La(SSB)

Autoantibodies to the La (SSB) antigen can be detected in the sera of up to 87% of patients with primary or secondary Sjögren’s syndrome. The presence of anti-La (SSB) autoantibodies usually coincides with the presence of anti-Ro (SSA) autoantibodies, however the fact that anti-Ro autoantibodies are far more common in other rheumatological conditions such as systemic lupus erythematosus (SLE) and mixed connective tissue disease (MCTD) suggests that anti-La is more specific for primary and secondary Sjögren’s than anti-Ro.

La (SSB) is a highly phosphorylated protein which migrates at about 50 kDa in SDS-polyacrylamide gel electrophoresis. Phosphorylated residues are present at the carboxy-terminal part of the protein. At least 8 isoelectric forms (pI range 6 to 7) have been identified.

The amino acid sequences of both human and bovine La (SSB) antigen have been determined by cDNA cloning and sequencing. Comparison of the two sequences shows 22 largely conservative amino acid substitutions with a total of 95% identity. Three regions of the La molecule (amino acids 1-107, 111-242 and 346-408) are thought to contain the major epitopes reactive with human anti-La sera. The broad cross-reactivity of patient sera with La (SSB) from diverse mammalian species indicates the presence of conserved epitopes. 

 

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