IL6 and rheumatoid arthritis: Since a previous study showed that IL-1β could stimulate the expression of FABP4 in ATDC5 cells (a chondrogenic cell line),50 we hypothesized that FABP4 upregulation in RA chondrocytes was induced by excessive proinflammatory factors (IL-1β, IL-6 and TNF-α) derived from synovial M1 macrophages, FLSs and ECs.