Inflammation, ageing and diseases of the lung: Potential therapeutic strategies from shared biological pathways

Lung diseases disproportionately affect elderly individuals. The lungs form a unique environment: a highly elastic organ with gaseous exchange requiring the closest proximity of inhaled air containing harmful agents and the circulating blood. The lungs are highly susceptible to senescence, with age and ‘inflammageing’ creating a pro‐inflammatory environment with a reduced capacity to deal with challenges. While lung diseases may have disparate causes, the burden of ageing and inflammation provides a common process that can exacerbate seemingly unrelated pathologies. However, these shared pathways may also provide a common route to treatment, with increased interest in drugs that target ageing processes across respiratory diseases. In this review, we will examine the evidence for the increased burden of lung disease in older adults, the structural and functional changes seen with advancing age and assess what our expanding knowledge of inflammation and ageing pathways could mean for the treatment of lung disease.


| The epidemiology of lung disease in the elderly
While age is a risk factor for most diseases, the lungs have been described as the worst affected organ for disease in the elderly (Budinger et al., 2017). Older people are at a greater risk of developing acute and chronic lung diseases and suffer worse outcomes from these conditions. Approximately 1% of the UK population is diagnosed with community acquired pneumonia (CAP) annually (The National Institute for Health and Care Excellence, 2021). Over 70% of hospital-treated CAP occur in those aged over 65, and the 30-day mortality is approximately 20% in this age group (Daniel et al., 2016;Grudzinska et al., 2017Grudzinska et al., , 2019. Across Europe, the annual incidence of CAP in adults is 1.2 per 1000 person-years but 14 per 1000 person-years in those aged over 65 years (Torres et al., 2013). In the United States, approximately 1.6 million adults are estimated to be hospitalised with CAP with a median age of 68 and a 1-year mortality as high as 30% (Ramirez et al., 2017).
The ongoing coronavirus disease 2019 (COVID-19) pandemic demonstrates the potential impact of respiratory viruses on the elderly. To date, there have been 235 million cases and 4.8 million deaths globally with over 73% of deaths occurring in those aged 65 or older (World Health Organisation, 2021). Approximately 70% of hospitalised COVID-19 patients in the United Kingdom are above 50 years, with 80% of deaths seen in those aged over 60 years of age (Escher et al., 2021;Sapey et al., 2020). A similar trend can be seen across Europe where there is a hospitalisation rate and hospitalised case fatality rate of approximately 80% in those over 60 years of age (European Centre for Disease Prevention and Control, 2021). In the United States, 80% of those who have died from COVID-19 are over the age of 65 (Centre for Disease Control and Prevention, 2021a).
The severity of infectious disease depends on the virulence of the infecting organism, the dose or route of infection and the response of the host. Given that exposure to infectious agents may be unchanged or reduced (with potentially less social contact) with advanced age, the increased burden and poorer outcomes seen from both CAP and COVID-19 suggest an increased susceptibility to becoming infected and developing severe disease. Indeed, in a recent systematic analysis across infectious diseases as diverse as tuberculosis, polio, typhoid, influenza, Middle East respiratory syndrome (MERS), severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), smallpox, chickenpox, measles, infectious mononucleosis, hepatitis and HIV, a higher prevalence, severity and increased deaths were reported in older adults, with a trend towards poorer outcomes starting after the age of 50 (Glynn & Moss, 2020).
Chronic lung diseases are also more common in older adults. This not only reflects that these diseases are incurable; onset is more common with older age (Li, Cao, et al., 2020). One such example is chronic obstructive pulmonary disease (COPD). The international GOLD group defines COPD as a common, preventable and treatable disease that is characterised by persistent respiratory symptoms and airflow limitation that is due to airways and/or alveolar abnormalities and lung and systemic inflammation (Global Initiative for Chronic Obstructive Lung Disease, 2020). According to the Global Burden of Disease (GBD) study, COPD is already the third leading cause of death worldwide, something that the World Health Organisation had not predicted to occur until 2030 (Lozano et al., 2012), and globally, the burden of COPD is projected to increase because of continued exposure to risk factors and our ageing population. In the United Kingdom, the average age of diagnosis of COPD is 67 years (James et al., 2014).
In the absence of screening, symptoms precede diagnostic tests, and the first symptoms suggestive of COPD are often described in patients in their late 40s or early 50s (Yip et al., 2021). The average age of hospitalised patients with COPD in Europe was 70 years, with an average inpatient mortality of 250 deaths per 100,000 inhabitants (Atsou et al., 2011). In the United States, as elsewhere, the death rate for COPD is highest in those aged over 75 years (Thannickal et al., 2015).
A similar pattern is seen with interstitial lung disease (ILD) where the risk of developing it is 6.9 times higher (95% confidence interval [CI]: 5.9-8.0) in those aged over 70 than those in their 40s (Choi et al., 2018). Idiopathic pulmonary fibrosis (IPF) is a progressive fibrosing interstitial lung disease with a median survival from diagnosis of 2 to 5 years (Strongman et al., 2018), although the course of the disease is highly variable. IPF is diagnosed mostly in older adults (Strongman et al., 2018), with approximately 75% of those diagnosed aged over 73 years, both in the United Kingdom (British Lung Foundation, 2020) and in the United States (Raghu et al., 2006;Thannickal et al., 2015). In registry studies, the age of symptom onset was reported to be in people aged 60 and over (Hoyer et al., 2019).
Asthma is often considered a disease of children, but mortality is currently greatest in those aged over 55. This group also experience the most symptoms, more severe disease, more emergency presentations and a worse quality of life compared with younger adults (Hoyer et al., 2019;Plaza et al., 2000). The impact of age on pulmonary malignancies has been widely described and will not be discussed further here.
These lung diseases have a huge global economic burden. The annual cost of lung disease in Europe is estimated to be €380 billion with pneumonia accounting for €46 billion and COPD alone accounting for €140 billion (Gibson et al., 2013). In the United States, the annual cost of pneumonia is estimated to be over $17 billion (File & Marrie, 2010), and the annual cost for COPD, $50 billion (Centre for Disease Control and Prevention, 2021b).
Acute, communicable lung diseases such as CAP and COVID-19 have recommended treatment regimes. The mainstay of CAP treatment is anti-microbial agents with national guidelines highlighting the most commonly identified pathogens and their resistance patterns (National Institute for Health and Care Excellence, 2021). Poor responses to these therapies in older patients do not appear to be due to differences in virulence or resistance patterns in different age groups, and in general, our ability to diagnose CAP and initiate supportive treatments has improved (Grudzinska et al., 2020). This suggests that the poorer outcomes are more likely to be due to host responses rather than the insult per se.
In chronic lung diseases, there are treatments to support compromised respiratory function and reduce symptomatic burden, and these appear effective across age groups (Ferguson et al., 2020;Hanania et al., 2021). However, there are few drugs that impact on the biological processes that drive these conditions. Examples of disease-modifying treatments are anti-IL-5 strategies for asthma. IL-5 is the main mediator of the inflammatory cascade in eosinophilic asthma, exerting its effects by binding to the α chain of the IL-5 receptor (IL-5R), and controlling eosinophil development and T A B L E 1 Summary of common lung mechanics and physiology tests and how they change with increasing age  (Stam et al., 1994) PaO 2 Partial pressure of oxygen in arterial blood

