Human autoantibodies recognizing a native macromolecular structure composed of Sm core proteins in U small nuclear rnp particles

Objective. Monoclonal antibody (mAb) F78 recognizes a heat-labile particle composed of Sm core proteins designated F78P. The objective of this study was to identify human autoantibodies recognizing the conformational structure of F78P. Methods. Immunoblots using HeLa cell extracts without heating prior to sodium dodecyl sulfatepolyacrylamide gel electrophoresis were used to identify autoantibodies recognizing F78P. To confirm reactivities with F78P, immunoprecipitates of mAb F78 were used as a substrate for immunoblots. To identify reactivities against the F78P structure in classic anti-Sm-positive sera, autoantibodies to individual Sm core proteins were absorbed with purified U1 small nuclear RNP before immunoblotting. Results. We identified 2 sera that, like F78, recognized only F78P and not its component polypeptides. When classic anti-Sm antibodies were preabsorbed, the presence of F78-1ike, particle-specific antibodies was revealed in all of the anti-Sm-positive sera tested. Conclusion. Autoantibodies against the F78P structure were commonly present in sera from patients with systemic rheumatic diseases, often in combination with anti-Sm autoantibodies.