vasculopathy. However, transfer of IFN-γ-deficient NK cells or host IFN-γ neutralization led to amelioration of these lesions. Use of either perforin-deficient NK cells or CD95 (Fas)-deficient donors alone did not alter development of vasculopathy, but simultaneous disruption of NK cell-derived perforin and allograft Fas expression resulted in prevention of these abnormalities. Therefore, both NK cell IFN-γ production and contact-dependent cytotoxic activity are rate-limiting effector pathways that contribute to this form of antibody-induced chronic allograft vasculopathy.

Adoptive Transfer of Tracer-Alloreactive CD4+ T Cell Receptor Transgenic T Cells Alters the Endogenous Immune Response to an Allograft

T cell receptor transgenic (TCR-Tg) T cells are often used as tracer populations of antigen-specific responses to extrapolate findings to endogenous T cells. The extent to which TCR-Tg T cells behave purely as tracer cells or modify the endogenous immune response is not clear. To test the impact of TCR-Tg T cell transfer on endogenous alloimmunity, recipient mice were seeded with CD4⁺ or CD8⁺ TCR-Tg or polyclonal T cells at the time of cardiac allograft transplantation. Only CD4⁺ TCR-Tg T cells accelerated rejection and, unexpectedly, led to a dose-dependent decrease in both transferred and endogenous T cells infiltrating the graft. In contrast, recipients of CD4⁺ TCR-Tg T cells exhibited enhanced endogenous donor-specific CD8⁺ T cell activation in the spleen and accelerated alloantibody production. Introduction of CD4⁺ TCR-Tg T cells also perturbed the intragraft accumulation of innate cell populations. Transferred CD4⁺ TCR-Tg T cells alter many aspects of endogenous alloimmunity, suggesting that caution should be used when interpreting experiments using these adoptively transferred cells because the overall nature of allograft rejection may be altered. These results also may have implications for adoptive CD4⁺ T cell immunotherapy in tumor and infectious clinical settings because cell infusion may have additional effects on natural immune responses.

Direct and indirect allograft recognition: pathways dictating graft rejection mechanisms

PURPOSE OF REVIEW

The T cell-dependent recognition of allogeneic tissues and organs is complicated by the fact that both donor and host antigen-presenting cells can present donor antigens to host T cells. As such, these pathways result in T cells that can be restricted to either donor ('direct') or host ('indirect') major histocompatibility complex (MHC). These pathways are well recognized, but how these distinct patterns actually dictate allograft recognition is less clear. Thus, the purpose of the review is to summarize results from preclinical animal models in an attempt to clarify the distinct forms of allograft rejection dictated by these recognition pathways.

RECENT FINDINGS

CD4 and CD8 donor MHC-restricted T cells are sufficient to reject allografts by a T-cell receptor-mediated direct ('cognate') interaction using a defined array of effector molecules. Conversely, 'noncognate' host MHC-restricted CD4 T cells must interact with intermediate host-type antigen-presenting cells and so greatly amplify the response by triggering antibody and inflammatory responses.

SUMMARY

Importantly, 'cognate' CD4 and CD8 T cells have strikingly similar requirements for rejection, suggesting that this effector mechanism is dictated by the nature of allograft recognition rather than by T-cell subset. Conversely, 'noncognate' allograft recognition drives an increasingly appreciated role for inciting innate immunity in mediating allograft injury.

T-bet Regulates Natural Regulatory T Cell Afferent Lymphatic Migration and Suppressive Function

T-bet is essential for natural regulatory T cells (nTreg) to regulate Th1 inflammation, but whether T-bet controls other Treg functions after entering the inflammatory site is unknown. In an islet allograft model, T-bet(-/-) nTreg, but not induced Treg, failed to prolong graft survival as effectively as wild-type Treg. T-bet(-/-) nTreg had no functional deficiency in vitro but failed to home from the graft to draining lymph nodes (dLN) as efficiently as wild type. T-bet regulated expression of adhesion- and migration-related molecules, influencing nTreg distribution in tissues, so that T-bet(-/-) nTreg remained in the grafts rather than migrating to lymphatics and dLN. In contrast, both wild-type and T-bet(-/-) CD4(+) conventional T cells and induced Treg migrated normally toward afferent lymphatics. T-bet(-/-) nTreg displayed instability in the graft, failing to suppress Ag-specific CD4(+) T cells and prevent their infiltration into the graft and dLN. Thus, T-bet regulates nTreg migration into afferent lymphatics and dLN and consequently their suppressive stability in vivo.

