Human autoantibody secreted by immortalized lymphocyte cell line against the 68K polypeptide of the U1 small nuclear ribonucleoprotein

Immunoreactivity between a monoclonal lupus autoantibody and the arginine/aspartic acid repeats within the U1-snRNP 70K autoantigen is conformationally restricted

Immunoreactivity of the arginine/aspartic acid (RD) repeats of the 70K protein of U1 small nuclear ribonucleoprotein (snRNP) was determined to be conformationally dependent. The monoclonal autoantibody 2.73, isolated from a lupus-prone MRL/n mouse model, is reactive with the RD repeat regions of U1 snRNP 70K protein. Immunochemical analysis of the antigenic determinants with use of chemically synthesized peptides characterized the 2.73 epitope as the RD repeat [Pelsue, S., et al. (1993) Autoimmunity, 15, 231-236] Analysis by circular dichroism (CD) and nuclear magnetic resonance spectroscopy indicates conformational preferences in the immunoreactive peptides. Computer analyses of CD spectra obtained on the RD-containing peptides predict beta-turns and beta-sheet to be the preferred conformations of the RD repeats. This structure was also predicted by the Chou-Fasman algorithm. The RD repeat is believed to be a conserved structural motif; however, the biological function is still unclear. Immunological and biochemical analysis of autoimmune antibodies and their respective antigenic determinants has helped to characterize the possible mechanisms that lead to autoimmune diseases. This is the first report of a conformationally dependent, linear epitope of an autoantibody.

Immunochemical analysis of an arginine-rich systemic lupus erythematosus autoepitope

The MRL mouse strain spontaneously produces antinuclear autoantibodies that recognize DNA and the small nuclear ribonucleoprotein (snRNP) antigens. The monoclonal antibody 2.73 was derived from the lupus prone MRL/n line and is reactive with the 70K protein of the U1 snRNP particle. The epitope recognized by 2.73 was characterized by peptide and inhibition ELISA analysis. Several arginine/aspartic acid (RD) repeats of varying lengths are found in the carboxyl terminus of the 70K protein and are responsible for immunoreactivity with 2.73. We investigated the contribution of charge and found that the immunoreactivity of 2.73 and the 70K protein is specific for the RD repeats. The presentation of the epitope may also contribute to the epitopes immunoreactivity with the 2.73 mouse monoclonal autoantibody.

Anti-Sm autoantibodies of systemic lupus erythematosus cross react with dietary plant proteins

Cross reactivity of patient lupus autoantibodies to the small nuclear ribonucleoprotein particles of many different types of animals is well documented. The aim of our research was to determine if any level of cross reactivity existed between proteins of common dietary plants and anti-Sm autoantibodies of lupus patient's sera, as has been found for scleroderma patient sera (Agris et al., Exptl. Cell Res. 189, 276-279, 1990). Protein extracts from soy bean, corn, spinach, and carrot were analyzed. At least one protein (molecular weight approximately 28,000 daltons) common to all the above protein extracts was recognized by most of the anti-Sm sera tested. Affinity purified antibody eluted from the 28 kilodalton plant protein specifically recognized the Sm proteins of a HeLa cell protein extract. Recognition of the 28 kilodalton dietary plant protein was found to be unique to anti-Sm lupus sera.

Small nuclear ribonucleoprotein particles contain glycoproteins recognized by rheumatic disease-associated autoantibodies

Proteins from U1 and U2 small nuclear ribonucleoprotein (snRNP) particles, which are common targets of autoantibodies found in some rheumatic diseases, were analysed for the presence of glycans. A glycan detection assay revealed that the U1-specific proteins 68K and A and the U2-specific protein B" are glycosylated. However, none of the Sm proteins, which are common to all the major snRNP particles, showed a detectable level of glycosylation. With the use of specific lectins, an analysis of the particular carbohydrate(s) attached to the U1 snRNP 68K protein demonstrated the presence of at least one N-linked oligosaccharide chain. Lectin detection of galactose, glucose, mannose and N-acetylglucosamine on 68K was confirmed by chemical analysis of the carbohydrates. The glycopeptide nature of these antigens may be important for understanding the role of autoantigens in the pathogenesis of autoimmune disorder.

Antigenic domains on the U1 small nuclear ribonucleoprotein-associated 70K polypeptide: a comparison of regions selectively recognized by human and mouse autoantibodies and by monoclonal antibodies

Antigenic regions on the U1 small nuclear ribonucleoprotein (snRNP)-associated 70K polypeptide recognized by human and mouse autoantibodies or by monoclonal antibodies were identified and compared. Using a set of 70K fusion proteins as antigen in enzyme-linked immunosorbent assay and immunoblotting revealed that serum autoantibodies of human and of MRL/Mp mouse origin recognized a common region of the 70K polypeptide. Monoclonal anti-70K antibodies derived from a patient with mixed connective tissue disease, from an autoimmune MRL/Mp mouse, and from a BALB/c mouse immunized with purified U1 snRNP were all shown to bind to a part of the 70K polypeptide rich in charged residues and different from the region recognized by most human and MRL/Mp mouse serum autoantibodies.

Recombinant 70-kD protein used for determination of autoantigenic epitopes recognized by anti-RNP sera

A panel of high- and low-titre anti-RNP-positive patient sera was tested for reactivity with human snRNP proteins. The U1 snRNP-specific 70-kD peptide was found to be the most prominent RNP antigen recognized by high-titre anti-RNP sera, mainly found in patients with mixed connective tissue disease (MCTD). The reactivity with the 70-kD protein was further analysed with recombinant fusion proteins containing different segments of the protein. One major and two minor antigenic regions were found. Most patient sera, both with high- and low-titre anti-RNP antibodies, only recognized the major region. The most aminoterminal region, showing partial sequence homology with a mouse retroviral p30 gag protein, contained an epitope that was recognized by one serum only.

Enzyme-linked immunosorbent assay using isolated (U) small nuclear ribonucleoprotein polypeptides as antigens to investigate the clinical significance of autoantibodies to these polypeptides

In order to investigate the clinical significance of autoantibodies to individual U small nuclear ribonucleoprotein (snRNP) polypeptides, an enzyme-linked immunosorbent assay (ELISA) using isolated 68K, A, B/B', and D polypeptides from purified U1 snRNP was developed. The ELISA levels of IgG antibodies were positively correlated with results of immunoblotting and hemagglutination. In patients positive for antibodies to ribonucleoprotein, IgG anti-68K reactivity was associated with active mixed connective tissue disease, and in particular with myositis and esophageal hypomotility. IgG B/B' and D polypeptide reactivities were associated with systemic lupus erythematosus and renal disorder. Raynaud's phenomenon was infrequent in patients with high IgG B/B' and D polypeptide reactivities. Pleuritis/pericarditis was associated with the IgG B/B' polypeptide reactivities. In longitudinal studies, ELISA levels of IgG antibodies against these polypeptides changed in parallel with disease activity.