Time between onset of apoptosis and release of nucleosomes from apoptotic cells: putative implications for systemic lupus erythematosus

Asbestos-induced autoimmunity in C57BL/6 mice

Environmental impacts on autoimmunity have significant public health implications. Epidemiological studies have shown associations between exposure to airborne silicates, such as crystalline silica or asbestos, and autoimmunity, but the etiology remains unclear. The purpose of this study was to test the hypothesis that asbestos could lead to a specific pattern of autoantibodies and pathology indicative of systemic autoimmune disease (SAID). Female C57Bl/6 mice were instilled intratracheally with 2 doses x 60 microg/mouse of amphibole asbestos (tremolite), wollastonite (a non-fibrogenic control fiber), or saline alone. Serum samples were collected and urine was checked for protein bi-weekly for 7 months. By 26 weeks, the asbestos-instilled animals had a significantly higher frequency of positive anti-nuclear antibody (ANA) tests compared to wollastonite and saline groups. The majority of positive ANAs showed homogeneous or combined homogeneous/speckled patterns, and tested positive for antibodies to dsDNA and SSA/Ro 52. Serum isotyping showed no significant changes in IgM, IgA, or IgG subclasses. However, there was an overall decrease in the mean IgG serum concentration in asbestos-instilled mice. IgG immune complex deposition was demonstrated in the kidneys of asbestos-instilled mice, with evidence of glomerular and tubule abnormalities suggestive of glomerulonephritis. Flow cytometry demonstrated moderate changes in the percentages of CD25+ T-suppressor cells and B1a B-cells in the superficial cervical lymph nodes of the asbestos-instilled mice. These data demonstrate that asbestos leads to immunologic changes consistent with the development of autoimmunity. This study provides a non-autoimmune prone murine model for use in future elucidation of mechanisms involved in asbestos-induced autoimmune disease.

Engulfed cell remnants, and not cells undergoing apoptosis, constitute the LE-cell phenomenon

The LE cell has been one of the first immunological signs of active systemic lupus erythematosus, included into the ACR criteria. LE cells consist of a phagocyte engulfing material of disputed origin, which was interpreted as either cellular remnants from necrotic cells or as early apoptotic cells. It is well established that LE cell formation is dependent on autoantibodies against the linker histone H1. In view of this fact, we investigated whether anti-histone H1 antibodies and LE cell positive sera bound to cells where apoptosis had been induced by gliotoxin or actinomycin D or which were necrotic after heating. Necrotic cell remnants, but not (early) apoptotic cells were bound by anti-histone H1 antibodies and LE cell positive sera, establishing that the process of LE cell formation, which is dependent on anti-H1 binding, leads to engulfment of necrotic (or late apoptotic) material, but not of early apoptotic cells.

Autoimmunity versus tolerance: can dying cells tip the balance?

Apoptosis is a physiological process of self-destruction for cells that are damaged or programmed to die. Apoptosis occurs through a series of regulated events that allow cellular debris to be contained and efficiently phagocytosed without initiating a proinflammatory immune response. Recent data have linked physiological apoptosis and the uptake of apoptotic cells by macrophages and some subsets of dendritic cells to the maintenance of peripheral immune tolerance. However, when cells die through necrosis, spilling their intracellular contents, or are infected with various pathogens, activation of antigen-presenting cells and induction of an immune response can occur. Receptors for extrinsic pathogen-associated structures, such as membrane bound Toll-like receptors (TLRs) or intracellular Nod-like receptors (NLRs) can also respond to cross-reactive host molecules from dying cells and may focus autoimmune responses onto these antigens. Several autoimmune disorders have been linked to defects in the apoptotic process. Defective apoptosis of immune cells leads to autoimmunity, as in autoimmune lymphoproliferative syndrome (ALPS) associated with mutations in the death receptor Fas. Defective clearance of apoptotic cell debris can also lead to autoantibody production. We will discuss how cell death and apoptotic cell clearance may affect the finely tuned balance between peripheral immune tolerance and autoimmunity.

