Sm

Anti-Smith (Sm) antibodies are found in the sera of approximately 25% of all patients with systemic lupus erythematosus (SLE) and their presence is considered to be a very specific marker of this disease. Autoantibodies to Sm are also known to often occur in combination with autoantibodies to other antigens, including DNA, histone, RNP, SSA(Ro) and SSB(La).

The term "Sm antigen" is now known to be synonymous with at least nine different polypeptides. These proteins are also known as the common or core proteins of snRNP particles. snRNPs are a group of nuclear particles comprised of several polypeptides associated with a small nuclear RNA molecule.  The most abundant snRNPs are involved in pre-mRNA-splicing. Since they bind to proteins common to different snRNP particles (B, D, E, F and G subunits), autoantibodies directed against Sm are able to precipitate a wide range of snRNAs.

While the major Sm autoantigen is believed to be represented by the D polypeptide, the B and D proteins are known to share at least one epitope based on monoclonal antibodies with multiple specificities. Sequence comparison has shown that all the known Sm proteins share two evolutionarily conserved structural sequence motifs, a possible explanation for their immunological cross-reactivity.

Sm D polypeptide subtypes represent the most abundant components of AROTEC's Sm antigen. It has been shown elsewhere that the D polypeptide exhibits a much higher specificity for Sm autoantibodies than the B polypeptide does. Other Sm subunits (E,F,G) are detectable. Human Sm D1, D2 and D3 are identical to the bovine sequences indicating that the antigens are highly conserved across mammalian species.

 

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