The road to tolerance: renal transplant tolerance induction in nonhuman primate studies and clinical trials

An effective approach to prevent immune rejection of human ESC-derived allografts

Human embryonic stem cells (hESCs) hold great promise for cell therapy as a source of diverse differentiated cell types. One key bottleneck to realizing such potential is allogenic immune rejection of hESC-derived cells by recipients. Here, we optimized humanized mice (Hu-mice) reconstituted with a functional human immune system that mounts a vigorous rejection of hESCs and their derivatives. We established knockin hESCs that constitutively express CTLA4-Ig and PD-L1 before and after differentiation, denoted CP hESCs. We then demonstrated that allogenic CP hESC-derived teratomas, fibroblasts, and cardiomyocytes are immune protected in Hu-mice, while cells derived from parental hESCs are effectively rejected. Expression of both CTLA4-Ig, which disrupts T cell costimulatory pathways, and PD-L1, which activates T cell inhibitory pathway, is required to confer immune protection, as neither was sufficient on their own. These findings are instrumental for developing a strategy to protect hESC-derived cells from allogenic immune responses without requiring systemic immune suppression.

Influence of cytotoxic T-lymphocyte antigen-4 polymorphisms on acute rejection onset of cadaveric renal transplants

We retrospectively examined in cadaveric renal transplants the association between acute rejection episodes (ARE) and single nucleotide polymorphisms (SNPs) localized in the cytotoxic T-lymphocyte antigen (CTLA)-4 promoter, -1147T/C and -318C/T, in exon 1 +49A/G and within the 3' untranslated region (UTR) CT60G/A. Each one of these SNPs may influence the cell surface expression of the CTLA-4 molecule. Seventy-two cadaveric renal transplant recipients with at least 6 month's follow-up were genotyped for CTLA-4 dimorphisms using direct sequencing of specific polymerase chain reaction products. Allele frequencies in both groups of patients with or without acute rejection (ARE and non-ARE) did not show significant differences in various nucleotide positions. At the level of genotype frequency we first noted a positive association to acute rejection of G/G genotypes (ARE af = 14.7%, non-ARE af = 5.9%) for the +49 (cod. 17), which was associated with decreased expression of the CTLA-4 full-length molecule. In contrast, the AG genotype seemed to be protective (61.8% vs 32.4%, P = .028; odds ratio [OR] = 0.2961). Regarding the CT60G/A dimorphism, noteworthy was the identification of a significantly higher incidence of CT60 A/A genotype in ARE compared with non-ARE group (29.7% vs 8.6%; Yates P = .035; OR = 4.51). Such association of protective AA genotype with ARE, as observed also in autoimmunity, was associated with an increased level of sCTLA-4 induced by the polymorphism, which blocks B7-flCTLA-4 interactions, enhancing T-cell reactivity by preventing transduction of inhibitory signals. Considering the various polymorphic sites in the haplotype, we observed a significant increase in ARE among patients of the CTLA4 +49A/CT60A (HF = 51.5% vs 29.5%; P = .014; OR = 2.545) and a reduction among the +49A/CT60G (17.6% vs 33.8%; P = .04; OR = 0.4193) 2-loci haplotype, As regards the -1147/-318/+49/CT60 CTLA-4 4-loci haplotypes, we observed a significantly higher frequency of the CCAA haplotype in ARE patients comparison with those free of rejection (HF = 51.8% vs 31.1%, P = .046 OR = 2.363). These findings are consistent with those observed in allogeneic stem cell transplantation, wherein patients with CT60 AA showed a major incidence of graft-versus-host disease. An association of protective AA genotype with ARE, as observed also in autoimmunity was associated with an increased level of sCTLA-4 induced by this polymorphism, which blocking the B7-flCTLA-4 interaction, would enhance T-cell reactivity by preventing transduction of inhibitory signals.

MHC-mismatched chimerism is required for induction of transplantation tolerance in autoimmune nonobese diabetic recipients

