The production of rabbit polyclonal anti-NP antibody was described previously [53] , and this antibody was used as a primary antibody for indirect immunofluorescence assay. A goat antirabbit IgG antibody conjugated to Alexa Fluor 488 or Alexa Fluor 568 was purchased from Invitrogen and used as a secondary antibody for indirect immunofluorescence assay. A polyclonal antibody against influenza A virus was obtained from 2-month-old female rabbit immunized with 250 mg of purified virions of influenza virus strain A/Puerto Rico/8/34 [54] . The generation of antibodies was boosted three times and used as neutralizing antibodies to block the influenza virus infection.