Nucleosomal DNA Fragments in Autoimmune Diseases

The inadequate response of immune cells to circulating apoptotic products, such as nucleosomal DNA fragments, is assumed to be a potent stimulus for the production of autoantibodies during the pathogenesis and progression of systemic lupus erythematosus (SLE). Here, we analyzed the levels of circulating nucleosomes, caspases, and C-reactive protein in sera of 244 individuals with various autoimmune diseases (155 with autoimmune hepatic disorders, 25 with ANCA-associated vasculitis, and 64 with various connective tissue diseases), and 32 healthy controls. Nucleosomes and caspase activities were significantly elevated in sera of patients with hepatic autoimmune diseases, connective tissue diseases, and particularly in ANCA-associated vasculitis when compared with healthy individuals. Nucleosomes showed a correlation with caspases, and caspases with C-reactive protein, but nucleosomes did not correlate with C-reactive protein. Serum levels of the apoptotic products, nucleosomes, and caspases are increased in various autoimmune diseases but may not be solely responsible for antinucleosome antibody production in SLE patients. It remains to be clarified whether qualitative changes in nucleosomes are linked with pathogenesis and disease progression in SLE.

Nucleosomes in colorectal cancer patients during radiochemotherapy

Apoptotic markers and tumor-associated antigens might be suitable to indicate the response to radiochemotherapy early. We analyzed the courses of nucleosomes, CEA, CA 19-9 and CYFRA 21-1 in 25 colorectal cancer patients during radiochemotherapy (4 postoperative, 13 preoperative, 8 local relapse therapy). Blood was taken before therapy, daily during the first week, once weekly during the following weeks, and at the end of the radiochemotherapy. After a temporary decline 6 h after the first irradiation, nucleosomes rose in most patients rapidly reaching a maximum during the first days which was followed by a subsequent decrease. In patients receiving postoperative therapy after complete resection of tumor, nucleosome levels generally were lower than in patients with preoperative or relapse therapy. Correspondingly, CEA, CA 19-9 and CYFRA 21-1 levels of postoperatively treated patients were the lowest whereas those with tumor relapse had the highest ones. During preoperative therapy, lower nucleosome concentrations were found in patients with response to therapy resulting in a smaller area under the curve of days 1-3 (AUC) than in those with progressive disease (p = 0.028). The other parameters did not indicate the response to therapy at the initial treatment phase. In conclusion, the course of nucleosomes (AUC) might be valuable for the early prediction of therapy response in preoperatively treated colorectal cancer patients.

Early prediction of therapy response in patients with acute myeloid leukemia by nucleosomal DNA fragments

BACKGROUND

Elevated levels of nucleosomal DNA fragments can be detected in plasma and sera of patients with malignant diseases.

METHODS

We investigated the course of nucleosomal DNA, thymidine kinase, lactate dehydrogenase and leukocytes in sera of 25 patients with acute myeloid leukemia during the first cycle of induction chemotherapy and tested their power to distinguish between patients with complete remission and those with no remission.

RESULTS

Almost all patients showed strongly decreasing levels of nucleosomal DNA during the first week, in some cases after initial peaks. In overall analysis of variance, DNA levels could clearly distinguish between patients with complete remission, who had higher DNA values, and those with insufficient response (p = 0.017). The area under the curve of DNA values of days 2-4 after start of therapy (AUC 2-4) discriminated between both groups with a sensitivity of 56% at a specificity of 100%. Further, pretherapeutic levels and AUC 2-4 of nucleosomal DNA correlated significantly with blast reduction after 16 days. A tendency to higher levels in patients with complete response was also found for thymidine kinase, lactate dehydrogenase and leukocytes, however the difference did not reach the level of significance (p = 0.542, p = 0.260, and p = 0.144, respectively).

CONCLUSION

Our results indicate that nucleosomal DNA fragments are valuable markers for the early prediction of therapeutic efficacy in patients with acute myeloid leukemia.

The struggle to detect circulating DNA

In various diseases, such as cancer, autoimmune disease, sepsis or myocardial infarction, elevated levels of circulating DNA can be measured. However, its predictive value is under debate. Circulating DNA in plasma is protein-bound (nucleosomal) DNA. Quantification of circulating DNA can be performed by real-time quantitative PCR or immunological methods such as ELISA. The diagnostic value of both methods can be impaired by inappropriate handling of the samples. Assessment of circulating DNA in patients admitted to the intensive care unit offers a tool for predicting morbidity and mortality.

