Comparison of chimpanzee and human leukocyte Ig-like receptor genes reveals framework and rapidly evolving genes

The leukocyte receptor complex (LRC) on human chromosome 19 contains relate d Ig superfamily killer cell Ig-like receptor (KIR) and leukocyte Ig-like r eceptor (LIR) genes. Previously, we discovered much difference in the KIR g enes between humans and chimpanzees, primate species estimated to have simi lar to 99.8% genomic sequence similarity. Here, the common chimpanzee LIR g enes are identified, characterized, and compared with their human counterpa rts. From screening a chimpanzee splenocyte cDNA library, clones correspond ing to nine different chimpanzee LIRs were isolated and sequenced. Analysis of genomic DNA from 48 unrelated chimpanzees showed 42 to have all nine LI R genes, and six animals to lack just one of the genes. In structural diver sity and functional type, the chimpanzee LIRs cover the range of human LIRs . Although both species have the same number of inhibitory LIRs, humans hav e more activating receptors, a trend also seen for KIRs. Four chimpanzee LI Rs are clearly orthologs of human LIRs. Five other chimpanzee LIRs have par alogous relationships with clusters of human LIRs and have undergone much r ecombination. Like the human genes, chimpanzee LIR genes appear to be organ ized into two duplicated blocks, each block containing two orthologous gene s. This organization provides a conserved framework within which there are clusters of faster evolving genes. Human and chimpanzee KIR genes have an a nalogous arrangement. Whereas both AIR and LIR genes can exhibit greater in terspecies differences than the genome average, within each species the LIR gene family is more conserved than the KIR gene family.