Mechanisms of internalization of apoptotic and necrotic L929 cells by a macrophage cell line studied by electron microscopy

Mechanobiology of Ferroptotic Cancer Cells as a Novel "Eat-Me" Signal: Regulating Efferocytosis through Layer-by-Layer Coating

The importance of the clearance of dead cells is shown to have a regulatory role for normal tissue homeostasis and for the modulation of immune responses. However, how mechanobiological properties of dead cells affect efferocytosis remains largely unknown. Here, it is reported that the Young's modulus of cancer cells undergoing ferroptosis is reduced. To modulate their Young's modulus a layer-by-layer (LbL) nanocoating is developed. Scanning electron and fluorescence microscopy confirm coating efficiency of ferroptotic cells while atomic force microscopy reveals encapsulation of the dead cells increases their Young's modulus dependent on the number of applied LbL layers which increases their efferocytosis by primary macrophages. This work demonstrates the crucial role of mechanobiology of dead cells in regulating their efferocytosis by macrophages which can be exploited for the development of novel therapeutic strategies for diseases where modulation of efferocytosis can be potentially beneficial and for the design of drug delivery systems for cancer therapy.

Resolution Potential of Necrotic Cell Death Pathways

During tissue damage caused by infection or sterile inflammation, not only damage-associated molecular patterns (DAMPs), but also resolution-associated molecular patterns (RAMPs) can be activated. These dying cell-associated factors stimulate immune cells localized in the tissue environment and induce the production of inflammatory mediators or specialized proresolving mediators (SPMs). Within the current prospect of science, apoptotic cell death is considered the main initiator of resolution. However, more RAMPs are likely to be released during necrotic cell death than during apoptosis, similar to what has been observed for DAMPs. The inflammatory potential of many regulated forms of necrotic cell death modalities, such as pyroptosis, necroptosis, ferroptosis, netosis, and parthanatos, have been widely studied in necroinflammation, but their possible role in resolution is less considered. In this review, we aim to summarize the relationship between necrotic cell death and resolution, as well as present the current available data regarding the involvement of certain forms of regulated necrotic cell death in necroresolution.

In Vitro Veritas: From 2D Cultures to Organ-on-a-Chip Models to Study Immunogenic Cell Death in the Tumor Microenvironment

Immunogenic cell death (ICD) is a functionally unique form of cell death that promotes a T-cell-dependent anti-tumor immune response specific to antigens originating from dying cancer cells. Many anticancer agents and strategies induce ICD, but despite their robust effects in vitro and in vivo on mice, translation into the clinic remains challenging. A major hindrance in antitumor research is the poor predictive ability of classic 2D in vitro models, which do not consider tumor biological complexity, such as the contribution of the tumor microenvironment (TME), which plays a crucial role in immunosuppression and cancer evasion. In this review, we describe different tumor models, from 2D cultures to organ-on-a-chip technology, as well as spheroids and perfusion bioreactors, all of which mimic the different degrees of the TME complexity. Next, we discuss how 3D cell cultures can be applied to study ICD and how to increase the translational potential of the ICD inducers. Finally, novel research directions are provided regarding ICD in the 3D cellular context which may lead to novel immunotherapies for cancer.

Immunogenic Cell Death and Role of Nanomaterials Serving as Therapeutic Vaccine for Personalized Cancer Immunotherapy

Immunogenic cell death (ICD) is a rapidly growing research area representing one of the emerging therapeutic strategies of cancer immunotherapy. ICD is an umbrella term covering several cell death modalities including apoptosis, necroptosis, ferroptosis and pyroptosis, and is the product of a balanced combination of adjuvanticity (damage-associated molecular patterns and chemokines/cytokines) and antigenicity (tumor associated antigens). Only a limited number of anti-cancer therapies are available to induce ICD in experimental cancer therapies and even much less is available for clinical use. To overcome this limitation, nanomaterials can be used to increase the immunogenicity of cancer cells killed by anti-cancer therapy, which in themselves are not necessarily immunogenic. In this review, we outline the current state of knowledge of ICD modalities and discuss achievements in using nanomaterials to increase the immunogenicity of dying cancer cells. The emerging trends in modulating the immunogenicity of dying cancer cells in experimental and translational cancer therapies and the challenges facing them are described. In conclusion, nanomaterials are expected to drive further progress in their use to increase efficacy of anti-cancer therapy based on ICD induction and in the future, it is necessary to validate these strategies in clinical settings, which will be a challenging research area.

Immunogenic ferroptosis and where to find it?

Ferroptosis is a recently discovered form of regulated cell death that is morphologically, genetically, and biochemically distinct from apoptosis and necroptosis, and its potential use in anticancer therapy is emerging. The strong immunogenicity of (early) ferroptotic cancer cells broadens the current concept of immunogenic cell death and opens up new possibilities for cancer treatment. In particular, induction of immunogenic ferroptosis could be beneficial for patients with cancers resistant to apoptosis and necroptosis. However, ferroptotic cancer cells may be a rich source of oxidized lipids, which contribute to decreased phagocytosis and antigen cross-presentation by dendritic cells and thus may favor tumor evasion. This could explain the non-immunogenicity of late ferroptotic cells. Besides the presence of lactate in the tumor microenvironment, acidification and hypoxia are essential factors promoting ferroptosis resistance and affecting its immunogenicity. Here, we critically discuss the crucial mediators controlling the immunogenicity of ferroptosis that modulate the induction of antitumor immunity. We emphasize that it will be necessary to also identify the tolerogenic (ie, immunosuppressive) nature of ferroptosis, which can lead to tumor evasion.

