Fig. 3. Caspase activity is required at late steps during influenza virus replication in several cell types independent of the type I interferon response. (A) MDCK cells were infected with influenza A virus (m.o.i. = 1) and incubated in the presence of the solvent DMSO, the inactive inhibitor analogue Z-FA-FMK or the active caspase inhibitor Z-DEVD-FMK for the times indicated. (B) A549 or Vero cells were infected with influenza A virus (m.o.i. = 1) in the presence of the a broad-range caspase inhibitor Z-VAD-FMK or the more specific inhibitor Z-DEVD-FMK. The inactive inhibitor analogue Z-FA-FMK and the solvent DMSO served as a control. Twenty-four hours p.i., virus titres were determined by plaque assay. The DMSO control was arbitrarily set as 100%. The data shown are a mean ± SE of triplicate plaque titrations which were independently confirmed.