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

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

PURPOSE OF REVIEW

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

RECENT FINDINGS

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

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.

The Immunogenicity of HLA Class II Mismatches: The Predicted Presentation of Nonself Allo-HLA-Derived Peptide by the HLA-DR Phenotype of the Recipient Is Associated with the Formation of DSA

The identification of permissible HLA class II mismatches can prevent DSA in mismatched transplantation. The HLA-DR phenotype of recipients contributes to DSA formation by presenting allo-HLA-derived peptides to T-helper cells, which induces the differentiation of B cells into plasma cells. Comparing the binding affinity of self and nonself allo-HLA-derived peptides for recipients' HLA class II antigens may distinguish immunogenic HLA mismatches from nonimmunogenic ones. The binding affinities of allo-HLA-derived peptides to recipients' HLA-DR and HLA-DQ antigens were predicted using the NetMHCIIpan 3.1 server. HLA class II mismatches were classified based on whether they induced DSA and whether self or nonself peptide was predicted to bind with highest affinity to recipients' HLA-DR and HLA-DQ. Other mismatch characteristics (eplet, hydrophobic, electrostatic, and amino acid mismatch scores and PIRCHE-II) were evaluated. A significant association occurred between DSA formation and the predicted HLA-DR presentation of nonself peptides (P = 0.0169; accuracy = 80%; sensitivity = 88%; specificity = 63%). In contrast, mismatch characteristics did not differ significantly between mismatches that induced DSA and the ones that did not, except for PIRCHE-II (P = 0.0094). This methodology predicts DSA formation based on HLA mismatches and recipients' HLA-DR phenotype and may identify permissible HLA mismatches to help optimize HLA matching and guide donor selection.

Reflections on HLA Epitope-Based Matching for Transplantation

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

Differences in immunogenicity of HLA antigens and the impact of cross-reactivity on the humoral response

BACKGROUND

Information about differences in immunogenicity of various HLA antigens may help guide donor selection and identify mismatches to avoid for patients likely to need retransplantation. To date, antibody responses to a wide array of individual mismatched antigens have not been evaluated.

METHODS

Frequencies of antibodies to mismatched HLA-A, HLA-B, HLA-DR, and HLA-DQ antigens were determined for 703 renal transplant patients who had no detectable donor-specific antibody before transplantation. The impact of cross-reactive group matching and production of antibodies cross-reactive with mismatched antigens were also assessed. Antibodies were identified using multiplexed bead assays.

RESULTS

The overall mean frequencies were similar for HLA-A (53.2%), HLA-DR (52.6%), and HLA-DQ (59.0%) antibodies, but significantly lower for HLA-B antibodies (42.4%). However, the response to individual antigens ranged from 15.0% to 76.2%. Antibody frequencies were reduced significantly for 54 of 62 specificities when the patient possessed an antigen cross-reactive with the donor mismatch, but the magnitude of the effect was variable and ranged from 8% to 83%. Moreover, there was directionality in the protective effect of cross-reactive group matching. Overall mean donor-specific antibody frequencies were comparable for men and women except for a significantly higher frequency of antibodies to HLA-DR among men (56.6% vs. 47.8%, P=0.004). Overall mean frequencies in blacks were higher than, or comparable to those of, whites, but differences were not significant.

CONCLUSION

There is considerable variability in the immunogenicity of different HLA antigens that is impacted by the presence or absence of cross-reactive antigens in the patient's phenotype. This information can be used to augment the immunologic evaluation of donor-recipient pairs.

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.

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).

Structural epitope matching for HLA-alloimmunized thrombocytopenic patients: a new strategy to provide more effective platelet transfusion support?

BACKGROUND

During the past three decades, HLA matching for platelet (PLT) transfusion of refractory thrombocytopenic patients has been based on serologic cross-reactivity between HLA-A and HLA-B antigens. Although many blood banks are using this matching strategy, the general experience is that such matched PLT transfusions are often ineffective.

STUDY DESIGN AND METHODS

This report describes a new HLA matching algorithm that considers structurally defined epitopes recognized by antibodies. HLAMatchmaker is a computer program that determines histocompatibility at the amino acid level initially designed as triplets (i.e., linear sequences of three residues in molecular surface-exposed positions) but now updated as eplets representing patches of antibody-accessible polymorphic residues surrounded by residues within a 3-A radius. The eplet version of HLAMatchmaker is also useful in the analysis of HLA antibody reactivity patterns of alloimmunized patients so that acceptable mismatches can be identified.

CONCLUSION

An HLA epitope-based matching protocol is proposed that may permit a more effective PLT transfusion management of refractory patients. This protocol includes high-resolution HLA-A, -B, and -C typing of patients and donors, serum screening to identify acceptable mismatches, and the identification of suitable donors in a donor database that incorporates HLAMatchmaker as a search engine. HLAMatchmaker programs can be downloaded from the Web site http://tpis.upmc.edu/tpis/HLAMatchmaker/.