Linker for Activation of T Cells Integrates Positive and Negative Signaling in Mast Cells

The transmembrane adapter linker for activation of T cells (LAT) is thought to couple immunoreceptors to intracellular signaling pathways. In mice, its intracytoplasmic domain contains nine tyrosines which, when phosphorylated upon receptor aggregation, recruit Src-homology 2 domain-containing cytosolic enzymes and adapters. The four distal tyrosines are critical for both TCR and FcepsilonRI signaling. Unexpectedly, knock-in mice expressing LAT with a point mutation of the first or of the last three of these tyrosines exhibited an abnormal T cell development characterized by a massive expansion of TH2-like alphabeta or gammadelta T cells, respectively. This phenotype suggests that, besides positive signals, LAT might support negative signals that normally regulate terminal T cell differentiation and proliferation. We investigated here whether LAT might similarly regulate mast cell activation, by generating not only positive but also negative signals, following FcR engagement. To this end, we examined IgE- and/or IgG-induced secretory and intracellular responses of mast cells derived from knock-in mice expressing LAT with combinations of tyrosine mutations (Y136F, Y(175, 195, 235)F, or Y(136, 175, 195, 235)F). A systematic comparison of pairs of mutants enabled us to dissect the respective roles played by the five proximal and the four distal tyrosines. We found that LAT tyrosines differentially contribute to exocytosis and cytokine secretion and differentially regulate biological responses of mucosal- and serosal-type mast cells. We also found that, indeed, both positive and negative signals may emanate from distinct tyrosines in LAT, whose integration modulates mast cell secretory responses.