Antigenic determinants reacting with rheumatoid factor: epitopes with different primary sequences share similar conformation.

Polyclonal or monoclonal human IgM rheumatoid factors (RF) react with eight antigenic sites on the CH3 IgG domain, four sites on CH2 and two on human beta 2-microglobulin. All 14 of these RF-reactive epitopes are linear 7-11 amino acid peptides with different primary sequence. We questioned whether RF reactivity with such a variety of epitopes showing no obvious sequence homology might result from conformational similarities shared by various RF-reactive regions. Strong support for this concept was obtained using rabbit antisera as well as mouse mAbs to individual CH3, CH2 or beta 2m RF-reactive peptides. Major cross-reactivity was demonstrated between most of the 14 different CH3, CH2, or beta 2m RF-reactive peptides using individual anti-epitope antibodies. Molecular modelling studies of these peptides showed striking similarities in three-dimensional shape among many RF-reactive peptides. Main-chain atoms rather than side chains seemed to contribute most directly to conformational similarity. Molecular simulation studies on control peptides showed no conformational similarities with RF-reactive peptides. Our studies indicate that autoantibodies such as RF recognize main-chain conformations of reactive epitopes and react with a number of antigenic determinants of quite different primary sequence but similar main chain conformations.