Respiratory Syncytial Virus and Human Metapneumovirus Infections in Three-Dimensional Human Airway Tissues Expose an Interesting Dichotomy in Viral Replication, Spread, and Inhibition by Neutralizing Antibodies.

Respiratory syncytial virus (RSV) and human metapneumovirus (HMPV) are two of the leading causes of respiratory infections in children and elderly and immunocompromised patients worldwide. There is no approved treatment for HMPV and only one prophylactic treatment against RSV, palivizumab, for high-risk infants. Better understanding of the viral lifecycles in a more relevant model system may help identify novel therapeutic targets. By utilizing three-dimensional (3-D) human airway tissues to examine viral infection in a physiologically relevant model system, we showed that RSV infects and spreads more efficiently than HMPV, with the latter requiring higher multiplicities of infection (MOIs) to yield similar levels of infection. Apical ciliated cells were the target for both viruses, but RSV apical release was significantly more efficient than HMPV. In RSV- or HMPV-infected cells, cytosolic inclusion bodies containing the nucleoprotein, phosphoprotein, and respective viral genomic RNA were clearly observed in human airway epithelial (HAE) culture. In HMPV-infected cells, actin-based filamentous extensions were more common (35.8%) than those found in RSV-infected cells (4.4%). Interestingly, neither RSV nor HMPV formed syncytia in HAE tissues. Palivizumab and nirsevimab effectively inhibited entry and spread of RSV in HAE tissues, with nirsevimab displaying significantly higher potency than palivizumab. In contrast, 54G10 completely inhibited HMPV entry but only modestly reduced viral spread, suggesting HMPV may use alternative mechanisms for spread. These results represent the first comparative analysis of infection by the two pneumoviruses in a physiologically relevant model, demonstrating an interesting dichotomy in the mechanisms of infection, spread, and consequent inhibition of the viral lifecycles by neutralizing monoclonal antibodies. Respiratory syncytial virus and human metapneumovirus are leading causes of respiratory illness worldwide, but limited treatment options are available. To better target these viruses, we examined key aspects of the viral life cycle in three-dimensional (3-D) human airway tissues. Both viruses establish efficient infection through the apical surface, but efficient spread and apical release were seen for respiratory syncytial virus (RSV) but not human metapneumovirus (HMPV). Both viruses form inclusion bodies, minimally composed of nucleoprotein (N), phosphoprotein (P), and viral RNA (vRNA), indicating that these structures are critical for replication in this more physiological model. HMPV formed significantly more long, filamentous actin-based extensions in human airway epithelial (HAE) tissues than RSV, suggesting HMPV may promote cell-to-cell spread via these extensions. Lastly, RSV entry and spread were fully inhibited by neutralizing antibodies palivizumab and the novel nirsevimab. In contrast, while HMPV entry was fully inhibited by 54G10, a neutralizing antibody, spread was only modestly reduced, further supporting a cell-to-cell spread mechanism.