Gas diffusion test
Older people demonstrate a 50% reduction in the response to hypoxia (Ebihara et al., 2016) PaCO 2 Partial pressure of carbon dioxide in the blood

Gas diffusion test
Older people demonstrate a 40% reduction in the response to hypercapnia (Ebihara et al., 2016) (Continues) maturation in the bone marrow, as well as mobilisation. Anti-IL-5 has been shown to be highly effective in subsets of patients with severe asthma and eosinophilia. Although anti-IL-5 was shown to be effective across all age groups, a systematic review of 10 studies demonstrated that, in subjects with a high blood eosinophil count, the efficacy of these therapies were reduced in older patients (Principe et al., 2019).
In summary, older age is associated with an increased susceptibility to lung disease, the outcomes of lung disease are worse and some treatments appear less efficacious in older adults. With our population ageing and diseases of older age likely to become even more common globally, it is important to understand why this is and then mitigate these factors where possible. There are a number of potential mechanisms that might make older adults more susceptible to the effects of acute and chronic lung insults and diseases, and these are often host based, including structural changes to the lungs, lung physiology, alterations to inflammatory pathways, changes to how immune cells function and how the host can repair damaged cells and tissues, which are impacted with age. Of importance, these mechanisms are often shared, irrespective of lung disease, offering the potential for shared therapeutic strategies, targeted more at the ageing process rather than the specific lung disease. These will be discussed, below.

