Epitopes of the p70 and p80 (Ku) lupus autoantigens

High titer autoantibodies to the Ku Ag, a DNA-protein complex containing 70- and approximately 80-kDa protein subunits (p70 and p80, respectively), are found in sera of certain patients with systemic lupus erythematosus and related disorders. Autoepitopes of the Ku Ag were identified and partially characterized by expressing fragments of the p70 and p80 cDNA as fusion proteins in bacteria. Systemic lupus erythematosus sera reacted on immunoblots with at least three epitopes of p70 (amino acids 560-609, 506-535, and 115-467), and three epitopes of p80 (amino acids 682-732, 558-681, and 1-374). These six antigenic regions had distinct amino acid sequences, and were also immunologically distinct, as determined by using immunoaffinity-purified auto-antibodies to particular epitopes. Detailed mapping of the strongly antigenic region near the C terminus of p70 revealed a complex conformational or discontinuous epitope, the antigenicity of which was abolished by deleting either amino acids 560-571 or 601-609. The C terminus of p80 may also contain a discontinuous or conformational epitope(s). Although only some sera reacted with p70 or p80 on immunoblots, all sera that immunoprecipitated the native Ku complex reacted with native Ku by ELISA, and inhibited the binding of mAb directed at epitopes of native Ku. Taken together, these studies indicate that anti-Ku autoantibodies target a diversity of independent epitopes located on p70, p80, and the intact Ku complex, and that a significant portion of the autoantibodies in most patients' sera is directed against conformational/discontinuous epitopes.

Epitopes of the p70 and p80 (Ku) lupus autoantigens

High titer autoantibodies to the Ku Ag, a DNA-protein complex containing 70- and approximately 80-kDa protein subunits (p70 and p80, respectively), are found in sera of certain patients with systemic lupus erythematosus and related disorders. Autoepitopes of the Ku Ag were identified and partially characterized by expressing fragments of the p70 and p80 cDNA as fusion proteins in bacteria. Systemic lupus erythematosus sera reacted on immunoblots with at least three epitopes of p70 (amino acids 560-609, 506-535, and 115-467), and three epitopes of p80 (amino acids 682-732, 558-681, and 1-374). These six antigenic regions had distinct amino acid sequences, and were also immunologically distinct, as determined by using immunoaffinity-purified auto-antibodies to particular epitopes. Detailed mapping of the strongly antigenic region near the C terminus of p70 revealed a complex conformational or discontinuous epitope, the antigenicity of which was abolished by deleting either amino acids 560-571 or 601-609. The C terminus of p80 may also contain a discontinuous or conformational epitope(s). Although only some sera reacted with p70 or p80 on immunoblots, all sera that immunoprecipitated the native Ku complex reacted with native Ku by ELISA, and inhibited the binding of mAb directed at epitopes of native Ku. Taken together, these studies indicate that anti-Ku autoantibodies target a diversity of independent epitopes located on p70, p80, and the intact Ku complex, and that a significant portion of the autoantibodies in most patients' sera is directed against conformational/discontinuous epitopes.

Onset and regulation of anti-lamin B autoantibody production is independent of the level of polyclonal activation

Anti-lamin B autoantibodies are associated with both systemic lupus erythematosus (SLE) and autoimmune liver disease. We examined the possibility that the underlying clinical feature in patients with anti-lamin B autoantibodies might be chronic autoimmune liver disease, and whether the hypergammaglobulinemia present in both disorders is involved in generating anti-lamin B autoantibodies. A lamin B fusion protein (MLB1), consisting of amino acids 77-533 of lamin B fused to TrpE, was used to screen sera from 84 patients with SLE for anti-lamin B autoantibodies. 3/4 prototype human lamin B antisera, 5/84 SLE sera (6%), and 0/30 sera from healthy individuals reacted with MLB1 on immunoblots at a 1:500 dilution. Of the 9 anti-lamin B autoantibody positive patients studied, all but 1 fulfilled at least four ARA criteria for SLE. None of the patients displayed evidence of chronic autoimmune liver disease, suggesting that autoimmune liver disease is not strongly associated with anti-lamin B antibodies in SLE. In SLE, as in "lupoid hepatitis", anti-lamin B autoantibodies are often produced transiently during periods of increased disease activity. Although polyclonal hypergammaglobulinemia is also associated with increased activity of both diseases, anti-lamin B autoantibody production in 2 patients was independent of total immunoglobulin levels, antibodies to irrelevant proteins, and production of some other autoantibodies. Thus, polyclonal activation is insufficient to explain either the initiation or regulation of anti-lamin B autoantibody production, supporting the hypothesis that antinuclear antibodies are antigen-selective.

