Secretion of S100A8 and S100A9 proteins from isolated synovial tissue (ST) cells of rheumatoid arthritis (RA) and osteoarthritis (OA). Freshly isolated ST cells (1 × 106 cells per ml in culture medium with 10% fetal calf serum) were incubated without any stimulation for 72 hours. Culture supernatants were measured in duplicate for the S100A8/A9 heterodimer by enzyme-linked immunosorbent assay. Values are the mean ± standard error of the mean. n, number of samples tested.