Mechanisms of Mixed Chimerism-Based Transplant Tolerance

Donor and recipient hematopoietic stem and progenitor cells mobilization in liver transplantation patients

BACKGROUND

Hematopoietic stem and progenitor cells (HSPCs) mobilize from bone marrow to peripheral blood in response to stress. The impact of alloresponse-induced stress on HSPCs mobilization in human liver transplantation (LTx) recipients remains under-investigated.

METHODS

Peripheral blood mononuclear cell (PBMC) samples were longitudinally collected from pre- to post-LTx for one year from 36 recipients with acute rejection (AR), 74 recipients without rejection (NR), and 5 recipients with graft-versus-host disease (GVHD). 28 PBMC samples from age-matched healthy donors were collected as healthy control (HC). Multi-color flow cytometry (MCFC) was used to immunophenotype HSPCs and their subpopulations. Donor recipient-distinguishable major histocompatibility complex (MHC) antibodies determined cell origin.

RESULTS

Before LTx, patients who developed AR after transplant contained more HSPCs in PBMC samples than HC, while the NR group patients contained fewer HSPCs than HC. After LTx, the HSPC ratio in the AR group sharply decreased and became less than HC within six months, and dropped to a comparable NR level afterward. During the one-year follow-up period, myeloid progenitors (MPs) biased differentiation was observed in all LTx recipients who were under tacrolimus-based immunosuppressive treatment. During both AR and GVHD episodes, the recipient-derived and donor-derived HSPCs mobilized into the recipient's blood-circulation and migrated to the target tissue, respectively. The HSPCs percentage in blood reduced after the disease was cured.

CONCLUSIONS

A preoperative high HSPC ratio in blood characterizes recipients who developed AR after LTx. Recipients exhibited a decline in blood-circulating HSPCs after transplant, the cells mobilized into the blood and migrated to target tissue during alloresponse.

Unlocking Transplant Tolerance with Biomaterials

For patients suffering from organ failure due to injury or autoimmune disease, allogeneic organ transplantation with chronic immunosuppression is considered the god standard in terms of clinical treatment. However, the true "holy grail" of transplant immunology is operational tolerance, in which the recipient exhibits a sustained lack of alloreactivity toward unencountered antigen presented by the donor graft. This outcome is resultant from critical changes to the phenotype and genotype of the immune repertoire predicated by the activation of specific signaling pathways responsive to soluble and mechanosensitive cues. Biomaterials have emerged as a medium for interfacing with and reprogramming these endogenous pathways toward tolerance in precise, minimally invasive, and spatiotemporally defined manners. By viewing seminal and contemporary breakthroughs in transplant tolerance induction through the lens of biomaterials-mediated immunomodulation strategies-which include intrinsic material immunogenicity, the depot effect, graft coatings, induction and delivery of tolerogenic immune cells, biomimicry of tolerogenic immune cells, and in situ reprogramming-this review emphasizes the stunning diversity of approaches in the field and spotlights exciting future directions for research to come.

Intercellular transfer of MHC molecules in T cell alloimmunity and allotransplantation

After transplantation of allogeneic tissues and organs, recognition by recipient T cells of donor MHC molecules initiates the pro-inflammatory adaptive immune response leading to allograft rejection. T cell allorecognition has long been known to be mediated via two distinct pathways: the direct pathway in which T cells recognize intact allogeneic MHC molecules displayed on donor cells and the indirect pathway whereby T cells recognize donor MHC peptides processed and presented by recipient antigen-presenting cells (APCs). It is believed that direct allorecognition is the driving force behind early acute allograft rejection while indirect allorecognition is involved in chronic allograft rejection, a progressive condition characterized by graft vasculopathy and tissue fibrosis. Recently, we and others have reported that after transplantation of allogeneic skin and organs, donor MHC molecules are transferred from donor cells to the host's APCs via trogocytosis or extracellular vesicles. Recipient APCs having captured donor MHC molecules can either present them to T cells in their intact form on their surface (semi-direct pathway) or the form of peptides bound to self-MHC molecules (indirect pathway). The present article provides an overview of recent studies evaluating the role of intercellular exchange of MHC molecules in T cell alloimmunity and its contribution to allograft rejection and tolerance.

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.

Coupling of a Major Allergen to the Surface of Immune Cells for Use in Prophylactic Cell Therapy for the Prevention of IgE-Mediated Allergy

Up to a third of the world's population suffers from allergies, yet the effectiveness of available preventative measures remains, at large, poor. Consequently, the development of successful prophylactic strategies for the induction of tolerance against allergens is crucial. In proof-of-concept studies, our laboratory has previously shown that the transfer of autologous hematopoietic stem cells (HSC) or autologous B cells expressing a major grass pollen allergen, Phl p 5, induces robust tolerance in mice. However, eventual clinical translation would require safe allergen expression without the need for retroviral transduction. Therefore, we aimed to chemically couple Phl p 5 to the surface of leukocytes and tested their ability to induce tolerance. Phl p 5 was coupled by two separate techniques, either by 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) or by linkage via a lipophilic anchor, 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-poly(ethylene glycol)-maleimide (DSPE-PEG-Mal). The effectiveness was assessed in fresh and cultured Phl p 5-coupled cells by flow cytometry, image cytometry, and immunofluorescence microscopy. Chemical coupling of Phl p 5 using EDC was robust but was followed by rapid apoptosis. DSPE-PEG-Mal-mediated linkage was also strong, but antigen levels declined due to antigen internalization. Cells coupled with Phl p 5 by either method were transferred into autologous mice. While administration of EDC-coupled splenocytes together with short course immunosuppression initially reduced Phl p 5-specific antibody levels to a moderate degree, both methods did not induce sustained tolerance towards Phl p 5 upon several subcutaneous immunizations with the allergen. Overall, our results demonstrate the successful chemical linkage of an allergen to leukocytes using two separate techniques, eliminating the risks of genetic modifications. More durable surface expression still needs to be achieved for use in prophylactic cell therapy protocols.

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.

Kidney Transplantation in Multiple Myeloma and Monoclonal Gammopathy of Renal Significance

Recent advances in the treatment of plasma cell disorders (PCDs) have provided a wealth of therapy alternatives and improved overall survival tremendously. Various types of PCDs are associated with kidney injury and end-stage kidney disease in a considerable number of patients. Kidney transplantation (KTx) is the best option for renal replacement therapy in select patients in terms of both quality of life parameters and overall survival. Even with modern therapies, all PCDs carry the risk of hematologic progression, whereas histologic recurrence and graft loss are other prevailing concerns in these patients. The risk of mortality is also higher in some of these disorders compared with KTx recipients who suffer from other causes of kidney disease. Unlike solid cancers, there is no well-defined "waiting time" after hematologic remission before proceeding to KTx. Thus, clinicians are usually reluctant to recommend KTx to patients who develop end-stage kidney disease due to PCDs. This review aims to provide the current evidence on KTx outcomes in patients with monoclonal gammopathy of renal significance and multiple myeloma. Although immunoglobulin light chain amyloidosis is a monoclonal gammopathy of renal significance subtype, KTx outcomes in this group are mentioned in another chapter of this issue.

