Heme exposure shifts shifts myeloid differentiation in bone marrow derived mixed cell cultures.. Heme exposure shifts shifts myeloid differentiation in bone marrow derived mixed cell cultures.

Heme is an erythrocyte-derived toxin that drives disease progression in hemolytic anemias. During hemolysis, specialized bone marrow-derived macrophages with a high heme-metabolism capacity orchestrate disease adaptation by removing damaged erythrocytes and heme-protein complexes from the blood and supporting iron recycling for erythropoiesis. Here, we performed single-cell RNA sequencing with RNA velocity analysis of GM-CSF-supplemented mouse bone marrow cultures to assess myeloid differentiation under heme stress. We found that heme-activated NRF2 signaling shifted the differentiation trajectories of cells towards antioxidant, iron-recycling macrophages at the expense of dendritic cells, as these cells were selectively deficient in heme-exposed bone marrow cultures. Heme eliminated the capacity of GM-CSF-supplemented bone marrow cultures to activate antigen-specific T cells. The generation of functionally competent dendritic cells was restored by NRF2 loss. The heme-induced phenotype was reproduced in hemolytic mice with sickle cell disease and spherocytosis and associated with reduced dendritic cell functions in the spleen. Our data provide a novel mechanistic underpinning how hemolytic stress may provoke hyposplenism-related secondary immunodeficiency, which is a critical determinant of mortality in patients with genetic hemolytic anemias. Overall design: M-CSF- and GM-CSF-supplemented BM cell cultures were stimulated with heme (300μM) or albumin (control) on day 3. On day 7, the cells were harvested, tagged with DNA-barcoded antibodies, and pooled into a single scRNA-seq sample for sequencing.