Here, significantly more IL-6 was produced in endothelial co-cultures with synovial fibroblasts than in co-cultures with dermal fibroblasts from the same RA donors in the absence of exogenous cytokines (2.92±0.52 ng/mL compared with 0.64±0.14 ng/mL, respectively; mean±SEM n=9, p<0.01 by paired t-test). Here, IL6 is linked to rheumatoid arthritis.