Virus infection
Mouse-adapted A(H1N1)pdm09 (strain: A/Narita/1/09) or seasonal H1N1 (strain: A/Puerto Rico) viruses were provided by the National Institute of Infectious Diseases (Tokyo, Japan). On day 31, influenza virus (concentration: 1×105 pfu/20 μL) or vehicle (mock-infection) was administered intranasally to the mice. The number of mice in each group ranged from 4 to 7.