Adaptive Mutations That Occurred during Circulation in Humans of H1N1 Influenza Virus in the 2009 Pandemic Enhance Virulence in Mice

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. 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 ...

Sweep Dynamics (SD) plots: Computational identification of selective sweeps to monitor the adaptation of influenza A viruses

“…The pandemic waves were defined based on large viral prevalence with a phase of low viral prevalence in between. Five of seven detected sweep-related changes were also described in this study as emerging changes in the first wave of the pandemic, with frequencies close to 100% in the second wave in 2010N, similar to the frequencies we observed in our analysis (frequencies between 91% and 99%, with slight differences most likely due to differing time frames and datasets in both analyses 36 . Moreover, all seven adaptive mutations were associated with enhanced respiratory droplet transmission in a ferret model, suggesting their crucial role to sustain with the mammalian host 34 .…”

Evolution of 2009 H1N1 influenza viruses during the pandemic correlates with increased viral pathogenicity and transmissibility in the ferret model

“…Recombinant 2009 pH1N1 viruses were generated by reverse genetics using the pHW2000 based 8-plasmid system5. All viruses, including the recombinant A/Netherlands/213/03 (H3N2) and A/Vietnam/11/94 (H5N1) strains were propagated on MDCK cells.…”

Reduced accumulation of defective viral genomes contributes to severe outcome in influenza virus infected patients

“…Special effort has been previously made to identify virulence determinants of AH1N1pdm09 viruses and AH1N1 seasonal viruses. As a result, some residues distributed all over the genome have been associated to increased virulence of specific viral isolates [6, 7]. These determinants map mainly to the polymerase genes (PB1, PB2, PA), the hemagglutinin (HA), neuraminidase (NA), and non-structural protein 1 (NS1) (reviewed in [8]).…”