Oral alloantigen exposure promotes donor-specific tolerance in a mouse model of minor-mismatched skin transplantation

Oral antigen exposure is a powerful, non-invasive route to induce immune tolerance to dietary antigens, and has been modestly successful at prolonging graft survival in rodent models of transplantation. To harness the mechanisms of oral tolerance for promoting long-term graft acceptance, we developed a mouse model where the antigen ovalbumin (OVA) was introduced orally prior to transplantation with skin grafts expressing OVA. Oral OVA treatment pre-transplantation promoted permanent graft acceptance and linked tolerance to skin grafts expressing OVA fused to the additional antigen 2W. Tolerance was donor-specific, as secondary donor-matched, but not third-party allografts were spontaneously accepted. Oral OVA treatment promoted an anergic phenotype in OVA-reactive CD4+ and CD8+ conventional T cells (Tconvs) and expanded OVA-reactive Tregs pre-transplantation. However, skin graft acceptance following oral OVA resisted partial depletion of Tregs and blockade of PD-L1. Mechanistically, we revealed a role for the proximal gut draining lymph nodes (gdLNs) in mediating this effect, as an intestinal infection that drains to the proximal gdLNs prevented tolerance induction. Our study extends previous work applying oral antigen exposure to transplantation and serves as proof of concept that the systemic immune mechanisms supporting oral tolerance are sufficient to promote long-term graft acceptance.

Genetically targeting the BATF family transcription factors BATF and BATF3 in the mouse abrogates effector T cell activities and enables long-term heart allograft survival

T cells must be activated and become effectors first before executing allograft rejection, a process that is regulated by diverse signals and transcription factors. In this study, we studied the basic leucine zipper ATF-like transcription factor (BATF) family members in regulating T cell activities in a heart transplant model and found that mice deficient for both BATF and BATF3 (Batf^-/- Batf3^-/- mice) spontaneously accept the heart allografts long-term without tolerizing therapies. Similarly, adoptive transfer of wild type T cells into Rag1^-/- hosts induced prompt rejection of heart and skin allografts, whereas the Batf^-/- Batf3^-/- T cells failed to do so. Analyses of graft-infiltrating cells showed that Batf^-/- Batf3^-/- T cells infiltrate the graft but fail to acquire an effector phenotype (CD44^high KLRG1⁺ ). Co-transfer experiments in a T cell receptor transgenic TEa model revealed that the Batf^-/- Batf3^-/- T cells fail to expand in vivo, retain a quiescent phenotype (CD62L⁺ CD127⁺ ), and unable to produce effector cytokines to alloantigen stimulation, which contrasted sharply to that of wild type T cells. Together, our data demonstrate that the BATF and BATF3 are critical regulators of T effector functions, thus making them attractive targets for therapeutic interventions in transplant settings.

Donor plasmacytoid dendritic cells modulate effector and regulatory T cell responses in mouse spontaneous liver transplant tolerance

We assessed the role of donor liver non-conventional plasmacytoid dendritic cells (pDCs) in spontaneous liver transplant tolerance in a fully MHC-mismatched (C57BL/6 (H2^b ) to C3H (H2^k )) mouse model. Compared with spleen pDCs, liver pDCs expressed higher levels of DNAX-activating protein of 12 kDa and its co-receptor, triggering receptor expressed by myeloid cells 2, and higher ratios of programed death ligand-1 (PD-L1):costimulatory CD80/CD86 in the steady state and after Toll-like receptor 9 ligation. Moreover, liver pDCs potently suppressed allogeneic CD4⁺ and CD8⁺ T cell proliferative responses. Survival of pDC-depleted livers was much poorer (median survival time: 25 days) than that of either untreated donor livers or pDC-depleted syngeneic donor livers that survived indefinitely. Numbers of forkhead box p3 (FoxP3)⁺ regulatory T cells in grafts and mesenteric lymph nodes of mice given pDC-depleted allogeneic livers were reduced significantly compared with those in recipients of untreated livers. Graft-infiltrating CD8⁺ T cells with an exhausted phenotype (programed cell death protein 1⁺ , T cell immunoglobulin and mucin domain-containing protein 3⁺ ) were also reduced in recipients of pDC-depleted livers. PD1-PD-L1 pathway blockade reversed the reduction in exhausted T cells. These novel observations link immunoregulatory functions of liver interstitial pDCs, alloreactive T cell exhaustion, and spontaneous liver transplant tolerance.

