Microenvironmental influences of apoptosis in vivo and in vitro. Apoptosis. 2010;15:1029-1049. [PMID: 20237956 DOI: 10.1007/s10495-010-0485-9]

Antitumor immune responses induced by ionizing irradiation and further immune stimulation

The therapy of cancer emerged as multimodal treatment strategy. The major mode of action of locally applied radiotherapy (RT) is the induction of DNA damage that triggers a network of events that finally leads to tumor cell cycle arrest and cell death. Along with this, RT modifies the phenotype of the tumor cells and their microenvironment. Either may contribute to the induction of specific and systemic antitumor immune responses. The latter are boosted when additional immune therapy (IT) is applied at distinct time points during RT. We will focus on therapy-induced necrotic tumor cell death that is immunogenic due to the release of damage-associated molecular patterns. Immune-mediated distant bystander (abscopal) effects of RT when combined with dendritic cell-based IT and the role of fractionation of radiation in the induction of immunogenic tumor cell death will be discussed. Autologous whole-tumor-cell-based vaccines generated by high hydrostatic pressure technology will be introduced and the influence of cytokines and the immune modulator AnnexinA5 on the ex vivo generated or in situ therapy-induced vaccine efficacy will be outlined. RT should be regarded as immune adjuvant for metastatic disease and as a tool for the generation of an in situ vaccine when applied at distinct fractionation doses or especially in combination with IT to generate immune memory against the tumor. To identify the most beneficial combination and chronology of RT with IT is presumably one of the biggest challenges of innovative tumor research and therapies.

Apoptosis in Living Animals Is Assisted by Scavenger Cells and Thus May Not Mainly Go through the Cytochrome C-Caspase Pathway

Because billions of cells die every day in their bodies, animals have evolutionarily developed apoptosis to preserve the tissue environment from adverse effects of dead cells, a process achieved via phagocytosis of the cell corpses by professional or amateur phagocytes that are collectively referred to as scavengers. Hence, apoptosis is a merger of two procedures separately occurring inside the dying and the scavenger cells, respectively. The task of apoptosis research is to study how these death procedures occur without hurting the host tissues, and recruitment of in vitro system into the study must be justified for this purpose. Cells in culture have no motivation to preserve the environment, and their death does not involve corpse clearance by scavengers. Therefore, programmed cell death in culture should be redefined, for example as stress-induced cell death, to avoid many sources of confusions, since the word “apoptosis” had already been defined, prior to the era of cell culture, as a silent and beneficial cell suicide with corpse clearance as a distinctive hallmark. We should start over again on apoptosis research by determining whether different physiological apoptotic procedures in animals involve the cytochrome c-caspase pathway, since it has been established from cultured cells as a central mechanism of “apoptosis” but whether it overarches any physiological apoptotic procedure in animals is still unclear. Probably, cells in living animals are programmed to use scavengers to assist their apoptosis but cells in culture have no scavengers to help and thus need to go mainly through the cytochrome c-caspase pathway.

The N-terminus of CD14 acts to bind apoptotic cells and confers rapid-tethering capabilities on non-myeloid cells

Cell death and removal of cell corpses in a timely manner is a key event in both physiological and pathological situations including tissue homeostasis and the resolution of inflammation. Phagocytic clearance of cells dying by apoptosis is a complex sequential process comprising attraction, recognition, tethering, signalling and ultimately phagocytosis and degradation of cell corpses. A wide range of molecules acting as apoptotic cell-associated ligands, phagocyte-associated receptors or soluble bridging molecules have been implicated within this process. The role of myeloid cell CD14 in mediating apoptotic cell interactions with macrophages has long been known though key molecules and residues involved have not been defined. Here we sought to further dissect the function of CD14 in apoptotic cell clearance. A novel panel of THP-1 cell-derived phagocytes was employed to demonstrate that CD14 mediates effective apoptotic cell interactions with macrophages in the absence of detectable TLR4 whilst binding and responsiveness to LPS requires TLR4. Using a targeted series of CD14 point mutants expressed in non-myeloid cells we reveal CD14 residue 11 as key in the binding of apoptotic cells whilst other residues are reported as key for LPS binding. Importantly we note that expression of CD14 in non-myeloid cells confers the ability to bind rapidly to apoptotic cells. Analysis of a panel of epithelial cells reveals that a number naturally express CD14 and that this is competent to mediate apoptotic cell clearance. Taken together these data suggest that CD14 relies on residue 11 for apoptotic cell tethering and it may be an important tethering molecule on so called 'non-professional' phagocytes thus contributing to apoptotic cell clearance in a non-myeloid setting. Furthermore these data establish CD14 as a rapid-acting tethering molecule, expressed in monocytes, which may thus confer responsiveness of circulating monocytes to apoptotic cell derived material.

