Complement-activating properties of IgM rheumatoid factors reacting with IgG subclasses

Summary To estimate the complement-activating property (CAP) of IgM rheumatoid factor (RF), which was purified from synovial fluids of patients with rheumatoid arthritis, in a reaction with each IgG subclass, the activation and binding of C4 in the classical pathway of complement by IgM RF was measured in an enzyme-linked immunosorbent assay using biotinylated F(ab')₂ antibody to human C4. The CAP of IgM RF reacting with IgG3 was significantly higher than that of IgM RFs bound to the other IgG subclasses (P<0.01). These results suggest that IgM RF reacting with IgG3 in synovial fluid could induce a greater degree of complement-dependent inflammation in RA synovium than IgM RF reacting with other IgG subclasses.