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

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.