HLAMatchmaker-Based Analysis of Human Monoclonal Antibody Reactivity Demonstrates the Importance of an Additional Contact Site for Specific Recognition of Triplet-Defined Epitopes

Using single antigen specificity magnetic beads for the isolation of specific antibodies against HLA antigens

The presence of multiple donor-specific antibodies (DSAs) targeting HLA antigens poses a challenge to transplantation. Various techniques, including the use of recombinant cell lines and crossmatch cells have been developed to isolate DSAs. To simplify the extraction of HLA-specific DSAs from complex sera, we introduced magnetic beads with single HLA specificity (MagSort). Sera were treated with MagSort, allowing HLA-specific antibodies to bind to the beads, and these specific antibodies were subsequently eluted. MagSort beads, coated with 59 different HLA variants, underwent testing through 1329 adsorption/elution processes, demonstrating their effectiveness and specificity in adsorbing and eluting HLA-specific antibodies. The MagSort method proves comparable to the cell method, showing similar isolated antibody binding patterns. The isolated antibody binding patterns from MagSort reveal both known eplets and unknown patterns, suggesting its utility for eplet discovery. Additionally, MagSort proved effective in extracting signals for flow cytometry cross-matching, offering a means to assess the binding capability of isolated antibodies against specific donor cells.

HLA antibody affinity determination: From HLA-specific monoclonal antibodies to donor HLA specific antibodies (DSA) in patient serum

Organs transplanted across donor-specific HLA antibodies (DSA) are associated with a variety of clinical outcomes, including a high risk of acute kidney graft rejection. Unfortunately, the currently available assays to determine DSA characteristics are insufficient to clearly discriminate between potentially harmless and harmful DSA. To further explore the hazard potential of DSA, their concentration and binding strength to their natural target, using soluble HLA, may be informative. There are currently a number of biophysical technologies available that allow the assessment of antibody binding strength. However, these methods require prior knowledge of antibody concentrations. Our objective within this study was to develop a novel approach that combines the determination of DSA-affinity as well as DSA-concentration for patient sample evaluation within one assay. We initially tested the reproducibility of previously reported affinities of human HLA-specific monoclonal antibodies and assessed the technology-specific precision of the obtained results on multiple platforms, including surface plasmon resonance (SPR), bio-layer interferometry (BLI), Luminex (single antigen beads; SAB), and flow-induced dispersion analysis (FIDA). While the first three (solid-phase) technologies revealed comparable high binding-strengths, suggesting measurement of avidity, the latter (in-solution) approach revealed slightly lower binding-strengths, presumably indicating measurement of affinity. We believe that our newly developed in-solution FIDA-assay is particularly suitable to provide useful clinical information by not just measuring DSA-affinities in patient serum samples but simultaneously delivering a particular DSA-concentration. Here, we investigated DSA from 20 pre-transplant patients, all of whom showed negative CDC-crossmatch results with donor cells and SAB signals ranging between 571 and 14899 mean fluorescence intensity (MFI). DSA-concentrations were found in the range between 11.2 and 1223 nM (median 81.1 nM), and their measured affinities fall between 0.055 and 24.7 nM (median 5.34 nM; 449-fold difference). In 13 of 20 sera (65%), DSA accounted for more than 0.1% of total serum antibodies, and 4/20 sera (20%) revealed a proportion of DSA even higher than 1%. To conclude, this study strengthens the presumption that pre-transplant patient DSA consists of various concentrations and different net affinities. Validation of these results in a larger patient cohort with clinical outcomes will be essential in a further step to assess the clinical relevance of DSA-concentration and DSA-affinity.

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.

Two Human Monoclonal HLA-Reactive Antibodies Cross-React with Mamu-B*008, a Rhesus Macaque MHC Allotype Associated with Control of Simian Immunodeficiency Virus Replication

MHC class I molecules play an important role in adaptive immune responses against intracellular pathogens. These molecules are highly polymorphic, and many allotypes have been characterized. In a transplantation setting, a mismatch between MHC allotypes may initiate an alloimmune response. Rhesus macaques (Macaca mulatta, Mamu) are valuable as a preclinical model species in transplantation research as well as to evaluate the safety and efficacy of vaccine candidates. In both lines of research, the availability of nonhuman primate MHC-reactive mAbs may enable in vitro monitoring and detection of presence of particular Mamu molecules. In this study, we screened a collection of thoroughly characterized HLA class I-specific human mAbs for cross-reactivity with rhesus macaque MHC class I allotypes. Two mAbs, OK4F9 and OK4F10, recognize an epitope that is defined by isoleucine (I) at amino acid position 142 that is present on the Indian rhesus macaque Mamu-B*008:01 allotype, which is an allotype known to be associated with elite control of SIV replication. The reactive pattern of a third mAb, MUS4H4, is more complex and includes an epitope shared on Mamu-A2*05:01 and -B*001:01-encoded Ags. This is the first description, to our knowledge, of human HLA-reactive mAbs that can recognize Mamu allotypes, and these can be useful tools for in vitro monitoring the presence of the relevant allelic products. Moreover, OK4F9 and OK4F10 can be powerful mAbs for application in SIV-related research.

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.

