Cell death processes are tightly regulated to safeguard successful resolution of inflammation. Nevertheless, dysregulation of cell death commonly occurs hampering the pro-resolution process. In the lung, numerous cell death processes govern inflammation. Understanding the mechanisms that regulate cell death in the lung will help enable identification of novel therapeutic targets to limit/resolve inflammation and restore homeostasis.
Apoptosis
Granulocyte apoptosis has been a subject of much interest over recent decades, and there is strong evidence that failure of inflammation resolution contributes to numerous chronic inflammatory conditions and with its manipulation, therefore offering potential novel therapeutic targets. Inflammatory cells have the potential to be incendiary in the host tissue environment and, in the absence of an appropriate inflammatory ‘threat’, can trigger host tissue damage secondary to release of histotoxic mediators such as proteases and reactive oxygen species. Perhaps one of the most critical mechanisms for resolution and restoration of tissue homeostasis following an acute inflammatory insult is the ability of accumulated migratory granulocytes to undergo immunologically silent programmed cell death, namely, apoptosis. This highly regulated, energy-dependent and complex process involves the coordinated destruction and packaging of inflammatory cell contents for phagocytic clearance in a manner that does not elicit a host immune response, facilitates healing, and promotes and maintains self-tolerance by the adaptive immune system to create immunological memory. In addition, this neat packaging of cell contents prevents the leakage of pro-inflammatory mediators and contains histotoxic weaponry, including proteases, reactive oxygen species production and lysozymes. Granulocyte apoptosis is a caspase-dependent process that proceeds following activation of one of two major pathways, the intrinsic and extrinsic [
1,
54]. It has become increasingly evident that the mutual exclusivity of these pathways is not as clear-cut as was previously assumed, and there is a degree of cross talk between the molecular component of their execution, with both ultimately dependent on the actions of caspases to initiate cell suicide. Caspases, a family of cysteine-aspartic proteases, are the critical intracellular mediators of apoptosis and are also implicated in inflammation and necrosis, thus offering a promising target for pharmacological manipulation [
55,
56].
The intrinsic, or mitochondrial, pathway occurs when the balance of pro- and anti-apoptotic mediators of the Bcl-2 family proteins tips in favour of cell death, which occurs in response to DNA damage or endoplasmic reticulum stress. In the mature granulocyte, the pro-apoptotic family members, Bax, Bad, Bak and Bid, are suppressed by their anti-apoptotic counterparts, Mcl-1, Bcl-xl and A1, thus maintaining cell viability. In the presence of sufficient cellular stress, they circumvent this suppression and translocate from cytoplasm to mitochondria, triggering development of mitochondrial outer membrane permeabilisation (MOMP). MOMP allows mitochondrial molecules cytochrome C, Smac/DIABLO, Omi/HtrA2 and serine proteases to enter the cytosol, where cytochrome C interacts with Apaf-1 to form the apoptosome, which is ultimately responsible for cell death via the activation of pro-caspase 9. The resultant caspase 9 causes cleavage of the ‘executioner’—caspase 3, leading to DNA fragmentation, cross-linking and degradation of intracellular proteins and membrane receptor switch. Conversely, apoptosis advancing via the extrinsic ‘death receptor’ pathway occurs in response to stimulation by extracellular mediators, primarily TNF, Fas ligand and tumour necrosis factor-alpha-related apoptosis-inducing ligand (TRAIL) [
57]. These intercellular messengers activate receptors on granulocyte plasma membranes—specifically TRAIL receptor (TRAIL-R), TNF receptor 1 (TNFR1) and Fas receptor (FasR), which upon binding with their corresponding ligand are prompted to coalesce. Assemblages of membrane proteins bind with internal adaptors, forming death domain proteins that attract clusters of cytosolic pro-caspase 8. The interactions of these proteins trigger an intracellular cascade, namely, the death-inducing signalling complex (DISC) that culminates in autocatalytic cleavage of pro-caspase 8, which then results in apoptosis of the cell again via cleavage of caspase 3. Caspase 8 generated in response to extracellular Fas ligand is the main executor of cross talk between the intrinsic and extrinsic pathways, as its release triggers MOMP via cleavage of Bid [
58,
59].