| INCREASED SUSCEPTIBILITY TO LUNG DISEASE: CHANGES IN LUNG STRUCTURE AND PHYSIOLOGY WITHIN AN AGEING HOST
There are broad structural and functional lung changes that occur during older age (lung senescence) that are implicated in the pathogenesis or progression of acute and chronic lung diseases. The changes impact on every aspect of lung function, reducing the resilience of the host when dealing with challenges. The impact of ageing on lung structure, including senile emphysema, has been recognised for over 60 years (Rappaport & Mayer, 1954), and a number of reviews have commented on the causes and consequences of structural lung changes with age (Janssens et al., 1999;Sharma & Goodwin, 2006), but a non-exhaustive list of changes and potential consequences is described in after the age of 70 years) (Sharma & Goodwin, 2006). This would amplify the airflow obstruction seen with COPD or asthma. Another example is the reduction in gas transfer across the alveolar-capillary membrane in old age, which would further exacerbate hypoxia in the presence of pneumonia or diseases of ventilation/perfusion mismatch such as a pulmonary embolus or emphysema (Stam et al., 1994). Further, the reduced response to hypoxia seen in the ageing host (with less compensatory increase in the minute volume) might further exacerbate hypoxia, placing additional strain on end-organs (Polkey et al., 1997). There are known differences in lung function with age by sex (Becklake & Kauffmann, 1999), but less is known about how sex might impact on the mechanisms underlying the ageing lung and therefore will not be discussed further in the context of this review.

| Cough reflex and aspirations
As well as the structural and functional changes described above, elderly people may lose protective facets for lung health. This includes the cough reflex and the mucociliary escalator.
The respiratory muscles involved in the cough reflex are weaker in the elderly (Kim et al., 2009). The 'urge to cough' decreases with normal ageing and is severely reduced in the frail elderly, even with strong stimuli (Ebihara et al., 2016). A reduced cough reflex places the person at risk of aspiration. Aspiration is the movement of gastrointestinal content from the mouth or stomach into the respiratory tract, caused by poor swallowing (dysphagia) of food and oropharyngeal secretions. Inflammation is thought to contribute to the risk of chronic aspiration seen in the elderly (Ebihara et al., 2016). It has been proposed that chronic micro-aspiration induces inflammation in the lung and the recruitment of leukocytes to airways containing aspirate, which secrete VEGF-related cytokines and TNF-α, among other mediators (Costa et al., 2007). These elevated inflammatory cytokines are associated with a reduction in muscle mass and strength, termed Note: A non-exhaustive list of changes in lung mechanics and physiology with advancing age. Abbreviations: DLCO, diffusion capacity of carbon monoxide (a measure of the conductance or ease of transfer for CO molecules from alveolar gas to the Hb of the red blood cells in the pulmonary circulation); FEV 1 , forced expiratory volume in 1 s; FVC, forced vital capacity; MV, minute ventilation; PaCO 2 , partial pressure of carbon dioxide; PaO 2 , partial pressure of oxygen; PEF, peak expiratory flow; RV, residual volume; TLC, total lung capacity; VC, vital capacity; VO 2 max, maximal oxygen consumption.
sarcopenia, which is associated with frailty in the elderly (Wilson et al., 2020). Sarcopenia of swallowing muscles is directly associated with dysphagia and aspiration (Maeda & Akagi, 2016).
Dysphagia is very common among patients with chronic respiratory disease and can manifest as aspiration pneumonia (Verin et al., 2017). Dysphagia is prevalent in COPD, for example, and is associated with frequent exacerbations (Terada et al., 2010).