TGFβ-dependent expression of PD-1 and PD-L1 controls CD8(+) T cell anergy in transplant tolerance

CD8⁺ T cell anergy is a critical mechanism of peripheral tolerance, poorly investigated in response to immunotherapy. Here, using a pancreatic islet allograft model and CD3 antibody therapy, we showed, by single cell gene profiling, that intragraft CD8⁺ lymphocytes coexpressing granzyme B and perforin were selectively depleted through the Fas/FasL pathway. This step led to long-standing anergy of the remaining CD8⁺ T cells marked by the absence of cytotoxic/inflammatory gene expression also confirmed by transcriptome analysis. This sustained unresponsiveness required the presence of the alloantigens. Furthermore, tissue-resident CD8⁺ lymphocytes produced TGFβ and expressed the inhibitory receptors PD-1 and PD-L1. Blockade of TGFβ downregulated PD-1 and PD-L1 expression and precipitated graft rejection. Neutralizing PD-1, PD-L1 or TGFβRII signaling in T cells also abrogated CD3 antibody-induced tolerance. These studies unravel novel mechanisms underlying CD8⁺ T cell anergy and reveal a cell intrinsic regulatory link between the TGFβ and the PD-1/PD-L1 pathways.

DOI:http://dx.doi.org/10.7554/eLife.08133.001

The immune system is always on guard for signs of infection or cells that have become diseased. When these signs are identified, a subset of white blood cells called CD8⁺ T cells leap into action, multiply in number and then act to eliminate the potential threat. While this response is essential to fighting off infections and other diseases like cancer, it can backfire in people with an organ transplant. Indeed, the CD8⁺ T cells can target and attack the cells of the transplanted organ causing the body to reject the organ.

One way to avoid transplant rejection would be to turn off CD8⁺ T cells that have learned to recognize cells from the transplant. In fact, studies in 2012 and 2013 showed that treating transplanted animals with an antibody that binds T cells protects a transplanted organ from attack. This treatment had to be given after the CD8⁺ T cells had recognized and began targeting the transplanted organ to be effective. But it was not clear exactly how this antibody treatment protected the transplant.

Now, Baas, Besançon et al. – including some of the same researchers involved in the earlier studies – show that the antibodies used in the treatment selectively target and eliminate the attacking CD8⁺ T cells. This leaves behind only inactive CD8⁺ T cells that don’t harm the transplant. To do this, Baas, Besançon et al. transplanted pancreatic cells from mice into other mice with a diabetes-like disorder. Next, the experiments compared gene expression in CD8⁺ T cells found within the transplanted tissue in mice that were treated with the antibody and those that were not treated. The expression of many genes for toxic molecules was stopped after treatment with the antibody leaving the CD8⁺ T cells in an inactive state.

In addition, the treated CD8⁺ T cells expressed more of a certain type of receptor (called PD-1 and PD-L1) that acts as inhibitory checkpoint for the immune system. So, Baas, Besançon et al. treated transplanted mice with both the T cell-eliminating antibody and antibodies that block these inhibitory receptors to see what would happen. The transplanted organs were quickly attacked and rejected. This shows that the inhibitory receptors play a crucial role in helping to shut down attacking CD8⁺ T cells in the initial antibody treatment and allowed long-term survival of the transplanted organs. Blocking another protein called TGFβ in antibody-treated mice also caused organ rejection. The findings help explain how these antibodies protect transplanted organs and may help scientists trying to develop new anti-transplant rejection drugs in the future.

DOI:http://dx.doi.org/10.7554/eLife.08133.002

Arsenic Trioxide Induces T Cell Apoptosis and Prolongs Islet Allograft Survival in Mice

BACKGROUND

T cell-mediated immune rejection is a key barrier to islet transplantation. Preliminary studies have shown that arsenic trioxide (As2O3) can inhibit T cell responses and prolong heart allograft survival. Here, we sought to investigate the possibility of using As2O3 to prolong islet allograft survival in an acute rejection model of Balb/c to C57B/6 mice.

METHODS

Recipient mice were treated with As2O3 and/or rapamycin after islet allograft transplantation. At day 10 after transplantation, the graft, spleen, lymph nodes, and blood of the recipient mice were recovered for analysis. In vitro, to further examine the mechanism underlying As2O3 protection of islet allografts against T cell-mediated rejection, mixed lymphocyte reaction and apoptosis analyses of T cells were performed. The phosphorylation levels of IκBα and p38 were also evaluated to confirm the proliferation and apoptosis of As2O3-treated T cells.

