Radioimmunoassay for antibodies to cytoplasmic ribosomes in human serum

Autoantibodies directed against ribosomal P proteins: use of a multiple antigen peptide as the coating agent in ELISA

Autoantibodies directed against the ribosomal proteins P0, P1 and P2 (P proteins) are specific for systemic lupus erythematosus (SLE) and there are some evidences that they could be related to the neuropsychiatric manifestations of the disease. In this study, a multiple antigen peptide (MAP) carrying four copies of the C-terminal peptide (13 residues) of the P2 protein, which is a common epitope of the three P proteins, was prepared for use in an ELISA assay. It was employed to detect antibodies directed against the ribosomal P proteins in 102 SLE patients and the results were compared with those obtained using immunoblotting (IB). With this new ELISA, antiribosomal P protein antibodies were found in 15/102 SLE sera. These results correlated well with the results of IB. Furthermore, we confirmed that naturally occurring antiribosomal P protein antibodies are directed mainly against the epitope containing the C-terminal sequence and shared by the three P proteins. MAP appears to be an excellent coating agent for ELISA assays designed to detect anti-P antibodies. Further experiments showed the superiority of MAP, compared to the free peptide, in the detection of weakly positive sera. This ELISA can also be used for the serological follow-up of SLE patients.

Heterogeneity of ribosomal autoantibodies from human, murine and canine connective tissue diseases

Antiribosomal auto-antibodies (anti-Rib.Ab) have been studied in connective tissue diseases (human, dog and mouse) by immunoblotting after one-dimensional (1D) or two-dimensional (2D) gel electrophoresis of rat ribosomes. Anti-Rib.Ab could be found in systemic lupus erythematosus (SLE), rheumatoid arthritis (RA) and other connective tissue diseases (progressive systemic sclerosis, PSS; Sjögren syndrome, SjS; mixed connective tissue disease, MCTD; and dermatomyositis, DM with the frequencies 41.7%, 54.6% and 33%, respectively. Immunoblotting after 1D gel electrophoresis showed the great heterogeneity of ribosomal proteins recognized by the anti-Rib.Ab. In the SLE, however, the most frequent antibodies stained bands of the 40S subunit: 30 kDa (34% of positive sera), 19.5 kDa (24.5%) and 43 kDa (17%). In RA, the 25-kDa band of the 60S subunit was the most common (54% of positive sera). In the other human connective tissue diseases, there was no particular predominance. In the MRL/1, anti-Rib.Ab were very frequent (92.6%). The 43-kDa band of the 40S subunit was found in 100% of positive sera. Seventeen out of nineteen dogs with SLE gave positive results on immunoblot, and all of them stained the 43-kDa band of the 40S subunit. 2D gel electrophoresis gave identification of Po, L7, L5, Sb, S19, S13 and L2 proteins in SLE, S3 and SjS, L35a and L37a in RA, and L7, S6 and/or L7a in MRL/1.

Clinical and serologic associations of the antiribosomal P protein antibody

Antibodies to the ribosomal P proteins (anti-P) were detected, by Western blot analysis, in the sera of 20 of 114 patients with various autoimmune disorders. Eighty-five percent of the patients with anti-P had systemic lupus erythematosus (SLE). Of 93 randomly selected patients, the frequency of anti-P was 7 of 59 SLE patients (12%) and 0 of 34 non-SLE patients. Approximately one-third of the patients with anti-P antibodies were male; approximately half were black. In contrast to the findings of some previous studies which used isolated ribosomes as antigen, an increased frequency of renal disease was not observed. Although the overall frequency of central nervous system lupus was similar in SLE patients with and those without anti-P, 6 of 6 patients with psychosis had anti-P antibodies. Western blotting was the most sensitive and specific method for the detection of anti-P antibodies; counter-immunoelectrophoresis and cytoplasmic indirect immunofluorescence were positive in only 47% and 65% of the anti-P-positive patients, respectively. Although 53% of the SLE patients with anti-P had concomitant anti-Ro antibodies, none had anti-La (as detected by counterimmunoelectrophoresis). Anti-P antibodies, therefore, appear to be relatively specific serologic markers for SLE and may be detected in the serum even when antibodies to double-stranded DNA are not found.

