First report on the antibody verification of HLA-ABC epitopes recorded in the website-based HLA Epitope Registry

Relevance of Human Leukocyte Antigen Class C Donor-Specific Antibodies in Kidney Transplant

Kidney transplant is well known to be the best possible therapy for patients with end-stage kidney failure; however, allograft rejection remains a major obstacle despite the advent of modern immunosuppression regimens. Despite the well-established role of donorspecific antibodies directed at anti-HLA-A, -B, -DR, and -DQ antigens, the particular role of anti-HLA-C donorspecific antibodies in allograft longevity is not yet clear. Recently, preformed anti-native HLA-C donorspecific antibodies were reported to be possibly linked to poor allograft outcome. In addition, inclusion of HLA-C in all transplant allocation regimens has been suggested. Moreover, possible relevance of HLA-C has been shown in other fields (eg, transfusion and obstetrics). Its reduced expression could explain the diminished immunogenicity of the anti-HLA-C antibodies with subsequent lowered strength and prevalence. Furthermore, the"missed self" theory has gained interest. Here, we investigated HLA-C donorspecific antibody immunogenicity, pathogenicity, cellsurface expression, antibody heterogenicity, and possible management tools.

HLA topography enforces shared and convergent immunodominant B cell and antibody alloresponses in transplant recipients

Donor-specific antibody (DSA) responses against human leukocyte antigen (HLA) proteins mismatched between kidney transplant donors and recipients cause allograft loss. The rules governing the immunogenicity of non-self donor HLA are poorly understood. Using single-cell, molecular, structural, and proteomic techniques, we profiled the HLA-specific B cell response in the kidney and blood of a transplant recipient with antibody-mediated rejection (AMR). We observed an immunodominant B cell antibody response focused on topographically exposed, solvent-accessible mismatched HLA residues along the peptide-binding groove - a subregion comprising only 20% of the HLA molecule. We further demonstrated that, even within a diverse cohort of transplant recipients, the B cell alloresponse consistently converges on this same immunodominant subregion on the crown of the HLA molecule. Based on these findings, we propose that B cell immunodominance in transplant rejection relies on antigenic topography, and we suggest that this link could be exploited for organ matching and therapeutics.

Implications of MHC-restricted immunopeptidome in transplantation

The peptide presentation by donor and recipient major histocompatibility complex (MHC) molecules is the major driver of T-cell responses in transplantation. In this review, we address an emerging area of interest, the application of immunopeptidome in transplantation, and describe the potential opportunities that exist to use peptides for targeting alloreactive T cells. The immunopeptidome, the set of peptides presented on an individual's MHC, plays a key role in immune surveillance. In transplantation, the immunopeptidome is heavily influenced by MHC-derived peptides, delineating a key subset of the diverse peptide repertoire implicated in alloreactivity. A better understanding of the immunopeptidome in transplantation has the potential to open up new approaches to identify, characterize, longitudinally quantify, and therapeutically target donor-specific T cells and ultimately support more personalized immunotherapies to prevent rejection and promote allograft tolerance.

A Short History of B-Cell HLA Epitopes

Background

HLA epitopes are currently in the focus of transplantation immunogenetics. The main reason is the complexity of the HLA system with >38,000 alleles, the number of which increases steadily. These alleles are determined by the current state-of-the art typing methods like second- and third-generation sequencing. Screening for HLA antibodies is hampered by the lack of specific target beads with all possible alleles described.

Summary

A way to circumvent the problem is to define HLA epitopes. The number of antibody-confirmed epitopes, on the other hand, was found to be 72 for HLA class I and 74 for HLA class II. Here, we elaborate on the current knowledge on these HLA epitopes. Absolute definitions of these structures are not yet available.

Key Messages

Making use of eplets is a comparable way allowing statistical analyses. However, one should keep in mind that the results obtained are approximative or perhaps better associative. Continuous collaboration is needed for the full understanding of the HLA epitopes. The reactivity toward epitopes remains patient-specific.

Anti-HLA-B7/HLA-B44 strong cross immunoreactivity observed in flow cytometry HLA-B27 immunotyping

Cross reactivities are known for human leukocyte antigen inside HLA-B7 related Cross-Reactive Group (B7CREG). Some CE-IVD flow-cytometry kits use double monoclonal antibodies (mAb) to distinguish HLA-B27 and HLA-B7 but practice reveals more complexes results. This study explores the performances of this test. Analysis of 466 consecutive cases using HLA-B27 IOTest™ kit on a Navios™ cytometer from Beckman-Coulter, partially compared to their genotypes. Expected haplotypes HLA-B27-/HLA-B7- (undoubtedly HLA-B27 negative) and HLA-B27+/HLA-B7- (undoubtedly HLA-B27+) were clearly identified according to the manufacturer's instructions. On the opposite, patients strongly labeled with anti-HLA-B7 showed three different phenotypes regarding anti-HLA-B27 labeling: (1) most of the cases were poorly labeled in accordance with cross reactivity inside B7CREG (HLA-B27-/HLA-B7+ haplotype); (2) rare cases had strong B7 and B27 labeling corresponding to HLA-B27+/HLA-B7+ haplotype; (3) even less cases had strong labeling by anti-HLA-B7 but non for anti-HLA-B27, all expressing HLA-B44 and no B7CREG molecules. Surprisingly, more cases were not labeled with anti-HLA-B7 antibody but partially labeled with anti-HLA-B27 suggesting another cross reactivity out of B7CREG. mAb HLA typing suggests new, cross reactivities of anti-HLA-B27 antibody to more molecules out of B7CREG and of anti-HLA-B7 antibody but not anti-HLA-B27 to HLA-B44 molecule also out of B7CREG. HLA-B27 could surely be excluded in most samples labeled with HLA-B27, below a "grey zone" on intermediate intensity. More comparison is needed in future studies.