RIPK3 signaling and its role in the pathogenesis of cancers

RIPK3 (receptor-interacting protein kinase 3) is a serine/threonine-protein kinase. As a key component of necrosomes, RIPK3 is an essential mediator of inflammatory factors (such as TNFα-tumor necrosis factor α) and infection-induced necroptosis, a programmed necrosis. In addition, RIPK3 signaling is also involved in the regulation of apoptosis, cytokine/chemokine production, mitochondrial metabolism, autophagy, and cell proliferation by interacting with and/or phosphorylating the critical regulators of the corresponding signaling pathways. Similar to apoptosis, RIPK3-signaling-mediated necroptosis is inactivated in most types of cancers, suggesting RIPK3 might play a critical suppressive role in the pathogenesis of cancers. However, in some inflammatory types of cancers, such as pancreatic cancers and colorectal cancers, RIPK3 signaling might promote cancer development by stimulating proliferation signaling in tumor cells and inducing an immunosuppressive response in the tumor environment. In this review, we summarize recent research progress in the regulators of RIPK3 signaling, and discuss the function of this pathway in the regulation of mixed lineage kinase domain-like (MLKL)-mediated necroptosis and MLKL-independent cellular behaviors. In addition, we deliberate the potential roles of RIPK3 signaling in the pathogenesis of different types of cancers and discuss the potential strategies for targeting this pathway in cancer therapy.

Vaccination with early ferroptotic cancer cells induces efficient antitumor immunity

Background

Immunotherapy represents the future of clinical cancer treatment. The type of cancer cell death determines the antitumor immune response and thereby contributes to the efficacy of anticancer therapy and long-term survival of patients. Induction of immunogenic apoptosis or necroptosis in cancer cells does activate antitumor immunity, but resistance to these cell death modalities is common. Therefore, it is of great importance to find other ways to kill tumor cells. Recently, ferroptosis has been identified as a novel, iron-dependent form of regulated cell death but whether ferroptotic cancer cells are immunogenic is unknown.

Methods

Ferroptotic cell death in murine fibrosarcoma MCA205 or glioma GL261 cells was induced by RAS-selective lethal 3 and ferroptosis was analyzed by flow cytometry, atomic force and confocal microscopy. ATP and high-mobility group box 1 (HMGB1) release were detected by luminescence and ELISA assays, respectively. Immunogenicity in vitro was analyzed by coculturing of ferroptotic cancer cells with bone-marrow derived dendritic cells (BMDCs) and rate of phagocytosis and activation/maturation of BMDCs (CD11c⁺CD86⁺, CD11c⁺CD40⁺, CD11c⁺MHCII⁺, IL-6, RNAseq analysis). The tumor prophylactic vaccination model in immune-competent and immune compromised (Rag-2^−/−) mice was used to analyze ferroptosis immunogenicity.

Results

Ferroptosis can be induced in cancer cells by inhibition of glutathione peroxidase 4, as evidenced by confocal and atomic force microscopy and inhibitors’ analysis. We demonstrate for the first time that ferroptosis is immunogenic in vitro and in vivo. Early, but not late, ferroptotic cells promote the phenotypic maturation of BMDCs and elicit a vaccination-like effect in immune-competent mice but not in Rag-2^−/− mice, suggesting that the mechanism of immunogenicity is very tightly regulated by the adaptive immune system and is time dependent. Also, ATP and HMGB1, the best-characterized damage-associated molecular patterns involved in immunogenic cell death, have proven to be passively released along the timeline of ferroptosis and act as immunogenic signal associated with the immunogenicity of early ferroptotic cancer cells.

Conclusions

These results pave the way for the development of new therapeutic strategies for cancers based on induction of ferroptosis, and thus broadens the current concept of immunogenic cell death and opens the door for the development of new strategies in cancer immunotherapy.

Deep learning with digital holographic microscopy discriminates apoptosis and necroptosis

Regulated cell death modalities such as apoptosis and necroptosis play an important role in regulating different cellular processes. Currently, regulated cell death is identified using the golden standard techniques such as fluorescence microscopy and flow cytometry. However, they require fluorescent labels, which are potentially phototoxic. Therefore, there is a need for the development of new label-free methods. In this work, we apply Digital Holographic Microscopy (DHM) coupled with a deep learning algorithm to distinguish between alive, apoptotic and necroptotic cells in murine cancer cells. This method is solely based on label-free quantitative phase images, where the phase delay of light by cells is quantified and is used to calculate their topography. We show that a combination of label-free DHM in a high-throughput set-up (~10,000 cells per condition) can discriminate between apoptosis, necroptosis and alive cells in the L929sAhFas cell line with a precision of over 85%. To the best of our knowledge, this is the first time deep learning in the form of convolutional neural networks is applied to distinguish-with a high accuracy-apoptosis and necroptosis and alive cancer cells from each other in a label-free manner. It is expected that the approach described here will have a profound impact on research in regulated cell death, biomedicine and the field of (cancer) cell biology in general.

