Intrathymic deletion of alloreactive T cells in mixed bone marrow chimeras prepared with a nonmyeloablative conditioning regimen

Differentiation of regulatory myeloid and T-cells from adult human hematopoietic stem cells after allogeneic stimulation

Introduction

Donor hematopoietic stem cell (DHSC) infusions are increasingly being studied in transplant patients for tolerance induction.

Methods

To analyze the fate of infused DHSCs in patients, we developed an in vitro culture system utilizing CD34+DHSCs stimulated with irradiated allogeneic cells in cytokine supplemented medium long-term.

Results

Flow cytometric analyses revealed loss of the CD34 marker and an increase in CD33+ myeloid and CD3+ T-cell proportion by 10.4% and 72.7%, respectively, after 21 days in culture. T-cells primarily expressed TcR-αβ and were of both CD4+ and CD8+ subsets. Approximately 80% of CD3+ T cells lacked expression of the co-stimulatory receptor CD28. The CD4+ compartment was predominated by CD4+CD25+CD127-FOXP3+ Tregs (>50% CD4+CD127- compartment) with <1% of all leukocytes exhibiting a CD4+CD127+ phenotype. Molecular analyses for T-cell receptor excision circles showed recent and increased numbers of TcR rearrangements in generated T cells over time suggesting de novo differentiation from DHSCs. CD33+ myeloid cells mostly expressed HLA-DR, but lacked expression of co-stimulatory receptors CD80 and CD83. When studied as modulators in primary mixed lymphocyte reactions where the cells used to stimulate the DHSC were used as responders, the DHSC-lines and their purified CD8+, CD4+, CD33+ and linage negative subsets inhibited the responses in a dose-dependent and non-specific fashion. The CD8+ cell-mediated inhibition was due to direct lysis of responder cells.

Discussion

Extrapolation of these results into the clinical situation would suggest that DHSC infusions into transplant recipients may generate multiple subsets of donor "chimeric" cells and promote recipient Treg development that could regulate the anti-donor immune response in the periphery. These studies have also indicated that T cell maturation can occur in vitro in response to allogeneic stimulation without the pre-requisite of a thymic-like environment or NOTCH signaling stimulatory cell line.

Immunological barriers to haematopoietic stem cell gene therapy

Cell and gene therapies using haematopoietic stem cells (HSCs) epitomize the transformative potential of regenerative medicine. Recent clinical successes for gene therapies involving autologous HSC transplantation (HSCT) demonstrate the potential of genetic engineering in this stem cell type for curing disease. With recent advances in CRISPR gene-editing technologies, methodologies for the ex vivo expansion of HSCs and non-genotoxic conditioning protocols, the range of clinical indications for HSC-based gene therapies is expected to significantly expand. However, substantial immunological challenges need to be overcome. These include pre-existing immunity to gene-therapy reagents, immune responses to neoantigens introduced into HSCs by genetic engineering, and unique challenges associated with next-generation and off-the-shelf HSC products. By synthesizing these factors in this Review, we hope to encourage more research to address the immunological issues associated with current and next-generation HSC-based gene therapies to help realize the full potential of this field.

High Throughput Human T Cell Receptor Sequencing: A New Window Into Repertoire Establishment and Alloreactivity

Recent advances in high throughput sequencing (HTS) of T cell receptors (TCRs) and in transcriptomic analysis, particularly at the single cell level, have opened the door to a new level of understanding of human immunology and immune-related diseases. In this article, we discuss the use of HTS of TCRs to discern the factors controlling human T cell repertoire development and how this approach can be used in combination with human immune system (HIS) mouse models to understand human repertoire selection in an unprecedented manner. An exceptionally high proportion of human T cells has alloreactive potential, which can best be understood as a consequence of the processes governing thymic selection. High throughput TCR sequencing has allowed assessment of the development, magnitude and nature of the human alloresponse at a new level and has provided a tool for tracking the fate of pre-transplant-defined donor- and host-reactive TCRs following transplantation. New insights into human allograft rejection and tolerance obtained with this method in combination with single cell transcriptional analyses are reviewed here.

Unraveling the Crucial Roles of FoxP3+ Regulatory T Cells in Vascularized Composite Allograft Tolerance Induction and Maintenance

BACKGROUND

The role of regulatory T cells (Treg) in tolerance induction of vascularized composite allotransplantation (VCA) remains unclear. This study was designed to examine characteristics of Treg after VCA and their capacity to rescue allografts from rejection.

METHODS

Osteomyocutaneous allografts were transplanted from Balb/c to C57BL/6 mice. All mice received costimulatory blockade and a short course of rapamycin. To elucidate the role of Treg for tolerance induction, Treg depletion was performed at postoperative day (POD) 0, 30, or 90. To assess capacity of Treg to rescue allografts from rejection, an injection of 2 × 106 Treg isolated from tolerant mice was applied.

RESULTS

Eighty percent of VCA recipient mice using costimulatory blockade and rapamycin regimen developed tolerance. The tolerant recipients had a higher ratio of circulating Treg to effector T cells and elevated interleukin-10 at POD 30. A significantly higher rejection rate was observed when Treg were depleted at POD 30. But Treg depletion at POD 90 had no effect on tolerance. Treg from tolerant recipients showed stronger suppressive potential and the ability to rescue allografts from rejection. Furthermore, transplanted Treg-containing skin grafts from tolerant mice delayed rejection elicited by adoptively transferred effector T cells to Rag2-/- mice.

CONCLUSIONS

Circulating Treg are crucial for inducing VCA tolerance in the early posttransplant phase, and allograft-residing Treg may maintain tolerance. Treg may, therefore, serve as a potential cellular therapeutic to improve VCA outcomes.

A Prospective Controlled Trial to Evaluate Safety and Efficacy of in vitro Expanded Recipient Regulatory T Cell Therapy and Tocilizumab Together With Donor Bone Marrow Infusion in HLA-Mismatched Living Donor Kidney Transplant Recipients (Trex001)

Background: The induction of donor-specific immunological tolerance could improve outcome after kidney transplantation. However, no tolerance protocol is available for routine clinical use. Chimerism-based regimens hold promise, but their widespread application is impeded in part by unresolved safety issues. This study tests the hypothesis that therapy with polyclonal recipient regulatory T cells (Tregs) and anti-IL6R (tocilizumab) leads to transient chimerism and achieves pro-tolerogenic immunomodulation in kidney transplant recipients also receiving donor bone marrow (BM) without myelosuppressive conditioning of the recipient.

