Processed MHC class I alloantigen as the stimulus for CD4+ T-cell dependent antibody-mediated graft rejection

Transcriptional Changes in Kidney Allografts with Histology of Antibody-Mediated Rejection without Anti-HLA Donor-Specific Antibodies

BACKGROUND

Circulating donor-specific anti-HLA antibodies (HLA-DSAs) are often absent in serum of kidney allograft recipients whose biopsy specimens demonstrate histology of antibody-mediated rejection (ABMR). It is unclear whether cases involving ABMR histology without detectable HLA-DSAs represent a distinct clinical and molecular phenotype.

METHODS

In this multicenter cohort study, we integrated allograft microarray analysis with extensive clinical and histologic phenotyping from 224 kidney transplant recipients between 2011 and 2017. We used the term ABMR histology for biopsy specimens that fulfill the first two Banff 2017 criteria for ABMR, irrespective of HLA-DSA status.

RESULTS

Of 224 biopsy specimens, 56 had ABMR histology; 26 of these (46.4%) lacked detectable serum HLA-DSAs. Biopsy specimens with ABMR histology showed overexpression of transcripts mostly related to IFNγ-induced pathways and activation of natural killer cells and endothelial cells. HLA-DSA-positive and HLA-DSA-negative biopsy specimens with ABMR histology displayed similar upregulation of pathways and enrichment of infiltrating leukocytes. Transcriptional heterogeneity observed in biopsy specimens with ABMR histology was not associated with HLA-DSA status but was caused by concomitant T cell-mediated rejection. Compared with cases lacking ABMR histology, those with ABMR histology and HLA-DSA had higher allograft failure risk (hazard ratio [HR], 7.24; 95% confidence interval [95% CI], 3.04 to 17.20) than cases without HLA-DSA (HR, 2.33; 95% CI, 0.85 to 6.33), despite the absence of transcriptional differences.

CONCLUSIONS

ABMR histology corresponds to a robust intragraft transcriptional signature, irrespective of HLA-DSA status. Outcome after ABMR histology is not solely determined by the histomolecular presentation but is predicted by the underlying etiologic factor. It is important to consider this heterogeneity in further research and in treatment decisions for patients with ABMR histology.

T Follicular Regulatory Cells and Antibody Responses in Transplantation

De novo donor-specific antibody (DSA) formation is a major problem in transplantation, and associated with long-term graft decline and loss as well as sensitization, limiting future transplant options. Forming high-affinity, long-lived antibody responses involves a process called the germinal center (GC) reaction, and requires interaction between several cell types, including GC B cells, T follicular helper (Tfh) and T follicular regulatory (Tfr) cells. T follicular regulatory cells are an essential component of the GC reaction, limiting its size and reducing nonspecific or self-reactive responses.An imbalance between helper function and regulatory function can lead to excessive antibody production. High proportions of Tfh cells have been associated with DSA formation in transplantation; therefore, Tfr cells are likely to play an important role in limiting DSA production. Understanding the signals that govern Tfr cell development and the balance between helper and regulatory function within the GC is key to understanding how these cells might be manipulated to reduce the risk of DSA development.This review discusses the development and function of Tfr cells and their relevance to transplantation. In particular how current and future immunosuppressive strategies might allow us to skew the ratio between Tfr and Tfh cells to increase or decrease the risk of de novo DSA formation.

Antibody Subclass Repertoire and Graft Outcome Following Solid Organ Transplantation

Long-term outcomes in solid organ transplantation are constrained by the development of donor-specific alloantibodies (DSA) against human leukocyte antigen (HLA) and other targets, which elicit antibody-mediated rejection (ABMR). However, antibody-mediated graft injury represents a broad continuum, from extensive complement activation and tissue damage compromising the function of the transplanted organ, to histological manifestations of endothelial cell injury and mononuclear cell infiltration but without concurrent allograft dysfunction. In addition, while transplant recipients with DSA as a whole fare worse than those without, a substantial minority of patients with DSA do not experience poorer graft outcome. Taken together, these observations suggest that not all DSA are equally pathogenic. Antibody effector functions are controlled by a number of factors, including antibody concentration, antigen availability, and antibody isotype/subclass. Antibody isotype is specified by many integrated signals, including the antigen itself as well as from antigen-presenting cells or helper T cells. To date, a number of studies have described the repertoire of IgG subclasses directed against HLA in pretransplant patients and evaluated the clinical impact of different DSA IgG subclasses on allograft outcome. This review will summarize what is known about the repertoire of antibodies to HLA and non-HLA targets in transplantation, focusing on the distribution of IgG subclasses, as well as the general biology, etiology, and mechanisms of injury of different humoral factors.

Germinal center alloantibody responses are mediated exclusively by indirect-pathway CD4 T follicular helper cells

The durable alloantibody responses that develop in organ transplant patients indicate long-lived plasma cell output from T-dependent germinal centers (GCs), but which of the two pathways of CD4 T cell allorecognition is responsible for generating allospecific T follicular helper cells remains unclear. This was addressed by reconstituting T cell-deficient mice with monoclonal populations of TCR-transgenic CD4 T cells that recognized alloantigen only as conformationally intact protein (direct pathway) or only as self-restricted allopeptide (indirect pathway) and then assessing the alloantibody response to a heart graft. Recipients reconstituted with indirect-pathway CD4 T cells developed long-lasting IgG alloantibody responses, with splenic GCs and allospecific bone marrow plasma cells readily detectable 50 d after heart transplantation. Differentiation of the transferred CD4 T cells into T follicular helper cells was confirmed by follicular localization and by acquisition of signature phenotype. In contrast, IgG alloantibody was not detectable in recipient mice reconstituted with direct-pathway CD4 T cells. Neither prolongation of the response by preventing NK cell killing of donor dendritic cells nor prior immunization to develop CD4 T cell memory altered the inability of the direct pathway to provide allospecific B cell help. CD4 T cell help for GC alloantibody responses is provided exclusively via the indirect-allorecognition pathway.

Circulating alloreactive T cells correlate with graft function in longstanding renal transplant recipients

Monitoring for alloreactive memory T cells after organ transplantation may allow individualization of immunosuppression. Two pathways of T cell allorecognition have been implicated in chronic graft dysfunction: Direct (recipient T cells respond to donor peptides presented by donor antigen-presenting cells) and indirect (donor peptides are processed and presented by recipient antigen-presenting cells). Previous studies have assessed these alloresponses only during the first 2 yr after kidney transplantation,so this study correlated the presence of circulating donor-reactive memory/effector T cells, primed by both pathways, in 34 longstanding living-donor renal transplant recipients using the highly sensitive IFN-gamma Elispot assay. Remarkably, 59% of patients had directly primed donor-reactive T cells, and their presence correlated directly with serum creatinine (P = 0.001) and inversely with estimated GFR (P = 0.042). Multivariate analysis revealed that hyporesponsiveness of direct, donor-specific T cells was the only variable that significantly correlated with graft function and that antidonor indirect alloreactivity was the only variable that significantly correlated with proteinuria. Interestingly, when both allorecognition pathways were considered together, patients with undetectable direct alloreactivity had better longterm graft function, independent of allosensitization by the indirect pathway. In conclusion, circulating donor-specific alloreactive T cells primed by both pathways are detectable long after transplantation and are associated with graft injury. Assessment of alloreactive memory/effector T cells might be helpful to tailor individual immunosuppression regimens for transplant recipients in the future.