Novel insights into non-HLA alloimmunity in kidney transplantation

Recognition of non‐self structures on donor cells represents the main immunological barrier in solid organ transplantation. The human leukocyte antigens (HLA) are considered the most important non‐self (allo)antigens in transplantation. Long‐term graft attrition is mainly caused by the formation of alloreactive antibodies that are directed against non‐self structures (i.e., epitopes) on cell surface proteins. Recently published data provided evidence for a similar importance of non‐HLA mismatches between donors and recipients in acute rejection as well as long‐term kidney allograft survival. These data suggest a broader concept of immunological non‐self that goes beyond HLA incompatibility and expands the current concept of polymorphic non‐self epitopes on cell surface molecules from HLA to non‐HLA targets. Amino acid substitutions caused by single nucleotide variants in protein‐coding genes or complete loss of gene expression represent the basis for polymorphic residues in both HLA and non‐HLA molecules. To better understand these novel insights in non‐HLA alloimmunity, we will first review basic principles of the alloimmune response with a focus on the HLA epitope concept in donor‐specific antibody formation before discussing key publications on non‐HLA antibodies.

Recombinant human monoclonal HLA antibodies of different IgG subclasses recognising the same epitope: Excellent tools to study differential effects of donor-specific antibodies

In the field of transplantation, the humoural immune response against mismatched HLA antigens of the donor is associated with inferior graft survival, but not in every patient. Donor‐specific HLA antibodies (DSA) of different immunoglobulin G (IgG) subclasses may have differential effects on the transplanted organ. Recombinant technology allows for the generation of IgG subclasses of a human monoclonal antibody (mAb), while retaining its epitope specificity. In order to enable studies on the biological function of IgG subclass HLA antibodies, we used recombinant technology to generate recombinant human HLA mAbs from established heterohybridomas. We generated all four IgG subclasses of a human HLA class I and class II mAb and showed that the different subclasses had a comparable affinity, normal human Fc glycosylation, and retained HLA epitope specificity. For both mAbs, the IgG1 and IgG3 isotypes were capable of binding complement component 3d (C3d) and efficient in complement‐dependent cell lysis against their specific targets, while the IgG2 and IgG4 subclasses were not able to induce cytotoxicity. Considering the fact that the antibody‐binding site and properties remained unaffected, these IgG subclass HLA mAbs are excellent tools to study the function of individual IgG subclass HLA class I and class II‐specific antibodies in a controlled fashion.

Mapping and definition of HLA class I and II serologic epitopes using an unbiased reverse engineering strategy

Current models describing HLA epitopes are both theoretical and empirical. Each has limitations yielding discordant results and increasingly complex modeling. The models make a priori assumptions that epitopes must be present only on the mature protein, solvent accessible, on the 'top' (peptide binding surface) of the molecule, restricted to the same class as the antibody, and in the same position on the target allele if reactive to more than one locus. Results obtained counter to these assumptions are routinely discounted. For the 17th International Histocompatibility and Immunogenetics Workshop, we developed a reverse engineering algorithm to define epitopes without these assumptions on a cohort of 332 primary transplant pairs. Complete NGS typing of the transcribed (including leader) genomic DNA for 11 HLA loci of donor and recipient and DSA assignment by single antigen beads was performed. Our results show that, when grouped by 16 class I and II allele specific DSA, uniform clusters and 172 specific amino acid target epitopes are recognized by recipients despite originating from disparate HLA pairs. Data also show that these targets can be in the leader, alpha 3, transmembrane and cytoplasmic domains, thus calling into question current assumptions regarding immunogenic epitopes. Comparisons of amino acid epitopes defined by the Terasaki and Duquesnoy groups (TerEp and EpRegistry) are given.

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.

Usefulness of the ElliPro epitope predictor program in defining the repertoire of HLA-ABC eplets

HLA matching at the epitope level offers new opportunities to identify suitable donors for transplant patients. The International HLA Epitope Registry (www.Epregistry.com.br) describes for the various HLA loci, repertoires of eplets including those that correspond to epitopes experimentally verified with specific antibodies. There are also many eplets which have remained as theoretical entities because no informative antibodies have been found. Which of them have immunogenic potential or conversely, might be considered as non-epitopes that cannot elicit specific antibody responses? This question is important for the application of epitope-based HLA matching in clinical transplantation. Correct predictions of B-cell epitopes on antigenic proteins are essential to the effective design of microbial vaccines and the development of specific antibodies used in immunotherapy and immunodiagnostics but prediction programs based on structural and physiochemical properties of amino acid residues are generally ineffective. Recent prediction programs based on three-dimensional structures of antigen-antibody complexes are more promising. One such program is called ElliPro developed by Ponomarenko. This report describes studies demonstrating that ElliPro can predict alloantibody responses to HLA-ABC eplets. Antibody-verified eplets have amino acid residues with much higher ElliPro scores than eplets for which no specific antibodies have been found. The latter group includes residues with very low ElliPro scores; they appear to represent eplets that might be classified as non-epitopes. In conclusion, ElliPro offers a new approach to characterize epitope repertoires that are clinically relevant in HLA matching.

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.

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.

Usefulness of the Nonself-Self Algorithm of HLA Epitope Immunogenicity in the Specificity Analysis of Monospecific Antibodies Induced during Pregnancy

BACKGROUND

HLAMatchmaker is a program to analyze the epitope specificities of HLA antibodies. It considers each HLA allele as a string of eplets. Intralocus and interlocus comparisons between donor and recipient alleles offer a structural assessment of compatibility and an analysis of allele panel reactivity patterns can generate information about epitope specificities of HLA antibodies. However, HLAMatchmaker cannot always generate conclusive interpretations of reactivity patterns of all monospecific antibodies, which by definition recognize single epitopes.

HYPOTHESIS

We have therefore developed a new antibody analysis approach that utilizes the nonself-self algorithm of HLA epitope immunogenicity. It is based on the concept that HLA antibodies originate from B-cells with immunoglobulin receptors to self-HLA epitopes on one given allele and which can be activated by epitopes defined by a few nonself residue differences whereas the remainder of the structural epitope of the immunizing allele consists of self residues.