Antibody verification of HLA class I and class II eplets by human monoclonal HLA antibodies

In solid organ transplantation, formation of de novo donor-specific HLA antibodies is induced by mismatched eplets on donor HLA molecules. While several studies have shown a strong correlation between the number of eplet mismatches and inferior outcomes, not every eplet mismatch is immunogenic. Eplets are theoretically defined entities, necessitating formal proof that they can be recognised and bound by antibodies. This antibody verification is pivotal to ensure that clinically relevant eplets are considered in studies on molecular matching. Recombinant human HLA-specific monoclonal antibodies (mAbs) were generated from HLA-reactive B cell clones isolated from HLA immunised individuals using recombinant HLA molecules. Subsequently, the reactivity patterns of the mAbs obtained from single antigen bead assay were analysed using HLA-EMMA software to identify single or configurations of solvent accessible amino acids uniquely present on the reactive HLA alleles and were mapped to eplets. Two HLA class I and seven HLA class II-specific human mAbs were generated from four individuals. Extensive mAb reactivity analysis, led to antibody verification of three HLA-DR-specific eplets, and conversion of five eplets (one HLA-A, one HLA-B, two HLA-DR, and one HLA-DP), from provisionally verified to truly antibody-verified. Finally, one HLA-DQ-specific eplet was upgraded from level A2 to level A1 verification evidence. The generation of recombinant human HLA-specific mAbs with different specificities contributes significantly to the antibody verification of eplets and therefore is instrumental for implementation of eplet matching in the clinical setting.

Improving HLA typing imputation accuracy and eplet identification with local next-generation sequencing training data

Assessing donor/recipient HLA compatibility at the eplet level requires second field DNA typings but these are not always available. These can be estimated from lower-resolution data either manually or with computational tools currently relying, at best, on data containing typing ambiguities. We gathered NGS typing data from 61,393 individuals in 17 French laboratories, for loci A, B, and C (100% of typings), DRB1 and DQB1 (95.5%), DQA1 (39.6%), DRB3/4/5, DPB1, and DPA1 (10.5%). We developed HaploSFHI, a modified iterative maximum likelihood algorithm, to impute second field HLA typings from low- or intermediate-resolution ones. Compared with the reference tools HaploStats, HLA-EMMA, and HLA-Upgrade, HaploSFHI provided more accurate predictions across all loci on two French test sets and four European-independent test sets. Only HaploSFHI could impute DQA1, and solely HaploSFHI and HaploStats provided DRB3/4/5 imputations. The improved performance of HaploSFHI was due to our local and nonambiguous data. We provided explanations for the most common imputation errors and pinpointed the variability of a low number of low-resolution haplotypes. We thus provided guidance to select individuals for whom sequencing would optimize incompatibility assessment and cost-effectiveness of HLA typing, considering not only well-imputed second field typing(s) but also well-imputed eplets.

Specificity of HLA monoclonal antibodies and their use to determine HLA expression on lymphocytes and peripheral blood stem cells

HLA Class I and II expression are known to differ locus-to-locus, however, HLA expression on the cell-surface is frequently reported as the total amount of HLA Class I or II antigens. This is despite evidence that indicates the differential expression of HLA can influence patient outcomes post-transplantation. Although numerous commercially available HLA monoclonal antibodies (mAbs) exist to characterize HLA expression, there is currently a lack of detailed information regarding their reactivities to HLA specificities. The specificities of locus-specific HLA mAbs (nine Class I and four Class II mAbs) were evaluated by two solid-phase Luminex single antigen bead assays. The reactivity patterns of these mAbs were then confirmed by flow cytometry using lymphocytes and PBSCs (peripheral blood stem cells). Out of the 13 HLA mAbs tested, only four (one Class I and three Class II mAbs) displayed intra-locus reactivity without also reacting to inter-locus specificities. Epitope analysis revealed the presence of shared epitopes across numerous HLA loci, explaining much of the observed inter-locus reactivity. The specificity of the HLA mAbs seen in solid-phase assays was confirmed against PBSCs and lymphocytes by flow cytometry. Using this method, we observed differences in the cell surface expression of HLA-C, HLA-DR, HLA-DQ, and HLA-DP between PBSCs and lymphocytes. Our results emphasize the need to characterize the reactivity patterns of HLA mAbs using solid-phase assays before their use on cells. Through understanding the reactivity of these HLA mAbs, the cellular expression of HLA can be more accurately assessed in downstream assays.

BSHI and BTS UK guideline on the detection of alloantibodies in solid organ (and islet) transplantation

Solid organ transplantation represents the best (and in many cases only) treatment option for patients with end-stage organ failure. The effectiveness and functioning life of these transplants has improved each decade due to surgical and clinical advances, and accurate histocompatibility assessment. Patient exposure to alloantigen from another individual is a common occurrence and takes place through pregnancies, blood transfusions or previous transplantation. Such exposure to alloantigen's can lead to the formation of circulating alloreactive antibodies which can be deleterious to solid organ transplant outcome. The purpose of these guidelines is to update to the previous BSHI/BTS guidelines 2016 on the relevance, assessment, and management of alloantibodies within solid organ transplantation.

Epitope mismatch at HLA-DRB1 associates with reduced relapse risk in cord blood transplant for standard-risk hematological malignancy

BACKGROUND

Cord blood transplantation (CBT) is an attractive therapeutic option for patients with hematological malignancies. CBT tolerates HLA mismatches between donors and recipients, but which HLA mismatches generate graft-versus-tumor (GVT) effects is unknown.

OBJECTIVE

Given that HLA molecules contain epitopes comprising polymorphic amino acids that determine their immunogenicity, we investigated associations between epitope-level HLA mismatches and relapse following single-unit CBT.

STUDY DESIGN

A total of 492 patients with hematological malignancies who received single unit, T cell replete CBT were included in this multi-center retrospective study. HLA epitope mismatches (EM) were quantified using HLA matchmaker software from donor and patient HLA-A, B, C and DRB1 allele data. Patients were dichotomized by median EM value and divided into groups transplanted in complete/partial remission (standard stage: 62.4%) and others (advanced stage: 37.6%).

RESULTS

Median EM numbers in the graft-versus-host direction (GVH-EM) at HLA-class I and HLA-DRB1 were 3 (range, 0-16) and 1 (range, 0-7), respectively. Higher HLA-class I GVH-EM increased non-relapse mortality (NRM) in the advanced stage group (adjusted hazard ratio [HR], 2.12; p=0.021), with no significant advantage for relapse in either stage. On the other hand, higher HLA-DRB1 GVH-EM was associated with better disease-free survival in the standard stage group (adjusted HR, 0.63; p=0.020), which was attributed to lower relapse risk (adjusted HR, 0.46; p=0.014). These associations were also observed even within HLA-DRB1 allele-mismatched transplants in the standard stage group, indicating that EM might have impacts on relapse risk independently of allele mismatch. High HLA-DRB1 GVH-EM did not increase NRM in either stage.

