Clearance deficiency and systemic lupus erythematosus (SLE)

The influence of a 6 mT static magnetic field on apoptotic cell phagocytosis depends on monocyte/macrophage differentiation

In a previous work we showed that a 6 mT static magnetic field (SMF) interferes with monocyte/macrophage 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced differentiation of promonocytes (U937 cells) and monocytes (THP-1 cells). In this study we investigated whether in the same cells and under the same conditions, phagocytosis of apoptotic cells is influenced by 6 mT SMF exposure. Fluid phase endocytosis and phagocytosis of latex particles were also analyzed for comparison. The results indicate that SMF exposure has effects on phagocytosis but not on fluid phase endocytosis, and that these effects are greater at the late stages of macrophage differentiation (THP-1 > U937 cells). The phagocytosis index and rate of phagocytosis decreased under SMF exposure while the number of latex particles bound to the plasma membrane of TPA-differentiated U937 and THP-1 cells increased. Conversely, the rate of phagocytosis of apoptotic cells increased under SMF exposure, while the number of apoptotic cells bound to the plasma membrane of isolated human Kupffer cells, Raw 264.7 macrophages and TPA-differentiated THP-1 and U937 cells decreased. In non-differentiated U937 and THP-1 cells, the SMF exposure enhanced the number of cell-surface bound apoptotic cells and latex beads.

Carthamus tinctorius Enhances the Antitumor Activity of Dendritic Cell Vaccines via Polarization toward Th1 Cytokines and Increase of Cytotoxic T Lymphocytes

Carthamus tinctorius (CT), also named safflower, is a traditional Chinese medicine widely used to improve blood circulation. CT also has been studied for its antitumor activity in certain cancers. To investigate the effects of CT on the dendritic cell (DC)-based vaccine in cancer treatment, cytokine secretion of mouse splenic T lymphocytes and the maturation of DCs in response to CT were analyzed. To assess the antitumor activity of CT extract on mouse CD117⁺ (c-kit)-derived DCs pulsed with JC mammal tumor antigens, the JC tumor was challenged by the CT-treated DC vaccine in vivo. CT stimulated IFN-γ and IL-10 secretion of splenic T lymphocytes and enhanced the maturation of DCs by enhancing immunological molecule expression. When DC vaccine was pulsed with tumor antigens along with CT extract, the levels of TNF-α and IL-1β were dramatically increased with a dose-dependent response and more immunologic and co-stimulatory molecules were expressed on the DC surface. In addition, CT-treated tumor lysate-pulsed DC vaccine reduced the tumor weight in tumor-bearing mice by 15.3% more than tumor lysate-pulsed DC vaccine without CT treatment. CT polarized cytokine secretion toward the Th1 pathway and also increased the population of cytotoxic T lymphocytes ex vivo. In conclusion, CT activates DCs might promote the recognition of antigens and facilitate antigen presentation to Th1 immune responses.

Complement factor H deficiency accelerates development of lupus nephritis

Complement factor H (CfH) is a key regulator of the alternative pathway, and its presence on mouse platelets and podocytes allows the processing of immune complexes. Because of the role of immune complexes in the pathophysiology of lupus nephritis, we studied the role of CfH in the development of nephritis in MRL-lpr mice, an animal model of lupus. At 12 weeks, CfH-deficient MRL-lpr mice had significantly more albuminuria and higher BUN levels than MRL-lpr controls. Cfh-deficient MRL-lpr mice also experienced earlier mortality: at 14 weeks, 6 of 9 CfH-deficient MRL-lpr mice had died of renal failure, whereas all 11 littermate CfH-sufficient MRL-lpr mice were alive (P ≤ 0.001). Histologically, CfH-deficient MRL-lpr mice developed severe diffuse lupus nephritis by 12 weeks (glomerulonephritis scores of 2.6 ± 0.4 versus 0.4 ± 0.2 in littermate controls, P = 0.001). Similar to other CfH-deficient mouse models on nonautoimmune backgrounds, immunofluorescence staining showed extensive linear C3 staining along glomerular capillary walls. IgG was present in the mesangium and peripheral capillary walls along with excessive infiltration of macrophages and neutrophils. Ultrastructurally, there were subendothelial and subepithelial immune deposits and extensive podocyte foot process effacement. In summary, the loss of CfH accelerates the development of lupus nephritis and recapitulates the functional and structural features of the human disease. This illustrates the critical role of complement regulation and metabolism of immune complexes in the pathogenesis of lupus nephritis.

Surface-exposed calreticulin in the interaction between dying cells and phagocytes

Phagocytosis is essential for pathogen elimination and for the removal of apoptotic corpses, a process that has been long considered immunologically silent. The phagocytic uptake of apoptotic/necrotic cells involves a plethora of molecules, including immunoglobulins, lectins, components of the complement system (all of which act as opsonins), as well as the phospholipid phosphatidylserine (PS) and the endoplasmic reticulum chaperone calreticulin (CRT), both of which can be exposed on the surface of dying cells. For a long time, surface-exposed CRT was believed to participate in phagocytosis, mostly as a (co)receptor for specific opsonins. Recently, this view has been challenged by the observations that, similar to PS, CRT acts as a facultative recognition ligand on apoptotic cells, and that cytotoxic agents such as anthracyclines induce the exposure of CRT on the surface of dying tumor cells, thereby generating an engulfment signal that stimulates the uptake of apoptotic corpses and the presentation of the corresponding antigens by dendritic cells. Here, we summarize the current knowledge on the role of CRT and CRT-interacting proteins during corpse removal.

