COVID-19, caused by SARS-CoV-2, has spread worldwide with a dire disaster situation.To urgently investigate the pathogenicity of COVID-19 and develop vaccines and therapeutics, animal models that are highly susceptible to SARS-CoV-2 infection are needed. In the present study, we established an animal model highly susceptible to SARS-CoV-2 via the intratracheal tract infection in CAG-promoter-driven human angiotensin-converting enzyme 2 transgenic (CAG-hACE2) mice. The CAG-hACE2 mice showed several severe symptoms of SARS-CoV-2 infection, with definitive weight loss and subsequent death. Acute lung injury with elevated cytokine and chemokine levels was observed at an early stage of infection in CAG-hACE2 mice infected with SARS-CoV-2. The analysis of the hACE2 gene in CAG-hACE2 mice revealed that more than 15 copies of hACE2 genes were tandemly integrated into the mouse genome, supporting the high susceptibility to SARS-CoV-2. In the developed model, immunization with viral antigen or injection of plasma from immunized mice prevented body weight loss and lethality due to infection with SARS-CoV-2. These results indicate that a highly susceptible model of SARS-CoV-2 infection in CAG-hACE2 mice via the intratracheal tract is suitable for evaluating vaccines and therapeutic medicines.

Angiotensin-converting enzyme 2 (ACE2) is a receptor for cell entry of SARS-CoV-2, and recombinant soluble ACE2 protein inhibits SARS-CoV-2 infection as a decoy. ACE2 is the carboxypeptidase to degrade angiotensin II (Ang II) to angiotensin 1-7 and improves the pathologies of cardiovascular disease and acute lung injury. To address whether the carboxypeptidase enzymatic activity of ACE2 is protective against COVID-19, we investigated the effects of B38-CAP, an ACE2-like enzyme, on SARS-CoV-2-induced lung injury. Expression of ACE2 protein was significantly downregulated in the lungs of SARS-CoV-2-infected hamsters. Recombinant S1 domain or receptor-binding domain (RBD) of the SARS-CoV-2 Spike protein also directly downregulated ACE2 expression and elevated Ang II levels and considerably worsened acid-induced lung injury in hamsters. Treatment with B38-CAP downregulated Spike RBDinduced high Ang II levels, severe inflammation and pulmonary edema through its ACE2-like enzymatic activity. Consistently, elevated cytokine mRNA levels and impaired lung functions were improved by B38-CAP treatment. Moreover, in SARS-CoV-2-infected humanized ACE2 transgenic mice, B38-CAP significantly improved the pathologies of lung injury, alleviated the cytokine storms and downregulated viral RNA levels. These results provide the first experimental in vivo evidence that increasing ACE2-like enzymatic activity is a potential and powerful therapeutic strategy for lung pathologies in COVID-19.