Impaired clearance of apoptotic cells in systemic lupus erythematosus: challenge of T and B cell tolerance

Serum GAS6, sAXL, IL-10, NO, and BCL-2 levels are decreased in patients with Behçet's disease

PURPOSE

Behçet's disease (BD) is an autoimmune chronic systemic inflammatory disease characterized by a versatile clinical spectrum. Growth arrest specific protein 6 (GAS6)/soluble AXL (sAXL) signaling pathway draws attention in the resolution of inflammation, and its deficiency is associated with chronic inflammatory, autoimmune diseases, as well as clearance of apoptotic cells by phagocytes - efferocytosis. In this study, it was aimed to investigate whether GAS6/sAXL, interleukin (IL)-10, nitric oxide (NO), and BCL-2 levels were associated with inflammation and efferocytosis contributes to the pathogenesis of BD.

METHODS

A total of 37 Behçet patients with ocular involvement and 30 healthy control subjects were included in this study. GAS6, sAXL, IL-10, NO, and BCL-2 levels were quantified using enzyme-linked immunosorbent assay (ELISA) method.

RESULTS

Serum GAS6, sAXL, IL-10, NO, and BCL-2 levels were significantly lower in patients with BD compared to the controls (P < 0.005, P < 0.001, P < 0.001, P < 0.001, and P < 0.001, respectively). In correlation analysis, research parameters decreased in patients with BD was significantly correlated with each other: GAS6-IL-10 (r = 0.585, P < 0.001), GAS6-BCL-2 (r = 0.541, P < 0.001), sAXL-BCL-2 (r = 0.696, P < 0.001), IL-10-NO (r = 0.717, P < 0.001), IL-10-BCL-2 (r = 0.759, P < 0.001), and NO-BCL-2 (r = 0.541, P < 0.001).

CONCLUSION

In conclusion, decreased serum BCL-2 level may be an indicator of increased apoptosis in these patients and decreased levels of GAS6/sAXL, IL-10, and NO may indicate insufficient clearance of apoptotic bodies released as a result of increased apoptosis in BD patients.

The role of efferocytosis in neuro-degenerative diseases

Efferocytosis has a critical role in maintaining tissues and organs' homeostasis by removing apoptotic cells. It is essential for human health, and disturbances in efferocytosis may result indifferent illnesses. In case of inadequate clearance of the dead cells, the content in the cells would be released. In fact, it induces some damages to the tissue and leads to the prolonged inflammation, so unsuitable phagocytosis of the apoptotic cells is involved in occurrence as well as expansion of numerous human chronic inflammatory diseases. Studies have shown age dependence of the neuro-degenerative diseases, which are largely due to the neuro-inflammation and the loss of neurons and thus cause the brain's functional disorders. Efferocytosis is coupled to anti-inflammatory responses that contribute to the elimination of the dying neurons in neuro-degenerative diseases, so its disruption may make a risk factor in numerous human chronic inflammatory diseases such as multiple sclerosis, Alzheimer's disease, glioblastoma, and Rett syndrome. This study is a review of the efferocytosis molecular pathways and their role in neuro-degenerative diseases in order to discover a new treatment option to cure patients.

Cell Death in the Kidney

Apoptotic cell death is usually a response to the cell's microenvironment. In the kidney, apoptosis contributes to parenchymal cell loss in the course of acute and chronic renal injury, but does not trigger an inflammatory response. What distinguishes necrosis from apoptosis is the rupture of the plasma membrane, so necrotic cell death is accompanied by the release of unprocessed intracellular content, including cellular organelles, which are highly immunogenic proteins. The relative contribution of apoptosis and necrosis to injury varies, depending on the severity of the insult. Regulated cell death may result from immunologically silent apoptosis or from immunogenic necrosis. Recent advances have enhanced the most revolutionary concept of regulated necrosis. Several modalities of regulated necrosis have been described, such as necroptosis, ferroptosis, pyroptosis, and mitochondrial permeability transition-dependent regulated necrosis. We review the different modalities of apoptosis, necrosis, and regulated necrosis in kidney injury, focusing particularly on evidence implicating cell death in ectopic renal calcification. We also review the evidence for the role of cell death in kidney injury, which may pave the way for new therapeutic opportunities.

