KRT9 is required for GBP5 suppression of human respiratory syncytial virus.

Unlabelled: Respiratory syncytial virus (RSV) infections continue to plague infants, young children, and older individuals worldwide. Since there is no specific treatment for RSV, characterizing the interactions between RSV and host factors remains crucial for the eventual development of robust therapeutic interventions. In our previous study, guanylate binding protein 5 (GBP5) was shown to promote excessive RSV-small hydrophobic (RSV-SH) protein secretion by microvesicles and inhibited viral replication. In this study, using affinity mass spectrometry, keratin (KRT) 9 was identified to be required for GBP5 to trigger RSV-SH transport. Silencing KRT9 expression reduced the antiviral effects of GBP5 and interferon-γ. A direct interaction was detected between KRT9 and GBP5, but not RSV-SH; a GBP5 binding domain was identified on KRT9. Our results suggest that GBP5, as a bridge, interacts with KRT9 and RSV-SH, after which KRT9 triggers RSV-SH transport. The mechanism underlying the interaction between KRT9 and GBP5 explains the inability of the GBP5-C583A and GBP5-△C mutants in inhibiting RSV replication. Conversely, KRT1, KRT5, and KRT6C, which were identified as potential partners of KRT9, did not affect the GBP5 anti-RSV process. Overall, our study provides evidence for KRT9 involvement in host innate immunity for the first time.

Importance: RSV causes severe acute lower respiratory tract infections, which have posed serious health and safety risks to children and older adults worldwide. Although some RSV interventions are available, the longer-lasting monoclonals, which are expensive, are required to be injected before RSV infection, and their protection is observed only up to one RSV infection season; vaccines are currently only available to the elderly but are not suitable for application in infants and young children. As specific drug treatments are absent, a systematic and in-depth mechanism for research is essential. In our study, KRT9 was identified to play an important role in the GBP5 anti-RSV process for the first time. This investigation improved the interaction mechanism between GBP5 and RSV, provided new evidence for the synergistic effect between keratin transport and innate immunity, and opened a new research direction with GBP5 and the keratin transport system as the main subjects.