Biomarkers and possible mechanisms of operational tolerance in kidney transplant patients

Harnessing Mechanisms of Immune Tolerance to Improve Outcomes in Solid Organ Transplantation: A Review

Survival after solid organ transplantation (SOT) is limited by chronic rejection as well as the need for lifelong immunosuppression and its associated toxicities. Several preclinical and clinical studies have tested methods designed to induce transplantation tolerance without lifelong immune suppression. The limited success of these strategies has led to the development of clinical protocols that combine SOT with other approaches, such as allogeneic hematopoietic stem cell transplantation (HSCT). HSCT prior to SOT facilitates engraftment of donor cells that can drive immune tolerance. Recent innovations in graft manipulation strategies and post-HSCT immune therapy provide further advances in promoting tolerance and improving clinical outcomes. In this review, we discuss conventional and unconventional immunological mechanisms underlying the development of immune tolerance in SOT recipients and how they can inform clinical advances. Specifically, we review the most recent mechanistic studies elucidating which immune regulatory cells dampen cytotoxic immune reactivity while fostering a tolerogenic environment. We further discuss how this understanding of regulatory cells can shape graft engineering and other therapeutic strategies to improve long-term outcomes for patients receiving HSCT and SOT.

Continuous function of 80 primary renal allografts for 30-47 years with maintenance prednisone and azathioprine/mycophenolate mofetil therapy: A clinical mosaic of long-term successes

Eighty primary renal allograft recipients, 61 living-related and 19 deceased donor, transplanted from 1963 through 1984 had continuous graft function for 30-47 years. They were treated with three different early immunosuppression programs (1963-1970: thymectomy, splenectomy, high oral prednisone; 1971-1979: divided-dose intravenous methylprednisolone; and 1980-1984: antilymphocyte globulin) each with maintenance prednisone and azathioprine, and no calcineurin inhibitor. Long-term treatment often included the anti-platelet medication, dipyridamole. Although both recipient and donor ages were young (27.2 ± 9.5 and 33.1 ± 12.0 years, respectively), six recipients with a parent donor had >40-year success. At 35 years, death-censored graft survival was 85.3% and death with a functioning graft 84.2%; overall graft survival was 69.5% (Kaplan-Meier estimate). Biopsy-documented early acute cellular and highly probable antibody-mediated rejections were reversed with divided-dose intravenous methylprednisolone. Complications are detailed in an integrated timeline. Hypogammaglobulinemia identified after 20 years doubled the infection rate. An association between a monoclonal gammopathy of undetermined significance and non-plasma-cell malignancies was identified. Twenty-seven azathioprine-treated patients tested after 37 years had extremely low levels of T1/T2 B lymphocytes representing a "low immunosuppression state of allograft acceptance (LISAA)". The lifetime achievements of these patients following a single renal allograft and low-dose maintenance immunosuppression are remarkable. Their success evolved as a clinical mosaic.

Regulatory/inflammatory cellular response discrimination in operational tolerance

BACKGROUND

Antigen-specific cellular response is essential in immune tolerance. We tested whether antigen-specific cellular response is differentially modulated in operational tolerance (OT) in renal transplantation with respect to critical antigenic challenges in allotransplantation-donor antigens, pathogenic antigens and self-antigens.

METHODS

We analysed the profile of immunoregulatory (REG) and pro-inflammatory (INFLAMMA) cytokines for the antigen-specific response directed to these three antigen groups, by Luminex.

RESULTS

We showed that, in contrast to chronic rejection and healthy individuals, OT gives rise to an immunoregulatory deviation in the cellular response to donor human leucocyte antigen DR isotype peptides, while preserving the pro-inflammatory response to pathogenic peptides. Cellular autoreactivity to the N6 heat shock protein 60 (Hsp60) peptide also showed a REG profile in OT, increasing IL4, IL-5, IL-10 and IL-13.

CONCLUSIONS

The REG shift of donor indirect alloreactivity in OT, with inhibition of interleukin (IL)-1B, IL-8, IL-12, IL-17, granulocyte colony-stimulating factor, Interferon-γ and monocyte chemoattractant protein-1, indicates that this may be an important mechanism in OT. In addition, the differential REG profile of cellular response to the Hsp60 peptide in OT suggests that REG autoimmunity may also play a role in human transplantation tolerance. Despite cross-reactivity of antigen-specific T cell responses, a systemic functional antigen-specific discrimination takes place in OT.