Antigenic determinants of the Ku (p70/p80) autoantigen are poorly conserved between species

The Ku (p70/p80) autoantigen is a DNA-protein complex recognized by sera from certain patients with SLE and related diseases. Although human autoantibodies react with at least eight different epitopes of the human Ku complex, they had little reactivity with rodent Ku Ag on immunoblots. Small amounts of 70- and 80-kDa proteins were immunoprecipitated from murine cell extracts, however, suggesting that the Ku particle is not unique to human cells. This was confirmed by isolating cDNA clones encoding murine Ku Ag by plaque hybridization with a human p70 cDNA probe. The murine p70 cDNA clones had a deduced amino acid sequence 82.9% identical to that of human p70, and comparable amounts of murine and human p70 mRNA were detected in 3T3 and K562 cells, respectively. The poor reactivity of human autoantibodies with murine p70 was attributable to specific amino acid substitutions in an immunodominant conformational epitope located on amino acids 560-609 of human p70. Several amino acids critical for antigenicity of this region were defined by mutagenesis studies. Other conformational epitopes of Ku were also antigenically poorly conserved among species. Species-specific epitopes recognized by lupus autoantibodies are unusual but not unique to Ku. In general, poorly conserved autoepitopes have been conformational, rather than sequential, suggesting that the antigenicity of conformational epitopes may be particularly sensitive to evolutionary change.

Antigenic determinants of the Ku (p70/p80) autoantigen are poorly conserved between species

The Ku (p70/p80) autoantigen is a DNA-protein complex recognized by sera from certain patients with SLE and related diseases. Although human autoantibodies react with at least eight different epitopes of the human Ku complex, they had little reactivity with rodent Ku Ag on immunoblots. Small amounts of 70- and 80-kDa proteins were immunoprecipitated from murine cell extracts, however, suggesting that the Ku particle is not unique to human cells. This was confirmed by isolating cDNA clones encoding murine Ku Ag by plaque hybridization with a human p70 cDNA probe. The murine p70 cDNA clones had a deduced amino acid sequence 82.9% identical to that of human p70, and comparable amounts of murine and human p70 mRNA were detected in 3T3 and K562 cells, respectively. The poor reactivity of human autoantibodies with murine p70 was attributable to specific amino acid substitutions in an immunodominant conformational epitope located on amino acids 560-609 of human p70. Several amino acids critical for antigenicity of this region were defined by mutagenesis studies. Other conformational epitopes of Ku were also antigenically poorly conserved among species. Species-specific epitopes recognized by lupus autoantibodies are unusual but not unique to Ku. In general, poorly conserved autoepitopes have been conformational, rather than sequential, suggesting that the antigenicity of conformational epitopes may be particularly sensitive to evolutionary change.

Preferential use of lambda L chain in lamin B autoantibodies

The generation of lamin B autoantibodies was examined in a patient with SLE. Lamin B autoantibodies in this individual's serum were almost entirely IgG1 lambda at the onset of the autoimmune response. There was no evidence of IgM to IgG class switching, and no apparent diversification of the autoantibody response over the next 2 yr. The kappa/lambda L chain ratio of the lamin B autoantibodies was 0.13 by ELISA, with a kappa/lambda ratio of 0.70 for the patient's total IgG. In contrast, anti-histone autoantibodies in the patient's serum contained primarily kappa L chain. The kappa/lambda L chain ratios of lamin B autoantibodies from sera of four other patients with SLE were also consistent with restricted heterogeneity of lamin autoantibodies. In two, lambda L chain predominated, and in two others, the lamin B autoantibodies contained predominantly kappa L chains. The restricted heterogeneity of lamin B autoantibodies could be related to targeting of a single immunodominant autoepitope, to the existence of a particular L chain gene or genes that preferentially undergo somatic mutation, leading to the production of lamin B reactive clones, or to a combination of both mechanisms.