| The mucociliary escalator
The epithelial surface of the respiratory tract is continually exposed to pathogens and particulates. In health, the airways produce approximately 20-30 ml of secretions each day. The airway surfaces are lined by ciliated epithelial cells and covered with an airway surface layer (ASL), which consists of a mucus layer that entraps inhaled particles/ foreign pathogens, and a low viscosity periciliary layer (PCL) that lubricates airway surfaces and facilitates ciliary beating to enable mucus clearance (Shei et al., 2018).
Mucociliary clearance decreases with age (Proença de Oliveira- Maul et al., 2013). In elderly individuals, the frequency of cilia beating is slower and the time taken to clear mucus is longer (Ho et al., 2001).
Environmental exposure to cigarette smoke (Pavia et al., 1971), pollution (Pedersen, 1990), repeated infections (Look et al., 2001) and chronic lung diseases (Lam et al., 2013) have all been shown to impact on ciliogenesis. The mechanisms underpinning this appear again associated with inflammation and degranulation by recruited immune cells.
This process is exacerbated in lung diseases, which are often associated with submucosal gland hypertrophy and goblet cell hyperplasia, impairing mucociliary clearance, leading to mucostasis (Williams et al., 2006).  (Helander et al., 2003). sIgA also mediates bacterial translocation to dendritic cells for immune-mediated responses including those leading to pathogen clearance and those leading to immune tolerance of that specific inhaled particle (Diana et al., 2013).
Alveolar macrophages line the alveoli and interstitium, where they phagocytose organisms and release inflammatory or antiinflammatory mediators to control immune response (Belchamber & Donnelly, 2020). There is considerable cellular crosstalk involving alveolar macrophages as the resident sentinel immune cell and dendritic cells that project their dendrites into the airway lumen. These However, ligands for pattern recognition receptors are also produced by commensal microbiota during healthy colonisation, and here, they enable an immune response that is thought to regulate commensal microbes by preventing over growth, thus maintaining tissue integrity (Rakoff-Nahoum et al., 2004).
When a pathogen is detected and local immune responses require systemic reinforcement, neutrophils respond swiftly to pathogen-and damage-associated molecular pattern molecules (PAMPs and DAMPs), chemokines, cytokines and lipid mediators and are recruited to sites of inflammation, alongside monocytes and other trafficking inflammatory cells. Neutrophils are effective phagocytes, but their granular contents have the ability to cause immense local damage. Figure 1 contains a non-exhaustive overview of the contents of neutrophil granules and their substrates, as examples of the tissue damaging potential of these cells. This is more fully reviewed elsewhere .
All of the immune facets described above are impacted by host ageing (termed immunosenescence), impairing the ability to respond as effectively to pathogens and also to tolerate benign inhalants. This has been reviewed in detail elsewhere , but three examples are given, below.
The lung microbiota is known to change in composition with age.
Lower microbiome diversity and the presence of specific microbial taxa are associated with ageing and decreased lung function in pathogenic disease states. Ageing is associated with a reduction in the rela- function was noted, with a significant loss in diversity (Rampelli et al., 2013). There are known associations of microbiome dysbiosis and ill-health, including a loss of tolerance, decreased resistance to and reduced containment of potential pathogens, leading to infections such as pneumonia.
While the production of anti-microbial peptides appears to be pre- Lung cells cannot repair elastic fibres damaged by elastin degradation (Shifren & Mecham, 2006), and this leads to permanently compromised lung function as elastic tissue is replaced with non-elastic, scar formation. The adaptive immune system is also compromised in ageing. The relative number of CD8+ T cells decreases with age without a corresponding change in CD4+ T cells; there is also a very pronounced age-dependent loss of CD45RA+ naïve T cells and dysregulation of Tcell/B-cell interactions (Lazuardi et al., 2005). This supports the hypothesis that older people are less able to respond to new pathogens or mount an effective response to pathogens faced before. Further, regulatory T cell function may be impaired in the elderly, reducing immune tolerance and impairing the containment of inflammation.
The inter-related hallmarks of ageing. Diagrammatic representation of the connection between the hallmarks of ageing that starts with damage to the DNA that can translate to intracellular modifications that affect DNA replication and transcription as well as RNA translation and post-translation modification of proteins. Cellular damage results in dysfunction and altered phenotype, which is represented by the hallmarks outside the cell dysregulation of ECM, has also been described in lung ageing due to the impact of the inability to replace damaged elastin fibres . These hallmarks, though distinct, are interconnected and arise over time as a result of exposure to sources of damage that ultimately result in loss of cellular function and dysregulated tissue homeostasis ( Figure 2). Although an emerging field, there is evidence of most of these processes in lung diseases, which most commonly affect the elderly.

