Confluent MDCK cell monolayer was prepared on transwell inserts (BD Falcon, pore size 0.4 mm) and infected with the NAdeficient influenza virus at MOI of 0.0001. After virus adsorption at 37uC for 1 hour, the cell monolayer was washed with serum-free MEM, and maintenance medium was added into both sides within the transwells. The neutralizing antibody to influenza A virus was added into the inside or the outside of transwell inserts with the maintenance medium. Subsequently, cells were incubated at 37uC for 36 hours followed by analyses using the fluorescence microscopy.