METHODS

Three human monoclonal class I antibodies from HLA typed women sensitized during pregnancy were tested in Ig-binding assays with single alleles on a Luminex platform.

FINDINGS

Three new HLA epitopes were identified; they are defined by combinations of nonself- and self-residues for one allele of the antibody producer.

CONCLUSION

The nonself-self paradigm of HLA epitope immunogenicity offers a second approach to analyze HLA antibody specificities.

HLA epitopes as viewed by antibodies: what is it all about?

The need for new approaches to define HLA antibodies, in the context of organ transplantation, is intensely debated among HLA professionals. In this review, we sought to provide background and perspective to current understanding of the immunogenicity of HLA mismatches with respect to the humoral alloimmune response and the definition of B cell epitopes. Initial data suggest that epitope matching not only assists in defining better matches for the current transplant, but also minimizes the risk of developing de novo HLA-donor-specific-antibodies posttransplant. In other words, other than lowering the risk of current graft rejection, epitope matching is likely to lower overall future sensitization levels and thus increases the likelihood of finding a compatible donor when the need for a retransplantation arises. More detailed knowledge of epitopes makes it possible to investigate what constitutes permissible versus non-permissible HLA mismatches. The currently available evidence suggest that epitope matching is the most rational way to decrease the risk of HLA-linked transplant rejection. This review is aimed at stimulating further and more intense collaborative effort in this field.

Human leukocyte antigen epitope antigenicity and immunogenicity

PURPOSE OF REVIEW

Human leukocyte antigen (HLA) antibodies are now recognized as being specific for epitopes which can be defined structurally with amino acid differences between HLA alleles. This article addresses two general perspectives of HLA epitopes namely antigenicity, that is their reactivity with antibody and immunogenicity, that is their ability of eliciting an antibody response.

RECENT FINDINGS

Single-antigen bead assays have shown that HLA antibodies recognize epitopes that are equivalent to eplets or corresponding to eplets paired with other residue configurations. There is now a website-based Registry of Antibody-Defined HLA Epitopes (http://www.epregistry.com.br). Residue differences within eplet-defined structural epitopes may also explain technique-dependent variations in antibody reactivity determined in Ig-binding, C1q-binding and lymphocytotoxicity assays.HLA antibody responses correlate with the numbers of eplets on mismatched HLA antigens, and the recently proposed nonself-self paradigm of epitope immunogenicity may explain the production of epitope-specific antibodies.

SUMMARY

These findings support the usefulness of HLA matching at the epitope level, including the identification of acceptable mismatches for sensitized patients and permissible mismatching for nonsensitized patients aimed to reduce HLA antibody responses.

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

The International Registry of Antibody-Defined HLA Epitopes ( http://www.epregistry.com.br) has been recently established as a tool to understand humoral responses to human leukocyte antigen (HLA) mismatches. These epitopes are defined structurally by three-dimensional molecular modeling and amino acid sequence differences between HLA antigens. So-called eplets represent essential components of HLA epitopes and they are defined by polymorphic residues. A major goal is to identify HLA epitopes that have been verified experimentally with informative antibodies. Our analysis has also included data in many publications. As of 1 November 2013, 95 HLA-ABC antibody-verified epitopes have been recorded, 62 correspond to eplets and 33 are defined by eplets paired with other residue configurations. The Registry is still a work-in-progress and will become a useful resource for HLA professionals interested in histocompatibility testing at the epitope level and investigating antibody responses to HLA mismatches in transplant patients.

Epitope analysis of HLA-DQ antigens: what does the antibody see?

BACKGROUND

Human leukocyte antigen (HLA)-DQ has emerged as the alloantibody most frequently associated with the generation of de novo donor-specific antibody (DSA), antibody-mediated-rejection, and unfavorable transplantation outcome.

METHODS

The generation of HLA-DQ de novo DSA was interrogated in 40 transplant recipients who were immunologically naive before their failed transplantation. Eplet and epitope analyses were performed using HLAMatchmaker and Cn3D software.

RESULTS

Ten DQA and thirteen DQB eplets or eplet combinations were identified. All but one revealed an epitope footprint that includes both the DQα and DQβ chains. Four examples are illustrated in detail, representing a range of different epitope landscapes. A disparity between antigen density and mean fluorescence intensity values for some alleles within an eplet group was noted, with mean fluorescence intensity values of the lowest fluorescence bead being one tenth of the highest fluorescence bead, despite the fact that the amount of antigen on these beads were not significantly different.

CONCLUSION

Our data support the need for changing the manner in which HLA-DQ antigens and antibodies are evaluated for organ transplantation. The current nomenclature system does not reflect the true nature of HLA-DQ polymorphism. Moreover, epitope immunogenicity likely involves more than the mere presence of a specific eplet. Because our field contemplates the use of epitope matching as an approach to improve organ allocation and overall outcomes, it is imperative to have accurate characterization of the immunogenicity of each epitope. This will pave the way to identifying acceptable mismatches and will allow risk stratification for generating de novo HLA-DSA after transplantation.

HLA epitope based matching for transplantation

As important risk factors for transplant rejection and failure, HLA antibodies are now recognized as being specific for epitopes which can be defined structurally with amino acid differences between HLA alleles. Donor-recipient compatibility should therefore be assessed at the epitope rather than the antigen level. HLAMatchmaker is a computer algorithm that considers each HLA antigen as a series of small configurations of polymorphic residues referred to as eplets as essential components of HLA epitopes. It includes epitopes on antigens encoded by all HLA-A, B, C, DR, DQ and DP loci as well as MICA. HLA epitopes have two characteristics namely antigenicity, i.e. the reactivity with antibody and immunogenicity, i.e. the ability of eliciting an antibody response. This article addresses the relevance of determining epitope-specificities of HLA antibodies, the effect of epitope structure on technique-dependent antibody reactivity and the identification of acceptable mismatches for sensitized patients considered for transplantation. Permissible mismatching for non-sensitized patients aimed to prevent or reduce HLA antibody responses could consider epitope loads of mismatched antigens and the recently developed nonself-self paradigm of epitope immunogenicity.

