Episodic evolution and turnover of HLA-B in the indigenous human populations of the Americas

Nucleotide sequences were determined for the HLA-A, B and C alleles of three populations of Amerindians: the Havasupai tribe from North America, and the Guarani and Kaingang tribes from South America. All 15 Havasupai alleles are found in Eastern Hemisphere populations, whereas the Guarani and Kaingang each have six alleles that appear to be present only in the Western Hemisphere. Nine of the ''new'' alleles come from HLA-B, one comes from HLA-A and one from HLA-C: ten appear to be the result of recombination and one the result of point substitution. Of the 14 Guarani alleles and 16 Kaingang alleles, only four are held in common. Despite their differences, the three tribes possess comparable numbers of HLA class I alleles, revealing a trend for ''allele turnover'', in which new alleles tends to supplant older alleles rather than supplement them. Although many new HLA-B alleles have been produced in Latin America, their net effect has been to differentiate populations, not to increase allele diversity within a population. From sequence comparisons, the Amerindian subset of HLA class I allotypes appears to cover the overall ranges of peptide binding specificity, natural killer-cell interactions, and CD8 interactions, that are found in all HLA class I. The recombinations that produced the new alleles of the Kaingang and Guarani class I are predicted to have modulated these functional properties rather than radically change them. Exchange of Bw4 and Bw6 motifs by recombination are noticeably absent in the events forming new alleles in America, whereas they have been the most common of recombinations elsewhere.