Molecular interactions between human IgG, IgM rheumatoid factor and streptococcal IgG Fc receptors.
Group A streptococci type M15 were previously shown to bind both human IgG via the Fc component and a purified monoclonal IgM kappa rheumatoid factor (IgM RF). Using 125I-labelled IgG and 125I-labelled IgM RF, the present study gave association constants of 2.2 x 10(7) and 2.9 x 10(8) M-1, respectively. The binding of 125I-IgG to the streptococci was inhibited by unlabelled IgG, IgG Fc and fragment D of staphylococcal protein A but not by the IgM RF or F(ab')2 of anti-idiotype antibodies to RF (anti-Id RF). Inversely, unlabelled IgM RF and anti-Id RF inhibited the binding of 125I-IgM RF markedly and unlabelled human IgG and IgG Fc only slightly or moderately, respectively. Thus, group A streptococci type M15 showed different binding sites for IgG Fc and the antibody combining sites of a human monoclonal RF. The findings were still more complex on a background of previous reports showing that streptococcal IgG Fc receptors and RFs bind to the same amino acids on the Fc molecule. This complex pattern may play a role in the pathogenesis of rheumatoid arthritis.
Schröder
A K
AK
Department of Rheumatology, University of Lund, Sweden.
Gharavi
A E
AE
Christensen
P
P
D001666:Binding Sites, Antibody; D006801:Humans; D007074:Immunoglobulin G / Q000378:metabolism*; D007075:Immunoglobulin M / Q000378:metabolism*; D007457:Iodine Radioisotopes; D011961:Receptors, Fc / Q000378:metabolism*; D012217:Rheumatoid Factor / Q000378:metabolism*; D013205:Staphylococcal Protein A / Q000037:antagonists & inhibitors; D013211:Staphylococcus aureus / Q000648:ultrastructure; D013297:Streptococcus pyogenes / Q000378:metabolism / Q000648:ultrastructure*
D007074:Immunoglobulin G; D007075:Immunoglobulin M; D007457:Iodine Radioisotopes; D011961:Receptors, Fc; D013205:Staphylococcal Protein A; D012217:Rheumatoid Factor