Clone REA281 recognizes the human C-X3-C motif chemokine 1 (CX3CL1) antigen, a single-pass type I membrane protein, which is also known as fractalkine or neurotactin. CX3CL1 is expressed in a number of cell types including neurons, intestinal epithelium, and activated vascular endothelium and is structurally distinct from other chemokines. The membrane-anchored form of CX3CL1 induces tight, shear-resistant endothelial adhesion of leukocytes in a manner that is independent of integrin and Gi protein activation. Soluble forms of CX3CL1 generated by proteolytic cleavage with the disintegrin and metalloproteinases CD156c (ADAM10) and CD156b (ADAM17) were shown to promote conventional chemotaxis of T cells and monocytes, but not of neutrophils. The sole receptor for CX3CL1 is CX3CR1, which is widely expressed by leukocytes, including CD14

cells of the monocyte-macrophage DC lineage and subsets of tissue-resident DCs and epidermal Langerhans cells. In addition to supporting leukocyte extravasation from the blood, the CX3CL1–CX3CR1 axis has been implicated in pathogenesis of inflammatory diseases such as atherosclerosis, rheumatoid arthritis, and renal fibrosis. CX3CL1 was also shown to promote extension of transepithelial dendrites by monocytes and/or DCs within intestinal lamina propria, for the purpose of antigen sensing in the intestinal lumen.