FIG. 2.

Influenza virus-specific total serum IgG antibody responses. Groups of mice were immunized twice with 40 μg VLPs (A) or three times with 10 μg VLPs (B). Mice (24 BALB/c mice per group) were intranasally immunized with influenza HA VLPs or M1 VLPs at 3-week intervals. Blood samples were collected individually at 2 weeks after each immunization. Sera diluted 100-fold were used to determine PR8-specific total IgG by ELISAs. Optical densities (OD) were read at 450 nm, and results are expressed as the arithmetic means plus standard deviations (error bars). Immunization groups: 40 μg VLP, 40 μg influenza VLPs containing HA and M1 at weeks 0 and 3; 40 μg M1 VLP, 40 μg HA-negative M1 VLPs (M1 VLPs) at weeks 0 and 3; 10 μg VLP, 10 μg influenza VLPs containing HA and M1 at weeks 0, 3, and 6; 10 μg M1 VLP, 10 μg M1 VLPs at weeks 0, 3, and 6. Statistical significance is indicated for the difference between mice immunized with influenza HA VLPs and M1 VLPs (*, P < 0.005). (C) Comparison of immune sera from mice immunized with heat-treated VLPs and intact VLPs. Groups of mice (12 mice per group) were immunized with 10 μg of VLPs (heat treated or intact) containing PR8 HA at weeks 0, 2, and 4. The abilities of serum samples (sera diluted 200-fold) after the last immunization to bind to antibodies against inactivated A/PR8 (PR8), heat-treated VLPs (Heat-VLP), and intact VLPs (Intact-VLP) used as an ELISA coating antigen were compared.