Monoclonal antibodies derived from BALB/c mice immunized with apoptotic Jurkat T cells recognize known autoantigens

Specific post-translational histone modifications of neutrophil extracellular traps as immunogens and potential targets of lupus autoantibodies

Introduction

Autoreactivity to histones is a pervasive feature of several human autoimmune disorders, including systemic lupus erythematosus (SLE). Specific post-translational modifications (PTMs) of histones within neutrophil extracellular traps (NETs) may potentially drive the process by which tolerance to these chromatin-associated proteins is broken. We hypothesized that NETs and their unique histone PTMs might be capable of inducing autoantibodies that target histones.

Methods

We developed a novel and efficient method for the in vitro production, visualization, and broad profiling of histone-PTMs of human and murine NETs. We also immunized Balb/c mice with murine NETs and profiled their sera on autoantigen and histone peptide microarrays for evidence of autoantibody production to their immunogen.

Results

We confirmed specificity toward acetyl-modified histone H2B as well as to other histone PTMs in sera from patients with SLE known to have autoreactivity against histones. We observed enrichment for distinctive histone marks of transcriptionally silent DNA during NETosis triggered by diverse stimuli. However, NETs derived from human and murine sources did not harbor many of the PTMs toward which autoreactivity was observed in patients with SLE or in MRL/lpr mice. Further, while murine NETs were weak autoantigens in vivo, there was only partial overlap in the immunoglobulin G (IgG) and IgM autoantibody profiles induced by vaccination of mice with NETs and those seen in patients with SLE.

Conclusions

Isolated in vivo exposure to NETs is insufficient to break tolerance and may involve additional factors that have yet to be identified.

Autoantigenesis: the evolution of protein modifications in autoimmune disease

Protein targets in autoimmune disease vary in location, originating within cells as in system lupus erythematosus (SLE), or found on cell surfaces or in extracellular spaces. The term 'autoantigenesis' is first defined here as the changes that arise in self-proteins as they break self-tolerance and trigger autoimmune B and/or T cell responses. As illustrated in many studies, between 50 and 90% of the proteins in the human body acquire post-translational modification. In some cases, it may be that these modifications are necessary for the biological functions of proteins of the cells in which they reside or as extracellular mediators. Summarized herein, it is clear that some post-translational modifications can create new self-antigens by altering immunologic processing and presentation. While many protein modifications exist, we will focus on those created, amplified, or altered in the context of inflammation or other immune system responses. Finally, we will address how post-translational modifications in self-antigens may affect the analyses of B and T cell specificity, current diagnostic techniques, and/or the development of immunotherapies for autoimmune diseases.

Improving seroreactivity-based detection of glioma

Seroreactivity profiling emerges as valuable technique for minimal invasive cancer detection. Recently, we provided first evidence for the applicability of serum profiling of glioma using a limited number of immunogenic antigens. Here, we screened 57 glioma and 60 healthy sera for autoantibodies against 1827 Escherichia coli expressed clones, including 509 in-frame peptide sequences. By a linear support vector machine approach, we calculated mean specificity, sensitivity, and accuracy of 100 repetitive classifications. We were able to differentiate glioma sera from sera of the healthy controls with a specificity of 90.28%, a sensitivity of 87.31% and an accuracy of 88.84%. We were also able to differentiate World Health Organization grade IV glioma sera from healthy sera with a specificity of 98.45%, a sensitivity of 80.93%, and an accuracy of 92.88%. To rank the antigens according to their information content, we computed the area under the receiver operator characteristic curve value for each clone. Altogether, we found 46 immunogenic clones including 16 in-frame clones that were informative for the classification of glioma sera versus healthy sera. For the separation of glioblastoma versus healthy sera, we found 91 informative clones including 26 in-frame clones. The best-suited in-frame clone for the classification glioma sera versus healthy sera corresponded to the vimentin gene (VIM) that was previously associated with glioma. In the future, autoantibody signatures in glioma not only may prove useful for diagnosis but also offer the prospect for a personalized immune-based therapy.

