Binding of IgM rheumatoid factor to group A, C and G streptococci with IgG Fc receptors

The possibility that IgM rheumatoid factors bind to streptococci was studied. Using a sequence of Sephadex G200 gel filtration, protein A-Sepharose CL-4B chromatography and preparatory electrophoresis, IgM was isolated from the sera of 2 patients with rheumatoid arthritis and then radiolabelled with 125I. There was significant binding of radiolabelled IgM to group-A streptococci types M1, M15 and M22, and to a group-C and a group-G strain, all expressing IgG Fc receptors, but none to Staphylococcus aureus, Escherichia coli or to 11 other strains of streptococci without IgG Fc receptors. The radiolabelled IgM was separated by affinity chromatography on a column containing human IgG. Types M1 and M15 bound the fraction retained on the column, whereas M22 bound both this fraction and the non-retained fraction. Commercial human IgG, even at high concentrations, did not inhibit binding. The binding reaction, which is perhaps triggered either by the IgM rheumatoid factor or by IgG complexed with rheumatoid factor, could be a useful tool for removal of anti-immunoglobulin from the blood of patients with rheumatoid arthritis.