Cell-Based Influenza A/H1N1pdm09 Vaccine Viruses Containing Chimeric Hemagglutinin with Improved Membrane Fusion Ability

The H1N1 influenza pandemic vaccine has been developed from the A/California/07/09 (Cal) virus and the well-known high-yield A/Puerto Rico/8/34 (PR8) virus by classical reassortment and reverse genetics (RG) in eggs. Previous studies have suggested that Cal-derived chimeric hemagglutinin (HA) and neuraminidase (NA) improve virus yields. However, the cell-based vaccine of the H1N1 pandemic virus has been less investigated. RG viruses that contained Cal-derived chimeric HA and NA could be rescued in Madin–Darby canine kidney cells that expressed α2,6-sialyltransferase (MDCK-SIAT1). The viral growth kinetics and chimeric HA and NA properties were analyzed. We attempted to generate various RG viruses that contained Cal-derived chimeric HA and NA, but half of them could not be rescued in MDCK-SIAT1 cells. When both the 3′- and 5′-terminal regions of Cal HA viral RNA were replaced with the corresponding regions of PR8 HA, the RG viruses were rescued. Our results were largely consistent with those of previous studies, in which the N- and C-terminal chimeric HA slightly improved virus yield. Importantly, the chimeric HA, compared to Cal HA, showed cell fusion ability at a broader pH range, likely due to amino acid substitutions in the transmembrane region of HA. The rescued RG virus with high virus yield harbored the chimeric HA capable of cell fusion at a broader range of pH.