Donor major histocompatibility complex (MHC) peptides are presented by recipient MHC molecules during graft rejection

Activation and Regulation of Indirect Alloresponses in Transplanted Patients With Donor Specific Antibodies and Chronic Rejection

Following transplantation, human CD4+T cells can respond to alloantigen using three distinct pathways. Direct and semi-direct responses are considered potent, but brief, so contribute mostly to acute rejection. Indirect responses are persistent and prolonged, involve B cells as critical antigen presenting cells, and are an absolute requirement for development of donor specific antibody, so more often mediate chronic rejection. Novel in vitro techniques have furthered our understanding by mimicking in vivo germinal centre processes, including B cell antigen presentation to CD4+ T cells and effector cytokine responses following challenge with donor specific peptides. In this review we outline recent data detailing the contribution of CD4+ T follicular helper cells and antigen presenting B cells to donor specific antibody formation and antibody mediated rejection. Furthermore, multi-parametric flow cytometry analyses have revealed specific endogenous regulatory T and B subsets each capable of suppressing distinct aspects of the indirect response, including CD4+ T cell cytokine production, B cell maturation into plasmablasts and antibody production, and germinal centre maturation. These data underpin novel opportunities to control these aberrant processes either by targeting molecules critical to indirect alloresponses or potentiating suppression via exogenous regulatory cell therapy.

T-cell receptor sequencing reveals selected donor-reactive CD8+ T cell clones resist antithymocyte globulin depletion after kidney transplantation

High frequencies of donor-reactive memory T cells in the periphery of transplant candidates prior to transplantation are linked to the development of posttransplant acute rejection episodes and reduced allograft function. Rabbit antithymocyte globulin (rATG) effectively depletes naïve CD4+ and CD8+ T cells for >6 months posttransplant, but rATG's effects on human donor-reactive T cells have not been carefully determined. To address this, we performed T cell receptor β-chain sequencing on peripheral blood mononuclear cells aliquots collected pretransplant and serially posttransplant in 7 kidney transplant recipients who received rATG as induction therapy. We tracked the evolution of the donor-reactive CD4+ and CD8+ T cell repertoires and identified stimulated pretransplant, CTV-(surface dye)-labeled, peripheral blood mononuclear cells from each patient with donor cells or third-party cells. Our analyses showed that while rATG depleted CD4+ T cells in all tested subjects, a subset of donor-reactive CD8+ T cells that were present at high frequencies pretransplant, consistent with expanded memory cells, resisted rATG depletion, underwent posttransplant expansion and were functional. Together, our data support the conclusion that a subset of human memory CD8+ T cells specifically reactive to donor antigens expand in vivo despite induction therapy with rATG and thus have the potential to mediate allograft damage.

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.

Activation and regulation of alloreactive T cell immunity in solid organ transplantation

Transplantation is the only curative treatment for patients with kidney failure but it poses unique immunological challenges that must be overcome to prevent allograft rejection and ensure long-term graft survival. Alloreactive T cells are important contributors to graft rejection, and a clearer understanding of the mechanisms by which these cells recognize donor antigens - through direct, indirect or semi-direct pathways - will facilitate their therapeutic targeting. Post-T cell priming rejection responses can also be modified by targeting pathways that regulate T cell trafficking, survival cytokines or innate immune activation. Moreover, the quantity and quality of donor-reactive memory T cells crucially shape alloimmune responses. Of note, many fundamental concepts in transplant immunology have been derived from models of infection. However, the programmed differentiation of allograft-specific T cell responses is probably distinct from that of pathogen-elicited responses, owing to the dearth of pathogen-derived innate immune activation in the transplantation setting. Understanding the fundamental (and potentially unique) immunological pathways that lead to allograft rejection is therefore a prerequisite for the rational development of therapeutics that promote transplantation tolerance.

Role of Exosomes in Islet Transplantation

Exosomes are known for their ability to transport nucleic acid, lipid, and protein molecules, which allows for communication between cells and tissues. The cargo of the exosomes can have a variety of effects on a wide range of targets to mediate biological function. Pancreatic islet transplantation is a minimally invasive cell replacement therapy to prevent or reverse diabetes mellitus and is currently performed in patients with uncontrolled type 1 diabetes or chronic pancreatitis. Exosomes have become a focus in the field of islet transplantation for the study of diagnostic markers of islet cell viability and function. A growing list of miRNAs identified from exosomes collected during the process of isolating islets can be used as diagnostic biomarkers of islet stress and damage, leading to a better understanding of critical steps of the isolation procedure that can be improved to increase islet yield and quality. Exosomes have also been implicated as a possible contributor to islet graft rejection following transplantation, as they carry donor major histocompatibility complex molecules, which are then processed by recipient antigen-presenting cells and sensed by the recipient immune cells. Exosomes may find their way into the therapeutic realm of islet transplantation, as exosomes isolated from mesenchymal stem cells have shown promising results in early studies that have seen increased viability and functionality of isolated and grafted islets in vitro as well as in vivo. With the study of exosomes still in its infancy, continued research on the role of exosomes in islet transplantation will be paramount to understanding beta cell regeneration and improving long-term graft function.

N-Octanoyl-Dopamine inhibits cytokine production in activated T-cells and diminishes MHC-class-II expression as well as adhesion molecules in IFNγ-stimulated endothelial cells

IFNγ enhances allograft immunogenicity and facilitates T-cell mediated rejection. This may cause interstitial fibrosis and tubular atrophy (IFTA), contributing to chronic allograft loss. We assessed if inhibition of T-cell activation by N-octanoyl dopamine (NOD) impairs adherence of activated T-cells to endothelial cells and the ability of activated T-cells to produce IFNγ. We also assessed if NOD affects IFNγ mediated gene expression in endothelial cells. The presence of NOD during T-cell activation significantly blunted their adhesion to unstimulated and cytokine stimulated HUVEC. Supernatants of these T-cells displayed significantly lower concentrations of TNFα and IFNγ and were less capable to facilitate T-cell adhesion. In the presence of NOD VLA-4 (CD49d/CD29) and LFA-1 (CD11a/CD18) expression on T-cells was reduced. NOD treatment of IFNγ stimulated HUVEC reduced the expression of MHC class II transactivator (CIITA), of MHC class II and its associated invariant chain CD74. Since IFTA is associated with T-cell mediated rejection and IFNγ to a large extent regulates immunogenicity of allografts, our current data suggest a potential clinical use of NOD in the treatment of transplant recipients. Further in vivo studies are warranted to confirm these in vitro findings and to assess the benefit of NOD on IFTA in clinically relevant models.

