Nucleotides released by apoptotic cells act as a find-me signal to promote phagocytic clearance

Leukocyte migratory responses to apoptosis: the attraction and the distraction

An expanding body of evidence demonstrates that cells undergoing apoptosis send out a selection of molecular navigational signals including proteins, lipids and nucleotides that serve to recruit phagocytes to the dying targets which are subsequently engulfed and removed. This homeostatic process is essentially non-phlogistic, contrasting markedly with the acute inflammatory responses elicited in phagocytes by damaging or infectious agents. The "professional" scavengers of apoptotic cells are mononuclear phagocytes-the macrophages-and sites of high-rate apoptosis are clearly characterized by macrophages associated with the apoptotic cells. By contrast, members of the other class of professional phagocytes-the granulocytes-are not recruited to sites of apoptosis as a direct consequence of the cell-death program. Indeed, recent work indicates that apoptotic cells release a mixture of migratory cues to leukocytes in order to selectively attract mononuclear phagocytes but not granulocytes through functional balancing of positive and negative signals. Here we discuss these molecular mechanisms which not only serve as migratory cues but also may activate responding phagocytes to engulf apoptotic cells effectively. Finally, we speculate upon new therapeutic opportunities these mechanisms offer for a range of pathological conditions, including inflammatory disorders and cancer.

Expression and functional characterization of P2X receptors in mouse alveolar macrophages

Alveolar macrophages (AM) are crucial for pulmonary host defense, and evidence emerges that ATP-gated P2X receptors are involved in inflammatory processes. This study focuses on the expression and functional characterization of P2X receptors in AM from mouse. In RT-PCR experiments, transcripts encoding the P2X₁, P2X₃, P2X₄, P2X₅, and P2X₇ receptors were detected. In whole-cell patch-clamp recordings, ATP (1 mM) evoked an inward current (mouse and human AM) that was reversible upon washout, and the reversal potential was ~5 mV, indicating the activation of a non-selective conductance-a fingerprint of P2X receptors. Further characterization (mouse AM) revealed that the current was not desensitized by a second ATP application. The ATP-induced current was increased by the removal of extracellular Ca²⁺ (in human and mouse AM), and EC₅₀ in mouse AM were determined with ~1 mM ATP, in the presence as well as in the absence of extracellular Ca²⁺. Pharmacological characterization of mouse AM revealed that the effect was augmented by BzATP and pre-application with ivermectin, but no effect with α,β-meATP was observed. Further, the ATP effect was reduced by PPADS (300 μM), brilliant blue G (5 μM), and about A438079 (10 μM). Although different P2X receptor transcripts were detected in mouse AM, the observed functional and pharmacological characteristics indicate primarily the participation of P2X₄ and P2X₇ receptors as mediators of the ATP-induced ion current in mouse AM. These suggestions were confirmed by experiments with AM from P2X₇ -deficient animals, indicating a contribution of P2X₄ and P2X₇ receptors in pulmonary immune function.

RahU: an inducible and functionally pleiotropic protein in Pseudomonas aeruginosa modulates innate immunity and inflammation in host cells

The aim of this study was to define the functional role of a recently identified RahU protein from Pseudomonas aeruginosa in macrophages and its role in bacterial defense. Recombinant (r)-RahU had no significant effect on cell apoptosis or cell viability in human monocytic THP-1 cells. Gene expression array of murine macrophage cells (RAW 264.7) stimulated with LPS showed modulation of common transcripts (by r-RahU and predisone) involved in inflammation. Functional cellular analysis showed RAW cells incubated with r-RahU at 1.0-10 μg/ml (0.06-0.6 μM) inhibited accumulation of nitric oxide (NO) in the presence of LPS by 10-50%. The IC(50) of r-RahU (0.6 μM) was distinct from the known inhibitors of NO production: prednisone (50 μM) and L-NMMA (100 μM). r-RahU also significantly inhibited chemotactic activity of THP-1 cells toward CCL2 or chemotactic supernatants from apoptotic T-cells. These reports show previously unknown pleiotropic properties of RahU in modulating both microbial physiology and host innate immunity.

Rho signaling regulates pannexin 1-mediated ATP release from airway epithelia

ATP released from airway epithelial cells promotes purinergic receptor-regulated mucociliary clearance activities necessary for innate lung defense. Cell swelling-induced membrane stretch/strain is a common stimulus that promotes airway epithelial ATP release, but the mechanisms transducing cell swelling into ATP release are incompletely understood. Using knockdown and knockout approaches, we tested the hypothesis that pannexin 1 mediates ATP release from hypotonically swollen airway epithelia and investigated mechanisms regulating this activity. Well differentiated primary cultures of human bronchial epithelial cells subjected to hypotonic challenge exhibited enhanced ATP release, which was paralleled by the uptake of the pannexin probe propidium iodide. Both responses were reduced by pannexin 1 inhibitors and by knocking down pannexin 1. Importantly, hypotonicity-evoked ATP release from freshly excised tracheas and dye uptake in primary tracheal epithelial cells were impaired in pannexin 1 knockout mice. Hypotonicity-promoted ATP release and dye uptake in primary well differentiated human bronchial epithelial cells was accompanied by RhoA activation and myosin light chain phosphorylation and was reduced by the RhoA dominant negative mutant RhoA(T19N) and Rho and myosin light chain kinase inhibitors. ATP release and Rho activation were reduced by highly selective inhibitors of transient receptor potential vanilloid 4 (TRPV4). Lastly, knocking down TRPV4 impaired hypotonicity-evoked airway epithelial ATP release. Our data suggest that TRPV4 and Rho transduce cell membrane stretch/strain into pannexin 1-mediated ATP release in airway epithelia.

