Development of live vaccine based on the Enterococcus faecium L3 vector for prevention influenza infections

Probiotic microorganisms are currently considered as a promising platform for the development of recombinant vaccines expressing foreign antigens. In this study we generated and evaluated the live mucosal recombinant vaccine by integrating genes encoding influenza virus neuraminidase (NA) of N2 subtype into the DNA of the probiotic strain Enterococcus faecium L3 (L3). We confirmed NA expression in the pili of L3 using immune electron microscopy. Mice were fed with a probiotic vaccine containing the NA gene (L3-NA) or pure L3. Oral administration of L3-NA caused detectable increase of virus-specific serum IgG and local IgA after the third feeding. A the same time, single spleen cell suspensions were stimulated with whole A(H1N1)pdm09 virus followed by flow cytometry. In mice received L3-NA, the content of cytotoxic T-lymphocytes was more pronounced compared to mice receiving pure L3. Immunization with L3-NA increased the survival rate by 34% when the mice were infected using A(H1N1)pdm09 influenza virus after the third feeding. After S. pneumonia post-influenza infection, the L3-NA immunized mice were 50% more protected from lethality in comparison with L3 fed mice. Thus, a live probiotic vaccine candidate based on L3 induced the formation of systemic and local immunity and provide protection against complicated influenza. The approach based on a probiotic vaccine expressing viral epitopes can allow repeated immunization during epidemic season.