Abrogation of antibody-mediated allograft rejection by regulatory CD4 T cells with indirect allospecificity

Alloantibody is an important effector mechanism for allograft rejection. In this study, we tested the hypothesis that regulatory T cells with indirect allospecificity can prevent humoral rejection by using a rat transplant model in which acute rejection of MHC class I-disparate PVG.R8 heart grafts by PVG.RT1(u) recipients is mediated by alloantibody and is dependent upon help from CD4 T cells that can recognize the disparate MHC alloantigen only via the indirect pathway. Pretransplant treatment of PVG.RT1(u) recipients with anti-CD4 mAb plus donor-specific transfusion abrogated alloantibody production and prolonged PVG.R8 graft survival indefinitely. Naive syngeneic splenocytes injected into tolerant animals did not effect heart graft rejection, suggesting the presence of regulatory mechanisms. Adoptive transfer experiments into CD4 T cell-reconstituted, congenitally athymic recipients confirmed that regulation was mediated by CD4 T cells and was alloantigen-specific. CD4 T cell regulation could be broken in tolerant animals either by immunizing with an immunodominant linear allopeptide or by depleting tolerant CD4 T cells, but surprisingly this resulted in neither alloantibody generation nor graft rejection. These findings demonstrate that anti-CD4 plus donor-specific transfusion treatment results in the development of CD4 regulatory T cells that recognize alloantigens via the indirect pathway and act in an Ag-specific manner to prevent alloantibody-mediated rejection. Their development is associated with intrinsic tolerance within the alloantigen-specific B cell compartment that persists after T cell help is made available.

Smooth muscle cell proliferation but not neointimal formation is dependent on alloantibody in a murine model of intimal hyperplasia

Transplant coronary artery disease is the pre-eminent cause of late cardiac allograft failure. It is primarily characterized by a concentric intimal hyperplasia, which we designate transplant intimal hyperplasia (TIH). Although the pathogenesis of TIH is predominately immune driven, the specific role of alloantibodies in the disease process remains undefined. In this study we investigated the contribution of alloantibodies to the development of TIH in a murine model. Orthotopic, carotid artery transplantation was performed between B10A(2R) (H-2(h2)) donor mice and B-cell deficient muMT(-/-) knockout or wild-type C57BL/6 (H-2(b)) recipients in the absence of immunosuppression. Grafts were harvested at 35 days and subjected to planimetry and immunohistochemistry. Alloantibodies were detectable in wild-type recipients within 7 days of transplantation and recipients developed marked TIH at 35 days. Allografts harvested from B-cell deficient recipient mice also developed TIH, which was comparable in severity with wild-type recipients. However, whereas allografts from wild-type recipients showed marked intimal smooth muscle cell (SMC) proliferation, the neointima in B-cell deficient recipients lacked SMCs. Post-transplantation administration of anti-donor serum to muMT(-/-) recipients restored neointimal SMC population but did not influence the severity of TIH. Significant neointimal formation occurs in the absence of alloantibodies but lacks a SMC component. Therefore, SMC migration and proliferation is antibody dependent.

CD4+ T-cell-dependent immune damage of liver parenchymal cells is mediated by alloantibody

BACKGROUND

Allogeneic hepatocytes initiate both CD4- and CD8-dependent rejection responses. The current studies address the hypothesis that acute damage of allogeneic liver parenchymal cells by the CD4-dependent pathway is alloantibody-mediated and examines immune conditions which promote activation of this pathway.

METHODS

The role of alloantibody in CD4-dependent hepatocyte rejection was evaluated by assessing hepatocyte (FVB/N, H-2q) survival in CD8-depleted B-cell knockout (KO) (H-2b) recipients and by monitoring hepatocyte survival in C57BL/6.SCID (H-2b) recipients transfused with donor-reactive alloantibody. The development of donor-reactive alloantibody in C57BL/6 (H-2b), CD8-depleted C57BL/6, CD8 KO (H-2b), IFN-gamma KO (H-2b), perforin KO (H-2b), and FasL mutant gld/gld (H-2b) hepatocyte recipients was assessed.

RESULTS

Hepatocyte rejection in B-cell KO mice was significantly delayed by CD8+ T-cell depletion (median survival time [MST], 35 days) when compared to untreated (MST, 8 days) and CD4-depleted (MST, 10 days) recipient mice. Transfusion of donor-reactive alloantibody into SCID recipients with functional hepatocellular allografts was sufficient to precipitate rejection in a dose-dependent fashion. Donor-reactive alloantibody was minimal in the serum of C57BL/6 hepatocyte recipients, but was produced in significant quantities in hepatocyte recipients genetically deficient in or depleted of CD8+ T cells and in recipients with impaired cytotoxic effector mechanisms. In addition, recipients with defects in Th1 immunity, such as IFN-gamma KO recipients, also produced readily detectable alloantibody.

CONCLUSIONS

Collectively, these data support the hypothesis that acute immune damage of allogeneic hepatocytes by the CD4-dependent pathway is mediated by alloantibody and that this pathway is favored when Th1- or cell-mediated cytotoxic effector immune mechanisms are impaired.

The HLA-DR phenotype of the responder is predictive of humoral response against HLA class I antigens

Recent studies suggest that the immunogenicity of an human leukocyte antigen (HLA) incompatibility should be considered in the context of the HLA phenotype of the recipient. The HLA-DR phenotype of the responder is thought to be predictive for the strength of the alloimmune response. In order to analyze the humoral response against HLA class I antigens in the context of the HLA-DR phenotype of the responder, we selected all HLA-DR homozygous Dutch patients that were present on the Eurotransplant waiting list between 1967 and 2000 (n=1,317 patients). By logistic regression it was determined whether antibody production against a specific HLA class I antigen is associated with a particular HLA-DR antigen in the patient. Furthermore, it was analyzed whether a patient, expressing a particular HLA-DR antigen, preferentially produces antibodies against particular HLA class I antigens. The results demonstrate that patients, homozygous for a certain HLA-DR antigen, cannot be considered high or low responders when analyzing the antibody response in terms of panel reactive antibody (PRA) value. However, a correlation can be found between the HLA-DR phenotype of the patient and the specific antibody response against HLA class I antigens. For example, antibodies against HLA-A10, -A11, -A19, and -B35 are produced more frequently by HLA-DR6 positive individuals, whereas antibodies against HLA-A3, -B5, -B7, -B8, and -B12 are produced more frequently by HLA-DR4 positive individuals. These data confirm that the HLA-DR phenotype of the responder plays a determinative role in the immunogenicity of mismatched HLA antigens. The results indicate that selection of HLA class I mismatches of the donor in the context of the HLA-DR phenotype of the responder might reduce the incidence of humoral graft rejection and minimize the sensitization grade of retransplant candidates.

Non-complement- and complement-activating antibodies synergize to cause rejection of cardiac allografts

Alloantibodies (AlloAbs) are a clinically significant component of the immune response to organ transplants. In our experimental model, B10.A (H-2a) cardiac transplants survived significantly longer in C57BL/6 (H-2b) immunoglobulin knock-out (IgKO) recipients than in their wild-type (WT) counterparts. Passive transfer of a single 50-200-microg dose of complement-activating IgG2b AlloAbs to IgKO recipients reconstituted acute rejection of cardiac allografts. Although passive transfer of a subthreshold dose of 25 microg of IgG2b or a single 100-200-microg dose of non-complement-activating IgG1 AlloAbs did not restore acute rejection to IgKO recipients, a combination of these AlloAbs did cause acute graft rejection. Histologically, rejection was accompanied by augmented release of von Willebrand factor from endothelial cells. IgG1 AlloAbs did not activate complement on their own and did not augment complement activation by IgG2b AlloAbs. However, IgG1 AlloAbs stimulated cultured mouse endothelial cells to produce monocyte chemotactic protein 1 (MCP-1) and neutrophil chemoattractant growth-related oncogene alpha (KC). TNF-alpha augmented IgG1 induced secretion of MCP-1 and KC. These findings indicate that non-complement-activating AlloAbs can augment injury to allografts by complement-activating AlloAbs. Non-complement-activating AlloAbs stimulate endothelial cells to produce chemokines and this effect is augmented in the milieu of proinflammatory cytokines.