Anti-HLA-A, -B, -DR, -DQB1 and -DQA1 antibodies reactive epitope determination with HLAMatchmaker in multipare awaiting list for heart transplant

Human leukocyte antigen (HLA) antibodies represent a significant risk factor for transplant failure. It is very important to characterize anti-HLA antibodies as epitopes rather than antigens so that this knowledge can be applied clinically. The aim of the study was to investigate the extra reactivity patterns in sensitized multipare. Here, we have used the HLAMatchmaker program, a theoretical algorithm, to explain these unexpected antibody reactivity patterns in multipare awaiting for heart transplant. The patient was sensitized during pregnancy by alleles HLA-A(*)24:02, HLA-DRB1(*)07:01, HLA-DRB4(*)01:01, DQB1(*)02:02 and DQA1(*)02:01 mismatches with development of respective antibodies. However, the patient' sera were shown an unexpected reactivity not directed toward HLA mismatches of daughters: A(∗)23:01, A(*)24:03 and B(*)15:12 for class I and DRB4(*)01:03, DRB1(*)09:02, DRB1(*)09:01, DQB1(*)03:01, DQB1(*)03:03, DQB1(*)03:02, DQB1(*)04:02, DQB1(*)04:01 and DQB1(*)02:01 for class II. By HLAMatchmaker analysis we found that these antibodies reacted with eplet shared by antigens in single allele Luminex panels. These eplets were: 62EE, 66GKH, 70KAH, 71HS, 127K, 113YH, 144KR, 150AAH, 151AHV, 163TG and 167DG for class I and 4Q, 74RRAE, 71RRA, 98KN, 120N, and 135G, 25FT, 34HE, 41ER, 47EK2, 48LF for class II. Thus, HLAMatchmaker software together with to solid phase techniques could open new horizons for a more precise characterization of the HLA-antibodies.

Structural aspects of HLA class I epitopes reacting with human monoclonal antibodies in Ig-binding, C1q-binding and lymphocytotoxicity assays

This study addresses the reactivity patterns of human cytotoxic HLA class I epitope-specific monoclonal antibodies in Ig-binding and complement component C1q-binding Luminex assays in comparison with complement-dependent lymphocytotoxicity data reported at the 13th International HLA Workshop. Some monoclonal antibodies reacted similarly with epitope-carrying alleles in all three assays but others showed different reactivity patterns. These reactivity differences were analyzed with HLAMatchmaker and we incorporated the concept that eplets are essential parts of structural epitopes which can contact the six Complementarity Determining Regions (CDRs) of antibody. The data show that technique-dependent reactivity patterns are associated with distinct differences between polymorphic amino acid configurations on eplet-defined structural epitopes. The findings have been viewed in context of antigen-antibody complex formation that results in the release of free energy necessary to stabilize binding and to induce conformational changes in the antibody molecule to expose the C1q binding site, the first step of complement activation. Moreover the amount of free energy should be sufficient to induce a conformational change of C1q thereby initiating the first stages of the classical complement cascade leading to lymphocytotoxicity. The complement-fixing properties of HLA antibodies require not only specific recognition of eplets but also depend on interactions of other CDRs with critical amino acid configurations within the structural epitope. Eplet-carrying alleles that lack such configurations may only bind with antibody. This concept is important to our understanding whether or not complement-fixing donor-specific HLA antibodies can initiate antibody-mediated rejection.

16th IHIW: a website for antibody-defined HLA epitope Registry

The concept that HLA antibodies are specific for epitopes rather than HLA antigens is important not only for the determination of mismatch acceptability for sensitized patients but also for a better understanding of the antibody response to an HLA mismatch. Numerous publications describe epitope-specific antibodies, but there is no standardized information about the repertoire of clinically relevant HLA epitopes. Under auspices of the 16th IHIW, we have developed a website-based registry of antibody-verified HLA epitopes. Epitope notations are based on HLA molecular modelling of amino acid residues in polymorphic sequence positions. Informative epitope-specific antibodies had been induced by a transplant, transfusion or pregnancy and were monoclonal antibodies or eluates of sera absorbed with single HLA alleles. Antibody reactivity was determined in binding assays with single-allele panels. Antibody producer/immunizer HLA types enhanced the characterization of specific epitopes. The Registry also includes epitopes described in original research publications. Based on the extent of antibody reactivity information, we assigned epitope status as confirmed (well documented) or provisional (more data are needed). At present, the Registry has 69 HLA-ABC, 53 DRB1/3/4/5, 17 DQ, 8 DP and 22 MICA antibody-verified epitopes and will be updated on a quarterly basis. Laboratories worldwide continue to submit data about previously unreported antibody-specific epitopes. For each epitope, the website shows its amino acid composition and HLA alleles that share the epitope. Links show antibody reactivity patterns, sensitization information and references. Other links show molecular modelling of corresponding structural epitopes and polymorphic residue information for epitope-carrying alleles. The website will also have a link to epitope frequency information in different populations. Search functions will list mismatched epitopes on mismatched alleles for selected HLA types. The HLA Epitope Registry will become a valuable resource for researchers interested in HLA compatibility at the epitope level and investigating antibody responses to HLA mismatches.

