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

First Impressions Matter: Immune Imprinting and Antibody Cross-Reactivity in Influenza and SARS-CoV-2

Many rigorous studies have shown that early childhood infections leave a lasting imprint on the immune system. The understanding of this phenomenon has expanded significantly since 1960, when Dr. Thomas Francis Jr first coined the term "original antigenic sin", to account for all previous pathogen exposures, rather than only the first. Now more commonly referred to as "immune imprinting", this effect most often focuses on how memory B-cell responses are shaped by prior antigen exposure, and the resultant antibodies produced after subsequent exposure to antigenically similar pathogens. Although imprinting was originally observed within the context of influenza viral infection, it has since been applied to the pandemic coronavirus SARS-CoV-2. To fully comprehend how imprinting affects the evolution of antibody responses, it is necessary to compare responses elicited by pathogenic strains that are both antigenically similar and dissimilar to strains encountered previously. To accomplish this, we must be able to measure the antigenic distance between strains, which can be easily accomplished using data from multidimensional immunological assays. The knowledge of imprinting, combined with antigenic distance measures, may allow for improvements in vaccine design and development for both influenza and SARS-CoV-2 viruses.

The MHC class I MICA gene is a histocompatibility antigen in kidney transplantation

The identity of histocompatibility loci, besides human leukocyte antigen (HLA), remains elusive. The major histocompatibility complex (MHC) class I MICA gene is a candidate histocompatibility locus. Here, we investigate its role in a French multicenter cohort of 1,356 kidney transplants. MICA mismatches were associated with decreased graft survival (hazard ratio (HR), 2.12; 95% confidence interval (CI): 1.45–3.11; P < 0.001). Both before and after transplantation anti-MICA donor-specific antibodies (DSA) were strongly associated with increased antibody-mediated rejection (ABMR) (HR, 3.79; 95% CI: 1.94–7.39; P < 0.001; HR, 9.92; 95% CI: 7.43–13.20; P < 0.001, respectively). This effect was synergetic with that of anti-HLA DSA before and after transplantation (HR, 25.68; 95% CI: 3.31–199.41; P = 0.002; HR, 82.67; 95% CI: 33.67–202.97; P < 0.001, respectively). De novo-developed anti-MICA DSA were the most harmful because they were also associated with reduced graft survival (HR, 1.29; 95% CI: 1.05–1.58; P = 0.014). Finally, the damaging effect of anti-MICA DSA on graft survival was confirmed in an independent cohort of 168 patients with ABMR (HR, 1.71; 95% CI: 1.02–2.86; P = 0.041). In conclusion, assessment of MICA matching and immunization for the identification of patients at high risk for transplant rejection and loss is warranted.

Analysis of a multicenter cohort of kidney transplants shows that mismatches in the MICA locus and the presence of anti-MICA donor-specific antibodies are associated with reduced graft survival and increased rejection.

Targeting MICA/B with cytotoxic therapeutic antibodies leads to tumor control

Background: MICA and MICB are tightly regulated stress-induced proteins that trigger the immune system by binding to the activating receptor NKG2D on cytotoxic lymphocytes. MICA and MICB are highly polymorphic molecules with prevalent expression on several types of solid tumors and limited expression in normal/healthy tissues, making them attractive targets for therapeutic intervention. Methods: We have generated a series of anti-MICA and MICB cross-reactive antibodies with the unique feature of binding to the most prevalent isoforms of both these molecules. Results: The anti-MICA and MICB antibody MICAB1, a human IgG1 Fc-engineered monoclonal antibody (mAb), displayed potent antibody-dependent cellular cytotoxicity (ADCC) and antibody-dependent cellular phagocytosis (ADCP) of MICA/B-expressing tumor cells in vitro. However, it showed insufficient efficiency against solid tumors in vivo, which prompted the development of antibody-drug conjugates (ADC). Indeed, optimal tumor control was achieved with MICAB1-ADC format in several solid tumor models, including patient-derived xenografts (PDX) and carcinogen-induced tumors in immunocompetent MICAgen transgenic mice. Conclusions: These data indicate that MICA and MICB are promising targets for cytotoxic immunotherapy.