Secondary necrosis: the natural outcome of the complete apoptotic program

The predominant definition of apoptosis considers that the elimination of the apoptosing cell is by heterolytic degradation following phagocytosis by an assisting scavenger (efferocytosis). However, an alternative and largely underestimated outcome of apoptosis is secondary necrosis, an autolytic process of cell disintegration with release of cell components that occurs when there is no intervention of scavengers and the full apoptotic program is completed. Secondary necrosis is the typical outcome of apoptosis in unicellular eukaryotes but, importantly, it may also occur in multicellular animals and has been implicated in the genesis of important human pathologies. Secondary necrosis is a mode of cell elimination with specific molecular and morphological features and should be considered the natural outcome of the complete apoptotic program.

Cell death in the neighbourhood: direct microenvironmental effects of apoptosis in normal and neoplastic tissues

Here we consider the impact of the physiological cell-death programme on normal tissue homeostasis and on disease pathogenesis, with particular reference to evolution and progression of neoplasia. We seek to describe the direct contributions played by apoptosis in creating the microenvironments of normal and malignant tissues and to discuss the molecular mechanisms underlying the elements of the '3Rs' that define the meaning of apoptosis: recognition, response, and removal. Apoptotic cells elicit responses in other cell types-both phagocytic and non-phagocytic-through short- and long-range signalling modes that range from direct contact to intercellular communication via membrane-bound microparticles. Such cellular responses include migration, proliferation, and differentiation, as well as production of immunomodulatory and anti-inflammatory mediators together with, in the case of phagocytes, engulfment, and breakdown of apoptotic cells. In normal tissues, the removal of apoptotic cells is rapid and typically non-phlogistic. We discuss the importance of this clearance process in tissue homeostasis and the consequences of its failure in disease pathogenesis. Using the typical cell culture environment in vitro as an illustrative example in which apoptosis occurs commonly in the absence of the removal mechanisms, we also discuss the inhibitory effects of persistent apoptotic cells on their otherwise viable neighbours. Since apoptosis is a common and sustained event in high-grade malignancies, we hypothesize on its purposeful role in conditioning the tumour microenvironment. We propose that apoptosis subserves several pro-tumour functions-trophic, anti-inflammatory, and immunomodulatory-and we identify strategies targeting host responses to apoptotic cells as promising modes of future therapies that could be applied to multiple cancer types in which tumour-cell apoptosis is active.

Impaired apoptotic cell clearance in the germinal center by Mer-deficient tingible body macrophages leads to enhanced antibody-forming cell and germinal center responses

Germinal centers (GCs) are specialized microenvironments that generate high-affinity Ab-forming cells (AFCs) and memory B cells. Many B cells undergo apoptosis during B cell clonal selection in GCs. Although the factors that regulate the AFC and GC responses are not precisely understood, it is widely believed that dysregulated AFCs and GCs contribute to autoimmunity. The Mer receptor tyrosine kinase (Mer) facilitates macrophage clearance of apoptotic cells. The Tyro-3, Axl, and Mer receptors, including Mer, suppress TLRs and cytokine-mediated inflammatory responses. We report in this study that tingible body macrophages (TBMφs) in GCs express Mer. Compared to C57BL/6 (B6) controls, Mer-deficient (Mer(-/-)) mice had significantly higher AFC, GC, and Th1-skewed IgG2 Ab (especially IgG2c) responses against the T cell-dependent Ag (4-hydroxy-3-nitrophenyl) acetyl-chicken γ globulin. Mer(-/-) mice had a significantly higher percentage of GC B cells on days 9, 14, and 21 postimmunization compared with B6 controls. Significantly increased numbers of apoptotic cells accumulated in Mer(-/-) GCs than in B6 GCs, whereas the number of TBMφs remained similar in both strains. Our data are the first, to our knowledge, to demonstrate a critical role for Mer in GC apoptotic cell clearance by TBMφs and have interesting implications for Mer in the regulation of B cell tolerance operative in the AFC and GC pathways.

Monocyte activation by apoptotic cells removal in systemic lupus erythematosus patients

Decreased apoptotic cells (ACs) removal has been described as relevant in systemic lupus erythematosus (SLE) pathogenesis. Binding/phagocytosis of ACs was decreased in SLE patients. Blocking experiments suggested a role for CD36 in ACs clearance in healthy controls, not observed in SLE patients. Binding/phagocytosis of ACs induced the production of IL-6, CXCL8 and CCL22 in patients and controls and IL-1β, TNF-α and CCL3 only in healthy controls. ACs clearance induced an increase in CD80 and a decrease in CD86 expression in healthy controls and atherosclerotic patients. However, SLE patients did not up-regulate CD80 expression. The number and expression of CD36 and CD163 in monocytes was not different between the groups. ACs removal induced a down-regulation of CD36 expression in adherent HLA-DR(+) cells in SLE patients but not healthy controls. The decreased binding/phagocytosis of ACs observed in SLE patients, induces a distinct immune response compared with healthy controls.

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.

Phagocytosis of dying cells: influence of smoking and static magnetic fields

It is becoming evident that failure in the removal of dying cells causes and/or promotes the onset of chronic diseases. Impairment of phagocytosis of apoptotic cells can be due not only to genetic or molecular malfunctioning but also to external/environmental factors. Two of these environmental factors have been recently reported to down regulate the clearance of apoptotic cells: cigarette smoke and static magnetic fields. Cigarette smoke contains highly reactive carbonyls that modify proteins which directly/indirectly affects cellular function. Human macrophages interacting with carbonyl or cigarette smoke modified extracellular matrix (ECM) proteins dramatically down regulated their ability to phagocytose apoptotic neutrophils. It was postulated that changes in the ECM environment as a result of cigarette smoke affect the ability of macrophages to remove apoptotic cells. This decreased phagocytic activity was as a result of sequestration of receptors involved in the uptake of apoptotic cells towards that of recognition of carbonyl adducts on the modified ECM proteins leading to increased macrophage adhesion. Downregulation of the phagocytosis of apoptotic cells was also described when performed in presence of static magnetic fields (SMFs) of moderate intensity. SMFs have been reported to perturb distribution of membrane proteins and glycoproteins, receptors, cytoskeleton and trans-membrane fluxes of different ions, especially calcium [Ca(2+)]i, that in turn, interfere with many different physiological activities, including phagocytosis. The effects of cigarette smoke and SMF on the phagocytosis of dying cells will be here discussed.