Structural aspects of human leukocyte antigen class I epitopes detected by human monoclonal antibodies

This study addresses the concept that human leukocyte antigen (HLA) class I-specific alloantibodies are specific for epitopes that correspond to HLAMatchmaker-defined eplets. Eplets are essential parts of so-called structural epitopes that make contact with the 6 complementarity determining regions of an antibody. From published molecular models of crystallized protein antigen-antibody complexes, we have calculated that contact residues on structural HLA epitopes should reside within a 15-Å radius of a mismatched eplet. This study addresses the structural basis of high-frequency HLA class I epitopes reacting with human monoclonal antibodies (mAbs) derived from women sensitized during pregnancy. All mAbs were tested in Luminex assays with single HLA allele panels. The HLAMatchmaker algorithm was used to determine their specificity in context with eplet sharing between the immunizing allele and antibody-reactive alleles. To assess the autoreactive B cell origin of these antibodies, we have applied the recently developed nonself-self paradigm of epitope immunogenicity to analyze residue differences between the immunizer and the alleles of the antibody producer. A total of 9 mAbs were specific for epitopes associated with the 41T, 80NRG, 163LW, 69AA, or 80ERILR eplets. In each case, the immunizing allele had within 15 Å of the mismatched eplet, no residue differences with 1 of the alleles of the antibody producer. This observation is consistent with the concept that these mAbs originated from B cells with self HLA immunoglobulin receptors. Eplet-carrying alleles exhibited different levels of reactivity, which, when compared with the immunizing allele, ranged from high to intermediate to very low. In many cases, lower reactivities were associated with differences from self to nonself residues in surface locations within 15 Å of the specific eplet. Apparently, such locations may serve as critical contact sites for the antibody. In other cases, other residue differences did not appear to affect binding with the antibody, suggesting that these locations do not play a major role in antibody binding. For these mAbs we did not obtain convincing evidence that residue differences in hidden positions below the molecular surface had significant effects on antibody binding. These findings have increased our understanding of the structural basis of the immunogenicity and antigenicity of HLA class I epitopes and provide a basis for interpreting HLA antibody reactivity patterns in Luminex assays with single alleles.

Humoral alloimmunity in transplantation: relevance of HLA epitope antigenicity and immunogenicity

HLA mismatching is an important risk factor for antibody-mediated rejection and transplant failure. With the realization HLA antibodies recognize epitopes rather than antigens, it has become apparent that donor-recipient compatibility should be assessed at the epitope level. Recent developments have increased our understanding of the structural basis of HLA antigenicity, i.e., the reactivity with specific antibody and, immunogenicity, i.e., the ability to induce an antibody response. HLAMatchmaker is a computer algorithm that considers each HLA antigen as a series of small configurations of polymorphic residues referred to as eplets as essential components of HLA epitopes. This article addresses the relevance of determining epitope-specificities of HLA antibodies in the identification of acceptable mismatches for sensitized patients considered for transplantation. Permissible mismatching for non-sensitized patients aimed to prevent or reduce HLA antibody responses could consider epitope loads of mismatched antigens and the recently developed non-self–self paradigm of epitope immunogenicity.

The antibody response to an HLA mismatch: a model for nonself-self discrimination in relation to HLA epitope immunogenicity

Antibodies to HLA mismatches are specific for epitopes rather than antigens. HLAMatchmaker considers each HLA antigen as a string of eplets that represent key elements of epitopes. Certain antibodies are specific for single eplets, but recent studies have demonstrated that epitopes defined by eplet pairs always involve one nonself-eplet and a self-eplet shared between the immunizing antigen and the antibody producer. This suggests an autoreactive component of the alloantibody response to an HLA mismatch and this report expands this concept. During B-cell development, V(H) and V(L) gene rearrangements produce a diversity of Ig receptors that can recognize epitopes on autologous proteins. It is hypothesized that B cells carry low-affinity receptors for self-HLA antigens. Their interactions with self-HLA proteins will not lead to B-cell activation or antibody production. In contrast, exposure to HLA mismatches induces often strong alloantibody responses. The activation of self-HLA-specific B cell by a nonself-eplet may require that the remainder of the structural epitope of the immunizing antigen has considerable structural similarity with one of the antibody producer's alleles. This hypothesis has been tested in molecular modelling studies with six epitopes defined by human monoclonal antibodies. In each case, one allele of the antibody producer had no or few differences with the immunizing allele in antibody-accessible positions defined by a 15 Ångstrom radius of the mismatched eplet. The other alleles of the antibody producer showed significantly greater numbers of residue differences with the immunizer (5.8 ± 2.0 versus 1.0 ± 0.6, P < 0.0001). These data support the concept that HLA antibodies originate from B cells with self-HLA immunoglobulin receptors that recognize mismatched eplets as nonself entities on immunizing antigens. The nonself-self paradigm provides a new insight of HLA epitope immunogenicity and may explain why sensitized patients have antibodies to a restricted number of mismatched epitopes.

High-resolution, three-dimensional modeling of human leukocyte antigen class I structure and surface electrostatic potential reveals the molecular basis for alloantibody binding epitopes

The potential of human leukocyte antigens (HLA) to stimulate humoral alloimmunity depends on the orientation, accessibility and physiochemical properties of polymorphic amino acids. We have generated high-resolution structural and physiochemical models of all common HLA class I alleles and analyzed the impact of amino acid polymorphisms on surface electrostatic potential. Atomic resolution three-dimensional structural models of HLA class I molecules were generated using the MODELLER computer algorithm. The molecular surface electrostatic potential was calculated using the DelPhi program. To confirm that electrostatic surface topography reflects known HLA B cell epitopes, we examined Bw4 and Bw6 and ascertained the impact of amino acid polymorphisms on their tertiary and physiochemical composition. The HLA protein structures generated performed well when subjected to stereochemical and energy-based testing for structural integrity. The electrostatic pattern and conformation of Bw4 and Bw6 epitopes are maintained among HLA molecules even when expressed in a different structural context. Importantly, variation in epitope amino acid composition does not always translate into a different electrostatic motif, providing an explanation for serologic cross-reactivity. Mutations of critical amino acids that abrogate antibody binding also induce distinct changes in epitope electrostatic properties. In conclusion, high-resolution structural modeling provides a physiochemical explanation for serologic patterns of antibody binding and provides novel insights into HLA immunogenicity.

