Immunogenetic study of a new HLA allele, B*2723

A novel HLA-B*27 allele (B*2723) detected by irregular serological and PCR-SSP typing results was identified by nucleotide sequencing of exons 2 and 3. B*2723 differs from B*27052 by nine nucleotides which encode seven amino acid changes at positions 63 (Glu to Asn), 67 (Cys to Phe), 69 (Ala to Thr), 70 (Lys to Asn), 71 (Ala to Thr), 74 (Asp to Tyr) and 77 (Asp to Ser) in the alpha1 helix. All these substitutions are possessed by B*35 alleles suggesting that B*2723 was created by a gene conversion-like event involving B*27052 and a B*35 allele. Using the HLA-A*26 and DRB1*12 alleles of the B*2723-bearing haplotype as 'markers', two further examples of B*2723 were found in 29,851 blood donors. Therefore, B*2723 has a 'minimum' gene frequency of 0.000034 (phenotype frequency 0.0067%) in blood donors resident in Wales. In all three families, B*2723 was present on a haplotype with: A*26; Cw*0202; DRB1*1201/6/7; DRB3*02; DQA1*05; DQB1*0301. The B*2723 product failed to react with HLA-B27 antisera and reacted weakly or not at all with Bw4 antisera. Lack of the ECAKA motif at amino acid positions 63, 67, 69-71 probably accounts for lack of the B27 specificity while the amino acid combination 74Y, 77S, 80T, 81L may cause aberrant Bw4 reactivity.