Heme inhibits B-cell activation. Purified naïve B cells from HDs were CFSE labeled and stimulated with B-cell activation cocktail for 7 days. (A-B) By using the gating strategy for analysis of proliferated B cells and CD38⁺ plasmablasts (A), we showed the frequency of proliferated B cells in total B cells and of CD38⁺ plasmablasts in the absence or presence of 2.5 μM heme (B). (C) Fold change in proliferated B-cell and CD38⁺ plasmablast frequencies in the presence of increasing doses of heme relative to no heme treatment. (D) Fold change in CD38⁺ plasmablast frequencies in the presence of different doses of RBC lysate or heme (left) or a given RBC lysate dose or 2.5 μM heme preincubated or not with 2.5 μM hemopexin (HPX; right). (E) Cell-free heme levels just before transfusion or within 4 hours after transfusion in plasma from allo-positive and allo-negative SCD patients. (F) CD38⁺ plasmablast frequencies in stimulated naïve B cells from HDs and allo-positive and allo-negative SCD patients. (G) Fold change in CD38⁺ plasmablast frequencies in the presence of 2.5 μM or 5 μM heme. Data represent means ± standard error of the mean. *P < .05; **P < .01. CON, control.