Human monoclonal antibody reactivity with human leukocyte antigen class I epitopes defined by pairs of mismatched eplets and self-eplets

AIM

Humoral sensitization affects transplant outcome, and it is now apparent that human leukocyte antigen (HLA) antibodies are specific for epitopes rather than antigens. Such epitopes can be structurally defined by HLAMatchmaker, an algorithm that considers eplets as critical elements of epitopes recognized by alloantibodies. This study addressed the question how mismatched HLA antigens induce specific antibodies in context with eplet differences with the antibody producer.

METHODS

HLA class I-specific human monoclonal antibodies derived from women sensitized during pregnancy were tested in Luminex assays with single allele panels. Their epitope specificity was determined from reactivity patterns and eplet differences between immunizing antigen and the antibody producer.

RESULTS

This study focuses on the reactivity patterns of 10 monoclonal antibodies specific for epitopes defined by a mismatched eplet paired with a self-eplet shared between immunizing HLA antigens and HLA antigens of the antibody producer. The eplets in these pairs are between 7 and 16 Å apart, a sufficient distance for contact by two separate complementarity-determining regions of antibody.

CONCLUSIONS

These findings demonstrate that immunizing antigens have mismatched eplets that can form antibody-reactive epitopes with self-configurations on the molecular surface. They seem to suggest that HLA antibodies can be produced by autoreactive B cells that have undergone receptor editing to accommodate the recognition of nonself-eplets, the driving force of the humoral alloresponse. This concept enhances our understanding of structural epitope immunogenicity and the interpretation of antibody reactivity patterns with HLA panels.

Detection of antibodies against HLA-C epitopes in patients with rejected kidney transplants

BACKGROUND

Although HLA-C matching is not considered in kidney transplantation, several reports have shown that anti-HLA-C antibodies are associated with rejection and graft failure. DNA-based typing methods can now accurately determine HLA-C compatibility and sensitive assays such as Luminex with single alleles can identify HLA-C antibodies. HLA-C displays considerable amino acid polymorphism that can be translated into a structurally defined epitope repertoire.

METHODS

We have analyzed post-allograft nephrectomy sera from 45 HLA-C mismatched cases submitted by 15 laboratories worldwide participating in the 15th International Histocompatibility Workshop. All of them had HLA class I antibodies detected by a Luminex-based solid phase method using single-allele beads. This study addressed the determination of antibodies against donor HLA-C mismatches. Analysis of antibody reactivity patterns was performed using HLAMatchmaker, a structurally based matching program that considers 56 HLA-C eplets to define antibody-reactive epitopes. Many eplets shared by groups of HLA-C antigens, whereas others are also shared with HLA-A and/or HLA-B antigens.

RESULTS

Twenty-seven patients (60%) had donor-specific HLA-C antibodies, significantly less than the donor-specific antibodies induced by HLA-A and HLA-B mismatches. HLA-C antibody responses correlated with the eplet loads of the HLA-C mismatches. There were 352 instances whereby a donor HLA-C eplet was mismatched and for 84 (24%) of them there was antibody reactivity with a particular eplet (69 instances) or an eplet pair (15 instances). The latter generally consisted of mismatched eplets paired with self-eplets shared between the immunizing HLA-C alleles and HLA alleles of the patient. Several HLA-C eplets exhibited a relatively high immunogenicity as evidenced by their frequencies of specific antibodies.

CONCLUSION

These findings demonstrate the importance of HLA-C mismatching in humoral sensitization and that HLA-C epitopes can induce specific antibodies. They illustrate the usefulness of HLAMatchmaker in understanding donor-specific antibody reactivity patterns and the determination of HLA mismatch acceptability when transplantation is considered.

Human monoclonal HLA antibodies reveal interspecies crossreactive swine MHC class I epitopes relevant for xenotransplantation

Crossreactivity of anti-HLA antibodies with SLA alleles may limit the use of pig xenografts in some highly sensitized patients. An understanding of the molecular basis for this crossreactivity may allow better selection of xenograft donors. We have tested 68 human monoclonal HLA class I antibodies (mAbs) for reactivity with pig lymphocytes from SLA defined pigs and found nine to be crossreactive. Eight of nine were broadly HLA reactive IgM-mAbs. The putative HLA epitopes for seven mAbs. were conserved in the aminoacid sequence of the SLA alleles studied. The lack of reactivity of a large number of mAbs largely correlated with the absence of the putative epitopes in the SLA alleles studied. We conclude that most patients with anti-HLA class I antibodies should be able to find pig donors lacking SLA antigens that cross react with their antibodies and that many of the crossreacting epitopes can be defined by analysis of shared epitopes in the aminoacid sequence of human and pig MHC antigens.