Nucleic Acid Immunity in the Pathogenesis of Cutaneous Lupus Erythematosus

Cutaneous lupus erythematosus can be a devastating painful and mutilating disease that is associated with an inflammatory response in the skin driven by type I interferon activation. Clearance defects in the extra- and intracellular space lead to an enhanced prevalence of nucleic acids that represent danger signals for the innate immune system. Self nucleic acids can stimulate DNA and RNA sensors that have originally evolved to ensure viral defense. Their activation can induce a type I interferon dominated response in resident skin cells, macrophages and dendritic cells that subsequently progresses to adaptive immune stimulation. The genetic exploration of rare monogenic type I interferon driven diseases helped to identify these pathogenic concepts. Based on a genetic susceptibility lupus patients are more vulnerable to environmental trigger factors such as UV-irradiation that can provoke inflammation with local tissue destruction and eventually systemic disease. Understanding of these pathogenic concepts is a prerequisite for development of targeted therapies.

The Role of Efferocytosis in Autoimmune Diseases

Apoptosis happens continuously for millions of cells along with the active removal of apoptotic debris in order to maintain tissue homeostasis. In this respect, efferocytosis, i.e., the process of dead cell clearance, is orchestrated through cell exposure of a set of "find me," "eat me," and "tolerate me" signals facilitating the engulfment of dying cells through phagocytosis by macrophages and dendritic cells. The clearance of dead cells via phagocytes is of utmost importance to maintain the immune system tolerance to self-antigens. Accordingly, this biological activity prevents the release of autoantigens by dead cells, thus potentially suppressing the undesirable autoreactivity of immune cells and the appearance of inflammatory autoimmune disorders as systemic lupus erythematous and rheumatoid arthritis. In the present study, the apoptosis pathways and their immune regulation were reviewed. Moreover, efferocytosis process and its impairment in association with some autoimmune diseases were discussed.

Promoter polymorphisms of the TIM-4 gene are correlated with disease activity in patients with systemic lupus erythematosus

Although the TIM gene family plays important roles in immune responses, little is known about TIM regulation in the development of systemic lupus erythematosus (SLE). This study aimed to investigate the association of two TIM-4 single nucleotide polymorphisms (SNPs) rs6874202 (-1419G>A) and rs62382402 (-1609G>A) with SLE susceptibility in a Chinese Han population. The results showed no significant differences between patients with SLE and control group for rs6874202 and rs62382402 (p = .72, .53 respectively). However, the anti-dsDNA levels in serum from SLE patients with GG genotype of TIM-4 gene at -1419 site were significantly higher than those with GA and AA genotype (p = .0335), and C3 levels of SLE patients with GG and GA genotype were much lower than those with AA genotypes (p = .0187). Moreover, the apoptotic cell levels of SLE patients with AA and GG genotypes were significantly higher than those with GA genotypes in patients with SLE (p = .0393). In addition, the C3 concentration of SLE patients with the GG genotype of TIM-4 gene at -1609 site was found to be significantly higher than those with the GA genotype (p = .0129). The results imply that GG genotype of the TIM-4 gene at -1419 site might be associated with the disease activity of SLE.

Increased expression of Mer tyrosine kinase in circulating dendritic cells and monocytes of lupus patients: correlations with plasma interferon activity and steroid therapy

INTRODUCTION

The requirement for the immunoregulatory Mer tyrosine kinase (Mer) for optimal removal of apoptotic cells prompted us to look at its expression in systemic lupus erythematosus (SLE), in which apoptotic cell clearance is abnormal. We compared the levels of expression of Mer in normal human subjects and in patients with SLE.

METHODS

We used flow cytometry of isolated peripheral blood mononuclear cells to compare the levels of Mer on leukocyte subsets. We used a Mer-specific enzyme-linked immunosorbent assay (ELISA) to quantify soluble Mer (sMer) in plasmas.

RESULTS

Monocytes, CD1c⁺ myeloid dendritic cells (mDCs), and plasmacytoid dendritic cells (pDCs) from both normal individuals and from SLE patients expressed Mer. In both normal and SLE patients, the CD14⁺⁺CD16⁺ subpopulation of monocytes expressed the highest levels of Mer, with somewhat lower levels on the CD14(int)CD16⁺ population. Mer levels on CD1c⁺ mDCs and pDCs, and sMer levels in blood were increased in SLE patients compared with controls. In patients, Mer levels on CD14(int)CD16⁺, CD14⁺⁺CD16⁻ monocytes, and CD1c⁺ dendritic cells correlated positively with type I interferon (IFN-I) activity detected in blood. In SLE patients treated with corticosteroids, Mer expression on monocytes correlated with prednisone dose, CD1c⁺ myeloid dendritic cells in patients treated with prednisone had higher levels of Mer expression than those in patients not receiving prednisone.

