Microvascular Endothelial Cells Express Phosphatidylserine Receptor: A Functionally Active Receptor for Phosphatidylserine-Positive Erythrocytes.

Phosphatidylserine (PS)-positive erythrocytes adhere to both endothelial cells and the sub-endothelial matrix components. While thrombospondin mediates these interactions, it is not known whether PS-associated erythrocyte-endothelial adhesion occurs in the absence of plasma ligands. Using ionophore-treated PS-expressing control erythrocytes, we demonstrate that PS-positive erythrocytes adhered directly to human lung micro-endothelial cells in the absence of plasma ligands, that this adhesion was further enhanced following endothelial activation with IL-1α, TNF-α, LPS, and hypoxia (2.5- to 8-fold increase), and that this adhesive interaction was selective to erythrocyte-PS. We next explored whether micro-endothelial cells express an adhesion receptor that recognizes cell surface expressed PS (PSR) similar to that expressed on activated macrophages. Using RT-PCR and Western blotting, we demonstrate constitutive expression of both PSR mRNA and protein which were up-regulated in a time-dependent manner following endothelial activation (with maximal increases of 3-fold in mRNA and 2-fold in protein noted at 4 and 6 hour, respectively). While minimal PSR expression was noted on un-stimulated cell surface (8% positivity), endothelial activation up-regulated surface expression of this receptor (35–40% positivity in IL-1α and TNF-α activated cultures). In antibody blocking studies, using both artificially generated PS-positive erythrocytes and also using PS-positive erythrocytes from patient with sickle cell disease, we demonstrate that PSR was functionally active supporting PS-mediated erythrocyte adhesion to activated endothelial cells. Our results demonstrate the existence of a novel functional adhesion receptor for PS on the micro-endothelium which is up-regulated by such pathologically relevant agonists as hypoxia and cytokines.