Preferential use of lambda L chain in lamin B autoantibodies

The generation of lamin B autoantibodies was examined in a patient with SLE. Lamin B autoantibodies in this individual's serum were almost entirely IgG1 lambda at the onset of the autoimmune response. There was no evidence of IgM to IgG class switching, and no apparent diversification of the autoantibody response over the next 2 yr. The kappa/lambda L chain ratio of the lamin B autoantibodies was 0.13 by ELISA, with a kappa/lambda ratio of 0.70 for the patient's total IgG. In contrast, anti-histone autoantibodies in the patient's serum contained primarily kappa L chain. The kappa/lambda L chain ratios of lamin B autoantibodies from sera of four other patients with SLE were also consistent with restricted heterogeneity of lamin autoantibodies. In two, lambda L chain predominated, and in two others, the lamin B autoantibodies contained predominantly kappa L chains. The restricted heterogeneity of lamin B autoantibodies could be related to targeting of a single immunodominant autoepitope, to the existence of a particular L chain gene or genes that preferentially undergo somatic mutation, leading to the production of lamin B reactive clones, or to a combination of both mechanisms.

Role of antigen selectivity in autoimmune responses to the Ku (p70/p80) antigen

Levels of anti-Ku (p70/p80) antibodies were measured longitudinally in sera from four individuals with systemic lupus erythematosus or related disorders. Antibodies to the native Ku antigen (p70/p80 complex) varied over a range of up to 577-fold. Large fluctuations were also observed in the levels of autoantibodies to several distinct epitopes of the Ku (p70/p80) antigen. Levels of these individual autoantibody populations generally paralleled one another, suggesting that they are coordinately regulated. A similar pattern of anti-DNA antibody fluctuation was seen in some sera. To examine the possibility that these autoantibodies were generated by polyclonal B cell activation, the levels of anti-Ku (p70/p80) and anti-DNA antibodies were compared to the levels of antibodies to Escherichia coli proteins, tetanus toxoid, and bovine insulin, transferrin, cytochrome c, serum albumin, and thyroglobulin. In sera from the same individual, anti-Ku (p70/p80) antibodies were sometimes produced in the complete absence of polyclonal activation, and at other times were accompanied by increased polyclonal activation. Anti-DNA antibody levels more closely paralleled the level of polyclonal activation than did the anti-Ku (p70/p80) levels. These studies suggest that anti-Ku (p70/p80) antibodies are generated by an antigen-selective mechanism, but that polyclonal activation frequently, although not invariably, accompanies autoantibody production. This observation is consistent with the possibility that polyclonal activation might be secondary to autoantibody production.

Acquired congenital heart block. Pattern of maternal antibody response to biochemically defined antigens of the SSA/Ro-SSB/La system in neonatal lupus

The molecular basis of autoantibody reactivity with components of the SSA/Ro-SSB/La particle exhibited by sera of mothers of infants with severe and permanent manifestations of neonatal lupus (NLE) was investigated using immunoblotting and immunoprecipitation. The characteristics of NLE that were studied included congenital complete heart block (CCHB), second degree heart block, and hepatic fibrosis. Antibodies specific for one or more components of the SSA/Ro-SSB/La particle were found in sera from all 20 mothers of permanently affected infants. However, no antibody specific for a single peptide of this particle was common to all sera. Using tissue extracts from a human cell substrate, 80% of these sera had antibodies to one or more components of the SSA/Ro particle demonstrable by immunoblotting. The predominant antibody response in the NLE group was to the newly recognized 52-kD SSA/Ro peptide component. In contrast, antibodies to the 60-kD SSA/Ro component although present, were the least represented and not significantly increased in frequency among mothers of these infants, compared with a group of 31 mothers with autoimmune diseases such as systemic lupus erythromatosus (SLE) but who had healthy offspring. Antibodies directed to the 48-kD SSB/La antigen were demonstrated in 90% of the NLE mothers often accompanying antibodies against the 52-kD SSA/Ro component. The combination of antibodies to 48- and 52-kD structures was significantly increased in the NLE group, with an odds ratio of 35. The type of cell or tissue substrate was shown to influence detectability of antibodies. The 52-kD SSA/Ro peptide and the 48-kD SSB/La peptide were abundant in cardiac tissues from fetuses aged 18-24 wk, further supporting the possible relevance of these peptides to heart block.

