Antigenic structure of histone H2B

Antibodies to H2a and H2b histones from the sera of HIV-infected patients catalyze site-specific degradation of these histones

Histones and their post-translational modifications have key roles in chromatin remodeling and gene transcription. Besides intranuclear functions, histones act as damage-associated molecules when they are released into the extracellular space. Administration of histones to animals leads to systemic inflammatory and toxic responses. Autoantibodies with enzymatic activities (abzymes) are distinctive features of some autoimmune and viral diseases. Electrophoretically homogeneous IgGs containing no canonical enzymes were isolated from the sera of HIV-infected patients by chromatography on several affinity sorbents including anti-histone Sepharose. In contrast to canonical proteases (trypsin, chymotrypsin, proteinase K), IgGs from HIV-infected patients specifically hydrolyzed only histones but not many other tested globular proteins. Using MALDI mass spectrometry the sites of H2a and H2b histone cleavage by anti-histone IgGs were determined for the first time. One cluster of H2a hydrolysis contains two major (↕) and four moderate (↓) cleavage sites: 31-H↓R↓L↓L↓R↕K G↕N-38. One major and two moderate sites of cleavage were revealed in the second cluster: 14-A↕KSRS↓SRA↓G-22. The third cluster corresponding to the H2a C-terminal part contains only five minor (†) sites of cleavage: 82-H†LQLAIRNDEELN†KLLG†RV†T†I-102. It was shown that two major and four moderate sites of cleavage were present in the main cluster of H2b hydrolysis: 46-K↕QvhpD↓TgiS↓SkA↓M↕GiM↓N-63. Two moderate sites of cleavage correspond to a relatively short 6-mer cluster: 12-K↓GskK↓A-17. The third relatively long 9-mer cluster contains one major and two minor sites of H2b cleavage: 80-L↕AHYN†KRS†T-88. In the nucleosome core particle, most of the major and moderate cleavage sites are located at the H2a/H2b interaction interface. Minor cleavage sites of H2a are involved in binding with H3 in the nucleosome core. Two moderate cleavage sites of H2b and one major cleavage site of H2a are located in the disordered N-terminal region interacting with DNA. According to the crystal structure of the nucleosome core particle, all identified cleavage sites are expected to affect H2a and H2b folding, nucleosome assembly, and binding of H2a and H2b with DNA. The existence of H2a and H2b hydrolyzing abzymes may be very important for the further understanding of unknown possibilities of immune systems and biological functions of antibodies.

Cytokine production but lack of proliferation in peripheral blood mononuclear cells from chronic Chagas' disease cardiomyopathy patients in response to T. cruzi ribosomal P proteins

Background

Trypanosoma cruzi ribosomal P proteins, P2β and P0, induce high levels of antibodies in patients with chronic Chagas' disease Cardiomyopathy (CCC). It is well known that these antibodies alter the beating rate of cardiomyocytes and provoke apoptosis by their interaction with β1-adrenergic and M2-muscarinic cardiac receptors. Based on these findings, we decided to study the cellular immune response to these proteins in CCC patients compared to non-infected individuals.

Methodology/Principal findings

We evaluated proliferation, presence of surface activation markers and cytokine production in peripheral blood mononuclear cells (PBMC) stimulated with P2β, the C-terminal portion of P0 (CP0) proteins and T. cruzi lysate from CCC patients predominantly infected with TcVI lineage. PBMC from CCC patients cultured with P2β or CP0 proteins, failed to proliferate and express CD25 and HLA-DR on T cell populations. However, multiplex cytokine assays showed that these antigens triggered higher secretion of IL-10, TNF-α and GM-CSF by PBMC as well as both CD4+ and CD8+ T cells subsets of CCC subjects. Upon T. cruzi lysate stimulation, PBMC from CCC patients not only proliferated but also became activated within the context of Th1 response. Interestingly, T. cruzi lysate was also able to induce the secretion of GM-CSF by CD4+ or CD8+ T cells.

Conclusions/Significance

Our results showed that although the lack of PBMC proliferation in CCC patients in response to ribosomal P proteins, the detection of IL-10, TNF-α and GM-CSF suggests that specific T cells could have both immunoregulatory and pro-inflammatory potential, which might modulate the immune response in Chagas' disease. Furthermore, it was possible to demonstrate for the first time that GM-CSF was produced by PBMC of CCC patients in response not only to recombinant ribosomal P proteins but also to parasite lysate, suggesting the value of this cytokine to evaluate T cells responses in T. cruzi infection.

