A new HLA-A9 subtype lacking the Bw4 epitope. Ancestral or revertant allele?

The HLA-A9 group has been subdivided into three serologically defined splits, A23, A24, and A2403. We have found a new HLA-A9 split antigen, tentatively called A24AK, in the Japanese population. Sequence analysis of A24AK (officially assigned A*2404) showed that this new allele was different from HLA-A*2402, which codes for the common A24 antigen, by seven nucleotides, and the two alleles could be discriminated by the PCR-SSCP method. These nucleotide substitutions are predicted to result in substitution of six amino acid residues at positions 76, 79, 80, 81, 82, and 83. In all HLA-A9-group alleles described to date, this region is known to code for the Bw4 epitope, which is usually localized on HLA-B molecules. However, the new allele lacks the Bw4 coding sequence. Sequencing results are supported by results showing that the lymphocytes from A24AK-positive individuals did not react with anti-Bw4 antiserum. The nucleotide sequence of A*2404 in this region was identical to that of A*0101, A*2601, A*2602, and several other alleles. These findings suggest several possible paths of evolution of this new allele.