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We examined by RT-PCR the effect of a number of immunomodulatory compounds on cytokine gene expression at the level of mRNA in the HMC-1 human leukemic mast cell line. Resting cells expressed relatively low constitutive levels of mRNA for the cytokine genes IL-3, IL-4 and IL-8, and mRNA levels for each of these cytokines were significantly enhanced after 4-h stimulation with the calcium ionophore ionomycin. Treatment of the cells with the immunosuppressant CsA at 10(-5) M produced a significant inhibition of ionomycin-induced expression of IL-3, IL-4 and IL-8 mRNA, and at 10(-6) M produced a significant inhibition of induced expression of IL-3 and IL-8 but not IL-4. At both concentrations of CsA, expression of IL-3 was inhibited to a greater extent than that of the other two cytokines. Treatment of the cells with the corticosteroid DEX at 10(-5) M but not 10(-6) M significantly reduced the ionomycin-induced expression of IL-3 but not IL-4 or IL-8 mRNA. Progesterone and methotrexate were both inactive in modulation of induced cytokine expression in this cell line. In conclusion, this study shows that cytokine expression, particularly of IL-3, is inhibited in a human mast cell line by CsA and DEX. These findings may be relevant to the anti-allergic action of these drugs.