RESULTS

We found that As2O3 prolonged islet allograft survival by reducing inflammatory reactions, influencing cytokine synthesis and secretion and T-cell subset proportions, and inhibiting T-cell responses. Furthermore, As2O3 and rapamycin showed a synergistic effect in suppressing islet allotransplant rejection.

CONCLUSIONS

Arsenic trioxide may prevent allograft rejection by inhibiting T-cell proliferation and inducing T-cell apoptosis.

Targeting CD8 T-Cell Metabolism in Transplantation

Infiltration of effector CD8 T cells plays a major role in allograft rejection, and increases in memory and terminally differentiated effector memory CD8 T cells are associated with long-term allograft dysfunction. Alternatively, CD8 regulatory T cells suppress the inflammatory responses of effector lymphocytes and induce allograft tolerance in animal models. Recently, there has been a renewed interest in the field of immunometabolics and its important role in CD8 function and differentiation. The purpose of this review is to highlight the key metabolic pathways involved in CD8 T cells and to discuss how manipulating these metabolic pathways could lead to new immunosuppressive strategies for the transplantation field.

Cytotoxic effector function of CD4-independent, CD8(+) T cells is mediated by TNF-α/TNFR

BACKGROUND

Liver parenchymal cell allografts initiate both CD4-dependent and CD4-independent, CD8(+) T cell-mediated acute rejection pathways. The magnitude of allospecific CD8(+) T cell in vivo cytotoxic effector function is maximal when primed in the presence of CD4(+) T cells. The current studies were conducted to determine if and how CD4(+) T cells might influence cytotoxic effector mechanisms.

METHODS

Mice were transplanted with allogeneic hepatocytes. In vivo cytotoxicity assays and various gene-deficient recipient mice and target cells were used to determine the development of Fas-, TNF-α-, and perforin-dependent cytotoxic effector mechanisms after transplantation.

RESULTS

CD8(+) T cells maturing in CD4-sufficient hepatocyte recipients develop multiple (Fas-, TNF-α-, and perforin-mediated) cytotoxic mechanisms. However, CD8(+) T cells, maturing in the absence of CD4(+) T cells, mediate cytotoxicity and transplant rejection that is exclusively TNF-α/TNFR-dependent. To determine the kinetics of CD4-mediated help, CD4(+) T cells were adoptively transferred into CD4-deficient mice at various times posttransplant. The maximal influence of CD4(+) T cells on the magnitude of CD8-mediated in vivo allocytotoxicityf occurs within 48 hours.

CONCLUSION

The implication of these studies is that interference of CD4(+) T cell function by disease or immunotherapy will have downstream consequences on both the magnitude of allocytotoxicity as well as the cytotoxic effector mechanisms used by allospecific CD8(+) cytolytic T cells.

The effect of human umbilical cord blood cells on survival and cytokine production by post-ischemic astrocytes in vitro

Cerebral ischemia induces death of all neural cell types within the region affected by the loss of blood flow. We have shown that administering human umbilical cord blood cells after a middle cerebral artery occlusion in rats significantly reduces infarct size, presumably by rescuing cells within the penumbra. In this study we examined whether the cord blood cells enhanced astrocyte survival in an in vitro model of hypoxia with reduced glucose availability. Primary astrocyte cultures were incubated for 2 h in no oxygen (95% N, 5% CO(2)) and low glucose (1% compared to 4.5%) media. Cord blood mononuclear cells were added to half the cultures at the beginning of hypoxia. Astrocyte viability was determined using fluorescein diacetate/propidium iodide (FDA/PI) labeling and cytokine production by the astrocytes measured using ELISA. In some studies, T cells, B cells or monocytes/macrophages isolated from the cord blood mononuclear fraction with magnetic antibody cell sorting (MACS) were used instead to determine which cellular component of the cord blood mononuclear fraction was responsible for the observed effects. Co-culturing mononuclear cord blood cells with astrocytes during hypoxia stimulated production of IL-6 and IL-10 during hypoxia. The cord blood T cells decreased survival of the astrocytes after hypoxia but had no effect on the examined cytokines. Our data demonstrate that the tested cord blood fractions do not enhance astrocyte survival when delivered individually, suggesting there is either another cellular component that is neuroprotective or an interaction of all the cells is essential for protection.