In nonautoimmune recipients, induction of mixed and complete chimerism with hematopoietic progenitor cells from MHC (HLA)-matched or -mismatched donors are effective approaches for induction of organ transplantation immune tolerance in both animal models and patients. But it is still unclear whether this is the case in autoimmune recipients. With the autoimmune diabetic NOD mouse model, we report that, although mixed and complete MHC-mismatched chimerism provide immune tolerance to donor-type islet and skin transplants, neither mixed nor complete MHC-matched chimerism does. The MHC-mismatched chimerism not only tolerizes the de novo developed, but also the residual pre-existing host-type T cells in a mismatched MHC class II-dependent manner. In the MHC-mismatched chimeras, the residual host-type peripheral T cells appear to be anergic with upregulation of PD-1 and downregulation of IL-7Rα. Conversely, in the MHC-matched chimeras, the residual host-type peripheral T cells manifest both alloreactivity and autoreactivity; they not only mediate insulitis and sialitis in the recipient, but also reject allogeneic donor-type islet and skin grafts. Interestingly, transgenic autoreactive BDC2.5 T cells from Rag1(+/+), but not from Rag1(-/-), NOD mice show alloreactivity and mediate both insulitis and rejection of allografts. Taken together, MHC-mismatched, but not MHC-matched, chimerism can effectively provide transplantation immune tolerance in autoimmune recipients.

Requirement of B7-H1 in mesenchymal stem cells for immune tolerance to cardiac allografts in combination therapy with rapamycin

BACKGROUND

The potential of mesenchymal stem cells (MSCs) for immunosuppression has been tested in transplantation, but its mechanisms are not fully understood. This study investigated the role of MSC-expressing B7-H1 in the induction of immune tolerance to cardiac allografts by the combination therapy of MSCs and rapamycin (RAPA).

METHODS

The anti-alloimmunity of donor MSCs in the presence or absence of RAPA was examined in both mouse cardiac allograft model (C57BL/6 to BALB/c mice) and a variety of cultured immune cells. Immunohistochemical staining was used for the measurement of intragraft antibody deposition, and fluorescence-activated cell sorting (FACS) for the determination of serum alloantibodies and leukocyte phenotypes.

RESULTS

B7-H1 expression in cultured MSCs was up-regulated following IFN-γ stimulation. In transplant recipients, combination therapy of MSCs and RAPA induced immune tolerance to allografts, but blockade of B7-H1 on MSCs with monoclonal antibody abrogated the combination therapy-induced immune tolerance as heart allografts were rejected. The negative effect of MSC-expressing B7-H1 neutralization on graft survival was correlated with a reduction of regulatory immune cells (CD4(+)CD25(+)Foxp3(+) T cells, tolerogenic dendritic cells and IL-4(high)IL-10(High)CD83(low) B cells), and also with an increase in alloantibody (IgG and IgM) levels both inside the grafts and in the circulation as compared with un-neutralized controls. In vitro MSC-mediated suppression of antibody production and B cell proliferation depended on B7-H1 function and cell contact between CD19(+) B cells and MSCs.

CONCLUSION

These data suggest that MSC-expressing B7-H1 mediates the immune tolerance to cardiac allografts in recipients receiving MSC and RAPA combination therapy.

The influence of CTLA-4 gene polymorphism on long-term kidney allograft function in Caucasian recipients

The aim of the study was to examine whether CTLA-4 (CD152) and CD28 gene polymorphisms affect the outcome of kidney transplantation (KTx). Polymorphisms of the CTLA-4 gene (-318 C>T, +49 A>G, and the microsatellite polymorphism in the 3'UTR of exon 4 (AT)(n)) and a CD28 gene (IVS3 +17T>C) were investigated in 314 allograft recipients with a mean age of 41.9+/-12 years. The median time since KTx was 97.5 months. The genotypes of the SNPs were determined by SSP-PCR and (AT)(n) genotype by PCR and capillary electrophoresis (ABI Prism 310). In general, no relationship was found between the allele variants and acute rejection or graft function. Univariate and multivariate analyses showed no influence of CTLA-4 or CD28 polymorphism on graft/patient survival. In the individuals bearing the combination of the homozygous variant of low AT repeat number (82 bp) and the homozygous variant A (adenine) in CTLA-4 +49 A>G, higher eGFR was observed at one year after KTx, which was also maintained at 10 years. In summary, 24.2% of the studied patients carrying the "favorable" CTLA-4 genotype exhibited significantly higher allograft function than the 16.9% recipients with the "unfavorable" genotype up to 10 years post transplantation.