Genomic instability
Genomic instability is defined as decline in the DNA repair process in response to DNA damage. In murine and human lung tissue, DNA damage accumulates with age (Birch et al., 2015;Lee et al., 1999), With increasing age, and repeated chromosome replication, telomere length is naturally shortened (Daniali et al., 2013). Advanced age is associated with a higher frequency of cellular proliferative events, and once telomeres have reached a critical length, p53-dependent cell cycle arrest occurs (Saretzki et al., 1999). Telomere length varies from tissue to tissue, with less proliferative tissues such as muscle and fat having longer telomeres while highly proliferative tissues such as skin cells having shorter telomeres (Daniali et al., 2013). In the lung, environmental factors such as cigarette smoke, pollution and repeated infections can increase oxidative stress and inflammation, which accelerates telomere shortening due to increased cellular proliferation and DNA replication in response to damage (Daniali et al., 2013). Indeed, in a recent systematic review of 19 articles assessing pollution and telomere length in adults, both long-term and short-term exposure to PM2.5 showed an inverse association with telomere length, demonstrating the importance of air quality on lung senescence (Miri et al., 2019).
In COPD, telomere shortening appears to be tissue dependent.
There are inconsistent reports of telomere shortening in homogenised lung tissue (Birch et al., 2015;Everaerts et al., 2018), but telomeres do appear shortened in circulating leukocytes of COPD patients compared with healthy controls (Rutten et al., 2016;Savale et al., 2009).
There is an association between telomere length and declining lung function (Rutten et al., 2016).
Short telomere length is also a feature of IPF, and while it is more common in patients with genetic mutations in the telomerase genes In COPD, evidence suggests that alteration in the DNA methylation process is a contributing factor to disease pathology. DNA methylation genes in COPD are differentially expressed compared with patients without COPD (Vucic et al., 2014). Nuclear factor-E2-related factor 2 (Nrf2) drives the expression of numerous cytoprotective genes involved in xenobiotic metabolism, antioxidant responses and anti-inflammatory responses. In COPD, alterations in DNA methylation of genes involved in the Nrf2-mediated oxidative stress response pathway compromise the function of this pathway, enabling a more sustained inflammatory response to ROS, thereby exposing COPD airways to greater ROS-associated damage (Vucic et al., 2014).
Underlying genetic factors such as α-1 antitrypsin deficiency (AATD), which predisposes to COPD but at a younger age and with less/no cigarette smoke exposure, also contribute to epigenetic alterations. AATD patients can be heterogeneous in the age of onset, rate of decline and clinical manifestation of COPD despite a similar underlying genetic mutation of the SERPINA1 gene and similar AAT levels present (Sapey, 2020). Heterogeneity of CpG methylation between individuals with AATD may contribute to the varied clinical manifestations of AATD, highlighting the role of epigenetics in disease progression (Wang, Marek, et al., 2019).
Emerging evidence points to histone modification playing a role in the pathophysiology of IPF. Histone acetylation reduces the binding of histone to DNA, which expands chromatin and promotes transcription. Cell free nucleosomes associated with histone modification, HMGB1, mH2A1.1, H3K9Ac and H3K27Ac were significantly lower in IPF patients compared with healthy controls (Guiot et al., 2017). AATD causes a genetically driven loss of proteostasis with an accumulation of misfolded AAT in hepatocytes and macrophages, leading to low levels of functional AAT and cellular damage  due to endoplasmic reticulum (ER) stress responses. Patients with cystic fibrosis (CF) have a genetic dysfunction in the cystic fibrosis transmembrane regulator (CFTR) protein, leading to a decline in lung health, an increased susceptibility to infection, pancreatic dysfunction and infertility (Gibson et al., 2003). Protein misfolding of the cystic fibrosis transmembrane regulator results in a build-up of intracellular chloride ions, which is thought to draw in sodium ions and water down electrochemical and osmotic gradients, thickening secretions (Gibson et al., 2003). Environmental stress causing loss of proteostasis can be due to heat shock, ER stress or oxidative stress (L opez-Otín et al., 2013), all of which can cause acute lung injury by interfering with activity of heat shock proteins and the unfolded protein response (UPR) (Yang et al., 2021). Impaired activity of the UPR to restore homeostasis has been seen in IPF, and this response is a possible target for development of therapies aimed at reducing ER stress (Burman et al., 2018). For example, binding immunoglobulin protein (BiP) keeps the ER proteins of the UPR pathway, protein kinase R (PKR)-like endoplasmic reticulum kinase (PERK), activating transcription factor 6 (ATF6) and inositol requiring enzyme 1α (IRE1α) inactive, but there is evidence of increased expression of binding immunoglobulin protein, a marker of ER stress, in AECII of IPF patients (Burman et al., 2018), suggesting activation of UPR pathway due to ER stress. Crucially, ER stress in AECII appears to contribute to development of fibrosis after lung injury (Burman et al., 2018).

Deregulated nutrient-sensing
Nutritional regulation of cell growth, metabolism and proliferation is a central part of normal tissue function. Deregulated nutrient sensing is seen in the elderly and can manifest in the form of altered metabolism and/or increased inflammation.
The insulin and insulin-like growth factor 1 (IGF-1) pathway, when deregulated, can signal overexpression of mammalian target of rapamycin (mTOR) through the PI3K-Akt pathway, which in turn can accelerate the ageing of cells (L opez-Otín et al., 2013). Dietary restriction has been shown to increase life span in several models with this mechanism also evolutionarily conserved . In murine models of lung disease, mice on restricted calorie diet were recently shown to be protected from pulmonary injury and inflammation when exposed to particulate matter (Li, Chen, et al., 2020). Targeting the nutrient sensing mTOR pathway has been shown to reduce pulmonary fibrosis in mice with evidence of a reduction in lung collagen (characteristic of fibrosis) (Korfhagen et al., 2009), and in vitro a reduction in senescence-associated, pro-inflammatory markers in primary endothelial and smooth muscle cells taken from COPD patients (Houssaini et al., 2018).
The IGF/mTOR ageing pathway is linked to other pathways in lung diseases. In COPD, increased ROS leads to hypermetabolism with increased mTOR activation and reduced autophagy, via Akt inhibition of forkhead box O3A (FOXO3A), further exacerbating senescence and age-related disease progression (Mercado et al., 2015). The heightened metabolism increases the demand for more energy further exacerbating the ROS production and creating oxidative stress. The activity of mTOR also plays a role in maintaining protein homeostasis, perhaps through interaction with the UPR (discussed above) or regulation of autophagy via forkhead box O3A. Crucially, mTOR is involved in both pro-inflammatory and anti-inflammatory pathways. Activation by IGF via PI3K/Akt activates the nuclear factor κ-light-chainenhancer of activated B cells (NF-κB) via mTOR resulting in release of pro-inflammatory cytokines. Conversely, metformin is believed to exert its anti-inflammatory properties via mTOR perhaps through AMP-activated protein kinase (AMPK) (Mercado et al., 2015). This evidence shows mTOR as a central regulator of metabolism, with importance in maintaining energy and protein homeostasis and regulation of inflammation, confirming why it is a target of novel therapeutics as discussed below.

