Synovial fluid inhibits killing of Staphylococcus aureus by neutrophils

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Abstract
To evaluate the effect of synovial fluid on neutrophil bactericidal activity independent of opsonin activity, the effect of non-inflammatory osteoarthritic synovial fluid from osteoarthritic patients on serum-mediated neutrophil killing of S. aureus was examined. With an initial inoculum of 5 .times. 106 colony-forming units/ml, 1.47 .+-. 0.14% bacteria survived after 120 min of incubation with 10% serum and neutrophils; 4.07 .+-. 0.33% bacteria survived after incubation in serum plus synovial fluid (P < 0.001). This inhibitory effect was directly related to the concentration of synovial fluid in the incubation mixture. Increasing the concentration of synovial fluid resulted in an increased percent survival. Studies utilizing preopsonized bacteria and radiolabeled organisms demonstrated that synovial fluid did not interfere with opsonization or phagocytosis. Intracellular bactericidal activity was assayed separately from phagocytosis by utilizing a brief ingestion period followed by the removal of extracellular bacteria by differential centrifugation or lysostaphin treatment. The reincubation of cells and associated bacteria with serum or serum plus synovial fluid revealed that synovial fluid significantly inhibited the promoting effect of serum on neutrophil bactericidal activity. After 60 min of incubation with 10% serum, 13.0 .+-. 1.2% bacteria survived; 21.5 .+-. 2.3% survived after incubation in serum plus synovial fluid (P < 0.005). O2- production was not affected by the presence of synovial fluid. Apparently, the inhibitory effect of synovial fluid is due to an interaction between synovial fluid and the serum factors that promote intracellular killing.

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