Killer cell receptors: keeping pace with MHC class I evolution.

NK cells express receptors that bind to polymorphic determinants of MHC class I heavy chains. MHC ligands vary greatly between mammalian species, and the use of distinct molecular families of NK cell receptors by humans and mice suggests that the receptors too can be evolving rapidly. The KIR (killer cell inhibitory receptor) family of receptors are found in primates and recognize class I epitopes that are of relatively recent origin in primate evolution. Therefore, KIR molecules have probably evolved class I receptor function more recently than C-type lectins, which are represented in both humans and mice. Individual humans express NK cell receptors for which they have no class I ligand, demonstrating a looseness in the coupling of expression between the receptors and their ligands. However, study of a single donor suggests that every NK cell expresses at least one inhibitory receptor for a self-HLA class I allotype, consistent with the missing self hypothesis. Thus the NK-cell receptor-class I interaction appears to control the NK-cell repertoire during ontogeny of the individual and has the potential to be a selective factor influencing both MHC class I and NK cell receptor diversity in the evolution of populations and species.