Macrophages and kidney transplantation

Macrophages are present within the transplanted kidney in varying numbers throughout its lifespan. Because of their prominence during acute rejection episodes, macrophages traditionally have been viewed as contributors to T-cell-directed graft injury. With growing appreciation of macrophage biology, it has become evident that different types of macrophages exist within the kidney, subserving a range of functions that include promotion or attenuation of inflammation, participation in innate and adaptive immune responses, and mediation of tissue injury and fibrosis, as well as tissue repair. A deeper understanding of how macrophages accumulate within the kidney and of what factors control their differentiation and function may identify novel therapeutic targets in transplantation.

CD8+ T cells negatively regulate IL-4-dependent, IgG1-dominant posttransplant alloantibody production

We have previously reported that CD8(+) T cells significantly influence Ab production based on the observation that posttransplant alloantibody levels in CD8-deficient murine hepatocyte transplant recipients are markedly enhanced. However, the precise mechanisms contributing to enhanced alloantibody production in the absence of CD8(+) T cells is not understood. We hypothesized that alloactivated CD8(+) T cells inhibit Ab production by skewing toward a proinflammatory cytokine profile, whereas when these cells are absent, an anti-inflammatory cytokine profile shifts the alloimmune response toward alloantibody production. To investigate this possibility, alloantibody isotype profiles were examined in CD8-deficient and wild-type hepatocyte recipients. We found that IgG1 (IL-4-dependent isotype) was the dominant alloantibody isotype in wild-type recipients as well as in CD8-deficient recipients, although the amount of alloantibody in the latter group was substantially higher. Utilizing real-time PCR we found that CD4(+) T cells from wild-type recipients significantly upregulated IFN-γ but not IL-4 mRNA. In contrast, in the absence of CD8(+) T cells, CD4(+) T cells switched to significantly upregulate IL-4 mRNA, while IFN-γ was downregulated. IL-4 knockout mice do not produce any posttransplant alloantibody. However, adoptive transfer of wild-type CD4(+) T cells into CD8-depleted IL-4 knockout mice restores high alloantibody levels observed in CD8-depleted wild-type recipients. This suggests that IL-4-producing CD4(+) T cells are critical for posttransplant alloantibody production. Additionally, this CD8-mediated regulation of posttransplant alloantibody production is IFN-γ-dependent. Further elucidation of the mechanisms by which CD8(+) T cells influence Ab production will significantly contribute to development of therapies to manipulate humoral responses to Ag.

Selective depletion of cross-presenting dendritic cells enhances islet allograft survival

MHC class I presentation of peptides derived from exogenous antigens (not synthesized within the antigen-presenting cell) is called cross-presentation and is mediated by selective subsets of dendritic cells (DC). A proportion of both donor and host DC may cross-present. Although there has been many studies that have investigated the role of donor versus host DC (i.e., direct vs. indirect pathway), what role cross-presenting DC play in allograft rejection has not been determined. We recently identified an agent, cytochrome c (cytc), that selectively depletes cross-presenting DC in vivo. By administering cytc we were able to study the impact of cross-presenting DC on rejection of islets grafted into fully mismatched mice. We found that cytc protected about half of the islet allografts from rejection. Our results indicate that cross-presenting DC can make potent contributions to the immune response to islet allografts, and contend that agents such as cytc that selectively target DC heralds a novel method of immunosuppression.

Mechanisms of parenchymal injury and signaling pathways in ectatic ducts of chronic pancreatitis: implications for pancreatic carcinogenesis

The pathobiology of chronic pancreatitis (CP) remains enigmatic despite remarkable progress made recently in uncovering key mechanisms involved in the initiation and progression of the disease. CP is increasingly thought of as a multifactorial disorder. Apoptosis plays a role in parenchymal destruction, the pathological hallmark of CP. The apoptotic mechanisms preferentially target the exocrine compartment, leaving endocrine islets relatively intact for a prolonged period. Exocrine cells shed their 'immunoprivileged' status, express death receptors, and are rendered susceptible to apoptosis induced by death ligands on infiltrating lymphocytes, and released locally by activated pancreatic stellate cells. Islet cells retain their 'immunoprivileged' status and activate anti-apoptotic programs through NF-kappaB. Ductal changes, including distortion, dilatation, and pancreatic ductal hypertension in the setting of CP, induce genomic damage and increased cell turnover. In addition, signaling mechanisms that play a role in the development of embryonic pancreas are reinstated, thus, playing a role in repair, regeneration, and transformation. This, in turn, leads to acino-ductal metaplasia (ADM) and pancreatic intraepithelial neoplasia (PanIN). Some of these pathways are activated in pancreatic cancer. We attempt to integrate the current knowledge and major concepts in the pathogenesis of CP and to explain the mechanism of differential cell loss. We also discuss the possible implications of signaling pathway activation in pancreatic inflammation, relevant to the cellular transformation that leads to pancreatic neoplasia.