A functionally unique anti-CD3 monoclonal antibody cross-reactive with basal keratinocytes

Monoclonal antibodies (mAb) 147, 446, and 454 each recognize different epitopes of CD3. The CD3 epitope recognized by mAb 446 is functionally unique for the T cell. In contrast to mAb 147 and 454, mAb 446 induces modulation of surface CD3 with delayed kinetics and, hence, is impaired in inducing a refractory state in the T cell to subsequent anti-CD3-induced helper function. MAb 446 (but not other anti-CD3 mAb, including mAb 147, 454, OKT3, and anti-Leu4) recognizes a cytoplasmic determinant within basal keratinocytes. Extraction of keratinocytes with nonionic detergent and 2 M NaCl abolished subsequent staining with mAb 446 but enhanced subsequent staining with anti-keratin mAb, suggesting that this cross-reactive determinant is not keratin. Immunoprecipitation of internally labeled keratinocytes with the anti-CD3 mAb 147 and 446 failed to reveal specific bands, whereas these same mAb immunoprecipitated specific bands from internally labeled CD3+ Jurkat cells corresponding to previously identified CD3 subunits, suggesting that the cross-reactive determinant in keratinocytes is also not CD3. The cross-reactivity is not species specific, in that mAb 446 stained a mouse keratinocyte line, nor is it absolutely keratinocyte specific, in that mAb 446 stained one of the two nonkeratinocyte human epithelial cell lines tested. This study raises the possibility that perturbation of unique CD3 epitopes may have unique effects on T cell surface events and subsequent activation and function.

Molecular cloning of cDNA encoding the p70 (Ku) lupus autoantigen

The Ku (p70/p80) autoantigen consists of two phosphoproteins of molecular mass approximately 70,000 and 80,000 forming a macromolecular complex that binds DNA. Autoantibodies from a patient with systemic lupus erythematosus were used to isolate cDNA clones encoding the human approximately 70-kDa Ku antigen (p70) from a lambda gt11 expression library. The deduced amino acid sequence of p70 consisted of 609 amino acid residues and was confirmed by partial amino acid sequencing. The protein contains two acidic domains of 61 residues (31% Glu + Asp) and 19 residues (53% Glu + Asp) that are similar in size and charge to those found in a number of proteins involved in transcriptional activation. The 61-residue acidic region is rich in serine, raising the possibility that its charge might be modulated by phosphorylation. The predicted amino acid sequence also contains two regions with periodic repeats of either leucine alone, or leucine alternating with serine every seventh position. The latter repeat displays sequence and secondary structural similarities with the "leucine zipper" regions of the c-myc and v-myc oncogene products. The p70 antigen does not appear to have extensive sequence homology with the 80-kDa Ku autoantigen based on analysis of RNA blots and immunological criteria. A major antigenic determinant or determinants recognized by human autoantibodies is located near a leucine repeat on the carboxyl-terminal 190 amino acid residues of p70.

Multiple factors are required for poly(A) addition to a mRNA 3' end

Polyadenylation of pre-mRNAs in the nucleus involves a specific endonucleolytic cleavage, followed by the addition of approximately 200 adenylic acid residues. We have assayed HeLa nuclear extracts for the activity that catalyzes the poly(A) addition reaction. The authenticity of the in vitro assay was indicated by the observation that the poly(A) tract added in vitro is approximately 200 nucleotides in length. We have fractionated nuclear extracts in order to define components involved in specific poly(A) addition. No single fraction from DEAE-Sephacel chromatography of a HeLa nuclear extract possessed the specific poly(A) addition activity. However, if the various fractions were recombined, activity was restored, indicating the presence of multiple components. Further fractionation revealed the presence of at least two factors necessary for the poly(A) addition reaction. The reconstituted system retains the characteristics and specificity seen in the crude extract. Additional purification of one of the factors strongly suggests it to be a previously characterized poly(A) polymerase which, when assayed in the absence of the other factor, can add AMP to an RNA terminus but without specificity. Thus, the other component of the reaction may provide specificity to the process. In contrast to the 3' cleavage reaction, the poly(A) addition machinery does not possess an essential RNA component, as assayed by micrococcal nuclease digestion, nor do anti-Sm sera inhibit the reaction. Thus, the total process of formation of a polyadenylated mRNA 3' end is complex and requires the concerted action of distinct nuclear components.

