Growth factors/supplements
The following reagents were used at indicated concentrations: recombinant human (rhu) BMP-6 (1 μg/ml, if not specified otherwise), rhu BMP-RIB/ALK-6/Fc Chimera (5 μg/ml), rhu BMPR-II/Fc Chimera (5 μg/ml), and recombinant mouse Noggin (5 μg/ml) were purchased from R&D Systems (Abingdon, UK); Anti-IgM F(ab)2 fragments of rabbit polyclonal antibodies to human IgM heavy chain (37.5 μg/ml) was obtained from Dako, Copenhagen, Denmark and rhu CD40 ligand (CD40L, 10 ng/ml) was a gift from Immunex Corp. (Seattle, WA).