Exploring the inhibition of human natural killer cells: Co-operation of KIR and ILT2

Natural Killer (NK) cells function in defense against viruses and cancerous cells. They express combinations of activating and inhibitory receptors in a stochastic manner during maturation. Generally, each NK cell expresses an activating receptor and at least one inhibitory receptor for self-MHC-I to mediate self-tolerance and detect altered MHC-I expression on cells. The precise way signals from inhibitory and activating NK receptors are integrated is not fully understood.; Inhibitory Killer-cell Ig-like Receptors (KIR) signal by recruiting the tyrosine phosphatase SHP-1 to immunoreceptor tyrosine-based inhibitory motifs (ITIMs). The majority of my thesis work has been centered on the surprising observation that KIRs lacking ITIMs signal for inhibition in human NK cells. Signalling by ITIM-deficient KIR is weaker than wildtype KIR, does not require the transmembrane or cytoplasmic tail of KIR, and is blocked by over-expression of a catalytically inactive SHP-1 molecule. Antibody blocking studies revealed that ITIM-deficient KIR signalling requires, in addition to a KIR and MHC-I interaction, an Ig-like transcript 2 (ILT2) interaction with the alpha-3 domain of MHC-I. ILT2, an inhibitory receptor present on a variety of cells, is found to be expressed at low levels on a subset of NK cells. On its own, ILT2 is insufficient to signal in response to HLA-C. My studies with ITIM-deficient KIR and HLA-C have revealed the contribution of ILT2 to KIR signalling. The endogenous level of ILT2 on human NK cells can signal in a KIR-dependent manner and perhaps ILT2 and KIR co-expression compensates for weaker KIR and MHC-I interactions.; Another portion of this thesis explores the effect of Vaccinia virus (VV) infection on NK cell function. NK cells are implicated in the response to poxviruses, but the interaction between NK and infected cells is not well characterized. Downregulation of MHC-I by VV sensitize cells to lysis by NK cells. Also, NK cells become infected after co-culture with infected target cells. NK cell infection leads to decreased cytotoxicity and renders NK cells more sensitive to inhibitory signals.; Collectively, this body of work illustrates the intricate network of events between viruses, NK cells and the receptors and ligands that regulate them.