The DESCARTES-Nantes survey of kidney transplant recipients displaying clinical operational tolerance identifies 35 new tolerant patients and 34 almost tolerant patients

Frontiers and Controversies in De Novo Gastrointestinal Tumors After Organ Transplantation: Current Progress and Future Directions

The increasing success of organ transplantation has significantly improved survival for patients with end-stage diseases, yet it introduces a complex dilemma: the elevated risk for the development of de novo gastrointestinal (GI) tumors. The sustained immunosuppression required to maintain graft function paradoxically undermines the body's natural defenses against cancer, leading to a higher incidence, aggressive progression, and atypical presentations of GI tumors among transplant recipients compared with the general population. This presents a pressing challenge: balancing the dual imperatives of preventing graft rejection and effectively managing malignancies. Current treatment paradigms, including surgical approaches, chemotherapy, radiation therapy, and the emerging role of immunotherapy, are fraught with complexities due to the altered immune landscape in these patients. This review underscores the critical need to understand the multifaceted relationship between post-transplantation immunosuppression and tumorigenesis, providing a comprehensive exploration of epidemiologic shifts, pathophysiologic insights, and the intricacies of the tumor microenvironment in this unique patient population. Understanding and managing GI tumors in transplant recipients is not only a clinical challenge, but also a necessary frontier in transplant oncology, promising to refine therapeutic strategies and improve the longevity and quality of life for this growing patient cohort.

More than four decades of graft survival in pediatric kidney transplant recipients

BACKGROUND

Early in the history of kidney transplantation, short-term graft survival was low. Yet some have had excellent long-term survival. Herein, we describe characteristics of pediatric recipients with > 40 years of graft survival currently alive with a functioning first graft.

METHODS

We reviewed all pediatric (age < 18 years) kidney transplants performed at the University of Minnesota between January 1, 1970, and December 31, 1979 (n = 148), to identify all recipients currently alive with a functioning first graft. Data are presented as medians with interquartile ranges (IQR) and proportions.

RESULTS

We identified 10 recipients with > 40-year graft survival (median follow-up: 45.0 years (IQR: 43.1, 48.1)). The median age at transplant was 13.8 years (IQR: 5.1, 16.3). All recipients were white; half were male. Of the 10, 4 had glomerulonephritis, 2 had congenital anomalies of the kidney and the urinary tract, 2 had congenital nephrotic syndrome, 1 had Alport syndrome, and 1 had cystic kidney disease as kidney failure cause. Nine patients received a living-related donor transplant, and 1 patient received a deceased-donor transplant. The median estimated glomerular filtration rate at 20 years post-transplant was 79.9 (IQR: 72.3, 98.4); at 30 years, 67.7 (IQR: 63.2, 91.8); and at 40 years, 80.3 ml/min/1.73 m2 (IQR: 73.7, 86.0). None developed rejection, 5 developed hypertension, 2 developed dyslipidemia, 1 developed diabetes, and 7 patients developed malignancy (4 skin cancer, 2 breast cancer, and 1 post-transplant lymphoproliferative disease).

CONCLUSION

Pediatric kidney transplant recipients may achieve > 4 decades of graft survival. Cancer is a common complication warranting vigilant screening.

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

INTRODUCTION

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

MATERIALS AND METHODS

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

RESULTS

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

CONCLUSIONS

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

Intragraft regulatory T cells in the modern era: what can high-dimensional methods tell us about pathways to allograft acceptance?

Replacement of diseased organs with transplanted healthy donor ones remains the best and often only treatment option for end-stage organ disease. Immunosuppressants have decreased the incidence of acute rejection, but long-term survival remains limited. The broad action of current immunosuppressive drugs results in global immune impairment, increasing the risk of cancer and infections. Hence, achievement of allograft tolerance, in which graft function is maintained in the absence of global immunosuppression, has long been the aim of transplant clinicians and scientists. Regulatory T cells (Treg) are a specialized subset of immune cells that control a diverse array of immune responses, can prevent allograft rejection in animals, and have recently been explored in early phase clinical trials as an adoptive cellular therapy in transplant recipients. It has been established that allograft residency by Tregs can promote graft acceptance, but whether intragraft Treg functional diversification and spatial organization contribute to this process is largely unknown. In this review, we will explore what is known regarding the properties of intragraft Tregs during allograft acceptance and rejection. We will summarize recent advances in understanding Treg tissue residency through spatial, transcriptomic and high-dimensional cytometric methods in both animal and human studies. Our discussion will explore properties of intragraft Tregs in mediating operational tolerance to commonly transplanted solid organs. Finally, given recent developments in Treg cellular therapy, we will review emerging knowledge of whether and how these adoptively transferred cells enter allografts in humans. An understanding of the properties of intragraft Tregs will help lay the foundation for future therapies that will promote immune tolerance.

