BIOMARKERS OF OPERATIONAL TOLERANCE IN SOLID ORGAN TRANSPLANTATION

New insights in immunomodulation for intestinal transplantation

Tolerance is the Holy Grail of solid organ transplantation (SOT) and remains its primary challenge since its inception. In this topic, the seminal contributions of Thomas Starzl at Pittsburgh University outlined foundational principles of graft acceptance and tolerance, with chimerism emerging as a pivotal factor. Immunologically, intestinal transplantation (ITx) poses a unique hurdle due to the inherent characteristics and functions of the small bowel, resulting in increased immunogenicity. This necessitates heavy immunosuppression (IS) while IS drugs side effects cause significant morbidity. In addition, current IS therapies fall short of inducing clinical tolerance and their discontinuation has been proven unattainable in most cases. This underscores the unfulfilled need for immunological modulation to safely reduce IS-related burdens. To address this challenge, the Leuven Immunomodulatory Protocol (LIP), introduced in 2000, incorporates various pro-tolerogenic interventions in both the donor to the recipient, with the aim of facilitating graft acceptance and improving outcome. This review seeks to provide an overview of the current understanding of tolerance in ITx and outline recent advances in this domain.

Innate protection against intrarectal SIV acquisition by a live SHIV vaccine

Identifying immune correlates of protection is a major challenge in AIDS vaccine development. Anti-Envelope antibodies have been considered critical for protection against SIV/HIV (SHIV) acquisition. Here, we evaluated the efficacy of an SHIV vaccine against SIVmac251 challenge, where the role of antibody was excluded, as there was no cross-reactivity between SIV and SHIV envelope antibodies. After 8 low-dose intrarectal challenges with SIVmac251, 12 SHIV-vaccinated animals demonstrated efficacy, compared with 6 naive controls, suggesting protection was achieved in the absence of anti-envelope antibodies. Interestingly, CD8+ T cells (and some NK cells) were not essential for preventing viral acquisition, as none of the CD8-depleted macaques were infected by SIVmac251 challenges. Initial investigation of protective innate immunity revealed that protected animals had elevated pathways related to platelet aggregation/activation and reduced pathways related to interferon and responses to virus. Moreover, higher expression of platelet factor 4 on circulating platelet-leukocyte aggregates was associated with reduced viral acquisition. Our data highlighted the importance of innate immunity, identified mechanisms, and may provide opportunities for novel HIV vaccines or therapeutic strategy development.

Clinical application of immune repertoire sequencing in solid organ transplant

Background

Measurement of T cell receptor (TCR) or B cell receptor (BCR) gene utilization may be valuable in monitoring the dynamic changes in donor-reactive clonal populations following transplantation and enabling adjustment in therapy to avoid the consequences of excess immune suppression or to prevent rejection with contingent graft damage and to indicate the development of tolerance.

Objective

We performed a review of current literature to examine research in immune repertoire sequencing in organ transplantation and to assess the feasibility of this technology for clinical application in immune monitoring.

Methods

We searched MEDLINE and PubMed Central for English-language studies published between 2010 and 2021 that examined T cell/B cell repertoire dynamics upon immune activation. Manual filtering of the search results was performed based on relevancy and predefined inclusion criteria. Data were extracted based on study and methodology characteristics.

Results

Our initial search yielded 1933 articles of which 37 met the inclusion criteria; 16 of these were kidney transplant studies (43%) and 21 were other or general transplantation studies (57%). The predominant method for repertoire characterization was sequencing the CDR3 region of the TCR β chain. Repertoires of transplant recipients were found to have decreased diversity in both rejectors and non-rejectors when compared to healthy controls. Rejectors and those with opportunistic infections were more likely to have clonal expansion in T or B cell populations. Mixed lymphocyte culture followed by TCR sequencing was used in 6 studies to define an alloreactive repertoire and in specialized transplant settings to track tolerance.

Conclusion

Methodological approaches to immune repertoire sequencing are becoming established and offer considerable potential as a novel clinical tool for pre- and post-transplant immune monitoring.

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.

