Involvement of Toll-like Receptors 2 and 4 in Cellular Activation by High Mobility Group Box 1 Protein

Alarmins: chemotactic activators of immune responses

The recruitment and activation of antigen-presenting cells are critical early steps in mounting an immune response. Many microbial components and endogenous mediators participate in this process. Recent studies have identified a group of structurally diverse multifunctional host proteins that are rapidly released following pathogen challenge and/or cell death and, most importantly, are able to both recruit and activate antigen-presenting cells. These potent immunostimulants, including defensins, cathelicidin, eosinophil-derived neurotoxin, and high-mobility group box protein 1, serve as early warning signals to activate innate and adaptive immune systems. We propose to highlight these proteins' unique activities by grouping them under the novel term 'alarmins', in recognition of their role in mobilizing the immune system.

Heat shock response inhibits release of high mobility group box 1 protein induced by endotoxin in murine macrophages

The purpose of this study was to evaluate the kinetic changes and the localization of high-mobility group box 1 protein (HMGB1) and to observe the effect of heat shock response (HSR) on the expression and release of HMGB1 in lipopolysaccharide (LPS)-activated murine macrophage-like RAW 264.7 cells. Reverse transcriptase (RT)-PCR and Western blot were used to examine HMGB1 expression after LPS treatment. The intracellular localization of HMGB1 in normal or LPS-activated cells was investigated by immunocytochemical analysis and HMGB1 released from cultured macrophages by Western blot. HSR was performed by incubating RAW 264.7 cells at 42.5 degrees C for 1 h then recovery at 37 degrees C for 12 h. The effect of HSR on expression and release of HMGB1 was observed. The results showed that a decrease of HMGB1 mRNA expression was observed at 18 h after LPS (500 ng/mL) treatment, although the total intracellular HMGB1 protein levels were not affected. A visible translocation of HMGB1 from the nuclear to the cytoplasm was observed at 20 h after stimulation with LPS (500 ng/mL). Furthermore, HMGB1 was released into the medium by LPS-activated RAW 264.7 cells in a time- and dose-dependent manner. Heat shock pretreatment significantly inhibited LPS-induced release of HMGB1 and the translocation of HMGB1 from the nucleus to the cytoplasm in RAW 264.7 cells. These findings suggest that the release of HMGB1 by LPS-activated macrophages is a late event in the pathogenesis of sepsis and that HSR could inhibit the release and translocation of HMGB1 induced by LPS.

High-mobility group box 1 protein is an inflammatory mediator in necrotizing enterocolitis: protective effect of the macrophage deactivator semapimod

High-mobility group box 1 (HMGB1) is a late mediator of endotoxemia known to stimulate the production of proinflammatory cytokines that are putative mediators of intestinal inflammation associated with necrotizing enterocolitis (NEC). We hypothesized that HMGB1 is also involved in the pathogenesis of NEC. We examined the expression of HMGB1 and the effect of the novel drug semapimod on intestinal inflammation in an experimental model of NEC in neonatal rats. Newborn rats were subjected to hypoxia and fed a conventional formula by gavage (FFH) or were breast fed (BF). Rats were killed on day 4, and the distal ileum was harvested for morphological studies and Western blot analysis. FFH newborn rats but not BF controls developed intestinal inflammation similar to the histological changes observed in human NEC. We found that the expression of HMGB1 and its receptor for advanced glycation end products (RAGE) as well as that of other apoptosis/inflammation-related proteins (Bad, Bax, inducible nitric oxide synthase, and cyclooxygenase 2) was upregulated in the ileal mucosa of FFH newborn rats compared with BF animals. Administration of the drug semapimod inhibited the upregulation of those proteins and partially protected the animals against the FFH-induced intestinal injury. Elevated levels of HMGB1 were also found in ileal samples from infants undergoing intestinal resection for acute NEC. Our results implicate HMGB1 and RAGE as important mediators of enterocyte cell death and hypoxia-induced injury in NEC and support the hypothesis that inhibitors such as semapimod might play a therapeutic role in chronic intestinal inflammation characterized by this animal model.

