MBAFF strongly enhances BCR- but not CD40-induced proliferation of B cells. (A) BAFF strongly augments BCR-activated B-cell proliferation. B cells were incubated with the indicated doses of BAFF in the presence (▪) or absence (□) of goat anti-human IgM F(ab′)₂ fragments (10 μg/mL) for 72 hours. [³H]-thymidine incorporation was determined as described in Figure 1. Incubations were performed in triplicate, and results are presented as the mean ± SD from 1 of 4 experiments. (B) APRIL moderately increases BCR-induced B-cell proliferation. B cells were treated with APRIL in the presence (▪) or absence (□) of goat anti-human IgM F(ab′)₂ fragments (10 μg/mL) for 72 hours. [³H]-thymidine incorporation was determined as described in Figure 1. Assays were performed in triplicate, and results are presented as the mean ± SD from 1 of 2 similar experiments. (C) Effect of BAFF on CD40 ligand-stimulated B-cell proliferation. B cells were treated for 72 hours with increasing doses of BAFF in the absence (□) or presence of soluble human CD40L at the following concentrations: 20 ng/mL (▪), 50 ng/mL (○), or 100 ng/mL (•). [³H]-thymidine incorporation was determined as described in Figure 1. Assays were performed in triplicate, and results are presented as the mean ± SD from 1 of 3 similar experiments.