Necroptosis in Immuno-Oncology and Cancer Immunotherapy

Immune-checkpoint blockers (ICBs) have revolutionized oncology and firmly established the subfield of immuno-oncology. Despite this renaissance, a subset of cancer patients remain unresponsive to ICBs due to widespread immuno-resistance. To "break" cancer cell-driven immuno-resistance, researchers have long floated the idea of therapeutically facilitating the immunogenicity of cancer cells by disrupting tumor-associated immuno-tolerance via conventional anticancer therapies. It is well appreciated that anticancer therapies causing immunogenic or inflammatory cell death are best positioned to productively activate anticancer immunity. A large proportion of studies have emphasized the importance of immunogenic apoptosis (i.e., immunogenic cell death or ICD); yet, it has also emerged that necroptosis, a programmed necrotic cell death pathway, can also be immunogenic. Emergence of a proficient immune profile for necroptosis has important implications for cancer because resistance to apoptosis is one of the major hallmarks of tumors. Putative immunogenic or inflammatory characteristics driven by necroptosis can be of great impact in immuno-oncology. However, as is typical for a highly complex and multi-factorial disease like cancer, a clear cause versus consensus relationship on the immunobiology of necroptosis in cancer cells has been tough to establish. In this review, we discuss the various aspects of necroptosis immunobiology with specific focus on immuno-oncology and cancer immunotherapy.

Phagocytosis of Necrotic Debris at Sites of Injury and Inflammation

Clearance of cellular debris is required to maintain the homeostasis of multicellular organisms. It is intrinsic to processes such as tissue growth and remodeling, regeneration and resolution of injury and inflammation. Most of the removal of effete and damaged cells is performed by macrophages and neutrophils through phagocytosis, a complex phenomenon involving ingestion and degradation of the disposable particles. The study of the clearance of cellular debris has been strongly biased toward the removal of apoptotic bodies; as a result, the mechanisms underlying the removal of necrotic cells have remained relatively unexplored. Here, we will review the incipient but growing knowledge of the phagocytosis of necrotic debris, from their recognition and engagement to their internalization and disposal. Critical insights into these events were gained recently through the development of new in vitro and in vivo models, along with advances in live-cell and intravital microscopy. This review addresses the classes of "find-me" and "eat-me" signals presented by necrotic cells and their cognate receptors in phagocytes, which in most cases differ from the extensively characterized counterparts in apoptotic cell engulfment. The roles of damage-associated molecular patterns, chemokines, lipid mediators, and complement components in recruiting and activating phagocytes are reviewed. Lastly, the physiological importance of necrotic cell removal is emphasized, highlighting the key role of impaired debris clearance in autoimmunity.

Voices from the dead: The complex vocabulary and intricate grammar of dead cells

Of the roughly one million cells per second dying throughout the body, the vast majority dies by apoptosis, the predominant form of regulated cell death in higher organisms. Long regarded as mere waste, apoptotic cells are now recognized as playing a prominent and active role in homeostatic maintenance, especially resolution of inflammation, and in the sculpting of tissues during development. The activities associated with apoptotic cells are continually expanding, with more recent studies demonstrating their ability to modulate such vital functions as proliferation, survival, differentiation, metabolism, migration, and angiogenesis. In each case, the role of apoptotic cells is active, exerting their effects via new activities acquired during the apoptotic program. Moreover, the capacity to recognize and respond to apoptotic cells is not limited to professional phagocytes. Most, if not all, cells receive and integrate an array of signals from cells dying in their vicinity. These signals comprise a form of biochemical communication. As reviewed in this chapter, this communication is remarkably sophisticated; each of its three critical steps-encoding, transmission, and decoding of the apoptotic cell's "message"-is endowed with exquisite robustness. Together, the abundance and intricacy of the variables at each step comprise the vocabulary and grammar of the language by which dead cells achieve their post-mortem voice. The combinatorial complexity of the resulting communication network permits dying cells, through the signals they emit and the responses those signals elicit, to partake of an expanded role in homeostasis, acting as both sentinels of environmental change and agents of adaptation.

