Importin-α7 is Required for Enhanced Influenza A Virus Replication in the Alveolar Epithelium and Severe Lung Damage in Mice

Influenza A viruses recruit components of the nuclear import pathway to enter host cell nucleus and promote viral replication. Here, we analyzed the role of the nuclear import factor importin-α7 in H1N1 influenza virus pulmonary tropism using various ex vivo imaging techniques (magnetic resonance imaging, confocal laser-scanning microscopy and correlative light-electron microscopy). We infected importin-α7 gene deficient mice (α7(-/-)) with a recombinant H1N1 influenza virus and assessed in vivo viral kinetics in comparison to wild type (WT) mice. In WT mice, influenza virus replication occurred in bronchial and alveolar epithelium already a few days after infection. Accordingly, extensive mononuclear infiltration and alveolar destruction was present in lungs of infected WT mice followed by 100% lethality. Conversely, in α7(-/-) mice, virus replication was mostly restricted to bronchial epithelium with marginal alveolar infection resulting in significantly reduced lung damage and enhanced animal survival. To investigate the host immune response during alveolar virus replication, we studied the role of primary macrophages in virus propagation and clearance. The ability of macrophages to support or clear virus infection as well as the host cellular immune responses did not significantly differ between WT and α7(-/-) mice. However, cytokine and chemokine responses were generally elevated in WT mice likely reflective of increased viral replication in the lung. In summary, these data show that a cellular factor, importin-α7, is required for enhanced virus replication in the alveolar epithelium resulting in elevated cytokine and chemokine levels, extensive mononuclear infiltration and thus, severe pneumonia and enhanced virulence in mice.

IMPORTANCE: Influenza A viruses are respiratory pathogens which may cause pneumonia in humans. Viral infection and replication in the alveoli of the respiratory tract is believed to be crucial for the development of the acute respiratory distress syndrome associated with fatal outcome in influenza virus infected patients. Here, we report the requirement of a cellular factor, importin-α7, for efficient virus replication in the alveolar epithelium of mice. Using complementary ex vivo imaging approaches, we show that influenza virus replication is restricted to the bronchial epithelium followed by enhanced survival in importin-α7 deficient mice. In contrast, presence of this gene results in enhanced virus replication in the alveoli, elevated cytokine and chemokine responses, mononuclear infiltration, alveolar destruction and 100% lethality in wild type mice. Taken together, we show that importin-α7 is particularly required for virus replication in the alveolar epithelium associated with severe pneumonia and death in mice.