Contrasting CD25hiCD4+T cells/FOXP3 patterns in chronic rejection and operational drug-free tolerance

B Cell-mediated Immune Regulation and the Quest for Transplantation Tolerance

Pathophysiologic function of B cells in graft rejection has been well recognized in transplantation. B cells promote alloantigen-specific T-cell response and secrete antibodies that can cause antibody-mediated graft failures and rejections. Therefore, strategies targeting B cells, for example, B-cell depletion, have been used for the prevention of both acute and chronic rejections. Interestingly, however, recent mounting evidence indicates that subsets of B cells yet to be further identified can display potent immune regulatory functions, and they contribute to transplantation tolerance and operational tolerance in both experimental and clinical settings, respectively. In this review, we integrate currently available information on B-cell subsets, including T-cell Ig domain and mucin domain 1-positive transitional and T-cell immunoreceptor with Ig and immunoreceptor tyrosine-based inhibitory motif domain-positive memory B cells, displaying immune regulatory functions, with a focus on transplantation tolerance, by analyzing their mechanisms of action. In addition, we will discuss potential T-cell Ig domain and mucin domain 1-positive and T-cell immunoreceptor with Ig and immunoreceptor tyrosine-based inhibitory motif domain-positive B cell-based strategies for the enhancement of operational tolerance in transplantation patients.

The balance between memory and regulatory cell populations in kidney transplant recipients with operational tolerance

Donor-reactive memory cells represent a barrier to long-term kidney graft survival. A better understanding of regulatory mechanisms that counterbalance alloreactive memory responses may help to identify patients with operational tolerance. This prospective study investigated the equilibrium between memory T-cell subsets and regulatory T or B cells (Tregs, Bregs) in peripheral blood of kidney transplant recipients with operational tolerance (N = 8), chronic rejection (N = 8), and different immunosuppressive treatment regimens (N = 81). Patients on hemodialysis and healthy individuals served as controls (N = 50). In addition, the expression of Treg- and Breg-associated molecule genes was analyzed. Patients with chronic rejection showed a disrupted memory T-cell composition with a significantly higher frequency of circulating CD8+ terminally differentiated effector memory (TEMRA) T cells than patients with operational tolerance, patients on hemodialysis, or healthy controls (P < 0.001). Low frequency of CD8+ TEMRA and high frequency of Tregs and transitional Bregs were found in operationally tolerant patients. Consequently, operationally tolerant patients showed, as compared to all other transplant recipients with different immunosuppressive regiments, the lowest ratios between CD8+ TEMRA T cells and Tregs or Bregs (for both P < 0.001). Moreover, a specific peripheral blood transcription pattern was found in operationally tolerant patients with an increased expression of Breg- and Treg-associated genes CD22 and FoxP3 and a decreased FcγRIIA/FcγRIIB transcript ratio (for all P < 0.001). In conclusion, monitoring the balance between circulating CD8+ TEMRA T cells and regulatory cell subsets and their transcripts may help to distinguish transplant recipients with operational tolerance from recipients at risk of graft loss.

FOXP3 full length splice variant is associated with kidney allograft tolerance

Background

Progressive decline of allograft function leads to premature graft loss. Forkhead box P3 (FOXP3), a characteristic gene of T-regulatory cells, is known to be essential for auto-antigen tolerance. We assessed the hypothesis that low FOXP3 mRNA splice variant levels in peripheral blood cells early after transplantation are associated with progressive allograft injury.

Methods

Blood samples were prospectively collected from 333 incident kidney transplant recipients on the first and 29th postoperative day. We used quantitative polymerase chain reaction to determine transcripts of 3 isotypes of FOXP3 splice variants, including pre-mature FOXP3 and full length FOXP3 (FOXP3fl). We investigated the association between FOXP3 splice variant levels and the declines in estimated glomerular filtration rate (eGFR) of more than 5ml/min/1.73m2 within the first-year post-transplant using logistic regression.

Results

We observed lower FOXP3fl levels in recipients with declining eGFR (N = 132) than in recipients with stable eGFR (N = 201), (logarithmic value -4.13 [IQR -4.50 to -3.84] vs -4.00 [4.32 to -3.74], p=0.02). In ad hoc analysis pre-transplant FOXP3fl levels were similar in both groups. The association between FOXP3fl and declining eGFR was confirmed by multivariable analysis adjusted for potential confounding factors (Odds Ratio 0.51, 95% confidence interval 0.28 to 0.91: p=0.02). When stratifying FOXP3fl levels into quartiles, recipients with lower day1 FOXP3fl had the highest rate of declining eGFR (p=0.04).

Conclusion

Low FOXP3fl splice variant levels at the first postoperative day in kidney transplant recipients were associated with severe decline of eGFR, a well-known surrogate for hard endpoints.

Discovery of cellular and genetic signatures of immune tolerance in kidney transplant recipients through single cell RNA sequencing analysis

The objective of this study was to uncover distinct cellular and genetic signatures of transplant operational tolerance (TOT) in kidney transplant recipients (KTRs) through single cell RNA sequencing (scRNA-seq) using peripheral blood mononuclear cells (PBMCs). PBMCs were isolated from 12 KTRs, including those with TOT (TOT, n = 4), stable allograft function on maintenance immunosuppression (STA, n = 4) and biopsy-proven allograft rejection (BPAR, n = 4). ScRNA-seq of PBMCs was analyzed using 20 cell surface marker antibody sequencing to annotate clusters and 399 immune response panel to identify gene expression. Differences in cellular distribution and gene expression were compared among the three groups. Heatmap hierarchical clustering showed that overall cellular distribution pattern was distinct in TOT in comparison with those in the other two groups, with the proportion of B cells being higher in TOT, attributed to immature B cell fraction (TOT vs. STA vs. BPAR: 4.61% vs. 1.27% vs. 2.53%, p = 0.01). Transcript analysis of B cells revealed that genes involved in allo-immune pathway were downregulated in TOT. In T cell subset analysis, the proportion of naïve T cells and regulatory T cells (Tregs) was increased. In transcript analysis, genes associated with inflammation were decreased, while expression levels of CCR6 in Tregs were increased in TOT. Proportions of NKT and NK cells were increased in TOT than in the other two groups. This study showed that TOT has distinct cellular and genetic signatures such as increases of immature B cells, naïve T cells and Tregs and high expression levels of CCR6 in Tregs.

Radiolabelling of Polyclonally Expanded Human Regulatory T Cells (Treg) with 89Zr-oxine for Medium-Term In Vivo Cell Tracking

Regulatory T cells (Tregs) are a promising candidate cell therapy to treat autoimmune diseases and aid the longevity of transplanted solid organs. Despite increasing numbers of clinical trials using human Treg therapy, important questions pertaining to their in vivo fate, distribution, and function remain unanswered. Treg accumulation in relevant tissues was found to be crucial for Treg therapy efficacy, but existing blood-borne biomarkers are unlikely to accurately reflect the tissue state. Non-invasive Treg tracking by whole-body imaging is a promising alternative and can be achieved by direct radiolabelling of Tregs and following the radiolabelled cells with positron emission tomography (PET). Our goal was to evaluate the radiolabelling of polyclonal Tregs with 89Zr to permit their in vivo tracking by PET/CT for longer than one week with current preclinical PET instrumentation. We used [89Zr]Zr(oxinate)4 as the cell-labelling agent and achieved successful radiolabelling efficiency of human Tregs spanning 0.1-11.1 Bq 89Zr/Treg cell, which would be compatible with PET tracking beyond one week. We characterized the 89Zr-Tregs, assessing their phenotypes, and found that they were not tolerating these intracellular 89Zr amounts, as they failed to survive or expand in a 89Zr-dose-dependent manner. Even at 0.1 Bq 89Zr per Treg cell, while 89Zr-Tregs remained functional as determined by a five-day-long effector T cell suppression assay, they failed to expand beyond day 3 in vitro. Moreover, PET imaging revealed signs of 89Zr-Treg death after adoptive transfer in vivo. In summary, 89Zr labelling of Tregs at intracellular radioisotope amounts compatible with cell tracking over several weeks did not achieve the desired outcomes, as 89Zr-Tregs failed to expand and survive. Consequently, we conclude that indirect Treg labelling is likely to be the most effective alternative method to satisfy the requirements of this cell tracking scenario.

