Deciphering innate immunity and tolerance from single molecules to systemic levels – NK Tolerance

Self versus non-self discrimination is a central theme in biology, extending from plants to vertebrates. Natural killer (NK) cells in the mammalian innate immune system constitute an experimental model system for investigating the molecular and cellular mechanisms involved in these complex biological processes. Indeed, NK cells are tolerant to normal self tissues, but kill and produce cytokines in response to tumors or virus-infected cells. - NK cells recognize the absence of self MHC class I molecules as a mechanism of discriminating normal cells from cells in distress. This missing self recognition is ensured by inhibitory receptors which dampen NK cell activation upon interaction with their ligands. Inhibitory receptors are germline-encoded by a set of polymorphic genes that segregate independently from their ligands and are not uniformly expressed on NK cells. A subset of NK cells lacking inhibitory receptors for self MHC class I molecules has been described in healthy organisms. These cells have a mature phenotype and, despite the expression of normal levels of activating receptors, they are hyporesponsive to stimuli involving the engagement of these receptors. Contrary to expectations, NK cells lacking inhibitory receptors are thus impaired in their functions. Our results in human and other studies in the mouse have shown that self MHC class I recognition is required for the acquisition of NK cell functional competence. However, the education process involved in the calibration of NK cell effector capacities and its role in the subsequent missing self recognition need to be further elucidated. We therefore propose to elucidate the mechanisms of NK cell education to self versus non-self discrimination. - An understanding of such complex biological systems, from the molecular scale to higher level tissue architecture and beyond, to the physiology of whole organisms, requires multiple approaches at various levels. We thus propose to work up from the molecular level (nanotechnology approaches) to the whole organism scale (random germline mutagenesis in the mouse, genome-wide transcriptome analysis, NK cell imaging in tissues). The imaging of single molecules has made it possible to measure parameters, such as the number of molecules, diffusion coefficients, spatial distributions and temporal fluctuations, essential for the construction of quantitative models. We will also try to develop an understanding of the system as a whole, using the large-scale approaches listed above. We will generate, in particular, a number of new mouse mutants. - By determining the mechanisms adjusting the balance of receptor signaling appropriately so as to allow both self-tolerance and a capacity to respond to diseased cells in NK cells, we should be able to improve our understanding of developmental mechanisms that may operate broadly in the vertebrate innate immune system. - - ...