Dendritic cells exposed to MVA-based HIV-1 vaccine induce highly functional HIV-1-specific CD8(+) T cell responses in HIV-1-infected individuals

Currently, MVA virus vectors carrying HIV-1 genes are being developed as HIV-1/AIDS prophylactic/therapeutic vaccines. Nevertheless, little is known about the impact of these vectors on human dendritic cells (DC) and their capacity to present HIV-1 antigens to human HIV-specific T cells. This study aimed to characterize the interaction of MVA and MVA expressing the HIV-1 genes Env-Gag-Pol-Nef of clade B (referred to as MVA-B) in human monocyte-derived dendritic cells (MDDC) and the subsequent processes of HIV-1 antigen presentation and activation of memory HIV-1-specific T lymphocytes. For these purposes, we performed ex vivo assays with MDDC and autologous lymphocytes from asymptomatic HIV-infected patients. Infection of MDDC with MVA-B or MVA, at the optimal dose of 0.3 PFU/MDDC, induced by itself a moderate degree of maturation of MDDC, involving secretion of cytokines and chemokines (IL1-ra, IL-7, TNF-α, IL-6, IL-12, IL-15, IL-8, MCP-1, MIP-1α, MIP-1β, RANTES, IP-10, MIG, and IFN-α). MDDC infected with MVA or MVA-B and following a period of 48 h or 72 h of maturation were able to migrate toward CCL19 or CCL21 chemokine gradients. MVA-B infection induced apoptosis of the infected cells and the resulting apoptotic bodies were engulfed by the uninfected MDDC, which cross-presented HIV-1 antigens to autologous CD8⁺ T lymphocytes. MVA-B-infected MDDC co-cultured with autologous T lymphocytes induced a highly functional HIV-specific CD8⁺ T cell response including proliferation, secretion of IFN-γ, IL-2, TNF-α, MIP-1β, MIP-1α, RANTES and IL-6, and strong cytotoxic activity against autologous HIV-1-infected CD4⁺ T lymphocytes. These results evidence the adjuvant role of the vector itself (MVA) and support the clinical development of prophylactic and therapeutic anti-HIV vaccines based on MVA-B.

Cell death signaling and anticancer therapy

For a long time, it was commonly believed that efficient anticancer regimens would either trigger the apoptotic demise of tumor cells or induce a permanent arrest in the G₁ phase of the cell cycle, i.e., senescence. The recent discovery that necrosis can occur in a regulated fashion and the increasingly more precise characterization of the underlying molecular mechanisms have raised great interest, as non-apoptotic pathways might be instrumental to circumvent the resistance of cancer cells to conventional, pro-apoptotic therapeutic regimens. Moreover, it has been shown that some anticancer regimens engage lethal signaling cascades that can ignite multiple oncosuppressive mechanisms, including apoptosis, necrosis, and senescence. Among these signaling pathways is mitotic catastrophe, whose role as a bona fide cell death mechanism has recently been reconsidered. Thus, anticancer regimens get ever more sophisticated, and often distinct strategies are combined to maximize efficacy and minimize side effects. In this review, we will discuss the importance of apoptosis, necrosis, and mitotic catastrophe in the response of tumor cells to the most common clinically employed and experimental anticancer agents.

P2Y(6) agonist uridine 5'-diphosphate promotes host defense against bacterial infection via monocyte chemoattractant protein-1-mediated monocytes/macrophages recruitment

Extracellular nucleotides are important messengers involved in series crucial physiological functions through the activation of P2 purinergic receptors. The detailed function and mechanism of the P2Y family in regulating immune response against invaded pathogens still remains unknown. In this study, the activation of purinoreceptor P2Y(6) by UDP was found to play a crucial role in promoting host defense against invaded bacteria through monocytes/macrophages recruitment. The expression level of P2Y(6) was much higher than other purinoreceptors in RAW264.7 cells, bone marrow macrophages, and peritoneal macrophages determined by real-time PCR. The supernatant of UDP (P2Y(6)-specific agonist)-treated RAW264.7 cells exhibited direct chemotaxis to monocytes/macrophages in vitro through Boyden Chambers assay. Meanwhile, the releasing of MCP-1 (MCP-1/CCL2) was enhanced obviously by UDP both in mRNA and protein level. Furthermore, the activation of P2Y(6) receptor by UDP also promotes ERK phosphorylation and AP-1 activation in a concentration- and time-dependent manner in RAW264.7 cells. This UDP-induced activation could be inhibited by P2Y(6) selectivity antagonist (MRS2578), MEK inhibitor (U0126), and MCP-1 blocking Ab, respectively. Moreover, i.p. injection with UDP resulted in a more efficacious clearance of invaded Escherichia coli and lower mortality in peritonitis mouse model. Together, our studies demonstrate that P2Y(6) receptor could be a novel mediator in upregulating innate immune response against the invaded pathogens through recruiting monocytes/macrophages.

Axonal regeneration proceeds through specific axonal fusion in transected C. elegans neurons

Functional neuronal recovery following injury arises when severed axons reconnect with their targets. In Caenorhabditis elegans following laser-induced axotomy, the axon still attached to the cell body is able to regrow and reconnect with its separated distal fragment. Here we show that reconnection of separated axon fragments during regeneration of C. elegans mechanosensory neurons occurs through a mechanism of axonal fusion, which prevents Wallerian degeneration of the distal fragment. Through electron microscopy analysis and imaging with the photoconvertible fluorescent protein Kaede, we show that the fusion process re-establishes membrane continuity and repristinates anterograde and retrograde cytoplasmic diffusion. We also provide evidence that axonal fusion occurs with a remarkable level of accuracy, with the proximal re-growing axon recognizing its own separated distal fragment. Thus, efficient axonal regeneration can occur by selective reconnection and fusion of separated axonal fragments beyond an injury site, with restoration of the damaged neuronal tract.