Kidney allograft rejection is associated with an imbalance of B cells, regulatory T cells and differentiated CD28-CD8+ T cells: analysis of a cohort of 1095 graft biopsies

Introduction

The human immune system contains cells with either effector/memory or regulatory functions. Besides the well-established CD4+CD25hiCD127lo regulatory T cells (Tregs), we and others have shown that B cells can also have regulatory functions since their frequency and number are increased in kidney graft tolerance and B cell depletion as induction therapy may lead to acute rejection. On the other hand, we have shown that CD28-CD8+ T cells represent a subpopulation with potent effector/memory functions. In the current study, we tested the hypothesis that kidney allograft rejection may be linked to an imbalance of effector/memory and regulatory immune cells.

Methods

Based on a large cohort of more than 1000 kidney graft biopsies with concomitant peripheral blood lymphocyte phenotyping, we investigated the association between kidney graft rejection and the percentage and absolute number of circulating B cells, Tregs, as well as the ratio of B cells to CD28-CD8+ T cells and the ratio of CD28-CD8+ T cells to Tregs. Kidney graft biopsies were interpreted according to the Banff classification and divided into 5 biopsies groups: 1) normal/subnormal, 2) interstitial fibrosis and tubular atrophy grade 2/3 (IFTA), 3) antibody-mediated rejection (ABMR), 4) T cell mediated-rejection (TCMR), and 5) borderline rejection. We compared group 1 with the other groups as well as with a combined group 3, 4, and 5 (rejection of all types) using multivariable linear mixed models.

Results and discussion

We found that compared to normal/subnormal biopsies, rejection of all types was marginally associated with a decrease in the percentage of circulating B cells (p=0.06) and significantly associated with an increase in the ratio of CD28-CD8+ T cells to Tregs (p=0.01). Moreover, ABMR, TCMR (p=0.007), and rejection of all types (p=0.0003) were significantly associated with a decrease in the ratio of B cells to CD28-CD8+ T cells compared to normal/subnormal biopsies. Taken together, our results show that kidney allograft rejection is associated with an imbalance between immune cells with effector/memory functions and those with regulatory properties.

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.

Impacts of dosing and drug withdrawal period on tacrolimus-based triple therapy in a non-human primate renal transplantation model

Non-human primate (NHP) renal transplantation models are widely used vivo models for researching new immunosuppressive therapies including allograft tolerance strategies. To enroll animals into a tolerance study, an immunosuppressive regimen that efficiently establishes stable renal function in NHPs is needed. Here, we assessed the effect of triple therapy comprising 2.0 mg/kg tacrolimus, mycophenolate mofetil and a steroid and its success rate for achieving stable renal function. In addition, to predict the pathophysiological consequences of withdrawing immunosuppressants, an indispensable process after induction of tolerance, we also assessed changes in the stable renal state maintained by triple therapy after drug withdrawal. Six cynomolgus monkeys were used. The median survival time was >176 days over the dosing period and 45 days after drug withdrawal. The triple therapy successfully induced stable graft function without calcineurin inhibitor nephrotoxicity in three of six recipients, although adopting trough-dependent tacrolimus dose adjustment rather than a preset dose regimen could improve on the present strategy. Further, drug withdrawal led to deterioration of renal function, de novo donor specific antibody production and increased the memory/naïve T cell ratio within two weeks post drug withdrawal. We expect that these findings contribute to establish one of the choices for animal model for evaluating future tolerance therapy for renal transplantation.

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.

Utilization of Treg Cells in Solid Organ Transplantation

Organ transplants have been a life-saving form of treatment for many patients who are facing end stage organ failure due to conditions such as diabetes, hypertension, various congenital diseases, idiopathic diseases, traumas, and other end-organ failure. While organ transplants have been monumental in treatment for these conditions, the ten year survival and long-term outcome for these patients is poor. After receiving the transplant, patients receive a multi-drug regimen of immunosuppressants. These drugs include cyclosporine, mTOR inhibitors, corticosteroids, and antibodies. Polyclonal antibodies, which inhibit the recipient’s B lymphocytes, and antibodies targeting host cytokine inhibitors which prevent activation of B cells by T cells. Use of these drugs suppresses the immune system and increases the risk of opportunistic pathogen infections, tumors, and further damage to the transplanted organs and vasculature. Many regulatory mechanisms are present in organs to prevent the development of autoimmune disease, and Tregs are central to these mechanisms. Tregs secrete suppressive cytokines such as IL-10, TGF-B, and IL-35 to suppress T cells. Additionally, Tregs can bind to target cells to induce cell cycle arrest and apoptosis and can inhibit induction of IL-2 mRNA in target T cells. Tregs also interact with CTLA-4 and CD80/CD86 on antigen presenting cells (APCs) to prevent their binding to CD28 present on T cells. Due to their various immunosuppressive capabilities, Tregs are being examined as a possible treatment for patients that receive organ transplants to minimize rejection and prevent the negative outcomes. Several studies in which participants were given Tregs after undergoing organ transplantations were reviewed to determine the efficacy and safety of using Tregs in solid organ transplantation to prevent adverse outcomes.

A New Generation of Cell Therapies Employing Regulatory T Cells (Treg) to Induce Immune Tolerance in Pediatric Transplantation

Kidney transplantation is the most common solid organ transplant and the preferred treatment for pediatric patients with end-stage renal disease, but it is still not a definitive solution due to immune graft rejection. Regulatory T cells (Treg) and their control over effector T cells is a crucial and intrinsic tolerance mechanism in limiting excessive immune responses. In the case of transplants, Treg are important for the survival of the transplanted organ, and their dysregulation could increase the risk of rejection in transplanted children. Chronic immunosuppression to prevent rejection, for which Treg are especially sensitive, have a detrimental effect on Treg counts, decreasing the Treg/T-effector balance. Cell therapy with Treg cells is a promising approach to restore this imbalance, promoting tolerance and thus increasing graft survival. However, the strategies used to date that employ peripheral blood as a Treg source have shown limited efficacy. Moreover, it is not possible to use this approach in pediatric patients due to the limited volume of blood that can be extracted from children. Here, we outline our innovative strategy that employs the thymus removed during pediatric cardiac surgeries as a source of therapeutic Treg that could make this therapy accessible to transplanted children. The advantageous properties and the massive amount of Treg cells obtained from pediatric thymic tissue (thyTreg) opens a new possibility for Treg therapies to prevent rejection in pediatric kidney transplants. We are recruiting patients in a clinical trial to prevent rejection in heart-transplanted children through the infusion of autologous thyTreg cells (NCT04924491). If its efficacy is confirmed, thyTreg therapy may establish a new paradigm in preventing organ rejection in pediatric transplants, and their allogeneic use would extend its application to other solid organ transplantation.

