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NK (natural killer) cells are lymphocytes distinct from B and T cells that induce perforin-mediated lysis of tumor cells and virus-infected cells. NK cell-mediated cytotoxicity is activated by glycoproteins on the cell surface (activating receptors) and inhibited by MHC-1 with self-peptide bound. The MHC-1 inhibitory signal through Ig-family or lectin receptors prevents NK cells from killing normal cells. Abnormal MHC-1 expression in infected or tumor cells results in the release of perforin, the lysis of the abnormal cell and the release of cytokines that stimulate the immune response. MAP kinase inhibitors, but not ras inhibitors, are able to block NK cell cytotoxicity, indicating that the pathway can function by a ras-independent manner that involves the MAP kinase pathway. This pathway includes phosphoinositide-3-kinase (PI3K) as a key component, followed by Rac1 and the exchange factor Vav. The tyrosine kinase SYK and LAT may provide an additional pathway for activation of MAP kinases leading to NK cell activation, and also Pyk-2 activation by integrins. The protein tyrosine phosphatase SHP-1 appears to mediate the cytotoxicity inhibitory signal that blocks lysis of normal cells. The balance of these positive and negative signaling pathways regulates the role of NK cells in the immune response. (This definition may be outdated - see the DesignNote.)
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