Cell Therapy as a Tool for Induction of Immunological Tolerance after Liver Transplantation

Transplantation of solid organs, including liver, induces a number of serious complications related to immune incompatibility and requiring long-term use of immunosuppressive drugs. Finding the ways to inducing recipient immunological tolerance to the grafts is a top priority in organ transplantation and immunology. Along with the search for immunosupressive therapy, the development of alternative approaches to induction of immunological tolerance based on cell technologies is now in progress. In this regard, studies of the so-called spontaneous operational tolerance observed in ~20% patients after orthotopic liver transplantation is a promising trend. Understanding of this phenomenon can shed light on the mechanisms of immunological tolerance to allografts and will help to identify specific tolerance biomarkers and cell types with the aptitude for the induction of tolerance to liver allografts.

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.

Immunosuppression in the pediatric transplant recipient

The field of pediatric solid-organ transplantation has significantly evolved since its beginnings in the early 20^th century. As advancements have led to the development of innovative surgical techniques and novel medication regimens, transplantation has now become a routine practice leading to an increase in the rates of organ recipients worldwide. The care of pediatric solid-organ transplant recipients differs from adults in several areas not only due to technically challenging surgeries, but mostly due to the complexity of their immunosuppression management. Although there is large variation of pediatric immunosuppression regimens worldwide, the use of calcineurin inhibitors, either tacrolimus or cyclosporine, still forms the backbone of immunosuppression regimens after solid-organ transplantation. Both medications are relatively well tolerated but are known to have long-term side effects, especially nephrotoxicity and neurotoxicity. The goal of care in long-term pediatric survivors of solid-organ transplant now aims to safely minimize exposure to immunosuppression and to achieve long-term graft tolerance.

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.

A New Window into the Human Alloresponse

Alloreactive T lymphocytes are the primary mediators of allograft rejection. The size and diversity of the HLA-alloreactive T cell repertoire has thus far precluded the ability to follow these T cells and thereby to understand their fate in human transplant recipients. This review summarizes the history, challenges, and recent advances in the study of alloreactive T cells. We highlight the historical development of assays to measure alloreactivity and discuss how high-throughput T cell receptor (TCR) sequencing-based assays can provide a new window into the fate of alloreactive T cells in human transplant recipients. A specific approach combining a classical in vitro assay, the mixed lymphocyte reaction, with deep T cell receptor sequencing is described as a tool to track the donor-reactive T cell repertoire for any specific HLA-mismatched donor-recipient pair. This assay can provide mechanistic insights and has potential as a noninvasive, highly specific biomarker for rejection and tolerance.

Application of Operational Tolerance Signatures Are Limited by Variability and Type of Immunosuppression in Renal Transplant Recipients: A Cross-Sectional Study

Supplemental digital content is available in the text.

Background

Renal transplant recipients (RTR) frequently develop complications relating to chronic immunosuppression. Identifying RTR who could safely reduce immunosuppression is therefore highly desirable. We hypothesized that “signatures” described in RTR who have stopped immunosuppression but maintained stable graft function (“operational tolerance”) may enable identification of immunosuppressed RTR who are candidates for immunosuppression minimization. However, the effect of immunosuppression itself on these signatures and circulating B-cell populations is currently unknown.

Methods

We undertook a cross-sectional study of 117 RTR to assess the effect of immunosuppression upon circulating B cell populations, humoral alloresponse and 2 previously published “signatures” of operational tolerance.

Results

Immunosuppression associated with alterations in both published “signatures.” Azathioprine associated with a decrease in transitional and naive B-cell numbers and calcineurin inhibition associated with an increase in the number of circulating plasmablasts. However, only azathioprine use associated with the presence of donor-specific anti-HLA IgG antibodies. Calcineurin inhibition associated with an increase in total serum IgM but not IgG. Data were corrected for age, time since last transplant, and other immunosuppression.

Conclusions

Current signatures of operational tolerance may be significantly affected by immunosuppressive regimen, which may hinder use in their current form in clinical practice. Calcineurin inhibition may prevent the development of long-lasting humoral alloresponses, whereas azathioprine therapy may be associated with donor specific antibody development.

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.

Failure of Calcineurin Inhibitor (Tacrolimus) Weaning Randomized Trial in Long-Term Stable Kidney Transplant Recipients

Long-term renal transplant outcome is limited by side effects of immunosuppressive drugs, particularly calcineurin inhibitor (CNI). We assumed that some patients selected for a "low immunological risk of rejection" could be eligible and benefit from a CNI weaning strategy. We designed a prospective, randomized, multicenter, double-blind placebo-controlled clinical study (Eudract: 2010-019574-33) to analyze the benefit-risk ratio of tacrolimus weaning on highly selected patients (≥4 years of transplantation, normal histology, stable graft function, no anti-HLA immunization). The primary endpoint was improvement of renal function. Fifty-two patients were scheduled in each treatment arm, placebo compared to the CNI maintenance arm. Only 10 patients were eligible and randomized. Five patients were assigned to the placebo arm and five were assigned to the tacrolimus maintenance arm. In the tacrolimus maintenance arm, all patients maintained stable graft function and no immunological events occurred. Contrastingly, in the placebo arm, all five patients had to reintroduce a full dose of tacrolimus since three of them presented an acute rejection episode (one humoral, one mixed, and one borderline) and two displayed anti-HLA antibodies without histological lesion (one donor-specific antibodies [DSA] and one non-DSA). Clearly, tacrolimus withdrawal must be avoided even in long-term highly selective stable kidney recipients.

