Long-term prevention of chronic allograft rejection by regulatory T-cell immunotherapy involves host Foxp3-expressing T cells

Chimeric antigen receptor T cell and regulatory T cell therapy in non-oncology diseases: A narrative review of studies from 2017 to 2023

Recently, the remarkable success of chimeric antigen receptor T cell (CAR-T) therapy in treating certain tumors has led to numerous studies exploring its potential application to treat non-oncology diseases. This review discusses the progress and evolution of CAR-T cell therapies for treating non-oncology diseases over the past 5 years. Additionally, we summarize the advantages and disadvantages of CAR-T cell therapy in treating non-oncological diseases and identify any difficulties that should be overcome. After conducting an in-depth analysis of the most recent studies on CAR-T technology, we discuss the key elements of CAR-T therapy, such as developing an effective CAR design for non-oncological diseases, controlling the rate and duration of response, and implementing safety measures to reduce toxicity. These studies provide new insights into different delivery strategies, the discovery of new target molecules, and improvements in the safety of CAR-T therapy for non-oncological diseases.

Immunopathological insights into villitis of unknown etiology on the basis of transplant immunology

Villitis of unknown etiology (VUE) is an inflammatory disease characterized by the infiltration of maternal CD8 +T cells into the placental villi. Although the pathogenesis of VUE is still debated, dysregulation of the immune system appears to be an important factor in the development of the disease. Interaction of maternal T cells with the fetal antigens seems to be the trigger for the VUE onset. In this context, graft vs host disease (GVHD) and allographic rejection seem to share similarities in the VUE immunopathological mechanism, especially those related to immunoregulation. In this review, we compared the immunological characteristics of VUE with allograft rejection, and GVHD favoring a better knowledge of VUE pathogenesis that may contribute to VUE therapeutics strategies in the future.

The role of regulatory T cells in liver transplantation

The liver is considered a tolerogenic organ that can induce peripheral tolerance. The exact mechanisms of tolerance in the liver remain undefined. Regulatory T cells (Tregs) have been demonstrated to be involved in inducing and maintaining peripheral tolerance. They play an important role in the prevention of immune responses and autoimmunity. The main focus of this review is the role of Tregs and their subpopulation in liver transplantation. More specifically, this manuscript will highlight the recent findings about using Treg cells as a biomarker in liver transplantation. There are some reports and animal models about the role of Tregs in the process of rejection of liver transplantation. Previous reports and studies have suggested that by increasing the number of Tregs better liver transplant outcomes will be accomplished by enhancing tolerance. It has been shown that the levels of CD4 + CD25 + FOXP3+ Treg cells correlate with the inhibition of acute allograft rejection in liver transplantation; however, further studies must be done to address the potential role of Treg cells in chronic rejection. Indeed, in the future, Treg cells may have potential use as a beneficial biomarker to screen long-term graft function.

Post-transplantation cyclophosphamide for chimerism-based tolerance

High-dose cyclophosphamide given post-transplant (PTCy) successfully enables tolerance induction in HLA-mismatched related blood or marrow transplantation (haploBMT) manifested by low rates of graft failure, severe acute graft-versus-host disease (GVHD), and chronic GVHD. When proceeded by nonmyeloablative conditioning, PTCy has also been associated with a low incidence of nonrelapse mortality. The safety of this platform has garnered interest in expanding its use to non-malignant indications for allogeneic blood or marrow transplantation (alloBMT). After success in a preliminary Phase I/II trial, use of a PTCy-based haploBMT platform is now being explored in a large Blood and Marrow Transplant Clinical Trials Network (BMT CTN) study for sickle cell disease. These emerging data in patients with hemoglobinopathies provided the rationale for exploring the use of PTCy in combined solid organ and BM transplantation as a means of tolerance induction through donor hematopoietic chimerism with a goal to obviate the need for a lifetime of immunosuppression. Several case reports, series, and small clinical trials have now been published of combined solid organ and alloBMT in patients with hematologic malignancies who had organ failure that would have been preclusive of alloBMT in the absence of solid organ transplantation. Here we will review the pre-clinical and clinical studies supporting the use of PTCy for chimerism-based tolerance induction.

