The innate immune system recognizes influenza A virus via TLR 7 or retinoic acid-inducible gene I in a cell-type specific manner in vitro, however, physiological function (s) of the MyD88-or interferon-β promoter stimulator 1 (IPS-1)-dependent signaling pathways in antiviral responses in vivo remain unclear. In this study, we show that although either MyD88-or IPS-1-signaling pathway was sufficient to control initial antiviral responses to intranasal influenza A virus infection, mice lacking both pathways failed to show antiviral responses, resulting in increased viral load in the lung. By contrast, induction of B cells or CD4 T cells specific to the dominant hemagglutinin or nuclear protein Ags respectively, was strictly dependent on MyD88 signaling, but not IPS-1 signaling, whereas induction of nuclear protein Ag-specific CD8 T cells was not impaired in the absence of either MyD88 or IPS-1. Moreover, vaccination of TLR7-and MyD88-deficient mice with inactivated virus failed to confer protection against a lethal live virus challenge. These results strongly suggest that either the MyD88 or IPS-1 signaling pathway is sufficient for initial antiviral responses, whereas the protective adaptive immune responses to influenza A virus are governed by the TLR7-MyD88 pathway.