Methods/design: A prospective, open-label, controlled, single-center, phase I/IIa academic study is performed in HLA-mismatched living donor kidney transplant recipients.

Study group: Recipients of the study group receive in vitro expanded recipient Tregs and a donor bone marrow cell infusion within 3 days after transplantation and tocilizumab for the first 3 weeks post-transplant. In addition they are treated with thymoglobulin, belatacept, sirolimus, and steroids as immunosuppression. Starting 6 months post-transplant, sirolimus and steroids are withdrawn in a step-wise manner in stable patients.

Control group: Recipients of the control group are treated with thymoglobulin, belatacept, sirolimus, and steroids as immunosuppression. Co-primary endpoints of safety (impaired graft function [eGFR <35 mL/min/1.73 m²], graft-vs.-host disease or patient death by 12 months) and efficacy (total leukocyte donor chimerism within 28 days post-transplant) are assessed. Secondary endpoints include frequency of biopsy-proven acute rejection episodes and subclinical rejection episodes on surveillance biopsies, assessment of kidney graft function, and the evaluation whether the study protocol leads to detectable changes in the immune system indicative of pro-tolerogenic immune modulation.

Discussion: The results of this trial will provide evidence whether treatment with recipient Tregs and donor BM is feasible, safe and efficacious in leading to transient chimerism. If successful, this combination cell therapy has the potential to become a novel treatment option for immunomodulation in organ transplantation without the toxicities associated with myelosuppressive recipient conditioning.

Trial registration: European Clinical Trials Database EudraCT Nr 2018-003142-16 and clinicaltrials.gov NCT03867617.

Cultured thymus tissue implantation promotes donor-specific tolerance to allogeneic heart transplants

Eighty-six infants born without a thymus have been treated with allogeneic cultured thymus tissue implantation (CTTI). These infants, who lack T cells and are profoundly immunodeficient at birth, after CTTI from an unmatched donor develop T cells similar to those of recipient that are tolerant to both their own major histocompatibility antigens and those of the donor. We tested use of CTTI with the goal of inducing tolerance to unmatched heart transplants in immunocompetent rats. We thymectomized and T cell-depleted Lewis rats. The rats were then given cultured thymus tissue from F1 (Lewis × Dark Agouti ) under the kidney capsule and vascularized Dark Agouti (DA) heart transplants in the abdomen. Cyclosporine was administered for 4 months. The control group did not receive CTTI. Recipients with CTTI showed repopulation of naive and recent thymic emigrant CD4 T cells; controls had none. Recipients of CTTI did not reject DA cardiac allografts. Control animals did not reject DA grafts, due to lack of functional T cells. To confirm donor-specific unresponsiveness, MHC-mismatched Brown Norway (BN) hearts were transplanted 6 months after the initial DA heart transplant. LW rats with LWxDA CTTI rejected the third-party BN hearts (mean survival time 10 days); controls did not. CTTI recipients produced antibody against third-party BN donor but not against the DA thymus donor, demonstrating humoral donor-specific tolerance. Taken together, F1(LWxDA) CTTI given to Lewis rats resulted in specific tolerance to the allogeneic DA MHC expressed in the donor thymus, with resulting long-term survival of DA heart transplants after withdrawal of all immunosuppression.

Donor Lymphocyte-Derived Natural Killer Cells Control MHC Class I-Negative Melanoma

Natural killer (NK) cells provide a natural defense against MHC-I-negative tumors, such as melanoma. Donor lymphocyte infusion (DLI) containing NK cells, a form of adoptive immunotherapy used after allogenic bone marrow transplantation (allo-BMT), promotes antitumor immune responses but is often associated with life-threatening complications such as graft-versus-host disease (GvHD). Here, we showed that without prior allo-BMT, DLI provoked melanoma control associated with the infiltration and persistence of the transferred NK cells. This allograft acceptance did not correlate with an increase of GvHD; instead it correlated with the expansion and activation of tumor-infiltrating NK cells that expressed the cytotoxic molecules (e.g., IFNγ and granzyme B) and maturation signatures (e.g., CD11b^hiCD27^lo and KLRG^hi/CD43^hi). The development of beneficial tumor-infiltrating NK cells of DLI origin required host CD4⁺ T-cell help in part by producing IL2, as well as by limiting regulatory CD4⁺ T cells (Treg). IL2 blockade impaired the NK-dependent melanoma control, which could not be rescued by IL2 administration beyond CD4⁺ T-cell help. Our findings linked NK allograft acceptance-CD4⁺ T-cell help crosstalk to melanoma development without the need of allo-BMT. We thereby helped define that tumor-infiltrating NK cells of DLI origin may serve as effective therapeutic targets for controlling melanoma.

Vasculogenically conditioned peripheral blood mononuclear cells inhibit mouse immune response to induced pluripotent stem cell-derived allogeneic cardiac grafts

Allogeneic transplantation of induced pluripotent stem cell (iPSC)-derived cardiomyocytes is apromising treatment for cardiac diseases, although immune rejection by the recipient poses a concern. In this study, we aimed to investigate whether concomitant transplantation of vasculogenically conditioned peripheral blood mononuclear cells, which are otherwise immunosuppressive, may enhance graft survival. Luciferase-transduced, iPSC-derived cardiomyocytes from C57BL/6 mice were transplanted to the dorsal subcutaneous space of syngeneic C57BL/6 mice (n = 19), allogeneic Balb/c mice treated with (n = 20) or without (n = 20) immunosuppressants, and those injected with vasculogenically conditioned peripheral blood mononuclear cells (n = 20). Although graft survival, assessed by bioluminescence, was comparable among the groups initially, it improved significantly at days 7 and 10 in allogeneic transplanted mice treated with vasculogenically conditioned peripheral blood mononuclear cells than in others (P < 0.01). Our results proved that cell-based immunosuppression may boost clinical outcomes from allogeneic cell therapy.

Incognito: Are Microchimeric Fetal Stem Cells that Cross Placental Barrier Real Emissaries of Peace?

Chimerism occurs naturaly throughout gestation and can also occur as a consequence of transfusion and transplantation therapy. It consists of the acquisition and long-term persistence of a genetically distinct population of allogenic cells inside another organism. Previous reports have suggested that feto-maternal microchimerism could exert a beneficial effect on the treatment of hematological and solid tumors in patients treated by PBSCT. In this review we report the mechanism of transplacental fetal stem cell trafficking during pregnancy and the effect of their long-term persistence on autoimmunity, GVHD, PBSCT, cancer and stem cell treatment.