CONCLUSIONS

High HLA-DRB1 GVH-EM may lead to potent GVT effects and favorable prognosis following CBT especially in the patients transplanted in the standard stage. This approach may facilitate appropriate unit selection and improve the overall prognosis of patients with hematological malignancies who receive CBT.

Site-directed mutagenesis of HLA molecules reveals the functional epitope of a human HLA-A1/A36-specific monoclonal antibody

Eplet 44KM is currently listed in the HLA Epitope Registry but does not adhere to the eplet definition of an amino acid configuration within a 3.5 Å radius. Eplet 44KM has been previously redefined to the antibody-verified reactivity pattern 44K/150V/158V, based on reactivity analysis of monoclonal antibody VDK1D12. Since the three residues are always simultaneously present on common HLA alleles, methods to define which residue is crucial for antibody-induction and binding are limited. In this proof-of-concept study, we performed site-directed mutagenesis to narrow down the antibody-verified reactivity pattern 44K/150V/158V to a single amino acid and defined 44K as the eplet or functional epitope of mAb VDK1D12.

Virtual crossmatching and epitope analysis in kidney transplantation: What the physician involved in kidney transplantation should know?

Solid-phase single antigen bead (SAB) assay for detection of anti-human leukocyte antigen (HLA) antibodies and high-resolution HLA typing have enabled tremendous progress in virtual crossmatch (VXM) technology in recent years. However, misinterpretation of the SAB assay may result in detrimental consequences after kidney transplantation. Meanwhile, epitope analysis could be an effective method to estimate immunizing eplets, which may provide ancillary information for better understanding of the SAB assay. To perform epitope analysis appropriately, it is necessary to understand the basic principles related to histocompatibility testing and the characteristics of the SAB assay. Therefore, knowledge of the properties and limitations of the SAB assay is critical. In this review, we aim to describe the fundamental concepts regarding immunobiological assessment, including HLA, anti-HLA antibodies, and SAB assay, and explain epitope analysis using examples.

Eplet-Predicted Antigens: An Attempt to Introduce Eplets into Unacceptable Antigen Determination and Calculated Panel-Reactive Antibody Calculation Facilitating Kidney Allocation

(1) Calculated panel-reactive antibody (CPRA) is a measure of sensitization based on unacceptable antigens (UAs). Determination of UAs based on single-antigen bead assays at allele or antigen levels may be inappropriate. We aimed to introduce eplets for better assessment of sensitization; (2) 900 recipients and 1427 donors were enrolled for candidate or donor pools, respectively. Eplets were from the HLA Epitope Registry. UAs were determined by anti-HLA antibodies identified using LIFECODES Single Antigen (LSA) kits. CPRA values were calculated using a simplified method of donor filtering; (3) HLA antigens containing all eplets of an HLA antigen in LSA kits (LSA antigen) were defined as eplet-predicted (EP) antigens, the reactivity of which could be predicted by that LSA antigen. High reactivity concordance was found between LSA and EP antigens. More HLA antigens were covered by EP antigens in the population than LSA antigens. CPRA values at the EP level were higher than at the allele level and lower than at the antigen level. The EP antigens facilitated UA determination for non-LSA antigens and avoided acute rejection; (4) UA determination using EP antigens can lead to more accurate assessment of sensitization, enabling a high probability of compatible organs and a low risk of adverse outcomes.

Antigen-guided depletion of anti-HLA antibody-producing cells by HLA-Fc fusion proteins

Platelet transfusion and transplantation of allogeneic stem cells and solid organs are life-saving therapies. Unwanted alloantibodies to nonself human leukocyte antigens (HLAs) on donor cells increase the immunological barrier to these therapies and are important causes of platelet transfusion refractoriness and graft rejection. Although the specificities of anti-HLA antibodies can be determined at the allelic level, traditional treatments for antibody-mediated rejection nonselectively suppress humoral immunity and are not universally successful. We designed HLA-Fc fusion proteins with a bivalent targeting module derived from extracellular domains of HLA and an Fc effector module from mouse IgG2a. We found that HLA-Fc with A2 (A2Fc) and B7 (B7Fc) antigens lowered HLA-A2- and HLA-B7-specific reactivities, respectively, in sera from HLA-sensitized patients. A2Fc and B7Fc bound to B-cell hybridomas bearing surface immunoglobulins with cognate specificities and triggered antigen-specific and Fc-dependent cytotoxicity in vitro. In immunodeficient mice carrying HLA-A2-specific hybridoma cells, A2Fc treatment lowered circulating anti-HLA-A2 levels, abolished the outgrowth of hybridoma cells, and prolonged survival compared with control groups. In an in vivo anti-HLA-A2-mediated platelet transfusion refractoriness model, A2Fc treatment mitigated refractoriness. These results support HLA-Fc being a novel strategy for antigen-specific humoral suppression to improve transfusion and transplantation outcomes. With the long-term goal of targeting HLA-specific memory B cells for desensitization, further studies of HLA-Fc's efficacy in immune-competent animal models are warranted.

A new strategy for systematically classifying HLA alleles into serological specificities

HLA serological specificities were defined by the reactivity of HLA molecules with sets of sera and monoclonal antibodies. Many recently identified alleles defined by molecular typing lack their serotype assignment. We surveyed the literature describing the correlation of the reactivity of serologic reagents with AA residues. 20 - 25 AA residues determining epitopes (DEP) that correlated with 82 WHO serologic specificities were identified for HLA class I loci. Thirteen DEP each located in the beta-1 domains that correlated with 24 WHO serologic specificities were identified for HLA-DRB1 and -DQB1 loci. The designation of possible HLA-DPB1, -DQA1, -DPA1, and additional serological specificities that result from epitopes defined by residues located at both -DQA1 and -DQB1 subunits were also examined. HATS software was developed for automated serotype assignments to HLA alleles in one of the three hierarchical matching criteria: 1) all DEP (FULL); 2) selected DEP specific to each serological specificities (SEROTYPE); 3) one AA mismatch with one or more SEROTYPES (INCOMPLETE). Results were validated by evaluating the alleles whose serotypes do not correspond to the first field of the allele name listed in the HLA dictionary. Additional 85 and 21 DEP patterns that do not correspond to any WHO serologic specificities for common HLA class I and DRB1 alleles were identified, respectively. A comprehensive antibody identification panel would allow for accurate unacceptable antigen listing and compatibility predictions in solid organ transplantations. We propose that antibody-screening panels should include all serologic specificities identified in this study. This article is protected by copyright. All rights reserved.