Chronic Chagas' disease Cardiomyopathy (CCC) is the most frequent and severe consequence of the chronic infection by protozoan parasite T. cruzi. Patients with CCC develop high levels of antibodies against ribosomal P proteins of T. cruzi, called P2β and P0. These antibodies can cross-react with, and stimulate, the β1-adrenergic and M2 muscarinic cardiac receptors, inducing a functional and pathological response in cardiomyocytes. In this study, we focused on the cellular immune response developed by CCC patients in response to T. cruzi ribosomal P proteins. Peripheral blood mononuclear cells (PBMC) from CCC patients stimulated with both proteins neither proliferated nor induced the expression of activation markers on CD4+ and CD8+ T cells. However, these cells responded by the secretion of IL-10, TNF-α and GM-CSF, giving evidence that there is indeed a pool of specific T cells in the periphery responsive to these proteins. Interestingly, the cytokines profile was not related with those described to whole parasite lysate or other recombinant proteins, suggesting that each parasite protein may contribute differently to the complex immune response developed in patients with Chagas' disease.

Selective blockade of trypanosomatid protein synthesis by a recombinant antibody anti-Trypanosoma cruzi P2β protein

The ribosomal P proteins are located on the stalk of the ribosomal large subunit and play a critical role during the elongation step of protein synthesis. The single chain recombinant antibody C5 (scFv C5) directed against the C-terminal region of the Trypanosoma cruzi P2β protein (TcP2β) recognizes the conserved C-terminal end of all T. cruzi ribosomal P proteins. Although this region is highly conserved among different species, surface plasmon resonance analysis showed that the scFv C5 possesses very low affinity for the corresponding mammalian epitope, despite having only one single amino-acid change. Crystallographic analysis, in silico modelization and NMR assays support the analysis, increasing our understanding on the structural basis of epitope specificity. In vitro protein synthesis experiments showed that scFv C5 was able to specifically block translation by T. cruzi and Crithidia fasciculata ribosomes, but virtually had no effect on Rattus norvegicus ribosomes. Therefore, we used the scFv C5 coding sequence to make inducible intrabodies in Trypanosoma brucei. Transgenic parasites showed a strong decrease in their growth rate after induction. These results strengthen the importance of the P protein C terminal regions for ribosomal translation activity and suggest that trypanosomatid ribosomal P proteins could be a possible target for selective therapeutic agents that could be derived from structural analysis of the scFv C5 antibody paratope.

Crystal structure of the complex mAb 17.2 and the C-terminal region of Trypanosoma cruzi P2β protein: implications in cross-reactivity

Patients with Chronic Chagas' Heart Disease possess high levels of antibodies against the carboxyl-terminal end of the ribosomal P2ß protein of Trypanosoma cruzi (TcP2ß). These antibodies, as well as the murine monoclonal antibody (mAb) 17.2, recognize the last 13 amino acids of TcP2ß (called the R13 epitope: EEEDDDMGFGLFD) and are able to cross-react with, and stimulate, the ß1 adrenergic receptor (ß1-AR). Indeed, the mAb 17.2 was able to specifically detect human β1-AR, stably transfected into HEK cells, by flow cytometry and to induce repolarisation abnormalities and first degree atrioventricular conduction block after passive transfer to naïve mice. To study the structural basis of this cross-reactivity, we determined the crystal structure of the Fab region of the mAb 17.2 alone at 2.31 Å resolution and in complex with the R13 peptide at 1.89 Å resolution. We identified as key contact residues on R13 peptide Glu3, Asp6 and Phe9 as was previously shown by alanine scanning. Additionally, we generated a model of human β1-AR to elucidate the interaction with anti-R13 antibodies. These data provide an understanding of the molecular basis of cross-reactive antibodies induced by chronic infection with Trypanosoma cruzi.

Human recombinant antibodies againstTrypanosoma cruziribosomal P2βprotein

Patients with chronic Chagas' Heart Disease (cChHD) develop an antibody response that is suspected to be involved in the cardiac pathogenesis. The response againstTrypanosoma cruziribosomal P proteins is of particular interest, as these antibodies can cross-react with host cardiac receptors causing electrophysiological alterations. To better understand the humoral anti-P response we constructed a single-chain variable fragment library derived from a cChHD patient. The variable heavy and light regions were amplified from bone-marrow RNA and subcloned into the vector pComb3X. The phage library was subsequently panned againstT. cruziribosomal P2βprotein (TcP2β). We obtained 3 different human recombinant antibodies that specifically reacted with TcP2βin ELISA and Western blots. Two of them reacted with the C-terminal region of TcP2β, peptide R13, as the recombinant autoanti-P antibodies from Systemic Lupus Erythematosus (SLE) patients. Interestingly, the third one was specific for TcP2βbut did not recognize R13, confirming the specific nature of the anti-P response in Chagas disease. Neither sequence nor VH usage similarities between Chagas and SLE anti-P autoantibodies were observed. Herein, the first human mAbs against TcP2βhave been obtained and characterized showing that the humoral anti-P response is directed against the parasite and does not include an autoimmune component.

