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

Extracellular histones as damage-associated molecular patterns in neuroinflammatory responses

The four core histones H2A, H2B, H3, H4, and the linker histone H1 primarily bind DNA and regulate gene expression within the nucleus. Evidence collected mainly from the peripheral tissues illustrates that histones can be released into the extracellular space by activated or damaged cells. In this article, we first summarize the innate immune-modulatory properties of extracellular histones and histone-containing complexes, such as nucleosomes, and neutrophil extracellular traps (NETs), described in peripheral tissues. There, histones act as damage-associated molecular patterns (DAMPs), which are a class of endogenous molecules that trigger immune responses by interacting directly with the cellular membranes and activating pattern recognition receptors (PRRs), such as toll-like receptors (TLR) 2, 4, 9 and the receptor for advanced glycation end-products (RAGE). We then focus on the available evidence implicating extracellular histones as DAMPs of the central nervous system (CNS). It is becoming evident that histones are present in the brain parenchyma after crossing the blood-brain barrier (BBB) or being released by several types of brain cells, including neurons, microglia, and astrocytes. However, studies on the DAMP-like effects of histones on CNS cells are limited. For example, TLR4 is the only known molecular target of CNS extracellular histones and their interactions with other PRRs expressed by brain cells have not been observed. Nevertheless, extracellular histones are implicated in the pathogenesis of a variety of neurological disorders characterized by sterile neuroinflammation; therefore, detailed studies on the role these proteins and their complexes play in these pathologies could identify novel therapeutic targets.

EAE of Mice: Enzymatic Cross Site-Specific Hydrolysis of H2B Histone by IgGs against H1, H2A, H2B, H3, and H4 Histones and Myelin Basic Protein

Histones have vital roles in chromatin functioning and gene transcription. At the same time, they are pernicious in intercellular space because they stimulate systemic inflammatory and toxic responses. Myelin basic protein (MBP) is the major protein of the axon myelin–proteolipid sheath. Antibody-abzymes with various catalytic activities are specific features of some autoimmune diseases. IgGs against five individual histones (H2B, H1, H2A, H3, and H4) and MBP were isolated from the blood of experimental autoimmune encephalomyelitis-prone C57BL/6 mice by affinity chromatography. Abzymes corresponding to various stages of EAE development, including spontaneous EAE, myelin oligodendrocyte glycoprotein (MOG)- and DNA-histone complex-accelerated onset, as well as acute and remission stages, were analyzed. IgG-abzymes against MBP and five individual histones showed unusual polyreactivity in complex formation and enzymatic cross-reactivity in the specific hydrolysis of H2B histone. All IgGs against MBP and individual histones in 3-month-old mice (zero time) demonstrated from 4 to 11 different H2B hydrolysis sites. Spontaneous development of EAE during 60 days led to a significant change in the type and number of H2B hydrolysis sites by IgGs against the five histones and MBP. Mouse treatment with MOG and DNA-histone complex changed the type and number of H2B hydrolysis sites compared to zero time. The minimum number (3) of different H2B hydrolysis sites was found for IgGs against H3 20 days after mouse immunization with DNA-histone complex, whereas the maximum number (33) for anti-H2B IgGs was found 60 days after mouse treatment with DNA-histone complex. Overall, this is the first study to demonstrate that at different stages of EAE evolution, IgG-abzymes against five individual histones and MBP could significantly differ in the specific sites and number of H2B hydrolysis sites. Possible reasons for the catalytic cross-reactivity and significant differences in the number and type of histone H2B cleavage sites were analyzed.

On the possible use of exogenous histones in cell technology

The prospect of developing transport systems using histones for site-specific delivery of therapeutic agents that have poor penetration characteristics through cellular membranes and tissue barriers has been investigated. Histones immobilized on microspheres can also be used to modify surfaces intended for cell cultivation, facilitating adhesion, proliferation and network formation by interactions of cells through contacts with several microspheres. They can be applied to three-dimensional pore matrices that are designed for producing tissue-like structures in vitro.

