Virus-induced senescence is a driver and therapeutic target in COVID-19

Senescent cell fate can be triggered by many stressors, including DNA damage, oncogene activation, mitochondrial dysfunction, reactive metabolites, oxygen extremes, proteotoxic stress, extracellular signals, infections, mechanical or shear stress, resistance exercise, and factors secreted by other senescent cells.

“The senescent cell fate can be triggered by a number of stressors including DNA damage, cancerous mutations or oncogene activation, mitochondrial dysfunction, reactive metabolites, hyperoxia or hypoxia, proteotoxic stress, extracellular signals, infections, mechanical or shear stresses that deform cells, resistance exercise and factors secreted by other senescent cells18,38–47.”

Senescent cells accumulate with aging and skeletal injury in the bone microenvironment.

“There is considerable evidence that senescent cells accumulate with aging and skeletal injury in the bone microenvironment25,73–76.”

Transgenic mice with cell type-specific elimination of senescent cells were recently developed, p16-LOX-ATTAC mice.

“To address this limitation, transgenic mice with cell type-specific elimination of senescent cells were recently developed, p16-LOX-ATTAC mice75.”

SARS-CoV-2 worsens the senescence-associated secretory phenotype (SASP).

“SARS-CoV-2, like other viruses, can induce senescence and exacerbates the senescence-associated secretory phenotype (SASP)”

Virus-induced senescence appears to be a universal stress response in host cells and can be triggered by many virus species.

“VIS appears to be a universal stress response in host cells and can be induced by many different virus species.”

SARS-CoV-2 is able to induce cellular senescence.

“Recently, it was found that SARS-CoV-2 can evoke a form of cellular senescence, virus-induced senescence (VIS) (Box 1), through multiple mechanisms27–29”

Cells lacking functional p53 or overexpressing JMJD2c fail to enter VIS and lack a SASP after SARS-CoV-2 infection.

“in vitro experiments showed that cells that lack functional p53 or overexpress the H3K9me3 demethylase JMJD2c46,50 — which therefore cannot enter senescence in response to pro-senescent oncogenes such as Ras-G12V or Braf-V600E46,51 — also failed to enter VIS and lacked a SASP upon infection with SARS-CoV-2 (ref.28).”

Lung specimens from COVID-19 patients show much stronger signs of senescence than SARS-CoV-2 negative controls.

“Similar analyses of lung specimens from patients with COVID-19, consistent with multiple independent patient datasets29,44,52, also showed much stronger signs of senescence compared to SARS-CoV-2 negative controls28.”

A panel of senescence markers distinguishes nasopharyngeal mucosa from COVID-19 patients versus pre-pandemic controls, with higher reactivity in COVID-19 samples.

“when comparing samples of nasopharyngeal mucosa from patients with COVID-19 to pre-pandemic biopsy samples from individuals without a manifest respiratory tract infection, a panel of senescence markers was shown to robustly discriminate these two groups, with significantly higher reactivity in samples from patients with COVID-19 (refs.28,29,44,52).”

SARS-CoV-2 mRNA is detectable in many but not all VIS-positive samples, suggesting that VIS can persist after the initial infection.

“SARS-CoV-2 mRNA was detectable by in situ hybridization in many but not all VIS-positive samples, suggesting that VIS may persist beyond the initial infection28,61,62”

Senescence markers p16INK4a, p21CIP1, H3K9me3, lipofuscin, and IL-8 are markedly elevated in upper respiratory mucosa from COVID-19 patients.

“senescence markers, such as p16INK4a, p21CIP1, H3K9me3 and lipofuscin (detected by the GL13-SenTraGorTM reagent in fixed tissues60), as well as IL-8 (a component of the SASP) were markedly elevated in samples from the upper respiratory mucosa from patients with COVID-19 (ref.28)”

In vitro and in vivo experiments show that macrophages near SASP-producing virus-infected epithelial cells can undergo paracrine secondary senescence or acquire a senescence-like condition.

“in vitro and in vivo experiments have shown that resident and non-resident macrophages in the vicinity of SASP-producing virus-infected epithelial cells can undergo paracrine secondary senescence or acquire a senescence-like condition28,63,66,77,78.”

