Co-immunization with tandem repeat heterologous M2 extracellular proteins overcomes strain-specific protection of split vaccine against influenza A virus.

Current influenza vaccines are less efficacious against antigenically different influenza A viruses. This study presents an approach to overcome strain-specific protection, using a strategy of co-immunization with seasonal H3N2 split vaccine and yeast-expressed soluble proteins of a tandem repeat containing heterologous influenza M2 ectodomains (M2e5x). Co-immunization with both vaccines in mice was superior to either vaccine alone in inducing cross protection against heterologous H3N2 virus by raising M2e-specific humoral and cellular immune responses toward a T-helper type 1 profile inducing IgG2a isotype antibodies as well as interferon-γ-producing cells in systemic and mucosal sites. In addition, co-immunization sera were found to confer cross-protection against different subtypes of H1N1 and H5N1 influenza A viruses in naïve mice. A mechanistic study provides evidence that activation of dendritic cells by co-stimulation with M2e5x and split vaccine was associated with the proliferation of CD4(+) T cells. Our results suggest that a strategy of co-immunization with seasonal split and M2e5x protein vaccines could be a promising approach for overcoming the limitation of strain-specific protection by current influenza vaccination.