title: pH Optimum of Hemagglutinin-Mediated Membrane Fusion Determines Sensitivity of Influenza A Viruses to the Interferon-Induced Antiviral State and IFITMs.
abstract: The replication and pathogenicity of influenza A viruses (IAVs) critically depend on their ability to tolerate the antiviral interferon (IFN) response. To determine a potential role for the IAV hemagglutinin (HA) in viral sensitivity to IFN, we studied the restriction of IAV infection in IFN-β-treated human epithelial cells by using 2:6 recombinant IAVs that shared six gene segments of A/Puerto Rico/8/1934 virus (PR8) and contained HAs and neuraminidases of representative avian, human, and zoonotic H5N1 and H7N9 viruses. In A549 and Calu-3 cells, viruses displaying a higher pH optimum of HA-mediated membrane fusion, H5N1-PR8 and H7N9-PR8, were less sensitive to the IFN-induced antiviral state than their counterparts with HAs from duck and human viruses, which fused at a lower pH. The association between a high pH optimum of fusion and reduced IFN sensitivity was confirmed by using HA point mutants of A/Hong Kong/1/1968-PR8 that differed solely by their fusion properties. Furthermore, similar effects of the viral fusion pH on IFN sensitivity were observed in experiments with (i) primary human type II alveolar epithelial cells and differentiated cultures of human airway epithelial cells, (ii) nonrecombinant zoonotic and pandemic IAVs, and (iii) preparations of IFN-α and IFN-λ1. A higher pH of membrane fusion and reduced sensitivity to IFN correlated with lower restriction of the viruses in MDCK cells stably expressing the IFN-inducible transmembrane proteins IFITM2 and IFITM3, which are known to inhibit viral fusion. Our results reveal that the pH optimum of HA-driven membrane fusion of IAVs is a determinant of their sensitivity to IFN and IFITM proteins.IMPORTANCEThe IFN system constitutes an important innate defense against viral infection. Substantial information is available on how IAVs avoid detection by sensors of the IFN system and disable IFN signaling pathways. Much less is known about the ability of IAVs to tolerate the antiviral activity of IFN-induced cellular proteins. The IFN-induced proteins of the IFITM family block IAV entry into target cells and can restrict viral spread and pathogenicity. Here we show for the first time that the sensitivity of IAVs to the IFN-induced antiviral state and IFITM2 and IFITM3 proteins depends on the pH value at which the viral HA undergoes a conformational transition and mediates membrane fusion. Our data imply that the high pH optimum of membrane fusion typical of zoonotic IAVs of gallinaceous poultry, such as H5N1 and H7N9, may contribute to their enhanced virulence in humans.
authors.affiliation: Institute of Virology, Philipps University, Marburg, Germany.
authors.forename: Thomas
authors.initials: T
authors.lastname: Gerlach
authors.affiliation: Institute of Virology, Philipps University, Marburg, Germany.
authors.forename: Luca
authors.initials: L
authors.lastname: Hensen
authors.affiliation: Institute of Virology, Philipps University, Marburg, Germany.
authors.forename: Tatyana
authors.initials: T
authors.lastname: Matrosovich
authors.affiliation: Institute of Virology, Philipps University, Marburg, Germany.
authors.forename: Janina
authors.initials: J
authors.lastname: Bergmann
authors.affiliation: Deutsches Primatenzentrum, Göttingen, Germany.
authors.forename: Michael
authors.initials: M
authors.lastname: Winkler
authors.affiliation: Department of Internal Medicine II, Universities Gießen & Marburg Lung Center, member of the German Center for Lung Research (DZL), Gießen, Germany.
authors.forename: Christin
authors.initials: C
authors.lastname: Peteranderl
authors.affiliation: Institute of Virology, Philipps University, Marburg, Germany.
authors.forename: Hans-Dieter
authors.initials: HD
authors.lastname: Klenk
authors.affiliation: Institute of Virology, FB10-Veterinary Medicine, Justus-Liebig-Universität, Gießen, Germany.
authors.forename: Friedemann
authors.identifier: http://orcid.org/0000-0001-9737-337X
authors.initials: F
authors.lastname: Weber
authors.affiliation: Department of Internal Medicine II, Universities Gießen & Marburg Lung Center, member of the German Center for Lung Research (DZL), Gießen, Germany.
authors.forename: Susanne
authors.initials: S
authors.lastname: Herold
authors.affiliation: Deutsches Primatenzentrum, Göttingen, Germany.
authors.forename: Stefan
authors.identifier: http://orcid.org/0000-0001-6086-9136
authors.initials: S
authors.lastname: Pöhlmann
authors.affiliation: Institute of Virology, Philipps University, Marburg, Germany m.matrosovich@gmail.com.
authors.forename: Mikhail
authors.initials: M
authors.lastname: Matrosovich
mesh_terms: D000072283:A549 Cells; D000818:Animals; D000943:Antigens, Differentiation / Q000235:genetics / Q000378:metabolism; D002460:Cell Line; D004285:Dogs; D004372:Ducks; D004847:Epithelial Cells / Q000187:drug effects; D019267:Hemagglutinin Glycoproteins, Influenza Virus / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D054884:Host-Pathogen Interactions; D006801:Humans; D006863:Hydrogen-Ion Concentration; D053124:Influenza A Virus, H5N1 Subtype / Q000737:chemistry / Q000235:genetics / Q000502:physiology*; D064766:Influenza A Virus, H7N9 Subtype / Q000737:chemistry / Q000235:genetics / Q000502:physiology*; D016899:Interferon-beta / Q000276:immunology; D007372:Interferons / Q000276:immunology*; D061985:Madin Darby Canine Kidney Cells; D008561:Membrane Fusion; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism*; D016601:RNA-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D053586:Virus Internalization; D014779:Virus Replication
chemical_list: D000943:Antigens, Differentiation; D019267:Hemagglutinin Glycoproteins, Influenza Virus; C080081:IFITM2 protein, human; C080082:IFITM3 protein, human; D008565:Membrane Proteins; D016601:RNA-Binding Proteins; C062010:leu-13 antigen; D016899:Interferon-beta; D007372:Interferons
keywords: H5N1; H7N9; IFITM; fusion; hemagglutinin; influenza; interferon; restriction