2.4 Mouse experiments
All animal experiments were conducted in biosafety level 2-plus facilities at the Korea Research Institute of Bioscience and Biotechnology (Daejeon, South Korea). General animal care was provided as required by the Institutional Animal Care and Use committee. Six- to eight-week-old C57BL/6 mice were purchased from Koatech (Pyeongtaek, South Korea), and ten mice per group were used. The mice were inoculated via intranasal instillation with 3 times the 50% lethal dose (3 LD50) of the PR8 virus. The following day, the mice were treated intravenously with aprotinin (2 mg/kg/day), orally with oseltamivir (10 mg/kg/day), or intravenously with PBS (negative control) twice a day for 5 days. The mice were monitored daily for 14 days for weight change and mortality.