Interaction map of the Trypanosoma cruzi ribosomal P protein complex (stalk) and the elongation factor 2

The large subunit of the eukaryotic ribosome possesses a long and protruding stalk formed by the ribosomal P proteins. This structure is involved in the translation step of protein synthesis through interaction with the elongation factor 2 (EF-2). The Trypanosoma cruzi stalk complex is composed of four proteins of about 11 kDa, TcP1α, TcP1β, TcP2α, TcP2β and a fifth TcP0 of about 34 kDa. In a previous work, a yeast two-hybrid (Y2H) protein-protein interaction map of T. cruzi ribosomal P proteins was generated. In order to gain new insight into the assembly of the stalk, a complete interaction map was generated by surface plasmon resonance (SPR) and the kinetics of each interaction was calculated. All previously detected interactions were confirmed and new interacting pairs were found, such as TcP1β-TcP2α and TcP1β-TcP2β. Moreover P2 but not P1 proteins were able to homo-oligomerize. In addition, the region comprising amino acids 210-270 on TcP0 was identified as the region interacting with P1/P2 proteins, using Y2H and SPR. The interaction domains on TcP2β were also mapped by SPR identifying two distinct regions. The assembly order of the pentameric complex was assessed by SPR showing the existence of a hierarchy in the association of the different P proteins forming the stalk. Finally, the TcEF-2 gene was identified, cloned, expressed and refolded. Using SPR analysis we showed that TcEF-2 bound with similar affinity to the four P1/P2 ribosomal P proteins of T. cruzi but with reduced affinity to TcP0.

Structural basis of the cross-reaction between an antibody to the Trypanosoma cruzi ribosomal P2beta protein and the human beta1 adrenergic receptor

Antibodies from patients with Chagas heart disease and monoclonal antibodies (or mAb) to the carboxy-terminal end (B cell epitope R13) of the ribosomal P2beta protein of Trypanosoma cruzi (TcP2beta) cross-react with the beta1 adrenergic receptor (beta1-AR). Two single-chain Fv fragments (scFv) C5 and B7 derived from the variable regions of the anti-R13 mAb 17.2 were expressed. scFv C5 was a dimer and bound to TcP2beta with an affinity of K(d) = 8 nM, whereas scFv B7 was monomeric and had less affinity than scFv C5 for TcP2beta, K(d) = 46 nM. The affinity constant of scFv C5 to the second extracellular loop of the human beta1-AR was of 10 microM. Moreover, scFv C5 induced an increase in cAMP levels of CHO-K cells transfected with the human beta1-AR; scFv B7 had no effect but blocked isoproterenol stimulation. The agonist-like activity of scFv C5 and the antagonist activity of scFv B7 were both confirmed in vivo on heart beating frequency after their passive transfer to mice. Molecular modeling of the variable region of mAb 17.2 indicated which amino acids were likely to be involved in recognizing both peptide EDDDMGFGLF, derived from the R13 epitope of TcP2beta, and peptide ESDEARRCYN from the second extracellular loop of the human beta1-AR. It is plausible that the recently described cross-reaction of mAb 17.2 with rhodopsin can also be explained by this model. The physiological effects of this type of anti-T. cruzi antibodies may increase the liability of patients with Chagas disease.

The beta1 adrenergic effects of antibodies against the C-terminal end of the ribosomal P2beta protein of Trypanosoma cruzi associate with a specific pattern of epitope recognition

BALB/c mice immunized with recombinant Trypanosoma cruzi ribosomal P2beta protein (TcP2beta) develop a strong and specific antibody response against its 13 residue-long C-terminal epitope (peptide R13: EEEDDDMGFGLFD) that has a concomitant beta1-adrenergic stimulating activity. However, other animals that undergo similar immunizations seem tolerant to this epitope. To evaluate further the antibody response against the ribosomal P proteins, 25 BALB/c and 25 Swiss mice were immunized with TcP2beta. From the 50 animals, 31 developed a positive anti-R13 response, whereas 19 were non-responsive. From the 31 anti-R13 positive mice, 25 had anti-R13 antibodies that recognized the discontinuous motif ExDDxGF, and their presence correlated with the recording of supraventricular tachycardia. The other six had anti-R13 antibodies but with a normal electrocardiographic recording. These anti-R13 antibodies recognized the motif DDxGF shared by mammals and T. cruzi and proved to be a true anti-P autoantibody because they were similar to those elicited in Swiss, but not in BALB/c mice, by immunization with the C-terminal portion of the mouse ribosomal P protein. Our results show that the recognition of the glutamic acid in position 3 of peptide R13 defines the ability of anti-R13 antibodies to react with the motif AESDE of the second extracellular loop of the beta1-adrenergic receptor, setting the molecular basis for their pathogenic beta1 adrenoceptor stimulating activity.

