A novel, nonclassical MHC class I molecule specific to the common chimpanzee

All expressed human MHC class I genes (HLA-A, -B, -C, -E, -F, and -G) have functional orthologues in the MHC of the common chimpanzee (Pan troglodytes). In contrast, a nonclassical MHC class I gene discovered in the chimpanzee is not present in humans or the other African ape species. In exons and more so in introns, this Patr-AL gene is similar to the expressed A locus in the orangutan, Popy-A, suggesting they are orthologous. Pair-AL/Popy-A last shared a common ancestor with the classical MHC-A locus >20 million years ago. Population analysis revealed little Patr-AL polymorphism: just three allotypes differing only at residues 52 and 91. Patr-AL is expressed in PBMC and B cell lines, but at low level compared with classical MHC class I. The Patr-AL polypeptide is unusually basic, but its glycosylation, association with beta (2)-microglobulin, and antigenicity at the cell surface are like other MHC class I. No Patr-AL-mediated inhibition of polyclonal chimpanzee NK cells was detected. The Patr-AL gene is present in 50% of chimpanzee MHC haplotypes, correlating with presence of a 9.8-kb band in Southern blots. The flanking regions of Patr-AL contain repetitive/retroviral elements not flanking other class I genes. In sequenced HLA class I haplotypes, a similar element is present in the A*2901 haplotype but not the A*0201 or A*0301 haplotypes. This element, 6 kb downstream of A*2901, appears to be the relic of a human gene related to Patr-AL. Patr-AL has characteristics of a class I molecule of innate immunity with potential to provide common chimpanzees with responses unavailable to humans.