Senescent cells acutely reinforce systemic inflammatory responses to pathogens and facilitate the spread of senescence, as described by the SASP amplifier/rheostat model.

“We suspect that senescent cells acutely reinforce systemic inflammatory responses to pathogens and facilitate the spread of senescence, a hypothesis termed the ‘SASP amplifier/rheostat model’27,28.”

WHO-graded clinical severity of COVID-19 correlates positively with increasing levels of IL-6, SERPINA3, and D-Dimer-based clotting activity.

“WHO-graded clinical severity of COVID-19 correlated positively with increasing concentrations of the pro-inflammatory SASP cytokine IL-6, the coagulation-related SASP representative SERPINA3 and D-Dimer-based clotting activity28.”

SASP-typical cytokines, extracellular matrix-active proteases, and SASP-related pro-coagulant and complement-activating factors are strongly elevated in the serum of COVID-19 patients compared with healthy controls.

“were also strongly elevated in the serum of patients with COVID-19 compared to healthy controls28,52,67–70.”

Lungs from patients with severe COVID-19 show much higher infiltration by CD86+ macrophages and stronger SASP-reminiscent cytokine expression than controls.

“lungs from patients with severe COVID-19 exhibited much higher levels of infiltration by CD86+ macrophages and much stronger SASP-reminiscent cytokine expression compared to lungs from patients who died without notable signs of a respiratory infection and who were not infected with SARS-CoV-2 (refs.28,61).”

These effects were detected to a much lesser extent when supernatant from virus-infected but senescence-resistant cells was tested.

“These effects represent highly disease-relevant components of COVID-19-related immunothrombosis, which were detected to a much lesser extent if the supernatant from virus-infected but senescence-resistant cells was tested28 (Fig. 2).”

Conditioned medium from senescent cells rich in SASP-associated factors evoked paracrine senescence in endothelial cells or triggered complement-mediated cytolysis, promoted NET formation by neutrophils, activated platelets, and accelerated clotting.

“Specifically, conditioned medium from senescent cells that is rich in SASP-associated factors evoked paracrine senescence in endothelial cells or triggered their complement-mediated cytolysis, promoted NET formation by neutrophils, activated platelets, and accelerated clotting.”

In vitro observations suggest an essential, secretome-dependent role of VIS and senescence in general in the pathobiological manifestation of SARS-CoV-2 infection.

“Overall, these in vitro observations suggest an essential, secretome-dependent role of VIS and senescence in general in the pathobiological manifestation of the infection27–29.”

Culture supernatant from equally virus-infected but senescence-incapable cells did not evoke those macrophage effects.

“By contrast, the culture supernatant of equally virus-infected but senescence-incapable cells failed to evoke these cellular effects28.”

In animal models of fatal COVID-19, senolytic regimens based on flavonoids, such as the dasatinib and quercetin combination or fisetin, enhanced survival rates.

“In animal models of fatal COVID-19, senolytic regimens based on flavonoids, such as the dasatinib and quercetin combination or fisetin, enhanced survival rates28.”

Navitoclax, fisetin, and a combination of dasatinib and quercetin demonstrated significant cytotoxic activity against VIS cells in vitro, leaving uninfected or virus-infected but genetically senescence-incapable cells virtually unaffected.

“Navitoclax, fisetin, and a combination of dasatinib and quercetin demonstrated significant cytotoxic activity against VIS cells (including human nasal epithelial cells exposed to SARS-CoV-2) in vitro, leaving uninfected or virus-infected but genetically senescence-incapable cells virtually unaffected28.”

Navitoclax, fisetin, or a combination of dasatinib and quercetin quantitatively eliminated senescent cells in situ and improved at least some central histopathological findings of SARS-CoV-2 infection in the lung in all mentioned animal models.

“Navitoclax, fisetin, or a combination of dasatinib and quercetin were found to quantitatively eliminate senescent cells in situ and to improve at least some of the central histopathological findings of SARS-CoV-2 infection in the lung in all the animal models mentioned28.”

SARS-CoV-2 Alpha and Beta variants elicited a significantly stronger pro-inflammatory SASP upon infection compared to the ancestral strain, and these VIS cells exhibited excellent susceptibility to the aforementioned senolytics in vitro, comparable to cells infected with the ancestral virus.

