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

Poor kidney allograft survival associated with positive B cell - Only flow cytometry cross matches: a ten year single center study

The presence of donor specific antibody (DSA) to class 1 or class 2 HLA as detected respectively in T cell or B cell - only flow cytometry cross matches (FCXMs) are risk factors for renal allograft survival, though the comparative risk of these XMs has not been definitively established. Allograft survival and FCXM data in 624 microcytotoxicity (CDC) XM negative kidney transplants were evaluated. Short and long term allograft survival was significantly less in recipients with T(-) B(+) FCXMs (1 year, 74%, 10 year, 58%) compared to T(+) B(+) FCXMs (1 year, 84%, 10 year, 68%) and to T(-) B(-) FCXM (1 year, 90%, 10 year, 85%). Risk factors were positive FCXM, deceased donor (DD) transplantation and donor age, but not race, gender, recipient age or previous transplant. Recipients with T(-) B(+) and T(+) B(+) FCXMs were at 4.5 and 2.5 fold greater risk, respectively, of DD allograft failure compared to patients with T(-) B(-) FCXMs. The quantitative value of FCXM did not correlate with the duration of graft survival. We conclude that patients with DSA to class 2 HLA have a greater risk of early and late allograft failure compared to patients with DSA to class 1 HLA.

The road to transplant tolerance is paved with good dendritic cells

After transplantation, recipient T cells can recognize donor antigens either by interacting with MHC class II on donor bone marrow-derived cells (direct allorecognition), or by recognizing allogeneic peptides bound to self-MHC class II molecules on recipient antigen presenting cells (indirect allorecognition). The activation of pro-inflammatory T cells via either of these pathways leads to allograft rejection, so the suppression of both of these pathways is needed to achieve transplantation tolerance. A study in this issue of the European Journal of Immunology [Eur. J. Immunol. 2013. 43: 734-746] shows that allogeneic dendritic cells (DCs) modified to either lack expression of CD80/86 or over-express indoleamine 2,3-dioxygenase (IDO) are able to inhibit direct and/or indirect alloresponses in vitro and in vivo in mice. Notably, both allorecognition pathways were suppressed by the coexpression of self- and allo-MHC molecules on semi-allogeneic DCs. This Commentary discusses the challenges and potential of using genetically-modified DCs to suppress alloreactivity in the context of transplant tolerance.

Direct and indirect antigen presentation lead to deletion of donor-specific T cells after in utero hematopoietic cell transplantation in mice

In utero hematopoietic cell transplantation (IUHCTx) is a promising method to induce donor-specific tolerance but the mechanisms of antigen presentation that educate host T cells and the relative importance of deletion vs regulation in this setting are unknown. We studied the roles of direct and indirect antigen presentation (mediated by donor- and host-derived antigen-presenting cells [APCs], respectively) in a mouse model of IUHCTx. We found that IUHCTx leads to precocious maturation of neonatal host dendritic cells (DCs) and that there is early differentiation of donor-derived DCs, even after transplantation of a stem cell source without mature APCs. We next performed allogeneic IUHCTx into donor-specific T-cell receptor transgenic mice and confirmed that both direct and indirect antigen presentation lead to clonal deletion of effector T cells in chimeras. Deletion did not persist when chimerism was lost. Importantly, although the percentage of regulatory T cells (Tregs) after IUHCTx increased, there was no expansion in Treg numbers. In wild-type mice, there was a similar deletion of effector cells without expansion of donor-specific Tregs. Thus, tolerance induction after IUHCTx depends on both direct and indirect antigen presentation and is secondary to thymic deletion, without de novo Treg induction.

Translating tolerogenic therapies to the clinic - where do we stand?

Manipulation of the immune system to prevent the development of a specific immune response is an ideal strategy to improve outcomes after transplantation. A number of experimental techniques exploiting central and peripheral tolerance mechanisms have demonstrated success, leading to the first early phase clinical trials for tolerance induction. The first major strategy centers on the facilitation of donor-cell mixed chimerism in the transplant recipient with the use of bone marrow or hematopoietic stem cell transplantation. The second strategy, utilizing peripheral regulatory mechanisms, focuses on cellular therapy with regulatory T cells. This review examines the key studies and novel research directions in the field of immunological tolerance.

Relevance of regulatory T cell promotion of donor-specific tolerance in solid organ transplantation

Current clinical strategies to control the alloimmune response after transplantation do not fully prevent induction of the immunological processes which lead to acute and chronic immune-mediated graft rejection, and as such the survival of a solid organ allograft is limited. Experimental research on naturally occurring CD4⁺CD25^highFoxP3⁺ Regulatory T cells (Tregs) has indicated their potential to establish stable long-term graft acceptance, with the promise of providing a more effective therapy for transplant recipients. Current approaches for clinical use are based on the infusion of freshly isolated or ex vivo polyclonally expanded Tregs into graft recipients with an aim to redress the in vivo balance of T effector cells to Tregs. However mounting evidence suggests that regulation of donor-specific immunity may be central to achieving immunological tolerance. Therefore, the next stages in optimizing translation of Tregs to organ transplantation will be through the refinement and development of donor alloantigen-specific Treg therapy. The altering kinetics and intensity of alloantigen presentation pathways and alloimmune priming following transplantation may indeed influence the specificity of the Treg required and the timing or frequency at which it needs to be administered. Here we review and discuss the relevance of antigen-specific regulation of alloreactivity by Tregs in experimental and clinical studies of tolerance and explore the concept of delivering an optimal Treg for the induction and maintenance phases of achieving transplantation tolerance.

Are we ready for the use of foxp3(+) regulatory T cells for immunodiagnosis and immunotherapy in kidney transplantation?

The existence of T-cell subsets naturally committed to perform immunoregulation has led to enthusiastic efforts to investigate their role in the immunopathogenesis of transplantation. Being able to modulate alloresponses, regulatory T cells could be used as an immunodiagnostic tool in clinical kidney transplantation. Thus, the measurement of Foxp3 transcripts, the presence of regulatory T cells in kidney biopsies, and the phenotypic characterisation of the T-cell infiltrate could aid in the diagnosis of rejection and the immune monitoring and prediction of outcomes in kidney transplantation. Interestingly, the adoptive transfer of regulatory T cells in animal models has been proven to downmodulate powerful alloresponses, igniting translational research on their potential use as an immunomodulatory therapy. For busy transplant clinicians, the vast amount of information in the literature on regulatory T cells can be overwhelming. This paper aims to highlight the most applicable research findings on the use of regulatory T cells in the immune diagnosis and potential immunomodulatory therapy of kidney transplant patients. However, can we yet rely on differential regulatory T-cell profiles for the identification of rejection or to tailor patient's immunosuppression? Are we ready to administer regulatory T cells as inductive or adjunctive therapy for kidney transplantation?

