Induction of IgG-producing plasma cells from CD27⁺ B cells and IgM-producing plasma cells from CD27⁻ B cells. (A,C) Intracytoplasmic immunoglobulin staining was used to assess immunoglobulin production with each phase. Intracellular IgM increased in both (A) CD27⁺ and (C) CD27⁻ B cells by the end of phase II, and then declined after phase III. In contrast, CD27⁺ cells had a steady increase in intracellular IgG staining through terminal differentiation into CD138⁺ plasma cells. Figures are representative of 4 independent experiments with cells from 4 different blood donors. (B,D) IgG, IgM, and IgA were measured by ELISA from supernatants taken at the end of each culture phase for (B) CD27⁺ and (D) CD27⁻ B cells. Error bars are mean ± standard error for all plots. Experimental data are from 5 separate experiments each with different blood donors.