Why certain antibodies cross-react with HLA- A and HLA-G: Epitope mapping of two common MHC class I reagents

Antigen presentation at the maternal-fetal interface has been characterized by a reported lack of classical MHC class I products and the presence of a tissue-restricted, non-classical class I product with limited polymorphism, HLA-G. The lack of HLA-A, -B, and -C products at this interface would allow escape from T-cell mediated attack, while the presence of HLA-G may enable evasion of NK cell-mediated destruction. We provide evidence that in addition to HLA-G, the classical class I product HLA-C is also present in trophoblast. Specifically, cDNA from the trophoblast-derived JEG 3 cell line encodes the HLA-C-locus product, HLA-Cw∗0401. This protein, obtained by in vitro transcription\translation, has biochemical characteristics identical to MHC class I products immunoprecipitated directly from the same cells. These findings are in agreement with RNA analysis and immunohistochemistry on both cell lines and primary trophoblast tissues. We report here the preferential reactivity in JEG 3 cells of two widely used monoclonal anti-MHC class I heavy chain antibodies, HC10 and HCA2, with HLA-C and HLA-G, respectively. We have mapped the epitopes recognized by these reagents to distinct areas of the α 1 domain of the MHC class I heavy chain. HCA2 recognizes the motif xLxTLRGx spanning amino acids 77–84 present in both HLA-A and HLA-G. In contrast, HC10 may recognize a discontinuous epitope, with essential elements of the recognized motif surrounding residue 60 in the α1 domain of the class I heavy chain, as shown by truncation analysis. These results adequately explain the immunochemical cross-reactivity of HLA-A and HLA-G.