Apoptotic lymphocytes treated with IgG from Trypanosoma cruzi infection increase TNF-alpha secretion and reduce parasite replication in macrophages

Phagocytic removal of apoptotic lymphocytes exacerbates replication of Trypanosoma cruzi in macrophages. We investigated the presence of Ab against apoptotic lymphocytes in T. cruzi infection and the role of these Ab in parasite replication. Both control and chagasic serum contained IgG Ab that opsonized apoptotic lymphocytes. Treatment of apoptotic lymphocytes with purified IgG from chagasic, but not control serum, reduced T. cruzi replication in macrophages. The protective effect of chagasic IgG depended on Fcgamma receptors, as demonstrated by the requirement for the intact Fc portion of IgG, and the effect could be abrogated by treating macrophages with an anti-CD16/CD32 Fab fragment. Chagasic IgG displayed increased reactivity against a subset of apoptotic cell Ag, as measured by flow cytometry and immunoblot analyses. Apoptotic lymphocytes treated with chagasic IgG, but not control IgG, increased production of TNF-alpha, while decreasing production of TGF-beta1 by infected macrophages. Increased control of parasite replication required TNF-alpha production. Previous immunization with apoptotic cells or injection of apoptotic cells opsonized with chagasic IgG reduced parasitemia in infected mice. These results indicate that Ab raised against apoptotic cells could play a protective role in control of T. cruzi replication by macrophages.

Murine Lupus Autoantibodies Identify Distinct Subsets of Apoptotic Bodies

The specific modification of autoantigens and their redistribution into blebs at the surface of apoptotic cells contribute to the induction of autoimmune responses. Blebs containing fragments of the apoptotic nucleus separate from the remainder of the cell to form membrane-bound sub-cellular particles (SCPs), otherwise known as apoptotic bodies. To determine whether apoptotic bodies containing nuclear antigens represent a defined subset of SCPs, we examined the heterogeneity of particles generated by Jurkat cells following synchronization of the cell cycle by serum withdrawal and inhibition of topoisomerase I by camptothecin. Particles were purified by filtration, incubated in the presence of antinucleosome or anti-cardiolipin autoantibodies, annexin V, and Sytox Orange and analyzed by flow cytometry and confocal microscopy. We demonstrate that nuclear autoantigens are associated with one clearly defined subset of SCPs that can be distinguished from other products of late apoptosis. Our experiments represent an important step towards characterizing the heterogeneity of SCPs that are generated in late apoptosis and identifying their contributions to tolerance and autoimmunity.

Identification of annexin 1 as a novel autoantigen in acute exacerbation of idiopathic pulmonary fibrosis

Consistent with the hypothesis that pulmonary epithelial apoptosis is the key to the acute exacerbation of idiopathic pulmonary fibrosis (IPF), we conducted serological identification of Ags by recombinant expression cloning (SEREX) analysis using type II alveolar cell carcinoma (A549) cell lines to identify disease-related Abs. In a survey of Abs to the recombinant autoantigens identified by SEREX analysis, five Abs were identified as novel candidates for the acute exacerbation of IPF. Abs to annexin 1 were detected in 47 and 53% of the sera and bronchoalveolar lavage materials from patients with acute exacerbation of IPF. Some identical TCR Vbeta genes were identified in sequential materials obtained at 1-3 mo in all 10 acute exacerbation IPF cases, suggesting that some infiltrating CD4-positive T cells sharing limited epitopes expand by Ag-driven stimulation during disease extension. The CDR3 region of these identical TCR Vbeta genes showed high homology with the N-terminal portion of annexin 1, including in the HLA-DR ligand epitopes predicted by TEPITOPE analysis. By Western blotting analysis and observation of the CD4-positive T cell responses in bronchoalveolar lavage samples, the N-terminal portion of annexin 1 was cleaved and found to induce marked proliferative responses of CD4-positive T cells in three patients. Our study demonstrates that annexin 1 is an autoantigen that raises both Ab production and T cell response in patients with acute exacerbation of IPF, and that the N-terminal portion of annexin 1 plays some role in the pathogenesis of acute exacerbation in IPF patients.

Development of autoimmunity in mice lacking DNA topoisomerase 3beta

Mice lacking DNA topoisomerase 3beta are predisposed to a shortened lifespan, infertility, and lesions in multiple organs resulting from inflammatory responses. Examination of the immune system of 6- and 52-week-old top3beta(-/-) mice revealed no significant aberrations in their central and peripheral tolerance or in T lymphocyte activation. However, the older but not the younger cohort shows a high incidence of serum autoantibodies relative to their TOP3beta(+/+) age-mates. The mutant mice also show an increase in numerical aberrations of chromosomes in splenocytes and bone marrow cells, as well as an increase in apoptotic cells in the thymus. Thus, it appears plausible that the inflammatory lesions in top3beta(-/-) mice are caused by the development of autoimmunity as they age: Chromosomal abnormalities in top3beta(-/-) mice might lead to a persistent increase in apoptotic cells, which might in turn lead to the progression of autoimmunity.

