Effect of IL-12 on IFN-γ expression by primary B cells. (A-B) IFN-γ mRNA expression. Purified primary B cells were treated for up to 72 hours with IL-12 or with IL-12 plus IL-18. Corresponding cDNA was amplified with IFN-γ primers. Panel A shows the specific PCR bands for the 48-hour time point. In panel B, IFN-γ mRNA was quantified as indicated in “Materials and methods.” Results (means ± SEMs) are expressed as -fold increases in normalized cDNA values of IFN-γ versus the level at the 1-hour time point, because B cells did not express detectable IFN-γ at rest. (C-E) IFN-γ secretion in supernatants. (C-D) Purified B and T cells were treated for 48 hours with IL-12 or with IL-12 plus IL-18, as described in “Materials and methods.” (E) B cells were treated for 48 hours with SAC plus IL-12, as described in “Materials and methods.” Culture supernatants were harvested and assayed for IFN-γ by ELISA. Similar results were obtained in 3 other experiments.