The airway epithelium, a ciliated pseudo-stratified columnar epithelium, represents the first barrier against inhaled microbes and actively prevents the entry of respiratory pathogens. It consists of ciliated cells, basal cells and secretory goblet cells that together with the mucosal immune system, provide local defense mechanisms for the mucociliary clearance of inhaled microorganisms [56] . The polarized ciliated primary HAE, which is generated by growing isolated tracheobronchial epithelial cells at an ALI for on average one month, forms a pseudo-stratified, mucociliary epithelium and displays morphologic and phenotypic characteristics resembling those of the in vivo human cartilaginous airway epithelium of the lung [57, 58] . Recent studies have revealed that this model system recapitulates important characteristics of interactions between respiratory viruses and their host cells [41, 45, [59] [60] [61] [62] .