FOXP1 overexpression inhibits differentiation of IgG⁺ MBCs but cannot further repress differentiation of IgM⁺ MBCs. IgM⁺ (IgG⁻IgA⁻) and IgG⁺ (IgM⁻IgA⁻) MBCs were sorted from human peripheral blood and transduced with FOXP1-IRES-YFP, BCL6-IRES-GFP (except for IgG⁺ MBCs), or control empty vector and cultured under conditions that promote PC differentiation. (A) Equal numbers of sorted cells (50 000) were cultured with IL-21 and IL-2 for an additional 24 hours. Thereafter, the supernatants were collected, and IgM and IgG protein levels were analyzed by ELISA. Levels were normalized to levels in control-transduced cells. Means ± SD of 3 independent experiments are shown. (B) Equal numbers of sorted cells were plated onto membranes in serial dilutions and cultured with IL-21 and IL-2 for an additional 18 hours, after which numbers of IgM- or IgG-secreting cells were determined by ELISPOT. Spot numbers were normalized to numbers in stimulated, control-transduced cells. Means ± SD of 3 independent experiments are shown (1 sample t test, *P < .05; **P < .01; ***P < .001).