title: T cell receptor signal initiation induced by low-grade stimulation requires the cooperation of LAT in human T cells.
abstract: BACKGROUND
  One of the earliest activation events following stimulation of the T cell receptor (TCR) is the phosphorylation of the immunoreceptor tyrosine-based activation motifs (ITAMs) within the CD3-associated complex by the Src family kinase Lck. There is accumulating evidence that a large pool of Lck is constitutively active in T cells but how the TCR is connected to Lck and to the downstream signaling cascade remains elusive.
  
  
  RESULTS
  We have analyzed the phosphorylation state of Lck and Fyn and TCR signaling in human naïve CD4+ T cells and in the transformed T cell line, Hut-78. The latter has been shown to be similar to primary T cells in TCR-inducible phosphorylations and can be highly knocked down by RNA interference. In both T cell types, basal phosphorylation of Lck and Fyn on their activatory tyrosine was observed, although this was much less pronounced in Hut-78 cells. TCR stimulation led to the co-precipitation of Lck with the transmembrane adaptor protein LAT (linker for activation of T cells), Erk-mediated phosphorylation of Lck and no detectable dephosphorylation of Lck inhibitory tyrosine. Strikingly, upon LAT knockdown in Hut-78 cells, we found that LAT promoted TCR-induced phosphorylation of Lck and Fyn activatory tyrosines, TCRζ chain phosphorylation and Zap-70 activation. Notably, LAT regulated these events at low strength of TCR engagement.
  
  
  CONCLUSIONS
  Our results indicate for the first time that LAT promotes TCR signal initiation and suggest that this adaptor may contribute to maintain active Lck in proximity of their substrates.
authors.affiliation: Unit of Cytokine Signaling, Department of Immunology, Institut Pasteur, Paris, France.
authors.forename: Shen
authors.initials: S
authors.lastname: Dong
authors.forename: Béatrice
authors.initials: B
authors.lastname: Corre
authors.forename: Konstantina
authors.initials: K
authors.lastname: Nika
authors.forename: Sandra
authors.initials: S
authors.lastname: Pellegrini
authors.forename: Frédérique
authors.initials: F
authors.lastname: Michel
mesh_terms: D048868:Adaptor Proteins, Signal Transducing / Q000235:genetics / Q000276:immunology* / Q000378:metabolism; D015496:CD4-Positive T-Lymphocytes / Q000166:cytology / Q000276:immunology / Q000378:metabolism; D045744:Cell Line, Tumor; D002478:Cells, Cultured; D006801:Humans; D015151:Immunoblotting; D019860:Lymphocyte Specific Protein Tyrosine Kinase p56(lck) / Q000378:metabolism; D008565:Membrane Proteins / Q000235:genetics / Q000276:immunology* / Q000378:metabolism; D010766:Phosphorylation; D051076:Proto-Oncogene Proteins c-fyn / Q000378:metabolism; D034622:RNA Interference; D011948:Receptors, Antigen, T-Cell / Q000276:immunology* / Q000378:metabolism; D015398:Signal Transduction / Q000276:immunology; D013379:Substrate Specificity; D013601:T-Lymphocytes / Q000166:cytology / Q000276:immunology* / Q000378:metabolism; D014443:Tyrosine / Q000378:metabolism
chemical_list: D048868:Adaptor Proteins, Signal Transducing; C110810:LAT protein, human; D008565:Membrane Proteins; D011948:Receptors, Antigen, T-Cell; D014443:Tyrosine; C494957:FYN protein, human; D019860:Lymphocyte Specific Protein Tyrosine Kinase p56(lck); D051076:Proto-Oncogene Proteins c-fyn