Highly Purified Alloantigen-Specific Tregs From Healthy and Chronic Kidney Disease Patients Can Be Long-Term Expanded, Maintaining a Suppressive Phenotype and Function in the Presence of Inflammatory Cytokines

The adoptive transfer of alloantigen-specific regulatory T cells (^alloTregs) has been proposed as a therapeutic alternative in kidney transplant recipients to the use of lifelong immunosuppressive drugs that cause serious side effects. However, the clinical application of ^alloTregs has been limited due to their low frequency in peripheral blood and the scarce development of efficient protocols to ensure their purity, expansion, and stability. Here, we describe a new experimental protocol that allows the long-term expansion of highly purified allospecific natural Tregs (nTregs) from both healthy controls and chronic kidney disease (CKD) patients, which maintain their phenotype and suppressive function under inflammatory conditions. Firstly, we co-cultured CellTrace Violet (CTV)-labeled Tregs from CKD patients or healthy individuals with allogeneic monocyte-derived dendritic cells in the presence of interleukin 2 (IL-2) and retinoic acid. Then, proliferating CD4⁺CD25^hiCTV⁻ Tregs (allospecific) were sorted by fluorescence-activated cell sorting (FACS) and polyclonally expanded with anti-CD3/CD28-coated beads in the presence of transforming growth factor beta (TGF-β), IL-2, and rapamycin. After 4 weeks, ^alloTregs were expanded up to 2,300 times the initial numbers with a purity of >95% (CD4⁺CD25^hiFOXP3⁺). The resulting allospecific Tregs showed high expressions of CTLA-4, LAG-3, and CD39, indicative of a highly suppressive phenotype. Accordingly, expanded ^alloTregs efficiently suppressed T-cell proliferation in an antigen-specific manner, even in the presence of inflammatory cytokines (IFN-γ, IL-4, IL-6, or TNF-α). Unexpectedly, the long-term expansion resulted in an increased methylation of the specific demethylated region of Foxp3. Interestingly, ^alloTregs from both normal individuals and CKD patients maintained their immunosuppressive phenotype and function after being expanded for two additional weeks under an inflammatory microenvironment. Finally, phenotypic and functional evaluation of cryopreserved ^alloTregs demonstrated the feasibility of long-term storage and supports the potential use of this cellular product for personalized Treg therapy in transplanted patients.

The MHC-characterized Miniature Swine: Lessons Learned From a 40-Year Experience in Transplantation

Over the last 40 y, a specialized herd of miniature swine has been intentionally bred to develop lines of animals homozygous for the swine major histocompatibility complex (MHC), which have facilitated transplantation studies across reproducible MHC and minor antigen mismatch barriers. These MHC-characterized miniature swine (Mc-MS) have been used for the study of novel surgical techniques, various approaches to tolerance induction of solid organ and vascularized composite allografts, as well as studies of the immunobiology of allografts and xenografts. Mc-MS possess characteristics that are highly advantageous to these studies, and their continued use will likely continue to play an important role in bridging "bench-to-cage-to bedside" therapies in the field of transplantation. In this review, we highlight the seminal contributions of the Mc-MS model to the field and analyze their role in the broader context of large animal models in transplantation research.

Regulatory T-Cell Therapy in Liver Transplantation and Chronic Liver Disease

The constant exposure of the liver to gut derived foreign antigens has resulted in this organ attaining unique immunological characteristics, however it remains susceptible to immune mediated injury. Our understanding of this type of injury, in both the native and transplanted liver, has improved significantly in recent decades. This includes a greater awareness of the tolerance inducing CD4+ CD25+ CD127low T-cell lineage with the transcription factor FoxP3, known as regulatory T-Cells (Tregs). These cells comprise 5-10% of CD4+ T cells and are known to function as an immunological "braking" mechanism, thereby preventing immune mediated tissue damage. Therapies that aim to increase Treg frequency and function have proved beneficial in the setting of both autoimmune diseases and solid organ transplantations. The safety and efficacy of Treg therapy in liver disease is an area of intense research at present and has huge potential. Due to these cells possessing significant plasticity, and the potential for conversion towards a T-helper 1 (Th1) and 17 (Th17) subsets in the hepatic microenvironment, it is pre-requisite to modify the microenvironment to a Treg favourable atmosphere to maintain these cells' function. In addition, implementation of therapies that effectively increase Treg functional activity in the liver may result in the suppression of immune responses and will hinder those that destroy tumour cells. Thus, fine adjustment is crucial to achieve this immunological balance. This review will describe the hepatic microenvironment with relevance to Treg function, and the role these cells have in both native diseased and transplanted livers.

Regulatory B cells, A to Z

B cells play a central role in the immune system through the production of antibodies. During the past two decades, it has become increasingly clear that B cells also have the capacity to regulate immune responses through mechanisms that extend beyond antibody production. Several types of human and murine regulatory B cells have been reported that suppress inflammatory responses in autoimmune disease, allergy, infection, transplantation, and cancer. Key suppressive molecules associated with regulatory B-cell function include the cytokines IL-10, IL-35, and TGF-β as well as cell membrane-bound molecules such as programmed death-ligand 1, CD39, CD73, and aryl hydrocarbon receptor. Regulatory B cells can be induced by a range of different stimuli, including microbial products such as TLR4 or TLR9 ligands, inflammatory cytokines such as IL-6, IL-1β, and IFN-α, as well as CD40 ligation. This review provides an overview of our current knowledge on regulatory B cells. We discuss different types of regulatory B cells, the mechanisms through which they exert their regulatory functions, factors that lead to induction of regulatory B cells and their role in the alteration of inflammatory responses in different diseases.

Inflammation Determines the Capacity of Allogenic Endothelial Cells to Regulate Human Treg Expansion

During allotransplantation, the endothelium acts as semi-professional antigen-presenting cells with the ability to activate proliferation and to promote differentiation of CD4⁺-T subsets. These abilities are dependent on the luminal expression of HLA class II antigens by microvascular endothelial cells, which is regulated by inflammatory cytokines. The upregulation of HLA-DR and HLA-DQ during rejection implies significant intragraft inflammation. Furthermore, the microvascular inflammation is an independent determinant for renal allograft failure. In this study, the potential of inflammation to modify endothelial regulation of peripheral CD4⁺ Treg cells was examined. Microvascular endothelial cells were exposed to pro-inflammatory cytokines for varying durations before co-culture with PBMC from non-HLA matched donors. Proliferation and expansion of CD4⁺Treg and soluble factor secretion was determined. Early interactions were detected by phosphorylation of Akt. Video microscopy was used to examine spatial and temporal endothelial-CD4⁺T interactions. Highly inflammatory conditions led to increased endothelial expression of HLA-DR, the adhesion molecule ICAM-1, the costimulatory molecule PD-L1 and de novo expression of HLA-DQ. Treg differentiation was impaired by exposure of endothelial cells to a high level of inflammation. Neither IL-6, IL-2 nor TGFβ were implicated in reducing Treg numbers. High PD-L1 expression interfered with early endothelial cell interactions with CD4⁺T lymphocytes and led to modified TCR signaling. Blocking endothelial PD-L1 resulted in a partial restoration of Treg. The allogenic endothelial cell-mediated expansion of Treg depends on a critical threshold of inflammation. Manipulation of the PD-L1/PD-1 pathway or endothelial activation post-transplantation may promote or interfere with this intrinsic mechanism of allospecific Treg expansion.

