Senolytics Combat COVID-19 in Aging

Abstract

Aging increases vulnerability to respiratory viral-infections, including SARS-CoV-2. Delval et al. established a causal role for age-related pre-existing senescent cells, in the severity of COVID-19 symptoms in an ageing hamster model. Selective depletion of senescent cells, using senolytics, was shown to mitigate the risk of severe COVID-19 symptoms linked to aging.

Aging considerably influences the severity of COVID-19, with older individuals experiencing heightened illness with increased hospitalizations and mortality rates. Cellular senescence has been proposed as a potential contributor to the severity of COVID-19 infection in older individuals^1,2; however, empirical evidence substantiating this connection in-vivo is currently lacking. Virus-induced senescent cells have been implicated in SARS-CoV-2 infections², yet the specific contribution of age-related senescent cells remains unclear. In this issue of Nature Aging, Delval and colleagues ³ present compelling evidence linking age-related senescent cells, including those pre-existing before the infection, via a causal relationship to the pathogenic severity of experimental COVID-19 in an aging hamster model (Fig. 1). These findings underscore the urgent need for clinical trials focusing on senolytics, a group of small molecules that selectively kill senescent cells, to mitigate the immediate and long-term health impacts of SARS-CoV-2 infection.

Figure 1. Senescent cells facilitate SARS-CoV-2 infection, whereas senolytic agents inhibit its progression.

In aged hamsters, the accumulation of senescent cells with aging (pre-existing senescent cells) and the Senescence-associated secretory phenotype (SASP) factors increases angiotensin-converting enzyme 2 (ACE2) receptor expression in the lung tissues, which predisposes the lung tissue to be more susceptible to SARS-CoV-2 virus entry and increase viral load. Meanwhile, the virus infection actively induces more senescent cells (virus-induced senescent cells) in aged hamsters than in young ones, thus reinforcing a detrimental positive feedback loop. Together, these events contribute to the development of severe infection sequelae. Conversely, removing senescent cells in aged hamsters with the senolytic agent ABT-263 decreases viral load, lowers SASP factor levels, and improves outcomes.

Cellular senescence is a recognized causal factor of aging^4,5. This cellular process is a lasting cell cycle cessation in response to stress or damage, characterized by altered cellular function and a unique senescence-associated secretory phenotype (SASP). In an earlier study by Camell and colleagues, senescent cells demonstrated an amplified inflammatory response, impaired antiviral defense mechanisms, and heightened expression of viral entry proteins to viral infection⁶. However, this study lacked direct in-vivo evidence linking cellular senescence to SARS-CoV-2 infection.

In the new study by Delval et al.³, the relationship between cellular senescence and SARS-CoV-2 infection was elucidated in aged hamsters. The authors first compared the burden of age-related pre-existing senescent cells within the lungs of aged hamsters to young counterparts. Senescence-associated genes were notably upregulated in the older cohort, including the common senescent cell markers p16^INK4a (termed as p16) and SASP factors. Furthermore, aged hamster lungs exhibited a significant increase in p16-positive senescent cells, senescence-associated beta-galactosidase (SA-β-Gal) activity, and B-cell lymphoma-2 (Bcl-2) pathway levels, a known senescent cell anti-apoptotic pathway⁷. These findings highlight the possible role of pre-existing age-related senescent cells in increasing susceptibility to SARS-CoV-2 infection in aged hamsters.

To examine the impact of age-related senescent cells on SARS-CoV-2 infection susceptibility, Delval and the team employed senolytics to selectively deplete senescent cells in the lungs of aged hamsters. They utilized the Bcl2-family inhibitor ABT-263, previously established as an effective senolytic in murine models^8,9 and demonstrated that ABT-263 treatment reduced p16-high expressing senescent cells within the lungs of the aged hamsters, as evidenced by immunohistochemistry, immunofluorescence, and SA-β-Gal activity. These findings suggest that aged hamsters could serve as a pertinent model for investigating the effects of senolytics, the targeted removal of senescent cells, in COVID-19.

Viral load is a widely used parameter to evaluate viral infection, including SARS-CoV-2 ¹⁰. Delval et al. found a higher viral load at 3- and 7-days post-infection (dpi) in aged hamsters than in their younger counterparts, and aged hamsters retained infectious viruses at 7 dpi. This heightened viral load may be attributable to increased viral entry⁶. The researchers subsequently assessed the expression of ACE2 and found it was elevated in the lungs of aged hamsters. They observed a colocalization of ACE2 with p16, indicating an upregulation of ACE2 in senescent cells. Additionally, it is possible that the surrounding epithelial cells may also contribute as a source of ACE2 when stimulated with the SASP ⁶, as illustrated in Figure 1.