Achievement of operational tolerance in a pediatric liver transplant recipient following successful hematopoietic stem cell transplantation from a different donor

Hematopoietic stem cell transplantation (HSCT)-based approaches are increasingly investigated strategies to induce tolerance in recipients of solid allografts. However, in the majority of cases, these approaches rely on the infusion of hematopoietic stem cells recovered from the same solid organ donor. In this report, we describe the case of a boy who received liver transplantation from a deceased donor, who had successfully underwent allogeneic HSCT from an unrelated donor for hepatitis-associated aplastic anemia. In this patient, it was possible to permanently withdraw post-HSCT immune suppression without causing any sign of liver graft dysfunction. To the best of our knowledge, this is the first case of operational tolerance documented in a patient who received combined liver transplantation and HSCT from different donors.

Novel Human Umbilical Di-Chimeric (HUDC) cell therapy for transplantation without life-long immunosuppression

Background

Cell-based therapies are promising for tolerance induction in bone marrow (BM), solid organs, and vascularized composite allotransplantation (VCA). The toxicity of bone marrow transplantation (BMT) protocols precludes this approach from routine clinical applications. To address this problem, we developed a new therapy of Human Umbilical Di-Chimeric (HUDC) cells for tolerance induction in transplantation. This study established in vitro characterization of the created HUDC cells.

Methods

We performed sixteen ex vivo polyethylene glycol (PEG)-mediated fusions of human umbilical cord blood (UCB) cells from two unrelated donors. Fusion feasibility was confirmed in vitro by flow cytometry (FC) and confocal microscopy (CM). The HUDC cells' genotype was assessed by lymphocytotoxicity test and short tandem repeat-polymerase chain reaction (STR-PCR) analysis, phenotype by FC, viability by LIVE/DEAD® assay, and apoptosis level by Annexin V staining. We used COMET assay to assess HUDC cells' genotoxicity after the fusion procedure. Clonogenic properties of HUDC cells were evaluated by colony forming unit (CFU) assay. Mixed lymphocyte reaction (MLR) assay assessed immunogenic and tolerogenic properties of HUDC cells.

Results

We confirmed the creation of HUDC cells from two unrelated human donors of UCB cells by FC and CM. Human leukocyte antigen (HLA) class I and II typing, and STR-PCR analysis of HUDC cells confirmed the presence of alleles and loci from both unrelated UCB donors (donor chimerism: 49%±8.3%, n=4). FC confirmed the hematopoietic phenotype of HUDC cells. We confirmed high HUDC cells' viability (0.47% of dead cells) and a low apoptosis level of fused HUDC cells (15.9%) compared to positive control of PKH-stained UCB cells (20.4%) before fusion. COMET assay of HUDC cells revealed a lack of DNA damage. CFU assay confirmed clonogenic properties of HUDC cells, and MLR assay revealed a low immunogenicity of HUDC cells.

Conclusions

This study confirmed creation of a novel HUDC cell line by ex vivo PEG-mediated fusion of UCB cells from two unrelated donors. The unique concept of creating a HUDC cell line, representing the genotype and phenotype of both, transplant donor and the recipient, introduces a promising approach for tolerance induction in BM, solid organs, and VCA transplantation.

Donor programmed cell death 1 ligand 1 is required for organ transplant tolerance in major histocompatibility complex-mismatched mixed chimeras although programmed cell death 1 ligand 1 and major histocompatibility complex class II are not required for inducing chimerism

Induction of major histocompatibility complex (MHC) human leukocyte antigen (HLA)-mismatched mixed chimerism is a promising approach for organ transplantation tolerance; however, human leukocyte antigen-mismatched stable mixed chimerism has not been achieved in the clinic. Tolerogenic dendritic cell (DC) expression of MHC class II (MHC II) and programmed cell death 1 ligand 1 (PD-L1) is important for immune tolerance, but whether donor-MHC II or PD-L1 is required for the induction of stable MHC-mismatched mixed chimerism and transplant tolerance is unclear. Here, we show that a clinically applicable radiation-free regimen can establish stable MHC-mismatched mixed chimerism and organ transplant tolerance in murine models. Induction of MHC-mismatched mixed chimerism does not require donor cell expression of MHC II or PD-L1, but donor-type organ transplant tolerance in the mixed chimeras (MC) requires the donor hematopoietic cells and the organ transplants to express PD-L1. The PD-L1 expressed by donor hematopoietic cells and the programmed cell death 1 expressed by host cells augment host-type donor-reactive CD4+ and CD8+ T cell anergy/exhaustion and differentiation into peripheral regulatory T (pTreg) cells in association with the organ transplant tolerance in the MC. Conversely, host-type Treg cells augment the expansion of donor-type tolerogenic CD8+ DCs that express PD-L1. These results indicate that PD-L1 expressed by donor-type tolerogenic DCs and expansion of host-type pTreg cells in MHC-mismatched MCs play critical roles in mediating organ transplant tolerance.

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.

After Bone Marrow Transplantation, the Cell-Intrinsic Th2 Pathway Promotes Recipient T Lymphocyte Survival and Regulates Graft-versus-Host Disease

Recipient T cells can aggravate or regulate lethal and devastating graft-versus-host disease (GVHD) after bone marrow transplantation (BMT). In this context, we have shown before that intestinal immune conditioning with helminths is associated with survival of recipient T cells and Th2 pathway-dependent regulation of GVHD. We investigated the mechanism of survival of recipient T cells and their contribution to GVHD pathogenesis in this helminth infection and BMT model after myeloablative preparation with total body irradiation in mice. Our results indicate that the helminth-induced Th2 pathway directly promotes the survival of recipient T cells after total body irradiation. Th2 cells also directly stimulate recipient T cells to produce TGF-β, which is required to regulate donor T cell-mediated immune attack of GVHD and can thereby contribute to recipient T cell survival after BMT. Moreover, we show that recipient T cells, conditioned to produce Th2 cytokines and TGF-β after helminth infection, are fundamentally necessary for GVHD regulation. Taken together, reprogrammed or immune-conditioned recipient T cells after helminth infection are crucial elements of Th2- and TGF-β-dependent regulation of GVHD after BMT, and their survival is dependent on cell-intrinsic Th2 signaling.

Human kidney-derived hematopoietic stem cells can support long-term multilineage hematopoiesis

Long-term multilineage hematopoietic donor chimerism occurs sporadically in patients who receive a transplanted solid organ enriched in lymphoid tissues such as the intestine or liver. There is currently no evidence for the presence of kidney-resident hematopoietic stem cells in any mammal species. Graft-versus-host-reactive donor T cells promote engraftment of graft-derived hematopoietic stem cells by making space in the bone marrow. Here, we report full (over 99%) multilineage, donor-derived hematopoietic chimerism in a pediatric kidney transplant recipient with syndromic combined immune deficiency that leads to transplant tolerance. Interestingly, we found that the human kidney-derived hematopoietic stem cells took up long-term residence in the recipient's bone marrow and gradually replaced their host counterparts, leading to blood type conversion and full donor chimerism of both lymphoid and myeloid lineages. Thus, our findings highlight the existence of human kidney-derived hematopoietic stem cells with a self-renewal ability able to support multilineage hematopoiesis.

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.

