Molecular Profiling and Clonal Tracking of Secreted Rheumatoid Factors in Primary Sjögren's Syndrome

Objective Rheumatoid factors (RFs) are associated with systemic disease in primary Sjögren's syndrome (SS) and may be pathogenic as mixed cryoglobulins. Current detection methods cannot resolve RFs at a molecular level. This study was undertaken to perform the first proteomic and transcriptomic analysis of secreted and membrane‐bound IgM‐RF in primary SS and identify unique heavy‐chain peptide signatures for RF clonotype tracking. Methods Purified heavy chains of serum RFs from 15 patients with primary SS were subjected to de novo mass spectrometric sequencing. The circulating B cell Ig repertoire was determined by massively parallel sequencing of IGH RNA from matched peripheral blood mononuclear cells (n = 7). RF‐specific heavy‐chain third complementarity‐determining region (CDR3) peptides were identified by searching RF heavy‐chain peptide sequences against the corresponding IGH RNA sequence libraries. Heavy‐chain CDR3 peptides were used as biomarkers to track serum RF clonotypes using quantitative multiple reaction monitoring. Results Serum RFs were clonally restricted and composed of shared sets of IgM heavy‐chain variable region (Ig VH) 1–69, 3–15, 3–7, and 3–74 subfamilies. Cryoprecipitable RFs from patients with mixed cryoglobulinemia (MC) were distinguishable from nonprecipitating RFs by a higher frequency of amino acid substitutions and identification of stereotypic heavy‐chain CDR3 transcripts. Potentially pathogenic RF clonotypes were detected in serum by multiple reaction monitoring years before patients presented with MC. Levels of Ig VH4–34 IgM‐RF decreased following immunosuppression and remission of MC. Conclusion Cryoprecipitable RF clonotypes linked to vasculitis in primary SS have different molecular profiles than nonprecipitating RFs, suggesting different underlying mechanisms of production. The combined omics workflow presented herein provides molecular biomarkers for tracking and removal of pathogenic RF clones.