B Cell Immunity in Solid Organ Transplantation

The contribution of B cells to alloimmune responses is gradually being understood in more detail. We now know that B cells can perpetuate alloimmune responses in multiple ways: (i) differentiation into antibody-producing plasma cells; (ii) sustaining long-term humoral immune memory; (iii) serving as antigen-presenting cells; (iv) organizing the formation of tertiary lymphoid organs; and (v) secreting pro- as well as anti-inflammatory cytokines. The cross-talk between B cells and T cells in the course of immune responses forms the basis of these diverse functions. In the setting of organ transplantation, focus has gradually shifted from T cells to B cells, with an increased notion that B cells are more than mere precursors of antibody-producing plasma cells. In this review, we discuss the various roles of B cells in the generation of alloimmune responses beyond antibody production, as well as possibilities to specifically interfere with B cell activation.

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.

Pediatric liver transplantation: Personal perspectives on historical achievements and future challenges

This review presents the author's personal perspective and contributions to the first steps, the development, the current status, and the remaining issues of pediatric liver transplantation (LT). Innumerable children around the world who have undergone LT have reached adulthood. The techniques have reached maturity. As shown by my own group's experience, grafts donated by living donors might provide the best short-term and longterm results. Debate persists about the optimal immunosuppression (IS), although the place of tacrolimus remains unchallenged. Tolerance induction protocols aiming to induce microchimerism have been tried in clinical transplantation without convincing results. Withdrawal of maintenance IS is possible in some children who underwent liver transplantation who have excellent clinical status and normal liver function tests but is not without risk of rejection and subsequent worsening of histology. The current trend favored by the Brussels' group is to minimize IS as soon after transplant as possible, aiming to obtain a state of "prope" or "almost" tolerance. Liver grafts are threatened in the long term by increasing hepatitis-related fibrosis, resulting most likely from immunological assault. Nowadays, the focus is on the longterm survival, quality of life (growth, academic performance, employment, self-fulfillment, fertility, raising a family, etc.), induction of tolerance, prevention of risks bound to decades of IS (nephrotoxicity and neurotoxicity, cardiovascular risk, de novo malignancies, etc.), and prevention of graft fibrosis. All these issues are fertile fields for younger scientists. Liver Transplantation 22 1284-1294 2016 AASLD.

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.

Extracorporeal photopheresis after heart transplantation

The addition of extracorporeal photopheresis (ECP) to a standard immunosuppressive drug therapy after heart transplantation in clinical studies has shown to be beneficial, for example, by reducing acute rejection, allograft vasculopathy or CMV infection. However, the protocols varied considerably, have a predetermined finite number of ECP treatments and adjuvant immunosuppressive regimens used in combination with ECP have differed significantly. Furthermore, there are scarce data to guide which patients should be treated with ECP and when or who would benefit further if ECP were to be continued long term to increase the safety by reducing immunosuppressive drug toxicities without losing efficacy. The knowledge of the tolerance-inducing effects of ECP-like upregulation of regulatory T cells and of dendritic cells may allow to develop a strategy to monitor immunomodulation effects of ECP to further identify ECP responders, the optimal individual ECP schedule and whether ECP therapy can replace or reduce immunosuppressive drug therapy.

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.

Association between a gain-of-function variant of PTPN22 and rejection in liver transplantation

BACKGROUND

The protein tyrosine phosphatase nonreceptor 22 gene (PTPN22) encodes a strong T-cell regulator called lymphoid protein tyrosine phosphatase. Previously, PTPN22 was described as a susceptibility gene for autoimmunity because it contains single nucleotide polymorphisms (SNPs) associated with several autoimmune diseases. One SNP (rs2476601; 1858G>A) has emerged as a particularly potent risk factor for autoimmunity. We address the question whether PTPN22 polymorphisms are also associated with acute rejection after liver transplantation.

METHODS

We investigated the influence of six PTPN22 SNPs on the susceptibility to acute liver allograft rejection. Consequently, we carried out a retrospective study genotyping 345 German liver recipients at six SNP loci, which include rs2488457 (-1123G>C), rs33996649 (788C>T), rs2476601 (1858G>A), rs1310182 (-852A>G), rs1217388 (-2200G>A), rs3789604 (64434T>G). Our study enrolled 165 recipients who did not develop rejection, 123 who showed one rejection episode, and 57 patients who suffered from multiple acute rejections after transplantation.