Antibodies against the carboxyl-terminal end of the Trypanosoma cruzi ribosomal P proteins are pathogenic

Sera from patients with chronic Chagas heart disease recognize the carboxyl-terminal regions of the Trypanosoma cruzi ribosomal P proteins defined by B cell epitopes P013 (EDDDDDFGMGALF) and R13 (EEEDDDMGFGLFD) corresponding to the T. cruzi ribosomal P0 (TcP0) and P2beta (TcP2beta) proteins, respectively. It has been hypothesized that both epitopes may induce antibodies that cross-react and stimulate the beta1-adrenoreceptor. However, no proof as to their pathogenicity has been obtained. We investigated the consequences of immunizing mice with either TcP0 or TcP2beta proteins. Of 24 immunized animals, 16 generated antibodies against the carboxyl-terminal end of the corresponding protein, 13 of which showed an altered ECG (P<0.001, 81%). Immunization with TcP0 induced anti-P013 antibodies that bind to and stimulate cardiac G-protein-coupled receptors and are linked to the induction of supraventricular arrhythmia, repolarization, and conduction abnormalities as monitored by serial electrocardiographic analysis. In contrast, immunization with TcP2beta generated anti-R13 antibodies with an exclusive beta1-adrenergic-stimulating activity whose appearance strictly correlated with the recording of supraventricular tachycardia and death. These findings demonstrate that anti-P antibodies are arrhythmogenic in the setting of a normal heart, since no inflammatory lesions or fibrosis were evident to light microscopic examination.

Molecular and structural properties of three autoimmune IgG monoclonal antibodies to histone H2B

In systemic autoimmune diseases such as lupus the immune system produces autoantibodies to nuclear antigens including DNA and histone molecules. In the present study, we describe three monoclonal IgG antibodies that have been obtained from lupus-prone MRL/lpr mice. These three antibodies react with the amino terminus of histone H2B, a region of the molecule that is accessible in chromatin. Using a series of overlapping H2B synthetic peptides and structural analogues, we have mapped the different epitopes recognized by these antibodies. We have also sequenced the combining sites (variable regions) of the antibodies and modeled their interactions with the corresponding epitopes. Overall, the data suggest that the mechanisms of interaction with antigen are different for each of the three antibodies, even though they all react with the amino-terminal domain of the histone H2B molecule. The results also suggest that the binding between these antibodies and histone H2B is different from that between most antibodies and conventional protein antigens since the heavy chain complementarity-determining region 3 appears to play only a limited role in the three antibodies tested. The study of the interaction between self-antigens and spontaneously occurring autoantibodies may help us elucidate the mechanisms driving the expansion of self-reactive lymphocytes.

Antibodies to ribosomal P proteins of Trypanosoma cruzi in Chagas disease possess functional autoreactivity with heart tissue and differ from anti-P autoantibodies in lupus

Anti-P antibodies present in sera from patients with chronic Chagas heart disease (cChHD) recognize peptide R13, EEEDDDMGFGLFD, which encompasses the C-terminal region of the Trypanosoma cruzi ribosomal P1 and P2 proteins. This peptide shares homology with the C-terminal region (peptide H13 EESDDDMGFGLFD) of the human ribosomal P proteins, which is in turn the target of anti-P autoantibodies in systemic lupus erythematosus (SLE), and with the acidic epitope, AESDE, of the second extracellular loop of the beta1-adrenergic receptor. Anti-P antibodies from chagasic patients showed a marked preference for recombinant parasite ribosomal P proteins and peptides, whereas anti-P autoantibodies from SLE reacted with human and parasite ribosomal P proteins and peptides to the same extent. A semi-quantitative estimation of the binding of cChHD anti-P antibodies to R13 and H13 using biosensor technology indicated that the average affinity constant was about 5 times higher for R13 than for H13. Competitive enzyme immunoassays demonstrated that cChHD anti-P antibodies bind to the acidic portions of peptide H13, as well as to peptide H26R, encompassing the second extracellular loop of the beta1 adrenoreceptor. Anti-P antibodies isolated from cChHD patients exert a positive chronotropic effect in vitro on cardiomyocytes from neonatal rats, which resembles closely that of anti-beta1 receptor antibodies isolated from the same patient. In contrast, SLE anti-P autoantibodies have no functional effect. Our results suggest that the adrenergic-stimulating activity of anti-P antibodies may be implicated in the induction of functional myocardial impairments observed in cChHD.

