P131 Description of a new antibody-defined epitope on denatured HLA class I molecules

Abstract

The purpose of this report is to describe a new epitope present on denatured HLA class I molecules. Serum from a kidney transplant candidate, male, and with only one previous blood transfusion was tested with the IgG-binding assay with single-antigen beads on a Luminex platform (LSA) (One Lambda - OL). The following positive reactions were observed: HLA-B- 13, 18, 37, 41, 44, 45, 47, 48, 49, 50, 52, 60, 61, 62, 72, 75, 76, 77. This reaction pattern could be explained by the HLA-Matchmaker eplet 66IS (65Q66I67S69T) which is considered an exposed eplet ( http://www.epregistry.com.br ). The serum was tested with flow-cytometry using three different cells carrying one of these specificities: HLA-B37 (Ac with MFI of 5250), B60 (Ac with MFI of 3661) or B50 (Ac com MFI of 4752) and the results were negative. In addition, no positive reactions were observed when the serum was tested with a LSA kit from another vendor. In order to investigate whether the OL Luminex reactions were false-positive, the serum was tested with OL beads bearing HLA molecules denatured by acid treatment and the same reactions were observed, although with higher MFI values. The reaction pattern observed on the beads with denatured HLA could not be explained by any cryptic epitope described by El-Awar, 2009. In order to search for the targeted epitope on the denatured HLA molecules, we analyzed the amino acid sequences and we found that the position 67S, cryptic in the HLA molecule, was present only in the molecules with which the serum reacted. In fact, this position is included in the polymorphic residues of HLAMatchMaker eplet 66IS. This finding underlines that not all reactions that can be explained by the HLA-Matchmaker are true-positive reactions.