Early expansion of donor-specific Tregs in tolerant kidney transplant recipients

Allograft tolerance, in which a graft is accepted without long-term immunosuppression, could overcome numerous obstacles in transplantation. Human allograft tolerance has been intentionally induced across HLA barriers via combined kidney and bone marrow transplantation (CKBMT) with a regimen that induces only transient chimerism. Tregs are enriched early after CKBMT. While deletional tolerance contributes to long-term tolerance, the role of Tregs remains unclear. We have optimized a method for identifying the donor-specific Treg repertoire and used it to interrogate the fate of donor-specific Tregs after CKBMT. We expanded Tregs with several different protocols. Using functional analyses and T cell receptor sequencing, we found that expanding sorted Tregs with activated donor B cells identified the broadest Treg repertoire with the greatest potency and donor specificity of suppression. This method outperformed both alloantigen stimulation with CTLA4Ig and sequencing of CFSElo cells from the primary mixed lymphocyte reaction. In 3 tolerant and 1 nontolerant CKBMT recipients, we sequenced donor-specific Tregs before transplant and tracked them after transplant. Preexisting donor-specific Tregs were expanded at 6 months after CKBMT in tolerant patients and were reduced in the nontolerant patient. These results suggest that early expansion of donor-specific Tregs is involved in tolerance induction following CKBMT.

Murine models of transplantation tolerance through mixed chimerism: advances and roadblocks

Organ transplantation is the treatment of choice for patients with end-stage organ failure, but chronic immunosuppression is taking its toll in terms of morbidity and poor efficacy in preventing late graft loss. Therefore, a drug-free state would be desirable where the recipient permanently accepts a donor organ while remaining otherwise fully immunologically competent. Mouse studies unveiled mixed chimerism as an effective approach to induce such donor-specific tolerance deliberately and laid the foundation for a series of clinical pilot trials. Nevertheless, its widespread clinical implementation is currently prevented by cytotoxic conditioning and limited efficacy. Therefore, the use of mouse studies remains an indispensable tool for the development of novel concepts with potential for translation and for the delineation of underlying tolerance mechanisms. Recent innovations developed in mice include the use of pro-apoptotic drugs or regulatory T cell (Treg ) transfer for promoting bone marrow engraftment in the absence of myelosuppression and new insight gained in the role of innate immunity and the interplay between deletion and regulation in maintaining tolerance in chimeras. Here, we review these and other recent advances in murine studies inducing transplantation tolerance through mixed chimerism and discuss both the advances and roadblocks of this approach.

A New Window into the Human Alloresponse

Alloreactive T lymphocytes are the primary mediators of allograft rejection. The size and diversity of the HLA-alloreactive T cell repertoire has thus far precluded the ability to follow these T cells and thereby to understand their fate in human transplant recipients. This review summarizes the history, challenges, and recent advances in the study of alloreactive T cells. We highlight the historical development of assays to measure alloreactivity and discuss how high-throughput T cell receptor (TCR) sequencing-based assays can provide a new window into the fate of alloreactive T cells in human transplant recipients. A specific approach combining a classical in vitro assay, the mixed lymphocyte reaction, with deep T cell receptor sequencing is described as a tool to track the donor-reactive T cell repertoire for any specific HLA-mismatched donor-recipient pair. This assay can provide mechanistic insights and has potential as a noninvasive, highly specific biomarker for rejection and tolerance.

Cutaneous graft-versus-host disease after hematopoietic stem cell transplant - a review

Graft-versus-host disease (GVHD) is a major complication of allogeneic hematopoietic stem cell transplants (allo-HSCT) associated with significant morbidity and mortality. The earliest and most common manifestation is cutaneous graft-versus-host disease. This review focuses on the pathophysiology, clinical features, prevention and treatment of cutaneous graft-versus-host disease. We discuss various insights into the disease's mechanisms and the different treatments for acute and chronic skin graft-versus-host disease.

Bone marrow chimerism as a strategy to produce tolerance in solid organ allotransplantation

PURPOSE OF REVIEW

Clinical transplant tolerance has been most successfully achieved combining hematopoietic chimerism with kidney transplantation. This review outlines this strategy in animal models and human transplantation, and possible clinical challenges.

RECENT FINDINGS

Kidney transplant tolerance has been achieved through chimerism in several centers beginning with Massachusetts General Hospital's success with mixed chimerism in human leukocyte antigen (HLA)-mismatched patients and the Stanford group with HLA-matched patients, and the more recent success of the Northwestern protocol achieving full chimerism. This has challenged the original view that stable mixed chimerism is necessary for organ graft tolerance. However, among the HLA-mismatched kidney transplant-tolerant patients, loss of mixed chimerism does not lead to renal-graft rejection, and the development of host Foxp3+ regulatory T cells has been observed. Recent animal models suggest that graft tolerance through bone marrow chimerism occurs through both clonal deletion and regulatory immune cells. Further, Tregs have been shown to improve chimerism in animal models.

SUMMARY

Animal studies continue to suggest ways to improve our current clinical strategies. Advances in chimerism protocols suggest that tolerance may be clinically achievable with relative safety for HLA-mismatched kidney transplants.

Hematopoietic stem cells and solid organ transplantation

Solid organ transplantation provides lifesaving therapy for patients with end stage organ disease. In order for the transplanted organ to survive, the recipient must take a lifelong cocktail of immunosuppressive medications that increase the risk for infections, malignancies and drug toxicities. Data from many animal studies have shown that recipients can be made tolerant of their transplanted organ by infusing stem cells, particularly hematopoietic stem cells, prior to the transplant. The animal data have been translated into humans and now several clinical trials have demonstrated that infusion of hematopoietic stem cells, along with specialized conditioning regimens, can permit solid organ allograft survival without immunosuppressive medications. This important therapeutic advance has been made possible by understanding the immunologic mechanisms by which stem cells modify the host immune system, although it must be cautioned that the conditioning regimens are often severe and associated with significant morbidity. This review discusses the role of hematopoietic stem cells in solid organ transplantation, provides an understanding of how these stem cells modify the host immune system and describes how newer information about adaptive and innate immunity might lead to improvements in the use of hematopoietic stem cells to induce tolerance to transplanted organs.

