Virulence of and inflammatory response to a virus expressing the 1918 PB1-F2. An otherwise isogenic virus engineered to express the PB1-F2 protein from the 1918 pandemic strain (1918) was compared to the wt PR8 strain (WT). (A) Survival of groups of 4 mice infected with 500 or 1000 TCID50 of wt PR8 or PR8-PB1-F21918. (B) Viral titers from lung homogenates taken from groups of 6–12 mice at multiple timepoints after infection with wt PR8 or PR8-PB1-F21918. Error bars indicate standard deviation of the measurements and an asterisk (*) indicates a significant difference at that timepoint by ANOVA compared to the wt PR8. In a separate experiment, mice were infected with PR8 wt or PR8-PB1-F21918 and were assayed for number of (C) neutrophils, (D) macrophages, (E) T-cells, and (F) B-cells from broncho-alveolar lavage fluid samples 1, 3, and 7 days after infection. The 25th – 75th percentiles are represented by the shaded box-plots with the horizontal bar indicating the mean value. Error bars indicate the standard deviation of the measurements. An asterisk (*) indicates a significant difference (p < 0.05) by ANOVA compared to wt PR8 at that timepoint.