Enhancement of oxidative cleavage of DNA by the binding sites of two anti-double-stranded DNA antibodies

Anti-DNA antibodies cross-reactive with β2-glycoprotein I induce monocyte tissue factor through the TLR9 pathway

Antibodies specific for cardiolipin (CL)-β₂-glycoprotein I (β₂GPI) are known to induce tissue factor (TF) expression by monocytes and endothelial cells which leads to a prothrombotic state in antiphospholipid syndrome (APS), but the mechanism is not fully elucidated. Previously, we reported that the mouse monoclonal anti-CL-β₂GPI antibody WB-6 cross-reacts with DNA, enters monocytes via binding to cell surface DNA, and induces TF expression. The current study aimed to identify the intracellular signaling pathways involved in this process. The binding of WB-6 to CL-β₂GPI or DNA, and endocytosis was not prevented by chloroquine, but pre-treatment of the cells with chloroquine significantly suppressed TF expression. TLR9 inhibitory oligodeoxynucleotide also suppressed the WB-6-induced TF expression, suggesting a pivotal role of the TLR9 pathway in TF production. Serum antibodies obtained from a patient with APS accompanying systemic lupus erythematosus (SLE) bound to both CL-β₂GPI and DNA, and induced TF in normal monocytes. This effect was suppressed by chloroquine, and abolished by removal of the DNA-binding activity. These results suggest that induction of TF expression results from TLR9 activation by DNA which was internalized together with cross-reactive antibodies produced in secondary APS accompanying SLE.

Ion Binding Properties of a Naturally Occurring Metalloantibody

LT1009 is a humanized version of murine LT1002 IgG1 that employs two bridging Ca²⁺ ions to bind its antigen, the biologically active lipid sphingosine-1-phosphate (S1P). We crystallized and determined the X-ray crystal structure of the LT1009 Fab fragment in 10 mM CaCl₂ and found that it binds two Ca²⁺ in a manner similar to its antigen-bound state. Flame atomic absorption spectroscopy (FAAS) confirmed that murine LT1002 also binds Ca²⁺ in solution and inductively-coupled plasma-mass spectrometry (ICP-MS) revealed that, although Ca²⁺ is preferred, LT1002 can bind Mg²⁺ and, to much lesser extent, Ba²⁺. Isothermal titration calorimetry (ITC) indicated that LT1002 binds two Ca²⁺ ions endothermically with a measured dissociation constant (K_D) of 171 μM. Protein and genome sequence analyses suggested that LT1002 is representative of a small class of confirmed and potential metalloantibodies and that Ca²⁺ binding is likely encoded for in germline variable chain genes. To test this hypothesis, we engineered, expressed, and purified a Fab fragment consisting of naïve murine germline-encoded light and heavy chain genes from which LT1002 is derived and observed that it binds Ca²⁺ in solution. We propose that LT1002 is representative of a class of naturally occurring metalloantibodies that are evolutionarily conserved across diverse mammalian genomes.

Monoclonal anti-dsDNA antibody 2C10 escorts DNA to intracellular DNA sensors in normal mononuclear cells and stimulates secretion of multiple cytokines implicated in lupus pathogenesis

There have been many studies on the mechanisms of internalization of DNA–anti‐DNA immune complexes by cells, including the one used for rheumatoid factor‐expressing mouse B cells. In parallel, studies on the role of intracellular DNA sensors in the pathogenesis of systemic lupus erythematosus (SLE) have been conducted, including the one using a mouse model lacking one of the sensors. These and other data have established a framework for understanding the pathogenic role of anti‐DNA antibodies, but studies on normal cells are limited. Here, we used the monoclonal anti‐dsDNA antibody 2C10, 2‐kbp dsDNA and healthy human peripheral blood mononuclear cells (PBMCs) to test whether and how 2C10 and/or DNA cause pathology in normal cells. We found that on culture with PBMCs, 2C10 preferentially entered monocytes and that DNA enhanced this internalization. In contrast, DNA alone was not significantly internalized by monocytes, but 2C10 facilitated its internalization. This was suppressed by cytochalasin D, but not by methyl‐β‐cyclodextrin, chloroquine or an Fc blocker, suggesting the involvement of macropinocytosis in this process. Internalization of 2C10 and DNA together resulted in production of interferon (IFN)‐α, IFN‐γ, tumor necrosis factor (TNF)‐α, monocyte chemoattractant protein‐1 (MCP‐1), interleukin (IL)‐1β, IL‐6, IL‐10 and IL‐33 by PBMCs. Cytokine production was suppressed by chloroquine and shikonin, but not by RU.521, suggesting dependence on activation of the Toll‐like receptor (TLR)‐9 and absent in melanoma 2 (AIM‐2) pathways. These results established a simple model to demonstrate that anti‐DNA antibodies can cause dysregulation of cytokine network mimicking systemic lupus erythematosus in culture of normal PBMCs, and emphasize again the importance of maintaining anti‐DNA antibodies at low levels by treatment.

