Inhibition of JNK activity using SP600125 causes a reduction in BCR-induced proliferation of primary splenic B cells. (A) Splenic B cells from DBA/2 mice were treated with 50 μg/mL anti-IgM alone (□ and - - -) or 10 μg/mL anti-IgM + IL-4 20 U/mL (▴ and —) in the presence or absence of indicated doses of SP600125 for 2 days. Results were expressed as percentage of control response (mean ± SD of triplicate cultures) when compared with cells that were not treated with SP600125 (the actual counts are 81 032 ± 12 381 for anti-IgM 50 μg/mL, 16 489 ± 3815 for anti-IgM plus IL-4). (B) Human peripheral blood B cells were treated with 10 μg/mL anti-IgM in the absence (□) or presence (♦) of indicated doses of SP600125 for 3 days. The actual counts were 4942 ± 61 for anti-IgM (10 μg/mL) + IL-2 (100 U/mL) and 4717 ± 208 for DMSO equivalent. Proliferation was measured as described in “Materials and methods.” Results are presented as mean ± SE of triplicate cultures. Results are representative of 4 experiments in panel A and of 2 experiments in panel B.