IgG antibodies to the histone complex H2A-H2B characterize procainamide-induced lupus

Patients treated with procainamide and other drugs commonly develop antinuclear antibodies and occasionally symptoms of lupus erythematosus. However, the pathological events which lead to clinical symptoms in some patients but only abnormal serology in others have not been established. The present study examines the incidence, amount, immunoglobulin class, and antigen-binding specificity of anti-histone and anti-denatured DNA (anti-dDNA) antibodies in three groups of patients. These comprised a prospective study of patients treated with procainamide, patients with clinical drug-induced lupus symptoms, and a group undergoing therapy for many years without any symptoms. Procainamide elicited IgG and IgM anti-dDNA antibodies concordantly. Anti-histone IgM antibodies also appeared de novo during this period but IgG anti-histone antibodies were detected less frequently. Asymptomatic patients tended to have an antibody profile consisting of highly elevated anti-dDNA, IgM antibodies reactive with all histones and IgG antibodies specific for only one or two histone classes. In contrast symptomatic patients usually had little anti-dDNA or antibodies to individual histones but had pronounced IgG antibodies to the histone complex H2A-H2B. This unique antibody was characteristics of procainamide-induced lupus and was not detected in patients whose disease was induced by hydralazine. Anti-(H2A-H2B) decreased after procainamide was discontinued, concomitant with subsidence of symptoms. The finding that autoantibodies elicited by procainamide in patients with lupus symptoms have a characteristic immunoglobulin class and specificity may be of pathogenic significance and suggests that patients susceptible to procainamide-induced lupus have a unique immune response. In addition, this information could be of diagnostic value in predicting which procainamide-treated patients will develop overt symptoms of lupus.     

New Zealand white rabbits immunized with RNA-complexed total histones develop an autoimmune-like response.

The antibody response of rabbits immunized with a total histone mixture containing randomly coiled H1/H5, H2A, H2B, H3 and H4 devoid of DNA was investigated in direct and competitive ELISA. The antisera were tested with isolated histones and chromatin and with a series of overlapping synthetic peptides covering the entire sequences of the four core histones and two peptides of H1. It was found that the New Zealand (NZ) white rabbits immunized with the total histone (TH) mixture complexed with RNA produced IgG antibodies reacting with histones and with a number of histone peptides but not with chromatin. The antisera also contained IgG antibodies which bound components that correspond to common target antigens in autoimmune diseases such as native dsDNA, peptides of Sm-D antigen, ubiquitin, branched peptides of ubiquitinated H2A and poly(ADP-ribose). By competition experiments, it was shown that these antibodies corresponded to non-crossreacting antibody populations. New Zealand rabbits immunized with TH in the absence of RNA or random outbred rabbits immunized with the RNA-complexed histone fraction produced antibodies reacting with histone, chromatin and very few histone peptides, while no activity with non-related antigens was observed. The pattern of reactivity of antisera raised in NZ rabbits with RNA-complexed TH was found to be very similar to that observed in sera of patients with systemic lupus erythematosus while, in contrast, the antibody response was very different in NZ or outbred rabbits immunized with various native nuclear particles and with individual histones. Altered nucleosome particles rather than native nucleosomes may represent the antigenic stimulus giving rise to autoantibodies.     

Relationship between autoepitope and DNA-binding site on a histone H1 molecule.

Autoepitope and DNA-binding domain on a histone H1 molecule were compared using truncated histone H1 peptides as antigens. At least two epitopes (epitope A, N-terminal side; epitope B, C-terminal side) were found both of which were composed of approximately 20 amino acids. IgM from all 17 anti-histone H1-positive SLE sera reacted with epitope A. IgG from 12 sera reacted with epitope A and IgG from 4 sera reacted with epitope B. In one case, no IgG anti-histone H1 reactivities were found while IgM from the same patient reacted with epitope A. Epitope A had the ability to bind DNA. The reactivities against histone H1 of affinity-purified antiepitope A autoantibodies were inhibited by DNA. These data suggest that some anti-histone H1 antibodies are directed against a histone H1 DNA-binding site, raising the possibility that an idiotype/anti-idiotype network, at least in part, is involved in the generation of anti-histone H1 autoantibodies.     