Macrophages engulf apoptotic and primary necrotic thymocytes through similar phosphatidylserine-dependent mechanisms

One of the major roles of professional phagocytes is the removal of dead cells in the body. We know less about the clearance of necrotic cells than apoptotic cell phagocytosis, despite the fact that both types of dead cells need to be cleared together and necrotic cells appear often in pathological settings. In the present study, we examined phagocytosis of heat‐ or H₂O₂‐killed necrotic and apoptotic thymocytes by mouse bone marrow‐derived macrophages (BMDMs) in vitro and found that the two cell types are engulfed at equal efficiency and compete with each other when added together to BMDMs. Phagocytosis of both apoptotic and necrotic thymocytes was decreased by (a) blocking phosphatidylserine on the surface of dying cells; (b) inhibition of Mer tyrosine kinase, Tim‐4, integrin β3 receptor signaling, or Ras‐related C3 botulinum toxin substrate 1 activity; or (c) using BMDMs deficient for transglutaminase 2. Stimulation of liver X, retinoid X, retinoic acid or glucocorticoid nuclear receptors in BMDMs enhanced not only apoptotic, but also necrotic cell uptake. Electron microscopic analysis of the engulfment process revealed that the morphology of phagosomes and the phagocytic cup formed during the uptake of dying thymocytes is similar for apoptotic and necrotic cells. Our data indicate that apoptotic and necrotic cells are cleared via the same mechanisms, and removal of necrotic cells in vivo can be facilitated by molecules known to enhance the uptake of apoptotic cells.

Necroptosis Signaling Pathways in Stroke: From Mechanisms to Therapies

It has been confirmed that apoptosis, autophagy and necrosis are the three major modes of cell death. For a long time, necrosis is regarded as a deranged or accidental cell demise. In recent years, there is evidence showing that necrotic cell death can be a well regulated and orchestrated event, which is also known as programmed cell death or “necroptosis”. Necroptosis can be triggered by a variety of external stimuli and regulated by a caspase-independent pathway. It plays a key role in the pathogenesis of some diseases including neurological diseases. In the past two decades, a variety of studies have revealed that the necroptosis related pathway is activated in stroke, and plays a crucial role in the pathogenesis of stroke. Moreover, necroptosis may serve as a potential target in the therapy of stroke because genetic or pharmacological inhibition of necroptosis has been shown to be neuroprotective in stroke in vitro and in vivo. In this review, we briefly summarize re-cent advances in necroptosis, introduce the mechanism and strategies targeting necroptosis in stroke, and finally propose some issues in the treatment of stroke by targeting necroptosis

Necroptotic cell death in anti-cancer therapy

Necroptosis is one the best-characterized forms of regulated necrosis. Necroptosis is mediated by the kinase activities of receptor interacting protein kinase-1 and receptor interacting protein kinase-3, which eventually lead to the activation of mixed lineage kinase domain-like. Necroptosis is characterized by rapid permeabilization of the plasma membrane, which is associated with the release of the cell content and subsequent exposure of damage-associated molecular patterns (DAMPs) and cytokines/chemokines. This release underlies the immunogenic nature of necroptotic cancer cells and their ability to induce efficient anti-tumor immunity. Triggering necroptosis has become especially important in experimental cancer treatments as an alternative to triggering apoptosis because one of the hallmarks of cancer is the blockade or evasion of apoptosis. In this review, we discuss recent advances in necroptosis research and the functional consequences of necroptotic cancer cell death, with focus on its immunogenicity and its role in the activation of anti-tumor immunity. Next, we discuss the molecular mechanisms of phosphatidylserine exposure during necroptosis and its role in the recognition of necroptotic cells. We also highlight the complex role of the necroptotic pathway in tumor promotion and suppression and in metastasis. Future studies will show whether necroptosis is truly a better strategy to overcome apoptosis resistance and provide the insights needed for development of novel treatment strategies for cancer.

Cell death in the pathogenesis of systemic lupus erythematosus and lupus nephritis

Nephritis is one of the most severe complications of systemic lupus erythematosus (SLE). One key characteristic of lupus nephritis (LN) is the deposition of immune complexes containing nucleic acids and/or proteins binding to nucleic acids and autoantibodies recognizing these molecules. A variety of cell death processes are implicated in the generation and externalization of modified nuclear autoantigens and in the development of LN. Among these processes, apoptosis, primary and secondary necrosis, NETosis, necroptosis, pyroptosis, and autophagy have been proposed to play roles in tissue damage and immune dysregulation. Cell death occurs in healthy individuals during conditions of homeostasis yet autoimmunity does not develop, at least in part, because of rapid clearance of dying cells. In SLE, accelerated cell death combined with a clearance deficiency may lead to the accumulation and externalization of nuclear autoantigens and to autoantibody production. In addition, specific types of cell death may modify autoantigens and alter their immunogenicity. These modified molecules may then become novel targets of the immune system and promote autoimmune responses in predisposed hosts. In this review, we examine various cell death pathways and discuss how enhanced cell death, impaired clearance, and post-translational modifications of proteins could contribute to the development of lupus nephritis.

How are necrotic cells recognized by their predators?

