Cellular Senescence: Friend or Foe to Respiratory Viral Infections?

Cellular Senescence: Definition and Evolutionary Origins

Cellular senescence describes a state of permanent replicative arrest in normally-proliferative cells. Originally discovered in vitro in human fibroblasts by Hayflick and colleagues, the concept of cellular senescence was met with controversy [1, 2]. Initially, some investigators were skeptical and believed that senescence was an in vitro artefact. But with accumulating in vivoevidence including the presence of senescence in aged human skin cells [3], scientists accepted senescence as a true biological phenomenon. Senescence is now known to contribute to a variety of age-related diseases including: type 2 diabetes, obesity, atherosclerosis, chronic obstructive pulmonary disease (COPD), pulmonary fibrosis, and others [4–6]. With aging, our cells continually divide, shorten their telomeres, accumulate mutations and DNA damage, all of which ultimately increase the likelihood of oncogenesis [7, 8].

Under typical physiological conditions, oncogene-induced senescence may hinder the development of certain cancers including lymphoma [9] and prostate cancer [10] and improve patient response to chemotherapeutics [11]. Senescent cells may also upregulate phagocytic receptors, therefore increasing phagocytosis and immune surveillance to reduce cancer development. Additionally, senescent cells secrete extra-cellular matrix proteins and growth factors that contribute to some restorative processes including wound healing [12]. Similarly, senescent cells have been shown to reduce liver [13, 14] and pancreatic fibrosis and promote wound healing [15]. Furthermore, fibroblasts made senescent by the matricellular protein CCN1 limit fibrosis in cutaneous wounds [16]. Senescent cells may also contribute to tissue growth during embryonic development [17, 18]. In summary, senescence likely evolved to respond to the acute stress of organ development, mitigate against the acute effects of damaged cells to return tissue to homeostasis, and ultimately to reduce the development of cancer [19].

With improvements in public health in the 20th century and subsequent extensions of human life-span, senescence has emerged as a contributing factor to several chronic age-associated diseases. Indeed, we now recognise senescence as one of the key hallmarks of aging, as senescence occurs as we age naturally [20]. Although a key hallmark of aging, senescence is not sufficient for all aging phenotypes [20], and senescence can occur in young hosts as a result of acute stressors (see next section). However, the importance of senescence in acute inflammatory conditions, especially respiratory viral infections, is less well appreciated. Here, we first briefly review how senescence contributes to chronic inflammation, particular in the context of lung diseases, and emerging therapies to treat senescence. Then, we discuss the role senescence plays in acute respiratory viral infections and implications for such a role during infection with SARS-CoV-2.

What leads to senescence and the resulting Senescence Associated Secretory Phenotype (SASP)

Senescence can arise from both repeated cellular division and cellular stressors. Replicative senescence arises from repeated cellular divisions, such as occur naturally when we age. Stress-induced senescence arises from cellular stressors such as increases in reactive oxygen species, which correlate with DNA damage [21, 22]. These can occur from irradiation, chemotherapeutic agents, chronic exposure to pollutants (cigarette smoke), exposure to pathogens, and some forms of accelerated aging syndromes, such as progeria [23–26]. Therefore, cells from both young and aged hosts can exhibit senescence. As previously mentioned, senescence is one of the key biological hallmarks of aging and contributes to aging via inflammation, tissue exhaustion and impaired stem cell renewal [20].

Importantly, stem cells can become senescent as we age. Specifically, mesenchymal stem cells isolated from older humans (>70 years of age) exhibit senescence including reduced proliferation and a chronic inflammatory secretome, which impairs hematopoietic stem and progenitor cell (HSPC) replication and increases HSPC MCP1 and IL-8 expression [27]. Consequently, senescent mesenchymal stem cells are not able to be used in stem cell therapeutics due to their inability to proliferate and their reduced expression of pro-angiogenic factors [28]. Additionally, senescent neural progenitor cells contribute to the development of multiple sclerosis [29]. Specifically, senescent neural progenitor cells inhibit oligodendrocyte differentiation and contribute to demyelination via the production of high mobility group box-1, which suppresses oligodendrocyte progenitor cell proliferation and induces pro-inflammatory transcriptomic changes in oligodendrocyte progenitor cells [29]. Thus, senescent stem cells impair wound healing, promote the development of inflammatory diseases, and limit the use of stem cells in therapeutic applications.

