FO B cells in mb.1-Rap1A17 Tg mice show normal function with even enhanced efficiency of GC B-cell generation. (A) FO (B220⁺ CD23^high) B cells sorted from Rap1A17^f (□) and mb.1-Rap1A17 (■) Tg littermates were plated on the fibronectin (FN)–coated wells for 30 minutes, and the adherent cell proportion was assessed (left). The aliquots of the cells were cultured in the presence of anti-IgM or anti-IgM plus anti-CD40 (5 μg/mL each) for 3 days followed by the pulse with ³H-TdR. The means and SEs of triplicate cultures are indicated. (B) FO B cells sorted from Rap1A17^f and mb.1-Rap1A17 Tg littermates before (fine lines) and after (solid lines) the culture in the presence of IL-4 (10 ng/mL) plus anti-CD40 or LPS (10 μg/mL each) for 4 days were analyzed for anti-IgG1 expression with FACSCalibur. The proportions of IgG1⁺ cells are indicated. (C) Rap1A17^f and mb.1-Rap1A17 Tg littermates were injected intraperitoneally with SRBCs (5 × 10⁸). Six days later, the spleens were immunostained with the indicated antibodies (left). The pictures at the widest transverse sections are indicated (PE–anti-IgD and Alexa Fluor 488–anti-GL7). Bars indicate 400 μm. The spleen cells were 3-color analyzed with anti-B220, anti-CD9, and PNA, and the proportions in total B220⁺ cells as well as the cell numbers of the indicated phenotypes are shown. □ indicate unimmunized normal mice; , immunized Rap1A17^f mice; ■, immunized mb.1-Rap1A17 mice. The means and SEs of 5 mice are indicated (right).