IFN-gamma dictates allograft fate via opposing effects on the graft and on recipient CD8 T cell responses

CD8 T cells are necessary for costimulation blockade-resistant rejection. However, the mechanism by which CD8 T cells mediate rejection in the absence of major costimulatory signals is poorly understood. IFN-gamma promotes CD8 T cell-mediated immune responses, but IFN-gamma-deficient mice show early graft loss despite costimulation blockade. In contrast, we found that IFN-gamma receptor knockout mice show dramatically prolonged graft survival under costimulation blockade. To investigate this paradox, we addressed the effects of IFN-gamma on T cell alloresponses in vivo independent of the effects of IFN-gamma on graft survival. We identified a donor-specific CD8 T cell breakthrough response temporally correlated with costimulation blockade-resistant rejection. Neither IFN-gamma receptor knockout recipients nor IFN-gamma-deficient recipients showed a CD8 breakthrough response. Graft death on IFN-gamma-deficient recipients despite costimulation blockade could be explained by the lack of IFN-gamma available to act on the graft. Indeed, the presence of IFN-gamma was necessary for graft survival on IFN-gamma receptor knockout recipients, as either IFN-gamma neutralization or the lack of the IFN-gamma receptor on the graft precipitated early graft loss. Thus, IFN-gamma is required both for the recipient to mount a donor-specific CD8 T cell response under costimulation blockade as well as for the graft to survive after allotransplantation.

Induction of tolerance to cardiac allografts using donor splenocytes engineered to display on their surface an exogenous fas ligand protein

The critical role played by Fas ligand (FasL) in immune homeostasis renders this molecule an attractive target for immunomodulation to achieve tolerance to auto- and transplantation Ags. Immunomodulation with genetically modified cells expressing FasL was shown to induce tolerance to alloantigens. However, genetic modification of primary cells in a rapid, efficient, and clinically applicable manner proved challenging. Therefore, we tested the efficacy of donor splenocytes rapidly and efficiently engineered to display on their surface a chimeric form of FasL protein (SA-FasL) for tolerance induction to cardiac allografts. The i.p. injection of ACI rats with Wistar-Furth rat splenocytes displaying SA-FasL on their surface resulted in tolerance to donor, but not F344 third-party cardiac allografts. Tolerance was associated with apoptosis of donor reactive T effector cells and induction/expansion of CD4(+)CD25(+)FoxP3(+) T regulatory (Treg) cells. Treg cells played a critical role in the observed tolerance as adoptive transfer of sorted Treg cells from long-term graft recipients into naive unmanipulated ACI rats resulted in indefinite survival of secondary Wistar-Furth grafts. Immunomodulation with allogeneic cells rapidly and efficiently engineered to display on their surface SA-FasL protein provides an effective and clinically applicable means of cell-based therapy with potential application to regenerative medicine, transplantation, and autoimmunity.

Cytotoxic T-lymphocyte-mediated killing of human pancreatic islet cells in vitro

Cytotoxic T lymphocytes (CTL) are believed to play an essential role in beta-cell destruction leading to development of type 1 diabetes and allogeneic islet graft failure. We aimed to identify the mechanisms used by CTL to kill human beta cells. CTL clones that recognize epitopes from influenza virus and Epstein-Barr virus restricted by human leukocyte antigen (HLA)-A0201 and -B0801, respectively, were used to investigate the susceptibility of human beta cells to CTL. In a short-term (5-hour) assay, CTL killed human islet cells of the appropriate major histocompatibility complex (MHC) class I type that had been pulsed with viral peptides. Killing was increased by pretreating islets with interferon gamma that increases MHC class I on target cells. Killing was abolished by incubation of CTL with the perforin inhibitor concanamycin A. The Fas pathway did not contribute to killing because blocking with neutralizing anti-Fas ligand antibody did not significantly reduce beta-cell killing. In conclusion, we report a novel way of investigating the interaction between CTL and human islets. Human islets were rapidly killed in vitro by MHC class I-restricted CTL predominantly by the granule exocytosis pathway.

