CCL2 and rheumatoid arthritis: Moreover, chemokine receptors CCR2 (CD192) of CD14+ monocytes and CX3CR1 (fractalkine receptor, GPR13) of CD16+ monocytes interact with the corresponding ligands CCL2 (MCP-1) and CX3CL1 (fractalkine) released from FLSs, and this process contributes to the migration of monocytes from circulation and their recruitment into the RA synovium [9].