Restricted epitope recognition by precipitin‐negative anti‐La/SS‐B‐positive sera

Objective. To determine whether anti–La/SS‐B–positive sera that are precipitin negative show a distinct B cell epitope pattern. Methods. Serum reactivity was tested with recombinant La/SS‐B fusion proteins. Results. Among the 18 precipitin‐negative anti‐La/SS‐B–positive sera, reactivity was confined to the full‐length recombinant protein (La33.3) in 8 (44%); 5 of 18 (28%) reacted only with La33.3 and with the first 107 N‐terminal amino acids (LaA), and 4 (22%) reacted with La33.3, LaA, and the middle region of the La molecule (LaC; amino acids 111–242). One serum reacted with La33.3 and LaC. None of the 18 precipitin‐negative sera was positive on a carboxy‐terminal fragment (LaL2/3; amino acids 346–408). In contrast, all 26 precipitin‐positive anti–La/SS‐B–positive sera reacted with La33.3, LaA, and LaC, and 92% reacted with LaL2/3. Rheumatoid factor and serum IgG levels were significantly lower in the precipitin‐negative group, providing further evidence of a distinct serologic subset. Conclusion. The restricted epitope recognition by these sera may explain the lack of precipitin formation and may represent an early autoantibody response to La/SS‐B.