Differential activation of human T cells to allogeneic endothelial cells, epithelial cells and fibroblasts in vitro

BACKGROUND

In the direct pathway, T cells recognize intact donor major histocompatability complexes and allogeneic peptide on the surface of donor antigen presenting cells (APCs). Indirect allorecognition results from the recognition of processed alloantigen by self MHC complexes on self APCs. In this study, we wished to evaluate the relative contribution of different intragraft cells to the alloactivation of nave and memory T cells though the direct and the indirect pathway of allorecognition.

METHODS

The processing of membrane fragments from IFN-treated single donor endothelial cells (EC), fibroblasts or renal epithelial cells (RPTEC) was evaluated by DiOC labeling of each cell type and flow cytometry following interaction with PBMC. Direct pathway activation of nave CD45RA+ or memory CD45RO+ CD4+ T cells was evaluated following coculture with IFN-treated and MHC class II-expressing EC, fibroblasts or RPTEC. Indirect pathway activation was assessed using CD45RA+ or CD45RO+ CD4+ T cells cocultured with autologous irradiated APCs in the absence or presence of sonicates derived from IFN-treated allogeneic EC, fibroblasts or RPTEC. Activation of T cells was assessed by [3H]thymidine incorporation and by ELISpot assays.

RESULTS

We find that CD14+ APCs readily acquire membrane fragments from fibroblasts and RPTEC, but fail to acquire membrane fragments from intact EC. However, APCs process membranes from EC undergoing apoptosis.There was a notable direct pathway alloproliferative response of CD45RO+ CD4+ T cells to IFN-treated EC, but not to fibroblasts or RPTEC. Also, there was a minimal direct pathway response of CD45RA+ CD4+ T cells to all cell types. In contrast, we found that both CD45RA+ and CD45RO+ CD4+ T cells proliferated following coculture with autologous APCs in the presence of sonicates derived from IFN-treated EC, fibroblasts or RPTEC. By ELISpot, we found that these T cells stimulated via the indirect pathway also produced the cytokines IFN, IL-2, IL-4 and IL-5.

CONCLUSIONS

Recipient APCs may readily process membrane fragments from allogeneic intragraft cells, but not from EC unless they are undergoing apoptosis. This processing is sufficient for indirect pathway alloactivation of both CD45RA+ and CD45RO+ CD4+ T cells. Only graft vascular EC mediate direct pathway reactivation of CD4+ T cells.

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.

Identification of a coordinated CD8 and CD4 T cell response directed against mismatched HLA Class I causing severe acute graft-versus-host disease

After HLA class I-mismatched stem cell transplantation, allo-HLA-directed CD8 T cell responses can be activated without the help of CD4 T cells if memory CD8 T cells cross-reactive against the allo-HLA class I are present or if naïve CD8 T cells are administered during inflammatory conditions. However, in the absence of inflammatory conditions, cooperation between CD4 and CD8 T cells likely is required for an effective primary CD8 T cell response directed against allo-HLA class I. In this study we investigated whether a coordinated response of CD8 and CD4 T cells could be demonstrated in an HLA class I-directed immune response in a patient who developed severe graft-versus-host disease (GVHD) after the administration HLA-A2-mismatched donor lymphocyte infusion in the absence of inflammatory conditions. A previously administered donor lymphocyte infusion from the same donor did not lead to an immune response, excluding the presence of a substantial pool of CD8 T cells cross-reactive against HLA-A2 within the memory T cell compartment of the donor. Analysis of isolated donor CD8 and CD4 T cell clones activated during the GVHD revealed a polyclonal CD8 T cell response directed against the mismatched HLA-A2 and a polyclonal CD4 T cell response recognizing HLA-A2-derived peptides presented in HLA class II. In addition, leukemic blasts present at the time of the emergence of GVHD expressed HLA-A2 and HLA class II and could activate both the CD4 and CD8 alloreactive T cells. Our results demonstrate that the GVHD was mediated by a cooperative CD4 and CD8 response directed against the mismatched HLA-A2 and suggest that leukemic blasts possibly activated this CD8 and CD4 T cell response.

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.

Regulatory T cell therapy for the induction of clinical organ transplantation tolerance

The pursuit of transplantation tolerance is the holygrail in clinical organ transplantation. It has been established that regulatory T cells (Tregs) can confer donor-specific tolerance in mouse models of transplantation. However, this is crucially dependent on the strain combination, the organ transplanted and most importantly, the ratio of Tregs to alloreactive effector T cells. The ex vivo expansion of Tregs is one solution to increase the number of alloantigen specific cells capable of suppressing the alloresponse. Indeed, ex vivo expanded, alloantigen specific murine Tregs are shown to preferentially migrate to, and proliferate in, the graft and draining lymph node. In human transplantation it has been proposed that depletion of the majority of direct pathway alloreactive T cells will be required to tip the balance in favour of regulation. Ex vivo expansion of alloantigen specific, indirect pathway human Tregs, which can cross regulate the residual direct pathway has been established. Rapid expansion of these cells is possible, whilst they retain antigen specificity, suppressive properties and favourable homing markers. Furthermore, considerable progress has been made to define which immunosuppressive drugs favour the expansion and function of Tregs. Currently a series of clinical trials of adoptive Treg therapy in combination with depletion of alloreactive T cells and short term immunosuppression are underway for human transplantation with the aim of minimizing immunosuppressive drugs and completely withdrawal.

KIR and HLA-C interactions promote differential dendritic cell maturation and is a major determinant of graft failure following kidney transplantation

BACKGROUND

HLA-C is an important ligand for killer immunoglobulin like receptors (KIR) that regulate natural killer (NK) cell function. Based on KIR specificity HLA-C molecules are allocated into two groups, HLA-C1 or HLA-C2; HLA-C2 is more inhibiting to NK cell function than HLA-C1. We studied the clinical importance of HLA-C genotypes on the long-term graft survival of 760 kidney transplants performed at our centre utilising a population based genetic study and cell culture model to define putative mechanisms.