Subnucleosome structures as substrates in enzyme-linked immunosorbent assays.

Histone preparations preserving the tertiary and quaternary structure of histone-histone complexes and histone-DNA complexes, as well as individual histones, were isolated or reconstituted. Various parameters were tested in order to determine the suitability of these complexes for use as substrates in enzyme-linked immunosorbent assays. The protein concentration required to saturate the solid phase was determined, and the amount of bound protein was quantified by the micro-bicinchoninic acid protein assay. In addition, the relative DNA content of solid phase antigen was measured by the binding of monoclonal anti-native DNA antibodies. Prototype sera representing different disease groups produced reproducible and unique patterns of reactivity on the panel of antigens, demonstrating the lack of substantial assay bias. Two substrates, the H2A-H2B dimer and the H2A-H2B-DNA complex, both appear to be oriented in a random manner on the solid phase, leaving a number of different epitopes exposed to the solution. This novel set of histone antigens can now be used to define the specificity of anti-histone antibodies in relation to the quaternary structure of chromatin.     

A Solid-Phase Radioimmunoassay for Anti-Histone Antibodies in Human Sera: Comparison with an Immunofluorescence Assay

A solid-phase assay for anti-histone antibodies is described, in which antibodies bound to calf thymus total histones, specific histone classes, or chromatin were detected by radioactive anti-human IgG, IgA or IgM. The reactivity of human sera from various rheumatic diseases was analysed by this assay and compared with results obtained from an indirect immunofluorescence assay (FANA) previously described. Sera that were positive in the FANA assay were usually positive in the solid-phase assay when total histones were the solid-phase antigen. However, many sera had IgM antibodies to total histones in the solid-phase assay but gave no detectable reaction in the FANA assay. Results from the solid-phase assay, using individual histones and chromatin, showed that the anti-histone antibodies in these sera were predominantly reactive with histones H3-H4 and did not bind well to histones complexed to DNA in the form of chromatin.     

Inhibition of histone/anti-histone reactivity by histone-binding serum components; differential effect on anti-H1 versus anti-H2B antibodies.

IgG fractions were purified on a protein G-agarose column from sera of both systemic lupus erythematosus (SLE) patients and healthy donors. All IgG fractions, after elution with 0.5 M acetic acid, reacted with histones in an anti-histone ELISA assay, and IgG anti-histone activity was in all instances higher in the IgG fraction than in the corresponding whole serum. This was shown to be due to the presence in serum of histone-binding components that inhibited IgG binding to histones. Both normal human and SLE patients' sera had these histone-binding components, and disparity between serum-positive and -negative anti-histone antibody (AHA) tests was not dependent on differences in the blocking capacity but on IgG antibody levels and avidity. Interaction of normal serum IgG fraction with all five histones was of low avidity, whereas interaction of IgG from AHA-positive SLE sera with both H1 and H2B had high avidity. Low-affinity antibodies to every histone fraction, but also high-affinity anti-H1 antibodies, were preferentially inhibited. Our data indicate that several serum protein components are inhibiting histone/anti-histone interaction and may play a protective role against both high-affinity anti-H1 antibodies present in SLE patients, and natural, low-affinity, anti-histone antibodies. As some acute phase proteins, notably C-reactive protein, bind to histones, it is conceivable that they play such a role. High-affinity anti-H2B antibodies, present in some SLE patients, and not inhibited by these serum components, may, on the other hand, participate in the pathogenesis of the disease.     

Antigenicity of the Leishmania infantum histones H2B and H4 during canine viscerocutaneous leishmaniasis

In this study we show that sera from dogs naturally infected with Leishmania infantum contain antibodies that specifically react against the parasite H2B and H4 histones. The Leishmania H2B and the amino-terminal region of the histone H4, expressed as fusion proteins, when confronted with sera from canine viscerocutaneous leishmaniasis (VCL) dogs, were recognized by 63% and 47%, respectively. No reactivity was detected when sera from dogs naturally infected with pathogens other than Leishmania were used. Using a collection of synthetic peptides covering the complete sequence of both proteins, we have determined that the main linear antigenic determinants are located in the amino-terminal domains of these histones. The humoral response against histones H2B and H4 induced during canine leishmaniasis was found to be specific for Leishmania histones, since no cross-reactivity of the VCL sera with mammal histones was observed. Also, a comparative study of the prevalence of antibodies among VCL sera against the four core histones of L. infantum was performed. Although a large heterogeneity of the humoral responses against these proteins was found, histones H2A and H3 seem to be more prevalent immunogens than histones H2B and H4 during canine natural leishmaniasis. The origin of the anti-histone humoral response and its possible implications in the pathogenesis of Leishmania infection are discussed.     