Cell surface expression of intermediate filament proteins vimentin and lamin B1 in human neutrophil spontaneous apoptosis

Neutrophils represent an important source of autoantigens for antineutrophil cytoplasmic antibody associated with vasculitis. To date, two cytoskeletal proteins, vinculin and vimentin, have been reported to be expressed on the cell surfaces of activated macrophages, platelets, and apoptotic T lymphocytes. However, such cell surface expression has never been studied in human neutrophils. As we recently demonstrated that different cytoskeletal proteins were cleaved in apoptotic neutrophils, we hypothesized that some of these were expressed on the cell surface of apoptotic neutrophils. Herein, we found that among vinculin, paxillin, gelsolin, vimentin, lamin B1, alpha-tubulin, and beta-tubulin, only the two intermediate filament (INFIL) proteins, vimentin and lamin B1, are expressed on the cell surface of 24-h aged neutrophils [spontaneous apoptosis (SA)]. By monitoring intracellular expression of vimentin and lamin B1 during SA, we found that these two proteins were cleaved and that such cleavage was reversed by the pan caspase inhibitor N-benzyloxy-carbonyl-V-A-D-O-methylfluoromethyl ketone (z-VAD-fmk). When neutrophil apoptosis was delayed or suppressed by lipopolysaccharide or the cytokines granulocyte-colony stimulating factor (G-CSF), granulocyte macrophage (GM)-CSF, or interleukin-4, the loss of intracellular expression of vimentin and lamin B1 was prevented. The INFIL proteins were absent from the cell surface when neutrophil apoptosis was delayed. Addition of z-VAD-fmk significantly decreased the cell surface expression of vimentin and lamin B1 during SA. This study provides the first evidence that apoptotic neutrophils express cytoskeletal proteins on their surface, opening the possibility that these cells may participate in the development of autoantibodies directed against cytoskeletal proteins, a condition frequently reported in several inflammatory diseases.

Clinical proteomics in lung diseases

Proteomics is a relatively new approach for understanding the pathology and pathogenesis of various diseases. It has also been used for characterizing the modifications in protein expression during the development of interstitial lung diseases, in lung tumors, or following exposure to exogenous stress signals. We compared the protein composition of primary human lung fibroblasts derived from patients with lung fibrosis and control fibroblasts of unaffected lung tissues. We found a predominant modulation in proteins related to the cytoskeleton, including decreased expression of vimentin and lamin A/C, and increased expression of moesin. Furthermore, we observed lower levels of components of the antioxidative system, such as omega class glutathione S-transferase and an up-regulation of an intracellular chloride channel. In fibroblasts obtained from fibrotic lungs, the expression of a major histocompatibility complex class I C was decreased, and so was the expression of tripeptidyl-peptidase-I-precursor, a collagen-degrading exopeptidase. Our results and the studies reviewed in this paper represent just the beginning of detailed studies that should unravel the relevance and the functional consequences of differential protein expressions in the diseased lung.

Analysis of the autoantibody repertoire in Burkitt's lymphoma patients: frequent response against the transcription factor ATF-2

In the last few years, serological identification of tumour-associated antigens (TAAs) by recombinant cDNA expression cloning (SEREX) has enabled the mapping of humoral immune responses against TAAs in various types of cancer. The present paper describes the application of SEREX to Burkitt's lymphoma (BL), a malignancy not previously characterized by SEREX. By using a cDNA library from a BL cell line that does not express IgG, technical difficulties related to background immunoglobulin clones were overcome. Screening with sera from three BL patients revealed immunoreactivity against seven different gene products, six of which represent known human genes. Five proteins had previously been characterized by SEREX in other malignancies or identified as targets of autoantibodies in autoimmune disease. Seroreactivity against ATF-2, a member of the AP-1 transcription factor family, was validated by enzyme-linked immunosorbent assay (ELISA) and Western blot analysis using recombinant ATF-2 protein. Autoantibody responses against ATF-2 were detected by ELISA in 6 of 8 BL patients, compared with 6 of 13 patients with T-cell non-Hodgkin's lymphoma (T-NHL), 5 of 23 patients with follicular lymphoma and 2 of 27 diffuse large B-cell lymphoma patients. In contrast, reactivity was found in only 1 of 50 healthy volunteers. Next, we showed by immunohistochemistry that the activated form of ATF2 (ATF-2pp) was highly expressed in six different BL samples. We conclude that the SEREX approach with a B-cell cDNA source is applicable in NHL. Furthermore, we identified genes with possible involvement in the pathogenesis of BL using this technique.

