The level of B7h on plasma B cells regulates the number of antibody secreting cells generated by immunization. Transgenic and control mice on the B7h+/+ background were immunized with NP-CGG and analyzed after 14 days. (A) The relative concentration of anti-NP IgG1 antibodies in supernatants from overnight splenocyte cultures and in serum were measured by ELISA. Each symbol represents one mouse and bars indicate the mean of each group. The data were normalized such that the mean of the control group had a value of 1. P values are shown where statistically significant differences were found between transgenic and control groups. (B) The number of anti-NP IgG1-secreting splenocytes was determined using an ELISPOT assay. Symbols represent the population mean at each dilution and error bars show the standard error of the mean for each group. Statistical significance was found between control mice (n = 6) and B-B7hTg mice (n = 5), but not DC-B7hTg mice (n = 6). (C) The number of B220+IgDneg-GL7+Fas+ germinal center B cells was determined by flow cytometry. Each symbol represents one mouse and bars indicate the mean of each group. The data are representative of at least 3 independent experiments.