Dynamics of lung-infiltrating virus-specific T cells associated with age-dependent SARS-CoV-2 pneumonia severity

Coronavirus disease 2019 (COVID-19) pneumonia is prevalent in the elderly infected with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2); however, the mechanisms underlying its age-dependent pathogenesis remain unclear. In this study, we established a mouse-adapted SARS-CoV-2 strain infected Nr4a3-Tocky mouse model to examine T-cell dynamics associated with disease severity. Nr4a3-Tocky mice allow the analysis of the dynamics and induction of antigen-reactive T cells following antigen recognition in vivo using fluorescent Timer protein. SARS-CoV-2-infected adult mice exhibited transient body weight loss and recovery, whereas aged mice developed severe pneumonia. BALF viral RNA was comparable between 1-4 days post-inoculation (d.p.i.), but declined in adults at 5 d.p.i. Aged mice displayed stronger inflammation as indicated by scRNA-seq, and higher levels of inflammatory cytokines (TNF-α, CCL2, CXCL10 and IL-6) in BALF correlated with weight loss. Timer analysis revealed induction of antigen-reactive T cells in the adult lungs at 5 and 8 d.p.i., which inversely correlated with disease severity. Additionally, S-specific IFN-γ ⁺ CD8 ⁺ T cells were detected at 5 d.p.i. in adults, whereas detection of antigen-specific T cells was delayed in aged mice. These results suggest that the coexistence of age-related lung inflammation and delayed induction of antigen-specific T cells is linked to more severe pneumonia, while earlier T-cell responses are associated with improved viral control and milder disease. In this study, we utilized a novel mouse model enabling characterization of antigen-reactive T cells in the local tissue, and investigated inflammatory responses in the lung together with lung-infiltrating virus-specific T cells, finding the dynamics of these immunological parameters associated with the age of the mice. Our analysis provides new insights into understanding how age-related T-cell dysfunction is associated with the severity of SARS-CoV-2 pneumonia.