Compromised efferocytosis during aging is related to COVID-19 severity in mice

Aging is one of the greatest risk factors for morbidity caused by the coronavirus disease 2019 (COVID-19). In older individuals, a dysregulated immune response to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection contributes to disease severity; however, the underlying mechanism remains elusive. In this study, we established an aging mouse model of COVID-19, successfully replicating the development of a relatively severe disease in older adults. Further single-cell transcriptome analysis revealed a distinct immune cell landscape in the infected lungs, accompanied by an over-activated inflammatory response, especially in aging mice. Compared to young mice, aging mice showed extensive neutrophil activation, NETosis, and a dramatic decrease in the number of alveolar macrophages (AMs). Moreover, as important executors of efferocytosis, AMs exhibited a low efferocytotic gene signature and downregulation of multiple efferocytosis receptors in aged mice. Further analysis indicated that the efferocytosis of neutrophils, whether undergoing apoptosis or NETosis, was compromised after SARS-CoV-2 infection. Since efferocytosis is a key process in inflammatory resolution, impaired efferocytosis may contribute to hyperinflammation in aging lungs. Our study reveals the characteristics and role of efferocytosis in aging mice after SARS-CoV-2 infection and provides valuable insights for the potential treatment of COVID-19.