Novel transcriptomic signatures associated with premature kidney allograft failure

BACKGROUND

The power to predict kidney allograft outcomes based on non-invasive assays is limited. Assessment of operational tolerance (OT) patients allows us to identify transcriptomic signatures of true non-responders for construction of predictive models.

METHODS

In this observational retrospective study, RNA sequencing of peripheral blood was used in a derivation cohort to identify a protective set of transcripts by comparing 15 OT patients (40% females), from the TOMOGRAM Study (NCT05124444), 14 chronic active antibody-mediated rejection (CABMR) and 23 stable graft function patients ≥15 years (STA). The selected differentially expressed transcripts between OT and CABMR were used in a validation cohort (n = 396) to predict 3-year kidney allograft loss at 3 time-points using RT-qPCR.

FINDINGS

Archetypal analysis and classifier performance of RNA sequencing data showed that OT is clearly distinguishable from CABMR, but similar to STA. Based on significant transcripts from the validation cohort in univariable analysis, 2 multivariable Cox models were created. A 3-transcript (ADGRG3, ATG2A, and GNLY) model from POD 7 predicted graft loss with C-statistics (C) 0.727 (95% CI, 0.638-0.820). Another 3-transcript (IGHM, CD5, GNLY) model from M3 predicted graft loss with C 0.786 (95% CI, 0.785-0.865). Combining 3-transcripts models with eGFR at POD 7 and M3 improved C-statistics to 0.860 (95% CI, 0.778-0.944) and 0.868 (95% CI, 0.790-0.944), respectively.

INTERPRETATION

Identification of transcripts distinguishing OT from CABMR allowed us to construct models predicting premature graft loss. Identified transcripts reflect mechanisms of injury/repair and alloimmune response when assessed at day 7 or with a loss of protective phenotype when assessed at month 3.

FUNDING

Supported by the Ministry of Health of the Czech Republic under grant NV19-06-00031.

An exome-wide study of renal operational tolerance

Background

Renal operational tolerance is a rare and beneficial state of prolonged renal allograft function in the absence of immunosuppression. The underlying mechanisms are unknown. We hypothesized that tolerance might be driven by inherited protein coding genetic variants with large effect, at least in some patients.

Methods

We set up a European survey of over 218,000 renal transplant recipients and collected DNAs from 40 transplant recipients who maintained good allograft function without immunosuppression for at least 1 year. We performed an exome-wide association study comparing the distribution of moderate to high impact variants in 36 tolerant patients, selected for genetic homogeneity using principal component analysis, and 192 controls, using an optimal sequence-kernel association test adjusted for small samples.

Results

We identified rare variants of HOMER2 (3/36, FDR 0.0387), IQCH (5/36, FDR 0.0362), and LCN2 (3/36, FDR 0.102) in 10 tolerant patients vs. 0 controls. One patient carried a variant in both HOMER2 and LCN2. Furthermore, the three genes showed an identical variant in two patients each. The three genes are expressed at the primary cilium, a key structure in immune responses.

Conclusion

Rare protein coding variants are associated with operational tolerance in a sizable portion of patients. Our findings have important implications for a better understanding of immune tolerance in transplantation and other fields of medicine.ClinicalTrials.gov, identifier: NCT05124444.

Treg Therapy for the Induction of Immune Tolerance in Transplantation-Not Lost in Translation?

