Regulation of NK cell activity : studies of DAP12-associated receptors in immune synapse formation and in responses to cytomegalovirus infection

Abstract: Natural killer (NK) cell effector functions are important for innate resistance against tumor cells and viral infections. NK cells display a broad range of inhibitory and activating receptors on the cell surface, which ensure specificity. Several activating NK cell receptors are co-expressed with and function through the immunoreceptor tyrosinebased activation motif (ITAM)-bearing molecule DAP12. The general aim of this thesis was to address the role of a specific signaling pathway for activating NK cell receptors, the DAP12 pathway, in complex situations such as cellular or hostpathogen interactions. The redistribution of inhibiting and activating receptors and their ligands to the NK cell immune synapse have recently been investigated. To determine the role of DAP12 signaling in activating NK cell immune synapse formation, we established a system based on in vitro co-incubation with mouse NK cells and ligand-expressing target cells, using NK cells from mice bearing DAP12 molecules with non-functional ITAMs. We showed that the recruitment of the DAP12-associated activating NK cell receptor Ly49D to the NK cell immune synapse upon ligand-interaction was independent of DAP12 signaling. Signaling was however crucial for ligand induced down-modulation of Ly49D, similar to TCR-downmodulation. To address specific activating pathways in the regulation of NK cells in the hostpathogen interaction, we studied the role of DAP12 in the early response to murine cytomegalovirus (MCMV). In DAP12 mutant mice bearing a non-functional ITAM, we found a considerable increase in viral titers in the spleen (30-40 fold) and in the liver (2-5 fold). The difference compared to wild type mice could be attributed to NK cells. Moreover, the percentage of hepatic NK cells producing IFN-