METHODS AND FINDINGS

Genotyping was performed using conventional DNA PCR techniques and correlations made to clinical outcomes. We found that transplant recipients with HLA-C2 had significantly better long-term graft survival than transplant recipients with HLA-C1 (66% versus 44% at 10 years, log-rank p = 0.002, HR = 1.51, 95%CI = 1.16-1.97). In in-vitro NK and dendritic cell (DC) co-culture model we made several key observations that correlated with the population based genetic study. We observed that donor derived NK cells, on activation with IL-15, promoted differential HLA-C genotype dependent DC maturation. In NK-DC co-culture, the possession of HLA-C2 by DC was associated with anti-inflammatory cytokine production (IL-1RA/IL-6), diminished DC maturation (CD86, HLA-DR), and absent CCR7 expression. Conversely, possession of HLA-C1 by DC was associated with pro-inflammatory cytokine synthesis (TNF-α, IL-12p40/p70), enhanced DC maturation and up-regulation of CCR7 expression. By immunohistochemistry the presence of donor NK cells was confirmed in pre-transplant kidneys.

CONCLUSIONS

We propose that after kidney transplantation IL-15 activated donor derived NK cells interact with recipient DC with less activation of indirect allo-reactivity in HLA-C2 positive recipients than HLA-C1 positive recipients; this has implications for long-term graft survival. Early events following kidney transplantation involving NK-DC interaction via KIR and HLA-C immune synapse may have a central role in long-term kidney transplant outcomes.

Biomarkers of tolerance: searching for the hidden phenotype

Induction of transplantation tolerance remains the ideal long-term clinical and logistic solution to the current challenges facing the management of renal allograft recipients. In this review, we describe the recent studies and advances made in identifying biomarkers of renal transplant tolerance, from study inceptions, to the lessons learned and their implications for current and future studies with the same goal. With the age of biomarker discovery entering a new dimension of high-throughput technologies, here we also review the current approaches, developments, and pitfalls faced in the subsequent statistical analysis required to identify valid biomarker candidates.

The indirect alloimmune response causes microvascular endothelial dysfunction-a possible role for alloantibody

BACKGROUND

The causes of endothelial dysfunction after cardiac transplantation are unknown. Here, we have investigated whether the indirect alloimmune response mediates endothelial dysfunction in a major histocompatibility complex class I mismatch model.

METHODS

PVG.RT1 rat hearts were transplanted into thymectomized CD8 T-cell-depleted allogeneic (PVG.R8) or syngeneic (PVG.RT1) recipients. Alloantibody was assessed at 2, 4, and 8 weeks. Cardiac allograft vasculopathy, the nature of the inflammatory infiltrate, and origin of endothelial cells were examined at 1, 2, 4, and 8 weeks. Endothelial function was assessed by Langendorff preparations at 1, 2, and 4 weeks.

RESULTS

Recipients produced alloantibody and showed luminal occlusion at 1 (17.7%±8.0%), 2 (23.2%±4.9%), 4 (34.3%±5.0%), and 8 weeks (58.1%±1.8%) posttransplantation. The major inflammatory features of the allografts consisted of CD11b monocytes, CD4 T cells, and C4d deposition. At 1 week, the basal coronary flow and the vasodilator response to 5-hydroxytrytamine of syngeneic and allografted hearts were inhibited compared with normal hearts. At 4 weeks, the basal coronary flow of allografts was 54% lower than syngrafts (P<0.01), and 5- hydroxytrytamine and sodium nitroprusside did not evoke an increase in coronary flow in the allograft heart compared with syngeneic controls (P<0.01). Culture of aortic rings with antibody to major histocompatibility complex class I inhibited endothelium-dependent vasodilation to acetylcholine.

CONCLUSION

Transient microvascular endothelial dysfunction occurred in syngeneic and allogeneic cardiac grafts after transplantation. Syngeneic but not allogeneic grafts recovered, suggesting the indirect immune response, consisting of CD4 T cells, monocytes, and antibody, mediates endothelial dysfunction. A possible role for alloantibody in endothelial dysfunction is discussed.

Detailed kinetics of the direct allo-response in human liver transplant recipients: new insights from an optimized assay

Conventional assays for quantification of allo-reactive T-cell precursor frequencies (PF) are relatively insensitive. We present a robust assay for quantification of PF of T-cells with direct donor-specificity, and establish the kinetics of circulating donor-specific T cells after liver transplantation (LTx). B cells from donor splenocytes were differentiated into professional antigen-presenting cells by CD40-engagement (CD40-B cells). CFSE-labelled PBMC from LTx-recipients obtained before and at several time points after LTx, were stimulated with donor-derived or 3rd party CD40-B cells. PF of donor-specific T cells were calculated from CFSE-dilution patterns, and intracellular IFN-γ was determined after re-stimulation with CD40-B cells. Compared to splenocytes, stimulations with CD40-B cells resulted in 3 to 5-fold higher responding T-cell PF. Memory and naïve T-cell subsets responded equally to allogeneic CD40-B cell stimulation. Donor-specific CD4+ and CD8+ T-cell PF ranged from 0.5 to 19% (median: 5.2%). One week after LTx, PF of circulating donor-specific CD4+ and CD8+ T cells increased significantly, while only a minor increase in numbers of T cells reacting to 3rd party allo-antigens was observed. One year after LTx numbers of CD4+ and CD8+ T cells reacting to donor antigens, as well as those reacting to 3rd party allo-antigens, were slightly lower compared to pre-transplant values. Moreover, CD4+ and CD8+ T cells responding to donor-derived, as well as those reacting to 3rd party CD40-B cells, produced less IFN-γ. In conclusion, our alternative approach enables detection of allo-reactive human T cells at high frequencies, and after application we conclude that donor-specific T-cell PF increase immediately after LTx. However, no evidence for a specific loss of circulating T-cells recognizing donor allo-antigens via the direct pathway up to 1 year after LTx was obtained, underscoring the relative insensitiveness of previous assays.

