Segment 2 from influenza A(H1N1)pdm09 viruses confers temperature sensitive HA yield on candidate vaccine virus growth in eggs that is complemented by PB2 701D

Candidate vaccine viruses (CVVs) for seasonal influenza A virus are made by reassortment of the antigenic virus with a high-yielding egg-adapted strain, typically A/Puerto Rico/8/34 (PR8). Many 2009 H1N1 pandemic (pdm09) high-growth reassortants (HGRs) selected by this process contain pdm09 segment 2 in addition to the antigenic genes. To investigate this, we made CVV mimics by reverse genetics (RG) that were either 6:2 or 5:3 reassortants between PR8 and two pdm09 strains, A/California/7/2009 (Cal7) and A/England/195/2009, differing in the source of segment 2. The 5:3 viruses replicated better in MDCK-SIAT1 cells than the 6:2 viruses, but the 6:2 CVVs gave higher HA antigen yields from eggs. This unexpected phenomenon reflected temperature sensitivity conferred by pdm09 segment 2, as HA yields from eggs for the 5:3 viruses improved substantially when viruses were grown at 35°C compared with 37.5°C, whereas 6:2 virus yield did not. Authentic 5:3 pdm09 HGRs, X-179A and X-181, were not markedly temperature-sensitive however, despite their PB1 sequences being identical to that of Cal7, suggestive of compensatory mutations elsewhere in the genome. Sequence comparisons of the PR8-derived backbone genes identified single changes in PB2 and NP, 5 in NS1, and 1 in NS2. PB2 N701D but not NP T130A affected the temperature dependency of viral transcription. Furthermore, introducing the PB2 701D change into a 5:3 CVV mimic improved and drastically reduced the temperature sensitivity of HA yield. We conclude that RG PR8 backbones used for vaccine manufacture in eggs should contain PB2 701D to maximise virus yield.