Creation of human hematopoietic chimeric cell (HHCC) line as a novel strategy for tolerance induction in transplantation

Background

Cell-based and chimerism-based therapies represent a promising approach for tolerance induction in transplantation. We propose a new cell therapy of the ex vivo created human hematopoietic chimeric cells (HHCC) as an alternative approach to bone marrow (BM)-based therapies in support of solid organ and vascularized composite allotransplantation (VCA). This study aimed to characterize in vitro the phenotype, genotype, clonogenic, and tolerogenic properties of HHCC.

Methods

Thirty ex vivo fusions of CD34⁺ cells from two unrelated human BM donors were performed. CD34⁺ cells were stained separately with PKH26 and PKH67 membrane dyes and fused using polyethylene glycol (PEG). Creation of human HHCC and chimeric state was confirmed by flow cytometry (FC), confocal microscopy (CM) and electron microscopy (EM). HHCC's phenotype (CD34, CD133, CD117, CD4, CD19, CD4/CD25) was assessed by FC, viability by Trypan Blue, LIVE/DEAD and apoptosis by AnnexinV/Sytox Blue and TUNEL assay, while mixed lymphocyte reaction (MLR) assay assessed HHCC's immunogenicity and tolerogenic properties. HHCC differentiation, proliferation and clonogenic potential were assessed by the colony forming unit (CFU). Polyploidy was evaluated by fluorescence in situ hybridization (FISH), whereas polymerase chain reaction-reverse sequence-specific oligonucleotide probe (PCR-rSSOP) and short tandem repeats-polymerase chain reaction (STR-PCR) assessed HHCC's genotype, and chimerism. Reverse transcription polymerase chain reaction (RT-PCR) analyzed cytokines secretion [interleukin (IL)-10, transforming growth factor-β (TGF-β) and tumor necrosis factor-α (TNF-α)] up to 14 days post-fusion.

Results

FC and CM confirmed creation of HHCC by fusion of CD34⁺ cells from two unrelated human donors. After fusion, maintenance of hematopoietic markers and increased expression of Treg-cells (CD4/CD25) was confirmed. Moreover, high HHCC viability (99%) and a low apoptosis rate (1.2%) were revealed HHCC presented decreased immunogenicity by MLR, and significant, 40-fold increase of IL-10 the pro-tolerogenic cytokine at 21 days after fusion (RT-PCR) P<0.0001. The number of polyploid cells was negligible (0.48%). PCR-rSSOP of HHCC after fusion confirmed presence of human leukocyte antigen (HLA) class I and class II-alleles and presence of the loci specific for both CD34⁺ cells BM donors by STR-PCR.

Conclusions

We have created a new hematopoietic cell line of HHCC from two unrelated human donors, and have successfully characterized in vitro, viability, phenotype, genotype, clonogenic, and tolerogenic properties of HHCC. These unique immunomodulatory and tolerogenic properties introduce HHCC as a novel therapeutic approach for tolerance induction in VCA and solid organ transplantation.

Curative islet and hematopoietic cell transplantation in diabetic mice without toxic bone marrow conditioning

Mixed hematopoietic chimerism can promote immune tolerance of donor-matched transplanted tissues, like pancreatic islets. However, adoption of this strategy is limited by the toxicity of standard treatments that enable donor hematopoietic cell engraftment. Here, we address these concerns with a non-myeloablative conditioning regimen that enables hematopoietic chimerism and allograft tolerance across fully mismatched major histocompatibility complex (MHC) barriers. Treatment with an αCD117 antibody, targeting c-Kit, administered with T cell-depleting antibodies and low-dose radiation permits durable multi-lineage chimerism in immunocompetent mice following hematopoietic cell transplant. In diabetic mice, co-transplantation of donor-matched islets and hematopoietic cells durably corrects diabetes without chronic immunosuppression and no appreciable evidence of graft-versus-host disease (GVHD). Donor-derived thymic antigen-presenting cells and host-derived peripheral regulatory T cells are likely mediators of allotolerance. These findings provide the foundation for safer bone marrow conditioning and cell transplantation regimens to establish hematopoietic chimerism and islet allograft tolerance.

CD3e-immunotoxin spares CD62Llo Tregs and reshapes organ-specific T-cell composition by preferentially depleting CD3ehi T cells

CD3-epsilon(CD3e) immunotoxins (IT), a promising precision reagent for various clinical conditions requiring effective depletion of T cells, often shows limited treatment efficacy for largely unknown reasons. Tissue-resident T cells that persist in peripheral tissues have been shown to play pivotal roles in local and systemic immunity, as well as transplant rejection, autoimmunity and cancers. The impact of CD3e-IT treatment on these local cells, however, remains poorly understood. Here, using a new murine testing model, we demonstrate a substantial enrichment of tissue-resident Foxp3+ Tregs following CD3e-IT treatment. Differential surface expression of CD3e among T-cell subsets appears to be a main driver of Treg enrichment in CD3e-IT treatment. The surviving Tregs in CD3e-IT-treated mice were mostly the CD3e^dimCD62L^lo effector phenotype, but the levels of this phenotype markedly varied among different lymphoid and nonlymphoid organs. We also found notable variations in surface CD3e levels among tissue-resident T cells of different organs, and these variations drive CD3e-IT to uniquely reshape T-cell compositions in local organs. The functions of organs and anatomic locations (lymph nodes) also affected the efficacy of CD3e-IT. The multi-organ pharmacodynamics of CD3e-IT and potential treatment resistance mechanisms identified in this study may generate new opportunities to further improve this promising treatment.

RETRACTED: The impact of second-donor lymphocyte infusion on secondary graft failure after allogeneic hematopoietic stem cell transplantation through activation of Foxp3 and regulatory T cells

This article has been retracted: please see Elsevier Policy on Article Withdrawal (http://www.elsevier.com/locate/withdrawalpolicy). The Editor of Cytotherapy has retracted this article. The article duplicates significant parts of a paper that had already appeared in Transplantation and Cellular Therapy, Volume 28, Issue 3, 2022, Pages 152.e1-152.e7, https://doi.org/10.1016/j.jtct.2021.12.017 In accordance with Committee on Publication Ethics (COPE) and Elsevier's policies, the authors have been contacted. After considering the author's response, the decision has been made to retract the paper. Redundant publications overweigh the relative importance of published findings and distort the academic record of the authors. One of the conditions of submission of a paper for publication is therefore that authors declare explicitly that the paper has not been previously published and is not under consideration for publication elsewhere. As such this article represents a misuse of the scientific publishing system. The scientific community takes a very strong view on this matter and apologies are offered to readers of the journal that this was not detected during the submission process.