Dendritic cells in systemic lupus erythematosus

Systemic lupus erythematosus (SLE) persists as a chronic inflammatory autoimmune disease and is characterized by the production of autoantibodies and immune complexes that affect multiple organs. The underlying mechanism that triggers and sustains disease are complex and involve certain susceptibility genes and environmental factors. There have been several immune mediators linked to SLE including cytokines and chemokines that have been reviewed elsewhere [ 1-3 ]. A number of articles have reviewed the role of B cells and T cells in SLE [ 4-10 ]. Here, we focus on the role of dendritic cells (DC) and innate immune factors that may regulate autoreactive B cells.

Blood dendritic cells in systemic lupus erythematosus exhibit altered activation state and chemokine receptor function

BACKGROUND

Dendritic cells (DCs) have a pivotal role in the pathogenesis of systemic lupus erythematosus (SLE). Reduced numbers of blood DCs and the accumulation of DCs at inflammatory sites have been observed in SLE. One crucial feature of DCs is their ability to migrate.

OBJECTIVE

To analyse the maturation/activation state and the migratory capacity of different DC precursor subsets in SLE to further elucidate their role in autoimmunity.

METHODS

Plasmacytoid DCs (pDCs), myeloid DCs (mDCs) and monocytes from patients with SLE, healthy volunteers and healthy volunteers immunised with tetanus/diphtheria were examined by flow cytometry for expression of subset-specific antigens (BDCA-2, CD11c, CD14, HLA-DR), activation/maturation markers (CD83, CD86, CD40, BLyS) and chemokine receptors (CCR1, CCR5, CCR7, ChemR23). Additionally, migratory capacity to chemokine receptors was investigated in vitro using the chemokines RANTES, CCL19 and chemerin.

RESULTS

SLE monocytes and mDCs had higher CD86 and B-lymphocyte stimulatory factor (BLyS) expression levels. ChemR23 expression was lower in SLE pDCs and mDCs. Basal and CCL19-specific migration levels were higher in SLE pDCs. Altered DC function in SLE had no correlative changes in chemokine receptor expression, whereas immunisation-induced blood DC migration patterns in healthy donors were accompanied by changes in chemokine receptor expression.

CONCLUSIONS

The phenotypic and migratory disturbances observed in SLE blood DCs could result in altered distribution of DCs in peripheral tissues, contributing to dysregulated immune responses and autoimmunity.

Bacteria-induced phagocyte secondary necrosis as a pathogenicity mechanism

Triggering of phagocyte apoptosis is a major virulence mechanism used by some successful bacterial pathogens. A central issue in the apoptotic death context is that fully developed apoptosis results in necrotic cell autolysis (secondary necrosis) with release of harmful cell components. In multicellular animals, this occurs when apoptosing cells are not removed by scavengers, mainly macrophages. Secondary necrotic lysis of neutrophils and macrophages may occur in infection when extensive phagocyte apoptosis is induced by bacterial cytotoxins and removal of apoptosing phagocytes is defective because the apoptotic process exceeds the available scavenging capacity or targets macrophages directly. Induction of phagocyte secondary necrosis is an important pathogenic mechanism, as it combines the pathogen evasion from phagocyte antimicrobial activities and the release of highly cytotoxic molecules, particularly of neutrophil origin, such as neutrophil elastase. This pathogenicity mechanism therefore promotes the unrestricted multiplication of the pathogen and contributes directly to the pathology of several necrotizing infections, where extensive apoptosis and necrosis of macrophages and neutrophils are present. Here, examples of necrotizing infectious diseases, where phagocyte secondary necrosis is implicated, are reviewed.

Can lupus flares be associated with tuberculosis infection?

Systemic lupus erythematosus (SLE) is an autoimmune disease that frequently requires treatment with high doses of corticosteroids and immunosuppressive drugs. Primary defects in the innate immunity also contribute to an increased susceptibility to infections. Patients with SLE are at an increased risk for infections with several pathogens, among them Mycobacterium tuberculosis, which is a significant cause of morbidity and mortality, especially in endemic regions. TB infection requires awareness for several reasons: first, TB infection thrives under conditions of immunosuppression, may it be secondary to the disease itself or its treatment. Second, shared antigens by mycobacteria and autoantigens have been described, which may be targets for autoantibodies. We present four Brazilian patients, in whom a diagnosis of tuberculosis was determined during or following persistent flares of their disease. The association of SLE and TB is discussed, as well as different aspects of the tuberculosis infection in this selected population, and its possible role in the course of SLE.

The role of dendritic cells in the pathogenesis of systemic lupus erythematosus

The etiology of the autoimmune disease systemic lupus erythematosus is not known, but aberrant apoptosis and/or insufficient clearance of apoptotic material have been assigned a pivotal role. During apoptosis, nucleosomes and several endogenous danger-associated molecular patterns are incorporated in blebs. Recent data indicate that apoptotic blebs induce maturation of myeloid dendritic cells, resulting in IL-17 production by T cells. In this review we summarize current knowledge on the role of dendritic cells in the pathogenesis of systemic lupus erythematosus with special emphasis on the uptake of apoptotic blebs by dendritic cells, and the subsequent induction of Th17 cells.

