Cleavage of influenza virus hemagglutinin (HA) by host proteases is essential for virus infectivity. HA of most influenza A and B (IAV/IBV) viruses is cleaved at a monobasic motif by trypsin-like proteases. Previous studies have reported that transmembrane serine protease 2 (TMPRSS2) is essential for activation of H7N9 and H1N1pdm IAV in mice, but that H3N2 IAV and IBV activation is independent of TMPRSS2 and carried out by as-yet-undetermined protease(s). Here, to identify additional H3 IAV- and IBV-activating proteases, we used RNA-Seq to investigate the protease repertoire of murine lower airway tissues, primary type II alveolar epithelial cells (AECII), and the mouse lung cell line ... More
Cleavage of influenza virus hemagglutinin (HA) by host proteases is essential for virus infectivity. HA of most influenza A and B (IAV/IBV) viruses is cleaved at a monobasic motif by trypsin-like proteases. Previous studies have reported that transmembrane serine protease 2 (TMPRSS2) is essential for activation of H7N9 and H1N1pdm IAV in mice, but that H3N2 IAV and IBV activation is independent of TMPRSS2 and carried out by as-yet-undetermined protease(s). Here, to identify additional H3 IAV- and IBV-activating proteases, we used RNA-Seq to investigate the protease repertoire of murine lower airway tissues, primary type II alveolar epithelial cells (AECII), and the mouse lung cell line MLE-15. Among 13 candidates identified, TMPRSS4, TMPRSS13, hepsin, and prostasin activated H3 and IBV HA in vitro. IBV activation and replication was reduced in AECII from Tmprss2/Tmprss4-deficient mice compared with wild-type or Tmprss2-deficient mice, indicating that murine TMPRSS4 is involved in IBV activation. Multicycle replication of H3N2 IAV and IBV in AECII of Tmprss2/Tmprss4-deficient mice varied in sensitivity to protease inhibitors, indicating that different, but overlapping, sets of murine proteases facilitate H3 and IBV HA cleavages. Interestingly, human hepsin and prostasin orthologs did not activate H3, but did activate IBV HA in vitro. Our results indicate that TMPRSS4 is an IBV-activating protease in murine AECII and suggest that TMPRSS13, hepsin, and prostasin cleave H3 and IBV HA in mice. They further show that hepsin and prostasin orthologs might contribute to the differences observed in TMPRSS2-independent activation of H3 in murine and human airways.