Anti-β2 -glycoprotein I antibody with DNA binding activity enters living monocytes via cell surface DNA and induces tissue factor expression

Autoantibodies characteristic for anti‐phospholipid syndrome (APS) and systemic lupus erythematosus (SLE) are anti‐β₂‐glycoprotein I (β₂GPI) antibodies and anti‐DNA antibodies, respectively, and almost half of APS cases occur in SLE. Anti‐β₂GPI antibodies are recognized to play a pivotal role in inducing a prothrombotic state, but the precise mechanism has not been fully elucidated. In a widely accepted view, binding of anti‐β₂GPI antibodies to cell surface β₂GPI in monocytes and endothelial cells triggers the Toll‐like receptor 4‐myeloid differentiation primary response 88 (TLR)‐4‐MyD88) signaling pathway which leads to activation of p38 mitogen‐activated protein kinase (MAPK), mitogen‐activated protein kinase kinase 1/extracellular signal‐regulated kinases (MEK‐1/ERK) and/or nuclear factor kappa B (NF‐κB) and expression of tissue factor (TF). However, resting cells do not express substantial amounts of TLR‐4. Previously, we generated a mouse monoclonal anti‐β₂GPI antibody WB‐6 and showed that it induced a prothrombotic state – including TF expression on circulating monocytes – in normal mice. In the current study, we aimed to clarify the mechanism of interaction between WB‐6 and resting monocytes, and found that WB‐6 exhibits binding activity to DNA and enters living monocytes or a monocytic cell line and, to a lesser extent, vascular endothelial cells. Treatment of the cells with DNase I reduced the internalization, suggesting the involvement of cell surface DNA in this phenomenon. Monocytes harboring internalized WB‐6 expressed TF and tumor necrosis factor (TNF)‐α which, in turn, stimulated endothelial cells to express intercellular adhesion molecule 1 (ICAM‐I) and vascular cell adhesion molecule 1 (VCAM‐I). These results suggest the possibility that a subset of anti‐β₂GPI antibodies with dual reactivity to DNA possesses ability to stimulate DNA sensors in the cytoplasm, in addition to the cell surface receptor‐mediated pathways, leading to produce proinflammatory and prothrombotic states.

Impacts of Anti-dsDNA Antibody on In Vitro Fertilization-Embryo Transfer and Frozen-Thawed Embryo Transfer

Our purpose is to explore whether anti-dsDNA antibody, which was demonstrated to enter living cells and induced apoptosis, could adversely affect reproductive outcomes. A total of 259 women receiving the in vitro fertilization-embryo transfer (IVF) cycle were enrolled in this study, including 52 women with positive ANA and anti-dsDNA (ANA+/anti-dsDNA+ group), 86 women with positive ANA and negative anti-dsDNA (ANA+/anti-dsDNA− group), and 121 women with negative ANA and anti-dsDNA (ANA−/anti-dsDNA− group). 136 nonpregnant women among 259 patients in the IVF-ET cycle were enrolled in the hormone replacement therapy frozen-thawed embryo transfer (HRT-TET) cycle. We compared basic characters and IVF outcomes among three groups in fresh embryo transfer and frozen-thawed embryo transfer cycle, respectively. The number of retrieved oocytes, available embryos, and high-quality embryos in the ANA+/anti-dsDNA+ group was lower than those in the other two groups in the fresh embryo transfer cycle. The rates of fertilization, implantation, and clinical pregnancy in the ANA+/anti-dsDNA+ group were the lowest, while the early miscarriage rate was the highest in the ANA+/anti-dsDNA+ group both in the fresh embryo transfer cycle and in the frozen-thawed embryo transfer cycle. Our data suggested that anti-dsDNA antibody may be the essential marker for defective oocytes or embryos in infertile women with any type of ANA.

