The cells grown in complete Macrophage-SFM medium were washed tree times with PBS and the media was changed to RPMI growth media supplemented with 1 mM HEPES, L-Glutamine and antibiotics. The RPMI media has lower content of initial media proteins than Macrophage-SFM medium. The cathepsin B inhibitors, Ca-074 Me (Calbiochem) and z-FA-fmk (Calbiochem) were added 0.5 h before influenza A virus infection and used at final concentration of 40 mM and 50 mM, respectively.