HLA-DQ-Specific Recombinant Human Monoclonal Antibodies Allow for In-Depth Analysis of HLA-DQ Epitopes

HLA-DQ donor-specific antibodies (DSA) are the most prevalent type of DSA after renal transplantation and have been associated with eplet mismatches between donor and recipient HLA. Eplets are theoretically defined configurations of surface exposed amino acids on HLA molecules that require verification to confirm that they can be recognized by alloantibodies and are therefore clinically relevant. In this study, we isolated HLA-DQ specific memory B cells from immunized individuals by using biotinylated HLA-DQ monomers to generate 15 recombinant human HLA-DQ specific monoclonal antibodies (mAb) with six distinct specificities. Single antigen bead reactivity patterns were analyzed with HLA-EMMA to identify amino acids that were uniquely shared by the reactive HLA alleles to define functional epitopes which were mapped to known eplets. The HLA-DQB1*03:01-specific mAb LB_DQB0301_A and the HLA-DQB1*03-specific mAb LB_DQB0303_C supported the antibody-verification of eplets 45EV and 55PP respectively, while mAbs LB_DQB0402_A and LB_DQB0602_B verified eplet 55R on HLA-DQB1*04/05/06. For three mAbs, multiple uniquely shared amino acid configurations were identified, warranting further studies to define the inducing functional epitope and corresponding eplet. Our unique set of HLA-DQ specific mAbs will be further expanded and will facilitate the in-depth analysis of HLA-DQ epitopes, which is relevant for further studies of HLA-DQ alloantibody pathogenicity in transplantation.

A Comprehensive Evaluation of the Antibody-Verified Status of Eplets Listed in the HLA Epitope Registry

Matching strategies based on HLA eplets instead of HLA antigens in solid organ transplantation may not only increase the donor pool for highly sensitized patients, but also decrease the incidence of de novo donor-specific antibody formation. However, since not all eplets are equally capable of inducing an immune response, antibody verification is needed to confirm their ability to be bound by antibodies, such that only clinically relevant eplets are considered. The HLA Epitope Registry has documented all theoretically defined HLA eplets along with their antibody verification status and has been the foundation for many clinical studies investigating eplet mismatch in transplantation. The verification methods for eplets in the Registry range from polyclonal sera from multi- and uni-parous women to murine and human monoclonal antibodies (mAbs), and antibodies purified by adsorption and elution from sera of HLA immunized individuals. The classification of antibody verification based on different methods for validation is problematic, since not all approaches represent the same level of evidence. In this study, we introduce a classification system to evaluate the level of evidence for the antibody-verified status of all eplets in the HLA Epitope Registry. We demonstrate that for a considerable number of eplets, the antibody-verified status is solely based on polyclonal serum reactivity of multiparous women or on reactivity of murine mAbs. Furthermore, we noted that a substantial proportion of patient sera analyses and human mAb data presented in the HLA Epitope Registry Database has never been published in a peer-reviewed journal. Therefore, we tested several unpublished human HLA-specific mAbs by luminex single antigen beads assay to analyze their HLA reactivity for eplet antibody verification. Although the majority of analyzed mAbs indeed verified their assigned eplets, this was not the case for a number of eplets. This comprehensive overview of evidence for antibody verification of eplets in the HLA Epitope Registry is instrumental for future investigations towards eplet immunogenicity and clinical studies considering antibody-verified eplet mismatch in transplantation and warrants further standardization of antibody verification using high quality data.

Implementation of molecular matching in transplantation requires further characterization of both immunogenicity and antigenicity of individual HLA epitopes

Over the past decade, high HLA epitope mismatch scores have been associated with inferior transplant outcomes using several tools, of which HLAMatchmaker is most well-known. This software uses theoretically defined polymorphic amino acid configurations, called eplets, for HLA compatibility analysis. Although consideration of eplet mismatch loads has potential for immunological risk stratification of transplant patients, the use of eplet matching in organ allocation algorithms is hindered by lacking knowledge of the immunogenicity of individual eplets, and the possibility that single mismatched amino acids, rather than complete eplets, are responsible for HLA antibody induction. There are several approaches to define eplet immunogenicity, such as antibody verification of individual eplets, and data-driven approaches using large datasets that correlate specific eplet mismatches to donor specific antibody formation or inferior transplant outcomes. Data-driven approaches can also be used to define whether single amino acid mismatches may be more informative than eplet mismatches for predicting HLA antibody induction. When using epitope knowledge for the assignment of unacceptable antigens, it important to realize that alleles sharing an eplet to which antibodies have formed are not automatically all unacceptable since multiple contact sites determine the binding strength and thus biological function and pathogenicity of an antibody, which may differ between reactive alleles. While the future looks bright for using HLA epitopes in clinical decision making, major steps need to be taken to make this a clinical reality.

Improve in-depth immunological risk assessment to optimize genetic-compatibility and clinical outcomes in child and adolescent recipients of parental donor kidney transplants: protocol for the INCEPTION study

Background

Parental donor kidney transplantation is the most common treatment option for children and adolescents with kidney failure. Emerging data from observational studies have reported improved short- and medium-term allograft outcomes in recipients of paternal compared to maternal donors. The INCEPTION study aims to identify potential differences in immunological compatibility between maternal and paternal donor kidneys and ascertain how this affects kidney allograft outcomes in children and adolescents with kidney failure.

Methods

This longitudinal observational study will recruit kidney transplant recipients aged ≤18 years who have received a parental donor kidney transplant across 4 countries (Australia, New Zealand, United Kingdom and the Netherlands) between 1990 and 2020. High resolution human leukocyte antigen (HLA) typing of both recipients and corresponding parental donors will be undertaken, to provide an in-depth assessment of immunological compatibility. The primary outcome is a composite of de novo donor-specific anti-HLA antibody (DSA), biopsy-proven acute rejection or allograft loss up to 60-months post-transplantation. Secondary outcomes are de novo DSA, biopsy-proven acute rejection, acute or chronic antibody mediated rejection or Chronic Allograft Damage Index (CADI) score of > 1 on allograft biopsy post-transplant, allograft function, proteinuria and allograft loss. Using principal component analysis and Cox proportional hazards regression modelling, we will determine the associations between defined sets of immunological and clinical parameters that may identify risk stratification for the primary and secondary outcome measures among young people accepting a parental donor kidney for transplantation. This study design will allow us to specifically investigate the relative importance of accepting a maternal compared to paternal donor, for families deciding on the best option for donation.