Prevention of chronic rejection with immunoregulatory cells induced by intrathymic immune modulation with class I allopeptides

Intrathymic immune modulation with RT1.Aa allopeptides in the PVG.R8-to-PVG.1 U rat strain combination leads to long-term survival of cardiac allografts. This regimen, however, does not induce transplantation tolerance, since most long-surviving allografts undergo chronic rejection. We investigated recipients with chronic rejection for donor-specific immune nonresponsiveness and immunoregulatory cells as possible mechanisms responsible for long-term graft survival. There was a significant reduction in the proliferative response of T cells from long-term allograft recipients to donor alloantigens as compared with that of naïve T cells. Adoptive transfer of splenocytes from intrathymically manipulated primary long-term graft survivors into minimally irradiated secondary hosts resulted in indefinite survival of > 80% of allografts, providing evidence for immunoregulatory cells. Secondary recipients had total absence of donor-reactive cellular and humoral responses. Immunoregulation was also transferable from secondary to tertiary graft recipients. More importantly, there was a significant reduction in the incidence of chronic rejection in secondary hosts (> 85%) and complete prevention of acute and chronic rejection in tertiary hosts. This study demonstrates that intrathymic immunomodulation with class I allopeptides results in the generation of immunoregulatory cells that do not block chronic rejection in primary hosts where they develop, but prevent both acute and chronic allograft rejection when adoptively transferred into secondary and tertiary recipients.

Helper T-cell type 1 or type 2 function of xeno-MHC-restricted T-cell clones in a direct xenoantigen recognition

There have been several reports that xeno-MHC-restricted T-cells have a cytotoxic function through a direct xenoantigen recognition, but yet no report that they have a helper function. Previously we showed that both xeno-MHC-restricted CD4(+) and CD8(+) T-cells recognized xenoantigens directly in a mouse anti-rat combination. In this study, we investigated whether or not xeno-MHC-restricted T-cells had a helper function. Mouse T-cell clones recognizing rat antigens directly were derived from T-cell lines using the limiting dilution method. Phenotype, cytotoxic activity and cytokine production of these clones were analyzed by flow cytometry, 51Cr release assay and ELISA, respectively. Rat-MHC class I-restricted mouse CD8(+) T-cell clones showed a specific cytotoxic activity against rat antigens. One CD4(+) clone produced IL-4 and IL-10, and the other CD4(+) clone produced not T-helper (Th) 2 cytokine but TNF-alpha. Our results suggested that xeno-MHC class I-restricted CD8(+) T-cells should have a cytotoxic function, and xeno-MHC class II-restricted CD4(+) T-cells should have either Th1 or Th2 function.

Chronic cardiac allograft rejection in a rat model disparate for one single class I MHC molecule is associated with indirect recognition by CD4(+) T cells

T-cell mediated immune responses play a critical role in chronic allograft dysfunction. The complex nature of allograft rejection, particularly with respect to the vast repertoire of alloantigens and their mode of recognition by T cells, presents a major challenge for the design of well-controlled studies into the immunobiology of chronic rejection. The purpose of this study was to develop a rat model with restricted antigenic specificity that develops chronic rejection without any immunologic manipulation to study the T-cell response. PVG.1U allogeneic hearts disparate for one single class I antigen, RT.1A(u), were transplanted into PVG.R8 rat recipients. Grafts from PVG.R8 were used as syngeneic controls. Chronic rejection was studied by histological analysis of the grafted hearts at various time points posttransplantation (20-100 days). Donor specific alloreactive response was studied in a mixed lymphocyte reaction assay. All allografts survived more than 90 days and showed extensive evidence of chronic rejection, which was characterized by interstitial fibrosis, vasculitis, and occlusive myointimal thickening. Chronic rejection was evident by day 20 and most extensive by day 100 posttransplantation. In marked contrast, syngeneic grafts remained free of chronic lesions. Lymphocytes harvested from graft recipients showed a more vigorous proliferative response to allogeneic splenocytes as compared with that of lymphocytes from nai;ve animals. The proliferative response was primarily mediated by CD4(+) T cells recognizing the RT1.A(a) molecule via the indirect pathway. A single class I disparity in this model generates chronic rejection associated with potent CD4(+) T-cell responses induced by the indirect recognition pathway. The use of this antigenically restricted model may facilitate the design of well-controlled studies for the characterization of immune mechanisms responsible for chronic rejection.

Concomitant allorecognition of mismatched donor HLA class I- and class II-derived peptides in pediatric lung transplant recipients with bronchiolitis obliterans syndrome

BACKGROUND

The authors' previous studies with 2 different adult patient populations demonstrated a correlation between indirect allorecognition of mismatched donor HLA Class I- and Class II-derived peptides and the development of bronchiolitis obliterans syndrome (BOS) after lung transplantation. The aim of this study was to determine whether a parallel allorecognition of mismatched donor HLA Class I- and Class II-derived peptides occurs after lung transplantation and to determine its correlation with the development of BOS after lung transplantation in a group of pediatric patients.

METHODS

Peripheral blood mononuclear cells from 7 BOS-positive and 6 BOS-negative pediatric lung transplant recipients (age, 11.5 +/- 4.4 years) were cultured in the presence of synthetic peptides corresponding to the alpha-chain hypervariable regions of a mismatched donor HLA Class I molecule and the beta-chain hypervariable region of a mismatched donor HLA-DR molecule. The frequencies of HLA Class I and Class II alloreactive T cells were determined using limiting dilution analysis.

RESULTS

A significant increase (p = 0.025) in HLA Class I-alloreactive T cells was observed in BOS-positive patients (7.1 x 10(-5) +/- 4.3 x 10(-5)) compared with BOS-negative patients (2.1 x 10(-5) +/- 1.8 x 10(-6)). In addition, a significant increase (p = 0.033) in HLA Class II-alloreactive T cells also was observed in BOS-positive patients (9.6 x 10(-5) +/- 7.9 x 10(-5)) compared with BOS-negative patients (1.3 x 10(-5) +/- 2.1 x 10(-6)).

CONCLUSIONS

This study indicates that a parallel CD4+ T-cell alloreactivity to both donor HLA Class I and Class II molecules may play a role in the pathogenesis of BOS both in adult and pediatric lung transplant recipients.

Multiple pathways to allograft rejection

Allograft rejection results from a complex process involving both the innate and acquired immune systems. The innate immune system predominates in the early phase of the allogeneic response, during which chemokines and cell adhesion play essential roles, not only for leukocyte migration into the graft but also for facilitating dendritic and T-cell trafficking between lymph nodes and the transplant. This results in a specific and acquired alloimmune response mediated by T cells. Subsequently, T cells and cells from innate immune system function synergistically to reject the allograft through nonexclusive pathways, including contact-dependent T cell cytotoxicity, granulocyte activation by either Th1 or Th2 derived cytokines, NK cell activation, alloantibody production, and complement activation. Blockade of individual pathways generally does not prevent allograft rejection, and long-term allograft survival is achieved only after simultaneous blockade of several of them. In this review, we explore each of these pathways and discuss the experimental evidence highlighting their roles in allograft rejection.