Differential role of naïve and memory CD4 T-cell subsets in primary alloresponses

The T cell response to major histocompatibility complex (MHC) alloantigens occurs via two main pathways. The direct pathway involves the recognition of intact allogeneic MHC:peptide complexes on donor cells and provokes uniquely high frequencies of responsive T cells. The indirect response results from alloantigens being processed like any other protein antigen and presented as peptide by autologous antigen-presenting cells. The frequencies of T cells with indirect allospecificity are orders of magnitude lower and comparable to other peptide-specific responses. In this study, we explored the contributions of naïve and memory CD4(+) T cells to these two pathways. Using an adoptive transfer and skin transplantation model we found that naive and memory CD4(+) T cells, both naturally occurring and induced by sensitization with multiple third-party alloantigens, contributed equally to graft rejection when only the direct pathway was operative. In contrast, the indirect response was predominantly mediated by the naïve subset. Elimination of regulatory CD4(+)CD25(+) T cells enabled memory cells to reject grafts through the indirect pathway, but at a much slower tempo than for naïve cells. These findings have implications for better targeting of immunosuppression to inhibit immediate and later forms of alloimmunity.

Characterization of immune responses to cardiac self-antigens myosin and vimentin in human cardiac allograft recipients with antibody-mediated rejection and cardiac allograft vasculopathy

BACKGROUND

Herein we study the role of donor-specific antibodies (DSA) to mismatched human leukocyte antigen (HLA) and antibodies (Abs) to the cardiac self-antigens myosin (MYO) and vimentin (VIM) in the pathogenesis of acute antibody-mediated rejection (AMR) in the early post-transplant period (EP, <12 months) and cardiac allograft vasculopathy (CAV) in the late post-transplant period (LP, >12 months) after heart transplantation (HTx).

METHODS

One hundred forty-eight HTx recipients (65 in EP, 83 in LP) were enrolled in the study. Development of DSA was determined by Luminex. Circulating Abs against MYO and VIM in sera were measured using enzyme-linked immunoassay (ELISA). Frequency of CD4+ T-helper cells (CD4+ Th) secreting interferon (IFN)-γ, interleukin (IL)-17, IL-10 or IL-5 specific to either MYO or VIM were analyzed in vitro using ELISpot assays.

RESULTS

AMR patients were more likely DSA positive (AMR-: 15%; AMR+: 70%; p = 0.03) and demonstrated increased Abs to MYO (AMR-: 144 ± 115 μg/ml; AMR+: 285 ± 70 μg/ml; p = 0.033) and VIM (AMR-: 37 ± 19 μg/ml; AMR+: 103 ± 43 μg/ml; p = 0.014). AMR patients demonstrated increased IL-5 CD4+ Th cells specific to MYO (5.2 ± 0.9 fold, p = 0.003) and VIM (7.3 ± 2.9-fold, p = 0.004) and decreased IL-10 CD4+ Th cells specific to MYO (2.2 ± 0.4-fold, p = 0.009) and VIM (1.7 ± 0.2-fold, p = 0.03). CAV patients were more likely DSA positive (CAV-): 25%; CAV+: 79%; p = 0.03) and demonstrated increased Abs to MYO (CAV-: 191 ± 120 μg/ml; CAV+: 550 ± 98 μg/ml; p = 0.025) and VIM (CAV-: 55 ± 25 μg/ml; CAV+: 255 ± 49 μg/ml; p = 0.001). CAV patients demonstrated increased IL-17 CD4+ Th cells specific to MYO (10.5 ± 7.3-fold, p = 0.002) and VIM (7.0 ± 3.9-fold, p = 0.003).

CONCLUSIONS

The presence of DSA in AMR and CAV is significantly associated with development of Abs to MYO and VIM in post-HTx patients. Induction of high CD4+ Th cells specific to cardiac self-antigens that secrete predominantly IL-5 and IL-17 plays a significant role in the development of Abs to self-antigens leading to AMR and CAV, respectively.

Role of T cells in graft rejection and transplantation tolerance

Transplantation is the most effective treatment for end-stage organ failure, but organ survival is limited by immune rejection and the side effects of immunosuppressive regimens. T cells are central to the process of transplant rejection through allorecognition of foreign antigens leading to their activation, and the orchestration of an effector response that results in organ damage. Long-term transplant acceptance in the absence of immunosuppressive therapy remains the ultimate goal in the field of transplantation and many studies are exploring potential therapies. One promising cellular therapy is the use of regulatory T cells to induce a state of donor-specific tolerance to the transplant. This article first discusses the role of T cells in transplant rejection, with a focus on the mechanisms of allorecognition and the alloresponse. This is followed by a detailed review of the current progress in the field of regulatory T-cell therapy in transplantation and the translation of this therapy to the clinical setting.

Split immunological tolerance to trophoblast

Split immunological tolerance refers to states in which an individual is capable of mounting certain types of immune responses to a particular antigenic challenge, but is tolerant of the same antigen in other compartments of the immune system. This concept is applicable to the immunological relationship between mother and fetus, and particularly relevant in equine pregnancy. In pregnant mares, antibody responses to paternal foreign Major Histocompatibility Complex class I antigens are robust, while anti-paternal cytotoxic T cell responses are diminished compared to those mounted by non-pregnant mares. Here, we compared the distribution of the major lymphocyte subsets, the percentage of lymphocytes expressing Interferon Gamma (IFNG) and Interleukin 4 (IL4) and the level of expression of the immunoregulatory transcription factor FOXP3 between pregnant and non-pregnant mares, and between peripheral blood and the endometrium during pregnancy. In a cohort of mares in which peripheral blood lymphocytes were tested during early pregnancy and in the non-pregnant state, there were only slight changes observed during pregnancy. In contrast, comparison of peripheral blood lymphocytes with lymphocytes isolated from the endometrial cups of pregnant mares revealed striking differences in lymphocyte sub-populations. The endometrial cups contained higher numbers of IFNG+ lymphocytes, and lower numbers of lymphocytes expressing IL4. The endometrial cup lymphocytes also had higher numbers of FOXP3+ cells compared to peripheral blood lymphocytes. Taken together, these results strengthen the evidence for a state of split tolerance to trophoblast, and furthermore define sharp differences in immune reactivity during equine pregnancy between peripheral blood lymphocytes and lymphocytes at the maternal-fetal interface.