Caspase-9, caspase-3 and caspase-7 have distinct roles during intrinsic apoptosis

Background

Apoptosis is a form of programmed cell death that is regulated by the Bcl-2 family and caspase family of proteins. The caspase cascade responsible for executing cell death following cytochrome c release is well described; however the distinct roles of caspases-9, -3 and -7 during this process are not completely defined.

Results

Here we demonstrate several unique functions for each of these caspases during cell death. Specific inhibition of caspase-9 allows for efficient release of cytochrome c, but blocks changes in mitochondrial morphology and ROS production. We show that caspase-9 can cleave Bid into tBid at amino acid 59 and that this cleavage of Bid is required for ROS production following serum withdrawal. We also demonstrate that caspase-3-deficient MEFs are less sensitive to intrinsic cell death stimulation, yet have higher ROS production. In contrast, caspase-7-deficient MEFs are not resistance to intrinsic cell death, but remain attached to the ECM.

Conclusions

Taken together, these data suggest that caspase-9 is required for mitochondrial morphological changes and ROS production by cleaving and activating Bid into tBid. After activation by caspase-9, caspase-3 inhibits ROS production and is required for efficient execution of apoptosis, while effector caspase-7 is required for apoptotic cell detachment.

Blebs produced by actin-myosin contraction during apoptosis release damage-associated molecular pattern proteins before secondary necrosis occurs

Apoptosis is a fundamental homeostatic mechanism essential for the normal growth, development and maintenance of every tissue and organ. Dying cells have been defined as apoptotic by distinguishing features, including cell contraction, nuclear fragmentation, blebbing, apoptotic body formation and maintenance of intact cellular membranes to prevent massive protein release and consequent inflammation. We now show that during early apoptosis limited membrane permeabilization occurs in blebs and apoptotic bodies, which allows release of proteins that may affect the proximal microenvironment before the catastrophic loss of membrane integrity during secondary necrosis. Blebbing, apoptotic body formation and protein release during early apoptosis are dependent on ROCK and myosin ATPase activity to drive actomyosin contraction. We identified 231 proteins released from actomyosin contraction-dependent blebs and apoptotic bodies by adapted SILAC (stable isotope labeling with amino acids in cell culture) combined with mass spectrometry analysis. The most enriched proteins released were the nucleosomal histones, which have previously been identified as damage-associated molecular pattern proteins (DAMPs) that can initiate sterile inflammatory responses. These results indicate that limited membrane permeabilization occurs in blebs and apoptotic bodies before secondary necrosis, leading to acute and localized release of immunomodulatory proteins during the early phase of active apoptotic membrane blebbing. Therefore, the shift from apoptosis to secondary necrosis is more graded than a simple binary switch, with the membrane permeabilization of apoptotic bodies and consequent limited release of DAMPs contributing to the transition between these states.

