Tolerance and chimerism after renal and hematopoietic-cell transplantation

Combination cell therapy leads to clonal deletion of donor-specific T cells in kidney transplant recipients

BACKGROUND

Induction of donor-specific tolerance is a promising approach to achieve long-term graft patency in transplantation with little to no maintenance immunosuppression. Changes to the recipient's T cell receptor (TCR) repertoire are understood to play a pivotal role in the establishment of a robust state of tolerance in chimerism-based transplantation protocols.

METHODS

We investigated changes to the TCR repertoires of patients participating in an ongoing prospective, controlled, phase I/IIa trial designed to evaluate the safety and efficacy of combination cell therapy in living donor kidney transplantation. Using high-throughput sequencing, we characterized the repertoires of six kidney recipients who also received bone marrow from the same donor (CKBMT), together with an infusion of polyclonal autologous Treg cells instead of myelosuppression.

FINDINGS

Patients undergoing combination cell therapy exhibited partial clonal deletion of donor-reactive CD4+ T cells at one, three, and six months post-transplant, compared to control patients receiving the same immunosuppression regimen but no cell therapy (p = 0.024). The clonality, R20 and turnover rates of the CD4+ and CD8+ TCR repertoires were comparable in both groups, showing our protocol caused no excessive repertoire shift or loss of diversity. Treg clonality was lower in the case group than in control (p = 0.033), suggesting combination cell therapy helps to preserve Treg diversity.

INTERPRETATION

Overall, our data indicate that combining Treg cell therapy with CKBMT dampens the alloimmune response to transplanted kidneys in humans in the absence of myelosuppression.

FUNDING

This study was funded by the Vienna Science and Technology Fund (WWTF).

Chimeric Cell Therapies as a Novel Approach for Duchenne Muscular Dystrophy (DMD) and Muscle Regeneration

Chimerism-based strategies represent a pioneering concept which has led to groundbreaking advancements in regenerative medicine and transplantation. This new approach offers therapeutic potential for the treatment of various diseases, including inherited disorders. The ongoing studies on chimeric cells prompted the development of Dystrophin-Expressing Chimeric (DEC) cells which were introduced as a potential therapy for Duchenne Muscular Dystrophy (DMD). DMD is a genetic condition that leads to premature death in adolescent boys and remains incurable with current methods. DEC therapy, created via the fusion of human myoblasts derived from normal and DMD-affected donors, has proven to be safe and efficacious when tested in experimental models of DMD after systemic-intraosseous administration. These studies confirmed increased dystrophin expression, which correlated with functional and morphological improvements in DMD-affected muscles, including cardiac, respiratory, and skeletal muscles. Furthermore, the application of DEC therapy in a clinical study confirmed its long-term safety and efficacy in DMD patients. This review summarizes the development of chimeric cell technology tested in preclinical models and clinical studies, highlighting the potential of DEC therapy in muscle regeneration and repair, and introduces chimeric cell-based therapies as a promising, novel approach for muscle regeneration and the treatment of DMD and other neuromuscular disorders.

Biodistribution and Safety of Human Multi-Chimeric Cells After Systemic Intraosseous and Intravenous Administration in the Experimental Mouse Model

Cellular therapies provide promising options for inducing tolerance in transplantation of solid organs, bone marrow, and vascularized composite allografts. However, novel tolerance-inducing protocols remain limited, despite extensive research. We previously introduced and characterized a human multi-chimeric cell (HMCC) line, created through ex vivo fusion of human umbilical cord blood (UCB) cells derived from three unrelated donors. In this study, we assessed in vivo biodistribution and safety of HMCCs in the NOD.Cg-PrkdcscidIl2rgtm1Wjl/SzJ NOD scid gamma (NSG) mouse model. Twenty-four NSG mice were randomly assigned to four groups (n = 6/group) and received intraosseous (IO.) or intravenous (IV.) injections of 0.6 × 106 donor UCB cells or fused HMCC: Group 1-UCB (IO.), Group 2-UCB (IV.), Group 3-HMCC (IO.), and Group 4-HMCC (IV.). Hematopoietic phenotype maintenance and presence of human leukocyte antigens (HLA), class I antigens, in the selected lymphoid and nonlymphoid organs were assessed by flow cytometry. Weekly evaluation and magnetic resonance imaging (MRI) assessed HMCC safety. Comparative analysis of delivery routes revealed significant differences in HLA class I percentages for IO.: 1.83% ± 0.79%, versus IV. delivery: 0.04% ± 0.01%, P < 0.01, and hematopoietic stem cell marker percentages of CD3 (IO.: 1.41% ± 0.04%, vs. IV.: 0.07% ± 0.01%, P < 0.05) and CD4 (IO.: 2.74% ± 0.31%, vs. IV.: 0.59% ± 0.11%, P < 0.01). Biodistribution analysis after IO. delivery confirmed HMCC presence in lymphoid organs and negligible presence in nonlymphoid organs, except for lung (IO.: 0.19% ± 0.06%, vs. IV.: 6.33% ± 0.56%, P < 0.0001). No evidence of tumorigenesis was observed by MRI at 90 days following IO. and IV. administration of HMCC. This study confirmed biodistribution and safety of HMCC therapy in the NSG mouse model, both following IO. and IV. administration. However, IO. delivery route confirmed higher efficacy of engraftment and safety profile, introducing HMCCs as a novel cell-based therapeutic approach with promising clinical applications in solid organ, bone marrow, and vascularized composite allotransplantation transplantation.

Combining Treg Therapy With Donor Bone Marrow Transplantation: Experimental Progress and Clinical Perspective

Donor-specific tolerance remains a goal in transplantation because it could improve graft survival and reduce morbidity. Cotransplantation of donor hematopoietic cells to achieve chimerism is a promising approach for tolerance induction, which was successfully tested in clinical trials. However, current protocols are associated with side effects related to the myelosuppressive recipient conditioning, which makes it difficult to introduce them as standard therapy. More recently, adoptive cell therapy with polyclonal or donor-specific regulatory T cells (Treg) proved safe and feasible in several transplant trials, but it is unclear whether it can induce tolerance on its own. The combination of both approaches-Treg therapy and hematopoietic cell transplantation-leads to chimerism and tolerance without myelosuppressive treatment in murine models. Treg therapy promotes engraftment of allogeneic hematopoietic cells, reducing conditioning requirements and enhancing regulatory mechanisms maintaining tolerance. This review discusses possible modes of action of transferred Treg in experimental chimerism models and describes translational efforts investigating the potent synergy of Treg and chimerism.

Tolerance in intestinal transplantation

Intestinal transplantation (ITx) is highly immunogenic, resulting in the need for high levels of immunosuppression, with frequent complications along with high rejection rates. Tolerance induction would provide a solution to these limitations. Detailed studies of alloreactive T cell clones as well as multiparameter flow cytometry in the graft and peripheral tissues have provided evidence for several tolerance mechanisms that occur spontaneously following ITx, which might provide targets for further interventions. These include the frequent occurrence of macrochimerism and engraftment in the recipient bone marrow of donor hematopoietic stem and progenitor cells carried in the allograft. These phenomena are seen most frequently in recipients of multivisceral transplants and are associated with reduced rejection rates. They reflect powerful graft-vs-host responses that enter the peripheral lymphoid system and bone marrow after expanding within and emigrating from the allograft. Several mechanisms of tolerance that may result from this lymphohematopoietic graft-vs-host response are discussed. Transcriptional profiling in quiescent allografts reveals tolerization of pre-existing host-vs-graft-reactive T cells that enter the allograft mucosa and become tissue-resident memory cells. Dissection of the pathways driving and maintaining this tolerant tissue-resident state among donor-reactive T cells will allow controlled tolerance induction through specific therapeutic approaches.

Early allogeneic immune modulation after establishment of donor hematopoietic cell-induced mixed chimerism in a nonhuman primate kidney transplant model

Background

Mixed lymphohematopoietic chimerism is a proven strategy for achieving operational transplant tolerance, though the underlying immunologic mechanisms are incompletely understood.

