A North American H7N3 influenza virus supports reassortment with 2009 pandemic H1N1 and induces disease in mice without prior adaptation

Reassortment between H5 or H9 subtype avian and mammalian influenza A viruses (IAV) can generate a novel virus that causes disease and transmits between mammals. Such information is currently not available for H7 subtype viruses. We evaluated the ability of a low-pathogenicity North American avian H7N3 virus (A/shorebird/Delaware/22/2006) to reassort with mammalian or avian viruses using a plasmid-based competition assay. In addition to genome segments derived from an avian H7N9 virus, the H7N3 virus reassorted efficiently with the PB2, NA, andMsegments from the 2009 pandemic H1N1 (_PH1N1) virus. In vitro and in vivo evaluation of the H7N3:_PH1N1 (7 + 1) reassortant viruses revealed that the PB2, NA, orMsegments from _PH1N1 largely do not attenuate the H7N3 virus, whereas the PB1, PA, NP, or NS genome segments from _PH1N1 do. Additionally, we assessed the functionality of the H7N3:_PH1N1 7 + 1 reassortant viruses by measuring the inflammatory response in vivo. We found that infection with wild-type H7N3 resulted in increased inflammatory cytokine production relative to that seen with the _PH1N1 strain and that the increase was further exacerbated by substitution of _PH1N1 PB2 but not NA or M. Finally, we assessed if any adaptations occurred in the individually substituted segments after in vivo inoculation and found no mutations, suggesting that _PH1N1 PB2, NA, and Mare genetically stable in the background of this H7N3 virus. Taking the data together, we demonstrate that a North American avian H7N3 IAV is genetically and functionally compatible with multiple gene segments from the 2009 pandemic influenza virus strain without prior adaptation.