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
Authors: Pons-Grífols Anna , Tarrés-Freixas Ferran , Pérez Mònica , Riveira-Muñoz Eva , Raïch-Regué Dàlia , Pérez-Zsolt Daniel , Muñoz-Basagoiti Jordana , Tondelli Barbara , Izquierdo-Useros Nuria , Capdevila Sara , Vergara-Alert Júlia , Urrea Victor , Carrillo Jorge , Ballana Ester , Forrow Stephen , Clotet Bonaventura , Segalés Joaquim , Trinité Benjamin , Blanco Julià 
Published: June 2024

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 compared to the K18-hACE2 transgenic model, high levels of viral RNA were detected in lungs, oropharyngeal swab, and nasal turbinate. Conversely, in sharp contrast to K18-hACE2 transgenic mice, no viral replication was detected in the brains of Col1a1-K18-hACE2 animals at any timepoint, explaining the reduced severity of clinical signs. At 14 dpi, while infection was cleared in the lungs, increased lesions and residual inflammation were detected. 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.

Importance

K18-hACE2 mice express high levels of the human protein ACE-2, the receptor for SARS-CoV-2, and therefore are 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 rapidly die after infection with initial SARS-CoV-2 variants 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 and inflaming the lungs; however, in contrast to K18-hACE2 mice, no brain infection was observed, and most animals recovered from infection. This new model could be instrumental for the study of specific disease aspects such as post-COVID condition, sequelae, and susceptibility to reinfection.