CX3C-Chemokine, Fractalkine-Enhanced Adhesion of THP-1 Cells to Endothelial Cells Through Integrin-Dependent and -Independent Mechanisms

Abstract

Leukocyte adhesion and trafficking at the endothelium requires both cellular adhesion molecules and chemotactic factors. A newly identified CX3C chemokine, fractalkine, expressed on activated endothelial cells, plays an important role in leukocyte adhesion and migration. We examined the functional effects of fractalkine on β1 and β2 integrin-mediated adhesion using a macrophage-like cell line, THP-1 cells. In this study, we report that THP-1 cells express mRNA encoding a receptor for fractalkine, CX3CR1, determined by Northern blotting. Scatchard analysis using fractalkine-SEAP (secreted form of placental alkaline phosphatase) chimeric proteins revealed that THP-1 cells express a single class of CX3CR1 with a dissociation constant of 30 pM and a mean expression of 440 sites per cell. THP-1 cells efficiently adhered, in a fractalkine-dependent manner, to full-length of fractalkine immobilized onto plastic and to the membrane-bound form of fractalkine expressed on ECV304 cells or TNF-α-activated HUVECs. Moreover, soluble-fractalkine enhanced adhesion of THP-1 cells to fibronectin and ICAM-1 in a dose-dependent manner. Pertussis toxin, an inhibitor of Gi, inhibited the fractalkine-mediated enhancement of THP-1 cell adhesion to fibronectin and ICAM-1. Finally, we found that soluble-fractalkine also enhanced adhesion of freshly separated monocytes to fibronectin and ICAM-1. These results indicate that fractalkine may induce firm adhesion between monocytes and endothelial cells not only through an intrinsic adhesion function itself, but also through activation of integrin avidity for their ligands.

Enhanced Recruitment of CX3CR1+ T Cells by Mucosal Endothelial Cell–Derived Fractalkine in Inflammatory Bowel Disease

“…In view of these results, it seems justified to conclude that FKN acts a major adhesion molecule involved in leukocyte recruitment into IBD mucosa. Our findings are in keeping with other studies demonstrating the capacity of FKN to capture and adhere various CX3CR1-expressing leukocyte subsets in other experimental systems, like immobilized FKN or FKN fusion proteins 21, 43, 46, 58-60 , FKN-transfected endothelial cells 46,58,60 ,58,60 . In addition to its intrinsic adhesive function, there is indirect evidence that FKN also cooperates with other adhesion molecules, presumably by augmenting integrin affinity 43, 58 . VCAM-1 is essential for leukocyte adhesion to endothelium, but so far there is no direct evidence that FKN upregulates the active form of β1 integrin 38, 49 , a component of the VCAM-1 counterreceptor α 4 β 1 integrin.…”

Human fractalkine mediates leukocyte adhesion but not capture under physiological shear conditions; a mechanism for selective monocyte recruitment

“…Chemokines can activate integrins to bind their ligand with high affinity. Indeed, soluble fractalkine has been shown to induce LFA-1-mediated leukocyte adhesion to ICAM-1 [18]. Here, we demonstrate that fractalkine can also induce adhesion to VCAM-1 in a PTX-sensitive manner.…”

“…A soluble form of fractalkine also exists through cleavage with an endogenous protease [12] and this form may act more like a conventional chemokine, inducing integrinmediated adhesion [18] and chemotaxis [16]. However, it is the membrane-bound form that has generated the most interest as it potentially combines the direct action of an adhesion molecule with the specificity of a chemokine.…”

Soluble Fractalkine Prevents Monocyte Chemoattractant Protein-1-Induced Monocyte Migration via Inhibition of Stress-Activated Protein Kinase 2/p38 and Matrix Metalloproteinase Activities

“…In its membrane form, FKN behaves essentially as an adhesion molecule that interacts with its coligand CX 3 CR1 expressed at the surface of T lymphocytes, NK cells, monocytes (18,22), dendritic cells (23,24), microglial cells (63), and neurons (27). The G-coupled CX 3 CR1 also transduces, in addition to the intrinsic adhesive function of FKN, signals that augment the integrin avidity for their ligands (64). The membraneanchored form of FKN can be cleaved from the cell membrane by proteolysis to yield s-FKN (18,20,21).…”