Preemptive CD20+ B cell depletion attenuates cardiac allograft vasculopathy in cyclosporine-treated monkeys

Chronic rejection currently limits the long-term efficacy of clinical transplantation. Although B cells have recently been shown to play a pivotal role in the induction of alloimmunity and are being targeted in other transplant contexts, the efficacy of preemptive B cell depletion to modulate alloimmunity or attenuate cardiac allograft vasculopathy (CAV) (classic chronic rejection lesions found in transplanted hearts) in a translational model has not previously been described. We report here that the CD20-specific antibody (alphaCD20) rituximab depleted CD20+ B cells in peripheral blood, secondary lymphoid organs, and the graft in cynomolgus monkey recipients of heterotopic cardiac allografts. Furthermore, CD20+ B cell depletion therapy combined with the calcineurin inhibitor cyclosporine A (CsA) prolonged median primary graft survival relative to treatment with alphaCD20 or CsA alone. In animals treated with both alphaCD20 and CsA that achieved efficient B cell depletion, alloantibody production was substantially inhibited and the CAV severity score was markedly reduced. We conclude therefore that efficient preemptive depletion of CD20+ B cells is effective in a preclinical model to modulate pathogenic alloimmunity and to attenuate chronic rejection when used in conjunction with a conventional clinical immunosuppressant. This study suggests that use of this treatment combination may improve the efficacy of transplantation in the clinic.

Targeting MHC class I monomers to dendritic cells inhibits the indirect pathway of allorecognition and the production of IgG alloantibodies leading to long-term allograft survival

T cell depletion strategies are an efficient therapy for the treatment of acute rejections and are an essential part of tolerance induction protocols in various animal models; however, they are usually nonselective and cause wholesale T cell depletion leaving the individual in a severely immunocompromised state. So far it has been difficult to selectively delete alloreactive T cells because the majority of protocols either delete all T cells, subsets of T cells, or subpopulations of T cells expressing certain activation markers, ignoring the Ag specificity of the TCR. We have developed a model in which we were able to selectively deplete alloreactive T cells with an indirect specificity by targeting intact MHC molecules to quiescent dendritic cells using 33D1 as the targeting Ab. This strategy enabled us to inhibit the indirect alloresponse against MHC-mismatched skin grafts and hence the generation of IgG alloantibodies, which depends on indirectly activated T cells. In combination with the temporary abrogation of the direct alloresponse, we were able to induce indefinite skin graft survival. Importantly, the targeting strategy had no detrimental effect on CD4(+)CD25(+)FoxP3(+) T cells, which could potentially be used as an adjunctive cellular therapy. Transplantation tolerance depends on the right balance between depletion and regulation. For the former this approach may be a useful tool in the development of future tolerance induction protocols in non-sensitized patients.

Deficient TNF-alpha and IFN-gamma production correlates with nondetectable donor-specific cytotoxicity after clinical kidney transplantation

BACKGROUND

We previously reported that no cytotoxic T-lymphocyte precursor frequencies (CTLpf) were found in 60% of patients on azathioprine or mycophenolate mofetil+Pred long after kidney transplantation. We questioned whether the absence of donor-specific CTLpf was associated with low levels of stimulatory Th1 (tumor necrosis factor [TNF]-alpha, interferon [IFN]-gamma) or high levels of regulatory Th2 (interleukin [IL]-4, IL-6, IL-10) cytokines.

METHODS

In this study, peripheral blood monocyte cells (PBMC) were stimulated with irradiated donor cells. After 7 days, cytokine production was determined by cytokine bead array, and CTLpf by limiting dilution assay.

RESULTS

Patients with detectable CTLpf (> or =10/10(6) PBMC) had significantly higher levels of TNF-alpha (P=0.04) and IFN-gamma (P=0.02) than patients with nondetectable CTLpf (<10/10(6) PBMC). Donor-reactive IL-4, IL-6, and IL-10 production was comparable in both patient groups. Additionally, CTLpf was positively correlated with TNF-alpha (rs=0.54, P=0.0003) and IFN-gamma (rs=0.64, P<0.0001) production.

CONCLUSION

The absence of donor-specific CTLp after transplantation correlates with low levels of stimulatory cytokines, not with elevated levels of regulatory cytokines.

Preferential priming of alloreactive T cells with indirect reactivity

The relative contributions of the direct and indirect pathways in alloimmune responses have not been fully elucidated. We report a novel murine TCR transgenic system that can simultaneously track the CD4-direct (CD4-d), CD4-indirect (CD4-i) and CD8-direct (CD8-d) pathways after transplantation. Using this system, we have observed a profoundly greater proliferation of CD4-i T cells relative to CD4-d and CD8-d T cells after transplantation. Furthermore, a much larger proportion of CD4-i T cells attain an effector phenotype. We also analyzed endogenous, wild-type T cells using enzyme-linked immunospot analysis. In naïve mice, T cells with indirect reactivity were undetectable, but T cells with direct reactivity were abundant. However, 10 days after skin or heterotopic heart transplantation, CD4-i T cells comprised approximately 10% of the CD4+ response. Consistent with increased priming of the CD4-i pathway, we observed that the CD4-i T cells were further enriched in the effector cells migrating to the allograft and in memory-like T cells persisting after rejection. Thus, priming of the CD4-i pathway is favored after transplantation, allowing a rare population to rapidly become a major component of the CD4+ T-cell response in acute allograft rejection. The generalizability of this observation to other models remains to be determined.

Tolerance-inducing immunosuppressive strategies in clinical transplantation: an overview

The significant development of immunosuppressive drug therapies within the past 20 years has had a major impact on the outcome of clinical solid organ transplantation, mainly by decreasing the incidence of acute rejection episodes and improving short-term patient and graft survival. However, long-term results remain relatively disappointing because of chronic allograft dysfunction and patient morbidity or mortality, which is often related to the adverse effects of immunosuppressive treatment. Thus, the induction of specific immunological tolerance of the recipient towards the allograft remains an important objective in transplantation. In this article, we first briefly describe the mechanisms of allograft rejection and immune tolerance. We then review in detail current tolerogenic strategies that could promote central or peripheral tolerance, highlighting the promises as well as the remaining challenges in clinical transplantation. The induction of haematopoietic mixed chimerism could be an approach to induce robust central tolerance, and we describe recent encouraging reports of end-stage kidney disease patients, without concomitant malignancy, who have undergone combined bone marrow and kidney transplantation. We discuss current studies suggesting that, while promoting peripheral transplantation tolerance in preclinical models, induction protocols based on lymphocyte depletion (polyclonal antithymocyte globulins, alemtuzumab) or co-stimulatory blockade (belatacept) should, at the current stage, be considered more as drug-minimization rather than tolerance-inducing strategies. Thus, a better understanding of the mechanisms that promote peripheral tolerance has led to newer approaches and the investigation of individualized donor-specific cellular therapies based on manipulated recipient regulatory T cells.

