Molecular mimicries between human IgG, IgM rheumatoid factor and streptococcal IgG Fc receptors.

A monoclonal IgM rheumatoid factor (RF) from a patient with mixed essential cryoglobulinaemia was found to bind to type M15 group A streptococci with high affinity. The reaction was exerted via the antibody combining sites since it was inhibited by F(ab')2 fragments of anti-idiotypic (Id) antibodies but not by F(ab')2 of anti-IgMFc or normal rabbit IgG. The streptococcal strain also possessed receptors for the Fc part of IgG. IgMRF and IgG attached to different sites on the streptococcal surface, since e.g. anti-Id did not inhibit the IgG Fc-receptor binding. Furthermore, it was found that the interaction between the streptococcal IgG Fc receptor and IgG--like the RF/IgG interaction--took place in the interface between C gamma 2 and C gamma 3 and that histidine 435, tyrosine 436 and/or one both of histidine 433 and 310 were involved.