Elsevier

Human Immunology

Volume 82, Issue 3, March 2021, Pages 147-154
Human Immunology

Research article
Incidence and impact of allele-specific anti-HLA antibodies and high-resolution HLA genotyping on assessing immunologic compatibility

https://doi.org/10.1016/j.humimm.2021.01.002Get rights and content

Abstract

The ability to identify specific HLA molecules against which a patient has alloantibodies has revolutionized assessment of immunologic compatibility. Anti-HLA antibodies are typically evaluated as reactive against well-defined serologic antigen groups. Thus, donor HLA genotyping is aimed at defining HLA at the serologic split-antigen level to avoid incompatible antigen-antibody combinations. However, anti-HLA antibodies can have reactivities not accurately described by well-defined serologic antigens. While existence of these antibodies is acknowledged, their precise impact on clinical practice is not clear. We performed a single-center review of 2 years of pre-and post-transplant anti-HLA antibody testing data combined with high-resolution HLA genotyping data for living and deceased organ donors to evaluate the clinical impact of anti-HLA antibodies with reactivities outside of commonly defined serologic antigen groups. We find approximately 15% of patients awaiting transplantation have alloantibodies with differential reactivity for HLA proteins encoded by specific alleles within a serologic antigen group. Allele-specific antibodies are associated with positive cellular crossmatches not accurately predicted by standard donor HLA genotyping and can manifest as post-transplant donor-specific antibodies. Our data highlights the importance of evaluating anti-HLA antibodies at the allele-level and provides evidence supporting utility for high-resolution HLA genotyping in solid organ transplantation.

Introduction

The importance of antibodies against allogeneic Human Leukocyte Antigen (HLA) molecules as a primary barrier to allogeneic solid-organ transplantation is well established. Antibodies against mismatched donor HLA present at the time of transplantation can mediate hyperacute rejection while donor-specific antibodies (DSA) present after transplantation can drive antibody-mediated rejection [1], [2], [3]. Detection and definition of anti-HLA antibodies in transplant patients has undergone significant refinement since the initial discovery of their pathogenic potential in the 1960s. In particular, introduction of multiplexed solid-phase single-antigen bead (SAB) assays in the 2000s [4], [5] revolutionized the ability to define the specific reactivity of alloantibodies. SAB assays are designed to provide representative coverage for the 165 commonly recognized serologic antigens [6], [7]. This is achieved by the inclusion of one or more different HLA molecules from each of the serologic antigen groups. For several serologically-defined antigens, SAB assays contain HLA molecules encoded by multiple alleles; for instance, an assay may include beads with molecules encoded by the A*02:01, A*02:03, and A*02:06 alleles to represent the A2 serologic antigen. SAB assays are most directly interpreted based on the average reactivity against all beads containing molecules within a serologic antigen group. Differential reactivity among beads is sometimes interpreted as possible false positive or false negative reactions. However, similar to the well-defined concept of serologic-split antigens, HLA allele-specific reactivity is directed at epitopes differentially present among alleles within a broad serologic group [8], [9], [10], [11].

Emerging evidence for allele-specific antibodies has prompted debate of the importance of these antibodies in evaluation of donor-recipient immunologic compatibility. In the pre-transplant setting, this evaluation increasingly relies on virtual crossmatch analysis (VCXM) [12], [13], [14]. VCXM is performed by examination of donor HLA genotype for the presence of antigens against which the recipient has alloantibodies. This process is facilitated by registry systems such as UNET and the National Kidney Registry that utilize patient anti-HLA antibody information and donor HLA genotyping to direct organ offers to recipients who are most likely to be immunologically compatible. The potential importance of anti-HLA antibodies with reactivity outside of standard serologic antigens has been recognized by updates to these systems enabling reporting of antibodies against specific HLA alleles, including DQA1 and DPB1 which are not well-represented by the classically defined serologic antigens. However, standards for donor genotyping currently require only typing to the level necessary to determine serologic split antigen equivalents, potentially limiting the ability to accurately assess immunologic compatibility for patients with HLA allele-specific alloantibodies. This has spurred debate as to the necessity and appropriateness of high-resolution allele-level genotyping of donors [11], [15], [16], [17].

While demonstration of potential clinical impact in individual cases has been reported [9], [11], the prevalence of allele-specific antibodies is not well-defined. Understanding the prevalence of allele-specific antibodies in the context of existing ambiguity in donor genotyping is necessary for evaluating the costs and benefits of potentially updating donor HLA typing requirements. To this end, we performed a retrospective analysis of 2 years of HLA antibody testing for patients listed for transplant at our center combined with examining the ambiguity and accuracy of standard of care low-resolution donor HLA typing to evaluate the clinical impact of allele-specific antibodies. We also examined the prevalence of allele-specific DSA in post-transplant monitoring during the same period to evaluate the benefit of high-resolution donor genotyping information in this context.

Section snippets

Human subjects

All testing was performed as standard of care. Collection and analysis of de-identified data is considered exempt from institutional review board approval.

Anti-HLA antibody testing

Anti-HLA antibody testing was performed between 5/1/2018 and 4/30/2020 at the University of California San Diego Immunogenetics and Transplantation Laboratory as standard of care. Testing included pre-transplant testing of 4547 samples from 1939 patients listed for heart, lung, or kidney transplantation or post-transplant testing and

Allele-specific alloantibodies are frequent among patients awaiting transplantation

To evaluate the potential impact of allele-specific antibodies, we first assessed the frequency of allele-specific antibodies among patients awaiting solid organ transplantation. Allele-specific antibodies were defined as selective reactivity in SAB testing against HLA-A, B, C, DRB1, DRB3/4/5, DQB1, or DPB1 alleles within a defined serologic antigen. Selective reactivity was defined as positive reactivity against 1 or more beads within a serologic antigen group ≥3000 MFI with 1 or more beads

Discussion

The data presented here demonstrate that HLA allele-specific antibodies are relatively common among HLA-sensitized patients and should be considered as an important element of evaluating a patient’s anti-HLA alloantibody profile. These data are a single-center 2-year retrospective review of standard clinical testing of patients listed for heart, lung, and kidney transplantation, and are representative of a typical pre-transplant patient population. Approximately 15% of total pre-transplant

Declaration of Competing Interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: UCSD Immunogenetics and Transplantation Laboratory has received educational funds from Thermo-Fisher and CareDx. G.P.M. has received speaker honorarium and travel support from Thermo-Fisher.

Acknowledgement

The authors would like to thank Bing Yang (UCSD) and Arnold Jasman (UC Irvine) for technical assistance related to this project.

Authorship

D.Z., J.A., and P.R. performed data collection and analysis, and participated in writing the manuscript. G.P.M. designed the study, performed data collection and analysis, and participated in writing the manuscript.

Funding

This study was not funded by any sponsor.

References (25)

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