Correlations between Terasaki's HLA class II epitopes and HLAMatchmaker-defined eplets on HLA-DR and -DQ antigens

Human leukocyte antigen (HLA) class II-specific antibodies increase the risk of transplant failure, and their characterization must consider epitopes rather than antigens. There are two strategies to determine HLA epitope structure. Terasaki's group has analyzed antibody reactivity patterns with single antigen panels with a computer program based on shared amino acid residues of reactive alleles. HLAMatchmaker is a theoretical algorithm that predicts HLA epitopes on the HLA molecular surface from stereochemical modeling of epitope-paratope interfaces of antigen-antibody complexes. Our epitope repertoire is based on so-called 'eplets' representing 3-A patches of at least one polymorphic residue on the molecular surface. This report describes how 49 of 53 Terasaki's HLA-DR epitopes correspond to HLAMatchmaker-defined eplets. Most of them are equivalent to single eplets (n = 33) or two or more possible eplets (n = 10), but six had corresponding eplet pairs. There were 10 cases whereby eplets have permissible residue combinations, and in 5 cases, we found that eplet specificity might be influenced by nearby hidden residues. We could assign corresponding eplets to 17 of 18 Terasaki's HLA-DQ epitopes. This study demonstrates how the HLAMatchmaker interpretation of amino acid residues shared between antibody-reactive antigens can increase our understanding of the structural basis of HLA epitopes.

Correlations between Terasaki's HLA class I epitopes and HLAMatchmaker-defined eplets on HLA-A, -B and -C antigens

Although the determination of human leukocyte antigen (HLA) antibody specificity has traditionally been directed toward HLA antigens, there is now increasing attention to structurally defined HLA epitopes. An understanding of the HLA epitope repertoire is important to acceptable mismatching for sensitized patients and to a new epitope-based matching algorithm aimed to reduce antibody-mediated rejection. There are two strategies to determine the HLA epitope repertoire. Terasaki's group has used an empirical method to analyze the reactivity of single allele Luminex panels with mouse monoclonal antibodies (mAbs) and absorbed/eluted alloantibodies with a computer program based on shared residues in the amino acid sequences of reactive alleles. HLAMatchmaker is a theoretical algorithm that predicts HLA epitopes on the HLA molecular surface from stereochemical modeling of epitope-paratope interfaces of antigen-antibody complexes. Our epitope repertoire is based on so-called 'eplets' representing 3-A patches of at least one polymorphic residue on the molecular surface. A comparative analysis has shown that 81/103 Terasaki's HLA class I epitopes are equivalent to individual eplets (n = 50) or pairs of eplets (n = 31) separated far enough to serve as potential contact sites for two complementarity-determining regions of antibody. An additional 12 Terasaki's epitopes (TerEps) correspond to eplets with permissible residue combinations that do not seem to affect epitope specificity. We could not identify corresponding eplets for the remaining 10 TerEps, including 8 that might be considered xeno-epitopes defined by mouse mAbs. Conversely, HLAMatchmaker has 38 additional eplets in well-exposed surface positions that do not have equivalent TerEps, and for many of them, we have found specific antibodies. These findings strengthen the concept that eplets are essential basic units of HLA epitopes and that they provide a better understanding of HLA immunogenicity (i.e. ability to induce an antibody response) and antigenicity (i.e. reactivity with specific antibody).

Why can sensitization by an HLA-DR2 mismatch lead to antibodies that react also with HLA-DR1?

HLAMatchmaker is a matching algorithm that can be used to characterize antibodies specific for structurally defined epitopes. Under the auspices of the 15(th) International Histocompatibility Workshop, we are conducting a multilaboratory collaborative project to characterize these epitopes and also to determine how often they induce specific antibodies in patients with rejected kidney transplants. This report addresses the reactivity of post-allograft nephrectomy sera tested for DRB antibodies with Luminex assays using single alleles. This analysis was performed for 19 informative kidney transplant cases contributed by 13 laboratories worldwide. There were 11 cases with a single DR2 mismatch (DR15 or DR16), and nine of them (82%) showed antibodies with both DR2 and DR1. Although these antigens might share an epitope recognized by these antibodies, this interpretation is incorrect. The HLAMatchmaker analysis offers a clearly different explanation that involves antibodies induced by DR51, which commonly associates with DR2. DR51 has an epitope defined by the 96EV eplet, which is also present on DR1 but no other DR antigen. This means that the reactivity with DR51 and DR1 reflects the presence of 96EV-specific antibodies. Conversely, we analyzed eight patients sensitized by a single DR1 mismatch that has no associated DR51. All of these patients reacted also with DR51, and this could be explained only by antibodies against the shared 96EV eplet. These findings demonstrate that 96EV represents a highly immunogenic epitope that can induce cross-sensitization between antigens encoded by the different DRB loci, and also that DR51 is important in determining DRB mismatch acceptability of potential donors. This analysis has also demonstrated that antibody responses are restricted to a few epitopes on these immunizing DR antigens. For DR2 they are 142M3 (unique for DR2), 71QAA (shared with DB5*02), and 96QV (shared with DR10). DR51 mismatches appear to have three immunogenic eplets: 96EV (shared with DR1), 108T3 (unique for DR51), and 40HFD (shared with DR9). Immunogenic eplets on DR1 are 12LKF2 (unique for DR1), 14FEH (shared with DR9 and DR10), and 25HRL (shared with DR10).

Structurally based epitope analysis of major histocompatibility complex class I-related chain A (MICA) antibody specificity patterns

Recent studies have suggested a clinical significance to the detection of anti-major histocompatibility complex class I-related chain A (MICA) antibodies in transplantation. We have developed an eplet-based version of the HLAMatchmaker algorithm to assess the epitope specificity of these antibodies. Molecular viewing of the MICA structure and the determination of amino acid sequence differences between MICA alleles has yielded a repertoire of 38 potentially immunogenic MICA eplets. These eplets are based on the functional epitope structure that considers a configuration of amino acids within a 3-Angstrom radius of an antibody-accessible polymorphic residue on the molecular surface. In this study MICA-reactive sera were screened in Luminex assays with single MICA allele panels and analyzed with HLAMatchmaker. We identified reactivity patterns that correspond to eplet-specific antibodies to identify of unacceptable MICA mismatches including those alleles that have not been tested with the serum. In conclusion, HLAMatchmaker is a useful algorithm to analyze the reactivity patterns of anti-MICA antibodies and the determination of MICA mismatch acceptability at the structural level.