Methods

A post-transplant, non-myeloablative, tomotherapy-based total lymphoid (TLI) irradiation protocol combined with anti-thymocyte globulin and T cell co-stimulatory blockade (belatacept) induction was applied to a 3-5 MHC antigen mismatched rhesus macaque kidney and hematopoietic cell transplant model. Mechanistic investigations of early (60 days post-transplant) allogeneic immune modulation induced by mixed chimerism were conducted.

Results

Chimeric animals demonstrated expansion of circulating and graft-infiltrating CD4+CD25+Foxp3+ regulatory T cells (Tregs), as well as increased differentiation of allo-protective CD8+ T cell phenotypes compared to naïve and non-chimeric animals. In vitro mixed lymphocyte reaction (MLR) responses and donor-specific antibody production were suppressed in animals with mixed chimerism. PD-1 upregulation was observed among CD8+ T effector memory (CD28-CD95+) subsets in chimeric hosts only. PD-1 blockade in donor-specific functional assays augmented MLR and cytotoxic responses and was associated with increased intracellular granzyme B and extracellular IFN-γ production.

Conclusions

These studies demonstrated that donor immune cell engraftment was associated with early immunomodulation via mechanisms of homeostatic expansion of Tregs and early PD-1 upregulation among CD8+ T effector memory cells. These responses may contribute to TLI-based mixed chimerism-induced allogenic tolerance.

Hematopoietic cell-based and non-hematopoietic cell-based strategies for immune tolerance induction in living-donor renal transplantation: A systematic review

INTRODUCTION

Despite its use to prevent acute rejection, lifelong immunosuppression can adversely impact long-term patient and graft outcomes. In theory, immunosuppression withdrawal is the ultimate goal of kidney transplantation, and is made possible by the induction of immunological tolerance. The purpose of this paper is to review the safety and efficacy of immune tolerance induction strategies in living-donor kidney transplantation, both chimerism-based and non-chimerism-based. The impact of these strategies on transplant outcomes, including acute rejection, allograft function and survival, cost, and immune monitoring, will also be discussed.

MATERIALS AND METHODS

Databases such as PubMed, Scopus, and Web of Science, as well as additional online resources such as EBSCO, were exhaustively searched. Adult living-donor kidney transplant recipients who developed chimerism-based tolerance after concurrent bone marrow or hematopoietic stem cell transplantation or those who received non-chimerism-based, non-hematopoietic cell therapy using mesenchymal stromal cells, dendritic cells, or regulatory T cells were studied between 2000 and 2021. Individual sources of evidence were evaluated critically, and the strength of evidence and risk of bias for each outcome of the transplant tolerance study were assessed.

RESULTS

From 28,173 citations, 245 studies were retrieved after suitable exclusion and duplicate removal. Of these, 22 studies (2 RCTs, 11 cohort studies, 6 case-control studies, and 3 case reports) explicitly related to both interventions (chimerism- and non-chimerism-based immune tolerance) were used in the final review process and were critically appraised. According to the findings, chimerism-based strategies fostered immunotolerance, allowing for the safe withdrawal of immunosuppressive medications. Cell-based therapy, on the other hand, frequently did not induce tolerance except for minimising immunosuppression. As a result, the rejection rates, renal allograft function, and survival rates could not be directly compared between these two groups. While chimerism-based tolerance protocols posed safety concerns due to myelosuppression, including infections and graft-versus-host disease, cell-based strategies lacked these adverse effects and were largely safe. There was a lack of direct comparisons between HLA-identical and HLA-disparate recipients, and the cost implications were not examined in several of the retrieved studies. Most studies reported successful immunosuppressive weaning lasting at least 3 years (ranging up to 11.4 years in some studies), particularly with chimerism-based therapy, while only a few investigators used immune surveillance techniques. The studies reviewed were often limited by selection, classification, ascertainment, performance, and attrition bias.

CONCLUSIONS

This review demonstrates that chimerism-based hematopoietic strategies induce immune tolerance, and a substantial number of patients are successfully weaned off immunosuppression. Despite the risk of complications associated with myelosuppression. Non-chimerism-based, non-hematopoietic cell protocols, on the other hand, have been proven to facilitate immunosuppression minimization but seldom elicit immunological tolerance. However, the results of this review must be interpreted with caution because of the non-randomised study design, potential confounding, and small sample size of the included studies. Further validation and refinement of tolerogenic protocols in accordance with local practice preferences is also warranted, with an emphasis on patient selection, cost ramifications, and immunological surveillance based on reliable tolerance assays.

Impact of T Lymphocytes Isolated from Liver Perfusate of Deceased Brain Donors on Kidney Transplantation: Preliminary Evidence and Future Directions

BACKGROUND

Ischemia/reperfusion injury (IRI), acute rejection (AR), and delayed graft function (DGF) might occur as major complications following kidney transplantation. Thus, the identification of biomarkers for the IRI, AR, and/or DGF development becomes crucial as it may help to guide post-transplant management. Natural killer (NK) cells, hepatic interstitial T-lymphocytes (T-Li), and NK-T cells are crucial in both innate and adaptive immunity after abdominal solid organ transplantation. Hence, the aim of this study was to evaluate the impact of the immune system after graft reperfusion during KT in adults in order to identify predictive biomarkers.

METHODS

The NK, T-Li, and NK-T phenotypes and concentrations were retrospectively analyzed in a consecutive series of liver perfusates obtained after organ procurement flushing the abdominal cavity recovered from deceased brain donors (DBDs). Their percentage was compared with the renal transplant recipients' characteristics with kidneys taken from the same DCDs. The hepatic perfusate cells were purified by density gradient centrifugation. Flow cytometric investigation was used to determine their phenotype with the following immunological markers in order to determine the relative percentage of T-Li, NK-T, and NK cells: CD3, CD4, CD8, and CD56.

RESULTS

42 DBDs' liver perfusates were analyzed. The related clinical outcomes of kidney transplant recipients from 2010 to 2020 performed at our Institute were evaluated. Time in days of delayed functional recovery of transplanted kidneys (DGF) (p = 0.02) and the onset of secondary infection from a cytomegalovirus (p = 0.03) were significantly associated with the T-Li percentage. An increased relative risk (HR) of organ survival was significantly associated with the percent cell concentration of T-Li and time to DGF, on COX analysis, were (HR = 1.038, p = 0.04; and HR = 1.029, p = 0.01, respectively). None relevant clinical outcomes in kidney transplant patients were associated with the specificity of the NK and NK-T cell proportions.

CONCLUSIONS

A new potential role of T-Li cells was detected in the context of hepatic perfusate from DBDs. It could detect potential impacts in organ allocation, surgical procuring techniques, and in the analysis of IRI pathophysiological events.

Congenic hematopoietic stem cell transplantation promotes survival of heart allografts in murine models of acute and chronic rejection

The ability to induce tolerance would be a major advance in the field of solid organ transplantation. Here, we investigated whether autologous (congenic) hematopoietic stem cell transplantation (HSCT) could promote tolerance to heart allografts in mice. In an acute rejection model, fully MHC-mismatched BALB/c hearts were heterotopically transplanted into C57BL/6 (CD45.2) mice. One week later, recipient mice were lethally irradiated and reconstituted with congenic B6 CD45.1 Lin-Sca1+ckit+ cells. Recipient mice received a 14-day course of rapamycin both to prevent rejection and to expand regulatory T cells (Tregs). Heart allografts in both untreated and rapamycin-treated recipients that did not undergo HSCT were rejected within 33 days (median survival time = 8 days for untreated recipients, median survival time = 32 days for rapamycin-treated recipients), whereas allografts in HSCT-treated recipients had a median survival time of 55 days (P < 0.001 vs. both untreated and rapamycin-treated recipients). Enhanced allograft survival following HSCT was associated with increased intragraft Foxp3+ Tregs, reduced intragraft B cells, and reduced serum donor-specific antibodies. In a chronic rejection model, Bm12 hearts were transplanted into C57BL/6 (CD45.2) mice, and congenic HSCT was performed two weeks following heart transplantation. HSCT led to enhanced survival of allografts (median survival time = 70 days vs. median survival time = 28 days in untreated recipients, P < 0.01). Increased allograft survival post-HSCT was associated with prevention of autoantibody development and absence of vasculopathy. These data support the concept that autologous HSCT can promote immune tolerance in the setting of allotransplantation. Further studies to optimize HSCT protocols should be performed before this procedure is adopted clinically.