Mitochondrial dysfunction
Mitochondrial dysfunction is the loss of mitochondrial homeostasis that alters the energy creating activity of this organelle. The bioenergetic function of mitochondria can reduce with age due to an accumulation of mutations in mitochondrial DNA, defective mitophagy, electron transport destabilisation and reduced mitochondria replenishment (L opez- Otín et al., 2013).
In response to ER stress and age, mitochondria in AECIIs were shown to have altered respiration and bioenergetics and impaired fission and fusion using murine and cellular models, with AECIIs from IPF lungs also demonstrating an accumulation of dysfunctional mitochondria (Bueno et al., 2015). Reduced expression of the PINK1, a regulator of mitophagy and mitochondrial homeostasis, was observed in IPF lungs and ER stressed mice with PINK deficiency showed a pro-fibrotic phenotype (Bueno et al., 2015).
Broadly, dysfunction in mitochondria can drive changes in metabolism of lipids, glucose and other nutrients that can promote a fibrotic phenotype in the lung (Bueno et al., 2020). Senescent markers have been reported in human lung fibroblasts from IPF patients ( Alvarez et al., 2017), while Serpine-1 was shown to increase senescence in AECII (Jiang et al., 2017). Cellular senescence in lung fibroblasts, epithelial and endothelial cells by cigarette smoke is known to contribute to COPD and COPD co-morbidities (Ahmad et al., 2015) and cigarette smoke induces expression of p16 and p21 in a murine model of COPD (Rashid et al., 2018). In aged mice, high lung expression of p16 increases adhesion of bacterial ligands and increases susceptibility to pneumococcal pneumonia (Shivshankar et al., 2011). Logically, the higher population of senescent cells in older people may be a contributing factor to the increased susceptibility to the SARS-Cov-2.

Stem cell exhaustion
Stem cell exhaustion refers to depletion of the stem cell reserve due to excessive proliferation and differentiation. In health, the adult lung is relatively quiescent with a slow cell turnover. However, after insult, injury or infection, the lung can rapidly respond, replenishing damaged tissue using tissue-specific lung stem/progenitor cells with selfrenewal and differentiation potential (Volckaert & De Langhe, 2014).
Studies have shown that the differentiation potential of lung basal stem cells decreases with ageing in human patients (Wang, Lu, et al., 2019).
The FGF FGF10-FGFR2B signalling pathway has been shown to help maintain basal cells and can promote differentiation of basal cells to AECIIs, which is important for epithelial regeneration (Yuan et al., 2019). In COPD, airway basal progenitor cells are depleted, have reduced self-renewal and multipotentiality and mostly differentiated to basal and mucus epithelial cells and less to ciliated cells when compared with progenitor cells from non-COPD patients (Ghosh et al., 2018). A lack of ciliated cells can impair mucociliary escalator function in these patients, contributing to mucus retention and airway plugging. Further, evidence from COPD patients describe that airway progenitor variant clones contribute to the squamous and goblet cell metaplasia seen in patients (Rao et al., 2020). has also been demonstrated where it contributes to corticosteroid resistance . While PI3K inhibition has no effect on phagocytosis of macrophages from COPD patients (Bewley et al., 2016), up-regulation of PI3K, which negatively correlates with phosphatase and tensin homologue (PTEN), increases the release of pro-inflammatory cytokines (Yanagisawa et al., 2017), suggesting that targeting this altered pathway is a potential target for treating agerelated lung disease. Interestingly, inhibition of mTOR in PBMCs from COPD patients has shown promise in restoration of corticosteroid sensitivity (Mitani et al., 2016). The NF-κB pathway is also a site of altered cellular communication in lung disease that can lead to increased inflammation.