Autoantibodies closely relate to the elevation level of in vivo hydrogen peroxide and tissue damage in systemic lupus erythematosus

Recent studies have shown that antibodies efficiently catalyze the conversion of molecular singlet oxygen (1O2) plus water to hydrogen peroxide (H2O2). H2O2 is toxic to cells and is a cause of further free radical generation that are implicated in the pathogenesis of a variety of diseases. Systemic lupus erythematosus (SLE) is one of the most serious autoimmune diseases which are characterized by the production of various autoantibodies and subsequent tissue damage. However, the correlation of autoantibodies, H2O2, and tissue lesion in SLE has not been yet investigated. To address this issue, in the present study, we induced autoantibodies and kidney tissue damage by using SLE animal model as described previously. We detected the level of H2O2 in SLE mice and found the increase of in vivo H2O2 was accompanied and closely correlated with the production of anti-dsDNA and antihistone antibodies. Importantly, there was onefold increase of H2O2 in the mice kidneys with apparent glomerulonephritis and IgG deposits. These results suggest that the induced autoantibodies possess catalytic activity. The produced autoantibodies lead to the production and elevation of H2O2, which results in subsequent renal damage and the pathogenesis of SLE. Our findings provide an insight into the understanding of SLE mechanism and provide a potential approach for therapeutic intervention of SLE.

Pathogenesis of lupus nephritis: an update

Lupus nephritis (LN) is a prototypic autoimmune disease. Its immunopathogenesis is characterized by the loss of self-tolerance. In this article, we review our current understanding of the disease mediators of LN. There is ample evidence to suggest a pathogenic role of nephritogenic autoantibodies. These antibodies cross react with nucleosomal epitopes, and the in vivo generation of nucleosomes requires apoptosis. Furthermore, there is an intriguing and paradoxical relationship between complement and systemic lupus erythematosus (SLE). Immune complex-mediated activation of complement through the classic pathway is traditionally believed to be a major mechanism by which tissue injury occurs. In contrast, hereditary deficiencies of complement components increase the risk of SLE. Finally, the roles of reactive nitrogen and oxygen species are emphasized.

New approaches to the renal pathogenicity of anti-DNA antibodies in systemic lupus erythematosus

Autoantibodies against double stranded (ds) DNA are not only a helpful serological marker for diagnosis of systemic lupus erythematosus (SLE), but have also been shown to be crucial in the pathogenesis of lupus nephritis. However, the question of how anti-dsDNA antibodies contribute to renal damage is unresolved. Many authorities believe that indirect binding (mediated by nuclear antigens) or direct cross-reactivity of anti-dsDNA antibodies with kidney antigens are important determinants of anti-dsDNA nephritogenicity. An alternative hypothesis for the renal pathogenicity of anti-dsDNA antibodies was proposed more than 20 years ago, namely that certain autoantibodies could penetrate into living cells and thus induce damage. Work from several laboratories has recently provided firm support for this iconoclastic theory, which contradicted prevailing immunologic dogma that cell interiors are inaccessible to antibodies. Here, we review the evidence that anti-dsDNA antibodies may penetrate into living cells, and discuss which intracellular events may follow from binding of anti-dsDNA antibodies to the cell surface and subsequent intracellular penetration. Determining the mechanism by which anti-dsDNA antibodies induce renal injury is important for understanding a major disease manifestation of lupus, and may lead to the development of novel approaches to the treatment of lupus renal disease.

