Guo, Z.; Banas, V. S.; He, Y.; Weiland, E.; Xu, J.; Tan, Y.; Xiao, Z.; Brody, S. L.; Boon, A. C. M.; Janetka, J. W.; Vahey, M. D.

Influenza viruses utilize host proteases to activate the viral fusion protein, hemagglutinin (HA), into its fusion-competent form. Although proteolytic activation of HA is essential for virus replication, the cell-type dependence of HA activation within the airway epithelium and the subcellular location(s) in which it occurs are not well-established. To address these questions, we investigated the proteolytic activation of HA in differentiated human airway epithelial cells using contemporary and historical H1N1 and H3N2 strains. We find that activation is efficient across viral strains and subtypes but depends on cellular tropism, with ciliated cells activating HA more effectively than non-ciliated cells. Similar to prior observations in immortalized cell lines, we find that HA activation occurs intracellularly, constraining the antiviral activity of host-directed protease inhibitors. These results establish that HA activation within the airway epithelium depends on cellular tropism, and identify important considerations for the development of protease inhibitors as antivirals.