We have previously demonstrated that the globular head of the
hemagglutinin (HA)
antigen fused to
flagellin of Salmonella typhimurium fljB (STF2, a TLR5
ligand) elicits protective immunity to H1N1 and H5N1 lethal
influenza infections in mice (Song et al., 2008, PLoS ONE 3, e2257; Song et al., 2009,
Vaccine 27, 5875-5888). These fusion
proteins can be efficiently and economically manufactured in E. coli fermentation systems as next generation pandemic and seasonal
influenza vaccines. Here we report immunogenicity and efficacy results of three
vaccine candidates in which the HA globular head of A/California/07/2009 (H1N1) was fused to STF2 at the C-terminus (STF2.HA1), in replace of domain 3 (STF2R3.HA1), or in both positions (STF2R3.2xHA1). For all three
vaccines, two subcutaneous immunizations of BALB/c mice with doses of either 0.3 or 3 µg elicit robust neutralizing (HAI)
antibodies, that lead to > = 2 Log(10) unit reduction in day 4 lung virus titer and full protection against a lethal A/California/04/2009 challenge. Vaccination with doses as low as 0.03 µg results in partial to full protection. Each candidate, particularly the STF2R3.HA1 and STF2R3.2xHA1 candidates, elicits robust
neutralizing antibody responses that last for at least 8 months. The STF2R3.HA1 candidate, which was intermediately protective in the challenge models, is more immunogenic than the H1N1 components of two commercially available trivalent inactivated
influenza vaccines (TIVs) in mice. Taken together, the results demonstrate that all three
vaccine candidates are highly immunogenic and efficacious in mice, and that the STF2R3.2xHA1 format is the most effective candidate
vaccine format.