Allorecognition

Disulfiram, an Anti-alcoholic Drug, Targets Macrophages and Attenuates Acute Rejection in Rat Lung Allografts

Macrophages contribute to post-transplant lung rejection. Disulfiram (DSF), an anti-alcoholic drug, has an anti-inflammatory effect and regulates macrophage chemotactic activity. Here, we investigated DSF efficacy in suppressing acute rejection post-lung transplantation. Male Lewis rats (280-300 g) received orthotopic left lung transplants from Fisher 344 rats (minor histocompatibility antigen-mismatched transplantation). DSF (0.75 mg/h) monotherapy or co-solvent only (50% hydroxypropyl-β-cyclodextrin) as control was subcutaneously administered for 7 days (n = 10/group). No post-transplant immunosuppressant was administered. Grades of acute rejection, infiltration of immune cells positive for CD68, CD3, or CD79a, and gene expression of monocyte chemoattractant protein and pro-inflammatory cytokines in the grafts were assessed 7 days post-transplantation. The DSF-treated group had significantly milder lymphocytic bronchiolitis than the control group. The infiltration levels of CD68+ or CD3+ cells to the peribronchial area were significantly lower in the DSF than in the control groups. The normalized expression of chemokine ligand 2 and interleukin-6 mRNA in allografts was lower in the DSF than in the control groups. Validation assay revealed interleukin-6 expression to be significantly lower in the DSF than in the control groups. DSF can alleviate acute rejection post-lung transplantation by reducing macrophage accumulation around peripheral bronchi and suppressing pro-inflammatory cytokine expression.

Pathways of Antigen Recognition by T Cells in Allograft Rejection

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

Modeling resistance of colorectal peritoneal metastases to immune checkpoint blockade in humanized mice

BACKGROUND

The immunogenic nature of metastatic colorectal cancer (CRC) with high microsatellite instability (MSI-H) underlies their responsiveness to immune checkpoint blockade (ICB). However, resistance to ICB is commonly observed, and is associated with the presence of peritoneal-metastases and ascites formation. The mechanisms underlying this site-specific benefit of ICB are unknown.

METHODS

We created a novel model for spontaneous multiorgan metastasis in MSI-H CRC tumors by transplanting patient-derived organoids (PDO) into the cecum of humanized mice. Anti-programmed cell death protein-1 (PD-1) and anti-cytotoxic T-lymphocytes-associated protein 4 (CTLA-4) ICB treatment effects were analyzed in relation to the immune context of primary tumors, liver metastases, and peritoneal metastases. Immune profiling was performed by immunohistochemistry, flow cytometry and single-cell RNA sequencing. The role of B cells was assessed by antibody-mediated depletion. Immunosuppressive cytokine levels (interleukin (IL)-10, transforming growth factor (TGF)b1, TGFb2, TGFb3) were determined in ascites and serum samples by ELISA.

RESULTS

PDO-initiated primary tumors spontaneously metastasized to the liver and the peritoneum. Peritoneal-metastasis formation was accompanied by the accumulation of ascites. ICB completely cleared liver metastases and reduced primary tumor mass but had no effect on peritoneal metastases. This mimics clinical observations. After therapy discontinuation, primary tumor masses progressively decreased, but peritoneal metastases displayed unabated growth. Therapy efficacy correlated with the formation of tertiary lymphoid structures (TLS)-containing B cells and juxtaposed T cells-and with expression of an interferon-γ signature together with the B cell chemoattractant CXCL13. B cell depletion prevented liver-metastasis clearance by anti-CTLA-4 treatment. Peritoneal metastases were devoid of B cells and TLS, while the T cells in these lesions displayed a dysfunctional phenotype. Ascites samples from patients with cancer with peritoneal metastases and from the mouse model contained significantly higher levels of IL-10, TGFb1, TGFb2 and TGFb3 than serum samples.

CONCLUSIONS

By combining organoid and humanized mouse technologies, we present a novel model for spontaneous multiorgan metastasis by MSI-H CRC, in which the clinically observed organ site-dependent benefit of ICB is recapitulated. Moreover, we provide empirical evidence for a critical role for B cells in the generation of site-dependent antitumor immunity following anti-CTLA-4 treatment. High levels of immunosuppressive cytokines in ascites may underlie the observed resistance of peritoneal metastases to ICB.

Designing biomaterials for the modulation of allogeneic and autoimmune responses to cellular implants in Type 1 Diabetes

The effective suppression of adaptive immune responses is essential for the success of allogeneic cell therapies. In islet transplantation for Type 1 Diabetes, pre-existing autoimmunity provides an additional hurdle, as memory autoimmune T cells mediate both an autoantigen-specific attack on the donor beta cells and an alloantigen-specific attack on the donor graft cells. Immunosuppressive agents used for islet transplantation are generally successful in suppressing alloimmune responses, but dramatically hinder the widespread adoption of this therapeutic approach and fail to control memory T cell populations, which leaves the graft vulnerable to destruction. In this review, we highlight the capacity of biomaterials to provide local and nuanced instruction to suppress or alter immune pathways activated in response to an allogeneic islet transplant. Biomaterial immunoisolation is a common approach employed to block direct antigen recognition and downstream cell-mediated graft destruction; however, immunoisolation alone still permits shed donor antigens to escape into the host environment, resulting in indirect antigen recognition, immune cell activation, and the creation of a toxic graft site. Designing materials to decrease antigen escape, improve cell viability, and increase material compatibility are all approaches that can decrease the local release of antigen and danger signals into the implant microenvironment. Implant materials can be further enhanced through the local delivery of anti-inflammatory, suppressive, chemotactic, and/or tolerogenic agents, which serve to control both the innate and adaptive immune responses to the implant with a benefit of reduced systemic effects. Lessons learned from understanding how to manipulate allogeneic and autogenic immune responses to pancreatic islets can also be applied to other cell therapies to improve their efficacy and duration. STATEMENT OF SIGNIFICANCE: This review explores key immunologic concepts and critical pathways mediating graft rejection in Type 1 Diabetes, which can instruct the future purposeful design of immunomodulatory biomaterials for cell therapy. A summary of immunological pathways initiated following cellular implantation, as well as current systemic immunomodulatory agents used, is provided. We then outline the potential of biomaterials to modulate these responses. The capacity of polymeric encapsulation to block some powerful rejection pathways is covered. We also highlight the role of cellular health and biocompatibility in mitigating immune responses. Finally, we review the use of bioactive materials to proactively modulate local immune responses, focusing on key concepts of anti-inflammatory, suppressive, and tolerogenic agents.

