Killer cell immunoglobulin-like receptor diversity: balancing signals in the natural killer cell response.

Killer cell immunoglobulin-like receptors (KIR) are a family of inhibitory and activating receptors that are expressed by most natural killer (NK) cells and by small subpopulations of T cells. In humans the KIR genes form part of the leukocyte receptor complex (LRC) on chromosome 19. Within an individual NK cell clones are distinguished by the combinations of KIR genes they express, patterns which are established during NK cell development and remain stable. Within the human population KIR haplotypes and genotypes differ in their gene content, in the balance between genes encoding activating and inhibitory receptors, and by allelic polymorphism at the individual KIR genes. Investigation of the KIR gene family in non-human primate species shows that species-specific genes outnumber the conserved genes. The KIR gene family is seen to be both highly diverse and rapidly evolving. Such characteristics suggest that novel KIR variants provide competitive advantages in primate survival or reproduction, which are of short duration on the evolutionary time-scale.