Antinuclear antibodies as probes to explore the structural organization of the genome

Certain DNA binding proteins are thought to organize the mammalian genome into distinct 3 dimensional structures, each characteristic of a given differentiated state. Autoantibodies to 2 types of DNA binding protein complexes, the nuclear lamina and p70/p80 (Ku), were identified in sera of patients with collagen vascular diseases. The intranuclear distribution, DNA binding, and behavior during mitosis of these antigens were examined using autoimmune sera and murine monoclonal antibodies. In vivo, the antigens have different intranuclear distributions and solubility characteristics. However, both antigens appear to reversibly bind to DNA during interphase and to rapidly dissociate from DNA during mitosis. Although the binding affinity of p70/p80 to DNA is heterogeneous, the interaction between p70/p80 and DNA in vitro is stable over 2 h or more. The rapid dissociation of p70/p80 from DNA during mitosis may therefore be mediated by a modification in either chromatin structure or in the p70/p80 antigen itself. Other proteins that reversibly interact with DNA, such as the lamins and nuclear pores, may have a role in the organization of DNA into transcribable euchromatin and nontranscribable heterochromatin. Autoantibodies to these proteins, and possibly those reactive with p70/p80, or other DNA binding proteins may be useful probes for studying both chromatin organization and the causes of autoimmune diseases such as systemic lupus erythematosus.

Description and partial characterization of a nucleolar RNA-associated autoantigen defined by a human monoclonal antibody

B lymphocytes from a patient with systemic lupus erythematosus (SLE) and several circulating autoantibodies (including antinucleolar antibodies) were immortalized by fusion with a hypoxanthine/guanine phosphoribosyl transferase (HGPRT)-deficient human B cell line. Multiple human monoclonal antibodies (mAb) were obtained which, in solid-phase enzyme immunoassay, were reactive with DNA. One mAb was of special interest because it reacted strongly with both single-stranded DNA and an extractable nuclear antigen found in rabbit thymus extract (RTE). In an immunofluorescent assay using fixed human cells, the latter mAb also bound predominantly to cell nucleoli. A combination of enzyme digestion and metabolic inhibitor studies of the target cells in this immunofluorescent assay suggested that the antigen(s) bound by the mAb was an RNA-associated protein or a ribonucleoprotein that is distinct from intact RNA polymerase I and not associated with the transcriptional units of the nucleolus. In other experiments, using fractions of RTE isolated by ion-exchange chromatography, the antigens bound by the mAb were shown to be highly negatively charged molecules. Immunoprecipitation and SDS-PAGE analyses of labeled cell extracts bound by the mAb revealed a doublet of 17 and 18 kD. Since the original patient's serum autoantibodies also bound to both an RNase-sensitive, acidic, extractable nuclear antigen and to nucleoli, and immunoprecipitated proteins of similar molecular masses in SDS-PAGE, it appears that the described mAb is a product of an immortalized autoantibody-producing B cell clone from the SLE patient's peripheral blood. This mAb probably defines a novel RNA-associated autoantigen residing predominantly in the nucleolus or, less likely, a variant of either RNA polymerase I or the ribosomal autoantigens (P proteins).

