A limited number of HLA epitopes are recognized from HLA class I-specific antibodies detected in the serum of sensitized patients

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).

Immunological profile of patients awaiting a renal transplant

BACKGROUND

A renal transplant is the best possible treatment for patients with terminal renal failure. Advances in the development of techniques of screening of pre-formed antibodies have contributed to a notable improvement in the results obtained with allogenic transplants.

METHODS

The aim of the present work is to study the nature, class, isotype and specificity of antibodies detected in patients awaiting renal transplantation at Complejo Hospitalario Universitario Juan Canalejo, as well as their relation with the level of anti-human leucocyte antigen (HLA) sensitization.

RESULTS

In all patient groups, there was a predominance of IgG. The distribution of anti-HLA antibody class showed that the most frequent pattern corresponded to a mixture of class I and class II antibodies for all groups. The study of specificity of anti-HLA antibody showed that of the patients with at least one previous transplant, 72% developed specific anti-HLA antibodies against some of the incompatible antigens of the donor, 12% against HLA antigens not related with the phenotype of the donor and in 16% it was not possible to determine their specificity. Most patients developed antibodies against antigens of locus B, probably because of that the number of incompatibilities contributed by the donors is also greater for locus B.

CONCLUSIONS

The exhaustive study of sera of patients on a waiting list for transplantation with respect to the nature, isotype, class and specificity of the antibody is important since it is possible that different antibodies can bring about, in the transplant, events that will have different consequences for the survival of the graft.

Incidence and specificity of HLA antibodies in multitransfused patients with acquired aplastic anemia

BACKGROUND

This study aimed to establish the prevalence and characteristics of anti-HLA in antibody acquired aplastic anemia patients following cessation of antithymocyte globulin therapy and to characterize antibody in terms of epitope specificity.

STUDY DESIGN AND METHODS

One hundred and fifty multitransfused, untransplanted patients from eight European centers were investigated by serologic methods.

RESULTS

Sixty-two percent were antibody positive. Eighteen HLA-Class-I-specific antibodies (15 IgG, 3 IgM) were identified in 13 patients; 13 antibodies were specific for HLA-A epitopes and 5 for HLA-B. Epitope analysis identified significant correlation between serum reactivity and amino acid substitutions associated with HLA-Class-I epitopes. An excess of antibodies to HLA-A1-associated cross-reactive groups was identified. There was no significant difference in antibody frequency in patients taking cyclosporine compared to those who were not.

CONCLUSION

Data suggested a contribution from B cell memory of alloantigens introduced during pregnancy. In some cases, antibody production continued many years after the last transfusion, and although the target varied between individual patients, the antibody to HLA was focused on a few specific Class I epitopes, the majority of which mapped to the HLA-A molecule.

Definition of permissible and immunogenic HLA antigens based on epitope analysis of the HLA specific antibodies produced in sensitized patients

The goal of this study was to develop an accurate protocol whereby detection of acceptable HLA-A and -B mismatches is based on epitope analysis of HLA class I specific antibodies detected in the serum of highly sensitized patients awaiting a kidney retransplant. A total of 400 serum samples from 44 highly sensitized patients with panel reactive antibodies (PRA) of > or = 60% were collected during a 3-year follow-up period. All patients had been sensitized from a previous graft. In order to define the specificities of the HLA class I specific antibodies, two techniques were used in parallel: the antihuman globulin augmented complement-dependent cytotoxicity (CDC) technique and an enzyme-linked immunoabsorbent assay (ELISA) technique. Epitope identification was based on class I HLA antigen sequencing, where the unique epitope configuration on one HLA antigen represented the private epitope of the specific HLA antigen, and epitopes shared by more than one HLA antigen represented public determinants. The epitope prediction for the immunogenic HLA epitopes was based on an MHC database. For each highly sensitized patient, antibody specificities against actual and 'at risk' epitopes were defined. Following epitope analysis, all HLA antigens that did not express the actual and/or 'at risk' immunogenic epitopes were considered as acceptable mismatches of epitope analysis. The cytotoxicity of highly sensitized patients was determined using two different panels of selected, separated T lymphocytes. HLA class I specific IgG antibodies against 69 actual and 86 'at risk' epitopes were detected. In all patients, a large number of acceptable mismatches were defined. These included a large number of HLA antigens, corresponding to both HLA-A and -B loci. Our study introduces an accurate protocol for the detection of acceptable mismatches in highly sensitized patients. According to this protocol, the detailed description of immunogenic HLA specific epitope targets, against which HLA class I specific antibodies are directed, is a useful tool for the detection of acceptable mismatches in highly sensitized patients. This may lead to reduced production of HLA class I specific antibodies and, consequently, improved graft survival.