Immune tolerance induced by hematopoietic stem cell infusion after HLA identical sibling kidney transplantation

After the first attempt to induce operational tolerance, it has taken decades to implement it in clinical practice. Recipients with Human leukocyte antigen (HLA) identical sibling donors were enrolled. Hematopoietic stem cells (HSCs) infusion was done after HLA identical sibling kidney transplantation (KTx). Three cases included were followed up for over 8 years. The perioperative conditioning protocol included anti-CD20, rabbit anti-thymocyte globulin (ATG), total lymphoid irradiation (TLI), and cyclophosphamide. Infusion of CD3⁺ cells and CD34⁺ cells was conducted. The withdrawal of immunosuppression was determined by mixed lymphocyte reaction (MLR) and graft biopsy. Case 1 and Case 2 showed persistent chimerism, while chimerism was not detected in Case 3. All three recipients showed a low-level response to donor-specific stimulation. Case 1 and Case 3 met the withdrawal rules at 16 and 32 months after transplantation, respectively. Graft function was stable, and no rejection signs were observed in routine biopsies until 94 and 61 months after transplantation. Case 2 was diagnosed with graft-versus-host disease (GVHD) 9 months after transplantation and recovered after an enhanced immunosuppression therapy. Steroids were withdrawn after 1 year, and 0.5 mg tacrolimus twice a day is currently the only immunosuppression at 8 years and 8 months. In conclusion, our clinical experience indicated the efficacy of non-myeloablative conditioning protocol for tolerance induction in HLA identical patients. Complete chimerism might be a risk factor for GVHD.

The Inability of Ex Vivo Expanded Mesenchymal Stem/Stromal Cells to Survive in Newborn Mice and to Induce Transplantation Tolerance

An encounter of the developing immune system with an antigen results in the induction of immunological areactivity to this antigen. In the case of transplantation antigens, the application of allogeneic hematopoietic cells induces a state of neonatal transplantation tolerance. This tolerance depends on the establishment of cellular chimerism, when allogeneic cells survive in the neonatally treated recipient. Since mesenchymal stem/stromal cells (MSCs) have been shown to have low immunogenicity and often survive in allogeneic recipients, we attempted to use these cells for induction of transplantation tolerance. Newborn (less than 24 h old) C57BL/6 mice were injected intraperitoneally with 5 × 10⁶ adipose tissue-derived MSCs isolated from allogeneic donors and the fate and survival of these cells were monitored. The impact of MSC application on the proportion of cell populations of the immune system and immunological reactivity was assessed. In addition, the survival of skin allografts in neonatally treated recipients was tested. We found that in vitro expanded MSCs did not survive in neonatal recipients, and the living MSCs were not detected few days after their application. Furthermore, there were no significant changes in the proportion of individual immune cell populations including CD4⁺ cell lineages, but we detected an apparent shift to the production of Th1 cytokines IL-2 and IFN-γ in neonatally treated mice. However, skin allografts in the MSC-treated recipients were promptly rejected. These results therefore show that in vitro expanded MSCs do not survive in neonatal recipients, but induce a cytokine imbalance without induction of transplantation tolerance.

The Effect of Narrow-Band Ultraviolet B Irradiation on the Vascularized Composite Allotransplantation Rat Model

INTRODUCTION

Vascularized composite allotransplantation (VCA) allows functional and esthetic reconstruction for patients with complex anatomical defects. However, acute and chronic graft rejections are significant obstacles to VCA. Ultraviolet light is an oncogenic environmental hazard. However, ultraviolet B (UVB) has an immunomodulation effect. Therefore, this study aims to elucidate the impact of UVB irradiation on the VCA rat model.

METHODS

The rat vascularized bone marrow allotransplantation model was used. A vascularized bone marrow from a Brown Norway rat (RT1Ac) was transplanted into a Lewis rat (RT1Ab). The allograft and surrounding abdominal skin were exposed to narrow-band ultraviolet B (NB-UVB) (311 nm) radiation with an energy of 1350 mJ/cm2 3 times a week until the end of the study period. There were 5 study groups: syngeneic transplantation (group 1), allogeneic transplantation (group 2), allogenic transplantation-NB-UVB (group 3), allogenic transplantation-antilymphocyte serum (ALS)-tacrolimus (group 4), and allogenic transplantation-antilymphocyte serum-tacrolimus-NB-UVB (group 5).

RESULTS

Group 5 had decreased graft survival compared with group 4. In the donor cell chimerism analysis, donor cell chimerism decreased significantly after UVB irradiation and was unresponsive to the administered immunosuppressants. After UVB irradiation, the CD8 T-cell ratio was increased, and the regulatory T-cell ratio was decreased.

CONCLUSIONS

The preliminary data showed that NB-UVB irradiation of the VCA rat model may decrease graft survival. However, further studies are needed to elucidate the possible mechanisms of this phenomenon.

Chimerism-Based Tolerance to Kidney Allografts in Humans: Novel Insights and Future Perspectives

Chronic rejection and immunosuppression-related toxicity severely affect long-term outcomes of kidney transplantation. The induction of transplantation tolerance – the lack of destructive immune responses to a transplanted organ in the absence of immunosuppression – could potentially overcome these limitations. Immune tolerance to kidney allografts from living donors has been successfully achieved in humans through clinical protocols based on chimerism induction with hematopoietic cell transplantation after non-myeloablative conditioning. Notably, two of these protocols have led to immune tolerance in a significant fraction of HLA-mismatched donor-recipient combinations, which represent the large majority of cases in clinical practice. Studies in mice and large animals have been critical in dissecting tolerance mechanisms and in selecting the most promising approaches for human translation. However, there are several key differences in tolerance induction between these models and humans, including the rate of success and stability of donor chimerism, as well as the relative contribution of different mechanisms in inducing donor-specific unresponsiveness. Kidney allograft tolerance achieved through durable full-donor chimerism may be due to central deletion of graft-reactive donor T cells, even though mechanistic data from patient series are lacking. On the other hand, immune tolerance attained with transient mixed chimerism-based protocols initially relies on Treg-mediated suppression, followed by peripheral deletion of donor-reactive recipient T-cell clones under antigenic pressure from the graft. These conclusions were supported by data deriving from novel high-throughput T-cell receptor sequencing approaches that allowed tracking of alloreactive repertoires over time. In this review, we summarize the most important mechanistic studies on tolerance induction with combined kidney-bone marrow transplantation in humans, discussing open issues that still need to be addressed and focusing on techniques developed in recent years to efficiently monitor the alloresponse in tolerance trials. These cutting-edge methods will be instrumental for the development of immune tolerance protocols with improved efficacy and to identify patients amenable to safe immunosuppression withdrawal.

Operational tolerance after hematopoietic stem cell transplantation is characterized by distinct transcriptional, phenotypic, and metabolic signatures

The mechanisms underlying operational tolerance after hematopoietic stem cell transplantation in humans are poorly understood. We studied two independent cohorts of patients who underwent allogeneic hematopoietic stem cell transplantation from human leukocyte antigen-identical siblings. Primary tolerance was associated with long-lasting reshaping of the recipients' immune system compared to their healthy donors with an increased proportion of regulatory T cell subsets and decreased T cell activation, proliferation, and migration. Transcriptomics profiles also identified a role for nicotinamide adenine dinucleotide biosynthesis in the regulation of immune cell functions. We then compared individuals with operational tolerance and nontolerant recipients at the phenotypic, transcriptomic, and metabolomic level. We observed alterations centered on CD38⁺-activated T and B cells in nontolerant patients. In tolerant patients, cell subsets with regulatory functions were prominent. RNA sequencing analyses highlighted modifications in the tolerant patients' transcriptomic profiles, particularly with overexpression of the ectoenzyme NT5E (encoding CD73), which could counterbalance CD38 enzymatic functions by producing adenosine. Further, metabolomic analyses suggested a central role of androgens in establishing operational tolerance. These data were confirmed using an integrative approach to evaluating the immune landscape associated with operational tolerance. Thus, balance between a CD38-activated immune state and CD73-related production of adenosine may be a key regulator of operational tolerance.