T and B cell hyperactivity and autoimmunity associated with niche-specific defects in apoptotic body clearance in TIM-4-deficient mice

TIM-4, a member of the TIM family expressed on antigen-presenting cells, binds to phosphatidylserine exposed on the surface of apoptotic bodies. However, the significance of this interaction in vivo remains unknown because other receptors have been implicated in the clearance of apoptotic bodies and could compensate for the TIM-4 deficiency in vivo. In this study, we describe the generation of TIM-4-deficient mice and address whether TIM-4 serves a unique function in vivo. We show that TIM-4(-/-) peritoneal macrophages and B-1 cells do not efficiently engulf apoptotic bodies in vitro, or clear apoptotic bodies in vivo. TIM-4-deficient mice have hyperactive T and B cells, elevated levels of serum Ig, and develop antibodies to double-stranded DNA. Taken together, we show that TIM-4 is critical for the clearance of apoptotic bodies in vivo, and that lack of TIM-4 results in aberrant persistence of apoptotic bodies leading to dysregulated lymphocyte activation and signs of systemic autoimmunity.

Chemerin peptides promote phagocytosis in a ChemR23- and Syk-dependent manner

Chemerin peptides represent a recently identified component of the endogenous anti-inflammatory network that act via the G protein-coupled receptor ChemR23. The role of the chemerin peptide/ChemR23 pathway in phagocytosis, the clearance of apoptotic cells (efferocytosis), and the resolution of inflammation is unknown. In this article, we report that low picomolar concentrations of the chemerin peptide chemerin15 (C15) enhance macrophage (MPhi) phagocytosis of microbial particles and apoptotic cells by up to 360% in vitro. These prophagocytic effects of C15 are significantly impaired in ChemR23(-/-) MPhis and are associated with increased actin polymerization and localization of F-actin to the phagocytic cup. Importantly, pharmacological inhibition of Syk activity completely abrogates the prophagocytic activities of C15 and associated changes in actin polymerization and phagocytic cup formation, suggesting that C15 promotes phagocytosis by facilitating phagocytic cup development in a Syk-dependent manner. During peritoneal inflammation, C15 administration (8 pg/mouse) enhances microbial particle clearance and apoptotic neutrophil ingestion by MPhis in wild-type but not ChemR23(-/-) mice, such that levels of apoptotic and necrotic cells at the inflammatory site are profoundly reduced. In contrast, neutralization of endogenous chemerin species during peritoneal inflammation significantly impairs MPhi ingestion of apoptotic neutrophils and zymosan. Our data identify a key role of the chemerin peptide/ChemR23 axis in the efficient clearance of foreign material, efferocytosis, and, hence, the resolution of inflammation. Manipulation of the chemerin peptide/ChemR23 axis may represent a novel therapeutic approach for the treatment of inflammatory pathologies, especially if failure to efficiently clear phagocytic targets has been implicated in their pathogenesis.

Infection and apoptosis as a combined inflammatory trigger

While inflammatory phagocytosis of microbial pathogens and non-inflammatory phagocytosis of apoptotic cells have each been studied extensively, the consequences of innate immune recognition of host cells undergoing apoptosis as a direct result of infection are unclear. In this situation, the innate immune system is confronted with mixed signals, those from apoptotic cells and those from the infecting pathogen. Nuclear receptor activation has been implicated downstream of apoptotic cell recognition while Toll-like receptors are the prototypical inflammatory receptors engaged during infection. When the two signals combine, a new set of events takes place beginning with transrepression of a subset of inflammatory-response genes and ending with the induction of a T helper-17 adaptive immune response. This response is best suited for clearing the infecting pathogen and repairing the damage that occurred to the host tissue during infection.

Definition of human autoimmunity--autoantibodies versus autoimmune disease

The critical function of the immune system is to discriminate self from non-self. Tolerance against self-antigens is a highly regulated process and, in order to maintain it, the immune system must be able to distinguish self-reactive lymphocytes as they develop. The presence of autoantibodies is the consequence of breakdown of tolerance and, although they are an important serological feature of autoimmune diseases, their presence is not exclusive of these conditions. Antibodies against self-antigens are also found in cancer, during massive tissue damage and even in healthy subjects. Natural autoantibodies provide immediate protection against infection and also prevent inflammation by facilitating the clearance of oxidized lipids, oxidized proteins, and apoptotic cells; their role in development of autoimmunity is still unclear. Detection of serum autoantibodies in clinical practice has become more available to clinicians worldwide while providing a powerful diagnostic tool. This review discusses the clinical significance of autoantibodies, their pathogenic mechanisms in autoimmune diseases and, finally, illustrates the technology available for appropriate autoantibody detection.

The endoplasmic reticulum stress-inducible protein, Herp, is a potential triggering antigen for anti-DNA response

Anti-dsDNA Abs are highly specific indicators of systemic lupus erythematosus (SLE) and play a pathogenic role in lupus nephritis. Human anti-dsDNA Abs are most likely generated by an Ag-driven mechanism. However, the Ag responsible for triggering anti-dsDNA Ab production has not been identified. To search for proteins that are cross-reactive with anti-dsDNA Abs, we screened a cDNA library from a patient with SLE with single-chain Fv of O-81 human anti-ss/dsDNA mAb by using a two-hybrid system. Homocysteine-induced ER protein (Herp), an endoplasmic reticulum (ER) stress-inducible ER membrane protein, was identified and shown to bind to original O-81 Ab and human lupus anti-dsDNA Abs. Some IgG purified from patients with active SLE by Herp-immobilized affinity chromatography bound to dsDNA. BALB/c mice immunized with Herp showed IgG anti-dsDNA Abs, IgG anti-nucleosome Abs, and glomerular IgG deposition. Herp reactivity was strongly positive in a proportion of PBLs from patients with active SLE, but undetectable in those from healthy controls. Moreover, activation of caspases was observed in the Herp-positive cells, implying that ER stress-induced apoptosis likely occurs in patients with active SLE. Herp is exposed on blebs of ER stress-induced apoptotic cells, suggesting that Herp can be recognized by immune cells. These results indicate that Herp mimics structural determinants of DNA immunologically and can be immunogenic in vivo. Thus, Herp represents a candidate autoantigen for anti-DNA Abs. This study may help explain how common environmental factors induce the production of anti-DNA Abs and contribute the development of SLE.