Alloreactive (CD4-Independent) CD8+ T cells jeopardize long-term survival of intrahepatic islet allografts

Despite success of early islet allograft engraftment and survival in humans, late islet allograft loss has emerged as an important clinical problem. CD8+ T cells that are independent of CD4+ T cell help can damage allograft tissues and are resistant to conventional immunosuppressive therapies. Previous work demonstrates that islet allografts do not primarily initiate rejection by the (CD4-independent) CD8-dependent pathway. This study was performed to determine if activation of alloreactive CD4-independent, CD8+ T cells, by exogenous stimuli, can precipitate late loss of islet allografts. Recipients were induced to accept intrahepatic islet allografts (islet 'acceptors') by short-term immunotherapy with donor-specific transfusion (DST) and anti-CD154 mAb. Following the establishment of stable long-term islet allograft function for 60-90 days, recipients were challenged with donor-matched hepatocellular allografts, which are known to activate (CD4-independent) CD8+ T cells. Allogeneic islets engrafted long-term were vulnerable to damage when challenged locally with donor-matched hepatocytes. Islet allograft loss was due to allospecific immune damage, which was CD8- but not CD4-dependent. Selection of specific immunotherapy to suppress both CD4- and CD8-dependent immune pathways at the time of transplant protects islet allografts from both early and late immune damage.

The effects of tolerance on allograft damage caused by the innate immune system

BACKGROUND

It is not known whether tolerance can be induced in a strong proinflammatory milieu or whether the induction of tolerance can prevent interferon (IFN)-gamma-associated graft injury. To address these questions, we studied the effects of rIFN-gamma infusion on porcine cardiac allograft survival.

METHODS

Recombinant interferon (rIFN)-gamma was continuously infused into the left anterior descending artery of hearts transplanted into major histocompatibility complex-inbred miniature swine treated with a 12-day course of cyclosporine A. Group 1 recipients received a nearly syngeneic heart, group 2 recipients received a class I disparate heart, and group 3 recipients were cotransplanted with a class I-disparate heart and kidney, a procedure demonstrated to induce tolerance to both grafts. A fourth group of animals were not transplanted but received intracoronary rIFN-gamma infusion into the native heart.

RESULTS

rIFN-gamma perfusion not only accelerated the acute rejection of class I-disparate hearts (mean survival time, 19+/-7.21 vs. 38+/-8.19; P=0.025) but caused near-syngeneic heart transplants, which otherwise survived indefinitely, to reject within 35 days. In contrast, rIFN-gamma perfusion had no demonstrable effects on hearts grafts in tolerant recipients or on autologous hearts.

CONCLUSIONS

These results suggest that tolerance induction can occur in the presence of IFN-gamma-mediated inflammation, and that tolerance induction can prevent the tissue injury caused by the overproduction of IFN-gamma. This suggests that the beneficial effects of tolerance may include protection from nonspecific inflammatory responses, such as those produced by ischemia-reperfusion injury and brain death.

Islet allograft rejection by contact-dependent CD8+ T cells: perforin and FasL play alternate but obligatory roles

Though CD8(+) T lymphocytes are important cellular mediators of islet allograft rejection, their molecular mechanism of rejection remains unidentified. Surprisingly, while it is generally assumed that CD8(+) T cells require classic cytotoxic mechanisms to kill grafts in vivo, neither perforin nor FasL (CD95L) are required for acute islet allograft rejection. Thus, it is unclear whether such contact-dependent cytotoxic pathways play an essential role in islet rejection. Moreover, both perforin and CD95L have been implicated in playing roles in peripheral tolerance, further obscuring the role of these effector pathways in rejection. Therefore, we determined whether perforin and/or FasL (CD95L) were required by donor MHC-restricted ('direct') CD8(+) T cells to reject islet allografts in vivo. Islet allograft rejection by primed, alloreactive CD8(+) T cells was examined independently of other lymphocyte subpopulations via adoptive transfer studies. Individual disruption of T-cell-derived perforin or allograft Fas expression had limited impact on graft rejection. However, simultaneous disruption of both pathways prevented allograft rejection in most recipients despite the chronic persistence of transferred T cells at the graft site. Thus, while there are clearly multiple cellular pathways of allograft rejection, perforin and FasL comprise alternate and necessary routes of acute CD8(+) T-cell-mediated islet allograft rejection.