Repetitive gastric aspiration leads to augmented indirect allorecognition after lung transplantation in miniature swine

INTRODUCTION

Lung transplant recipients with documented gastroesophageal reflux disease (GERD) are at increased risk for graft dysfunction. Here, we present the first large-animal model of gastric aspiration after allogeneic lung transplantation and some preliminary data demonstrating the effect of chronic aspiration on the direct and indirect pathways of allorecognition.

METHODS

Left orthotopic lung transplants (n=3) were performed in miniature swine across a major histocompatibility complex class I disparity, followed by 12 days of high-dose cyclosporine A. At the time of transplantation, a transtracheal catheter was placed at the carina, above the bronchial anastomosis. A gastrostomy tube was placed for daily aspiration of gastric contents. Subsequently, graft lungs were instilled with gastric aspirate daily (3 mL/hrX8 hr/day) for 50 days. Recipients were followed up with daily complete blood count, scheduled chest radiographs, and biopsies. In vitro studies, including cell-mediated lympholysis, mixed lymphocyte reactions, and peptide proliferation assays, were performed. Results from these three recipients were compared with those of historical controls (n=6) who were treated identically, except for the tracheal cannulation and simulated gastric aspiration.

RESULTS

Two of the experimental animals were euthanized with nonviable lungs soon after the postoperative day 50 biopsy. In both cases the native lung was normal. The third animal survived over 180 days without the evidence of chronic rejection. After immunosuppressive treatment, all animals demonstrated donor-specific hyporesponsiveness by assays of direct alloresponse (cell-mediated lympholysis, mixed lymphocyte reaction). A significant response to synthetic donor-derived class I peptide, however, was seen in all animals. A more pronounced and diffuse response was seen in the animals rejecting their grafts. The historical controls showed medium-term graft survival with evidence of chronic rejection in the majority of animals, as previously reported.

CONCLUSION

In a model of GERD after lung transplantation, a spectrum of clinical outcomes was observed. The in vitro data suggest that acid reflux enhances the indirect alloresponse to processed donor class I antigen, giving mechanistic insight into the manner in which GERD may be deleterious to the transplanted lung.

Divergent role of donor dendritic cells in rejection versus tolerance of allografts

Little is known about heart tissue/donor dendritic cells, which play a key role in mounting alloimmune responses. In this report, we focus on three primary features of donor dendritic cells: their generation, their trafficking after transplantation, and their role in regulating tolerance versus rejection. Using transgenic mice as donors of heart allografts enabled us to monitor trafficking of donor dendritic cells after transplantation. Donor dendritic cells rapidly migrated into secondary lymphoid tissues within 3 h of transplantation. We found that the chemokine receptor CX3CR1 regulates the generation of heart tissue dendritic cells constitutively. Compared with wild-type hearts, CX3CR1(-/-) hearts contained fewer dendritic cells, and heart allografts from CX3CR1(-/-) donors survived significantly longer without immunosuppression. Unexpectedly, though, co-stimulatory blockade with anti-CD154 or CTLA4-Ig induced long-term survival for wild-type heart allografts but not for CX3CR1(-/-) heart allografts. Increasing the dendritic cell frequency in CX3CR1(-/-) hearts by treatment with Flt3L restored the anti-CD154-induced prolongation of CX3CR1(-/-) heart allograft survival. Compared with wild-type donors, depleting transgenic donors of dendritic cells before heart transplantation also markedly worsened chronic rejection under anti-CD154 treatment. These data indicate the importance of the CX3CR1 pathway in the generation of heart tissue dendritic cells and the divergent role of tissue/dendritic cells in rejection versus tolerance.

Immunosuppression for lung transplantation

As a result of advances in surgical techniques, immunosuppressive therapy, and postoperative management, lung transplantation has become an established therapeutic option for individuals with a variety of end-stage lung diseases. The current 1-year actuarial survival rate following lung transplantation is approaching 80%. However, the 5- year actuarial survival rate has remained virtually unchanged at approximately 50% over the last 15 years due to the processes of acute and chronic lung allograft rejection (1). Clinicians still rely on a vast array of immunosuppressive agents to suppress the process of graft rejection, but find themselves limited by an inescapable therapeutic paradox. Insufficient immunosuppression results in graft loss due to rejection, while excess immunosuppression results in increased morbidity and mortality from opportunistic infections and malignancies. Indeed, graft rejection, infection, and malignancy are the three principal causes of mortality for the lung transplant recipient. One should also keep in mind that graft loss in a lung transplant recipient is usually a fatal event, since there is no practical means of long-term mechanical support, and since the prospects of re-transplantation are low, given the shortage of acceptable donor grafts. This chapter reviews the current state of immunosuppressive therapy for lung transplantation and suggests alternative paradigms for the management of future lung transplant recipients.

In vitro suppression of xenoimmune-mediated macrophage activation by human CD4+CD25+ regulatory T cells

BACKGROUND

Macrophages are important effector cells in T cell-mediated xenograft rejection. The aim of this study was to determine whether CD4+CD25+ regulatory T cells (Tregs) were capable of suppressing macrophage activation in vitro.

METHODS

Porcine cell or xenoantigen-primed human peripheral blood mononuclear cells, CD4+ T cell-depleted peripheral blood mononuclear cells, or CD14+ macrophages plus autologous CD4+CD25- T cells were cultured with or without expanded autologous Tregs. Transwell cultures were used to separate the various components to determine the need for cell-cell contact.

