The PB1 segment of an influenza A virus H1N12009pdm isolate enhances the replication efficiency of specific influenza vaccine strains in cell culture and embryonated eggs

Influenza vaccine strains (IVSs) contain the haemagglutinin (HA) and neuraminidase (NA) genome segments of relevant circulating strains in the genetic background of influenza A/PR/8/1934 virus (PR8). Previous work has shown that the nature of the PB1 segment may be a limiting factor for the efficient production of IVSs. Here, we showed that the PB1 segment (PB1(Gi)) from the 2009 pandemic influenza A virus (IAV) A/Giessen/06/2009 (Gi wt, H1N1pdm) may help to resolve (some of) these limitations. We produced a set of recombinant PR8-derived viruses that contained (i) the HA and NA segments from representative IAV strains (H3N2, H5N1, H7N9, H9N2); (ii) the PB1 segment from PR8 or Gi wt, respectively; and (iii) the remaining five genome segments from PR8. Viruses containing the PB1(Gi) segment, together with the heterologous HA/NA segments and five PR8 segments (5 + 2 + 1), replicated to higher titres compared with their 6 + 2 counterparts containing six PR8 segments and the equivalent heterologous HA/NA segments. Compared with PB1(PR8)-containing IVSs, viruses with the PB1(Gi) segment replicated to higher or similar titres in both cell culture and embryonated eggs, most profoundly IVSs of the H5N1 and H7N9 subtype, which are known to grow poorly in these systems. IVSs containing either the PB1(Gi) or the cognate PB1 segment of the respective specific HA/NA donor strain showed enhanced or similar virus replication levels. This study suggests that substitution of PB1(PR8) with the PB1(Gi) segment may greatly improve the large-scale production of PR8-derived IVSs, especially of those known to replicate poorly in vitro.