Activated FLS secrete proinflammatory cytokines (tumor necrosis factor (TNF)-α and interleukin [IL]-1β), chemokines [monocyte chemoattractant protein (MCP)-1, also known as C–C motif chemokine ligand (CCL)-2], matrix metalloproteinases [(MMP)-3 and -9], and angiogenic factors (Yoshitomi, 2019) that enter the intra-articular synovial fluid and destroy cartilage and bone in RA (Ai et al., 2018). Here, CCL2 is linked to rheumatoid arthritis.