PIRCHE-II: an algorithm to predict indirectly recognizable HLA epitopes in solid organ transplantation

Human leukocyte antigen (HLA) mismatches between donors and recipients may lead to alloreactivity after solid organ transplantation. Over the last few decades, our knowledge of the complexity of the HLA system has dramatically increased, as numerous new HLA alleles have been identified. As a result, the likelihood of alloreactive responses towards HLA mismatches after solid organ transplantation cannot easily be assessed. Algorithms are promising solutions to estimate the risk for alloreactivity after solid organ transplantation. In this review, we show that the recently developed PIRCHE-II (Predicted Indirectly ReCognizable HLA Epitopes) algorithm can be used to minimize alloreactivity towards HLA mismatches. Together with the use of other algorithms and simulation approaches, the PIRCHE-II algorithm aims for a better estimated alloreactive risk for individual patients and eventually an improved graft survival after solid organ transplantation.

Characterization of the new HLA-DQB1*06:344 allele by next-generation sequencing in a Korean individual

DQB1*06:344 differs from DQB1*06:02:01:01 by one nucleotide substitution at codon 211 in exon 3.

Regulation of human T cell responses by dNP2-ctCTLA-4 inhibits human skin and microvessel graft rejection

Manipulation of human T cell functioning by delivery of macromolecules such as DNA, RNA, or protein is limited, unless the human T cells have been stimulated or electropermeabilized. To achieve successful adaptation and survival of a grafted organ, the alloreactive T cells that induce graft rejection must be regulated. Corticosteroids, calcineurin inhibitors, and mTOR inhibitors, which are systemic immunosuppressants, are currently used for transplantation, with significant side effects. In this study, we demonstrated that a cell-permeable peptide (CPP), dNP2, could efficiently deliver proteins into human CD4 and CD8 T cells. We confirmed regulatory functioning of the cytoplasmic domain of CTLA-4 conjugated with dNP2 (dNP2-ctCTLA-4) in human T cell activation, proliferation, and chemokine receptor expression. We utilized a human skin allograft system in SCID/beige mice to examine whether dNP2-ctCTLA-4 could inhibit allograft rejection by controlling T cell responses. The grafted skin tissue inflammation, allogeneic T cell infiltration, and blood cytokine level was markedly reduced by dNP2-ctCTLA-4, resulting in successful transplantation. In addition, it also inhibited T cell alloresponses against microvessels formed form Bcl-2-transduced human umbilical vein endothelial cells implanted into Balb/c Rag1^-/-/IL-2Rγ^-/- double knockout (DKO) mice, assessed as reduced T cell infiltration and granzyme B expression. These results collectively suggest that dNP2 peptide conjugation offers a valuable tool for delivering macromolecules like proteins into human T cells, and dNP2-ctCTLA-4 is a novel agent that shows potential in controlling human T cell responses to allow successful adaptation of grafted tissues.

Matching donor and recipient based on predicted indirectly recognizable human leucocyte antigen epitopes

The predicted indirectly recognizable human leucocyte antigen (HLA) epitopes (PIRCHE) algorithm is a novel in silico algorithm to determine donor-recipient compatibility. The PIRCHE algorithm determines donor-recipient compatibility by counting the number of mismatched HLA-derived epitopes that are involved in indirect T-cell alloimmune responses; these epitopes are designated as PIRCHE. Over the last few years, the PIRCHE algorithm has been investigated in both hematopoietic stem cell transplantation and solid organ transplantation. This review describes the theory of the algorithm, its application in transplantation, and highlights the future perspectives on the clinical application of the PIRCHE algorithm.

New Insights into Graft-Versus-Host Disease and Graft Rejection

Allogeneic transplantation of foreign organs or tissues has lifesaving potential, but can lead to serious complications. After solid organ transplantation, immune-mediated rejection mandates the use of prolonged global immunosuppression and limits the life span of transplanted allografts. After bone marrow transplantation, donor-derived immune cells can trigger life-threatening graft-versus-host disease. T cells are central mediators of alloimmune complications and the target of most existing therapeutic interventions. We review recent progress in identifying multiple cell types in addition to T cells and new molecular pathways that regulate pathogenic alloreactivity. Key discoveries include the cellular subsets that function as potential sources of alloantigens, the cross talk of innate lymphoid cells with damaged epithelia and with the recipient microbiome, the impact of the alarmin interleukin-33 on alloreactivity, and the role of Notch ligands expressed by fibroblastic stromal cells in alloimmunity. While refining our understanding of transplantation immunobiology, these findings identify new therapeutic targets and new areas of investigation.

The role of MHC genes in contagious cancer: the story of Tasmanian devils

The Tasmanian devil, a marsupial species endemic to the island of Tasmania, harbours two contagious cancers, Devil Facial Tumour 1 (DFT1) and Devil Facial Tumour 2 (DFT2). These cancers pass between individuals in the population via the direct transfer of tumour cells, resulting in the growth of large tumours around the face and neck of affected animals. While these cancers are rare, a contagious cancer also exists in dogs and five contagious cancers circulate in bivalves. The ability of tumour cells to emerge and transmit in mammals is surprising as these cells are an allograft and should be rejected due to incompatibility between Major Histocompatibility Complex (MHC) genes. As such, considerable research has focused on understanding how DFT1 cells evade the host immune system with particular reference to MHC molecules. This review evaluates the role that MHC class I expression and genotype plays in allowing DFT1 to circumvent histocompatibility barriers in Tasmanian devils. We also examine recent research that suggests that Tasmanian devils can mount an immune response to DFT1 and may form the basis of a protective vaccine against the tumour.

