Examining protective efficacy of influenza virus NA-containing virus-like particles (VLP) in mouse model of post-influenza bacterial pneumonia caused by Staphylococcus aureus

Currently, pneumonia resulting from post-influenza infection still remains a pressing issue. In particular, virtually no studies regarding a role of immune response against influenza virus neuraminidase in regulating host susceptibility to subsequent bacterial superinfection are now available. Virus-like particles represent one of the new and promising approaches in contemporary virology for developing influenza vaccines and studying influenza virus proteins. Upon that, it is possible to obtain virus-like particles carrying individual influenza virus-derived proteins such as neuraminidase and hemagglutinin. This allows to get closer insight into potential role of individual influenza virus proteins for host immune response as well as assess a risk of developing secondary bacterial pneumonia. In this study we examined an immune response against influenza virus NA protein and its impact in generating resistance to secondary bacterial pneumonia by using neuraminidase-bearing virus-like particles. We used an experimental model of secondary bacterial pneumonia induced by Staphylococcus aureus after influenza infection. Animals were preliminarily vaccinated with virus-like particles carrying neuraminidase, hemagglutinin, or both (a cocktail of virus-like particles). In this model vaccinated animals were infected 21 days later with influenza viruses A/Puerto Rico/8/34 (H1N1) and reassortant strain NIBRG-121xp (A/California/04/2009 (pndm H1N1 2009) X A/Puerto Rico/8/34 ( H1N1) bearing surface proteins hemagglutinin and neuraminidase derived from the A/California/04/2009 virus as well as the internal proteins derived from A/Puerto Rico/8/34. Next, 5 days after influenza infection mice were infected with Staphylococcus aureus. Moreover, animals in one group were simultaneously vaccinated and infected with A/Puerto Rico/8/34 (H1N1) followed by inoculation on day 21 with reassortant virus NIBRG- 121xp. The protective vaccine activity was assessed by measuring survival rate, life expectancy and decreased body weight loss. The data obtained showed that virus-like particles containing neuraminidase revealed no protective activity. However, a cocktail of virus-like particles, containing hemagglutinin and neuraminidase, protected animals from lethal outcome as well as body weight loss. Moreover, the increase virus-like particles containing neuraminidase in the cocktail of virus-like particles (virus-like particles containing neuraminidase + hemagglutinin) led to elevated protective effect after vaccination that was comparable or even superior to that one in mice with post-infectious immunity.