Toll-like receptors and chronic lung disease

TLRs (Toll-like receptors) comprise a family of proteins whose function is principally to facilitate the detection of, and response to, pathogens. Protozoa, helminths, viruses, bacteria and fungi can all activate TLR signalling, and these signals have important roles in the activation of host defence. TLRs may also respond to products of tissue damage, providing them with roles in infective and sterile inflammation. Their role as detectors of pathogens and pathogen-associated molecules provides molecular mechanisms to underpin the observations leading to the hygiene hypothesis. Targeting of TLR signalling has implications in the control of infection, vaccine design, desensitization to allergens and down-regulation of inflammation. This review will explore TLR history, molecular signalling and the potential roles of TLRs in chronic lung disease.

HMGB1: guiding immunity from within

Two of the main challenges that eukaryotic multicellular organisms faced during evolution were to eliminate and replace dying cells and to cope with invading microorganisms. The innate immune system evolved to handle both tasks: to scavenge cellular debris and to form the first line of defence against microbes. In this review, we focus on high mobility group box 1 (HMGB1) protein as a common signal that alerts the innate immune system to excessive or deregulated cell death and to microbial invasion. HMGB1, which is well known nuclear protein, has revealed unexpected facets as an extracellular mediator. The role of HMGB1 as an endogenous molecule that facilitates immune responses and has an important role in tissue homeostasis and disease will be highlighted here.

Requirement of HMGB1 and RAGE for the maturation of human plasmacytoid dendritic cells

Dendritic cells (DC) are key components of innate and adaptive immune responses. Plasmacytoid DC (PDC) are a specialized DC subset that produce high amounts of type I interferons in response to microbes. High mobility group box 1 protein (HMGB1) is an abundant nuclear protein, which acts as a potent pro-inflammatory factor when released extracellularly. We show that HMGB1 leaves the nucleus of maturing PDC following TLR9 activation, and that PDC express on the plasma membrane the best-characterized receptor for HMGB1, RAGE. Maturation and type I IFN secretion of PDC is hindered when the HMGB1/RAGE pathway is disrupted. These results reveal HMGB1 and RAGE as the first known autocrine loop modulating the maturation of PDC, and suggest that antagonists of HMGB1/RAGE might have therapeutic potential for the treatment of systemic human diseases.

Functional Connectivity between Immune Cells Mediated by Tunneling Nanotubules

Intercellular signals can be transmitted through neuronal synapses or through gap junctions, with the latter mediating transmission of calcium fluxes and small molecules between cells. We show here that a third form of communication between cells can be mediated by tunneling nanotubules (TNT). When myeloid-lineage dendritic cells and monocytes are triggered to flux calcium by chemical or mechanical stimulation, the signal can be propagated within seconds to other cells at distances hundreds of microns away via TNT. A complex and transient network of TNT is seen in live cells, with individual tubules exhibiting substantial variation in length and diameter. In addition to calcium fluxes, microinjected dye tracers can be transferred through these connections. Following TNT-mediated stimulation, spreading of lamellipodia occurs in dendritic cells characteristic of that seen during the phagocytic response to bacteria. These results demonstrate that nonneuronal cells can transmit signals to distant cells through a physically connected network.

Release of high mobility group box 1 by dendritic cells controls T cell activation via the receptor for advanced glycation end products

High mobility group box 1 (HMGB1) is an abundant and conserved nuclear protein that is released by necrotic cells and acts in the extracellular environment as a primary proinflammatory signal. In this study we show that human dendritic cells, which are specialized in Ag presentation to T cells, actively release their own HMGB1 into the extracellular milieu upon activation. This secreted HMGB1 is necessary for the up-regulation of CD80, CD83, and CD86 surface markers of human dendritic cells and for IL-12 production. The HMGB1 secreted by dendritic cells is also required for the clonal expansion, survival, and functional polarization of naive T cells. Using neutralizing Abs and receptor for advanced glycation end product-deficient (RAGE(-/-)) cells, we demonstrate that RAGE is required for the effect of HMGB1 on dendritic cells. HMGB1/RAGE interaction results in downstream activation of MAPKs and NF-kappaB. The use of an ancient signal of necrosis, HMGB1, by dendritic cells to sustain their own maturation and for activation of T lymphocytes represents a profitable evolutionary mechanism.

