Mouse-Adapted SARS-CoV-2 MA10 Strain Displays Differential Pulmonary Tropism and Accelerated Viral Replication, Neurodissemination, and Pulmonary Host Responses in K18-hACE2 Mice.

Several models were developed to study the pathogenicity of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) as well as the in vivo efficacy of vaccines and therapeutics. Since wild-type mice are naturally resistant to infection by ancestral SARS-CoV-2 strains, several transgenic mouse models expressing human angiotensin-converting enzyme 2 (hACE2) were developed. An alternative approach has been to develop mouse-adapted SARS-CoV-2 strains. Here, we compared the clinical progression, viral replication kinetics and dissemination, pulmonary tropism, and host innate immune response dynamics between the mouse-adapted MA10 strain and its parental strain (USA-WA1/2020) following intranasal inoculation of K18-hACE2 mice, a widely used model. Compared to its parental counterpart, the MA10 strain induced earlier clinical decline with significantly higher viral replication and earlier neurodissemination. Importantly, the MA10 strain also showed a wider tropism, with infection of bronchiolar epithelia. While both SARS-CoV-2 strains induced comparable pulmonary cytokine/chemokine responses, many proinflammatory and monocyte-recruitment chemokines, such as interleukin-6 (IL-6), tumor necrosis factor alpha (TNF-α), IP-10/CXCL10, and MCP-1/CCL2, showed an earlier peak in MA10-infected mice. Furthermore, both strains induced a similar downregulation of murine Ace2, with only a transient downregulation of Tmprss2 and no alterations in hACE2 expression. Overall, these data demonstrate that in K18-hACE2 mice, the MA10 strain has a pulmonary tropism that more closely resembles SARS-CoV-2 tropism in humans (airways and pneumocytes) than its parental strain. Its rapid replication and neurodissemination and early host pulmonary responses can have a significant impact on the clinical outcomes of infection and are, therefore, critical features to consider for study designs using these strains and mouse model. IMPORTANCE The COVID-19 pandemic, caused by SARS-CoV-2, is still significantly impacting health care systems around the globe. Refined animal models are needed to study SARS-CoV-2 pathogenicity as well as efficacy of vaccines and therapeutics. In line with this, thorough evaluation of animal models and virus strains/variants are paramount for standardization and meaningful comparisons. Here, we demonstrated differences in replication dynamics between the Wuhan-like USA-WA1/2020 strain and the derivative mouse-adapted MA10 strain in K18-hACE2 mice. The MA10 strain showed accelerated viral replication and neurodissemination, differential pulmonary tropism, and earlier pulmonary innate immune responses. The observed differences allow us to better refine experimental designs when considering the use of the MA10 strain in the widely utilized K18-hACE2 murine model.

Development of a Mouse-Adapted MERS Coronavirus.

First identified in 2012, Middle East respiratory syndrome coronavirus (MERS-CoV) is a novel virus that can cause acute respiratory distress syndrome (ARDS), multiorgan failure, and death, with a case fatality rate of ~35%. An animal model that supports MERS-CoV infection and causes severe lung disease is useful to study pathogenesis and evaluate therapies and vaccines. The murine dipeptidyl peptidase 4 (Dpp4) protein is not a functional receptor for MERS-CoV; thus, mice are resistant to MERS-CoV infection. We generated human DPP4 knock-in (hDPP4 KI) mice by replacing exons 10-12 at the mouse Dpp4 locus with exons 10-12 from the human DPP4 gene. The resultant human DPP4 KI mice are permissive to MERS-CoV (HCoV-EMC/2012 strain) infection but develop no disease. To generate a mouse model with associated morbidity and mortality from respiratory disease, we serially passaged HCoV-EMC/2012 strain in the lungs of young hDPP4 KI mice. After 30 in vivo passages, an adapted virus clone was isolated and designated MERSMA6.1.2. This virus clone produced significantly higher titers than the parental clone in the lungs of hDPP4 KI mice and caused diffuse lung injury and a fatal respiratory infection. In this chapter, we will describe in detail the procedures used to mouse adapt MERS-CoV by serial passage of the virus in lungs. We also describe the methods used to isolate virus clones and characterize virus infection.