title: RIG-I Signaling Is Essential for Influenza B Virus-Induced Rapid Interferon Gene Expression.
abstract: Influenza B virus causes annual epidemics and, along with influenza A virus, accounts for substantial disease and economic burden throughout the world. Influenza B virus infects only humans and some marine mammals and is not responsible for pandemics, possibly due to a very low frequency of reassortment and a lower evolutionary rate than that of influenza A virus. Influenza B virus has been less studied than influenza A virus, and thus, a comparison of influenza A and B virus infection mechanisms may provide new insight into virus-host interactions. Here we analyzed the early events in influenza B virus infection and interferon (IFN) gene expression in human monocyte-derived macrophages and dendritic cells. We show that influenza B virus induces IFN regulatory factor 3 (IRF3) activation and IFN-λ1 gene expression with faster kinetics than does influenza A virus, without a requirement for viral protein synthesis or replication. Influenza B virus-induced activation of IRF3 required the fusion of viral and endosomal membranes, and nuclear accumulation of IRF3 and viral NP occurred concurrently. In comparison, immediate early IRF3 activation was not observed in influenza A virus-infected macrophages. Experiments with RIG-I-, MDA5-, and RIG-I/MDA5-deficient mouse fibroblasts showed that RIG-I is the critical pattern recognition receptor needed for the influenza B virus-induced activation of IRF3. Our results show that innate immune mechanisms are activated immediately after influenza B virus entry through the endocytic pathway, whereas influenza A virus avoids early IRF3 activation and IFN gene induction.
  
  
  OBJECTIVE
  Recently, a great deal of interest has been paid to identifying the ligands for RIG-I under conditions of natural infection, as many previous studies have been based on transfection of cells with different types of viral or synthetic RNA structures. We shed light on this question by analyzing the earliest step in innate immune recognition of influenza B virus by human macrophages. We show that influenza B virus induces IRF3 activation, leading to IFN gene expression after viral RNPs (vRNPs) are released into the cytosol and are recognized by RIG-I receptor, meaning that the incoming influenza B virus is already able to activate IFN gene expression. In contrast, influenza A (H3N2) virus failed to activate IRF3 at very early times of infection, suggesting that there are differences in innate immune recognition between influenza A and B viruses.
authors.affiliation: Viral Infections Unit, Department of Infectious Diseases, National Institute for Health and Welfare, Helsinki, Finland sanna.makela@thl.fi.
authors.forename: Sanna M
authors.initials: SM
authors.lastname: Mäkelä
authors.affiliation: Viral Infections Unit, Department of Infectious Diseases, National Institute for Health and Welfare, Helsinki, Finland.
authors.forename: Pamela
authors.initials: P
authors.lastname: Österlund
authors.affiliation: Viral Infections Unit, Department of Infectious Diseases, National Institute for Health and Welfare, Helsinki, Finland.
authors.forename: Veera
authors.initials: V
authors.lastname: Westenius
authors.affiliation: Viral Infections Unit, Department of Infectious Diseases, National Institute for Health and Welfare, Helsinki, Finland.
authors.forename: Sinikka
authors.initials: S
authors.lastname: Latvala
authors.affiliation: Departments of Medicine, Pathology and Immunology, and Molecular Microbiology, Washington University School of Medicine, St. Louis, Missouri, USA.
authors.forename: Michael S
authors.initials: MS
authors.lastname: Diamond
authors.affiliation: Center for Innate Immunity and Immune Disease, Department of Immunology, University of Washington School of Medicine, Seattle, Washington, USA.
authors.forename: Michael
authors.initials: M
authors.lastname: Gale
authors.affiliation: Viral Infections Unit, Department of Infectious Diseases, National Institute for Health and Welfare, Helsinki, Finland Department of Virology, University of Turku, Turku, Finland.
authors.forename: Ilkka
authors.initials: I
authors.lastname: Julkunen
mesh_terms: D000818:Animals; D002460:Cell Line; D000071457:DEAD Box Protein 58; D053487:DEAD-box RNA Helicases / Q000378:metabolism*; D003713:Dendritic Cells / Q000276:immunology; D005786:Gene Expression Regulation / Q000276:immunology*; D054884:Host-Pathogen Interactions; D006801:Humans; D015151:Immunoblotting; D009981:Influenza B virus / Q000276:immunology*; D007251:Influenza, Human / Q000276:immunology*; D050838:Interferon Regulatory Factor-3 / Q000235:genetics / Q000378:metabolism*; D007372:Interferons; D007378:Interleukins / Q000276:immunology / Q000378:metabolism*; D008264:Macrophages / Q000276:immunology; D051379:Mice; D018345:Mice, Knockout; D008856:Microscopy, Fluorescence; D060888:Real-Time Polymerase Chain Reaction; D011971:Receptors, Immunologic; D015398:Signal Transduction / Q000276:immunology*; D000095022:Interferon Lambda
chemical_list: D000071457:DEAD Box Protein 58; D053487:DEAD-box RNA Helicases; D050838:Interferon Regulatory Factor-3; D007372:Interferons; D007378:Interleukins; D011971:Receptors, Immunologic; C000624353:interferon-lambda, human; D000095022:Interferon Lambda; C494232:IRF3 protein, human; C452820:RIGI protein, human