CX3C-chemokine, fractalkine-enhanced adhesion of THP-1 cells to endothelial cells through integrin-dependent and -independent mechanisms

Leukocyte adhesion and trafficking at the endothelium requires both cellula r adhesion molecules and chemotactic factors. A newly identified CX3C chemo kine, fractalkine, expressed on activated endothelial cells, plays an impor tant role in leukocyte adhesion and migration. We examined the functional e ffects of fractalkine on beta(1) and beta(2) integrin-mediated adhesion usi ng a macrophage-like cell line, THP-1 cells. In this study, we report that THP-1 cells express mRNA encoding a receptor for fractalkine, CX(3)CR1, det ermined by Northern blotting, Scatchard analysis using fractalkine-SEAP (se creted form of placental alkaline phosphatase) chimeric proteins revealed t hat THP-1 cells express a single class of CX(3)CR1 with a dissociation cons tant of 30 pM and a mean expression of 440 sites per cell. THP-1 cells effi ciently adhered, in a fractalkine-dependent manner, to full-length of fract alkine immobilized onto plastic and to the membrane-bound form of fractalki ne expressed on ECV304 cells or TNF-alpha-activated HUVECs. Moreover, solub le-fractalkine enhanced adhesion of THP-1 cells to fibronectin and ICAM-1 i n a dose-dependent manner. Pertussis toxin, an inhibitor of G(i), inhibited the fractalkine-mediated enhancement of THP-1 cell adhesion to fibronectin and ICAM-1, Finally, we found that soluble-fractalkine also enhanced adhes ion of freshly separated monocytes to fibronectin and ICAM-1, These results indicate that fractalkine may induce firm adhesion between monocytes and e ndothelial cells not only through an intrinsic adhesion function itself, bu t also through activation of integrin avidity for their ligands.