Monoclonal autoantibodies to histones from autoimmune NZB/NZW F1 mice

Fusion of spleen cells from autoimmune NZB/NZW female mice with drug-resistant myeloma cells (clones NSI/1, X63-Ag8.653 and NSO/1) produced hybrid clones which secreted antibodies to various nuclear components. Roughly 50% of the anti-nuclear hybridomas produced antibodies reacting with DNA, 20% with RNA and 30% reacted with other nuclear antigens. Two hybridomas of the latter group were cloned and studied in detail. They secreted antibodies which produced bright fluorescence staining of nuclei and metaphase chromosomes. The specificity of the antibodies was determined by testing them in an enzyme-linked immunosorbent assay and a radioimmunoassay against individual acid- and salt-extracted histones, against histones mixed two and three at a time and against histone complexes isolated as such from chromatin. One of the monoclonal antibodies was specific for histone H2B and reacted with the histone free in solution or when present as a H2A-H2B complex. The second monoclonal antibody recognized a specific conformation in the H3-H4 complex that was present only when the complex was obtained from chromatin by salt extraction. The same conformation, however, could be induced by adding histone H2B to a mixture of acid-extracted H3 and H4. Our findings show that the autoimmune syndrome in NZB/NZW mice resembles human systemic lupus erythematosus not only in the incidence of antibodies to DNA and RNA, but also in the production of autoantibodies to histones.     

Non-specific binding of heat-aggregated IgG to histone detected by ELISA

Anti-histone antibodies are currently detected by micro ELISA in systemic lupus erythematosus sera from humans, mice and dogs. Here we show that the control-heated sera may bind non-specifically to the whole histones and histone fractions. The heated immunoglobulins binding to histones are mainly IgG and to a lesser extent IgA, but never IgM. These false positive ELISA reactions occurred only with aggregated IgG which binds to histones via Fc; IgM rheumatoid factor prevented their fixation. Immune complexes do not seem to interfere significantly in the detection of anti-histone antibodies with the ELISA test.     

Individual variation in the isotype profile of anti-histone autoantibodies in systemic lupus erythematosus

Using a solid-phase radioimmunoassay we have measured levels of anti-histone autoantibodies of the IgG, IgA and IgM heavy chain classes in 40 patients with systemic lupus erythematosus. Twenty-two patients (55%) had significantly elevated levels of at least one anti-histone isotype. Our results reveal four characteristics of the anti-histone response. (1) There is wide variation between patients in the isotype profile of anti-histone antibodies and these isotype profiles are a consistent individual characteristic. (2) There is no significant correlation between the level of IgG, IgA and IgM anti-histone in individual patients and a marked tendency for a single isotype (either IgG, IgA or IgM) to predominate in any one patient. (3) IgG anti-histone antibodies are predominantly of the IgG1 subclass. (4) Among 12 patients tested, IgG and IgA antibodies showed a preference for histones 1 and 2B whereas IgM antibodies showed no consistent preference for individual histones.     

Relationships among antinuclear antibodies from autoimmune MRL mice reacting with histone H2A-H2B dimers and DNA