Helical TomoTherapy Total Lymphoid Irradiation and Hematopoietic Cell Transplantation for Kidney Transplant Tolerance in Rhesus Macaques

Development of a post-transplant kidney transplant tolerance induction protocol involving a novel total lymphoid irradiation (TLI) conditioning method in a rhesus macaque model is described. We examined the feasibility of acheiving tolerance to MHC 1-haplotype matched kidney transplants by establishing a mixed chimeric state with infusion of donor hematopoietic cells (HC) using TomoTherapy TLI. The chimeric state was hypothesized to permit the elimination of all immunosuppressive (IS) medications while preserving allograft function long-term without development of graft-versus-host-disease (GVHD) or rejection. An experimental group of 11 renal transplant recipients received the tolerance induction protocol and outcomes were compared to a control group (n = 7) that received the same conditioning but without donor HC infusion. Development of mixed chimerism and operational tolerance was accomplished in two recipients in the experimental group. Both recipients were withdrawn from all IS and continued to maintain normal renal allograft function for 4 years without rejection or GVHD. None of the animals in the control group achieved tolerance when IS was eliminated. This novel experimental model demonstrated the feasibility for inducing of long-term operational tolerance when mixed chimerism is achieved using a TLI post-transplant conditioning protocol in 1-haplotype matched non-human primate recipients of combined kidney and HC transplantation.

Biomaterials-based immunomodulation enhances survival of murine vascularized composite allografts

Vascularized composite allotransplantation (VCA) is a restorative option for patients suffering from severe tissue defects not amenable to conventional reconstruction. However, the toxicities associated with life-long multidrug immunosuppression to enable allograft survival and induce immune tolerance largely limit the broader application of VCA. Here, we investigate the potential of targeted immunomodulation using CTLA4-Ig combined with a biological porcine-derived extracellular matrix (ECM) scaffold that elicits a pro-regenerative Th2 response to promote allograft survival and regulate the inflammatory microenvironment in a stringent mouse orthotopic hind limb transplantation model (BALB/c to C57BL/6). The median allograft survival time (MST) increased significantly from 15.0 to 24.5 days (P = 0.0037; Mantel-Cox test) after adding ECM to the CTLA4-Ig regimen. Characterization of the immune infiltration shows a pro-regenerative phenotype prevails over those associated with inflammation and rejection including macrophages (F4/80^hi+CD206^hi+MHCII^low), eosinophils (F4/80^lowSiglec-F⁺), and T helper 2 (Th2) T cells (CD4⁺IL-4⁺). This was accompanied by an increased expression of genes associated with a Type 2 polarized immune state such as Il4, Ccl24, Arg1 and Ym1 within the graft. Furthermore, when ECM was applied along with a clinically relevant combination of CTLA4-Ig and Rapamycin, allograft survival was prolonged from 33.0 to 72.5 days (P = 0.0067; Mantel-Cox test). These studies implicate the clinical exploration of combined regimens involving local application of pro-regenerative, immunomodulatory biomaterials in surgical wound sites with targeted co-stimulatory blockade to reduce adverse effects of immunosuppression and enhance graft survival in VCA.

Biased IL-2 signals induce Foxp3-rich pulmonary lymphoid structures and facilitate long-term lung allograft acceptance in mice

Transplantation of solid organs can be life-saving in patients with end-stage organ failure, however, graft rejection remains a major challenge. In this study, by pre-conditioning with interleukin-2 (IL-2)/anti-IL-2 antibody complex treatment biased toward IL-2 receptor α, we achieved acceptance of fully mismatched orthotopic lung allografts that remained morphologically and functionally intact for more than 90 days in immunocompetent mice. These allografts are tolerated by the actions of forkhead box p3 (Foxp3)⁺ regulatory T (Treg) cells that home to the lung allografts. Although counts of circulating Treg cells rapidly return to baseline following cessation of IL-2 treatment, Foxp3⁺ Treg cells persist in peribronchial and peribronchiolar areas of the grafted lungs, forming organized clusters reminiscent of inducible tertiary lymphoid structures (iTLS). These iTLS in lung allografts are made of Foxp3⁺ Treg cells, conventional T cells, and B cells, as evidenced by using microscopy-based distribution and neighborhood analyses. Foxp3-transgenic mice with inducible and selective deletion of Foxp3⁺ cells are unable to form iTLS in lung allografts, and these mice acutely reject lung allografts. Collectively, we report that short-term, high-intensity and biased IL-2 pre-conditioning facilitates acceptance of vascularized and ventilated lung allografts without the need of immunosuppression, by inducing Foxp3-controlled iTLS formation within allografts.

Donor derived hematopoietic stem cell niche transplantation facilitates mixed chimerism mediated donor specific tolerance

Compelling experimental evidence confirms that the robustness and longevity of mixed chimerism (MC) relies on the persistence and availability of donor-derived hematopoietic stem cell (HSC) niches in recipients. Based on our prior work in rodent vascularized composite allotransplantation (VCA) models, we hypothesize that the vascularized bone components in VCA bearing donor HSC niches, thus may provide a unique biologic opportunity to facilitate stable MC and transplant tolerance. In this study, by utilizing a series of rodent VCA models we demonstrated that donor HSC niches in the vascularized bone facilitate persistent multilineage hematopoietic chimerism in transplant recipients and promote donor-specific tolerance without harsh myeloablation. In addition, the transplanted donor HSC niches in VCA facilitated the donor HSC niches seeding to the recipient bone marrow compartment and contributed to the maintenance and homeostasis of stable MC. Moreover, this study provided evidences that chimeric thymus plays a role in MC-mediated transplant tolerance through a mechanism of thymic central deletion. Mechanistic insights from our study could lead to the use of vascularized donor bone with pre-engrafted HSC niches as a safe, complementary strategy to induce robust and stable MC-mediated tolerance in VCA or solid organ transplantation recipients.

Into the multi-omics era: Progress of T cells profiling in the context of solid organ transplantation

T cells are the common type of lymphocyte to mediate allograft rejection, remaining long-term allograft survival impeditive. However, the heterogeneity of T cells, in terms of differentiation and activation status, the effector function, and highly diverse T cell receptors (TCRs) have thus precluded us from tracking these T cells and thereby comprehending their fate in recipients due to the limitations of traditional detection approaches. Recently, with the widespread development of single-cell techniques, the identification and characterization of T cells have been performed at single-cell resolution, which has contributed to a deeper comprehension of T cell heterogeneity by relevant detections in a single cell - such as gene expression, DNA methylation, chromatin accessibility, surface proteins, and TCR. Although these approaches can provide valuable insights into an individual cell independently, a comprehensive understanding can be obtained when applied joint analysis. Multi-omics techniques have been implemented in characterizing T cells in health and disease, including transplantation. This review focuses on the thesis, challenges, and advances in these technologies and highlights their application to the study of alloreactive T cells to improve the understanding of T cell heterogeneity in solid organ transplantation.

Clinical trials for renal allograft tolerance induction through combined hematopoietic stem cell transplantation: A narrative review

During the last four decades, development of effective immunosuppressants has significantly improved short-term results of organ transplantation. However, long-term results have not been satisfactory due to chronic rejection or complications caused by immunosuppressive drugs. Therefore, induction of immunological tolerance of the transplanted organ is considered essential to improve the long-term results. Despite numerous tolerance strategies that have been successful in murine models, inducing hematopoietic chimerism through donor hematopoietic stem cell transplantation is the only method that reproducibly induces kidney allograft tolerance in nonhuman primates or humans. Combining kidney and hematopoietic stem cell transplantation to achieve allograft tolerance has now been attempted with different chimerism strategies. This review summarizes the status of current clinical trials on the induction of allograft tolerance. We also summarize recent studies to extend the chimerism approach to deceased donor transplant recipients.

