Inhibitory effect of ribavirin and combination with siS1 and siS2. A) A549 cells were infected with HAZV at a MOI of 0.1 and treated with different concentrations of ribavirin (ranging from 0 to 200 μM) for 48 hrs. Results are presented as a ratio between virus titer obtained in ribavirin treated wells and titer determined in non-treated cells. B) A549 cells were transfected with 1 nM or 10 nM of siS1 or C) with 1 nM or 10 nM siS2. Cells were infected with HAZV at a MOI of 0.1 and then incubated for two days in medium containing 25 μM of ribavirin. Viral titers were determined 48 hrs post-infection. The reduction of virus titer is determined after treatment with either ribavirin or siS1 (or siS2) and with the combination of ribavirin and siS1 or siS2. The white bar represents the theoretical sum of antiviral activity obtained with each compound. The synergistic effect of the combination of ribavirin and each siRNAs is indicated by the grey bar. Nb: no synergy was observed when siS2 was used at 10 nM with ribavirin. * Significant differences between additive and synergistic effect: Student's t-test; P < 0.05. Errors bars represent the standard deviation (SD) of the means for at least two independent experiments performed in quadruplicates.