Modelling the onset of Type 1 diabetes: can impaired macrophage phagocytosis make the difference between health and disease?

A wave of apoptosis (programmed cell death) occurs normally in pancreatic beta-cells of newborn mice. We previously showed that macrophages from non-obese diabetic (NOD) mice become activated more slowly and engulf apoptotic cells at a lower rate than macrophages from control (Balb/c) mice. It has been hypothesized that this low clearance could result in secondary necrosis, escalating inflammation and self-antigen presentation that later triggers autoimmune, Type 1 diabetes (T1D). We here investigate whether this hypothesis could offer a reasonable and parsimonious explanation for onset of T1D in NOD mice. We quantify variants of the Copenhagen model (Freiesleben De Blasio et al. 1999 Diabetes 48, 1677), based on parameters from NOD and Balb/c experimental data. We show that the original Copenhagen model fails to explain observed phenomena within a reasonable range of parameter values, predicting an unrealistic all-or-none disease occurrence for both strains. However, if we take into account that, in general, activated macrophages produce harmful cytokines only when engulfing necrotic (but not apoptotic) cells, then the revised model becomes qualitatively and quantitatively reasonable. Further, we show that known differences between NOD and Balb/c mouse macrophage kinetics are large enough to account for the fact that an apoptotic wave can trigger escalating inflammatory response in NOD, but not Balb/c mice. In Balb/c mice, macrophages clear the apoptotic wave so efficiently, that chronic inflammation is prevented.

Accelerated macrophage apoptosis induces autoantibody formation and organ damage in systemic lupus erythematosus

Increased monocyte/macrophage (Mphi) apoptosis occurs in patients with systemic lupus erythematosus (SLE) and is mediated, at least in part, by an autoreactive CD4(+) T cell subset. Furthermore, autoreactive murine CD4(+) T cells that kill syngeneic Mphi in vitro induce a lupus-like disease in vivo. However, it is unclear whether increased Mphi apoptosis in SLE per se is sufficient to accelerate/promote autoimmunity. We have investigated whether increased Mphi apoptosis in vivo, induced by the administration of clodronate liposomes, can exacerbate the autoimmune phenotype in NZB x SWR (SNF(1)) lupus-prone mice, and induce autoantibody production in haplotype-matched BALB/c x DBA1 (DBF(1)) non-lupus-prone mice. Lupus-prone mice SNF(1) mice that were treated with clodronate liposomes, but not mice treated with vehicle, developed significant increases in autoantibodies to dsDNA, nucleosomes, and the idiotypically related family of nephritic Abs Id(LN)F(1), when compared with untreated SNF(1) mice. Furthermore, clodronate treatment hastened the onset of proteinuria and worsened SNF(1) lupus nephritis. When compared with vehicle-treated controls, clodronate-treated non-lupus-prone DBF(1) mice developed significantly higher levels of anti-nucleosome and Id(LN)F(1) Abs but did not develop lupus nephritis. We propose that Mphi apoptosis contributes to the pathogenesis of autoantibody formation and organ damage through both an increase in the apoptotic load and impairment in the clearance of apoptotic material. This study suggests that mechanisms that induce scavenger cell apoptosis, such as death induced by autoreactive cytotoxic T cells observed in SLE, could play a pathogenic role and contribute to the severity of the disease.

The abnormal apoptosis of T cell subsets and possible involvement of IL-10 in systemic lupus erythematosus