RESULTS

Pig cell primed CD14+ macrophages required the presence of CD4+CD25- T cells for activation and increased expression of CD40, interleukin-12, and tumor necrosis factor-alpha. This up-regulated expression of macrophage activation markers was reduced substantially in the presence of autologous Tregs. Coculture with Tregs did not alter macrophage viability but reduced the capacity of macrophages to stimulate proliferation of responder T cells. Tregs required direct contact with CD4+CD25- T cells to inhibit macrophage activation but activated macrophage phenotype was not altered by separating the stimulated human peripheral blood mononuclear cells or CD14+ macrophages from Tregs in a transwell system. Macrophages did not require direct cell contact with porcine stimulator cells for full activation by CD4+CD25- T cells.

CONCLUSIONS

Human Tregs were able to suppress xenoantigen-primed and CD4+ T-cell-mediated macrophage activation and antigen-presenting cell function. However, Tregs had no direct effect on macrophages in vitro.

Conferring indirect allospecificity on CD4+CD25+ Tregs by TCR gene transfer favors transplantation tolerance in mice

T cell responses to MHC-mismatched transplants can be mediated via direct recognition of allogeneic MHC molecules on the cells of the transplant or via recognition of allogeneic peptides presented on the surface of recipient APCs in recipient MHC molecules - a process known as indirect recognition. As CD4(+)CD25(+) Tregs play an important role in regulating alloresponses, we investigated whether mouse Tregs specific for allogeneic MHC molecules could be generated in vitro and could promote transplantation tolerance in immunocompetent recipient mice. Tregs able to directly recognize allogeneic MHC class II molecules (dTregs) were obtained by stimulating CD4(+)CD25(+) cells from C57BL/6 mice (H-2(b)) with allogeneic DCs from BALB/c mice (H-2(d)). To generate Tregs that indirectly recognized allogeneic MHC class II molecules, dTregs were retrovirally transduced with TCR genes conferring specificity for H-2K(d) presented by H-2A(b) MHC class II molecules. The dual direct and indirect allospecificity of the TCR-transduced Tregs was confirmed in vitro. In mice, TCR-transduced Tregs, but not dTregs, induced long-term survival of partially MHC-mismatched heart grafts when combined with short-term adjunctive immunosuppression. Further, although dTregs were only slightly less effective than TCR-transduced Tregs at inducing long-term survival of fully MHC-mismatched heart grafts, histologic analysis of long-surviving hearts demonstrated marked superiority of the TCR-transduced Tregs. Thus, Tregs specific for allogeneic MHC class II molecules are effective in promoting transplantation tolerance in mice, which suggests that such cells have clinical potential.

Minimization of calcineurin inhibitors to improve long-term outcomes in kidney transplantation

Long-term outcomes after kidney transplantation remain suboptimal, despite the great achievements observed in recent years with the use of modern immunosuppressive drugs. Currently, the calcineurin inhibitors (CNI) cyclosporine and tacrolimus remain the cornerstones of immunosuppressive regimens in many centers worldwide, regardless of their well described side-effects, including nephrotoxicity. In this article, we review recent CNI-minimization strategies in kidney transplantation, while emphasizing on the importance of long-term follow-up and patient monitoring. Finally, accumulating data indicate that low-dose CNI-based regimens would provide an interesting balance between efficacy and toxicity.

The importance of the indirect pathway of allorecognition in clinical transplantation

The immune system mounts a response to non-self transplanted tissue through a number of mechanisms. The indirect pathway of allorecognition, in which cells of the adaptive immune system recognize MHC alloantigen-derived peptide on self-MHC molecules, has emerged as a potent inducer of allograft rejection. In particular, recent evidence convincingly connects the indirect pathway with chronic rejection, including antibody-mediated and CD8(+) T cell-mediated rejection. However, the indirect pathway can also promote the generation of regulatory T cells, which have emerged as crucial suppressors of the alloresponse, and hold much promise in the quest for clinical tolerance. An improved understanding of the indirect pathway is likely to bring important benefits to transplant recipients.

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.

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.

Specific alloantigen self-control by regulatory T cells in organ transplantation: a review

Multidrug immunosuppressive protocols have increased short-term patient and graft survival rates from 50% to 90% in the past two decades. Unfortunately, chronic graft rejection still remains the main cause of long-term failure and patients must undergo lifelong immunosuppression. The severe side effects such as life-threatening infections, secondary malignancies, and cardiovascular dysfunction all together include roughly 50% of deaths among kidney transplant patients with functioning grafts. Therefore, it should be of crucial importance to reduce immunosuppression and seek induction of specific tolerance to donor alloantigens. Several investigations have suggested that the acquisition of tolerance to self and/or foreign antigens is dependent on the number and function of naturally occurring and acquired regulatory T cells, which can control all aggressive T cells. The regulatory T cells together with their receptors, costimulatory molecules, cytokines, chemokines, and growth factors all contribute to maintain an equilibrium between aggressive and suppressive effector immune responses. As a consequence of increased knowledge, new immunosuppressive approaches based on either alloantigen-specific regulatory T-cell expansion in vivo or in vitro have been proposed to achieve donor-specific transplantation tolerance in kidney allograft recipients. This contribution attempted to summarize knowledge about regulatory T cells and developing methods to induce specific tolerance in kidney transplantation.

Allopeptide-specific CD4(+) T cells facilitate the differentiation of directly alloreactive graft-infiltrating CD8(+) T Cells

To investigate the mechanism of CD4(+) T-cell help during the activation and differentiation of directly alloreactive CD8(+) T cells, we examined the development of obliterative airways disease (OAD) following transplantation of airways into fully mismatched recipient mice deficient in CD4(+) T cells. BALB/c trachea allografts became fibrosed significantly less frequently in B6 CD4(-/-) recipients as compared to wildtype controls. Furthermore, class I-directed cytotoxicity failed to develop in the absence of CD4(+) T cells. The infiltration of graft tissue by primed L(d)-specific directly alloreactive 2C CD8(+) T cells was not found to depend on the presence of CD4(+) T cells. Nevertheless, graft-infiltrating 2C CD8(+) T cells failed to express CD69 and granzyme B when CD4(+) T-cell help was unavailable. Importantly, reconstitution of B6 CD4(-/-) recipient mice with graft peptide-specific TCR-Tg CD4(+) T cells (OT-II or TEa) capable of recognizing antigen only on recipient APC allowed for full expression of CD69 and granzyme B by the directly alloreactive CD8(+) T cells and restored the capacity of recipients to reject their allografts. These results demonstrate that indirectly alloreactive CD4(+) T cells ensure the optimal activation and differentiation of graft-infiltrating directly alloreactive CD8(+) T cells independent of donor APC recognition.

