CCL2 and rheumatoid arthritis: Among of them, RA fibroblast-like synoviocytes (RAFLS) secrete various pro-inflammatory cytokines, such as TNF-α, interleukin-1β (IL-1β), IL-6, and IL-17, chemokines, such as monocyte chemotactic protein-1 (MCP-1) and IL-8, and vascular endothelial growth factor (VEGF) to promote and maintain joint inflammation.