Motofumi Shimizu, Satomi Yanase, Chang Myint OO, Masatoshi Okamatsu, Yoshihiro Sakoda, Hiroshi Kida and Hiroshi Takaku
Background: Intermittent outbreaks of highly pathogenic avian influenza virus (flu) infections illustrate the potential for pandemic spread of this deadly disease, thus making the development of sufficient supplies of safe vaccines a necessity. Influenza virus-like particles (Vlps) have been suggested as a promising non-egg or nonmammalian cell culture-based candidate for vaccination against flu. Vlps containing hemagglutinin have previously been shown to promote protection against homologous viral strains. In this report, we describe the development of an H5N1 flu Vlp vaccine involving only three flu viral structural proteins, (i.e., HA, NA, and M1), which were derived from an avian flu A/duck/Hokkaido/vac-1/2004 (H5N1) virus. The H5N1 Vlps produced from insect cells exhibited hemagglutination and neuraminidase activities and generated an immune response in BALB/c mice. We additionally performed viral challenge studies using chickens. Methodology and results: Vlps consisting of the hemagglutinin (HA), neuraminidase (NA), and matrix 1 (M1) proteins of A/duck/Hokkaido/vac-1/2004 (H5N1) were transferred using baculovirus within Spodoptera frugiperda (Sf9) cells. Mice were first immunized with Vlps, and the immune response was compared between animals vaccinated with HA and NA-HA-negative M1 Vlps. The IgG levels of HA-M1 Vlp- and NA-M1 Vlp-treated groups were observed, and 5-fold higher levels of H5N1-specific antibodies were induced in the groups of mice immunized with HA-NA-M1 Vlp. The HA-NA and HA-NA-M1 Vlps vaccines induced IgG2a and IgG2b antibodies as well as IgG1 antibodies, indicating that both Th1 and Th2 immune responses were induced. Furthermore, NA-M1 immunization induced IgG and IgG1 isotype antibodies and led to low levels of IgG2a and IgG2b. Additionally, all chickens immunized with HA-NA-M1 Vlp were protected against deadly infections with the highly pathogenic avian flu virus A/ chicken/Yamaguchi/4/2004 (H5N1). Conclusions: Intramuscular administration of H5 Vlps conferred immunity against a deadly viral challenge. The H5 Vlp vaccine was more successful at raising Th1-biased protective responses, including IgG2a production. Thus, flu Vlps offer an important approach for immunization, especially if a pandemic occurs. Recognizing the current state of vaccination strategies, it is imperative to investigate the associated immunogenicity and defensive capabilities of H5 Vlps in comparison with inactivated whole virus and attenuated live H5N1 viruses. These results support the continued development of Vlps as a formulation for a vaccine against flu infection.