Induced regulatory T cells in allograft tolerance via transient mixed chimerism

Successful induction of allograft tolerance has been achieved in nonhuman primates (NHPs) and humans via induction of transient hematopoietic chimerism. Since allograft tolerance was achieved in these recipients without durable chimerism, peripheral mechanisms are postulated to play a major role. Here, we report our studies of T cell immunity in NHP recipients that achieved long-term tolerance versus those that rejected the allograft (AR). All kidney, heart, and lung transplant recipients underwent simultaneous or delayed donor bone marrow transplantation (DBMT) following conditioning with a nonmyeloablative regimen. After DBMT, mixed lymphocyte culture with CFSE consistently revealed donor-specific loss of CD8⁺ T cell responses in tolerant (TOL) recipients, while marked CD4⁺ T cell proliferation in response to donor antigens was found to persist. Interestingly, a significant proportion of the proliferated CD4⁺ cells were FOXP3⁺ in TOL recipients, but not in AR or naive NHPs. In TOL recipients, CD4⁺FOXP3⁺ cell proliferation against donor antigens was greater than that observed against third-party antigens. Finally, the expanded Tregs appeared to be induced Tregs (iTregs) that were converted from non-Tregs. These data provide support for the hypothesis that specific induction of iTregs by donor antigens is key to long-term allograft tolerance induced by transient mixed chimerism.

Combined Anti-CD154/CTLA4Ig Costimulation Blockade-Based Therapy Induces Donor-Specific Tolerance to Vascularized Osteomyocutaneous Allografts

Tolerance induction by means of costimulation blockade has been successfully applied in solid organ transplantation; however, its efficacy in vascularized composite allotransplantation, containing a vascularized bone marrow component and thus a constant source of donor-derived stem cells, remains poorly explored. In this study, osteomyocutaneous allografts (alloOMCs) from Balb/c (H2(d) ) mice were transplanted into C57BL/6 (H2(b) ) recipients. Immunosuppression consisted of 1 mg anti-CD154 on day 0, 0.5 mg CTLA4Ig on day 2 and rapamycin (RPM; 3 mg/kg per day from days 0-7, then every other day for 3 weeks). Long-term allograft survival, donor-specific tolerance and donor-recipient cell trafficking were evaluated. Treatment with costimulation blockade plus RPM resulted in long-term graft survival (>120 days) of alloOMC in 12 of 15 recipients compared with untreated controls (median survival time [MST] ≈10.2 ± 0.8 days), RPM alone (MST ≈33 ± 5.5 days) and costimulation blockade alone (MST ≈45.8 ± 7.1 days). Donor-specific hyporesponsiveness in recipients with viable grafts was demonstrated in vitro. Evidence of donor-specific tolerance was further assessed in vivo by secondary donor-specific skin graft survival and third-party graft rejection. A significant increase of Foxp3(+) regulatory T cells was evident in tolerant animals. Donor cells populated peripheral blood, thymus, and both donor and recipient bone marrow. Consequently, combined anti-CD154/CTLA4Ig costimulation blockade-based therapy induces donor-specific tolerance in a stringent murine alloOMC transplant model.

Immune tolerance in recipients of combined haploidentical bone marrow and kidney transplantation

The success of allogeneic hematopoietic cell transplantation (HCT) has been limited by transplant-associated toxicities related to the conditioning regimens used and to graft-vs-host disease (GVHD). The frequency and severity of GVHD observed when extensive HLA barriers are transgressed has greatly impeded the routine use of extensively HLA-mismatched HCT. Allogeneic HCT also has potential as an approach to organ allograft tolerance induction, but this potential has not been previously realized because of the toxicity associated with traditional conditioning. This paper reviews an approach to HCT involving reduced intensity conditioning that demonstrated sufficient safety in patients with hematologic malignancies, even in the HLA-mismatched transplant setting, to be applied for the induction of kidney allograft tolerance in humans with no other indication for HCT. These studies provided the first successful example of intentional organ allograft tolerance induction across HLA barriers in humans. Current data and hypotheses on the mechanisms of tolerance in these patients are reviewed.

Tracking donor-reactive T cells: Evidence for clonal deletion in tolerant kidney transplant patients

T cell responses to allogeneic major histocompatibility complex antigens present a formidable barrier to organ transplantation, necessitating long-term immunosuppression to minimize rejection. Chronic rejection and drug-induced morbidities are major limitations that could be overcome by allograft tolerance induction. Tolerance was first intentionally induced in humans via combined kidney and bone marrow transplantation (CKBMT), but the mechanisms of tolerance in these patients are incompletely understood. We now establish an assay to identify donor-reactive T cells and test the role of deletion in tolerance after CKBMT. Using high-throughput sequencing of the T cell receptor B chain CDR3 region, we define a fingerprint of the donor-reactive T cell repertoire before transplantation and track those clones after transplant. We observed posttransplant reductions in donor-reactive T cell clones in three tolerant CKBMT patients; such reductions were not observed in a fourth, nontolerant, CKBMT patient or in two conventional kidney transplant recipients on standard immunosuppressive regimens. T cell repertoire turnover due to lymphocyte-depleting conditioning only partially accounted for the observed reductions in tolerant patients; in fact, conventional transplant recipients showed expansion of circulating donor-reactive clones, despite extensive repertoire turnover. Moreover, loss of donor-reactive T cell clones more closely associated with tolerance induction than in vitro functional assays. Our analysis supports clonal deletion as a mechanism of allograft tolerance in CKBMT patients. The results validate the contribution of donor-reactive T cell clones identified before transplant by our method, supporting further exploration as a potential biomarker of transplant outcomes.

Immune Reconstitution Kinetics following Intentionally Induced Mixed Chimerism by Nonmyeloablative Transplantation

Establishing mixed chimerism is a promising approach for inducing donor-specific transplant tolerance. The establishment and maintenance of mixed chimerism may enable long-term engraftment of organ transplants while minimizing the use of immunosuppressants. Several protocols for inducing mixed chimerism have been reported; however, the exact mechanism underlying the development of immune tolerance remains to be elucidated. Therefore, understanding the kinetics of engraftment during early post-transplant period may provide insight into establishing long-term mixed chimerism and permanent transplant tolerance. In this study, we intentionally induced allogeneic mixed chimerism using a nonmyeloablative regimen by host natural killer (NK) cell depletion and T cell-depleted bone marrow (BM) grafts in a major histocompatibility complex (MHC)-mismatched murine model and analyzed the kinetics of donor (C57BL/6) and recipient (BALB/c) engraftment in the weeks following transplantation. Donor BM cells were well engrafted and stabilized without graft-versus-host disease (GVHD) as early as one week post-bone marrow transplantation (BMT). Donor-derived thymic T cells were reconstituted four weeks after BMT; however, the emergence of newly developed T cells was more obvious at the periphery as early as two weeks after BMT. Also, the emergence and changes in ratio of recipient- and donor-derived NKT cells and antigen presenting cells (APCs) including dendritic cells (DCs) and B cells were noted after BMT. Here, we report a longitudinal analysis of the development of donor- and recipient-originated hematopoietic cells in various lymphatic tissues of intentionally induced mixed chimerism mouse model during early post-transplant period. Through the understanding of immune reconstitution at early time points after nonmyeloablative BMT, we suggest guidelines on intentionally inducing durable mixed chimerism.

