FIGURE 1

Cross-protective antibody responses against A/PR8 HA. Five mice per group were IN (a) or IM (b) immunized with A/PR8 (H1N1), A/Yamagata (H1N1), A/Guizhou (H3N2), or B/Ibaraki HA vaccine with -GalCer as an adjuvant twice with a 4-week interval. The nasal wash and serum samples were collected 2 weeks after the second immunization, and anti-A/PR8 HA-specific IgA and IgG titers and serum HI titers to A/PR8 virus were measured. Another set of mice was IN infected with 1,000 PFU of A/PR8 virus in 1 l PBS 2 weeks after the second immunization. The nasal wash fluid was collected 3 days after virus challenge. The virus titer was measured by plaque assay. In the influenza virus-induced lethal pneumonia model, the immunized mice were challenged with a lethal dose (1,000 PFU in 20 l: 40 LD₅₀) of A/PR8 virus at 2 weeks after the second immunization. The body weight and survival rate of the mice were then estimated daily. The survival rate at day 14 is indicated. For the body-weight change, each point represents the relative ratio of initial body weight (mean) of five mice for each day after the challenge. One mouse (no. 1; 20%) recovered thereafter. (c) Five mice per group were IN immunized twice with A/PR8 (H1N1) plus -GalCer. Serum HI titers to the indicated viruses and nasal virus titers after challenge of the indicated viruses are shown. "No HA vaccine" group received only -GalCer. All the experiments were repeated independently at least three times, with similar results. Each column represents the means.d. *P<0.01 vs. group of no HA vaccine. HA, hemagglutinin; HI, hemagglutination inhibition; IN, intranasal; ND, not detected; PBS, phosphate-buffered saline.