Hemolysis, with the release of extracellular free heme and hemin, is thought to play a major role in the deadly pulmonary complications of sickle cell disease. Acute chest syndrome is the second most common cause of hospitalization for sickle cell disease patients and the leading cause of death. Pulmonary hypertension complicates sickle cell disease in up to 10% of patients and is associated with significant morbidity and mortality. Current therapies, including hydroxyurea and blood transfusions, are mostly supportive and have been used for decades, while specific therapeutics targeting the pulmonary circulation have been elusive. We hypothesize that hemin, as a breakdown product of cell-free hemoglobin, has direct effects on the pulmonary endothelium and that concentration specific effects contribute to both the acute and chronic complications of sickle cell disease. In this proposal, we will evaluate the specific effects of chronic, low concentration hemin in the development of pulmonary vascular remodeling and pulmonary hypertension. We propose direct effects on pulmonary artery endothelial cells through Toll-like receptor 4 lead to endothelial-to-mesenchymal transition. Endothelial cells undergoing trans-differentiation will have increased proliferative and migratory properties as well as cytoskeletal rearrangement leading to an invasive, dysfunctional phenotype. In animal models of chronic hemolysis, this will manifest as pulmonary vascular remodeling with the development and progression of pulmonary hypertension. Similarly, high concentration hemin will have effects on the pulmonary endothelium through Toll-like receptor 4, but overwhelming stimulation will lead to activation of necroptosis and programmed necrosis. This necroptotic cell death will result in endothelial barrier dysfunction and acute lung injury in cell and animal models as a mechanism for the development of non-cardiogenic pulmonary edema and acute chest syndrome. The concentration dependent effects of hemin on pulmonary artery endothelial cells and animal models of hemolysis in our work mimic the spectrum of pulmonary complications in sickle cell disease patients. Therefore, this mechanistic work will complement a translational approach associating circulating hemin abundance with pulmonary complications in sickle cell disease patients and utilizing RNA sequencing technology to identify differentially expressed genes and genomic signatures associated with the presence of pulmonary hypertension and acute chest syndrome. Our long-term goals are to utilize our understanding of the mechanisms of endothelial dysfunction in both acute and chronic pulmonary complications of sickle cell disease and other chronic hemolytic syndrome in order to identify novel molecular targets, develop innovative therapies, impact health and survivorship of patients through our research and clinical care.