title: Accumulation of human-adapting mutations during circulation of A(H1N1)pdm09 influenza virus in humans in the United Kingdom.
abstract: The influenza pandemic that emerged in 2009 provided an unprecedented opportunity to study adaptation of a virus recently acquired from an animal source during human transmission. In the United Kingdom, the novel virus spread in three temporally distinct waves between 2009 and 2011. Phylogenetic analysis of complete viral genomes showed that mutations accumulated over time. Second- and third-wave viruses replicated more rapidly in human airway epithelial (HAE) cells than did the first-wave virus. In infected mice, weight loss varied between viral isolates from the same wave but showed no distinct pattern with wave and did not correlate with viral load in the mouse lungs or severity of disease in the human donor. However, second- and third-wave viruses induced less alpha interferon in the infected mouse lungs. NS1 protein, an interferon antagonist, had accumulated several mutations in second- and third-wave viruses. Recombinant viruses with the third-wave NS gene induced less interferon in human cells, but this alone did not account for increased virus fitness in HAE cells. Mutations in HA and NA genes in third-wave viruses caused increased binding to α-2,6-sialic acid and enhanced infectivity in human mucus. A recombinant virus with these two segments replicated more efficiently in HAE cells. A mutation in PA (N321K) enhanced polymerase activity of third-wave viruses and also provided a replicative advantage in HAE cells. Therefore, multiple mutations allowed incremental changes in viral fitness, which together may have contributed to the apparent increase in severity of A(H1N1)pdm09 influenza virus during successive waves.
  
  
  OBJECTIVE
  Although most people infected with the 2009 pandemic influenza virus had mild or unapparent symptoms, some suffered severe and devastating disease. The reasons for this variability were unknown, but the numbers of severe cases increased during successive waves of human infection in the United Kingdom. To determine the causes of this variation, we studied genetic changes in virus isolates from individual hospitalized patients. There were no consistent differences between these viruses and those circulating in the community, but we found multiple evolutionary changes that in combination over time increased the virus's ability to infect human cells. These adaptations may explain the remarkable ability of A(H1N1)pdm09 virus to continue to circulate despite widespread immunity and the apparent increase in severity of influenza over successive waves of infection.
authors.affiliation: Section of Virology, Faculty of Medicine, Imperial College London, London, United Kingdom.
authors.forename: Ruth A
authors.initials: RA
authors.lastname: Elderfield
authors.affiliation: Wellcome Trust Sanger Institute, Hinxton, United Kingdom.
authors.forename: Simon J
authors.initials: SJ
authors.lastname: Watson
authors.affiliation: Section of Leucocyte Biology, Faculty of Medicine, Imperial College London, London, United Kingdom.
authors.forename: Alexandra
authors.initials: A
authors.lastname: Godlee
authors.affiliation: West of Scotland Specialist Virology Centre, Gartnavel General Hospital, Glasgow, United Kingdom.
authors.forename: Walt E
authors.initials: WE
authors.lastname: Adamson
authors.affiliation: Virus Reference Department, Public Health England, London, United Kingdom.
authors.forename: Catherine I
authors.initials: CI
authors.lastname: Thompson
authors.affiliation: Section of Respiratory Medicine, Faculty of Medicine, Imperial College London, London, United Kingdom.
authors.forename: Jake
authors.initials: J
authors.lastname: Dunning
authors.affiliation: Section of Virology, Faculty of Medicine, Imperial College London, London, United Kingdom.
authors.forename: Mirian
authors.initials: M
authors.lastname: Fernandez-Alonso
authors.affiliation: Section of Virology, Faculty of Medicine, Imperial College London, London, United Kingdom.
authors.forename: Deena
authors.initials: D
authors.lastname: Blumenkrantz
authors.affiliation: Section of Leucocyte Biology, Faculty of Medicine, Imperial College London, London, United Kingdom.
authors.forename: Tracy
authors.initials: T
authors.lastname: Hussell
authors.affiliation: Virus Reference Department, Public Health England, London, United Kingdom.
authors.forename: Maria
authors.initials: M
authors.lastname: Zambon
authors.affiliation: Section of Respiratory Medicine, Faculty of Medicine, Imperial College London, London, United Kingdom.
authors.forename: Peter
authors.initials: P
authors.lastname: Openshaw
authors.affiliation: Wellcome Trust Sanger Institute, Hinxton, United Kingdom.
authors.forename: Paul
authors.initials: P
authors.lastname: Kellam
authors.affiliation: Section of Virology, Faculty of Medicine, Imperial College London, London, United Kingdom w.barclay@imperial.ac.uk.
authors.forename: Wendy S
authors.initials: WS
authors.lastname: Barclay
mesh_terms: D000220:Adaptation, Biological; D000293:Adolescent; D000328:Adult; D000818:Animals; D002648:Child; D002675:Child, Preschool; D004195:Disease Models, Animal; D005260:Female; D016679:Genome, Viral; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D053118:Influenza A Virus, H1N1 Subtype / Q000235:genetics* / Q000302:isolation & purification; D007251:Influenza, Human / Q000453:epidemiology / Q000821:virology*; D007372:Interferons / Q000378:metabolism; D008168:Lung / Q000276:immunology / Q000473:pathology / Q000821:virology; D008297:Male; D008807:Mice, Inbred BALB C; D009154:Mutation; D009976:Orthomyxoviridae Infections / Q000276:immunology / Q000473:pathology / Q000821:virology; D010802:Phylogeny; D012367:RNA, Viral; D017422:Sequence Analysis, DNA; D006113:United Kingdom / Q000453:epidemiology; D053585:Virus Attachment; D014779:Virus Replication; D055815:Young Adult
chemical_list: D012367:RNA, Viral; D007372:Interferons