. Activated neutrophilmediated sickle red blood cell adhesion to lung vascular endothelium: role of phosphatidylserine-exposed sickle red blood cells. Am J Physiol Heart Circ Physiol 291: H1679 -H1685, 2006. First published May 19, 2006 doi:10.1152/ajpheart.00256.2006.-Activated neutrophils (ANs) increase sickle red blood cell (SRBC) retention/adhesion in the pulmonary circulation. This study investigates the role of neutrophil activation and SRBC retention/adhesion in the pulmonary circulation through a mechanism that involves increasing phosphatidylserine (PS) exposure on the external membrane surface of the SRBCs (PS-exposed). With the use of flow cytometry, double-labeling studies were performed with a calcium-dependent phospholipidbinding protein, annexin V-fluorescein isothiocyanate fluorescence, and the erythroid-specific marker glycophorin A to assess for the percentage of PS-exposed normal and SRBCs at baseline and after coincubation with ANs. Additional studies were performed that assessed retention/adhesion of SRBCs in the isolated rat lung using 51 Cr-labeled SRBC alone, SRBC ϩ AN, SRBC ϩ AN ϩ zileuton, and SRBC ϩ AN ϩ annexin V. Specific activities of lung and perfusate were measured, and the number of retained SRBCs per gram lung was calculated. Flow cytometry demonstrated that ANs increased the percentage of PS-exposed normal and SRBCs. The 5-lipoxygenase inhibitor zileuton attenuated AN-mediated increases in PS-exposed SRBCs and decreased SRBC retention/adherence in the lung on histological sections. Similarly, in the isolated perfused lung and in histological lung sections, retention/adherence of SRBCs cloaked with annexin V was attenuated in the presence of ANs. We conclude that ANs enhance the adhesion of SRBCs to vascular endothelium by increasing red blood cell membrane externalization of PS. Zileuton attenuation of AN-mediated SRBC PS externalization suggests that a 5-lipoxygenase product(s), secreted by the AN, plays a vital role in altering the adhesive properties of PS-exposed SRBCs to vascular endothelium. zileuton; microvascular occlusion; pulmonary circulation INCREASED SICKLE red blood cell (SRBC) adhesion to vascular endothelial cells (EC) is well documented in cell culture systems (19 -21, 23, 24, 34) and is thought to be central to vasoocclusion by the SRBC. In isolated rat lung perfused with SRBC suspensions, we have shown that the retention/adherence of SRBCs was 3.5 times greater than that seen in lung perfused with normal (hemoglobin A) human red blood cells (RBCs). In isolated perfused lung, products of activated neutrophils (ANs) further enhanced SRBC retention/adherence. This effect of ANs on RBC adhesion is attenuated when neutrophils are pretreated with a 5-lipoxygenase inhibitor (16). Whereas these in vitro results suggest the involvement of leukotriene products, the cellular mechanism involved in neutrophil-mediated increased SRBC adhesion to EC has yet to be elucidated.Adhesion molecules and their ligands on the RBC membrane surface, on EC, and in plasma have been im...