Alveolar type II cells harbouring SARS-CoV-2 show senescence with a proinflammatory phenotype

Background: COVID-19 disease primarily affects the respiratory system but can evolve to a multisystem inflammatory syndrome. Increased mortality is observed in elderly and in patients with age-related comorbidities. Cellular senescence is characterised by cell-cycle arrest and a proinflammatory 'Senescence-Associated-Secretory-Phenotype' (SASP). Viral infection has been associated with senescence. We have studied whether SARS-CoV-2 may be associated with senescence and the SASP phenotype. Methods: Lung tissues from ten COVID-19 and ten age-matched control patients were analysed by immunohistochemistry for: SARS-CoV-2, ACE-2, TTF-1, CD68, p16^INK4, IL-1β, IL-6 and the novel cellular senescence marker SenTraGor. Results: SARS-CoV-2 was detected in alveolar type-II (AT2) pneumocytes of all COVID-19 patients using a monoclonal antibody we developed against the spike protein. Electron microscopy confirmed the presence of the virus within AT2 cells. We observed that a proportion of SARS-CoV-2 AT2 infected cells exerted features of senescence, displaying strong reactivity to SenTraGor. Senescent phenotype was further confirmed with p16^INK4 co-staining. Subsequently, we found that SenTraGor-positive cells in COVID-19 samples co-expressed IL-1β and IL-6, which were absent in non-COVID-19 cases. Conclusion: This is the first evidence of senescence and expression of SASP components in SARS-CoV-2 infected human lung cells. Further studies need to determine whether senescence pre-exists at the time of infection, making cells susceptible to the virus, or is triggered as an antiviral response. Our findings justify the application of senotherapeutics for treatment or prevention of COVID-19 patients.