Apoptosis: opening PANdora's BoX

Extracellular nucleotides have been reported to act as a 'find-me' signal in the context of phagocyte recruitment by apoptotically dying cells. A new study now examines the mechanisms of nucleotide release during apoptosis and describes the hemichannel-forming protein pannexin 1 as a crucial player in this scenario.

International Union of Basic and Clinical Pharmacology. LXXXI. Nomenclature and classification of adenosine receptors--an update

In the 10 years since our previous International Union of Basic and Clinical Pharmacology report on the nomenclature and classification of adenosine receptors, no developments have led to major changes in the recommendations. However, there have been so many other developments that an update is needed. The fact that the structure of one of the adenosine receptors has recently been solved has already led to new ways of in silico screening of ligands. The evidence that adenosine receptors can form homo- and heteromultimers has accumulated, but the functional significance of such complexes remains unclear. The availability of mice with genetic modification of all the adenosine receptors has led to a clarification of the functional roles of adenosine, and to excellent means to study the specificity of drugs. There are also interesting associations between disease and structural variants in one or more of the adenosine receptors. Several new selective agonists and antagonists have become available. They provide improved possibilities for receptor classification. There are also developments hinting at the usefulness of allosteric modulators. Many drugs targeting adenosine receptors are in clinical trials, but the established therapeutic use is still very limited.

Disturbances of apoptotic cell clearance in systemic lupus erythematosus

Systemic lupus erythematosus is a multifactorial autoimmune disease with an as yet unknown etiopathogenesis. It is widely thought that self-immunization in systemic lupus is driven by defective clearance of dead and dying cells. In lupus patients, large numbers of apoptotic cells accumulate in various tissues including germinal centers. In the present review, we discuss the danger signals released by apoptotic cells, their triggering of inflammatory responses, and the breakdown of B-cell tolerance. We also review the pathogenic role of apoptotic cell clearance in systemic lupus erythematosus.

Emerging role of damage-associated molecular patterns derived from mitochondria in inflammation

Cell death and injury often lead to release or exposure of intracellular molecules called damage-associated molecular patterns (DAMPs) or cell death-associated molecules. These molecules are recognized by the innate immune system by pattern recognition receptors - the same receptors that detect pathogen-associated molecular patterns, thus revealing similarities between pathogen-induced and non-infectious inflammatory responses. Many DAMPs are derived from the plasma membrane, nucleus, endoplasmic reticulum and cytosol. Recently, mitochondria have emerged as other organelles that function as a source of DAMPs. Here, we highlight the significance of mitochondrial DAMPs and discuss their contribution to inflammation and development of human pathologies.

Immune cell regulation by autocrine purinergic signalling

Stimulation of almost all mammalian cell types leads to the release of cellular ATP and autocrine feedback through a diverse array of purinergic receptors. Depending on the types of purinergic receptors that are involved, autocrine signalling can promote or inhibit cell activation and fine-tune functional responses. Recent work has shown that autocrine signalling is an important checkpoint in immune cell activation and allows immune cells to adjust their functional responses based on the extracellular cues provided by their environment. This Review focuses on the roles of autocrine purinergic signalling in the regulation of both innate and adaptive immune responses and discusses the potential of targeting purinergic receptors for treating immune-mediated disease.

TLR-2 and TLR-9 are sensors of apoptosis in a mouse model of doxorubicin-induced acute inflammation

Anthracycline antibiotics are inducers of an immunogenic form of apoptosis that has immunostimulatory properties because of the release of damage-associated molecular patterns. To study the mechanisms used by the innate immune system to sense this immunogenic form of cell death, we established an in vivo model of cell death induced by intraperitoneal injection of doxorubicin, a prototype of anthracyclines. The acute sterile inflammation in this model is characterized by rapid influx of neutrophils and increased levels of IL-6 and monocyte chemotactic protein-1. We demonstrate that acute inflammation induced by doxorubicin is associated with apoptosis of monocytes/macrophages and that it is specific for doxorubicin, an immunogenic chemotherapeutic. Further, the inflammatory response is significantly reduced in mice deficient in myeloid differentiation primary response gene 88 (MyD88), TLR-2 or TLR-9. Importantly, a TLR-9 antagonist reduces the recruitment of neutrophils induced by doxorubicin. By contrast, the acute inflammatory response is not affected in TRIF(Lps2) mutant mice and in TLR-3, TLR-4 and caspase-1 knockout mice, which shows that the inflammasome does not have a major role in doxorubicin-induced acute inflammation. Our findings provide important new insights into how the innate immune system senses immunogenic apoptotic cells and clearly demonstrate that the TLR-2/TLR-9-MyD88 signaling pathways have a central role in initiating the acute inflammatory response to this immunogenic form of apoptosis.

Structure based design of a Ca2+-sensitive RhoA protein that controls cell morphology

The Rho proteins are important regulators of cell morphology, and the prototypical protein RhoA is known to regulate contraction, blebbing and bleb retraction. We have identified and experimentally confirmed that RhoA has a binding site for calmodulin, a ubiquitous transducer of the Ca(2+) second messenger. Using structural modeling, a fusion protein was designed wherein RhoA activity was controlled by Ca(2+) via calmodulin. Living cells transfected with this synthetic protein underwent Ca(2+) sensitive and calmodulin-dependent bleb retraction within minutes. Further, the modularity of Ca(2+) signaling was exploited to induce bleb retraction in response to blue light (using channelrhodopsin-2) or exogenous chemicals (with acetylcholine receptor), showing input signal versatility. The widespread use of Ca(2+) signaling in nature suggests that fully exploring its signaling potential may allow powerful applications to other synthetic biological systems.

