On the clinical relevance of using complete high-resolution HLA typing for an accurate interpretation of posttransplant immune-mediated graft outcomes

Clinical Applicability of 2-Field High-Resolution and Extended HLA-Allele Typing in Deceased Donor Kidney Allocation

HLA-compatibility remains an important triage test for deceased donor kidney allocation. Low-intermediate resolution donor HLA-typing is typically available at allocation, but its accuracy in assigning pre-transplant donor-specific anti-HLA antibody (DSA) and HLA mismatches compared to 2-field high-resolution typing is poorly characterised. Consecutive deceased donor/recipient pairs from a single centre between 2016 and 2020 were included. Majority of donor typing at HLA-ABDRB1 loci were performed at low-intermediate resolution, with 2-field high-resolution NGS typing across extended loci performed by NGS-technique post-transplantation. We compared the two typing methods for (1) accuracy of pre-transplant DSA assignment; (2) misassignment of HLA-antigen/allele mismatches and performance of each model for acute rejection and (3) proportion of recipients who developed de novo DSA (dnDSA) when matched at antigen but mismatched at allele level. Of 179 deceased donor/recipient pairs, 157 donors had low-intermediate resolution typing and 22 with high-resolution ONT typing. Sixty-two recipients (35%) had potential pre-transplant DSAs, with incorrect assignment of allele-specific Class I and II actual DSAs in 31% and 53% of cases, respectively. NGS typing identified 59 (33%) additional HLA-DRB1 allele mismatches. ONT typing accurately assigned pre-transplant DSAs and allele mismatches in all cases. Seven (4%) recipients with antigen/allele level discordance developed dnDSAs, majority HLA-DQ antibodies. Two-field high-resolution donor HLA typing may provide a more accurate transplant immunological risk assessment and identify those at risk of developing dnDSA to matched HLA antigen.

Two-field resolution on-call HLA typing for deceased donors using nanopore sequencing

The current practice of HLA genotyping in deceased donors poses challenges due to limited resolution within time constraints. Nevertheless, the assessment of compatibility between anti-HLA sensitized recipients and mismatched donors remains a critical medical need, particularly when dealing with allele-specific (second field genotyping level) donor-specific antibodies. In this study, we present a customized protocol based on the NanoTYPE® HLA typing kit, employing the MinION® sequencer, which enables rapid HLA typing of deceased donors within a short timeframe of 3.75 h on average at a three-field resolution with almost no residual ambiguities. Through a prospective real-time analysis of HLA typing in 18 donors, we demonstrated the efficacy and precision of our nanopore-based method in comparison to the conventional approach and without delaying organ allocation. Indeed, this duration was consistent with the deceased donor organ donation procedure leading to organ allocation via the French Biomedicine Agency. The improved resolution achieved with our protocol enhances the security of organ allocation, particularly benefiting highly sensitized recipients who often present intricate HLA antibody profiles. By overcoming technical challenges and providing comprehensive genotyping data, this approach holds the potential to significantly impact deceased donor HLA genotyping, thereby facilitating optimal organ allocation strategies.

Unrepresented human leucocyte antigen alleles in single-antigen bead assays: A single-centre cohort study

Human leucocyte antigen (HLA) alleles may generate antibodies that are undetectable by routine single-antigen beads (SABs) assays if their unique epitopes are unrepresented. We aimed to describe the prevalence and explore the potential impact of unrepresented HLA alleles in standard SAB kits in our cohort. All individuals who had undergone two-field HLA typing (HLA-A/B/C/DRB1/DQA1/-DQB1/-DPA1/-DPB1) from February 2021 to July 2023 were included. Two-field HLA-DRB3/4/5 typing was imputed. Each unrepresented allele was compared with the most similar represented allele in the standard LABScreen, LABScreen ExPlex (One Lambda) and the LIFECODES (Immucor) SAB kits. Differences in eplet expression (HLA Eplet Registry) were identified. Differences in three-dimensional molecular structures were visualized using generated models (SWISS-MODEL). Two-field HLA typing was performed for 116 individuals. Overall, 16.7% of all HLA alleles, found in 36.2% of individuals, were unrepresented by all SAB test kits. Four eplets, found in 12.9% of individuals, were unrepresented in at least 1 SAB kit. Non-Chinese individuals were more likely to have unrepresented HLA alleles and eplets than Chinese individuals. There were differences in HLA allele and eplet representation amongst the different SAB test kits. Use of supplementary SAB test kits may improve HLA allele and eplet representation. Although some HLA alleles were unrepresented, most epitopes were represented in current SAB kits. However, some unrepresented alleles may contain epitopes which may generate undetectable antibodies. Further studies may be needed to investigate the potential clinical impact of these unrepresented alleles and eplets, especially in certain ethnic populations or at-risk individuals.