Slipped (CTG).(CAG) repeats of the myotonic dystrophy locus: surface probing with anti-DNA antibodies

At least 15 human diseases have been associated with the length-dependent expansion of gene-specific (CTG).(CAG) repeats, including myotonic dystrophy (DM1) and spinocerebellar ataxia type 1 (SCA1). Repeat expansion is likely to involve unusual DNA structures. We have structurally characterized such DNA, with (CTG)(n).(CAG)(n) repeats of varying length (n=17-79), by high-resolution gel electrophoresis, and have probed their surfaces with anti-DNA antibodies of known specificities. We prepared homoduplex S-DNAs, which are (CTG)x.(CAG)y where x=y, and heteroduplex SI-DNAs, which are hybrids where x>y or x<y. S-DNAs formed many different species of slipped isomers, as indicated by its multiple electrophoretic species. In contrast, SI-DNAs formed distinct structures, as indicated by the limited electrophoretic species for all possible repeat length pairings. Sister SI-DNAs with an excess of CAG repeats always migrated slower than their sister SI-DNAs with an excess of CTG repeats. Strikingly, both the propensity to form slipped structures and the pattern of S-DNAs, but not SI-DNAs, varied for similar lengths of CTG/CAG repeats between the DM1 and SCA1 loci, highlighting a role for flanking cis-elements in S-DNA but not SI-DNA formation. Slipped structures bound structure and nucleotide-specific anti-DNA antibodies. Binding of anti-B-DNA antibodies was reduced for both S-DNAs and SI-DNAs relative to their linear forms. SI-DNAs bound anti-Z-DNA antibodies, while both S and SI-DNAs bound anti-cruciform antibodies, revealing shared characteristics between the corresponding DNA structures and slipped DNAs. Such features of the repeats may be recognized by cellular proteins known to bind such structures.

Modulation of fine specificity of anti-DNA antibody by CDR3L residues

The light chain of 2C10, an anti-double stranded DNA (dsDNA) autoantibody, is not favorable for DNA binding and it was suggested that the light chain might modulate the specificity of the antibody in DNA binding. We studied several mutant scFvs expressing mutated VL and normal VH of 2C10 to explore the role of the light chain in determining the fine specificity of the antibody, which we define as the preferential binding to a specific sequence of bases or a helical conformation compared to dsDNA from calf thymus. The wild-type Fab and scFv of 2C10 bind to poly(dA-dC).(dG-dT) better than to dsDNA. However, in the absence of the light chain domain, the VH domain bound dsDNA better than poly(dA-dC).(dG-dT), indicating the possible involvement of the light chain in determining the fine specificity in DNA binding. The mutations we studied were located in either CDR1L or CDR3L of the antibody. The CDR1 mutants, D28A, D30A, D31A, and D32A have been previously shown to cause an increase in the affinity of 2C10 scFv to DNA. The fine specificity of 2C10 was not affected by the CDR1 mutants which bound to poly(dA-dC).(dG-dT) better than dsDNA. However, CDR3L mutants, D92A and N93A, which had been shown to be involved in direct interaction with DNA, preferred dsDNA to poly(dA-dC).(dG-dT) in their binding. Our results indicate that the fine specificity of 2C10 in DNA binding is modulated primarily by Asp at 92 and Asn at 93 in CDR3L. The effects of CDR1L mutations indicate that this region affects only the affinity but not the fine specificity of 2C10.

Induction of anti-DNA antibodies by immunization with anti-DNA antibodies: mechanism and characterization

Two well-characterized IgG monoclonal antibodies, reactive with double-stranded (ds) DNA and nucleosomes, were administered to normal BALB/c mice to examine the reproducibility and the biology of a previously reported model of anti-DNA antibody induction by immunization with anti-DNA antibodies. The monoclonal antibodies were purified either with or without a high-salt wash to remove nucleosomal antigens bound to them during the cell culture. Both monoclonal antibodies, but not normal IgG, induced significant IgG anti-dsDNA antibody production from 1 week to 25 weeks after the last immunization. The antibodies produced in this manner possess different binding preferences to ds synthetic polynucleotides than the antibodies used for the immunization, and they did not react with nucleosomes. The monoclonal antibodies purified with the high-salt wash were more effective in anti-DNA antibody induction than those purified without the high-salt wash. Even when bound to these monoclonal antibodies, neither dsDNA, nucleosomes, or ds synthetic polynucleotides exert significant antigenicity. For example, anti-DNA antibodies produced by mice immunized with an immune complex formed by poly(dA-dT) and one of the monoclonal antibodies that has a high affinity to this polynucleotide did not show an increased affinity to poly(dA-dT). Together, these results suggest that anti-DNA antibody molecules or processed antibody peptides, and not DNA/nucleosomes carried by anti-DNA antibodies, play a role in this model of anti-DNA antibody production.

