ICOS costimulation mediated by plasma B cells and DCs selectively regulate the magnitude and isotype usage of antibody responses. Transgenic mice on the B7h+/+ background and non-transgenic littermate controls were immunized with NP-CGG in alum, and serum antibody responses were analyzed by ELISA. Relative isotype-specific anti-NP serum antibody concentrations at day 7 are shown for B-B7hTg (A) and DC-B7hTg (B) mice, compared with wild-type controls. (C) The relative serum concentrations of anti-NP IgG, IgG1, IgG2a, and the IgG2a/IgG1 ratio are shown at day 14 for transgenic and wild-type mice. Each symbol represents an individual immunized mouse and the mean of each group is indicated by a solid bar. The data were normalized such that control groups have mean value of 1. Asterisks and p values are shown where statistical significance (Student’s t test, p < 0.05) was detected between transgenic and control groups. The data are representative of at least 3 independent experiments.