Acceptable mismatching at the class II epitope level: the Canadian experience

Antibody-mediated rejection diagnosed in early protocol biopsies in high immunological risk kidney transplant recipients

BACKGROUND

Renal transplant recipients with donor-specific anti-HLA antibodies are at an increased risk of antibody-mediated rejection (ABMR). Early protocolized renal biopsies may serve as a strategy to improve diagnosis in this patient population.

METHODS

We evaluated 155 highly sensitized renal transplant recipients with cPRA class I + II >90% pre-transplant from 2015 to 2022. Patients with protocol biopsies within the first 2 weeks post-transplant were included.

RESULTS

A total of 122 patients were included in the study. Of these, 13 (10.6%) were diagnosed with very early antibody-mediated rejection (veABMR) within the first 2 weeks post-transplant. This corresponds to 52% (13/25 patients) of all ABMR cases reported during the follow-up of this population. The graft survival rates at 1 and 3 years were significantly lower in patients with veABMR (P < .001) compared with patients without rejection in the early protocol biopsy. In terms of severity, the veABMR cohort exhibited a hazard ratio (HR) of 10.33 (95% confidence interval 3.23-33.06, P < .001) for graft failure. The presence of donor-specific antibodies class II on the day of transplantation and a higher percentage of eplet mismatch (EpMM), particularly EpMM DQA1, correlated with the development of veABMR.

CONCLUSION

Early protocol biopsies play a pivotal role in the early detection of veABMR in high-risk immunological patients. Patients with veABMR face significant risks of graft loss, despite early treatment of rejection.

Impact of HLA eplet mismatch load in immunological outcomes after living donor kidney transplantation

INTRODUCTION

HLA eplets mismatches (eMM) have been associated with negative kidney outcomes after transplantation, such as the development of de novo donor-specific antibody (dnDSA), antibody-mediated rejection (ABMR), and early graft loss. This study aimed to evaluate the clinical effects of the HLA eMM load on dnDSA development, ABMR, renal function, allograft survival and graft loss.

MATERIAL AND METHODS

This retrospective study involved 159 living donor kidney transplant patients categorized into groups based on antigen HLA mismatches assessed traditionally and HLA eMM load. Patients had followed for at least one year. The EpViX online program was used to evaluate the HLA eMM load. Cox models were constructed to assess the risk of graft loss. Kaplan-Meier survival curves were carried out. The analyses had performed using the R program and p < 0.05 was considered significant.

RESULTS

From all 159 patients, 28 (17.6%) lost their allografts. Rejection episodes occurred in 37.1% of patients, 13.6% of whom were ABMR. Patients with rejection episodes had higher HLA-AB (p = 0.032) and HLA-DR (p = 0.008) HLA eMM load, HLA-AB (p = 0.006) and HLA-DR (p = 0.009) antigens mismatches, and higher proportions of the following eMM in the HLA-DR locus: 70R eMM (p = 0.015), 70RE (p = 0.015), 74E (p = 0.015) and 48Q (p = 0.047). In multiple models, the presence of HLA-DR 70qq eMM (HR 3.75, 95% CI 1.47; 9.55) add an increase in creatinine levels at 1-year (HR 3.87, 95% CI 2.30, 6.53) were associated with the risk of graft loss.

CONCLUSION

The HLA eMM load was related to episodes of rejection and allograft loss. The HLA-DR eMM was most strongly associated with a worse immunologic outcome than eMM mismatches for HLA-AB.

Antibody-mediated rejection: prevention, monitoring and treatment dilemmas

PURPOSE OF REVIEW

Antibody-mediated rejection (AMR) has emerged as the leading cause of late graft loss in kidney transplant recipients. Donor-specific antibodies are an independent risk factor for AMR and graft loss. However, not all donor-specific antibodies are pathogenic. AMR treatment is heterogeneous due to the lack of robust trials to support clinical decisions. This review provides an overview and comments on practical but relevant dilemmas physicians experience in managing kidney transplant recipients with AMR.

RECENT FINDINGS

Active AMR with donor-specific antibodies may be treated with plasmapheresis, intravenous immunoglobulin and corticosteroids with additional therapies considered on a case-by-case basis. On the contrary, no treatment has been shown to be effective against chronic active AMR. Various biomarkers and prediction models to assess the individual risk of graft failure and response to rejection treatment show promise.