Surface architecture of histoplasma capsulatum

The dimorphic fungal pathogen Histoplasma capsulatum is the most frequent cause of clinically significant fungal pneumonia in humans. H. capsulatum virulence is achieved, in part, through diverse and dynamic alterations to the fungal cell surface. Surface components associated with H. capsulatum pathogenicity include carbohydrates, lipids, proteins, and melanins. Here, we describe the various structures comprising the cell surface of H. capsulatum that have been associated with virulence and discuss their involvement in the pathobiology of disease.

[Specificities and clinical significance of autoantibodies directed against histones]

Systemic lupus erythematosus (SLE) is an autoimmune disease characterized by the occurrence of numerous autoantibodies directed against nuclear antigens. Anti-histone antibodies (AHA) are as prevalent as their anti-dsDNA counterparts in SLE. Despite their frequency and potential importance, there have not been given much attention to AHA until recently. Nucleosomes, the fundamental repeating units of the chromatin, are formed of complexes of histones and DNA. The nucleosome core particle is composed of a central tetramer of 2 molecules each of H3 and H4 flanked by 2 dimers of H2A and H2B and surrounded by 2 superhelical turns of approximately 146 base pairs of DNA. The full nucleosome contains a molecule of H1 located at the point where DNA enters and exits the nucleosome. Recent studies have shown that the post transcriptional modification of histone changes chromatin structure to regulate transcription and the concept of this mechanism "epigenetics" has become center of attention in the field of basic cell biology. There have been described diverging specificities of AHA. Many attempts to locate antigenic determinants recognized by AHA have been made and H1 and H2B have been thought as common targets in lupus patients. Studies on murine models of lupus have shown several interesting findings. The universal epitope is located on H2B in (NZBxNZW)F1 mice. In addition to core histones, MRL-MP/Fas(lpr) mice develop high titers of autoantibodies to H1. Autoimmunity to chromatin regularly involves humoral immune responses directed against H1. These histones appear to be an early (possibly initial trigger) autoantigen for this autoimmune response in lupus.

Antibodies to a cell surface histone-like protein protect against Histoplasma capsulatum

A protective role for antibodies has not previously been described for host defense against the pathogenic fungus Histoplasma capsulatum (Hc). Mouse mAb's were generated from mice immunized with Hc yeast that binds the cell surface of Hc. Administration of mAb's before Hc infection reduced fungal burden, decreased pulmonary inflammation, and prolonged survival in a murine infection model. Protection mediated by mAb's was associated with enhanced levels of IL-4, IL-6, and IFN-gamma in the lungs of infected mice. The mAb's increased phagocytosis of yeast by J774.16 cells through a CR3-dependent process. Ingestion of mAb-opsonized Hc by J774.16 macrophage-like cells was associated with yeast cell growth inhibition and killing. The mAb's bound to a 17-kDa antigen expressed on the surface of Hc. The antigen was identified as a histone H2B-like protein. This study establishes that mAb's to a cell surface protein of Hc alter the intracellular fate of the fungus and mediate protection in a murine model of lethal histoplasmosis, and it suggests a new candidate antigen for vaccine development.