Kinetics of transplant arteriosclerosis in MHC-Class I mismatched and fully allogeneic mouse aortic allografts

INTRODUCTION

Transplant arteriosclerosis is still the major complication for long-term allograft survival in clinical transplantation. The aim of our study was to investigate the impact of MHC disparity on the kinetics of the development of transplant arteriosclerosis.

METHODS

MHC-class I mismatched CBK (H2k+Kb) or fully allogeneic C57BL/10 (H2b) aortic allografts were transplanted into CBA.CA (H2k) recipients; syngeneic grafts were used as controls. Aortic grafts were analyzed on days 7, 14, and 30 after transplantation by performing morphometry, immunohistochemistry and quantitative reverse-transcriptase polymerase chain reaction (RT-PCR) for the detection of intragraft cytokine mRNA production. Donor specific alloantibody production was measured by FACS analysis.

RESULTS

Intimal proliferation developed more rapidly in fully allogeneic grafts (direct and indirect allorecognition by CD4+ T cells) compared to MHC-class I mismatched grafts (indirect allorecognition only by CD4+ T cells) (day 7: 6+/-7 vs. 2+/-3%; day 14: 17+/-8 vs. 5+/-1%; day 30: 65+/-5 vs. 38+/-7% (C57BL/10 vs. CBK). However, by day 60, the level of intimal proliferation in the MHC-class I mismatched grafts was equivalent to that observed with fully allogeneic grafts on day 30. There was also a marked delay in the kinetics of graft infiltration by CD4+, CD8+, CD11b+, and CD40+ leukocytes and alloantibody production when CD4+ T cells were only activated via indirect presentation (MHC-class I mismatched grafts). Expression of interferon-gamma, interleukin-2, and interleukin-4 correlated with the kinetics of leukocyte infiltration, whereas interleukin-10, interleukin-12p40, iNOS, and TGF-beta1 showed a distinct pattern of expression.

CONCLUSIONS

These data demonstrate that the degree of MHC incompatibility between donor and recipient markedly influences the kinetics of the development of transplant arteriosclerosis. The onset of disease was delayed when grafts were mismatched for only MHC-class I antigens, but ultimately reached the same levels as seen in fully allogeneic grafts. The pattern of leukocyte infiltration and the kinetics of cytokine production suggest that in the MHC-class I mismatched grafts CD4+ T cells responding via the indirect pathway might play an important role in the development of transplant arteriosclerosis.

Indirect allorecognition can play an important role in the development of transplant arteriosclerosis

BACKGROUND

Indirect allorecognition has been implicated in the initiation of chronic allograft dysfunction. Our aim was to develop an animal model that allowed the contribution of the direct and indirect pathway of allorecognition in the evolution of transplant arteriosclerosis, the main feature of chronic allograft rejection, to be evaluated.

METHODS

Aortic allografts mismatched for a single MHC class I antigen were transplanted into athymic NUDE or RAG (-/-) mice. Immunodeficient mice were reconstituted with either CD4(+) (indirect) or CD8(+) (direct + indirect) T cells in the presence or absence of depleting antibodies specific for the opposite T-cell subset. Aortic grafts were analyzed by performing morphometry, immunohistochemistry, and quantitative reverse transcriptase-polymerase chain reaction for the detection of cytokine mRNA production. Donor-specific alloantibody production was measured by fluorescence-activated cell sorter analysis.

RESULTS

Reconstitution of athymic nude mice with 4 x 10(7) purified CD4(+) T cells resulted in vascular rejection of MHC class I mismatched aortic grafts. Intimal proliferation was 24+/-8% and did not decrease when nude-derived endogenous CD8(+) T cells were depleted from the nude recipients (intimal proliferation, 21+/-7%). Transplant arteriosclerosis initiated by CD4+ T cells was associated with the presence of intragraft mRNA for interferon-gamma, tumor necrosis factor-alpha, inducible nitric oxide synthase, and interleukin 12. Reconstitution of RAG-1(-/-) mice with 4 x 10(7) purified CD4(+) T cells resulted in a similar degree of transplant arteriosclerosis (intimal proliferation, 20+/-9%) in MHC class I mismatched aortic grafts in the absence of alloantibody production.

CONCLUSION

Indirect recognition of donor MHC class I molecules by CD4(+) T cells can play an important role in the process of transplant arteriosclerosis. CD8(+) T-cell effector function and alloantibody production in this model are dependent on CD4(+) T-cell help after indirect allorecognition.

Loss of direct and maintenance of indirect alloresponses in renal allograft recipients: implications for the pathogenesis of chronic allograft nephropathy

Chronic allograft nephropathy (CAN) is the principal cause of late renal allograft failure. This complex process is multifactorial in origin, and there is good evidence for immune-mediated effects. The immune contribution to this process is directed by CD4(+) T cells, which can be activated by either direct or indirect pathways of allorecognition. For the first time, these pathways have been simultaneously compared in a cohort of 22 longstanding renal allograft recipients (13 with good function and nine with CAN). CD4(+) T cells from all patients reveal donor-specific hyporesponsiveness by the direct pathway according to proliferation or the secretion of the cytokines IL-2, IL-5, and IFN-gamma. Donor-specific cytotoxic T cell responses were also attenuated. In contrast, the frequencies of indirectly alloreactive cells were maintained, patients with CAN having significantly higher frequencies of CD4(+) T cells indirectly activated by allogeneic peptides when compared with controls with good allograft function. An extensive search for alloantibodies has revealed significant titers in only a minority of patients, both with and without CAN. In summary, this study demonstrates widespread donor-specific hyporesponsiveness in directly activated CD4(+) T cells derived from longstanding recipients of renal allografts, whether they have CAN or not. However, patients with CAN have significantly higher frequencies of CD4(+) T cells activated by donor Ags in an indirect manner, a phenomenon resembling split tolerance. These findings provide an insight into the pathogenesis of CAN and also have implications for the development of a clinical tolerance assay.

IFN-gamma alters the pathology of graft rejection: protection from early necrosis

We studied the effect of host IFN-gamma on the pathology of acute rejection of vascularized mouse heart and kidney allografts. Organs from CBA donors (H-2k) were transplanted into BALB/c (H-2d) hosts with wild-type (WT) or disrupted (GKO, BALB/c mice with disrupted IFN-gamma genes) IFN-gamma genes. In WT hosts, rejecting hearts and kidneys showed mononuclear cell infiltration, intense induction of donor MHC products, but little parenchymal necrosis at day 7. Rejecting allografts in GKO recipients showed infiltrate but little or no induction of donor MHC and developed extensive necrosis despite patent large vessels. The necrosis was immunologically mediated, since it developed during rejection, was absent in isografts, and was prevented by immunosuppressing the recipient with cyclosporine or mycophenolate mofetil. Rejecting kidneys in GKO hosts showed increased mRNA for heme oxygenase 1, and decreased mRNA for NO synthase 2 and monokine inducible by IFN-gamma (MIG). The mRNA levels for CTL genes (perforin, granzyme B, and Fas ligand) were similar in rejecting kidneys in WT and GKO hosts, and the host Ab responses were similar. The administration of recombinant IFN-gamma to GKO hosts reduced but did not fully prevent the effects of IFN-gamma deficiency: MHC was induced, but the prevention of necrosis and induction of MIG were incomplete compared with WT hosts. Thus, IFN-gamma has unique effects in vascularized allografts, including induction of MHC and MIG, and protection against parenchymal necrosis, probably at the level of the microcirculation. This is probably a local action of IFN-gamma produced in large quantities in the allograft.