SUMMARY

The ability to personalize management for a given kidney transplant recipient and identify treatments that will improve their long-term outcome remains a critical unmet need. Earlier identification of AMR with noninvasive biomarkers and prediction models to assess the individual risk of graft failure should be considered. Enrolling patients with AMR in clinical trials to assess novel therapeutic agents is highly encouraged.

Not all eplet mismatches are created equal - A cohort study illustrating implications to long-term graft outcomes

We assessed implications of various eplet-compatibility strategies to death-censored graft failure (DCGF), defined as return to dialysis or re-transplantation, in a base-case scenario from the Scientific Registry of Transplant Recipients. To inform personalized care, we evaluated how recipient, donor, and transplant characteristics affect DCGF by ascending categories of eplet mismatches (EMM), and derived adjusted hazard ratios (HR). The base-case analysis demonstrated 15-year estimated survival probabilities of 77.1%, 75.4%, 73.6%, 72.2%, 74.9%, and 73.5% for the lowest EMM categories (complete epitype: 0-19, antibody-verified (AbVer) epitype and class II eplets: 0-9, class II AbVer eplets: 0-4, 55 high-risk eplets associated with DCGF: 0-3, and subset of 15 high-risk eplets validated in an independent subcohort: 0 EMM, respectively). Beyond the lowest EMM categories, the Epi15 strategy allowed better differentiation of change in DCGF risk per EMM, with additional 5.2%, 3.9% and 4.1% decrease in estimated graft survival for each additional EMM (1, 2, and ≥ 3, respectively). Recipients < 25 years, donors > 55 years, and immunosuppression regimens excluding calcineurin inhibitors and steroids, demonstrated higher HR for DCGF. High-risk EMM allowed better differentiation between DCGF probabilities per EMM, suggesting that recipients at higher risk for graft failure could benefit most from allocation schemes ensuring compatibility on these eplets.

Human leukocyte antigen eplet mismatches and long-term clinical outcomes in pediatric renal transplantation: A pragmatic, registry-based study

BACKGROUND

HLA epitope-based matching offers the potential to improve immunological risk prediction and management in children receiving renal allografts; however, studies demonstrating the association between systems for defining epitope mismatches and clinical end-points are lacking in this population.

METHODS

We conducted a pragmatic, retrospective, registry-based study of pediatric recipients of primary renal allografts in Victoria, Australia between 1990 and 2014 to determine the association between HLA EpMM and clinical outcomes including graft failure, re-transplantation and dnDSA formation.

RESULTS

A total of 196 patients were included in the analysis with a median age of 11 years. Median follow-up period was 15 years during which time 108 (55%) primary grafts failed and 72 patients were re-transplanted. HLA class I but not class II EpMM was a significant predictor of graft failure on univariate analysis but not in adjusted models. EpMM was associated with reduced likelihood of re-transplantation in univariate but not adjusted analysis. Within the limitations of the study, class-specific EpMM was a strong predictor of dnDSA formation. Associations were stronger when considering only the subset of antibody-verified EpMM.

CONCLUSION

Associations between HLA EpMM and clinical outcomes in pediatric renal allograft recipients seen on univariate analysis were attenuated following adjustment for confounders. These findings are inconclusive but suggest that HLA EpMM may provide one tool for assessing long-term risk in this population while highlighting the need for further clinical studies.

Assessment of human leukocyte antigen immunogenicity: current methods, challenges and opportunities

Purpose of review

Donor–recipient human leukocyte antigen (HLA) matching improves outcomes after solid-organ transplantation, but current assessment of HLA incompatibility is inadequate as it does not consider the relative immunogenicity of individual HLA mismatches. In this article, we review existing strategies for assessing HLA immunogenicity and discuss current challenges and future opportunities in this field.

Recent findings

Current HLA immunogenicity algorithms focus primarily on the humoral component of the alloimmune response and aim to determine a measure of ‘dissimilarity’ between donor and recipient HLA. This can be achieved by deriving information from comparison of donor and recipient HLA at the amino acid sequence, structural and/or the physicochemical level, accounting for both B-cell and T-cell pathways of alloreactivity. Substantial evidence now supports the superiority of this molecular definition of HLA incompatibility, over conventional enumeration of HLA antigenic differences, for assessing the risk of humoral alloimmunity and for predicting graft outcomes after transplantation.