Necrosis is a type of cell death often caused by cell injury and is linked to human diseases including neuron degeneration, stroke, and cancer. Cells undergoing necrosis are engulfed and degraded by engulfing cells, their predators. The mechanisms by which necrotic cells are recognized and removed remain elusive. Here we comment on our recent findings that reveal new molecular mechanisms of necrotic-cell recognition. Through studying the C. elegans touch neurons undergoing excitotoxic necrosis, we identified a receptor/ligand pair that enables engulfing cells to recognize necrotic neurons. The phagocytic receptor CED-1 is activated through interaction with its ligand phosphatidylserine (PS), exposed on the surface of necrotic cells. Furthermore, against the common belief that necrotic cells have ruptured plasma membrane, we found that necrotic C. elegans touch neurons actively present PS on their outer surfaces while maintaining plasma membrane integrity. We further identified 2 mechanisms governing the presentation of PS, one of which is shared with cells undergoing apoptosis, a “cell suicide” event, whereas the other is unique to necrotic neurons. The influx of Ca²⁺, a key necrosis-triggering factor, is implicated in activating a neuronal PS-scramblase for PS exposure. We propose that the mechanisms controlling PS-exposure and necrotic-cell recognition by engulfing cells are likely conserved from worms to humans.

In vivo characterization of microglial engulfment of dying neurons in the zebrafish spinal cord

Microglia are specialized phagocytes in the vertebrate central nervous system (CNS). As the resident immune cells of the CNS they play an important role in the removal of dying neurons during both development and in several neuronal pathologies. Microglia have been shown to prevent the diffusion of damaging degradation products of dying neurons by engulfment and ingestion. Here we describe a live imaging approach that uses UV laser ablation to selectively stress and kill spinal neurons and visualize the clearance of neuronal remnants by microglia in the zebrafish spinal cord. In vivo imaging confirmed the motile nature of microglia within the uninjured spinal cord. However, selective neuronal ablation triggered rapid activation of microglia, leading to phagocytic uptake of neuronal debris by microglia within 20-30 min. This process of microglial engulfment is highly dynamic, involving the extension of processes toward the lesion site and consequently the ingestion of the dying neuron. 3D rendering analysis of time-lapse recordings revealed the formation of phagosome-like structures in the activated microglia located at the site of neuronal ablation. This real-time representation of microglial phagocytosis in the living zebrafish spinal cord provides novel opportunities to study the mechanisms of microglia-mediated neuronal clearance.

Necrotic Cells Actively Attract Phagocytes through the Collaborative Action of Two Distinct PS-Exposure Mechanisms

Necrosis, a kind of cell death closely associated with pathogenesis and genetic programs, is distinct from apoptosis in both morphology and mechanism. Like apoptotic cells, necrotic cells are swiftly removed from animal bodies to prevent harmful inflammatory and autoimmune responses. In the nematode Caenorhabditis elegans, gain-of-function mutations in certain ion channel subunits result in the excitotoxic necrosis of six touch neurons and their subsequent engulfment and degradation inside engulfing cells. How necrotic cells are recognized by engulfing cells is unclear. Phosphatidylserine (PS) is an important apoptotic-cell surface signal that attracts engulfing cells. Here we observed PS exposure on the surface of necrotic touch neurons. In addition, the phagocytic receptor CED-1 clusters around necrotic cells and promotes their engulfment. The extracellular domain of CED-1 associates with PS in vitro. We further identified a necrotic cell-specific function of CED-7, a member of the ATP-binding cassette (ABC) transporter family, in promoting PS exposure. In addition to CED-7, anoctamin homolog-1 (ANOH-1), the C. elegans homolog of the mammalian Ca(2+)-dependent phospholipid scramblase TMEM16F, plays an independent role in promoting PS exposure on necrotic cells. The combined activities from CED-7 and ANOH-1 ensure efficient exposure of PS on necrotic cells to attract their phagocytes. In addition, CED-8, the C. elegans homolog of mammalian Xk-related protein 8 also makes a contribution to necrotic cell-removal at the first larval stage. Our work indicates that cells killed by different mechanisms (necrosis or apoptosis) expose a common "eat me" signal to attract their phagocytic receptor(s); furthermore, unlike what was previously believed, necrotic cells actively present PS on their outer surfaces through at least two distinct molecular mechanisms rather than leaking out PS passively.

Necroptosis: the release of damage-associated molecular patterns and its physiological relevance

Regulated necrosis, termed necroptosis, is negatively regulated by caspase-8 and is dependent on the kinase activity of RIPK1 and RIPK3. Necroptosis leads to rapid plasma membrane permeabilization and to the release of cell contents and exposure of damage-associated molecular patterns (DAMPs). We are only beginning to identify the necroptotic DAMPs, their modifications, and their potential role in the regulation of inflammation. In this review, we discuss the physiological relevance of necroptosis and its role in the modulation of inflammation. For example, during viral infection, RIPK3-mediated necroptosis acts as a backup mechanism to clear pathogens. Necroptosis is also involved in apparently immunologically silent maintenance of T cell homeostasis. In contrast, the induction of necroptosis in skin, intestine, systemic inflammatory response syndrome, and ischemia reperfusion injury provoke a strong inflammatory response, which might be triggered by emission of DAMPs from necroptotic cells, showing the detrimental side of necroptosis.

