Definition of a discontinuous immunodominant epitope at the NH2 terminus of the La/SS-B ribonucleoprotein autoantigen.

Abstract

High-titer IgG autoantibodies to the La/SS-B ribonucleoprotein (RNP) are a hallmark of patients with primary Sjogren's syndrome. Anti-La/SS-B-positive human sera bind to multiple epitopes on recombinant La/SS-B, although the initial response is against an immunodominant epitope within the first 107 NH2-terminal amino acids (aa). Sequence analysis has identified a striking homology between aa 88-101 in this NH2-terminal region of La/SS-B and a feline retroviral gag polypeptide suggesting the anti-La/SS-B response may be initiated by cross-reactivity with an exogenous agent. In the present study, detailed mapping of this NH2-terminal epitope, using recombinant La/SS-B purified from the expression of overlapping DNA fragments spanning aa 1-107, has shown that this immunodominant epitope is a complex conformational or discontinuous epitope dependent upon both aa 12-28 and 82-99 for expression, even though these regions share no homology with each other. This requirement questions the significance of the homology between La/SS-B and a retroviral gag polypeptide in the generation of the B cell response to La/SS-B and is in accord with the general concept that B cells recognize conformational epitopes on antigens rather than small linear peptide sequences. The finding also reinforces the notion that native autoantigen could be the initiator of the autoimmune response. (J. Clin. Invest. 1992. 89:1652-1656

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

“…It is also possible that sera reactive with the LaL2/3 fragment play a particularly important role in the cross-linking necessary for precipitin formation. Preliminary results from serially obtained serum samples have suggested that the LdSS-B autoantibody response is initially directed against LaA, but extends over time to involve L a c and LaL2/3 (8). This pattern of reactivity is consistent with that observed among the precipitin-negative sera in the present study, in that the LaA fragment was the most commonly recognized, followed by Lac, with no sera reacting with the LaL2/3 fragment.…”

“…Of these 18 sera, 8 reacted with La33.3 alone, and did not bind to any of the La fragments. Five of the remaining sera reacted with La33.3 and LaA (amino acids 1-107), 4 sera reacted with La33.3, LaA, and Lac (amino acids 111-2421, and 1 serum reacted with La33.3 and Lac.…”

“…Fulllength LdSS-B (La33.3) and LaA, Lac, and LaL2/3 subfragments were expressed in Escherichia coli as soluble glutathione-S-transferase (GST) fusion proteins, using the pGEX-2 vector (8). Recombinant protein was prepared by affinity purification, as described by Smith and Johnson (1 I).…”

“…By testing anti-LdSS-B-positive sera for reactivity with recombinant LdSS-B fragments, several investigators have identified multiple B cell epitopes on the La/SS-B molecule (2)(3)(4)(5)(6), and levels of specific anti-LdSS-B antibody directed against these epitopes are often in the milligram-per-milliliter range (7). Two groups (2,H) have shown that the majority of anti-LdSS-B precipitin-positive sera react with 3 non-overlapping regions: LaA (amino acids 1-107; 99% of sera); Lac (amino acids 11 1-242; 91%); and LaL2/3 (amino acids 346408, 91%); with preliminary evidence suggesting that the initial antibody response to La/SS-B may be directed to the N-terminal epitope, LaA (8).…”

Identification of a human-specific epitope in a conserved region of the La/SS-B autoantigen.

“…An immunodominant, discontinuous epitope has been identified at the NH2 terminus (LaA; amino acids [aa ]' -107) and may be recognized early in the response (8). The majority of anti-La/SS-B sera also react with a putative RNA-recognition motif( RRM; aa 112-187) located in the middle region of the molecule (LaC; aa 11 (1,7).…”