Donor-Specific Regulatory T Cell-Mediated Immune Tolerance in an Intrahepatic Murine Allogeneic Islet Transplantation Model with Short-Term Anti-CD154 mAb Single Treatment

Anti-CD154 blockade-based regimens remain unequaled in prolonging graft survival in various organ transplantation models. Several studies have focused on transplantation tolerance with the anti-CD154 blockade, but none of these studies has investigated the mechanisms associated with its use as the sole treatment in animal models, delaying our understanding of anti-CD154 blockade-mediated immune tolerance. The purpose of this study was to investigate the mechanism underlying the anti-CD154 monoclonal antibody (mAb) blockade in inducing immune tolerance using an intrahepatic murine allogeneic islet transplantation model. Allogeneic BALB/c AnHsd (BALB/c) islets were infused into the liver of diabetic C57BL/6 (B6) mice via the cecal vein. Anti-CD154 mAb (MR1) was administered on -1, 0, 1, 3, 5, and 7 d posttransplantation at 0.5 mg per mouse. We showed that short-term MR1 monotherapy could prolong the allogeneic islet grafts to more than 250 d in the murine intrahepatic islet transplantation model. The second islet grafts transplanted under the kidney capsule of the recipients were protected from rejection. We also found that rejection of same-donor skin grafts transplanted to the tolerant mice was modestly delayed. Using a DEREG mouse model, FoxP3+ regulatory T (Treg) cells were shown to play important roles in transplantation tolerance. In mixed lymphocyte reactions, Treg cells from the tolerant mice showed more potency in suppressing BALB/c splenocyte-stimulated Teff cell proliferation than those from naïve mice. In this study, we demonstrated for the first time that a short-term anti-CD154 mAb single treatment could induce FoxP3+ Treg cell-mediated immune tolerance in the intrahepatic murine allogeneic islet transplantation model.

Systemic Complement Activation in Donation After Brain Death Versus Donation After Circulatory Death Organ Donors

OBJECTIVES

Complement activation in organs from deceased donors is associated with allograft injury and acute rejection. Because use of organs from donors after circulatory death is increasing, we characterized relative levels of complement activation in organs from donors after brain death and after circulatory death and examined associations between donor complement factor levels and outcomes after kidney and liver transplant.

MATERIALS AND METHODS

Serum samples from 65 donors (55 donations after brain death, 10 donations after circulatory death) were analyzed for classical, lectin, alternative, and terminal pathway components by Luminex multiplex assays. Complement factor levels were compared between groups, and associations with posttransplant outcomes were explored.

RESULTS

Serum levels of the downstream complement activation product C5a were similar in organs from donors after circulatory death versus donors after brain death. In organs from donors after circulatory death, complement activation occurred primarily via the alternative pathway; the classical, lectin, and alternative pathways all contributed in organs from donors after brain death. Donor complement levels were not associated with outcomes after kidney transplant. Lower donor complement levels were associated with need for transfusion, reintervention, hospital readmission, and acute rejection after liver transplant.

CONCLUSIONS

Complement activation occurs at similar levels in organs donated from donors after circulatory death versus those after brain death. Lower donor complement levels may contribute to adverse outcomes after liver transplant. Further study is warranted to better understand how donor complement activation contributes to posttransplant outcomes.

Cardiac allograft vasculopathy: Insights on pathogenesis and therapy

Cardiac allograft vasculopathy (CAV) is a unique accelerated form of coronary vascular disease affecting heart transplant recipients. This complication is a significant contributor to medium- to long-term post-transplant morbidity and mortality. There is a high prevalence of CAV with approximately one in three patients developing CAV by 5 years post-transplant. Morphologically, CAV is characterized by concentric coronary intimal hyperplasia in both the epicardial arteries and intramural microvasculature. Although several immune and non-immune factors have been identified, their precise pathogenic mechanisms, interactions, and relative importance in the development of CAV are not well defined. The advent of improved imaging surveillance modalities has resulted in earlier detection during the disease process. However, overall management of CAV remains challenging due to paucity of treatment. This review aims to discuss key concepts on the pathogenesis of CAV and current management strategies, focusing on the use of mammalian target of rapamycin inhibitors.

Standardisation of flow cytometry for whole blood immunophenotyping of islet transplant and transplant clinical trial recipients

Understanding the immunological phenotype of transplant recipients is important to improve outcomes and develop new therapies. Immunophenotyping of whole peripheral blood (WPB) by flow cytometry is a rapid method to obtain large amounts of data relating to the outcomes of different transplant treatments with limited patient impact. Healthy individuals and patients with type 1 diabetes (T1D) enrolled in islet transplantation were recruited and WPB was collected. 46 fluorochrome-conjugated mouse-anti-human antibodies were used (43 of 46 antibodies were titrated). BD cytometer setup and tracking beads were used to characterize and adjust for cytometer performance. Antibody cocktails were pre-mixed <60 minutes before staining. Multicolour panels were designed based on fluorochrome brightness, antigen density, co-expression, and fluorochrome spillover into non-primary detectors in each panel on a 5 laser flow cytometer. WPB sample staining used 50–300 μl WPB for each panel and was performed within 2 hours of blood sample collection. Samples were acquired on a BD-LSRFortessa. The operating procedures, including specimen collection, antibody cocktails, staining protocol, flow-cytometer setup and data analysis, were standardized. The staining index of 43 antibodies and the spillover spreading matrix for each panel was calculated. The final concentrations for the 46 antibodies used was determined for staining of WPB samples. Absolute cell-count and 7 leukocyte profiling panels consisting of subsets and/or status of granulocytes, monocytes, dendritic, B, NK, and T cells including regulatory T cells (Tregs) and NKT were designed and established on a 5 laser BD-LSR Fortessa. 13 T1D patients, including 4 islet transplant recipients and 8 healthy controls, were evaluated. The ability to reproducibly measure immune subsets and immune-profiles of islet transplant patients up to 18 months post transplantation has been established as a tool to measure immune cell reconstitution after transplantation.

Are donor lymphocytes a barrier to transplantation tolerance?