Interplay between Misplaced Müllerian-Derived Stem Cells and Peritoneal Immune Dysregulation in the Pathogenesis of Endometriosis

In the genetic regulation of Müllerian structures development, a key role is played by Hoxa and Wnt clusters, because they lead the transcription of different genes according to the different phases of the organogenesis, addressing correctly cell-to-cell interactions, allowing, finally, the physiologic morphogenesis. Accumulating evidence is suggesting that dysregulation of Wnt and/or Hox genes may affect cell migration during organogenesis and differentiation of Müllerian structures of the female reproductive tract, with possible dislocation and dissemination of primordial endometrial stem cells in ectopic regions, which have high plasticity to differentiation. We hypothesize that during postpubertal age, under the influence of different stimuli, these misplaced and quiescent ectopic endometrial cells could acquire new phenotype, biological functions, and immunogenicity. So, these kinds of cells may differentiate, specializing in epithelium, glands, and stroma to form a functional ectopic endometrial tissue. This may provoke a breakdown in the peritoneal cavity homeostasis, with the consequent processes of immune alteration, documented by peripheral mononuclear cells recruitment and secretion of inflammatory cytokines in early phases and of angiogenic and fibrogenic cytokines in the late stages of the disease.

Preliminary study on XAGE-1b gene and its mechanism for promoting tumor cell growth

The XAGE-1b gene has been identified in numerous malignancies in the human body. However, little is known regarding its mechanism for promoting tumorigenesis in adenoid cystic carcinoma. The aim of this study was to explore the correlation between tumor cell growth and the XAGE-1b gene. The constructed PCMV-Myc plasmid vector containing the XAGE-1b gene and transfected adenoid cystic carcinoma (ACC)-2 cells was applied to study cell cycle alterations and anti-apoptotic effects. These were assessed by flow cytometry with PI staining and the measurement of cell content at its Sub-G₁ phase, respectively. The fluorescence intensity representing the regulation of XAGE-1b on the transcription factors located downstream of the signaling pathway using the Mercury pathway profiling system was also detected. XAGE-1b over expression promoted cell growth by shortening G₀-G₁ and prolonging the G₂-M phase. Additionally, XAGE-1b overexpression enhanced the anti-apoptotic effects induced by tumor necrosis factor-α (TNF-α) and serum deprivation in ACC-2 cells. The results of the present study suggested that XAGE-1b gene is crucial in the tumorigenesis of ACC, and its mechanism should be further investigated.

High levels of cleaved caspase-3 in colorectal tumour stroma predict good survival

Aim:

The primary aim was to determine the prognostic significance of apoptosis in colorectal tumour cells and tumour-associated stroma. A secondary aim was to determine whether apoptosis was related to immune surveillance.

Methods:

Immunohistochemistry was performed using monoclonal antibodies recognising cleaved caspase-3 (CC3), cleaved poly (ADP-ribose) polymerase (PARP), p53, Bcl2, MHC-II, B cells (CD16), macrophages (CD68) and T cells (CD3), on a tissue microarray of 462 colorectal tumours.

Results:

Kaplan–Meier analysis demonstrated that patients with high expression of CC3 in the tumour or CC3 or cleaved PARP in tumour-associated stroma have a good prognosis. This suggests that tumour stroma is promoting tumourigenesis and that high levels of death within the stroma breaks this link. CC3 levels in the tumour correlated with cleaved PARP and MHC-II expression but not with CD16, CD68, CD3, p53 or Bcl2 expression. CC3 levels on tumour-associated stroma also correlated with cleaved PARP and MHC-II expression but not with CD16, CD68, CD3, p53 or Bcl2 expression. Tumour cells express MHC-II in response to IFN-γ, suggesting that this may be one of the initiators of apoptosis within the good prognosis tumours. Although 73% of the MHC-II-positive tumour had high levels of apoptosis, many tumours had high levels of apoptosis in the absence of MHC-II, implying that this is only one of many causes of apoptosis within tumours. On multivariate analysis, using Cox's proportional hazards model, tumour stage, vascular invasion and expression of CC3 in tumour-associated stroma were shown to be independent markers of prognosis.

Conclusion:

This study shows that a high level of apoptosis within colorectal tumour-associated stroma is an independent marker of good prognosis.