Autoimmunity induced by adjuvant hydrocarbon oil components of vaccine

Adjuvant oils such as Bayol F (Incomplete Freund's adjuvant: IFA) and squalene (MF59) have been used in human and veterinary vaccines despite poor understanding of their mechanisms of action. Several reports suggest an association of vaccination and various autoimmune diseases, however, few were confirmed epidemiologically and the risk of vaccination for autoimmune diseases has been considered minimal. Microbial components, not the adjuvant components, are considered to be of primary importance for adverse effects of vaccines. We have reported that a single intraperitoneal injection of the adjuvant oils pristane, IFA or squalene induces lupus-related autoantibodies to nRNP/Sm and -Su in non-autoimmune BALB/c mice. Induction of these autoantibodies appeared to be associated with the hydrocarbon's ability to induce IL-12, IL-6, and TNF-alpha, suggesting a relationship with hydrocarbon's adjuvanticity. Whether this is relevant in human vaccination is a difficult issue due to the complex effects of vaccines and the fact that immunotoxicological effects vary depending on species, route, dose, and duration of administration. Nevertheless, the potential of adjuvant hydrocarbon oils to induce autoimmunity has implications in the use of oil adjuvants in human and veterinary vaccines as well as basic research.

Nucleosomes in the pathogenesis of systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is characterized by the development of a large array of autoantibodies that primarily are directed against the whole chromatin (antinucleosome) and its individual components, dsDNA and histones. Apoptotic defects and impaired removal of apoptotic cells could contribute to an overload of autoantigens (and in particular of nucleosomes) in circulation or in target tissues that could become available to initiate an autoimmune response. In susceptible individuals, this can lead to autoantibody-mediated tissue damage. In addition to intrinsic or secondary apoptosis/apoptotic cell removal defects, certain apoptotic stimuli (eg, UV, viruses) could lead to posttranscriptional modifications that generate autoantigen cryptic fragments for which cells of the immune system have not been tolerized. Besides their role as a major immunogen in lupus, nucleosomes participate in antibody-mediated renal pathogenicity and act as a bridging molecule that recognizes heparin sulfate/collagen V components of the glomerular basement membrane. New tools that were developed to detect antinucleosome antibodies in the serum of patients (by ELISA) have shown the specificity and the high sensitivity of antinucleosome antibody reactivity in SLE. In particular, antinucleosome could be a useful marker of patients who have SLE and lack anti-dsDNA antibodies, a prognosis marker for imminent relapse, and a diagnosis marker of lupus nephritis.

Apoptosis as a mechanism for cell surface expression of the autoantigen pyruvate dehydrogenase complex

A number of antigens implicated in the pathogenesis of autoimmune diseases including Sjogren's syndrome and systemic lupus erythematosus (SLE) are expressed aberrantly by apoptotic cells. It is also known that apoptogenic proteins are released from the mitochondrial intermembrane space at an early stage during the induction and development of apoptosis. Combination of this evidence led us to test the hypothesis that apoptotic mechanisms provide an explanation for the abnormal expression of the inner mitochondrial enzyme, pyruvate dehydrogenase complex (PDC), observed on the surface of some cells in patients with the autoimmune liver disease primary biliary cirrhosis (PBC). Using one murine and two human cell lines it was found that the induction of apoptosis led to early detection of PDC within the cytoplasm. However, cytochrome c oxidase subunit 4 (COX 4), which is also present on the inner surface of the inner mitochondrial membrane, remained within the mitochondria. Immunoreactive PDC was also detected on the outer surface of the intact plasma membrane of cells sampled after the induction of apoptosis. Serial release of PDC to the cytoplasm and then onto the external surface of the plasma membrane provides direct evidence that the antigen on the cell surface is of mitochondrial origin. Immunoreactivity specific for PDC is strongly implicated in the pathogenesis of PBC, but this autoantigen is normally concealed from the immune system by three membrane systems. Release of PDC onto the cell surface during apoptosis provides a possible route for recognition of this antigen by the immune system which could contribute to both afferent and efferent phases of the disease process.

Apoptosis and immune responses to self

The mystery that surrounds autoimmunity revolves around how the immune system of patients who have systemic autoimmune diseases becomes primed to recognize intracellular antigens, how the autoantibodies thus produced contribute to the pathogenesis of the disease, and how those autoantibodies access their target proteins. By examining the mechanisms that are involved in the normal cellular process of apoptosis, we are beginning to unravel this mystery. The intracellular autoantigen targets of many systemic autoimmune diseases become altered during apoptosis in ways that may change how they are perceived by the immune system. High concentrations of self-antigens, or in the case of viral infection, complexes of foreign and self-antigens, are packaged during generation of apoptotic cells. The packages also may contain altered fragments of self-antigens that have not been encountered previously by the immune system. Under normal circumstances, apoptotic cells are cleared rapidly by macrophages and DCs. The normal consequence of that clearance is that the apoptosis-altered self-antigens are either ignored by the immune system or tolerance to those antigens is maintained. Clearance is achieved through complex mechanisms that enable macrophages and DCs to recognize apoptotic cells as nonthreatening "self" particles. Defects in this process that cause a delay in clearance could change the appearance of apoptotic cells and cause them to be recognized as "foreign invaders," thereby stimulating an inflammatory response that, in turn, activates an immune response to self-antigens. By studying the mechanisms that are involved in recognition and clearance of apoptotic cells, we are uncovering clues to the defects that may underlie the development of systemic autoimmunity.