CD4+CD25+ T regulatory cells in renal transplantation

The immune response to an allograft activates lymphocytes with the capacity to cause rejection. Activation of CD4+CD25+Foxp3+T regulatory cells (Treg) can down-regulate allograft rejection and can induce immune tolerance to the allograft. Treg represent <10% of peripheral CD4+T cells and do not markedly increase in tolerant hosts. CD4+CD25+Foxp3+T cells include both resting and activated Treg that can be distinguished by several markers, many of which are also expressed by effector T cells. More detailed characterization of Treg to identify increased activated antigen-specific Treg may allow reduction of non-specific immunosuppression. Natural thymus derived resting Treg (tTreg) are CD4+CD25+Foxp3+T cells and only partially inhibit alloantigen presenting cell activation of effector cells. Cytokines produced by activated effector cells activate these tTreg to more potent alloantigen-activated Treg that may promote a state of operational tolerance. Activated Treg can be distinguished by several molecules they are induced to express, or whose expression they have suppressed. These include CD45RA/RO, cytokine receptors, chemokine receptors that alter pathways of migration and transcription factors, cytokines and suppression mediating molecules. As the total Treg population does not increase in operational tolerance, it is the activated Treg which may be the most informative to monitor. Here we review the methods used to monitor peripheral Treg, the effect of immunosuppressive regimens on Treg, and correlations with clinical outcomes such as graft survival and rejection. Experimental therapies involving ex vivo Treg expansion and administration in renal transplantation are not reviewed.

Reduced Satb1 expression predisposes CD4+ T conventional cells to Treg suppression and promotes transplant survival

Limiting CD4+ T cell responses is important to prevent solid organ transplant rejection. In a mouse model of costimulation blockade-dependent cardiac allograft tolerance, we previously reported that alloreactive CD4+ conventional T cells (Tconvs) develop dysfunction, losing proliferative capacity. In parallel, induction of transplantation tolerance is dependent on the presence of regulatory T cells (Tregs). Whether susceptibility of CD4+ Tconvs to Treg suppression is modulated during tolerance induction is unknown. We found that alloreactive Tconvs from transplant tolerant mice had augmented sensitivity to Treg suppression when compared with memory T cells from rejector mice and expressed a transcriptional profile distinct from these memory T cells, including down-regulated expression of the transcription factor Special AT-rich sequence-binding protein 1 (Satb1). Mechanistically, Satb1 deficiency in CD4+ T cells limited their expression of CD25 and IL-2, and addition of Tregs, which express higher levels of CD25 than Satb1-deficient Tconvs and successfully competed for IL-2, resulted in greater suppression of Satb1-deficient than wild-type Tconvs in vitro. In vivo, Satb1-deficient Tconvs were more susceptible to Treg suppression, resulting in significantly prolonged skin allograft survival. Overall, our study reveals that transplantation tolerance is associated with Tconvs' susceptibility to Treg suppression, via modulated expression of Tconv-intrinsic Satb1. Targeting Satb1 in the context of Treg-sparing immunosuppressive therapies might be exploited to improve transplant outcomes.

Transplant Tolerance, Not Only Clonal Deletion

The quest to understand how allogeneic transplanted tissue is not rejected and how tolerance is induced led to fundamental concepts in immunology. First, we review the research that led to the Clonal Deletion theory in the late 1950s that has since dominated the field of immunology and transplantation. At that time many basic mechanisms of immune response were unknown, including the role of lymphocytes and T cells in rejection. These original observations are reassessed by considering T regulatory cells that are produced by thymus of neonates to prevent autoimmunity. Second, we review "operational tolerance" induced in adult rodents and larger animals such as pigs. This can occur spontaneously especially with liver allografts, but also can develop after short courses of a variety of rejection inhibiting therapies. Over time these animals develop alloantigen specific tolerance to the graft but retain the capacity to reject third-party grafts. These animals have a "split tolerance" as peripheral lymphocytes from these animals respond to donor alloantigen in graft versus host assays and in mixed lymphocyte cultures, indicating there is no clonal deletion. Investigation of this phenomenon excludes many mechanisms, including anti-donor antibody blocking rejection as well as anti-idiotypic responses mediated by antibody or T cells. This split tolerance is transferred to a second immune-depleted host by T cells that retain the capacity to effect rejection of third-party grafts by the same host. Third, we review research on alloantigen specific inhibitory T cells that led to the first identification of the CD4+CD25+T regulatory cell. The key role of T cell derived cytokines, other than IL-2, in promoting survival and expansion of antigen specific T regulatory cells that mediate transplant tolerance is reviewed. The precise methods for inducing and diagnosing operational tolerance remain to be defined, but antigen specific T regulatory cells are key mediators.

Establishment of Chimerism and Organ Transplant Tolerance in Laboratory Animals: Safety and Efficacy of Adaptation to Humans

The definition of immune tolerance to allogeneic tissue and organ transplants in laboratory animals and humans continues to be the acceptance of the donor graft, rejection of third-party grafts, and specific unresponsiveness of recipient immune cells to the donor alloantigens in the absence of immunosuppressive treatments. Actively acquired tolerance was achieved in mice more than 60 years ago by the establishment of mixed chimerism in neonatal mice. Once established, mixed chimerism was self-perpetuating and allowed for acceptance of tissue transplants in adults. Successful establishment of tolerance in humans has now been reported in several clinical trials based on the development of chimerism after combined transplantation of hematopoietic cells and an organ from the same donor. This review examines the mechanisms of organ graft acceptance after establishment of mixed chimerism (allo-tolerance) or complete chimerism (self-tolerance), and compares the development of graft versus host disease (GVHD) and graft versus tumor (GVT) activity in complete and mixed chimerism. GVHD, GVT activity, and complete chimerism are also discussed in the context of bone marrow transplantation to treat hematologic malignancies. The roles of transient versus persistent mixed chimerism in the induction and maintenance of tolerance and organ graft acceptance in animal models and clinical studies are compared. Key differences in the stability of mixed chimeras and tolerance induction in MHC matched and mismatched rodents, large laboratory animals, and humans are examined to provide insights into the safety and efficacy of translation of results of animal models to clinical trials.

Biomarkers of immune tolerance in kidney transplantation: an overview

Kidney failure, one of the most prevalent diseases in the world and with increasing incidence, is associated with substantial morbidity and mortality. Currently available modes of kidney replacement therapy include dialysis and kidney transplantation. Though kidney transplantation is the preferred and ideal mode of kidney replacement therapy, this modality, however, is not without its risks. Kidney transplant recipients are constantly at risk of complications associated with immunosuppression, namely, opportunistic infections (e.g., Epstein-Barr virus and cytomegalovirus infections), post-transplant lymphoproliferative disorder^, and complications associated with immunosuppressants (e.g., calcineurin inhibitor- and corticosteroid-associated new onset diabetes after transplantation and calcineurin inhibitor-associated nephrotoxicity). Transplantation tolerance, an acquired state in which immunocompetent recipients have developed donor-specific unresponsiveness, may be the Holy Grail in enabling optimal allograft survival and obviating the risks associated with immunosuppression in kidney transplant recipients. This review aims to discuss the biomarkers available to predict, identify, and define the transplant immune tolerant state and various tolerance induction strategies. Regrettably, pediatric patients have not been included in any tolerance studies and this should be the focus of future studies.

Lung Regeneration by Transplantation of Allogeneic Lung Progenitors Using a Safer Conditioning Regimen and Clinical-grade Reagents

Over the last decades, several studies demonstrated the possibility of lung regeneration through transplantation of various lung progenitor populations. Recently, we showed in mice that fetal or adult lung progenitors could potentially provide donor cells for transplantation, provided that the lung stem cell niche in the recipient is vacated of endogenous lung progenitors by adequate conditioning. Accordingly, marked lung regeneration could be attained following i.v. infusion of a single cell suspension of lung cells into recipient mice conditioned with naphthalene (NA) and 6Gy total body irradiation (TBI). As clinical translation of this approach requires the use of allogenic donors, we more recently developed a novel transplantation modality based on co-infusion of hematopoietic and lung progenitors from the same donor. Thus, by virtue of hematopoietic chimerism, which leads to immune tolerance toward donor antigens, the lung progenitors can be successfully engrafted without any need for post-transplant immune suppression. In the present study, we demonstrate that it is possible to replace NA in the conditioning regimen with Cyclophosphamide (CY), approved for the treatment of many diseases and that a lower dose of 2 GY TBI can successfully enable engraftment of donor-derived hematopoietic and lung progenitors when CY is administered in 2 doses after the stem cell infusion. Taken together, our results suggest a feasible and relatively safe protocol that could potentially be translated to clinical transplantation of lung progenitors across major MHC barriers in patients with terminal lung diseases.