Resolution of inflammation: mechanisms and opportunity for drug development

Inflammation is a beneficial host reaction to tissue damage and has the essential primary purpose of restoring tissue homeostasis. Inflammation plays a major role in containing and resolving infection and may also occur under sterile conditions. The cardinal signs of inflammation dolor, calor, tumor and rubor are intrinsically associated with events including vasodilatation, edema and leukocyte trafficking into the site of inflammation. If uncontrolled or unresolved, inflammation itself can lead to further tissue damage and give rise to chronic inflammatory diseases and autoimmunity with eventual loss of organ function. It is now evident that the resolution of inflammation is an active continuous process that occurs during an acute inflammatory episode. Successful resolution requires activation of endogenous programs with switch from production of pro-inflammatory towards pro-resolving molecules, such as specific lipid mediators and annexin A1, and the non-phlogistic elimination of granulocytes by apoptosis with subsequent removal by surrounding macrophages. These processes ensure rapid restoration of tissue homeostasis. Here, we review recent advances in the understanding of resolution of inflammation, highlighting the pharmacological strategies that may interfere with the molecular pathways which control leukocyte survival and clearance. Such strategies have proved beneficial in several pre-clinical models of inflammatory diseases, suggesting that pharmacological modulation of the resolution process may be useful for the treatment of chronic inflammatory diseases in humans.

Noncanonical roles of the immune system in eliciting oncogene addiction

Cancer is highly complex. The magnitude of this complexity makes it highly surprising that even the brief suppression of an oncogene can sometimes result in rapid and sustained tumor regression, illustrating that cancers can be 'oncogene addicted' [1-10]. The essential implication is that oncogenes may not only fuel the initiation of tumorigenesis, but in some cases must be excessively activated to maintain a neoplastic state [11]. Oncogene suppression acutely restores normal physiological programs that effectively overrides secondary genetic events and a cancer collapses [12,13]. Oncogene addiction is the description of the dramatic and sustained regression of some cancers upon the specific inactivation of a single oncogene [1-13,14(••),15,16(••)], that can occur through tumor intrinsic [1,2,4,12], but also host immune mechanisms [17-23]. Notably, oncogene inactivation elicits a host immune response that involves specific immune effectors and cytokines that facilitate a remodeling of the tumor microenvironment including the shut down of angiogenesis and the induction of cellular senescence of tumor cells [16(••)]. Hence, immune effectors are not only critically involved in tumor prevention, initiation [17-19], and progression [20], but also appear to be essential to tumor regression upon oncogene inactivation [21,22(••),23(••)]. Understanding how the inactivation of an oncogene elicits a systemic signal in the host that prompts a deconstruction of a tumor could have important implications. The combination of oncogene-targeted therapy together with immunomodulatory therapy may be ideal for the development of both robust tumor intrinsic and immunological responses, effectively leading to sustained tumor regression.

Inflammation and cancer revisited: an hypothesis on the oncogenic potential of the apoptotic tumor cell

It is well known that an important property of apoptosis is the prevention of cancer. However, it is becoming increasingly clear that apoptotic cells have the capacity to activate reparative and regenerative responses, which have the potential to make significant impact on pathological processes underlying autoimmune disease and cancer. Those properties of apoptotic cells that influence their immediate microenvironment are reviewed here in the context of cancer progression and treatment.

Innate recognition of apoptotic cells: novel apoptotic cell-associated molecular patterns revealed by crossreactivity of anti-LPS antibodies

Cells dying by apoptosis are normally cleared by phagocytes through mechanisms that can suppress inflammation and immunity. Molecules of the innate immune system, the pattern recognition receptors (PRRs), are able to interact not only with conserved structures on microbes (pathogen-associated molecular patterns, PAMPs) but also with ligands displayed by apoptotic cells. We reasoned that PRRs might therefore interact with structures on apoptotic cells - apoptotic cell-associated molecular patterns (ACAMPs) - that are analogous to PAMPs. Here we show that certain monoclonal antibodies raised against the prototypic PAMP, lipopolysaccharide (LPS), can crossreact with apoptotic cells. We demonstrate that one such antibody interacts with a constitutively expressed intracellular protein, laminin-binding protein, which translocates to the cell surface during apoptosis and can interact with cells expressing the prototypic PRR, mCD14 as well as with CD14-negative cells. Anti-LPS cross reactive epitopes on apoptotic cells colocalised with annexin V- and C1q-binding sites on vesicular regions of apoptotic cell surfaces and were released associated with apoptotic cell-derived microvesicles (MVs). These results confirm that apoptotic cells and microbes can interact with the immune system through common elements and suggest that anti-PAMP antibodies could be used strategically to characterise novel ACAMPs associated not only with apoptotic cells but also with derived MVs.

