Differences in the fine specificity of ANTI–Ro (SS‐A) in relation to the presence of other precipitating autoantibodies

In sera from patients with systemic lupus erythematosus or Sjögren's syndrome, we determined the fraction of antibody that remained reactive with human Ro (SS‐A) after absorption with bovine spleen extract, and the reactivity with the 60‐kd and 54‐kd red blood cell Ro (SS‐A) bands by Western blot. Of the 3 groups of sera studied, those containing anti–Ro (SS‐A) alone had the highest degree of reactivity with human Ro (SS‐A) after absorption with bovine spleen extract, followed, in descending order, by sera containing anti–Ro (SS‐A) and anti–La (SS‐B), and sera containing anti–Ro (SS‐A) and anti–nuclear RNP. The groups of sera could be distinguished on this basis. Sera with anti–Ro (SS‐A) and anti–nuclear RNP could also be distinguished from the other 2 types of sera by their uniform and preferential reactivity with the 60‐kd red blood cell Ro (SS‐A), by Western blot analysis. These findings indicate that there are both qualitative and quantitative differences, associated with the presence of other autoantibodies, in the fine specificity of anti–Ro (SS‐A) sera.