Discussion

The INCEPTION study findings will explore potentially differential immunological risks of maternal and paternal donor kidneys for transplantation among children and adolescents. Our study will provide the evidence base underpinning the selection of parental donor in order to achieve the best projected long-term kidney transplant and overall health outcomes for children and adolescents, a recognized vulnerable population.

Trial registration

The INCEPTION study has been registered with the Australian New Zealand Clinical Trials Registry, with the trial registration number of ACTRN12620000911998 (14th September 2020).

BSHI/BTS guidance on crossmatching before deceased donor kidney transplantation

All UK H&I laboratories and transplant units operate under a single national kidney offering policy, but there have been variations in approach regarding when to undertake the pre‐transplant crossmatch test. In order to minimize cold ischaemia times for deceased donor kidney transplantation we sought to find ways to be able to report a crossmatch result as early as possible in the donation process. A panel of experts in transplant surgery, nephrology, specialist nursing in organ donation and H&I (all relevant UK laboratories represented) assessed evidence and opinion concerning five factors that relate to the effectiveness of the crossmatch process, as follows: when the result should be ready for reporting; what level of donor HLA typing is needed; crossmatch sample type and availability; fairness and equity; risks and patient safety. Guidelines aimed at improving practice based on these issues are presented, and we expect that following these will allow H&I laboratories to contribute to reducing CIT in deceased donor kidney transplantation.

Characterization of the novel HLA-B*35:460Q allele by next-generation sequencing

B*35:460Q differs from B*35:03:01:01 by one nucleotide substitution at position 406 in exon 3.

Generation and reactivity analysis of human recombinant monoclonal antibodies directed against epitopes on HLA-DR

In kidney transplantation, eplet mismatches between donor and recipient have been associated with de novo donor-specific antibody development. Eplets are theoretically defined configurations of polymorphic amino acids and require experimental verification to establish whether they can be bound by alloantibodies. Human HLA-specific monoclonal antibodies (mAbs) have been instrumental for this purpose but are largely lacking for HLA class II. In this study, we isolated single HLA-DR-specific memory B cells from peripheral blood of immunized individuals (n = 3) using HLA class II tetramers to generate recombinant human HLA-DR antigen-reactive mAbs (n = 5). Comparison of the amino acid composition of the reactive HLA alleles in relation to the antibody reactivity patterns led to identification of 3 configurations, 70Q 73A, 31F 32Y 37Y, and 14K 25Q recognized, respectively, by HLA-DRB1*01:01, HLA-DRB1*04:01, and HLA-DRB1*07:01 antigen-reactive mAbs. The first 2 correspond to eplets 70QA and 31FYY and can now be considered antibody verified. The latter indicates that eplet 25Q needs to be redefined before being considered as antibody verified. Generation and reactivity analysis of human HLA-DR mAbs allowed for identification of amino acid configurations corresponding to known eplets, whereas the other patterns may be used to redefine eplets with similar, but not identical predicted amino acid composition.

Clinical relevance of donor-specific antibodies directed at HLA-C: A long road to acceptance

In solid organ transplantation (SOT), the clinical relevance of donor-specific antibodies (DSA) directed at anti-HLA-A, -B, -DR and -DQ antigens is largely recognized while it is still a matter of debate for DSA directed at HLA-C. In this review, we summarize the peculiarities of HLA-C among class I HLA antigens as well as their immunogenicity, which underlie the clinical relevance of HLA-C locus and anti-HLA-C DSA in SOT. Many factors, both intrinsic and extrinsic to the HLA-C gene and HLA-C protein, explain its lower expression in comparison with HLA-A and -B. This lower expression can explain the apparent lower immunogenicity of HLA-C leading to a lower prevalence and strength of anti-HLA-C antibodies. Nevertheless, HLA-C antigens are truly immunogenic and preformed anti-HLA-C DSA are clinically relevant. Indeed, anti-HLA-C DSA are able to bind donor cells and to activate the complement pathway both ex vivo and in vivo. In line with this, numerous clinical studies now show that preformed DSA directed at native HLA-C molecules induce poorer graft outcomes. We then plead for the inclusion of HLA-C in all transplant allocation systems and we propose a strategy to cope with anti-HLA-C DSA in SOT. Beyond SOT, anti-HLA-C antibodies generate a growing interest in the allo-HCT, transfusion and obstetrics fields, while new concepts such as the role of the "missing-self" in solid organ rejection places HLA-C as an inescapable actor in transplant tolerance.

Characterization of the novel HLA-DRB1*01:106 allele by next-generation sequencing

DRB1*01:106 differs from DRB1*01:01:01:01 by one nucleotide substitution at position 355 in exon 2.

Eplet Mismatch Load and De Novo Occurrence of Donor-Specific Anti-HLA Antibodies, Rejection, and Graft Failure after Kidney Transplantation: An Observational Cohort Study

BACKGROUND

In kidney transplantation, evaluating mismatches of HLA eplets-small patches of surface-exposed amino acids of the HLA molecule-instead of antigen mismatches might offer a better approach to assessing donor-recipient HLA incompatibility and improve risk assessment and prediction of transplant outcomes.

METHODS

To evaluate the effect of number of eplet mismatches (mismatch load) on de novo formation of donor-specific HLA antibodies (DSAs) and transplant outcomes, we conducted a cohort study that included consecutive adult kidney recipients transplanted at a single center from March 2004 to February 2013. We performed retrospective high-resolution genotyping of HLA loci of 926 transplant pairs and used the HLAMatchmaker computer algorithm to count HLA eplet mismatches.

RESULTS

De novo DSAs occurred in 43 (4.6%) patients. Multivariable analysis showed a significant independent association between antibody-verified eplet mismatch load and de novo DSA occurrence and graft failure, mainly explained by DQ antibody-verified eplet effects. The association with DQ antibody-verified eplet mismatches was linear, without a safe threshold at which de novo DSA did not occur. Odds for T cell- or antibody-mediated rejection increased by 5% and 12%, respectively, per antibody-verified DQ eplet mismatch.

