Predominance of one T-cell antigen receptor BV haplotype in African populations

The human T-cell antigen receptor (TCR) is the counter-receptor for the HLA /peptide complex displayed on the surface of antigen-presenting cells. It c onfers antigen specificity on T lymphocytes and therefore plays a central r ole in pathogen recognition and host response. The most frequently used for m of the TCR is a heterodimer composed of variable alpha and beta chains. W e investigated allele frequencies for four variable-region gene segments of the beta chain (2S1, 3S1, 8S3, and 15S1) in 146 Caucasians and 165 African s. The results reveal significant unexpected differences between the two po pulations for allele frequencies, phenotypes, genotypes, and haplotypes. Am ong Caucasians. there are 43 phenotypes, whereas there are 31 among the Afr icans studied. There are 17 haplotypes in the Caucasian sample but only 10 in Africans. This loss of diversity is largely due to the high frequency of one haplotype in the African sample which represents 65% of the informativ e chromosomes. At least one copy of this haplotype is present in 90% of inf ormative individuals. As a result, 29% of Africans are homozygous for the c ommon haplotype. Less genetic diversity at TCRBV is unexpected, since Afric ans usually show greater genetic diversity than other ethnic groups. For ex ample, there are approximately twice as many HLA haplotypes in Africans com pared to Caucasians. Homozygosity is also unexpected because it reduces the number of TCR variants available to recognize HLA pathogen-derived peptide complexes.