Successful Induction of Specific Immunological Tolerance by Combined Kidney and Hematopoietic Stem Cell Transplantation in HLA-Identical Siblings

Induction of immunological tolerance has been the holy grail of transplantation immunology for decades. The only successful approach to achieve it in patients has been a combined kidney and hematopoietic stem cell transplantation from an HLA-matched or -mismatched living donor. Here, we report the first three patients in Europe included in a clinical trial aiming at the induction of tolerance by mixed lymphohematopoietic chimerism after kidney transplantation. Two female and one male patient were transplanted with a kidney and peripherally mobilized hematopoietic stem cells from their HLA-identical sibling donor. The protocol followed previous studies at Stanford University: kidney transplantation was performed on day 0 including induction with anti-thymocyte globulin followed by conditioning with 10x 1.2 Gy total lymphoid irradiation and the transfusion of CD34+ cells together with a body weight-adjusted dose of donor T cells on day 11. Immunosuppression consisted of cyclosporine A and steroids for 10 days, cyclosporine A and mycophenolate mofetil for 1 month, and then cyclosporine A monotherapy with tapering over 9–20 months. The 3 patients have been off immunosuppression for 4 years, 19 months and 8 months, respectively. No rejection or graft-versus-host disease occurred. Hematological donor chimerism was stable in the first, but slowly declining in the other two patients. A molecular microscope analysis in patient 2 revealed the genetic profile of a normal kidney. No relevant infections were observed, and the quality of life in all three patients is excellent. During the SARS-CoV-2 pandemic, all three patients were vaccinated with the mRNA vaccine BNT162b2 (Comirnaty^®), and they showed excellent humoral and in 2 out 3 patients also cellular SARS-CoV-2-specific immunity. Thus, combined kidney and hematopoietic stem cell transplantation is a feasible and successful approach to induce specific immunological tolerance in the setting of HLA-matched sibling living kidney donation while maintaining immune responsiveness to an mRNA vaccine (ClinicalTrials.gov: NCT00365846).

Emerging Concepts in Managing Malignancy in Kidney Transplant Patients

Cancer remains a significant cause of morbidity and mortality in kidney transplant recipients, due to long-term immunosuppression. Salient issues to consider in decreasing the burden of malignancy among kidney transplant recipients include pretransplant recipient evaluation, post-transplant screening and monitoring, and optimal treatment strategies for the kidney transplant recipients with cancer. In this review, we address cancer incidence and outcomes, approaches to cancer screening and monitoring pretransplant and post-transplant, as well as treatment strategies, immunosuppressive management, and multidisciplinary approaches in the kidney transplant recipients with cancer.

Abdominal Wall Vascularized Composite Allotransplantation: A Scoping Review

BACKGROUND

 Abdominal wall vascularized composite allotransplantation (AW-VCA) is a novel reconstructive technique used for large abdominal wall defects in combination with intestinal transplantation (ITx) or multivisceral abdominal transplantation (MVTx). Since the introduction of this procedure, several studies have been published reporting their experience. This study aims to present a scoping review looking at all available evidence-based medicine information to understand the most current surgical techniques and clinical outcomes.

METHODS

 This scoping review followed the Preferred Reporting Items for Systematic Reviews and Meta-analysis (PRISMA) extension for scoping reviews checklist. A comprehensive research strategy of several databases was conducted.

RESULTS

 A total of 31 studies were included in this review, which comprised animal, cadaveric, and human studies. In human studies, four surgical techniques with high flap survival rates and low complication rates were found. In cadaveric studies, it was shown that the use of iliofemoral cuff-based flaps provided adequate tissue perfusion to the abdominal wall graft. Also, the use of thoracolumbar nerves have been described to provide functionality to the AW-VCA and prevent long-term muscle atrophy.

CONCLUSION

 AW-VCA is a safe and efficient alternative for patients with large and complex abdominal wall defects. The future holds a promising evolution of a functional AW-VCA, though surgeons must face and overcome the challenge of distorted anatomy frequently present in this population. Forthcoming studies with a better level of evidence are required to evaluate functionality and differences between surgical techniques.

Induction of Immune Tolerance in Islet Transplantation Using Apoptotic Donor Leukocytes

Allogeneic islet transplantation has become an effective treatment option for severe Type 1 diabetes with intractable impaired awareness due to hypoglycemic events. Although current immunosuppressive protocols effectively prevent the acute rejection associated with initial T cell activation in recipients, chronic rejection has remained an obstacle for achieving long-term allogeneic islet engraftment. The development of donor-specific immune tolerance to the allograft is the ultimate goal given its potential ability to overcome chronic rejection and disregard the need for maintenance immunosuppression, which may be toxic to islet grafts. Recently, a breakthrough in tolerance induction during allogeneic islet transplantation using apoptotic donor lymphocytes (ADLs) in a non-human primate model had been reported. Several studies have suggested that the clonal depletion, anergy, and expansion of the antigen-specific regulatory immune network are the mechanisms for donor-specific tolerance with ADLs, which act synergistically to induce robust transplant tolerance. This achievement represents a huge step forward toward the clinical application of immune tolerance induction. We herein summarize the reported operational induction therapies in islet transplantation using the ADLs. Moreover, a few obstacles for the engraftment of transplanted islets, such as islet immunogenicity and instant blood-mediated response, which need to be resolved in the future, are also discussed.

A systematic review of immunomodulatory strategies used in skin-containing preclinical vascularized composite allotransplant models

BACKGROUND

Acute rejection remains a vexing problem in vascularized composite allotransplantation (VCA). Available immunosuppressive regimens are successful at minimizing alloimmune response and allowing VCA in humans. However, repeated rejection episodes are common, and systemic side effects of the current standard regimen (Tacrolimus, MMF, Prednisone) are dose limiting. Novel immunomodulatory approaches to improve allograft acceptance and minimize systemic toxicity are continuously explored in preclinical models. We aimed to systematically summarize past and current approaches to help guide future research in this complex field.

METHODS

We conducted a systematic review of manuscripts listed in the MEDLINE and PubMed databases. For inclusion, articles had to primarily investigate the effect of a therapeutic approach on prolonging the survival of a skin-containing preclinical VCA model. Non-VCA studies, human trials, anatomical and feasibility studies, and articles written in a language other than English were excluded. We followed the preferred reporting items for systematic reviews and meta-analyses (PRISMA) guidelines.

RESULTS

The search retrieved 980 articles of which 112 articles were ultimately included. The majority of investigations used a rat model. An orthotopic hind limb VCA model was used in 53% of the studies. Cell and drug-based approaches were investigated 58 and 52 times, respectively. We provide a comprehensive review of immunomodulatory strategies used in VCA preclinical research over a timeframe of 44 years.

CONCLUSION

We identify a transition from anatomically non-specific to anatomical models mimicking clinical needs. As limb transplants have been most frequently performed, preclinical research focused on using the hind limb model. We also identify a transition from drug-based suppression therapies to cell-based immunomodulation strategies.

Cellular Therapies in Solid Organ Allotransplantation: Promise and Pitfalls

Donor specific transfusions have been the basis of tolerance inducing protocols since Peter Medawar showed that it was experimentally feasible in the 1950s. Though trials of cellular therapies have become increasingly common in solid organ transplantation, they have not become standard practice. Additionally, whereas some protocols have focused on cellular therapies as a method for donor antigen delivery—thought to promote tolerance in and of itself in the correct immunologic context—other approaches have alternatively focused on the intrinsic immunosuppressive properties of the certain cell types with less emphasis on their origin, including mesenchymal stem cells, regulatory T cells, and regulatory dendritic cells. Regardless of intent, all cellular therapies must contend with the potential that introducing donor antigen in a new context will lead to sensitization. In this review, we focus on the variety of cellular therapies that have been applied in human trials and non-human primate models, describe their efficacy, highlight data regarding their potential for sensitization, and discuss opportunities for cellular therapies within our current understanding of the immune landscape.

