Repression of the plasma cell gene expression program by Spi-B. CD19⁺ B cells were retrovirally transduced with constructs expressing Spi-B, BCL-6, or control-GFP. Five days after transduction and culturing in conditions promoting plasma cell differentiation (as in Figure 1), GFP⁺ cells were sorted. (A) Gene expression levels of Spi-B, BCL-6, BLIMP-1, XBP-1, IRF-4, and PAX-5 were analyzed by quantitative RT-PCR. Expression levels in Spi-B– and BCL-6–transduced cells were normalized to expression levels in control transduced cells. Mean plus or minus SD values of 4 independent experiments are shown (Student t test, **P < .01). (B) Cell lysates from sorted GFP⁺ cells were subjected to immunoblot analysis using antibodies directed against Spi-B, BCL-6, BLIMP-1, IRF-4, or PAX-5. Actin levels were determined as loading control. Representative results of 2 independent experiments are shown.