HB-LTCs initiated with CD34⁺CD38⁻ stem cells lead to production of IgM⁺ B cells capable of secreting of Ig after transfer to the CD40L system. HB-LTCs were established in microwells using FACS-sorted CD34⁺CD38⁻ cord blood cells. (A) Total cultured cell populations (adherent and nonadherent cells) from independent wells were collected at weeks 12 and 13 and evaluated by FACS after gating on the lymphoid-sized population. (B) Decreasing numbers CD19⁺ cells from cultures initiated with either CD34⁺CD38⁺ (0.125 to 2 × 10⁴ CD19⁺ cells/well) or CD34⁺CD38⁻ input cells (5 × 10³ CD19⁺ CD19⁺ cells/well; 4 independent wells shown) were transferred to CD40L system with IL-4 and IL-10. Production of IgM was evaluated by ELISA after 14 days. Results shown are representative of three independent experiments.