Live attenuated vaccination protects aged chimeric ACE2 mice from severe SARS-CoV-2 pathogenicity in vivo

Advanced age is one of the greatest risk factors for a severe outcome of COVID-19. Although mRNA vaccines were highly successful in protecting the elderly, the strongest increase in morbidity and mortality upon infection with emerging SARS-CoV-2 variants was among the elderly. To better understand SARS-CoV-2 pathogenicity and to thoroughly evaluate novel vaccination strategies, better models reliably reproducing human SARS-CoV-2 pathogenicity are needed. Here, we generated mice expressing a human-mouse chimera of ACE2 (chACE2) by CRISPR/Cas9-mediated gene editing in C57BL/6 mouse zygotes. ChACE2 mice express the chimeric viral receptor at physiological levels, enabling efficient SARS-CoV-2 infection without the heightened mortality seen in K18-hACE2 mice due to neuroinvasion. We used the chACE2 model to analyze SARS-CoV-2 infection as well as antiviral immune responses in vitro and in vivo. Similar to SARS-CoV-2 in elderly humans, aged chACE2 mice suffered from a highly aggravated disease. In addition, we found that a live attenuated vaccine candidate, LAVNsp16, induces robust mucosal and systemic immune responses in these mice despite being highly attenuated. The immunization with LAVNsp16 protected aged chACE2 mice from otherwise severe pathogenicity of SARS-CoV-2 by blocking viral replication of homologous and heterologous SARS-CoV-2 variants. The newly developed chACE2 model allows for longer observation periods of SARS-CoV-2 infection in mice, which is essential for assessing the immunogenicity of novel vaccine designs or monitoring viral pathogenicity over time. Immunization with LAVNsp16 induced robust and protective immune responses in young and aged mice, making viruses lacking 2'-O-methyltrasferse activity promising candidates for future live attenuated vaccine development.