Hemolytic diseases, such as
sickle cell anemia and
thalassemia, are characterized by enhanced release of
hemoglobin and
heme into the circulation,
heme-
iron loading of reticulo-endothelial system macrophages, and chronic
inflammation. Here we show that in addition to activating the vascular endothelium,
hemoglobin and
heme excess alters the macrophage phenotype in
sickle cell disease. We demonstrate that exposure of cultured macrophages to hemolytic aged red blood cells,
heme, or
iron causes their functional phenotypic change toward a proinflammatory state. In addition,
hemolysis and macrophage
heme/
iron accumulation in a mouse model of sickle disease trigger similar proinflammatory phenotypic alterations in hepatic macrophages. On the mechanistic level, this critically depends on
reactive oxygen species production and activation of the
Toll-like receptor 4 signaling pathway. We further demonstrate that the
heme scavenger
hemopexin protects reticulo-endothelial macrophages from
heme overload in
heme-loaded Hx-null mice and reduces production of
cytokines and
reactive oxygen species. Importantly, in sickle mice, the administration of human exogenous
hemopexin attenuates the inflammatory phenotype of macrophages. Taken together, our data suggest that therapeutic administration of
hemopexin is beneficial to counteract
heme-driven macrophage-mediated
inflammation and its pathophysiologic consequences in
sickle cell disease.