IFNG and infection: MEF cells were treated with mouse IFN-β at 1, 10, 50 or 100 U/ml for 16 hrs before and after virus infection, meanwhile Vero cells and A549 cells were treated with human IFN-α, IFN-β or IFN-γ for 16 hrs before and after infection at 125, 250, 500 or 1000 U/ml (Fig 1).