The clinical success of solid organ transplantation is still limited by the insufficiency of immunosuppressive regimens to control chronic rejection and late graft loss. Moreover, serious side effects caused by chronic immunosuppressive treatment increase morbidity and mortality in transplant patients. Regulatory T cells (Tregs) have proven to be efficient in the induction of allograft tolerance and prolongation of graft survival in numerous preclinical models, and treatment has now moved to the clinics. The results of the first Treg-based clinical trials seem promising, proving the feasibility and safety of Treg therapy in clinical organ transplantation. However, many questions regarding Treg phenotype, optimum dosage, antigen-specificity, adjunct immunosuppressants and efficacy remain open. This review summarizes the results of the first Treg-based clinical trials for tolerance induction in solid organ transplantation and recapitulates what we have learnt so far and which questions need to be resolved before Treg therapy can become part of daily clinical practice. In addition, we discuss new strategies being developed for induction of donor-specific tolerance in solid organ transplantation with the clinical aims of prolonged graft survival and minimization of immunosuppression.

Pharmacotherapeutic options for the prevention of kidney transplant rejection: the evidence to date

INTRODUCTION

Although early rejection episodes are successfully controlled, the problem of unrecognized production of de novo anti HLA antibodies and associated chronic rejection still persists.

AREAS COVERED

In addition to the standard induction and maintenance therapy, we present a couple of new drugs as induction (Alemtuzumab), CNI free protocol (Belatacept, Sirolimus, Everolimus), maintenance treatment in transplant patients with various type of malignancies (T cell targeted immunomodulators blocking the immune checkpoints CTLA-4, PD1/PDL1) and TMA (aHUS) -eculizimab, and IL6 receptor antagonists in antibody mediated rejection (AMR).

EXPERT OPINION

There are couple of issues still preventing improvement in kidney transplant long-term outcomes with current and anticipated future immunosuppression: patient more susceptible to infection and CNI nephrotoxicity in kidneys obtained from elderly donors, highly sensitized patients with limited chances to get appropriate kidney and a higher risk for late AMR. A lower rate of CMV/BK virus infections has been observed in everolimus treated patients. Belatacept use has been justified only in EBV seropositive kidney transplants due to the increased risk of PTLD. Eculizumab upon recurrence of aHUS is a sole cost-effective option. A new IL-6 blocking drug (clazakizumab/tocilizumab) is promising option for prevention/treatment of AMR. Clinical experience in tailoring immunosuppression for as long as possible graft and patient survival is inevitable.

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.

Urinary metabolomic profiling from spontaneous tolerant kidney transplanted recipients shows enrichment in tryptophan-derived metabolites

Background

Operational tolerance is the holy grail in solid organ transplantation. Previous reports showed that the urinary compartment of operationally tolerant recipients harbor a specific and unique profile. We hypothesized that spontaneous tolerant kidney transplanted recipients (KTR) would have a specific urinary metabolomic profile associated to operational tolerance.

Methods

We performed metabolomic profiling on urine samples from healthy volunteers, stable KTR under standard and minimal immunosuppression and spontaneous tolerant KTR using liquid chromatography in tandem with mass spectrometry. Supervised and unsupervised multivariate computational analyses were used to highlight urinary metabolomic profile and metabolite identification thanks to workflow4metabolomic platform.

Findings

The urinary metabolome was composed of approximately 2700 metabolites. Raw unsupervised clustering allowed us to separate healthy volunteers and tolerant KTR from others. We confirmed by two methods a specific urinary metabolomic signature in tolerant KTR mainly driven by kynurenic acid independent of immunosuppressive drugs, serum creatinine and gender.

Interpretation

Kynurenic acid and tryptamine enrichment allowed the identification of putative pathways and metabolites associated with operational tolerance like IDO, GRP35 and AhR and indole alkaloids.

Funding

This study was supported by the ANR, IRSRPL and CHU de Nantes.

Microbiota and immunoregulation: A focus on regulatory B lymphocytes and transplantation

The microbiota plays a major role in the regulation of the host immune functions thus establishing a symbiotic relationship that maintains immune homeostasis. Among immune cells, regulatory B cells (Bregs), which can inhibit effector T cell responses, may be involved in the intestinal homeostasis. Recent works suggest that the interaction between the microbiota and Bregs appears to be important to limit autoimmune diseases and help to maintain tolerance in transplantation. Short-chain fatty acids (SCFAs), recognized as major metabolites of the microbiota, seem to be involved in the generation of a pro-tolerogenic environment in the gut, particularly through the regulation of B cell differentiation, limiting mature B cells and promoting the function of Bregs. In this review, we show that this B cells-microbiota interaction may open a path toward new potential therapeutic applications not only for patients with autoimmune diseases but also in transplantation.