Apoptotic-cell-derived membrane vesicles induce an alternative maturation of human dendritic cells which is disturbed in SLE

The clearance of apoptotic cells occurs in a non-inflammatory context. Defects in this clearance process have been linked to the emergence of human autoimmune diseases like systemic lupus erythematosus (SLE). A characteristic of apoptotic cell death is the shedding of membrane coated vesicles from the cellular surfaces. Those vesicles have recently been recognized as mediators of intercellular communication or as adjuvant in the pathogenesis of autoimmune diseases. We analyzed the interactions between these apoptotic cell-derived membrane vesicles and professional antigen presenting cells. These vesicles were engulfed by monocyte-derived dendritic cells (mDC) and stimulated their maturation towards a phenotype comprising an upregulation of CD80, CD83, CD86, and a remarkable downregulation of MHC class II molecules. We observed only a minor release of proinflammatory cytokines from these mDC when compared to LPS stimulation. mDC stimulated by apoptotic vesicles did not cause significant T-cell expansion. Interestingly, when compared to normal healthy donors SLE patients-derived dendritic cells showed a significantly different phenotype lacking the downregulation of MHC class II, which correlated to disease activity.

MFGE8 does not orchestrate clearance of apoptotic neurons in a mouse model of Parkinson's disease

Parkinson's disease (PD) is an age-related neurodegenerative disorder characterized by a loss of dopaminergic neurons (DN) in the substantia nigra (SN). Several lines of evidence suggest that apoptotic cell death of DN is driven in part by non-cell autonomous mechanisms implicating microglial cells and inflammatory processes. Yet, how apoptotic DNs get removed by professional phagocytes and how this process modulates inflammatory processes are still unresolved issues. In this study, we investigated the role of MFGE8, a soluble factor involved in phagocytic recognition, in apoptotic DN clearance and neuroinflammation in PD. We report that glial expression of MFGE8 is enhanced in post-mortem PD brains compared to control individuals. Then, in vivo functional analysis of Mfge8 was assessed in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-intoxicated mouse model of PD using wild-type (WT) and Mfge8-deficient mice. Neuropathological analysis consisted in evaluating (i) the loss of nigral DN and striatal DN terminals, (ii) the extent of glial cell activation and (iii) the number of apoptotic profiles. In vivo microglial phagocytic activity was further assessed by measuring the engulfment of apoptotic DN preloaded with fluorescent latex beads. Here we show that Mfge8 deficiency neither impact the phagocytic clearance of apoptotic bodies nor change the overall neuropathological parameters (DN cell loss and glial cell activation). In summary, our data argue that MFGE8 is not likely involved in the phagocytic clearance of neuronal debris associated with nigrostriatal pathway injury.

Radiation-induced bone marrow apoptosis, inflammatory bystander-type signaling and tissue cytotoxicity

PURPOSE

A study of irradiated (0.25-2 Gy) murine bone marrow has investigated the relationships between apoptotic responses of cells exposed in vivo and in vitro and between in vivo apoptosis and tissue cytotoxicity.

MATERIALS AND METHODS

The time course of reduction in bone marrow cellularity in vivo was determined by femoral cell counts and apoptosis measurements obtained using three commonly used assays. Inflammatory pro-apoptotic cytokine production at 24 h post-exposure in vivo was investigated using a bystander protocol.

RESULTS

In vivo, there is a dose- and time-dependent non-linear reduction in bone marrow cellularity up to 24 h post- irradiation not directly represented by apoptosis measurements. The majority of cells are killed within 6 h but there is on-going cell loss in vivo up to 24 h post-irradiation in the absence of elevated levels of apoptosis and associated with the induction of cytokines produced in response to the initial tumor protein 53 (p53)-dependent apoptosis.