The histone H2A-H2B dimer is a component of nucieosomes in chromatinand a frequent target of autoantibodies in spontaneous and drug-inducedlupus. We obtained a panel of several lgG mAbs reacting withH2A-H2B or DNA from MRL mice which develop a spontaneous iupus–likesyndrome. Several of these antibodies do not react with individualhistones, but bind strongly to the H2A-H2B dimer and some bindeven more strongly to the H2A-H2B-DNA complex. Moreover, theseantibodies not only bind to H2A-H2B dimers in the absence ofDNA, but also exhibit significant binding to DNA in the absenceof histones, indicating an overlap between the anti-histoneand anti-DNA specificities. The analysis of the variable regiongene sequences of these antibodies shows a recurrent usage ofsimilar V_H genes, suggesting a dominant role for the heavy chainin determining binding specificity. The heavy chain third complementaritydetermining regions of these antibodies are also remarkablefor their frequency of D-D fusions and of D segments read inunusual reading frames and for many arginlne residues that maycontribute to DNA binding. In addition, several antibodies obtainedfrom an individual mouse are clonally related and some differthrough somatic mutations, indicating that autoreactlve clonesare positively selected by nuclear antigens.     

Specificity of Monoclonal Anti-nucleosome Auto-antibodies Derived from Lupus Mice

Recently, anti-nucleosome antibodies, which do not bind to DNA or to individual histones, have been identified in longitudinal studies in lupus mice. These anti-nucleosome antibodies occur early in spontaneous SLE and are formed prior to other anti-nuclear specificities. However, nucleosomal epitopes are yet to be fully characterized. We selected a panel of six monoclonal anti-nucleosome antibodies (mAbs) (#2, #32, #34, PL2-6, LG8-1 and LG10-1) derived from lupus mice. These mAbs were tested in ELISA on subnucleosome structures and on a panel of 53 histone peptides, covering the entire sequence of the five histones. Two mAbs reacted with one of these peptides, but the reactivity hardly exceeded the background reactivity. Based on the nucleosome and subnucleosome ELISA we identified different recognition patterns. Three mAbs showed the highest reactivity towards the intact nucleosome. For two of them (#32 and LG8-1) the nucleosomal epitope was primarily located on H2A-H2B/DNA, whereas for mAb #34 this primary epitope was located on H3/H4/DNA. Two mAbs (#2 and PL2-6) showed the highest reactivity with H2A-H2B/DNA and one mAb (LG10-1) recognized H3-H4/DNA. In the subnucleosome ELISA all but one (mAb #32) recognized more than one epitope, including DNA complexed to a variety of cationic molecules. Comparing these reactivities we identified for all mAbs one specific nucleosomal epitope, whereas reactivity with other subnucleosomes was comparable to the reactivity towards DNA complexed with cationic molecules. In inhibition experiments both in ELISA and in immunofluorescence it was found that only one of the mAbs (i.e. PL2-6), recognizing an epitope on H2A-H2B/DNA as primary epitope, could be inhibited by H2A-H2B/DNA in fluid phase. The two mAbs recognizing an epitope on H3-H4/DNA as primary epitope could be inhibited by H3-H4/DNA in fluid phase. From these analyses, we conclude first that for these nucleosome specific mAbs linear histone peptides are not very important. Second, that these mAbs all recognize different epitopes on both H2A/H2B-DNA and H3/H4-DNA and third that some solid phase H2A/H2B-DNA epitopes are not expressed on fluid phase H2A/H2B-DNA. Our findings suggest that in SLE the nucleosome can act as auto-antigen and that there is no immunodominant β cell epitope within the nucleosome.     

Comparison between autoantibodies in malaria and leprosy with lupus.

Sera from 16 patients with falciparum malaria, 16 patients with vivax malaria and 31 patients with leprosy were tested for autoantibodies to intracellular proteins and nucleic acids. Precipitating antibodies to soluble protein extracts were not detected in any serum. Sera from malaria patients showed prominent immunofluorescence staining of the HEP2 nuclear membrane as well as frequent 75% (24/32) and intense Western blot reactivity. In contrast, only 20% and 36% of patients with leprosy had positive immunofluorescence or positive immunoblots respectively, and reactivity was weak in most cases. Neither the malaria nor leprosy sera contained autoantibodies with specificities similar to the characteristic lupus autoantibodies such as double stranded DNA (dsDNA), Ro/SSA, La/SSB, Sm, RNP and P proteins. Low levels of antibodies to single stranded (ssDNA) were however found in 11 (34%) malaria sera and in seven (23%) leprosy sera. Thirteen percent of patients with leprosy had anti-histone antibodies. These findings demonstrate considerable differences in the capacity of infectious agents to induce autoantibodies and also the infrequency with which autoantibodies characteristic of idiopathic systemic lupus erythematosus are induced.     

