Figure 1.

Virulence is increased by each of the 103L and 106I NS1 mutations in the H5N1-NS gene on human and mouse adapted virus backbones as well as for 103S and 106I PR8 NS1 gene mutations in PR8 virus. a and b. Groups of 3 CD-1 mice were infected intranasally with 5 × 10⁶ pfu of rHK viruses possessing wt (103L + 106I) or mutant H5N1 NS1 genes that differed due to the indicated mutations at positions 103 and 106. c and d. Groups of 3 CD-1 mice were infected intranasally with 1 × 10⁴ pfu with the different rPR8 viruses possessing wt (103L + 106I) or mutant H5N1 NS1 genes that differed due to the indicated mutations at positions 103 and 106. e and f, Groups of 5 BALB/c mice were infected intranasally with 1 × 10⁴ pfu with wt rPR8 or mutants that differed due to the indicated mutations at positions 103 and 106. The percent of body weight loss was calculated as the mice body weight loss was recorded daily throughout the whole course of the experiment. Values are shown as average +/− standard deviation (*p<0.05, **p<0.01, *** p<0.001; two-tailed student’s t-test).