Elucidating the origin of HLA-B*73 allelic lineage: Did modern humans benefit by archaic introgression?

A previous study reported that some of the human leukocyte antigen (HLA) alleles and haplotypes in present-day humans were acquired by admixture with archaic humans; specifically, an exceptionally diverged HLA-B*73 allele was proposed to be transmitted from Denisovans, although the DNA sequence of HLA-B*73 has not been detected in the Denisovan genome. Here, we argue against the hypothesis that HLA-B*73 introgressed from Denisovans into early modern humans. A phylogenetic analysis revealed that HLA-B*73:01 formed a monophyletic group with a chimpanzee MHC-B allele, strongly suggesting that the HLA-B*73 allelic lineage has been maintained in humans as well as in chimpanzees since the divergence of humans and chimpanzees. The global distribution of HLA-B*73 allele showed that the population frequency of HLA-B*73 in west Asia (0.24 %)-a possible site of admixture with Denisovans-is lower than that in Europe (0.72 %) and in south Asia (0.69 %). Furthermore, HLA-B*73 is not observed in Melanesia even though the Melanesian genome contains the highest proportion of Denisovan ancestry in present-day human populations. Single nucleotide polymorphisms in HLA-A*11-HLA-C*12:02 or HLA-A*11-C*15 haplotypes, one of which was assumed to be transmitted together with HLA-B*73 from Denisovans by the study of Abi-Rached and colleagues, were not differentiated from those in other HLA-A-C haplotypes in modern humans. These results do not support the introgression hypothesis. Thus, we conclude that it is highly likely that HLA-B*73 allelic lineage has been maintained in the direct ancestors of modern humans.