Cross-protection Mechanisms of Influenza Virus Vaccines

Since current influenza vaccine strategy is effective in conferring protection against vaccine strain-matched influenza viruses but not against antigenically different viruses, the development of broad cross-protective vaccines is of a high priority to improve vaccination efficacy and to prevent future pandemic outbreaks. In this study, I investigated the cross-protective efficacy and the immune mechanism of three different target antigens including hemagglutinin (HA) based inactivated virus vaccines, neuraminidase (NA) protein vaccines, and tandem repeat extracellular domains of the ion channel protein M2 (M2e5x) on virus-like particle (M2e5x VLP) vaccines. Anti-NA antibodies could confer better cross-protection against multiple heterologous influenza viruses correlating with NA inhibition activity compared to anti-HA antibodies. Whereas anti-HA antibodies were superior to NA in conferring homologous protection. Anti-NA and M2e antibodies showed comparable survival protection. To better understand cross-protective vaccine efficacy, M2e and HA vaccines were tested in different genetic backgrounds. BALB/c mice showed higher IgG responses and cross-protection than C57BL/6 mice after M2e vaccine immunization. M2e vaccine immune mice after primary challenges developed strong immunity to a secondary heterosubtypic virus as a future pandemic. The classical complement pathway is activated to eliminate antigen-antibody immune complexes, subsequently followed by complement-dependent cytotoxicity in addition to Fc receptor-mediated antibody-dependent cell-mediated cytotoxicity. However, the role of complement system remains largely unknown in influenza virus M2e-mediated cross protective immunity. This study demonstrated that complement protein C3 is essential in inducing immune responses to influenza M2e5x VLP vaccination and influenza virus infection, which include M2e-specific isotype-switched antibody production and M2e-specific effector CD4 and CD8 T cell responses. C3 deficient (KO) mice showed lower levels of M2e-specific IgG isotype antibodies after M2e5x VLP vaccination, no control of lung viral replication, and severe weight loss upon challenge infection compared to those in wild type (WT) mice. Whereas, C3 KO mice were protected against homologous virus after immunization with hemagglutinin-based virus vaccine despite lower levels of neutralizing antibodies than those in WT mice. In addition, C3 KO mice showed impaired recruitment of macrophages and different subsets of dendritic cells. The findings in this study suggest that C3 is a key regulator in developing protective immunity by non-neutralizing antibody-based vaccination