Chimerism, graft survival, and withdrawal of immunosuppressive drugs in HLA matched and mismatched patients after living donor kidney and hematopoietic cell transplantation

Thirty-eight HLA matched and mismatched patients given combined living donor kidney and enriched CD34(+) hematopoietic cell transplants were enrolled in tolerance protocols using posttransplant conditioning with total lymphoid irradiation and anti-thymocyte globulin. Persistent chimerism for at least 6 months was associated with successful complete withdrawal of immunosuppressive drugs in 16 of 22 matched patients without rejection episodes or kidney disease recurrence with up to 5 years follow up thereafter. One patient is in the midst of withdrawal and five are on maintenance drugs. Persistent mixed chimerism was achieved in some haplotype matched patients for at least 12 months by increasing the dose of T cells and CD34(+) cells infused as compared to matched recipients in a dose escalation study. Success of drug withdrawal in chimeric mismatched patients remains to be determined. None of the 38 patients had kidney graft loss or graft versus host disease with up to 14 years of observation. In conclusion, complete immunosuppressive drug withdrawal could be achieved thus far with the tolerance induction regimen in HLA matched patients with uniform long-term graft survival in all patients.

Depletion of foxp3(+) T cells abrogates tolerance of skin and heart allografts in murine mixed chimeras without the loss of mixed chimerism

The relative contribution of central and peripheral mechanisms to the generation and maintenance of allograft tolerance is of considerable interest. Here, we present new evidence that regulatory T cells (Foxp3(+) ) maintain skin and heart allograft tolerance in mixed hematopoietic chimeric mice. Transient depletion of both donor- and recipient-derived Foxp3(+) cells was necessary and sufficient to induce decisive rejection of long-accepted skin and heart allografts. In contrast, stable hematopoietic chimerism remained, and there was no detectable induction of donor-specific reactivity to hematopoietic cells. Foxp3(+) cell depletion did not result in the rejection of skin grafts of only MHC-disparate donors (B6.C-H2(d) /bByJ), indicating that MHC antigens were not the target in the graft. We conclude that two different mechanisms of tolerance are present in mixed chimeras. Hematopoietic chimerism, resistant to Foxp3(+) depletion, is probably due to deletional tolerance to MHC antigens, as supported by previous studies. In contrast, regulatory tolerance mechanisms involving Foxp3(+) cells are required to control reactivity against non-MHC antigens not present on hematopoietic lineages.

Improving allogeneic islet transplantation by suppressing Th17 and enhancing Treg with histone deacetylase inhibitors

Islet transplantation is a new treatment for achieving insulin independence for patients with severe diabetes. However, major drawbacks of this treatment are the long graft survival, the necessity for immunosuppressive drugs, and the efficacy of transplantation. Donor-specific transfusion (DST) has been shown to reduce rejection after organ transplantation, potentially through enhanced regulatory T-cell (Treg) activity. However, recent findings have shown that activated Treg can be converted into Th17 cells. We focused on histone deacetylase inhibitors (HDACi) because it was reported that inhibition of HDAC activity prevented Treg differentiation into IL17-producing cells. We therefore sought to enhance Treg while suppressing Th17 cells using DST with HDACi to prolong graft survival. To stimulate Treg by DST, we used donor splenocytes. In DST with HDACi group, Foxp3 mRNA expression and Treg population increased in the thymus and spleen, whereas Th17 population decreased. qPCR analysis of lymphocyte mRNA indicated that Foxp3, IL-10, and TGF-b expression increased. However, interleukin 17a, Stat3 (Th17), and IFN-g expression decreased in DST + HDACi group, relative to DST alone. Moreover, DST treated with HDACi prolonged graft survival relative to controls in mice islet transplantation. DST with HDACi may therefore have utility in islet transplantation.

Indirect immune recognition of mouse embryonic stem cell-derived hematopoietic progenitors in vitro

The clinical use of embryonic stem cell (ESC)-derived hematopoietic progenitors (ESHPs) requires the generation of ESHPs that produce mature hematopoietic cells and do not induce immune rejection after transplantation. We compared the developmental maturity and immunogenicity of ESHPs generated using two methods: embryoid body (EB) formation and culture of ESCs with the OP9 bone marrow stromal cell line (ESC-OP9). ESHPs derived from EBs displayed an immature hematopoietic phenotype and were devoid of immunogenicity marker expression. In contrast, ESHPs derived via ESC-OP9 displayed a mature phenotype and expressed high levels of some immunostimulatory molecules. ESHPs alone could not stimulate CD4(+) T lymphocyte proliferation directly. However, preferential phagocytosis of ESHPs and T cell proliferation were observed in the presence of antigen-presenting cells, consistent with a model of indirect immune recognition of ESHPs. These results suggest that depletion of host CD4(+) T lymphocytes or antigen-presenting cells may be necessary for successful ESHP transplantation.

Induction of tolerance through mixed chimerism

"Mixed chimerism" refers to a state in which the lymphohematopoietic system of the recipient of allogeneic hematopoietic stem cells comprises a mixture of host and donor cells. This state is usually attained through either bone marrow or mobilized peripheral blood stem cell transplantation. Although numerous treatment regimens have led to transplantation tolerance in mice, the induction of mixed chimerism is currently the only treatment modality that has been successfully extended to large animals and to the clinic. Here we describe and compare the use of mixed chimerism to establish transplantation tolerance in mice, pigs, monkeys, and in the clinic. We also attempt to correlate the mechanisms involved in achieving tolerance with the nature of the tolerance that has resulted in each case.

