Immune Correlates of Protection Induced by Virus-Like Particles Containing 2009 H1N1 Pandemic Influenza HA, NA or M1 Proteins

Influenza virus-like particle (VLPs) vaccines are a promising alternative to conventional egg-based vaccines. Evaluation of vaccine efficacy induced by HA-M1 VLPs, NA-M1 VLPs or M1 VLPs against virus challenge infection would provide important insight into vaccine design strategy. In this study, we generated VLPs containing hemagglutinin (HA), neuraminidase (NA) or M1 proteins derived from the A/California/04/09. Mice were immunized intramuscularly with HA-M1, NA-M1 or M1 VLPs and protective immunity was evaluated by assessing lung virus loads against low (5LD50) or high (100LD50) lethal dose of homologous virus challenges. High levels of virus-specific serum IgG antibody responses were induced in mice after HA-M1 VLPs immunization, whereas low or no IgG antibody responses were detected from immunization with NA-M1 VLPs or M1 VLPs, independently. Mice that were immunized with HA-M1 VLPs showed below the limit of detection on lung virus loads against low dose (5LD50) of challenge and significant reduction against high dose (100LD50) of challenge infection. Mice that were immunized with NA-M1 or M1 VLPs also displayed reduced lung viral loads compared to naïve control. In vitro cultures of cells from mouse spleen and bone marrow revealed that HA-M1 VLPs and NA-M1 VLPs induced higher levels of antibody-secreting cell (ASC) responses compared to naïve control, whereas M1 VLPs showed no ASC responses. HA-M1, NA-M1 or M1 VLPs immunization demonstrated varying degree of protection with respect to body weight changes and survival rates, which are consistent with the levels of antibody responses in sera and ASC responses from spleen and bone marrow.