Effect of donor lymphocyte infusion from two types of donors on Mixed Chimerism with Secondary Graft Failure after allogeneic haematopoietic stem cell transplantation

Mixed chimerism (MC) and secondary graft failure (SGF) with recipient-or donor-type chimerism is a major obstacle in allogeneic hematopoietic stem cell transpl- antation (HSCT). Donor lymphocyte infusion(DLI) can eradicate minimal residual disease or be used to rescue a hematologic relapse, being able to induce durable remissions after HSCT.This study aimed to analyse the efficacy and immune mecha- nism of DLI from the original and alternative donor for patients of mixed donor chimerism with SGF . The alternative donor refers to the candidate relative donor who did not initially provide stem cells include HLA-matched sibling donor(MSD) or HLA- haploidentical donor (HID). We conducted a retrospective study of 246 patients with a median age of 37 (9-58) years who were regularly detected MC, complete donor chimera (CC) and regulatory T cells (Treg). The median diagnosis time of SGF was 69 (39-141) days after transplantation . Sixteen patients of SGF received DLI from the alternative donor, including 3 patients who chose DLI from the original donor with no initial response and 13 patients who directly chose DLI from the alternative donor. Sixteen patients with SGF exsisted mixed chimerism synchronously and the rate calculated overall chimerism of MC was 63% (range, 42%-85%) after transplantation. The proportion of Treg decreased significantly in SGF patients from a median of (2.66% ±0.80%) to (0.93%±0.57%) at a time point after transplantation (p=0.02).The DLI of the alternative donor in 14 patients achieved complete response and MC gradually convert to CC state, simultaneously there was significant increase in the Treg fraction [SGF vs CR: (0.93% ± 0.57%) vs (3.61%±0.82%), p=0.01)].For the clinical nonres- ponders from two types of donor there was no significant change in MC and Treg cells. The OS and DFS at 2 years after DLI were 69.7%±3.19 % and 61.3%±4.80%, respectively. DLI from the alternative donor may be an effective treatment for MC with SGF and the mechanism is closely related to the activation of Treg cells level.

Self-Centered Function of Adaptive Immunity in Regulation of Immune Responses and in Tolerance

The search for common mechanisms underlying the pathogenesis of chronic inflammatory conditions has crystalized the concept of continuous dual resetting of the immune repertoire (CDR) as a basic principle of the immune system function. Consequently, outlined was the first dynamic comprehensive picture of the immune system function. The goal of this study is to elaborate on regulation of immune responses and mechanisms of tolerance, particularly focusing on adaptive immunity. It is well established that the T/B cell repertoire is selected and maintained based on interactions with self. However, their activation also requires interaction with a self-specific major histocompatibility complex (MHC) “code,” i.e., the context of MHC molecules. Therefore, not only repertoire selection and maintenance but also the T/B cell activation and function are self-centered. Thus, adaptive effectors may be primarily focused on the state of self and maintenance of integrity of the self, and only to a certain degree on elimination of the foreign. As examples of such function are used immunologically poorly understood MHC-disparate settings typical for transplantation and pregnancy. Transplantation represents an extreme setting of strong systemic compartment-level adaptive/MHC-restricted immune responses. Described are clinically identified conditions for operational tolerance of MHC-disparate tissues/living systems in allotransplantation, which are in line with the CDR-proposed self-centered regulatory role of T/B cells. In contrast, normal pregnancy is coexistence of semiallogeneic or entirely allogeneic mother and fetus, but without alloreactivity akin to transplantation settings. Presented data support the notion that maintenance of pregnancy is a process that relies predominantly on innate/MHC-independent immune mechanisms. By the inception of hemotrophic stage of pregnancy (second and third trimester), both mother and child are individual living systems, with established adaptive immune repertoires. Although mother-fetus interactions at that point become indirect systemic compartment-level communications, their interactions throughout gestation remain within the innate realm of molecular-level adaptations.

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.

Expansion of CD45RA-FOXP3++ regulatory T cells is associated with immune tolerance in patients with combined kidney and bone marrow transplantation

Objectives

Simultaneous transplantation of a solid organ and bone marrow from the same donor is a possible means of achieving transplant tolerance. Here, we attempted to identify biomarkers that indicate transplant tolerance for discontinuation of immunosuppressants in combined kidney and bone marrow transplantation (CKBMT).

Methods

Conventional kidney transplant (KT) recipients (n = 20) and CKBMT recipients (n = 6) were included in this study. We examined various immunological parameters by flow cytometry using peripheral blood mononuclear cells (PBMCs), including the frequency and phenotype of regulatory T (Treg) cell subpopulations. We also examined the suppressive activity of the Treg cell population in the setting of mixed lymphocyte reaction (MLR) with or without Treg cell depletion.

Results

Among six CKBMT recipients, three successfully discontinued immunosuppressants (tolerant group) and three could not (non‐tolerant group). The CD45RA⁻FOXP3⁺⁺ Treg cell subpopulation was expanded in CKBMT recipients compared to conventional kidney transplant patients, and this was more obvious in the tolerant group than the non‐tolerant group. In addition, high suppressive activity of the Treg cell population was observed in the tolerant group. The ratio of CD45RA⁻FOXP3⁺⁺ Treg cells to CD45RA⁻FOXP3⁺ cells indicated good discrimination between the tolerant and non‐tolerant groups.

Conclusion

Thus, our findings propose a biomarker that can distinguish CKBMT patients who achieve transplant tolerance and are eligible for discontinuation of immunosuppressants and may provide insight into tolerance mechanisms in CKBMT.

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.

Engineering Strategies for Allogeneic Solid Tissue Acceptance

Advances in allogeneic transplantation of solid organs and tissues depend on our understanding of mechanisms that mediate the prevention of graft rejection. For the past decades, clinical practice has established guidelines to prevent allograft rejection, which mostly rely on the intake of nontargeted immunosuppressants as the gold standard. However, such lifelong regimens have been reported to trigger severe morbidities and commonly fail in preventing late allograft loss. In this review, the biology of allogeneic rejection and self-tolerance is analyzed, as well as the mechanisms of cellular-based therapeutics driving suppression and/or tolerance. Bioinspired engineering strategies that take advantage of cells, biomaterials, or combinations thereof to prevent allograft rejection are addressed, as well as biological mechanisms that drive their efficacy.

Human pluripotent stem cell-derived insulin-producing cells: A regenerative medicine perspective

Tremendous progress has been made over the last two decades in the field of pancreatic beta cell replacement therapy as a curative measure for diabetes. Transplantation studies have demonstrated therapeutic efficacy, and cGMP-grade cell products are currently being deployed for the first time in human clinical trials. In this perspective, we discuss current challenges surrounding the generation, delivery, and engraftment of stem cell-derived islet-like cells, along with strategies to induce durable tolerance to grafted cells, with an eye toward a functional cellular-based therapy enabling insulin independence for patients with diabetes.

