SARS-CoV-2 N protein represses IFN-β production induced by poly(I:C) and Sendai virus. (A,B) HEK293T cells were transfected with IFN-β luciferase reporter pIFN-β-Luc and pPRL-TK for 24 h and then infected with SeV (0.1 MOI) for 16 h (A) or transfected with cytoplasmic poly(I:C) (2 μg/mL) for 16 h (B). Cell lysates were harvested, IFN-β-Luc reporter activity was determined by dual luciferase reporter assays (top), and HA-N was detected by Western blotting (bottom). (C,D) A549 cells were transfected with pFlag-N for 24 h and infected with SeV (MOI = 0.1) for 16 h (C) or transfected with poly(I:C) (1 μg/mL) for 16 h (D). IFN-β mRNA was determined by q-PCR (top) and Flag-N was confirmed by Western blotting (bottom). (E,F) HEK293T cells were transfected with pHA-N for 24 h and infected with SeV (MOI = 0.1) for 16 h (E) or transfected with poly(I:C) (2 μg/mL) for 16 h (F). IFN-β mRNA was determined by q-PCR (top) and HA-N was confirmed by Western blotting (bottom). Data in A–F were expressed as the mean ± SEM of at least three independent experiments. * p < 0.05, ** p < 0.01, *** p < 0.001.