Nucleosomes bind to cell surface proteoglycans

Material on the surface of activated T-cells was displaced following incubation with a sulfated polysaccharide, dextrin 2-sulfate (D2S), and purified by anion-exchange chromatography. This revealed a complex comprising histones H2A, H2B, H3, and H4 and DNA fragmented into 180-base pair units characteristic of mono-, di-, tri, and polynucleosomes, a pattern of fragmentation similar to that found in apoptotic cells. An antibody raised against the purified nucleosome preparation bound to the plasma membrane of activated T-cells confirming the surface location of nucleosomes. The interaction of sulfated polysaccharides with nucleosomes was investigated using a biotinylated derivative of D2S. It was found that sulfated polysaccharides bound to nucleosomes via the N termini of histones, especially H2A and H2B. Treatment of T-cells with either heparinase or heparitinase abolished nucleosome binding to plasma membranes. This suggests that nucleosomes are anchored to the surface of T-cells by heparan sulfate proteoglycans through an ionic interaction with the basic N-terminal residues in the histones. Furthermore, nucleosomes bound to the cell surface in this manner are then able to bind other sulfated polysaccharides, such as D2S, heparin, or dextran sulfate, through unoccupied histone N termini forming a complex comprising cell surface heparan sulfate proteoglycans, nucleosomes, and sulfated polysaccharides.

Extracellularly occurring histone H1 mediates the binding of thyroglobulin to the cell surface of mouse macrophages

Thyroglobulin is the major secretory protein of thyroid epithelial cells. Part of thyroglobulin reaches the circulation of vertebrates by transcytosis across the epithelial wall of thyroid follicles. Clearance of thyroglobulin from the circulation occurs within the liver via internalization of thyroglobulin by macrophages. Here we have analyzed the interaction of thyroglobulin with the cell surface of J774 macrophages with the aim to identify the possible thyroglobulin-binding sites on macrophages. Binding of thyroglobulin to J774 cells was saturated at approximately 100 nM thyroglobulin with a Kd of 50 nM, and it was competed by the ligand itself. Preincubation of J774 cells with thyroglobulin resulted in downregulation of thyroglobulin-binding sites, indicating internalization of thyroglobulin and its binding proteins. By affinity chromatography, two proteins from J774 cells were identified as thyroglobulin-binding proteins with an apparent molecular mass of approximately 33 kD. Unexpectedly, both proteins were identified as histone H1 by protein sequencing. The occurrence of histone H1 at the plasma membrane was further proven by biotinylation or immunolabeling of J774 cells. The in vitro interaction between histone H1 and thyroglobulin was analyzed by surface plasmon resonance that revealed a Kd at 46 nM. In situ, histone H1 was colocalized to FITC-Tg-containing endocytic compartments of Kupffer cells, i.e., liver macrophages. We conclude that histone H1 is detectable at the cell surface of macrophages where it serves as a thyroglobulin-binding protein and mediates thyroglobulin endocytosis.

Panel of anti-gp120 monoclonal antibodies reacts with same nuclear proteins in uninfected cells as those recognized by autoantibodies from patients with systemic lupus erythematosus

Mouse monoclonal antibodies (MAbs) directed against the HIV-1 gp120 envelope protein were screened for their reactivity with intracellular antigens expressed in normal uninfected monocytes by indirect immunofluorescence and immunoblotting. Some of these MAbs reacted with the nuclei of normal uninfected cells, producing three main staining patterns by indirect immunofluorescence. Western blot analysis showed that these monoclonal antibodies recognize peptides of various molecular weights present in nuclei preparations of normal monocytes. Reactivity with peptides of similar molecular weight was also detected in sera from both HIV-infected individuals and patients with systemic lupus erythematosus. This evidence for antigenic similarities between HIV-1 gp120 and nuclear antigens represents a novel example of molecular mimicry of self-antigens by HIV envelope proteins, which supports the involvement of mechanisms of autoimmunity in HIV disease pathogenesis through recruitment of autoimmune responses to self-structures by HIV antigens.

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.

Nucleosome-specific antibody from an autoimmune MRL/Mp-lpr/lpr mouse

OBJECTIVE

To characterize the binding properties and variable-region sequences of LG4-1, a monoclonal antibody from an autoimmune MRL/Mp-lpr/lpr mouse that reacts specifically with nucleosome core particles and represents a new antinuclear antibody specificity.

METHODS

The reactivity of the antibody against various nuclear substrates was determined using an enzymatic immunoassay, and the variable-region genes were sequenced from messenger RNA, using the dideoxy chain termination method.

