60-kDa Ro protein autoepitopes identified using recombinant polypeptides.

The human Ro ribonucleoprotein is a clinically important yet poorly understood autoantigen. The contribution of Ro autoantibodies to pathogenesis of autoimmune disease remains unclear, as do the stimuli that initiate and maintain the response. Recent evidence suggests that patient anti-Ro responses target individual proteins, including a 60- and a 52-kDa species, within the complex in a disease-specific manner. However, Ro antisera retain considerable heterogeneity in their recognition of both continuous and discontinuous epitopes on the protein components. Previous characterization of Ro autoepitopes has primarily involved solid phase assays, which are of limited value in identifying discontinuous epitopes. To address the heterogeneity of Ro and the issue of discontinuous autoepitopes, we have generated 10 overlapping recombinant polypeptides of the human 60-kDa Ro protein and compared their reactivities using a soluble immunoprecipitation assay. Seven different epitopes, both continuous and discontinuous, were distinguished and seven distinct patterns of reactivity were discerned among the sera from 12 patients. These patterns of reactivity showed no relationship to clinical diagnosis but did correlate with the titer of Abs against recombinant 60-kDa Ro and with the concomitant presence of Abs directed against recombinant 52-kDa Ro protein. Sera that immunoprecipitated only the full-length 60-kDa protein had low or undetectable anti-60 kDa titers by ELISA and immunoblot using recombinant Ag, demonstrating a predominant recognition of discontinuous epitopes. These data indicate that autoantibody responses to the 60-kDa Ro Ag can preferentially target discontinuous epitopes and that the ability to recognize continuous epitopes is accompanied by the appearance of 52-kDa Ro autoantibodies.

Molecular composition of Ro small ribonucleoprotein complexes in human cells. Intracellular localization of the 60- and 52-kD proteins.

Ro small ribonucleoprotein complexes (RoRNPs) are thought to comprise several proteins, including the 60-kD Ro and the 52-kD Ro proteins, and several small RNAs, designated Y RNAs. Although RoRNPs are fairly ubiquitous in nature, their precise composition remains unknown, their function has been elusive, and their intracellular localization has been controversial. We have analyzed HeLa cell extracts by glycerol density gradient fractionation in order to determine the distribution of the individual protein and RNA components of RoRNPs. We found that 52-kD Ro was not detectable in an RNP complex with the 60-kD protein under a variety of conditions. Pretreatment of cell extracts with ribonuclease affected gradient migration of the 60-kD but not the 52-kD protein, suggesting that the latter is not complexed with RNA. The migration of the hY RNAs in these gradients closely followed that of 60-kD and not 52-kD Ro. Immunofluorescence analysis of two different cell lines with monospecific antibodies against 52- and 60-kD proteins strongly suggests that these two proteins are not present on overlapping sets of structures in vivo. We conclude that the 52-kD Ro protein is not a detectable component of the RoRNP complex under these conditions despite its reactivity with Ro autoimmune antisera.

Molecular biology of nuclear autoantigens.

This article provides a historical overview of the application of molecular and immunologic techniques to the analysis of autoantigenic structure and function, as well as to autoantibody recognition of protein and nucleic acid autoantigens. Examples presented here illustrate the role of autoantibodies as tools in the elucidation of the autoimmune components of cellular ribonucleoproteins. In turn, the subsequent molecular dissection of autoantigenic ribonucleoproteins has advanced understanding of autoantibody specificities. The nature of autoantibodies reactive with various proteins and nucleic acids will be the subject of the following articles in this issue. Taken together, these studies of antibody-antigen interactions that arise during the autoimmune response have revealed novel mechanisms of molecular recognition within the RNP autoantigens. These findings are of general importance for understanding basic cellular processes and have contributed to our knowledge of the underlying mechanisms of immunoregulatory abnormalities that arise in autoimmune diseases.