Targeting MICA/B with cytotoxic therapeutic antibodies leads to tumor control

Background: MICA and MICB are tightly regulated stress-induced proteins that trigger the immune system by binding to the activating receptor NKG2D on cytotoxic lymphocytes. MICA and MICB are highly polymorphic molecules with prevalent expression on several types of solid tumors and limited expression in normal/healthy tissues, making them attractive targets for therapeutic intervention. Methods: We have generated a series of anti-MICA and MICB cross-reactive antibodies with the unique feature of binding to the most prevalent isoforms of both these molecules. Results: The anti-MICA and MICB antibody MICAB1, a human IgG1 Fc-engineered monoclonal antibody (mAb), displayed potent antibody-dependent cellular cytotoxicity (ADCC) and antibody-dependent cellular phagocytosis (ADCP) of MICA/B-expressing tumor cells in vitro. However, it showed insufficient efficiency against solid tumors in vivo, which prompted the development of antibody-drug conjugates (ADC). Indeed, optimal tumor control was achieved with MICAB1-ADC format in several solid tumor models, including patient-derived xenografts (PDX) and carcinogen-induced tumors in immunocompetent MICAgen transgenic mice. Conclusions: These data indicate that MICA and MICB are promising targets for cytotoxic immunotherapy.

Reviewing the pathogenesis of antibody-mediated rejection and renal graft pathology after kidney transplantation

The clinicopathological context of rejection after kidney transplantation was well recognized. Banff conferences greatly contributed to elucidate the pathogenesis and to establish the pathologic criteria of rejection after kidney transplantation. The most important current problem of renal transplantation is de novo donor-specific antibody (DSA) production leading chronic rejection and graft loss. Microvascular inflammation is considered as a reliable pathological marker for antibody-mediated rejection (AMR) in the presence of DSA. Electron microscopic study allowed us to evaluate early changes in peritubular capillaries in T-lymphocyte mediated rejection and transition to antibody-mediated rejection. Severe endothelial injuries with edema and activated lymphocyte invaded into subendothelial space with early multi-layering of peritubular capillary basement membrane suggest T-lymphocyte mediated rejection induce an unbounded chain of antibody-mediated rejection. The risk factors of AMR after ABO-incompatible kidney transplantation are important issues. Anti-ABO blood type antibody titre of IgG excess 32-fold before transplant operation is the only predictable factor for acute AMR. Characteristics of chronic active antibody-mediated rejection (CAAMR) are one of the most important problems. Light microscopic findings and C4d stain of peritubular capillary and glomerular capillary are useful diagnostic criteria of CAAMR. Microvascular inflammation, double contour of glomerular capillary and thickening of peritubular capillary basement are good predictive factors of the presence of de novo DSA. C4d stain of linear glomerular capillary is a more sensitive marker for CAAMR than positive C4d of peritubular capillary. Early and sensitive diagnostic attempts of diagnosing CAAMR are pivotal to prevent chronic graft failure.

MICA and NKG2D: Is There an Impact on Kidney Transplant Outcome?

This paper aims to present an overview of MICA and natural killer group 2 member D (NKG2D) genetic and functional interactions and their impact on kidney transplant outcome. Organ transplantation has gone from what can accurately be called a “clinical experiment” to a routine and reliable practice, which has proven to be clinically relevant, life-saving and cost-effective when compared with non-transplantation management strategies of both chronic and acute end-stage organ failures. The kidney is the most frequently transplanted organ in the world (transplant-observatory¹). The two treatment options for end-stage renal disease (ESRD) are dialysis and/or transplantation. Compared with dialysis, transplantation is associated with significant improvements in quality of life and overall longevity. A strong relationship exists between allograft loss and human leukocyte antigens (HLA) antibodies (Abs). HLA Abs are not the only factor involved in graft loss, as multiple studies have shown that non-HLA antigens are also involved, even when a patient has a good HLA matche and receives standard immunosuppressive therapy. A deeper understanding of other biomarkers is therefore important, as it is likely to lead to better monitoring (and consequent success) of organ transplants. The objective is to fill the void left by extensive reviews that do not often dive this deep into the importance of MICA and NKG2D in allograft acceptance and their partnership in the immune response. There are few papers that explore the relationship between these two protagonists when it comes to kidney transplantation. This is especially true for the role of NKG2D in kidney transplantation. These reasons give a special importance to this review, which aims to be a helpful tool in the hands of researchers in this field.

