The structure and expression of genes encoding serologically undetected HLA-C locus antigens

Approximately 20 to 50% individuals in every race are untypable by human alloantisera for at least one allele of HLA-C locus and the surface expression of HLA-C locus Ag in such an individual (HLA-C blank Ag) remains unknown. To investigate the structure and the surface expression of HLA-C blank Ag, two genes (Cb-1 and Cb-2) encoding HLA-C blank Ag were cloned and their primary structures were determined and compared with other HLA-C locus genes. The similarity of amino acids between Cb-1 and Cw1 was the highest among HLA-C locus genes previously published. Five amino acid substitutions between these molecules were shown to be located on the beta-strand of alpha 1 and alpha 2 domains, suggesting that they might change the conformational allodeterminants on the alpha-helical region of Cw1 which were recognized by antibodies. On the other hand, Cb-2 was the closest to Cw2.2. Six of nine amino acid substitutions between these molecules were observed on alpha 1 and alpha 2 domains, whereas three other substitutions were located on the leader peptide, the alpha 3 domain and the transmembrane. Two substitutions (residues 73 and 163) of the alpha-helical region of the alpha 1 and alpha 2 domains and one (residue 16) of exposed loop may make new allodeterminants which are not recognized by anti-Cw2 sera as well as other alloantisera. The surface expression of these genes was examined on transfected mouse L cells and human B cell line. Both gene products were expressed stably on the surface of these cells. These results suggest that HLA-C blank Ag are most probably expressed on cells in HLA-C blank individuals and that the primary structures of these Ag, which were not detectable by the available alloantisera, may be incapable of generating corresponding alloantibodies.