Reprogrammed CD4+ T Cells That Express FoxP3+ Control Inhibitory Antibody Formation in Hemophilia A Mice

Coagulation Factor VIII (FVIII) replacement therapy in hemophilia A patients is complicated by the development of inhibitory antibodies, which often render the treatment ineffective. Previous studies demonstrated a strong correlation between induction of regulatory T cells (Treg) and tolerance to the therapeutic protein. We, therefore, set out to evaluate whether the adoptive transfer of FVIII-specific CD4⁺ Treg cells prevents inhibitor response to FVIII protein therapy. To this end, we first retrovirally transduced FoxP3⁺ into FVIII-specific CD4⁺ cells, which resulted in cells that stably express FoxP3, are phenotypically similar to peripherally induced Tregs and are antigen specific suppressors, as judged by in vitro assays. Upon transfer of the FVIII-specific CD4⁺ FoxP3⁺ cells into hemophilia A mice, development of inhibitory antibodies in response to administering FVIII protein was completely suppressed. Suppression was extended for 2 months, even after transferred cells were no longer detectable in the secondary lymphoid organs of recipient animals. Upon co-transfer of FoxP3⁺-transduced cells with the B cell depleting anti-CD20 into mice with pre-existing inhibitory antibodies to FVIII, the escalation of inhibitory antibody titers in response to subsequent FVIII protein therapy was dramatically reduced. We conclude that reprogramed FoxP3 expressing cells are capable of inducing the in vivo conversion of endogenous FVIII peripheral Tregs, which results in sustained suppression of FVIII inhibitors caused by replacement therapy in recipient hemophilia A animals.

Evaluation of the impact of conventional immunosuppressant on the establishment of murine transplantation tolerance - an experimental study

Regulatory T cells (Tregs) play a significant role in immune tolerance. Since Treg function deeply depends on Interleukin-2 signaling, calcineurin inhibitors could affect their suppressive potentials, whereas mammalian target of rapamycin (mTOR) inhibitors may have less impact, as mTOR signaling is not fundamental to Treg proliferation. We previously reported a novel mixed hematopoietic chimerism induction regimen that promotes Treg proliferation by stimulating invariant natural killer T cells under CD40 blockade. Here, we use a mouse model to show the impact of tacrolimus (TAC) or everolimus (EVL) on the establishment of chimerism and Treg proliferation in the regimen. In the immunosuppressive drug-dosing phase, peripheral blood chimerism was comparably enhanced by both TAC and EVL. After dosing was discontinued, TAC-treated mice showed gradual graft rejection, whereas EVL-treated mice sustained long-term robust chimerism. Tregs of TAC-treated mice showed lower expression of both Ki67 and cytotoxic T lymphocyte antigen-4 (CTLA-4), and lower suppressive activity in vitro than those of EVL-treated mice, indicating that TAC negatively impacted the regimen by interfering with Treg proliferation and activation. Our results suggest that the usage of calcineurin inhibitors should be avoided if utilizing the regimen to induce Tregs in vivo for the establishment of mixed hematopoietic chimerism.

Therapeutic application of T regulatory cells in composite tissue allotransplantation

With growing number of cases in recent years, composite tissue allotransplantation (CTA) has been improving the quality of life of patient who seeks reconstruction and repair of damaged tissues. Composite tissue allografts are heterogeneous. They are composed of a variety of tissue types, including skin, muscle, vessel, bone, bone marrow, lymph nodes, nerve, and tendon. As a primary target of CTA, skin has high antigenicity with a rich repertoire of resident cells that play pivotal roles in immune surveillance. In this regard, understanding the molecular mechanisms involved in immune rejection in the skin would be essential to achieve successful CTA. Although scientific evidence has proved the necessity of immunosuppressive drugs to prevent rejection of allotransplanted tissues, there remains a lingering dilemma due to the lack of specificity of targeted immunosuppression and risks of side effects. A cumulative body of evidence has demonstrated T regulatory (Treg) cells have critical roles in induction of immune tolerance and immune homeostasis in preclinical and clinical studies. Presently, controlling immune susceptible characteristics of CTA with adoptive transfer of Treg cells is being considered promising and it has drawn great interests. This updated review will focus on a dominant form of Treg cells expressing CD4+CD25+ surface molecules and a forkhead box P3 transcription factor with immune tolerant and immune homeostasis activities. For future application of Treg cells as therapeutics in CTA, molecular and cellular characteristics of CTA and immune rejection, Treg cell development and phenotypes, Treg cell plasticity and stability, immune tolerant functions of Treg cells in CTA in preclinical studies, and protocols for therapeutic application of Treg cells in clinical settings are addressed in this review. Collectively, Treg cell therapy in CTA seems feasible with promising perspectives. However, the extreme high immunogenicity of CTA warrants caution.