CONCLUSIONS

We found no global defect in Mer expression in lupus blood. In contrast, we observed increased levels of Mer expression in DC populations, which could represent a response to increased IFN-I in SLE patients. Enhanced Mer expression induced by corticosteroids may contribute to its beneficial effects in SLE.

Epigenetic modulation of RFC1, MHC2TA and HLA-DR in systemic lupus erythematosus: association with serological markers and six functional polymorphisms of one-carbon metabolic pathway

The current study was conducted to elucidate the effect of genetic variations in one-carbon metabolism on the epigenetic regulation of major histocompatibility complex II transactivator (MHC2TA), reduced folate carrier 1 (RFC1/SLC19A1) and human leukocyte antigen (HLA)-DR in systemic lupus erythematosus (SLE). PCR-RFLP/AFLP, bisulfite-sequencing and real-time PCR approaches were used for genetic, epigenetic and expression analysis respectively. SLE cases exhibited elevated plasma homocysteine levels compared to healthy controls (24.93 ± 1.3 vs. 11.67 ± 0.48 μmol/l), while plasma folate levels showed no association (7.10 ± 2.49 vs. 7.64 ± 2.09 ng/ml). The RFC1 80G>A polymorphism showed 1.32-fold risk (95% CI: 1.02-1.72) for SLE, while glutamate carboxypeptidase II (GCPII) 1561C>T showed reduced risk (OR: 0.47, 95% CI: 0.24-0.90). The expression of RFC1 (0.37 ± 0.09 vs. 0.60 ± 0.17) and HLA-DR (0.68 ± 0.17 vs. 0.98 ± 0.02) was down regulated in the SLE cases. The hypermethylation of RFC1 as observed in the current study may contribute for its down regulation. Plasma folate and thymidylate synthase (TYMS) 5'-UTR 28 bp tandem repeat showed an inverse association with methylation of RFC1 and MHC2TA. SLE cases with hypocomplementemia showed hypermethylation of RFC1, hypomethylation/up regulation of MHC2TA and down regulation of HLA-DR. The hypermethylation of MHC2TA and down regulation of RFC1, MHC2TA and HLA-DR were observed in anti-cardiolipin antibody positive SLE cases. The up regulation of RFC1 and HLA-DR was observed in anti-dsDNA antibody positive SLE cases. The hypomethylation/upregulation of RFC1 and MHC2TA was observed in anti-RNP antibody positive cases. To conclude, one-carbon genetic variants influence epigenetic of MHC2TA and RFC1, thus contributing to phenotypic heterogeneity of SLE.

Surface code--biophysical signals for apoptotic cell clearance

Apoptotic cell death and the clearance of dying cells play an important and physiological role in embryonic development and normal tissue turnover. In contrast to necrosis, apoptosis proceeds in an anti-inflammatory manner. It is orchestrated by the timed release and/or exposure of so-called 'find-me', 'eat me' and 'tolerate me' signals. Mononuclear phagocytes are attracted by various 'find-me' signals, including proteins, nucleotides, and phospholipids released by the dying cell, whereas the involvement of granulocytes is prevented via 'stay away' signals. The exposure of anionic phospholipids like phosphatidylserine (PS) by apoptotic cells on the outer leaflet of the plasma membrane is one of the main 'eat me' signals. PS is recognized by a number of innate receptors as well as by soluble bridging molecules on the surface of phagocytes. Importantly, phagocytes are able to discriminate between viable and apoptotic cells both exposing PS. Due to cytoskeleton remodeling PS has a higher lateral mobility on the surfaces of apoptotic cells thereby promoting receptor clustering on the phagocyte. PS not only plays an important role in the engulfment process, but also acts as 'tolerate me' signal inducing the release of anti-inflammatory cytokines by phagocytes. An efficient and fast clearance of apoptotic cells is required to prevent secondary necrosis and leakage of intracellular danger signals into the surrounding tissue. Failure or prolongation of the clearance process leads to the release of intracellular antigens into the periphery provoking inflammation and development of systemic inflammatory autoimmune disease like systemic lupus erythematosus. Here we review the current findings concerning apoptosis-inducing pathways, important players of apoptotic cell recognition and clearance as well as the role of membrane remodeling in the engulfment of apoptotic cells by phagocytes.