CONCLUSIONS

Eplet mismatches in HLA-DQ confer substantial risk for de novo DSA formation, graft rejection, and graft failure after kidney transplantation. Mismatches in other loci seem to have less effect. The results suggest that antibody-verified HLA-DQ eplet mismatch load could be used to guide personalized post-transplant immunosuppression. Adoption of molecular matching for DQA₁ and DQB₁ alleles could also help to minimize de novo DSA formation and potentially improve transplant outcomes.

HLA-EMMA: A user-friendly tool to analyse HLA class I and class II compatibility on the amino acid level

In renal transplantation, polymorphic amino acids on mismatched donor HLA molecules can lead to the induction of de novo donor‐specific antibodies (DSA), which are associated with inferior graft survival. To ultimately prevent de novo DSA formation without unnecessarily precluding transplants it is essential to define which polymorphic amino acid mismatches can actually induce an antibody response. To facilitate this, we developed a user‐friendly software program that establishes HLA class I and class II compatibility between donor and recipient on the amino acid level. HLA epitope mismatch algorithm (HLA‐EMMA) is a software program that compares simultaneously the HLA class I and class II amino acid sequences of the donor with the HLA amino acid sequences of the recipient and determines the polymorphic solvent accessible amino acid mismatches that are likely to be accessible to B cell receptors. Analysis can be performed for a large number of donor‐recipient pairs at once. As proof of principle, a previously described study cohort of 191 lymphocyte immunotherapy recipients was analysed with HLA‐EMMA and showed a higher frequency of DSA formation with higher number of solvent accessible amino acids mismatches. Overall, HLA‐EMMA can be used to analyse compatibility on amino acid level between donor and recipient HLA class I and class II simultaneously for large cohorts to ultimately determine the most immunogenic amino acid mismatches.

A curious case of de novo anti-HLA-C antibody-mediated humoral rejection and Fabry-like zebra bodies in a renal transplant recipient

Detection of donor-specific antibodies (DSA) is an essential part of diagnosing antibody-mediated renal allograft rejection (ABMR). The role of solitary preformed, or post-transplant HLA-C antigens in solid organ transplantation is unclear, due to the less sensitive nature of the historical assays, lack of data, low expression level on the cell surface, and their co-existence with other anti-HLA DSA. Herein, we present the case of a 39-year-old African American man, without prior history of pre-transplant sensitization that was diagnosed with biopsy-proven ABMR due to de novo donor-specific anti-HLA-C antibodies. This case report illustrates the role of HLA-C antibodies in causing ABMR if generated toward immunogenic-shared epitopes and demonstrates the need for their recognition in the pre- and post-transplant period. Another interesting aspect of this case is the incidental finding of Fabry-like zebra bodies, which we eventually determined to be of unclear etiology.

Identification of risk epitope mismatches associated with de novo donor-specific HLA antibody development in cardiothoracic transplantation

The development of de novo donor-specific HLA antibodies (dnDSA) after transplantation is associated with graft failure, mortality, and cost. There is no effective therapeutic intervention to prevent dnDSA or ameliorate associated injury. The aims of this study were to identify specific HLA factors associated with dnDSA development and to propose primary prevention strategies that could reduce the incidence of dnDSA without prohibitively limiting access to transplant. The investigation cohort included heart transplant recipients from 2008 to 2015 (n = 265). HLA typing was performed and HLA antibody testing was undertaken before and after transplantation. HLAMatchmaker analysis was performed for persistent dnDSA to identify potentially more immunogenic eplet differences. Validation was performed in recipients of lung transplants from 2008 to 2013 (n = 433). The majority of recipients with dnDSA had antibodies to identical eplet positions on DQ2 and DQ7. A high-risk epitope mismatch (found in DQA1*05 +  DQB1*02/DQB1*03:01(7)) was associated with a 4.2- and 4.9-fold increased risk of dnDSA in heart and lung recipients respectively. HLA electrostatic potential modeling provided a plausible explanation for this observed immunogenicity. A theoretical allocation algorithm avoiding high-risk epitope mismatches was generated and predicted to reduce dnDSA by up to 72% without additional testing, eplet analysis, or cost.

The long and winding road towards epitope matching in clinical transplantation

Recent data suggest that HLA epitope matching is beneficial for the prevention of de novo donor specific antibody (DSA) formation after transplantation. In this review, different approaches to predict the immunogenicity of an HLA mismatch will be discussed. The parameters used in these models are often called epitopes but the actual antibody epitope is far more complex. Exact knowledge of the antibody epitope is crucial if epitope matching is also used as a tool to select compatible donors for (highly) sensitized patients. Evidence is provided that it is not always possible to give an exact definition of an antibody epitope. We conclude that HLA "epitope" matching is superior over HLA antigen matching with respect to the prevention of de novo DSA formation and will enhance the prediction of acceptable HLA mismatches for sensitized patients. However, epitope matching at our current level of knowledge will not solve all histocompatibility problems as unexpected antibody reactivity still may occur.

Human Leukocyte Antigen Epitope Matching in Solid Organ Transplantation

HLA epitope matching provides a better approach to stratify patients at risk of developing antibody-mediated rejection compared with counting HLA mismatches. However, several immunologic parameters are not incorporated into these algorithms used to assess HLA epitopes, raising questions about the predictive value of these programs. Therefore, it is imperative to obtain more 3D structural data of antibody-antigen binding to "train" these computer algorithms. Also, mechanistic studies should be performed to prove these theoretic "epitopes." Most important, more information is needed to ensure these predictive computer algorithms are equitable and safe to use in clinical diagnostics before wide-scale implementation.

