SARS-CoV-2 Rapidly Adapts in Aged BALB/c Mice and Induces Typical Pneumonia

Age is a risk factor for coronavirus disease 2019 (COVID-19)-associated morbidity and mortality in humans; hence, in this study, we compared the course of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection in young and aged BALB/c mice. We found that SARS-CoV-2 isolates replicated in the respiratory tracts of 12-month-old (aged) mice and caused pathological features of pneumonia upon intranasal infection. In contrast, rapid viral clearance was observed 5 days following infection in 2-month-old (young) mice with no evidence of pathological changes in the lungs. Infection with SARS-CoV-2 elicited significantly upregulated production of cytokines, especially interleukin 6 and interferon gamma, in aged mice, whereas this response was much weaker in young mice. Subsequent challenge of infected aged BALB/c mice with SARS-CoV-2 resulted in neutralized antibody responses, a significantly reduced viral burden in the lungs, and inflammation mitigation. Deep sequencing showed a panel of mutations potentially associated with the enhanced infection in aged BALB/c mice, such as the Q498H mutation, which is located at the receptor-binding domain (RBD) of the spike (S) protein. We further found that the isolates cannot only multiply in the respiratory tract of mice, but also cause disease in aged mice. Overall, viral replication and rapid adaption in aged BALB/c mice were associated with pneumonia, confirming that the age-related susceptibility to SARS-CoV-2 in mice resembled that in humans. IMPORTANCE Aged BALB/c mice are in use as a model of disease caused by SARS-CoV-2. Our research demonstrated SARS-CoV-2 can rapidly adapt in aged BALB/c mice through causing mutations at the RBD of the S protein. Moreover, SARS-CoV-2-infected aged BALB/c mice indicated that alveolar damage, interstitial pneumonia, and inflammatory immune responses were similar to the clinical manifestations of human infections. Therefore, our aged BALB/c challenge model will be useful for further understanding the pathogenesis of SARS-CoV-2 and for testing vaccines and antiviral agents.

Automated summary

This study showed that aged BALB/c mice infected with SARS-CoV-2 exhibited rapid viral replication leading to pneumonia, while young mice quickly cleared the virus with no pathological changes in the lungs. Aged mice also had stronger cytokine responses, particularly interleukin 6 and interferon gamma, compared to young mice. The research further demonstrated the potential of using aged BALB/c mice as a model for understanding SARS-CoV-2 pathogenesis and testing vaccines and antiviral agents.