ECM dysregulation
This hallmark of ageing has been proposed specifically for the lungs.
The ECM provides support to help maintain lung integrity and biomechanics. Growing evidence suggests that ECM dysregulation is an important hallmark of lung ageing, mostly as a result of proteinase/ anti-proteinase imbalance . This is exemplified in AATD, where ECM degradation is a hallmark of the disease (Crossley et al., 2019). ECM degradation is closely associated with proteinases, and although neutrophil serine proteinases are the quintessential proteinase for ECM degradation (Butler et al., 2018), others are implicated. For example, the MMP inducer, CD147, is elevated in serum of patients with COPD (Berg et al., 2018) and metalloproteinase ECM degradation is seen in COPD with ECM fragments increased in COPD lung (Bihlet et al., 2017).
In IPF, high levels of circulating MMPs and tissue inhibitor of MMPs (TIMPs) suggest high ECM turnover in these patients (Todd et al., 2020). MMPs enhance the development of IPF through dysregulation of ECM homeostasis (Craig et al., 2015). Ongoing advances in the study of ECM in vitro such as the use of decellularized or acellular lung scaffolds could provide valuable avenue towards use of ECM components to reengineer and remodel damaged lung tissue clinically (Wagner et al., 2014).

| THERAPEUTICS THAT TARGET THE AGEING PROCESS FOR THE TREATMENT OF LUNG DISEASE
Many therapies in lung diseases treat symptoms, by enhancing bronchodilation or enabling more effective mucus clearance. Some conditions, such as asthma and IPF, have disease-modifying drugs, but these remain limited in scope and available to only a subset of patients.
There has been increasing interest in the concept of multi-ceuticals or using one treatment to target a central driver of pathology that might be present across a number of disease processes. Drugs that target the hallmarks of ageing might form such a therapeutic strategy for lung disease and could be used across any lung (or other organ) pathology, where they are present. There are a number of approaches under investigation, with three areas described to provide key examples, below.

| Senolytics and drugs impacting on epigenetic changes
Targeting the senescence pathway in age-related disease is of significant interest and the subject of ongoing therapeutic investigations.
Senolytics are drugs that target senescent cells. Ex vivo treatment of fibrotic primary mouse AECIIs with the senolytic drugs dasatinib plus quercetin (DQ) resulted in a reduction in expression of senescent marker P16, an increase in apoptosis and reduction in expression of SASP markers Mmp12, Serpine1 and Spp1 (Lehmann et al., 2017). An open-label pilot study in 14 IPF patients showed DQ improved physical performance, but this needs to be confirmed in larger randomised control clinical trials (Justice et al., 2019). However, it is promising that DQ has been proven to be effective in reducing senescence burden in a bleomycin mice model of IPF. This landmark study showed that secretome of senescent fibroblasts, which were selectively killed by DQ, was fibrogenic and that DQ-mediated removal of senescent cells improved pulmonary function and physical health, although lung fibrosis was visibly unaltered (Schafer et al., 2017). Findings from a recent open-label trial in patients with diabetic and chronic kidney disease further demonstrate the promise of this drug combination, as it demonstrated a reduction in senescence-associated markers and cells after treatment with DQ (Hickson et al., 2019). Metformin, a drug used for decades to improve diabetes control, has also been shown to inhibit the SASP by interfering with inhibitor of NF-κB kinase (IKK)/ NF-κB activation (Moiseeva et al., 2013) and has been shown to exert antifibrotic effects in the lung by inhibiting TGFβ1 action, suppressing collagen formation and activating PPARγ signalling in lung fibroblasts derived from IPF patients (Kheirollahi et al., 2019).
Targeting underlying epigenetic modifications can help reduce senescence. In a murine bleomycin model of IPF, the inhibition of histone deacetylase using suberoylanilide hydroxamic acid (SAHA) (which has been approved for clinical use in cancer) induced apoptosis of IPF-promoting myofibroblasts, an effect that was mediated, at least in part, by up-regulation of the pro-apoptotic gene Bak and downregulation of the anti-apoptotic gene Bcl-xL (Sanders et al., 2014). In the same study, suberoylanilide hydroxamic acid (SAHA)-treated mice displayed increased lung function compared with the bleomycin-only group (Sanders et al., 2014). Further evidence of epigenetic drugs as novel therapeutics for lung disease has been extensively reviewed (Comer et al., 2015). For instance, the expression of the miRNA miR-17-92 cluster, important in homeostasis of lung epithelial cells, is reduced in IPF lung tissue and fibroblasts with a corresponding increase in expression of DNA methyl transferase 1 (DNMT-1) (Dakhlallah et al., 2013). Treatment with 5 0 -aza-2 0 -deoxycytidine, a demethylating agent, in murine bleomycin IPF model reduced fibrosis and resulted in increased expression of miR-17-92 demonstrating therapeutic potential (Dakhlallah et al., 2013).
In addition, histone deacytelases (HDAC) are dysregulated in COPD and the use of quercetin and theophylline to up-regulate HDAC has shown some success in both murine and clinical models (Comer et al., 2015). In particular, it was demonstrated that corticosteroid resistance, which can limit the use of steroids as an anti-inflammatory, in COPD patients was ameliorated by treatment with low-dose theophylline by up-regulating HDAC2 activity via inhibition of PI3K δ . This suggests that targeting HDAC2 up-regulation can be a useful strategy for COPD patients, especially in patients with steroid resistance where long-term use can be detrimental through physical weight gain and excessive suppression of the immune system.

