SARS-CoV-2 Delta requires human ACE2 but not human TMPRSS2 to infect mice and elicits greater lung injury and adaptive immune response than Omicron in human ACE2 knock-in mice

SARS-CoV-2 pathogenesis remains poorly understood in large part due to lack of knowledge about the mechanisms of entry and behaviors of SARS-CoV-2 variants in vivo. We generated single and double knock-in (KI) mice expressing human ACE2 (hACE2) and/or human TMPRSS2 (hTMPRSS2) under endogenous promoters in place of murine ACE2 (mACE2) and TMPRSS2 on the C57BL/6 and BALAB/c genetic backgrounds to evaluate Delta vs Omicron BA.1 infection, disease, and immune response. Delta replication was observed in lungs of mice expressing hACE2 but not hTMPRSS2 or mACE2 following intranasal inoculation, and similar levels of Delta replication were observed in lungs of hACE2-KI and hACE2xhTMPRSS2-KI mice. Thus, Delta requires hACE2 but not hTMPRSS2 to infect mice. In contrast, BA.1 established similar levels of replication in lungs of single and double KI mice, demonstrating that BA.1 requires neither hACE2 nor hTMPRSS2 to infect mice. Although no significant differences in viral burden were observed in hACE2-KI mice infected with Delta vs BA.1, Delta-infected hACE2-KI mice exhibited increased histopathologic lung injury and higher SARS-CoV-2-specific CD4+ and CD8+ T cell responses (spleen) and anti-SARS-CoV-2 spike IgG titers (serum). Additionally, hACE2-KI mice on the C57BL/6 background showed more severe lung disease and stronger Th1 response than BALB/c. These results associate the severity of lung disease with the magnitude of Th1-dominant immune responses, and set a foundation for dissecting mechanisms of COVID-19 pathogenesis in hACE2-KI mice representing Th1- vs Th2-dominant genetic backgrounds.