Role of HO-1 enzyme activity in heme-mediated B-cell activation. (A) HO-1 analysis of HD unstimulated peripheral blood B cells (left) and after 7-day stimulation of purified naïve B cells in proliferated B cells (gate showing CD38⁺ plasmablasts). (B) Fold change in HO-1 expression in plasma cells in 7-day stimulated purified HD B cells treated with 2.5 μM or 5 μM heme relative to no heme treatment. (C-D) Absolute and fold change of frequency of B-cell proliferation (C) and CD38⁺ plasmablasts in the presence of SnPPIX (2.5 μM) without or with (2.5 μM) heme (D). (E) Fold change in B-cell proliferation and CD38⁺ plasmablasts 7-day stimulated naïve B cells from allo-positive and allo-negative SCD patients in the absence or presence of SnPPIX (2.5 μM) and heme (2.5 μM) relative to untreated media control (Con). (F) Fold change in B-cell proliferation and plasma B cells in 7-day stimulated HD naïve B cells in the presence of different doses of heme degradation byproducts, carbon monoxide releasing molecule 3 (CORM-3) and biliverdin, relative to untreated cultures. Data represent means ± standard error of the mean. *P < .05; **P < .01.