PURPOSE OF REVIEW

Following solid organ transplantation (SOT), populations of donor lymphocytes are frequently found in the recipient circulation. Their impact on host alloimmunity has long been debated but remains unclear, and it has been suggested that transferred donor lymphocytes may either promote tolerance to the graft or hasten its rejection. We discuss possible mechanisms by which the interaction of donor passenger lymphocytes with recipient immune cells may either augment the host alloimmune response or inhibit it.

RECENT FINDINGS

Recent work has highlighted that donor T lymphocytes are the most numerous of the donor leukocyte populations within a SOT and that these may be transferred to the recipient after transplantation. Surprisingly, graft-versus-host recognition of major histocompatibility complex class II on host B cells by transferred donor CD4 T cells can result in marked augmentation of host humoral alloimmunity and lead to early graft failure. Killing of donor CD4 T cells by host natural killer cells is critical in preventing this augmentation.

SUMMARY

The ability of passenger donor CD4 T cells to effect long-term augmentation of the host humoral alloimmune response raises the possibility that ex-vivo treatment or modification of the donor organ prior to implantation may improve long-term transplant outcomes.

Nanocarriers for spleen targeting: anatomo-physiological considerations, formulation strategies and therapeutic potential

There are several clinical advantages of spleen targeting of nanocarriers. For example, enhanced splenic concentration of active agents could provide therapeutic benefits in spleen resident infections and hematological disorders including malaria, hairy cell leukemia, idiopathic thrombocytopenic purpura, and autoimmune hemolytic anemia. Furthermore, spleen delivery of immunosuppressant agents using splenotropic carriers may reduce the chances of allograft rejection in organ transplantation. Enhanced concentration of radiopharmaceuticals in the spleen may improve visualization of the organ, which could provide benefit in the diagnosis of splenic disorders. Unique anatomical features of the spleen including specialized microvasculature environment and slow blood circulation rate enable it an ideal drug delivery site. Because there is a difference in blood flow between spleen and liver, splenic delivery is inversely proportional to the hepatic uptake. It is therefore desirable engineering of nanocarriers, which, upon intravenous administration, can avoid uptake by hepatic Kupffer cells to enhance splenic localization. Stealth and non-spherical nanocarriers have shown enhanced splenic delivery of active agents by avoiding hepatic uptake. The present review details the research in the field of splenotropy. Formulation strategies to design splenotropic drug delivery systems are discussed. The review also highlights the clinical relevance of spleen targeting of nanocarriers and application in diagnostics.

Immunological challenges associated with artificial skin grafts: available solutions and stem cells in future design of synthetic skin

The repair or replacement of damaged skins is still an important, challenging public health problem. Immune acceptance and long-term survival of skin grafts represent the major problem to overcome in grafting given that in most situations autografts cannot be used. The emergence of artificial skin substitutes provides alternative treatment with the capacity to reduce the dependency on the increasing demand of cadaver skin grafts. Over the years, considerable research efforts have focused on strategies for skin repair or permanent skin graft transplantations. Available skin substitutes include pre- or post-transplantation treatments of donor cells, stem cell-based therapies, and skin equivalents composed of bio-engineered acellular or cellular skin substitutes. However, skin substitutes are still prone to immunological rejection, and as such, there is currently no skin substitute available to overcome this phenomenon. This review focuses on the mechanisms of skin rejection and tolerance induction and outlines in detail current available strategies and alternatives that may allow achieving full-thickness skin replacement and repair.

Ablation of Transcription Factor IRF4 Promotes Transplant Acceptance by Driving Allogenic CD4+ T Cell Dysfunction

CD4⁺ T cells orchestrate immune responses and destruction of allogeneic organ transplants, but how this process is regulated on a transcriptional level remains unclear. Here, we demonstrated that interferon regulatory factor 4 (IRF4) was a key transcriptional determinant controlling T cell responses during transplantation. IRF4 deletion in mice resulted in progressive establishment of CD4⁺ T cell dysfunction and long-term allograft survival. Mechanistically, IRF4 repressed PD-1, Helios, and other molecules associated with T cell dysfunction. In the absence of IRF4, chromatin accessibility and binding of Helios at PD-1 cis-regulatory elements were increased, resulting in enhanced PD-1 expression and CD4⁺ T cell dysfunction. The dysfunctional state of Irf4-deficient T cells was initially reversible by PD-1 ligand blockade, but it progressively developed into an irreversible state. Hence, IRF4 controls a core regulatory circuit of CD4⁺ T cell dysfunction, and targeting IRF4 represents a potential therapeutic strategy for achieving transplant acceptance.

Overview of Immunosuppressive Therapy in Solid Organ Transplantation

Mechanisms of rejection, new pharmacologic approaches, and genomic medicine are major foci for current research in transplantation. It is hoped that these new agents and personalized immunosuppression will provide for less toxic regimens that are effective in preventing both acute and chronic allograft rejection. Until new agents are available, practitioners must use various combinations of currently approved agents to find the best regimens for improved long-term outcomes.

Human plasmacytoid dendritic cells acquire phagocytic capacity by TLR9 ligation in the presence of soluble factors produced by renal epithelial cells

Plasmacytoid dendritic cells (pDCs) are antigen presenting cells specialized in viral recognition through Toll-like receptor (TLR)7 and TLR9, and produce vast amounts of interferon alpha upon ligation of these TLRs. We had previously demonstrated a strong influx of pDCs in the tubulointerstitium of renal biopsies at the time of acute rejection. However, the role of human pDCs in mediating acute or chronic allograft rejection remains elusive. pDCs are thought to have a limited capacity to ingest apoptotic cells, critical for inducing CD4⁺ T cell activation via indirect antigen presentation and subsequent activation of antibody producing B cells. Here we tested whether the function of pDCs is affected by their presence within the graft. Maturation and interferon alpha production by pDCs was enhanced when cells were activated in the presence of viable HK2 renal epithelial cells. Importantly, soluble factors produced by cytomegalovirus-infected (primary) epithelial or endothelial cells enhanced pDC activation and induced their capacity to phagocytose apoptotic cells. Phagocytosis was not induced by free virus or soluble factors from non-infected cells. Activated pDCs showed an enhanced CD4⁺ and CD8⁺ T cell allostimulatory capacity as well as a potent indirect alloantigen presentation. Granulocyte Macrophage-Colony Stimulating Factor is one of the soluble factors produced by renal epithelial cells that, combined with TLR9 ligation, induced this functional capacity. Thus, pDCs present in the rejecting allograft can contribute to alloimmunity and potentially act as important orchestrators in the manifestation of acute and chronic rejection.