Apoptotic cells at the crossroads of tolerance and immunity

Clearance of apoptotic cells by phagocytes can result in either anti-inflammatory and immunosuppressive effects or prostimulatory consequences through presentation of cell-associated antigens to T cells. The differences in outcome are due to the conditions under which apoptosis is induced, the type of phagocytic cell, the nature of the receptors involved in apoptotic cell capture, and the milieu in which phagocytosis of apoptotic cells takes place. Preferential ligation of specific receptors on professional antigen-presenting cells (dendritic cells) has been proposed to induce potentially tolerogenic signals. On the other hand, dendritic cells can efficiently process and present antigens from pathogen-infected apoptotic cells to T cells. In this review, we discuss how apoptotic cells manipulate immunity through interactions with dendritic cells.

The effects of CpG DNA on HMGB1 release by murine macrophage cell lines

DNA containing cytosine-guanine dinucleotide (CpG) motifs (CpG DNA) has potent immunostimulatory activities that resemble those of lipopolysaccharide (LPS) in its effects on the innate immune system. Among its activities, LPS can induce the release of high mobility group protein (HMGB1) by macrophages, a dual function molecule that can mediate the late effects of LPS. To determine whether CpG DNA can also induce HMGB1 release, the effects of a synthetic CpG oligonucleotide (ODN) on HMGB1 release from RAW 264.7 and J774A.1 cells were assessed by Western blotting of culture supernatants. Under conditions in which the CpG ODN activated the cell lines, as assessed by stimulation of tumor necrosis factor alpha and interleukin-12, it failed to cause HMGB1 release into the media. Although unable to induce HMGB1 release by itself, the CpG ODN nevertheless potentiated the action of LPS. With RAW 264.7 cells, lipoteichoic acid and polyinosinic-polycytidylic acid, like LPS, stimulated HMGB1 release as well as cytokine production. These results indicate that the effects of CpG DNA on macrophages differ from other ligands of Toll-like receptors and may lead to a distinct pattern of immune cell activation in the context of infection or its use as an immunomodulatory agent.

Toll-like receptors, endogenous ligands, and systemic autoimmune disease

The critical role of Toll-like receptors (TLRs) as mediators of pathogen recognition by the innate immune system is now firmly established. Such recognition results in the initiation of an inflammatory immune response and subsequent instruction of the adaptive immune system, both of which are designed to rid the host of the invading pathogen. More controversial is the potential role of TLRs in the recognition of endogenous ligands and what effect this might have on the consequent development of autoimmune or other chronic sterile inflammatory disorders. An increasing number of studies implicate TLRs as being involved in the immune response to self-molecules that have in some way been altered from their native state or accumulate in non-physiologic sites or amounts, although questions have been raised about possible contaminants in certain of these studies. In this review, we discuss the evidence for endogenous ligand-TLR interactions with particular emphasis on mammalian chromatin, systemic lupus erythematosus, and atherosclerosis. Overall, the data support the general concept of a role for TLRs in the recognition of endogenous ligands. However, the precise details of the interactions and the extent to which they may contribute to the pathogenesis of human disease remain to be clarified.

Necrosis is associated with IL-6 production but apoptosis is not

Due to loss of cell membrane integrity, necrotic cells passively release several cytosolic factors that can activate antigen presenting cells and other immune cells. In contrast, cells dying by apoptosis do not induce an inflammatory response. Here we show that necrotic cell death induced by several stimuli, such as TNF, anti-Fas or dsRNA, coincides with NF-kappaB-and p38MAPK-mediated upregulation and secretion of the pro-inflammatory cytokine IL-6. This event is greatly reduced or absent in conditions of apoptotic cell death induced by the same stimuli. This demonstrates that besides the capacity of necrotic cells to induce an inflammatory response due to leakage of cellular contents, necrotic dying cells themselves are involved in the expression and secretion of inflammatory cytokines. Moreover, inhibition of NF-kappaB and p38MAPK activation does not affect necrotic cell death in all conditions tested. This suggests that the activation of inflammatory pathways is distinct from the activation of necrotic cell death sensu strictu.

