Synergistic effect of BAFF CD19 phosphorylation by BCR. (A) After coculture of human B-cell lines or murine splenic B cells with mitomycin-treated transfectants at a ratio of 10:2 for 3 hours, BCR ligation was induced by antihuman or antimouse IgM Ab (0.5 μg/mL). After 5 minutes, the reaction was terminated and cell lysates were prepared for immunoblot analysis using antiphospho CD19 (Tyr513) Ab. The expression levels of phophorylated CD19 are presented in the histogram in panel B. CD19 was detected as a loading control. For the blocking assay, TACI-Ig fusion protein was used at 0.4 μg/mL. Data shown are representative of 3 different experiments and are presented as mean ± SD; ^*P < .05; ^**P < .01.