Immunobiology of Cellular Transplantation

The goal of cellular transplantation is to allow long-term function of the grafted cells using minimal host immunosuppression. To this end, the major strategies to implant cells and tissues are through: (i) the pretreatment of the graft to reduce tissue immunogenicity; (ii) the application of immunoisolation technologies to prevent host sensitization to implanted cells; and (iii) the induction of immunological tolerance to the donor tissues. Further, a major dilemma facing clinical tissue grafting is the shortage of donor tissue for transplantation. This problem requires the consideration of tissues from other species (xenografts) as a potential source of donor material. In light of these issues, the focus of this discussion is on the T cell-dependent response to allogeneic and xenogeneic transplants and the implications of this reactivity on the field of cellular replacement therapy.

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

B lymphocytes contribute to acute and chronic allograft rejection through their production of donor-specific antibodies (DSAs). In addition, B cells present allopeptides bound to self-MHC class II molecules and provide costimulation signals to T cells, which are essential to their activation and differentiation into memory T cells. On the other hand, both in laboratory rodents and patients, the concept of effector T cell regulation by B cells is gaining traction in the field of transplantation. Specifically, clinical trials using anti-CD20 monoclonal antibodies to deplete B cells and reverse DSA had a deleterious effect on rates of acute cellular rejection; a peculiar finding that calls into question a central paradigm in transplantation. Additional work in humans has characterized IL-10-producing B cells (IgM memory and transitional B cells), which suppress the proliferation and inflammatory cytokine productions of effector T cells in vitro. Understanding the mechanisms of regulating the alloresponse is critical if we are to achieve operational tolerance across transplantation. This review will focus on recent evidence in murine and human transplantation with respect to non-traditional roles for B cells in determining clinical outcomes.

Autologous and Allogenous Antibodies in Lung and Islet Cell Transplantation

The field of organ transplantation has undoubtedly made great strides in recent years. Despite the advances in donor-recipient histocompatibility testing, improvement in transplantation procedures, and development of aggressive immunosuppressive regimens, graft-directed immune responses still pose a major problem to the long-term success of organ transplantation. Elicitation of immune responses detected as antibodies to mismatched donor antigens (alloantibodies) and tissue-restricted self-antigens (autoantibodies) are two major risk factors for the development of graft rejection that ultimately lead to graft failure. In this review, we describe current understanding on genesis and pathogenesis of antibodies in two important clinical scenarios: lung transplantation and transplantation of islet of Langerhans. It is evident that when compared to any other clinical solid organ or cellular transplant, lung and islet transplants are more susceptible to rejection by combination of allo- and autoimmune responses.

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

Transplantation of allogeneic organs and tissues represents a lifesaving procedure for a variety of patients affected with end-stage diseases. Although current immunosuppressive therapy prevents early acute rejection, it is associated with nephrotoxicity and increased risks for infection and neoplasia. This stresses the need for selective immune-based therapies relying on manipulation of lymphocyte recognition of donor antigens. The passenger leukocyte theory states that allograft rejection is initiated by recipient T cells recognizing donor major histocompatibility complex (MHC) molecules displayed on graft leukocytes migrating to the host's lymphoid organs. We revisited this concept in mice transplanted with allogeneic skin, heart, or islet grafts using imaging flow cytometry. We observed no donor cells in the lymph nodes and spleen of skin-grafted mice, but we found high numbers of recipient cells displaying allogeneic MHC molecules (cross-dressed) acquired from donor microvesicles (exosomes). After heart or islet transplantation, we observed few donor leukocytes (100 per million) but large numbers of recipient cells cross-dressed with donor MHC (>90,000 per million). Last, we showed that purified allogeneic exosomes induced proinflammatory alloimmune responses by T cells in vitro and in vivo. Collectively, these results suggest that recipient antigen-presenting cells cross-dressed with donor MHC rather than passenger leukocytes trigger T cell responses after allotransplantation.

Sensitivity of Dendritic Cells to Microenvironment Signals

Dendritic cells are antigen-presenting cells capable of either activating the immune response or inducing and maintaining immune tolerance. They do this by integrating stimuli from the environment and changing their functional status as a result of plasticity. The modifications suffered by these cells have consequences in the way the organism may respond. In the present work two opposing situations known to affect dendritic cells are analyzed: tumor growth, leading to a microenvironment that favors the induction of a tolerogenic profile, and organ transplantation, which leads to a proinflammatory profile. Lessons learned from these situations may help to understand the mechanisms of modulation resulting not only from the above circumstances, but also from other pathologies.

Extracellular matrix-based biomaterial scaffolds and the host response

Extracellular matrix (ECM) collectively represents a class of naturally derived proteinaceous biomaterials purified from harvested organs and tissues with increasing scientific focus and utility in tissue engineering and repair. This interest stems predominantly from the largely unproven concept that processed ECM biomaterials as natural tissue-derived matrices better integrate with host tissue than purely synthetic biomaterials. Nearly every tissue type has been decellularized and processed for re-use as tissue-derived ECM protein implants and scaffolds. To date, however, little consensus exists for defining ECM compositions or sources that best constitute decellularized biomaterials that might better heal, integrate with host tissues and avoid the foreign body response (FBR). Metrics used to assess ECM performance in biomaterial implants are arbitrary and contextually specific by convention. Few comparisons for in vivo host responses to ECM implants from different sources are published. This review discusses current ECM-derived biomaterials characterization methods including relationships between ECM material compositions from different sources, properties and host tissue response as implants. Relevant preclinical in vivo models are compared along with their associated advantages and limitations, and the current state of various metrics used to define material integration and biocompatibility are discussed. Commonly applied applications of these ECM-derived biomaterials as stand-alone implanted matrices and devices are compared with respect to host tissue responses.

Exosomes: The missing link between microchimerism and acquired tolerance?

It has become increasingly clear that the immune system of viviparous mammals is much more in the business of acquiring tolerance to non-self antigens, than it is in rejecting cells that express them (for a recent review, highlighting the role of Treg cells, see ref. (1) ). It is also clear that both self-tolerance, and acquired tolerance to non-self is a dynamic process, with a natural ebb and flow. As has been often said of an effective team defense in sports, tolerance will "bend but does not break." How microchimerism, defined as the presence of extremely rare [1/10(4)-1/10(6)] cells of a genetically different individual, can induce either new immunogenetic pressures that push self-tolerance to the breaking point, or alternatively, provide relief from pre-existing immunogenetic risk, preventing development of autoimmune disease, remains a mystery. Indeed, the inability to directly correlate DNA-level microchimerism detected in blood samples by qPCR, with naturally occurring regulation to minor H and MHC alloantigens expressed by the rare cells themselves, has been frustrating to researchers in this field. (2) [Haynes, W.J. et al, this issue] However, recent developments in the areas of transplantation and reproductive immunology offer clues to how the effects of microchimerism can be amplified, and how a disproportionate immune impact might occur from a very limited cell source.