Long-term survival of transplanted allogeneic cells engineered to express a T cell chemorepellent

BACKGROUND

Alloantigen specific T cells have been shown to be required for allograft rejection. The chemokine, stromal cell derived factor-1 (SDF-1) at high concentration, has been shown to act as a T-cell chemorepellent and abrogate T-cell infiltration into a site of antigen challenge in vivo via a mechanism termed fugetaxis or chemorepulsion. We postulated that this mechanism could be exploited therapeutically and that allogeneic cells engineered to express a chemorepellent protein would not be rejected.

METHODS

Allogeneic murine insulinoma beta-TC3 cells and primary islets from BALB/C mice were engineered to constitutively secrete differential levels of SDF-1 and transplanted into allogeneic diabetic C57BL/6 mice. Rejection was defined as the permanent return of hyperglycemia and was correlated with the level of T-cell infiltration. The migratory response of T-cells to SDF-1 was also analyzed by transwell migration assay and time-lapse videomicroscopy. The cytotoxicity of cytotoxic T cell (CTLs) against beta-TC3 cells expressing high levels of SDF-1 was measured in standard and modified chromium-release assays in order to determine the effect of CTL migration on killing efficacy.

RESULTS

Control animals rejected allogeneic cells and remained diabetic. In contrast, high level SDF-1 production by transplanted cells resulted in increased survival of the allograft and a significant reduction in blood glucose levels and T-cell infiltration into the transplanted tissue.

CONCLUSIONS

This is the first demonstration of a novel approach that exploits T-cell chemorepulsion to induce site specific immune isolation and thereby overcomes allograft rejection without the use of systemic immunosuppression.

Dichotomy of fates of pancreatic epithelia in chronic pancreatitis: apoptosis versus survival

Chronic pancreatitis is now thought to have a multifactorial etiology. New concepts integrating cellular, molecular and genetic knowledge of the disease have been proposed to explain its pathogenesis. However, the mechanisms responsible for early exocrine parenchymal destruction and preservation of endocrine islets were unexplored until recently. In the course of chronic inflammation, pancreatic acini lose their "immunoprotective" status by neo-expressing death receptors. Therefore, they become susceptible to apoptosis that is triggered by their respective ligands expressed on lymphocytes and released by pancreatic stellate cells. By contrast, islets retain their immunoprotective status and activate nuclear factor-kappaB (NF-kappaB)-induced anti-apoptotic factors, thus enabling survival. This knowledge might be exploited for devising therapeutic approaches to retard acinar loss and to prolong islet survival.

A critical role for granzyme B, in addition to perforin and TNFalpha, in alloreactive CTL-induced mouse pancreatic beta cell death

BACKGROUND

The precise effector mechanisms and molecular mediators used by alloreactive cytotoxic T lymphocytes to kill transplanted pancreatic beta cells are poorly defined. We have used mouse (H2b-anti-d) CTLs raised in strains deficient in various key cytotoxic effector molecules to assess the importance of the various signaling pathways mobilized to kill primary mouse pancreatic islet cells, the beta cell line NIT-1, and NIT-1 cells overexpressing dominant-negative FADD and Bcl-2.

METHODS

Death of target cells was assessed using 51Cr release assays.

RESULTS

In short-term assays (<5 hours) beta cell death did not require a functional FasL/Fas pathway, and was not inhibited by Bcl-2. However, the absence of either perforin or granzyme B resulted in cell survival. By contrast, a crucial role for granzyme B was not seen when hematopoietic P815 cells were used as targets, indicating differential regulation of apoptosis. Interestingly, coincubation with CTL for 24 hours revealed an additional but less potent "late phase" of beta cell death that did not require perforin. This delayed death was blocked by dominant-negative FADD, but not by Bcl-2, and was likely to be due to TNFalpha secretion.

CONCLUSIONS

This study suggests that strategies to protect beta cells from allogeneic CTL attack will need to inhibit the perforin/granzyme and probably also the TNFalpha pathway. As there are no known pharmacological approaches to blocking perforin, therapeutic approaches based on overexpressing both dominant negative FADD and an inhibitor of granzyme B may hold promise in prolonging beta cell survival in the allogeneic setting.