Influence of microbial environment on autoimmunity

During protective immune responses, the adaptive arm of the immune system requires activation by signals provided by innate immunity and driven by microbial stimuli. Whether the same rules apply to autoimmune diseases involving clonal self-reactive T and B lymphocytes--a process referred to here as 'adaptive autoimmunity'--is not quite clear. Nevertheless, in these diseases, the innate-adaptive connection is likely to be influenced by the microbial environment. This review integrates the results of experiments analyzing autoimmunity in sterile versus nonsterile conditions and experiments testing the role of innate immune receptor signaling in autoimmunity. It proposes that autoimmune diseases can be divided into two groups, the pathogenesis of which either follows the rules of innate-adaptive connection or does not.

Phospholipids: key players in apoptosis and immune regulation

Phosphatidylserine (PS), a phospholipid predominantly found in the inner leaflet of eukaryotic cellular membranes, plays important roles in many biological processes. During apoptosis, the asymmetric distribution of phospholipids of the plasma membrane gets lost and PS is translocated to the outer leaflet of the plasma membrane. There, PS acts as one major “eat me” signal that ensures efficient recognition and uptake of apoptotic cells by phagocytes. PS recognition of activated phagocytes induces the secretion of anti-inflammatory cytokines like interleukin-10 and transforming grow factor-beta. Deficiencies in the clearance of apoptotic cells result in the occurrence of secondarily necrotic cells. The latter have lost the membrane integrity and release immune activating danger signals, which may induce inflammatory responses. Accumulation of dead cells containing nuclear autoantigens in sites of immune selection may provide survival signals for autoreactive B-cells. The production of antibodies against nuclear structures determines the initiation of chronic autoimmunity in systemic lupus erythematosus. Since PS on apoptotic cells is an important modulator of the immune response, natural occurring ligands for PS like annexinA5 have profound effects on immune responses against dead and dying cells, including tumour cells. In this review we will focus on the role of PS exposure in the clearance process of dead cells and its implications in clinical situations where apoptosis plays a relevant role, like in cancer, chronic autoimmunity, and infections. Relevance of other phospholipids during the apoptosis process is also discussed.

Defensins in systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is an autoimmune disease, resulting in inflammation and tissue damage. The precise etiology for SLE is so far unknown. It has been shown that the innate immunity plays a role in SLE pathogenesis. The innate immune system confers broad protection against pathogens by the secretion of broad-spectrum antibacterial and immunomodulatory substances named defensins. Recently, alpha-defensin, the products of neutrophils have been found to be upregulated at the mRNA and protein levels in SLE patients. In addition, increased antidefensin antibodies were found in sera of patients with SLE, but these levels decreased after therapy with corticosteroids. These recent findings suggest a role for defensins in the pathogenesis of SLE. Thus, activation and degranulation of neutrophils leads to alpha-defensin secretion in SLE patients. Given their immunomodulatory role, alpha-defensin secretion might activate the adaptive immune system leading to the stimulation of the immune system, as is manifested in SLE.

Extracorporeal photopheresis-induced immune tolerance: a focus on modulation of antigen-presenting cells and induction of regulatory T cells by apoptotic cells

PURPOSE OF REVIEW

This review is intended to introduce recent advances in the research surrounding extracorporeal photopheresis (ECP) with a focus on how apoptotic cells modulate antigen-presenting cells and induce regulatory T cells, given that ECP therapy induces apoptosis of leukocytes collected through leukapheresis.

RECENT FINDINGS

It has been suggested that ECP therapy, unlike other immunosuppressive regimens, does not cause global immunosuppression, but induces immune tolerance. Recent clinical and animal studies demonstrate that ECP therapy induces antigen-specific regulatory T cells, including CD4+CD25+FoxP3+ T cells and IL-10-producing Tr1 cells, that may arise secondarily to the induction of tolerogenic antigen-presenting cells (APCs) by infusion of apoptotic cells. It has also been suggested that ECP therapy may induce IL-10-producing regulatory B cells and regulatory CD8+ T cells. Finally, several recent studies, which examined the cellular elements involved in the uptake of apoptotic cells, demonstrated that apoptotic cells modulate APCs through binding to specific receptors, particularly TAM receptors that provide inhibitory signals that block APC activation.

SUMMARY

ECP therapy induces immune tolerance through modulation of antigen-presenting cells as well as induction of regulatory T cells. ECP therapy has great potential in the management of allogeneic transplantation and autoimmune diseases.