High-mobility group box 1 protein (HMGB1): nuclear weapon in the immune arsenal

High-mobility group box 1 protein (HMGB1), which previously was thought to function only as a nuclear factor that enhances transcription, was recently discovered to be a crucial cytokine that mediates the response to infection, injury and inflammation. These observations have led to the emergence of a new field in immunology that is focused on understanding the mechanisms of HMGB1 release, its biological activities and its pathological effects in sepsis, arthritis, cancer and other diseases. Here, we discuss these features of HMGB1 and summarize recent advances that have led to the preclinical development of therapeutics that modulate HMGB1 release and activity.

Skeletal muscle cytokines: regulation by pathogen-associated molecules and catabolic hormones

PURPOSE OF REVIEW

This review will update clinicians and basic scientists who study the molecular mechanisms of muscle wasting associated with infection, trauma, cancer cachexia, and AIDS. A special emphasis is placed on recent studies that examine the interaction of insulin-like growth factor 1 and proinflammatory cytokines as positive and negative regulators of muscle mass.

RECENT FINDINGS

Potential mediators of the wasting syndromes include catabolic hormones, such as glucocorticoids, as well as the inflammatory cytokines tumour necrosis factor, IL-1, and IL-6. Cytokines may function either systemically or locally within muscle per se. Lipopolysaccharide and other pathogen-associated molecules stimulate cytokine expression in muscle. The failure to clear pathogen-associated molecules or the introduction of muscle damage may initiate a protracted activation of enzymes and transcription factors that orchestrate a genetic programme that ultimately produces muscle wasting.

SUMMARY

This review highlights recent advances in our understanding of the expression of the afferent and efferent limbs of the innate immune system in skeletal muscle. A special emphasis is placed on the recognition of pathogen-associated molecules by skeletal muscle cells and how these molecules regulate the expression of inflammatory cytokines and other muscle genes to result in muscle wasting, and when sustained, the erosion of lean body mass.

Monocytes promote natural killer cell interferon gamma production in response to the endogenous danger signal HMGB1

Substantial attention has been paid to the role of the toll-like receptor (TLR) ligands of late and their role in regulating the innate immune response. They serve as exogenous danger signals important in informing and driving the distal adaptive immune response to pathogens. Less clear has been the role of the nominal endogenous danger signals released and recognized in stressed cells following genotoxic or metabolic stress as occurs in progressively growing tumors. HMGB1 (high-mobility group B1) is a nuclear protein well characterized for its ability to modify DNA access to transcriptional proteins that is released from necrotic cells as well as secreted through the endosomal route from hematopoietic cells, serving as a late mediator of sepsis. It interacts with high-affinity RAGE (receptor for advanced glycation end products) and TLR2 receptors. Here we show that HMGB1 enhances interferon gamma release from macrophage (but not dendritic cell)-stimulated NK cells. This is effective only when coupled with other pro-inflammatory cytokines particularly with IL-2 in combination with IL-1 or IL-12. We have used this information to suggest that HMGB1, which also promotes epithelial migration and proliferation, drives repair in the absence or inhibition of other factors but enhances inflammation in their presence. The implications for tumorigenesis and tumor progression are quite important as they may be for other states of chronic inflammation.

Extracellular HMGB1 as a proinflammatory cytokine

High mobility group box-1 protein (HMGB1, formerly known as HMG-1), a highly conserved ubiquitous protein, has been for a long time described as a nuclear DNA-binding protein involved in nucleosome stabilization and gene transcription. Recent discoveries indicate that HMGB1 is released from activated innate immune cells or necrotic cells and functions as an important mediator of endotoxemia, sepsis, arthritis, and local inflammation. Therapeutic agents that inhibit HMGB1 release or action confer significant protection against endotoxemia, sepsis, and arthritis in animal models and thus hold potential for the clinical management of various inflammatory diseases.