Autoimmunity and chronic inflammation - two clearance-related steps in the etiopathogenesis of SLE

Systemic lupus erythematosus (SLE) is an autoimmune disease with very prominent chronic inflammatory aspects that render into multiple symptoms and clinical signs. The precise etiology of SLE remains elusive; however, it is known that its etiopathogenesis is of multifactorial nature. The production of autoantibodies (AAb) targeting double stranded DNA (dsDNA) and other nuclear autoantigens is the main characteristic of this disease. These target antigens are often modified and/or translocated when apoptotic cells undergo secondary necrosis as a consequence of the clearance deficiency in patients with SLE. In healthy individuals, dead and dying cells are rapidly removed by macrophages in an anti-inflammatory context; this does not elicit immune responses. In SLE, apoptotic cells are often not properly cleared; autoantigens leak out, and are subsequently presented to B cells by follicular dendritic cells (FDC) in secondary lymphoid tissues. This defect challenges the peripheral self-tolerance. Autoreactive B cell activation and production of anti-nuclear AAb result as the first step in the etiopathogenesis of SLE. The second step is the formation of immune complexes (IC) with apoptotic cell-derived nuclear remnants either in situ or deposited in various tissues. Nucleic acid-containing IC may also be ingested by phagocytes, which subsequently produce pro-inflammatory cytokines. Both processes result in chronic organ and tissue damage, development and maintenance of the systemic autoimmune disease. In conclusion, clearance deficiency may contribute to SLE in two ways: first, in germinal centres it enables the affinity maturation of autoreactive B cells and second, in peripheral tissues it leads to the accumulation of accessible nuclear autoantigens. Chronic inflammation in SLE is consequently promoted by the persistently binding of AAb with their cognate autoantigens forming a binary weapon: the nucleic acid-containing IC.

Renal Dnase1 enzyme activity and protein expression is selectively shut down in murine and human membranoproliferative lupus nephritis

Background

Deposition of chromatin-IgG complexes within glomerular membranes is a key event in the pathogenesis of lupus nephritis. We recently reported an acquired loss of renal Dnase1 expression linked to transformation from mild to severe membranoproliferative lupus nephritis in (NZBxNZW)F1 mice. As this may represent a basic mechanism in the progression of lupus nephritis, several aspects of Dnase1 expression in lupus nephritis were analyzed.

Methodology/Principal Findings

Total nuclease activity and Dnase1 expression and activity was evaluated using in situ and in vitro analyses of kidneys and sera from (NZBxNZW)F1 mice of different ages, and from age-matched healthy controls. Immunofluorescence staining for Dnase1 was performed on kidney biopsies from (NZBxNZW)F1 mice as well as from human SLE patients and controls. Reduced serum Dnase1 activity was observed in both mesangial and end-stage lupus nephritis. A selective reduction in renal Dnase1 activity was seen in mice with massive deposition of chromatin-containing immune complexes in glomerular capillary walls. Mice with mild mesangial nephritis showed normal renal Dnase1 activity. Similar differences were seen when comparing human kidneys with severe and mild lupus nephritis. Dnase1 was diffusely expressed within the kidney in normal and mildly affected kidneys, whereas upon progression towards end-stage renal disease, Dnase1 was down-regulated in all renal compartments. This demonstrates that the changes associated with development of severe nephritis in the murine model are also relevant to human lupus nephritis.

Conclusions/Significance

Reduction in renal Dnase1 expression and activity is limited to mice and SLE patients with signs of membranoproliferative nephritis, and may be a critical event in the development of severe forms of lupus nephritis. Reduced Dnase1 activity reflects loss in the expression of the protein and not inhibition of enzyme activity.

