CX3CL1 [Fractalkine] is expressed on TNF-alpha and IL-1 activated endothelial cells (ECs) and binds to CX3CR1. CX3CL1 can exist in 2 forms, as membrane-anchored or as a shed 95-kDa glycoprotein. The soluble CX3CL1 has potent chemoattractant activity for T cells and monocytes, and the cell surface bound protein, which is induced on activated primary endothelial cells, promotes strong adhesion of those leukocytes. CX3CL1 may play a role in regulating leukocyte adhesion and migration processes at the endothelium.