Lung tissue sections taken on day 6 p.i. were stained for influenza A virus antigens. An anti-influenza nucleoprotein monoclonal antibody (AA5H; Abcam, Hong Kong) was used to identify influenza A virus nucleoprotein in sections. Secondary antibody (Millipore, Billerica, MA, USA) against the primary antibody was labeled with horseradish peroxidase, and the color reaction was developed with a horseradish peroxidase reaction kit (diaminobenzidine-tetrahydrochloride; Sigma, St. Louis, MO, USA).