Promotion of plasma cell differentiation using the MS40L-low cell line along with IL-2, IL-10, and CpG. (A) MS5 cells were transduced by FUW-CD40L virus, and 2 single-cell clones (MS40L-high and MS40L-low) were selected based on the expression level of CD40L detected by flow cytometry. The shaded peaks represent parental MS5 cells; open peaks, transduced cells. (B) Stage 2-derived cells carrying the GFP transgene were transferred onto MS40L stable lines (MS40L-high and MS40L-low) and incubated in the presence of IL-2 (10 ng/mL), IL-10 (100 ng/mL), and CpG (2 μM) for 18 days. The ability of MS40L cell lines to support B-cell activation was compared with that of soluble CD40L (1 μg/mL). Soluble CD40L did not support the survival of B cells, leading to nearly no detection of CD19⁺GFP⁺ cells. In contrast, the MS40L cell lines supported the survival and expansion of B cells. (C) The abilities of MS40L-high and MS40L-low to induce plasma cell differentiation were compared as indicated by decreased CD19 and surface IgM expression (top panels), appearance of CD20⁻CD38⁺ cells (middle panels), and increased CD27 expression (bottom panels). (D) Naive B cells isolated from human peripheral blood were cultured in the presence of MS40L-low and the indicated stimuli (CpG, IL-2, IL-10, and/or IL-6 [50 ng/mL]) for 9 days and stained for flow cytometric analysis of plasma cell differentiation (middle panels, CD20⁻CD38⁺ cells; right panels, CD19⁻CD27⁺ and CD19^lowCD27^+/high cells).