Thrombospondin-1 as a Regulator of Corneal Inflammation and Lymphangiogenesis: Effects on Dry Eye Disease and Corneal Graft Immunology

Thrombospondin-1 (TSP-1) is a matricellular glycoprotein that belongs to a family of evolutionary highly conserved calcium-binding proteins consisting of 5 members (TSP-1-TSP-5). In the eye, TSP-1 is expressed by several ocular cell types and is also detectable in the aqueous humor and the vitreous body. So far, TSP-1 is one of the major activators of TGFβ, suggesting a strong influence on various important cellular functions and interactions such as differentiation, migration, and wound healing. TSP-1 is also a key endogenous inhibitor of hem- and lymphangiogenesis. Several lines of evidence indicate a crucial role of TSP-1 in maintaining the ocular immune and angiogenic privilege, for example, by regulating T lymphocytes and the tolerance-promoting properties of ocular antigen-presenting cells. This review discusses the role of TSP-1 in dry eye disease and corneal graft rejection through its effects on hem- and lymphangiogenesis, as well as on the underlying immune responses. Recent work will be reviewed showing by which molecular mechanism TSP-1 modulates inflammatory processes during ocular diseases. This opens potential new treatment avenues in inflammatory and (lymph)angiogenic ocular diseases.

Immunology of naturally transmissible tumours

Naturally transmissible tumours can emerge when a tumour cell gains the ability to pass as an infectious allograft between individuals. The ability of these tumours to colonize a new host and to cross histocompatibility barriers contradicts our understanding of the vertebrate immune response to allografts. Two naturally occurring contagious cancers are currently active in the animal kingdom, canine transmissible venereal tumour (CTVT), which spreads among dogs, and devil facial tumour disease (DFTD), among Tasmanian devils. CTVT are generally not fatal as a tumour-specific host immune response controls or clears the tumours after transmission and a period of growth. In contrast, the growth of DFTD tumours is not controlled by the Tasmanian devil's immune system and the disease causes close to 100% mortality, severely impacting the devil population. To avoid the immune response of the host both DFTD and CTVT use a variety of immune escape strategies that have similarities to many single organism tumours, including MHC loss and the expression of immunosuppressive cytokines. However, both tumours appear to have a complex interaction with the immune system of their respective host, which has evolved over the relatively long life of these tumours. The Tasmanian devil is struggling to survive with the burden of this disease and it is only with an understanding of how DFTD passes between individuals that a vaccine might be developed. Further, an understanding of how these tumours achieve natural transmissibility should provide insights into general mechanisms of immune escape that emerge during tumour evolution.

Both rejection and tolerance of allografts can occur in the absence of secondary lymphoid tissues

In this study, we showed that aly/aly mice, which are devoid of lymph nodes and Peyer's patches, acutely rejected fully allogeneic skin and heart grafts. They mounted potent inflammatory direct alloresponses but failed to develop indirect alloreactivity after transplantation. Remarkably, skin allografts also were rejected acutely by splenectomized aly/aly (aly/aly-spl(-)) mice devoid of all secondary lymphoid organs. In these recipients, the rejection was mediated by alloreactive CD8(+) T cells presumably primed in the bone marrow. In contrast, cardiac transplants were not rejected by aly/aly-spl(-) mice. Actually, aly/aly-spl(-) mice that spontaneously accepted a heart allotransplant and displayed donor-specific tolerance also accepted skin grafts from the same, but not a third-party, donor via a mechanism involving CD4(+) regulatory T cells producing IL-10 cytokine. Therefore, direct priming of alloreactive T cells, as well as rejection and regulatory tolerance of allogeneic transplants, can occur in recipient mice lacking secondary lymphoid organs.

A polymeric conjugate foreignizing tumor cells for targeted immunotherapy in vivo

Antigen-specific CD8(+) cytotoxic T lymphocytes (CTLs) are key elements of immunological rejection in transplantation as well as cancer immunotherapy. Most tumors, however, are not immunologically rejected because they have self antigens, which are not recognized as the foreigner by CTLs. In this study, we hypothesized that "foreignizing" tumor cells by delivering non-self foreign antigens into the tumors would result in rejection by foreign antigen-reactive CTLs. As the model system to foreignize the tumors, we prepared a polymeric conjugate consisting of hyaluronic acid as the CD44(+) tumor-targeting ligand and ovalbumin (OVA) as a foreign antigen. When the conjugate was treated with CD44(high) TC-1 tumor cells, it was effectively taken up and allowed for displaying of antigenic OVA257-264 peptide at MHC class I on the surface of the cells. In addition, the conjugate was effectively accumulated into tumor tissue after its systemic administration to mice which are immunized with a vaccine for a vaccinia virus expressing OVA to generate OVA257-264 specific CTLs, resulting in substantial inhibition of tumor growth. Overall, these results suggest that the polymeric conjugates bearing foreign antigens may be innovative and promising cancer immunotherapeutic agents by foreignizing tumor cells, leading to immunological rejection.

Primary vascularization of allografts governs their immunogenicity and susceptibility to tolerogenesis

We investigated the influence of allograft primary vascularization on alloimmunity, rejection, and tolerance in mice. First, we showed that fully allogeneic primarily vascularized and conventional skin transplants were rejected at the same pace. Remarkably, however, short-term treatment of mice with anti-CD40L Abs achieved long-term survival of vascularized skin and cardiac transplants but not conventional skin grafts. Nonvascularized skin transplants triggered vigorous direct and indirect proinflammatory type 1 T cell responses (IL-2 and IFN-γ), whereas primarily vascularized skin allografts failed to trigger a significant indirect alloresponse. A similar lack of indirect alloreactivity was also observed after placement of different vascularized organ transplants, including hearts and kidneys, whereas hearts placed under the skin (nonvascularized) triggered potent indirect alloresponses. Altogether, these results suggest that primary vascularization of allografts is associated with a lack of indirect T cell alloreactivity. Finally, we show that long-term survival of vascularized skin allografts induced by anti-CD40L Abs was associated with a combined lack of indirect alloresponse and a shift of the direct alloresponse toward a type 2 cytokine (IL-4, IL-10)-secretion pattern but no activation/expansion of Foxp3(+) regulatory T cells. Therefore, primary vascularization of allografts governs their immunogenicity and tolerogenicity.