“Importantly, the SARS-CoV-2 Alpha and Beta variants elicited a significantly stronger pro-inflammatory SASP upon infection compared to the ancestral SARS-CoV-2 strain, and these VIS cells exhibited excellent susceptibility to the aforementioned senolytics in vitro, comparable to cells infected with the ancestral virus28.”

Targeting the cGAS-STING axis with the small-molecule STING inhibitor H-151 had a SASP-suppressing, senomorphic effect and led to reduced lung pathology and prolonged survival in a human ACE2-transgenic mouse model of COVID-19.

“Targeting of the cGAS–STING axis, which induces type I interferons and other SASP factors, with the small-molecule STING inhibitor H-151 had a SASP-suppressing, senomorphic effect28,121,128,129 and led to reduced lung pathology and prolonged survival in a human ACE2-transgenic mouse model of COVID-19 (ref.59).”

Broadly elevated levels of SASP-typical cytokines and interleukins, including G-CSF, IFNγ, IL-1α, IL-7, IL-17A, TNF and VEGF, in serum of infected animals were reduced by all agents tested to levels virtually indistinguishable from uninfected animals.

“Strikingly, the broadly elevated levels of SASP-typical cytokines and interleukins — among them granulocyte colony-stimulating factor (G-CSF), IFNγ, IL-1α, IL-7, IL-17A, TNF and vascular endothelial growth factor (VEGF) — that were detected (despite the challenge to do this at the protein level in hamsters) in the serum of infected animals, were reduced by all agents tested to levels that were”

Expression of BCL-xL and BCL-w and activity of SRC, AKT and p38 kinases were elevated in retrovirus-infected fibroblast models of VIS, in SARS-CoV-2-infected hamsters, and in patients with COVID-19.

“Indeed, the expression of BCL-xL and BCL-w and the activity of SRC, AKT and p38 kinases were found to be elevated in retrovirus-infected fibroblast models of VIS, in SARS-CoV-2-infected hamsters and in patients with COVID-19 (ref.28).”

In the Roborovski dwarf hamster model, treatment with dasatinib and quercetin resulted in significantly attenuated disease features regarding overall pneumonia score, bronchial epithelial hyperplasia, and alveolar damage.

“Particularly pronounced effects were observed in the Roborovski dwarf hamster model, where treatment with a combination of dasatinib and quercetin resulted in significantly attenuated disease features with respect to the overall pneumonia score, bronchial epithelial hyperplasia and alveolar damage28.”

A collective analysis of both trials demonstrated a significantly better outcome in terms of avoiding hospitalization, need for oxygen, referral to the intensive care unit and death.

“Specifically, a collective analysis of both trials demonstrated a significantly better outcome in terms of avoiding hospitalization, need for oxygen, referral to the intensive care unit and death28.”

Rapidly expanding evidence implicates cellular senescence as a host cell response to viral infection in general and to SARS-CoV-2 in particular.

“Rapidly expanding evidence implicates cellular senescence as a host cell response to viral infection in general and to SARS-CoV-2 in particular, similar to other cellular insults such as telomere dysfunction, oncogene activation or chemotherapy27–29.”

A large variety of viruses have been shown to induce senescence in vitro and in vivo, whereas no VIS phenotype was observed upon infection with the mild seasonal influenza A (H1N1) virus in Syrian hamsters.

“A large variety of viruses have been shown to induce senescence in vitro and in vivo27,28,38, whereas no VIS phenotype was observed upon infection with the mild seasonal influenza A (H1N1) virus in Syrian hamsters62, implying that not all virus infections might be susceptible to senolytic strategies.”

Senolytic treatment in animal models of COVID-19 eliminates virus-induced senescent cells and reduces inflammation and mortality, suggesting senescent cells are a potential therapeutic target in viral infections.

“Notably, senolytic treatment in animal models of COVID-19 eliminates virus-induced senescent cells and reduces inflammation and mortality, suggesting that senescent cells are a potential therapeutic target in viral infections (Camell et al., 2021; Lee et al., 2021).”