Complement activating rheumatoid factors in rheumatoid arthritis studied by haemolysis in gel: relation to antibody class and response to treatment with podophyllotoxin derivatives.

Complement (C) activating rheumatoid factors (RF) were measured in 16 patients with rheumatoid arthritis (RA) by a simple haemolysis in gel (HIG) assay. IgM-RF, but not IgG-RF or IgA-RF, measured by an enzyme linked immuno-sorbent assay, were closely correlated with C activating RF (r = 0.86). In neither assay did the concentrations of RF appear to be directly related to C1 activation as expressed by serum concentrations of C1r-C1s-C1 inactivator complexes or of C4. In the sera, C1q binding substances, measured by C1q binding assay, were markedly correlated with C activating RF (r = 0.88), whereas C1q binding substances detected by the C1q deviation test were not. Treatment with podophyllotoxin derivatives for 6 months clearly reduced patients' RF concentrations. The decrease of IgG-RF, IgA-RF and IgM-RF was significantly more pronounced than that of the IgG, IgA and IgM concentrations, respectively.