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Recently, antibodies that bind citrulline\u2010containing IgG epitopes were discovered in RA, suggesting that additional undiscovered IgG epitopes could exist and that IgG could be a shared antigen for RFs and AMPAs. This study was undertaken to reveal new IgG epitopes in rheumatic disease and to determine if multireactive AMPAs bind IgG.<\/jats:p><\/jats:sec><jats:sec><jats:title>Methods<\/jats:title><jats:p>Using sera from patients with RA, systemic lupus erythematosus, Sj\u00f6gren's disease (SjD), or spondyloarthropathy, IgG binding to native, citrulline\u2010containing, and homocitrulline\u2010containing linear epitopes of the IgG constant region was evaluated by peptide array, with highly bound epitopes further evaluated by enzyme\u2010linked immunosorbent assay (ELISA). Binding of monoclonal AMPAs to IgG\u2010derived peptides and IgG Fc was also evaluated by ELISA.<\/jats:p><\/jats:sec><jats:sec><jats:title>Results<\/jats:title><jats:p>Seropositive RA sera showed high IgG binding to multiple citrulline\u2010 and homocitrulline\u2010containing IgG\u2010derived peptides, whereas anti\u2010SSA+ sera from SjD patients showed consistent binding to a single linear native epitope of IgG in the hinge region. Monoclonal AMPAs bound citrulline\u2010 and homocitrulline\u2010containing IgG peptides and modified IgG Fc.<\/jats:p><\/jats:sec><jats:sec><jats:title>Conclusion<\/jats:title><jats:p>The repertoire of epitopes bound by AMPAs includes modified IgG epitopes, positioning IgG as a common antigen that connects the otherwise divergent reactivities of RFs and AMPAs.<\/jats:p><\/jats:sec>","DOI":"10.1002\/art.42064","type":"journal-article","created":{"date-parts":[[2022,1,9]],"date-time":"2022-01-09T21:59:35Z","timestamp":1641765575000},"page":"984-991","update-policy":"https:\/\/doi.org\/10.1002\/crossmark_policy","source":"Crossref","is-referenced-by-count":14,"title":["Rheumatoid Factor and Anti\u2013Modified Protein Antibody Reactivities Converge on IgG Epitopes"],"prefix":"10.1002","volume":"74","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-8177-9161","authenticated-orcid":false,"given":"Aisha M.","family":"Mergaert","sequence":"first","affiliation":[{"name":"University of Wisconsin\u2010Madison"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Zihao","family":"Zheng","sequence":"additional","affiliation":[{"name":"University of Wisconsin\u2010Madison"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Michael F.","family":"Denny","sequence":"additional","affiliation":[{"name":"University of Wisconsin\u2010Madison"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Maya F.","family":"Amjadi","sequence":"additional","affiliation":[{"name":"University of Wisconsin\u2010Madison"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"S. 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