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.

Animal models have been instrumental in elucidating the pathogenesis of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection and in testing coronavirus disease 2019 (COVID-19) vaccines and therapeutics. Wild-type (WT) mice are not susceptible to many SARS-CoV-2 variants, and therefore, transgenic K18-hACE2 mice have emerged as a standard model system. However, this model is characterized by a 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 type I alpha chain (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 pre-established humane endpoint, showing a progressive weight gain 9 days postinfection (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 the lungs, oropharyngeal swab, and nasal turbinates. Moreover, the remaining lesions and inflammation in the lungs were still observed 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 angiotensin-converting enzyme 2 (ACE2), the receptor for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), and are therefore susceptible to infection by this virus. These animals have been crucial to understanding viral pathogenesis and to testing coronavirus disease 2019 (COVID-19) vaccines and antiviral drugs. However, K18-hACE2 often dies 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 (KI) that allows for the targeted insertion of a gene into a mouse, and we have generated a new human ACE2 (hACE2) mouse. We have characterized this new animal model demonstrating that, upon challenge with SARS-CoV-2, 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 in this new model. After 14 days, most animals recovered from the infection, although lung tissue lesions were still observed. This new model could be instrumental for the study of specific disease aspects such as post-COVID-19 condition, sequelae, and susceptibility to reinfection.