Structural interpretation of P2X receptor mutagenesis studies on drug action

P2X receptors for ATP are ligand gated cation channels that form from the trimeric assembly of subunits with two transmembrane segments, a large extracellular ligand binding loop, and intracellular amino and carboxy termini. The receptors are expressed throughout the body, involved in functions ranging from blood clotting to inflammation, and may provide important targets for novel therapeutics. Mutagenesis based studies have been used to develop an understanding of the molecular basis of their pharmacology with the aim of developing models of the ligand binding site. A crystal structure for the zebra fish P2X4 receptor in the closed agonist unbound state has been published recently, which provides a major advance in our understanding of the receptors. This review gives an overview of mutagenesis studies that have led to the development of a model of the ATP binding site, as well as identifying residues contributing to allosteric regulation and antagonism. These studies are discussed with reference to the crystal to provide a structural interpretation of the molecular basis of drug action.

Respiratory and gastrointestinal epithelial modulation of the immune response during viral infection

Respiratory and enteric viral infections cause significant morbidity and mortality world-wide and represent a major socio-economic burden. Many of these viruses have received unprecedented public and media interest in recent years. A popular public misconception is that viruses are a threat to which the human body has only limited defences. However, the majority of primary and secondary exposures to virus are asymptomatic or induce only minor symptoms. The mucosal epithelial surfaces are the main portal of entry for viral pathogens and are centrally involved in the initiation, maintenance and polarisation of the innate and adaptive immune response to infection. This review describes the defences employed by the epithelium of the respiratory and gastrointestinal tracts during viral infections with focus on epithelial modulation of the immune response at the innate/adaptive interface.

Molecular pharmacology, physiology, and structure of the P2Y receptors

The P2Y receptors are a widely expressed group of eight nucleotide-activated G protein-coupled receptors (GPCRs). The P2Y(1)(ADP), P2Y(2)(ATP/UTP), P2Y(4)(UTP), P2Y(6)(UDP), and P2Y(11)(ATP) receptors activate G(q) and therefore robustly promote inositol lipid signaling responses. The P2Y(12)(ADP), P2Y(13)(ADP), and P2Y(14)(UDP/UDP-glucose) receptors activate G(i) leading to inhibition of adenylyl cyclase and to Gβγ-mediated activation of a range of effector proteins including phosphoinositide 3-kinase-γ, inward rectifying K(+) (GIRK) channels, phospholipase C-β2 and -β3, and G protein-receptor kinases 2 and 3. A broad range of physiological responses occur downstream of activation of these receptors ranging from Cl(-) secretion by epithelia to aggregation of platelets to neurotransmission. Useful structural models of the P2Y receptors have evolved from extensive genetic analyses coupled with molecular modeling based on three-dimensional structures obtained for rhodopsin and several other GPCRs. Selective ligands have been synthesized for most of the P2Y receptors with the most prominent successes attained with highly selective agonist and antagonist molecules for the ADP-activated P2Y(1) and P2Y(12) receptors. The widely prescribed drug, clopidogrel, which results in irreversible blockade of the platelet P2Y(12) receptor, is the most important therapeutic agent that targets a P2Y receptor.

Autoinflammation by endogenous DNA

In various mammalian developmental processes such as programmed cell death, erythropoiesis, and lens-cell differentiation, chromosomal DNA is degraded into nucleotides by a set of specific nucleases. If this process does not proceed smoothly, the undigested DNA causes various problems. For example, when chromosomal DNA is not degraded in the lens cells, cataracts form. In other cases, undigested DNA in macrophages activates the innate immune system, like a DNA virus, and causes strong inflammation, resulting in anemia, arthritis, and lymphopenia. Here, we discuss when, where, and how DNA is degraded to maintain mammalian homeostasis.

Purinergic regulation of airway inflammation

The immune and inflammatory responses initiated by the interaction of a pathogen with airway surfaces constitute vital mechanisms to eradicate an infection. Sentinel dendritic cells embedded in the mucosa migrate to the lymph nodes to induce immune responses, whereas epithelial cells release chemokines to recruit inflammatory cells engaged in the active destruction of the intruder. All immune and inflammatory cells are regulated by customized purinergic networks of receptors and ectonucleotidases. The general concept is that bacterial products induce ATP release, which activates P2 receptors to initiate an inflammatory response, and is terminated by the conversion of ATP into adenosine (ADO) to initiate P1 receptor-mediated negative feedback responses. However, this chapter exposes a far more complex purinergic regulation of critical functions, such as the differentiation of naive lymphocytes and the complex maturation and secretion of pro-cytokines (i.e. IL-1β) by the "inflammasome". This material also reconciles decades of research by exposing the specificity and plasticity of the signaling network expressed by each immune and inflammatory cell, which changes through cell differentiation and in response to infectious or inflammatory mediators. By the end of this chapter, the reader will have a new appreciation for this aspect of airway defenses, and several leads in terms of therapeutic applications for the treatment of chronic respiratory diseases.

Liver X receptors as regulators of macrophage inflammatory and metabolic pathways

The liver X receptors (LXRα and LXRβ) are members of the nuclear receptor family of transcription factors that play essential roles in the transcriptional control of lipid metabolism. LXRs are endogenously activated by modified forms of cholesterol known as oxysterols and control the expression of genes important for cholesterol uptake, efflux, transport, and excretion in multiple tissues. In addition to their role as cholesterol sensors, a number of studies have implicated LXRs in the modulation of innate and adaptive immune responses. Both through activation and repression mechanisms, LXRs regulate diverse aspects of inflammatory gene expression in macrophages. The ability of LXRs to coordinate metabolic and immune responses constitutes an attractive therapeutic target for the treatment of chronic inflammatory disorders. This article is part of a Special Issue entitled: Translating nuclear receptors from health to disease.