Connection of BANK1, Tolerance, Regulatory B cells, and Apoptosis: Perspectives of a Reductionist Investigation

BANK1 transcript is upregulated in whole blood after kidney transplantation in tolerant patients. In comparison to patients with rejection, tolerant patients display higher level of regulatory B cells (Bregs) expressing granzyme B (GZMB⁺) that have the capability to prevent effector T cells proliferation. However, BANK1 was found to be decreased in these GZMB⁺ Bregs. In this article, we investigated seven different transcriptomic studies and mined the literature in order to make link between BANK1, tolerance and Bregs. As for GZMB⁺ Bregs, we found that BANK1 was decreased in other subtypes of Bregs, including IL10⁺ and CD24^hiCD38^hi transitional regulatory B cells, along with BANK1 was down-regulated in activated/differentiated B cells, as in CD40-activated B cells, in leukemia and plasma cells. Following a reductionist approach, biological concepts were extracted from BANK1 literature and allowed us to infer association between BANK1 and immune signaling pathways, as STAT1, FcγRIIB, TNFAIP3, TRAF6, and TLR7. Based on B cell signaling literature and expression data, we proposed a role of BANK1 in B cells of tolerant patients that involved BCR, IP3R, and PLCG2, and a link with the apoptosis pathways. We confronted these data with our experiments on apoptosis in total B cells and Bregs, and this suggests different involvement for BANK1 in these two cells. Finally, we put in perspective our own data with other published data to hypothesize two different roles for BANK1 in B cells and in Bregs.

Progress in Translational Regulatory T Cell Therapies for Type 1 Diabetes and Islet Transplantation

Regulatory T cells (Tregs) have become highly relevant in the pathophysiology and treatment of autoimmune diseases, such as type 1 diabetes (T1D). As these cells are known to be defective in T1D, recent efforts have explored ex vivo and in vivo Treg expansion and enhancement as a means for restoring self-tolerance in this disease. Given their capacity to also modulate alloimmune responses, studies using Treg-based therapies have recently been undertaken in transplantation. Islet transplantation provides a unique opportunity to study the critical immunological crossroads between auto- and alloimmunity. This procedure has advanced greatly in recent years, and reports of complete abrogation of severe hypoglycemia and long-term insulin independence have become increasingly reported. It is clear that cellular transplantation has the potential to be a true cure in T1D, provided the remaining barriers of cell supply and abrogated need for immune suppression can be overcome. However, the role that Tregs play in islet transplantation remains to be defined. Herein, we synthesize the progress and current state of Treg-based therapies in T1D and islet transplantation. We provide an extensive, but concise, background to understand the physiology and function of these cells and discuss the clinical evidence supporting potency and potential Treg-based therapies in the context of T1D and islet transplantation. Finally, we discuss some areas of opportunity and potential research avenues to guide effective future clinical application. This review provides a basic framework of knowledge for clinicians and researchers involved in the care of patients with T1D and islet transplantation.

Spatiotemporal in vivo tracking of polyclonal human regulatory T cells (Tregs) reveals a role for innate immune cells in Treg transplant recruitment

Regulatory T cells (Tregs) are emerging as a new cell-based therapy in solid organ transplantation. Adoptive transfer of Tregs has been shown preclinically to protect from graft rejection, and the safety of Treg therapy has been demonstrated in clinical trials. Despite these successes, the in vivo distribution and persistence of adoptively transferred Tregs remained elusive, which hampers clinical translation. Here we isolated human Tregs using a GMP-compatible protocol and lentivirally transduced them with the human sodium iodide symporter to render them traceable in vivo by radionuclide imaging. Engineered human Tregs were characterized for phenotype, survival, suppressive capacity, and reporter function. To study their trafficking behavior, they were subsequently administered to humanized mice with human skin transplants. Traceable Tregs were quantified in skin grafts by non-invasive nano-single-photon emission computed tomography (nanoSPECT)/computed tomography (CT) for up to 40 days, and the results were validated ex vivo. Using this approach, we demonstrated that Treg trafficking to skin grafts was regulated by the presence of recipient Gr-1+ innate immune cells. We demonstrated the utility of radionuclide reporter gene-afforded quantitative Treg in vivo tracking, addressing a fundamental need in Treg therapy development and offering a clinically compatible methodology for future Treg therapy imaging in humans.

Cellular Immunotherapies in Preclinical Large Animal Models of Transplantation

Hematopoietic stem cell (HSC) transplantation and solid organ transplantation remain the only curative options for many hematologic malignancies and end-stage organ diseases. Unfortunately, the sequelae of long-term immunosuppression, as well as acute and chronic rejection, carry significant morbidities, including infection, malignancy, and graft loss. Numerous murine models have demonstrated the efficacy of adjunctive cellular therapies using HSCs, regulatory T cells, mesenchymal stem cells, and regulatory dendritic cells in modulating the alloimmune response in favor of graft tolerance; however, translation of such murine approaches to other preclinical models and in the clinic has yielded mixed results. Large animals, including nonhuman primates, swine, and canines, provide a more immunologically rigorous model in which to test the clinical translatability of these cellular therapies. Here, we highlight the contributions of large animal models to the development and optimization of HSCs and additional cellular therapies to improve organ transplantation outcomes.

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

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

Decreased Foxp3 and function of Tregs caused immune imbalance and liver injury in patients with autoimmune liver diseases post-liver transplantation

Background

Autoimmune liver diseases (AILD) is a type of autoimmune disease which may cause end-stage liver failure and require liver transplantation. Regulatory T cells (Tregs) play an irreplaceable role in maintaining immunological homeostasis.

Methods

In this study, we made a comparative analysis of the immune balance and graft function between AILD patients’ post-transplantation and the patients who have had liver failure with hepatitis B virus (HBV) infection post-transplantation. Immune cell phenotype of two groups were analyzed. We sorted CD4+CD25+CD127-Tregs both in vitro and vivo and did TSDR methylation status assay to explore further possible mechanisms.

Results

Our data showed that there is a worse prognosis with severe graft function in liver transplant patients with AILD compared to patients with HBV-induced liver failure. Immune cell phenotype analysis showed that more Tregs could be detected in AILD patients compared with HBV patients’ post-transplantation. We sorted CD4+CD25+CD127-Tregs in vivo and showed that Tregs presented decreased function both in vitro and vivo. Mechanism study also proved that modulation of the phosphorylation level of STAT1 and STAT3 as well as the methylation level of TSDR in Foxp3 might partially result in the function loss of Tregs.

Conclusions

These results suggest that loss of Foxp3 expression and suppressive function of Tregs may be the critical factor that causes graft loss for liver transplant patients after AILD.

Reduced CD4+ CD25++ CD45RA- Foxp3hi activated regulatory T cells and its association with acute rejection in patients with kidney transplantation

BACKGROUND

It was found that regulatory T cells (Tregs) importantly affect the maintenance of the kidney graft. However, Tregs are a heterogeneous population with less to more suppressive activity. The aim of this study was to determine the effects of different subsets of Tregs, as well as their ratio to effector T cells (Teff), on kidney transplantation outcomes.

METHODS

A total of 58 participants were enrolled in this study and divided into four groups: (i) first kidney transplant recipients (stable 1); (ii) second kidney transplant recipients (stable 2); (iii) transplant recipients with acute rejection (AR); and (iv) healthy control subjects. By using flow cytometer, the frequencies of CD4⁺ CD25⁺⁺ CD45RA^- Foxp3^hi activated Tregs (aTregs), CD4⁺ CD25⁺ CD45RA⁺ Foxp3^lo resting Tregs (rTregs), CD4⁺ CD25⁺ CD45RA^- Foxp3^lo non-suppressive T cells, CD4⁺ CD25⁺ Foxp3^- cells Teff, and total Tregs were analyzed in all subjects.

RESULTS

The frequency of aTregs (as well as the ratio of aTregs/Tregs) was significantly lower in the AR patients than the other three groups. In contrast to AR patients, stables 1 and 2 had a higher aTreg/Treg ratio than those in the control group. Although patients with AR had a significantly lower total Tregs than the other three groups, the balance of total Tregs and Teff was similar between patients with and without AR.