Early renal graft function deterioration in recipients with preformed anti-MICA antibodies: partial contribution of complement-dependent cytotoxicity

BACKGROUND

We previously reported that preformed anti-MHC class I-related chain A (MICA) antibodies increase the risk for renal graft rejection and enhance the deleterious effect of PRA(+) status early after transplantation.

METHODS

We studied 727 kidney recipients. Days to reach optimal serum creatinine level, estimated glomerular filtration rate (eGFR) at Month 3 and chronic kidney disease (CKD) stages were recorded. Anti-MICA specificities and C1q binding were tested by solid-phase assay. Complement-dependent cytotoxicity (CDC) and flow cytometry (FC) cross-matches with HeLa and PMA/CD28-T-blasts were performed.

RESULTS

PRA(+)MICA(+) recipients exhibited longer time to reach optimal serum creatinine level after transplantation (P = 0.005) and had the lowest eGFR at Month 3 (P = 0.006). PRA(+)MICA(+) status independently increased the risk for CKDT stage 5 at Month 3 [hazard ratio (HR) 4.92, P = 0.030]. Pre-transplant anti-MICA antibodies were polyspecific and showed stronger reactions when coexisting with anti-HLA antibodies (mean standard fluorescent intensity 112 157 ± 44 426 in HLA(+)MICA(+) sera versus 49 680 ± 33 116 in HLA(-)MICA(+) sera, P = 0.0006). Anti-AYVE supereplet reactivity was significantly higher in HLA(+)MICA(+) versus HLA(-)MICA(+) patients (P < 0.001) and significantly superior than anti-CMGWS supereplet within HLA(+)MICA(+) patients (P = 0.001). Three of 13 anti-MICA(+) pre-transplant sera were positive for the C1q binding assay; one of them (serum 3) exclusively recognized AYVE supereplet with a strong reactivity against MICA*027 antigen (same as MICA*008). Anti-MICA antibodies in anti-HLA-absorbed serum 3 bound native MICA molecules in MICA*008(+) HeLa and PMA/CD28-T-blasts and mediated cell death by activating complement.

CONCLUSION

Preformed anti-MICA antibodies may occasionally be cytotoxic by fixing and activating complement. This way they might contribute to worse early kidney graft function.

Acute Antibody-Mediated Rejection in Presence of MICA-DSA and Successful Renal Re-Transplant with Negative-MICA Virtual Crossmatch

The presence of donor-specific alloantibodies (DSAs) against the MICA antigen results in high risk for antibody-mediated rejection (AMR) of a transplanted kidney, especially in patients receiving a re-transplant. We describe the incidence of acute C4d+ AMR in a patient who had received a first kidney transplant with a zero HLA antigen mismatch. Retrospective analysis of post-transplant T and B cell crossmatches were negative, but a high level of MICA alloantibody was detected in sera collected both before and after transplant. The DSA against the first allograft mismatched MICA*018 was in the recipient. Flow cytometry and cytotoxicity tests with five samples of freshly isolated human umbilical vein endothelial cells demonstrated the alloantibody nature of patient’s MICA-DSA. Prior to the second transplant, a MICA virtual crossmatch and T and B cell crossmatches were used to identify a suitable donor. The patient received a second kidney transplant, and allograft was functioning well at one-year follow-up. Our study indicates that MICA virtual crossmatch is important in selection of a kidney donor if the recipient has been sensitized with MICA antigens.