Antigen Specific Regulatory T Cells in Kidney Transplantation and Other Tolerance Settings

Kidney transplantation is the most common solid organ transplant and the best current therapy for end-stage kidney failure. However, with standard immunosuppression, most transplants develop chronic dysfunction or fail, much of which is due to chronic immune injury. Tregs are a subset of T cells involved in limiting immune activation and preventing autoimmune disease. These cells offer the potential to provide tolerance or to allow reduction in immunosuppression in kidney transplants. The importance of Tregs in kidney transplantation has been shown in a number of seminal mouse and animal studies, including those with T cell receptors (TCRs) transgenic Tregs (TCR-Tregs) or Chimeric Antigen Receptor (CAR) Tregs (CAR-Tregs) showing that specificity increases the potency of Treg function. Here we outline the animal and human studies and clinical trials directed at using Tregs in kidney transplantation and other tolerance settings and the various modifications to enhance allo-specific Treg function in vivo and in vitro.

Tomotherapy Applied Total Lymphoid Irradiation and Allogeneic Hematopoietic Cell Transplantation Generates Mixed Chimerism in the Rhesus Macaque Model

Development of a new methodology to induce immunological chimerism after allogeneic hematopoietic cell (HC) transplantation in a rhesus macaque model is described. The chimeric state was achieved using a non-myeloablative, helical tomotherapy-based total lymphoid irradiation (TomoTLI) conditioning regimen followed by donor HC infusions between 1-haplotype matched donor/recipient pairs. The technique was tested as a feasibility study in an experimental group of seven rhesus macaques that received the novel TomoTLI tolerance protocol and HC allo-transplants. Two tomotherapy protocols were compared: TomoTLI (n = 5) and TomoTLI/total-body irradiation (TBI) (n = 2). Five of seven animals developed mixed chimerism. Three of five animals given the TomoTLI protocol generated transient mixed chimerism with no graft-versus-host disease (GVHD) with survival of 33, 152 and >180 days. However, the inclusion of belatacept in addition to a single fraction of TBI resulted in total chimerism and fatal GVHD in both animals, indicating an unacceptable conditioning regimen.

Donor Recipient Chimeric Cells Induce Chimerism and Extend Survival of Vascularized Composite Allografts

This study evaluated the efficacy of donor recipient chimeric cell (DRCC) therapy created by fusion of donor and recipient derived bone marrow cells (BMC) in chimerism and tolerance induction in a rat vascularized composite allograft (VCA) model. Twenty-four VCA (groin flaps) from MHC-mismatched ACI (RT1^a) donors were transplanted to Lewis (RT1^l) recipients. Rats were randomly divided into (n = 6/group): Group 1—untreated controls, Groups 2—7-day immunosuppression controls, Group 3—DRCC, and Group 4—DRCC with 7-day anti-αβTCR monoclonal antibody and cyclosporine A protocol. DRCC created by polyethylene glycol-mediated fusion of ACI and Lewis BMC were cultured and transplanted (2–4 × 10⁶) to VCA recipients via intraosseous delivery route. Flow cytometry assessed peripheral blood chimerism while fluorescent microscopy and PCR tested the presence of DRCC in the recipient’s blood, bone marrow (BM), and lymphoid organs at the study endpoint (VCA rejection). No complications were observed after DRCC intraosseous delivery. Group 4 presented the longest average VCA survival (79.3 ± 30.9 days) followed by Group 2 (53.3 ± 13.6 days), Group 3 (18 ± 7.5 days), and Group 1 (8.5 ± 1 days). The highest chimerism level was detected in Group 4 (57.9 ± 6.2%) at day 7 post-transplant. The chimerism declined at day 21 post-transplant and remained at 10% level during the entire follow-up period. Single dose of DRCC therapy induced long-term multilineage chimerism and extended VCA survival. DRCC introduces a novel concept of customized donor-recipient cell-based therapy supporting solid organ and VCA transplants.

Fetal interventions for congenital renal anomalies

Congenital abnormalities of the kidney and urinary tract (CAKUT) represent 20% of prenatally diagnosed congenital abnormalities. Although the majority of these abnormalities do not require intervention either pre or postnatally, there is a subset of patients whose disease is so severe that it may warrant intervention prior to delivery to prevent morbidity and mortality. These cases consist of patients with moderate lower urinary tract obstruction (LUTO) in which vesicocentesis, shunting or cystoscopy are options and patients with early pregnancy renal anhydramnios (EPRA) in whom amnioinfusion therapy may be an option. The main causes of EPRA are congenital bilateral renal agenesis (CoBRA), cystic kidney disease (CKD) and severe LUTO. Untreated, EPRA is universally fatal secondary to anhydramnios induced pulmonary hypoplasia. The evidence regarding therapy for LUTO is limited and the stopped early PLUTO (Percutaneous Shunting in Lower Urinary Tract Obstruction) trial was unable to provide definitive answers about patient selection. Evidence for EPRA therapy is also scant. Serial amnioinfusions have shown promise in cases of EPRA due to CoBRA or renal failure and this treatment modality forms the basis of the ongoing NIH funded RAFT (Renal Anhydramnios Fetal Therapy) trial. At present, there is consensus that treatment for EPRA should only occur in the setting of a clinical trial.

Lymphohematopoietic graft-versus-host responses promote mixed chimerism in patients receiving intestinal transplantation

In humans receiving intestinal transplantation (ITx), long-term multilineage blood chimerism often develops. Donor T cell macrochimerism (≥4%) frequently occurs without graft-versus-host disease (GVHD) and is associated with reduced rejection. Here we demonstrate that patients with macrochimerism had high graft-versus-host (GvH) to host-versus-graft (HvG) T cell clonal ratios in their allografts. These GvH clones entered the circulation, where their peak levels were associated with declines in HvG clones early after transplant, suggesting that GvH reactions may contribute to chimerism and control HvG responses without causing GVHD. Consistently, donor-derived T cells, including GvH clones, and CD34+ hematopoietic stem and progenitor cells (HSPCs) were simultaneously detected in the recipients' BM more than 100 days after transplant. Individual GvH clones appeared in ileal mucosa or PBMCs before detection in recipient BM, consistent with an intestinal mucosal origin, where donor GvH-reactive T cells expanded early upon entry of recipient APCs into the graft. These results, combined with cytotoxic single-cell transcriptional profiles of donor T cells in recipient BM, suggest that tissue-resident GvH-reactive donor T cells migrated into the recipient circulation and BM, where they destroyed recipient hematopoietic cells through cytolytic effector functions and promoted engraftment of graft-derived HSPCs that maintain chimerism. These mechanisms suggest an approach to achieving intestinal allograft tolerance.

Multi-lineage Lung Regeneration by Stem Cell Transplantation across Major Genetic Barriers

Induction of lung regeneration by transplantation of lung progenitor cells is a critical preclinical challenge. Recently, we demonstrated that robust lung regeneration can be achieved if the endogenous stem cell niches in the recipient's lung are vacated by sub-lethal pre-conditioning. However, overcoming MHC barriers is an additional requirement for clinical application of this attractive approach. We demonstrate here that durable tolerance toward mis-matched lung progenitors and their derivatives can be achieved without any chronic immune suppression, by virtue of co-transplantation with hematopoietic progenitors from the same donor. Initial proof of concept of this approach was attained by transplantation of fetal lung cells comprising both hematopoietic and non-hematopoietic progenitors. Furthermore, an even higher rate of blood and epithelial lung chimerism was attained by using adult lung cells supplemented with bone marrow hematopoietic progenitors. These results lay the foundation for repair of lung injury through a procedure akin to bone marrow transplantation.

Apoptotic Donor Cells in Transplantation

Despite significant advances in prevention and treatment of transplant rejection with immunosuppressive medications, we continue to face challenges of long-term graft survival, detrimental medication side effects to both the recipient and transplanted organ together with risks for opportunistic infections. Transplantation tolerance has so far only been achieved through hematopoietic chimerism, which carries with it a serious and life-threatening risk of graft versus host disease, along with variability in persistence of chimerism and uncertainty of sustained tolerance. More recently, numerous in vitro and in vivo studies have explored the therapeutic potential of silent clearance of apoptotic cells which have been well known to aid in maintaining peripheral tolerance to self. Apoptotic cells from a donor not only have the ability of down regulating the immune response, but also are a way of providing donor antigens to recipient antigen-presenting-cells that can then promote donor-specific peripheral tolerance. Herein, we review both laboratory and clinical evidence that support the utility of apoptotic cell-based therapies in prevention and treatment of graft versus host disease and transplant rejection along with induction of donor-specific tolerance in solid organ transplantation. We have highlighted the potential limitations and challenges of this apoptotic donor cell-based therapy together with ongoing advancements and attempts made to overcome them.