HLA-DQ antibodies in alloimmunity, what makes them different?

PURPOSE OF REVIEW

De novo HLA-DQ antibodies are the most frequently observed after solid-organ allotransplantation; and are associated with the worse adverse graft outcomes compared with all other HLA antibodies. However, the biological explanation for this observation is not yet known. Herein, we examine unique characteristics of alloimmunity directed specifically against HLA-DQ molecules.

RECENT FINDINGS

While investigators attempted to decipher functional properties of HLA class II antigens that may explain their immunogenicity and pathogenicity, most early studies focused on the more expressed molecule - HLA-DR. We here summarize up-to-date literature documenting specific features of HLA-DQ, as compared to other class II HLA antigens. Structural and cell-surface expression differences have been noted on various cell types. Some evidence suggests variations in antigen-presenting function and intracellular activation pathways after antigen/antibody interaction.

SUMMARY

The clinical effects of donor-recipient incompatibility at HLA-DQ, the risk of generating de novo antibodies leading to rejection, and the inferior graft outcomes indicate increased immunogenicity and pathogenicity that is unique to this HLA antigen. Clearly, knowledge generated for HLA-DR cannot be applied interchangeably. Deeper understanding of features unique to HLA-DQ may support the generation of targeted preventive-therapeutic strategies and ultimately improve solid-organ transplant outcomes.

Extended genomic HLA typing identifies previously unrecognized mismatches in living kidney transplantation

Introduction

Antibody mediated rejection (ABMR) is the most common cause of long-term allograft loss in kidney transplantation (KT). Therefore, a low human leukocyte antigen (HLA) mismatch (MM) load is favorable for KT outcomes. Hitherto, serological or low-resolution molecular HLA typing have been adapted in parallel. Here, we aimed to identify previously missed HLA mismatches and corresponding antibodies by high resolution HLA genotyping in a living-donor KT cohort.

Methods

103 donor/recipient pairs transplanted at the University of Leipzig Medical Center between 1998 and 2018 were re-typed using next generation sequencing (NGS) of the HLA loci -A, -B, -C, -DRB1, -DRB345, -DQA1, -DQB1, -DPA1, and -DPB1. Based on these data, we compiled HLA MM counts for each pair and comparatively evaluated genomic HLA-typing with pre-transplant obtained serological/low-resolution HLA (=one-field) typing results. NGS HLA typing (=two-field) data was further used for reclassification of de novo HLA antibodies as "donor-specific".

Results

By two-field HLA re-typing, we were able to identify additional MM in 64.1% (n=66) of cases for HLA loci -A, -B, -C, -DRB1 and -DQB1 that were not observed by one-field HLA typing. In patients with biopsy proven ABMR, two-field calculated MM count was significantly higher than by one-field HLA typing. For additional typed HLA loci -DRB345, -DQA1, -DPA1, and -DPB1 we observed 2, 26, 3, and 23 MM, respectively. In total, 37.3% (69/185) of de novo donor specific antibodies (DSA) formation was directed against these loci (DRB345 ➔ n=33, DQA1 ➔ n=33, DPA1 ➔ n=1, DPB1 ➔ n=10).

Conclusion

Our results indicate that two-field HLA typing is feasible and provides significantly more sensitive HLA MM recognition in living-donor KT. Furthermore, accurate HLA typing plays an important role in graft management as it can improve discrimination between donor and non-donor HLA directed cellular and humoral alloreactivity in the long range. The inclusion of additional HLA loci against which antibodies can be readily detected, HLA-DRB345, -DQA1, -DQB1, -DPA1, and -DPB1, will allow a more precise virtual crossmatch and better prediction of potential DSA. Furthermore, in living KT, two-field HLA typing could contribute to the selection of the immunologically most suitable donors.