Novel mechanisms of innate immunity

Over decades, one of the central dogmas of immunity has been that immune cells can be classified as parts of either the adaptive or innate immune system. In recent years, however, the lines between these two systems have become increasingly blurred. For example, subsets of innate natural killer (NK) cells have been demonstrated to display memory phenotypes, while recently identified mucosal-associated invariant CD8 T cells (MAIT cells) realize innate cytotoxicity. MAIT and NK cells both constitute predominant populations in human blood and other organs and share a high expression of CD161. NK cells detect virus-infected and abnormally transformed cells, and MAIT cells are able to identify and directly lyse bacterial-infected cells. Although NK cell activation is well defined, the underlying signaling networks are still obscure. In the first part of this thesis, the role of CaMKII and PKD kinases in NK cell activity was investigated. It could be shown that chemical inhibition leads to a significant decrease of NK cell cytotoxicity and cytokine release, underscoring the importance of CaMKII and PKDs in NK cell activation. Notably, these experiments also indicate significant donor-variations, inhibitory pathways and further involved cell types, which concertedly bias drug-modulated NK responses. In the second part, new mechanisms of innate immunity were investigated for the first time in primary human MAIT cells. Currently, the exact role of MAIT cells in health and disease remains widely undefined, and proteomic data about this innate-like T cell subset is missing. Primary human MAIT were proteomically compared with innate NK cells and conventional CD8 T cells by mass spectrometry. In total, 5500 proteins could be identified from five representative human donors. These data reveal a functional inventory of MAIT cells that integrates phenotypes from both NK and CD8 cells. Donor-dependent analyses then defined a MAIT-specific profile of effector molecules, and regulators of exocytosis and proliferation, respectively. Furthermore, this thesis provides candidates that can now be considered as novel MAIT markers and/or likely contribute to their anti-bacterial unique phenotype. In this context, the first observation of the immunological synapse (IS) of a MAIT cell interacting with a bacterial infected target cell is presented and recruitment of candidate protein S100A4 to the site of anti-microbial activity is confirmed.