TREX1 acts in degrading damaged DNA from drug-treated tumor cells

The major mammalian exonuclease TREX1 has been proposed to play a role in DNA repair and drug resistance. However, no cellular evidence substantiates this claim. Recent reports indicate TREX1's involvement in autoimmunity. To further understand its role, we studied TREX1 expression and functionality in anticancer drug-treated tumor cells. We report that the expression and localization of TREX1 are cell-type dependent. Camptothecin and other DNA damaging agents induced both TREX1 protein and its mRNA in a dose- and time-dependent manner. Using a TREX1-inducible cell line, we performed clonogenic assays and found no change in sensitivity of the cells to the agents upon TREX1 induction, suggesting that TREX1 may not play a role in DNA repair or drug sensitivity. Nevertheless, TREX1 serves as a key enzyme in the degradation of DNA from dying cells leading to less cellular DNA. Ubiquitously expressed in normal tissues, TREX1 may act in degrading DNA in all cell types undergoing a dying process before phagocytosis occurs.

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.

Upregulated BclG(L) expression enhances apoptosis of peripheral blood CD4+ T lymphocytes in patients with systemic lupus erythematosus

Increased lymphocyte apoptosis has been suggested to contribute to the development of systemic lupus erythematosus (SLE), but the critical factors involved in the apoptotic pathways are still unknown. By long serial analysis of gene expression (LongSAGE) profiles and microarray analyses, a novel apoptosis-related gene BclG(L) expression was found significantly increased in peripheral blood CD4+ T cells of SLE patients, which was correlated with the enhanced CD4+ T cells apoptosis, anti-nuclear antibody (ANA) titer and proteinuria. In vitro, BclG(L) expression could be specially upregulated by SLE serum stimulation and positively correlated with induced CD4+ T cell apoptosis. Enforcing BclG(L) overexpression by lentivirus could directly enhance CD4+ T cell apoptosis, but these apoptosis-inducing effects could be partially inhibited by knockdown of BclG(L) expression. Collectively, these results indicate that increased BclG(L) expression may contribute to the aberrant CD4+ T cell apoptosis which causes an inappropriate immune response and impaired homeostasis in SLE.

Endothelial-derived thrombospondin-1 promotes macrophage recruitment and apoptotic cell clearance

Rapid apoptotic cell engulfment is crucial for prevention of inflammation and autoimmune diseases and is conducted by special immunocompetent cells like macrophages or immature dendritic cells. We recently demonstrated that endothelial cells (ECs) also participate in apoptotic cell clearance. However, in contrast to conventional phagocytes they respond with an inflammatory phenotype. To further confirm these pro-inflammatory responses human ECs were exposed to apoptotic murine ECs and changes in thrombospondin-1 (TSP-1) expression and in activation of intracellular signalling cascades were determined by real-time qPCR, immunoblotting and immunocytochemistry. Human primary macrophages or monocytic lymphoma cells (U937) were incubated with conditioned supernatant of human ECs exposed to apoptotic cells and changes in activation, migration and phagocytosis were monitored. Finally, plasma levels of TSP-1 in patients with anti-neutrophil cytoplasmic antibody(ANCA)-associated vasculitis (AAV) were determined by ELISA. We provided evidence that apoptotic cells induce enhanced expression of TSP-1 in human ECs and that this increase in TSP-1 is mediated by the mitogen-activated protein kinases (MAPK) ERK1 and 2 and their upstream regulators MEK and B-Raf. We also showed that plasma TSP-1 levels are increased in patients with AAV. Finally, we showed that conditioned supernatant of ECs exposed to apoptotic cells induces pro-inflammatory responses in monocytes or U937 cells and demonstrated that increased TSP-1 expression enhances migration and facilitates engulfment of apoptotic cells by monocyte-derived macrophages or U937 cells. These findings suggest that under pathological conditions with high numbers of uncleared dying cells in the circulation endothelial-derived elevated TSP-1 level may serve as an attraction signal for phagocytes promoting enhanced recognition and clearance of apoptotic cells.

Immunogenic and tolerogenic cell death

The immune system is routinely exposed to dead cells during normal cell turnover, injury and infection. Mechanisms must exist to discriminate between different forms of cell death to correctly eliminate pathogens and promote healing while avoiding responses to self, which can result in autoimmunity. However, an effective immune response against host tissue is often needed to eliminate tumours following treatment with chemotherapeutic agents that trigger tumour cell death. Consequently, a central problem in immunology is to understand how the immune system determines whether cell death is immunogenic, tolerogenic or 'silent'.

Lack of association of the TP53 Arg72Pro SNP and the MDM2 SNP309 with systemic lupus erythematosus in Caucasian, African American, and Asian children and adults

The p53 tumour suppressor is the central regulator of apoptosis. Previously, the functional TP53 Arg72Pro polymorphism was found to be associated with systemic lupus erythematosus (SLE) in Koreans but not Spaniards. MDM2 is the major negative regulator of p53. An intronic polymorphism in MDM2, the SNP309, attenuates p53 activity and is associated with accelerated tumour development in premenopausal women. Polymorphic variation in MDM2 has never been studied in SLE. The aim of this study is to further assess the contribution of p53-pathway genetic variation to SLE by testing the association of the TP53 Arg72Pro polymorphism and the MDM2 SNP309 with SLE in a well-characterised and ethnically diverse cohort of patients with both childhood- and adult-onset SLE (n = 314). No association was found between the TP53 Arg72Pro polymorphism and SLE in patients of European descent, Asian descent or in African Americans, nor was an association found between the MDM2 SNP309 and SLE in patients of European descent or in African Americans. In addition, there was no correlation between either variant and early-onset disease or nephritis, an index of severe disease. It is concluded that neither the TP53 Arg72Pro polymorphism nor the MDM2 SNP309 contributes significantly to either susceptibility or disease severity in SLE.

Peripheral B cell abnormalities and disease activity in systemic lupus erythematosus

Autoreactive B cells and plasma cells appear to be of central importance in the pathogenesis of systemic lupus erythematosus (SLE) characterized by a plethora of autoantibodies. Recent insights into abnormalities of B cell and plasma cell compartments in human SLE have identified a number of cellular disturbances within these compartments that in part correlate with the disease activity. This review discusses these findings and the potential underlying extrinsic and/or intrinsic influences apparently driving general B cell activation in this entity.

