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.
IMPORTANCEIn 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.T he first pandemic of the 21st century was caused by a novel influenza A virus strain of the H1N1 subtype that contained gene segments from both North American and Eurasian swine lineages (1-3). In the beginning, the 2009 H1N1 influenza pandemic was considered to be relatively mild as the majority of cases underwent an uncomplicated or even an asymptomatic infection course. However, this was partially revoked since infection attack rates were highest among the younger age groups, in contrast to seasonal influenza, where mostly the elderly are affected (4-7). During the pandemic in 2009, a disproportionately high number of young adults were hospitalized due to pneumonia and eventually died (6-9). Retrospective modeling estimates that during the first 12 months of the pandemic, approximately 80% of the overall 201,200 respiratory and additional 83,300 cardiovascular deaths occurred in people younger than 65 years (10). This age-specific mortality pattern among younger individuals has been captured by the "years-of-life-lost" metric (11) as a more accurate parameter to measure pandemic burden. In the United States, the number of years of life lost within the first months of the pandemic was estimated to range between the impact of the more virulent H3N2 influenza epidemics and that of the 1968 Hong Kong pandemic (12).Subsequently, it was postulated that ...