SR proteins are autoantigens in patients with systemic lupus erythematosus - Importance of phosphoepitopes

Objective. To determine whether members of the highly phosphorylated SR protein family are autoantigens and, if so, to determine the frequency and molecular basis of antigen recognition. Methods. Native human SR proteins were purified to homogeneity from HeLa cells, and an enzyme-linked immunosorbent assay (ELISA) was developed. Further studies employed immunoblotting of both phosphorylated and dephosphorylated SR proteins. Results. Anti-SR protein reactivity was frequently detected in the sera of patients with systemic lupus erythematosus (SLE), Sera from 52% of the SLE patients in a group of patients with a variety of auto-immune and other disorders (n = 137) and from 50% of the SLE patients in a separate group (n = 102) were positive in an ELISA. In contrast, sera from patients with other disorders, such as rheumatoid arthritis and primary antiphospholipid syndrome, reacted infrequently. Reactivity with double-stranded DNA (dsDNA), used in the diagnosis of SLE, did not correlate with SR protein reactivity. Anti-SR autoantisera did not bind highly charged unphosphorylated peptides related to the SR domain, which is rich in arginine and phospho-serine residues. Surprisingly, many of the epitopes were influenced by the presence or absence of SR protein phosphorylation. In immunoblots, some patient sera lost reactivity upon SR protein dephosphorylation, while others significantly gained reactivity. Conclusion. We have identified a novel set of autoantigens in SLE, the SR protein family of non-small nuclear RNP pre-messenger RNA splicing factors. Anti-SR autoantibodies are distinct from those which bind dsDNA, The identification of this new set of autoantigens and the observation that the auto-epitope(s) involves posttranslational modification offer new possibilities for understanding autoimmunity and its development.