Assays for the Sm and RNP autoantigens: the requirement for RNA and influence of the tissue source.

The disease systemic lupus erythematosus is characterized by antibodies directed against two nuclear antigens, designated Sm and RNP. The two antigens from any one tissue can be distinguished on the basis of the polypeptides recognized by anti-Sm and anti-RNP specific sera. Multiple polypeptides were associated with RNP isolated from rabbit thymus. To determine if these polypeptides were species-specific, a comparison was made of this antigen among different mammalian species. We report that the native RNP antigen has a m.w. of 70,000 and is readily susceptible to proteolysis generating smaller polypeptides that still retain antigenicity. Such an analysis also demonstrated multiple Sm-specific polypeptides, including species of 13,000 and 29,000 m.w. Because the immunoreactive species are protein in nature, the role of RNA for the detection of Sm and RNP by assay procedures such as immunodiffusion and counterimmunoelectrophoresis was examined. RNP, while still antigenically active, becomes insoluble after RNAse treatment; Sm remains largely soluble after such treatment. Both antigens can be reconstituted from their separated protein and RNA moieties with restoration of precipitin reactivity.