A Large-Scale Bank of Organ Donor Bone Marrow and Matched Mesenchymal Stem Cells for Promoting Immunomodulation and Transplant Tolerance

Induction of immune tolerance for solid organ and vascular composite allografts is the Holy Grail for transplantation medicine. This would obviate the need for life-long immunosuppression which is associated with serious adverse outcomes, such as infections, cancers, and renal failure. Currently the most promising means of tolerance induction is through establishing a mixed chimeric state by transplantation of donor hematopoietic stem cells; however, with the exception of living donor renal transplantation, the mixed chimerism approach has not achieved durable immune tolerance on a large scale in preclinical or clinical trials with other solid organs or vascular composite allotransplants (VCA). Ossium Health has established a bank of cryopreserved bone marrow (BM), termed "hematopoietic progenitor cell (HPC), Marrow," recovered from deceased organ donor vertebral bodies. This new source for hematopoietic cell transplant will be a valuable resource for treating hematological malignancies as well as for inducing transplant tolerance. In addition, we have discovered and developed a large source of mesenchymal stem (stromal) cells (MSC) tightly associated with the vertebral body bone fragment byproduct of the HPC, Marrow recovery process. Thus, these vertebral bone adherent MSC (vBA-MSC) are matched to the banked BM obtained from each donor, as opposed to third-party MSC, which enhances safety and potentially efficacy. Isolation and characterization of vBA-MSC from over 30 donors has demonstrated that the cells are no different than traditional BM-MSC; however, their abundance is >1,000-fold higher than obtainable from living donor BM aspirates. Based on our own unpublished data as well as reports published by others, MSC facilitate chimerism, especially at limiting hematopoietic stem and progenitor cell (HSPC) numbers and increase safety by controlling and/or preventing graft-vs.-host-disease (GvHD). Thus, vBA-MSC have the potential to facilitate mixed chimerism, promote complementary peripheral immunomodulatory functions and increase safety of BM infusions. Both HPC, Marrow and vBA-MSC have potential use in current VCA and solid organ transplant (SOT) tolerance clinical protocols that are amenable to "delayed tolerance." Current trials with HPC, Marrow are planned with subsequent phases to include vBA-MSC for tolerance of both VCA and SOT.

Memory T Cells in Transplantation: Old Challenges Define New Directions

Immunologic memory is the ability of adaptive immune system to quickly and specifically recognize previously encountered antigens and initiate an effector response. Alloreactive memory cells can mount rapid and robust responses to the transplanted organ resulting in allograft injury. Thus preexisting humoral or cellular memory alloresponses are typically associated with poor graft outcomes in experimental and clinical transplantation. While both B and T lymphocytes exhibit memory responses, this review discusses recent updates on the biology of memory T cells and their relevance to the field of transplantation. Three major areas of focus are the emergence and characterization of tissue resident memory T cells, manipulation of T cell metabolic pathways, and the latest promising approaches to targeting detrimental T cell memory in the settings of organ transplantation.

Dynamic adoption of anergy by antigen-exhausted CD4+ T cells

Exhausted immune responses to chronic diseases represent a major challenge to global health. We study CD4⁺ T cells in a mouse model with regulatable antigen presentation. When the cells are driven through the effector phase and are then exposed to different levels of persistent antigen, they lose their T helper 1 (Th1) functions, upregulate exhaustion markers, resemble naturally anergic cells, and modulate their MAPK, mTORC1, and Ca²⁺/calcineurin signaling pathways with increasing dose and time. They also become unable to help B cells and, at the highest dose, undergo apoptosis. Transcriptomic analyses show the dynamic adjustment of gene expression and the accumulation of T cell receptor (TCR) signals over a period of weeks. Upon antigen removal, the cells recover their functionality while losing exhaustion and anergy markers. Our data suggest an adjustable response of CD4⁺ T cells to different levels of persisting antigen and contribute to a better understanding of chronic disease.

In Utero Hematopoietic Cell Transplantation Does Not Alter the Germ Line

BACKGROUND

In utero hematopoietic cell transplantation (IUHCT) has been demonstrated to reliably generate chimeric offspring. This technique introduces transplanted cells into a fetus while the immune system is still developing, allowing for engraftment without the need for myeloablation. However, little is known about the effect of engraftment on the gonadal tissue or within the germ line of the resultant chimeras.

MATERIALS AND METHODS

BALB/cJ mice pups were injected with B6-green fluorescent protein mononuclear bone marrow (BM) cells at gestational ages E13 or E14. Two female and two male chimeras were then crossbred with untreated mice. The gonadal tissue of the chimeras was evaluated with fluorescent stereomicroscopy and green fluorescent protein histologic staining. The progeny of the cross-bred mice was analyzed using flow cytometric evaluation of both the peripheral blood and BM.

RESULTS

Although transplanted cells engrafted within the gonads, no evidence of chimerism was found in oocytes or spermatogonia of female and male mice treated with IUHCT, respectively. Crossbreeding chimeric mice with untreated mice generated progeny without evidence of chimerism in peripheral blood and BM.

CONCLUSIONS

IUHCT yields chimeric mice that have engrafted cells within the gonads but not within the germ line itself. Correspondingly, progeny from the unaltered germ line has no detectable chimerism. This has clinical implications as the offspring of future patients treated with IUHCT would carry the disease for which their parents were treated with IUHCT.

Immune Checkpoint Inhibitors in Transplantation-A Case Series and Comprehensive Review of Current Knowledge

Cancer is a leading cause of morbidity and deaths in solid organ transplant recipients. In immunocompetent patients, cancer prognosis has been dramatically improved with the development of immune checkpoint inhibitors (ICI), as programmed cell death protein 1/programmed death-ligand 1 and cytotoxic T lymphocyte-associated antigen 4 inhibitors, that increase antitumor immune responses. ICI has been developed outside of the scope of transplantation because of the theoretical risk of graft rejection, which has later been confirmed by the publication of several cases and small series. The use of ICI became unavoidable for treating advanced cancers including in organ transplant patients, but their management in this setting remains highly challenging, as to date no strategy to adapt the immunosuppression and to prevent graft rejection has been defined. In this article, we report a monocentric series of 5 solid organ transplant recipients treated with ICI and provide a comprehensive review of current knowledge of ICI management in the setting of solid organ transplantation. Strategies warranted to increase knowledge through collecting more exhaustive data are also discussed.

Haploidentical mixed chimerism cures autoimmunity in established type 1 diabetic mice

Clinical trials are currently testing whether induction of haploidentical mixed chimerism (Haplo-MC) induces organ transplantation tolerance. Whether Haplo-MC can be used to treat established autoimmune diseases remains unknown. Here, we show that established autoimmunity in euthymic and adult-thymectomized NOD (H-2g7) mice was cured by induction of Haplo-MC under a non-myeloablative anti-thymocyte globulin-based conditioning regimen and infusion of CD4+ T cell-depleted hematopoietic graft from H-2b/g7 F1 donors that expressed autoimmune-resistant H-2b or from H-2s/g7 F1 donors that expressed autoimmune-susceptible H-2s. The cure was associated with enhanced thymic negative selection, increased thymic Treg (tTreg) production, and anergy or exhaustion of residual host-type autoreactive T cells in the periphery. The peripheral tolerance was accompanied by expansion of donor- and host-type CD62L-Helios+ tTregs as well as host-type Helios-Nrp1+ peripheral Tregs (pTregs) and PD-L1hi plasmacytoid DCs (pDCs). Depletion of donor- or host-type Tregs led to reduction of host-type PD-L1hi pDCs and recurrence of autoimmunity, whereas PD-L1 deficiency in host-type DCs led to reduction of host-type pDCs and Helios-Nrp1+ pTregs. Thus, induction of Haplo-MC reestablished both central and peripheral tolerance through mechanisms that depend on allo-MHC+ donor-type DCs, PD-L1hi host-type DCs, and the generation and persistence of donor- and host-type tTregs and pTregs.

