Analyses of endogenous Viperin activity. (A) Viperin expression in U937 cells stably transduced with empty pGIPZ shRNA vector (Ctl) or Viperin targeting shRNA (Vip) was analyzed by Western blot twenty hours after interferon β1b treatment (500 IU/ml). (B) Spreading infection of wildtype HIVLai or HIVLaiΔNef was quantified by p24 ELISA at indicated time points after infecting shRNA-transduced U937 cells at an moi of 0.5. Cells were maintained in interferon β1b (500 IU/ml) for the duration of the experiment. The data are representative of at least three independent experiments.