In addition to the direct effects on stem cells, there are two potential pathways by which senescence may contribute to disease. First, senescent cells stop proliferating, which prevents tissue repair after injury. Second, as senescent cells accumulate, they produce, for unclear reasons, low levels of inflammatory mediators, a phenomenon which has been termed “inflammaging” particularly when evident in older people [30–33]. Senescent cells contribute to chronic inflammation via the senescence-associated secretory phenotype (SASP), the collective term for the proinflammatory chemokines and cytokines released by senescent cells [34]. The SASP is comprised of a number of proinflammatory molecules, including TNF-α, IL-1α/β, IL-6, IL-8, CCL-2, and others [35–38]. The SASP seems to be a double-edged sword. On the one hand, long-term exposure of SASP inflammatory mediators can lead to over-recruitment of damaging immune cells, resulting in chronic inflammatory diseases and even fibrosis [5, 39]. On the other hand, in the short term, the SASP promotes wound healing that is required to respond to acute cellular damage [12, 13, 40].

Importantly, the specific characteristics of the SASP may be dependent on the cell type and the mechanism of senescence induction [41]. For example, the SASP associated with fibroblasts contains significantly more SASP factors than the SASP associated with renal epithelial cells [41]. Some SASP pathways are shared between fibroblasts and epithelial cells, while some pathways are distinct to each cell type. Specifically, senescent epithelial cells upregulate pathways associated with protein translation and degradation; senescent fibroblasts upregulate pathways associated with apoptosis, ROS generation, and extracellular matrix reorganisation; and both senescent fibroblasts and epithelial cells upregulated pathways associated with vesicle transport, metabolism, and detoxification [41]. Note that in this particular study, senescence was induced in vitro in human cell lines. However, the investigators identified several secreted proteins in their in vitro models which were also found in the plasma of older subjects, suggesting that the findings of the study could be translatable in vivo. In vitro senescence is associated with the accumulation of sterile inflammatory mediators, like HMBG1, a nuclear protein that can activate inflammation [41]. Furthermore, SASP mediators such as TNF-α can induce senescence, which likely further enhances the SASP in a feed-forward loop [42]. Thus, one of the mechanisms by which senescence leads to chronic inflammation may be by the release of sterile inflammatory mediators, also known as damage associated molecular patterns. Additionally, the SASP is temporally dynamic and the dynamics change depending on the cell type [43]. Overall, senescence induces distinct cellular programs that lead to the secretion of proteins that promote chronic inflammation. Clearly, dissecting the physiological from the pathophysiological aspects of senescence will be required for the development of effective therapies targeting senescence to alleviate age-related diseases.

Senescence-Targeted Therapeutics

Recently, there has been a focus on alleviating the negative effects of cellular senescence using senescence-targeted therapeutics. Such therapeutics fall into two primary categories: SASP inhibition and senescent cell removal. SASP inhibition involves downregulating various components of the SASP, or inhibiting the inflammatory pathways that SASP factors engages, with the goal of ameliorating the deleterious effects of SASP factors. This strategy is successful in a variety of contexts. For example, Janus kinase (JAK) inhibition alleviates inflammation, both locally in the adipose tissue and systemically, and reduces frailty in aged mice [44]. Evidence that the SASP may be related to the pathogenesis of age-related diseases comes from reports that pharmacological tools to inhibit NF-κB signalling ameliorate age-related diseases. Inhibition of the mammalian target of rapamycin (mTOR) by rapamycin selectively reduces NF-κB signalling (a major inflammation signalling hub) in senescent cells through suppression of IL-1α activity, reduces age-associated cognitive decline, improves immune function in older people, and increases the lifespan of mice [45–49]. Metformin, a commonly-used type-2 diabetes drug, also inhibits NF-κB resulting in improved lifespan in mice, and reduced all-cause mortality and age-related disease in humans [50–53]. However, inhibiting the SASP results in off-target side effects including neutrophilia, nephrotoxicity, and changes in T cell phenotype due to the myriad of interconnected pathways involved in inflammatory signalling [45] Specifically, inhibition of the NF-κB pathway, either by genetic deletion or small-molecule inhibition, has been shown to inhibit T cell, B cell, and lymphoid progenitor cell development [55]; induce apoptosis of thymocytes, B cells, and macrophages [56, 57]; impair lymphocyte growth and cytokine production [58]; and potentially increase susceptibility to various infections [59]. All of these strategies target inflammatory pathways that senescence may activate but do not specifically target senescent cells.

