antiSm

Anti-Sm, or anti-Sm antibodies, are autoantibodies directed against components of the Sm ribonucleoprotein core of small nuclear ribonucleoproteins (snRNPs), which participate in pre-mRNA splicing. The Sm core comprises seven proteins (SmB/B', SmD1, SmD2, SmD3, SmE, SmF, SmG). Anti-Sm antibodies target conformational epitopes within these snRNP complexes and are generated in the context of autoimmunity, often after apoptotic release of nuclear antigens.

These antibodies are highly specific for systemic lupus erythematosus (SLE), rarely occurring in isolation in healthy individuals or other rheumatic diseases. Their presence supports an SLE diagnosis in the appropriate clinical setting, but they are only moderately sensitive for SLE, detected in roughly a quarter to two-fifths of patients depending on the cohort and assay used. A positive anti-Sm test is commonly reported as part of an extended autoimmune serology panel, alongside anti-nuclear antibodies (ANA) and other autoantibodies.

Detection and interpretation: anti-Sm antibodies can be identified by immunoprecipitation, enzyme-linked immunosorbent assays (ELISA), line immunoassays, or immunodiffusion techniques. Immunoprecipitation remains a traditional reference method. In clinical practice, a positive anti-Sm result adds specificity for SLE but does not by itself establish disease, and a negative result does not exclude SLE.

Clinical relevance: beyond diagnosis, anti-Sm positivity has been variably associated with certain clinical features in SLE, but it is not routinely used to gauge disease activity or prognosis.