Sesquiterpene lactones as drugs with multiple targets in cancer treatment: focus on parthenolide

Sesquiterpene lactones (SLs) constitute a large and diverse group of biologically active plant compounds that possess anti-inflammatory and antitumor activity. The subclass germacranolides is one of the major groups of SLs. It includes parthenolide, a highly cytotoxic SL that is being tested in clinical trials as an anti-cancer agent. In this review, we focus on SL antitumor activity related to cell-cycle arrest, differentiation, apoptosis induction through the intrinsic pathway, and sensitization of the extrinsic pathway. We also address the regression of tumors in response to cotreatment with conventional chemotherapeutics. We review the nuclear factor-κB-targeted anti-inflammatory activity in vitro and in vivo and relate it to the SL structural features involved in the molecular mechanisms. It is obvious that SLs are emerging as promising anticancer agents, but more investigations are required to fully understand the molecular mechanisms of known SLs in different cell death modalities and how these mechanisms contribute toward the potent antitumor and anti-inflammatory activities of SLs.

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.

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.

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.

Hovering between death and life: natural apoptosis and phagocytes in the blastogenetic cycle of the colonial ascidian Botryllus schlosseri

Colonies of the compound ascidian Botryllus schlosseri undergo recurrent generation changes during which massive, natural apoptosis occurs in zooid tissues: for this reason the species is emerging as an interesting model of invertebrate chordate, phylogenetically related to vertebrates, for studies of apoptosis during development. In the present work, we carried out a series of morphological, cytofluorimetrical and biochemical analyses, useful for a better characterization of Botryllus apoptosis. Results are consistent with the following viewpoints: (i) both intrinsic and extrinsic pathways, probably connected by the BH3-only protein Bid, are involved in cell death induction; (ii) phagocytes, once loaded with senescent cells, frequently undergo apoptosis, probably as a consequence of oxidative stress caused by prolonged respiratory burst, and (iii) senescent phagocytes are easily recognized and ingested by other phagocytes, responsible for their clearance. In addition, results suggest the conservation of apoptosis induction mechanisms throughout chordate evolution.

Tyro3 receptor tyrosine kinases in the heterogeneity of apoptotic cell uptake

Mononuclear phagocytes comprise a mobile, broadly dispersed and highly adaptable system that lies at the very epicenter of host defense against pathogens and the interplay of the innate and adaptive arms of immunity. Understanding the molecular mechanisms that control the response of mononuclear phagocytes to apoptotic cells and the anti-inflammatory consequences of that response is an important goal with implications for multiple areas of biomedical sciences. This review details current understanding of the heterogeneity of apoptotic cell uptake by different members of the mononuclear phagocyte family in humans and mice. It also recounts the unique role of the Tyro3 family of receptor tyrosine kinases, best characterized for Mertk, in the signal transduction leading both to apoptotic cell ingestion and the anti-inflammatory effects that result.

Apoptosis and necrosis: detection, discrimination and phagocytosis

Three major morphologies of cell death have been described: apoptosis (type I), cell death associated with autophagy (type II) and necrosis (type III). Apoptosis and cell death associated with autophagy can be distinguished by certain biochemical events. However, necrosis is characterized mostly in negative terms by the absence of caspase activation, cytochrome c release and DNA oligonucleosomal fragmentation. A particular difficulty in defining necrosis is that in the absence of phagocytosis apoptotic cells become secondary necrotic cells with many morphological features of primary necrosis. In this review, we present a selection of techniques that can be used to identify necrosis and to discriminate it from apoptosis. These techniques rely on the following cell death parameters: (1) morphology (time-lapse and transmission electron microscopy and flow fluorocytometry); (2) cell surface markers (phosphatidylserine exposure versus membrane permeability by flow fluorocytometry); (3) intracellular markers (oligonucleosomal DNA fragmentation by flow fluorocytometry, caspase activation, Bid cleavage and cytochrome c release by western blotting); (4) release of extracellular markers in the supernatant (caspases, HMGB-1 and cytokeratin 18). Finally, we report on methods that can be used to examine interactions between dying cells and phagocytes. We illustrate a quantitative method for detecting phagocytosis of dying cells by flow fluorocytometry. We also describe a recently developed approach based on the use of fluid phase tracers and different kind of microscopy, transmission electron and fluorescence microscopy, to characterize the mechanisms used by phagocytes to internalize dying cells.

The innate immune component ficolin 3 (Hakata antigen) mediates the clearance of late apoptotic cells

OBJECTIVE

Ficolin 3 (Hakata antigen), a collagen-like defense molecule, is a known autoantigen in patients with systemic lupus erythematosus (SLE). Recent studies have shown that other collagen-like defense molecules, such as C1q, mannose-binding lectin (MBL) and ficolin 2, bind to apoptotic cells and mediate their clearance by phagocytic cells. Dysfunction in this mechanism is regarded as an important contributor to the pathophysiology of SLE. Thus, we sought to determine whether ficolin 3 participates in the clearance of apoptotic cells.

METHODS

A Jurkat T cell line was used as the source of dying host cells. The cells were rendered apoptotic or necrotic by incubation with etoposide or by heat shocking, respectively. Binding of ficolin 3 to the cells was analyzed by flow cytometry. The apoptotic cells were incubated with human monocyte-derived macrophages, and the effect of ficolin 3 on the adhesion/uptake was examined by flow cytometry.

