Project Summary Human respiratory tissues have a unique cellular architecture and a mucus-based clearance system that, together, constitute an impressive barrier to viruses that use the airway epithelium as their portal of entry. The mucus-based clearance system acts as a physical, innate defense mechanism. Nasal epithelial cells have two classes of protrusions on their surfaces—long motile cilia and short microvilli. Motile cilia are specialized organelles that extend from the cell into the airway to drive mucociliary clearance. However, the mechanisms used by respiratory viruses to breach this barrier and infect the respiratory epithelium are not well understood. Our previous work has highlighted the crucial role of cilia in respiratory virus infection. Using cultured human primary nasal epithelial cells, we have demonstrated that SARS-CoV-2 and RSV specifically attach to motile cilia during infection. Importantly, inhibiting the interaction between SARS-CoV-2 or RSV and cilia can effectively suppress viral infections, indicating the critical role of the virus-cilia interaction. Furthermore, treatment with an inhibitor of ciliary protein trafficking significantly decreased SARS-CoV-2 infection in nasal epithelial cells. These findings underscore the critical role of cilia during viral entry. Based on these findings, we propose specific aims to understand how RSV and SARS-CoV-2 hijack nasal cilia for viral entry using the primary nasal epithelium as a model. By comparing the infection mechanisms of RSV and SARS-CoV-2, we aim to gain a clearer understanding of the differences between these viruses and the significance of nasal cilia in their respective contexts.