FcγRIIB cross-linking is required for the attenuation of BLyS receptor expression. (A) CD23⁺ B cells from FcγRIIB knockout (KO) or BALB/c wild-type (WT) mice were cultured with the indicated stimuli (concentrations of stimuli are indicated in “B-cell subset isolation, culture, and stimulation”). After 24 hours, the cells were harvested and analyzed by FACS for BR3 and TACI expression. Gray histograms indicate isotype controls; unfilled white histograms, expression by unstimulated cells; filled black histograms, expression by stimulated cells. Median fluorescence intensities (MFI) of respective populations are shown in bar graphs at bottom. Plots are representative of at least 3 experiments; in all cases, n equals 3 or more. Significance levels are indicated by P values. (B) Proliferation and death profiles of CD23⁺ B cells from FcγRIIB KO or WT mice 72 hours after stimulation. B cells were loaded with CFSE and cultured with indicated stimuli as in Figure 1. At 1 minute before analysis, TO-PRO-3 was added as a vital dye. CFSE profiles of live (TO-PRO-3⁻) cells are shown at right. Plots are representative of at least 3 independent experiments.