CONCLUSION

Patients with AR had poorer immunoregulatory properties than those with normal graft functioning, as well as those in the control group. These reduced immunoregulatory properties in patients with AR could lead to graft rejection.

Differential gene expression in non-adherent heart transplant survivors: Implications for regulatory T-cell expression

Survival despite prolonged non-adherence with immunosuppression is rare but has been reported in kidney, lung, and liver transplantation. Its occurrence in heart transplantation is quite rare. Our study was prompted by an index patient who survived despite prolonged medication non-adherence. Prospective consent and blood collection were conducted for seven additional patients who presented in a similar fashion. The blood of patients who were diagnosed with rejection, stable early post-transplant, and stable more than 5 years post-transplant were all compared with a custom gene array focusing on T-regulatory cell processes. The two genes that were differentially expressed in every comparison were TGF beta and RNASEN with very low expression in the rejector group. The prolonged non-adherent group had the maximum expression for TGF beta but average RNASEN expression as compared to the low expression for rejectors and high for post-5 years patients. The patients presented survived for varying lengths of time without immunosuppression. The gene array analysis showed intriguing differences between these rare patients and important patient cohorts. Further efforts should be directed to finding and studying more patients who survive despite lack of prescribed immunosuppression. The mechanisms underlying this phenomenon may inform future advances in transplant immunosuppression.

HLA-DQ alloantibodies directly activate the endothelium and compromise differentiation of FoxP3high regulatory T lymphocytes

Development of donor-specific antibodies is associated with reduced allograft survival in renal transplantation. Recent clinical studies highlight the prevalence of human leukocyte antigen (HLA)-DQ antibodies amongst de novo donor-specific antibodies (DSAs), yet the specific contribution of these DSAs to rejection has not been examined. Antibody-mediated rejection primarily targets the microvasculature, so this study explored how patient HLA-DQ alloantibodies can modulate endothelial activation and so immunoregulation. HLA-DQ antibodies phosphorylated Akt and S6 kinase in microvascular endothelial cells. This activation prior to culture with alloreactive lymphocytes increased IL-6 and RANTES secretion. The antibody-mediated upregulation of IL-6 was indeed Akt-dependent. The binding of HLA-DQ antibodies to endothelial cells selectively reduced T cell alloproliferation and FoxP3^high Treg differentiation. In clinical studies, detection of HLA-DQ DSAs with other DSAs is associated with worse graft survival than either alone. Endothelial cells stimulated with HLA-DR and HLA-DQ antibodies showed a synergistic increase in pro-inflammatory cytokine secretion and a decrease in Treg expansion. HLA-DQ antibodies strongly promote pro-inflammatory responses in isolation and in combination with other HLA antibodies. Thus, our data give new insights into the pathogenicity of HLA-DQ DSAs.

Regulatory T-cell Number in Peripheral Blood at 1 Year Posttransplant as Predictor of Long-term Kidney Graft Survival

Supplemental digital content is available in the text.

Background

Regulatory T (Treg) cells play a role in limiting kidney transplant rejection and can potentially promote long-term transplant tolerance. There are no large prospective studies demonstrating the utility of peripheral blood Treg cells as biomarkers for long-term graft outcome in kidney transplantation. The aim of our study was to analyze the influence of the absolute number of peripheral blood Treg cells after transplantation on long-term death-censored graft survival.

Methods

We monitored the absolute numbers of Treg cells by flow cytometry in nonfrozen samples of peripheral blood in 133 kidney transplant recipients, who were prospectively followed up to 2 years after transplantation. Death-censored graft survival was determined retrospectively in January 2017.

Results

The mean time of clinical follow-up was 7.4 ± 2.9 years and 24.1% patients suffered death-censored graft loss (DCGL). Patients with high Treg cells 1 year after transplantation and above the median value (14.57 cells/mm³), showed better death-censored graft survival (5-year survival, 92.5% vs 81.4%, Log-rank P = .030). One-year Treg cells showed a receiver operating characteristic - area under curve of 63.1% (95% confidence interval, 52.9–73.2%, P = 0.026) for predicting DCGL. After multivariate Cox regression analysis, an increased number of peripheral blood Treg cells was a protective factor for DCGL (hazard ratio, 0.961, 95% confidence interval, 0.924–0.998, P = 0.041), irrespectively of 1-year proteinuria and renal function.

Conclusions

Peripheral blood absolute numbers of Treg cells 1 year after kidney transplantation predict a better long-term graft outcome and may be used as prognostic biomarkers.

Transient increase of activated regulatory T cells early after kidney transplantation

Regulatory T cells (Tregs) are crucial in controlling allospecific immune responses. However, studies in human kidney recipients regarding the contribution of polyspecific Tregs have provided differing results and studies on alloreactive Tregs are missing completely. In this retrospective study, we specifically analyzed activated CD4⁺CD25^highFOXP3⁺GARP⁺ Tregs in 17 patients of a living donor kidney transplantation cohort longitudinally over 24 months by flow cytometry (FOXP3: forkhead box protein 3, GARP: glycoprotein A repetitions predominant). We could demonstrate that Tregs of patients with end-stage renal disease (ESRD) are already pre-activated when compared to healthy controls. Furthermore, even though total CD4⁺CD25^highFOXP3⁺ Treg numbers decreased in the first three months after transplantation, frequency of activated Tregs increased significantly representing up to 40% of all peripheral Tregs. In a cohort of living donor kidney transplantation recipients with stable graft function, frequencies of activated Tregs did not correlate with the occurrence of acute cellular rejection or chronic graft dysfunction. Our results will be important for clinical trials using adoptive Treg therapy after kidney transplantation. Adoptively transferred Tregs could be important to compensate the Treg loss at month 3, while they have to compete within the Treg niche with a large number of activated Tregs.

Kidney Transplant Outcome Is Associated with Regulatory T Cell Population and Gene Expression Early after Transplantation

Successful long-term kidney allograft survival with parallel reduction of complications resulting from prolonged immunosuppressive treatment is a goal in kidney transplantation. We studied the immune changes in cell phenotypes and gene expression induced by kidney transplantation. Our goal was to find a phenotypic and/or transcriptional pattern that might be considered prognostic for the kidney transplant outcome. The analysis was performed prospectively on 36 KTx recipients sampled during the first year and followed for five years after transplantation and on 40 long-term KTx recipients (7.9 ± 2.2 y. post-KTx). The research involved flow cytometry assessment of lymphocyte subpopulations (including Tregs and CD3⁺CD8⁺CD28⁻ lymphocytes) and gene expression analysis of immune-related genes (CD4, CD8, CTLA4, GZMB, FOXP3, IL10, IL4, ILR2A, NOTCH, PDCD1, PRF1, TGFB, and TNFA). The analysis of patterns observed over the first post-KTx year was confronted with control, pretransplant, and long-term transplant results. Treg counts at months one and three post-KTx correlated positively with the current and future allograft function. FOXP3 gene expression at month one post-KTx was also associated with long-term allograft function. The KTx-induced CD3⁺CD8⁺CD28⁻ population correlated with GZMB and PRF1 expression and suggested their cytotoxic properties. The size of the Treg population and regulatory FOXP3 gene expression in the early period after transplantation are associated with kidney transplant outcome. The outlined predictive power of the Treg population needs to be investigated further to be confirmed as one of the immune monitoring strategies that may help achieve the best long-term kidney allograft outcomes.

Clinical Operational Tolerance and Immunosuppression Minimization in Kidney Transplantation: Where Do We Stand?