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.

Biomarkers and immunopathology of tolerance

PURPOSE OF REVIEW

This article describes biomarkers capable of identifying and predicting operational tolerance in solid organ transplant recipients. We outline the utility of these biomarkers in distinguishing allograft recipients in whom toxic immunosuppressive therapies might safely be minimized or withdrawn, and discuss their value in the appraisal of tolerance induction strategies. Finally, we review the insights derived from biomarker discovery into the cellular mechanisms underlying allograft tolerance.

RECENT FINDINGS

Important progress has been made in the development of robust signatures of tolerance, in both renal and liver transplant settings. Methodological advances, including high-throughput sequencing and bioinformatic processes, have been brought to bear on biomarker discovery and have heralded improvements in the accuracy with which operational tolerance can be predicted. Although the immunopathological basis for donor-specific tolerance is increasingly recognized to involve a complex interplay between numerous cell types, we review new lines of evidence shedding light on these mechanisms.

SUMMARY

Significant recent progress in identifying robust tolerance biomarkers has been made. In recognition of the need for rigorous validation of these, the first biomarker-led prospective immunosuppression withdrawal trials are underway. Such projects promise further progress and refinement in tolerance biomarker discovery, and offer hope for the amelioration of the burden associated with immunosuppressive therapies.

Emergent Transcriptomic Technologies and Their Role in the Discovery of Biomarkers of Liver Transplant Tolerance

Liver transplantation offers a unique window into transplant immunology due, in part, to the considerable proportion of recipients who develop immunological tolerance to their allograft. Biomarkers are able to identify and predict such a state of tolerance, and thereby able to establish suitable candidates for the minimization of hazardous immunosuppressive therapies, are not only of great potential clinical benefit but might also shed light on the immunological mechanisms underlying tolerance and rejection. Here, we review the emergent transcriptomic technologies serving as drivers of biomarker discovery, we appraise efforts to identify a molecular signature of liver allograft tolerance, and we consider the implications of this work on the mechanistic understanding of immunological tolerance.

Novel biomarkers and functional assays to monitor cell-therapy-induced tolerance in organ transplantation

PURPOSE OF REVIEW

Cell-based immunotherapy offers a novel approach to minimize the need for immunosuppressive drugs and to promote a state of immunological tolerance to a transplanted organ. We review the most promising biomarkers and functional assays able to identify patients tolerant to their graft. Such a signature of tolerance is essential in the assessment of the efficacy with which trials of cellular therapies promote immunoregulation and minimize graft rejection.

RECENT FINDINGS

A multitude of novel cellular therapies have entered early-phase clinical trials in solid-organ transplant patients. Recent multicentre collaborations have enabled the determination of distinct tolerance profiles for both liver and kidney transplant recipients. These have been shown to be highly predictive of tolerance in certain settings and show utility in identifying patients in whom immunosuppressive drugs can be weaned or discontinued.

SUMMARY

In order to become a viable treatment option in solid-organ transplantation, the latest large, multicentre clinical trials of cellular therapies must utilize, validate and discover the biomarkers with the capacity to reliably identify a signature of immune tolerance.

The use of novel diagnostics to individualize immunosuppression following transplantation

Despite major improvements in short-term survival of organ allografts, long-term graft survival has not changed significantly. It is also known that toxic side effects of current immunosuppressive drugs (IS) especially calcineurin inhibitors (CNI) contribute to the unsatisfactory graft and patient survival following transplantation. Thus, clinicians strive to reduce or wean IS in potentially eligible patients. Research in the last 10 years has focussed on identification of biomarkers suitable for patient stratification in minimization or weaning trials. Most of the described biomarkers have been run retrospectively on samples collected within single-centre trials. Thus, often their performance has not been validated in other potentially multicentre clinical trials. Ultimately, the utility of biomarkers to identify potential weaning candidates should be investigated in large randomized prospective trials. In particular, for testing in such trials, we need more information about the accuracy, reproducibility, stability and limitations of the described biomarkers. Also, data repositories summarizing crucial information on biomarker performance in age- and gender-matched healthy individuals of different ethnicity are missing. This together with improved bioinformatics tools might help in developing better scores for patient stratification. Here, we will summarize the current results, knowledge and limitations on biomarkers for drug minimization or weaning trials.