Distinct roles for major and minor antigen barriers in chimerism-based tolerance under irradiation-free conditions

Eliminating cytoreductive conditioning from chimerism-based tolerance protocols would facilitate clinical translation. Here we investigated the impact of major histocompatibility complex (MHC) and minor histocompatibility antigen (MiHA) barriers on mechanisms of tolerance and rejection in this setting. Transient depletion of natural killer (NK) cells at the time of bone marrow (BM) transplantation (BMT) (20 × 10⁶ BALB/c BM cells → C57BL/6 recipients under costimulation blockade [CB] and rapamycin) prevented BM rejection. Despite persistent levels of mixed chimerism, BMT recipients gradually rejected skin grafts from the same donor strain. Extending NK cell depletion did not improve skin graft survival. However, F1 (C57BL/6×BALB/c) donors, which do not elicit NK cell-mediated rejection, induced durable chimerism and tolerance. In contrast, if F1 donors with BALB/c background only were used (BALB/c×BALB.B), no tolerance was observed. In the absence of MiHA disparities (B10.D2 donors, MHC-mismatch only), temporal NK cell depletion established stable chimerism and tolerance. Conversely, MHC identical BM (BALB.B donors, MiHA mismatch only) readily engrafted without NK cell depletion but no skin graft tolerance ensued. Therefore, we conclude that under CB and rapamycin, MHC disparities provoke NK cell-mediated BM rejection in nonirradiated recipients whereas MiHA disparities do not prevent BM engraftment but impede skin graft tolerance in established mixed chimeras.

Dominant regulation of long-term allograft survival is mediated by microRNA-142

Organ transplantation is often lifesaving, but the long-term deleterious effects of combinatorial immunosuppression regimens and allograft failure cause significant morbidity and mortality. Long-term graft survival in the absence of continuing immunosuppression, defined as operational tolerance, has never been described in the context of multiple major histocompatibility complex (MHC) mismatches. Here, we show that miR-142 deficiency leads to indefinite allograft survival in a fully MHC mismatched murine cardiac transplant model in the absence of exogenous immunosuppression. We demonstrate that the cause of indefinite allograft survival in the absence of miR-142 maps specifically to the T cell compartment. Of therapeutic relevance, temporal deletion of miR-142 in adult mice prior to transplantation of a fully MHC mismatched skin allograft resulted in prolonged allograft survival. Mechanistically, miR-142 directly targets Tgfbr1 for repression in regulatory T cells (TREG ). This leads to increased TREG sensitivity to transforming growth factor - beta and promotes transplant tolerance via an augmented peripheral TREG response in the absence of miR-142. These data identify manipulation of miR-142 as a promising approach for the induction of tolerance in human transplantation.

IL-22 is required for the induction of bronchus-associated lymphoid tissue in tolerant lung allografts

Long-term survival after lung transplantation remains profoundly limited by graft rejection. Recent work has shown that bronchus-associated lymphoid tissue (BALT), characterized by the development of peripheral nodal addressin (PNAd)-expressing high endothelial venules and enriched in B and Foxp3+ T cells, is important for the maintenance of allograft tolerance. Mechanisms underlying BALT induction in tolerant pulmonary allografts, however, remain poorly understood. Here, we show that the development of PNAd-expressing high endothelial venules within intragraft lymphoid follicles and the recruitment of B cells, but not Foxp3+ cells depends on IL-22. We identify graft-infiltrating gamma-delta (γδ) T cells and Type 3 innate lymphoid cells (ILC3s) as important producers of IL-22. Reconstitution of IL-22 at late time points through retransplantation into wildtype hosts mediates B cell recruitment into lymphoid follicles within the allograft, resulting in a significant increase in their size, but does not induce PNAd expression. Our work has identified cellular and molecular requirements for the induction of BALT in pulmonary allografts during tolerance induction and may provide a platform for the development of new therapies for lung transplant patients.

Prolongation of allograft survival by passenger donor regulatory T cells

Tissue resident lymphocytes are present within many organs, and are presumably transferred at transplantation, but their impact on host immunity is unclear. Here, we examine whether transferred donor natural regulatory CD4 T cells (nT-regs) inhibit host alloimmunity and prolong allograft survival. Transfer of donor-strain lymphocytes was first assessed by identifying circulating donor-derived CD4 T cells in 21 consecutive human lung transplant recipients, with 3 patterns of chimerism apparent: transient, intermediate, and persistent (detectable for up to 6 weeks, 6 months, and beyond 1 year, respectively). The potential for transfer of donor nT-regs was then confirmed by analysis of leukocyte filters recovered from ex vivo normothermic perfusion circuits of human kidneys retrieved for transplantation. Finally, in a murine model of cardiac allograft vasculopathy, depletion of donor CD4 nT-regs before organ recovery resulted in markedly accelerated heart allograft rejection and augmented host effector antibody responses. Conversely, adoptive transfer or purified donor-strain nT-regs inhibited host humoral immunity and prolonged allograft survival, and more effectively so than following administration of recipient nT-regs. In summary, following transplantation, passenger donor-strain nT-regs can inhibit host adaptive immune responses and prolong allograft survival. Isolated donor-derived nT-regs may hold potential as a cellular therapy to improve transplant outcomes.