Stem cell transplantation therapy for multifaceted therapeutic benefits after stroke

One of the exciting advances in modern medicine and life science is cell-based neurovascular regeneration of damaged brain tissues and repair of neuronal structures. The progress in stem cell biology and creation of adult induced pluripotent stem (iPS) cells has significantly improved basic and pre-clinical research in disease mechanisms and generated enthusiasm for potential applications in the treatment of central nervous system (CNS) diseases including stroke. Endogenous neural stem cells and cultured stem cells are capable of self-renewal and give rise to virtually all types of cells essential for the makeup of neuronal structures. Meanwhile, stem cells and neural progenitor cells are well-known for their potential for trophic support after transplantation into the ischemic brain. Thus, stem cell-based therapies provide an attractive future for protecting and repairing damaged brain tissues after injury and in various disease states. Moreover, basic research on naïve and differentiated stem cells including iPS cells has markedly improved our understanding of cellular and molecular mechanisms of neurological disorders, and provides a platform for the discovery of novel drug targets. The latest advances indicate that combinatorial approaches using cell based therapy with additional treatments such as protective reagents, preconditioning strategies and rehabilitation therapy can significantly improve therapeutic benefits. In this review, we will discuss the characteristics of cell therapy in different ischemic models and the application of stem cells and progenitor cells as regenerative medicine for the treatment of stroke.

Infectious pathogens may trigger specific allo-HLA reactivity via multiple mechanisms

Transplant recipients can be sensitized against allo-HLA antigens by previous transplantation, blood transfusion, or pregnancy. While there is growing awareness that multiple components of the immune system can act as effectors of the alloresponse, the role of infectious pathogen exposure in triggering sensitization and allograft rejection has remained a matter of much debate. Here, we describe that exposure to pathogens may enhance the immune response to allogeneic HLA antigens via different pathways. The potential role of allo-HLA cross-reactivity of virus-specific memory T cells, activation of innate immunity leading to a more efficient induction of the adaptive alloimmune response by antigen-presenting cells, and bystander activation of existing memory B cell activation will be discussed in this review.

Inhibition of allogeneic cytotoxic T cell (CD8+) proliferation via polymer-induced Treg (CD4+) cells

T cell-mediated immune rejection remains a barrier to successful transplantation. Polymer-based bioengineering of cells may provide an effective means of preventing allorecognition and the proliferation of cytotoxic (CD8⁺) T lymphocytes (CTL). Using MHC-disparate murine splenocytes modified with succinimidyl valerate activated methoxypoly(ethylene glycol) [SVA-mPEG] polymers, the effects of leukocyte immunocamouflage on CD8⁺ and CD4⁺ alloproliferation and T regulatory (Treg) cell induction were assessed in a mixed lymphocyte reaction (MLR) model. Polymer-grafting effectively camouflaged multiple leukocyte markers (MHC class I and II, TCR and CD3) essential for effective allorecognition. Consequent to the polymer-induced immunocamouflage of the cell membrane, both CD8⁺ and CD4⁺ T cell alloproliferation were significantly inhibited in a polymer dose-dependent manner. The loss of alloproliferation correlated with the induction of Treg cells (CD4⁺CD25⁺Foxp3⁺). The Tregs, surprisingly, arose primarily via differentiation of naive, non-proliferating, CD4⁺ cells. Of biologic importance, the polymer-induced Treg were functional and exhibited potent immunosuppressive activity on allogeneic CTL proliferation. These results suggest that immunocamouflage-mediated attenuation of alloantigen-TCR recognition can prevent the tissue destructive allogeneic CD8⁺ T cell response, both directly and indirectly, through the generation/differentiation of functional Tregs. Immunocamouflage induced tolerance could be clinically valuable in attenuating T cell-mediated transplant rejection and in the treatment of autoimmune diseases.

STATEMENT OF SIGNIFICANCE

While our previous studies have demonstrated that polymer-grafting to MHC disparate leukocytes inhibits CD4⁺ cell proliferation, the effects of PEGylation on the alloproliferation of CD8⁺ cytotoxic T cells (CTL) was not examined. As shown here, PEGylation of allogeneic leukocytes prevents the generation of the CTL response responsible for acute rejection. The loss of CTL proliferation is consequent to the polymer-based attenuation of allorecognition and the induction of T regulatory cells (Tregs). Interestingly, the Tregs are primarily generated via the differentiation of non-proliferating naive T cells. Importantly, the Tregs are functional and effectively induce a tolerogenic environment when transferred to an alloresponsive environment. The use of polymer-modified leukocytes provides a unique approach to effectively maximize the biologic production of functional Tregs both in vitro and in vivo. By using this approach it may be possible to attenuate unwanted alloresponses (e.g., graft rejection) or to treat autoimmune diseases.

Novel Insights Into the Immunoregulatory Function and Localization of Dendritic Cells

Dendritic cells (DCs) are antigen-presenting cells that normally play a critical role in stimulating T-cell-dependent immune responses. However, tolerogenic DCs (CD11cMHC-IICD80CD86) induce immune tolerance by stimulating regulatory T cells (Tregs: CD4CD25Foxp3). Although tolerogenic DCs are used to treat autoimmune diseases and to prevent transplantation rejection, the mechanisms by which they regulate alloimmunity are poorly understood. Here, we review our previous studies aiming to elucidate the mechanisms involved in immune rejection of corneal allografts using a corneal transplant model. We found that donor-derived tolerogenic DCs significantly prolonged corneal allograft survival by suppressing indirect allosensitization. We also reported the precise distribution of intraepithelial corneal DCs, termed Langerhans cells (LCs: CD11cLangerinMHC-II) in the cornea, which we maintain play a critical role in regulating corneal immunity. By confocal microscopy, we constructed 3-dimensional images of corneal LCs, which demonstrated that their cell bodies are present in the basal cell layer of the corneal epithelium. Furthermore, LC dendrites extend toward the ocular surface, but do not connect to epithelial tight junctions, indicating that they cannot directly interact with ocular surface antigens. We confirm the potential of DC therapy for corneal graft rejection and report the function of intraepithelial DCs (LCs) in the normal cornea.

