title: The transmembrane adaptor protein SIT inhibits TCR-mediated signaling.
abstract: Transmembrane adaptor proteins (TRAPs) organize signaling complexes at the plasma membrane, and thus function as critical linkers and integrators of signaling cascades downstream of antigen receptors. We have previously shown that the transmembrane adaptor protein SIT regulates the threshold for thymocyte selection. Moreover, T cells from SIT-deficient mice are hyperresponsive to CD3 stimulation and undergo enhanced lymphopenia-induced homeostatic proliferation, thus indicating that SIT inhibits TCR-mediated signaling. Here, we have further addressed how SIT regulates signaling cascades in T cells. We demonstrate that the loss of SIT enhances TCR-mediated Akt activation and increased phosphorylation/inactivation of Foxo1, a transcription factor of the Forkhead family that inhibits cell cycle progression and regulates T-cell homeostasis. We have also shown that CD4(+) T cells from SIT-deficient mice display increased CD69 and CD40L expression indicating an altered activation status. Additional biochemical analyses further revealed that suppression of SIT expression by RNAi in human T cells resulted in an enhanced proximal TCR signaling. In summary, the data identify SIT as an important modulator of TCR-mediated signaling that regulates T-cell activation, homeostasis and tolerance.
authors.affiliation: Institute of Molecular and Clinical Immunology, Otto-von-Guericke University Magdeburg, Magdeburg, Germany.
authors.forename: Börge
authors.initials: B
authors.lastname: Arndt
authors.forename: Tina
authors.initials: T
authors.lastname: Krieger
authors.forename: Thomas
authors.initials: T
authors.lastname: Kalinski
authors.forename: Anja
authors.initials: A
authors.lastname: Thielitz
authors.forename: Dirk
authors.initials: D
authors.lastname: Reinhold
authors.forename: Albert
authors.initials: A
authors.lastname: Roessner
authors.forename: Burkhart
authors.initials: B
authors.lastname: Schraven
authors.forename: Luca
authors.initials: L
authors.lastname: Simeoni
mesh_terms: D048868:Adaptor Proteins, Signal Transducing / Q000172:deficiency / Q000235:genetics / Q000378:metabolism*; D000818:Animals; D015496:CD4-Positive T-Lymphocytes / Q000166:cytology / Q000378:metabolism; D002453:Cell Cycle; D000071161:Forkhead Box Protein O1; D051858:Forkhead Transcription Factors / Q000378:metabolism; D006706:Homeostasis; D006801:Humans; D008562:Membrane Glycoproteins; D008565:Membrane Proteins / Q000172:deficiency / Q000235:genetics / Q000378:metabolism*; D051379:Mice; D010766:Phosphorylation; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism; D034622:RNA Interference; D011948:Receptors, Antigen, T-Cell / Q000378:metabolism*; D015398:Signal Transduction
chemical_list: D048868:Adaptor Proteins, Signal Transducing; C085126:FOXO1 protein, human; D000071161:Forkhead Box Protein O1; D051858:Forkhead Transcription Factors; D008562:Membrane Glycoproteins; D008565:Membrane Proteins; D011948:Receptors, Antigen, T-Cell; C118781:SIT1 protein, human; D051057:Proto-Oncogene Proteins c-akt