Enhancement of hydroxyl radical DNA cleavage by serum anti-dsDNA antibodies in SLE

Autoantibodies reactive with double-stranded (ds) DNA are a hallmark of systemic lupus erythematosus (SLE), but it is not fully elucidated how these antibodies contribute to the various pathological changes observed in lupus patients. In a previous study, we have found a significant enhancement effect of two murine monoclonal anti-dsDNA antibodies on DNA cleavage by the hydroxyl radical. We have extended this study and have found that purified polyclonal anti-dsDNA antibodies from sera of lupus patients and an MRL/lpr mouse, but not anti-ssDNA antibodies, also enhanced radical cleavage of DNA. The cleavage was inhibited by EDTA, DMSO and thiourea. The antibody preparations per se did not cleave DNA. These results suggest that such an enhancement effect on DNA cleavage is a feature of a significant part of anti-dsDNA antibody population and a possible clue in understanding the roles of anti-dsDNA antibodies in the pathophysiology of SLE under certain circumstances.

Isolation and characterization of apoptotic nucleosomes, free and complexed with lupus autoantibody generated during hybridoma B-cell apoptosis

Increasing evidence suggests that immune complexes made of anti-nuclear antibodies bound to nucleosomes released from dead cells play an important role in the pathogenesis of lupus nephritis. However, the nature and composition of apoptotic nucleosomes still remain elusive. Since large amounts of nucleosomes are released from cells undergoing apoptosis in hybridoma cell cultures, we used hybridomas secreting anti-DNA and anti-nucleosome antibodies grown in protein-free medium to generate nucleosome/anti-DNA and /anti-nucleosome immune complexes, as well as an irrelevant antibody hybridoma to generate free, non-complexed apoptotic nucleosomes. Hybridoma supernatants were fractionated by size-exclusion gel chromatography and eluted fractions with a ratio of A260/A280 >1.2 were pooled and analysed for DNA and histone profiles by gel electrophoresis and immunoblotting. When run on a 'native' gel, 'intact' apoptotic nucleosomes, free or within anti-nucleosome immune complexes, showed a strikingly reduced size compared with 'standard' nucleosomes prepared in vitro by endonuclease digestion of cell nuclei. Nucleosomal DNA (extracted from either free or complexed apoptotic nucleosomes) appeared as a major band of 160-180 bp, and had the size of 'standard' mononucleosome DNA, suggesting degradation of the histone moiety of apoptotic nucleosomes. Histone immunoblotting revealed degradation of histones H3 and H4, which was dramatically enhanced when apoptotic nucleosomes were complexed with an anti-nucleosome antibody. Our results provide direct evidence for abnormal histone composition of apoptotic nucleosomes and suggest that the fine specificity of the complexing antibody has an influence on complexed nucleosome composition.

Ferritin is a developmentally regulated nuclear protein of avian corneal epithelial cells

Previously, we generated monoclonal antibodies against chicken corneal cells (Zak, N. B., and Linsenmayer, T. F. (1983) Dev. Biol. 99, 373). We have now observed that one group of these antibodies reacts with a developmentally regulated component of corneal epithelial cell nuclei. This component is the heavy chain of ferritin, as determined by analyses of immunoisolated cDNA clones and immunoblotting of the protein. Immunoblotting also suggests that the nuclear ferritin may be in a supramolecular form that is similar to the iron-binding ferritin complex found in the cytoplasm of many cells. In vitro cultures and transfection studies show that the nuclear localization depends predominantly on cell type but can be altered by the in vitro environment. The appearance of nuclear ferritin is at least partially under translational regulation, as is known to be true for the cytoplasmic form of the molecule. The tissue and developmental distributions of the mRNA for the molecule are much more extensive than the protein itself, and the removal of iron from cultures of corneal epithelial cells with the iron chelator deferoxamine prevents the appearance of nuclear ferritin. At present the functional role(s) of nuclear ferritin remain unknown, but previous studies on cytoplasmic ferritin raise the possibility that it prevents damage due to free radical generation ("oxidative stress") by sequestering iron. Although it remains to be tested whether nuclear ferritin prevents oxidative damage, we find this an attractive possibility. Since the corneal epithelium is transparent and is constantly exposed to free radical-generating UV light, it is possible that the cells of this tissue have evolved a specialized mechanism to prevent oxidative damage to their nuclear components.