Following caspase activation, nuclear DNA forms nucleosomes, dense packages of genetic material. Occurring simultaneously is the alteration of the plasma membrane receptor profile. The pro-survival molecule suite, which includes CD47 and CD31, is replaced by a milieu of ‘find-me’ and ‘eat-me’ signals that trigger recognition and stimulate uptake of the dying cell by macrophages or other cells with phagocytic capacity [
10,
60]. Find-me signals are released from apoptotic cells which subsequently attract nearby phagocytes. In mammals, several find-me signals have been identified including fractalkine (CX3CL1), lysophosphatidycholine (lipid mediator), sphingosine 1-phosphate and nucleotides including adenosine triphosphate and uridine 5′ triphosphate [
61‐
64]. Eat-me signals allow the specific recognition of apoptotic cells via cell different cell surface changes which include exposure of phosphatidylserine (PS) to the outer membrane leaflet, intracellular adhesion moelecule-1 (ICAM1) epitope alteration, exposure of calreticulin and alteration of cell surface charge and glycosylation configurations (for review, see Gardai et al. [
65]). The best described and most evolutionarily conserved of these eat-me signals is the externalisation of PS to the outer membrane leaflet [
66,
67], which along with ICAM3 and annexin 1 promotes phagocytosis. Additionally, find-me signals such as nucleotides, fractalkine and lipid mediators attract not only professional phagocytes but can also facilitate uptake by neighbouring cells and other non-professional phagocytes including bronchial epithelial cells [
10,
68,
69]. In addition to preventing direct, though inadvertent, damage to host tissues, this mechanism of removal dampens the immune response and encourages resolution, allowing normal tissue homeostasis to resume. Apoptosis is an important clearance mechanism for effete cells and for the successful resolution of lung inflammation. Granulocyte apoptosis has been shown to be delayed in lung disease, and specific induction of granulocyte apoptosis can enhance the resolution of lung inflammation (for in-depth reviews, please refer to [
1,
14]).
Other cell death processes in the lung
In direct opposition to its well-tempered counterpart (apoptosis), necrosis results in loss of membrane integrity and the unrestrained release of intracellular contents following cell trauma. The release of toxic damage-associated molecular patterns (DAMPs) into the extracellular environment characteristically results in an acute inflammatory response with inflammatory cell influx, paracrine effects on surrounding cells with release of pro-inflammatory mediators and significant potential for host tissue destruction [
70,
71]. A variety of insults, including infection, chemicals, physical trauma and nutritional deficits, cause direct loss of membrane integrity—so-called primary necrosis. In situations where there is a failure in the timely and sufficient clearance of apoptotic cells by phagocytes, secondary necrosis occurs due to the inevitable disintegration of the apoptotic cell membrane, which may result in a late-phase inflammatory response, the nature of which is still debated [
10,
60,
68]. There is increasing recognition of the importance of DAMPs in the propagation of the acute inflammatory response within the lung through interaction with pathogen recognition receptors (PRRs) [
72]. Their activation promotes inflammation via transcription of pro-inflammatory cytokines and enhances the anti-microbial response. There is growing evidence to suggest that secondary necrosis, the necrotic fate of a cell following a failure of phagocytosis once apoptosis has occurred, contributes to persistent inflammation in a number of chronic conditions, and certainly evident that necrosis in the context of hyperactive acute response can result in significant long-term sequelae as a result of tissue damage and aberrant remodelling.
In 2004, a novel cell death process distinctly separate to necrosis and apoptosis was discovered when it was observed that human neutrophils could generate NETs [
73] for innate immune defence. NETs are composed of decondensed nuclear chromatin that is discharged to the extracellular environment in a controlled manner. Additionally, neutrophils can release mitochondrial DNA [
74]; however, mitochondrial DNA is 100,000 times less abundant on NETs than nuclear DNA [
75]. NETs are characterised by the nuclear membrane being entirely fragmented with most of the granules being dissolved, thus allowing direct contact and mixing of nuclear, cytoplasmic and granular components [
76]. Studded on the DNA backbone of NETs are nuclear, granule and cytosolic proteins. Nuclear proteins include citrullinated histones and anti-microbial peptides (AMPs) such as the cathelicidin, LL37; azurophilic (primary) granule proteins such as neutrophil elastase (NE), cathespin G, myeloperoxidase (MPO) and α-defensins; specific proteins from secondary and tertiary granules such as lactoferrin and gelatinase, respectively; or cytosolic proteins such as the cytosolic protein complex, calprotectin [
73,
77]. The core histones H2A, H2B, H3 and H4 account for 70 % of all NET-associated proteins [
77]. Histone hypercitrullination which mediates chromatin decondensation during NET formation is mediated via peptidylarginine deiminase 4 (PAD4) [
78].