The nuclear factor HMGB1 mediates hepatic injury after murine liver ischemia-reperfusion

High-mobility group box 1 (HMGB1) is a nuclear factor that is released extracellularly as a late mediator of lethality in sepsis as well as after necrotic, but not apoptotic, death. Here we demonstrate that in contrast to the delayed role of HMGB1 in the systemic inflammation of sepsis, HMGB1 acts as an early mediator of inflammation and organ damage in hepatic ischemia reperfusion (I/R) injury. HMGB1 levels were increased during liver I/R as early as 1 h after reperfusion and then increased in a time-dependent manner up to 24 h. Inhibition of HMGB1 activity with neutralizing antibody significantly decreased liver damage after I/R, whereas administration of recombinant HMGB1 worsened I/R injury. Treatment with neutralizing antibody was associated with less phosphorylation of c-Jun NH₂-terminal kinase and higher nuclear factor–κB DNA binding in the liver after I/R. Toll-like receptor 4 (TLR4)-defective (C3H/Hej) mice exhibited less damage in the hepatic I/R model than did wild-type (C3H/HeOuj) mice. Anti-HMGB1 antibody failed to provide protection in C3H/Hej mice, but successfully reduced damage in C3H/Ouj mice. Together, these results demonstrate that HMGB1 is an early mediator of injury and inflammation in liver I/R and implicates TLR4 as one of the receptors that is involved in the process.

Advanced glycation end products and RAGE: a common thread in aging, diabetes, neurodegeneration, and inflammation

The products of nonenzymatic glycation and oxidation of proteins and lipids, the advanced glycation end products (AGEs), accumulate in a wide variety of environments. AGEs may be generated rapidly or over long times stimulated by a range of distinct triggering mechanisms, thereby accounting for their roles in multiple settings and disease states. A critical property of AGEs is their ability to activate receptor for advanced glycation end products (RAGE), a signal transduction receptor of the immunoglobulin superfamily. It is our hypothesis that due to such interaction, AGEs impart a potent impact in tissues, stimulating processes linked to inflammation and its consequences. We hypothesize that AGEs cause perturbation in a diverse group of diseases, such as diabetes, inflammation, neurodegeneration, and aging. Thus, we propose that targeting this pathway may represent a logical step in the prevention/treatment of the sequelae of these disorders.

RAGE is the major receptor for the proinflammatory activity of HMGB1 in rodent macrophages

Abstract High-mobility group box chromosomal protein 1 (HMGB1) is a protein with both intranuclear functions and extracellular cytokine-like effects. In this report, we study possible candidate receptors for HMGB1 on macrophages (Mphi) and define pathways activated by HMGB1 binding. Bone marrow Mphi were prepared from Dark Agouti (DA) rats and stimulated in vitro with HMGB1. The kinetics of tumour necrosis factor (TNF) production, NO production, activation of p38 mitogen-activated protein kinase (MAPK), p44/42 MAPK- and SAPK/JNK-signalling pathways, nuclear translocation of nuclear factor kappa B (NF-kappaB) and HMGB1-induced upregulation of major histocompatibility complex (MHC) class II and CD86 were analysed. Mphi from interleukin (IL)-1 receptor type I-/-, Toll-like receptor 2 (TLR2-/-) and RAGE-/- mice were used to investigate the role of these receptors in HMGB1 signalling. HMGB1 induced TNF and NO production by Mphi, phosphorylation of all investigated MAP kinase pathways and NF-kappaB translocation, and expression of MHC class II was increased. Mphi from RAGE-/- mice produced significantly lower amounts of TNF, IL-1beta and IL-6, while IL-1RI-/- and TLR2-/- Mphi produced cytokine levels comparable with wildtype controls in response to HMGB1 stimulation. We conclude that HMGB1 has the potential to induce a proinflammatory phenotype in Mphi, with RAGE as the major activation-inducing receptor.