BACKGROUND

The 20th century represents a breakthrough in the transplantation era, since the first kidney transplantation between identical twins was performed. This was the first case of tolerance, since the recipient did not need immunosuppression. However, as transplantation became possible, an immunosuppression-free status became the ultimate goal, since the first tolerance case was a clear exception from the hard reality nowadays represented by rejection.

METHODS

A plethora of studies was described over the past decades to understand the molecular mechanisms responsible for rejection. This review focuses on the most relevant studies found in the literature where renal tolerance cases are claimed. Contrasting, and at the same time, encouraging outcomes are herein discussed and a glimpse on the main renal biomarkers analyzed in this field is provided.

RESULTS

The activation of the immune system has been shown to play a central role in organ failure, but also it seems to induce a tolerance status when an allograft is performed, despite tolerance is still rare to register. Although there are still overwhelming challenges to overcome and various immune pathways remain arcane; the immunosuppression minimization might be more attainable than previously believed.

CONCLUSION

. Multiple biomarkers and tolerance mechanisms suspected to be involved in renal transplantation have been investigated to understand their real role, with still no clear answers on the topic. Thus, the actual knowledge provided necessarily leads to more in-depth investigations, although many questions in the past have been answered, there are still many issues on renal tolerance that need to be addressed.

Ex vivo expanded natural regulatory T cells from patients with end-stage renal disease or kidney transplantation are useful for autologous cell therapy

Novel concepts employing autologous, ex vivo expanded natural regulatory T cells (nTreg) for adoptive transfer has potential to prevent organ rejection after kidney transplantation. However, the impact of dialysis and maintenance immunosuppression on the nTreg phenotype and peripheral survival is not well understood, but essential when assessing patient eligibility. The current study investigates regulatory T-cells in dialysis and kidney transplanted patients and the feasibility of generating a clinically useful nTreg product from these patients. Heparinized blood from 200 individuals including healthy controls, dialysis patients with end stage renal disease and patients 1, 5, 10, 15, 20 years after kidney transplantation were analyzed. Differentiation and maturation of nTregs were studied by flow cytometry in order to compare dialysis patients and kidney transplanted patients under maintenance immunosuppression to healthy controls. CD127 expressing CD4⁺CD25^highFoxP3⁺ nTregs were detectable at increased frequencies in dialysis patients with no negative impact on the nTreg end product quality and therapeutic usefulness of the ex vivo expanded nTregs. Further, despite that immunosuppression mildly altered nTreg maturation, neither dialysis nor pharmacological immunosuppression or previous acute rejection episodes impeded nTreg survival in vivo. Accordingly, the generation of autologous, highly pure nTreg products is feasible and qualifies patients awaiting or having received allogenic kidney transplantation for adoptive nTreg therapy. Thus, our novel treatment approach may enable us to reduce the incidence of organ rejection and reduce the need of long-term immunosuppression.

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

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

The number of FoxP3 regulatory T cells in the circulation may be a predictive biomarker for kidney transplant recipients: A multistage systematic review

BACKGROUND

The kinetics of the FoxP3 regulatory T-cell (Treg) population in kidney transplant recipients (KTR) are related to the clinical effect of immunosuppression based on mammalian Target Of Rapamycin inhibitors (mTORi) with/without belatacept (predictive biomarker).

METHODS

A multistage systematic review of published and unpublished literature is presented [registration IDs in the International Prospective Register of Systematic Reviews (PROSPERO): CRD42017057570, CRD42018085019, CRD42018084941, CRD42018085186]. A multidisciplinary supervision mechanism for contextualizing of search findings was required. The peripheral blood immune cell phenotypes encompassing all regulatory cells in KTRs were assessed in order to suggest new markers of acute rejection-associated acute allograft dysfunction (AR/AAD) events in KTRs treated with mTORi alone or combined to belatacept. Quantitative estimates and evaluation of the body of evidence are provided.

RESULTS

An increase in Tregs and other regulatory cell types in the circulation in KTRs under mTORi with/without belatacept were observed. Patients with increased Tregs presented a low frequency of AR/AAD events compared to those in which the number of Tregs remained unchanged or even diminished [Odds Ratio (OR)/95% confidence interval (95% CI)/I²/number of studies (n): 0.31/0.10-0.93/0%/6]. Nevertheless, there are too few trials to consider Tregs in the circulation as a predictive biomarker. Inadequate reporting prevents appreciating clinical relevance in such studies.

CONCLUSIONS

Despite advances, clinical qualification of potential predictive biomarkers continues to be difficult. Clinical evidence on Tregs in KTRs needs to be enlarged. Biomarkers should be able to evaluate the effect of medicines targeted to specific patient populations.

Immunological biomarkers of tolerance in human kidney transplantation: An updated literature review

The half-life of transplanted kidneys is <10 years. Acute or chronic rejections have a negative impact on transplant outcome. Therefore, achieving to allograft tolerance for improving long-term transplant outcome is a desirable goal of transplantation field. In contrast, there are evidence that distinct immunological characteristics lead to tolerance in some transplant recipients. In contrast, the main reason for allograft loss is immunological responses. Various immune cells including T cells, B cells, dendritic cells, macrophages, natural killer, and myeloid-derived suppressor cells damage graft tissue and, thereby, graft loss happens. Therefore, being armed with the comprehensive knowledge about either preimmunological or postimmunological characteristics of renal transplant patients may help us to achieve an operational tolerance. In the present study, we are going to review and discuss immunological characteristics of renal transplant recipients with rejection and compare them with tolerant subjects.

Reduced TCR Signaling Contributes to Impaired Th17 Responses in Tolerant Kidney Transplant Recipients

BACKGROUND

The development of spontaneous kidney transplant tolerance has been associated with numerous B cell-related immune alterations. We have previously shown that tolerant recipients exhibit reduced B-cell receptor signalling and higher IL-10 production than healthy volunteers. However, it is unclear whether cluster of differentiation (CD)4 T cells from tolerant recipients also display an anti-inflammatory profile that could contribute to graft maintenance.

METHODS

CD4 T cells were isolated from kidney transplant recipients who were identified as being tolerant recipients, patients with chronic rejection or healthy volunteers. CD4 T cells from the 3 groups were compared in terms of their gene expression profile, phenotype, and functionally upon activation.

RESULTS

Gene expression analysis of transcription factors and signalling proteins, in addition to surface proteins expression and cytokine production, revealed that tolerant recipients possessed fewer Th17 cells and exhibited reduced Th17 responses, relative to patients with chronic rejection or healthy volunteers. Furthermore, impaired T-cell receptor signalling and altered cytokine cooperation by monocytes contributed to the development of Th17 cells in tolerant recipients.

CONCLUSIONS

These data suggest that defective proinflammatory Th17 responses may contribute to the prolonged graft survival and stable graft function, which is observed in tolerant recipients in the absence of immunosuppressive agents.

B cells in operational tolerance

Transplantation is currently the therapy of choice for endstage organ failure even though it requires long-term immunosuppresive therapy, with its numerous side effects, for acceptance of the transplanted organ. In rare cases however, patients develop operational tolerance, that is, graft survival without immunosuppression. Studies conducted on these patients reveal genetic, phenotypic, and functional signatures. They provide a better understanding of the immunological mechanisms involved in operational tolerance and define biomarkers that could be used to adapt immunosuppressive treatment to the individual, safely reduce immunosuppression doses, and ideally and safely guide immunosuppression withdrawal. This review summarizes studies that suggest a role for B cells as biomarkers of operational tolerance and discusses the use of B cells as a predictive tool for immunologic risk.