Increased accessibility of the N-terminus of testis-specific histone TH2B to antibodies in elongating spermatids

Changes in chromatin structure during spermatogenesis were investigated using a monoclonal antibody that immunoreacts with the N-terminus of the testis-specific histone TH2B. This monoclonal antibody, which had been raised against rat tyrosine hydroxylase (TH), cross-reacted with TH2B because of sequence homology at the N-termini of TH and TH2B. The epitope was localized to the N-terminus of TH2B as trypsin-digested chromatin which lacked the N-terminal tail did not react with anti-TH and preincubating anti-TH with a synthetic peptide made from the homologous sequence between TH2B and TH inhibited its binding to TH and TH2B. In histological sections of rat testis, the primary spermatocytes and round spermatids immunoreacted weakly, whereas elongating spermatids at steps 10-12 immunoreacted intensely with anti-TH. Increased staining of elongating spermatids was also observed in mouse and hamster by immunohistochemistry. However, immunoblotting proteins extracted from separated rat testis cells showed no increase in the TH2B content of these late steps of spermatids. The apparent increase in the immunohistochemical staining corresponds to increased accessibility of the epitope in the elongating spermatids. This indicated that the N-terminus of TH2B is less tightly bound to DNA or to other proteins at this time in preparation for the removal of TH2B and other histones.

Structure and binding properties of monoclonal antibodies to core histones from autoimmune mice

Histones are frequent targets of self-reactive antibodies during autoimmune syndromes. We report the specificities and V region genes of three IgG anti-histone MAbs obtained from autoimmune mice. Each of the MAbs, named LG2-1, LG2-2 and BWA3, is directed against a different determinant located in the basic amino-terminal domain of core histones. LG2-1 reacts with a peptide from histone H3 (residues 30-45), LG2-2 recognizes the amino-terminus of H2B (residues 1-13) and BWA3 binds an epitope corresponding to a region of high sequence similarity between H2A and H4 (residues 1-20 and 1-29, respectively). The analysis of their V region sequences indicates that the H chain CDRs of these MAbs are remarkable for the presence of negatively charged amino acid residues that may play a role in the binding to cationic histones. The H chain importance in conferring reactivity to histones is corroborated by the observation that each of the VH gene segments of these MAbs is very similar to VH genes of previously described murine anti-histone antibodies.

Application and limitations of the multiple antigen peptide (MAP) system in the production and evaluation of anti-peptide and anti-protein antibodies

The multiple antigen peptide (MAP) system has been proposed as a novel and valuable approach for eliciting antibodies to peptides and developing synthetic vaccines. The MAP system consists of a small immunogenically inert core matrix of lysine residues with alpha- and epsilon-amino groups for anchoring multiple copies of the same or different synthetic peptides. Several MAP systems, each containing eight copies of 6-15 residue-long peptides derived from the terminal and central regions of various proteins were analyzed in this study. The immunogenicity of MAPs was compared to that of the same peptides linked to carrier protein by means of conventional conjugation procedures. The various peptide antisera were tested in ELISA with homologous peptides conjugated to a carrier protein via their C terminal (as in the MAP system) or their N terminal end, or with their parent proteins. The antigenic properties of MAPs were studied with anti-peptide sera obtained by classical methods and with anti-protein sera. The results showed that the MAP system was an efficient antigen in ELISA except when the peptide corresponded to a C terminal epitope. However, the value of MAPs for raising anti-peptide antibodies cross-reactive with the cognate protein appeared much more limited. In the case of one N terminal peptide, the MAP construction was not immunogenic while the conventionally conjugated peptide induced antibodies that reacted strongly with the corresponding protein. In the case of the two C terminal peptides tested, the antibodies raised against MAP constructs reacted well with homologous MAPs but did not cross-react with the whole protein. Only in the case of a peptide from an internal domain of histone H2A did immunization with a MAP generate antibodies that cross-reacted with the protein.

Molecular characterization of a Leishmania donovani infantum antigen identified as histone H2A

A Leishmania donovani infantum promastigote cDNA expression library was screened with a serum obtained from a dog naturally infected with this parasite. One of the positive clones obtained revealed nucleotide sequence similarities with the histone H2A genes from various organisms. Northern blot analyses and sequence data of three independently isolated cDNA clones indicated that the Leishmania H2A mRNAs are polyadenylated, as are the basal histone mRNAs of higher eukaryotes and the histone mRNAs of yeast. The analysis of the genomic distribution of the DNA coding for histone H2A suggested that, in L. d. infantum, there are at least four genes coding for the H2A protein. It is likely that there is a simultaneous expression of at least two of the H2A genes since differences in nucleotide sequence between two of the sequenced cDNAs were observed. Affinity-purified antibodies against the beta-galactosidase-fused H2A protein recognize specifically a Leishmania protein band with a molecular mass of 14 kDa.