Natural killer cells in rejection and tolerance of solid organ allografts

PURPOSE OF REVIEW

A series of recent studies defy conventional wisdom by showing that natural killer (NK) cells exert a powerful and long-lasting influence on the immune response to whole organ allografts. The early activation of NK cells following transplantation is associated with killing of allogeneic target cells and release of immunomodulatory chemokines and cytokines, which can contribute to either rejection or tolerance. Here, we review findings describing NK cell receptors, potential mediators and mechanisms underlying the dual influence of NK cells in solid organ transplantation.

RECENT FINDINGS

New studies show that NK cells can discriminate between self and foreign tissues and play a key role in the initiation and regulation of adaptive immune responses after solid organ transplantation. Depending upon the types of NK cell receptors engaged and the nature of cytokines released, early NK cell activation can promote either rejection or tolerance.

SUMMARY

Solid organ transplantation is associated with the early activation of NK cells, which are then licensed to kill allogeneic target cells directly or via antibody-dependent cellular cytotoxicity and release various chemokines and immunomodulatory cytokines. Depending upon the nature of NK cell subsets activated and their ability to kill allogeneic target cells and release certain types of cytokines, NK cells can promote the activation/expansion of pro-inflammatory Th1 cells or regulatory Th2/Treg cells thus tilting the balance of alloimmunity towards rejection or tolerance. An in-depth understanding of these mechanisms will be necessary in order to design therapies targeting NK cells in human transplantation.

Mixed chimerism and split tolerance: mechanisms and clinical correlations

Establishing hematopoietic mixed chimerism can lead to donor-specific tolerance to transplanted organs and may eliminate the need for long-term immunosuppressive therapy, while also preventing chronic rejection. In this review, we discuss central and peripheral mechanisms of chimerism induced tolerance. However, even in the long-lasting presence of a donor organ or donor hematopoietic cells, some allogeneic tissues from the same donor can be rejected; a phenomenon known as split tolerance. With the current goal of creating mixed chimeras using clinically feasible amounts of donor bone marrow and with minimal conditioning, split tolerance may become more prevalent and its mechanisms need to be explored. Some predisposing factors that may increase the likelihood of split tolerance are immunogenicity of the graft, certain donor-recipient combinations, prior sensitization, location and type of graft and minimal conditioning chimerism induction protocols. Additionally, split tolerance may occur due to a differential susceptibility of various types of tissues to rejection. The mechanisms involved in a tissue's differential susceptibility to rejection include the presence of polymorphic tissue-specific antigens and variable sensitivity to indirect pathway effector mechanisms. Finally, we review the clinical attempts at allograft tolerance through the induction of chimerism; studies that are revealing the complex relationship between chimerism and tolerance. This relationship often displays split tolerance, and further research into its mechanisms is warranted.

Modeling rejection immunity

Background

Transplantation is often the only way to treat a number of diseases leading to organ failure. To overcome rejection towards the transplanted organ (graft), immunosuppression therapies are used, which have considerable side-effects and expose patients to opportunistic infections. The development of a model to complement the physician’s experience in specifying therapeutic regimens is therefore desirable. The present work proposes an Ordinary Differential Equations model accounting for immune cell proliferation in response to the sudden entry of graft antigens, through different activation mechanisms. The model considers the effect of a single immunosuppressive medication (e.g. cyclosporine), subject to first-order linear kinetics and acting by modifying, in a saturable concentration-dependent fashion, the proliferation coefficient. The latter has been determined experimentally. All other model parameter values have been set so as to reproduce reported state variable time-courses, and to maintain consistency with one another and with the experimentally derived proliferation coefficient.

Results

The proposed model substantially simplifies the chain of events potentially leading to organ rejection. It is however able to simulate quantitatively the time course of graft-related antigen and competent immunoreactive cell populations, showing the long-term alternative outcomes of rejection, tolerance or tolerance at a reduced functional tissue mass. In particular, the model shows that it may be difficult to attain tolerance at full tissue mass with acceptably low doses of a single immunosuppressant, in accord with clinical experience.

Conclusions

The introduced model is mathematically consistent with known physiology and can reproduce variations in immune status and allograft survival after transplantation. The model can be adapted to represent different therapeutic schemes and may offer useful indications for the optimization of therapy protocols in the transplanted patient.

Innate immunity and resistance to tolerogenesis in allotransplantation

The development of immunosuppressive drugs to control adaptive immune responses has led to the success of transplantation as a therapy for end-stage organ failure. However, these agents are largely ineffective in suppressing components of the innate immune system. This distinction has gained in clinical significance as mounting evidence now indicates that innate immune responses play important roles in the acute and chronic rejection of whole organ allografts. For instance, whereas clinical interest in natural killer (NK) cells was once largely confined to the field of bone marrow transplantation, recent findings suggest that these cells can also participate in the acute rejection of cardiac allografts and prevent tolerance induction. Stimulation of Toll-like receptors (TLRs), another important component of innate immunity, by endogenous ligands released in response to ischemia/reperfusion is now known to cause an inflammatory milieu favorable to graft rejection and abrogation of tolerance. Emerging data suggest that activation of complement is linked to acute rejection and interferes with tolerance. In summary, the conventional wisdom that the innate immune system is of little importance in whole organ transplantation is no longer tenable. The addition of strategies that target TLRs, NK cells, complement, and other components of the innate immune system will be necessary to eventually achieve long-term tolerance to human allograft recipients.

Bidirectional alloreactivity: A proposed microchimerism-based solution to the NIMA paradox

The NIMA paradox is the observation that in transplants of allogeneic kidneys or hematopoietic stem cells, siblings benefit from re-exposure to non-inherited maternal antigens (NIMA), whereas re-exposure to a transplant from mother herself, theoretically the ideal "NIMA" donor, does not yield clinical results superior to a father-donated allograft. Recent observations of bidirectional alloreactivity in kidney and cord blood transplantation offer a possible solution to this paradox. If correct, the proposed solution points the way to clinical applications of microchimerism in solid organ and hematopoetic transplants.