Atherosclerosis and restenosis: is there a role for RAGE?

Diabetic vascular complications are a major cause of morbidity and mortality. Furthermore, such vascular disease is only incompletely explained by "traditional" risk factors in the nondiabetic complications. This situation has prompted the search for factors contributing to the pathogenesis of accelerated and more severe vascular disease in patients with diabetes. We review evidence that receptor for advanced glycation end products (RAGE), via its interaction with ligands, serves as a cofactor exacerbating diabetic vascular disease. RAGE is a member of the immunoglobulin superfamily of cell surface molecules with a diverse repertoire of ligands reminiscent of pattern recognition receptors. In the diabetic milieu, two classes of RAGE ligands, products of nonenzymatic glycoxidation and S100 proteins, appear to drive receptor-mediated cellular activation and, potentially, acceleration of vascular disease.

Activation of human umbilical vein endothelial cells leads to relocation and release of high-mobility group box chromosomal protein 1

The nuclear protein high-mobility group box chromosomal protein 1 (HMGB1) was recently described to act as a pro-inflammatory cytokine and as a late mediator of severe sepsis and septic shock. The protein is released from monocytes in response to endotoxin and activates monocytes and endothelial cells through nuclear factor kappa B. We have previously demonstrated that the B-box of HMGB1 mediates a pro-inflammatory effect on endothelial cells including the upregulation of cell-adhesion molecules and release of interleukin (IL)-8 and granulocyte colony-stimulating factor. Here, we report that HMGB1 is released from human umbilical vein endothelial cells (HUVEC) in response to lipopolysaccharide (LPS) and tumour necrosis factor (TNF)-alpha. A nuclear relocation of HMGB1 to the cytoplasm was seen at 4 h. Subsequently, high amounts of HMGB1 could be seen in the supernatants from stimulated cells after 16 h. It was also observed that the pro-inflammatory activity of HMGB1 is sensitive to dexamethasone. Interestingly, the HMGB1-induced TNF-alpha release from monocytes could be inhibited by either the A-box of the protein or the p38 inhibitor CNI-1493, but neither had any inhibitory effects on the HMGB1-dependent upregulation of cell-adhesion molecules on HUVEC. Altogether, these results suggest that HUVEC may be an important source of HMGB1 secretion in response to systemic infection and that endothelial cells and monocytes may use different signalling pathways.

Agonists of toll-like receptors 2 and 4 activate airway smooth muscle via mononuclear leukocytes

RATIONALE

Toll-like receptors 2 and 4 (TLR2, TLR4) enable cellular responses to bacterial lipoproteins, LPS, and endogenous mediators of cell damage. They have an established role in the activation of leukocytes, endothelial cells, and some smooth muscle cell types, but their roles in airway smooth muscle are uncertain.

OBJECTIVES

To determine the roles of TLRs in activation of airway smooth muscle.

METHODS

Airway smooth muscle cells were cultured with TLR agonists, in the presence or absence of mononuclear leukocytes.

MEASUREMENTS AND MAIN RESULTS

We observed expression of TLR2 and TLR4 mRNAs, which could be upregulated by treatment with proinflammatory cytokines in primary human airway smooth muscle, but no important functional responses to agonists of these TLRs were seen. Coincubation of airway smooth muscle with peripheral blood mononuclear cells, at concentrations as low as 250 mononuclear cells/ml, resulted in a marked cooperative response to TLR stimuli, and synergistic production of cytokines, including chemokines (interleukin [IL-]-8) and IL-6. This cooperative response was greater when monocytes were enriched and was transferable using supernatants from LPS-stimulated peripheral blood mononuclear cells. Activation of cocultures required IL-1 generation from mononuclear cells, and was blocked by IL-1 receptor antagonist, though IL-1 generation alone was not sufficient to account for the magnitude of mononuclear cell-dependent coculture activation.

CONCLUSIONS

These data indicate that potent amplification of inflammation induced by TLR agonists, such as LPS, may be achieved by cooperativity between airway smooth muscle and leukocytes involved in immune surveillance or inflammation.