Progression of murine lupus nephritis is linked to acquired renal Dnase1 deficiency and not to up-regulated apoptosis

The accumulation of apoptotic cells has been suggested as a possible mechanism of nucleosome conversion into self-antigens that may both initiate autoimmune responses and participate in immune complex deposition in lupus nephritis. In this study, we analyzed both the rate of transcription of apoptosis-related genes and the presence of activated apoptotic factors within kidneys of lupus-prone (NZBxNZW) F1 mice during disease progression. The results of this study demonstrated no activation of apoptotic pathways in kidneys of these lupus-prone mice at the time of appearance of anti-double standard DNA antibodies in serum, as well as the formation of mesangial immune deposits in glomeruli. In contrast, the transition of mesangial into membranoproliferative lupus nephritis coincided with an accumulation of activated caspase 3-positive cells in kidneys, in addition to a dramatic decrease in Dnase1 gene transcription. Highly reduced expression levels of the Dnase1 gene may be responsible for the accumulation of large chromatin-containing immune complexes in glomerular capillary membranes. Thus, the initiation of lupus nephritis is not linked to increased apoptotic activity in kidneys. The combined down-regulation of Dnase1 and the increased number of apoptotic cells, which is possibly due to their reduced clearance in affected kidneys, may together be responsible for the transformation of mild mesangial lupus nephritis into severe membranoproliferative, end-stage organ disease.

Apoptosis in the pathogenesis of systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is a prototype inflammatory autoimmune disease resulting from autoimmune responses against nuclear autoantigens. During apoptosis many lupus autoantigens congregate inside the cells and are susceptible to modifications. Modified nuclear constituents are considered foreign and dangerous. Therefore, apoptotic cells have to has to be efficiently removed to avoid the accumulation of apoptotic debris and the subsequently development of autoimmune responses. Hence, apoptosis and clearance of apoptotic cells/material are considered key processes in the aetiology of SLE. Clearance deficiencies may account for the development of autoimmunity by inducing a loss of tolerance in lymphoid tissues. Furthermore, phagocytosis of apoptotic cells may lead to a pro-inflammatory response in the presence of autoantibodies. This may sustain inflammatory conditions and the pathology found in overt lupus.

Lupus nephritis: the central role of nucleosomes revealed

Systemic lupus erythematosus (SLE) is an autoimmune syndrome characterized by autoantibodies to nuclear constituents. Some of these antibodies are diagnostically important, whereas others act as disease-modifying factors. One clinically important factor is autoantibodies against dsDNA and nucleosomes, which have overlapping diagnostic and nephritogenic impact in SLE. Although a scientific focus for 5 decades, the molecular and cellular origin of these antibodies, and why they are associated with lupus nephritis, is still not fully understood. A consensus has, however, evolved that antibodies to dsDNA and nucleosomes are central pathogenic factors in the development of lupus nephritis. In contrast, no agreement has been reached as to which glomerular structures are bound by nephritogenic anti-nucleosome antibodies in vivo. Mutually contradictory paradigms and models have evolved simply because we still lack precise and conclusive data to provide definitive insight into how autoantibodies induce lupus nephritis and which specificity is critical in the nephritic process(es). In this review, data demonstrating the central role of nucleosomes in inducing and binding potentially nephritogenic antibodies to DNA and nucleosomes are presented and discussed. These autoimmune-inducing processes are discussed in the context of Matzinger's danger model (Matzinger P: Friendly and dangerous signals: is the tissue in control? Nat Immunol 2007, 8:11-13; Matzinger P: The danger model: a renewed sense of self. Science 2002, 296:301-305; Matzinger P: Tolerance, danger, and the extended family. Annu Rev Immunol 1994, 12:991-1045) and Medzhitov's and Janeway's (Medzhitov R, Janeway CA Jr: Decoding the patterns of self and nonself by the innate immune system. Science 2002, 296:298-300; Medzhitov R, Janeway CA Jr: How does the immune system distinguish self from nonself? Semin Immunol 2000, 12:185-188; Janeway CA Jr, Medzhitov R: Innate immune recognition. Annu Rev Immunol 2002, 20:197-216) distinction of noninfectious self (NIS) and infectious nonself (INS). The mechanisms leading to production of potentially nephritogenic anti-nucleosome antibodies and to overt lupus nephritis are interpreted in the context of these paradigms.

