A multidisciplinary analysis of the function of the LAT adaptor : a major and highly vulnerable T-cell signaling hub. – ADAPT

T cells are at the basis of adaptive immunity. After recognition of peptides bound to major histocompatibility complex (MHC) molecules, the tyrosine-based motifs found in the CD3 chains of the T cell antigen receptor (TCR) complex are phosphorylated by the Src family kinase Lck. This allows the recruitment of the protein tyrosine kinase ZAP-70 that in turn phosphorylates a number of substrates including the transmembrane adaptor protein LAT (linker for activation of T cells). Upon tyrosine phosphorylation, LAT nucleates the assembly of a multiprotein complex – the LAT signalosome – that links the T cell-specific and the ubiquitous components of the signalling pathways that control T cell development and function. We have demonstrated that partial-loss-of-function mutations affecting the Lat gene result in stereotyped and devastating lymphoproliferative disorders with excessive production of T helper type 2 (Th2) cytokines. The extreme vulnerability of LAT likely results from the fact that it constitutes a highly connected signalling hub of the TCR signalling cassette that exerts both positive and negative regulatory roles that are mandatory to proper T cell homeostasy. The goal of the present project is to elucidate the mechanisms through which during physiological, antigen-driven T cell responses LAT leads first to activation of intracellular effectors involved in cell metabolism, cell division, cell motility, cell survival and differentiation into effectors T cells and then exerts with a temporal delay a feedback inhibition that leads to rapid attenuation of the TCR signalling pathway. In the absence of such negative feedback, T cell responses evolves into chronic pro-inflammatory responses that perpetuate themselves in a TCR-independent manner and induce the production of massive amounts of antibodies and autoantibodies in an MHC-independent, « quasi-mitogenic » mode. The proposed study has a multidisciplinary character and relies on several approaches involving the development of innovative mouse models, the identification of modifier genes via ENU mutagenesis and proteomic studies of the LAT signalosome and of the Lck kinase. Approaches for all of which proof-of-concepts have been recently obtained in the frame of previous, non ANR-funded, research contracts. The present study will provide deeper mechanistic insight into the complexity of T cell activation and TCR signaling in normal and pathological conditions and may have far reaching implications for our understanding of the structure of signaling networks in general and the mechanisms that regulate cell homeostasy. The present project will also permit to determine whether some pathological conditions collectively called « autoimmune » on the basis of the presence of autoantibodies do not result from the presence of self-reactive T cells but from defect in the cell-intrinsic mechanisms that normally keep in check physiologic T cell responses. Our laboratory has an extensive experience and a record of contributions in the genetic analysis of the components of the TCR signalling cassette and of their role during T cell development and T cell function. The present multidisciplinary project involves collaborators that work in Switzerland (Ruedi Aebersold, ETH, Zurich) and Japan (Sho Yamasaki, Medical Institute of Bioregulation, Fukuoka). In conclusion, we expect that owing to its multidisciplinary character this project will allow us to understand how can mutations that destroy LAT, a major signaling hub that bridges the T cell-specific and the ubiquitous components of the TCR signaling pathway, result in signals that drive the expansion and differentiation of full-fledged effectors that that are capable of giving rise to an early-onset lymphoproliferative disorder? A fascinating paradox we have been confronted to since the time we and another group discovered the existence of these fascinating immunopathology in 2002.