Heme inhibits human plasma B cell differentiation through blocking DOCK8/STAT3 signaling pathway and by modulating HO-1 enzymatic activity.
B cells from alloimmunized SCD patients are resistant to heme inhibition but resistance can be reversed with cotreatment with quinine.
Red blood cell alloimmunization remains a barrier for safe and effective transfusions in sickle cell disease (SCD), but the associated risk factors remain largely unknown. Intravascular hemolysis, a hallmark of SCD, results in the release of heme with potent immunomodulatory activity, although its effect on SCD humoral response, specifically alloimmunization, remains unclear. Here, we found that cell-free heme suppresses human B cell plasmablast/plasma cell differentiation by inhibiting the DOCK8/STAT3 signaling pathway, which is critical for B cell activation, as well as by upregulating heme oxygenase 1 (HO-1) through its enzymatic byproducts, carbon monoxide and biliverdin. Whereas non-alloimmunized SCD B cells were inhibited by exogenous heme, B cells from the alloimmunized group were non-responsive to heme inhibition and readily differentiated into plasma cells. Consistent with a differential B cell response to hemolysis, we found elevated B cell basal levels of DOCK8 and higher HO-1-mediated inhibition of activated B cells in non-alloimmunized compared to alloimmunized SCD patients. To overcome the alloimmunized B cell heme insensitivity, we screened several heme-binding molecules and identified quinine as a potent inhibitor of B cell activity, reversing the resistance to heme suppression in alloimmunized patients. B cell inhibition by quinine only occurred in the presence of heme and through HO-1 induction. Altogether, these data suggest that hemolysis can dampen the humoral B cell response and that B cell heme responsiveness maybe a determinant of alloimmunization risk in SCD. Quinine by restoring B cell heme sensitivity may have therapeutic potential to prevent and inhibit alloimmunization in SCD patients.