Passive transfer of alloantibodies restores acute cardiac rejection in IgKO mice

BACKGROUND

Alloantibody is an intrinsic component of the immune response to organ transplants. Although alloantibodies have been correlated with decreased graft survival, the mechanisms of alloantibody-mediated injury remain largely undefined in vivo. In the present study, we have established a model of alloantibody-mediated graft injury using B10.A (H-2a) hearts transplanted to wild type (WT) or immunoglobulin knock out (IgKO) C57BL-Igh-6 (H-2b) mice.

METHODS

Alloantibodies were measured in the circulation and graft by flow cytometry and in immunofluorescence staining, respectively. Intragraft cytokine mRNA expression was evaluated using a competitive template reverse transcriptase polymerase chain reaction (RT-PCR) technique. P-selectin and von Willebrand factor expression were localized by immunoperoxidase staining. The capacity of alloantibodies to restore acute cardiac allograft rejection was tested by passive transfer of monoclonal antibodies (mAbs) against donor major histocompatibility complex (MHC) class I antigens to IgKO recipients.

RESULTS

B10.A cardiac allografts are rejected acutely by WT C57BL/6 recipients, but over 50% of the cardiac allografts survived more than 50 days after transplantation in IgKO mice. Competitive template RT-PCR on the cardiac transplants demonstrated similar levels of IL-1-alpha, IL-12 (p40), TNF-alpha, IL-2, IFN-gamma, IL-4, and IL-10 mRNA in WT and IgKO recipients 8-10 days after transplantation, indicating that macrophage- and T-cell-dependent immune responses were intact in IgKO recipients. The rejection of B10.A hearts in WT recipients was characterized by interstitial and perivascular cellular infiltration; IgG, IgM, and complement (C3) deposition; vascular cell injury and intravascular platelet aggregation; and release of von Willebrand factor and P-selectin. In IgKO recipients the lower degree of vascular injury in the absence of alloantibody responses was reflected by the lack of release of von Willebrand factor and P-selectin, which remained confined to cytoplasmic storage granules of endothelial cells and platelets. Acute rejection of cardiac allografts was restored to IgKO recipients by passive transfer of proinflammatory IgG2b mAbs against donor MHC; recipients injected with isotype-matched control mAbs did not reject. In contrast, passive transfer of IgG1 mAbs against donor MHC failed to restore acute rejection of cardiac allografts to IgKO recipients. Passive transfer of IgG2b, but not IgG1 mAbs was associated with endothelial cell activation and plate. let aggregation together with the release of preformed von Willebrand factor and P-selectin from storage granules.

CONCLUSIONS

Acute rejection of cardiac allografts can be reconstituted in IgKO recipients by passive transfer of IgG2b, but not IgG1 antibody. This model allows the mechanism of alloantibody-mediate graft injury to be dissected in vivo.

Synthetic HLA-A2 derived peptides are recognized and presented in renal graft recipients

Indirect presentation of allogeneic MHC antigen is an important pathway by which allografts are rejected and tolerance maintained by regulatory CD4(+) T cells. In this study HLA-A2 derived synthetic peptides were used to determine whether T cells of non-HLA-A2 renal graft recipients, which had been HLA-A2 mismatched to their organ donors, recognize some of the HLA-A2-derived peptides. Among the HLA-A2 mismatched patients, 60% recognized residues 56--69, 65--79, and 75--89. Peripheral blood lymphocytes derived from healthy individuals showed low reactivity towards allopeptides, indicating that sensitization towards HLA-A2 induced response towards HLA-A2 derived peptides. The response to the peptides was blocked by antibodies to HLA-DR, -DQ, and CD4. Depletion of antigen presenting cells abrogated response towards the allopeptides, confirming that the observed proliferation was mediated by the indirect pathway. Interestingly, although none of the HLA-A2 mismatched patients had any signs for either acute or chronic rejection, considerable response to allo-derived HLA-A2 was observed.

Coronary arteries from human cardiac allografts with chronic rejection contain oligoclonal T cells: persistence of identical clonally expanded TCR transcripts from the early post-transplantation period (endomyocardial biopsies) to chronic rejection (coronary arteries)

Chronic cardiac allograft rejection presents pathologically as graft arteriosclerosis (GA) characterized by recipient T cell and monocyte infiltration. To determine whether oligoclonal T cells are present in coronary arteries of cardiac allografts from patients with GA, we conducted sequencing analysis of beta-chain TCR transcripts from these explanted coronary arteries using the nonpalindromic adaptor-PCR. Substantial proportions of identical beta-chain TCR transcripts in three of five patients were observed, clearly demonstrating the presence of oligoclonal T cells. TCR transcripts from the arteries of two other patients were relative heterogeneous. High proportions of identical CDR3 beta-chain TCR motifs were found in each patient. GENEBANK/EMBL/SWISS PROT database comparison of all sequences revealed that these beta-chain TCR transcripts were novel. Using Vbeta-specific PCR (independent amplification), we found in patient GA03 that the TCR transcript that was clonally expanded in the left anterior descending artery after nonpalindromic adaptor-PCR was also clonally expanded in the right coronary artery of the same allograft. These results demonstrate that this TCR transcript was clonally expanded at different anatomic sides of the cardiac allograft in a systemic manner. In two patients identical beta-chain TCR transcripts that were found to be clonally expanded in the coronary arteries of their explanted cardiac allografts were also found to be clonally explanted in endomyocardial biopsies collected 17 and 21 mo earlier from each patient. The presence of oligoclonal populations of T cells in the rejected graft suggest that these T cells have undergone specific Ag-driven proliferation and clonal expansion early on within the graft and persist throughout the post-transplantation period.

Twenty-five-year panorama of corneal immunology: emerging concepts in the immunopathogenesis of microbial keratitis, peripheral ulcerative keratitis, and corneal transplant rejection

PURPOSE

To describe the most recent advances in our understanding of the cellular and molecular mechanisms involved in the immunopathogenesis of corneal immunoinflammatory disorders including microbial keratitis, peripheral ulcerative keratitis. and allograft rejection.

METHODS

Review of the published peer-reviewed literature that has contributed significantly to our modern understanding of corneal immunology. In addition, the authors have summarized the information in conceptual diagrams that highlight the critical cellular and molecular pathways that lead to corneal immune responses in the two most thoroughly studied corneal immune disorders, herpes simplex keratitis (HSK) and transplant rejection.

RESULTS

In spite of the wide array of molecular and cellular factors that mediate corneal immunity, critical mechanistic facets are shared by the various corneal immunoinflammatory disorders. These include activation and migration of local antigen-presenting cells (APCs), including Langerhans cells (LCs), upregulation in pleiotropic proinflammatory cytokines such as interleukin-1 (IL-1) and tumor necrosis factor-alfa (TNF-alpha) that can mediate a wide array of immune functions in addition to up-regulating protease expression. and chemokines that play a critical role on the one hand in attracting nonantigen-specific inflammatory cells such as neutrophils and on the other in attracting CD4+ T helper type 1 (Th1) cells that mediate most of the destruction in the cornea.

CONCLUSIONS

In the last 25 years, we have seen our field develop from a descriptive stage into a new phase where the fundamental processes that mediate and effect corneal immunity are being accurately deciphered. It is anticipated that this new knowledge will allow development of specific molecular and genetic therapeutic strategies that could target critical steps in the immunopathogenesis of disease without the untoward side-effects of nonspecific generalized immune suppression that still remains the standard of care today.

High-risk corneal allografts are capable of stimulating complement dependent cytolytic antibodies

PURPOSE

To prospectively characterize serum antibody responses in high-risk corneal allografts.