Lupus autoantibodies target ribosomal P proteins

All nine SLE (systemic lupus erythematosus) sera with antiribosomal antibody activity targeted the same three ribosomal protein antigens, of molecular masses 38 and 17/19 kD when analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blotting. One serum reacted with an additional protein of approximately kD. Ribosomal subunit fractionation by composite gel electrophoresis and sucrose density ultracentrifugation showed that these proteins were part of the large subunit. Isoelectric focusing in agarose, and two-dimensional polyacrylamide gel electrophoresis revealed that the antigens had pI between 4.5 and 6.5, but that the 17/19 kD antigens were more acidic than the 38 kD antigen. Similarities in the molecular masses, charges, as well as the presence of highly conserved crossreactive epitopes, failure to bind to carboxymethylcellulose at pH 4.2, and extractability of the 17/19 kD proteins by 400 mM NH4Cl-ethanol at 0 degrees C indicated that these antigens were analogous to the proteins P0 (38 kD) and P1/P2 (17/19 kD) described previously (25, 36). Co-identity was confirmed using reference antibodies and antigen. Although antibodies to these proteins were only found in 5-10% of more than 50 sera screened by radioimmunoassay or Western blotting, the selective production of antibodies to epitopes on three (out of a total of more than 80) ribosomal proteins may provide further clues to autoantibody induction of SLE.

Anticytoplasmic antibodies in antinuclear antibody-negative lupus erythematosus. Correlation with clinical course

A patient with clinical manifestations of systemic lupus erythematosus (SLE) and without antinuclear antibodies was found to have anticytoplasmic antibodies. These anticytoplasmic antibodies were directed against ribosomal ribonucleoprotein, and the titer of anticytoplasmic and anti-ribosomal ribonucleoprotein antibodies correlated with the clinical course of the patient's illness. The importance of detecting anticytoplasmic antibodies and their role in producing disease in patients with SLE is discussed.

Studies on the specificity and clinical correlation of antiribosomal antibodies in systemic lupus erythematosus sera

The specificity of antibodies reactive with cytoplasmic ribosomes detected in systemic lupus erythematosus (SLE) sera was studied by a radioimmunoassay procedure with 3H labeled HeLa cell polysomes. The antibodies were directed primarily against RNA or a ribosomal RNA-protein complex (rRNP). Anti-RNA antibodies exhibited comparable reactivity with ribosomal RNA, nuclear RNA, structural RNA, and a slightly lower degree of reactivity with low molecular weight RNA. Anti-rRNP antibodies may be detected by precipitation in agar gel and appear to require both the RNA and protein moieties of ribosomes for reactivity. SLE sera containing only anti-rRNP antibodies are limited to patients with active disease. Sera from patients with both active and inactive SLE contain either anti-RNA or a mixture of anti-RNA and anti-rRNP antibodies. The possible participation of anti-ribosomal antibodies in immune complex formation is discussed.

Antibodies reacting with ribosomal ribonucleoprotein in connective tissue diseases

Precipitating antibody to an antigen present in cytoplasm was detected in the sera of 7 patients with systemic lupus erythematosus, 1 patient with an overlap toid arthritis, and 1 patient with mixed connective tissue disease. By physicochemical and enzymatic studies, the antigen was shown to have the properties of ribosomal ribonucleoprotein (gamma-RNP). Cytoplasmic staining in immunofluorescence was observed with all the 9 sera containing antibodies to gamma-RNP. In certain cases, cytoplasmic staining was associated with nucleolar staining. Antibody to gamma-RNP is different in immunologic specificity and clinical significance from antibody to nuclear RNP and is present primarily in patients with systemic lupus erythematosus.

Immunochemical analysis of the structure of eukaryotic ribosomes: antigenic properties of rat liver ribosomes and ribosomal proteins and characterization of the antisera

Antibodies were prepared in rabbits and sheep to rat liver ribosomes, ribosomal subunits, and to mixtures of proteins from the particles. The antisera were characterized by quantitative immunoprecipitation, by passive hemagglutination, by immunodiffusion on Ouchterlony plates, and by immunoelectrophoresis. While all the antisera contained antibodies specific for ribosomal proteins, none had precipitating antibodies against ribosomal RNA. Rat liver ribosomal proteins were more immunogenic in sheep than rabbits, and the large ribosomal subunit and its proteins were more immunogenic than those of the 40S subparticle. Antisera specific for one or the other ribosomal subunit could be prepared; thus it is unlikely that there are antigenic determinants common to the proteins of the two subunits. When ribosomes, ribosomal subunits, or mixtures of proteins were used as antigens the sera contained antibodies directed against a large number of the ribosomal proteins.