Histopathological findings in lungs of control pigs at 3 days post inoculation (DPI), in nasal mucosa and tracheal tissues at 3 DPI of infected pigs, and in tracheal tissues at 14 DPI of infected pigs.

A) Lung tissue from a control pig at 3 DPI, showing organized bronchus-associated lymphoid tissue (BALT) (arrows) throughout. B) Lung tissue from a different control pig at 3 DPI, showing infiltration of macrophages (arrows) and neutrophils (arrowheads). C) Nasal mucosa from an swH1N1pdm09-infected pig at 3 DPI, showing exudation of neutrophils (arrowhead), necrosis (star), and desquamation of epithelial cells (arrow). D) Trachea from an swH1N1pdm09-infected pig at 3 DPI, showing exocytosis of neutrophils (arrowhead) and loss of cilia (arrow). E) Trachea from an mxH1N1pdm09-infected pig at 3 DPI, showing neutrophils present in the lamina propria and lamina epithelialis (arrowheads). F) Lung tissue from an huH1N1pdm09-infected pig at 14 DPI, showing macrophages (arrows) and a few neutrophils (arrowhead). H&E stained. (TIF)