C5aR1 regulates migration of suppressive myeloid cells required for costimulatory blockade-induced murine allograft survival

Costimulatory blockade-induced murine cardiac allograft survival requires intragraft accumulation of CD11b⁺ Ly6C^lo Ly6G^- regulatory myeloid cells (Mregs) that expand regulatory T cells (Tregs) and suppress effector T cells (Teffs). We previously showed that C5a receptor (C5aR1) signaling on T cells activates Teffs and inhibits Tregs, but whether and/or how C5aR1 affects Mregs required for transplant survival is unknown. Although BALB/c hearts survived >60 days in anti-CD154 (MR1)-treated or cytotoxic T-lymphocyte associated protein 4 (CTLA4)-Ig-treated wild-type (WT) recipients, they were rejected at ~30 days in MR1-treated or CTLA4-Ig-treated recipients selectively deficient in C5aR1 restricted to myeloid cells (C5ar1^fl/fl xLysM-Cre). This accelerated rejection was associated with ~2-fold more donor-reactive T cells and ~40% less expansion of donor-reactive Tregs. Analysis of graft-infiltrating mononuclear cells on posttransplant day 6 revealed fewer Ly6C^lo monocytes in C5ar1^fl/fl xLysM-Cre recipients. Expression profiling of intragraft Ly6C^lo monocytes showed that C5aR1 deficiency downregulated genes related to migration/locomotion without changes in genes associated with suppressive function. Cotransfer of C5ar1^fl/fl and C5ar1^fl/fl xLysM-Cre myeloid cells into MR1-treated allograft recipients resulted in less accumulation of C5ar1^-/- cells within the allografts, and in vitro assays confirmed that Ly6C^hi myeloid cells migrate to C5a/C5aR1-initiated signals. Together, our results newly link myeloid cell-expressed C5aR1 to intragraft accumulation of myeloid cells required for prolongation of heart transplant survival induced by costimulatory blockade.

UV light crosslinking regresses mature corneal blood and lymphatic vessels and promotes subsequent high-risk corneal transplant survival

Immunologic graft rejection is the main complication after corneal transplant into pathologically prevascularized so-called high-risk eyes. The aim of this study was to evaluate whether ultraviolet (UV) light crosslinking can regress pathologic corneal blood and lymphatic vessels and thereby improve subsequent graft survival. Using the murine model of suture-induced corneal neovascularization, we found that corneal crosslinking with UVA light and riboflavin regressed both preexisting blood and lymphatic vessels significantly via induction of apoptosis in vascular endothelial cells. In addition, macrophages and CD45⁺ cell counts were significantly reduced. Consistently, corneal crosslinking reduced keratocyte density and corneal thickness without affecting corneal nonvascular endothelial cells, iris, and lens depending on the crosslinking duration. Furthermore, using the murine model of corneal transplant, long-term graft survival was significantly promoted (P < .05) and CD4⁺ CD25⁺ FoxP3⁺ T regulatory cells were upregulated (P < .01) in high-risk eyes preoperatively treated with crosslinking. Our results suggest UV light crosslinking as a novel method to regress both pathologic corneal blood and lymphatic vessels and to reduce CD45⁺ inflammatory cells. Furthermore, this study demonstrates for the first time that preoperative corneal crosslinking in prevascularized high-risk eyes can significantly improve subsequent graft survival and may become a promising novel therapy in the clinic.

Ex vivo allotransplantation engineering: Delivery of mesenchymal stem cells prolongs rejection-free allograft survival

Current pharmacologic regimens in transplantation prevent allograft rejection through systemic recipient immunosuppression but are associated with severe morbidity and mortality. The ultimate goal of transplantation is the prevention of allograft rejection while maintaining recipient immunocompetence. We hypothesized that allografts could be engineered ex vivo (after allotransplant procurement but before transplantation) by using mesenchymal stem cell-based therapy to generate localized immunomodulation without affecting systemic recipient immunocompetence. To this end, we evaluated the therapeutic efficacy of bone marrow-derived mesenchymal stem cells in vitro and activated them toward an immunomodulatory fate by priming in inflammatory or hypoxic microenvironments. Using an established rat hindlimb model for allotransplantation, we were able to significantly prolong rejection-free allograft survival with a single perioperative ex vivo infusion of bone marrow-derived mesenchymal stem cells through the allograft vasculature, in the absence of long-term pharmacologic immunosuppression. Critically, transplanted rats rejected a second, nonengineered skin graft from the same donor species to the contralateral limb at a later date, demonstrating that recipient systemic immunocompetence remained intact. This study represents a novel approach in transplant immunology and highlights the significant therapeutic opportunity of the ex vivo period in transplant engineering.