RESULTS

Ficolin 3 bound to a population of late apoptotic cells, while a strong and uniform binding to necrotic cells was observed. The binding properties differed from those of MBL and ficolin 2. Ficolin 3 binding to late apoptotic cells resulted in a significant increase in adhesion/uptake by macrophages.

CONCLUSION

Ficolin 3 mediates the clearance of late apoptotic cells, which suggests that this protein is involved in the maintenance of tissue homeostasis and might play a protective role against the development of autoimmunity.

Clearance of apoptotic and necrotic cells and its immunological consequences

The ultimate and most favorable fate of almost all dying cells is engulfment by neighboring or specialized cells. Efficient clearance of cells undergoing apoptotic death is crucial for normal tissue homeostasis and for the modulation of immune responses. Engulfment of apoptotic cells is finely regulated by a highly redundant system of receptors and bridging molecules on phagocytic cells that detect molecules specific for dying cells. Recognition of necrotic cells by phagocytes is less well understood than recognition of apoptotic cells, but an increasing number of recent studies, which are discussed here, are highlighting its importance. New observations indicate that the interaction of macrophages with dying cells initiates internalization of the apoptotic or necrotic targets, and that internalization can be preceded by "zipper"-like and macropinocytotic mechanisms, respectively. We emphasize that clearance of dying cells is an important fundamental process serving multiple functions in the regulation of normal tissue turnover and homeostasis, and is not just simple anti- or pro-inflammatory responses. Here we review recent findings on genetic pathways participating in apoptotic cell clearance, mechanisms of internalization, and molecules involved in engulfment of apoptotic versus necrotic cells, as well as their immunological consequences and relationships to disease pathogenesis.

Macrophages use different internalization mechanisms to clear apoptotic and necrotic cells

The present study characterized two different internalization mechanisms used by macrophages to engulf apoptotic and necrotic cells. Our in vitro phagocytosis assay used a mouse macrophage cell line, and murine L929sAhFas cells that are induced to die in a necrotic way by TNFR1 and heat shock or in an apoptotic way by Fas stimulation. Scanning electron microscopy (SEM) revealed that apoptotic bodies were taken up by macrophages with formation of tight fitting phagosomes, similar to the 'zipper'-like mechanism of phagocytosis, whereas necrotic cells were internalized by a macropinocytotic mechanism involving formation of multiple ruffles directed towards necrotic debris. Two macropinocytosis markers (Lucifer Yellow (LY) and horseradish peroxidase (HRP)) were excluded from the phagosomes containing apoptotic bodies, but they were present inside the macropinosomes containing necrotic material. Wortmannin (phosphatidylinositol 3'-kinase (PI3K) inhibitor) reduced the uptake of apoptotic cells, but the engulfment of necrotic cells remained unaffected. Our data demonstrate that apoptotic and necrotic cells are internalized differently by macrophages.

Phagocytosis of apoptotic bodies by hepatic stellate cells induces NADPH oxidase and is associated with liver fibrosis in vivo

Hepatic stellate cell activation is a main feature of liver fibrogenesis. We have previously shown that phagocytosis of apoptotic bodies by stellate cells induces procollagen alpha1 (I) and transforming growth factor beta (TGF-beta) expression in vitro. Here we have further investigated the downstream effects of phagocytosis by studying NADPH oxidase activation and its link to procollagen alpha1 (I) and TGF-beta1 expression in an immortalized human stellate cell line and in several models of liver fibrosis. Phagocytosis of apoptotic bodies in LX-1 cells significantly increased superoxide production both in the extracellular and intracellular milieus. By confocal microscopy of LX-1 cells, increased intracellular reactive oxygen species (ROS) were detected in the cells with intracellular apoptotic bodies, and immunohistochemistry documented translocation of the NADPH oxidase p47phox subunit to the membrane. NADPH oxidase activation resulted in upregulation of procollagen alpha1 (I); in contrast, TGF-beta1 expression was independent of NADPH oxidase activation. This was also confirmed by using siRNA to inhibit TGF-beta1 production. In addition, with EM studies we showed that phagocytosis of apoptotic bodies by stellate cells occurs in vivo. In conclusion, these data provide a mechanistic link between phagocytosis of apoptotic bodies, production of oxidative radicals, and the activation of hepatic stellate cells.

RGD-containing molecules induce macropinocytosis in ascidian hyaline amoebocytes

Phagocytes of the compound ascidian Botryllus schlosseri are capable of constitutive macropinocytosis (MP) at sites of membrane ruffling along the leading edge. This gives rise to the formation of initially irregular vesicles which then move to the inside of the cells and acquire a more regular morphology. Both phagocyte spreading and MP are enhanced by the recognition of molecules containing the sequence Arg-Gly-Asp (RGD): this suggests that, as in mammals, integrin activation is involved in the induction of both cell spreading and endocytosis. The occurrence of MP is associated with increased oxygen consumption and a rise in the production of superoxide anion, as indicated by nitroblue tetrazolium reduction, and ATP, as indicated by increased cytochrome oxidase activity. On the whole, our results indicate the conservation of common mechanisms of MP induction throughout the Chordate phylum.