CONCLUSION

The results demonstrate that small increases in measured apoptosis can reflect significant intramedullary cell death and with apoptotic processes being responsible for pro-inflammatory mechanisms that can contribute to additional on-going cell death. The findings demonstrate the importance of studying tissue responses when considering the mechanisms underlying the consequences of radiation exposures.

Immune response against Sporothrix schenckii in TLR-4-deficient mice

For many fungal diseases, macrophages are the major cell population implicated in host protection, primarily by their ability to eliminate the invading fungal pathogen through phagocytosis. In sporotrichosis, this remains true, because of macrophages’ ability to recognize Sporothrix schenckii through specific receptors for some of the fungus’ cellular surface constituents. Further confirmation for macrophages’ pivotal role in fungal diseases came with the identification of toll-like receptors, and the subsequent numerous associations found between TLR-4 deficiency and host susceptibility to diverse fungal pathogens. Involvement of TLR-4 in immune response against sporotrichosis has been conducted to investigate how TLR-4 signaling could affect inflammatory response development through evaluation of H2O2 production and IL-1β, IL-6 and TGF-β release during the course of S. schenckii infection on TLR-4-deficient mice. The results showed that macrophages are largely dependent on TLR-4 for inflammatory activation and that in the absence of TLR-4 signaling, increased TGF-β release may be one of the contributing factors for the abrogated inflammatory activation of peritoneal exudate cells during mice sporotrichosis.

Exosome mimetics: a novel class of drug delivery systems

The identification of extracellular phospholipid vesicles as conveyors of cellular information has created excitement in the field of drug delivery. Biological therapeutics, including short interfering RNA and recombinant proteins, are prone to degradation, have limited ability to cross biological membranes, and may elicit immune responses. Therefore, delivery systems for such drugs are under intensive investigation. Exploiting extracellular vesicles as carriers for biological therapeutics is a promising strategy to overcome these issues and to achieve efficient delivery to the cytosol of target cells. Exosomes are a well studied class of extracellular vesicles known to carry proteins and nucleic acids, making them especially suitable for such strategies. However, the considerable complexity and the related high chance of off-target effects of these carriers are major barriers for translation to the clinic. Given that it is well possible that not all components of exosomes are required for their proper functioning, an alternative strategy would be to mimic these vesicles synthetically. By assembly of liposomes harboring only crucial components of natural exosomes, functional exosome mimetics may be created. The low complexity and use of well characterized components strongly increase the pharmaceutical acceptability of such systems. However, exosomal components that would be required for the assembly of functional exosome mimetics remain to be identified. This review provides insights into the composition and functional properties of exosomes, and focuses on components which could be used to enhance the drug delivery properties of exosome mimetics.

The extracellular release of DNA and HMGB1 from Jurkat T cells during in vitro necrotic cell death

In innate immunity, dead and dying cells release internal constituents that can serve as damage-associated molecular patterns (DAMPs) or alarmins. This release occurs more abundantly during necrosis than apoptosis and may account for the differences in the immunologic properties of these death forms. To elucidate DAMP release in necrosis, we compared the levels of two nuclear molecules (DNA and HMGB1, a non-histone protein with alarmin activity) in media following necrosis of Jurkat T cells by freeze-thawing, ethanol, heat or hydrogen peroxide treatment. In our experiments, DNA release was measured by fluorimetry with the dye PicoGreen, while HMGB1 was measured by Western blotting. As the results of our study show, each form of necrosis is associated with a distinct pattern of DNA and HMGB1 release with respect to kinetics and amounts. Of these, freeze-thawing produced the highest and most rapid increase in HMGB1 and DNA levels, although the released DNA was subject to nuclease digestion; in addition, freeze-thawing led to the production of particles measured by flow cytometry. Together, these results indicate that experimental necrosis leads to diverse patterns of nuclear molecule release which could affect their immunologic activity.