Relationship of age and sex to autoantibody expression in MRL-+/+ and MRL-lpr/lpr mice: demonstration of an association between the expression of antibodies to histones, denatured DNA and Sm in MRL-+/+ mice.

Despite the protean nature of the clinical characteristics of systemic lupus erythematosus (SLE), autoantibodies represent an almost constant feature. Furthermore they are common to both human SLE and murine lupus. Nonetheless, the mechanism by which they arise has not been established. Amongst the several processes that have been proposed, evidence has emerged supporting specific antigen drive as a significant mechanism. We have documented the age- and sex-related differences in the prevalence of antibodies to both chromatin-related (histone and DNA) and non-chromatin-related (Sm) antigens in MRL mice. Our finding of an association between antihistone antibodies and anti-denatured DNA antibodies is consistent with chromatin being the putative antigen. Additionally, antibodies to the individual histones H1 and H2B, the most exposed histones in chromatin, were more prevalent than antibodies to the remaining histones (H2A, H3, H4). This, again, supports specific antigen drive as a mechanism for autoantibody production. However, associations were also found between antibodies to histone and DNA and antibodies to Sm. As Sm is a non-chromatin protein antigen, the associations between antibodies to Sm and those to histone and DNA suggest that mechanisms in addition to specific antigen drive are important in autoantibody production.     

Characterization of the antigenic determinants of the Leishmania infantum histone H3 recognized by antibodies elicited during canine visceral leishmaniasis

In the present study we show that sera from dogs naturally infected with the protozoan parasite Leishmania infantum contain antibodies that specifically react with the parasite histone H3. Using synthetic peptides covering the complete sequence of the protein we located the linear antigenic determinants within the 40 amino-terminal amino acids of the molecule. In addition to the complete form of the protein (rLiH3), two regions of the Leishmania histone H3 were expressed as recombinant proteins: the rLiH3-Nt fragment containing the 39 amino-terminal amino acids and the rLiH3-Ct fragment containing the 90 carboxyl-terminal residues. Competition experiments using the protein fragment rLiH3-Nt as competitor confirmed that the antigenic determinants of histone H3 are confined to the amino-terminal domain. This domain, which is believed to be exposed on the nucleosome surface, is also the most evolutionarily divergent region of the L. infantum histone H3. Visceral leishmaniasis (VL) sera do not react with mammalian histones, an indication that the anti-histone response elicited during Leishmania infection is triggered by the parasite histone. The results of the prevalence of anti-histone H3 antibodies in canine VL sera together with the sequence-specific characteristics of the amino-terminal region of L. infantum histone H3 indicate that the recombinant protein rLiH3-Nt may be of use for diagnosis of canine VL.     

Concomitant early appearance of anti-ribonucleoprotein and anti-nucleosome antibodies in lupus prone mice

OBJECTIVE: To gain insights on initial stages of the autoimmune response in lupus prone mice taking advantage of new sensitive and quantitative techniques for the detection of autoantibodies specific for RNA- (ribonucleoproteins) and DNA-protein (chromatin) complexes. METHODS: DNA and nucleosome antibodies were detected by ELISA, antibodies to SmB, U1A-RNP, Ro52, Ro60 and La by a new radioligand assay, using de novo synthesized radio-labeled antigens. RESULTS: Analysis of anti-chromatin (including anti-nucleosome, anti-dsDNA and anti-histone antibodies) and of anti-snRNP antibodies (including anti-U1A-RNP, anti-SmB, anti-Ro52, anti-Ro60, anti-La antibodies) was performed in sequential sera from B/W, MRL+/+, MRL Yaa and MRL lpr/lpr mice. In a cohort of 105 MRL+/+ mice of different ages, 59, 51, and 57 mice were positive for anti-nucleosome, anti-SmB and anti-U1A-RNP, respectively. None of them was positive for anti-dsDNA. Importantly, antibody positivities were not randomly distributed but were significantly clustered in individual mice. Appearance of DNA- and RNA-protein complex antibodies started at approximately 18-20 weeks of age, preceding that of the anti-dsDNA (or anti-histone) antibodies that only started at 30-32 weeks. Anti-nucleosome, anti-SmB and anti-U1A-RNP antibody responses did not display any cross-reactivity as demonstrated by inhibition and adsorption experiments. CONCLUSION: These data indicate that anti-nucleosome and anti-snRNP antibodies appear early and concomitantly in lupus prone mice even though they do not share any cross-reactivity. These results fit with the assumption that their production is triggered by tightly physically associated nucleosomes and snRNP autoantigens contained in the same apoptotic bodies.     