Impaired Clearance of Apoptotic Cells Induces the Activation of Autoreactive Anti-Sm Marginal Zone and B-1 B Cells

Since apoptotic cell Ags are thought to be a source of self-Ag in systemic lupus erythematosus, we have examined the role of apoptotic cells in the regulation and activation of B cells specific for Sm, a ribonucleoprotein targeted in human and murine lupus. Using Ig-transgenic mice that have a high frequency of anti-Sm B cells, we find that apoptotic cell injection induces a transient splenic B cell response, while simultaneously causing extensive splenic and peritoneal anti-Sm B cell death. In contrast, mice deficient in the clearance of apoptotic cells develop a chronic anti-Sm response beginning at 1-2 mo of age. These mice have expanded marginal zone and B-1 B cell populations and anti-Sm B cells of both types are activated to form Ab-secreting cells. This activation appears to be Ag-specific, suggesting that activation is due to increased availability of apoptotic cell Ags. Since marginal zone and B-1 cells are positively selected, these data suggest a loss of ignorance rather than a loss of tolerance.

Identification of an autoantigen on the surface of apoptotic human T cells as a new protein interacting with inflammatory group IIA phospholipase A2

One of the most studied secreted phospholipases A2 (sPLA2), the group IIA sPLA2, is found at high levels in inflammatory fluids of patients with autoimmune diseases. A characteristic of group IIA sPLA2 is its preference for negatively charged phospholipids, which become exposed on the extracellular leaflet of apoptotic cell membranes. We recently showed that low molecular weight heparan sulfate proteoglycans (HSPGs) and uncharacterized detergent-insoluble binding site(s) contribute to the enhanced binding of human group IIA PLA2 (hGIIA) to apoptotic human T cells. Using matrix-assisted laser desorption/ionization time of flight (MALDI-TOF) mass spectrometry we now identify vimentin as the major HSPG-independent binding protein of hGIIA on apoptotic primary T lymphocytes. Vimentin is partially exposed on the surface of apoptotic T cells and binds hGIIA via its rod domain in a calcium-independent manner. Studies with hGIIA mutants showed that specific motifs in the interfacial binding surface are involved in the interaction with vimentin. The sPLA2 inhibitor LY311727, but not heparin, inhibited this interaction. In contrast, heparin but not LY311727 abrogated the binding of hGIIA to cellular HSPGs. Importantly, vimentin does not inhibit the catalytic activity of hGIIA. Altogether, the results show that vimentin, in conjunction with HSPGs, contributes to the enhanced binding of hGIIA to apoptotic T cells.

CD4+ T cells from (New Zealand Black x New Zealand White)F1 lupus mice and normal mice immunized against apoptotic nucleosomes recognize similar Th cell epitopes in the C terminus of histone H3

We have previously reported that peptide 88-99 of histone H4 represents a minimal T cell epitope recognized by Th cells from nonautoimmune BALB/c (H-2(d/d)) mice immunized with nucleosomes. In this study, we tested a panel of overlapping peptides spanning the whole sequences of H4 and H3 for recognition by CD4(+) T cells from unprimed (New Zealand Black (NZB) x New Zealand White (NZW))F(1) lupus mice (H-2(d/z)). None of the 11 H4 peptides was recognized by CD4(+) T cells from (NZB x NZW)F(1) mice. In contrast, these cells proliferated and secreted IL-2, IL-10, and IFN-gamma upon ex vivo stimulation with H3 peptides representing sequences 53-70, 64-78, and 68-85. Peptides 56-73 and 61-78 induced the production of IFN-gamma and IL-10, respectively, without detectable proliferation, suggesting that they may act as partial agonist of the TCR. Th cells from unprimed BALB/c mice and other lupus-prone mice such as SNF(1) (H-2(d/q)) and MRL/lpr (H-2(k/k)) mice did not recognize any peptides present within the H3 region 53-85. We further demonstrated that immunization of normal BALB/c mice with syngeneic liver nucleosomes and spleen apoptotic cells, but not with nonapoptotic syngeneic cells, induced Th cell responses against several peptides of the H3 region 53-85. Moreover, we found that this conserved region of H3, which is accessible at the surface of nucleosomes, is targeted by Abs from (NZB x NZW)F(1) mice and lupus patients, and contains motifs recognized by several distinct HLA-DR molecules. It might thus be important in the self-tolerance breakdown in lupus.