Expression of a Chimeric Antigen Receptor Specific for Donor HLA Class I Enhances the Potency of Human Regulatory T Cells in Preventing Human Skin Transplant Rejection

Regulatory T cell (Treg) therapy using recipient-derived Tregs expanded ex vivo is currently being investigated clinically by us and others as a means of reducing allograft rejection following organ transplantation. Data from animal models has demonstrated that adoptive transfer of allospecific Tregs offers greater protection from graft rejection compared to polyclonal Tregs. Chimeric antigen receptors (CAR) are clinically translatable synthetic fusion proteins that can redirect the specificity of T cells toward designated antigens. We used CAR technology to redirect human polyclonal Tregs toward donor-MHC class I molecules, which are ubiquitously expressed in allografts. Two novel HLA-A2-specific CARs were engineered: one comprising a CD28-CD3ζ signaling domain (CAR) and one lacking an intracellular signaling domain (ΔCAR). CAR Tregs were specifically activated and significantly more suppressive than polyclonal or ΔCAR Tregs in the presence of HLA-A2, without eliciting cytotoxic activity. Furthermore, CAR and ΔCAR Tregs preferentially transmigrated across HLA-A2-expressing endothelial cell monolayers. In a human skin xenograft transplant model, adoptive transfer of CAR Tregs alleviated the alloimmune-mediated skin injury caused by transferring allogeneic peripheral blood mononuclear cells more effectively than polyclonal Tregs. Our results demonstrated that the use of CAR technology is a clinically applicable refinement of Treg therapy for organ transplantation.

Allorecognition by T Lymphocytes and Allograft Rejection

Recognition of donor antigens by recipient T cells in secondary lymphoid organs initiates the adaptive inflammatory immune response leading to the rejection of allogeneic transplants. Allospecific T cells become activated through interaction of their T cell receptors with intact allogeneic major histocompatibility complex (MHC) molecules on donor cells (direct pathway) and/or donor peptides presented by self-MHC molecules on recipient antigen-presenting cells (APCs) (indirect pathway). In addition, recent studies show that alloreactive T cells can also be stimulated through recognition of allogeneic MHC molecules displayed on recipient APCs (MHC cross-dressing) after their transfer via cell-cell contact or through extracellular vesicles (semi-direct pathway). The specific allorecognition pathway used by T cells is dictated by intrinsic and extrinsic factors to the allograft and can influence the nature and magnitude of the alloresponse and rejection process. Consequently, various organs and tissues such as skin, cornea, and solid organ transplants are recognized differently by pro-inflammatory T cells through these distinct pathways, which may explain why these grafts are rejected in a different fashion. On the other hand, the mechanisms by which anti-inflammatory regulatory T cells (Tregs) recognize alloantigen and promote transplantation tolerance are still unclear. It is likely that thymic Tregs are activated through indirect allorecognition, while peripheral Tregs recognize alloantigens in a direct fashion. As we gain insights into the mechanisms underlying allorecognition by pro-inflammatory and Treg cells, novel strategies are being designed to prevent allograft rejection in the absence of ongoing immunosuppressive drug treatment in patients.

Renal transplant immunology in the last 20 years: A revolution towards graft and patient survival improvement

To deride the hope of progress is the ultimate fatuity, the last word in poverty of spirit and meanness of mind. There is no need to be dismayed by the fact that we cannot yet envisage a definitive solution of our problems, a resting-place beyond which we need not try to go. -P.B. Medawar, 1969 * Thomas E. Starlz, also known as the Father of Clinical Transplantation, once said that organ transplantation was the supreme exception to the rule that most major advances in medicine spring from discoveries in basic science [Starzl T. The mystique of organ transplantation. J Am Coll Surg 2005 Aug;201(2):160-170]. In fact, the first successful identical-twin kidney transplantation performed by Murray's team in December 1954 (Murray J et al. Renal homotransplantations in identical twins. Surg Forum 1955;6:432-436) was the example of an upside down translation medicine: Human clinical transplantation began and researchers tried to understand the underlying immune response and how to control the powerful rejection pathways through experimental models. In the last 20 years, we have witnessed an amazing progress in the knowledge of immunological mechanisms regarding alloimmune response and an outstanding evolution on the identification and characterization of major and minor histocompatibility antigens. This review presents an historical and clinical perspective of those important advances in kidney transplantation immunology in the last 20 years, which contributed to the improvement in patients' quality of life and the survival of end-stage renal patients. In spite of these significant progresses, some areas still need substantial progress, such as the definition of non-invasive biomarkers for acute rejection; the continuous reduction of immunosuppression; the extension of graft survival, and finally the achievement of real graft tolerance extended to HLA mismatch donor: recipient pairs.

Alloantigen presentation and graft-versus-host disease: fuel for the fire

Allogeneic stem cell transplantation (SCT) is a unique procedure, primarily in patients with hematopoietic malignancies, involving chemoradiotherapy followed by the introduction of donor hematopoietic and immune cells into an inflamed and lymphopenic environment. Interruption of the process by which recipient alloantigen is presented to donor T cells to generate graft-versus-host disease (GVHD) represents an attractive therapeutic strategy to prevent morbidity and mortality after SCT and has been increasingly studied in the last 15 years. However, the immune activation resulting in GVHD has no physiological equivalent in nature; alloantigen is ubiquitous, persists indefinitely, and can be presented by multiple cell types at numerous sites, often on incompatible major histocompatibility complex, and occurs in the context of intense inflammation early after SCT. The recognition that alloantigen presentation is also critical to the development of immunological tolerance via both deletional and regulatory mechanisms further adds to this complexity. Finally, GVHD itself appears capable of inhibiting the presentation of microbiological antigens by donor dendritic cells late after SCT that is mandatory for the establishment of effective pathogen-specific immunity. Here, we review our current understanding of alloantigen, its presentation by various antigen-presenting cells, subsequent recognition by donor T cells, and the potential of therapeutic strategies interrupting this disease-initiating process to modify transplant outcome.

