A serum antibody in patients with rheumatoid arthritis stimulates cathepsin B activity in peritoneal mouse macrophages.

Mouse peritoneal macrophages were stimulated by sera from patients with active rheumatoid arthritis (RA) to increased intracellular cathepsin B activity. By gel filtration of three RA sera, the stimulatory activity was found in the IgG and to a lesser extent in the IgM containing fraction. The DEAE-cellulose purified IgG preparations of five additional RA patients stimulated intracellular cathepsin B activity significantly above IgG from healthy controls. IgG and IgM antibodies to macrophages were detected in sera from RA patients but not from controls by indirect immunofluorescence (IIF) technique. Pepsin F (ab')2 fragments of IgG from the RA patients also gave clearcut membrane fluorescent staining of the macrophages which demonstrated the antibody nature of the binding. A good correlation between the cathepsin B assay and the IIF was found when serial dilutions of serum were compared.