The effects of brain death and ischemia on tolerance induction are organ-specific

We have previously shown that 12 days of high-dose calcineurin inhibition induced tolerance in MHC inbred miniature swine receiving MHC-mismatched lung, kidney, or co-transplanted heart/kidney allografts. However, if lung grafts were procured from donation after brain death (DBD), and transplanted alone, they were rejected within 19-45 days. Here, we investigated whether donor brain death with or without allograft ischemia would also prevent tolerance induction in kidney or heart/kidney recipients. Four kidney recipients treated with 12 days of calcineurin inhibition received organs from donors rendered brain dead for 4 hours. Six heart/kidney recipients also treated with calcineurin inhibition received organs from donors rendered brain dead for 4 hours, 8 hours, or 4 hours with 4 additional hours of cold storage. In contrast to lung allograft recipients, all isolated kidney or heart/kidney recipients that received organs from DBD donors achieved long-term survival (>100 days) without histologic evidence of rejection. Proinflammatory cytokine gene expression was upregulated in lungs and hearts, but not kidney allografts, after brain death. These data suggest that the deleterious effects of brain death and ischemia on tolerance induction are organ-specific, which has implications for the application of tolerance to clinical transplantation.

The Effect of MHC Antigen Matching Between Donors and Recipients on Skin Tolerance of Vascularized Composite Allografts

The emergence of skin-containing vascularized composite allografts (VCAs) has provided impetus to understand factors affecting rejection and tolerance of skin. VCA tolerance can be established in miniature swine across haploidentical MHC barriers using mixed chimerism. Because the deceased donor pool for VCAs does not permit MHC antigen matching, clinical VCAs are transplanted across varying MHC disparities. We investigated whether sharing of MHC class I or II antigens between donors and recipients influences VCA skin tolerance. Miniature swine were conditioned nonmyeloablatively and received hematopoietic stem cell transplants and VCAs across MHC class I (n = 3) or class II (n = 3) barriers. In vitro immune responsiveness was assessed, and VCA skin-resident leukocytes were characterized by flow cytometry. Stable mixed chimerism was established in all animals. MHC class II-mismatched chimeras were tolerant of VCAs. MHC class I-mismatched animals, however, rejected VCA skin, characterized by infiltration of recipient-type CD8⁺ lymphocytes. Systemic donor-specific nonresponsiveness was maintained, including after VCA rejection. This study shows that MHC antigen matching influences VCA skin rejection and suggests that local regulation of immune tolerance is critical in long-term acceptance of all VCA components. These results help elucidate novel mechanisms underlying skin tolerance and identify clinically relevant VCA tolerance strategies.

Prevention of Allograft Rejection by Use of Regulatory T Cells With an MHC-Specific Chimeric Antigen Receptor

CD4⁺ CD25^high FOXP3⁺ regulatory T cells (Tregs) are involved in graft-specific tolerance after solid organ transplantation. However, adoptive transfer of polyspecific Tregs alone is insufficient to prevent graft rejection even in rodent models, indicating that graft-specific Tregs are required. We developed a highly specific chimeric antigen receptor that recognizes the HLA molecule A*02 (referred to as A2-CAR). Transduction into natural regulatory T cells (nTregs) changes the specificity of the nTregs without alteration of their regulatory phenotype and epigenetic stability. Activation of nTregs via the A2-CAR induced proliferation and enhanced the suppressor function of modified nTregs. Compared with nTregs, A2-CAR Tregs exhibited superior control of strong allospecific immune responses in vitro and in humanized mouse models. A2-CAR Tregs completely prevented rejection of allogeneic target cells and tissues in immune reconstituted humanized mice in the absence of any immunosuppression. Therefore, these modified cells have great potential for incorporation into clinical trials of Treg-supported weaning after allogeneic transplantation.

Tolerance of Vascularized Islet-Kidney Transplants in Rhesus Monkeys

We previously reported that transplantation (Tx) of prevascularized donor islets as composite islet-kidneys (IK) reversed diabetic hyperglycemia in both miniature swine and baboons. In order to enhance this strategy's potential clinical applicability, we have now combined this approach with hematopoietic stem cell (HSC) Tx in an attempt to induce tolerance in nonhuman primates. IKs were prepared by isolating islets from 70% partial pancreatectomies and injecting them beneath the autologous renal capsule of five rhesus monkey donors at least 3 months before allogeneic IK Tx. HSC Tx was performed after mobilization and leukapheresis of the donors and conditioning of the recipients with total body irradiation, T cell depletion, and cyclosporine. One IK was harvested for histologic analysis and four were transplanted into diabetic recipients. IK Tx was performed either 20-22 (n = 3) or 208 (n = 1) days after HSC Tx. All animals accepted IKs without rejection. All recipients required >20 U/day insulin before IK Tx to maintain <200 mg/dL, whereas after IK Tx, three animals required minimal doses of insulin (1-3 U/day) and one animal was insulin free. These results constitute a proof-of-principle that this IK tolerance strategy may provide a cure for both end-stage renal disease and diabetes without the need for immunosuppression.

