Influenza virus infection of well-differentiated human airway epithelial cells by infectious aerosols: insights into the earliest stages of infection

Background: Influenza virus is a major human pathogen, yet surprisingly little data is available on the earliest stage of infection. We have developed a novel method to study natural transmission influenza infection by aerosol and to observe the effects of early infection on the ciliated airway epithelium using high-speed video microscopy. Methods: Primary human ciliated epithelial cultures were infected with influenza A (H1N1), delivered either by aerosol or by liquid immersion. Cells were stained for viral antigens and the level of inflammatory mediators, and the number of motile ciliated cells and ciliary beat frequency and pattern was measured. Results: Infection by aerosol and liquid inoculums of influenza virus was shown to be trophic for ciliated cells. Infection by both methods also led to a significant decrease in the number of cells with motile cilia over the first 24 hours; however, the ciliary beat frequency and beat pattern of the remaining cilia was maintained over 24 hours. Conclusions: Influenza virus aerosols readily infect human ciliated nasal epithelial cells resulting in early loss of motile ciliated cells. Delivery of the virus by aerosol elicited an anti-inflammatory Th2 response, which was distinct from cells exposed to virus by liquid immersion delivery. This suggests our aerosol model may provide a more clinically relevant model for studying the early effects of influenza infection.