Summary

Significant progress has been made in developing computational HLA immunogenicity algorithms that offer exciting opportunities for a more rational approach to determining the degree of donor–recipient HLA incompatibility and to defining HLA-related immunological risk. A number of challenges now need to be overcome to enable their implementation into clinical practice.

Epitope-based human leukocyte antigen matching for transplantation: a personal perspective of its future

PURPOSE OF REVIEW

This study reflects my personal experience with the characterization of human leukocyte antigen (HLA) epitopes and their significance in HLA matching for transplantation. It offers a subjective assessment what further studies are needed to have this concept be applied in the clinical setting.

RECENT FINDINGS

This study addresses the structural characteristics of antibody-reactive HLA epitopes determined by different methods, eplet-associated antibody analysis and acceptable mismatching for sensitized patients and eplet immunogenicity and determination of mismatch permissibility. BASIC IMPLICATIONS: for clinical practice and research consider the need for further studies of the structural basis of antibody-verified HLA epitopes determined in different techniques and their clinical relevance, the biological basis of epitope immunogenicity and determinations of permissible mismatches and a computerized clinical transplant database with an Artificial Intelligence component that can generate evidence-based information for the practical application of epitope-based HLA matching.

Summary of 2017 FDA Public Workshop: Antibody-mediated Rejection in Kidney Transplantation

Despite major advances in understanding the pathophysiology of antibody-mediated rejection (AMR); prevention, diagnosis and treatment remain unmet medical needs. It appears that early T cell-mediated rejection, de novo donor-specific antibody (dnDSA) formation and AMR result from patient or physician initiated suboptimal immunosuppression, and represent landmarks in an ongoing process rather than separate events. On April 12 and 13, 2017, the Food and Drug Administration sponsored a public workshop on AMR in kidney transplantation to discuss new advances, importance of immunosuppressive medication nonadherence in dnDSA formation, associations between AMR, cellular rejection, changes in glomerular filtration rate, and challenges of clinical trial design for the prevention and treatment of AMR. Key messages from the workshop are included in this summary. Distinction between type 1 (due to preexisting DSA) and type 2 (due to dnDSA) phenotypes of AMR needs to be considered in patient management and clinical trial design. Standardization and more widespread adoption of routine posttransplant DSA monitoring may permit timely diagnosis and understanding of the natural course of type 2 and chronic AMR. Clinical trial design, especially as related to type 2 and chronic AMR, has specific challenges, including the high prevalence of nonadherence in the population at risk, indolent nature of the process until the appearance of graft dysfunction, and the absence of accepted surrogate endpoints. Other challenges include sample size and study duration, which could be mitigated by enrichment strategies.

HLA epitope matching in pediatric renal transplantation

Chronic graft loss due to antibody-mediated rejection (AMR) and the difficulty of re-transplanting highly sensitized patients are two of the major long-term challenges in pediatric renal transplantation. Treatments for AMR are often ineffective and desensitization protocols can be a high risk, making prevention a highly appealing strategy. Insights into the structural determinants of humoral alloantigenicity present an exciting opportunity to reassess our current paradigm of tissue matching and potentially preventing these complications. We review the theory behind human leukocyte antigen (HLA) B cell epitopes and the various systems that have been proposed to define them, including eplets. There is a growing body of clinical evidence suggesting that epitope-based tissue matching may be superior to traditional HLA antigen matching at predicting a range of clinical outcomes. However, additional studies are required to better understand the biological relevance of these systems of defining epitopes and their role in pediatric transplantation.

Are We Ready for Epitope-Based HLA Matching in Clinical Organ Transplantation?