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.

Inducing Transient Mixed Chimerism for Allograft Survival Without Maintenance Immunosuppression With Combined Kidney and Bone Marrow Transplantation: Protocol Optimization

BACKGROUND

Tolerance induction is an important goal in the field of organ transplantation. We have sequentially modified our conditioning regimen for induction of donor-specific tolerance in recipients of major histocompatibility complex-mismatched combined kidney and bone marrow transplantation (CKBMT).

METHODS

From December 2011 to May 2017, 8 major histocompatibility complex-mismatched patients received CKBMT. The initial conditioning regimen (protocol 1) consisted of cyclophosphamide (CP), rituximab, rabbit antithymocyte globulin, and thymic irradiation. Tacrolimus and steroids were used for the maintenance of immunosuppression (IS).

RESULTS

This regimen was complicated by transient acute kidney injury, which has been the major clinical feature of engraftment syndrome and side effects of CP, although one of 2 subjects successfully discontinued his IS for 14 months. The conditioning regimen was modified by reducing the CP dose and adding fludarabine (protocol 2). The final modification was reducing the fludarabine and rabbit antithymocyte globulin doses (protocol 3). Mixed chimerism, detected by the short tandem repeat method, was achieved transiently in all subjects for 3-20 weeks. Among the 3 subjects treated with protocol 2, IS was successfully discontinued for >35 months in one subject, but the other 2 subjects suffered from severe BK virus-associated nephritis. All 3 subjects treated with protocol 3 tolerated the protocol well and have successfully discontinued IS for >4-41 months. Interestingly, de novo donor-specific antibody was not detected in any subject during all the follow-up periods.

CONCLUSIONS

Our clinical trial has shown that long-term renal allograft survival without maintenance IS can be achieved by induction of mixed chimerism following CKBMT.

Current status and future perspectives of HLA-edited induced pluripotent stem cells

In 2007, Human-induced pluripotent stem cells (iPSCs) were generated by transducing four genes (Oct3/4, Sox2, Klf4, c-Myc). Because iPSCs can differentiate into any types of cells in the body and have fewer ethical issues compared to embryonic stem (ES) cells, application of iPSCs for regenerative medicine has been actively examined. In fact, iPSCs have already been used for clinical applications, but at present, only autologous iPSC-derived grafts or HLA homozygous iPSC-derived grafts are being transplanted into patients following HLA matching. HLA is an important molecule that enables the immune system differentiates between self and non-self-components; thus, HLA mismatch is a major hurdle in the transplantation of iPSCs. To deliver inexpensive off-the-shelf iPSC-derived regenerative medicine products to more patients, it is necessary to generate universal iPSCs that can be transplanted regardless of the HLA haplotypes. The current strategy to generate universal iPSCs has two broad aims: deleting HLA expression and avoiding attacks from NK cells, which are caused by HLA deletion. Deletion of B2M and CIITA genes using the CRISPR/Cas9 system has been reported to suppress the expression of HLA class I and class II, respectively. Transduction of NK inhibitory ligands, such as HLA-E and CD47, has been used to avoid NK cell attacks. Most recently, the HLA-C retaining method has been used to generate semi-universal iPSCs. Twelve haplotypes of HLA-C retaining iPSCs can cover 95% of the global population. In future, studying which types of universal iPSCs are most effective for engraftment in various physiological conditions is necessary.

Donor cell microchimerism in kidney transplantation: Implications for graft function

Given the uncertainty regarding the relationship between donor cells at microchimeric levels and its influence on graft function and clinical outcome, we explored the extent and importance of donor microchimerism in kidney transplantation. Twenty patients with chronic kidney disease who had received allografts from living donors were studied. We examined peripheral whole blood samples from the recipients one month after the transplant, applying mitochondrial DNA variant-specific polymerase chain reaction (PCR) to identify and quantify donor cells in relation to allograft function and survival during three years of follow-up. Higher quantities of donor-derived cell microchimerism in the peripheral blood correlated with better graft function in the early postoperative period at 1 month (R²  = .536, p = .001) and predicted improved graft function 1 year following the transplant (R²  = .430, p = .008). Furthermore, early post-transplant quantities of donor cell microchimerism were an important predictor of improved kidney function 3 years after transplantation (R²  = .397, p = .021). However, donor cell microchimerism failed to predict patient and graft survival after 3 years (odds ratio = 0.536, p = .860). Our findings suggest that donor cell microchimerism plays an immunoregulatory role in kidney transplantation and contributes to donor-specific immune hypo-responsiveness and graft acceptance.

The Immunological Basis of Liver Allograft Rejection

Liver allograft rejection remains a significant cause of morbidity and graft failure in liver transplant recipients. Rejection is caused by the recognition of non-self donor alloantigens by recipient T-cells. Antigen recognition results in proliferation and activation of T-cells in lymphoid tissue before migration to the allograft. Activated T-cells have a variety of effector mechanisms including direct T-cell mediated damage to bile ducts, endothelium and hepatocytes and indirect effects through cytokine production and recruitment of tissue-destructive inflammatory cells. These effects explain the histological appearances of typical acute T-cell mediated rejection. In addition, donor specific antibodies, most typically against HLA antigens, may give rise to antibody-mediated rejection causing damage to the allograft primarily through endothelial injury. However, as an immune-privileged site there are several mechanisms in the liver capable of overcoming rejection and promoting tolerance to the graft, particularly in the context of recruitment of regulatory T-cells and promotors of an immunosuppressive environment. Indeed, around 20% of transplant recipients can be successfully weaned from immunosuppression. Hence, the host immunological response to the liver allograft is best regarded as a balance between rejection-promoting and tolerance-promoting factors. Understanding this balance provides insight into potential mechanisms for novel anti-rejection therapies.

T cell exhaustion is associated with antigen abundance and promotes transplant acceptance

Exhaustion of T cells limits their ability to clear chronic infections or eradicate tumors. Here, in the context of transplant, we investigated whether T cell exhaustion occurs and has a role in determining transplant outcome. A peptide/MHC tetramer-based approach was used to track exhausted CD8⁺ T cells in a male-to-female skin transplant model. Transplant of large whole-tail skins, but not small tail skins (0.8 cm × 0.8 cm), led to exhaustion of anti-male tetramer⁺ CD8⁺ T cells and subsequently the acceptance of skin grafts. To study CD4⁺ T cell exhaustion, we used the TCR-transgenic B6 TEa cells that recognize a major transplant antigen I-Eα from Balb/c mice. TEa cells were adoptively transferred either into B6 recipients that received Balb/c donor skins or into CB6F1 mice that contained an excessive amount of I-Eα antigen. Adoptively transferred TEa cells in skin-graft recipients were not exhausted. By contrast, virtually all adoptively transferred TEa cells were exhausted in CB6F1 mice. Those exhausted TEa cells lost ability to reject Balb/c skins upon further transfer into lymphopenic B6.Rag1^-/- mice. Hence, T cell exhaustion develops in the presence of abundant antigen and promotes transplant acceptance. These findings are essential for better understanding the nature of transplant tolerance.