Dangerous attraction: phagocyte recruitment and danger signals of apoptotic and necrotic cells

Tissue homeostasis in metazoa requires the rapid and efficient clearance of dying cells by professional or semi-professional phagocytes. Impairment of this finely regulated, fundamental process has been implicated in the development of autoimmune diseases, such as systemic lupus erythematosus. Various studies have provided us a detailed understanding of the interaction between dying cells and phagocytes as well as the current concept that apoptotic cell removal leads to a non- or anti-inflammatory response, whereas necrotic cell removal stimulates a pro-inflammatory reaction. In contrast, our knowledge about the soluble factors released from dying cells is rather limited, although meanwhile it is generally accepted that not only the dying cell itself but also the substances liberated during cell death contribute to the process of corpse clearance and the subsequent immune response. This review article is intended as an up-to-date survey over attraction and danger signals of apoptotic, primary and secondary necrotic cells, their function as chemoattractants in phagocyte recruitment, additional effects on the immune system, and the receptors, which are engaged in this scenario.

Mechanisms of failed apoptotic cell clearance by phagocyte subsets in cardiovascular disease

Recent evidence in humans indicate that defective phagocytic clearance of dying cells is linked to progression of advanced atherosclerotic lesions, the precursor to atherothrombosis, ischemic heart disease, and leading cause of death in the industrialized world. During atherogenesis, apoptotic cell turnover in the vascular wall is counterbalanced by neighboring phagocytes with high clearance efficiency, thereby limiting cellularity and maintaining lesion integrity. However, as lesions mature, phagocytic removal of apoptotic cells (efferocytosis) becomes defective, leading to secondary necrosis, expansion of plaque necrotic cores, and susceptibility to rupture. Recent genetic causation studies in experimental rodents have implicated key molecular regulators of efferocytosis in atherosclerotic progression. These include MER tyrosine kinase (MERTK), milk fat globule-EGF factor 8 (MFGE8), and complement C1q. At the cellular level, atheromata are infiltrated by a heterogenous population of professional phagocytes, comprised of monocytes, differentiated macrophages, and CD11c(+) dendritic-like cells. Each cell type is characterized by disparate clearance efficiencies and varying activities of key phagocytic signaling molecules. It is in this context that we outline a working model whereby plaque necrosis and destabilization is jointly promoted by (1) direct inhibition of core phagocytic signaling pathways and (2) expansion of phagocyte subsets with poor clearance capacity. Towards identifying targets for promoting efficient apoptotic cell clearance and resolving inflammation in atherosclerosis and during ischemic heart disease and post myocardial infarction, this review will discuss potential in vivo suppressors of efferocytosis at each stage of clearance and how these putative interventional targets may differentially affect uptake at the level of vascular phagocyte subsets.

A model to die for: signaling to apoptotic cell removal in worm, fly and mouse

Programmed cell death is used during developmental morphogenesis to eliminate superfluous cells or cells with inappropriate developmental potential (e.g., self-reactive immune cells, tumorigenic cells). Recent work in genetic models has led to a number of key observations, revealing signal transduction pathways and identifying new roles for genes previously studied in corpse removal (e.g., removal of broken synapses in the nervous system). Further, studies using mouse models have suggested a role for removal of apoptotic cells in the establishment or maintenance of immune tolerance. In this review, we survey current knowledge of phagocytic pathways derived from studies in the nematode (Caenorhabditis elegans), the fly (Drosophila melanogaster), and mouse (Mus musculus) model systems.

Microenvironmental influences of apoptosis in vivo and in vitro

The apoptosis program of physiological cell death elicits a range of non-phlogistic homeostatic mechanisms-"recognition, response and removal"-that regulate the microenvironments of normal and diseased tissues via multiple modalities operating over short and long distances. The molecular mechanisms mediate intercellular signaling through direct contact with neighboring cells, release of soluble factors and production of membrane-delimited fragments (apoptotic bodies, blebs and microparticles) that allow for interaction with host cells over long distances. These processes effect the selective recruitment of mononuclear phagocytes and the specific activation of both phagocytic and non-phagocytic cells. While much evidence is available concerning the mechanisms underlying the recognition and responses of phagocytes that culminate in the engulfment and removal of apoptotic cell bodies, relatively little is yet known about the non-phagocytic cellular responses to the apoptosis program. These responses regulate inflammatory and immune cell activation as well as cell fate decisions of proliferation, differentiation and death. Here, we review current knowledge of these processes, considering especially how apoptotic cells condition the microenvironments of normal and malignant tissues. We also discuss how apoptotic cells that persist in the absence of phagocytic clearance exert inhibitory effects over their viable neighbors, paying particular attention to the specific case of cell cultures and highlighting how new cell-corpse-clearance devices-Dead-Cert Nanoparticles-can significantly improve the efficacy of cell cultures through effective removal of non-viable cells in the absence of phagocytes in vitro.