Allospecific CD4(+) T cells retain effector function and are actively regulated by Treg cells in the context of transplantation tolerance

Although donor-specific transfusion (DST) plus CD154 blockade represents a robust protocol for inducing transplantation tolerance, the underlying mechanisms are incompletely understood. In a murine T-cell adoptive transfer model, we have visualized alloantigen-specific, TCR-transgenic for H2-A(b) /H2-K(d) 54-68 epitope (TCR75) CD4(+) T cells with indirect allospecificity during the course of tolerance induction. Three main observations were made. First, although the majority of TCR75 CD4(+) T cells were deleted following DST plus CD154 blockade, the surviving TCR75 CD4(+) T cells were capable of making IL-2, upregulating CD44, and undergoing cell division, suggesting that they were functionally active. Indeed, residual TCR75 CD4(+) T cells reisolated from the primary recipients given DST plus CD154 blockade were fully capable of rejecting allografts upon secondary transfer. Second, in tolerant mice, TCR75 CD4(+) T cells were not induced to express Foxp3 in the graft-draining lymph node. TCR75 CD4(+) T cells were also absent in accepted graft tissues in which endogenous Treg cells were enriched. Finally, DST plus CD154 blockade resulted in an abortive expansion of TCR75 CD4(+) T cells, a process that required the presence of endogenous Treg cells. Collectively, surviving TCR75 CD4(+) T cells are immunocompetent but kept in check by an endogenous immunosuppressive network induced by DST plus CD154 blockade.

Dendritic Cells in Kidney Transplant Biopsy Samples Are Associated with T Cell Infiltration and Poor Allograft Survival

Progress in long-term renal allograft survival continues to lag behind the progress in short-term transplant outcomes. Dendritic cells are the most efficient antigen-presenting cells, but surprisingly little attention has been paid to their presence in transplanted kidneys. We used dendritic cell-specific intercellular adhesion molecule-3-grabbing nonintegrin as a marker of dendritic cells in 105 allograft biopsy samples from 105 kidney transplant recipients. High dendritic cell density was associated with poor allograft survival independent of clinical variables. Moreover, high dendritic cell density correlated with greater T cell proliferation and poor outcomes in patients with high total inflammation scores, including inflammation in areas of tubular atrophy. We then explored the association between dendritic cells and histologic variables associated with poor prognosis. Multivariate analysis revealed an independent association between the densities of dendritic cells and T cells. In biopsy samples with high dendritic cell density, electron microscopy showed direct physical contact between infiltrating lymphocytes and cells that have the ultrastructural morphologic characteristics of dendritic cells. The origin of graft dendritic cells was sought in nine sex-mismatched recipients using XY fluorescence in situ hybridization. Whereas donor dendritic cells predominated initially, the majority of dendritic cells in late allograft biopsy samples were of recipient origin. Our data highlight the prognostic value of dendritic cell density in allograft biopsy samples, suggest a new role for these cells in shaping graft inflammation, and provide a rationale for targeting dendritic cell recruitment to promote long-term allograft survival.

Tolerance in organ transplantation: from conventional immunosuppression to extracellular vesicles

Organ transplantation is often the unique solution for organ failure. However, rejection is still an unsolved problem. Although acute rejection is well controlled, the chronic use of immunosuppressive drugs for allograft acceptance causes numerous side effects in the recipient and do not prevent chronic allograft dysfunction. Different alternative therapies have been proposed to replace the classical treatment for allograft rejection. The alternative therapies are mainly based in pre-infusions of different types of regulatory cells, including DCs, MSCs, and Tregs. Nevertheless, these approaches lack full efficiency and have many problems related to availability and applicability. In this context, the use of extracellular vesicles, and in particular exosomes, may represent a cell-free alternative approach in inducing transplant tolerance and survival. Preliminary approaches in vitro and in vivo have demonstrated the efficient alloantigen presentation and immunomodulation abilities of exosomes, leading to alloantigen-specific tolerance and allograft acceptance in rodent models. Donor exosomes have been used alone, processed by recipient antigen-presenting cells, or administered together with suboptimal doses of immunosuppressive drugs, achieving specific allograft tolerance and infinite transplant survival. In this review, we gathered the latest exosome-based strategies for graft acceptance and discuss the tolerance mechanisms involved in organ tolerance mediated by the administration of exosomes. We will also deal with the feasibility and difficulties that arise from the application of this strategy into the clinic.

Coronary microvasculopathy in heart transplantation: Consequences and therapeutic implications

Despite the progress made in the prevention and treatment of rejection of the transplanted heart, cardiac allograft vasculopathy (CAV) remains the main cause of death in late survival transplanted patients. CAV consists of a progressive diffuse intimal hyperplasia and the proliferation of vascular smooth muscle cells, ending in wall thickening of epicardial vessels, intramyocardial arteries (50-20 μm), arterioles (20-10 μm), and capillaries (< 10 μm). The etiology of CAV remains unclear; both immunologic and non-immunologic mechanisms contribute to endothelial damage with a sustained inflammatory response. The immunological factors involved are Human Leukocyte Antigen compatibility between donor and recipient, alloreactive T cells and the humoral immune system. The non-immunological factors are older donor age, ischemia-reperfusion time, hyperlipidemia and CMV infections. Diagnostic techniques that are able to assess microvascular function are lacking. Intravascular ultrasound and fractional flow reserve, when performed during coronary angiography, are able to detect epicardial coronary artery disease but are not sensitive enough to assess microvascular changes. Some authors have proposed an index of microcirculatory resistance during maximal hyperemia, which is calculated by dividing pressure by flow (distal pressure multiplied by the hyperemic mean transit time). Non-invasive methods to assess coronary physiology are stress echocardiography, coronary flow reserve by transthoracic Doppler echocardiography, single photon emission computed tomography, and perfusion cardiac magnetic resonance. In this review, we intend to analyze the mechanisms, consequences and therapeutic implications of microvascular dysfunction, including an extended citation of relevant literature data.

Uterine NK cells: active regulators at the maternal-fetal interface

Pregnancy presents an immunological conundrum because two genetically different individuals coexist. The maternal lymphocytes at the uterine maternal-fetal interface that can recognize mismatched placental cells are T cells and abundant distinctive uterine NK (uNK) cells. Multiple mechanisms exist that avoid damaging T cell responses to the fetus, whereas activation of uNK cells is probably physiological. Indeed, genetic epidemiological data suggest that the variability of NK cell receptors and their MHC ligands define pregnancy success; however, exactly how uNK cells function in normal and pathological pregnancy is still unclear, and any therapies aimed at suppressing NK cells must be viewed with caution. Allorecognition of fetal placental cells by uNK cells is emerging as the key maternal-fetal immune mechanism that regulates placentation.