Immunoelectron microscopical distribution of histones H2B and H3 and protamines in the course of mouse spermiogenesis

We have followed the fine structural distribution of two nucleosomal core histones, H2B and H3, and of protamines in the course of mouse spermiogenesis by means of specific antibodies and ultrastructural immunocytochemistry. Our results demonstrate that the nuclear labeling density of histone H2B decreases during steps 6-8 and then increases again in step 9-10 spermatids, while the labeling for histone H3 is constant throughout this period. In step 12 spermatids, the anti-H2B antibody labels mainly the central area of the nucleus. The first signs of protamine labeling are present in step 12 spermatids, where the gold grains can be found over the periphery of the nucleus. Later on, protamine labeling constantly increases and, by the end of spermiogenesis, the whole nucleus is labeled. We suggest that the morphological and structural differences between the central area and the periphery of mouse spermatids are, at least partly, due to a difference in the protein moiety associated with DNA. The central area, which is peculiar to the mouse and has been previously considered as a focus of chromatin condensation, represents, however, the last nuclear region containing histones and consequently the last area where the substitution of histones by protamines takes place.

Epitope recognition in histone H1 by SLE autoantibodies in the presence of a DNA-ligand

To investigate the specificity of anti H1 antibodies peptides from the N- and C-domain of H1 and the synthetic oligonucleotide (AT)6 were complexed. Circular dichroism (CD) spectroscopy indicated that the free peptides H1(1-16), H1(204-218) and C(121-210) in low salt buffer assume a random structure but become helical when bound to the oligonucleotide. The structured and unstructured H1 fragments were then analyzed by enzyme linked immunosorbent assay (ELISA) with anti-H1 antibodies in sera from patients with systemic lupus erythematosis (SLE) and with the monoclonal anti-H1 antibody MRA-12 derived from MLR lpr/lpr autoimmune mice. Binding of these antibodies to H1(204-218) and C was inhibited to a level of 50% when these H1 peptides were complexed with (AT)6. When the same antibody was tested with H1 fragment GC(34-210), attachment to oligonucleotide (AT)6 did not influence antibody binding. Competition studies with liquid phase GC and C antigen against solid phase GC and C indicated that liquid phase GC was more efficient in displacing antibody binding reactivity than liquid phase C. The displacement effect of both liquid phase antigens was greatest against solid phase C. We conclude that anti-H1 autoantibodies are directed against an epitope located near the junction of the G- and C-domain which is exposed and not masked when H1 is bound to DNA.

The striated body: a new structure in the sperm head of the rabbit spermatozoon

The striated body, a new structure lying in the perinuclear substance of rabbit spermatozoa, is described. It is composed of protein cords displaying a perpendicular periodic arrangement of dark and light bands. It appears in late spermatids, immediately prior to spermiation and is still present in epididymal spermatozoa, mostly, but not exclusively close to the perforatorium. The nature of this component has been analysed using a series of cytochemical techniques, as well as various antibodies and immunocytochemistry. The absence of RNA and of DNA favours a nonviral origin for this structure. The striated body is devoid of actin, differing in that respect from the rest of the perinuclear substance.

Immunogenicity of free histones and of histones complexed with RNA

Histone antibodies have been obtained by immunizing rabbits with histones H1, H2A, H2B, H3, H4 and triacetylated H4, uncomplexed to RNA. The reactivity of these antibodies was investigated by ELISA using as antigen isolated histones and chromatin as well as thirty-five different synthetic peptides covering the entire sequence of the four core histones, two peptides of H1 and two acetylated peptides of H4. The binding of these antibodies to histones was also measured in immunoblotting and in microcomplement fixation (MCF) tests. In parallel experiments using the same assays the various antigens were tested with antisera raised against histones complexed with RNA. Antibodies induced in the absence of RNA did not react with histones in MCF tests nor with chromatin in ELISA but reacted with the histones in ELISA, although the antibody titers were somewhat lower than in the case of antisera to histone-RNA complexes. Antibodies to RNA-histone complexes reacted with histones in both ELISA and MCF tests. When they were tested with peptide-coated microtiter plates in a direct binding ELISA format, antibodies induced with uncomplexed histones recognized very few fragments which were mainly located in the N- and C-terminal ends of the histones.

