MAPK activation in CD19-deficient cells following anti-RP105 or LPS stimulation. Wild-type and CD19^– A20 cells were incubated with anti-RP105 Abs for the indicated times. Cell lysates were subjected to SDS-PAGE and transferred to nitrocellulose membranes for anti-phosphoERK (A) or anti-phosphoJNK (B) immunoblotting. (C) JNK1 (left panel) and JNK2 (right panel) phosphorylation levels were quantified and shown by mean percentages ± SEM with maximum phosphorylation levels in wild-type cells defined as 100%. ▪ indicates wild-type cells; and □, CD19^– cells. (D) JNK phosphorylation following anti-RP105 stimulation (5 μg/mL) in spleen B cells from wild-type and CD19^–/– mice. (E) JNK phosphorylation following LPS stimulation (10 μg/mL) in wild-type and CD19^– A20 cells. These results are representative of those obtained in 3 independent experiments.