Molecular characterization of HLA-A28*, a novel HLA product, and its relationship to HLA-A28 and HLA-A2.

The HLA-A28* molecule expressed by the B-cell line IDF is serologically distinct and intermediate between HLA-A28 and HLA-A2. Comparative tryptic peptide mapping of biosynthetically labeled HLA-A28*, A28, and A2 molecules showed that HLA-A28* is also chemically distinct. Reverse-phase high pressure liquid chromatographic analysis of tryptic peptides labeled with 3H-arginine and 3H-lysine revealed that A28*, A28, and A2 share approximately 65% of their tryptic peptides. Multiple differences were observed between A28* and both A28 and A2. No peptides unique to A28* were detected and 25 peptides were shared with both A28 and A2. These results show that A28* is a novel HLA product that is closely related to A28 and A2. Tryptic peptide map comparisons of these molecules labeled separately with 11 amino acids confirm these results. The data suggest that HLA-A28* may have arisen from a genetic exchange event involving HLA-A28 and -A2. These data are consistent with the hypothesis that A28* is identical with A28 in the first extracellular domain (alpha 1) and identical with A2 in the second domain (alpha 2).