Perioperative Perfusion of Allografts with Anti-Human T-lymphocyte Globulin Does Not Improve Outcome Post Liver Transplantation-A Randomized Placebo-Controlled Trial

Due to the lack of suitable organs transplant surgeons have to accept unfavorable extended criteria donor (ECD) organs. Recently, we demonstrated that the perfusion of kidney organs with anti-human T-lymphocyte globulin (ATLG) prior to transplantation ameliorates ischemia-reperfusion injury (IRI). Here, we report on the results of perioperative ATLG perfusion in a randomized, single-blinded, placebo-controlled, feasibility trial (RCT) involving 30 liver recipients (LTx). Organs were randomly assigned for perfusion with ATLG/Grafalon^® (AP) (n = 16) or saline only (control perfusion = CP) (n = 14) prior to implantation. The primary endpoint was defined as graft function reflected by aspartate transaminase (AST) values at day 7 post-transplantation (post-tx). With respect to the primary endpoint, no significant differences in AST levels were shown in the intervention group at day 7 (AP: 53.0 ± 21.3 mg/dL, CP: 59.7 ± 59.2 mg/dL, p = 0.686). Similarly, exploratory analysis of secondary clinical outcomes (e.g., patient survival) and treatment-specific adverse events revealed no differences between the study groups. Among liver transplant recipients, pre-operative organ perfusion with ATLG did not improve short-term outcomes, compared to those who received placebo perfusion. However, ATLG perfusion of liver grafts was proven to be a safe procedure without the occurrence of relevant adverse events.

TCL1A, B Cell Regulation and Tolerance in Renal Transplantation

Despite much progress in the management of kidney transplantation, the need for life-long immunosuppressive therapies remains a major issue representing many risks for patients. Operational tolerance, defined as allograft acceptance without immunosuppression, has logically been subject to many investigations with the aim of a better understanding of post-transplantation mechanisms and potentially how it would be induced in patients. Among proposed biomarkers, T-cell Leukemia/Lymphoma protein 1A (TCL1A) has been observed as overexpressed in the peripheral blood of operational tolerant patients in several studies. TCL1A expression is restricted to early B cells, also increased in the blood of tolerant patients, and showing regulatory properties, notably through IL-10 secretion for some subsets. TCL1A has first been identified as an oncogene, overexpression of which is associated to the development of T and B cell cancer. TCL1A acts as a coactivator of the serine threonine kinase Akt and through other interactions favoring cell survival, growth, and proliferation. It has also been identified as interacting with others major actors involved in B cells differentiation and regulation, including IL-10 production. Herein, we reviewed known interactions and functions of TCL1A in B cells which could involve its potential role in the set up and maintenance of renal allograft tolerance.

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.

Clinical and immunological follow-up of very long-term kidney transplant recipients treated with calcineurin inhibitors indicates dual phenotypes

Operationally tolerant kidney transplant recipients harbor an immunological signature, associated with low rejection risk, and focused on B lymphocytes. Here, we investigated whether patients with long-term transplantation and still on immunosuppressive therapy would present such a signature of low immunological rejection risk, compared to more recently transplanted patients. Of 114 kidney transplant recipients enrolled, 38 with more than 25 years of graft survival and stable graft function under calcineurin inhibitors, were matched with two different groups of transplanted patients (10-15 and 5-7 years after transplantation). Three phenotypes associated with low immunological rejection risk (Tfh, B and regulatory T cells), initially found in operationally tolerant kidney transplant recipients, and the composite score of tolerance (combination of six transcriptomic markers, age at transplantation and age at sampling) were analyzed. We found that very long-term patients were characterized by a significantly lower percentage of total B cells, a significantly higher proportion of CD24^HiCD38^Lo memory B cells, significantly fewer CD24^LoCD38^Lo naive B cells, and a significantly lower proportion of PD1^HiCCR7^Lo Tfh lymphocytes than more recently transplanted patients. This phenotype is associated with a positive composite score of tolerance in patients transplanted for more than 25 years. Thus, our study suggests a dual phenotype in very long-term kidney transplanted patients with an immunological profile associated with low rejection risk.

Development and validation of the first consensus gene-expression signature of operational tolerance in kidney transplantation, incorporating adjustment for immunosuppressive drug therapy

BACKGROUND

Kidney transplant recipients (KTRs) with "operational tolerance" (OT) maintain a functioning graft without immunosuppressive (IS) drugs, thus avoiding treatment complications. Nevertheless, IS drugs can influence gene-expression signatures aiming to identify OT among treated KTRs.

