The CD 3 ~ " Cytoplasmic Domain Mediates CD 2 - induced T Cell . Activation

CD2-mediated T lymphocyte activation requires surface expression of CD3-Ti, the T cell receptor (TCR) for antigen major histocompatibility complex protein. Given the importance of CD3~" in TCR signaling, we have directly examined the ability of the CD3~" cytoplasmic domain to couple CD2 to intracellular signal transduction pathways. A cDNA encoding a chimeric protein consisting of the human CD3~" cytoplasmic domain (amino acid residues 31-142) fused to the CDSoL extracellular and transmembrane domains (amino acid residues 1-187) was transfected into a CD2+CD3-CD8 variant of the human T cell line Jurkat. The resulting transfectants expressed the CD8c~/CD3~" chimeric receptor at the cell surface in the absence of other TCR subunits. Stimulation of these transfectants with anti-T112 + anti-T113 monoclonal antibodies (mAbs) initiated both a prompt cytosolic free calcium ([Ca2+]i) rise and protein tyrosine kinase activation. Stimulation with either intact anti-T112 + anti-T113 mAbs or purified F(ab')2 fragments resulted in interleukin 2 (Ib2) secretion. In contrast, control cell lines transfected with a cDNA encoding wild-type CDSol, and thus lacking surface expression of the CD3~'cytoplasmic domain, failed to show any [Ca2+]i rise, protein tyrosine kinase activation, or IL-2 secretion after identical stimulation. These data directly establish the CD3~" cytoplasmic domain as a necessary and sufficient component of the CD3-Ti complex involved in T lymphocyte activation through CD2. Moreover, they show that CD2 signaling can function in the absence of Fc receptors.