3.1 Mouse Model:
Mice are the most widely used animal models in infectious disease research. Many inbred mouse strains have been tested for SARS-CoV infections. BALB/c mice, C57BL/6 (B6) and 129SvEv mice were challenged intranasally with Urbani strain (Glass, Subbarao, Murphy, & Murphy, 2004; Roberts et al., 2005). This resulted in a productive infection with peak titers on day 3 but there was an early clearance of the virus. Furthermore, mice did not lose weight, display signs of clinical disease, or develop pulmonary pathology that were observed in humans (Yin & Wunderink, 2018). Mutant mice such as B6, Beige, and CD1−/− (lack NK cell function and NK-T cells) and RAG1−/− mice (lack T and B lymphocytes) did not develop clinical disease (Glass et al., 2004). STAT1−/− mice showed some clinical signs of weight loss and bronchiolitis but were unable to show the pathological signs and mortality as observed in human cases (Frieman et al., 2010). Examination of the amino acid sequence of mouse and rat revealed differences in the amino acids at positions 353 and 82. Mice have histidine and humans have lysine at position 353 (F. Li, Li, Farzan, & Harrison, 2005) which may explain the reason why SARS-CoV replicated less efficiently in murine cells (W. Li et al., 2004). At position 82, mice ACE2 has serine whereas human ACE2 has methionine, but this difference did not prevent the S-protein of SARS-CoV to bind mice cells and internalize but reduced permissiveness of these cells to infection, thus, making it semi-permissive to the SARS-CoV. Therefore, there is a need to generate ”transgenic” mice or mouse should be adapted to become permissive to SARS-CoV infection.
Various transgenic mouse models like mice expressing the human ACE2 (hACE2) receptor and transgenic hACE2 under the control of an epithelial cell-specific promoter K18, showed infectivity with disease pathogenesis similar to that in humans (Bao et al., 2020; McCray et al., 2007), making them suitable for pathogenesis studies and evaluation of vaccines and other therapeutics against SARS-CoVs. The genetically engineered mice expressing the human ACE-2 (hACE2) were suitable for SARS-CoV infection and produced mortality (Dediego et al., 2008). Different contract research companies have produced hACE-2 expressing mice models and are already supplying it for SARS-CoV-2 studies. Taconic Biosciences, Rensselaer, New York, is supporting CoV research by providing ACE2-expressing mice that have been transplanted with human leukocyte antigen (HLA). The Jackson Laboratory (JAX), Maine, Florida is working to provide transgenic mouse expressing hACE-2 that were originally developed by Dr. Stanley Perlman at the University of Iowa. A study by Bao et. al . have used the mice model for SARS-CoV-2 and reported that transgenic hACE2 mice, when inoculated with BetaCoV/Wuhan/IVDC-HB-01/2020|EPI_ISL_402119 strain, developed clinical signs of COVID-19, including weight loss and interstitial pneumonia with histopathological signs in lungs(Table 1) (Bao et al., 2020). Another study by Sheahan et. al. showed that an orally bioavailable broad-spectrum, ribonucleoside analog β-D-N4-hydroxycytidine (NHC, EIDD-1931), is able to inhibit SARS-CoV-2 replication. Following intranasal challenge with a mouse-adapted 2019-nCoV/USA-WA1/2020 strain of SARS-CoV-2 virus; resulted in such as nasal congestion & difficulty in breathing that have been reported in COVID-19 patients (Sheahan et al., 2020).(Table 1) . Researchers at the University of Pittsburgh School of Medicine have developed a vaccine candidate “PittCoVacc” (Pittsburgh CoV Vaccine) using in vitro -generated fragments of viral protein. Administration of the vaccine to transgenic BABL/c mice resulted in the successful production of neutralizing antibodies against SARS-CoV-2 (E. Kim et al., 2020). Researchers at the NIH tested a mRNA vaccine developed by Moderna in Phase I clinical trials and confirmed immune responses by natural infection, which is now in its Phase II trial (”Moderna Moderna announces funding award from CEPI to accelerate development of messenger RNA (mRNA) vaccine against novel coronavirus; 2020. [accessed 2020 February15]
,”). administration of an inactivated SARS-CoV-2 vaccine, PiCoVacc, to ten BALB/c mice at multiple doses on day-0 and day-7 produced a good amount of SARS-CoV-2 spike and Receptor Binding Domain -specific immunoglobulin G (Ig G) responses (Q. Gao et al., 2020). All these studies signify the importance of mouse models in the testing of vaccines against SARS-CoV-2.
The major limitation with mice is that they are partially permissive to SARS-CoV infection due to the differences in the residues in ACE2 that are required for the cellular attachment and entry of SARS-CoV (F. Li et al., 2005). Mice adapted SARS-CoV-2 strains are suitable for use in pathogenesis, vaccine and therapeutic studies (Table 2) . Altogether certain genetically engineered mice models can be useful in pathogenesis studies and testing vaccines and antivirals because of their small size, body weight, easy availability, and cost effectiveness for initial studies including high throughput screening.