Human autoantibodies directed against the RNA recognition motif of La (SS-B) bind to a conformational epitope present on the intact La (SS-B)/Ro (SS-A) ribonucleoprotein particle

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Abstract
In systemic autoimmunity, the human B cell response to the La (SS‐B) autoantigen is polyclonal and directed to both conserved and human‐specific epitopes. This study has further characterized the B cell epitope(s) present within the conserved central region of the La protein, LaC (amino acids 111–242) containing the RNA recognition motif (RRM, aa 111–187). Ten overlapping and non‐overlapping protein fragments spanning LaC were expressed in bacteria as NH2‐terminal fusions with glutathione‐S‐transferase. The fusion proteins were tested by ELISA for reactivity with a panel of human anti‐La sera in order to define the nature of the epitopes. Ninety‐two percent of patient sera containing anti‐La antibodies reacted with the region of La containing the RRM. Fine mapping of this reactivity using deletion mutants indicated that the deletion of 19 amino acids from either the NH2‐terminal or COOH‐terminal region of the RRM was associated with loss of antibody reactivity, suggesting that the immunodominant epitope expressed in this region is discontinuous. Autoantibodies affinity‐purified from the La RRM fragment to remove other specificities immunoprecipitated newly synthesized native La (SS‐B)/Ro (SS‐A) complexes, providing additional evidence that autoantibodies were recognizing a conformational epitope. The findings indicate that the human autoantibody response to La involves recognition of a conformational determinant involving the conserved RRM region without necessarily interfering with the RNA‐dependent association of the La/Ro ribonucleoprotein.