The formation of NETs is dependent upon generation of ROS via activation of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, actin filament polymerisation, as well as requiring activation of protein kinase C (PKC) pathways upstream of NADPH oxidase [
76,
79,
80]. NETs have the ability to capture and kill both Gram-positive/negative bacteria, viruses, fungi and larger parasites [
73,
81‐
83]; however, it is now widely regarded that NETs are more efficient at trapping microorganisms as opposed to killing. Having said that, bacteria do have the ability to escape and degrade NETs via numerous mechanisms, for example, polysaccharide formation which causes electrochemical repulsion of AMPs or DNAse generation aiding degradation of chromatin [
84,
85]. This process was first termed NETosis [
86] as it was thought to be exclusive to neutrophils. However, this cell death pathway is now commonly referred to as ETosis [
87] and can be found in a number of different immune cell types, as well as in haemocytes of lower invertebrates [
88]. The early origin of ETosis helps explain some of its pathological effects in mammals where ETosis can be viewed as a double-edged sword.
ETosis is implicated in a number of chronic lung inflammatory disorders, including ALI and ARDS, influenza pneumonia, cystic fibrosis, asthma, COPD and tuberculosis. A hallmark of infection-related ALI/ARDS and in sterile injury is the activation and subsequent mass migration of neutrophils into the alveolar space, which is initiated by chemokines released from macrophages, neutrophils and epithelial cells [
89]. Neutrophil activation and NET formation in the alveolar space are initiated by a highly localised concentration of stimulating factors. Injury to alveolar epithelial cells increases permeability of the barrier between the alveolar space and blood vessels, which also promotes the epithelium to release pro-inflammatory IL-8. This can result in leakage of edema fluid containing high infiltrating numbers of neutrophils into the alveolar space. Within the alveoli, NETs are released in response to host-derived factors such as granulocyte/macrophage colony-stimulating factor (GM-CSF), complement factor 5a (C5a), activated platelets and singlet oxygen. NETs then cause secondary epithelial cell damage via release of NET proteins and ROS generation, which results in chronic inflammation. Potent lung injury factors released by NETs include NE, which cleaves endothelial cytoskeleton, as well as E-cadherin and VE-cadherin that increase the permeability of the alveolar-capillary barrier [
90]. Other NET-derived components such as cathespin G can degrade anti-inflammatory proteins via pro-inflammatory protein production, LL-37 promotes apoptosis in epithelial and endothelial cells and ROS produced by MPO causes both apoptosis and necrosis in epithelial cells [
90]. Moreover, extracellular histones (H3 and H4) released from NETs are implicated as pivotal effectors of C5aR- and C5L2-mediated (C5a receptors) ALI in humans, rats and mice [
91]. NETs are also found in models of sterile injury such as transfusion-related ALI (TRALI) [
92]. NETs have been linked to ALI in influenza pneumonitis where NETs caused lung injury via association with alveoli in areas of tissue injury [
93]. NETs are found in the sputum of CF patients [
94]. The majority of extracellular DNA found in the sputum of CF patients is in fact NET derived, as the DNA complexes are consistent with neutrophil ETosis and share a similar protein signature [
95]. Extracellular DNA leads to an increase in sputum viscosity that correlates with a high concentration of neutrophils and NET accumulation in CF airways that consequently aids microbial colonisation, proliferation and biofilm formation causing chronic inflammation correlating with increased pulmonary obstruction [
96,
97]. Yet, why more ETosis is occurring in CF airways remains unclear. However, it is likely that NETs are formed in response to host bacteria, such as opportunistic
Pseudomonas aeruginosa, one of the main pathogens to colonise the CF lung which is also a common pathogen known to induce NETs [
97]. Both EETs and NETs are found in the airways of human atopic asthma patients in vivo [
98], whereas NETs decorated with NE, histone H1 and citrullinated histone H3 are found in sputum of COPD patients [
99,
100]. Interestingly, both
Mycobacterium genotypes
M. tuberculosis (cause of most types of tuberculosis) and
M. canetti induced NET formation and ROS production in a time-dependent manner [
101].