A Clinical Trial With Adoptive Transfer of Ex Vivo-induced, Donor-specific Immune-regulatory Cells in Kidney Transplantation-A Second Report

BACKGROUND

Although the outcome of kidney transplantation (KTx) has improved, various adverse effects of immunosuppressants and chronic rejection aggravate the long-term prognosis of patients. Therefore, the induction of immune tolerance may be an effective therapeutic strategy.

METHODS

A clinical trial aiming at immune tolerance induction was conducted in kidney transplant recipients from HLA mismatched living donors by infusing autologous donor-specific regulatory T cells (Treg). To obtain Treg, recipient's peripheral blood mononuclear cells were cocultured with irradiated donor cells in the presence of anti-CD80/CD86 monoclonal antibody for 2 weeks. For preconditioning, splenectomy + cyclophosphamide (CP) was employed in the first series (group A; n = 9). In group B, splenectomy was substituted by preadministration of rituximab (group B; n = 3). In the latest cases, rituximab + rabbit antithymocyte globulin was administered instead of cyclophosphamide (group C; n = 4). Twelve days after KTx, the cultured cells were intravenously infused, and immunosuppressants were gradually tapered thereafter.

RESULTS

Although mixed lymphocyte reaction was remarkably suppressed in a donor-specific fashion, 6 out of 9 patients from group A, 1 out of 3 from group B, and 1 out of 4 from group C developed acute rejection within 1 year after KTx. Complete cessation of immunosuppression was not achieved, and a small dose of immunosuppressants was continued.

CONCLUSIONS

The adoptive transfer of autologous ex vivo-expanded Treg is 1 of the options to possibly induce alloimmune hyporesponsiveness. However, in the present study, further regimen optimization is still required and should be the focus of future investigations.

Tregs and Mixed Chimerism as Approaches for Tolerance Induction in Islet Transplantation

Pancreatic islet transplantation is a promising method for the treatment of type 1 and type 3 diabetes whereby replacement of islets may be curative. However, long-term treatment with immunosuppressive drugs (ISDs) remains essential for islet graft survival. Current ISD regimens carry significant side-effects for transplant recipients, and are also toxic to the transplanted islets. Pre-clinical efforts to induce immune tolerance to islet allografts identify ways in which the recipient immune system may be reeducated to induce a sustained transplant tolerance and even overcome autoimmune islet destruction. The goal of these efforts is to induce tolerance to transplanted islets with minimal to no long-term immunosuppression. Two most promising cell-based therapeutic strategies for inducing immune tolerance include T regulatory cells (Tregs) and donor and recipient hematopoietic mixed chimerism. Here, we review preclinical studies which utilize Tregs for tolerance induction in islet transplantation. We also review myeloablative and non-myeloablative hematopoietic stem cell transplantation (HSCT) strategies in preclinical and clinical studies to induce sustained mixed chimerism and allograft tolerance, in particular in islet transplantation. Since Tregs play a critical role in the establishment of mixed chimerism, it follows that the combination of Treg and HSCT may be synergistic. Since the success of the Edmonton protocol, the feasibility of clinical islet transplantation has been established and nascent clinical trials testing immune tolerance strategies using Tregs and/or hematopoietic mixed chimerism are underway or being formulated.

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.

Phase I trial of donor-derived modified immune cell infusion in kidney transplantation

BACKGROUNDPreclinical experiments have shown that donor blood cells, modified in vitro by an alkylating agent (modified immune cells [MICs]), induced long-term specific immunosuppression against the allogeneic donor.METHODSIn this phase I trial, patients received either 1.5 × 106 MICs per kg BW on day -2 (n = 3, group A), or 1.5 × 108 MICs per kg BW on day -2 (n = 3, group B) or day -7 (n = 4, group C) before living donor kidney transplantation in addition to post-transplantation immunosuppression. The primary outcome measure was the frequency of adverse events (AEs) until day 30 (study phase) with follow-up out to day 360.RESULTSMIC infusions were extremely well tolerated. During the study phase, 10 treated patients experienced a total of 69 AEs that were unlikely to be related or not related to MIC infusion. No donor-specific human leukocyte antigen Abs or rejection episodes were noted, even though the patients received up to 1.3 × 1010 donor mononuclear cells before transplantation. Group C patients with low immunosuppression during follow-up showed no in vitro reactivity against stimulatory donor blood cells on day 360, whereas reactivity against third-party cells was still preserved. Frequencies of CD19+CD24hiCD38hi transitional B lymphocytes (Bregs) increased from a median of 6% before MIC infusion to 20% on day 180, which was 19- and 68-fold higher, respectively, than in 2 independent cohorts of transplanted controls. The majority of Bregs produced the immunosuppressive cytokine IL-10. MIC-treated patients showed the Immune Tolerance Network operational tolerance signature.CONCLUSIONMIC administration was safe and could be a future tool for the targeted induction of tolerogenic Bregs.TRIAL REGISTRATIONEudraCT number: 2014-002086-30; ClinicalTrials.gov identifier: NCT02560220.FUNDINGFederal Ministry for Economic Affairs and Technology, Berlin, Germany, and TolerogenixX GmbH, Heidelberg, Germany.

Tolerance induction with donor hematopoietic stem cell infusion in kidney transplantation: a single-center experience in China with a 10-year follow-up

Background

Immunosuppressive therapy after life-saving kidney transplantation increases the risk of infection, cardiovascular diseases, metabolic diseases, and cancer. To date, four centers (three in the USA and one in South Korea) have reported clinical tolerance trials in kidney transplantation. We performed the first Chinese clinical trial in which kidney transplantation was combined with donor hematopoietic stem cell (DHSC) infusion to induce tolerance. This study summarizes the 10-year follow-up results.

Methods

From 2009 to 2017, 11 donor/recipient pairs underwent living-related kidney transplantation combined with DHSC infusion. Two of the pairs were human leukocyte antigen (HLA)-matched, and nine were HLA-mismatched. DHSCs were mobilized using granulocyte colony-stimulating factor (G-CSF) and harvested 1 day before transplantation. The recipients received consecutive total lymphoid irradiation (TLI) for 3 days before kidney transplantation. The induction drug was anti-thymocyte globulin (ATG). DHSCs were infused on days 2, 4, and 6 post surgery. All patients were followed-up until Dec 2019. Routine laboratory examinations, chimerism, biopsies, and mixed lymphocyte reactions were performed.

Results

One HLA-matched recipient had 30-50% chimerism, while the other patients had less than 1% chimerism. Recipients had donor-specific hyporesponsiveness (DSH) while sustaining normal reactivity to non-donors in mixed lymphocyte reactions. All recipients were followed up for 717-3,918 days. One recipient lost allograft function, and 10 recipients had stable renal function. None of the 11 recipients developed myelosuppression or graft-versus-host disease (GVHD) post transplantation. Our protocol did not increase the risk of infection. Allograft biopsy confirmed that one patient had mild rejection with Banff grade IA, while the other ten recipients did not develop rejection. Five patients were able to reduce the dose of their immunosuppressive therapy.

Conclusions

Our immune tolerance induction protocol, which used DHSC infusion and TLI, achieved low dose immunosuppression with long-term stable kidney allograft survival in Chinese patients.

Mixed chimerism established by hematopoietic stem cell transplantation is maintained by host and donor T regulatory cells

Transplantation is an effective treatment of many clinical disorders, but the mechanisms that regulate immunological tolerance are uncertain and remain central to improving patient outcome. Hemopoietic stem cell transplantation (SCT) often establishes "mixed chimerism" in which immune cells from both the donor and patient coexist in vivo in a setting of immunological tolerance. We studied immune function in 69 patients within 2 months following SCT; 37 were fully donor and 32 displayed mixed chimerism. The proportion of T regulatory (Treg) cells was increased during mixed chimerism and comprised equal numbers of donor and host-derived regulatory cells. This was associated with a tolerogenic PD-L1⁺ profile on dendritic cells. Importantly, effector T cells from patients with mixed chimerism exhibited reduced cytotoxicity against host target cells in vitro, but this was restored following depletion of CD4⁺ Treg cells. These data show that Treg cells play a major role in sustaining immunological tolerance during mixed chimerism. These insights should help to guide novel interventions to improve clinical transplantation.

