FIG. 3.
Results of wild-type and mutant 1918 HA-expressing influenza virus infection experiments. (A) Weight loss of mice infected with rescued influenza viruses. Ten- to 12-week-old female BALB/c mice were intranasally inoculated with 2 × 105 PFU of indicated viruses and weighed daily. Only the 1918-SC- and 1918-NY-infected mice had uniform significant weight loss leading to a lethal infection (n = 5 mice per virus per group), while 1918-NYmut-infected animals showed similar early weight loss with significantly lower mortality (n = 15 mice per virus per group). (B) Kaplan-Meier survival curve of 10- to 12-week-old female BALB/c mice intranasally inoculated with 2 × 105 PFU of virus. The 1918-SC and 1918-NY viruses caused uniformly lethal infections typically by 6 dpi, while 33% of the mice infected with the 1918-NYmut virus had lethal outcomes (n = 15). (C) Virus titers in the lungs of infected BALB/c mice (n = 2 or 3) described in PFU per gram of lung tissue as determined by plaque assay (B.D. indicates below the detection threshold). Two-tailed P values were calculated with an unpaired t test.