METHODS

Orthotopic and heterotopic C3H (H-2k) corneas were transplanted to CB6F1 (H-2b/d) recipient mice. Individual serum samples were collected at days 7, 14, 21, and 28 from orthotopically and heterotopically grafted mice. Samples were tested for anti-C3H antibodies using an enzyme-linked immunosorbent assay (ELISA). Serum samples were also tested for the ability to lyse target cells in the presence of complement.

RESULTS

In high-risk donor-host combinations, 100% corneal allograft rejection occurs in the absence of IgG1 production. Both orthotopic and heterotopic corneal allografts induced cytolytic complement fixing antibody isotypes.

CONCLUSIONS

Allospecific corneal antibody may be involved in corneal graft rejection in high-risk donor-host combinations.

Effects of antibody reactivity to major histocompatibility complex (MHC) and non-MHC alloantigens on graft endothelial cells in heart allograft rejection

BACKGROUND

To gain insight in the pathogenesis of vascular lesions in heart allograft rejection, we investigated effects of allosera reactive with major histocompatibility complex (MHC) or non-MHC alloantigens on graft endothelial cells (EC) in a rat transplantation model.

METHODS

Anti-MHC and anti-non-MHC allosera were obtained from Brown Norway (RT.1(n)) recipients of a Lewis (RT.1(1)) or congenic LEW.1N (RT.1(n)) heart allograft respectively. Reactivity with endothelial alloantigens was studied in vitro using a series of three rat heart endothelial cell (RHEC) lines of Lewis origin. Phenotypic studies of MHC and non-MHC alloantigen expression, and adhesion molecule induction on EC were performed by immunostaining and fluorescence-activated cell sorting analysis. Complement-mediated cytotoxicity of allosera was studied using a 51Cr release assay.

RESULTS

Both anti-MHC allosera and anti-non-MHC allosera showed reactivity with all three RHEC lines. EC stimulation with tumor necrosis factor-alpha and interferon-y resulted in increased reactivity of anti-MHC but not of anti-non-MHC allosera. Anti-MHC allosera showed complement-mediated cytotoxicity for EC, which was strongly increased when cytokine-stimulated EC were used. With anti-non-MHC allosera, only minor cytotoxicity was measured, irrespective of the activation of EC. Anti-MHC and anti-non-MHC allosera from the day of rejection (days 7-8 and days 29-35, respectively) had similar subclass profiles of allospecific IgG, except for allospecific IgM, which was only detected in anti-MHC allosera. Complement-mediated cytotoxicity of anti-MHC allosera from the day of rejection was effected mainly by IgM alloantibodies, whereas, in allosera taken 4 days after rejection, a predominance of cytotoxic alloantibodies of the IgG class was observed. No indications were found that either alloantibody reactivity alone or in combination with complement activation led to EC activation processes relevant to intercellular adhesion molecule-1 or vascular cell adhesion molecule-1 induction.

CONCLUSIONS

Our data show that, in heart allograft rejection, MHC but also non-MHC alloantigens on EC are target structures in the alloantibody response. Alloantibodies reactive with endothelial MHC, but not those reactive with non-MHC alloantigens, may significantly contribute to vasculopathy by complement-mediated cytotoxicity. Although no evidence was found that alloantibodies reactive with graft EC induce adhesion molecule expression, they may trigger other EC mechanisms relevant to graft vasculopathy.

Chronic allograft rejection. Do the Th2 cells preferentially induced by indirect alloantigen recognition play a dominant role?

Chronic rejection has been the major obstacle to the long-term allograft survival in the clinic. Although the etiology of this rejection reaction is multifactorial, alloantigen-specific immune activation plays the most critical role. We herein hypothesize that CD4+ Th2 cells that are preferentially induced by the indirect recognition of allogeneic histocompatibility antigens late in transplantation may play the most critical role in the initiation and/or maintenance of chronic allograft rejection. Immunosuppression used to prevent acute rejection and the nature of antigen-presenting cells and alloligands in the graft may all contribute to immune deviation to the Th2 response. This response may be further perpetuated by type 2 cytokines conceivably produced by activated macrophages, NK cells, and CD8+ T cells in the graft. Cytokines and growth factors induced by this type 2 response, in turn, allow for activation of B, endothelial, and smooth muscle cells that collectively contribute to the pathogenesis of chronic allograft rejection by producing alloantibodies and growth hormones required for interstitial fibrosis, extracellular matrix deposition, and vascular neointimal hyperplasia.

The humoral immune response against an HLA class I allodeterminant correlates with the HLA-DR phenotype of the responder

BACKGROUND

The genetic basis for control of alloantibody responses against foreign HLA histocompatibility antigens has never been delineated. The most likely postulate would be that HLA class II alloantigens of the host regulate the response through their ability to present processed HLA allopeptide fragments for the cognate interaction between CD4+ T lymphocytes and B lymphocytes that leads to IgG antibody synthesis.

METHODS

We have analyzed our allosensitized transplant patient population with regard to humoral responsiveness to a serologically defined public HLA class I epitope, Bw4. Peptides representing the linear sequence of the Bw4 epitope (amino acids 74-86) and the alternative Bw6 epitope were synthesized and assayed for binding to a panel of HLA homozygous lymphoblastoid B cells using a quantitative fluorescence binding assay.

RESULTS

We found that 73% of patients who have produced a HLA-Bw4-specific alloantibody express either the HLA-DRB1*01 or HLA-DRB1*03 alloantigen; 19% of the remaining responders expressed HLA-DRB1*04. Analysis of the United Network for Organ Sharing Transplant Registry indicated that the survival of cadaver renal allografts mismatched for Bw4 was significantly compromised in sensitized DRB1*01+ or DRB1*03+ recipients (P<0.01). In vitro, the Bw4 peptide bound strongly to DRB1*01+ and DRB1*03+ lymphoblastoid B cells; no similar binding was observed with Bw6 peptide. These findings were confirmed using murine fibroblast lines transfected with HLA-DR alpha/beta genes and by solid-phase enzyme-linked immunosorbent assay using purified HLA-DR alloantigen.

CONCLUSIONS

We conclude that there are at least two human Ir genes, HLA-DRB1*01 and HLA-DRB1*03, that confer a high risk for both humoral allosensitization and renal allograft failure in situations of HLA-Bw4 incompatibility. These findings may be of future benefit in devising new antigen matching strategies for reducing the risk of humoral HLA allosensitization and chronic allograft rejection.

Mice with Th2-Biased Immune Systems Accept Orthotopic Corneal Allografts Placed in “High Risk” Eyes

CD4+ T cells of the Th1 type play a central role in acute rejection of solid tissue grafts, including orthotopic corneal allografts. Th1 cells, which mediate delayed hypersensitivity, are the polar opposites of CD4+ Th2 cells, and the latter cells cross-regulate Th1 cells through the unique pattern of cytokines they secrete. As such, Th2 cells may have a useful role to play in preventing rejection of corneal allografts. To test this possibility, the immune systems of adult mice were biased toward Th2 responses by immunization with keyhole limpet hemocyanin plus IFA. When immunized subsequently with either OVA or allogeneic corneal tissue, these mice acquired Ag-specific primed T cells of the Th2 type. More important, allogeneic corneas grafted into neovascularized eyes of Th2-biased mice experienced significantly enhanced survival. To demonstrate that enhanced survival was promoted by donor-specific Th2 cells, lymphoid cells from keyhole limpet hemocyanin-immune mice bearing healthy corneal allografts suppressed orthotopic corneal allograft rejection when adoptively transferred into naive, syngeneic recipients. We conclude that acceptance of corneal allografts in neovascularized mouse eyes can be significantly enhanced by biasing the recipient immune system toward Th2 responses.

