Therapeutic intervention of inflammatory/immune diseases by inhibition of the fractalkine (CX3CL1)-CX3CR1 pathway

Inflammatory and immune responses are generated locally by the selective invasion and accumulation of the immune cells into the lesion site. The infiltration process of the immune cells into the tissue from the blood through the vascular endothelial cells is closely regulated by a number of chemotactic factors and cell adhesion molecules. Fractalkine (FKN)/CX3CL1 is a membrane-bound chemokine possessing a chemokine/mucin hybrid structure and a transmembrane domain and has a dual function as an adhesion molecule and a chemoattractant. FKN is mainly expressed on activated endothelial cells, activated fibroblasts, and osteoblasts. Its receptor, CX3CR1, is expressed on cytotoxic effector lymphocytes, monocytes/macrophages, and osteoclasts. To date, a lot of key functional aspects of the FKN-CX3CR1 axis has been identified: (1) the rapid capture and firm adhesion of immune cells to vascular endothelial cells, (2) chemotaxis, (3) the enhancement of the transmigration to other chemokines, (4) the crawling behavior of the monocytes that patrol on vascular endothelial cells, (5) the retention of monocytes as the accessory cells of the inflamed endothelium to recruit inflammatory cells, and (6) the survival of the macrophage. In this review, we will focus on the pathological role of FKN in rheumatoid arthritis (RA) and the physiological role of FKN on osteoclast differentiation. Furthermore, we will discuss the therapeutic potential of anti-FKN mAb for RA patients and its distinct mode of action from other cytokine inhibitors.