METHODS

We compared five published signatures of OT in peripheral blood samples from 18 tolerant, 183 stable, and 34 chronic rejector KTRs, using gene-expression levels with and without adjustment for IS drugs and regularised logistic regression.

FINDINGS

IS drugs explained up to 50% of the variability in gene-expression and 20-30% of the variability in the probability of OT predicted by signatures without drug adjustment. We present a parsimonious consensus gene-set to identify OT, derived from joint analysis of IS-drug-adjusted expression of five published signature gene-sets. This signature, including CD40, CTLA4, HSD11B1, IGKV4-1, MZB1, NR3C2, and RAB40C genes, showed an area under the curve 0⋅92 (95% confidence interval 0⋅88-0⋅94) in cross-validation and 0⋅97 (0⋅93-1⋅00) in six months follow-up samples.

INTERPRETATION

We advocate including adjustment for IS drug therapy in the development stage of gene-expression signatures of OT to reduce the risk of capturing features of treatment, which could be lost following IS drug minimisation or withdrawal. Our signature, however, would require further validation in an independent dataset and a biomarker-led trial.

FUNDING

FP7-HEALTH-2012-INNOVATION-1 [305147:BIO-DrIM] (SC,IR-M,PM,DSt); MRC [G0801537/ID:88245] (MPH-F); MRC [MR/J006742/1] (IR-M); Guy's&StThomas' Charity [R080530]&[R090782]; CONICYT-Bicentennial-Becas-Chile (EN-L); EU:FP7/2007-2013 [HEALTH-F5-2010-260687: The ONE Study] (MPH-F); Czech Ministry of Health [NV19-06-00031] (OV); NIHR-BRC Guy's&StThomas' NHS Foundation Trust and KCL (SC); UK Clinical Research Networks [portfolio:7521].

Why some organ allografts are tolerated better than others: new insights for an old question

PURPOSE OF REVIEW

There is great variability in how different organ allografts respond to the same tolerance induction protocol. Well known examples of this phenomenon include the protolerogenic nature of kidney and liver allografts as opposed to the tolerance-resistance of heart and lung allografts. This suggests there are organ-specific factors which differentially drive the immune response following transplantation.

RECENT FINDINGS

The specific cells or cell products that make one organ allograft more likely to be accepted off immunosuppression than another are largely unknown. However, new insights have been made in this area recently.

SUMMARY

The current review will focus on the organ-intrinsic factors that contribute to the organ-specific differences observed in tolerance induction with a view to developing therapeutic strategies to better prevent organ rejection and promote tolerance induction of all organs.

Unique and specific Proteobacteria diversity in urinary microbiota of tolerant kidney transplanted recipients

Host-microbiota interactions can modulate the immune system both at local and systemic levels, with potential consequences for organ transplantation outcomes. In this study, we hypothesized that differences in the urinary microbiome following kidney transplantation would be associated with posttransplantation status: stable, minimally immunosuppressed, or tolerant. One hundred thirteen urine samples from stable (n = 51), minimally immunosuppressed (n = 19), and spontaneously tolerant (n = 16) patients, paired with age-matched controls (n = 27) were profiled and compared to each other at a taxonomic level with special interest in the immunosuppressive regimen. All comparisons and correlations were adjusted on sex and time posttransplantation. Our results highlighted a unique and specific urinary microbiota associated with spontaneous tolerance characterized by a high diversity and a clear Proteobacteria profile. Finally, we report that this profile is (1) impacted by gender, (2) inversely correlated with immunosuppressive drugs (calcineurin inhibitors and mammalian target of rapamycin inhibitors), and (3) stable in time.

Kidney transplant tolerance associated with remote autologous mesenchymal stromal cell administration

Here we report the case of successful immune tolerance induction in a living‐donor kidney transplant recipient remotely treated with autologous bone marrow‐derived mesenchymal stromal cells (MSC). This case report, which to the best of our knowledge is the first in the world in this setting, provides evidence that the modulation of the host immune system with MSC can enable the safe withdrawal of maintenance immunosuppressive drugs while preserving optimal long‐term kidney allograft function.