To study the apoptosis of lymphocyte subpopulations in systemic lupus erythematosus (SLE) patients and the possible role of IL-10 in this apoptosis involved in the pathogenesis of SLE, three color fluorescence and flow cytometry were used to investigate the early apoptosis of lymphocyte subsets from freshly separated or cultured peripheral blood mononuclear cells (PBMCs). ELISA was employed to detect the levels of IL-10 in serum and the levels of sFas and sFasL in cultured PBMC supernatants, and the results of sFas and sFasL were confirmed by real-time PCR of Fas and FasL mRNA. The results showed that in cells from SLE patients, the apoptosis of CD3+, CD4+, and CD8+ T cells was distinctly increased, and the percentage of CD4+ cells and the CD4/CD8 ratio was significantly decreased, as compared with normal controls. The apoptosis of T lymphocytes cultured with SLE serum was markedly higher than that of cells cultured with control's serum. Blockade of interleukin-10 (IL-10) activation by an anti-IL-10 antibody reduced the SLE serum induced apoptosis of CD4+ and CD8+ T cells. The levels of sFas and sFasL in the culture supernatant and Fas and FasL mRNA expressions in cultured cells were significantly higher in the SLE serum-cultured groups, but decreased evidently in the presence of the anti-IL-10 antibody. Above findings suggested that SLE cells showed abnormally high apoptosis of T lymphocytes, especially of the CD4+ subpopulation, resulting in a decreased CD4/CD8 ratio. The high percentage of apoptotic T cells in SLE patients may be related to the high levels of IL-10 in SLE serum, as IL-10 may induce the abnormally activated T cells to trigger apoptosis via the Fas-FasL pathway.

Nucleosomes in pancreatic cancer patients during radiochemotherapy

Nucleosomes appear spontaneously in elevated concentrations in the serum of patients with malignant diseases as well as during chemo- and radiotherapy. We analyzed whether their kinetics show typical characteristics during radiochemotherapy and enable an early estimation of therapy efficacy. We used the Cell Death Detection Elisa plus (Roche Diagnostics) and investigated the course of nucleosomes in the serum of 32 patients with a local stage of pancreatic cancer who were treated with radiochemotherapy for several weeks. Ten of them received postsurgical therapy, 21 received primary therapy and 1 received therapy for local relapse. Blood was taken before the beginning of therapy, daily during the first week, once weekly during the following weeks and at the end of radiochemotherapy. The response to therapy was defined according to the kinetics of CA 19-9: a decrease of CA 19-9 > or =50% after radiochemotherapy was considered as 'remission'; an increase of > or =100% (which was confirmed by two following values) was defined as 'progression'. Patients with 'stable disease' ranged intermediately. Most of the examined patients showed a decrease of the concentration of nucleosomes within 6 h after the first dose of radiation. Afterwards, nucleosome levels increased rapidly, reaching their maximum during the following days. Patients receiving postsurgery, primary or relapse therapies did not show significant differences in nucleosome values during the time of treatment. Single nucleosome values, measured at 6, 24 and 48 h after the application of therapy, could not discriminate significantly between patients with no progression and those with progression of disease. However, the area under the curve of the first 3 days, which integrated all variables of the initial therapeutic phase, showed a significant correlation with the progression-free interval (p=0.008). Our results indicate that the area under the curve of nucleosomes during the initial phase of radiochemotherapy could be valuable for the early prediction of the progression-free interval.

Release of DNA from dead and dying lymphocyte and monocyte cell lines in vitro

DNA is a nuclear macromolecule that circulates in the blood where its levels can reflect the activity of inflammatory and malignant diseases. While dead and dying cells have usually been considered the source of blood DNA, the mechanisms for its release during apoptosis and necrosis are not well defined. To elucidate DNA release, an in vitro model system was used, assessing DNA in the media of living, apoptotic or necrotic Jurkat and U937 cells. Apoptosis was induced by etoposide, camptothecin or staurosporine, while necrosis was induced by heating at 56 degrees C. DNA release was measured by fluorometry with the dye PicoGreen while the extent of death was measured by fluorescence-activated cell sorter analysis with propidium iodide and annexin. Apoptotic Jurkat cells released significantly more DNA in the media than untreated cells while necrotic cells did not show significant DNA release. U937 cells showed similar findings. Pretreatment of Jurkat cells with z-VAD-fmk, a caspase inhibitor, reduced both apoptosis and DNA release. By gel electrophoresis, extracellular DNA from apoptotic cells showed laddering with low molecular weight fragments. These studies suggest that extracellular release of DNA is a consequence of apoptosis and may account for some of the DNA in the blood.