Modulation of allogeneic CD8+ T-cell response by DZNep controls GVHD while preserving hematopoietic chimerism

BACKGROUND

Allogeneic hematopoietic stem cell transplantation (allo-HSCT) combined with solid-organ transplantation is a feasible method to achieve long-lasting organ allograft tolerance through the induction of hematopoietic chimerism in recipients. However, the allo-HSCT engraftment puts recipients at risk of life-threatening graft-versus-host disease (GVHD). Novel immunomodulatory approaches are required to effectively control GVHD while preserving the status of hematopoietic chimerism. We have reported that histone methylation inhibitor 3-deazaneplanocin A (DZNep) can control ongoing GVHD in mice by selectively inducing apoptosis of alloreactive effector T cells.

METHODS

Using donor-derived CD8 T cell-mediated mouse GVHD model, we further investigated the effect of in vivo administration of DZNep on allogeneic CD8 T cell response and the hematopoietic chimerism in recipients.

RESULTS

We found that DZNep delayed the in vivo proliferation of donor-derived alloreactive CD8 T cells and also reduced the interleukin-2 production by these T cells. Moreover, DZNep treatment resulted in a significant decrease of interferon-γ, tumor necrosis factor-α, granzyme B, TRAIL, and Fas ligand expressing donor-derived CD8 T cells, suggesting a multilevel modulation role on T-cell survival and effect in vivo. Notably, DZNep treatment did not hamper the generation of hematopoietic chimerism in recipients.

CONCLUSIONS

These findings suggest that modulation of histone methylation through DZNep may be a potential strategy for the induction of hematopoietic chimerism to achieve donor-specific organ allograft tolerance through donor allo-HSCT combined with solid-organ transplantation.

Lessons and limits of mouse models

Seminal studies in rabbits and rodent transplantation models by Peter Medawar revealed that cellular processes, rather than humoral antibodies, are central to the acute rejection of transplanted organs, and much of basic transplantation research continues to be focused on the biology and control of these cells, which were subsequently shown to be T cells. However, the success of current immunosuppression at controlling T-cell-mediated rejection has resulted in an increasing awareness of antibody-mediated rejection in the clinic. This, in turn, has fueled an emerging interest in the biology of allospecific antibodies, the B cells that produce these antibodies, and the development of mouse models that allow their investigation. Here we summarize some of the more widely used mouse models that have been developed to study the immunobiology of alloreactivity, transplantation rejection and tolerance, and used to identify therapeutic strategies that modulate these events.

CD11b(+) cells in donor-specific transfusion prolonged allogenic skin graft survival through indoleamine 2,3-dioxygenase

The aim of this study is to show the effect of donor-specific transfusion (DST) in inducing immunological tolerance mediated by regulatory T cells (Treg) and indoleamine 2,3-dioxygenase (IDO). Skin grafts from H2(d) Balb/c were transplanted into H2(k) C3H/He 7days after the infusion of donor splenocytes, isolated each immune cell populations. Graft survival prolonged in recipients who received splenocytes, MHC class II(+) CD90(-) cells and CD3(-)CD19(-) cells (p<0.001, p<0.05 and p<0.01, respectively). CD11b(+) cell infusion resulted in prolongation of graft survival when compared to CD11c(+) cell infusion (p<0.01). Foxp3(+)CD4(+)CD25(+) T cells were increased after the transplant in recipients infused with CD11b(+) cells (p<0.05). The mixed lymphocyte reaction showed donor-specificity (p<0.001). High IDO expression was observed in CD11b(+) cell infusion group. Graft survival with DST using IDO antagonist (1MT) were not prolonged. In conclusion, DST allows induction of donor-specific tolerance which involves Foxp3(+)CD4(+)CD25(+) T cells and IDO expression.

Transplant tolerance: new insights and strategies for long-term allograft acceptance

One of the greatest advances in medicine during the past century is the introduction of organ transplantation. This therapeutic strategy designed to treat organ failure and organ dysfunction allows to prolong the survival of many patients that are faced with no other treatment option. Today, organ transplantation between genetically dissimilar individuals (allogeneic grafting) is a procedure widely used as a therapeutic alternative in cases of organ failure, hematological disease treatment, and some malignancies. Despite the potential of organ transplantation, the administration of immunosuppressive drugs required for allograft acceptance induces severe immunosuppression in transplanted patients, which leads to serious side effects such as infection with opportunistic pathogens and the occurrence of neoplasias, in addition to the known intrinsic toxicity of these drugs. To solve this setback in allotransplantation, researchers have focused on manipulating the immune response in order to create a state of tolerance rather than unspecific immunosuppression. Here, we describe the different treatments and some of the novel immunotherapeutic strategies undertaken to induce transplantation 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.

Mixed chimerism and split tolerance: mechanisms and clinical correlations

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

Targeting cells causing split tolerance allows fully allogeneic islet survival with minimal conditioning in NOD mixed chimeras

Donor-specific tolerance induced by mixed chimerism is one approach that may eliminate the need for long-term immunosuppressive therapy, while preventing chronic rejection of an islet transplant. However, even in the presence of chimerism it is possible for certain donor tissues or cells to be rejected whereas others from the same donor are accepted (split tolerance). We previously developed a nonmyeloablative protocol that generated mixed chimerism across full major histocompatability complex plus minor mismatches in NOD (nonobese diabetic) mice, however, these chimeras demonstrated split tolerance. In this study, we used radiation chimeras and found that the radiosensitive component of NOD has a greater role in the split tolerance NOD mice develop. We then show that split tolerance is mediated primarily by preexisting NOD lymphocytes and have identified T cells, but not NK cells or B cells, as cells that both resist chimerism induction and mediate split tolerance. Finally, after recognizing the barrier that preexisting T cells impose on the generation of fully tolerant chimeras, the chimerism induction protocol was refined to include nonmyeloablative recipient NOD T cell depletion which generated long-term mixed chimerism across fully allogeneic barriers. Furthermore, these chimeric NOD mice are immunocompetent, diabetes free and accept donor islet allografts.