Induction of mixed chimerism in mice by employing different conditioning protocols and bone marrow cell transplantation

Mixed chimerism has been suggested to produce allograft tolerance. Since this phenomenon is not fully understood, the aim of our study was to evaluate various protocols for chimerism induction in a mouse model. B6.SJL-Ptprc(a)Pep3(b) mice were injected with 20 to 30 x 10(6) bone marrow cells from Balb C mice. Conditioning consisted of total body gamma irradiation with 9.5, 5, and 3 Gy on "-1 day" of the experiment, with 200 mg/kg cyclophosphamide (CP) ("+2 day"). Additionally, one group of mice received blocking antibody against CD40L on days 0, 1, 4, and 7. The presence of mixed chimerism in peripheral blood was assessed at 1, 2, 3, 4, 6, and 8 weeks using flow cytometry to detect CD45.1 or CD45.2 antigen expression. Moreover, the chimerism was examined in CD4, CD8, CD45/B220, Mac-1alpha subpopulations in peripheral blood and bone marrow cells at 8 weeks. We also compared chimerism in peripheral blood, bone marrow, and spleen leukocyte populations. We observed that the most effective conditioning method with relatively low toxicity was based on concomitant use of 5 Gy total body irradiation and CP. The percentage of donor cells differed among peripheral blood subpopulations and bone marrow cells, but was similar in leukocyte populations derived from various sources. Our experiments sought to optimize the induction of stable mixed chimerism.

Dissociation between peripheral blood chimerism and tolerance to hindlimb composite tissue transplants: preferential localization of chimerism in donor bone

BACKGROUND

Mixed chimerism induces donor-specific tolerance to composite tissue allotransplants (CTAs). In the present studies, we used a nonmyeloablative conditioning approach to establish chimerism and promote CTA acceptance.

METHODS

Wistar Furth (RT1A(u)) rats were conditioned with 600 to 300 cGy total body irradiation (TBI, day-1), and 100 x 10(6) T-cell-depleted ACI (RT1A(abl)) bone marrow cells were transplanted on day 0, followed by a 11-day course of tacrolimus and one dose of antilymphocyte serum (day 10). Heterotopic osteomyocutaneous flap transplantation was performed 4 to 6 weeks after bone marrow transplantation.

RESULTS

Mixed chimerism was initially achieved in almost all recipients, but long-term acceptance of CTA was only achieved in rats treated with 600 cGy TBI. When anti-alphabeta-T-cell receptor (TCR) monoclonal antibody (mAb) (day-3) was added into the regimens, donor chimerism was similar to recipients preconditioned without anti-alphabeta-TCR mAb. However, the long-term CTA survival was significantly improved in chimeras receiving more than or equal to 300 cGy TBI plus anti-alphabeta-TCR mAb. Higher levels of donor chimerism were associated with CTA acceptance. The majority of flap acceptors lost peripheral blood chimerism within 6 months. However, donor chimerism persisted in the transplanted bone at significantly higher levels compared with other hematopoietic compartments. The compartment donor chimerism may be responsible for the maintenance of tolerance to CTA. Long-term acceptors were tolerant to a donor skin graft challenge even in the absence of peripheral blood chimerism.

CONCLUSIONS

Mixed chimerism established by nonmyeloablative conditioning induces long-term acceptance of CTA, which is associated with persistent chimerism preferentially in the transplanted donor bone.

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.

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

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

Short-term cyclosporine therapy and cotransplantation of donor splenocytes: effects on graft rejection and survival rates in pigs subjected to renal transplantation

BACKGROUND

Donor-specific allogeneic loading can prolong the survival of solid organ transplants by inducing a state known as acceptance. Several populations of cells are known to be involved in this process, but their exact roles have yet to be defined. The aim of this study was to assess the effects of portal-vein transfusion of donor-specific splenocytes (DST) after short-term cyclosporine A (CyA) therapy in pigs subjected to renal transplantation.

METHODS

Four groups of unrelated swine underwent renal transplantation with removal of the native kidneys. Antirejection protocols consisted in portal-vein DST (3 x 10(8) cells/kg) (Group 2, n = 7); intravenous CyA (9 mg/kg/d) on postoperative days 1-12 (Group 3, n = 14); and DST + CyA (as described above) (Group 4, n = 13). Results (through postoperative day 90) were compared with those obtained in untreated control recipients (Group 1, n = 7).

RESULTS

Compared with animals of Groups 1, 2, and 3, Group 4 recipients presented significantly longer survival (mean: 90 days, P < 0.01 in Kaplan-Meier analysis) and better renal function (P < 0.05). Graft histology revealed preserved parenchyma.

CONCLUSION

The role of spleen cells in the immune response has probably been underestimated. Cotransplantation of donor splenocytes seems to induce a certain degree of acceptance toward the renal allograft. The route of administration (portal-vein infusion in this study) may be crucial for developing favorable mechanisms of recognition.