The emerging role of regulatory T cells following lung transplantation

Regulatory T cells (Treg) have proven to be a powerful immunologic force in nearly every organ system and hold therapeutic potential for a wide range of diseases. Insights gained from non-transplant pathologies, such as infection, cancer, and autoimmunity, are now being translated to the field of solid organ transplantation, particularly for livers and kidneys. Recent insights from animal models of lung transplantation have established that Tregs play a vital role in suppressing rejection and facilitating tolerance of lung allografts, and such discoveries are being validated in human studies and preclinical trials. Given that long-term outcomes following lung transplantation remain profoundly limited by chronic rejection, Treg therapy holds the potential to significantly improve patient outcomes and should be aggressively investigated.

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.

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.

Mechanisms and biomarkers of immune quiescence in kidney transplantation

This review discusses the current understanding of biomarkers of immune quiescence based on reviews of published literature in kidney transplant operational tolerance and mechanistic studies based on a better characterization of the stable, well-functioning renal allograft.

Messengers of tolerance

The use of immunosuppressant drugs after organ transplantation has brought great success in the field of organ transplantation with respect to short-term outcome. However, major challenges (i.e., limited improvement of long-term survival, immunosuppressant toxicity, infections and carcinoma) demand alternate treatment approaches that minimizes the use of immunosuppressants. Interestingly, few studies have identified groups of transplant patients who developed operational tolerance and thereby keep their allograft without complications in absence of immunosuppressants. These rare groups of patients are of particular interest as study subjects for understanding mechanisms of graft tolerance that could be leveraged in future for inducing tolerance and for understanding mechanisms involved in improving long-term allograft outcomes. Also, biomarkers from these studies could benefit the larger transplant population by their application in immunosuppressant tailoring and identification of tolerant patients among patients with stably functioning allografts. This review compiles several gene expression studies performed in samples from tolerant patients in different solid organ transplantations to identify key genes and associated molecular pathways relevant to tolerance. This review is aimed at putting forth all this important work done thus far and to identify research gaps that need to be filled, in order to achieve the greater purpose of these studies.

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.

Broad Impairment of Natural Killer Cells from Operationally Tolerant Kidney Transplanted Patients

The role of natural killer (NK) cells in organ transplantation is controversial. This study aims to decipher their role in kidney transplant tolerance in humans. Previous studies highlighted several modulated genes involved in NK cell biology in blood from spontaneously operationally tolerant patients (TOLs; drug-free kidney-transplanted recipients with stable graft function). We performed a phenotypic, functional, and genetic characterization of NK cells from these patients compared to kidney-transplanted patients with stable graft function under immunosuppression and healthy volunteers (HVs). Both operationally TOLs and stable patients harbored defective expression of the NKp46 activator receptor and lytic molecules perforin and granzyme compared to HVs. Surprisingly, NK cells from operationally TOLs also displayed decreased expression of the CD16 activating marker (in the CD56^Dim NK cell subset). This decrease was associated with impairment of their functional capacities upon stimulation, as shown by lower interferon gamma (IFNγ) production and CD107a membranous expression in a reverse antibody-dependent cellular cytotoxicity (ADCC) assay, spontaneous lysis assays, and lower target cell lysis in the ⁵¹Cr release assay compared to HVs. Conversely, despite impaired K562 cell lysis in the ⁵¹Cr release assay, patients with stable graft function harbored a normal reverse ADCC and even increased amounts of IFNγ⁺ NK cells in the spontaneous lysis assay. Altogether, the strong impairment of the phenotype and functional cytotoxic capacities of NK cells in operationally TOLs may accord with the establishment of a pro-tolerogenic environment, despite remaining highly activated after transplantation in patients with stable graft function.

Transplantation tolerance after allograft rejection

PURPOSE OF REVIEW

Although elusive for many decades, transplantation tolerance can now be achieved in the clinic. This has prompted follow-up investigations into its stability and longevity, as well as into barriers to its induction, which include memory T and B cells.

RECENT FINDINGS

Clinical observations reveal that transplantation tolerance can be induced in adult recipients and that even episodes of acute rejection do not preclude successful weaning from immunosuppression to reveal tolerance. These observations appear to conflict with the currently accepted notion that adult transplant recipients harbor high frequencies of memory human leukocyte antigen-specific T cells that are a barrier to transplantation tolerance. We discuss how these observations may be rationalized, by proposing the generation of helpless effector CD8 T cells that cannot develop into memory, and by highlighting recent findings on the ability of transplantation tolerance to be spontaneously restored after rejection. We speculate that in individuals who develop tolerance while on immunosuppression and then experience rejection, it is this restored tolerance that is revealed upon successful weaning of immunosuppression.