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.

Sercarzian immunology--In memoriam. Eli E. Sercarz, 1934-2009

During his long career as a principal investigator and educator, Eli Sercarz trained over 100 scientists. He is best known for developing hen egg white lysozyme (HEL) as a model antigen for immunologic studies. Working in his model system Eli furthered our understanding of antigen processing and immunologic tolerance. His work established important concepts of how the immune system recognizes antigenic determinants processed from whole protein antigens; specifically he developed the concepts of immunodominance and crypticity. Later in his career he focused more on autoimmunity using a variety of established animal models to develop theories on how T cells can circumvent tolerance induction and how an autoreactive immune response can evolve over time. His theory of "determinant spreading" is one of the cornerstones of our modern understanding of autoimmunity. This review covers Eli's entire scientific career outlining his many seminal discoveries.

Contributions of direct and indirect alloresponses to chronic rejection of kidney allografts in nonhuman primates

The relative contribution of direct and indirect allorecognition pathways to chronic rejection of allogeneic organ transplants in primates remains unclear. In this study, we evaluated T and B cell alloresponses in cynomolgus monkeys that had received combined kidney/bone marrow allografts and myeloablative immunosuppressive treatments. We measured donor-specific direct and indirect T cell responses and alloantibody production in monkeys (n = 5) that did not reject their transplant acutely but developed chronic humoral rejection (CHR) and in tolerant recipients (n = 4) that never displayed signs of CHR. All CHR recipients exhibited high levels of anti-donor Abs and mounted potent direct T cell alloresponses in vitro. Such direct alloreactivity could be detected for more than 1 y after transplantation. In contrast, only two of five monkeys with CHR had a detectable indirect alloresponse. No indirect alloresponse by T cells and no alloantibody responses were found in any of the tolerant monkeys. Only one of four tolerant monkeys displayed a direct T cell alloresponse. These observations indicate that direct T cell alloresponses can be sustained for prolonged periods posttransplantation and result in alloantibody production and chronic rejection of kidney transplants, even in the absence of detectable indirect alloreactivity.

Two amino acid changes located in the alpha 1 domain specify the novel HLA-B*27:67 allele affecting the peptide-binding-site characteristics

The novel HLA-B*27:67 contains three nucleotide substitutions in exon 2 leading to two amino acid changes.

Loss of tolerance to self after transplant

Organ transplantation is the widely accepted treatment for end-stage organ failure. Since the first successful kidney transplant from an identical twin donor in 1954, researchers have been studying the effects of the immune system on transplantation outcomes. Although the surgery is technically successful, the majority of grafts from genetically disparate donors are rejected due to a number of factors that stimulate recipient immune responses, ultimately resulting in graft loss despite the chronic use of immunosuppressive (IS) drugs. Unfortunately, while short-term success has greatly improved with the development of novel IS drugs, the long-term graft survival of solid organs has not improved significantly over the last few decades. The problem of late graft loss is mainly attributed to development of chronic rejection. Therefore, understanding all of the immune mechanisms involved in transplant rejection is important to prevent graft dysfunction, and eventually, graft loss. In this review, we will give an overview of allograft rejection, the progression from acute to chronic rejection, and in addition, the recent discovery of a critical role for loss of self-tolerance and development of IL-17-dependent autoimmunity in chronic rejection.

Phenotype, distribution and alloreactive properties of memory T cells from cynomolgus monkeys

The high frequency of memory T cells present in primates is thought to represent a major barrier to tolerance induction in transplantation. Therefore, it is crucial to characterize these memory T cells and determine their functional properties. High numbers of memory T cells were detected in peripheral blood and all lymphoid tissues except lymph nodes, which were essentially the site of naïve T cells. The majority of CD8(+) memory T cells were effector memory cells located in the blood and bone marrow while most CD4(+) memory T cells were central memory cells present in the spleen. Next, memory T cells from over 100 monkeys were tested for their response to alloantigens by ELISPOT. Memory alloreactivity mediated via direct but not indirect allorecognition was detected in all animals. The frequency of allospecific memory T cells varied dramatically depending upon the nature of the responder/stimulator monkey combination tested. MHC gene matching was generally associated with a low-memory alloreactivity. Nevertheless, low anamnestic alloresponses were also found in a significant number of fully MHC-mismatched monkey combinations. These results show that selected donor/recipient combinations displaying a low memory alloresponsiveness can be found. These combinations may be more favorable for transplant tolerance induction.

Roles of Host Nonhematopoietic Cells in Autoimmunity and Donor Cell Engraftment in Graft-versus-host Disease

Background

Graft-versus-host disease (GVHD) is initiated when alloreactive donor T cells are primed by host APCs to undergo clonal expansion and maturation. Since there is a controversy regarding the role of nonhematopoietic cells in GVHD, we wanted to investigate the influence of MHC disparity on nonhematopoietic cells on the pathogenesis of GVHD in the MHC-haplomismatched C57BL/6 (H-2^b) or DBA/2 (H-2^d)→unirradiated (C57BL/6×DBA/2) F₁(BDF₁; H-2^b/d) murine model of acute GVHD (aGVHD) or chronic GVHD (cGVHD).

Methods

We generated (BDF₁→C57BL/6), (BDF₁→DBA/2), and (BDF₁→BDF₁) chimeras and examined GVHD-related parameters and donor cell engraftment in those chimeras.

Results

Using this experimental system, we found that 1) severe aGVHD across MHC Ag barrier depends on the expression of nonhematopoietically rather than hematopoietically derived alloAgs for maximal GVHD manifestations; 2) host APCs were sufficient to break B cell tolerance to self molecules in cGVHD, whereas host APCs were insufficient to induce autoimmunity in aGVHD; 3) donor cell engraftment was greatly enhanced in the host with MHC-matched nonhematopoietic cells.

Conclusion

Taken together, our results provide an insight into how MHC disparity on GVHD target organs contribute to the pathogenesis of GVHD.