Erosion of Transplantation Tolerance After Infection

Recent clinical studies suggest that operational allograft tolerance can be persistent, but long-term surviving allografts can be rejected in a subset of patients, sometimes after episodes of infection. In this study, we examined the impact of Listeria monocytogenes (Lm) infection on the quality of tolerance in a mouse model of heart allograft transplantation. Lm infection induced full rejection in 40% of tolerant recipients, with the remaining experiencing a rejection crisis or no palpable change in their allografts. In the surviving allografts on day 8 postinfection, graft-infiltrating cell numbers increased and exhibited a loss in the tolerance gene signature. By day 30 postinfection, the tolerance signature was broadly restored, but with a discernible reduction in the expression of a subset of 234 genes that marked tolerance and was down-regulated at day 8 post-Lm infection. We further demonstrated that the tolerant state after Lm infection was functionally eroded, as rejection of the long-term surviving graft was induced with anti-PD-L1 whereas the same treatment had no effect in noninfected tolerant mice. Collectively, these observations demonstrate that tolerance, even if initially robust, exists as a continuum that can be eroded following bystander immune responses that accompany certain infections.

Role of Bone Marrow Maturity, Insulin-Like Growth Factor 1 Receptor, and Forkhead Box Protein N1 in Thymic Involution and Rejuvenation

Thymic involution is associated with age-related changes of the immune system. Utilizing our innovative technique of transplantation of a thymus as an isolated vascularized graft in MHC-inbred miniature swine, we have previously demonstrated that aged thymi are rejuvenated after transplantation into juvenile swine. Here we have studied the role of insulin-like growth factor (IGF) and forkhead-box protein-N1 (FOXN1) as well as bone marrow (BM) in thymic rejuvenation and involution. We examined thymic rejuvenation and involution by means of histology and flow cytometry. Thymic function was assessed by the ability to induce tolerance of allogeneic kidneys. Aged thymi were rejuvenated in a juvenile environment, and successfully induced organ tolerance, while juvenile thymi in aged recipients involuted and had a limited ability to induce tolerance. However, juvenile BM inhibited the involution process of juvenile thymi in aged recipients. An elevated expression of both FOXN1 and IGF1 receptors (IGF-1R) was observed in juvenile thymi and rejuvenated thymi. Juvenile BM plays a role in promoting the local thymic milieu as indicated by its ability to inhibit thymic involution in aged animals. The expression of FOXN1 and IGF-1R was noted to increase under conditions that stimulated rejuvenation, suggesting that these factors are involved in thymic recovery.

Ex Vivo Costimulatory Blockade to Generate Regulatory T Cells From Patients Awaiting Kidney Transplantation

Short-term outcomes of kidney transplantation have improved dramatically, but chronic rejection and regimen-related toxicity continue to compromise overall patient outcomes. Development of regulatory T cells (Tregs) as a means to decrease alloresponsiveness and limit the need for pharmacologic immunosuppression is an active area of preclinical and clinical investigation. Nevertheless, the immunomodulatory effects of end-stage renal disease on the efficacy of various strategies to generate and expand recipient Tregs for kidney transplantation are incompletely characterized. In this study, we show that Tregs can be successfully generated from either freshly isolated or previously cryopreserved uremic recipient (responder) and healthy donor (stimulator) peripheral blood mononuclear cells using the strategy of ex vivo costimulatory blockade with belatacept during mixed lymphocyte culture. Moreover, these Tregs maintain a CD3(+) CD4(+) CD25(+) CD127(lo) surface phenotype, high levels of intracellular FOXP3 and significant demethylation of the FOXP3 Treg-specific demethylation region on allorestimulation with donor stimulator cells. These data support evaluation of this simple, brief Treg production strategy in clinical trials of mismatched kidney transplantation.

Clonal Deletion Established via Invariant NKT Cell Activation and Costimulatory Blockade Requires In Vivo Expansion of Regulatory T Cells

Recently, the immune-regulating potential of invariant natural killer T (iNKT) cells has attracted considerable attention. We previously reported that a combination treatment with a liposomal ligand for iNKT cells and an anti-CD154 antibody in a sublethally irradiated murine bone marrow transplant (BMT) model resulted in the establishment of mixed hematopoietic chimerism through in vivo expansion of regulatory T cells (Tregs). Herein, we show the lack of alloreactivity of CD8(+) T cells in chimeras and an early expansion of donor-derived dendritic cells (DCs) in the recipient thymi accompanied by a sequential reduction in the donor-reactive Vβ-T cell receptor repertoire, suggesting a contribution of clonal deletion in this model. Since thymic expansion of donor DCs and the reduction in the donor-reactive T cell repertoire were precluded with Treg depletion, we presumed that Tregs should preform before the establishment of clonal deletion. In contrast, the mice thymectomized before BMT failed to increase the number of Tregs and to establish CD8(+) T cell tolerance, suggesting the presence of mutual dependence between the thymic donor-DCs and Tregs. These results provide new insights into the regulatory mechanisms that actively promote clonal deletion.

