Figure 4.

The fitness advantage conferred by 3′ UTR deletion in murine cells is independent of the type I interferon response and confers a fitness advantage in a mouse model of CHIKV disease. (A) In vitro viral fitness competitions of marked and unmarked CHIKV performed through five serial passages (p5) in Ifnar1^−/− MEFs. Data are from 1 experiment performed in triplicate. (B–D) For in vivo viral fitness competitions, mice were inoculated with 1000 PFU of each virus. Data are from 1–2 independent experiments performed with 4 mice per experiment. (B) Virus competition in WT C57BL/6 mice; virus proportions in spleen and contralateral ankle determined at 4 days post inoculation. (C,D) Virus competition in Ifnar1^−/− C57BL/6 mice; virus proportions in spleen and contralateral ankle determined at 3 days post inoculation. (A–D) Virus identity was determined by the presence or absence of a synonymous genetic marking in the nsP4 gene (*) that confers susceptibility to ApaI/PspOMI restriction digest. Input indicates starting ratios of marked to unmarked virus in initial inoculum. Statistics are unpaired t-test (A) or one-way ANOVA (B–D); ****, p < 0.0001; ***, p < 0.001; **, p < 0.01; ns, not significant.