Microvascular endothelial cells express a phosphatidylserine receptor: a functionally active receptor for phosphatidylserine-positive erythrocytes

Abstract

Phosphatidylserine (PS)-positive erythrocytes adhere to endothelium and subendothelial matrix components. While thrombospondin mediates these interactions, it is unknown whether PSassociated erythrocyte-endothelial adhesion occurs in the absence of plasma ligands. Using ionophore-treated PSexpressing control HbAA erythrocytes, we demonstrate that PS-positive erythrocytes adhered to human lung microendothelial cells in the absence of plasma ligands, that this adhesion was enhanced following endothelial activation with IL-1␣, TNF-␣, LPS, hypoxia, and heme, and that this adhesive interaction was selective to erythrocyte PS. We next explored whether microendothelial cells express an adhesion receptor that recognizes cell surface-expressed PS (PSR) similar to that expressed on activated macrophages. We demonstrate constitutive expression of both PSR mRNA and protein that were up-regulated in a time-dependent manner following endothelial activation. While minimal PSR expression was noted on unstimulated cells, endothelial activation up-regulated PSR surface expression. In antibody-blocking studies, using PSpositive erythrocytes generated either artificially via ionophore treatment of control erythrocytes or from patients with sickle cell disease, we demonstrate that PSR was functional, supporting PSmediated erythrocyte adhesion to activated endothelium. Our results demonstrate the existence of a novel functional adhesion receptor for PS on the microendothelium that is up-regulated by such pathologically relevant agonists as hypoxia, cytokines, and heme. IntroductionThe anionic phospholipid phosphatidylserine (PS), present exclusively in the inner leaflet of the plasma membrane of a normal cell, is externalized following cell activation with both physiologic and pathologic stimuli such as activation of blood platelets resulting from vessel wall injury and cells that are undergoing apoptosis. 1,2 PS exposure on the red cell surface occurs in patients with various hemolytic anemias 3-9 activating numerous pathobiologic processes, of which one is red cell-endothelial cell adhesion. 10,11 The mechanism(s) underlying this PS-mediated adhesion process is not well defined. Interaction of PS with immobilized subendothelial matrix thrombospondin has been documented. 10 In addition, soluble thrombospondin-mediated adhesion of PS-positive erythrocytes to endothelium can occur via the constitutively expressed endothelial vitronectin receptor (VnR). 10 Whether PS-related erythrocyte adhesion to endothelium can occur in the absence of plasma ligands is not known. In the present study, we explore this possibility, and hypothesize that endothelial cells express novel adhesion receptor(s) that can interact directly with PS-positive erythrocytes in the absence of plasma ligands. One such candidate is a receptor that recognizes cell surface-expressed membrane-associated phosphatidylserine, referred hitherto as the phosphatidylserine receptor or PSR, originally described by Fadok et al on the surface of activated macrophages that recogni...

RRx-001: a systemically non-toxic M2-to-M1 macrophage stimulating and prosensitizing agent in Phase II clinical trials

“…RRx-001-mediated vascular normalization is thought primarily to be due to repolarization of TAMs; this repolarization leads to an inhibition of VEGF and MMP-9, two critically important mediators of TAM angiogenic potential; a reduction in VEGF and MMP-9 has been observed in immunohistochemically stained patient biopsies. In addition, because RRx-001-modified red blood cells bind to the hypoxic endothelium [28], inducing a microvascular 'traffic jam', blood flow is shunted or redistributed through the remaining and more normally functioning (i.e. less leaky, disordered and hypoxic) capillary system, akin to a vascular normalization effect, with a net gain in overall perfusion ( Figure 5(c)).…”

Specific Binding of Red Blood Cells to Endothelial Cells Is Regulated by Nonadsorbing Macromolecules

“…It has been demonstrated that PS is a major component of this interaction in that PS can directly interact with EC features such as CD-36 and PS receptors (6,10,15,24) or bind to EC via plasma ligands such as TSP and lactadherin (33,34). This involvement of PS is also demonstrated in Fig.…”

“…Usually this anionic phospholipid is located exclusively on the inner leaflet of the RBC membrane but is translocated to the outer leaflet in hemoglobinopathies and oxidative stress states (9). The role of PS in adhesion has not been fully understood, but RBC with enhanced PS exposure have been identified to establish specific interactions with EC or the endothelial matrix via several receptors including thrombospondin, ␣ V ␤ 3 , CD36, and PS receptor (6,10,11).…”

Mechanism for phosphatidylserine-dependent erythrophagocytosis in mouse liver

“…Recently, it was reported that microvascular endothelial cells interact with PS-positive RBCs and express a functional receptor for the PS-specific interaction, 30,31 indicating that PS-specific interactions occur between erythrocytes and the endothelium. Growing evidence has revealed that HSECs play an important role in the blood clearance of macromolecules and apoptotic cells.…”