Donor-targeted serotherapy as a rescue therapy for steroid-resistant acute GVHD after HLA-mismatched kidney transplantation

Acute graft-versus-host disease (GVHD) is a rare but frequently lethal complication after solid organ transplantation. GVHD occurs in unduly immunocompromised hosts but requires the escalation of immunosuppression, which does not discriminate between host and donor cells. In contrast, donor-targeted therapy would ideally mitigate graft-versus-host reactivity while sparing recipient immune functions. We report two children with end-stage renal disease and severe primary immune deficiency (Schimke syndrome) who developed severe steroid-resistant acute GVHD along with full and sustained donor T cell chimerism after isolated kidney transplantation. Facing a therapeutic dead end, we used a novel strategy based on the adoptive transfer of anti-HLA donor-specific antibodies (DSAs) through the transfusion of highly selected plasma. After approval by the appropriate regulatory authority, an urgent nationwide search was launched among more than 3800 registered blood donors with known anti-HLA sensitization. Adoptively transferred DSAs bound to and selectively depleted circulating donor T cells. The administration of DSA-rich plasma was well tolerated and notably did not induce antibody-mediated rejection of the renal allografts. Acute GVHD symptoms promptly resolved in one child. This report provides a proof of concept for a highly targeted novel therapeutic approach for solid organ transplantation-associated GVHD.

Xenogeneic cross-circulation for extracorporeal recovery of injured human lungs

Patients awaiting lung transplantation face high wait-list mortality, as injury precludes the use of most donor lungs. Although ex vivo lung perfusion (EVLP) is able to recover marginal quality donor lungs, extension of normothermic support beyond 6 h has been challenging. Here we demonstrate that acutely injured human lungs declined for transplantation, including a lung that failed to recover on EVLP, can be recovered by cross-circulation of whole blood between explanted human lungs and a Yorkshire swine. This xenogeneic platform provided explanted human lungs a supportive, physiologic milieu and systemic regulation that resulted in functional and histological recovery after 24 h of normothermic support. Our findings suggest that cross-circulation can serve as a complementary approach to clinical EVLP to recover injured donor lungs that could not otherwise be utilized for transplantation, as well as a translational research platform for immunomodulation and advanced organ bioengineering.

The importance of MHC class II in allogeneic bone marrow transplantation and chimerism-based solid organ tolerance in a rat model

Mixed hematopoietic chimerism enables donor-specific tolerance for solid organ grafts. This study evaluated the influence of different serological major histocompatibility complex disparities on chimerism development, graft-versus-host disease incidence and subsequently on solid organ tolerance in a rat model. For bone marrow transplantation conditioning total body irradiation was titrated using 10, 8 or 6 Gray. Bone marrow transplantation was performed across following major histocompatibility complex mismatched barriers: complete disparity, MHC class II, MHC class I or non-MHC mismatch. Recipients were clinically monitored for graft-versus-host disease and analyzed for chimerism using flow cytometry. After a reconstitution of 100 days, composition of peripheral leukocytes was determined. Mixed chimeras were challenged with heart grafts from allogeneic donor strains to define the impact of donor MHC class disparities on solid organ tolerance on the basis of stable chimerism. After myeloablation with 10 Gray of total body irradiation, chimerism after bone marrow transplantation was induced independent of MHC disparity. MHC class II disparity increased the incidence of graft-versus-host disease and reduced induction of stable chimerism upon myelosuppressive total body irradiation with 8 and 6 Gray, respectively. Stable mixed chimeras showed tolerance towards heart grafts from donors with MHC matched to either bone marrow donors or recipients. Isolated matching of MHC class II with bone marrow donors likewise led to stable tolerance as opposed to matching of MHC class I. In summary, MHC class II disparity was critically associated with the onset of graft-versus host disease and was identified as obstacle for successful development of chimerism after bone marrow transplantation and subsequent donor-specific solid organ tolerance.

Manipulating the TCR signaling network for cellular immunotherapy: Challenges & opportunities

T cells can help confer protective immunity by eliminating infections and tumors or drive immunopathology by damaging host cells. Both outcomes require a series of steps from the activation of naïve T cells to their clonal expansion, differentiation and migration to tissue sites. In addition to specific recognition of the antigen via the T cell receptor (TCR), multiple accessory signals from costimulatory molecules, cytokines and metabolites also influence each step along the progression of the T cell response. Current efforts to modify effector T cell function in many clinical contexts focus on the latter - which encompass antigen-independent and broad, contextual regulators. Not surprisingly, such approaches are often accompanied by adverse events, as they also affect T cells not relevant to the specific treatment. In contrast, fine tuning T cell responses by precisely targeting antigen-specific TCR signals has the potential to radically alter therapeutic strategies in a focused manner. Development of such approaches, however, requires a better understanding of functioning of the TCR and the biochemical signaling network coupled to it. In this article, we review some of the recent advances which highlight important roles of TCR signals throughout the activation and differentiation of T cells during an immune response. We discuss how, an appreciation of specific signaling modalities and variant ligands that influence the function of the TCR has the potential to influence design principles for the next generation of pharmacologic and cellular therapies, especially in the context of tumor immunotherapies involving adoptive cell transfers.

Cytomegalovirus Latency and Reactivation: An Intricate Interplay With the Host Immune Response

CMV is an ancient herpesvirus that has co-evolved with its host over millions of years. The 236 kbp genome encodes at least 165 genes, four non-coding RNAs and 14 miRNAs. Of the protein-coding genes, 43-44 are core replication genes common to all herpesviruses, while ~30 are unique to betaherpesviruses. Many CMV genes are involved in evading detection by the host immune response, and others have roles in cell tropism. CMV replicates systemically, and thus, has adapted to various biological niches within the host. Different biological niches may place competing demands on the virus, such that genes that are favorable in some contexts are unfavorable in others. The outcome of infection is dependent on the cell type. In fibroblasts, the virus replicates lytically to produce infectious virus. In other cell types, such as myeloid progenitor cells, there is an initial burst of lytic gene expression, which is subsequently silenced through epigenetic repression, leading to establishment of latency. Latently infected monocytes disseminate the virus to various organs. Latency is established through cell type specific mechanisms of transcriptional silencing. In contrast, reactivation is triggered through pathways activated by inflammation, infection, and injury that are common to many cell types, as well as differentiation of myeloid cells to dendritic cells. Thus, CMV has evolved a complex relationship with the host immune response, in which it exploits cell type specific mechanisms of gene regulation to establish latency and to disseminate infection systemically, and also uses the inflammatory response to infection as an early warning system which allows the virus to escape from situations in which its survival is threatened, either by cellular damage or infection of the host with another pathogen. Spontaneous reactivation induced by cellular aging/damage may explain why extensive expression of lytic genes has been observed in recent studies using highly sensitive transcriptome analyses of cells from latently infected individuals. Recent studies with animal models highlight the potential for harnessing the host immune response to blunt cellular injury induced by organ transplantation, and thus, prevent reactivation of CMV and its sequelae.