Removal of dying cells and systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is a very heterogeneous systemic autoimmune disease, in which autoantibody synthesis against nuclear constituents is the main immunological characteristic. These autoantibodies underwent affinity maturation and isotype switching. Additionally, T-cell tolerance against nuclear autoantigens should be affected in these autoimmune patients. Nuclear material derived from apoptotic and/or necrotic cells may serve as an important source of autoantigens. However, dead and dying cells as well as cellular debris are rapidly removed from tissues by phagocytes without eliciting inflammation or immune responses under healthy conditions. During apoptosis nuclear components are strongly modified through enzymatic reactions. If these cells are not timely cleared, those autoantigens may be released, taken up, and presented by dendritic cells in tissues or presented by follicular dendritic cells in lymph nodes to T and B cells, respectively. This could be a mechanism for breaking the peripheral self-tolerance. In this article we focus on the deficient clearance of apoptotic cells in SLE patients and its importance in development of this autoimmune disease.

Clearance deficiency and systemic lupus erythematosus (SLE)

Systemic lupus erythematosus (SLE) is a fairly heterogeneous autoimmune disease. Impaired clearance functions for dying cells may explain accumulation of nuclear autoantigens in various tissues of SLE patients. Our data show that in a subgroup of patients with SLE, apoptotic cells accumulated in the germinal centres of the lymph nodes. Apoptotic material was attached to the surfaces of follicular dendritic cells. Furthermore, we found an accumulation of apoptotic cells in the skin of patients with cutaneous lupus after UV exposure. Granulocytes and monocytes in whole blood of SLE patients showed a reduced uptake of albumin- and polyglobin-coated beads. Furthermore, we analysed sera from SLE patients in migration assays and observed that the attraction signals for macrophages were reduced by sera of approximately 25% of the SLE patients. Analyses of high-affinity DNA binding IgG autoantibodies of SLE patients revealed that those antibodies had gained their DNA reactivity in a germinal centre reaction. We suggest a stepwise maturation from a non-anti-DNA reactive B cell to an anti-dsDNA autoreactive B cell. We conclude that impaired clearance in early phases of apoptosis leads to a secondary necrotic status of the cells. Danger signals are released; modified autoantigens are accessible, favouring an autoimmune reaction.

Macrophages from patients with SLE and rheumatoid arthritis have defective adhesion in vitro, while only SLE macrophages have impaired uptake of apoptotic cells

BACKGROUND

It has been suggested that defective handling of apoptotic cells by macrophages plays a key role in the development of systemic lupus erythematosus (SLE). The relative contribution of intrinsic defects and serum factors remains controversial.

OBJECTIVE

To compare monocytes from SLE patients, patients with rheumatoid arthritis, and healthy controls for their ability to differentiate in vitro into macrophages and to bind/engulf apoptotic cells.

METHODS

Peripheral blood derived monocytes from healthy donors or from patients with SLE or rheumatoid arthritis were allowed to differentiate into macrophages. The in vitro uptake of apoptotic cells by macrophages was evaluated by a flow cytometry assay that allowed discrimination between binding and internalisation.

RESULTS

Monocytes from SLE and rheumatoid patients showed a striking defect in adherence to plastic compared with healthy donors. Absence or heat inactivation of serum resulted in a reduction in the binding and engulfment of apoptotic cells by macrophages. Macrophages from rheumatoid and SLE patients had similar percentages of apoptotic cells bound to their surface compared with normal controls. However, macrophages from SLE patients showed a significant defect in the internalisation of apoptotic cells compared with those from healthy controls, even in the presence of normal human serum.

CONCLUSIONS

Monocytes from patients with SLE and rheumatoid arthritis have a similar defect in their capacity to adhere to plastic. However, only macrophages from SLE patients showed an impaired ability to engulf apoptotic cells, which indicates that an intrinsic cellular defect may be responsible for this phenomenon.

SLE—a disease of clearance deficiency?

Systemic lupus erythematosus (SLE) is a multifactorial disease and its pathogenesis and precise aetiology remain unknown. Under physiological conditions, neither apoptotic nor necrotic cell material is easily found in tissues because of its quick removal by a highly efficient scavenger system. Autoantigens are found in apoptotic and necrotic material and they are recognized by autoimmune sera from SLE patients. The clearance of dying cells is finely regulated by a highly redundant system of receptors on phagocytic cells and bridging molecules, which detect molecules specific for dying cells. Changes on apoptotic and necrotic cell surfaces are extremely important for their recognition and further disposal. Some SLE patients seem to have an impaired ability to clear such apoptotic material from tissues, and this could cause the breakdown of central and peripheral mechanisms of tolerance against self-antigens. In this article, we address the cells, receptors and molecules involved in the clearance process and show how deficiencies in this process may contribute to the aetiopathogenesis of SLE.