Anti-histone antibodies in the serum of autoimmune MRL and NZB/NZW1 F1 mice

Anti-histone antibodies have been reported in a number of human autoimmune diseases, most notably idiopathic and drug-induced lupus erythematosus. In the current study, anti-histone antibody activity was detected using ELISA and electroblotting techniques in sera from autoimmune NZB/W, MRL-lpr, and MRL-(+)/+ mice. Anti-histone activity increased with age, maturing earlier in females, in both NZB/W and MRL-lpr mice. Testosterone treatment decreased anti-histone activity in NZB/W mice and estrogen treatment from 2 weeks of age increased anti-histone activity in MRL-lpr mice, suggesting that gonadal hormones modified the expression of autoantibodies recognizing these protein antigens. Estrogen also increased serum IgG levels in MRL-lpr mice. Sex hormones affected expression of antibodies recognizing soy milk proteins but not ovalbumin in a similar manner. Nitrocellulose Western blots of SDS gels probed with sera from both types of autoimmune mice most often demonstrated reactivity with histone1. Some mice, usually mature females, also recognized histone4, histone3, and histone2.     

Anti-histone antibodies (ELISA and immunoblot) in canine lupus erythematosus.

In the canine systemic lupus erythematosus (SLE), anti-double stranded DNA (ds-DNA) antibodies (enzyme linked immunosorbent assay (ELISA) or indirect immunofluorescence on Crithidia luciliae) are rare whereas anti-histone antibodies are often found: 61.7% with ELISA and 74% with immunoblot. In canine SLE the pattern of anti-histone antibodies on immunoblot is different from anti-histone antibodies in human SLE. Indeed, histone fractions which are most often recognized by the canine antibodies are by order of frequency H3, H4 and H2A, whereas in man this order is H1, H2B then H3. In the diagnostic criteria of canine SLE, we suggest replacing the anti-ds-DNA antibodies by the anti-histone antibodies detected by immunoblot.     

Binding to histone of an anomalous IgG from patients with SLE and drug-induced lupus

The mechanism of attachment of circulating immune complexes (CIC) to glomerular basement membranes (GBM) in systemic lupus erythematosus (SLE) has not yet been elucidated. One difficulty is that CIC must be strongly cationic for such deposition to occur, which is opposite to the anionic nature of putative DNA-anti-DNA immune complexes (DNA-IC). The strongly cationic histone has been proposed as a potential "planted antigen"; it would decorate the GBM to function as a ligand for DNA in the DNA-IC. However, DNA-IC, aggregated IgG and most of the IgG "anti-histone antibodies" in SLE patient sera bind to histone on a solid phase not through DNA, but through the Fcgamma. Here, we investigated the nature of the anti-histone "antibody" in sera of 18 patients with SLE and 57 with drug-induced lupus (DIL). The binding to nucleosomes of IgG from these patients was mainly pepsin-resistant and F(ab')(2)-dependent, whereas the binding to histone was mainly pepsin-sensitive and Fcgamma-dependent. Surprisingly, after molecular sieving of 12 of these sera, the pepsin-sensitive histone-binding IgG was located mainly in the 150-kDa monomeric IgG peak. The binding to nucleosomes was only in the 150-kDa peak. These findings are consistent with the existence of an anomalous IgG in SLE and DIL sera, capable, like aggregated IgG, DNA-IC and other CIC, of binding to histone-decorated structures. We propose that this anomalous IgG plays an essential role in the pathogenesis of lupus nephritis and other related inflammatory conditions. These observations also explain the large discrepancies in the reports on anti-histone autoantibodies in autoimmune conditions.