Hematopoietic chimerism and transplantation tolerance: a role for regulatory T cells

The immunosuppressive regimens currently used in transplantation to prevent allograft destruction by the host's immune system have deleterious side effects and fail to control chronic rejection processes. Induction of donor-specific non-responsiveness (i.e., immunological tolerance) to transplants would solve these problems and would substantially ameliorate patients' quality of life. It has been proposed that bone marrow or hematopoietic stem-cell transplantation, and resulting (mixed) hematopoietic chimerism, lead to immunological tolerance to organs of the same donor. However, a careful analysis of the literature, performed here, clearly establishes that whereas hematopoietic chimerism substantially prolongs allograft survival, it does not systematically prevent chronic rejection. Moreover, the cytotoxic conditioning regimens used to achieve long-term persistence of chimerism are associated with severe side effects that appear incompatible with a routine use in the clinic. Several laboratories recently embarked on different studies to develop alternative strategies to overcome these issues. We discuss here recent advances obtained by combining regulatory T cell infusion with bone-marrow transplantation. In experimental settings, this attractive approach allows development of genuine immunological tolerance to donor tissues using clinically relevant conditioning regimens.

Translational studies in hematopoietic cell transplantation: treatment of hematologic malignancies as a stepping stone to tolerance induction

Allogeneic hematopoietic cell transplantation (HCT) has most commonly been used to treat hematologic malignancies, where it is often the only potentially curative option available. The success of HCT has been limited by transplant-associated toxicities related to the conditioning regimens used and to the common immunologic consequence of donor T cell recognition of recipient alloantigens, graft-vs-host disease (GVHD). The frequency and severity of GVHD observed when extensive HLA barriers are transgressed has essentially precluded the routine use of extensively HLA-mismatched HCT. Allogeneic HCT also has potential as an approach to organ allograft tolerance induction, but this potential has not been previously realized because of the toxicity associated with traditional conditioning. In this paper we review two approaches to HCT involving reduced intensity conditioning regimens that have been associated with improvements in safety in patients with hematologic malignancies, even in the HLA-mismatched transplant setting. These strategies have been applied in the first successful pilot studies for the induction of organ allograft tolerance in humans. Thus, we summarize an example of vertical translational research between animal models and humans and horizontal translation between two separate goals that culminated in the use of HCT to achieve allograft tolerance in humans.

Immuno-intervention for the induction of transplantation tolerance through mixed chimerism

The induction of transplantation tolerance could liberate organ transplant recipients from the complications of life-long chronic immunosuppression. The original description of tolerance induction through mixed hematopoietic chimerism in mice utilized lethal whole body irradiation as the preparative regimen for achieving mixed chimerism. While such a regimen might be acceptable for treatment of patients with malignancies, which might also respond to the therapeutic effects of radiation, its toxicity would be unacceptable for patients in need only of an organ transplant. Graft-vs.-host disease, which is frequently a complication of mismatched bone marrow transplantation, would likewise be unacceptable for ordinary clinical transplantation. Therefore, as we have extended the use of this modality for tolerance induction from mice to large animal models, we have attempted to design preparative regimens that avoid both of these complications. In this article, we review our studies of mixed chimerism in mice, miniature swine and monkeys, as well as the results of our recent clinical studies that have extended this treatment modality to a series of kidney transplant patients who have been successfully weaned from all immunosuppression while maintaining stable renal function for up to 8 years.

Bringing transplantation tolerance into the clinic: lessons from the ITN and RISET for the Establishment of Tolerance consortia

PURPOSE OF REVIEW

Fundamental discoveries during the 1990s revolutionized our understanding of transplantation tolerance and our ability to create it in animal models. The Immune Tolerance Network (ITN) and Reprogramming the Immune System for the Establishment of Tolerance (RISET) consortia were created to leverage these advances and work towards the goal of achieving clinical tolerance in transplantation. This article highlights their accomplishments and challenges during the past decade.

RECENT FINDINGS

In interventional trials, renal allograft tolerance has been achieved using bone marrow transplantation with nonmyeloablative protocols to induce transient hematopoietic chimerism. Drug minimization in renal transplantation was achieved with Campath-1H induction therapy and also with cellular therapy using 'transplant acceptance inducing cells'. Successful drug withdrawal was accomplished in long-term stable pediatric liver transplant recipients. Finally, 'registry' trials of tolerant kidney recipients revealed a B-cell signature of tolerance, which will form the basis for future investigations of its use as a biomarker for drug minimization or withdrawal in selected patients.

SUMMARY

Although transplantation tolerance is not yet reliably achieved in a clinical setting, collaborative efforts, such as those of the ITN and RISET networks, are an effective means to synergize intellectual and financial resources to bring this goal closer to reality.

Reversal of age-related thymic involution by an LHRH agonist in miniature swine

BACKGROUND AND AIMS OF STUDY: We have previously demonstrated a requirement for the presence of a juvenile thymus for the induction of transplantation tolerance to renal allografts by a short-course of calcineurin inhibition in miniature swine. We have also shown that aged, involuted thymi can be rejuvenated when transplanted as vascularized thymic lobes into juvenile swine recipients. The present studies were aimed at elucidating the extrinsic factors facilitating this restoration of function in the aged thymus. In particular, we tested the impact of sex steroid blockade by Luteinizing Hormone-Releasing Hormone (LHRH).

MATERIALS AND METHODS

30 naive animals (25 males and 5 females) were used for measurement of serum testosterone levels. 3 mature male pigs (aged at 22, 22 and 29 months old) were used to test the effects of Lupron (LHRH analog) injection at 45 mg (per 70-80 kg body weight) as a 3-month depot on testosterone levels and thymic rejuvenation. Thymic rejuvenation was assessed by histology, flow cytometric analysis, morphometric analysis and TREC assays.

RESULTS

Hormonal alterations were induced by Lupron and resulted in macroscopic and histologic regeneration of the thymus of aged animals within 2 months, as evidenced by restoration of juvenile thymus architecture and increased cellularity. Two animals that were evaluated for TREC both showed increased levels in the periphery following Lupron treatment.

CONCLUSION

Treatment of aged animals with Lupron leads to thymic rejuventaion in adult miniature swine. This result could expand the applicability of thymus-dependent tolerance-inducing regimens to adult recipients.

Should microchimerism turn into rejection prophylactics?

Non-self cells can circulate in the body of an individual after any sort of contact with an allogeneic source of cells, thus creating a situation of chimerism that can be transient or prolonged over time. This situation may appear after stem cell transplantation, pregnancy, transfusion or transplantation. Concerning transplantation, many hypotheses have been formulated regarding the existence, persistence and role of these circulating cells in the host. We will review the principal hypotheses that have been formulated for years since the first description of non-self circulating cells in mammals to the utilization of artificially induced chimerism protocols for the achievement of tolerance.