M. tuberculosis-induced NETs were decorated with key ETotic markers such as histone H2A, H2B and NE and were able to trap but not kill
M. tuberculosis [
101]. Granulomas are an important and hallmark feature of tuberculosis and are generally caused by mycobacterial or fungal infections. These prominent structures represent a key immune response to foreign material that is too large to be cleared by other immune defence processes. For an in-depth review of the role of ETosis during lung inflammation, refer to Cheng and Palaniyar [
102]. Interestingly, there appears to be a link between NADPH oxidase activation, ETosis and apoptosis in immune defence against infectious agents. This has been highlighted by studies involving neutrophils obtained from patients with chronic granulomatous disease (CGD; a rare inherited disorder of NADPH oxidase) and mouse models of CGD, where in both instances, the ETotic response is severely diminished [
76,
103]. Furthermore, following phagocytosis (in vitro), neutrophil apoptosis is compromised in CGD sufferers [
104]. Failed resolution of inflammation in patients with CGD can lead to a number of inflammatory lung conditions including pneumonia, pulmonary fibrosis and lung abscesses, and specifically, in CGD mice, ALI can result as a consequence of impaired tryptophan catabolism (a superoxide-dependent process) [
105].
Additional cell death processes play important roles during lung inflammation; these include autophagy and necroptosis. Autophagy entails the intracellular degradation of cellular components, which are then delivered to the lysosome for enzymatic degradation. Autophagy can play opposing roles during chronic lung inflammatory disorders and lung cancer. An increase in autophagy markers, such as autophagosome formation, and levels of LC3B-II (autophagosome-associated protein) are found in the pulmonary epithelium after induction of ALI in mice after extended exposure to hyperoxia [
106]. During tuberculosis, autophagy can assist in the generation of anti-virulence factors [
107], whereas during influenza A, infection autophagy is induced with viral replication dependent upon autophagosome formation [
108]. Mitophagy (selective degradation of mitochondria via autophagy) can, in certain instances, aggravate the severity of COPD by activating additional cell death processes, whereas during pulmonary hypertension, autophagy can regulate cell death facilitating host defence [
106]. Furthermore, autophagic degradation and clearance of cilia (ciliophagy) result in COPD-associated cilium dysfunction [
109]. Impairment of autophagy can escalate the severity of cystic fibrosis and idiopathic pulmonary fibrosis, and in lung cancer, it can reduce carcinogenesis; yet it can also promote tumour cell survival. Therefore, autophagy can control the effectiveness of certain cancer therapies [
106]. Conversely, necroptosis (programmed necrosis) is known to augment lung inflammation in several murine models. In a model of erythrocyte transfusion and LPS-induced lung inflammation, necroptosis of lung endothelial cells is induced via high mobility group box 1 (HMGB1) protein [
110].
Staphylococcus aureus toxins can induce necroptosis via receptor-interacting protein kinases (RIP) 1 and 2 which bind to pro-necrotic mixed lineage kinase domain-like (MLKL) protein via RIP1/RIP2/MLKL signalling, which results in depletion of alveolar macrophages as well as IL-1β expression leading to pulmonary damage [
111]. Necroptosis was also observed in bronchial epithelial cells in vitro via induction by cigarette smoke, which also triggered the release of DAMPs and pro-inflammatory cytokines (IL-8, IL-6) [
112]. In vivo, cigarette smoke caused neutrophilic airway inflammation as evidenced by increased the number of neutrophils present in the BAL fluid, which was significantly reduced by treatment with the necroptosis inhibitor, necrostatin-1 [
112].