Central memory CD8+ T lymphocytes mediate lung allograft acceptance

Memory T lymphocytes are commonly viewed as a major barrier for long-term survival of organ allografts and are thought to accelerate rejection responses due to their rapid infiltration into allografts, low threshold for activation, and ability to produce inflammatory mediators. Because memory T cells are usually associated with rejection, preclinical protocols have been developed to target this population in transplant recipients. Here, using a murine model, we found that costimulatory blockade-mediated lung allograft acceptance depended on the rapid infiltration of the graft by central memory CD8+ T cells (CD44(hi)CD62L(hi)CCR7+). Chemokine receptor signaling and alloantigen recognition were required for trafficking of these memory T cells to lung allografts. Intravital 2-photon imaging revealed that CCR7 expression on CD8+ T cells was critical for formation of stable synapses with antigen-presenting cells, resulting in IFN-γ production, which induced NO and downregulated alloimmune responses. Thus, we describe a critical role for CD8+ central memory T cells in lung allograft acceptance and highlight the need for tailored approaches for tolerance induction in the lung.

Evolving concepts and treatment strategies for cardiac allograft vasculopathy

OPINION STATEMENT

The central event in the development of allograft vasculopathy is the inflammatory response to immune-mediated and nonimmune-mediated endothelial damage. This response is characterized by the release of inflammatory cytokines, upregulation of cell-surface adhesion molecules, and subsequent binding of leukocytes. Growth factors stimulate smooth muscle cell proliferation and circulating progenitor cells are recruited to sites of arterial injury leading to neointima formation. Because of its diffuse nature, intravascular ultrasound is more sensitive than angiography for early diagnosis. Proliferation signal inhibitors (PSIs) have the capacity to slow vasculopathy progression by inhibiting smooth muscle cell proliferation, but its side effects profile makes its use as a first line agent difficult. Retransplantation is still the only definitive therapy but is available only in selected cases. The current hope is that immunomodulation at the time of transplant could induce long-term tolerance and graft accommodation, leading to less vasculopathy.

Selective Depletion of Alloreactive T Cells Leads to Long-Term Islet Allograft Survival across a Major Histocompatibility Complex Mismatch in Diabetic Mice

Islet cell transplantation as a therapy for type 1 diabetes has been limited by progressive graft loss. Significant immunosuppression including T-cell ablation has been used in an attempt to limit islet rejection. Here, we show that CD3+ lymphocytes depleted of alloreactive T cells selected from a mixed lymphocyte reaction (MLR), where responder BALB/c splenocytes stained with carboxyfluorescein succinimidyl ester (CFSE) were stimulated with irradiated C57BL/6 splenocytes for 5 days, infused into diabetic immunodeficient mice are capable of restoring a broad T-cell repertoire and specifically do not reject islet transplants from the strain (C57BL/6) used in the original depletion. These mice demonstrate reconstitution with CD4+ and CD8+ T cells, the capacity to reject third-party grafts (CBA), and restoration of interferon-γ (IFN-γ) responses to third-party alloantigens. Over time, both forkhead box P3-positive (Foxp3+) T regulatory cells (Tregs) and γδ T cells expand, suggesting a role for peripheral tolerance, in addition to the initial depletion of alloreactive T cells, in long-term islet graft survival. Our results suggest that immune restoration with CD3+ lymphocytes where alloreactive T cells are removed can restore cognate immunity without islet allograft loss and recurrence of diabetes.

New perspectives in transplantation through intravital microscopy imaging

PURPOSE OF REVIEW

Rejection of transplanted organs is a complex and highly dynamic immune process. Two-photon laser-scanning intravital microscopy (LSIM) allows for real-time, deep tissue, high-resolution imaging in physiological conditions. The recent application of this technology to study organ rejection started to provide a clearer picture of the spatiotemporal immunological dynamics of organ rejection.

RECENT FINDINGS

To date, LSIM has been applied to transplanted skin, islet, and kidney in mice, as well to constantly moving organs such as transplanted lung and heart. To characterize the dynamics of innate and adaptive immune cell infiltration, time-lapse imaging of various fluorescent-reporter mice was performed. Overall, these studies revealed differences between the anatomical location of infiltrating neutrophils and monocytes in various transplanted organs. In addition, the dynamics of lymphocytic infiltration revealed different transendothelial migration routes in vascularized versus nonvascularized transplanted tissues.

SUMMARY

LSIM is a very powerful tool that can be used to carefully dissect the immune cells dynamics in rejection and in tolerance induction in transplantation. Many dynamic biological processes can only be investigated using LSIM. Thus, LSIM promises to greatly enhance our knowledge in transplantation immunobiology and will help tailoring specific therapeutics in relation to the transplanted tissue.

Regulation of allograft survival by inhibitory FcγRIIb signaling

Fcγ receptors (FcγR) provide important immunoregulation. Targeting inhibitory FcγRIIb may therefore prolong allograft survival, but its role in transplantation has not been addressed. FcγRIIb signaling was examined in murine models of acute or chronic cardiac allograft rejection by transplanting recipients that either lacked FcγRIIb expression (FcγRIIb(-/-)) or overexpressed FcγRIIb on B cells (B cell transgenic [BTG]). Acute heart allograft rejection occurred at the same tempo in FcγRIIb(-/-) C57BL/6 (B6) recipients as wild type recipients, with similar IgG alloantibody responses. In contrast, chronic rejection of MHC class II-mismatched bm12 cardiac allografts was accelerated in FcγRIIb(-/-) mice, with development of more severe transplant arteriopathy and markedly augmented effector autoantibody production. Autoantibody production was inhibited and rejection was delayed in BTG recipients. Similarly, whereas MHC class I-mismatched B6.K(d) hearts survived indefinitely and remained disease free in B6 mice, much stronger alloantibody responses and progressive graft arteriopathy developed in FcγRIIb(-/-) recipients. Notably, FcγRIIb-mediated inhibition of B6.K(d) heart graft rejection was abrogated by increasing T cell help through transfer of additional H2.K(d)-specific CD4 T cells. Thus, inhibitory FcγRIIb signaling regulates chronic but not acute rejection, most likely because the supra-optimal helper CD4 T cell response in acute rejection overcomes FcγRIIb-mediated inhibition of the effector B cell population. Immunomodulation of FcγRIIb in clinical transplantation may hold potential for inhibiting progression of transplant arteriopathy and prolonging transplant survival.

