Mapping of epitopes on the La(SS-B) autoantigen of primary Sjögren's syndrome: identification of a cross-reactive epitope.

Autoepitopes on the ribonucleoprotein La(SS-B) were identified by using recombinant La(SS-B) polypeptides and sera from 166 patients with the antinuclear autoantibody anti-La(SS-B). The La(SS-B) polypeptides were encoded by polymerase chain reaction-derived overlapping or nonoverlapping fragments of the La(SS-B) gene, which encodes a protein of 408 amino acids (aa). Of the 166 sera tested, 99% reacted with a fusion protein comprising the first 107 N-terminal aa (LaA); 91% reacted with a fusion protein comprising aa 111 to 242 (LaC), and 91% reacted with a fusion protein comprising aa 346 to 408 (LaL2/3) at the C terminus of La(SS-B). The order of immunodominance as assessed by the number of sera reacting with each epitope and the strength of the reactivity was LaA (aa 1 to 107) greater than LaC (aa) 111 to 242) much greater than LaL2/3 (aa 346 to 408). Cross-reactivity was observed between antibodies eluted from LaC (aa 111 to 242) and LaL2/3 (aa 346 to 408), but there was no significant primary sequence homology between the two regions. The LaC region contained at least two epitopes, one encompassing a putative RNA-binding motif (aa 112 to 187) which was recognized by 83% of patient sera. Serial serum samples from three patients showed that the antibody response to La(SS-B) was initially directed to the N terminus (LaA, aa 1 to 107), but over a period of time all three major epitopes, including that encompassing the putative RNA-binding motif, were recognized. This result suggests that the primary immune response to La(SS-B) is restricted to an immunodominant epitope. As the specificity of the autoantibody response broadens, it includes the RNA-binding motif, which may have important implications for the expression of disease.