The role of complement in the development of systemic lupus erythematosus

Complement has both beneficial and deleterious roles in the pathogenesis of systemic lupus erythematosus (SLE). On the one hand, patients with SLE present with decreased complement levels and with complement deposition in inflamed tissues, suggestive of a harmful role of complement in the effector phase of disease. On the other hand, homozygous deficiency of any of the classical pathway proteins is strongly associated with the development of SLE. There are two main hypotheses to explain these observations. The first invokes an important role for complement in the physiological waste-disposal mechanisms of dying cells and immune complexes. The second hypothesis is based around the role of complement in determining the activation thresholds of B and T lymphocytes, with the proposal that complement deficiency causes incomplete maintenance of peripheral tolerance. These two hypotheses are not mutually exclusive. In addition, there is evidence for a contribution from other genetic factors in determining the phenotype of disease in the absence of complement.

DNA as a marker of cell death in systemic lupus erythematosus

DNA circulates in the blood in systemic lupus erythematosus, among other conditions, and plays a role in immunopathogenesis in the form of immune complexes. As shown in experiments in mice, blood DNA levels rise following treatments to induce apoptosis and the administration of cells made apoptotic or necrotic in vitro. In mice lacking macrophage function, however, blood levels do not rise following administration of dead cells. These results indicate that circulating DNA may be a marker of cell death, although its levels likely reflect a complex process involving the interactions of macrophages with dead and dying cells.

Accumulation of plasma nucleosomes upon treatment with anti-tumour necrosis factor-alpha antibodies

OBJECTIVE

Patients undergoing anti-tumour necrosis factor-alpha (TNF-alpha) treatment often develop autoantibodies. Apoptotic cell antigens are a potential initiating stimulus for autoantibodies. Our goal was to verify whether anti-cytokine therapy causes the release of nucleosomes, a major autoantigen generated during cell death.

DESIGN

Laboratory research study with comparison group.

SETTING

Clinical Immunology Unit and Lab, H San Raffaele University Hospital, Italy.

SUBJECTS

Eleven healthy controls and 87 rheumatic patients were studied, including 51 with rheumatoid arthritis (RA) and 33 patients with systemic lupus erythematosus (SLE).

INTERVENTIONS

Vein blood samples were taken via the antecubital vein. Blood was retrieved from 11 patients before and after injection of anti-TNF-alpha humanized antibodies. Nucleosomes were measured with an enzyme-linked immunosorbent assay. Cell death induced by anti-TNF-alpha antibodies and by the soluble cytokine was assessed in vitro.

MAIN OUTCOME MEASURES

Nucleosome level by treatment.

RESULTS

Enzyme-linked immunosorbent assay effectively detected nucleosomes either released by dying cells in vitro or circulating in the plasma. SLE but not RA patients had circulating nucleosomes at the steady state. Eight of 11 patients had significantly higher levels of plasma nucleosomes after infliximab. Minute amounts of TNF-alpha enabled infliximab to induce cell death in vitro.

CONCLUSIONS

The accumulation of nucleosomes possibly fosters the development of autoantibodies in subjects with appropriate genetic backgrounds.

The putative role of apoptosis-modified histones for the induction of autoimmunity in Systemic Lupus Erythematosus

In recent years, it has become evident that Systemic Lupus Erythematosus (SLE) is a disease characterized by an array of autoantibodies directed against the native nucleosome, its DNA component and/or its histone component. Nuclear antigens are generated and released in vivo during apoptosis. A hallmark of apoptosis is the cleavage of chromatin by caspase-activated DNase. This fragmentation occurs at the internucleosomal level and leads to DNA ladder formation classically associated with apoptosis. Thus, dysregulation of DNA fragmentation might be directly linked to the induction of autoimmunity in SLE. In our studies, activated human lymphoblasts contain high amounts of core histones in their cell lysates after apoptosis induction. This accumulation correlated highly with markers of early apoptosis (Annexin V positive, propidium iodide negative), but not with markers of late apoptosis or necrosis. Interestingly, accumulation of core histones or nucleosomes in cell lysates was detected as early as 30 or 60 min after UV irradiation, whereas phosphatidylserine externalization occurred 2 hr after apoptosis induction. Our results suggest that extranuclear accumulation of core histones is a very early event in apoptosis, preceding the externalization of phagocytosis signals on the outer membrane surface of apoptotically dying lymphoblasts. The following review will discuss these results in a broader perspective which includes our hypothesis of how apoptosis dysregulation during early phases may contribute to the induction of autoimmunity against nuclear autoantigens as seen in SLE.