Healing the Broken Heart; The Immunomodulatory Effects of Stem Cell Therapy

Cardiovascular Disease (CVD) is a leading cause of mortality within the United States. Current treatments being administered to patients who suffered a myocardial infarction (MI) have increased patient survival, but do not facilitate the replacement of damaged myocardium. Recent studies demonstrate that stem cell-based therapies promote myocardial repair; however, the poor engraftment of the transferred stem cell populations within the infarcted myocardium is a major limitation, regardless of the cell type. One explanation for poor cell retention is attributed to the harsh inflammatory response mounted following MI. The inflammatory response coupled to cardiac repair processes is divided into two distinct phases. The first phase is initiated during ischemic injury when necrosed myocardium releases Danger Associated Molecular Patterns (DAMPs) and chemokines/cytokines to induce the activation and recruitment of neutrophils and pro-inflammatory M1 macrophages (MΦs); in turn, facilitating necrotic tissue clearance. During the second phase, a shift from the M1 inflammatory functional phenotype to the M2 anti-inflammatory and pro-reparative functional phenotype, permits the resolution of inflammation and the establishment of tissue repair. T-regulatory cells (Tregs) are also influential in mediating the establishment of the pro-reparative phase by directly regulating M1 to M2 MΦ differentiation. Current studies suggest CD4+ T-lymphocyte populations become activated when presented with autoantigens released from the injured myocardium. The identity of the cardiac autoantigens or paracrine signaling molecules released from the ischemic tissue that directly mediate the phenotypic plasticity of T-lymphocyte populations in the post-MI heart are just beginning to be elucidated. Stem cells are enriched centers that contain a diverse paracrine secretome that can directly regulate responses within neighboring cell populations. Previous studies identify that stem cell mediated paracrine signaling can influence the phenotype and function of immune cell populations in vitro, but how stem cells directly mediate the inflammatory microenvironment of the ischemic heart is poorly characterized and is a topic of extensive investigation. In this review, we summarize the complex literature that details the inflammatory microenvironment of the ischemic heart and provide novel insights regarding how paracrine mediated signaling produced by stem cell-based therapies can regulate immune cell subsets to facilitate pro-reparative myocardial wound healing.

Outstanding questions in transplantation: Tolerance

In 2017, the American Society of Transplantation (AST) launched the Outstanding Questions in Transplantation Research forum to stimulate a community-wide discussion of how the field is evolving and to help identify areas where a better dialogue between clinicians and researchers could result in great advancements. Tolerance emerged as a topic of great interest to the AST community. This minireview provides an overview of clinical transplantation tolerance. Historical background followed by a review of the current status of attempts to establish tolerance in the clinic, highlighting the dynamic online discussion surrounding this important topic from the AST Transplantation Research forum, is provided.

Mixed chimerism and acceptance of kidney transplants after immunosuppressive drug withdrawal

Preclinical studies have shown that persistent mixed chimerism is linked to acceptance of organ allografts without immunosuppressive (IS) drugs. Mixed chimerism refers to continued mixing of donor and recipient hematopoietic cells in recipient tissues after transplantation of donor cells. To determine whether persistent mixed chimerism and tolerance can be established in patients undergoing living donor kidney transplantation, we infused allograft recipients with donor T cells and hematopoietic progenitors after posttransplant lymphoid irradiation. In 24 of 29 fully human leukocyte antigen (HLA)-matched patients who had persistent mixed chimerism for at least 6 months, complete IS drug withdrawal was achieved without subsequent evidence of rejection for at least 2 years. In 10 of 22 HLA haplotype-matched patients with persistent mixed chimerism for at least 12 months, reduction of IS drugs to tacrolimus monotherapy was achieved. Withdrawal of tacrolimus during the second year resulted in loss of detectable chimerism and subsequent rejection episodes, unless tacrolimus therapy was reinstituted. Posttransplant immune reconstitution of naïve B cells and B cell precursors was more rapid than the reconstitution of naïve T cells and thymic T cell precursors. Robust chimerism was observed only among naïve T and B cells but not among memory T cells. No evidence of rejection was observed in all surveillance graft biopsies obtained from mixed chimeric patients withdrawn from IS drugs, and none developed graft-versus-host disease. In conclusion, persistent mixed chimerism established in fully HLA- or haplotype-matched patients allowed for complete or partial IS drug withdrawal without rejection.

Apoptotic cell-based therapies for promoting transplantation tolerance

PURPOSE OF REVIEW

This article is aimed to provide readers with an updated review on the applicability, efficacy, and challenges of employing donor apoptotic cell-based therapies to promote transplantation tolerance in various experimental and clinical settings.

RECENT FINDINGS

Recently, donor apoptotic cell-based therapies have been employed in various models of cell (including pancreatic islets and bone marrow hematopoietic stem cells) and solid organ (heart and kidney) transplantation to promote donor-specific tolerance. Published data, thus far, have revealed a high potential of this approach in inducing robust transplantation tolerance. Recent clinical trials have also underscored the safety and potential efficacy of this approach in alleviating graft-versus-host disease (GVHD) in bone marrow transplantation (BMT). Host factors including prior allo-sensitization and opportunistic infections pose major obstacles in establishing transplantation tolerance employing this strategy. However, emerging data provide strategies for overcoming such obstacles in these clinically relevant settings.

SUMMARY

Donor apoptotic cell therapy is an emerging strategy in promoting transplantation tolerance, with recent data emphasizing its efficacy and applicability for transplantation tolerance in the clinic.

HLA in transplantation

The human major histocompatibility complex is a family of genes that encodes HLAs, which have a crucial role in defence against foreign pathogens and immune surveillance of tumours. In the context of transplantation, HLA molecules are polymorphic antigens that comprise an immunodominant alloreactive trigger for the immune response, resulting in rejection. Remarkable advances in knowledge and technology in the field of immunogenetics have considerably enhanced the safety of transplantation. However, access to transplantation among individuals who have become sensitized as a result of previous exposure to alloantigens is reduced proportional to the breadth of their sensitization. New approaches for crossing the HLA barrier in transplantation using plasmapheresis, intravenous immunoglobulin and kidney paired donation have been made possible by the relative ease with which even low levels of anti-HLA antibodies can now be detected and tracked. The development of novel protocols for the induction of tolerance and new approaches to immunomodulation was also facilitated by advances in HLA technology. Here, we review the progress made in understanding HLAs that has enabled organ transplantation to become a life-saving endeavour that is accessible even for sensitized patients. We also discuss novel approaches to desensitization, immunomodulation and tolerance induction that have the potential to further improve transplantation access and outcomes.

Long-term tolerance of islet allografts in nonhuman primates induced by apoptotic donor leukocytes

Immune tolerance to allografts has been pursued for decades as an important goal in transplantation. Administration of apoptotic donor splenocytes effectively induces antigen-specific tolerance to allografts in murine studies. Here we show that two peritransplant infusions of apoptotic donor leukocytes under short-term immunotherapy with antagonistic anti-CD40 antibody 2C10R4, rapamycin, soluble tumor necrosis factor receptor and anti-interleukin 6 receptor antibody induce long-term (≥1 year) tolerance to islet allografts in 5 of 5 nonsensitized, MHC class I-disparate, and one MHC class II DRB allele-matched rhesus macaques. Tolerance in our preclinical model is associated with a regulatory network, involving antigen-specific Tr1 cells exhibiting a distinct transcriptome and indirect specificity for matched MHC class II and mismatched class I peptides. Apoptotic donor leukocyte infusions warrant continued investigation as a cellular, nonchimeric and translatable method for inducing antigen-specific tolerance in transplantation.

Injection of donor apoptotic cells induces graft tolerance in mice. Here the authors combine this approach with short immunosuppressive therapy to achieve long-term tolerance to allogeneic islets and restoration of normoglycemia in diabetic nonhuman primates, and delineate cellular and molecular correlates of tolerance induction.