High Ro52 expression in spontaneous and UV-induced cutaneous inflammation

Ro52 is an E3 ubiquitin ligase with a recently identified regulatory role in inflammation. The protein is targeted by autoantibodies in rheumatic diseases, and Ro52 autoantibodies are specifically associated with cutaneous lupus erythematosus (CLE) and photosensitivity. The aim of this study was to investigate cutaneous Ro52 expression in CLE patients and to examine whether UVR might modulate Ro52. Ro52 expression was assessed by immunohistochemistry in biopsies derived from CLE lesions (n=25), nonlesional (n=7), and healthy control skin using four anti-Ro52 mAbs generated by us. Ro52 expression was also analyzed in psoriatic, lichenoid, and eczematous lesions. It was increased in the epidermis of spontaneous CLE lesions as compared with unaffected skin of patients and healthy controls. High epidermal Ro52 expression was also observed in other inflammatory dermatoses investigated. Ro52 was upregulated in experimentally photoprovoked CLE lesions as observed by immunohistochemistry in sequential biopsies, which was confirmed in vitro both at the mRNA and protein levels by exposing cultured patient-derived primary keratinocytes to UVR. In conclusion, Ro52 expression is upregulated in keratinocytes in inflammatory skin conditions and in response to UVR. High Ro52 expression might lead to the breaking of tolerance and the generation of Ro52 autoantibodies in genetically susceptible subjects. Further, the upregulation of Ro52 in keratinocytes after sun exposure might also be a triggering factor for skin lesions in patients with Ro52 antibodies.

Systemic Lupus Erythematosus: A Therapeutic Update

Systemic lupus erythematosus (SLE) is a complex autoimmune disease with variable clinical manifestations that is characterized by flares and periods of relative quiescence. The disease occurs approximately 10 times more frequently in women and is more prevalent among certain ethnic groups. The etiology is complex and dependent upon an interaction of genetic, hormonal, and environmental factors. Corticosteroids and immunosuppressive agents have transformed the outlook for patients with lupus. Unfortunately, the increased lifespan unmasked an accelerated process of atherosclerosis and cardiovascular disease. Early mortality is usually attributable to active lupus, but deaths late in the disease process are often secondary to thrombotic events. Advancements in the understanding of molecular and cellular mechanisms involved in the pathogenesis have resulted in development of novel therapies. Immunomodulatory drugs developed for other diseases are being investigated for use in specific manifestations of lupus. Individualization of treatment and lifelong monitoring are required in most patients.

Epigenetic regulation and the pathogenesis of systemic lupus erythematosus

The pathogenesis of systemic lupus erythematosus (SLE) is incompletely understood. Studies in both lupus animal models and human disease indicate a clear role for epigenetic defects, particularly DNA methylation, in the pathogenesis of lupus. T-cell DNA from active lupus patients is hypomethylated, which results in overexpression of methylation-regulated genes, T-cell autoreactivity, and autoimmunity in vivo. Inducing an extracellular signal-regulated kinase (ERK) signaling defect in T cells using a transgenic mouse model resulted in reduced DNA methyltransferase 1 (DNMT1) expression, overexpression of methylation-sensitive genes, and anti-double-stranded DNA (anti-dsDNA) antibody production. ERK signaling is known to be defective in lupus T cells, and this defect is now explained by impaired T-cell protein kinase C (PKC) delta activation. Herein, we discuss how defective epigenetic regulation is involved in the pathogenesis of lupus, which includes both DNA methylation and histone modification changes.

Sunlight triggers cutaneous lupus through a CSF-1-dependent mechanism in MRL-Fas(lpr) mice

Sunlight (UVB) triggers cutaneous lupus erythematosus (CLE) and systemic lupus through an unknown mechanism. We tested the hypothesis that UVB triggers CLE through a CSF-1-dependent, macrophage (Mø)-mediated mechanism in MRL-Fas(lpr) mice. By constructing mutant MRL-Fas(lpr) strains expressing varying levels of CSF-1 (high, intermediate, none), and use of an ex vivo gene transfer to deliver CSF-1 intradermally, we determined that CSF-1 induces CLE in lupus-susceptible MRL-Fas(lpr) mice, but not in lupus-resistant BALB/c mice. UVB incites an increase in Møs, apoptosis in the skin, and CLE in MRL-Fas(lpr), but not in CSF-1-deficient MRL-Fas(lpr) mice. Furthermore, UVB did not induce CLE in BALB/c mice. Probing further, UVB stimulates CSF-1 expression by keratinocytes leading to recruitment and activation of Møs that, in turn, release mediators, which induce apoptosis in keratinocytes. Thus, sunlight triggers a CSF-1-dependent, Mø-mediated destructive inflammation in the skin leading to CLE in lupus-susceptible MRL-Fas(lpr) but not lupus-resistant BALB/c mice. Taken together, CSF-1 is envisioned as the match and lupus susceptibility as the tinder leading to CLE.

Evidence for a pathogenetic role of interleukin-18 in cutaneous lupus erythematosus

OBJECTIVE

Cutaneous manifestations are the most common clinical features of lupus erythematosus (LE). The aim of this study was to analyze differences in the inflammatory response of keratinocytes from patients with cutaneous LE (CLE) compared with healthy controls.

METHODS

Keratinocytes from LE patients and controls were cultured from epidermal stem cells of the hair follicle of anagen head hairs. Functional responses of keratinocytes to cytokine stimulation were determined by flow cytometry and enzyme-linked immunosorbent assay. Biopsy samples of lesional skin were analyzed by immunohistochemistry.

