Signal transducing mechanisms involved in human T cell activation via surface T44 molecules. Comparison with signals transduced via the T cell receptor complex*
Antibodies against the T44 surface molecule have been shown to activate human T cells to produce interleukin 2. The role of Ca2+ in the triggering of the interleukin 2producing Jurkat T cell line by anti-T44 monoclonal antibody has been investigated. We show that activation is initiated by an increase in the concentration of free cytoplasmic calcium ions [Ca2+Ii. Subsequently, we have investigated the mechanism by which perturbation of T44 molecules induces increases of [Ca2+Ii in Jurkat cells. We show that the anti-T44-mediated increase in [Ca2+Ii can occur only in presence of extracellular Ca2+, since no increment is detectable when extracellular Ca2+ is depleted by EGTA. Thus, it appears that perturbation of T44 molecules, unlike that of T3-Ti antigen receptor complex, fails to mobilize Ca2+ from intracellular stores. As inositol triphosphate is considered the putative mobilizer of Ca2+ from internal stores, we measured the levels of inositol triphosphate and of the other inositol phosphate compounds in Jurkat cells after stimulation with anti-T44 antibodies. In contrast to the stimulation via the T3-Ti antigen receptor complex, stimulation via T44 molecule does not induce increments of all three inositol phosphates. Taken together, these data indicate that stimulation mediated by the T44 molecule proceeds via a mechanism independent from the typical inositol lipid metabolism which does not involve mobilization of Ca2+ from internal stores.