Autoepitopes reactive with anti-SS-B/La

Sera from 120 patients with suspected autoimmune rheumatic disease and antinuclear antibodies of anti-SS-B/La specificity were examined by Western blotting for reactivity with the SS-B/La polypeptide of HeLa cells and recombinant SS-B/La derived from a 1.4 kilobase (kb) cDNA encoding approximately 90% of the SS-B/La molecule. All sera reacted with the HeLa cell and the recombinant SS-B/La. One hundred and fourteen (95%) reacted with a set of three Staph. aureus V8 protease-resistant peptides of Mr 30,000, 29,00 and 28,000 from a methionine-rich region of HeLa cell SS-B/La designated the X domain, and 98 (82%) reacted with another set of two protease-resistant peptides of Mr 24,000 and 23,000 from a phosphorylated region of HeLa cell La designated the Y domain. One reacted weakly with the Y domain only. All sera that reacted with X and Y reacted more strongly with X, suggesting that X was the major epitope. Antibodies affinity purified from the X domain reacted strongly with the X peptides but not with the Y peptides and conversely, antibodies affinity purified from the Y domain reacted with the Y peptides but not with the X peptides. Both antibodies reacted with a fusion protein comprising 102 amino acids at the carboxyl terminus of the SS-B/La molecule. This protein contained no methionine, demonstrating that methionines were not involved in the antibody-binding site. Over 80% of patients whose only criteria for selection was the presence of anti-SS-B/La had the clinical, histologic, serologic and phenotypic features of Sjögren's syndrome whilst the remaining 20% had at least two of the features.(ABSTRACT TRUNCATED AT 250 WORDS)