A subset of anti-HLA antibodies induces FcγRIIa-dependent platelet activation

HLA antibodies are associated with refractoriness to platelet transfusion, leading to rapid platelet clearance, sometimes coinciding with clinical side effects such as fever and chills. The presence of HLA antibodies is not always manifested by clinical symptoms. It is currently unclear why refractoriness to platelet transfusion is only observed in a subset of patients. Here, we utilized the availability of a unique panel of human monoclonal antibodies to study whether these were capable of activating platelets. Three out of eight human HLA-specific monoclonal antibodies induced activation of HLA-matched platelets from healthy donors as evidenced by enhanced α-granule release, aggregation, and α_IIbb₃ activation. The propensity of HLA monoclonal antibodies to activate platelets was independent of the HLA subtype to which they were directed, but was dependent on the recognized epitope. Activation was fully inhibited either by blocking FcγRIIa, or by blocking FcγRIIa-dependent signaling with Syk inhibitor IV. Furthermore, activation required the presence of the IgG-Fc part, as F(ab’)₂ fragments of HLA monoclonal antibodies were unable to induce platelet activation. Mixing experiments revealed that activation of platelets occurred in an intra-platelet dependent manner. Accordingly, a proportion of sera from refractory patients with HLA antibodies induced FcγRIIa-dependent platelet activation. Our data show that a subset of HLA antibodies is capable of crosslinking HLA and FcγRIIa thereby promoting platelet activation and enhancing these cells’ phagocytosis by macrophages. Based on these findings we suggest that FcγRIIa-dependent platelet activation may contribute to the decreased platelet survival in platelet-transfusion-dependent patients with HLA antibodies.

Correlation and agreement between eplet mismatches calculated using serological, low-intermediate and high resolution molecular human leukocyte antigen typing methods

Structural human leukocyte antigen (HLA) matching at the eplet level can be identified by HLAMatchmaker, which requires the entry of four-digit alleles. The aim of this study was to evaluate the agreement between eplet mismatches calculated by serological and two-digit typing methods compared to high-resolution four-digit typing. In a cohort of 264 donor/recipient pairs, the evaluation of measurement error was assessed using intra-class correlation to confirm the absolute agreement between the number of eplet mismatches at class I (HLA-A, -B, C) and II loci (HLA-DQ and -DR) calculated using serological or two-digit molecular typing compared to four-digit molecular typing methods. The proportion of donor/recipient pairs with a difference of >5 eplet mismatches between the HLA typing methods was also determined. Intra-class correlation coefficients between serological and four-digit molecular typing methods were 0.969 (95% confidence intervals [95% CI] 0.960–0.975) and 0.926 (95% CI 0.899–0.944), respectively; and 0.995 (95% CI 0.994–0.996) and 0.993 (95% CI 0.991–0.995), respectively between two-digit and four-digit molecular typing methods. The proportion of donor/recipient pairs with a difference of >5 eplet mismatches at class I and II loci was 4% and 16% for serological versus four-digit molecular typing methods, and 0% and 2% for two-digit versus four-digit molecular typing methods, respectively. In this small predominantly Caucasian population, compared with serology, there is a high level of agreement in the number of eplet mismatches calculated using two-compared to four-digit molecular HLA-typing methods, suggesting that two-digit typing may be sufficient in determining eplet mismatch load in kidney transplantation.

Novel aspects of epitope matching and practical application in kidney transplantation

This review describes the recent developments in the applicability and clinical significance of epitope matching in kidney transplantation. As incremental human leukocyte antigen (HLA) mismatches are associated with increased risk of rejection and allograft loss, HLA-matching remains one of the standard immunological triage tests to determine transplant suitability. Advancements in tissue-typing techniques have led to innovative concepts of HLA-matching at the epitope level. Epitopes are configurations of polymorphic amino acid residues that are recognized by B cells, and antibodies reactive with these epitopes lead to rejection and/or premature allograft loss. Even though there is substantial advancement in the characterization and understanding of epitopes, evidence supporting the added clinical benefit of epitope matching over traditional antigen matching and the ability to identify clinically relevant immunogenic epitopes remains poorly defined. We present an overview of the evidence surrounding the immunogenicity of mismatched donor epitopes and the applicability of HLA epitope matching in kidney transplantation. A better understanding of the immunogenicity and structural characteristics of HLA epitopes will guide clinicians to integrate epitope matching as an important parameter for donor selection in kidney transplantation.

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.

Humoral Reactivity of Renal Transplant-Waitlisted Patients to Cells From GGTA1/CMAH/B4GalNT2, and SLA Class I Knockout Pigs

BACKGROUND

Antipig antibodies are a barrier to clinical xenotransplantation. We evaluated antibody binding of waitlisted renal transplant patients to 3 glycan knockout (KO) pig cells and class I swine leukocyte antigens (SLA).

METHODS

Peripheral blood mononuclear cells from SLA identical wild type (WT), α1, 3-galactosyltransferase (GGTA1) KO, GGTA1/ cytidine monophosphate-N-acetylneuraminic acid hydroxylase (CMAH) KO, and GGTA1/ CMAH /b1,4 N-acetylgalactosaminyl transferase (B4GalNT2) KO pigs were screened for human antibody binding using flow cytometric crossmatch (FCXM). Sera from 820 patients were screened on GGTA1/CMAH/B4GalNT2 KO cells and a subset with elevated binding was evaluated further. FCXM was performed on SLA intact cells and GGTA1/SLA class I KO cells after depletion with WT pig RBCs to remove cell surface reactive antibodies, but leave SLA antibodies. Lastly, human and pig reactive antibodies were eluted and tested for cross-species binding and reactivity to single-antigen HLA beads.

RESULTS

Sequential glycan KO modifications significantly reduce antibody binding of waitlisted patients. Sera exhibiting elevated binding without reduction after depletion with WT RBCs demonstrate reduced binding to SLA class I KO cells. Human IgG, eluted from human and pig peripheral blood mononuclear cells, interacted across species and bound single-antigen HLA beads in common epitope-restricted patterns.

CONCLUSIONS

Many waitlisted patients have minimal xenoreactive antibody binding to the triple KO pig, but some HLA antibodies in sensitized patients cross-react with class I SLA. SLA class I is a target for genome editing in xenotransplantation.

The Humoral Theory of Transplantation: Epitope Analysis and the Pathogenicity of HLA Antibodies

Central to the humoral theory of transplantation is production of antibodies by the recipient against mismatched HLA antigens in the donor organ. Not all mismatches result in antibody production, however, and not all antibodies are pathogenic. Serologic HLA matching has been the standard for solid organ allocation algorithms in current use. Antibodies do not recognize whole HLA molecules but rather polymorphic residues on the surface, called epitopes, which may be shared by multiple serologic HLA antigens. Data are accumulating that epitope analysis may be a better way to determine organ compatibility as well as the potential immunogenicity of given HLA mismatches. Determination of the pathogenicity of alloantibodies is evolving. Potential features include antibody strength (as assessed by antibody titer or, more commonly and inappropriately, mean fluorescence intensity) and ability to fix complement (in vitro by C1q or C3d assay or by IgG subclass analysis). Technical issues with the use of solid phase assays are also of prime importance, such as denaturation of HLA antigens and manufacturing and laboratory variability. Questions and controversies remain, and here we review new relevant data.