The novel allele HLA-DQB1*0636 of Caucasian origin has a unique amino acid exchange at position 186 of the beta 2 region

The novel HLA-DQB1*0636 allele differs from HLA-DQB1*060401 in one nucleotide substitution at codon 186 in exon 3.

Dual effects of the alloresponse by Th1 and Th2 cells on acute and chronic rejection of allotransplants

The contribution of direct and indirect alloresponses by CD4(+) Th1 and Th2 cells in acute and chronic rejection of allogeneic transplants remains unclear. In the present study, we addressed this question using a transplant model in a single MHC class I-disparate donor-recipient mouse combination. BALB/c-dm2 (dm2) mutant mice do not express MHC class I L(d) molecules and reject acutely L(d+) skin grafts from BALB/c mice. In contrast, BALB/c hearts placed in dm2 mice are permanently accepted in the absence of chronic allograft vasculopathy. In this model, CD4(+) T cells are activated following recognition of a donor MHC class I determinant, L(d) 61-80, presented by MHC Class II A(d) molecules on donor and recipient APC. Pre-transplantation of recipients with L(d) 61-80 peptide emulsified in complete Freund's adjuvant induced a Th1 response, which accelerated the rejection of skin allografts, but it had no effect on cardiac transplants. In contrast, induction of a Th2 response to the same peptide abrogated the CD8(+) cytotoxic T cells response and markedly delayed the rejection of skin allografts while it induced de novo chronic rejection of heart transplants. This shows that Th2 cells activated via indirect allorecognition can exert dual effects on acute and chronic rejection of allogeneic transplants.

NK cell patrolling and elimination of donor-derived dendritic cells favor indirect alloreactivity

Direct presentation of foreign MHC molecules expressed by donor-derived dendritic cells (DCs) has generally been considered the dominant pathway of allorecognition in acute transplant rejection. However, recent studies implicate preferential activation of the indirect pathway by host DCs. The respective importance of each pathway and the mechanisms that determine their relative contributions remain to be clearly established. In this study, using two-photon microscopy, we visualized host NK cell interactions with syngeneic and allogeneic DCs within intact lymph nodes of mice. Upon contact with allogeneic DCs, NK cells formed prolonged interactions that led directly to target cell lysis. This rapid elimination limited the ability of allogeneic DCs to stimulate primary and recall T cell responses. To discriminate whether donor or host DCs are principally involved in presenting Ag derived from allografts, we used CD11c-diphtheria toxoid receptor mice to conditionally ablate CD11c(+) DCs and to show that direct presentation by donor DCs is alone insufficient to elicit acute allograft rejection. We thus propose that rapid elimination of allogeneic DCs limits direct Ag presentation and thereby favors the indirect pathway of alloreactivity.

Mechanism of cellular rejection in transplantation

The explosion of new discoveries in the field of immunology has provided new insights into mechanisms that promote an immune response directed against a transplanted organ. Central to the allograft response are T lymphocytes. This review summarizes the current literature on allorecognition, costimulation, memory T cells, T cell migration, and their role in both acute and chronic graft destruction. An in depth understanding of the cellular mechanisms that result in both acute and chronic allograft rejection will provide new strategies and targeted therapeutics capable of inducing long-lasting, allograft-specific tolerance.

Indirect CD4+ TH1 response, antidonor antibodies and diffuse C4d graft deposits in long-term recipients conditioned by donor antigens priming

Priming of recipients by DST induces long-term survival of mismatched allografts in adult rats. Despite these recipients developing inducible T regulatory cells able to transfer long-term graft survival to a secondary host, a state of chronic rejection is also observed. We revisited the molecular donor MHC targets of the cellular response in acute rejection and analyzed the cellular and humoral responses in recipients with long-term graft survival following transplantation. We found three immunodominant peptides, all derived from LEW.1W RT1.D(u) molecules to be involved in acute rejection of grafts from unmodified LEW.1A recipients. Although the direct pathway of allorecognition was reduced in DST-treated recipients, the early CD4+ indirect pathway response to dominant peptides was almost unimpaired. We also detected early and sustained antidonor class I and II antibody subtypes with diffuse C4d deposits on graft vessels. Finally, long-term accepted grafts displayed leukocyte infiltration, endarteritis and fibrosis, which evolved toward vascular narrowing at day 100. Altogether, these data suggest that the chronic graft lesions developed in long-term graft recipients are the result of progressive humoral injury associated with a persisting indirect T helper response. These features may represent a useful model for understanding and manipulating chronic active antibody-mediated rejection in human.

Differential roles of direct and indirect allorecognition pathways in the rejection of skin and corneal transplants

BACKGROUND

It is generally accepted that all transplants are not rejected in the same fashion. However, the extrinsic and intrinsic factors that control the recognition and rejection of a particular allograft by the host are not well characterized.

METHODS

We compared the mechanisms underlying the response with donor antigens by T cells activated after transplantation of fully allogeneic skin and corneal grafts in mice.

RESULTS

In corneal-transplanted mice, the CD4+ T-cell response was exclusively mediated by T cells recognizing minor antigens in an indirect fashion and producing low levels of interleukin-2. In contrast, skin grafts elicited both direct and indirect CD4+ T-cell responses primarily directed to major histocompatibility complex antigens and characterized by high interleukin-2 levels. Although CD8+ T-cells producing gamma interferon were activated directly in both skin- and corneal-grafted mice, only CD8+ T cells from skin-transplanted mice mounted a cytotoxic response. Next, we investigated whether failure of corneal transplants to induce a CD4+ direct alloresponse is due to their poor immunogenicity or due to the site of placement (eye). We observed that corneas transplanted under the skin and splenocytes transplanted in the eye were both capable of inducing direct CD4+ T-cell alloreactivity.

CONCLUSIONS

This shows that failure of orthotopic corneal allotransplants to elicit a CD4+ T-cell direct alloresponse is associated with the combination of two factors, their low immunogenicity and the immune-privileged properties of the eye.