| Metabolism and inflammation
Therapies targeting metabolism focus on the mTOR pathway due to its central role in metabolism. Evidence from studies in mice and clinical trials has demonstrated that use of rapamycin, an inhibitor of mTOR, can alleviate the progression of pulmonary fibrosis and the severity of pneumonia (Houssaini et al., 2018;Korfhagen et al., 2009).
The use of rapamycin as an adjuvant therapy with corticosteroids significantly improved PaO 2 /FiO 2 and lessened the requirement for ventilator support in patients with influenza virus-induced severe pneumonia and acute respiratory failure . Similar results were obtained in mice showing a reduction in lung injury and viral titres as well as reduced NLRP3 inflammasome activation via inhibition of the mTOR pathway in mice receiving adjuvant therapy with rapamycin and antiviral drugs (Jia et al., 2018). Upstream of mTOR, inhibition of PI3K can improve neutrophil migration in elderly patients . Large randomised clinical trials targeting this pathway in treatment of age-related lung disease would be very useful in providing definitive evidence of their efficacy.
Both steroids and non-steroids anti-inflammatory drugs are used to reduce lung inflammation. Notably, dexamethasone was the first drug proven effective at reducing mortality of severe COVID-19 (Horby et al., 2021), with other drugs targeting inflammation now proven to be effective (such as tocilizumab and remdesivir) and many more under investigation. Our recent study of elderly patients with CAP and sepsis demonstrated that using simvastatin as adjuvant therapy improved neutrophil function (chemotaxis and NETosis) and reduced neutrophil elastase in circulation while improving clinical endpoints such as hospital free mortality and SOFA scores Sapey et al., 2019).
Adjuvant therapy of steroids with PI3K δ inhibitors may also be a useful strategy in treatment of COPD as discussed earlier . PBMCs from COPD patients have also been demonstrated to show improved sensitivity to dexamethasone after treatment with rapamycin (Mitani et al., 2016). The use of steroids with adjuvant therapy may prove useful in the elderly as steroid use in this patient group requires caution due to side effects such as delirium, confusion, agitation and the presence of other co-morbidities.

| CONCLUDING REMARKS
Our population is ageing and there is a pressing need to ensure old age is associated with good health, not only for the individual but also for society, to support economies and ensure healthcare and social care services are sustainable and resilient. Age is associated with multi-morbidity and many of these conditions are driven by the hallmarks of ageing. The lungs appear particularly prone to age-associated tissue damage leading to dysfunction, and lung diseases are common with advancing age. Traditional management of chronic lung diseases aim for symptom management with a small number of diseasemodifying drugs for which there is limited access for specific groups of individuals. Treatments for acute lung infections, such as antibiotics, are not always effective in the elderly. More therapeutic options are urgently needed to reduce both mortality and morbidity and preserve lung function.
A new way of tackling these conditions may be to focus on shared underlying mechanisms. Here, one therapy might address a number of pathological outputs potentially within the same disease, the same organ and maybe across damaged organs within the host. The hallmarks of ageing have been described in many age-related illnesses including lung diseases. These result in immunesenescence, dysfunctional metabolism and excessive inflammation, all of which have been linked to lung damage.
There is already some evidence that targeting ageing can impact positively on aspects of lung disease, with preclinical and early phase clinical studies demonstrating clinical benefit. There are a suite of clinical therapies (both novel and repurposed) being investigated within this field.
Should these be successful, it is likely that drugs targeting ageing will improve multi-morbidity and increase the percentage of people who experience healthy ageing. With lung disease increasing globally, targeting ageing pathways to reduce the burden of age-related lung disease may offer therapeutic hope.

CONFLICT OF INTEREST
The authors declare no relevant conflicts in reference to this article.

AUTHOR CONTRIBUTIONS
AAF, MJH, AS, KBR and ES all wrote the manuscript. ES finalised the manuscript and edited prior to submission.