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We compared the effects of cyclosporin A (CSA) and a macrotetrolide antibiotic, dinactin, on human T-cell proliferation and cytokine production induced by stimulation of the T-cell receptor alone (monoclonal antibody [mAb] directed against CD3) or in combination with costimulatory signals (mAbs directed against CD3 and CD28). These agents were also examined in a murine model of interleukin (IL)-5-mediated pulmonary inflammation. Dinactin inhibited T-cell proliferation induced by IL-2, by mAb to CD3, and by mAbs to CD3 plus α-CD28 with identical dose-response curves (IC50 = 10-20 ng/ml). Dinactin inhibited cytokine production with IC50 values of 10 ng/ml for IL-4 and IL-5 and 30 or 60 ng/ml for interferon-γ or IL-2, respectively. Unlike CSA, exogenous IL-2 did not alter the dinactin-mediated effects on T cells, and nuclear run-on and steady-state messenger RNA (mRNA) analysis showed that dinactin inhibited cytokine production through a post-transcriptional mechanism. CSA selectively blocked T-cell receptor-induced T-cell proliferation and cytokine production (IC50 = 10 ng/ml). Under costimulatory conditions, IL-5 synthesis was only minimally inhibited by high concentrations of CSA, and at CSA concentrations of less than 125 ng/ml, IL-5 was significantly increased above control values. Dinactin and CSA reduced pulmonary eosinophilia when administered within 1 d of airway antigen challenge. Of the cytokine mRNAs examined in the lungs of CSA-pretreated, antigen-challenged mice, IL-5 mRNA levels were the least reduced, paralleling the resistance of IL-5 to CSA observed in vitro and suggesting a role for CD28 in the in vivo induction of IL-5.