Removing senescent cells involves directly targeting senescent cells by inducing apoptosis. The first therapeutics designed to directly kill senescent cells (termed “senolytics”) targeted the BCL-2 protein family, an anti-apoptotic pathway that is involved in oncogenesis. The most widely-studied therapeutic strategy, however, is a combination therapy involving the administration of the tyrosine kinase inhibitor dasatinib (D) and the flavonoid quercetin (Q). The “D+Q” combination reduces senescent cell burden and improves health span and lifespan in aged mice, reduces vascular stiffening, improves vasomotor function, and improves pulmonary function in murine models of pulmonary fibrosis [60–64]. Indeed, preliminary clinical trials have shown evidence that D+Q treatment reduces senescent cell burden and may improve physical function in patients with pulmonary fibrosis [65, 66]. However, we still do not fully appreciate the potential off-target effects of senolytic agents such as D+Q, as well as their mechanism of action in humans, which will require elucidation before they can be used to treat age-related diseases.

Cellular Senescence in the Lung

In addition to replicative senescence induced by aging, several other factors contribute to the development of stress-induced senescence in the lung, including radiation therapy, smoking, mechanical ventilation, and chronic use of supplemental oxygen (fig. 1). [24, 67–70], which supports the concept that other processes besides natural aging lead to senescence. Increasing numbers of senescent cells correlate with increasing concentrations of SASP factors. This is supported by the finding that the cytokines IL-6 and IL-10 and neutrophil counts are elevated in the bronchoalveolar lavage fluid (BALF) of older (note: “older” is designated>65 years of age in this review) donors, which may exhibit senescence in the lung [71]. This age-related phenotype correlates with age-related lung diseases such as COPD. Furthermore, patients with COPD exhibit increased concentrations of senescent type II alveolar epithelial cells (AECs) [72, 73] and lung fibroblasts [74], as well as elevated levels of inflammatory cytokines including IL-1β, IL-6, IL-8, IL-10, and CCL-2 [75–78]. While a causal link is difficult to prove, the low-grade inflammation caused by lung senescent cells may contribute to the development of and/or exacerbates COPD. Similar patterns of elevated inflammatory cytokine levels and senescent cells are linked to other chronic lung diseases such as asthma [79, 80]. Furthermore, the exuberant wound healing response mediated by SASP factors, combined with the inability to effectively repair tissue due to cessation of proliferation, and possibly impairments in stem cell functions, may promote fibrosis and contribute to idiopathic pulmonary fibrosis [81–84]. Thus, lung senescent cells likely contribute to the severity of many chronic lung diseases.

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FIGURE 1

Causes of lung senescence. Many factors contribute to the induction of senescence within lung cells. Apart from natural aging, other sources of senescence include chronic exposure to pollutants and cigarette smoke, chemotherapy, ionising radiation, respiratory viral infections, and the use of mechanical ventilators. Each of these sources may induce senescence to varying degrees, and it remains unclear whether senescence originating from different sources and in different cells contributes to pathology similarly or differently.

The potential contributions of lung cellular senescence in acute respiratory conditions remain unclear. In a murine model of acute lung injury (ALI), senescent alveolar macrophages exhibit enhanced activation and secretion of proinflammatory cytokines, resulting in enhanced disease severity [85]. Similarly, aged mice, which likely exhibit increased lung senescence, also exhibit enhanced lung permeability and increased levels of reactive oxygen species in response to an LPS challenge, which is linked to deficient Nox4 (a ROS-generating mitochondrial protein) ubiquination [86]. Additionally, older trauma patients are significantly more likely to develop acute respiratory distress syndrome (ARDS), and older patients with ARDS exhibit a higher mortality rate [87]. However, in all of these studies the pathological role of senescence was not tested. Also, a recent study found that lung cellular senescence protects against lung injury induced by mechanical ventilation by preventing apoptosis [68]. Clearly, the impact of cellular senescence on acute lung conditions requires future investigation.