Humoral Immune Response and Allograft Function in Kidney Transplantation

HLA antibodies can damage a kidney transplant. In January 2013, consensus guidelines from The Transplantation Society were published regarding technical aspects of HLA antibody determination, as well as their potential significance in the pre- and posttransplantation periods. During the past 2 years, new studies have been reported, but controversies remain. In this article, these new data related to HLA antibodies in kidney transplantation are reviewed and compared to relevant prior research. Pretransplantation sensitization issues are discussed, including the new more sensitive assays (flow cytometry and solid-phase immunoassays such as Luminex single-antigen bead assays). A positive complement-dependent cytotoxicity crossmatch remains an absolute contraindication to transplantation, although a positive flow cytometry crossmatch is only a relative contraindication. Positivity only by solid-phase assays increases the risk for acute rejection and transplant loss, but acceptable cutoffs are not defined. The sensitizing effect of red blood cell transfusions is substantiated. Following allograft failure, continued immunosuppression decreases the risk of sensitization, whereas overall, the effect of nephrectomy remains uncertain. Regarding the posttransplantation period, new data are available concerning the timing and significance of donor-specific antibodies (DSA). Whereas some centers report DSA appearance after years, others detect DSA within months. The prominence of class II DSA, especially DQ, in the posttransplantation period is noted. The relevance of non-HLA antibodies is discussed, including anti-endothelial cell antibodies, major histocompatibility complex class I chain-related protein A antibodies, and angiotensin II type 1 receptor autoantibodies.

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.

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.

16(th) IHIW: report of the MICA project

The MICA Project of the 16th International HLA and Immunogenetics Workshop was planned to improve the detection of specific antibodies against MICA antigens that develop in organ transplant recipients. It was organized as a serum exchange with 18 laboratories located in seven countries as participants. Each laboratory used the MICA screening reagents available to them. It was found that there were four different kits of Luminex beads conjugated with MICA antigens that could be used in these experiments. They were, kit BL, produced by Rainer Blasczyk in Hannover, Germany, containing 12 MICA antigens, the kit from Gen-Probe (GP), which included 28 MICA antigens, the beads from One Lambda (OL), consisting of ten antigens, and the beads produced in the laboratory of the organizers (ZS) consisting of 11 MICA antigens. The sera were all from transplant recipients and represented a spectrum of MICA-specific antibodies that are commonly found. All laboratories accurately could recognize which sera contained MICA antibodies. None failed to recognize the negative serum that was included in one of the shipments. While there were important differences in the specificities of MICA antigens recognized by the different kits, the results in laboratories using the same beads were remarkably similar. Feedback was given to the participants, and especially to the producers of the antibody detection kits, after each set of four sera was sent, and results were collected. Certain beads were replaced and results improved as can be seen in the results of the third shipment of sera. It is important for laboratories to be able to accurately determine the specificity of antibodies against MICA to know whether the antibodies are donor specific. Although complete consensus was not attained, some important errors were corrected, and a better understanding of how to accurately determine MICA allele-specific antibodies was developed.

Antibody-mediated rejection: pathogenesis, prevention, treatment, and outcomes

Antibody-mediated rejection (AMR) is a major cause of late kidney transplant failure. It is important to have an understanding of human-leukocyte antigen (HLA) typing including well-designed studies to determine anti-MHC-class-I-related chain A (MICA) and antibody rejection pathogenesis. This can allow for more specific diagnosis and treatment which may improve long-term graft function. HLA-specific antibody detection prior to transplantation allows one to help determine the risk for AMR while detection of DSA along with a biopsy confirms it. It is now appreciated that biopsy for AMR does not have to include diffuse C4d, but does require a closer look at peritubular capillary microvasculature. Although plasmapheresis (PP) is effective in removing alloantibodies (DSAs) from the circulation, rebound synthesis of alloantibodies can occur. Splenectomy is used in desensitization protocols for ABO incompatible transplants as well as being found to treat AMR refractory to conventional treatment. Also used are agents targeted for plasma cells, B cells, and the complement cascade which are bortezomib rituximab and eculizumab, respectively.

Screening for antibodies against MICA by Luminex flow cytometry

Antibodies against MICA have been found in organ transplant recipients and were found to be associated with decreased survival of kidney allografts. The MICA antibody screening assay is a Luminex-based solid phase immunoassay designed to detect IgG antibodies binding to beads pre-coated with recombinant preparations of MICA alleles. These beads coated with soluble MICA recombinant proteins including 11 common alleles have been produced in our laboratory and similar preparations have been available from commercial sources. Here, we describe the procedure of MICA antibody screening with a prepared kit, in which all the reagents were optimized and standardized. We also review how to document the quality of single MICA antigen beads using MICA-specific monoclonal antibodies, as well as quality control of the procedure and data analysis.

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.

