FIG 4.

Induction of interferon by wt or mutant influenza viruses. (A) Virus progeny from individual A549/pr(IFN-β).GFP cells that were sorted as GFP positive and infected at low multiplicity was recovered after limiting-dilution infections in MDCK-V2 cells. The ability of these viruses to induce GFP expression after infection of A549/pr(IFN-β).GFP cells was quantified by FACS, using wt and ΔNS1 viruses as references. The results are representative of 3 independent experiments, in which 10,000 events were measured for each sample. (B) The ability of mutant viruses to induce the secretion of antiviral factors was tested by infecting of A549 cells at 5 PFU/cell, using wt and ΔNS1 viruses as references. Culture supernatants were collected at 24 hpi, and their ability to interfere with EMCV infection of A549/BVDV-Npro cells was determined by endpoint dilution, as described in Materials and Methods. ****, P < 0.0001.