CD154 blockade and donor-specific transfusions in DLA-identical marrow transplantation in dogs conditioned with 1-Gy total body irradiation

Stable mixed donor/host chimerism has been reliably established in dogs given a sublethal dose (2 Gy) of total body irradiation (TBI) before and immunosuppression with mycophenolate mofetil (MMF) or rapamycin combined with cyclosporine (CSP) after marrow transplantation from dog leukocyte antigen (DLA)-identical littermates (hematopoietic cell transplantation [HCT]). When TBI was reduced to 1 Gy, only transient engraftment was observed. Here we investigated whether stable engraftment after 1-Gy TBI could be accomplished by reducing host-versus-donor immune responsiveness through preceding CD154 blockade and infusion of donor peripheral blood mononuclear cells (PBMCs). We found that the anti-human CD154 antibody, 5c8, cross-reacted with canine lymphocytes and blocked alloimmune responses in vitro. Based on pharmacokinetic studies, 6 dogs received a single intravenous injection of 5 mg/kg anti-CD154 antibody (on day −5), followed 1 day later by donor PBMCs. On day 0, the dogs were given 1 Gy of TBI and underwent DLA-identical marrow grafts. Postgraft immunosuppression consisted of MMF and CSP. All 6 dogs demonstrated initial engraftment; 3 dogs sustained the engraftment for >26 weeks, whereas 3 dogs rejected their grafts, after 9, 22, and 24 weeks, and survived with autologous recovery. Graft survival was significantly improved over that in 11 historical controls conditioned with 1-Gy TBI and given either MMF or rapamycin with CSP after HCT, all of which rejected their grafts between 3 and 12 weeks (P = .03). Preceding donor PBMC infusion and CD154 blockade improved survival of DLA-identical marrow grafts after 1-Gy TBI.

Chronic allograft nephropathy in paediatric renal transplantation

Chronic allograft nephropathy (CAN) is now the leading cause of renal transplant loss in paediatric transplant recipients. Despite improvements in immunosuppression, which have significantly reduced the incidence of acute rejection, the rates of chronic kidney loss have remained unchanged in paediatric transplant patients over the last 20 years. Chronic allograft nephropathy is a pathological diagnosis of which the key features are tubular atrophy and interstitial fibrosis. More consistent definitions and grading of these through the Banff classification have allowed more rigorous study of the development of chronic allograft nephropathy along with further identification of specific lesions associated with the underlying aetiologies. While initially thought to be primarily due to immune injury, it is now evident that CAN is the end result of a variety of immune and non-immune injuries including ischaemia reperfusion injury, calcineurin inhibitor (CNI) toxicity and infections. Protocol biopsy studies have demonstrated rates of CAN development in children similar to those in adults with comparable underlying pathological processes. This review outlines the current knowledge of CAN within the context of paediatric renal transplantation.

From current immunosuppressive strategies to clinical tolerance of allografts

In order to prevent allograft rejection, most current immunosuppressive drugs nonspecifically target T-cell activation, clonal expansion or differentiation into effector cells. Experimental models have shown that it is possible to exploit the central and peripheral mechanisms that normally maintain immune homeostasis and tolerance to self-antigens, in order to induce tolerance to alloantigens. Central tolerance results from intrathymic deletion of T cells with high avidity for thymically expressed antigens. Peripheral tolerance to nonself-molecules can be achieved by various mechanisms including deletion of activated/effector T cells, anergy induction and active regulation of effector T cells. In this article, we briefly discuss the pathways of allorecognition and their relevance to current immunosuppressive strategies and to the induction of transplantation tolerance (through haematopoietic mixed chimerism, depleting protocols, costimulatory blockade and regulatory T cells). We then review the prospect of clinical applicability of these protocols in solid organ transplantation.

Costimulation blockade and its possible future use in clinical transplantation

The nonimmune effects of currently used immunosuppressive drugs result in a high incidence of late graft loss due to nephrotoxicity and death of patients. As an immune-specific alternative to conventional immunosuppressants, new biotechnology tools can be used to block the costimulation signals of T-cell activation. Many experimental studies--particularly preclinical studies in nonhuman primates--have focused on blocking the 'classical' B7/CD28 and CD40/CD40L pathways, which are critical in primary T-cell activation. Here, we review the limitations, the recent advances and the first large-scale clinical application of the CTLA4-Ig fusion protein to block the B7/CD28 costimulation pathway. We also focus on new B7/CD28 and tumor necrosis factor (TNF)/TNF-R family costimulatory molecules that can deliver positive or negative costimulation signals regulating the alloimmune response. Strategies that use single agents to block costimulation have often proved to be insufficient. Given the diversity of the different costimulation molecules, future strategies for human transplantation may involve the simultaneous blockade of several selected pathways or the simultaneous use of conventional immunosuppressants.

