CD40L culture enhances the proliferation of HB-LTC–derived CD19⁺ progenitors. A representative experiment in which CD19⁺ progenitors from an 8-week-old HB-LTC were seeded in microwells (10⁵ cells/well) containing S17 stromal cells or CD40L fibroblasts in medium containing no added cytokines, IL-4, IL-10, or both cytokines. (A) Cell recovery was determined by trypan blue dye exclusion and expressed as the mean number of viable cells. (B) Cell cycle analysis of CD19⁺cells removed from the CD40L system. (Left) Dot plot of IgM versus DNA content analyzed by propidium iodide staining. (Right) Histogram illustrating DNA content in the IgM⁺ (R1) versus the IgM⁻ (R2) populations. Numbers indicate the percentage of cells in S/G2/M.