14th International HLA and Immunogenetics Workshop: report on the structural basis of HLA compatibility

Under auspices of the 14th International Human Leukocyte Antigen (HLA) and Immunogenetics Workshop, multilaboratory collaborative studies were organized addressing the following projects: (1) identification of structurally defined class I and class II HLA epitopes by human monoclonal antibodies, (2) assessment of the relative immunogenicity of structurally defined donor HLA mismatches in kidney transplantation, and (3) evaluation of the usefulness of structurally based HLA matching in platelet transfusion support of alloimmunized thrombocytopenic patients. This progress report describes a preliminary HLAMatchmaker-based data analysis in each project. The findings support the clinical usefulness of structural HLA matching.

HLA Class I Epitopes: Recognition of Binding Sites by mAbs or Eluted Alloantibody Confirmed With Single Recombinant Antigens

Development of beads coated with single recombinant HLA antigens has permitted the confirmation and further definition of HLA class I epitopes. In this study, monoclonal antibodies (mAbs) or alloantibodies eluted from recombinant cell lines were tested for reactivity with Luminex beads individually coated with 79 recombinant HLA class I single antigen (rHLA SA). Published amino acid sequences were used to map epitopes common to sets of antigens reactive with each antibody. While several epitopes have already been demonstrated, this study confirmed them by adsorption of allosera with transfectants or SA beads having a single HLA antigen and specific binding of the eluted antibody on SA beads. The allosera and mAbs used in this study recognized a total of at least 58 HLA class I epitopes, as demonstrated by their different adsorption/reactivity patterns. Of these, 25 epitopes were characterized by a single unique common amino acid, 30 shared 2 signature amino acids in close proximity, and 3 epitopes involved 3 specific amino acids in a non-linear sequence. Since these epitopes may be targets for antibody-mediated allograft rejection, epitope analysis should complement HLA and CREG assignment for defining complex antibodies and identifying suitable donors for highly sensitized transplant patients.

HLAMatchmaker: a molecularly based algorithm for histocompatibility determination. V. Eplet matching for HLA-DR, HLA-DQ, and HLA-DP

This report describes the design of the eplet version of HLAMatchmaker to determine class II compatibility at the structural level. This matching algorithm is based on the hypothesis, developed from molecular modeling of crystallized antigen-antibody complexes, that functional epitopes are represented by patches of surface-exposed nonself-amino acid residues surrounded by residues within a 3-A radius. Patch determinations with a molecular viewer of crystalline structural models downloaded from the Entrez Molecular Modeling Database Web site led to the identification of 44 DRB, 33DQB, 29 DQA, 20 DPB, and 9 DPA unique combinations of polymorphic positions. The residue compositions of these patches were then determined from amino acid sequences. This analysis resulted in a repertoire of 146 DRB, 74 DQB, 58 DQA, 45 DPB, and 19 DPA eplets. In many eplets, the residues are in short linear sequences, but many other eplets have discontinuous sequences of residues that cluster on or near the molecular surface. This analysis has also shown that all serologically defined DR and DQ antigens detectable by monospecific antibodies have unique eplets. Other eplets are present in groups of class II antigens, many of which appear as cross-reacting. The eplet version of HLAMatchmaker should be considered as a hypothetical model for the structural assessment of donor-recipient compatibility and the determination of mismatch acceptability for sensitized patients. This computer algorithm can be downloaded from the HLA Matchmaker Webside at http://tpis.upmc.edu.

A structurally based approach to determine HLA compatibility at the humoral immune level

HLAMatchmaker is a structurally based matching program. Each HLA antigen is viewed as a string of epitopes represented by short sequences (triplets) involving polymorphic amino acid residues in antibody-accessible positions. HLAMatchmaker determines which triplets are different between donor and recipient, and this algorithm is clinically useful in determining HLA mismatch acceptability. Triplets provide however an incomplete description of the HLA epitope repertoire and expanded criteria must be used including longer sequences and polymorphic residues in discontinuous positions. Such criteria should consider the structural basis of antibody-antigen interactions including contact areas and binding energy, the essence of antigenicity. This report describes the development of a structurally defined HLA epitope repertoire based on stereochemical modeling of crystallized complexes of antibodies and different protein antigens. This analysis considered also data in the literature about contributions of amino acid residues to antigen-antibody binding energy. The results have led to the concept that HLA antigens like other antigenic proteins have structural epitopes consisting of 15-22 residues that constitute the binding face with alloantibody. Each structural epitope has a functional epitope of about 2-5 residues that dominate the strength and specificity of binding with antibody. The remaining residues of a structural epitope provide supplementary interactions that increase the stability of the antigen-antibody complex. Each functional epitope has one or more non-self residues and the term "eplet" is used to describe polymorphic HLA residues within 3.0-3.5 A of a given sequence position on the molecular surface. Many eplets represent short linear sequences identical to those referred to as triplets but others have residues in discontinuous sequence positions that cluster together on the molecular surface. Serologically defined HLA determinants correspond well to eplets. The eplet version of HLAMatchmaker represents therefore a more complete repertoire of structurally defined HLA epitopes and provides a more detailed assessment of HLA compatibility.