Double reactivity of monoclonal and polyclonal rheumatoid factors for IgG and histones: mapping of binding sites by means of histone synthetic peptides and anti-Id antibodies.

Polyreactive antibodies able to bind various apparently unrelated structures represent a frequent antibody population in autoimmune diseases. In this work, the structural basis of the double reactivity of such autoantibodies was investigated using as models polyclonal and monoclonal human rheumatoid factors (RF) reacting with histones. Both direct ELISA binding and competitive inhibition experiments were performed. A more precise delineation of the histone regions recognized by the RFs was made by means of 27 synthetic peptides of these proteins. Anti-idiotope (Id) murine antibodies were used to map the binding sites involved on RF in the interaction with IgG and histones. Among the 13 polyclonal and six monoclonal RFs tested, four and two respectively were found to cross-react with IgG and histones. The fragments shown to be the most frequently recognized by RFs were located in residues 1-16 and 204-218 of H1, 1-20 and 65-85 of H2A, and 1-21 of H3. The results obtained by competitive ELISA assays using IgG, histone peptides and anti-Id monoclonal antibodies led us to confirm and characterize more precisely our previous finding suggesting the existence of topographically distinct binding sites for the different targets recognized by RFs.