Evidence of indirect allorecognition in long-term human renal transplantation

The purpose of this study was to investigate indirect alloreactivity in the peripheral blood of long-term renal transplanted patients. We evaluated the T cell proliferative response to a whole pool of donor cell-derived allopeptides, processed and presented by host antigen-presenting cells (APC), rather than to synthetic peptides. For the indirect pathway, proliferation assays were performed using APC-depleted donor cells. Indirect alloreactivity was detected in 57% (8/14) of the patients, 6 of whom presented no evidence of rejection, but 2 patients had a diagnosis of chronic rejection. In 4 of 8 positive cases (50%), proliferation was detected with 5 days of culture, and sometimes indirect alloresponse was the dominant route. We present evidence that the indirect alloproliferative response to a pool of naturally processed donor peptides is present in the peripheral blood of long-term renal transplanted patients irrespective of rejection.

Predominant expression of T helper 2 cytokines and altered expression of T helper 1 cytokines in long-term allograft survival induced by intrathymic immune modulation with donor class I major histocompatibility complex peptides

BACKGROUND

We have recently demonstrated that three synthetic peptides corresponding to the alpha-helices of the alpha1 and alpha2 domains of the donor class I RT1.Aa molecule served as efficient CD4+ T-cell epitopes for indirect recognition of this molecule during cardiac allograft rejection in the PVG.R8-toPVG.1U rat strain combination. These peptides induce long-term graft survival when injected into the thymus 7 days before transplantation under the cover of transient immunosuppression with anti-rat lymphocyte serum. In this study, we analyzed intragraft cytokine gene expression to test whether immune deviation to the T helper (Th) 2 response is associated with long-term allograft survival in this model.

METHODS

Intragraft cytokine gene expression was analyzed using a competitive reverse transcription polymerase chain reaction method we developed for this study. Cytokine gene expression was quantified in control allografts (n=5) with acute rejection and allografts from intrathymically manipulated recipients with acute rejection (n=5), delayed rejection (n=7), or no rejection (n=8).

RESULTS

Long-surviving allografts expressed high levels of interleukin (IL)-4, IL-10, transforming growth factor (TGF)-beta, interferon (IFN)-gamma, and undetectable levels of IL-2. Allografts that were rejected in a delayed fashion expressed mostly IL-2, IFN-gamma, and TGF-beta with low or undetectable levels of IL-4 and IL-10. Acutely rejected allografts from unmanipulated controls or peptide-manipulated recipients expressed high levels of IL-2, IFN-gamma, TGF-beta and undetectable levels of IL-4 or IL-10. All allografts also expressed T-cell receptor Cbeta gene, providing evidence for the presence of T-cell infiltrates in the grafts.

CONCLUSIONS

These observations demonstrate that acute graft rejection in this model is associated with the expression of Th1 cytokines, IL-2, and IFN-gamma, whereas long-term survival is associated with predominant expression of Th2 cytokines, IL-4, and IL-10. The expression of IFN-gamma in long-surviving allografts in the absence of IL-2 provides evidence for altered activation of the Th1 response in this intrathymic immune modulation model.

Indirect T Cell Allorecognition and Alloantibody-Mediated Rejection of MHC Class I-Disparate Heart Grafts

Recent studies in the rat have identified a role for T cell-dependent alloantibody in rejection of MHC class I-disparate allografts. RT1Aa-disparate PVG.R8 heart grafts are rejected acutely in naive, and hyperacutely in sensitized, PVG.RT1u recipients by CD4 T cell-dependent alloantibody. Here, we examined the T cell Ag recognition pathways responsible and show that direct injection into skeletal muscle of plasmid DNA, encoding a water-soluble form of the RT1Aa MHC class I heavy chain (pcmu-tAa), stimulates IgG2b cytotoxic alloantibody and markedly accelerates rejection of PVG.R8 heart grafts (median survival time 2 days). pcmu-tAa injection did not induce CTL to Aa, arguing against direct allorecognition of soluble Aa. Treatment with mAbs confirmed that the alloimmune response to pcmu-tAa injection depended on CD4, not CD8, T cells. Priming T cells for indirect allorecognition by injection of 15-mer peptides spanning the α1 and α2 domains of Aa failed to stimulate anti-Aa Ab but caused an accelerated Ab response to a PVG.R8 heart and a modest acceleration in graft rejection (median survival time 4 days). These results suggest that both soluble MHC class I and allopeptides prime CD4 T cells by the indirect pathway, but that soluble class I is a more effective immunogen for humoral alloimmunity because its tertiary protein structure provides B cell epitopes. We propose that priming humoral alloimmunity, like CTL priming, requires recognition of intact MHC on donor cells, but essential T cell help can be provided by CD4 T cells recognizing allogeneic class I exclusively by the indirect pathway.

Identification of an indirectly presented epitope in a mouse model of skin allograft rejection

BACKGROUND

The indirect pathway of allorecognition involves the processing and presentation of donor molecules by recipient antigen-presenting cells to alloreactive CD4+ T cells. Our objective was to assess the occurrence and significance of the indirect presentation of allogeneic major histocompatibility complex molecules in the rejection of major histocompatibility complex class I-disparate skin.

METHODS

A mouse model of allograft rejection was developed in which tail skin from C57.BL/10 (H2b) donors was transplanted onto B10.A(5R) recipients resulting in an allogeneic mismatch at the D locus. T-cell depletion studies were used to characterize T-cell subset involvement in rejection. B10.A(5R) mice were immunized with pools of overlapping peptides spanning the polymorphic region of Db in order to identify Db-derived epitopes involved in rejection. The relevance of these epitopes was assessed through immunization of recipient mice with peptides before skin grafting to observe the effect of presensitization on the kinetics of rejection.

RESULTS

Rejection of Db-disparate skin by B10.A(5R) was delayed by CD4 and CD8+ T-cell depletion, indicating the significance of both cell types in rejection. At least six immunogenic peptides were identified, all of which contained a cryptic T-cell epitope. One peptide, however, was able to accelerate the rejection of Db-disparate skin. Presensitization of B10.A(5R) mice with this peptide also resulted in an increase in alloantibody, indicating the presence of a physiological as well as a cryptic epitope. Presensitization of mice with a peptide containing a distinct cryptic epitope, however, failed to influence rejection.

CONCLUSIONS

These findings demonstrate a significant role for the indirect pathway of antigen presentation in allograft rejection.

Assessment of the contribution that direct allorecognition makes to the progression of chronic cardiac transplant rejection in humans

BACKGROUND

Two populations of T cells contribute to allograft rejection. T cells with direct allospecificity are activated after recognition of intact MHC alloantigens displayed at the surface of donor passenger leukocytes carried within the graft. In contrast, T cells with indirect allospecificity recognize donor alloantigens as processed peptides associated with self (recipient)-MHC class II molecules. In small animal models of transplantation, direct pathway T cells dominate the acute rejection process and are rendered tolerant to the graft after the loss of donor passenger leukocytes. It has been argued that indirect pathway T cells contribute substantially to continual graft damage after passenger cell loss. The purpose of this study was to determine whether donor-specific tolerance could be detected in T cells with direct anti-donor allospecificity in human heart transplant recipients after prolonged graft residence.

