H5 N-terminal β sheet promotes oligomerization of H7-HA1 that induces better antibody affinity maturation and enhanced protection against H7N7 and H7N9 viruses compared to inactivated influenza vaccine.

Initiation of mass vaccination is critical in response to influenza pandemic. There is an urgent need of a simple, rapid method for production of influenza vaccine that is more effective than current traditional influenza vaccines. Recent H7N9 transmissions to humans in China with high morbidity/mortality initiated extensive vaccine evaluation. We produced the HA1 domains (amino acids 1-320) from H7N9 and H7N7 strains in E. coli. Both were found to contain primarily monomers/trimers with low oligomeric content. However, when residues from the N-terminal β sheet (first 8 amino acid) of H7 HA1 domains were swapped with the corresponding amino acids from H5N1, functional oligomeric H7 HA1 were produced (HA1-DS), demonstrating strong receptor binding and hemagglutination. In rabbits, the HA1-DS from either H7N9 or H7N7 generated high neutralization titers against both homologous and heterologous H7 strains, superior to the unmodified H7 HA1 proteins. In ferrets, HA1-DS from H7N7 elicited higher (and faster) HI titers, better protected ferrets from lethality, weight loss, and reduced viral loads following challenge with wild-type highly pathogenic H7N7 virus compared with inactivated H7N7 subunit vaccine. HA1-DS vaccinated ferrets were also better protected from weight loss after challenge with the heterologous H7N9 virus compared with inactivated H7N7 subunit vaccine. Importantly, the H7N7 HA1-DS vaccine induced antibody affinity maturation far superior to the inactivated H7N7 subunit vaccine, which strongly correlated with control of viral loads in the nasal washes after challenge with either H7N7 or H7N9 strains. We conclude that N-terminus β sheet domain-swap can be used to produce stable functional oligomeric forms of better recombinant HA1 vaccines in simple, inexpensive bacterial system for rapid response to emerging pandemic threat for the global population.