The anti-Sm immune response in autoimmunity and cell biology

Anti-Sm antibodies are found in greater than 30% of the patients with systemic lupus erythematosus (SLE) and are diagnostic of SLE. The Sm autoantigens are the small nuclear ribonucleoprotein (snRNP) common core proteins. The seven core proteins, B, D1, D2, D3, E, F and G, shared by a majority of the snRNP particles, form a heptamer ring approximately 20 nm in diameter, with the snRNA passing through the center. The Sm epitopes are distributed on the outside surface of the ring. A repeated proline rich motif with homology to an Epstein bar nuclear antigen in the B protein and a gly–arg–gly motif including a symmetrical dimethylarginine post translational modification in the B, D1 and D3 proteins are major Sm epitopes. The anti-Sm response has features typical of an antigen driven immune response. SnRNP proteins share several characteristics with other autoantigens including their assembly into ribonucleoprotein particles, homologies to known viral proteins, presence of post translational modifications, a high abundance and great stability and the presence of repeated motifs. Current work on the snRNP particles is attempting to identify the features that predispose the common core proteins to become autoantigens in vulnerable individuals.