Major Trypanosoma cruzi antigenic determinant in Chagas' heart disease shares homology with the systemic lupus erythematosus ribosomal P protein epitope

A Trypanosoma cruzi lambda gt11 cDNA clone, JL5, expressed a recombinant protein which was found to react predominantly with chronic Chagas' heart disease sera. The cloned 35-residue-long peptide was identified as the carboxyl-terminal portion of a T. cruzi ribosomal P protein. The JL5 13 carboxyl-terminal residues shared a high degree of homology with the systemic lupus erythematosus (SLE) ribosomal P protein epitope. Synthetic peptides comprising the 13 (R-13), 10 (R-10), and 7 (R-7) carboxyl-terminal residues of the JL5 protein were used to study, by enzyme-linked immunosorbent assay, the specificity of the Chagas' disease anti-JL5 and SLE anti-P antibodies. The R-13 peptide defined a linear antigenic determinant of the JL5 recombinant protein. As was proved for JL5, R-13 defined antibody specificities which were significantly increased in chronic Chagas' heart disease patients. Only SLE anti-P positive sera were found to react with JL5 and R-13. Fine epitope mapping showed that Chagas' disease anti-JL5 and SLE anti-P antibodies define similar epitopes within the R-13 peptide. The binding of the SLE sera to JL5 was completely blocked by the R-13 peptide, indicating that the shared specificity between anti-JL5 and anti-P autoantibodies was exclusively limited to the conserved linear epitope(s) within the R-13 peptide. The prevalence of high anti-R-13 antibody titers in Chagas' heart disease patients supports the hypothesis that postulates the existence of autoimmune disorders in Chagas' heart disease.

The relationship between CD5-expressing B lymphocytes and serologic abnormalities in rheumatoid arthritis patients and their relatives

The influence of genetic factors on the expression of CD5+ B lymphocytes and their relationship to a broad spectrum of autoantibodies was investigated in a study of 12 patients with rheumatoid arthritis (RA) and 52 of their healthy first-degree relatives. The proportion of CD5+ B cells was significantly higher in RA patients (mean +/- SEM 23.9 +/- 2.7%) compared with that in their relatives (18.3 +/- 1.1%, P less than 0.05) and compared with that in a group of healthy control subjects (16.1 +/- 1.8%; P less than 0.05). Much more striking, however, were the high levels of CD5+ B cells found in the patients and their relatives in 5 of the families studied. Increases in total immunoglobulin levels and autoantibody levels were frequently observed in RA patients (approximately 20-40%) and their relatives (approximately 10-20%). Furthermore, a statistically significant correlation (P less than 0.01) between IgM rheumatoid factor and the percentage of B lymphocytes expressing CD5 was observed.

Predicting antigenic determinants of autoantigens

Predicting antigenic determinants of foreign proteins from their amino acid sequence and/or conformation is of growing importance in the production of synthetic vaccines and antigens. Unlike foreign antigenic proteins, little is known of the suitability of predictive techniques for defining antigenic regions of self proteins recognised by autoantibodies. In this study we describe our use of two computer programmes (HYDRO 3 and ACROPHILICITY [ACRO], Hopp, 1986) for the prediction of antigenic determinants of autoantigens of the cell nucleus. Using the amino acid sequence of the protein, HYDRO 3 and ACRO respectively, provide information on the hydrophilic and surface regions of the protein. Both methods were used to predict the antigenic determinants of known autoantigens, including histones, the A, B", E and 70 kD proteins of snRNPs, SS-B/La, proliferating cell nuclear antigen (PCNA) and others. Our analysis of the antigenic determinants of histones agreed with other studies which have used antihistone antibodies and fragments of histones to show that autoantibody reactive sites reside in the terminal portions of these proteins, particularly the amino terminus. A detailed study of histone 2B correctly identified most regions recognised by antibodies, particularly autoantibodies. In addition the recently described epitope of the autoantigen ribosomal protein P2 was predicted by this analysis. From these observations we hypothesize that linear antigenic sites of self proteins can be predicted. Our hypothesis can be proven experimentally by demonstrating specific interaction between autoantibodies and synthetic peptides homologous with the predicted determinant.

A branched, synthetic octapeptide of ubiquitinated histone H2A as target of autoantibodies

Two peptides of eight (T2) and 10 (T1) residues corresponding to the branched moiety of ubiquitinated histone H2A have been synthesized and used for raising specific antibodies in rabbits. Antisera to peptide T1 reacted in ELISA with T1 and with H2A but not with ubiquitin; antisera to peptide T2 reacted with T2 but not with H2A or ubiquitin. When tested in immunoblotting, both peptide antisera reacted with ubiquitinated H2A but not with unconjugated H2A or with ubiquitin. Sera from patients with systemic lupus erythematosus (SLE) were shown previously to react with ubiquitin in ELISA and immunoblotting. When tested for their ability to react in ELISA with synthetic peptides T1 and T2, 96% of the SLE sera (diluted 1:500) that recognized ubiquitin also reacted with peptide T2. Of the SLE sera that did not react with ubiquitin, only 13% possessed antibodies able to bind peptide T2. Antibodies from seven SLE sera, purified on a T2-immunoadsorbent column, were also able to react either with H2A, and in three cases also with ubiquitin.

