A human-ACE2 knock-in mouse model for SARS-CoV-2 infection recapitulates respiratory disorders but avoids neurological disease associated with the transgenic K18-hACE2 model.

Abstract

Animal models have been instrumental in elucidating the pathogenesis of SARS-CoV-2 infection and testing COVID-19 vaccines and therapeutics. Wild-type (WT) mice are not susceptible to many SARS-CoV-2 variants, therefore transgenic K18-hACE2 mice have emerged as a standard model system. However, this model is characterized by severe disease, particularly associated with neuroinfection, which leads to early humane endpoint euthanasia. Here, we established a novel knock-in (KI) mouse model by inserting the original K18-hACE2 transgene into the collagen COL1A1 locus using a recombinase mediated cassette exchange (RMCE) system. Once the Col1a1-K18-hACE2 mouse colony was established, animals were challenged with a B.1 SARS-CoV-2 (D614G) isolate and were monitored for up to 14 days. Col1a1-K18-hACE2 mice exhibited an initial weight loss similar to the K18-hACE2 transgenic model but did not develop evident neurologic clinical signs. The majority of Col1a1-K18-hACE2 mice did not reach the preestablished humane endpoint, showing progressive weight gain after 9 days post-infection (dpi). Importantly, despite this apparent milder pathogenicity of the virus in this mouse model compared to the K18-hACE2 transgenic model, high levels of viral RNA were detected in lungs, oropharyngeal swab, and nasal turbinate. Remaining lesions and inflammation in lungs were still observed after 14 dpi. In contrast, although low level viral RNA could be detected in a minority of Col1a1-K18- hACE2 animals, no brain lesions were observed at any timepoint. Overall, Col1a1-K18- hACE2 mice constitute a new model for investigating SARS-CoV-2 pathogenesis and treatments, with potential implications for studying long-term COVID-19 sequelae. ImportanceK18-hACE2 mice express high levels of the human protein ACE2, the receptor for SARS- CoV-2, and can therefore be infected by this virus. These animals have been crucial to understand viral pathogenesis and to test COVID-19 vaccines and antiviral drugs. However, K18-hACE2 often die after infection with initial SARS-CoV-2 variants likely due to a massive brain infection that does not occur in humans. Here, we used a technology known as knock-in that allows for the targeted insertion of a gene into a mouse and we have generated a new hACE2-mouse. We have characterized this new animal model demonstrating that the virus replicates in the respiratory tract, damaging lung tissue and causing inflammation. In contrast to K18-hACE2 mice, only limited or no brain infection could be detected, and most animals recovered from infection with remaining lung lesions. This new model could be instrumental for the study of specific disease aspects such as post-COVID condition, sequelae, and susceptibility to reinfection.