Inclusions were demonstrated in normal polymorphonuclear cells incubated with rheumatoid factor-positive synovial fluid. These stained positively for IgG, IgM, and the ß1C component of complement. When normal polymorphonuclear cells were incubated with factor-negative rheumatoid fluid, inclusions were not obtained. However, after addition of IgM rheumatoid factor to such fluids, discrete inclusions were observed with similar staining properties. In a minority of nonrheumatoid fluids, inclusions were also obtained after addition of IgM rheumatoid factor. The phagocytosis of inclusions from synovial fluids after addition of IgM rheumatoid factor suggests the presence of aggregated IgG in these fluids.

Immunofluorescent staining of phagocytic synovial lining cells, isolated from synovial tissue by trypsin digestion, demonstrated a diffuse staining pattern for IgG and the ß1C component of complement in cells from both seropositive and seronegative patients. In seropositive patients, discrete inclusions were also observed which stained positively for IgG, IgM, and ß1C. These findings provide evidence of in vivo phagocytosis of immune complexes from the synovial fluid.

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