title: The nucleoprotein of newly emerged H7N9 influenza A virus harbors a unique motif conferring resistance to antiviral human MxA.
abstract: Interferon-induced Mx proteins show strong antiviral activity against influenza A viruses (IAVs). We recently demonstrated that the viral nucleoprotein (NP) determines resistance of seasonal and pandemic human influenza viruses to Mx, while avian isolates retain Mx sensitivity. We identified a surface-exposed cluster of amino acids in NP of pandemic A/BM/1/1918 (H1N1), comprising isoleucine-100, proline-283, and tyrosine-313, that is essential for reduced Mx sensitivity in cell culture and in vivo. This cluster has been maintained in all descendant seasonal strains, including A/PR/8/34 (PR/8). Accordingly, two substitutions in the NP of PR/8 [PR/8(mut)] to the Mx-sensitive amino acids (P283L and Y313F) led to attenuation in Mx1-positive mice. Serial lung passages of PR/8(mut) in Mx1 mice resulted in a single exchange of tyrosine to asparagine at position 52 in NP (in close proximity to the amino acid cluster at positions 100, 283, and 313), which partially compensates loss of Mx resistance in PR/8(mut). Intriguingly, the NP of the newly emerged avian-origin H7N9 virus also contains an asparagine at position 52 and shows reduced Mx sensitivity. N52Y substitution in NP results in increased sensitivity of the H7N9 virus to human Mx, indicating that this residue is a determinant of Mx resistance in mammals. Our data strengthen the hypothesis that the human Mx protein represents a potent barrier against zoonotic transmission of avian influenza viruses. However, the H7N9 viruses overcome this restriction by harboring an NP that is less sensitive to Mx-mediated host defense. This might contribute to zoonotic transmission of H7N9 and to the severe to fatal outcome of H7N9 infections in humans.
  
  
  OBJECTIVE
  The natural host of influenza A viruses (IAVs) are aquatic birds. Occasionally, these viruses cross the species barrier, as in early 2013 when an avian H7N9 virus infected humans in China. Since then, multiple transmissions of H7N9 viruses to humans have occurred, leaving experts puzzled about molecular causes for such efficient crossing of the species barrier compared to other avian influenza viruses. Mx proteins are known restriction factors preventing influenza virus replication. Unfortunately, some viruses (e.g., human IAV) have developed some resistance, which is associated with specific amino acids in their nucleoproteins, the target of Mx function. Here, we demonstrate that the novel H7N9 bird IAV already carries a nucleoprotein that overcomes the inhibition of viral replication by human MxA. This is the first example of an avian IAV that is naturally less sensitive to Mx-mediated inhibition and might explain why H7N9 viruses transmitted efficiently to humans.
authors.affiliation: Institute of Virology, University Medical Center, Freiburg, Germany.
authors.forename: David
authors.initials: D
authors.lastname: Riegger
authors.affiliation: Department of Microbiology, Icahn School of Medicine at Mount Sinai, New York, New York, USA Global Health and Emerging Pathogens Institute, Icahn School of Medicine at Mount Sinai, New York, New York, USA.
authors.forename: Rong
authors.initials: R
authors.lastname: Hai
authors.affiliation: Institute of Virology, University Medical Center, Freiburg, Germany.
authors.forename: Dominik
authors.initials: D
authors.lastname: Dornfeld
authors.affiliation: Institute of Virology, University Medical Center, Freiburg, Germany.
authors.forename: Benjamin
authors.initials: B
authors.lastname: Mänz
authors.affiliation: Department of Microbiology, Icahn School of Medicine at Mount Sinai, New York, New York, USA.
authors.forename: Victor
authors.initials: V
authors.lastname: Leyva-Grado
authors.affiliation: Department of Microbiology, Icahn School of Medicine at Mount Sinai, New York, New York, USA.
authors.forename: Maria T
authors.initials: MT
authors.lastname: Sánchez-Aparicio
authors.affiliation: Department of Microbiology, Icahn School of Medicine at Mount Sinai, New York, New York, USA Global Health and Emerging Pathogens Institute, Icahn School of Medicine at Mount Sinai, New York, New York, USA.
authors.forename: Randy A
authors.initials: RA
authors.lastname: Albrecht
authors.affiliation: Department of Microbiology, Icahn School of Medicine at Mount Sinai, New York, New York, USA Global Health and Emerging Pathogens Institute, Icahn School of Medicine at Mount Sinai, New York, New York, USA.
authors.forename: Peter
authors.initials: P
authors.lastname: Palese
authors.affiliation: Institute of Virology, University Medical Center, Freiburg, Germany.
authors.forename: Otto
authors.initials: O
authors.lastname: Haller
authors.affiliation: Institute of Virology, University Medical Center, Freiburg, Germany.
authors.forename: Martin
authors.initials: M
authors.lastname: Schwemmle
authors.affiliation: Department of Microbiology, Icahn School of Medicine at Mount Sinai, New York, New York, USA Global Health and Emerging Pathogens Institute, Icahn School of Medicine at Mount Sinai, New York, New York, USA Division of Infectious Diseases, Department of Medicine, Icahn School of Medicine at Mount Sinai, New York, New York, USA.
authors.forename: Adolfo
authors.initials: A
authors.lastname: García-Sastre
authors.affiliation: Institute of Virology, University Medical Center, Freiburg, Germany georg.kochs@uniklinik-freiburg.de mirco.schmolke@unige.ch.
authors.forename: Georg
authors.initials: G
authors.lastname: Kochs
authors.affiliation: Department of Microbiology, Icahn School of Medicine at Mount Sinai, New York, New York, USA Global Health and Emerging Pathogens Institute, Icahn School of Medicine at Mount Sinai, New York, New York, USA georg.kochs@uniklinik-freiburg.de mirco.schmolke@unige.ch.
authors.forename: Mirco
authors.initials: M
authors.lastname: Schmolke
mesh_terms: D000818:Animals; D001717:Birds; D002460:Cell Line; D002681:China; D006801:Humans; D057131:Immune Evasion; D064766:Influenza A Virus, H7N9 Subtype / Q000254:growth & development / Q000276:immunology*; D005585:Influenza in Birds / Q000821:virology*; D007251:Influenza, Human / Q000276:immunology*; D008810:Mice, Inbred C57BL; D008969:Molecular Sequence Data; D064547:Myxovirus Resistance Proteins / Q000276:immunology*; D019590:Nucleocapsid Proteins; D012367:RNA, Viral / Q000235:genetics; D016601:RNA-Binding Proteins / Q000235:genetics / Q000276:immunology*; D017422:Sequence Analysis, DNA; D014758:Viral Core Proteins / Q000235:genetics / Q000276:immunology*; D015047:Zoonoses / Q000635:transmission / Q000821:virology
chemical_list: C579071:MX1 protein, human; D064547:Myxovirus Resistance Proteins; C048842:NP protein, Influenza A virus; D019590:Nucleocapsid Proteins; D012367:RNA, Viral; D016601:RNA-Binding Proteins; D014758:Viral Core Proteins