Btk overexpression specifically enhances BCR-mediated B-cell activation. (A) WT (black line) and CD19-hBtk (red line) B cells were stimulated with α-IgM F(ab′)₂ fragments and Ca²⁺ mobilization was monitored and normalized for maximum influx on stimulation with ionomycin. Plots are representative for 4 mice of each genotype. (B) Flow cytometric analysis of in vivo expression of CD86, CD80, and MHCII on WT (gray) and CD19-hBtk (red line) CD19⁺ B cells. Graphs below summarize the MFI levels of these expression markers on WT (black bars) and CD19-hBtk cells (red bars; n = 4 per group). (C) Flow cytometric analysis of MACS-purified WT (gray) and CD19-hBtk (red line) naive B cells that were cultured in the presence of the indicated stimuli for 24 hours. Data are displayed as histogram overlays of the indicated markers; mice were 10 to 14 weeks old.