HMGB1 contributes to the development of acute lung injury after hemorrhage

High mobility group box 1 (HMGB1) is a novel late mediator of inflammatory responses that contributes to endotoxin-induced acute lung injury and sepsis-associated lethality. Although acute lung injury is a frequent complication of severe blood loss, the contribution of HMGB1 to organ system dysfunction in this setting has not been investigated. In this study, HMGB1 was detected in pulmonary endothelial cells and macrophages under baseline conditions. After hemorrhage, in addition to positively staining endothelial cells and macrophages, neutrophils expressing HMGB1 were present in the lungs. HMGB1 expression in the lung was found to be increased within 4 h of hemorrhage and then remained elevated for more than 72 h after blood loss. Neutrophils appeared to contribute to the increase in posthemorrhage pulmonary HMGB1 expression since no change in lung HMGB1 levels was found after hemorrhage in mice made neutropenic with cyclophosphamide. Plasma concentrations of HMGB1 also increased after hemorrhage. Blockade of HMGB1 by administration of anti-HMGB1 antibodies prevented hemorrhage-induced increases in nuclear translocation of NF-kappa B in the lungs and pulmonary levels of proinflammatory cytokines, including keratinocyte-derived chemokine, IL-6, and IL-1 beta. Similarly, both the accumulation of neutrophils in the lung as well as enhanced lung permeability were reduced when anti-HMGB1 antibodies were injected after hemorrhage. These results demonstrate that hemorrhage results in increased HMGB1 expression in the lungs, primarily through neutrophil sources, and that HMGB1 participates in hemorrhage-induced acute lung injury.

The macrophage and the apoptotic cell: an innate immune interaction viewed simplistically?

Macrophages play important roles in the clearance of dying and dead cells. Typically, and perhaps simplistically, they are viewed as the professional phagocytes of apoptotic cells. Clearance by macrophages of cells undergoing apoptosis is a non-phlogistic phenomenon which is often associated with actively anti-inflammatory phagocyte responses. By contrast, macrophage responses to necrotic cells, including secondarily necrotic cells derived from uncleared apoptotic cells, are perceived as proinflammatory. Indeed, persistence of apoptotic cells as a result of defective apoptotic-cell clearance has been found to be associated with the pathogenesis of autoimmune disease. Here we review the mechanisms by which macrophages interact with, and respond to, apoptotic cells. We suggest that macrophages are especially important in clearing cells at sites of histologically visible, high-rate apoptosis and that, otherwise, apoptotic cells are removed largely by non-macrophage neighbours. We challenge the view that necrotic cells, including persistent apoptotic cells are, of necessity, proinflammatory and immunostimulatory and suggest that, under appropriate circumstances, persistent apoptotic cells can provide a prolonged anti-inflammatory stimulus.

High mobility group box 1 protein, a cue for stem cell recruitment

High mobility group box 1 (HMGB1) is a non-histone protein required to maintain chromatin architecture. Recent observations demonstrated that HMGB1 can also act as a cytokine to regulate different biological processes such as inflammation, cell migration and metastasis. We showed previously that HMGB1 can be released passively by cells that die in a traumatic and unprogrammed way, and can serve a signal of tissue damage. More recently, we showed that HMGB1 can recruit stem cells: HMGB1 induces stem cell transmigration through an endothelial barrier; moreover, when beads containing HMGB1 are implanted into healthy muscle, they recruit stem cells injected into the general circulation. The inflammatory and tissue-regenerating roles of HMGB1 may be strictly interconnected, and are discussed here.