Immunology of Apoptosis and Necrosis

A complex of reactions regulating the number of cells in organs and tissues under normal and pathologic conditions is one of the most important systems of multicellular organisms. In this system, which controls both cell proliferation and clearance, clearance has been given special attention during the last three decades. Some stages of the clearance are known (the choice of "unwanted" cells, their destruction not affecting the surrounding tissue, and, finally, removal of the corpses), and undeniable progress has been achieved in the understanding of the second stage mechanisms, whereas mechanisms of elimination per se of cells or their fragments still continue to be terra incognita. The clearance of such cells is mainly determined by different components of natural and adaptive immunity: phagocytes, complement, opsonins, antigen-presenting cells, etc. Recently specific "danger signals", such as hydrolases, DNA, heat shock proteins, and other potential immunogens released by cells during their elimination have been discovered. Entering the extracellular space, these signals induce inflammation and injury of the surrounding tissues, i.e., autoimmune reactions. Heat shock proteins, in addition to chaperon activity, act as signaling, costimulating, and antigen-carrying molecules in the interactions of dying cells and the immune system.

Histidine-rich Glycoprotein Specifically Binds to Necrotic Cells via Its Amino-terminal Domain and Facilitates Necrotic Cell Phagocytosis

Cells that become necrotic or apoptotic through tissue damage or during normal cellular turnover are usually rapidly cleared from the circulation and tissues by phagocytic cells. A number of soluble proteins have been identified that facilitate the phagocytosis of apoptotic cells, but few proteins have been defined that selectively opsonize necrotic cells. Previous studies have shown that histidine-rich glycoprotein (HRG), an abundant (approximately 100 microg/ml) 75-kDa plasma glycoprotein, binds to cell surface heparan sulfate on viable cells and cross-links other ligands, such as plasminogen, to the cell surface. In this study we have demonstrated that HRG also binds very strongly, in a heparan sulfate-independent manner, to cytoplasmic ligand(s) exposed in necrotic cells. This interaction is mediated by the amino-terminal domain of HRG and results in enhanced phagocytosis of the necrotic cells by a monocytic cell line. In contrast, it was found that HRG binds poorly to and does not opsonize early stage apoptotic cells. Thus, HRG has the unique property of selectively recognizing necrotic cells and may play an important physiological role in vivo by facilitating the uptake and clearance of necrotic, but not apoptotic, cells by phagocytes.

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.

Counting the cost: markers of endothelial damage in hematopoietic stem cell transplantation

During hematopoietic stem cell transplantation (HSCT), endothelial damage is the pathological hallmark of veno-occlusive disease of the liver, thrombotic microangiopathy, capillary leak syndrome and graft-versus-host disease. Events prior to conditioning, the conditioning regimen itself as well as calcineurin inhibitors may all induce endothelial damage. Unfortunately, the relative importance of these factors and their interactions, the time frame of endothelial damage and individual susceptibility remain unknown. Moreover, it is conceivable that conditioning regimens differ markedly in their propensity to initiate endothelial damage. Monitoring endothelial damage and response to treatment is hampered by the current lack of suitable markers. In this regard, an ideal marker should be sensitive and specific and indicate the development of an endothelial disorder prior to the onset of symptoms and organ dysfunction. Soluble markers, such as thrombomodulin, are easily amenable with immunoassays; yet, the interpretation of their levels is hampered by the influence of comorbidity. Evaluation of circulating endothelial cells in HSCT demonstrated a marked and dose-dependent increase in cell numbers after conditioning. The challenge ahead is to establish and evaluate novel markers of endothelial damage to permit early detection of disease, monitor response to treatment and evaluate different conditioning regimens.

Phagocytosis of necrotic cells by macrophages is phosphatidylserine dependent and does not induce inflammatory cytokine production

Apoptotic cells are cleared by phagocytosis during development, homeostasis, and pathology. However, it is still unclear how necrotic cells are removed. We compared the phagocytic uptake by macrophages of variants of L929sA murine fibrosarcoma cells induced to die by tumor necrosis factor-induced necrosis or by Fas-mediated apoptosis. We show that apoptotic and necrotic cells are recognized and phagocytosed by macrophages, whereas living cells are not. In both cases, phagocytosis occurred through a phosphatidylserine-dependent mechanism, suggesting that externalization of phosphatidylserine is a general trigger for clearance by macrophages. However, uptake of apoptotic cells was more efficient both quantitatively and kinetically than phagocytosis of necrotic cells. Electron microscopy showed clear morphological differences in the mechanisms used by macrophages to engulf necrotic and apoptotic cells. Apoptotic cells were taken up as condensed membrane-bound particles of various sizes rather than as whole cells, whereas necrotic cells were internalized only as small cellular particles after loss of membrane integrity. Uptake of neither apoptotic nor necrotic L929 cells by macrophages modulated the expression of proinflammatory cytokines by the phagocytes.