CD147 blockade as a potential and novel treatment of graft rejection

Cluster of differentiation (CD)147 is highly involved in the T cell activation process. High CD147 expression is observed on the surfaces of activated T cells, particularly CD4+ T cells. In organ transplantation, it is important to prevent graft rejection resulting from the excessive activation of T cells, particularly CD4+ T cells, which exhibit a key role in amplifying the immune response. The present study aimed to investigate the effects of CD147 blockade in vitro and in vivo and used a transplant rejection system to assess the feasibility of utilizing CD147 antibody-based immunosuppressant drugs for the treatment of graft rejection. The effects of CD147 antibodies were evaluated on lymphocyte proliferation stimulated by phytohemagglutinin or CD3/CD28 magnetic beads and in a one-way mixed lymphocyte reaction (MLR) system in vitro. For the in vivo analysis, an allogeneic skin transplantation mouse model was used. CD147 antibodies were effective against lymphocytes, particularly CD4+T lymphocytes, and were additionally effective in the one-way MLR system. In the allogeneic skin transplantation mouse model, the survival of transplanted skin was extended in the CD147 antibody-treated group. Furthermore, the level of inflammatory cell infiltration in transplanted skin was reduced. CD147 blockade decreased the serum levels of interleukin (IL)-17 and the proportions of peripheral blood CD4+ and CD8+ memory T cells. The data demonstrated that CD147 blockade suppressed skin graft rejection, primarily by suppressing CD4+T and memory T cell proliferation, indicating that CD147 exhibits great potential as a target of immunosuppressant drugs.

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.

Expression of a Chimeric Antigen Receptor Specific for Donor HLA Class I Enhances the Potency of Human Regulatory T Cells in Preventing Human Skin Transplant Rejection

Regulatory T cell (Treg) therapy using recipient-derived Tregs expanded ex vivo is currently being investigated clinically by us and others as a means of reducing allograft rejection following organ transplantation. Data from animal models has demonstrated that adoptive transfer of allospecific Tregs offers greater protection from graft rejection compared to polyclonal Tregs. Chimeric antigen receptors (CAR) are clinically translatable synthetic fusion proteins that can redirect the specificity of T cells toward designated antigens. We used CAR technology to redirect human polyclonal Tregs toward donor-MHC class I molecules, which are ubiquitously expressed in allografts. Two novel HLA-A2-specific CARs were engineered: one comprising a CD28-CD3ζ signaling domain (CAR) and one lacking an intracellular signaling domain (ΔCAR). CAR Tregs were specifically activated and significantly more suppressive than polyclonal or ΔCAR Tregs in the presence of HLA-A2, without eliciting cytotoxic activity. Furthermore, CAR and ΔCAR Tregs preferentially transmigrated across HLA-A2-expressing endothelial cell monolayers. In a human skin xenograft transplant model, adoptive transfer of CAR Tregs alleviated the alloimmune-mediated skin injury caused by transferring allogeneic peripheral blood mononuclear cells more effectively than polyclonal Tregs. Our results demonstrated that the use of CAR technology is a clinically applicable refinement of Treg therapy for organ transplantation.

Donor bone marrow cells are essential for iNKT cell-mediated Foxp3+ Treg cell expansion in a murine model of transplantation tolerance

Mixed chimerism induction is the most reliable method for establishing transplantation tolerance. We previously described a novel treatment using a suboptimal dose of anti-CD40 ligand (anti-CD40L) and liposomal formulation of a ligand for invariant natural killer T cells administered to sub-lethally irradiated recipient mice after donor bone marrow cell (BMC) transfer. Recipient mice treated with this regimen showed expansion of a Foxp3-positive regulatory T(Treg) cell phenotype, and formation of mixed chimera. However, the mechanism of expansion and bioactivity of Treg cells remains unclear. Here, we examine the role of donor BMCs in the expansion of bioactive Treg cells. The mouse model was transplanted with a heart allograft the day after treatment. The results showed that transfer of spleen cells in place of BMCs failed to deplete host interferon (IFN)-γ-producing CD8⁺ T cells, expand host Ki67⁺ CD4⁺ CD25⁺ Foxp3⁺ Treg cells, and prolong graft survival. Severe combined immunodeficiency mice who received Treg cells obtained from BMC-recipients accepted skin grafts in an allo-specific manner. Myeloid-derived suppressor cells, which were a copious cell subset in BMCs, enhanced the Ki67 expression of Treg cells. This suggests that donor BMCs are indispensable for the expansion of host bioactive Treg cells in our novel treatment for transplant tolerance induction.

