The neuraminidase activity of influenza A virus determines the strain-specific sensitivity to neutralization by respiratory mucus.

Respiratory mucus lining the airway epithelium constitutes an important first line of defense against infection with influenza A virus (IAV). Previous studies suggested that the inhibitory potential of mucus relies not only on the presence of decoy receptors but also on the dense meshwork blocking virus access to the epithelium independent of sialic acid. Here, we report for a panel of human and avian IAV isolates that susceptibility to neutralization by respiratory mucus varies in a strain-dependent manner. Our data reveal that viral neuraminidase (NA) activity inversely correlates with the mucus sensitivity of a given strain. As NA cleaves sialic acid from glycan chains and thus releases IAV from decoy receptor binding, our results imply that virus inhibition by mucus is mainly driven by decoy receptor binding of IAV. Other inhibitory properties of mucus, such as size exclusion, non-covalent interactions with mucus components, or the presence of immunomodulatory molecules, likely play minor roles in neutralizing IAV. Inhibition of NA activity using oseltamivir was sufficient to increase the sensitivity of a seasonal IAV strain to mucus during infection in a cell-based neutralization assay and in differentiated primary human airway epithelial cultures. Overall, our data highlight the importance of NA for the initiation of virus infection besides its major function in release, especially in overcoming the host defense mechanism of mucociliary clearance in the airway epithelium. Furthermore, strain-specific differences in the ability to penetrate respiratory mucus may be the result of host adaptation and can have consequences for the transmissibility and infectivity of circulating viruses. IMPORTANCE The respiratory tract of humans is constantly exposed to potentially harmful agents, such as small particles or pathogens, and thus requires protective measures. Respiratory mucus that lines the airway epithelia plays a major role in the prevention of viral infections by limiting the mobility of viruses, allowing subsequent mucociliary clearance. Understanding the interplay between respiratory mucus and viruses can help elucidate host and virus characteristics that enable the initiation of infection. Here, we tested a panel of primary influenza A viruses of avian or human origin for their sensitivity to mucus derived from primary human airway cultures and found that differences between virus strains can be mapped to viral neuraminidase activity. We also show that binding of influenza A viruses to decoy receptors on highly glycosylated mucus components constitutes the major inhibitory function of mucus against influenza A viruses.