Anti-Sm Autoantibodies in Systemic Lupus Target Highly Basic Surface Structures of Complexed Spliceosomal Autoantigens

Abstract

Autoantibodies directed against spliceosomal proteins are a common and specific feature of systemic lupus erythematosus. These autoantibodies target a collection of proteins, including Sm B, B′, D1, D2, and D3. We define the common antigenic targets of Sm D2 and D3 and examine their role in spliceosomal autoimmunity. Our results define nine major common epitopes, five on Sm D2 and four on Sm D3. These epitopes have significantly higher (more basic) isoelectric points than do nonantigenic regions. In fact, this association is of sufficient power to make isoelectric point an excellent predictor of spliceosomal antigenicity. The crystallographic structure of Sm D2 and D3 is now partially described. The anti-Sm D2 and D3 antigenic targets are located on the surface of the respective three-dimensional complexed proteins, thereby suggesting that these epitopes are accessible in the native configuration. All but one of these nine epitopes conspicuously avoid the specific regions involved in intermolecular interactions within the spliceosomal complex. One of the D3 epitopes (RGRGRGMGR) has significant sequence homology with a major antigenic region of Sm D1 (containing a carboxyl-terminal glycine-arginine repeat), and anti-D3 Abs cross-react with this epitope of Sm D1. These results demonstrate that spliceosomal targets of autoimmunity are accessible on native structure surfaces and that cross-reactive epitopes, as well as structural associations of various spliceosomal Ags, may be involved in the induction of autoimmunity in systemic lupus.

Structural availability influences the capacity of autoantigenic epitopes to induce a widespread lupus-like autoimmune response

“…Sequences with multiple lysine residues have been shown to be very antigenic in the nRNP 70K molecule, whereas epitopes containing multiple arginine residues have been demonstrated to be highly antigenic in the Sm D1 protein. Additionally, a high isoelectric point has proven to be a fair indicator of autoantigenicity of epitopes in the snRNP spliceosomal system (35), and both the A3 and A6 peptides have relatively basic pIs (12.0 and 11.0, respectively). The difference is not due to species specificity, because we have demonstrated that the sequences of the epitopes in question are identical between human and rabbit.…”

Clinical criteria for systemic lupus erythematosus precede diagnosis, and associated autoantibodies are present before clinical symptoms

“…One microgram of Sm, nuclear RNP (nRNP), Ro, or La (ImmunoVision, Springdale AR) per well was plated. These methods have been previously described in detail (20,21). Anti-dsDNA antibodies were screened with a solid-phase assay (Varelisa; Pharmacia & Upjohn, Freiburg, Germany).…”

Improved Serological Differentiation between Systemic Lupus Erythematosus and Mixed Connective Tissue Disease by Use of an SmD3 Peptide-Based Immunoassay

“…In epitope-mapping studies, several linear and conformational epitopes have been mapped on the SmB and D proteins (1,7,9,12,14,17,19). On SmD1 and BBЈ, the major reactivity was predominantly found in the C-terminal extensions (7,8,19).…”