RESULTS

LG4-1 was found to react with nucleosome core particles but not with individual histones and DNA, or with various histone-histone and histone-DNA complexes. It was demonstrated that this antibody is encoded by a combination of variable-region genes and gene segments that have undergone few somatic mutations.

CONCLUSION

The nucleosome core particle expresses a unique conformational autoepitope(s) resulting from the ordered association of histones and DNA.

Anti-histone autoantibodies react specifically with the B cell surface

In an attempt to induce an immune response against Mls-1a antigens by immunizing C57B1/6 mouse (Mls-1b) with purified B cells from DBA/2 mouse (Mls-1a), we generated a panel of monoclonal antibodies from which the 5B9.6 mAb, taken as a representative antibody, was thoroughly investigated. This antibody specifically reacts with B cells from all mouse strains studied including C57Bl/6 mice as shown by FACS analysis of double-antibody labelled spleen cells. Using enzyme immunoassays and immunoprecipitation techniques, 5B9.6 mAb was found to be specific for histones. Amino acid sequence analysis of a peptide derived from a 5B9.6-immunoprecipitated polypeptide from DBA/2 B cells showed a 100% homology with a sequence within H2B histones. Furthermore, 5B9.6 mAb was able to interact with the cell surface of 7OZ/3 cell line, known as a typical pre-B cell line. The presence of histones can be modulated on the surface of 7OZ/3 cells since this antigen was upregulated after exposure of these cells to a cocktail of IL-1 and cAMP. Finally, 5B9.6 mAb was shown to interact with freshly isolated B cells from human peripheral blood.

Double reactivity of monoclonal and polyclonal rheumatoid factors for IgG and histones: Mapping of binding sites by means of histone synthetic peptides and anti-Id antibodies

Polyreactive antibodies able to bind various apparently unrelated structures represent a frequent antibody population in autoimmune diseases. In this work, the structural basis of the double reactivity of such autoantibodies was investigated using as models polyclonal and monoclonal human rheumatoid factors (RF) reacting with histones. Both direct ELISA binding and competitive inhibition experiments were performed. A more precise delineation of the histone regions recognized by the RFs was made by means of 27 synthetic peptides of these proteins. Anti-idiotope (Id) murine antibodies were used to map the binding sites involved on RF in the interaction with IgG and histones. Among the 13 polyclonal and six monoclonal RFs tested, four and two respectively were found to cross-react with IgG and histones. The fragments shown to be the most frequently recognized by RFs were located in residues 1-16 and 204-218 of H1, 1-20 and 65-85 of H2A, and 1-21 of H3. The results obtained by competitive ELISA assays using IgG, histone peptides and anti-Id monoclonal antibodies led us to confirm and characterize more precisely our previous finding suggesting the existence of topographically distinct binding sites for the different targets recognized by RFs.

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.

Intrinsic cell membrane antigens recognized by antichromatin autoantibodies. The membrane antigens do not derive from the nucleus

The main object of this study was to see whether or not chromatin constituents are present in cell membranes. The binding of antinuclear antibodies (ANA) to plasma membranes of leucocytes was studied by using autoantibodies and induced antibodies to different histones and histone peptides, and autoantibodies to double-stranded DNA (dsDNA) in adsorption-elution experiments. Eluates were subsequently tested for antinuclear antibodies by a solid-phase ELISA. No ANA activities in the eluates were observed, except when true cross-reacting ANA were employed in the study. Furthermore, no binding of these antibodies to plasma membranes could be visualized by indirect membrane fluorescence tests. The conclusion of these studies was that freshly isolated viable leucocytes did not carry detectable amounts of ectopic chromatin components at the level of plasma membranes. Thus, it seems fairly unlikely that chromatin autoantibodies in general exert some tissue damage by binding to homologous nuclear antigens associated with plasma membranes in vivo.