Immunosuppressive properties of bone marrow-derived and synovium-derived adherent cells. (a) Proliferative activity of T cells in a mixed lymphocyte reaction. Responding BALB/c splenocytes (105 cells) were incubated for 4 days with mitomycin-treated DBA/1 splenocytes (105) in the presence or absence of 105 adherent cells from bone marrow (BM) (n = 10; six healthy, two rheumatoid arthritis, two osteoarthritis) or synovium (n = 10; five rheumatoid arthritis, two osteoarthritis, three normal) or normal fibroblasts (fibro; n = 3). The proliferative response corresponding to the average counts per minute of triplicates of alloreactive T cells (allo) was assigned the value of 100% ± standard deviation (SD). (b) The indoleamine 2,3-dioxygenase (IDO) activity was detected as the tryptophan to kinurenin conversion measured at an optical density (OD) of 450 nm. Cells were cultured in the absence or presence of IFN-γ and/or tumor necrosis factor alpha (TNF-α) for 48 hours and the detection of kinurenin was measured by photometry. The IDO activity is expressed as the mean activity from four different samples of cells from each tissue ± SD.