Impaired immune response drives age-dependent severity of COVID-19

SARS-CoV-2 is a highly contagious respiratory virus and the causative agent for COVID-19. The severity of disease varies from mildly symptomatic to lethal and shows an extraordinary correlation with increasing age, which represents the major risk factor for severe COVID-19 1 . However, the precise pathomechanisms leading to aggravated disease in the elderly are currently unknown. Delayed and insufficient antiviral immune responses early after infection as well as dysregulated and overshooting immunopathological processes late during disease were suggested as possible mechanisms. Here we show that the age-dependent increase of COVID-19 severity is caused by the disruption of a timely and well-coordinated innate and adaptive immune response due to impaired interferon (IFN) responses. To overcome the limitations of mechanistic studies in humans, we generated a mouse model for severe COVID-19 and compared the kinetics of the immune responses in adult and aged mice at different time points after infection. Aggravated disease in aged mice was characterized by a diminished IFN-γ response and excessive virus replication. Accordingly, adult IFN-γ receptor-deficient mice phenocopied the age-related disease severity and supplementation of IFN-γ reversed the increased disease susceptibility of aged mice. Mimicking impaired type I IFN immunity in adult and aged mice, a second major risk factor for severe COVID-19 2–4 , we found that therapeutic treatment with IFN-λ in adult and a combinatorial treatment with IFN-γ and IFN-λ in aged Ifnar1 -/- mice was highly efficient in protecting against severe disease. Our findings provide an explanation for the age-dependent disease severity of COVID-19 and clarify the nonredundant antiviral functions of type I, II and III IFNs during SARS-CoV-2 infection in an age-dependent manner. Based on our data, we suggest that highly vulnerable individuals combining both risk factors, advanced age and an impaired type I IFN immunity, may greatly benefit from immunotherapy combining IFN-γ and IFN-λ.