Interaction of low molecular weight group IIA phospholipase A2 with apoptotic human T cells: role of heparan sulfate proteoglycans

Human group IIA phospholipase A2 (hIIA PLA2) is a 14 kDa secreted enzyme associated with inflammatory diseases. A newly discovered property of hIIA PLA2 is the binding affinity for the heparan sulfate proteoglycan (HSPG) glypican-1. In this study, the binding of hIIA PLA2 to apoptotic human T cells was investigated. Little or no exogenous hIIA PLA2 bound to CD3-activated T cells but significant binding was measured on activated T cells induced to undergo apoptosis by anti-CD95. Binding to early apoptotic T cells was greater than to late apoptotic cells. The addition of heparin and the hydrolysis of HSPG by heparinase III only partially inhibited hIIA PLA2 binding to apoptotic cells, suggesting an interaction with both HSPG and other binding protein(s). Two low molecular weight HSPG were coimmunoprecipitated with hIIA PLA2 from apoptotic T cells, but not from living cells. Treatment of CD95-stimulated T cells with hIIA PLA2 resulted in the release of arachidonic acid but not oleic acid from cells and this release was blocked by heparin and heparinase III. Altogether, these results suggest a role for hIIA PLA2 in the release of arachidonic acid from apoptotic cells through interactions with HSPG and its potential implication in the progression of inflammatory diseases.

Apoptosis in leukemia cells is accompanied by alterations in the levels and localization of nucleolin

Molecular defects in apoptotic pathways are thought to often contribute to the abnormal expansion of malignant cells and their resistance to chemotherapy. Therefore, a comprehensive knowledge of the mechanisms controlling induction of apoptosis and subsequent cellular disintegration could result in improved methods for prognosis and treatment of cancer. In this study, we have examined apoptosis-induced alterations in two proteins, nucleolin and poly(ADP-ribose) polymerase-1 (PARP-1), in U937 leukemia cells. Nucleolin is expressed at high levels in malignant cells, and it is a multifunctional and mobile protein that can shuttle among the nucleolus, nucleoplasm, cytoplasm, and plasma membrane. Here, we report our findings that UV irradiation or camptothecin treatment of U937 cells induced apoptosis and caused a significant change in the levels and localization of nucleolin within the nucleus. Additionally, nucleolin levels were dramatically decreased in extracts containing the cytoplasm and plasma membrane. These alterations could be abrogated by pre-incubation with an inhibitor of PARP-1 (3-aminobenzamide), and our data support a potential role for nucleolin in removing cleaved PARP-1 from dying cells. Furthermore, both nucleolin and cleaved PARP-1 were detected in the culture medium of cells undergoing apoptosis, associated with particles of a size consistent with apoptotic bodies. These results indicate that nucleolin plays an important role in apoptosis, and could be a useful marker for assessing apoptosis or detecting apoptotic bodies. In addition, the data provide a possible explanation for the appearance of nucleolin and PARP-1 autoantibodies in some autoimmune diseases.

Induction of cell death by sera from patients with acute brain injury as a mechanism of production of autoantibodies

OBJECTIVE

To investigate the capacity of blood draining from the central nervous system of patients with acute brain injury to induce cell death, and to determine whether this phenomenon could be a way to induce the production of autoantibodies.

METHODS

The induction of cell death of several human leukemia cell lines cultured in vitro in the presence of serum collected from the brain or the systemic circulation of patients with acute brain injury was analyzed by flow cytometry after staining with annexin V and propidium iodide. The percentages of apoptotic lymphocytes derived directly from the patients were also quantified. To investigate the mechanisms responsible for the induction of cell death, the expression of apoptosis-related molecules, as well as the effect of addition of several molecules known to interfere with apoptosis, was evaluated in the cell cultures. The presence of serum autoantibodies at the time of injury and 6 months later was studied.

RESULTS

Systemic serum and, especially, serum draining from the brain lesions induced the in vitro death of the leukemia cell lines used. Moreover, there were higher percentages of ex vivo dead lymphocytes in regional blood than in systemic blood 48 hours after injury. These effects seemed to be induced by an exogenous and/or endogenous opioid, since they were blocked by the opioid antagonist, naloxone. Furthermore, such effects were mediated by an increased expression of Bax. Importantly, apoptotic Jurkat cells were bound to autoantibodies, and patients with acute brain injury produced serum autoantibodies some months after the injury. However, they did not develop a full autoimmune disease at that time.