Endothelial dysfunction and cardiac allograft vasculopathy

Cardiac allograft vasculopathy remains a major challenge to long-term survival after heart transplantation. Endothelial injury and dysfunction, as a result of multifactorial immunologic and nonimmunologic insults in the donor and the recipient, are prevalent early after transplant and may be precursors to overt cardiac allograft vasculopathy. Current strategies for managing cardiac allograft vasculopathy, however, rely on the identification and treatment of established disease. Improved understanding of mechanisms leading to endothelial dysfunction in heart transplant recipients may provide the foundation for the development of sensitive screening techniques and preventive therapies.

Cellular immunotolerance in the transplant

In humans, a state of operational tolerance has been observed in some recipients who anecdotally or experimentally abandoned their immunosuppressive treatment. Besides, advances in the understanding of the immune response and the continuous appearance of new biological molecules have boosted the growing interest in transferring the knowledge concerning immune tolerance from experimental models to clinical transplantation. Most of the strategies for inducing tolerance target the T-lymphocytes, especially T CD4(+) since they play a central role in the regulation of the immune response. However, an effective tolerogenic treatment must also take into account the role of alloantibody producing B-lymphocytes, which have been shown to play a fundamental role in chronic rejection phenomena. There are multiple regulation and silencing mechanisms that operate both during lymphocyte ontogeny in the bone marrow and thymus (central tolerance) and in the periphery (peripheral tolerance). These regulatory mechanisms include the destruction of APCs by cytotoxic lymphocytes, suppressive cytokines, and activation-induced cell death, among others. However, the mechanism that in recent years has come to be attributed the greatest role has been the active suppression of the response by T-lymphocytes themselves. These lymphocytes are named as regulatory T cells that include Tregs CD4(+)CD25(+), Tr1 cells and Th3. The great therapeutic potential of regulatory lymphocyte populations for the control of allogeneic rejection is evident and several clinical trials in humans have been started to be implemented using populations of both Tregs and Tr1 cells for the prevention of allogeneic reactions.

Immune recognition and rejection of allogeneic skin grafts

The transplantation of allogeneic skin grafts is associated with a potent inflammatory immune response leading to the destruction of donor cells and the rejection of the graft. Shortly after transplantation, skin dendritic cells (DCs) migrate out of the graft through lymphatic vessels and infiltrate the recipient's draining lymph nodes where they present donor antigens via two mechanisms: the direct pathway, in which T cells recognize intact donor MHC antigens on donor DCs; and the indirect pathway, involving T-cell recognition of donor peptides bound to self-MHC molecules on recipient DCs. Some recent studies have suggested that T cells can become activated via recognition of donor MHC molecules transferred on recipient antigen-presenting cells (semidirect pathway). Activation of T cells via direct or indirect allorecognition is sufficient to trigger acute rejection of allogeneic skin grafts. In addition, allospecific antibodies contribute to the rejection process either by killing allogeneic targets in a complement-dependent fashion or by opsonizing donor cells and forming immune complexes. Finally, several studies demonstrate that NK cells, activated due to missing self-MHC class I molecules on allogeneic cells, are involved in allogeneic skin graft rejection via direct killing of donor cells and through the production of proinflammatory cytokines including IFN-γ and TNF-α.

Inhibition of arterial allograft intimal hyperplasia using recipient dendritic cells pretreated with B7 antisense peptide

Background. Low expression or absence of dendritic cell (DC) surface B7 molecules can induce immune tolerance or hyporesponse. Whether DCs could induce indirect allogeneic-specific cross-tolerance or hyporesponse to recipient T cells remains unclear. Methods. Generated from C3H/He mice bone marrow cells pulsed with donor antigen from C57BL/6 mice, recipient DCs were incubated with B7 antisense peptide (B7AP). Immune regulatory activities were examined in vitro by a series of mixed lymphocyte reactions. Murine allogeneic carotid artery orthotopic transplantation was performed from C57BL/6 to C3H/He. Recipients were given B7AP-treated DCs 7 days before transplantation. Allograft pathological analysis was done 2 months after transplantation. Results. B7AP-pretreated DCs markedly inhibited T-cell proliferation compared with untreated group. Pretreated T cells exhibited markedly reduced response to alloantigen versus third-party antigen. Pathological analysis of arterial allografts demonstrated significant reduction of intimal hyperplasia in B7-AP pretreated group versus control. Conclusion. Blockade of B7 molecules by B7AP could induce indirect allogeneic-specific hyporesponse and inhibit arterial allograft intimal hyperplasia, which may be involved in future strategies for human allograft chronic rejection.

Brain natriuretic peptide induces CD8+ T cell death via a caspase 3 associated pathway--implications following heart transplantation

BACKGROUND

Brain natriuretic peptide (BNP) remains elevated after cardiac transplantation despite replacement of the failing ventricle. Serum peaks are also seen during acute rejection episodes independent of intracardiac hemodynamic disturbance. High BNP levels are also reported during bacterial sepsis, burns, stroke and myocardial infarction. Given all of these conditions are linked by immune activation processes, we hypothesised that BNP is an immunoactive agent.

METHODS

Peripheral blood mononuclear cells (PBMCs) were isolated from whole blood of 40 cardiac transplant recipients. Cells were co-cultured for 72h in the presence or absence of BNP. Cells were then immunophenotyped using flow cytometry. Cell death pathways were determined using caspase 3 quantification and mitochondrial membrane assessment. Supernatants were analysed for cytokine, chemokine and growth factor production using luminex.

RESULTS

Co-culture of CD8+ T cells with BNP reduced cell number, and increased intracellular caspase 3. Supernatant analysis revealed that BNP reduced the expression of inflammatory cytokines including TNF-α, IL-1α and IL-6. However it preserved the production of anti-inflammatory and regulatory cytokines such as IL-4, 5 and 13.

CONCLUSION

Our findings provide evidence that BNP directly induces CD8+ T cell apoptosis via a caspase 3 associated mechanism from cardiac transplant patients. This may impart significant consequences on immune mediated disease processes, such as allograft rejection.