Fig 4.

PB1^SP and PA^SP viruses induce type I IFN production. (A) Stable MDCK-IFN-β reporter cells were infected with wild-type PR8, PB1^SP, or PA^SP viruses at two different MOIs (0.1 and 1.0). At 24 h postinfection, luciferase activity was determined per microgram of protein. The experiment was repeated three times. (B) Human DCs derived from PBMCs from three healthy donors were infected at an MOI of 1 with the indicated viruses or left uninfected as a control. At 24 h later, the supernatants were harvested and the type I IFN-α2 levels were determined by ELISA. (C) The amount of immunostimulatory RNAs bound to RIG-I was determined as previously described (1). Error bars represent standard errors. (D) Single-cycle replication curves were conducted in Vero cells to determine the effect of the SP mutations in the replication of the PB1^SP and PA^SP viruses in the absence of type I IFN.