Photodynamic therapy: illuminating the road from cell death towards anti-tumour immunity

Photodynamic therapy (PDT) utilizes the destructive power of reactive oxygen species generated via visible light irradiation of a photosensitive dye accumulated in the cancerous tissue/cells, to bring about their obliteration. PDT activates multiple signalling pathways in cancer cells, which could give rise to all three cell death modalities (at least in vitro). Simultaneously, PDT is capable of eliciting various effects in the tumour microenvironment thereby affecting the tumour-associated/-infiltrating immune cells and by extension, leading to infiltration of various immune cells (e.g. neutrophils) into the treated site. PDT is also associated to the activation of different immune phenomena, e.g. acute-phase response, complement cascade and production of cytokines/chemokines. It has also come to light that, PDT is capable of activating 'anti-tumour adaptive immunity' in both pre-clinical as well as clinical settings. Although the ability of PDT to induce 'anti-cancer vaccine effect' is still debatable, yet it has been shown to be capable of inducing exposure/release of certain damage-associated molecular patterns (DAMPs) like HSP70. Therefore, it seems that PDT is unique among other approved therapeutic procedures in generating a microenvironment suitable for development of systemic anti-tumour immunity. Apart from this, recent times have seen the emergence of certain promising modalities based on PDT like-photoimmunotherapy and PDT-based cancer vaccines. This review mainly discusses the effects exerted by PDT on cancer cells, immune cells as well as tumour microenvironment in terms of anti-tumour immunity. The ability of PDT to expose/release DAMPs and the future perspectives of this paradigm have also been discussed.

Common variants in P2RY11 are associated with narcolepsy

Growing evidence supports the hypothesis that narcolepsy with cataplexy is an autoimmune disease. We here report genome-wide association analyses for narcolepsy with replication and fine mapping across three ethnic groups (3,406 individuals of European ancestry, 2,414 Asians and 302 African Americans). We identify a SNP in the 3' untranslated region of P2RY11, the purinergic receptor subtype P2Y₁₁ gene, which is associated with narcolepsy (rs2305795, combined P = 6.1 × 10⁻¹⁰, odds ratio = 1.28, 95% CI 1.19-1.39, n = 5689). The disease-associated allele is correlated with reduced expression of P2RY11 in CD8(+) T lymphocytes (339% reduced, P = 0.003) and natural killer (NK) cells (P = 0.031), but not in other peripheral blood mononuclear cell types. The low expression variant is also associated with reduced P2RY11-mediated resistance to ATP-induced cell death in T lymphocytes (P = 0.0007) and natural killer cells (P = 0.001). These results identify P2RY11 as an important regulator of immune-cell survival, with possible implications in narcolepsy and other autoimmune diseases.

Cellular communications in bone homeostasis and repair

Cellular communication between the bone component cells osteoblasts, osteocytes and (pre-)osteoclasts is essential for bone remodeling which maintains bone integrity. As in the remodeling of other organs, cell death is a trigger for remodeling of bone. During the systematic process of bone remodeling, direct or indirect cell-cell communication is indispensable. Thus, osteoblasts induce migration and differentiation of preosteoclasts, which is followed by bone resorption (by mature multinuclear osteoclasts). After completion of bone resorption, apoptosis of mature osteoclasts and differentiation of osteoblasts are initiated. At this time, the osteoblasts do not support osteoclast differentiation but do support bone formation. Finally, osteoblasts differentiate to osteocytes in bone or to bone lining cells on bone surfaces. In this way, old bone areas are regenerated as new bone. In this review the role of cell-cell communication in bone remodeling is discussed.

Pannexin 1 channels mediate 'find-me' signal release and membrane permeability during apoptosis

Apoptotic cells release 'find-me' signals at the earliest stages of death to recruit phagocytes. The nucleotides ATP and UTP represent one class of find-me signals, but their mechanism of release is not known. Here, we identify the plasma membrane channel pannexin 1 (PANX1) as a mediator of find-me signal/nucleotide release from apoptotic cells. Pharmacological inhibition and siRNA-mediated knockdown of PANX1 led to decreased nucleotide release and monocyte recruitment by apoptotic cells. Conversely, PANX1 overexpression enhanced nucleotide release from apoptotic cells and phagocyte recruitment. Patch-clamp recordings showed that PANX1 was basally inactive, and that induction of PANX1 currents occurred only during apoptosis. Mechanistically, PANX1 itself was a target of effector caspases (caspases 3 and 7), and a specific caspase-cleavage site within PANX1 was essential for PANX1 function during apoptosis. Expression of truncated PANX1 (at the putative caspase cleavage site) resulted in a constitutively open channel. PANX1 was also important for the 'selective' plasma membrane permeability of early apoptotic cells to specific dyes. Collectively, these data identify PANX1 as a plasma membrane channel mediating the regulated release of find-me signals and selective plasma membrane permeability during apoptosis, and a new mechanism of PANX1 activation by caspases.

Parole terms for a killer: directing caspase3/CAD induced DNA strand breaks to coordinate changes in gene expression

In a series of discoveries over the preceding decade, a number of laboratories have unequivocally established that apoptotic proteins and pathways are well conserved cell fate determinants, which act independent of a cell death response. Within this context, the role for apoptotic proteins in the induction of cell differentiation has been widely documented. Despite these discoveries, little information has been forthcoming regarding a conserved mechanism by which apoptotic proteins achieve this non-death outcome. In the following discussion, we will explore the premise that the penultimate step in apoptosis, genome wide DNA damage/strand breaks act as a conserved genomic reprogramming event necessary for cell differentiation (Larsen et al. Proc Natl Acad Sci USA 2010; 107:4230-5). Moreover, we hypothesis that directed DNA damage, as mediated by known apoptotic proteins, may participate in numerous forms of regulated gene expression.

Intravascular danger signals guide neutrophils to sites of sterile inflammation

Neutrophils are recruited from the blood to sites of sterile inflammation, where they contribute to wound healing but may also cause tissue damage. By using spinning disk confocal intravital microscopy, we examined the kinetics and molecular mechanisms of neutrophil recruitment to sites of focal hepatic necrosis in vivo. Adenosine triphosphate released from necrotic cells activated the Nlrp3 inflammasome to generate an inflammatory microenvironment that alerted circulating neutrophils to adhere within liver sinusoids. Subsequently, generation of an intravascular chemokine gradient directed neutrophil migration through healthy tissue toward foci of damage. Lastly, formyl-peptide signals released from necrotic cells guided neutrophils through nonperfused sinusoids into the injury. Thus, dynamic in vivo imaging revealed a multistep hierarchy of directional cues that guide neutrophil localization to sites of sterile inflammation.

