Abstract
During the 2009 H1N1 influenza pandemic, infection attack rates were particularly high among young individuals who suffered from pneumonia with occasional death. Moreover, previously reported determinants of mammalian adaptation and pathogenicity were not present in 2009 pandemic H1N1 influenza A viruses. Thus, it was proposed that unknown viral factors might have contributed to disease severity in humans. In this study, we performed a comparative analysis of two clinical 2009 pandemic H1N1 strains that belong to the very early and later phases of the pandemic. We identified mutations in the viral hemagglutinin (HA) and the nucleoprotein (NP) that occurred during pandemic progression and mediate increased virulence in mice. Lethal disease outcome correlated with elevated viral replication in the alveolar epithelium, increased proinflammatory cytokine and chemokine responses, pneumonia, and lymphopenia in mice. These findings show that viral mutations that have occurred during pandemic circulation among humans are associated with severe disease in mice. IMPORTANCE: In this study, novel determinants of 2009 pandemic H1N1 influenza pathogenicity were identified in the viral hemagglutinin (HA) and the nucleoprotein (NP) genes. In contrast to highly pathogenic avian influenza viruses, increased virulence in mice did not correlate with enhanced polymerase activity but with reduced activity. Lethal 2009 pandemic H1N1 infection in mice correlated with lymphopenia and severe pneumonia. These studies suggest that molecular mechanisms that mediate 2009 pandemic H1N1 influenza pathogenicity are distinct from those that mediate avian influenza virus pathogenicity in mice.