Variation in the ligand binding domains of the CD94/NKG2 family of receptors in the squirrel monkey

Natural killer cells are regulated, in part, by cell surface expression of the inhibitory CD94/NKG2A heterodimer and the activating CD94/NKG2C heterodimer. In the present study, we characterize the CD94/NKG2 family in the squirrel monkey, a New World monkey species. Full-length CD94, NKG2A, and NKG2CE complementary deoxyribonucleic acid molecules were identified in three unrelated squirrel monkeys. Three alternatively spliced forms of CD94 were detected in which part of intron 4 was included in the mature transcript, suggesting evolutionary pressure for changes in the corresponding loop 3 region of the lectin domain in squirrel monkeys. Squirrel monkey NKG2A contains a three-nucleotide indel that results in an additional amino acid in the predicted NKG2A protein compared to NKG2A in other species. This NKG2A insertion tracks to loop five of the lectin domain, as is seen with the recently described marmoset NKG2CE indel. Transmembrane-deleted forms of CD94 and NKG2CE were also expressed in the squirrel monkey. Analysis of full-length squirrel monkey and additional primate CD94/NKG2 sequences demonstrated statistically significant increases in the Ka/Ks ratio in the putative major histocompatibility complex E (MHC-E) binding domain compared to the non-binding domain. Furthermore, positive selection was detected in the MHC-E binding domain of primate NKG2 family members, and purifying selection was detected in the primate CD94 binding domain. Purifying selection was also detected in the nonbinding domains of primate CD94 and NKG2 molecules.