This study tested the requirement of IRF3 in RV1B-induced IFN responses. Experiments showed that RV1B infection of BEAS-2B cells and primary tracheal epithelial cells led to IRF3 phosphorylation and dimerization, indicating IRF3 activation. UV-irradiated RV1B did not activate IRF3. Knocking down IRF3 with siRNA abolished RV1B-induced expression of IFN-β, IFN-λ1, IFN-λ2/3, IRF7 and IP-10. However, siRNA against IRF3 did not affect RV1B-induced IL-8 or GM-CSF expression. These results demonstrate that IRF3 is necessary for