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.

Regulatory T cells in kidney disease and transplantation

Regulatory T cells (Tregs) have been shown to be important in maintaining immune homeostasis and preventing autoimmune disease, including autoimmune kidney disease. It is also likely that they play a role in limiting kidney transplant rejection and potentially in promoting transplant tolerance. Although other subsets of Tregs exist, the most potent and well-defined Tregs are the Foxp3 expressing CD4(+) Tregs derived from the thymus or generated peripherally. These CD4(+)Foxp3(+) Tregs limit autoimmune renal disease in animal models, especially chronic kidney disease, and kidney transplantation. Furthermore, other subsets of Tregs, including CD8 Tregs, may play a role in immunosuppression in kidney disease. The development and protective mechanisms of Tregs in kidney disease and kidney transplantation involve multiple mechanisms of suppression. Here we review the development and function of CD4(+)Foxp3(+) Tregs. We discuss the specific application of Tregs as a therapeutic strategy to prevent kidney disease and to limit kidney transplant rejection and detail clinical trials in this area of transplantation.

Incomplete clonal deletion as prerequisite for tissue-specific minor antigen tolerization

Central clonal deletion has been considered the critical factor responsible for the robust state of tolerance achieved by chimerism-based experimental protocols, but split-tolerance models and the clinical experience are calling this assumption into question. Although clone-size reduction through deletion has been shown to be universally required for achieving allotolerance, it remains undetermined whether it is sufficient by itself. Therapeutic Treg treatment induces chimerism and tolerance in a stringent murine BM transplantation model devoid of myelosuppressive recipient treatment. In contrast to irradiation chimeras, chronic rejection (CR) of skin and heart allografts in Treg chimeras was permanently prevented, even in the absence of complete clonal deletion of donor MHC-reactive T cells. We show that minor histocompatibility antigen mismatches account for CR in irradiation chimeras without global T cell depletion. Furthermore, we show that Treg therapy-induced tolerance prevents CR in a linked suppression-like fashion, which is maintained by active regulatory mechanisms involving recruitment of thymus-derived Tregs to the graft. These data suggest that highly efficient intrathymic and peripheral deletion of donor-reactive T cells for specificities expressed on hematopoietic cells preclude the expansion of donor-specific Tregs and, hence, do not allow for spreading of tolerance to minor specificities that are not expressed by donor BM.

Missed, Not Missing: Phylogenomic Evidence for the Existence of Avian FoxP3

The Forkhead box transcription factor FoxP3 is pivotal to the development and function of regulatory T cells (Tregs), which make a major contribution to peripheral tolerance. FoxP3 is believed to perform a regulatory role in all the vertebrate species in which it has been detected. The prevailing view is that FoxP3 is absent in birds and that avian Tregs rely on alternative developmental and suppressive pathways. Prompted by the automated annotation of foxp3 in the ground tit (Parus humilis) genome, we have questioned this assumption. Our analysis of all available avian genomes has revealed that the foxp3 locus is missing, incomplete or of poor quality in the relevant genomic assemblies for nearly all avian species. Nevertheless, in two species, the peregrine falcon (Falco peregrinus) and the saker falcon (F. cherrug), there is compelling evidence for the existence of exons showing synteny with foxp3 in the ground tit. A broader phylogenomic analysis has shown that FoxP3 sequences from these three species are similar to crocodilian sequences, the closest living relatives of birds. In both birds and crocodilians, we have also identified a highly proline-enriched region at the N terminus of FoxP3, a region previously identified only in mammals.

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.

Use of hematopoietic cell transplants to achieve tolerance in patients with solid organ transplants

The goals of tolerance in patients with solid organ transplants are to eliminate the lifelong need for immunosuppressive (IS) drugs and to prevent graft loss due to rejection or drug toxicity. Tolerance with complete withdrawal of IS drugs has been achieved in recipients of HLA-matched and mismatched living donor kidney transplants in 3 medical centers using hematopoietic cell transplants to establish mixed or complete chimerism.

Autologous stem cell transplantation aids autoimmune patients by functional renewal and TCR diversification of regulatory T cells

Autologous HSCT induces functional renewal of regulatory T cells as well as a strong Treg TCR diversification in autoimmune patients. Adding regulatory T cells to the graft does not lead to additional clinical improvement but results in delayed donor T-cell reconstitution.

