Uncoupling activation-dependent HS1 phosphorylation from nuclear factor of activated T cells transcriptional activation in Jurkat T cells: Differential signaling through CD3 and the costimulatory receptors CD2 and CD28

CD3, CD2, and CD28 are functionally distinct receptors on T lymphocytes, Engagement of any of these receptors induces the rapid tyrosine phosphorylation of a shared group of intracellular signaling proteins, including Vav, Cbl, p85 phosphoinositide 3-kinase, and the Src family kinases Lck and Fyn, Ligation of CD3 also induces the tyrosine phosphorylation of HS1, a 75-kDa hematopoietic cell-specific intracellular signaling protein of unknown function, We have examined changes in HS1 phosphorylation after differential stimulation of CD3, CD2, and CD28 to elucidate its role in T cells and to further delineate the signaling pathways recruited by these receptors, Unlike ligation of CD3, stimulation with anti-CD28 mAb or CHO cells expressing the CD28 ligands CD80 or CD86 did not lead to tyrosine phosphorylation of HS1 in Jurkat T cells, Additionally, no tyrosine phosphorylation of HS1 was induced by mitogenic pairs of anti-CD2 mAbs capable of activating the transcription factor NFAT (nuclear factor of activated T cells), Costimulation through CD28 and/or CD2 did not modulate the CD3-dependent phosphorylation of HS1, Ln vivo studies indicated that CD3-induced HS1 phosphorylation was dependent upon both the Src family tyrosine kinase Lck and the tyrosine phosphatase CD45, did not require MEK1 kinase activity, and was regulated by protein kinase C activation, Thus, although CD3., CD28, and CD2 activate many of the same signaling molecules, they differed in their capacity to induce the tyrosine phosphorylation of HS1. Furthermore, activation-dependent tyrosine phosphorylation of HS1 was not required for NEAT transcriptional activation.