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Hemoglobin (Hb) is one of the most abundant proteins in the human body. When red blood cells rupture, cell-free Hb may initiate adverse pathophysiological reactions. Pathophysiology triggered by cell-free Hb plays an important role in modifying the phenotype of sickle cell disease (SCD). SCD is caused by a single nucleotide mutation of the β-globin gene resulting in Hemoglobin-S (HbS) instead of the normal HbA found in healthy individuals. Polymerization of HbS shortens the lifespan of sickle red blood cells and promotes intra- and extravascular hemolysis. In cell-free Hb ferrous Hb (Fe2+) is oxidized into ferric Hb (Fe3+) promoting the dissociation and transfer of heme into lipid compartments where it triggers lipid peroxidation and generation of cytotoxic and pro-inflammatory reaction products. These processes promote endothelial cell activation and damage. The endogenous plasma protein hemopexin exhibits the highest binding affinity for heme and binds heme in a 1:1 binding ratio. Heme bound to hemopexin is rendered relatively non-reactive and is delivered safely to hepatocytes for endocytosis and degradation. To investigate the endothelial-protective function of hemopexin based on its ability to scavenge heme, we exposed human umbilical vein endothelial cells (HUVEC) in vitro to heme(NaOH) in the presence or absence of different hemopexin doses. As a read-out, different markers for endothelial cell activation were analyzed by either flow cytometry or multiplexed particle-based flow cytometry (Luminex). Briefly, confluent HUVEC were preincubated with hemopexin at different concentrations for 5 min before stimulation with heme(NaOH) for 25 min. Following stimulation cells were analyzed by flow cytometry for expression of membrane bound P-Selectin, a robust marker of endothelial cell activation. Alternatively, heme(NaOH) stimulation of hemopexin-preincubated HUVEC was conducted for 16 h and cell culture supernatants were analyzed by Luminex for three additional well-characterized plasma markers of endothelial cell activation: pro-inflammatory cytokine IL-8, cell adhesion molecule VCAM-1 and glycoprotein Von Willebrand factor (vWF). In the absence of hemopexin, heme(NaOH) consistently induced robust cell surface expression of P-Selectin and elevated levels of soluble IL-8, VCAM-1 and vWF. However, hemopexin completely blocked the stimulatory potential of heme as HUVEC exposed to pre-formed heme:hemopexin complexes showed unchanged P-Selectin expression levels compared to negative control samples. We found that hemopexin reduced heme(NaOH)-mediated P-selectin expression on HUVEC in a dose-dependent fashion. Once an equimolar ratio between heme and hemopexin was reached, P-selectin expression was abolished as shown in figure 1. In addition to P-Selectin, hemopexin also had a strong effect to reduce the heme-induced expression of IL-8, VCAM-1 and vWF to background levels. Thus, the presented data underlines on the one hand the stimulatory capacity of heme(NaOH) on endothelial cells and demonstrates on the other hand the potential of hemopexin to efficiently neutralize free heme. In a stoichiometric fashion, hemopexin potently prevents the pro-inflammatory effect of heme on endothelial cells. Hence, our study suggests a protective role of hemopexin for endothelial cells exposed to elevated levels of cell-free heme due to intravascular hemolysis. Additional experiments are required to elucidate the effect of hemopexin on the endothelium in more detail. Combined with our other lines of data, our results further support the investigation of hemopexin as a potential therapeutic agent in the treatment of sickle cell disease. Disclosures Adam: CSL Behring AG: Current Employment. Gentinetta:CSL Behring: Current Employment. Diditchenko:CSL Behring AG: Current Employment. Schaub:CSL Behring AG: Current Employment. Kato:CSL Behring AG: Current Employment. Brinkman:CSL Behring: Current Employment. Zuercher:CSL Behring AG: Current Employment.