Since COVID-19 was declared a global pandemic in March 2020, a massive wave of mobilization has been seen from various companies and organizations at different fronts, to not only help fight the spreading of the virus but also treat the infected patients. From Formula 1 engineers teaming up with University research labs to develop Cpap devices, to college students participating in ventilator development challenges, many of these global efforts have been geared towards treating critically ill infected patients to reduce hospitalizations and mortality rates.
On another front, prominent infectious disease researchers are directing their efforts towards investigating the pathophysiology of SARS-CoV-2 and the varied symptoms associated with the COVID-19 disease it induces. In a recent paper published in Nature Immunology, Dr. Bailey and Dr. Diamond’s lab show that a humanized mouse model of SARS-CoV-2 from The Jackson Laboratory can effectively be infected with the virus and exhibit symptoms that closely match clinical manifestations of the severe form of COVID-19.
Animal models of COVID-19
Although animal models are effective tools to study infectious diseases, not all are susceptible to infection to the same degree. Some species demonstrate the ability to get infected with SARS-CoV-2 but do not present typical symptoms seen in the human disease. Hamsters, ferrets and NHP models, for example, develop moderate viral loads but have been shown to recover spontaneously. While these models remain useful to answer specific questions about the virus, developing one that is reproducible and faithful to the human manifestations is essential. Thus, allowing researchers to understand the pathophysiology of the disease better, test potential treatments and find effective vaccines.
Why humanizing mice is crucial for SARS-CoV-2 studies
Mice are among the most commonly used species for preclinical studies as they are easy to handle, breed and genetically modify. However, conventional strains of mice cannot easily be infected with SARS-CoV-2: mouse ACE2 receptors differ significantly from the human hACE2 and do not support the binding of the virus. While several techniques permit the introduction and expression of the hACE2 receptor in mice, that is not a sufficient condition to cause severe disease.
In their recent study, Dr. Bailey and Dr. Diamond’s lab uses a mouse model in which hACE2 expression is driven by the cytokeratins-18 (K18) gene promoter (K18-hACE2) and show that subjects can be effectively infected and recapitulate severe pulmonary disease phenotypes as seen in the clinical setting.
In their study, heterozygous K18-hACE c57BL/6J mice were administered 2.5 × 104 p.f.u. SARS-CoV-2 via intranasal inoculation. Following infection, histopathological and clinically relevant changes were measured up until 7 days post-infection (dpi). Their results show that the K18-hACE mice are highly susceptible to SARS-CoV-2 infection, presenting an important pro-inflammatory response marked by cellular infiltration leading to severe pneumonia. High viral titers (virus & virus RNA) are measured in the lungs, with the virus also spreading to other organs at lower levels. This response is accompanied by significant body weight loss (up to 25% after 7dpi). Histopathological findings show the accumulation of immune cells mainly in the alveolar spaces and peripheral tissues, which correlates with the sites of infection.
Among the clinically relevant measures, respiratory mechanics are evaluated with the flexiVent system to assess detailed pulmonary function at different time points post-intranasal inoculation of SARS-CoV-2. While various respiratory mechanics parameters remain stable until 4dpi, their results show a marked decline in respiratory function at 7dpi in all infected subjects relative to controls.
This notable decline is evidenced by reduced inspiratory capacity and stiffening of the lung parenchyma 7 days post-infection (dpi).
- Both Resistance (Rrs, airway constriction) and Elastance (Ers, airway stiffness) of the respiratory system significantly increase at that time point.
- Quasi-static compliance (Cst) shows decreased values, indicating reduced lung compliance and distensibility, which is indicative of structural changes and damage in the lungs.
- Measures of Tissue Damping (G) and Tissue Elastance (H), reflecting peripheral/tissue airway resistance and elastic recoil respectively, are both significantly increased in the infected group 7dpi, whereas Newtonian Resistance (Rn), reflecting the contribution of the large conducting airways, is largely unaffected in comparison to controls.
The results indicate SARS-CoV-2 infection targets disease induction in the alveoli and lung parenchyma (peripheral tissues). These findings are consistent with the histopathological findings in mice as well as the pulmonary function in humans, with viral pneumonia and respiratory failure, including COVID-19.
Results from this study show that K18-hACE2 mice are a relevant model to further investigate the pathogenesis of severe COVID-19. Dr. Bailey and Dr. Diamond’s lab work continues, as they now evaluate novel vaccines for COVID-19 and show their efficiency in protecting against infection in these humanized mice with just a single dose. Their more recent study demonstrates that a nasal delivery route is particularly more efficient than its intramuscular injection counterpart, providing a strong immune response in the body, and more importantly in both the nasal track and lower airways. They now aim to move towards testing this vaccine in non-human primates and humans, which will hopefully fast-track the development of efficient protection against the SARS-CoV-2 virus for the public.
If you would like to learn more about Dr. Diamond’s lab research, check out their lab website here.
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