Cellular Senescence and Respiratory Viral Infections

The COVID-19 pandemic continues to claim thousands of lives every day and disproportionately affects older people. Given this, how cellular senescence impacts the host response to acute respiratory viral infections is particularly relevant. Older people experience increased susceptibility to respiratory viruses such as influenza, respiratory syncytial virus (RSV), SARS, and now SARS-CoV-2 [88–92]. With COVID-19, the disease that results from SARS-CoV-2 infection, older people are ∼20 fold more likely to die than younger people [93]. This effect has been partially attributed to defects associated with the aging of the immune system, termed “immunosenescence”, which is distinct from classical cellular senescence as defined above and reviewed elsewhere [94–96]. Briefly, immunosenescence has pleiotropic effects on the immune system including: i) a decrease in the proliferative capacity of hematopoietic stem cells; ii) dysregulation of innate immunity; iii) reduced numbers of naïve T cells with thymic involution; iv) accumulation of memory T and B cells; and v) a general decline in both T and B cell function [94, 97–99]. Specifically, both CD4⁺ and CD8⁺ T cells from aged hosts (e.g., rodents and humans) exhibit a reduced proliferative capacity associated with downregulation of surface costimulatory receptors, CD27 and CD28, which is accompanied by shortened telomeres. Collectively, these alterations within the immune system with aging result in an impaired ability to fight viral infections and lead to reduced vaccine efficiency [90]. In addition to natural aging, chronic viral infections (e.g., HIV and cytomegalovirus infection) also induce a phenotype of reduced T cell function (i.e., reduced proliferation and cytokine secretion) that shares features with immunosenescence [100, 101]. Although aging of the immune system impairs host defense to acute respiratory viral infections [90], the role that cellular senescence of lung cells play in the pathophysiology of acute respiratory viral infections is not well-understood. Clearly, older people are highly susceptible to viral infections due to a myriad of effects exerted by aging on the immune system. But how much of this is contributed by cellular senescence, as defined above, remains to be elucidated. Indeed, it is not clear if cellular senescence, due to natural aging or due to an age-independent stressor, promotes or prevents acute respiratory viral infections. To the best of our knowledge, there are only a few studies on the role of cellular senescence and acute respiratory viral infections, with some showing contrasting results.

There are studies which claim that cellular senescence enhances anti-viral immunity [102], which is biologically plausible, as SASP factors, which includes chemokines (e.g., CxCL1/2) that recruit innate immune cells like neutrophils may augment host defense to viral infections [103]. Specifically, one study showed that cellular senescence inhibits vesicular stomatis virus (a RNA virus) replication in both primary murine fibroblasts with replication-induced senescence, and in a tumor cell line with chemotherapy-induced senescence [102], possibly due to SASP factors, including the release of type I interferons (IFN), important anti-viral cytokines [104]. Additionally, mice that were induced to exhibit senescence within the lung via bleomycin exposure were more resistant to vesicular stomatis virus infection in vivo than mice that were not exposed to bleomycin. This is compatible with in vitro studies that found that type I IFN, in addition to other SASP components such as IL-6 and IL-8, induces cellular senescence [105, 106], and vice versa. Additionally, cellular fusion caused by the fusogenic measles virus induces cellular senescence, including the release of SASP factors, in fibroblasts in vitro [107], although the in vivo implications of this finding are unclear. Collectively, these studies suggest that senescence that is induced acutely by viruses may be a part of the body's antiviral immune response [108, 109]. This defense mechanism may have evolved to allow virus-infected cells to secrete SASP factors to limit viral replication in neighboring cells and ultimately viral spread throughout the lung. Furthermore, the SASP factors, may enhance host defense by promoting immune cell recruitment to the lung. But if these SASP factors are produced exuberantly, immune pathology could ensue (fig. 2). Finally, some viruses, including human papillomavirus and hepatitis B, have evolved machinery specifically to overcome cellular senescence [110, 111], suggesting that senescence plays a role in antiviral defense in some contexts.

