The role of SHIP in natural killer cell licensing

Natural Killer (NK) cells comprise a unique subset of lymphocytes with innate ability to detect and kill abnormal cells. Inhibitory Ly49 receptors expressed on NK cells recognize major histocompatibility complex class I (MHC I) molecules and inhibit NK cell killing of normal cells that express high amounts of MHC I. In addition to maintaining self-tolerance of NK cells, the Ly49:MHC I interaction is required for the functional maturation of NK cells by a process termed “NK cell licensing.” NK cells from MHC I-deficient β2 microglobulin (β2m)-KO mice are not cytotoxic as they are not licensed. Our goal was to determine the mechanism through which NK cells of MHC-I deficient mice are kept hyporesponsive to NK-sensitive targets. We compared unlicensed NK cells from β2m-KO mice with licensed wild type (WT) NK cells in each step of NK cell cytotoxicity. We found that LFA-1 mediated adhesion to target and polarization of actin and talin are functional in these cells, yet unlicensed NK cells fail to polarize cytotoxic granules when bound to the prototypic NK target YAC-1, or beads coated with combinations of ligands for NK stimulatory receptors, namely the LFA-1 ligand ICAM-1, NKG2D ligand H60, and 2B4 ligand CD48. Thus, NKG2D and 2B4 mediated signalling seemed impaired in unlicensed NK cells. Ly49 receptors, upon tyrosine phosphorylation, are thought to recruit both Src homology 2-containing phosphatase 1 (SHP1) and Src homology 2 domain-containing inositol-5′-phosphatase (SHIP), whereas NK cell licensing is known to be SHP1-independent. Therefore, we tested whether SHIP is involved in NK cell licensing. In both WT and β2m-KO NK cells, antibody mediated cross-linking of inhibitory Ly49 receptors induced recruitment of SHIP rather than SHP1. When NK cells bound to MHC I-deficient YAC-1, SHIP was enriched at the contact site of unlicensed, but not in licensed, NK cells. These results suggest that during the process of NK cell licensing, Ly49C/I recruits SHIP and prevents SHIP’s association with activating NK cell receptors allowing stimulatory signalling and cytotoxicity to occur. These findings provide insight into the mechanism of NK cell licensing and signalling generated by the Ly49: MHC I interaction.Medicine, Faculty ofGraduat