The Phosphatidylserine Receptor-Phosphatidylserine Axis Promotes Sickle Erythrocyte Retention in the Lung Microcirculation.

Pulmonary vascular occlusion is a major cause of morbidity and mortality in sickle cell disease. Adhesion between sickle erythrocytes, leukocytes and the endothelium are intimately involved in pulmonary vascular occlusion, which contributes to the pathogenesis of the acute chest syndrome. Despite this significance, specific molecules and cognate mechanisms responsible for trapping sickle erythrocytes in the lung microcirculation are poorly understood. In this study, we show for the first time that products released by activated neutrophils act through the phosphatidyserine and phosphatidylserine receptor axis to promote retention of sickle erythrocytes in the lung microcirculation. We demonstrated that co-incubation with autologous activated neutrophils increased 2-fold the proportion of phosphatidylserine exposed sickle erythrocytes. This effect was abrogated by prior treatment of neutrophils with zileuton a pharmacological inhibitor of 5-lipoxygenase. We next determined that primary endothelial cells derived from the pulmonary vasculature expressed the stereospecific receptor for phosphatidylserine. Transcripts for phosphatidylserine receptor (PSR) were 2-fold more abundant in endothelial cells from the pulmonary microvasculature compared to those from the pulmonary artery. To determine the relevance of neutrophil activation in PSR expression, cultures of PMVECs were treated with supernatants of activated neutrophils from patients with sickle cell anemia, and this resulted in 2-fold increase in PSR transcripts. To unravel the pathophysiological relevance of these findings, isolated rat lungs were perfused with ⁵¹Cr labeled sickle erythrocytes pre-incubated with annexin V, and retention of sickle erythrocytes quantified by ⁵¹Cr radioactivity. The number of sickle erythrocytes trapped in isolated lungs was 2-fold lower compared to the value in control experiments performed with untreated sickle erythrocytes. In agreement with our previous findings activated neutrophils significantly increased sickle erythrocyte retention in the lung (P=0.0004), however cloaking of sickle erythrocytes with annexin V reduced retention by 3-fold. Collectively, these data show that activation of neutrophils increases expression of PSR and its cognate ligand on endothelial cells and sickle erythrocytes respectively, providing the molecular basis for an adhesion complex that increases retention of sickle erythrocytes in the pulmonary endothelium. The PSR-phosphatidylserine adhesion complex therefore offers a novel therapeutic target to reduce pulmonary vascular occlusive events in sickle cell disease.