TIGIT, a protein expressed by T and NK cells, binds to PVR and PVRL2 but not PVRL3, and inhibits NK cell cytotoxicity directly through its ITIM motif. This study shows that TIGIT is expressed by all human NK cells and inhibits NK-mediated killing of tumor cells while protecting normal cells from NK-mediated cytotoxicity, providing an "alternative self" mechanism for MHC class I inhibition. TIGIT interacts with PVR and PVRL2, and its inhibitory activity is dependent on the ITIM. TIGIT also functions as an inhibitory receptor on NK cells, counteracting the coactivating effects of DNAM-1 and CD96. The study demonstrates that TIGIT inhibits NK cell cytotoxicity by directly interacting with PVR and PVRL2, and that this interaction is more potent than the coactivating signals from DNAM-1 and CD96. TIGIT provides an alternative self-mechanism for MHC class I inhibition, preventing NK-mediated killing of normal cells. The findings suggest that TIGIT is a critical immunomodulator that controls NK and T cell activities.TIGIT, a protein expressed by T and NK cells, binds to PVR and PVRL2 but not PVRL3, and inhibits NK cell cytotoxicity directly through its ITIM motif. This study shows that TIGIT is expressed by all human NK cells and inhibits NK-mediated killing of tumor cells while protecting normal cells from NK-mediated cytotoxicity, providing an "alternative self" mechanism for MHC class I inhibition. TIGIT interacts with PVR and PVRL2, and its inhibitory activity is dependent on the ITIM. TIGIT also functions as an inhibitory receptor on NK cells, counteracting the coactivating effects of DNAM-1 and CD96. The study demonstrates that TIGIT inhibits NK cell cytotoxicity by directly interacting with PVR and PVRL2, and that this interaction is more potent than the coactivating signals from DNAM-1 and CD96. TIGIT provides an alternative self-mechanism for MHC class I inhibition, preventing NK-mediated killing of normal cells. The findings suggest that TIGIT is a critical immunomodulator that controls NK and T cell activities.