IL-17A increases cathelicidin through activation of a transcriptional mechanism. To investigate the mechanism of IL-17A-enhanced cathelicidin, NHEK were stimulated with IL-17A (10 ng/ml) alone or in combination with 1,25D3 (10−8 M). ActD (5 μg/ml) was added to inhibit active mRNA transcription and cathelicidin was measured by quantitative real-time PCR. A, ActD completely blocked the effect of IL-17A on cathelicidin. To evaluate the effect of IL-17A on cathelicidin mRNA stability, 5 cells were treated with 1,25D3 and the combination of IL-17A with 1,25D3. ActD was added 24 h after stimulation and cells were harvested at different time points. Cathelicidin expression was analyzed by quantitative real-time PCR and calculated relative to the time of ActD addition. Data are mean ± SD of a single experiment performed in triplicate and are representative of three independent experiments. **, p < 0.01, determined by Student’s t test.