Induction of tolerance to bone marrow allografts by donor-derived host nonreactive ex vivo-induced central memory CD8 T cells

Enabling engraftment of allogeneic T cell-depleted bone marrow (TDBM) under reduced-intensity conditioning represents a major challenge in bone marrow transplantation (BMT). Anti-third-party cytotoxic T lymphocytes (CTLs) were previously shown to be endowed with marked ability to delete host antidonor T cells in vitro, but were found to be less effective in vivo. This could result from diminished lymph node (LN) homing caused by the prolonged activation, which induces a CD44(+)CD62L(-) effector phenotype, and thereby prevents effective colocalization with, and neutralization of, alloreactive host T cells (HTCs). In the present study, LN homing, determined by imaging, was enhanced upon culture conditions that favor the acquisition of CD44(+)CD62L(+) central memory cell (Tcm) phenotype by anti-third-party CD8(+) cells. These Tcm-like cells displayed strong proliferation and prolonged persistence in BM transplant recipients. Importantly, adoptively transferred HTCs bearing a transgenic T-cell receptor (TCR) with antidonor specificity were efficiently deleted only by donor-type Tcms. All these attributes were found to be associated with improved efficacy in overcoming T cell-mediated rejection of TDBM, thereby enabling high survival rate and long-term donor chimerism, without causing graft-versus-host disease. In conclusion, anti-third-party Tcms, which home to recipient LNs and effectively delete antidonor T cells, could provide an effective and novel tool for overcoming rejection of BM allografts.

Type 1 regulatory T cells are associated with persistent split erythroid/lymphoid chimerism after allogeneic hematopoietic stem cell transplantation for thalassemia

BACKGROUND

Thalassemia major can be cured with allogeneic hematopoietic stem cell transplantation. Persistent mixed chimerism develops in around 10% of transplanted thalassemic patients, but the biological mechanisms underlying this phenomenon are poorly understood.

DESIGN AND METHODS

The presence of interleukin-10-producing T cells in the peripheral blood of eight patients with persistent mixed chimerism and five with full donor chimerism was investigated. A detailed characterization was then performed, by T-cell cloning, of the effector and regulatory T-cell repertoire of one patient with persistent mixed chimerism, who developed stable split erythroid/lymphoid chimerism after a hematopoietic stem cell transplant from an HLA-matched unrelated donor.

RESULTS

Higher levels of interleukin-10 were produced by peripheral blood mononuclear cells from patients with persistent mixed chimerism than by the same cells from patients with complete donor chimerism or normal donors. T-cell clones of both host and donor origin could be isolated from the peripheral blood of one, selected patient with persistent mixed chimerism. Together with effector T-cell clones reactive against host or donor alloantigens, regulatory T-cell clones with a cytokine secretion profile typical of type 1 regulatory cells were identified at high frequencies. Type 1 regulatory cell clones, of both donor and host origin, were able to inhibit the function of effector T cells of either donor or host origin in vitro.

CONCLUSIONS

Overall these results suggest that interleukin-10 and type 1 regulatory cells are associated with persistent mixed chimerism and may play an important role in sustaining long-term tolerance in vivo. These data provide new insights into the mechanisms of peripheral tolerance in chimeric patients and support the use of cellular therapy with regulatory T cells following hematopoietic stem cell transplantation.

Immunomodulatory effects of mixed hematopoietic chimerism: immune tolerance in canine model of lung transplantation

Long-term survival after lung transplantation is limited by acute and chronic graft rejection. Induction of immune tolerance by first establishing mixed hematopoietic chimerism (MC) is a promising strategy to improve outcomes. In a preclinical canine model, stable MC was established in recipients after reduced-intensity conditioning and hematopoietic cell transplantation from a DLA-identical donor. Delayed lung transplantation was performed from the stem cell donor without pharmacological immunosuppression. Lung graft survival without loss of function was prolonged in chimeric (n = 5) vs. nonchimeric (n = 7) recipients (p < or = 0.05, Fisher's test). There were histological changes consistent with low-grade rejection in 3/5 of the lung grafts in chimeric recipients at > or =1 year. Chimeric recipients after lung transplantation had a normal immune response to a T-dependent antigen. Compared to normal dogs, there were significant increases of CD4+INFgamma+, CD4+IL-4+ and CD8+ INFgamma+ T-cell subsets in the blood (p < 0.0001 for each of the three T-cell subsets). Markers for regulatory T-cell subsets including foxP3, IL10 and TGFbeta were also increased in CD3+ T cells from the blood and peripheral tissues of chimeric recipients after lung transplantation. Establishing MC is immunomodulatory and observed changes were consistent with activation of both the effector and regulatory immune response.

Mitomycin C-treated antigen-presenting cells as a tool for control of allograft rejection and autoimmunity: from bench to bedside

Cells have been previously used in experimental models for tolerance induction in organ transplantation and autoimmune diseases. One problem with the therapeutic use of cells is standardization of their preparation. We discuss an immunosuppressive strategy relying on cells irreversibly transformed by a chemotherapeutic drug. Dendritic cells (DCs) of transplant donors pretreated with mitomycin C (MMC) strongly prolonged rat heart allograft survival when injected into recipients before transplantation. Likewise, MMC-DCs loaded with myelin basic protein suppressed autoreactive T cells of MS patients in vitro and prevented experimental autoimmune encephalitis in mice. Comprehensive gene microarray analysis identified genes that possibly make up the suppressive phenotype, comprising glucocorticoid leucine zipper, immunoglobulin-like transcript 3, CD80, CD83, CD86, and apoptotic genes. Based on these findings, a hypothetical model of tolerance induction by MMC-treated DCs is delineated. Finally, we describe the first clinical application of MMC-treated monocyte-enriched donor cells in an attempt to control the rejection of a haploidentical stem cell transplant in a sensitized recipient and discuss the pros and cons of using MMC-treated antigen-presenting cells for tolerance induction. Although many questions remain, MMC-treated cells are a promising clinical tool for controlling allograft rejection and deleterious immune responses in autoimmune diseases.

Hematopoietic cell transplantation for tolerance induction: animal models to clinical trials

The induction of donor-specific immune tolerance is the "holy grail" of transplantation, as it would avoid the toxicities of chronic immunosuppressive therapies while preventing acute and chronic graft rejection. A large number of approaches to tolerance induction have been described in the experimental literature, but only hematopoietic cell transplantation has shown preliminary success for intentional tolerance induction in pilot clinical trials. This review summarizes the conditions that allow progress to be made in moving strategies for tolerance induction from the bench to the bedside and discuss the mechanisms by which tolerance may be achieved through hematopoietic cell transplantation.