Sphingosine 1-Phosphate Receptor 3-Deficient Dendritic Cells Modulate Splenic Responses to Ischemia-Reperfusion Injury

The plasticity of dendritic cells (DCs) permits phenotypic modulation ex vivo by gene expression or pharmacologic agents, and these modified DCs can exert therapeutic immunosuppressive effects in vivo through direct interactions with T cells, either inducing T regulatory cells (T(REG)s) or causing anergy. Sphingosine 1-phosphate (S1P) is a sphingolipid and the natural ligand for five G protein-coupled receptors (S1P1, S1P2, S1P3, S1P4, and S1P5), and S1PR agonists reduce kidney ischemia-reperfusion injury (IRI) in mice. S1pr3(-/-)mice are protected from kidney IRI, because DCs do not mature. We tested the therapeutic advantage of S1pr3(-/-) bone marrow-derived dendritic cell (BMDC) transfers in kidney IRI. IRI produced a rise in plasma creatinine (PCr) levels in mice receiving no cells (NCs) and mice pretreated with wild-type (WT) BMDCs. However, S1pr3(-/-) BMDC-pretreated mice were protected from kidney IRI. S1pr3(-/-) BMDC-pretreated mice had significantly higher numbers of splenic T(REG)s compared with NC and WT BMDC-pretreated mice. S1pr3(-/-) BMDCs did not attenuate IRI in splenectomized, Rag-1(-/-), or CD11c(+) DC-depleted mice. Additionally, S1pr3(-/-) BMDC-dependent protection required CD169(+)marginal zone macrophages and the macrophage-derived chemokine CCL22 to increase splenic CD4(+)Foxp3(+) T(REG)s. Pretreatment with S1pr3(-/-) BMDCs also induced T(REG)-dependent protection against IRI in an allogeneic mouse model. In summary, adoptively transferred S1pr3(-/-) BMDCs prevent kidney IRI through interactions within the spleen and expansion of splenic CD4(+)Foxp3(+) T(REG)s. We conclude that genetically induced deficiency of S1pr3 in allogenic BMDCs could serve as a therapeutic approach to prevent IRI-induced AKI.

Methyl-Guanine-Methyl-Transferase Transgenic Bone Marrow Transplantation Allows N,N-bis(2-chloroethyl)-Nitrosourea Driven Donor Mixed-Chimerism Without Graft-Versus-Host Disease, and With Donor-Specific Allograft Tolerance

BACKGROUND

Transplant tolerance has been achieved by mixed chimerism in animal models and in a limited number of kidney transplant patients. However, these mixed-chimerism strategies were limited either by loss of long-term mixed chimerism or risk of graft-versus-host disease (GVHD). Selective bone marrow (BM) engraftment using marrow protective strategies are currently reaching clinical use. In this study, we tested the utility of methyl-guanine-methyl-transferase (MGMT)-transgenic-C57BL/6 BM into a major histocompatibility complex mismatched-BALB/c model followed by N,N-bis(2-chloroethyl)-nitrosourea (BCNU) treatment to enhance donor-cell engraftment and then evaluated transplant tolerance induction.

METHODS

A single-dose of anti-CD8 antibody and busulfan was administered into BALB/c-host-mice at day 1. The BALB/c-mice also received costimulatory blockade through multiple-doses of anti-CD40L antibody. 10 × 10(6) BM-cells from MGMT-transgenic-mice were transplanted into host BALB/c mice at day 0. The BCNU was administered at 4 time points after BM transplantation (BMT). Heterotopic donor C57BL/6 cardiac allografts were performed at day 243 after BMT. Skin transplantation with third-party CBA, host BALB/c and donor C57BL/6 grafts was performed at day 358 after BMT.

RESULTS

The BALB/c-mice showed long-term stable and high-level donor-cell engraftment with MGMT transgenic C57BL/6 BMT after BCNU treatment, demonstrating full reconstitution and donor cardiac-allograft tolerance and no GVHD with expanded donor and host Foxp3 T regulatory cells. Further, skin grafts from donor, host, and third party showed good immune function with rejection of third-party grafts from all mice and benefit from enhanced chimerism after BCNU with less cell infiltrate and no chronic rejection in the donor skin grafts of BCNU treated mice compared no BCNU treated mice.

CONCLUSIONS

High-level mixed chimerism without GVHD can be achieved using MGMT transgenic BM in a mixed-chimerism model receiving BCNU across a major histocompatibility complex mismatch. Enhanced mixed chimerism leads to long-term donor-specific allograft tolerance.

