Chemokine that acts as a ligand for both CX3CR1 and integrins ITGAV:ITGB3 and ITGA4:ITGB1 (PubMed:10187784, PubMed:18971423). The CX3CR1-CX3CL1 signaling exerts distinct functions in different tissue compartments, such as immune response, inflammation, cell adhesion and chemotaxis (PubMed:10382755, PubMed:9177350, PubMed:10187784, PubMed:18971423). Regulates leukocyte adhesion and migration processes at the endothelium (PubMed:10382755, PubMed:9177350). Can activate integrins in both a CX3CR1-dependent and CX3CR1-independent manner (By similarity). In the presence of CX3CR1, activates integrins by binding to the classical ligand-binding site (site 1) in integrins (By similarity). In the absence of CX3CR1, binds to a second site (site 2) in integrins which is distinct from site 1 and enhances the binding of other integrin ligands to site 1 (By similarity). The soluble form is chemotactic for T-cells and monocytes, but not for neutrophils. The membrane-bound form promotes adhesion of those leukocytes to endothelial cells. Monomer (By similarity). Forms a ternary complex with CX3CR1 and ITGAV:ITGB3 or ITGA4:ITGB1 (By similarity). Highest levels in brain (PubMed:9177350, PubMed:9479488, PubMed:9845323). Lower levels in kidney, heart and lung (PubMed:9177350, PubMed:9479488). Also found in skeletal muscle and testis (PubMed:9177350, PubMed:9479488). Highly expressed in lesional smooth muscle cells, but not macrophages (PubMed:12569158). By 12-O-tetradecanoylphorbol-13-acetate (TPA) and lipopolysaccharides (LPS) in bone marrow stroma cells. A soluble short 80 kDa form may be released by proteolytic cleavage from the long membrane-anchored form. No visible phenotype (PubMed:11287620). Mice develop normally and have normal migration of leukocytes to lymphoid tissue and peripheral sites in several models of inflammation (PubMed:11287620). Belongs to the intercrine delta family.