CD40 ligand stimulation promotes the expansion and maturation of long-term cultured IgM⁺ B cells. Purified CD19⁺ progenitors from 8-week-old HB-LTC were transferred onto freshly plated S17 stroma or onto irradiated CD40L fibroblasts in medium supplemented by IL-4 and IL-10 and collected after 8 days in culture. (A) Wright-Giemsa staining of CD19⁺ cells (original magnification × 1,000) demonstrating plamacytoid appeareance of CD19⁺ cells in the CD40L system. (B) (Upper panel) FACS analysis demonstrating increase in IgM⁺ B cells after transfer to the CD40L system. (Lower panel) Results of anti-IgM staining after transfer to S17 versus the CD40L system using CD19⁺ cells from 7 independent HB-LTCs. (C) FACS analysis demonstrating the increase in IgM⁺, IgM⁺IgD⁺ populations, the increase in CD23 expression, and the decrease CD10 and CD38 expression in CD19⁺ progenitors after transfer to the CD40L system.