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FIGURE 2

Hypothesis: Senescence may play divergent roles during acute respiratory viral infections depending on host resilience. In young hosts, senescence induced by viral infection may play a positive role by recruiting neutrophils (PMNs) and other immune cells via the senescence associated secretory phenotype (SASP), resulting in viral clearance and tissue repair. In aged or vulnerable hosts, however, senescence induced by viral infection, in addition to the senescence already present, may tip the balance towards pathology. This pathology is characterised by an exuberant immune response, including high levels of SASP including cytokines and chemokines, causing exaggerated recruitment of PMNs and other immune cells, resulting in lung tissue damage.

However, there is evidence that senescence contributes to the pathophysiology of respiratory viral infections. RSV induces DNA damage resulting in senescence in both mononuclear cells and lung epithelia in young mice [25], which enhances airway tissue remodelling (exhibited by a loss of ciliated cells and an increase in secretory cells [112]), potentially leading to permanent tissue damage and fibrosis. Furthermore, senescent cells may promote viral replication. For example, the influenza virus replicates more efficiently within senescent human bronchial epithelial cells than non-senescent cells [113]. Similar results are found with varicella zoster virus, a herpes virus, which replicates more efficiently in senescent human dermal fibroblasts than non-senescent cells [113]. A potential explanation for this phenomenon may be that senescence induced by these viruses downregulates the antiviral type I IFN program, including key signalling proteins, e.g., STING and IRF-3, upon infection in vitro with varicella virus [113]. This downregulation could lead to reduced secretion of antiviral IFNs, promoting viral replication. Whether this occurs in vivo is not known, however. Differences between this study and the contrasting one above, which found that senescence reduces replication of vesicular stomatis virus [102], could be due to the different viruses employed (i.e., vesicular stomatitis versus influenza, varicella), different cell types (MCF7, A549 cell lines and primary murine embryo fibroblast cells in the vesicular stomatitis study versus primary human bronchial epithelial cells and primary human dermal fibroblasts in the influenza/varicella study), and different approaches to induce senescence (replicative senescence and bleomycin in the vesicular stomatitis study versus only replicative senescence in the influenza/varicella study). Clearly, future studies are required to determine if senescence is harmful or beneficial to pathologically relevant respiratory viruses.

Cellular Senescence: Friend or Foe in Respiratory Viral Infections?

The overall impact of cellular senescence on acute respiratory viruses may depend on host resilience factors (fig. 2). For example, the SASP may have evolved to enhance host defense by increasing the recruitment of immune cells. This is likely beneficial in resilient young hosts who do not overproduce SASP factors. Indeed, initial neutrophil recruitment to the lung is critical to host defense to influenza infection [103]. But in vulnerable aged hosts, the SASP factors may be produced at pathologically high levels to promote immune pathology. Specifically, CXCL-1, CXCL-2, and IL-17, are increased in the lungs of aged mice even before viral infection, and accompany excessive neutrophil recruitment into the lung to enhance mortality during acute influenza infection [103]. In this study, senescent alveolar epithelial cells were found to produce more neutrophil chemoattractants and thus increase neutrophil chemotaxis. Similarly, aged rhesus macaques express higher levels of CCL-2, IL-6, and IL-8 in the lungs in response to influenza infection, suggesting an exaggerated innate immune response [114]. Based on these studies, inhibiting the SASP or reducing lung senescent cells might be beneficial in aged hosts infected with acute respiratory viral infections. However, formal investigation is required to determine if removing senescent cells prior to infection is sufficient, or if treatment during infection is required. Importantly, vulnerable hosts, including older people and people with comorbidities such as diabetes, are more susceptible to SARS-COV-2, which emerging studies suggest is due to the cytokine storm and excessive neutrophil effector functions within the lung [90].

We still do not know whether senescent cells in the lung ultimately increase or reduce viral replication. Respiratory viruses may “hijack” the senescent machinery within the infected cell to downregulate the IFN pathway to promote viral packaging and ultimately enhance viral replication within the cell. It is not clear yet if this occurs in vivo and, if so, what underlying mechanisms are involved. In contrast, activation of the senescence machinery within a virally-infected cell may be a damage response mechanism that leads to the secretion of antiviral cytokines to limit viral replication, an issue that also requires future investigation. Alternatively, senescence may simply be a side-effect of viral infection; during and after infection, various inflammatory cytokines are upregulated, resulting in the induction of senescence in infected cells.