This overview describes recent developments demonstrating the significance of epitopes in HLA antibody responses and matching for organ transplantation. HLA epitopes are defined by molecular modeling and amino acid comparisons between HLA alleles and the HLAMatchmaker algorithm considers eplets as essential components. Each allele represents a distinct string of eplets and matching is done by aligning donor and recipient strings. Evidence is summarized how mismatched eplet loads affect antibody responses and transplant outcomes. Epitope-based matching has been applied not only to identify acceptable mismatches for sensitized transplant candidates but also to identify more suitably mismatched donors for nonsensitized patients. Three recently proposed theories will further our understanding of the immunogenicity of individual HLA eplets.It has become apparent that epitope-based matching is superior to antigen matching; we should be ready soon to apply this principle in the clinical transplant setting very soon.

Reflections on HLA Epitope-Based Matching for Transplantation

HLA antibodies are primary causes of transplant rejection; they recognize epitopes that can be structurally defined by eplets. There are many reviews about HLA epitope-based matching in transplantation. This article describes some personal reflections about epitopes including a historical perspective of HLA typing at the antigen and allele levels, the repertoires of antibody-verified HLA epitopes, the use of HLAMatchmaker in determining the specificities of antibodies tested in different assays, and, finally, possible strategies to control HLA antibody responses.

Should epitope-based HLA compatibility be used in the kidney allocation system?

The new kidney allocation system (KAS) still applies donor-recipient HLA compatibility mostly at the antigen level and although some four-digit alleles have been included. This system is used to record unacceptable mismatches for sensitized transplant candidates with serum HLA antibodies. Since the reactivities of such antibodies are specifically associated with epitopes rather than HLA antigens, a more scientifically accurate assessment of mismatch acceptability could be based on epitopes. HLA class I and class II epitope specificity analyses can now be readily performed with serum antibody assays with single allele panels. This report describes an epitope-based HLA compatibility system for KAS and involves recipient and donor HLA typing at the four-digit allele level. It focuses on sensitized patients who have serum antibodies specific for HLA epitopes that can be entered as unacceptable mismatches in the transplant candidate database. Newly developed software programs could readily identify compatible HLA types.

Antibody-defined epitopes on HLA-DQ alleles reacting with antibodies induced during pregnancy and the design of a DQ eplet map

The concept that HLA antibodies recognize epitopes is leading to new approaches of HLA matching at the epitope level. HLA-DQ plays an important role and many studies have identified structurally defined DQ epitopes specifically recognized by antibodies; they have been recorded in the International HLA Epitope Registry http://www.epregistry.com.br but the list is still incomplete. Pregnancy offers an attractive model to study antibody responses to HLA epitopes. The current analysis was done on 42 DQ-reactive post-pregnancy sera tested in binding assays with a panel of DQ heterodimers. The reactivity of 29 sera corresponded fully to the presence of antibody-verified DQA and DQB epitopes recorded in the Registry. Analysis of the remaining 13 sera led to the identification of additional antibody-defined DQB and DQA epitopes. We have designed the first version of an eplet map for DQ alleles which includes antibody-defined DQA and DQB epitopes and shows sequence positions with polymorphic residues which can be used in HLA epitology studies to identify new antibody-defined DQ epitopes.

The Synergistic Effect of Class II HLA Epitope-Mismatch and Nonadherence on Acute Rejection and Graft Survival

Predicting long-term outcomes in renal transplant recipients is essential to optimize medical therapy and determine the frequency of posttransplant histologic and serologic monitoring. Nonadherence and human leukocyte antigen (HLA) mismatch are risk factors that have been associated with poor long-term outcomes and may help individualize care. In the present study, class II HLA mismatches were determined at the HLA epitope level in 195 renal transplant recipients in whom medication adherence was prospectively measured using electronic monitors in medication vial caps. Recipients were grouped by medication adherence and high (≥10 HLA-DR, ≥17 HLA-DQ) or low epitope-mismatch load. We found that the combination of higher epitope mismatch and poor adherence acted synergistically to determine the risk of rejection or graft loss. Nonadherent recipients with HLA-DR epitope mismatch ≥10 had increased graft loss (35% vs. 8%, p < 0.01) compared to adherent recipients with low epitope mismatch. At the HLA-DQ locus nonadherent recipients with HLA-DQ epitope mismatch ≥17 had increased graft loss (33% vs. 10%, p < 0.01) compared to adherent recipients with low epitope mismatch. Subclinical nonadherence early posttransplant combined with HLA class II epitope mismatch may help identify recipients that could benefit from increased clinical, histologic, and serologic monitoring.