B7h expression on DCs, but not plasma B cells, contributes to germinal center formation and enhances IgG2a isotype usage. Transgenic mice on the B7h−/− background were immunized with NP-CGG in alum, and analyzed after 14–21 days. (A) Splenic cryosections were analyzed by immunohistochemistry on day 14 to reveal PNA+ germinal centers (indicated by arrow). Images are representative of larger fields shown in Additional file 2: Figure S2. Original total magnification was 64X. (B) The number of germinal center B cells measured by flow cytometry, the relative cross-sectional area of germinal centers determined by imaging, and serum anti-NP IgG responses measured by ELISA at day 14 are shown. The relative affinities of serum anti-NP antibody responses were measured by the ratio of NP1/NP15-binding serum IgG on day 21. (C) The ratios of NP-specific IgG2a and IgG1 to total NP-specific IgG in serum on day 14, determined by ELISA, are shown. For ELISA and flow cytometry data, each symbol represents an individual immunized mouse. For GC size analysis, each symbol represents an individual germinal center. The mean of each group is indicated by a solid bar, and p values are shown where significance was detected (Student’s t test, p < 0.05) between the B7h−/− group and transgenic or B7h+/+ groups. For ELISA data, the mean values from control B7h−/− groups were set to a value of 1. The data are representative of three independent experiments.