Activation of Human T Lymphocytes by Phorbol‐12,13‐Dibutyrate and Ionomycin

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Abstract
The calcium ionophore ionomycin and the phorbol ester phorbol‐12,13‐dibutyrate (PDBu) are shown to have a synergistic effect upon interleukin 2 (IL‐2) production, interleukin 2 receptor expression, and T‐lymphocyte proliferation.The proliferative response was inhibited by addition of a monoclonal antibody directed against the IL‐2R (Tac antigen) demonstrating that PDBu and ionomycin induce T‐cell growth through an IL‐2‐dependent autocrine pathway.Sequential stimulation with PDBu and ionomycin failed to induce IL‐2 production, IL‐2R expression, and consequently proliferation of the T cells, indicating that T‐cell activation requires simultaneous activation of protein kinase C (PKC) and elevation of cytosolic calcium.Exposure of T cells to both agents for different times resulted in IL‐2 production, IL‐2R expression, and proliferation in proportion to the duration of incubation with at least 4 h required for maximal T‐cell activation. Further, in the presence of PDBu maximal T‐cell activation was found to require stimulation with ionomycin for 4 h, indicating that a sustained increase in free cytoplasmic calcium of several hours' duration is essential for T‐cell activation. In contrast T cells incubated with ionomycin were induced to produce IL‐2 and express IL‐2Rs upon brief exposure to PDBu with a 2‐h incubation period being sufficient for maximal T‐cell activation. Thus transient activation of PKC seems to be sufficient for activation of the IL‐2 gene and IL‐2R gene. However, maximal T‐cell activation requires activation of PKC for at least 2 h.