Adoptive Transfer of Renal Allograft Tolerance in a Large Animal Model

Our recent studies in an inbred swine model demonstrated that both peripheral and intra-graft regulatory cells were required for the adoptive transfer of tolerance to a second, naïve donor-matched kidney. Here, we have asked whether both peripheral and intra-graft regulatory elements are required for adoptive transfer of tolerance when only a long-term tolerant (LTT) kidney is transplanted. Nine highly-inbred swine underwent a tolerance-inducing regimen to prepare LTT kidney grafts which were then transplanted to histocompatible recipients, with or without the peripheral cell populations required for adoptive transfer of tolerance to a naïve kidney. In contrast to our previous studies, tolerance of the LTT kidney transplants alone was achieved without transfer of additional peripheral cells and without strategies to increase the number/potency of regulatory T cells in the donor. This tolerance was systemic, since most subsequent, donor-matched challenge kidney grafts were accepted. These results confirm the presence of a potent tolerance-inducing and/or tolerance-maintaining cell population within LTT renal allografts. They suggest further that additional peripheral tolerance mechanisms, required for adoptive transfer of tolerance to a naïve donor-matched kidney, depend on peripheral cells that, if not transferred with the LTT kidney, require time to develop in the adoptive host.

Imaging Tolerance Induction in the Classic Medawar Neonatal Mouse Model: Active Roles of Multiple F1-Donor Cell Types

The immature immune system is uniquely susceptible to tolerance induction and thus an attractive target for immunomodulation strategies for organ transplantation. Newborn mice injected with adult semi-allogeneic lymphohematopoietic cells accept transplants without immunosuppressive drugs. Early in vivo/in situ events leading to neonatal tolerance remain poorly understood. Here, we show by whole body/organ imaging that injected cells home to lymphoid organs and liver where various F1-donor cell types selectively alter neonatal immunity. In host thymus, F1-donor dendritic cells (DC) interact with developing thymocytes and regulatory T cells suggesting a role in negative selection. In spleen and lymph nodes, F1-donor regulatory T/B cells associate with host alloreactive cells and by themselves prolong cardiac allograft survival. In liver, F1-donor cells give rise to albumin-containing hepatocyte-like cells. The neonatal immune system is lymphopenic, Th-2 immunodeviated and contains immature DC, suggesting susceptibility to regulation by adult F1-donor cells. CD8a T cell inactivation greatly enhances chimerism, suggesting that variable emerging neonatal alloreactivity becomes a barrier to tolerance induction. This comprehensive qualitative imaging study systematically shows contribution of multiple in vivo processes leading simultaneously to robust tolerance. These insights into robust tolerance induction have important implications for development of strategies for clinical application.

Kaempferol Promotes Transplant Tolerance by Sustaining CD4+FoxP3+ Regulatory T Cells in the Presence of Calcineurin Inhibitor

Calcineurin inhibitor cyclosporine is widely used as an immunosuppressant in clinic. However, mounting evidence has shown that cyclosporine hinders tolerance induction by dampening Tregs. Therefore, it is of paramount importance to overcome this pitfall. Kaempferol was reported to inhibit DC function. Here, we found that kaempferol delayed islet allograft rejection. Combination of kaempferol and low-dose, but not high-dose, of cyclosporine induced allograft tolerance in majority of recipient mice. Although kaempferol plus either dose of cyclosporine largely abrogated proliferation of graft-infiltrating T cells and their CTL activity, both proliferation and CTL activity in mice treated with kaempferol plus low-dose, but not high-dose, cyclosporine reemerged rapidly upon treatment withdrawal. Kaempferol increased CD4+FoxP3+ Tregs both in transplanted mice and in vitro, likely by suppressing DC maturation and their IL-6 expression. Reduction in Tregs by low dose of cyclosporine was reversed by kaempferol. Kaempferol-induced Tregs exhibited both allospecific and non-allospecific suppression. Administering IL-6 abrogated allograft tolerance induced by kaempferol and cyclosporine via diminishing CD4+FoxP3+ Tregs. Thus, for the first time, we demonstrated that kaempferol promotes transplant tolerance in the presence of low dose of cyclosporine, which allows for sufficient Treg generation while minimizing side effects, resulting in much-needed synergy between kaempferol and cyclosporine.

Severing corneal nerves in one eye induces sympathetic loss of immune privilege and promotes rejection of future corneal allografts placed in either eye

Less than 10% of corneal allografts undergo rejection even though HLA matching is not performed. However, second corneal transplants experience a threefold increase in rejection, which is not due to prior sensitization to histocompatibility antigens shared by the first and second transplants since corneal grafts are selected at random without histocompatibility matching. Using a mouse model of penetrating keratoplasty, we found that 50% of the initial corneal transplants survived, yet 100% of the subsequent corneal allografts (unrelated to the first graft) placed in the opposite eye underwent rejection. The severing of corneal nerves that occurs during surgery induced substance P (SP) secretion in both eyes, which disabled T regulatory cells that are required for allograft survival. Administration of an SP antagonist restored immune privilege and promoted graft survival. Thus, corneal surgery produces a sympathetic response that permanently abolishes immune privilege of subsequent corneal allografts, even those placed in the opposite eye and expressing a completely different array of foreign histocompatibility antigens from the first corneal graft.

