Swine-origin influenza virus like particle vaccine conferred complete protection against lethal viral challenge in BALB/C mice

Abstract

OBJECTIVE: To develop the novel swine-origin influenza virus-like particle (VLP) vaccine against influenza virus infection and compare the efficiency of this vaccine with the clinical inactivated Panenza vaccine. METHODS: Recombinant baculovirus genome encoded three structural proteins, hemagglutinin (HA), neuraminidase (NA), and matrix (M1) derived from Influenza A/Hong Kong /01/2009 (H1N1) virus and each was within its own expression cassette, downstream of the Autographa californica multiple nuclear polyhedrosis virus polyhedrin promoter and upstream of the SV40 polyadenylation signal. Spodoptera frugiperda Sf9 insect cells were infected with high MOI of recombinant baculoviruses. VLPs were purified from the supernatant by 20–60% sucrose density gradient ultracentrifugation and then characterized by western blotting and transmission electron microscopy. Female BALB/c mice were intramuscularly vaccinated with 3 μg VLPs and clinical Panenza vaccine respectively (based on HA content). The antibody titer in vaccinated mice sera was detected by ELISA and the immune responses were compared. After prime-boost vaccination, mice were challenged with 10LD50 of influenza A/Hong Kong/01/2009 (H1N1) virus. RESULTS: HA, NA and M1 proteins were co-expressed revealed by western blotting analysis. Biologically active VLPs were self-assembled and released into the culture medium at 72 h post-infection. All mice vaccinated with the clinical Panenza vaccine and VLPs were protected from morbidity and mortality against lethal viral challenge. Viral titer in homogenized lung tissues of survived mice was undetectable by real-time PCR and plaque forming unit assay. All vaccinated mice had high anti-HA antibodies regardless of the vaccine immunogen. However, the antibody titer in mice immunized with VLP was much higher than that in the clinical Panenza vaccinated mice. Moreover, the dominant IgG isotypes in Panenza vaccinated mice is IgG1, while in VLP-vaccinated mice is IgG1, IgG2a and IgG2b. VLPs could also stimulate T cell responses. Conclusions: These results suggest that highly immunogenic swine-origin influenza VLPs are feasible to be generated by Bac-to-Bac baculovirus expression system. Compared with the clinical Panenza vaccine, VLPs could induce broader immune responses. Influenza VLPs can be an effective and promising antigen for developing safe and alternative vaccine to control the spread influenza virus in the future.