In addition to viral load, Delval and colleagues examined age-dependent characteristics of COVID-19-like lung disease. By 3 dpi, aged hamsters developed bronchointerstitial pneumonia, congestion, and intra-alveolar and interstitial cell infiltration. By 7 dpi, symptoms intensified, with additional complications such as alveolar collapse, necrosis, hemorrhage, micro-vasculitis, and discrete thrombi. By 22 dpi, some inflammation and type II hyperplasia persisted, with significantly more inflammatory foci in aged hamsters. Aged hamsters also demonstrated higher collagen deposition and basal membrane disorganization around bronchi and blood vessels than younger hamsters. Despite both age groups exhibiting respiratory disease symptoms and weight loss post-infection, aged hamsters demonstrated slower weight recovery and failed to regain their initial body weight by 22 dpi. Collectively, aged hamsters exhibited higher viral load, increased basal ACE2 expression, and more severe post-acute lung sequelae, leading Delval et al. to explore the possible mechanism responsible for age-associated vulnerabilities.

Given the known implications of cellular senescence in viral infection and aging, Delval and colleagues assessed the senescence markers and found increased p16-positive lung cells during infection in aged hamsters. Detecting viral antigens within a subset of senescent cells suggests the coexistence of pre-existing senescent cells with those induced by the viral infection^2,11, notably in the context of aging. Next, the authors examined the influence of senescent cells on SARS-CoV-2 replication. Hamsters were pretreated daily with senolytic agent, ABT-263, before SARS-CoV2 infection until sacrifice. ABT-263 notably reduced viral load and ACE2 expression within the lungs of aged hamsters. These reductions suggest that eliminating pre-existing senescent cells before infection may mitigate the severity of SARS-CoV-2 infection. However, the precise role of pre-existing versus viral-induced senescent cells in viral replication remains unclear. Therefore, the exact molecular and cellular mechanisms underlying these beneficial effects warrant further exploration and comprehensive investigation.

Subsequently, the team explored the consequences of ABT-263 on the lung pathology of COVID-19. They found that ABT-263 notably decreased the extent of lung area deteriorating from subacute bronchointerstitial pneumonia in aged hamsters than in vehicle groups. No such effect was observed in younger hamsters. Furthermore, the overall severity of the disease, including factors like inflammation, hemorrhage, syncytia, and alveolar destruction, was significantly less in ABT-263-treated aged hamsters. From a systemic perspective, ABT-263 treatment in aged hamsters decreased several SASP factors, prothrombotic and inflammatory markers, and certain enzymes and transporters from various metabolic pathways in the bloodstream. These results indicate that senolytic treatment may help mitigate lung inflammation and lower systemic indicators of SASP in aged hamsters infected with SARS-CoV-2 during the early stages of the disease.

Furthermore, Delval and colleagues examined the long-term effects of senolytics on SARS-CoV-2 infection in aged hamsters. Although ABT-263 did not significantly improve weight recovery after infection, the treatment increased survival rate, attenuated inflammation, and diminished type II hyperplasia than in the control group. These beneficial effects were, again, absent in young hamsters. Collagen deposition was consistent across groups, but a slight reduction in basal membrane disorganization was observed in the senolytic-treated aged hamster. Proteomic analysis disclosed a significant decrease in the expression of numerous proteins implicated in chronic lung disorders, including specific components of cell adhesion, the ubiquitin-proteasome system, proteolysis, and the complement pathway upon senolytic treatment. These findings indicate that senolytics ameliorate acute and systemic manifestations akin to COVID-19 and alleviate the long-term repercussions of SARS-CoV-2 infection in aged hamsters.

Altogether, Delval and colleagues provide a valuable insight into the significant influence of aging-associated cellular senescence on the vulnerability and severity of COVID-19 in older adults and further underscores the broad implications of senescent cells in age-related infectious diseases. Exploring other underlying mechanisms, including the interplay between senescent cells, viral infection, and the immune system, is warranted. Moreover, this study highlights the translational potential of senolytics to mitigate the immediate and long-term health outcomes of SARS-CoV-2 infection in older individuals. Consequently, there is an urgent need for clinical trials to translate these findings into effective therapeutic strategies to manage COVID-19 in the aging population.