A Novel Humanized Lethal Mouse Model of SARS-CoV-2-Associated Disease

Mice are valuable small animal models for studying SARS-CoV-2 pathogenesis. Ancestral SARS-CoV-2 strains do not efficiently utilize murine Ace2, rendering wild-type mice resistant to infection. Although human ACE2 transgenic models such as K18-hACE2 have provided critical insights, they express multiple copies of both murine and human ACE2, and random transgene insertion can result in non-physiological receptor expression. To overcome these limitations, we employed a human ACE2 knock-in (hACE2-KI) model in which the murine Ace2 coding sequence is replaced with human ACE2 using CRISPR/Cas9 technology, generating an mAce2-null background. This design allows human ACE2 expression under endogenous regulatory control while eliminating murine Ace2 expression, thereby providing a more physiologically relevant platform to investigate SARS-CoV-2 pathogenesis and evaluate therapeutic and preventive strategies. In this study, SARS-CoV-2-associated disease was evaluated and compared among hACE2-KI, K18-hACE2 and C57BL/6J mice. Mice were intranasally inoculated with 10⁵ plaque-forming units of SARS-CoV-2 lineages B.1 or B.1.351. Both hACE2-KI and K18-hACE2 mice developed severe disease after SARS-CoV-2 infection. Following infection with B.1, both K18-hACE2 mice and hACE2-KI mice exhibited significant weight loss and mortality, with high viral loads detected in the lungs and brain. hACE2-KI mice infected with SARS-CoV-2 B.1.351 also showed significant weight loss and viral loads, resulting in high mortality. The pathology and inflammatory response within the lungs and brain of infected hACE2-KI mice revealed robust expression of viral nucleocapsid protein, histopathological changes, and upregulated cytokine and chemokine responses. Together, these findings demonstrate that the hACE2-KI knock-in mouse model supports robust SARS-CoV-2 replication and mimics severe COVID-19 disease.