Major histocompatibility complex class I-related chain A allele mismatching, antibodies, and rejection in renal transplantation

Even when kidney allografts are well matched for human leukocyte antigen (HLA) and anti-HLA antibodies are not detected, graft rejection can still occur. There is evidence that some patients who lose their graft have antibodies specific for major histocompatibility complex (MHC) class I-related chain A (MICA) antigens. We investigated whether mismatching MICA alleles associates with MICA antibody production and graft rejection or dysfunction. MICA and HLA antibody screening in 442 recipients was performed, and specificities were confirmed in a subgroup of 227 recipients using single-antigen multiplex technology. For assignment of MICA antibody specificity, we used three independent assays. In addition, MICA alleles of 227 recipients and donors were determined by DNA sequencing. In all, 17 patients (7.5%) had MICA antibodies, and 13 patients (6%) developed MICA donor-specific antibodies (DSA). Multivariate analysis revealed MICA mismatching, as an independent significant factor associated with the presence of MICA antibodies (p = 0.009), and 14 mismatched MICA residues significantly correlated with MICA antibody production. MICA and HLA antibodies significantly associated with acute rejection (AR) and MICA DSA and HLA DSA correlated with decreased graft function by univariate and multivariate analysis. We conclude that mismatching for MICA epitopes in renal transplantation is a mechanism leading to production of MICA antibodies that associate with AR and graft dysfunction.

Antibody-reactive epitope determination with HLAMatchmaker and its clinical applications

Antibodies against allogeneic human leukocyte antigen (HLA) molecules are important impediments to the success of different clinical procedures including transplantation and platelet transfusion. In these settings, characterization of the repertoire of immunogenic epitopes is important for permissible mismatch determination and the identification of acceptable mismatches for sensitized patients. HLAMatchmaker is a computer algorithm that considers small configurations of polymorphic residues referred to as eplets as essential components of HLA epitopes. This review critically elaborates the concepts underlying the HLAMatchmaker and describes the usefulness of HLAMatchmaker in the clinical setting. Recent developments have increased our understanding of structural basis of HLA antigenicity (i.e. reactivity with specific antibody) and immunogenicity (i.e. its ability to induce an antibody response).

Role of MICA in the immune response to transplants

Among the cell surface antigens that can elicit an immune response in transplant recipients MICA antigens occupy a special place. They are similar to human leukocyte antigens (HLAs) while being very different from them. They are not as polymorphic and their quantity is smaller. In consequence, the impact of MICA antigens in transplantation is not as dramatic. However, our early guess that these ligands of NKG2D could elicit antibodies and cell-mediated immunity has been definitely confirmed. Careful analysis with MICA transfectant cells, for absorption and elution, established the specificity of the epitopes involved. Typing of recipients and donors by sequencing the MICA alleles has established that de novo antibodies produced in kidney transplant recipients are directed at mismatched MICA epitopes and are associated with acute rejection and chronic transplant failure. Acute graft-versus-host disease was observed in stem cell recipients who were mismatched for MICA.

The role of major histocompatibility complex class I chain-related gene A antibodies in organ transplantation

PURPOSE OF REVIEW

Major histocompatibility complex class I chain-related gene A (MICA) antigens are expressed on the endothelium, they are polymorphic and have been shown to be recognized by antibodies produced by transplant recipients. Methods for detection of these antibodies have become available. In the 15th International Histocompatibility Workshop, a study for MICA antibody testing and of MICA genotyping was organized.

RECENT FINDINGS

Antibodies against MICA antigens have been determined either using cells transfected with MICA alleles or recombinant MICA antigens. MICA epitopes were characterized by empirical study of human sera and by correlation with MICA polymorphic amino acids. Sera were absorbed with cells transfected with MICA alleles and site-directed mutagenesis was employed to analyze complex sera. A number of clinical studies have shown associations of antibodies against MICA with decreased survival of kidney transplants and in one investigation with acute rejection in recipients of heart allografts.

SUMMARY

In addition to the HLA antigens, which elicit a strong immune response against allografted organs, the MICA antigens may be recognized as foreign and induce the production of MICA-specific antibodies. Antibodies against MICA have been associated with a decrease in the survival of organ allografts. The results suggest the MICA antigens are transplantation antigens that can induce an immune response associated with graft failure.

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).