Presence of antibodies to ubiquitin during the autoimmune response associated with systemic lupus erythematosus

Sera from patients with systemic lupus erythematosus (SLE) were shown to react with both ubiquitin and a synthetic fragment of it (residues 22-45) in an ELISA and with ubiquitin in immunoblotting experiments. Close to 80% of lupus patients possessed ubiquitin antibodies, whereas only 55% of them possessed native DNA antibodies, a marker of SLE. Less than 16% of patients with other rheumatic autoimmune diseases possessed antibodies to ubiquitin. Our results indicate that the combined measurement of antibodies to native DNA and to ubiquitin could appreciably increase the detection of SLE cases (up to 85% in our study). It is suggested that ubiquitin, a heat shock protein, could be involved in antibody formation against ubiquitin-protein conjugates present during cellular injury and that this represents a major characteristic of the autoimmune response in SLE.

Po, MBP, histone, and DNA levels in sciatic nerve. Postnatal accumulation studies in normal and trembler mice

We studied the quantitative changes in proteins (total, Po, MBP, and histones) and DNA from sciatic nerves of normal and Trembler mice during postnatal development. Polyacrylamide gel electrophoresis and immunoblotting procedures allowed an accurate characterization of Po, MBP, and histones, as well as the comparison of their respective amounts from d 2 to d 120 after birth. It was found that 1. The immunoblotting procedure ascertains the presence of Po in the sciatic nerve of Trembler. In the 2-d-old mice, Po is detected in essentially similar amounts in Trembler and normal PNS, whereas its level in adult mutant sciatic nerves is never greater than 20% of the control. The sharp increase in Po levels observed during the third week in the normal nerves is not observed in those of the mutant; 2. MBP species are at most 4% of the control in the 10- to 12-d-old Trembler mice, whereas they were not detectable in adult nerves. The distribution of the different MBP species is the same in both mutant and control mice; 3. In normal mice, Po and MBP accumulate at similar rates, but the 14 kDa MBP accumulates faster than the 18.5 kDa MBP; and 4. Histone and DNA contents decrease 3- to 5-fold in normal nerves, whereas they remain constant, or increase slightly, in the mutant.

Human antinuclear autoantibodies crossreacting with the plasma membrane and the N-terminal region of histone H2B

A subset of human antinuclear autoantibodies (denoted X-ANA) with affinity for mononucleosomes crossreacts with an antigen in the plasma membrane of viable leukocytes. The nature of the nuclear antigen recognized by these antibodies was studied in detail. In immunoblot experiments, it was shown that X-ANA recognizes the core histone H2B, but not the other histones. By using synthetic peptides corresponding to different parts of the core histones as antigens in ELISA, it was shown that X-ANA reacts with the N-terminal residues 6-18 of H2B. The binding of X-ANA to the synthetic 6-18 H2B peptide used as solid-phase antigen in ELISA was inhibited by nucleosomes and by the 6-18 H2B peptide but not by free H2B. These observations agree with earlier suggestions that the N-terminal tail of H2B is located at the surface of the nucleosome and indicate that the native structure of monomeric H2B is different from its structure when complexed within the nucleosomes.

Specificity of antibodies raised against triacetylated histone H4

Ten monoclonal antibodies (mAbs) were obtained by immunizing animals with triacetylated histone H4 from cuttle-fish. The fine specificity of these antibodies was studied using various populations of acetylated H4, (H3H4)2 tetramers and histone octamers as well as with acetylated and nonacetylated peptides of H4. None of these mAbs were found to recognize triacetylated H4. Only five of them bound to diacetylated, monoacetylated and nonacetylated H4. One antibody was specific for H4 associated in the form of histone octamers and did not bind to any nonacetylated or acetylated form of H4 monomers. Eight of the antibodies were specific for residues situated in the region 9-23 of H4. None of the mAbs was completely specific for acetylated forms of H4. In contrast, antisera raised in rabbits against triacetylated H4 reacted strongly with tri and diacetylated H4, weakly with monoacetylated H4 and barely or not at all with nonacetylated H4.

Comparison of different methods for localizing antigenic regions in histone H2A

Four different variations of enzyme-linked immunosorbent assays (ELISA) were used to analyze the antigenic structure of histone H2A. Eleven natural and 10 synthetic peptides of H2A were tested for their capacity to bind antibodies raised against the complete molecule in a direct binding assay. Results were compared to those obtained in a direct test using several peptide-BSA conjugates. The capacity of peptides to inhibit the reaction between H2A antibodies and the complete H2A molecule or large fragments of it was also measured. Inhibition assays detected antigenic activity in a large number of peptides than did direct binding assays. Antisera raised against eight synthetic, unconjugated peptides all reacted with histone H2A in ELISA. Using as probes peptides of 14-21 residues, at least 11 antigenic regions could be recognized, indicating that virtually the entire H2A polypeptide chain possessed antigenic activity.