Transplant Tolerance: Converging on a Moving Target

Enthusiasm for tolerance induction has been tempered by the realization that it is more difficult to achieve clinically than was predicted by experimental models. Unlike the view that the immune response to an allograft is ordered and thus predictable, we view alloimmunity as highly plastic and molded by previous and ongoing experiences with allogeneic and environmental antigens. This implies that an individual's response to an allograft changes over time and that responses of seemingly similar individuals may vary greatly. This variability highlights the need to develop assays for monitoring the recipient immune response as well as individualized methods for therapeutic immune modulation.

Transfer of regulatory T cells generated ex vivo modifies graft rejection through induction of tolerogenic CD4+CD25+ cells in the recipient

Certain CD4+CD25+ T cells can induce and maintain T-cell non-responsiveness to donor alloantigens and have therapeutic potential in solid organ transplantation. Peripheral CD4+CD25- cells alloactivated with IL-2 and transforming growth factor beta (TGF-beta) ex vivo express the transcription factor FoxP3, and become potent antigen-specific CD4+CD25- suppressor cells. Here we report that the transfer of TGF-beta-induced regulatory CD4+ and CD8+ T cells (Tregs) co-incident with transplantation of a histoincompatible heart resulted in extended allograft survival. To account for this result, we injected non-transplanted mice with a single dose of CD4+ and CD8+ Tregs and transferred donor cells every 2 weeks to mimic the continuous stimulation of a transplant. We observed increased splenic CD4+CD25+ cells that were of recipient origin. These cells rendered the animals non-responsive to donor alloantigens by an antigen-specific and cytokine-dependent mechanism of action. Both the increased number of CD4+CD25+ cells and their tolerogenic effect were dependent on continued donor antigen boosting. Thus, Tregs generated ex vivo can act like a vaccine that generates host suppressor cells with the potential to protect MHC-mismatched organ grafts from rejection.

Microchimerism maintains deletion of the donor cell-specific CD8+ T cell repertoire

Rare cases of stable allograft acceptance after discontinuation of immunosuppression are often accompanied by macrochimerism (> 1% donor cells in blood) or microchimerism (< 1% donor cells in blood). Here, we have investigated whether persistence of donor cells is the cause or the consequence of long-lasting CTL unresponsiveness. We found that engraftment of splenocytes bearing a single foreign MHC class I-restricted epitope resulted in lifelong donor cell microchimerism and specific CTL unresponsiveness. This status was reversed in a strictly time- and thymus-dependent fashion when the engrafted cells were experimentally removed. The results presented herein show that microchimerism actively maintains CTL unresponsiveness toward a minor histocompatibility antigen by deleting the specific repertoire and thus excluding dominant, T cell extrinsic mechanisms of CTL unresponsiveness independent of systemically persisting donor cell antigen.

Possible Association of Cytotoxic T-Lymphocyte Antigen 4 Gene Promoter Single Nucleotide Polymorphism With Acute Rejection of Allogeneic Kidney Transplant

Cytotoxic T-lymphocyte antigen 4 (CTLA-4) molecule is an important inhibitor of T-lymphocyte response. Polymorphisms in the CTLA-4 gene have been described to be associated with numerous autoimmune diseases. However, similar studies in solid organ transplantation have been scarce. Therefore, we examined the distribution of three single nucleotide dimorphisms, namely, -1147T/C, -318C/T, and +49A/G, in two groups of allogeneic kidney graft recipients: (1) those with at least one acute rejection episode ("rejectors"; n = 38) and (2) those with no signs of acute rejection ("nonrejectors"; n = 53). Allele frequencies in both groups of patients were similar in two positions, -1147T/C and +49A/G. However, rejectors showed slight differences from nonrejectors for allele and genotype frequencies in position -318. The -318T allele was two times less frequent among rejectors than nonrejectors, a difference that was close to statistical significance (P = .039; P corrected = .0583), and may reach it when greater numbers of patients are tested.

T cell tolerance induced by therapeutic antibodies

Ever since the discovery of Medawar, over 50 years ago, that immunological tolerance was an acquired phenomenon that could be manipulated in neonatal mice, the ability to induce therapeutic tolerance against autoantigens, allergens and organ grafts has been a major driving force in immunology. Within the last 20 years we have found that a brief treatment with monoclonal antibodies that block certain functional molecules on the surface of the T cell is able to reprogramme the established immune repertoire of the adult mouse, allowing indefinite acceptance of allografts or effective curing of autoimmune diseases. We are only now just beginning to define many of the regulatory mechanisms that induce and maintain the tolerant state with the aim of being able to safely and reliably apply these technologies to human clinical situations.