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An immunodominant La/SSB autoantibody proteome derives from public clonotypes

The La/SSB autoantigen is a major target of long‐term humoral autoimmunity in primary Sjögren's Syndrome (SS) and systemic lupus erythematosus. A majority of patients with linked anti‐Ro60/Ro52/La responses target an NH2‐terminal epitope designated LaA that is expressed on Ro/La ribonucleoprotein complexes and the surface membrane of apoptotic cells. In this study, we used high‐resolution Orbitrap mass spectrometry to determine the clonality, isotype and V‐region sequences of LaA‐specific autoantibodies in seven patients with primary SS. Anti‐LaA immunoglobulin (Ig)Gs purified from polyclonal sera by epitope‐specific affinity chromatography were analysed by combined database and de‐novo mass spectrometric sequencing. Autoantibody responses comprised two heavily mutated IgG1 kappa‐restricted monoclonal species that were shared (public) across unrelated patients; one clonotype was specified by an IGHV3‐30 heavy chain paired with IGKV3‐15 light chain and the second by an IGHV3‐43/IGKV3‐20 pairing. Shared amino acid replacement mutations were also seen within heavy and light chain complementarity‐determining regions, consistent with a common breach of B cell tolerance followed by antigen‐driven clonal selection. The discovery of public clonotypic autoantibodies directed against an immunodominant epitope on La, taken together with recent findings for the linked Ro52 and Ro60 autoantigens, supports a model of systemic autoimmunity in which humoral responses against protein–RNA complexes are mediated by public sets of autoreactive B cell clonotypes.