Hemin rescued plasma differentiation in Fip200 ^−/− B cells. (A) Quantification of the hemin level in WT (n = 2–3) and Fip200^−/− (n = 3–4) B cells at the naïve stage and day 3 after activation. P values were determined by an unpaired t test. **P = 0.0072. Three independent experiments were performed, with representative data shown for one. (B) Quantification of plasma cell fractions from WT and Fip200^−/− B cells cultured in the presence of IL4+CD40L (left), and IL4+LPS (right) with or without hemin (60 μM) for 3 days. Three independent experiments were performed; data from one experiment are shown (n = 1–3 per treatment/genotype). Left: ***P = 0.0004, right: ***P = 0.0004 (C) Representative plots of WT and Fip200^−/− B cells expressing Blimp-GFP were stimulated by IL4+CD40L (left) and IL4+LPS (right) without hemin (upper) or with hemin (60 μM, bottom) at day 3 by FACS. Three biological replicates were performed for each treatment; data from one experiment are shown. (D) mROS level in WT and Fip200^−/− B cells in the presence of IL4+LPS with or without hemin (60 μM). Two independent experiments were performed with three mice in each group; data from one experiment are shown.