SUMMARY

We have reviewed clinical and experimental data to explain how transplantation tolerance may be achieved in individuals who have experienced allograft rejection.

Fifty Shades of Tolerance: Beyond a Binary Tolerant/Non-Tolerant Paradigm

Purpose of review

It has long been considered that tolerance in a transplant recipient is a binary all-or-none state: either the graft is accepted without immunosuppression identifying the recipient as tolerant, or the recipient rejects the graft and is not tolerant. This tolerance paradigm, however, does not accurately reflect data emerging from animal models and patients and requires revision.

Recent Findings

It is becoming appreciated that there may be different gradations in the quality of tolerance based on underlying cellular mechanisms of immunological tolerance, and that individuals may enhance their tolerance by strengthening or combining different cellular mechanisms. Furthermore, evidence suggests that even if tolerance is lost, the loss may be only temporary, and in some circumstances tolerance can be restored.

Summary

Shifting our focus from an all-or-nothing tolerance paradigm to one with many shades may help us better understand how tolerance operates, and how this state may be tracked and enhanced for better patient outcomes.

Immune monitoring in renal transplantation: The search for biomarkers

It is now widely accepted that in order to improve long-term graft function and survival, a more personalized immunosuppressive treatment of transplant patients according to the individual anti-donor immune response status is needed. This applies to the identification of potentially "high-risk" patients likely to develop acute rejection episodes or display an accelerated decline of graft function, patients who might need immunosuppression intensification, and operationally tolerant patients suitable for immunosuppression minimization or weaning off. Such a patient stratification would benefit from biomarkers, which enable categorization into low and high risk or, ideally, identification of operational tolerant patients. Here, we report on recent developments regarding identification and performance analysis of noninvasive biomarkers such as mRNA and miRNA expression profiles, chemokines, or changes in immune cell subsets in either blood or urine of renal transplant patients. We will also discuss which future steps are needed to accelerate their clinical implementation.

Transplant trials with Tregs: perils and promises

Modern immunosuppression regimens effectively control acute rejection and decrease graft loss in the first year after transplantation; however, these regimens do not have a durable effect on long-term graft survival owing to a combination of drug toxicities and the emergence of chronic alloimmune responses. Eliminating drugs and their toxicities while maintaining graft acceptance has been the primary aim of cellular therapies. Tregs suppress both autoimmune and alloimmune responses and are particularly effective in protecting allografts in experimental transplant models. Further, Treg-based therapies are selective, do not require harsh conditioning, and do not have a risk of graft-versus-host disease. Trial designs should consider the distinct immunological features of each transplanted organ, Treg preparations, dose, and frequency, and the ability to detect and quantify Treg effects in a given transplant environment. In this Review, we detail the ongoing clinical trials of Treg therapy in liver and kidney transplantation. Integration of Treg biology gleaned from preclinical models and experiences in human organ transplantation should allow for optimization of trial design that will determine the potential efficacy of a given therapy and provide guidelines for further therapeutic development.

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.

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.

A composite score associated with spontaneous operational tolerance in kidney transplant recipients

New challenges in renal transplantation include using biological information to devise a useful clinical test for discerning high- and low-risk patients for individual therapy and ascertaining the best combination and appropriate dosages of drugs. Based on a 20-gene signature from a microarray meta-analysis performed on 46 operationally tolerant patients and 266 renal transplant recipients with stable function, we applied the sparse Bolasso methodology to identify a minimal and robust combination of six genes and two demographic parameters associated with operational tolerance. This composite score of operational tolerance discriminated operationally tolerant patients with an area under the curve of 0.97 (95% confidence interval 0.94-1.00). The score was not influenced by immunosuppressive treatment, center of origin, donor type, or post-transplant lymphoproliferative disorder history of the patients. This composite score of operational tolerance was significantly associated with both de novo anti-HLA antibodies and tolerance loss. It was validated by quantitative polymerase chain reaction using independent samples and demonstrated specificity toward a model of tolerance induction. Thus, our score would allow clinicians to improve follow-up of patients, paving the way for individual therapy.