Apoptosis and the airway epithelium

The airway epithelium functions as a barrier and front line of host defense in the lung. Apoptosis or programmed cell death can be elicited in the epithelium as a response to viral infection, exposure to allergen or to environmental toxins, or to drugs. While apoptosis can be induced via activation of death receptors on the cell surface or by disruption of mitochondrial polarity, epithelial cells compared to inflammatory cells are more resistant to apoptotic stimuli. This paper focuses on the response of airway epithelium to apoptosis in the normal state, apoptosis as a potential regulator of the number and types of epithelial cells in the airway, and the contribution of epithelial cell apoptosis in important airways diseases.

Apoptotic cell-derived ICAM-3 promotes both macrophage chemoattraction to and tethering of apoptotic cells

A wide range of molecules acting as apoptotic cell-associated ligands, phagocyte-associated receptors or soluble bridging molecules have been implicated within the complex sequential processes that result in phagocytosis and degradation of apoptotic cells. Intercellular adhesion molecule 3 (ICAM-3, also known as CD50), a human leukocyte-restricted immunoglobulin super-family (IgSF) member, has previously been implicated in apoptotic cell clearance, although its precise role in the clearance process is ill defined. The main objective of this work is to further characterise the function of ICAM-3 in the removal of apoptotic cells. Using a range of novel anti-ICAM-3 monoclonal antibodies (mAbs), including one (MA4) that blocks apoptotic cell clearance by macrophages, alongside apoptotic human leukocytes that are normal or deficient for ICAM-3, we demonstrate that ICAM-3 promotes a domain 1–2-dependent tethering interaction with phagocytes. Furthermore, we demonstrate an apoptosis-associated reduction in ICAM-3 that results from release of ICAM-3 within microparticles that potently attract macrophages to apoptotic cells. Taken together, these data suggest that apoptotic cell-derived microparticles bearing ICAM-3 promote macrophage chemoattraction to sites of leukocyte cell death and that ICAM-3 mediates subsequent cell corpse tethering to macrophages. The defined function of ICAM-3 in these processes and profound defect in chemotaxis noted to ICAM-3-deficient microparticles suggest that ICAM-3 may be an important adhesion molecule involved in chemotaxis to apoptotic human leukocytes.

Cell death in the pathogenesis of immune-mediated diseases: the role of HMGB1 and DAMP-PAMP complexes

Cell death is a ubiquitous process whose immunological consequences can influence the course of infectious, autoimmune and inflammatory diseases. While cell death has long been dichotomised in terms of apoptosis and necrosis, other forms of death can occur and they vary in their capacity to stimulate as well as inhibit inflammation. The pro-inflammatory activity of dead cells results from a wide variety of intracellular molecules that are released as cell permeability increases during death. These molecules have been termed as DAMPs (damage associated molecular patterns) or alarmins. Among these DAMPs, HMGB1, a non-histone nuclear protein, serves as the prototype. Although HMGB1 was originally thought to act alone as a cytokine, recent studies suggest that its immunological effects result from complexes of HMGB1 with either other DAMPs or with PAMPs (pathogen associated molecular patterns). Studies on the role of HMGB1 in pathogenesis suggest that the formation of extracellular complexes is an important mechanism for generating pro-inflammatory signals during cell death and therefore could be a potential target of new therapy.

Epithelial cell specificity and apotope recognition by serum autoantibodies in primary biliary cirrhosis

