Symmetrical dimethylarginine as the central antigenic determinant of anti-Smith autoantibodies in systemic lupus erythematosus

ABSTRACT Objective Systemic lupus erythematosus (SLE) is a chronic autoimmune disease causing multi-organ damage. The most specific autoantibody response in SLE, present in 20-30% of patients, targets the Sm-protein and has been shown to recognize a linear Sm-derived B cell epitope containing a post-translational modification (PTM) of arginine termed symmetrical dimethyl arginine (sDMA). As autoantibodies to other PTM-modified proteins are often promiscuous, we aimed to determine the specificity and cross-reactivity of anti-Sm antibodies. Methods Specificity and promiscuity/cross-reactivity of anti-Sm IgG were measured by ELISA in SLE patients and healthy donors using peptides containing either sDMA or unmodified arginine. Inhibition and cross-reactivity were determined using competitive ELISA and affinity purification. Recognition of endogenous EBNA1 by anti-Sm IgG was performed by Western blot using lysates of EBV-bearing lymphoblastoid cell lines. Results The sDMA residue is recognized by anti-Sm+ SLE patients regardless of the peptide amino acid sequence, with a modest impact of amino acids flanking sDMA on recognition. Most notably, IgGs targeting sDMA comprise the overall majority (∼90%) of the anti-Sm antibody repertoire and are highly cross-reactive between SmD3 108-122 and several sDMA-containing viral-derived epitopes, including full-length EBNA1. Conclusion Our data implicate that the majority of anti-Sm IgGs target the sDMA residue irrespective of its Sm-context, thus representing a prototypic anti-PTM response. These antibodies are highly promiscuous, recognizing several sDMA-modified targets, including naturally occurring viral sDMA-expressing epitopes. These findings suggest a new mechanism by which molecular mimicry of sDMA-modified viral proteins could contribute to a breach of tolerance in anti-Sm+ SLE patients. KEY MESSAGES What is already known on this topic Symmetrical dimethylarginine (sDMA) residues are present on RGG/RG repeat regions within the SmD1, SmD3, and SmB/B⍰ subunits of the Sm protein complex in vivo . The introduction of sDMA is essential for recognition of minimal antigenic epitopes spanning amino acids #95–119 on the SmD1 and #108–122 on the SmD3 subunits by a specific subset of anti-Sm autoantibodies. What this study adds We show that autoantibodies targeting sDMA residues are common in anti-Sm+ SLE patients and represent the majority of the anti-Sm autoantibody repertoire. We demonstrate that anti-sDMA autoantibodies are highly cross-reactive and can also bind to sDMA residues on other targets, including several virus-derived epitopes, thus representing a prototypic anti-modified protein antibody (AMPA) response directed against post-translationally modified proteins through symmetrical dimethylation of arginine. How this study might affect research, practice, or policy The high cross-reactivity of Sm antibodies to various sDMA-containing epitopes, combined with a dominant antigenic specificity, provides mechanistic insight in the potential link between viral infection and tolerance breach in anti-Sm+ SLE patients.