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.

Transplantation tolerance: don't forget about the B cells

Establishing a state of transplantation tolerance that leads to indefinite graft survival without the need for lifelong immunosuppression has been achieved successfully in limited numbers of transplant recipients in the clinic. These successes led to studies aimed at identifying potential biomarkers that diagnose allograft tolerance and identify the patients most amenable to drug minimization, and implicated an enriched B cell signature of tolerance. The emergence of a specialized subset of regulatory B cell (B_regs ), that possess immune-modulatory function in inflammation and autoimmune disease, raised the possibility that B_regs play critical roles in the promotion of transplantation tolerance and that B_regs are the underlying explanation for the B cell signature of tolerance. However, B cells are best known to play a key role in humoral immunity, and excessive production of donor specific antibodies has clear deleterious effects in transplantation. Thus, for tolerance to be persistent, alloantibody responses must also be curtailed, either through the suppression of T cell help or the induction of B cell-intrinsic dysfunction. Recent findings indicate a unique subset of follicular regulatory T cells (Tfr) that can suppress B cell function and induce epigenetic modifications that result in sustained defects in B cell differentiation and function. In this review, we summarize studies in animals and humans that suggest roles for B_regs and dysfunctional B cells in transplantation tolerance, and discuss how these insights may provide a roadmap for new approaches to diagnose, and new therapies to induce allograft tolerance.

Mechanisms of liver-induced tolerance

PURPOSE OF REVIEW

To highlight the results of the ongoing research on the mechanisms of liver-induced tolerance focusing on results from the last year.

RECENT FINDINGS

The liver is exposed to a massive antigenic burden of dietary and commensal products from the gastrointestinal tract via portal vein, most of which are necessary for survival. To prevent the immune system from destroying these foreign yet beneficial elements, the liver has developed unique mechanisms to suppress immune responses. It is thought that these mechanisms of acquired tolerance may also underlie the spontaneous acceptance of liver allografts observed after transplantation in many species. The fact that isolated hepatocyte transplants are acutely rejected, suggests that nonparenchymal liver cells play a critical role in spontaneous liver allograft acceptance. IFN-γ, a key inflammatory cytokine produced by T effector (Tef) cells, is paradoxically compulsory for spontaneous liver allograft acceptance. Analysis of IFN-γ signaling points to liver mesenchymal nonparenchymal liver cell that eliminate infiltrating Tef cells via expression of B7-H1, IL-10, and tumor growth factor-β, as well as the enhancement of Tregs and MDSCs. Thus, liver mesenchymal cells are thought to promote tolerance by eliminating alloreactive Tef cells and enhancing suppressor cells (T and B).

SUMMARY

The research during last year offered some key insights into the mechanisms of liver-induced tolerance. Through interactions with activated T cells and B cells via IFN-γ/B7-H1 pathways, liver mesenchymal cells have been shown to be critical components of liver-specific tolerance induction.

Tolerance in Kidney Transplantation: What Is on the B Side?

Regulatory B cells (Breg) are in the spotlight for their role in immune homeostasis maintenance and tolerance achievement as in the last years the correlation with functional and increased Breg numbers in autoimmune diseases and transplantation has been extensively proven. Their study is, however, in its infancy with still little knowledge and consensus on their origin, phenotype, and mechanism of action. All this hampers the pursuit of an effective Breg induction method for therapeutic purposes. In this review we aim to summarize the studies on human Breg and their implication in kidney transplantation and to further discuss the issues surrounding therapeutic applications of this cell subset.

Immunological characteristics of renal transplant tolerance in humans

Establishing allograft tolerance is a highly desirable therapeutic goal in kidney transplantation, from which recipients would greatly benefit by withdrawing or minimizing immunosuppression. Identifying biomarkers in predicting tolerance or early diagnosing rejection is essential to direct personalized management. Recent findings have revealed that multiple populations of immune cells have involved in promoting long-term graft function or inducing rejection in renal transplant recipients. Thus, roles of immune cells add another level to predict the renal tolerant state; tailoring their functional and/or phenotypic characteristics would provide insights into mechanism involved in transplant tolerance that may aid in designing new therapies. Here, we review these findings and discuss the current understanding immunological characteristics of renal transplant tolerance in humans, and their potential clinical translation to immune tolerance biomarkers.