Rapid evolution of NK cell receptor systems demonstrated by comparison of chimpanzees and humans

That NK cell receptors engage fast-evolving MHC class I ligands suggests th at they, too, evolve rapidly. To test this hypothesis, the structure and cl ass I specificity of chimpanzee KIR and CD94:NKG2 receptors were determined and compared to their human counterparts. The KIR families are divergent, with only three KIR conserved between chimpanzees and humans. By contrast, CD94:NKG2 receptors are conserved. Whereas receptors for polymorphic class I are divergent, those for nonpolymorphic class I are conserved. Although c himpanzee and human NK cells exhibit identical receptor specificities for M HC-C, they are mediated by nonorthologous KIR. These results demonstrate th e rapid evolution of NK cell receptor systems and imply that "catching up" with class I is not the only force driving this evolution.