Responding to infection and apoptosis--a task for TH17 cells

Two of the critical cytokines required for the differentiation of T helper 17 (T(H)17) cells from naive CD4 T cells are transforming growth factor-beta (TGF-β) and interleukin-6 (IL-6). Innate recognition of apoptotic cells in the presence of Toll-like receptor engagement directs the simultaneous synthesis of these cytokines by antigen-presenting cells (APCs), and as such provides a cytokine milieu that favors T(H)17 cell induction. In this situation, APCs are activated in response to ligands derived from apoptotic cells, but also to those from the infecting pathogen. Induction of a T(H)17 response against Citrobacter rodentium infection was dependent on the ability of Citrobacter to induce apoptosis of intestinal epithelial cells. In this review, we will discuss how simultaneous activation of inflammatory and noninflammatory pattern recognition receptors on APCs impacts T helper cell differentiation, and what relevance this effect has on the immune response generated against bacterial infections that cause host cell apoptosis.

Purinergic control of neutrophil activation

Neutrophils constitute the first line of defense against invading pathogens, but they also are responsible for tissue destruction in pathological conditions. In a recent paper published in Science Signaling, Junger and colleagues show that adenosine 5'-triphosphate release and signaling through purinergic P2 receptors upon neutrophil activation by bacterial and inflammatory stimuli represents an important mechanism for effective neutrophil functional response and host protection.

Comprehensive analysis of frequency and phenotype of T regulatory cells in HIV infection: CD39 expression of FoxP3+ T regulatory cells correlates with progressive disease

There are conflicting data about the frequency and role of regulatory T cells (Tregs) during the course of HIV infection. Peripheral blood of a large cohort of HIV-infected patients (n = 131) at different stages of disease, including 15 long-term nonprogressors and 21 elite controllers, was analyzed to determine the frequency and phenotype of Tregs, defined as CD4(+), CD25(high), CD127(low), FoxP3(high) cells. A significantly increased relative frequency of Tregs within the CD4(+) compartment of HIV(+) patients compared to that of healthy controls (P < 0.0001) was observed. Additionally, the relative frequency of Tregs directly correlated with HIV viral load and inversely with CD4(+) counts. However, the absolute Treg number was reduced in HIV-infected patients versus healthy controls (P < 0.0001), with the exception of elite controllers (P > 0.05). The loss of absolute Treg numbers coincided with rising markers of immune activation (P < 0.0006). The initiation of antiviral therapy significantly increased absolute Treg numbers (P < 0.0031). We find that the expression of CD39, a newly defined ectonucleotidase with immunomodulatory functions on Tregs, correlated with progressive HIV disease, HIV viral load, and immune activation. Of note, when tested in peripheral blood mononuclear cells of healthy volunteers, the in vitro capacity to suppress T-cell proliferation was limited to CD4(+), CD25(high), CD39(+) T cells. Interestingly, Tregs of elite controllers exhibited not only the highest expression of CCR5, CTLA-4, and ICOS but also the lowest level of CD39. The data presented here reconcile the seemingly contradictory results of previous studies looking at Tregs in HIV and highlight the complexity of Treg-mediated immunoregulation during human viral infections.

Review: Soluble innate immune pattern-recognition proteins for clearing dying cells and cellular components: implications on exacerbating or resolving inflammation

Soluble innate immune pattern-recognition proteins (sPRPs) identify non-self or altered-self molecular patterns. Dying cells often display altered-self arrays of molecules on their surfaces. Hence, sPRPs are ideal for recognizing these cells and their components. Dying cell surfaces often contain, or allow the access to different lipids, intracellular glycoproteins and nucleic acids such as DNA at different stages of cell death. These are considered as 'eat me' signals that replace the native 'don't eat me' signals such as CD31, CD47 present on the live cells. A programmed cell death process such as apoptosis also generates cell surface blebs that contain intracellular components. These blebs are easily released for effective clearance or signalling. During late stages of cell death, soluble components are also released that act as 'find me' signal (e.g. LysoPC, nucleotides). The sPRPs such as collectins, ficolins, pentraxins, sCD14, MFG-E8, natural IgM and C1q can effectively identify some of these specific molecular patterns. The biological end-point is different depending on sPRP, tissue, stage of apoptosis and the type of cell death. The sPRPs that reside in the immune-privileged surfaces such as lungs often act as opsonins and enhance a silent clearance of dying cells and cellular material by macrophages and other phagocytic cells. Although the recognition of these materials by complement-activating proteins could amplify the opsonic signal, this pathway may aggravate inflammation. Clear understanding of the involvement of specific sPRPs in cell death and subsequent clearance of dying cell and their components is essential for devising appropriate treatment strategies for diseases involving infection, inflammation and auto-antibody generation.

