Regulation of ASC differentiation by sCD40L is due to a block in Blimp-1, and XBP-1 transcription and not due to failure to exit the cell cycle. Proliferation of LPS-stimulated (30 μg/mL), CSFE-labeled C57BL/6 (A) and Sm-specific (2-12H/Vκ8) B cells (B), in the presence (—) or absence (▬) of rsCD40L (100 ng/mL), was determined on day 3 by FACs analysis. Data are representative of 5 experiments. C57BL/6 and Sm-specific (2-12H/Vκ8) B cells were stimulated with LPS (30 μg/mL) in the presence or absence of rsCD40L (100 ng/mL) for 3 days and the frequency of (C) intracellular IgM^hi cells or (D) ASCs was determined by fluorescence-activated cell-sorting analysis or ELISPOT, respectively. LPS-stimulated C57BL/6 cultures (100%) had 5.6 × 10⁴ to 7.8 × 10⁴ ASCs/10⁶ cells and 2-12H cultures had 7.6 × 10⁴ to 3.8 × 10⁵ ASCs/10⁶ cells. Blimp-1 (E) and XBP-1 (F) mRNA levels were measured by real-time PCR in Sm-specific B cells after 3 days of LPS stimulation (30 μg/mL) in the presence or absence of rsCD40L (100 ng/mL). Statistical analysis was performed using 1-sample t test by comparing treated B-cell cultures with untreated, LPS-stimulated, B-cell cultures (control). Data represent at least 3 experiments (*P ≤ .05). Error bars represent plus or minus SEM.