CONCLUSION

Serum factors from acute brain injuries induce cell death, both in vivo and in vitro. Apoptotic cells and, even more so, necrotic cells in acute brain injury are potential sources for autoantigen presentation that may stimulate autoimmune responses.

Association of autoantibodies to nuclear lamin B1 with thromboprotection in systemic lupus erythematosus: lack of evidence for a direct role of lamin B1 in apoptotic blebs

OBJECTIVE

To demonstrate the association between autoantibodies to nuclear lamin B1 (aLB1) and protection against thrombosis ("thromboprotection") in patients with systemic lupus erythematosus (SLE), and to elucidate the mechanism by which aLB1 cause thromboprotection in vivo. Since a number of autoantigens in SLE have been localized specifically to the external surface of apoptotic blebs, it was hypothesized that circulating aLB1 may block the procoagulant effect of apoptotic blebs by binding to LB1 displayed at the external bleb surface.

METHODS

A cross-sectional study was performed using serum samples obtained at first evaluation of 259 English Canadian and French Canadian patients from SLE registries at 3 hospitals. A case-control study was performed to analyze the relationship between aLB1 and lupus anticoagulant (LAC) status and thrombotic manifestations between onset of disease and last followup. Reactivity of aLB1 with Jurkat or endothelial cells, which had been induced to undergo apoptosis, was determined by indirect immunofluorescence. Localization of LB1 in apoptotic cells and blebs was analyzed by confocal microscopy and surface labeling of cell membrane proteins.

RESULTS

High-titer aLB1 was restricted to a subset of SLE patients (46 patients), with an overall frequency of 17.8% (range 11.6-24.3% in the 3 centers). LB1 antibodies were significantly associated with LAC but not with antibodies to cardiolipin (aCL) or beta(2)-glycoprotein I (anti-beta(2)GPI). The frequency of thrombosis differed markedly depending on aLB1 and LAC status, as follows: presence of LAC and absence of aLB1 50%, presence of both LAC and aLB1 22.7%, absence of both LAC and aLB1 25.5%, absence of LAC and presence of aLB1, 20.8%. Further subclassification of patients based on aCL and anti-beta(2)GPI status revealed that, in the presence of LAC but in the absence of aCL, anti-beta(2)GPI, and aLB1, the frequency of thrombosis was 40%, whereas in the presence of aLB1, it decreased strikingly, to 9.1%. LB1 was found to be translocated into surface membrane blebs during apoptosis and to be entirely enclosed within the apoptotic bleb plasma membrane of Jurkat and endothelial cells.

CONCLUSION

The presence of aLB1 in SLE patients with LAC essentially nullifies the strong prothrombotic risk associated with LAC. Hence, aLB1 is associated with thromboprotection. Reactivity of aLB1 with apoptotic blebs does not seem to play a direct role in mediating this protection, since LB1 is buried within apoptotic blebs and inaccessible to circulating aLB1. The mechanism by which aLB1 confers thromboprotection in SLE remains to be elucidated.

Orderly Pattern of Development of the Autoantibody Response in (New Zealand White × BXSB)F1 Lupus Mice: Characterization of Target Antigens and Antigen Spreading by Two-Dimensional Gel Electrophoresis and Mass Spectrometry

Immunoblots of a two-dimensional PAGE-separated HL-60 cell proteomic map and mass spectrometry were combined to characterize proteins targeted by autoantibodies produced by male (New Zealand White x BXSB)F(1) (WB) mice that develop lupus and anti-phospholipid syndrome. Analysis of sera sequentially obtained from seven individual mice at different ages showed that six proteins, vimentin, heat shock protein 60, UV excision-repair protein RAD23, alpha-enolase, heterogeneous nuclear ribonucleoprotein L, and nucleophosmin, were the targets of the B cell autoimmune response, and that autoantibodies to them were synthesized sequentially in an orderly pattern that recurred in all the male WB mice analyzed: anti-vimentin first and anti-nucleophosmin last, with anti-RAD23 and anti-heat shock protein 60, then anti-alpha-enolase and anti-heterogeneous nuclear ribonucleoprotein L Abs occuring concomitantly. Anti-vimentin reactivity always appeared before anti-cardiolipin and anti-DNA Abs, suggesting that vimentin is the immunogen initiating the autoimmune process. The pattern of HL-60 proteins recognized by female WB sera differed from that of male sera, indicating that the Y chromosome-linked autoimmune acceleration gene is not an accelerator but a strong modifier of the autoimmune response. Thus, 1) combining two-dimensional PAGE and mass spectrometry constitutes a powerful tool to identify the set of Ags bound by autoantibodies present in a single serum and the whole autoantibody pattern of an autoimmune disease; 2) the diversification of the autoimmune response in male WB mice occurs in a predetermined pattern consistent with Ag spreading, and thus provides a useful model to further our understanding of the development of the autoantibody response in lupus.