Increased degradation of ATP is driven by memory regulatory T cells in kidney transplantation tolerance

Regulatory T cells were recently proposed as the central actor in operational tolerance after renal transplantation. Tolerant patients harbor increased FoxP3hi memory Treg frequency and increased demethylation in the Foxp3 Treg-specific demethylated region when compared to stable kidney recipients and exhibit greater memory Treg suppressive capacities and higher expression of the ectonucleotidase CD39. However, in this particular and unique situation the mechanisms of action of Tregs were not identified. Thus, we analyzed the ability of memory Tregs to degrade extracellular ATP in tolerant patients, healthy volunteers, and patients with stable graft function under immunosuppression and determined the role of immunosuppressive drugs on this process. The conserved proportion of memory Tregs leads to the establishment of a pro-tolerogenic balance in operationally tolerant patients. Memory Tregs in tolerant patients display normal capacity to degrade extracellular ATP/ADP. In contrast, memory Tregs from patients with stable graft function do not have this ability. Finally, in vitro, immunosuppressive drugs may favor the lower proportion of memory Tregs in stable patients, but they have no effect on CD39-dependent ATP degradation and do not explain memory Treg lack of extracellular ATP/ADP degradation ability. Thus, intrinsic active regulatory mechanisms may act long after immunosuppressive drug arrest in operationally tolerant patients and may contribute to kidney allograft tolerance via the maintenance of CD39 Treg function.

The Role of the Endothelium during Antibody-Mediated Rejection: From Victim to Accomplice

Antibody-mediated rejection (AMR) of solid organ transplants is characterized by the activation and injury of the allograft endothelium. Histological and transcriptomic studies have associated microvascular inflammation and endothelial lesions with the severity of rejection and poor graft outcomes. The allograft endothelium forms the physical barrier between the donor organ and the recipient; this position directly exposes the endothelium to alloimmune responses. However, endothelial cells are not just victims and can actively participate in the pathogenesis of rejection. In healthy tissues, the endothelium plays a major role in vascular and immune homeostasis. Organ transplantation, however, subjects the endothelium to an environment of inflammation, alloreactive lymphocytes, donor-specific antibodies, and potentially complement activation. As a result, endothelial cells become activated and have modified interactions with the cellular effectors of allograft damage: lymphocytes, natural killer, and myeloid cells. Activated endothelial cells participate in leukocyte adhesion and recruitment, lymphocyte activation and differentiation, as well as the secretion of cytokines and chemokines. Ultimately, highly activated endothelial cells promote pro-inflammatory alloresponses and become accomplices to AMR.

Regulatory B cells: the cutting edge of immune tolerance in kidney transplantation

Kidney transplantation is the optimal treatment for end-stage renal diseases. Although great improvement has been achieved, immune tolerance is still the Holy Grail that every organ transplant practitioner pursues. The role of B cells in transplantation has long been considered simply to serve as precursors of plasma cells, which produce alloantibodies and induce antibody-mediated rejection. Recent research indicates that a specialized subset of B cells plays an important role in immune regulation, which has been well demonstrated in autoimmune diseases, infections, and cancers. This category of regulatory B cells (Bregs) differs from conventional B cells, and they may help develop a novel immunomodulatory therapeutic strategy to achieve immune tolerance in transplantation. Here, we review the latest evidence regarding phenotypes, functions, and effectors of Bregs and discuss their diverse effects on kidney transplantation.

Current status of tolerance in kidney transplantation

PURPOSE OF REVIEW

The attainment of tolerance remains a highly desirable goal in recipients of kidney transplants. Achievement of this goal would extend graft survival and eradicate toxicities related to long-term immunosuppression. Understanding mechanisms of tolerance and strategies to induce tolerance - their risk/benefit profiles - is essential for future success.

RECENT FINDINGS

Mechanistic studies of spontaneously tolerant kidney transplant recipients have uncovered potential roles for B or regulatory T cells, or both, in the maintenance of tolerance. Mixed hematopoietic chimerism has been the most commonly used approach to induce tolerance. Distinct protocols at three major transplant centers have led to successful withdrawal of immunosuppression in a subset of living donor kidney transplant recipients at the expense of complications such as infections and graft versus host disease. The addition of regulatory cell therapies to tolerance induction protocols could enhance success while minimizing complications.

SUMMARY

This review summarizes the features of spontaneous tolerance in kidney transplant recipients, the results of clinical trials of tolerance induction in the context of living donor kidney transplant, and potential measures to improve the safety and efficacy of tolerance induction strategies.

CD4+CD25Hi FoxP3+ regulatory T cells in long-term cardiac xenotransplantation

BACKGROUND

CD4+CD25^Hi FoxP3+ T (Treg) cells are a small subset of CD4+ T cells that have been shown to exhibit immunoregulatory function. Although the absolute number of Treg cells in peripheral blood lymphocytes (PBL) is very small, they play an important role in suppressing immune reactivity. Several studies have demonstrated that the number of Treg cells, rather than their intrinsic suppressive capacity, may contribute to determining the long-term fate of transplanted grafts. In this study, we analyzed Treg cells in PBL of long-term baboon recipients who have received genetically modified cardiac xenografts from pig donors.

METHODS

Heterotopic cardiac xenotransplantation was performed on baboons using hearts obtained from GTKO.hCD46 (n = 8) and GTKO.hCD46.TBM (n = 5) genetically modified pigs. Modified immunosuppression regimen included antithymocyte globulin (ATG), anti-CD20, mycophenolate mofetil (MMF), cobra venom factor (CVF), and costimulation blockade (anti-CD154/anti-CD40 monoclonal antibody). FACS analysis was performed on PBLs labeled with anti-human CD4, CD25, and FoxP3 monoclonal antibodies (mAb) to analyze the percentage of Treg cells in six baboons that survived longer than 2 months (range: 42-945 days) after receiving a pig cardiac xenograft.

RESULTS

Total WBC count was low due to immunosuppression in baboons who received cardiac xenograft from GTKO.hCD46 and GTKO.hCD46.hTBM donor pigs. However, absolute numbers of CD4+CD25^Hi FoxP3 Treg cells in PBLs of long-term xenograft cardiac xenograft surviving baboon recipients were found to be increased (15.13 ± 1.50 vs 7.38 ± 2.92; P < .018) as compared to naïve or pre-transplant baboons. Xenograft rejection in these animals was correlated with decreased numbers of regulatory T cells.

CONCLUSION

Our results suggest that regulatory T (Treg) cells may contribute to preventing or delaying xenograft rejection by controlling the activation and expansion of donor-reactive T cells, thereby masking the antidonor immune response, leading to long-term survival of cardiac xenografts.

Endothelial Cell Amplification of Regulatory T Cells Is Differentially Modified by Immunosuppressors and Intravenous Immunoglobulin

Immunosuppressive treatment is a prerequisite for both organ transplantation and tolerance of the allograft. However, long-term immunosuppression has been associated with a higher incidence of malignancies and infections. Immunosuppressors mainly target circulating immune cells and little is known of their “off-target” effects, such as their impact on endothelial cells (ECs). In chronic antibody-mediated rejection (AMR), the allograft endothelium is a target of damage, histologically detected as transplant glomerulopathy, and which correlates with poor graft survival. Under inflammatory conditions, EC expression of HLA class II antigens can lead to CD4⁺-T lymphocyte alloactivation and selective expansion of pro-inflammatory Th17 and pro-tolerance Treg subsets. This response can be modified and preactivation of the EC by HLA-DR antibody binding promoted a proinflammatory Th17 response. However, whether or not immunosuppressors alter EC immunogenicity has not been examined. In alloimmunized patients with AMR, cyclosporine A (CsA) and mycophenolic acid (MPA) are often combined with intravenous immunoglobulins (IVIgs). This study reports changes in the microvascular EC phenotype and function after treatment with CsA, MPA, or IVIg. Both CsA and MPA decreased HLA-DR and increased CD54 expression, whereas IVIg increased HLA-DR expression. Interleukin 6 secretion was reduced by all three immunomodulators. Preincubation of ECs with CsA or MPA limited, while IVIg amplified, Treg expansion. Because CsA, MPA, and IVIg are known for their ability to act upon leukocytes, we confirmed that ECs maintained their immunoregulatory role when allogeneic leukocytes were pretreated with CsA, MPA, or IVIg. The results reveal that individual immunosuppressors, used in the induction and maintenance of renal allograft tolerance, had direct and distinct effects on ECs. Results of experiments associating IVIg with either CsA or MPA underlined the differences observed using individual immunosuppressors. Paradoxically, CsA or MPA may increase EC mediated inflammatory responses and long-term exposure may contribute to limitation of allograft tolerance. In contrast, IVIg interaction with the endothelium may mediate some of its immunosuppressive effects through promotion of Treg expansion, contributing to the maintenance of allograft tolerance.