Now or never? The case for cell-based immunosuppression in kidney transplantation

By exploiting mechanisms of immunological regulation against donor alloantigen, it may be possible to reduce the dependence of kidney transplant recipients upon calcineurin inhibitor-based maintenance immunosuppression. One means to strengthen regulatory responses is treating recipients with preparations of regulatory cells obtained by ex vivo manipulation. This strategy, which is a well-established experimental method, has been developed to the point that early-phase clinical trials in kidney transplantation are now feasible. Cell-based therapies represent a radical departure from conventional treatment, so what grounds are there for this new approach? This article offers a three-part justification for trialing cell-based therapies in kidney transplantation: first, a clinical need for alternatives to standard immunosuppression is identified, based on the inadequacies of calcineurin inhibitor-based regimens in preventing late allograft loss; second, a mechanistic explanation of how cell-based therapies might address this clinical need is given; and third, the possible benefit to patients is weighed against the potential risks of cell-based immunosuppressive therapy. It is concluded that the safety of cell-based immunosuppressive therapy will not be greatly improved by further basic scientific and preclinical development. Only trials in humans can now tell us whether cell-based therapy is likely to benefit kidney transplant recipients, but these should be conservative in design to minimize any potential harm to patients.

'Default' generated neonatal regulatory T cells are hypomethylated at conserved non-coding sequence 2 and promote long-term cardiac allograft survival

Regulatory T (Treg) cells play an important role in the maintenance of immune self-tolerance and homeostasis. We previously reported that neonatal CD4(+) T cells have an intrinsic 'default' mechanism to become Treg (neoTreg) cells in response to T-cell receptor (TCR) stimulation. However, the underlying mechanisms are unclear and the effects of neoTreg cells on regulating immune responses remain unknown. Due to their involvement in Foxp3 regulation, we examined the role of DNA methyltransferase 1 (DNMT1) and DNMT3b during the induction of neoTreg cells in the Foxp3(gfp) mice. The function of neoTreg cells was assessed in an acute allograft rejection model established in RAG2(-/-) mice with allograft cardiac transplantation and transferred with syngeneic CD4(+) effector T cells. Following ex vivo TCR stimulation, the DNMT activity was increased threefold in adult CD4(+) T cells, but not significantly increased in neonatal cells. However, adoptively transferred neoTreg cells significantly prolonged cardiac allograft survival (mean survival time 47 days, P < 0.001) and maintained Foxp3 expression similar to natural Treg cells. The neoTreg cells were hypomethylated at the conserved non-coding DNA sequence 2 locus of Foxp3 compared with adult Treg cells. The DNMT antagonist 5-aza-2'-deoxycytidine (5-Aza) induced increased Foxp3 expression in mature CD4(+) T cells. 5-Aza-inducible Treg cells combined with continuous 5-Aza treatment prolonged graft survival. These results indicate that the 'default' pathway of neoTreg cell differentiation is associated with reduced DNMT1 and DNMT3b response to TCR stimulus. The neoTreg cells may be a strategy to alleviate acute allograft rejection.

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.

Ex Vivo Expanded Autologous Polyclonal Regulatory T Cells Suppress Inhibitor Formation in Hemophilia

Adoptive cell therapy utilizing ex vivo expanded polyclonal CD4⁺CD25⁺FOXP3⁺ regulatory T cells (Treg) is in use in clinical trials for the treatment of type 1 diabetes and prevention of graft vs host disease in bone marrow transplantation. Here we seek to evaluate this approach in the treatment of inherited protein deficiencies, i.e. hemophilia, which is often complicated by antibody formation against the therapeutic protein. Treg from mice that express GFP-marked FoxP3 were highly purified by two-step magnetic/flow sorting and ex vivo expanded 50- to 80-fold over a 2-week culture period upon stimulation with antibody-coated microbeads. FoxP3 expression was maintained in >80% of expanded Treg, which also expressed high levels of CD62L and CTLA-4. Transplanted Treg suppressed inhibitory antibody formation against coagulation factors VIII and IX in protein and gene therapies in strain-matched hemophilia A and B mice, including in mice with pre-existing antibodies. Although transplanted Treg became undetectable within two weeks, suppression persisted for >2 months. Additional studies suggested that antigen-specific suppression emerged due to induction of endogenous Treg. The outcomes of these studies support the concept that cell therapy with ex vivo expanded autologous Treg can be used successfully to minimize immune responses in gene and protein replacement therapies.