Sequence-based typing provides a new look at HLA-C diversity

Although extensive HLA-A and HLA-B polymorphism is evident, the true diversity of HLA-C has remained hidden due to poor resolution of HLA-C Ags. To better understand the polymorphic nature of HLA-C molecules, 1823 samples from the National Marrow Donor Program research repository in North America have been typed by DNA sequencing and interpreted in terms of HLA-C diversification. Results show that HLA-Cw*0701 was the most common allele with a frequency of 16%, whereas 28% of the alleles typed as Cw12-18 (serologic blanks). The frequency of homozygotes was 9.8% as compared with previous studies of 18% for sequence-specific primers and 50% for serology. Most startling was the frequency at which new alleles were detected; 19 new HLA-C alleles were detected, representing a rate of approximately 1 in 100 samples typed. These new HLA-C alleles result from 29 nucleotide substitutions of which 4 are silent, such that coding substitutions concentrated about the Ag-binding groove predominate. Polymorphism at the HLA-C locus therefore resembles that at the HLA-A and HLA-B loci more than previously believed, indicating that antigenic stress is driving HLA-C evolution. However, sequence conservation in the alpha-helix of the first domain and a clustering of unique amino acids around the B pocket indicate that HLA-C alleles respond to antigenic pressures differently than HLA-A and HLA-B. Finally, because the samples characterized were predominantly from Caucasians, we hypothesize that HLA-C polymorphism will equal or exceed that of the HLA-A and -B loci as DNA sequence-based typing is extended to include more non-Caucasian individuals.