A major enigma of primary biliary cirrhosis (PBC) is the selective targeting of biliary cells. Our laboratory has reported that after apoptosis, human intrahepatic biliary epithelial cells (HiBECs) translocate the E2 subunit of the pyruvate dehydrogenase complex immunologically intact into apoptotic bodies, forming an apotope. However, the cell type and specificity of this reaction has not been fully defined. To address this issue, we investigated whether the E2 subunit of the pyruvate dehydrogenase complex, the E2 subunit of the branched chain 2-oxo acid dehydrogenase complex, the E2 subunit of the oxo-glutarate dehydrogenase complex, four additional inner mitochondrial enzymes, and four nuclear antigens remain immunologically intact with respect to postapoptotic translocation in HiBECs and three additional control epithelial cells. We report that all three 2-oxo acid dehydrogenase enzymes share the ability to remain intact within the apotope of HiBECs. Interestingly, the E2 subunit of the branched chain 2-oxo acid dehydrogenase complex also remained intact in the other cell types tested. We extended the data, using sera from 95 AMA-positive and 19 AMA-negative patients with PBC and 76 controls, by testing for reactivity against the seven mitochondrial proteins studied herein and also the ability of AMA-negative sera to react with HiBEC apotopes. Sera from 3 of 95 AMA-positive sera, but none of the controls, reacted with 2,4-dienoyl coenzyme A reductase 1, an enzyme also present intact only in the HiBEC apotope, but which has not been previously associated with any autoimmune disease. Finally, the specificity of HiBEC apotope reactivity was confined to AMA-positive sera.

CONCLUSION

We submit that the biliary specificity of PBC is secondary to the unique processes of biliary apoptosis.

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.

Mechanisms underlying vesicular stomatitis virus-induced apoptosis of HepG2 cells in vitro

AIM: To investigate the apoptosis-inducing effect of a laboratory-attenuated vesicular stomatitis virus (VSV) strain on HepG2 cells and to explore the underlying mechanisms.

METHODS: After HepG2 cells were infected with VSV at a multiplicity of infection (MOI) of 1.0, cell viability was determined by MTT assay; morphological assessment of apoptosis was performed by acridine orange (AO)/ethidium bromide (EB) and Hoechst/PI staining; apoptotic cells were quantified by annexin V/PI double-staining and cell cycle analysis; mitochondrial membrane potential (ΔΨm) was measured by JC-1 staining; and activation of caspase proteolytic cascade was measured with caspase-9, caspase-8 and -3 colorimetric assay kits.

RESULTS: The attenuated VSV strain could markedly inhibit HepG2 cell proliferation in a time-dependent manner. After HepG2 cells were exposed to VSV at an MOI of 1.0 for 24 h, the percentages of early apoptotic cells (26.46% ± 6.01% vs 4.86% ± 2. 28%, t = -5.817, P < 0.01) and cells in sub-G1 phase (14.07% ± 3.83% vs 3.99% ± 1.36%, t = -4.293, P < 0.05) were increased compared with mock-infected cells. VSV infection significantly decreased mitochondria membrane potential (ΔΨm) (t = -4.586, P < 0.05) and increased the activity of caspase-9 and caspase-3 (both P < 0.05).

CONCLUSION: Human hepatoma cell line HepG2 is highly susceptible to infection with oncolytic VSV. VSV can inhibit the proliferation of HepG2 cell and promote apoptosis through the intrinsic mitochondria pathway. VSV-induced collapse of the mitochondrial trans-membrane potential could exert a feedback effect to elicit caspase-9, and then lead to the activation of the key downstream factor caspase-3.

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.

Ex vivo- and in vivo-induced dead tumor cells as modulators of antitumor responses

Joint application of standard tumor therapies like radiotherapy and/or chemotherapy with immune therapy has long been considered not to fit. However, it has become accepted that immune responses may contribute to the elimination of cancer cells. We present how in vivo-induced tumor cell death by irradiation, chemotherapeutic agents, or hyperthermia can be rendered more immunogenic. High hydrostatic pressure is introduced as an innovative inactivation method for tumor cells used as vaccines. Annexin A5, being a natural occurring ligand for phosphatidylserine that is exposed by dying tumor cells, renders apoptotic tumor cells immunogenic and induces tumor regression. Combinations of irradiation with hyperthermia may also foster antitumor responses. For preparation of autologous tumor cell vaccines, high hydrostatic pressure is suitable to induce immunogenic cancer cell death. Future work will be aimed toward evaluating which combination and chronological sequence of radiotherapy, chemotherapy, hyperthermia, annexin A5, and/or autologous tumor cell vaccines will induce specific and long-lasting antitumor immunity.

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.