Histopathological analysis of infiltrating T cell subsets in acute T cell-mediated rejection in the kidney transplant

AIM

To compare the differential immune T cell subset composition in patients with acute T cell-mediated rejection in the kidney transplant with subset composition in the absence of rejection, and to explore the association of their respective immune profiles with kidney transplant outcomes.

METHODS

A pilot cross-sectional histopathological analysis of the immune infiltrate was performed using immunohistochemistry in a cohort of 14 patients with acute T cell-mediated rejection in the kidney transplant and 7 kidney transplant patients with no rejection subjected to biopsy to investigate acute kidney transplant dysfunction. All patients were recruited consecutively from 2012 to 2014 at the Singapore General Hospital. Association of the immune infiltrates with kidney transplant outcomes at up to 54 mo of follow up was also explored prospectively.

RESULTS

In comparison to the absence of rejection, acute T cell-mediated rejection in the kidney transplant was characterised by numerical dominance of cytotoxic T lymphocytes over Foxp3⁺ regulatory T cells, but did not reach statistical significance owing to the small sample size in our pilot study. There was no obvious difference in absolute numbers of infiltrating cytotoxic T lymphocytes, Foxp3⁺ regulatory T cells and Th17 cells between the two patient groups when quantified separately. Our exploratory analysis on associations of T cell subset quantifications with kidney transplant outcomes revealed that the degree of Th17 cell infiltration was significantly associated with shorter time to doubling of creatinine and shorter time to transplant loss.

CONCLUSION

Although this was a small pilot study, results support our suspicion that in kidney transplant patients the immune balance in acute T cell-mediated rejection is tilted towards the pro-rejection forces and prompt larger and more sophisticated studies.

Immune monitoring as prerequisite for transplantation tolerance trials

Ever since its first application in clinical medicine, scientists have been urged to induce tolerance towards foreign allogeneic transplants and thus avoid rejection by the recipient's immune system. This would circumvent chronic use of immunosuppressive drugs (IS) and thus avoid development of IS-induced side effects, which are contributing to the still unsatisfactory long-term graft and patient survival after solid organ transplantation. Although manifold strategies of tolerance induction have been described in preclinical models, only three therapeutic approaches have been utilized successfully in a still small number of patients. These approaches are based on (i) IS withdrawal in spontaneous operational tolerant (SOT) patients, (ii) induction of a mixed chimerism and (iii) adoptive transfer of regulatory cells. Results of clinical trials utilizing these approaches show that tolerance induction does not work in all patients. Thus, there is a need for reliable biomarkers, which can be used for patient selection and post-therapeutic immune monitoring of safety, success and failure. In this review, we summarize recent achievements in the identification and validation of such immunological assays and biomarkers, focusing mainly on kidney and liver transplantation. From the published findings so far, it has become clear that indicative biomarkers may vary between different therapeutic approaches applied and organs transplanted. Also, patient numbers studied so far are very small. This is the main reason why nearly all described parameters lack validation and reproducibility testing in large clinical trials, and are therefore not yet suitable for clinical practice.

Blood biomarkers of kidney transplant rejection, an endless search?

INTRODUCTION

The tailoring of immunosuppressive treatment is recognized as a promising strategy to improve long-term kidney graft outcome. To guide the standard care of transplant recipients, physicians need objective biomarkers that can identify an ongoing pathology with the graft or low intensity signals that will be later evolved to accelerated transplant rejection. The early identification of 'high-risk /low-risk' patients enables the adjustment of standard of caring, including managing the frequency of clinical visits and the immunosuppression dosing. Given their ease of availability and the compatibility with a large technical array, blood-based biomarkers have been widely scrutinized for use as potential predictive and diagnostic biomarkers. Areas covered: Here, the authors report on non-invasive biomarkers, such as modification of immune cell subsets and mRNA and miRNA profiles, identified in the blood of kidney transplant recipients collected before or after transplantation. Expert commentary: Combined with functional tests, the identification of biomarkers will improve our understanding of pathological processes and will contribute to a global improvement in clinical management.

Renal Operational Tolerance Is Associated With a Defect of Blood Tfh Cells That Exhibit Impaired B Cell Help

Renal operationally tolerant patients (TOL) display a defect in B cell differentiation, with a deficiency in plasma cells. Recently described, T follicular helper (Tfh) cells play a critical role in B cell differentiation. We analyzed blood Tfh subsets in TOL and transplanted patients with stable graft function under immunosuppression (STA). We observed a reduced proportion of blood activated and highly functional Tfh subsets in TOL, without affecting Tfh absolute numbers. Functionally, Tfh cells from TOL displayed a modified gene expression profile, failed to produce interleukin-21, and were unable to induce IgG production by naive B cells. This Tfh defect is linked to a low incidence of postgraft de novo donor-specific antibody (dnDSA) immunization, suggesting that the lack of Tfh cells in TOL may induce a protolerogenic environment with reduced risk of developing dnDSA. Finally, we showed that elevated Tfh in STA precedes the occurrence of dnDSA during an alloresponse. These data provide new insights into the mechanisms of antibody response in operational tolerance. Disrupted homeostasis and impaired Tfh function in TOL could lead to a reduced risk of developing dnDSA and suggest a predictive role of blood Tfh cells on the occurrence of dnDSA in transplant recipients.

Operational tolerance in kidney transplantation and associated biomarkers

In the 1960s, our predecessors won a historical battle against acute rejection and ensured that transplantation became a common life-saving treatment. In parallel with this success, or perhaps because of it, we lost the battle for long-lived transplants, being overwhelmed with chronic immune insults and the toxicities of immunosuppression. It is likely that current powerful treatments block acute rejection, but at the same time condemn the few circulating donor cells that would have been able to elicit immunoregulatory host responses towards the allograft. Under these conditions, spontaneously tolerant kidney recipients - i.e. patients who maintain allograft function in the absence of immunosuppression - are merely accidents; they are scarce, mysterious and precious. Several teams pursue the goal of finding a biomarker that would guide us towards the 'just right' level of immunosuppression that avoids rejection while leaving some space for donor immune cells. Some cellular assays are attractive because they are antigen-specific, and provide a comprehensive view of immune responses toward the graft. These seem to closely follow patient regulatory capacities. However, these tests are cumbersome, and require abundant cellular material from both donor and recipient. The latest newcomers, non-antigen-specific recipient blood transcriptomic biomarkers, offer the promise that a practicable and simple signature may be found that overcomes the complexity of a system in which an infinite number of individual cell combinations can lead possibly to graft acceptance. Biomarker studies are as much an objective - identifying tolerant patients, enabling tolerance trials - as a means to deciphering the underlying mechanisms of one of the most important current issues in transplantation.

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

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