Soluble antibodies against the T3/antigen receptor complex alone are not sufficient to induce proliferation and interleukin 2 expression by T lymphocytes. An additional requirement is the presence of accessory cells (AC). In this model, AC provide at least two functions required for T cell activation: 1) the surface interaction of T3 antibodies with Fc receptors on the AC surface and 2) the production of soluble mediators such as interleukin 1 (IL 1). In the experiments reported here, these stimuli are represented by T3 antibodies immobilized onto Sepharose beads and by recombinant IL 1. In this study we investigated differences in activation requirements in resting and activated T cells. Resting T cells were represented by AC-depleted peripheral blood mononuclear cells (PBMC) or the T cell line Jurkat, which phenotypically resembles a resting T cell. Activated T cells were represented by T cell clones and the T cell line HUT 78, which express the activation molecules Ia and the IL 2 receptor (Tac). In resting cells, activation required the presence of three different signals: perturbation of the T3/antigen receptor complex by T3 antibodies, surface redistribution of T3/antigen receptor complexes, and presence of IL 1. In contrast, activated T cells require only perturbation and redistribution of T3/antigen receptor complexes and not IL 1 for the induction of proliferation or IL 2 production. Possible mechanisms of intracellular signaling for these stimuli are discussed.

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