RESULTS

Keratinocytes from CLE patients expressed higher levels of IL-18 receptor on their cell surface in response to tumor necrosis factor alpha (TNFalpha) or interferon-gamma stimulation. In response to IL-18 stimulation, these cells produced large amounts of TNFalpha. Of note, in the presence of IL-18, CLE keratinocytes failed to express IL-12. IL-12 has previously been shown to protect keratinocytes from ultraviolet irradiation-induced apoptosis. Keratinocytes from LE patients were more prone to die upon exposure to IL-18, and this increased apoptosis was abrogated by blockade of endogenously produced TNFalpha as well as by the addition of exogenous IL-12. IL-18 was highly expressed in biopsy samples of lesional skin from CLE patients.

CONCLUSION

Our results demonstrate an intrinsic difference in the inflammatory response of keratinocytes and indicate an autocrine feedback loop involving TNFalpha, IL-18, and IL-12 family members. Our results suggest that IL-18 may occupy an important position in the cytokine hierarchy in CLE, indicating the potential benefit of a local agent that blocks IL-18 activity in the treatment of the manifestations of CLE.

Heat shock protein 70 (HSP70) induces cytotoxicity of T-helper cells

Heat shock protein 70 (HSP70) has gained plenty of attention because of its adjuvant capability to induce CD8(+) cytotoxic T lymphocyte and CD4(+) T-helper cell responses. We investigated the behavior of T-cell subsets stimulated with endotoxin-free HSP70 with respect to proliferation, cytokine expression, cytotoxicity against allogeneic B-lymphoblastoid cell line and K562 cells, as well as target-independent cytotoxicity. CD4(+) cells exhibited a strong increase in proliferation after stimulation with HSP70 (29%). In the presence of targets, a 35-fold up-regulation of granzyme B was observed after stimulation of CD4(+) T cells with HSP70 in combination with interleukin-7 (IL-7)/IL-12/IL-15. The target cell-independent secretion of granzyme B by CD4(+) cells was greatly augmented after stimulation with HSP70 plus IL-2 or IL-7/IL-12/IL-15. In this study, we showed that HSP70 is capable of inducing a cytotoxic response of T-helper cells in the absence of lipopolysaccharide. The granzyme B secretion and cytolytic activity of T-helper cells are induced in a target-independent way, whereas the cytotoxic activity of CD3(+) and CD8(+) T cells can be further enhanced in the presence of target cells. Our data provide novel insights into the role of extracellular HSP70 on T-cell immune response concerning the induction of target-independent T-helper cell cytotoxicity.

The consequences of apoptosis in autoimmunity

The clearance of apoptotic cells is a highly regulated mechanism, normally associated with anti-inflammatory response. During early stages of apoptosis the cell is promptly recognized and engulfed by professional phagocytes or tissue cells to avoid the outflow of intracellular content and limit the immunological reaction against released antigens. However, increasing evidences suggest that impairment in the uptake of apoptotic cell debris is linked to the development of autoimmunity. In fact, autoantigens have been demonstrated to be content within apoptotic bodies and apoptotic cells seems to be critical in the presentation of antigens, activation of innate immunity and regulation of macrophage cytokine secretion. We herein review the known mechanisms for regulating the uptake of the products of apoptosis in the development of autoimmunity.

Developments in the scientific understanding of lupus

Systemic lupus erythematosus is a systemic autoimmune disease characterized by the production of antinuclear antibodies (ANAs). Recent research into human and murine lupus suggests that disease susceptibility results from genetic polymorphisms regulating immune responses as well as impairing the clearance of apoptotic cells. Because the products of dead cells, including nucleic acids, have immunologic activity, this situation can promote antigen-driven ANA responses. Furthermore, immune complexes of ANAs can drive the production of proinflammatory cytokines, inducing the 'interferon signature', and intensifying disease. Together, these findings point to new genetic and immunologic markers of disease as well as targets for new therapies.

Apoptotic cell-mediated immunoregulation of dendritic cells does not require iC3b opsonization

A number of recent studies show that activation of CR3 on dendritic cells (DCs) suppresses TLR-induced TNF-alpha and IL-12 production and inhibits effective Ag presentation. Although the proposed physiologic role for these phenomena is immune suppression due to recognition of iC3b opsonized apoptotic cells by CR3, all of the aforementioned investigations used artificial means of activating CR3. We investigated whether iC3b opsonized apoptotic cells could induce the same changes reported with artificial ligands such as mAbs or iC3b-opsonized RBC. We explored the kinetics of iC3b opsonization in two models of murine cell apoptosis, gamma-irradiated thymocytes and cytokine deprivation of the IL-3 dependent cell line BaF3. Using a relatively homogenous population of early apoptotic cells (IL-3 deprived BaF3 cells), we show that iC3b opsonized apoptotic cells engage CR3, but this interaction is dispensable in mediating the anti-inflammatory effects of apoptotic cells. TLR-induced TNF-alpha and IL-12 production by bone marrow-derived DCs occurs heterogeneously, with apoptotic cells inhibiting only certain populations depending on the TLR agonist. In contrast, although apoptotic cells induced homogeneous IL-10 production by DCs, IL-10 was not necessary for the inhibition of TNF-alpha and IL-12. Furthermore, because the ability of iC3b opsonization to enhance phagocytosis of apoptotic cells has been controversial, we report that iC3b opsonization does not significantly affect apoptotic cell ingestion by DCs. We conclude that the apoptotic cell receptor system on DCs is sufficiently redundant such that the absence of CR3 engagement does not significantly affect the normal anti-inflammatory processing of apoptotic cells.