The role of macrophages in allograft rejection

Macrophage accumulation has long been recognized as a feature of allograft rejection, yet the role of macrophages in rejection remains underappreciated. Macrophages contribute to both the innate and acquired arms of the alloimmune response and thus may be involved in all aspects of acute and chronic allograft rejection. Recent advances in macrophage biology have allowed a better understanding of the mechanisms of macrophage accumulation, their state of activation and the pleuripotent roles they play in allograft rejection. Therapeutic attention to macrophages, in addition to T lymphocytes, may lead to improved outcomes in organ transplantation.

The X-linked inhibitor of apoptosis protein enhances survival of murine islet allografts

Allotransplantation of pancreatic islets represents a promising approach to treat type 1 diabetes. Destruction of beta-cells in islet allografts involves multiple immune mechanisms that lead to activation of caspases and apoptotic cell death. The X-linked inhibitor of apoptosis (XIAP) inhibits apoptosis induced by a variety of triggers, primarily by preventing the activation of caspases. To determine whether XIAP would protect beta-cells from apoptosis, we used a recombinant adenovirus to overexpress XIAP in transformed murine beta-cells and in freshly isolated islets. In vitro cytokine-induced beta-cell death was decreased to baseline levels in XIAP-transduced MIN-6 and NIT-1 cell lines compared with controls. To evaluate the potential of XIAP overexpression to prevent in vivo allogeneic graft rejection, we transduced Balb/c islets ex vivo with XIAP before transplantation into CBA mice with streptozotocin-induced diabetes. We observed that almost all mice receiving allografts of XIAP-expressing islets maintained normoglycemia until the experiment was terminated (45-72 days posttransplant), whereas control mice receiving islets transduced with adenovirus expressing LacZ were hyperglycemic by approximately 17 days posttransplantation due to graft rejection. Immunohistochemistry revealed preservation of beta-cells and clearance of infiltrating immune cells in the XIAP-expressing islet grafts. The in vitro allogeneic response of splenocytes isolated from recipients of XIAP-expressing grafts 8 weeks posttransplant was similar to that seen in nonprimed allogeneic mice, suggesting that XIAP overexpression may lead to the acceptance of islet allografts in diabetic recipients. Long-term protection of islet allografts by XIAP overexpression may enhance the survival of islet transplants in diabetes.

Perturbation of Leukocyte Function-Associated Antigen-1/Intercellular Adhesion Molecule-1 Results in Differential Outcomes in Cardiac Vs Islet Allograft Survival

BACKGROUND

Various studies indicate the requirements for tolerance induction may vary between different transplanted tissues and organs. Consequently, we compared the efficacy of anti-leukocyte function-associated antigen-1 (LFA-1)/anti-intercellular adhesion molecule-1 (ICAM-1) monoclonal antibody therapy for facilitating cardiac vs islet long-term allograft acceptance in mice.

METHODS

BALB/c (H-2d) mouse cardiac or islet allografts were transplanted into recipient CBA/J (H-2k) mice. Monoclonal anti-body therapy with anti-LFA-1, anti-ICAM-1, the combination, or control rat immunoglobulin (Ig) was administered intraperitoneally on Days 0 to 5. Cardiac allograft function was assessed by palpation and islet graft function by blood glucose monitoring. Mixed lymphocyte assays were performed to assess proliferation of CD4 and CD8 T-cells under conditions of stimulator-cell ICAM-1 and/or LFA-1 deficiency.

RESULTS

Anti-ICAM-1 therapy resulted in a modest prolongation of cardiac allografts but in pronounced survival of islet allografts. Anti-LFA-1 therapy promoted significant long-term survival of both cardiac and islet allografts. Surprisingly, combined anti-LFA-1/anti-ICAM-1 therapy abrogated long-term islet, but not cardiac, allograft acceptance relative to either monotherapy. Mixed lymphocyte reactions demonstrated complete blockade of CD4 and CD8 T-cell proliferation under conditions of ICAM-1 deficiency alone or in combination with anti-LFA-1 therapy.

CONCLUSION

These results indicate that optimal therapies for some allografts (vascularized-heart) may not translate to other types of allografts (cellular-islet). Thus, the type of transplant represents an independent variable for optimizing strategies to promote indefinite allograft acceptance. Complete inhibition of CD4 and CD8 T-cell proliferation during ICAM-1/LFA-1 blockade suggests a threshold signal may be dependent upon ICAM-1/LFA-1 for regulatory tolerance to occur and that this signal may be lost under conditions of minimal graft cellular mass.