METHODS AND RESULTS

Alloreactive helper (HTLf) and cytotoxic (CTLf) T cells were enumerated by use of limiting dilution analysis. These assay systems were refined to make them specific for the direct pathway of allorecognition and more sensitive in the case of the HTLf assay. Recipient:anti-donor frequencies were generated in 10 long-term recipients of heart grafts with progressive chronic rejection and compared with those against equivalently HLA mismatched recipient:third-party controls. For HTLf, direct pathway donor-specific hyporesponsiveness was detected in 5 of the 10 recipients (HTLf<1:100,000). Of these 5 recipients, 4 also had low anti-donor CTLf (<1:100,000). In the 5th recipient, although the CTLf was >1:100,000, it was significantly lower than that estimated against the third-party control.

CONCLUSIONS

Donor-specific hyporesponsiveness is demonstrated in 50% of recipients in both the HTLf and CTLf compartments of the direct alloresponse. Direct allorecognition therefore appears unlikely to be responsible for the progression of chronic rejection, implicating indirect allorecognition as the predominant immunological driving force. Furthermore, these data have potential implications for graft outcome, adjustment of immunosuppression, and recipient monitoring.

Induction of allograft nonresponsiveness after intrathymic inoculation with donor class I allopeptides. II. Evidence for persistent chronic rejection despite high levels of donor microchimerism

We have recently demonstrated that three synthetic peptides corresponding to the donor class I RT1.Aa molecule induce long-term survival of cardiac allografts in the PVG.R8-to-PVG.1U rat strain combination (disparate for one isolated class I, RT1.A, molecule) when presented to the recipient immune system in the thymus. Long-term graft survivors had measurable levels of donor-reactive alloantibodies in their serum. In this study, we examined long-term allografts for the presence of chronic rejection and donor microchimerism to assess whether this regimen of immune modulation establishes true tolerance and whether this tolerance is dependent upon the presence of donor-recipient microchimerism. Histological examination of long-term heart grafts (>100 days) demonstrated chronic rejection, including a mild degree of myocardial infiltration by mononuclear cells, mild to moderate myocardial fibrosis, and various vascular changes ranging from focal intimal thickening to total vascular lumen blockade due to smooth muscle cell proliferation. In contrast, long-term syngeneic hearts transplanted under similar experimental conditions lacked these pathological manifestations. Donor microchimerism was analyzed using the polymerase chain reaction with a pair of oligonucleotides specific for the donor class I RT1.Aa gene and genomic DNA harvested from various tissues from graft recipients. We detected high levels of donor microchimerism in the heart, kidney, liver, skin, bone marrow, thymus, and lymph nodes of long-term graft recipients. Donor microchimerism was also detected in unmanipulated control graft recipients at rejection (7 days) and in intrathymically manipulated recipients that rejected allografts in a delayed fashion (12-82 days). These data clearly demonstrate that intrathymic inoculation of donor class I allopeptides induces long-term graft survival but does not prevent chronic rejection. Allograft rejection occurred despite high levels of donor microchimerism, providing direct evidence that donor-recipient microchimerism is not sufficient for the prevention of acute or chronic rejection in this model.

Induction of allograft nonresponsiveness after intrathymic inoculation with donor class I allopeptides. I. Correlation of graft survival with antidonor IgG antibody subclasses

We have recently demonstrated that cardiac allograft rejection in the PVG.R8-to-PVG.1U rat strain combination involves the recognition of a isolated class I (RT1.Aa) molecules as peptides in the context of the recipient MHC molecules. Three synthetic peptides (P1, P2, and P3) corresponding to the alpha-helices of the RT1.Aa molecule served as T-cell epitopes for graft rejection. In this study, we demonstrate that two of these peptides (P2 and P3) are sufficient to induce immune nonresponsiveness (median survival time >237 days) to cardiac allografts when presented to the recipient immune system in the thymus 7 days before transplantation. This effect was time dependent, as intrathymic inoculation 60 days before transplantation did not prolong graft survival (median survival time=12 days). Previous studies have demonstrated a critical role for alloantibody responses in mediating graft rejection in this rat strain combination. We, therefore, studied the role alloantibody responses may play in the observed immune nonresponsiveness. The titers of alloantibody in serum samples harvested from graft recipients at different times after transplantation were measured. We used recipient primary aortic endothelial cells genetically manipulated to express the donor RT1.Aa molecule as targets in an enzyme-linked immunosorbent assay. High titers of anti-RT1.Aa IgM antibody were detected in unmanipulated controls at the time of graft rejection. The IgM antibody switched to high IgG titers in intrathymically inoculated rats with accelerated or delayed rejection. Graft rejection in intrathymically manipulated recipients that had achieved a transient state of immunological nonresponsiveness correlated with higher titers of the IgG2b alloantibody. In marked contrast, the long-term graft survivors expressed undetectable or low levels of the IgG2b antibody and moderate to high levels of the IgG1 and IgG2a subclasses. These data suggest that the IgG2b alloantibody may contribute to the rejection reaction, whereas IgG1 and IgG2a may be involved in active enhancement of graft survival.

Inhibition of the alloantibody response by CD95 ligand

We investigated the effect of Fas/APO1-ligand (CD95L) gene transfer on allogeneic immune responses in vivo. A colon carcinoma cell line from BALB/c mice, CT26, was stably transfected with a vector encoding mouse CD95L and was inoculated into C57BL/6 mice. CD95L expression markedly reduced allogeneic cytotoxic T lymphocyte and helper T lymphocyte activity directed toward CT26. Strikingly, expression of CD95L on these allogeneic tumors completely inhibited the generation of alloantibodies of both IgM and IgG subclasses. Thus, CD95L inhibited alloantibody production and conferred localized immune suppression through this mechanism. These results provide insight into the role of CD95L in regulating the alloantibody response and the generation of local immune responses.

Reduction of alloantibody response to class I major histocompatibility complex by targeting synthetic allopeptides for presentation by B cells

BACKGROUND

PVG.RT1(u) rats develop a strong CD4 T cell-dependent alloantibody response to class I major histocompatibility complex (MHC) A(a) antigen, during which CD4 T helper cells recognize and respond to A(a)-derived peptides presented by recipient class II MHC (indirect allorecognition). On the basis of evidence that CD4 T cells that encounter antigen presented by resting B cells become tolerant, we have targeted synthetic A(a)-derived allopeptides for in vivo presentation to class I MHC-disparate CD4 T cells by resting recipient B cells.

METHODS

PVG.RT1(u) rats were treated with two peptides, P1 and P2, corresponding to the alpha-helical regions of A(a) (residues 57-80 and 143-163), which were conjugated via an N-terminal cysteine residue to monovalent Fab fragments of OX60 monoclonal antibody, which labels membrane IgD-positive B cells.

RESULTS

RT1(u) rats primed with free (nonconjugated) P1 or P2 emulsified in complete Freund's adjuvant produced strong peptide-specific antibody responses and a heightened anti-A(a) antibody response to an A(a)-disparate PVG.R8 heart graft, confirming that each peptide encompasses one or more major T cell determinant for B cell help. Pretreatment of PVG.RT1(u) rats with a mixture of OX60-Fab-P1/P2 conjugates markedly reduced their ability to mount an A(a) antibody response when challenged with either A(a)-disparate blood transfusion or an A(a)-disparate heart graft, although PVG.R8 heart graft survival was not prolonged.

CONCLUSIONS

In this report, we show that synthetic A(a)-derived allopeptides are able, when targeted for in vivo presentation to CD4 T cells by resting B cells, to impair the ability of RT1(u) rats to mount an antibody response to A(a) antigen. All subclasses of IgG anti-A(a) alloantibody were profoundly reduced, suggesting that the responsible mechanism is more likely to be CD4 T helper cell unresponsiveness rather than Th1/Th2 T cell polarization.