HMGB1 as a mediator of necrosis-induced inflammation and a therapeutic target in arthritis

For the second time in recent history, studies directed at the pathogenesis of infectious disease have led to the identification of an endogenous mediator of arthritis. HMGB1, a 30-kD nuclear and cytoplasmic protein widely studied as a DNA-binding protein, is a newly described cytokine and a necessary and sufficient mediator of lethal sepsis. HMGB1 is passively released during cell necrosis, but not apoptosis; it activates an inflammatory response to necrosis,but not apoptosis. Furthermore, HMGB1 can also be actively secreted by stimulated macrophages or monocytes in a process that requires acetylation of the molecule, enabling a translocation from the nucleus to secretory lysosomes. Recent evidence indicates that HMGB1 is a mediator of arthritis because of the following: (1) it is produced at the site of joint inflammation, (2) it causes the development of arthritis when applied to normal joints, and (3) therapies that inhibit HMGB1 prevent the progression of collagen-induced arthritis in rodents. Anti-HMGB1 may be studied in future clinical trials of diseases of excessive production of HMGB1, such as severe sepsis and arthritis.

HMGB1 is an endogenous immune adjuvant released by necrotic cells

Immune responses against pathogens require that microbial components promote the activation of antigen-presenting cells (APCs). Autoimmune diseases and graft rejections occur in the absence of pathogens; in these conditions, endogenous molecules, the so-called 'innate adjuvants', activate APCs. Necrotic cells contain and release innate adjuvants; necrotic cells also release high-mobility group B1 protein (HMGB1), an abundant and conserved constituent of vertebrate nuclei. Here, we show that necrotic HMGB1(-/-) cells have a reduced ability to activate APCs, and HMGB1 blockade reduces the activation induced by necrotic wild-type cell supernatants. In vivo, HMGB1 enhances the primary antibody responses to soluble antigens and transforms poorly immunogenic apoptotic lymphoma cells into efficient vaccines.

High mobility group box protein 1: an endogenous signal for dendritic cell maturation and Th1 polarization

High mobility group box protein 1 (HMGB1), a DNA binding nuclear and cytosolic protein, is a proinflammatory cytokine released by monocytes and macrophages. This study addressed the hypothesis that HMGB1 is an immunostimulatory signal that induces dendritic cell (DC) maturation. We show that HMGB1, via its B box domain, induced phenotypic maturation of DCs, as evidenced by increased CD83, CD54, CD80, CD40, CD58, and MHC class II expression and decreased CD206 expression. The B box caused increased secretion of the proinflammatory cytokines IL-12, IL-6, IL-1alpha, IL-8, TNF-alpha, and RANTES. B box up-regulated CD83 expression as well as IL-6 secretion via a p38 MAPK-dependent pathway. In the MLR, B box-activated DCs acted as potent stimulators of allogeneic T cells, and the magnitude of the response was equivalent to DCs activated by exposure to LPS, nonmethylated CpG oligonucleotides, or CD40L. Furthermore, B box induced secretion of IL-12 from DCs as well as IL-2 and IFN-gamma secretion from allogeneic T cells, suggesting a Th1 bias. HMGB1 released by necrotic cells may be a signal of tissue or cellular injury that, when sensed by DCs, induces and/or enhances an immune reaction.

Endogenous ligands of Toll-like receptors

Extensive work has suggested that a number of endogenous molecules such as heat shock proteins (hsp) may be potent activators of the innate immune system capable of inducing proinflammatory cytokine production by the monocyte-macrophage system and the activation and maturation of dendritic cells. The cytokine-like effects of these endogenous molecules are mediated via the Toll-like receptor (TLR) signal-transduction pathways in a manner similar to lipopolysaccharide (LPS; via TLR4) and bacterial lipoproteins (via TLR2). However, recent evidence suggests that the reported cytokine effects of hsp may be a result of the contaminating LPS and LPS-associated molecules. The reasons for previous failure to recognize the contaminant(s) being responsible for the putative TLR ligands of hsp include failure to use highly purified hsp free of LPS contamination; failure to recognize the heat sensitivity of LPS; and failure to consider contaminant(s) other than LPS. Whether other reported putative endogenous ligands of TLR2 and TLR4 are a result of contamination of pathogen-associated molecular patterns is not clear. It is essential that efforts should be directed to conclusively determine whether the reported putative endogenous ligands of TLRs are a result of the endogenous molecules or of contaminant(s), before exploring further the implication and therapeutic potential of these putative TLR ligands.