Dependency of the trans vivo delayed type hypersensitivity response on the action of regulatory T cells: implications for monitoring transplant tolerance

BACKGROUND

The trans vivo delayed-type hypersensitivity (DTH) assay has been used for monitoring the immune status of clinical transplant recipients. Here we tested the hypothesis that the assay can reveal control of allograft rejection by CD25CD4 regulatory T cells (Treg).

METHODS

CBA.Ca (H2) recipients of heterotopic C57BL/10 (H2, B10) heart transplants were untreated or pretreated with anti-CD4 antibody and donor-specific blood. This protocol has been shown previously to induce operational tolerance to alloantigens that is dependent on CD25CD4 Treg. Four weeks after transplantation leukocytes were harvested and used for the trans vivo DTH assay. Cells were stimulated with irradiated B10 leukocytes or subcellular antigen and injected into ear pinnae of immune deficient CB17.SCID.beige hosts.

RESULTS

Stimulation of leukocytes from recipients rejecting B10 cardiac allografts with recall alloantigen caused a "strong" swelling response, whereas similar stimulation of leukocytes from operationally tolerant mice resulted in significantly less swelling (n=17; P=0.003). When CD25 T cells were depleted from "tolerant" leukocytes, the swelling response triggered was similar to that obtained using cells from rejecting animals (P<0.001), while coinjection of purified CD25CD4 cells from tolerant recipients with rejecting cells suppressed the swelling response (P=0.007) to the level observed with cells from tolerant mice. Both the effector and the regulatory response were triggered by indirect allorecognition and blocking experiments showed that the regulation observed was dependent on cytoxic T lymphocyte antigen (CTLA) 4.

CONCLUSION

The trans vivo DTH assay can be used to reveal regulation mediated by CD25CD4 T cells and is CTLA4 dependent.

HA-1 Mismatch Has Significant Effect in Chronic Allograft Nephropathy in Clinical Renal Transplantation

BACKGROUND

The minor histocompatibility antigen HA-1 occurs in two allelic forms: H and R. The HA-1(H) form presented in the context of HLA A2 can elicit specific cytotoxic lymphocyte (CTL) responses and can cause graft-versus-host disease in marrow transplants. However, its significance in solid organ transplants is unknown. We determined whether incompatibility of the HA-1 resulted in enhanced rejection and whether HA-1 specific CTLs were generated.

MATERIALS AND METHODS

HLA A2-matched donor/recipient pairs were selected and typed for HA-1 antigens by polymerase chain reaction. Nineteen of 81 pairs were mismatched for HA-1. Peripheral blood mononuclear leucocytes from five recipients, HLA A2 DR-matched with donors, were stimulated for 3 days with third-party donor, matched for HLA A2 DR but mismatched for HA-1. Cells were stained for surface markers, HA-1(H)-specific tetramer reagent, and analyzed by flow cytometry. Controls were unstimulated cells; PBML from two patients never exposed to HA-1(H); immunoglobulin G isotype-matched controls. For all patients, acute rejection rates posttransplant was ascertained. Long-term data was available for 36 patients.

RESULTS AND CONCLUSIONS

There was no difference in acute rejection rates between the HA-1-matched and -mismatched groups, but there was a significant difference in chronic rejection rates, evidenced by increased graft failures during the follow-up period (P = .0024). Lymphocytes from five HA-1-mismatched recipients were stimulated in vitro with cells from HLA-A2 and DR-matched but HA-1-mismatched surrogate donor. Though there seemed to be an excess of tetramer-positive cells, anti-HA-1-specific CTL responses were not conclusively elicited in vitro.

Intercellular transfer of MHC and immunological molecules: molecular mechanisms and biological significance

The intercellular transfer of many molecules, including the major histocompatibility complexes (MHC), both class I and II, costimulatory and adhesion molecules, extracellular matrix organization molecules as well as chemokine, viral and complement receptors, has been observed between cells of the immune system. In this review, we aim to summarize the findings of a large body of work, highlight the molecules transferred and how this is achieved, as well as the cells capable of acquiring molecules from other cells. Although a physiological role for this phenomenon has yet to be established we suggest that the exchange of molecules between cells may influence the immune system with respect to immune amplification as well as regulation and tolerance. We will discuss why this may be the case and highlight the influence intercellular transfer of MHC molecules may have on allorecognition and graft 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.

Indirect recognition of T-cell epitopes derived from the alpha 3 and transmembrane domain of HLA-A2

Indirect allorecognition has been implicated in the mechanism of chronic rejection and alloantibody formation but precise definition of the epitopes involved has been limited. We have undertaken a detailed assessment of the antigenic properties of peptides derived from HLA-A2. Candidate epitopes were identified in vitro by assessment of MHC class II binding. The immune response to these epitopes was determined in patients awaiting a renal transplant by the assessment of PBMC activation using gamma-interferon ELISPOT. Twenty-two of fifty-five patients responded to peptides from HLA-A2 and this was associated with but not confined to those who had made antibody to HLA-A2 (14/18). Nineteen of twenty-two patients responded to peptides derived from the hypervariable alpha1 and alpha2 domains and 18/22 responded to peptides from the alpha 3 and transmembrane domain, the sequences of which show little polymorphism. In six patients, the sequence of these peptides was identical to self, that is, the response was autoimmune. The finding of indirect epitopes derived from regions of MHC class I that exhibit little polymorphism provides a novel perspective on the immune response to alloantigen and has potential implications for the development of specific therapies.

Tolerance in Heart Transplantation: The Holy Grail, or an Attainable Goal?

The father of cardiac transplantation, Norman Shumway, famously predicted that tolerance was the future of the field, and always would be. Although his prediction remains true to date, significant progress has been made toward this goal, the "Holy Grail" for transplant clinicians. Current efforts are fueled by disappointing long-term outcomes associated with chronic immunosuppression, and the promise that partial or complete tolerance will impact long-term results favorably. This article provides a clinical definition of tolerance primarily based on lessons learned from animal heart allograft models. It reviews several promising strategies for inducing tolerance and detecting its presence through the use of biomarkers in peripheral blood or the graft, and outlines a possible path toward making this vision a clinical reality.