High incidence of xenogenic bone marrow engraftment in pig-to-baboon intra-bone bone marrow transplantation

Previous attempts of α-1,3-galactocyltransferase knockout (GalTKO) pig bone marrow (BM) transplantation (Tx) into baboons have demonstrated a loss of macro-chimerism within 24 h in most cases. In order to achieve improved engraftment with persistence of peripheral chimerism, we have developed a new strategy of intra-bone BM (IBBM) Tx. Six baboons received GalTKO BM cells, with one-half of the cells transplanted into the bilateral tibiae directly and the remaining cells injected intravenously (IBBM/BM-Tx) with a conditioning immunosuppressive regimen. In order to assess immune responses induced by the combined IBBM/BM-Tx, three recipients received donor SLA-matched GalTKO kidneys in the peri-operative period of IBBM/BM-Tx (Group 1), and the others received kidneys 2 months after IBBM/BM-Tx (Group 2). Peripheral macro-chimerism was continuously detectable for up to 13 days (mean 7.7 days; range 3-13) post-IBBM/BM-Tx and in three animals, macro-chimerism reappeared at days 10, 14 and 21. Pig CFUs, indicating porcine progenitor cell engraftment, were detected in the host BM in four of six recipients on days 14, 15, 19 and 28. In addition, anti-pig unresponsiveness was observed by in vitro assays. GalTKO/pCMV-kidneys survived for extended periods (47 and 60 days). This strategy may provide a potent adjunct for inducing xenogeneic tolerance through BM-Tx.

TIM-1 signaling is required for maintenance and induction of regulatory B cells

Apart from their role in humoral immunity, B cells can exhibit IL-10-dependent regulatory activity (Bregs). These regulatory subpopulations have been shown to inhibit inflammation and allograft rejection. However, our understanding of Bregs has been hampered by their rarity, lack of a specific marker, and poor insight into their induction and maintenance. We previously demonstrated that T cell immunoglobulin mucin domain-1 (TIM-1) identifies over 70% of IL-10-producing B cells, irrespective of other markers. We now show that TIM-1 is the primary receptor responsible for Breg induction by apoptotic cells (ACs). However, B cells that express a mutant form of TIM-1 lacking the mucin domain (TIM-1(Δmucin) ) exhibit decreased phosphatidylserine binding and are unable to produce IL-10 in response to ACs or by specific ligation with anti-TIM-1. TIM-1(Δmucin) mice also exhibit accelerated allograft rejection, which appears to be due in part to their defect in both baseline and induced IL-10(+) Bregs, since a single transfer of WT TIM-1(+) B cells can restore long-term graft survival. These data suggest that TIM-1 signaling plays a direct role in Breg maintenance and induction both under physiological conditions (in response to ACs) and in response to therapy through TIM-1 ligation. Moreover, they directly demonstrate that the mucin domain regulates TIM-1 signaling.

Superiority of rapamycin over tacrolimus in preserving nonhuman primate Treg half-life and phenotype after adoptive transfer

Many critical issues remain concerning how best to deploy adoptive regulatory T cell (Treg) immunotherapy to the clinic. These include a determination of their pharmacokinetic characteristics, their optimal dose, their phenotypic stability and the best therapies with which to pair Tregs. By performing a CFSE-labeled autologous Treg pulse experiment, we determined that the accessible peripheral blood Treg pool in rhesus macaques is quite large (75 ± 11 × 10(6) Tregs/kg). Pharmacokinetic analysis revealed that Tregs have two phases of elimination: an α phase, with a T1/2 in the peripheral blood of 32.4 ± 11.3 h and a β phase with a T1/2 of 120.4 ± 19.7 h. In addition to their short initial half-life, Tregs underwent rapid phenotypic shifts after infusion, with significant loss of both CD25 and FoxP3 by day +6. While tacrolimus stabilized CD25 expression, it did not improve T1/2 , nor mitigate the loss of FoxP3. In contrast, rapamycin significantly stabilized both CD25 and FoxP3, and supported an increased half-life, with an α phase of 67.7 ± 6.9 h and a β phase of 252.1 ± 54.9 h. These results suggest that rapamycin may be a necessary addition to Treg immunotherapy, and that tacrolimus may be deleterious to Treg integrity posttransfer.

