Fractalkine and CX3CR1 Mediate a Novel Mechanism of Leukocyte Capture, Firm Adhesion, and Activation under Physiologic Flow

Abstract

Leukocyte migration into sites of inflammation involves multiple molecular interactions between leukocytes and vascular endothelial cells, mediating sequential leukocyte capture, rolling, and firm adhesion. In this study, we tested the role of molecular interactions between fractalkine (FKN), a transmembrane mucin-chemokine hybrid molecule expressed on activated endothelium, and its receptor (CX3CR1) in leukocyte capture, firm adhesion, and activation under physiologic flow conditions. Immobilized FKN fusion proteins captured resting peripheral blood mononuclear cells at physiologic wall shear stresses and induced firm adhesion of resting monocytes, resting and interleukin (IL)-2–activated CD8+ T lymphocytes and IL-2–activated NK cells. FKN also induced cell shape change in firmly adherent monocytes and IL-2–activated lymphocytes. CX3CR1-transfected K562 cells, but not control K562 cells, firmly adhered to FKN-expressing ECV-304 cells (ECV-FKN) and tumor necrosis factor α–activated human umbilical vein endothelial cells. This firm adhesion was not inhibited by pertussis toxin, EDTA/EGTA, or antiintegrin antibodies, indicating that the firm adhesion was integrin independent. In summary, FKN mediated the rapid capture, integrin-independent firm adhesion, and activation of circulating leukocytes under flow. Thus, FKN and CX3CR1 mediate a novel pathway for leukocyte trafficking.

Site‐directed mutagenesis of the chemokine receptor CXCR6 suggests a novel paradigm for interactions with the ligand CXCL16

“…This is also true for CX3CL1-mediated adhesion [36,37]. Interestingly, an N-terminally extended form of CXCL16 was still able to induce significant CXCR6 activation, suggesting that a model in which the chemokine N terminus interacts with an intrahelical binding pocket containing E274 of CXCR6 is unlikely to explain activation of this chemokine receptor.…”

Expression of fractalkine and its receptor, CX3CR1, in atopic dermatitis: Possible contribution to skin inflammation

“…FKN and CX 3 CR1 represent a novel type of leukocyte trafficking molecule that regulates both adhesive and chemotactic functions 17 . The membrane-bound FKN is able to mediate firm adhesion of CX 3 CR1-expressing leukocytes without requiring selectin-mediated rolling or activation of integrins 18,19 . Furthermore, FKN is released from the cell surface by proteolytic cleavage as soluble FKN (sFKN) that has potent chemoattractant activity for CX 3 CR1 + leukocytes and activates them 12,17,20 …”

IL-1β and IFN-γ induce the expression of diverse chemokines and IL-15 in human and rat pancreatic islet cells, and in islets from pre-diabetic NOD mice

“…It is a unique chemokine since it is located on a membrane bound mucin stalk and is in transmembrane and soluble shed forms [26]. Fractalkine has adhesive properties, promoting leukocyte migration and enhancing the cytotoxicity of NK cells [27] and cytotoxic T-lymphocytes [28]. Fractalkine has an important role in cardiac allograft rejection [29,30], and it is expressed in mononuclear cells during renal inflammation [31].…”