Lamin B autoantibodies in sera of certain patients with systemic lupus erythematosus

Sera from four patients with systemic lupus erythematosus containing antibodies that yield nuclear rim staining of HEp-2 cells by indirect immunofluorescence were identified and characterized. Each serum contained autoantibodies reacting strongly with lamin B on western blots. One of the four sera displayed weaker reactivity with lamins A and C, while the other three displayed only minimal reactivity with lamins A and C. Titers of antilamin antibodies ranged from 1:1,250 to 1:36,250. Two of the sera also reacted at a dilution of 1:20 with cytoplasmic filaments of PTK-2 cells, suggesting that a small fraction of the autoantibodies in these sera may bind to alpha-helical domains of the lamins that are homologous to those of intermediate filaments. The majority of the antilamin antibodies in these patients' sera are specific for portions of the lamin B molecule that are not homologous to lamins A and C, however. The findings suggest that autoantibodies to the nuclear lamina may, in some instances, be responsible for a rim pattern in the fluorescent antinuclear antibody assay. In addition, autoantibodies to the nuclear lamina in sera of certain patients with systemic lupus erythematosus may be useful for defining the molecular structure and biological functions of lamin B, as well as for studying mechanisms of autoimmunity.

Human autoantibodies reactive with the signal-recognition particle

We describe autoantibodies reactive with the signal-recognition particle (SRP) in serum of a patient with polymyositis. The serum specifically immunoprecipitated the SRP from human erythroleukemia (K562) cell extracts. Analysis of immunoblots revealed that the serum contained autoantibodies specific for the 54-kDa protein of the SRP but had little or no reactivity with its other five proteins. Indirect immunofluorescence of human laryngeal carcinoma (HEp-2) cells confirmed that the ribonucleoprotein immunoprecipitated by this serum is found mainly in the cytoplasm. This autoimmune serum may be useful for studying the function of the 54-kDa subunit of the SRP in binding to signal sequences or in interacting with other components of the translational machinery.

Interaction between anti-DNA and anti-DNA-binding protein autoantibodies in cryoglobulins from sera of patients with systemic lupus erythematosus

We have previously shown that sera from some patients with SLE and related disorders contain autoantibodies to a DNA-binding protein complex designated p70/p80. The present study shows that anti-p70/p80 autoantibodies are frequently accompanied by anti-DNA antibodies and cryoglobulins. When the cryoglobulins were isolated, they were found to be specifically enriched in both anti-p70/p80 and anti-DNA activities. The anti-p70/p80 and anti-DNA antibodies were found to be distinct populations of autoantibodies rather than a single crossreactive species, since they could be separated from one another by chromatography on DNA-cellulose. Certain human anti-DNA mAbs could inhibit the binding of autoimmune polyclonal anti-p70/p80 antibodies to p70/p80, suggesting that anti-DNA antibodies might also associate with the variable regions of some anti-p70/p80 antibodies in the cryoglobulins. Binding of one murine anti-p70/p80 mAb (111-12) also was inhibited by certain human anti-DNA mAbs, but the binding of another murine mAb (162-11) to a different epitope of p70/p80 was not. These studies suggest that certain anti-DNA antibodies may interact with the variable regions of a population of anti-p70/p80 antibodies. The cryoglobulins found in the sera containing both anti-p70/p80 and anti-DNA antibodies may represent immune complexes consisting, in part, of idiotype and antiidiotype.

Concept formation, problem-solving and the right hemisphere

The pioneering work of Sperry in the 1960s presented a challenge to the traditional view that the right hemisphere is functionally limited to the processing of nonlinguistic, visual-spatial information on a perceptual level. Yet, the recent position of Gazzaniga (1983) is in support of the traditional view. This investigation tested further the hypothesis of right hemisphere concept formation and problem-solving. Right hemispherically strong children were compared with normal controls in terms of linguistic and nonlinguistic information processing. The results were in support of the right hemisphere as capable of cognitive processes beyond the level of simple nonverbal perceptual tasks.

Temporal shifts from Sm to ribonucleoprotein reactivity in systemic lupus erythematosus

The Sm and RNP autoantibodies, found in the sera of many patients who have connective tissue diseases, recognize determinants on small nuclear ribonucleoprotein particles (snRNP). Numerous techniques have been used to distinguish between the subsets of snRNP proteins recognized by these two antibody systems. Using protein and snRNP immunoprecipitation, as well as a competitive enzyme-linked immunosorbent assay, antibodies in the sera of Sm patients have been observed to include variable quantities of RNP-like reactivity. To analyze changes in these autoantibodies, 2 patients with anti-Sm antibodies were followed temporally. The autoantibodies in the sera of both patients underwent shifts from predominant Sm reactivity to predominant RNP reactivity. In 1 patient the shift occurred gradually over several years, while in the other the shift occurred within 8 weeks.

