Identification and functional characterization of monocytes in rheumatoid synovial fluid

Mononuclear cell suspension from synovial fluids of 13 patients with rheumatoid arthritis and 5 patients with juvenile rheumatoid arthritis contained on average of 1.5 and 1.3% cytotoxic plaque-forming cells, respectively, when tested against a monolayer of antibody-sensitized sheep erythrocytes. The plaque-forming ability was completely abolished after removal of cells which had phagocytosed carbonyl iron. The highest proportion of plaque-forming cells was found in the cell suspensions that contained the highest percentage of peroxidase-positive mononuclear cells. The plaque-forming activity was almost completely inhibited by human IgG. Cell suspensions from patients treated with corticosteroids contained the highest proportions of plaque-forming cells and peroxidase-positive mononuclear cells compared with patients not receiving such treatment. Our findings indicate that the plaque-forming leukocytes in rheumatoid synovial fluid are monocytes.