Heme-stress activated NRF2 signaling skews fate trajectories of bone marrow cells from dendritic cells towards red pulp-like macrophages

Abstract

Heme is an erythrocyte-derived toxin that drives disease progression in hemolytic anemias, such as sickle cell disease. 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. Since chronic heme-stress is noxious for macrophages, erythrophagocytes in the spleen are continuously replenished from bone marrow-derived progenitors. Here, we hypothesized that adaptation to heme stress progressively shifts differentiation trajectories of BM progenitors to expand the capacity of heme-handling monocyte-derived macrophages at the expense of the homeostatic generation of dendritic cells, which emerge from shared myeloid precursors. This heme-induced redirection of differentiation trajectories may contribute to hemolysis-induced secondary immunodeficiency. We performed single-cell RNA sequencing with directional 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 of bone marrow cells towards antioxidant, iron-recycling macrophages, suppressing the generation of dendritic cells in heme-exposed bone marrow cultures. Heme eliminated the capacity of GM-CSF-supplemented bone marrow cultures to activate antigen-specific CD4 T cells. The generation of functionally competent dendritic cells was restored by NRF2 loss. The heme-induced phenotype of macrophage expansion with concurrent dendritic cell depletion 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 of hemolytic stress as a driver of hyposplenism-related secondary immunodeficiency.

Efferocytosis drives myeloid NLRP3 dependent inflammasome signaling secretion of IL-1β to promote tumor growth

“…Interestingly, we did not observe significant overlap of these two pathways in the 8 mononuclear phagocytic populations (Figure 6C). However, when we performed RNA velocity analysis that imputes cell fates (53,54,61,(84)(85)(86)(87)(88), we identified a robust directional flow from the efferocytosis high C1QC + macrophages towards the inflammasome rich NLRP3 + subset through an intermediate ISG15 + macrophage state (Figure 6E, Supplementary Figure S4, Supplementary Tables S6). We observed that another efferocytosis-rich cluster, SPP1 + , can also develop into the NLRP3 + macrophage subset (Figure 6E).…”

The clinical relevance of heme detoxification by the macrophage heme oxygenase system

“…The importance of PPP activity for heme clearance by macrophages suggests a multifaceted impact of G6PD deficiencynot only are patients at increased risk of hemolysis, but they may also be unable to degrade the free heme effectively. Increased free heme has also been shown to polarize myeloid cells toward heme-clearing phenotypes: In a genetic model of hereditary spherocytosis, chronic heme stress shifted the transcriptional profile of dendritic cells toward that of splenic red pulp macrophages through activation of NRF2 signaling (58), while in the same model liver macrophages had an anti-inflammatory, erythrophagocytic phenotype (21).…”