SARS-CoV S protein-specific humoral and mucosal immune responses in BALB/c mice intranasally immunized with PEI/pci-S complexes. The samples were collected on day 7 after the last immunization. Induction of (A) S-specific IgG subclasses in serum and (B) S-specific IgA in various mucosal samples were determined by ELISA. The results were expressed as means ± SEM for the group (n = 5 to 8). Significant differences compared with pci-S group were expressed as *P < 0.05 and **P < 0.01, respectively. (C) Proliferation activity of B220+ cells from spleen of mice immunized with PEI/pci-S complexes. Bar and number in each panel present the percentage of proliferated cells.