Delayed antiviral immune response in alveolar type 2 cells increases susceptibility to influenza virus in alveoli

The alveolar sac, which serves as the primary site for gas exchange, comprises alveolar type 1 (AT1) and alveolar type 2 (AT2) cells. These epithelial cells initiate immune responses upon encountering external stimuli. However, the influenza virus infection patterns and antiviral responses exhibited by different types of alveolar epithelial cells remain understudied. This study aimed to investigate the differential susceptibility and antiviral responses of AT1 and AT2 cells during influenza virus infection.

Immunofluorescence staining was performed to determine the spatial distribution of influenza A virus (IAV; H1N1). Primary murine epithelial in vitro culture models, including an AT2 sphere culture and an AT1 differentiation model, were established to elucidate the mechanisms underlying cellular susceptibility to IAV. Bulk RNA sequencing and quantitative polymerase chain reaction were performed to characterize the cellular responses to viral infection.

Mouse AT2 cells exhibited greater susceptibility to H1N1 than did AT1 cells. The in vitro primary epithelial cell culture models mimicked the in vivo cellular phenotype. They further reproduced the in vitro finding of relatively high viral replication in AT2 cells. Mechanistically, no significant difference was observed between AT1 and AT2 cells in the expression of α-2,3 or α-2,6 sialic acid (viral receptors). However, RNA sequencing revealed that gene ontology terms such as the response to viral infection, negative regulation of the viral life cycle, and regulation of type I interferon (IFN) production were enriched in AT1 cells. This finding was validated using quantitative polymerase chain reaction. From 12 to 48 h after infection, the expression levels of types I and III IFNs, chemokines, and IFN-stimulated genes remained lower in AT2 cells than in AT1 cells.

Our findings indicate the localization of IAV in alveoli and highlight the vulnerability of AT2 cells, which exhibit delayed immune responses, resulting in increased viral replication.