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.

[New aspects of HLA compatibility in organ transplantation]

In organ transplantation, HLA compatibility between a donor and a recipient is currently assessed through the comparison of their HLA antigens and the sole count of incompatible HLA antigens. Similarly, antibodies were originally described as antigenic-specific. With solid phase assays detection of anti-HLA antibodies and crystallographic studies, it is now recognized that anti-HLA antibodies are not specific for antigens, but for epitopes, i.e. short polymorphic sequences of amino acids that are more often in positions accessible to allo-antibodies. These epitopes, due to the distribution of HLA molecules polymorphism, may be shared by different HLA antigens. This explains why an immunization towards a given HLA antigen can lead to synthesis of antibodies reactive towards other antigens sharing one or more epitopes. Similarly, structural comparison of the HLA molecules of a recipient and his donor defines the epitope load, i.e. the number of incompatible epitopes. This epitope load is correlated with the risk of developing antibodies specific for the donor after transplantation. New tools, such as the HLAMatchmaker software, allow to determine the epitopic load and to analyze the epitopic specificity of alloantibodies. These tools will possibly lead to rethink the method of graft allocation, or at least to take differently into account HLA compatibility in allocation algorythms.

Extracorporeal photopheresis in solid organ transplant-associated acute graft-versus-host disease

BACKGROUND

Extracorporeal photopheresis (ECP) culls pathogenic T lymphocytes, be these the clones of cutaneous T-cell lymphoma, or mediators of chronic graft-versus-host disease (GVHD) after allogeneic bone marrow transplantation (BMT-GVHD). Whether or not ECP may have an effect in the rarer instances of solid organ transplantation-associated GVHD (SOT-GVHD) is unclear. Mortality rates in SOT-GVHD rival those of transfusion-associated GVHD, with fatalities preceded by pancytopenia and peripheral blood chimerism (PBC) levels exceeding 20%. ECP has been described in two SOT-GVHD cases to date, with one surviving.

STUDY DESIGN AND METHODS

Clinicolaboratory features (including HLA relationships) in a case of multivisceral transplantation were reviewed from the time of surgery to the onset and progression of SOT-GVHD. ECP, which was introduced as a less immunosuppressive and more selective intervention, was assessed for its effect on serial PBC (as measured by short-tandem-repeat analysis) and clinical outcome.

RESULTS

Multivisceral SOT-GVHD manifested with erythroderma, neutropenic sepsis, and PBC increasing from 6% on Posttransplant Day (PTD) 38 to 78% by PTD 60 (at a doubling time of 6 days despite corticosteroids). ECP was administered on PTDs 62 and 67 and was associated with the first evidence of PBC decay to 67% on PTD 69. Death nevertheless ensued on the last day of salvage antithymocyte globulin (PTDs 69-73) despite further PBC reduction to 41%.

CONCLUSION

Further study is needed to determine if the sooner or more frequent application of ECP might attenuate the high case fatality rates of SOT-GVHD.

Identification of epitopes on HLA-DRB alleles reacting with antibodies in sera from women sensitized during pregnancy

This report describes a HLAMatchmaker-based antibody analysis of post-pregnancy sera with antibodies against child-specific HLA-DR epitopes. These sera were reactive in IgG-binding assays with single allele bead (SAB) panels on a Luminex platform. The antibody specificity analysis focused on DRB epitopes that have been recorded in the International HLA Epitope Registry (http://www.epregistry.com.br) as experimentally verified with informative antibodies but we also considered other eplets that predict potential epitopes. The SAB panel has in several instances two or more alleles corresponding to the same serologically defined DR antigen and we selected six sera were with different reactivity patterns with DR1, DR4, DR13 and/or DR52 alleles. We demonstrate here how amino acid differences between these alleles can provide useful information in the determination of new epitope specificities of antibodies in these sera. Eight newly antibody-verified epitopes were identified including three that correspond to eplets paired with self-residue configurations. Epitope specificity information appears to be useful in the prediction of mismatch acceptability of non-SAB alleles within serological DR antigen groups.

EpViX: A cloud-based tool for epitope reactivity analysis and epitope virtual crossmatching to identify low immunologic risk donors for sensitized recipients

One of the challenges facing solid organ transplantation programs globally is the identification of low immunological risk donors for sensitized recipients by HLA allele genotype. Because recognition of donor HLA alleles by host antibodies is at the core of organ rejection, the objective of this work was to develop a new version of the EpHLA software, named EpViX, which uses an HLAMatchmaker algorithm and performs automated epitope virtual crossmatching at the initiation of the organ donation process. EpViX is a free, web-based application developed for use over the internet on a tablet, smartphone or computer. This program was developed using the Ruby programming language and the Ruby-on-Rails framework. To improve the user experience, the EpViX software interface was developed based on the best human–computer interface practices. To simplify epitope analysis and virtual crossmatching, the program was integrated with important available web-based resources, such as OPTN, IMGT/HLA and the International HLA Epitope Registry. We successfully developed a program that allows people to work collaboratively and effectively during the donation process by accurately predicting negative crossmatches, saving time and other resources.

Antibody-reactive class I epitopes defined by pairs of mismatched eplets and self-eplets

The identification of human leukocyte antigen (HLA) antibodies in the sera of candidates awaiting organ transplantation has evolved over time. This has been possible because of the introduction of more sensitive techniques and to the increasing focus on the structural aspects of the HLA epitopes. The use of the HLAMatchmaker algorithm in the analysis of positive sera and the verification of HLA ABC epitopes in the HLA Epitope Registry website provide new stimuli on the interpretation of antibody reactivity. The epitopes defined by eplet pairs often involve a nonself-eplet and a self-eplet (nonself-self paradigm), suggesting that the antibody response to an HLA mismatch must have an auto-reactive component. Here, we report an application of the nonself-self paradigm that provides a basis for better knowledge and interpretation of HLA-antibody reactivity in Luminex assays with single alleles.