The polymerase segments of zoonotic H7N7 avian influenza virus reduced virulence and transmission in poultry

Avian influenza viruses (AIV) of the subtype H7 can evolve from a low pathogenic (LP) precursor to a high pathogenic (HP) form in chickens, causing severe systemic infections and high mortality rates in infected flocks. A polybasic cleavage site (pCS) in the hemagglutinin surface protein of AIV is one of the main virulence determinants in high pathogenic AIV (HPAIV), whereas less is known about the role of the viral polymerase segments (PB2, PB1 and PA) in the transition of LP to HP AIV. AIV H7N7 are endemic in European wild birds and transmission to land-based poultry has been reported. In 2003, HPAIV H7N7 was isolated from poultry in the Netherlands and further spread to poultry farms in Germany and Belgium resulting in the death and culling of about 31 million birds. Furthermore, the HPAIV H7N7 infected more than 1000 humans. In humans, mutations in the HA and polymerase genes were the main virulence determinants, whereas little is known about the virulence determinants of this virus in poultry. Here, we constructed recombinant LP H7N7 viruses carrying the pCS (designated LP_Poly) with or without PB2, PB1 or PA from HP H7N7. The recombinant H7N7 viruses were characterized in vitro and in vivo in chickens. The pCS increased the virulence of LP H7N7, however, at lower levels compared to HPAIV H7N7. Reassortment of LP_Poly with HPAIV H7N7 PA reduced virus replication in avian cell culture. In chickens, the polymerase segments reduced the virulence of LP_Poly.