Tissue tropism of influenza A viruses in ex vivo cultures of human bronchus (A) and lung (B). Human bronchus and lung tissue sections infected with QY/HPAI H7N9, ck/YO28/H7N9, Sh2/H7N9, Ah1/H7N9, 734/H7N9, 483/H5N1, and 415742/H1N1pdm were formalin-fixed and paraffin-embedded and subjected to immunohistochemical staining at 24 hours postinfection. Sections were stained with a monoclonal antibody against the influenza nucleoprotein with positive cells identified by red color. Tissue tropism of influenza virus infection in ex vivo cultures of human bronchus and lung was also assessed by immunohistochemical staining. Magnification, ×200. (C) Quantification of tissue tropism of influenza virus infection of human bronchus and lung in immunohistochemistry IHC photos. Sparse: <1%; +, <10%; ++, 10%–50%; +++, >50% cells with positive staining. (D) Lung tissue sections were doubled stained with a monoclonal antibody against the influenza nucleoprotein with positive cells identified by brown color and a monoclonal antibody against prosurfactant-C with positive cells identified by pink color. The arrow indicated the type-II alveolar epithelial cells infected with influenza virus. Magnification, ×400.