The indirect alloresponse impairs the induction but not maintenance of tolerance to MHC class I-disparate allografts

We studied the effects of indirect allorecognition on the induction and maintenance phases of tolerance in miniature swine cotransplanted with heart and kidney allografts. MHC class I-mismatched heart and kidney grafts were cotransplanted in recipients receiving CyA for 12 days. Recipients were unimmunized or immunized with a set of donor-derived or control third-party MHC class I peptides either 21 days prior to transplantation or over 100 days after transplantation. T-cell proliferation, delayed type hypersensitivity reaction (DTH) and antibody production were assessed. All animals injected with donor MHC class I peptides developed potent indirect alloresponses specific to the immunizing peptides. While untreated recipients developed stable tolerance, all animals preimmunized with donor allopeptides rejected kidney-heart transplants acutely. In contrast, when peptide immunization was delayed until over 100 days after kidney-heart transplantation, no effects were observed on graft function or in vitro measures of alloimmunity. Donor peptide immunization prevented tolerance when administered to recipients pre transplantation but did not abrogate tolerance when administered to long-term survivors post transplantation. This suggests that the presence of T cells activated via indirect allorecognition represent a barrier to the induction but not the maintenance of tolerance.

Differential susceptibility of allogeneic targets to indirect CD4 immunity generates split tolerance

CD4 T cells frequently help to activate CD8 T and B cells that effect transplant rejection. However, CD4 T cells alone can reject transplants, either directly or indirectly. The relative effectiveness of indirect CD4 immunity in rejecting different types of allogeneic grafts is unknown. To address this, we used a TCR transgenic mouse model in which indirect CD4 alloimmunity alone can be studied. We challenged transgenic recipients with hematopoietic cells and shortly thereafter skin transplants that could only be rejected indirectly, and observed Ag-specific indirect donor B cell and skin rejection, but not T cell elimination, reflecting a state of split tolerance. Deficiency of indirect CD4 alloimmunity in donor T cell rejection was also apparent when acute indirect rejection of donor islets occurred despite generation and maintenance of mixed T cell chimerism, due to migration of the few passenger T cells into recipient circulation. Although passenger lymphocytes delayed indirect islet rejection, they enhanced rejection by a full repertoire capable of both direct and indirect reactivity. Interestingly, the persistence of chimerism was associated with the eventual development of tolerance, as demonstrated by acceptance of donor skin grafts given late to hematopoietic cell recipients, and hyporesponsiveness of transgenic T cells from islet recipients in vitro. Mechanistically, tolerance was recessive and associated with progressive down-regulation of CD4. Collectively, our data indicate that indirect CD4 immunity is not equally destructive toward different types of allogeneic grafts, the deficiency of which generates split tolerance. The futility of these responses can convert immunity into tolerance.

Interleukin-23 secretion by donor antigen-presenting cells is critical for organ-specific pathology in graft-versus-host disease

Damage to the gastrointestinal tract during graft-versus-host disease (GVHD) from the conditioning regimen in conjunction with alloreactive donor T cells plays a pivotal role in the pathogenesis of this disease. In this study, we have identified secretion of interleukin-23 (IL-23) by donor antigen-presenting cells (APCs) as a critical event in the induction of GVHD of the colon linking conditioning regimen-induced mucosal injury and lipopolysaccharide (LPS) translocation to subsequent proinflammatory cytokine production and GVHD-associated pathologic damage. In the absence of donor APC-derived IL-23 secretion, there is a selective and profound reduction in pathologic damage as well as a marked reduction in LPS and proinflammatory cytokine production in the colon microenvironment. The downstream proinflammatory effects of IL-23 are dependent upon donor-derived secretion of interferon-gamma (IFN-gamma), but are independent of donor IL-17 production. These findings define a novel organ-specific role for IL-23 in the pathophysiology of GVHD and demonstrate that IL-23 can direct tissue-specific pathology within the context of a systemic inflammatory disorder. Furthermore, these studies also identify IL-23 as a potential therapeutic target for the prevention of this life-threatening disorder.

Monitoring of indirect allorecognition: wishful thinking or solid data?

Monitoring of T cells involved in the alloimmune response after transplantation requires the availability of reliable in vitro assays for the detection of T cells with both direct and indirect allospecificity. While generally accepted assays exist to measure helper and cytotoxic T cells involved in direct allorecognition, consensus about an assay for monitoring indirect T-cell allorecognition in clinical transplantation is lacking. Many studies claim a relationship between the reactivity of T cells with indirect allospecificity and graft rejection, but different protocols are used and essential controls are often lacking. In this review, the disadvantages and pitfalls of the current approaches are discussed, in some cases supported by the results of our own in vitro experiments. We conclude that an international workshop is necessary to establish and validate a uniform, robust and reliable assay for the monitoring of transplant recipients and to study the actual role of indirect allorecognition in acute and chronic rejection.

Autoantibodies to vimentin cause accelerated rejection of cardiac allografts

Autoimmune responses to vimentin occur after solid organ transplantation, but their pathogenic effects are unclear. The aim of these studies was to investigate the effects of vimentin preimmunization on allogeneic and isografted hearts in a murine transplant model. Immunization of C57BL/6 mice with murine vimentin in complete Freund's adjuvant resulted in anti-vimentin antibodies and vimentin-reactive Th-1 cells. Transplantation of 129/sv hearts into vimentin-immunized C57BL/6 recipients resulted in accelerated rejection (8.4 +/- 1.5 days; n = 18), compared with hen egg lysozyme-immunized C57BL/6 (13.3 +/- 2.2 days; n = 10; P < 0.0001, log-rank test). In contrast, isografts continued to beat beyond 90 days. Immunohistochemical analysis of allografts from vimentin/complete Freund's adjuvant mice demonstrated increased numbers of T cells and enhanced microvascular deposition of C3d, CD41, and P-selectin compared with controls. Antibodies were necessary for accelerated rejection, shown by the fact that vimentin-immunized B-cell-deficient IgH6 mice did not show accelerated rejection of 129/sv allografts, but rejection was restored by adoptive transfer of serum containing anti-vimentin antibodies. Eluates from donor hearts placed in vimentin/complete Freund's adjuvant recipients contained anti-vimentin antibodies, shown by Western blotting. Confocal imaging of rejected hearts de-monstrated presence of vimentin and C3d on apoptosed leukocytes, endothelial cells, and platelet/leukocyte conjugates. These results demonstrate that autoantibodies to vimentin, in conjunction with the alloimmune response, have a pathogenic role in allograft rejection.