Molecular Interactions between Human IgG, IgM Rheumatoid Factor and Streptococcal IgG Fc Receptors

Abstract

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-la-belled IgM RF, the present study gave association constants of 2.2 × 10⁷ and 2.9 × 10⁸M^––1respectively. 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 RFor 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.

Molecular Mimicries between Human IgG, IgM Rheumatoid Factor and Streptococcal IgG Fc Receptors

“…IgM rheumatoid factor (RF) and streptococcal IgG Fc receptors may be of importance for the understanding of the mechanism of R F production and the pathogenesis of rheumatoid arthritis (1). IgM rheumatoid factor (RF) and streptococcal IgG Fc receptors may be of importance for the understanding of the mechanism of R F production and the pathogenesis of rheumatoid arthritis (1).…”