Bone marrow-derived hematopoietic stem and progenitor cells infiltrate allogeneic and syngeneic transplants

Lineage (CD3e, CD11b, GR1, B220 and Ly-76) negative hematopoietic stem cells (HSCs) and hematopoietic progenitor cells (HPCs) infiltrate islet allografts within 24 h posttransplantation. In fact, lineage(negative) Sca-1(+) cKit(+) ("LSK") cells, a classic signature for HSCs, were also detected among these graft infiltrating cells. Lineage negative graft infiltrating cells are functionally multi-potential as determined by a standard competitive bone marrow transplant (BMT) assay. By 3 months post-BMT, both CD45.1 congenic, lineage negative HSCs/HPCs and classic "LSK" HSCs purified from islet allograft infiltrating cells, differentiate and repopulate multiple mature blood cell phenotypes in peripheral blood, lymph nodes, spleen, bone marrow and thymus of CD45.2 hosts. Interestingly, "LSK" HSCs also rapidly infiltrate syngeneic islet transplants as well as allogeneic cardiac transplants and sham surgery sites. It seems likely that an inflammatory response, not an adaptive immune response to allo-antigen, is responsible for the rapid infiltration of islet and cardiac transplants by biologically active HSCs/HPCs. The pattern of hematopoietic differentiation obtained from graft infiltrating HSCs/HPCs, cells that are recovered from inflammatory sites, as noted in the competitive BMT assay, is not precisely the same as that of intramedullary HSCs. This does not refute the obvious multi-lineage potential of graft infiltrating HSCs/HPCs.

The mitochondrial protein TCAIM regulates activation of T cells and thereby promotes tolerance induction of allogeneic transplants

Primary T cell activation and effector cell differentiation is required for rejection of allogeneic grafts in naïve recipients. It has become evident, that mitochondria play an important role for T cell activation. Expression of several mitochondrial proteins such as TCAIM (T cell activation inhibitor, mitochondrial) is down-regulated upon T cell receptor triggering. Here we report that TCAIM inhibited spontaneous development of memory and effector T cells. CD4(+) T cells from Tcaim knock-in (KI) mice showed reduced activation, cytokine secretion and proliferation in vitro. Tcaim KI T cells tolerated allogeneic skin grafts upon transfer into Rag-1 KO mice. CD4(+) and CD8(+) T cells from these mice did not infiltrate skin grafts and kept a naïve or central memory phenotype, respectively. They were unable to acquire effector phenotype and functions. TCAIM altered T cell activation-induced mitochondrial distribution and reduced mitochondrial reactive oxygen species (mROS) production. Thus, TCAIM controls T cell activation and promotes tolerance induction probably by regulating TCR-mediated mitochondrial distribution and mROS production.

Use of CTLA4Ig for induction of mixed chimerism and renal allograft tolerance in nonhuman primates

We have previously reported successful induction of renal allograft tolerance via a mixed chimerism approach in nonhuman primates. In those studies, we found that costimulatory blockade with anti-CD154 mAb was an effective adjunctive therapy for induction of renal allograft tolerance. However, since anti-CD154 mAb is not clinically available, we have evaluated CTLA4Ig as an alternative agent for effecting costimulation blockade in this treatment protocol. Two CTLA4Igs, abatacept and belatacept, were substituted for anti-CD154 mAb in the conditioning regimen (low dose total body irradiation, thymic irradiation, anti-thymocyte globulin and a 1-month posttransplant course of cyclosporine [CyA]). Three recipients treated with the abatacept regimen failed to develop comparable lymphoid chimerism to that achieved with anti-CD154 mAb treatment and these recipients rejected their kidney allografts early. With the belatacept regimen, four of five recipients developed chimerism and three of these achieved long-term renal allograft survival (>861, >796 and >378 days) without maintenance immunosuppression. Neither chimerism nor long-term allograft survival were achieved in two recipients treated with the belatacept regimen but with a lower, subtherapeutic dose of CyA. This study indicates that CD28/B7 blockade with belatacept can provide a clinically applicable alternative to anti-CD154 mAb for promoting chimerism and renal allograft tolerance.

Abrogation of renal allograft tolerance in MGH miniature swine: the role of intra-graft and peripheral factors in long-term tolerance

We have previously demonstrated that long-term tolerance (LTT) of an MHC class-I mismatched renal allograft can be achieved with a short course of cyclosporine. In order to examine regulatory mechanisms underlying tolerance in this model, we assessed the contributions of factors within the graft and in the peripheral blood for their relative roles in the maintenance of stable tolerance. Twelve LTT recipients of MHC class-I mismatched primary kidneys were subjected to a treatment consisting of donor-specific transfusion followed by leukapheresis, in order to remove peripheral leukocytes, including putative regulatory T cells (Tregs). Following treatment, 2 controls were followed clinically and 10 animals had the primary graft removed and received a second, donor-MHC-matched kidney. Neither control animal showed evidence of rejection, while 8 of 10 retransplanted animals developed either rejection crisis or full rejection of the second transplant. In vitro assays confirmed that the removed leukocytes were suppressive and that CD4(+) Foxp3(+) Treg reconstitution in blood and kidney grafts correlated with return to normal renal function in animals experiencing transient rejection crises. These data indicate that components of accepted kidney grafts as well as peripheral regulatory components both contribute to the tolerogenic environment required for tolerance of MHC class-I mismatched allotransplants.