Figure 5.

The NS1 R38A/K41A virus exerted full infectivity but its replication is limited in mice. Mice (n = 6) were intranasally infected with 10³ PFU of the third passage NS1 R38A/K41A virus from the Tet-on NS1-expressing Vero cells, WT virus, or PBS, and then monitored daily for survival for 14 days. The weight curves (A) and survival rate (B) of mice were determined. Body weights were calculated as the percentage of the original body weight. The data are the mean body weights. The error bars represent the standard deviation of the mean. Three mice infected with different viruses or PBS were sacrificed at the indicated time points, and the lungs were collected. Lung indices (C) were calculated and virus titers in lungs (D) were determined by plaque assays using MDCK cells. IFN-α and IFN-β mRNA levels in lung tissues (E) of mice (n = 3) infected with WT or NS1 R38A/K41A virus for 0, 12, and 24 h p.i. were determined by real-time PCR. All data from three independent experiments were analyzed. Data are shown as means with SD. ^**p < 0.01 and ^***p < 0.001 (unpaired, two-tailed Student's t-test).