Animal Models of COVID-19: Transgenic Mouse Model

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the causative agent of coronavirus disease 2019 (COVID-19), emerged in December 2019 in Wuhan, China, and rapidly spread throughout the world, threatening global public health. An animal model is a valuable and a crucial tool that allows understanding of nature in the pathogenesis of SARS-CoV-2 and its associated COVID-19 disease. Here we introduce detailed protocols of SARS-CoV-2 infection and COVID-19 disease using C57BL/6 (B6) transgenic mice expressing the human angiotensin-converting enzyme 2 (hACE2) from the human cytokeratin 18 promoter (K18 hACE2). To mimic natural SARS-CoV-2 infection, K18 hACE2 transgenic mice are infected intranasally under anesthesia. Upon infection, viral pathogenesis is determined by monitoring changes in body weight (morbidity) and monitoring survival (mortality), cytokine/chemokine responses, gross-lung pathology, histopathology, and viral replication in tissues. The presence of the virus and viral replication is evaluated by immunohistochemistry (IHC) and viral titrations, respectively, from the upper (nasal turbinate) and the lower (lungs) respiratory tracts, and nervous system (brain). Also, the immune response to SARS-CoV-2 infection is measured by cytokine/chemokine enzyme-linked immunosorbent assay (ELISA) from lung, spleen and brain homogenates to characterize the cytokine storm that hallmarks as one of the major causes of death caused by SARS-CoV-2 infection. This small rodent animal model based on the use of K18 hACE2 transgenic mice represents an excellent option to understand the pathogenicity of natural SARS-CoV-2 strains and its recently described Variants of Concern (VoC), and will be applicable to the identification and characterization of prophylactic (vaccine) and therapeutic (antiviral and/or neutralizing monoclonal antibodies) strategies for the prevention or treatment of SARS-CoV-2 infection or its associated COVID-19 disease.

Contribution of SARS-CoV-2 Accessory Proteins to Viral Pathogenicity in K18 Human ACE2 Transgenic Mice

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the viral pathogen responsible for the current coronavirus disease 2019 (COVID-19) pandemic. As of 19 May 2021, John Hopkins University’s COVID-19 tracking platform reported 3.3 million deaths associated with SARS-CoV-2 infection. Currently, the World Health Organization has granted emergency use listing (EUL) to six COVID-19 vaccine candidates. However, much of the pathogenesis observed during SARS-CoV-2 infection remains elusive. To gain insight into the contribution of individual accessory open reading frame (ORF) proteins in SARS-CoV-2 pathogenesis, we used our recently described reverse-genetics system approach to successfully engineer recombinant SARS-CoV-2 (rSARS-CoV-2) constructs; we removed individual viral ORF3a, −6, −7a, −7b, and −8 proteins from them, and we characterized the resulting recombinant viruses in vitro and in vivo. Our results indicate differences in plaque morphology, with ORF-deficient (ΔORF) viruses producing smaller plaques than those of the wild type (rSARS-CoV-2/WT). However, growth kinetics of ΔORF viruses were like those of rSARS-CoV-2/WT. Interestingly, infection of K18 human angiotensin-converting enzyme 2 (hACE2) transgenic mice with the ΔORF rSARS-CoV-2s identified ORF3a and ORF6 as the major contributors of viral pathogenesis, while ΔORF7a, ΔORF7b, and ΔORF8 rSARS-CoV-2s induced pathology comparable to that of rSARS-CoV-2/WT. This study demonstrates the robustness of our reverse-genetics system to generate rSARS-CoV-2 constructs and the major role for ORF3a and ORF6 in viral pathogenesis, providing important information for the generation of attenuated forms of SARS-CoV-2 for their implementation as live attenuated vaccines for the treatment of SARS-CoV-2 infection and associated COVID-19.

IMPORTANCE Despite great efforts put forward worldwide to combat the current coronavirus disease 2019 (COVID-19) pandemic, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) continues to be a human health and socioeconomic threat. Insights into the pathogenesis of SARS-CoV-2 and the contribution of viral proteins to disease outcome remain elusive. Our study aims (i) to determine the contribution of SARS-CoV-2 accessory open reading frame (ORF) proteins to viral pathogenesis and disease outcome and (ii) to develop a synergistic platform combining our robust reverse-genetics system to generate recombinant SARS-CoV-2 constructs with a validated rodent model of infection and disease. We demonstrate that SARS-CoV-2 ORF3a and ORF6 contribute to lung pathology and ultimately disease outcome in K18 hACE2 transgenic mice, while ORF7a, ORF7b, and ORF8 have little impact on disease outcome. Moreover, our combinatory platform serves as a foundation for generating attenuated forms of the virus to develop live attenuated vaccines for the treatment of SARS-CoV-2.