Blebs and apoptotic bodies are B cell autoantigens

Mounting evidence suggests that systemic lupus erythematosus autoantigens are derived from apoptotic cells. To characterize the potential interactions between apoptotic cells and B cells, the D56R/S76R variant of 3H9, a murine autoantibody that binds to DNA, chromatin, and anionic phospholipids, was compared with DNA4/1, a human anti-DNA autoantibody. Flow cytometry revealed that only D56R/S76R bound to Jurkat cells treated with either of three distinct proapoptotic stimuli, Ab binding was dependent on caspase activity, and immunoreactivity developed subsequent to annexin V binding. Confocal microscopy established a structural basis for the distinct kinetics of binding. D56R/S76R preferentially bound to membrane blebs of apoptotic cells, whereas annexin V binding did not require blebs. Inhibition of ROCK I kinase, an enzyme that stimulates nuclear fragmentation and fragment distribution into blebs, significantly reduced Ab binding. Because members of the collectin and pentraxin families of serum proteins bind to blebs on apoptotic cells and assist in the clearance of cellular remains, our results suggest that Abs to blebs could affect the recognition of apoptotic cells by cells of the innate immune system and thus modify tolerance to nuclear Ags.

Posttranslational protein modifications: new flavors in the menu of autoantigens

Perhaps one of the most elusive areas of study in autoimmunity has been identifying the self-antigens that initially trigger the development of autoimmune responses. Recent work in this area has demonstrated that a number of biochemical modifications that arise in proteins after their translation induce autoimmune responses to otherwise ignored self-proteins. This article will describe those autoimmune diseases in which posttranslational modifications may play a role in initiation of disease, as well as identify how these modifications arise and contribute to the breakdown of immune tolerance. Lastly, we will address how posttranslational modifications in self-antigens affect current diagnostic techniques and the development of immunotherapies for autoimmune diseases.

Detection of antivimentin antibody in sera of patients with idiopathic pulmonary fibrosis and non-specific interstitial pneumonia

It has been suggested that the humoral immune system plays a role in the pathogenesis of non-specific interstitial pneumonia (NSIP). Although some circulating autoantibodies to cytoskeletal protein(s) have been suggested, the antimyofibroblast antibody has not been investigated in patients with idiopathic pulmonary fibrosis (IPF) and NSIP. The purpose of this study is to evaluate the existence of antimyofibroblast antibody in the sera of patients with IPF and NSIP. The MRC5 cell line was used as a model of myofibroblast. The anti-MRC5 cell antibody was characterized in a patient with NSIP using Western blotting. Since we found that one of the anti-MRC5 antibodies was an antivimentin antibody, we established an enzyme-linked immunosorbent assay (ELISA) to measure the levels of antivimentin antibody in the sera of patients with IPF (n = 12) and NSIP (n = 23). Initially, two anti-MRC5 cell antibodies were detected in the sera of patients with NSIP, one of which was characterized as the antivimentin antibody by Western blotting. The other was characterized as an antivimentin fragment antibody. We established an ELISA to measure the antivimentin antibody and found significantly higher levels in patients with IPF and NSIP than in normal volunteers. One of the anti-MRC5 cell antibodies in the serum of a patient with NSIP was against vimentin. The serum levels of antivimentin antibody were increased in patients with IPF and NSIP compared with that of normal volunteers. These results suggest that the antivimentin antibody may be involved in the process of lung injury in IPF and NSIP.

Induction of biologically active antineutrophil cytoplasmic antibodies by immunization with human apoptotic polymorphonuclear leukocytes

Translocation of intracellular components to the cell surface during the priming or apoptosis of polymorphonuclear leukocytes (PMN) is an important mechanism for interaction of antineutrophil cytoplasmic antibodies (ANCA) with these antigens. To test the capacity of apoptotic PMN to trigger production of ANCA, six groups of mice were immunized with either live or apoptotic lymphocytes, or with live, apoptotic, formalin-fixed, or lysed PMN. Mice immunized with both live and apoptotic neutrophils developed high titers of antibodies which gave a granular cytoplasmic immunofluorescent pattern. These antibodies were specific for lactoferrin and myeloperoxidase. Following a second intravenous infusion of apoptotic PMNs, mice developed anti-PR3 antibodies. Vasculitis lesions were not found in mice which developed ANCA. The ANCA-containing IgG fraction induced superoxide production by human PMNs. These results support the hypothesis that neutrophil-specific antigens presented on the cell membranes of apoptotic PMN may induce ANCA in the proper conditions.