Small nuclear ribonucleoprotein particle assembly in vivo: demonstration of a 6S RNA-free core precursor and posttranslational modification

The in vivo synthesis and assembly of human small nuclear ribonucleoproteins (snRNPs) have been studied using pulse/chase analysis. Antibodies derived from patients with systemic lupus erythematosus (SLE) and mixed connective tissue disease (MCTD) recognize distinguishable subsets of pulse-labeled snRNP peptides. These antibodies were used to immunoprecipitate sucrose gradient fractionated pulse-labeled and pulse/chased snRNP proteins. The results indicate that assembly of the U RNA-containing snRNPs is a multistep process involving prior assembly of an RNA-free 6S core particle. This precursor contains snRNP peptides D, E, F, and G, which are common to all the different U RNA-containing particles. Furthermore, a posttranslational modification of one of the U1 snRNP-specific peptides has been observed, and the kinetics of this process indicates that the modification occurs after particle assembly. Functional and structural implications of a protein core for snRNP particles are discussed.

Autoimmune sera reactive with Sm antigen contain high levels of RNP-like antibodies

Ribonucleoprotein particles containing Sm antigen were separated from particles containing both Sm and RNP antigens by ion-exchange chromatography to study the recognition of these antigens by autoimmune sera. By using the separated antigens, anti-Sm and/or anti-RNP antibodies were detected in approximately 60% of sera from systemic lupus erythematosus patients by both enzyme-linked immunosorbent assay and immunoprecipitation of radiolabeled antigens followed by analysis on sodium dodecyl sulfate-polyacrylamide gels. These antibodies were detected in 30% of the same sera using the standard passive hemagglutination technique. Competition experiments demonstrated that all of the sera tested that contained anti-Sm antibodies also had anti-RNP-like reactivity. This latter reactivity usually represented 80% or more of the total Sm and RNP binding activity in lupus sera. The binding to RNP-like determinants by several of the sera was uniquely resistant to treatment of the antigen with snake venom exonuclease. These studies indicate that humoral immunity against Sm and RNP antigens in systemic lupus erythematosus is directed primarily against a single type of ribonucleoprotein particle in which the two antigens are physically associated. The specific binding to a single type of ribonucleoprotein particle suggests that this particle may be especially immunogenic and that it might play an important role in induction of the humoral immune response to Sm and RNP.

Use of monoclonal antibodies for the characterization of novel DNA-binding proteins recognized by human autoimmune sera

Autoantibodies to a DNA-binding heterodimer consisting of 70,000 and 80,000 dalton subunits were identified in 30-50% of human autoimmune sera from patients with systemic lupus erythematosus (SLE), mixed connective tissue disease (MCTD), and scleroderma. Three murine monoclonal antibodies (mAb) against the heterodimer were produced in BALB/c mice by immunizing with isolated human B cell nuclei. By immunofluorescence, the mAb and autoimmune sera demonstrated both speckled nucleoplasmic staining and diffuse nucleolar staining in all human cell types examined. The nucleoplasmic staining was sensitive to DNase but not RNase pretreatment, while the nucleolar staining was sensitive to RNase but not DNase pretreatment. Biochemical characterization of the 70,000 and 80,000 dalton proteins using the mAb indicated that two forms of the antigen, with different mobilities on sucrose gradients, are present in human B cells. A 10 S form consists of the physically associated 70,000 and 80,000 dalton proteins, while a larger, 10-20 S form probably represents the same two proteins bound to DNA. Binding of the proteins to nucleolar RNA could not be confirmed in biochemical studies. These studies indicate that non-histone, DNA-binding proteins may be more frequently recognized by autoantibodies in SLE, MCTD, and scleroderma than has been previously recognized. Along with previous studies on RNA-binding proteins such as Sm, RNP, Ro, and La, the present findings suggest that nucleic acid-binding proteins, as a class, may be particularly frequent targets of autoimmunity in SLE and related disorders.