Distinct migratory and cytokine responses of human microglia and macrophages to ATP

Microglia and hematogenous myeloid cells are prominent components of inflammatory central nervous system (CNS) lesions associated with tissue injury. To help define the basis for recruitment of such cells into lesions and their contribution to the disease process, we characterized the migratory and cytokine responses of human adult and fetal microglia in the presence of extracellular ATP comparing them to monocytes and macrophages. Adult microglia showed increased migration in response to low ATP concentrations (1-10 μM) whereas fetal microglia also migrated in response to higher ATP dosages (100-300 μM). The enhanced migration of microglia was reproduced with 2-MeSADP, a P2Y1/12/13 agonist. In contrast, the chemokine CCL2 did not promote migration of microglia, but promoted the migration of monocytes. Monocyte migration was also enhanced with low concentrations of ATP, whereas higher concentrations of ATP mediated an inhibitory effect. ATP had only an inhibitory effect on macrophages, which was not reproduced with hydrolysis products ADP or adenosine. ATP led to a decrease in LPS-induced pro-inflammatory cytokine release (TNFα, IL-6) in both microglia and macrophages without suppression of an anti-inflammatory response (IL-10). These in vitro based results suggest that ATP can selectively favor the recruitment of microglia rather than hematogenous myeloid cells while promoting an anti-inflammatory state in both hematogenous and resident myeloid cells of the CNS. Our results highlight the importance of environmental signals in shaping the properties of the innate immune response to injury in the CNS.

Apoptosis as a mechanism for liver disease progression

Hepatocyte injury is ubiquitous in clinical practice, and the mode of cell death associated with this injury is often apoptosis, especially by death receptors. Information from experimental systems demonstrates that hepatocyte apoptosis is sufficient to cause liver hepatic fibrogenesis. The mechanisms linking hepatocyte apoptosis to hepatic fibrosis remain incompletely understood, but likely relate to engulfment of apoptotic bodies by professional phagocytic cells and stellate cells, and release of mediators by cells undergoing apoptosis. Inhibition of apoptosis with caspase inhibitors has demonstrated beneficial effects in murine models of hepatic fibrosis. Recent studies implicating Toll-like receptor 9 in liver injury and fibrosis are also of particular interest. Engulfment of apoptotic bodies is one mechanism by which the TLR9 ligand (CpG DNA motifs) could be delivered to this intracellular receptor. These concepts suggest therapy focused on interrupting the cellular mechanisms linking apoptosis to fibrosis would be useful in human liver diseases.

Adrenergic receptor activation involves ATP release and feedback through purinergic receptors

Formyl peptide receptor-induced chemotaxis of neutrophils depends on the release of ATP and autocrine feedback through purinergic receptors. Here, we show that adrenergic receptor signaling requires similar purinergic feedback mechanisms. Real-time RT-PCR analysis revealed that human embryonic kidney (HEK)-293 cells express several subtypes of adrenergic (α(1)-, α(2)-, and β-receptors), adenosine (P1), and nucleotide receptors (P2). Stimulation of G(q)-coupled α(1)-receptors caused release of cellular ATP and MAPK activation, which was blocked by inhibiting P2 receptors with suramin. Stimulation of G(i)-coupled α(2)-receptors induced weak ATP release, while G(s)-coupled β-receptors caused accumulation of extracellular ADP and adenosine. β-Receptors triggered intracellular cAMP signaling, which was blocked by scavenging extracellular adenosine with adenosine deaminase or by inhibiting A2a adenosine receptors with SCH58261. These findings suggest that adrenergic receptors require purinergic receptors to elicit downstream signaling responses in HEK-293 cells. We evaluated the physiological relevance of these findings using mouse aorta tissue rings. Stimulation of α(1)-receptors induced ATP release and tissue contraction, which was reduced by removing extracellular ATP with apyrase or in the absence of P2Y(2) receptors in aorta rings from P2Y(2) receptor knockout mice. We conclude that, like formyl peptide receptors, adrenergic receptors require purinergic feedback mechanisms to control complex physiological processes such as smooth muscle contraction and regulation of vascular tone.

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.

Supporting cells eliminate dying sensory hair cells to maintain epithelial integrity in the avian inner ear

Epithelial homeostasis is essential for sensory transduction in the auditory and vestibular organs of the inner ear, but how it is maintained during trauma is poorly understood. To examine potential repair mechanisms, we expressed β-actin-enhanced green fluorescent protein (EGFP) in the chick inner ear and used live-cell imaging to study how sensory epithelia responded during aminoglycoside-induced hair cell trauma. We found that glial-like supporting cells used two independent mechanisms to rapidly eliminate dying hair cells. Supporting cells assembled an actin cable at the luminal surface that extended around the pericuticular junction and constricted to excise the stereocilia bundle and cuticular plate from the hair cell soma. Hair bundle excision could occur within 3 min of actin-cable formation. After bundle excision, typically with a delay of up to 2-3 h, supporting cells engulfed and phagocytosed the remaining bundle-less hair cell. Dual-channel recordings with β-actin-EGFP and vital dyes revealed phagocytosis was concurrent with loss of hair cell integrity. We conclude that supporting cells repaired the epithelial barrier before hair cell plasmalemmal integrity was lost and that supporting cell activity was closely linked to hair cell death. Treatment with the Rho-kinase inhibitor Y-27632 did not prevent bundle excision but prolonged phagocytic engulfment and resulted in hair cell corpses accumulating within the epithelium. Our data show that supporting cells not only maintain epithelial integrity during trauma but suggest they may also be an integral part of the hair cell death process itself.

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.

Molecular mechanisms of necroptosis: an ordered cellular explosion

For a long time, apoptosis was considered the sole form of programmed cell death during development, homeostasis and disease, whereas necrosis was regarded as an unregulated and uncontrollable process. Evidence now reveals that necrosis can also occur in a regulated manner. The initiation of programmed necrosis, 'necroptosis', by death receptors (such as tumour necrosis factor receptor 1) requires the kinase activity of receptor-interacting protein 1 (RIP1; also known as RIPK1) and RIP3 (also known as RIPK3), and its execution involves the active disintegration of mitochondrial, lysosomal and plasma membranes. Necroptosis participates in the pathogenesis of diseases, including ischaemic injury, neurodegeneration and viral infection, thereby representing an attractive target for the avoidance of unwarranted cell death.