RESULTS

The 1858A allele containing genotypes (GA+AA) and the 1858A allele had a significantly higher frequency in the group of patients with multiple rejection episodes (35.1% and 18.4%) compared to rejection-free patients (15.8% and 7.9%; P=0.022 and 0.023). In contrast, we could not detect any association between rejection and the other tested SNPs. Additionally, we identified one haplotype contributing to risk of multiple rejections, however, exhibiting no stronger impact than the 1858A allele alone.

CONCLUSION

We conclude that the 1858G>A SNP may confer susceptibility to multiple acute liver transplant rejections in the German population.

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.

Monitoring B cell subsets and alloreactivity in kidney transplantation

B cells are the precursors of antibody producing plasma cells that can give rise to the formation of donor-specific antibodies. However, recent data suggest that besides their role in antibody production, B cells participate in antibody-independent responses, potentially leading to allograft rejection or allograft tolerance. The presence of CD20(+) B cells in kidney graft biopsies has been shown during severe acute rejection episodes and during chronic rejection. Furthermore, operationally tolerant kidney transplant recipients showed a clear B cell dominated fingerprint of tolerance. Several techniques exist to study B cells on different levels. Numerous classification schemes allow for the distinction of many different B cell subsets using flow cytometry. Regardless, data on B cell subsets during stable graft function, rejection or tolerance remain scarce. To obtain a complete picture of the role of B cells during transplantation, antigen specific B cell assays may be required. Therefore, techniques have now been developed that allow for studying the specificity and frequency of HLA specific B cells. Here, we present an overview of the existent assays, panels and techniques intended to characterize peripheral B cells, and the currently available HLA specific B cell functional assays that may allow for monitoring the humoral alloimmune response in transplant recipients.

B Cell Markers of Operational Tolerance Can Discriminate Acute Kidney Allograft Rejection From Stable Graft Function

BACKGROUND

Recently, several B cell-related markers have been described to be upregulated during operational tolerance in kidney allograft recipients. Little data exist on these markers during allograft rejection.

METHODS

In this study, we investigated regulation-associated B-cell phenotypes in peripheral blood mononuclear cells (PBMCs) of kidney transplant recipients with (n=21) and without (n=22) acute rejection (AR). We also determined expression levels of the B cell-related genes, MS4A1, TCL1A, and CD79B, in PBMCs and isolated B cells. Patient samples were analyzed before transplantation at discharge and at time of AR before initiation of antirejection therapy or at matching timepoints in patients with stable graft function.

RESULTS

On transplantation, the peripheral CD19CD24CD38 transitional B cell subset strongly declined, regardless of the subsequent occurrence of AR. In contrast, the CD19CD27CD24 subset remained stable after transplantation in both patients groups. MS4A1 gene expression levels in PBMC were comparable between patient groups at all timepoints. In contrast, TCL1A expression levels increased in stable patients, but decreased in patients at the time of AR in both PBMC and isolated B cells. CD79B expression levels in stable patients were unaltered after transplantation in PBMC but showed an increase in the B cell fraction at discharge. At the time of AR, CD79B gene expression was significantly lower compared to stable patients, being most apparent in the B-cell fraction.

CONCLUSION

These results suggest that, in addition to being markers for immunologic unresponsiveness, gene expression levels of TCL1A and CD79B may also identify immune activation in the setting of kidney transplantation.

Transplantation tolerance

Although transplantation has been a standard medical practice for decades, marked morbidity from the use of immunosuppressive drugs and poor long-term graft survival remain important limitations in the field. Since the first solid organ transplant between the Herrick twins in 1954, transplantation immunology has sought to move away from harmful, broad-spectrum immunosuppressive regimens that carry with them the long-term risk of potentially life-threatening opportunistic infections, cardiovascular disease, and malignancy, as well as graft toxicity and loss, towards tolerogenic strategies that promote long-term graft survival. Reports of "transplant tolerance" in kidney and liver allograft recipients whose immunosuppressive drugs were discontinued for medical or non-compliant reasons, together with results from experimental models of transplantation, provide the proof-of-principle that achieving tolerance in organ transplantation is fundamentally possible. However, translating the reconstitution of immune tolerance into the clinical setting is a daunting challenge fraught with the complexities of multiple interacting mechanisms overlaid on a background of variation in disease. In this article, we explore the basic science underlying mechanisms of tolerance and review the latest clinical advances in the quest for transplantation tolerance.