On the day of infection, AM were inoculated with live PR/8 at a designated multiplicity of infection (MOI) or with the same amount of UV-inactivated PR/8 for 1 h. After inoculation, cells were washed and then cultured until harvest. Influenza infection was verified by immuno-fluorescent staining with goat antibody to the hemagglutinin of PR/8 (kindly provided by BEI Resources, Manassas, VA). For NY/238 infection, AM was inoculated with a MOI of 0.1 instead of 0.5 due to the limitation of viral titer and infection was confirmed by immuno-fluorescent staining with mouse antibody to influenza nucleoprotein (Millipore, Billerica, MA).