K18-hACE2 mice develop respiratory disease resembling severe COVID-19

Abstract

SARS-CoV-2 emerged in late 2019 and resulted in the ongoing COVID-19 pandemic. Several animal models have been rapidly developed that recapitulate the asymptomatic to moderate disease spectrum. Now, there is a direct need for additional small animal models to study the pathogenesis of severe COVID-19 and for fast-tracked medical countermeasure development. Here, we show that transgenic mice expressing the human SARS-CoV-2 receptor (angiotensin-converting enzyme 2 [hACE2]) under a cytokeratin 18 promoter (K18) are susceptible to SARS-CoV-2 and that infection resulted in a dose-dependent lethal disease course. After inoculation with either 104 TCID50 or 105 TCID50, the SARS-CoV-2 infection resulted in rapid weight loss in both groups and uniform lethality in the 105 TCID50 group. High levels of viral RNA shedding were observed from the upper and lower respiratory tract and intermittent shedding was observed from the intestinal tract. Inoculation with SARS-CoV-2 resulted in upper and lower respiratory tract infection with high infectious virus titers in nasal turbinates, trachea and lungs. The observed interstitial pneumonia and pulmonary pathology, with SARS-CoV-2 replication evident in pneumocytes, were similar to that reported in severe cases of COVID-19. SARS-CoV-2 infection resulted in macrophage and lymphocyte infiltration in the lungs and upregulation of Th1 and proinflammatory cytokines/chemokines. Extrapulmonary replication of SARS-CoV-2 was observed in the cerebral cortex and hippocampus of several animals at 7 DPI but not at 3 DPI. The rapid inflammatory response and observed pathology bears resemblance to COVID-19. Taken together, this suggests that this mouse model can be useful for studies of pathogenesis and medical countermeasure development.

Comparison of transgenic and adenovirus hACE2 mouse models for SARS-CoV-2 infection

“…We, and others, detected hACE2 expression predominantly on alveolar epithelial cells, with some expression in bronchiolar epithelial cells following transduction with a dose of 2.5x10 8 PFU Ad-hACE2 (26,56). In this study, we also detected some transduction in endothelial cells and alveolar macrophages, which is consistent with the known in vivo cellular tropism for Ad vectors based on HAdV-C5 in mice (30,33,40).The K18-hACE2 transgenic model has been well characterized for SARS-CoV-1 (20,35), but until more recently had not been comprehensively for SARS-CoV-2 infection, in terms of measuring morbidity and viral dissemination with subsequent replication in extra-pulmonary organs (41,58). In this study, we wanted to perform a head-tohead comparison of the K18-hACE2 model and the Ad-hACE2 murine model in supporting SARS-CoV-2 viral replication in both BALB/c and B6 mice in parallel.…”

“…The brains of K18-hACE2 mice on D5 and D6 had strong red (Nova Red) labelling of neuronal cytoplasm within the cerebral cortex, which included strong labelling of the cell bodies as well as the dendrites and axons (Fig.4D). The timing of N expression in the brain is consistent with other studies which observed little expression prior to D3 post-challenge, but increased staining after D3 (41). Sections of tissues imaged at 20X magnification are shown in Supplementary.Fig.2.…”

“…Studies in which the homologs of these receptors were delivered either to non-permissive cells in vitro (35), or in vivo to the mouse lung by overexpression via viral vectors (25,34), could sensitize to infection. Since the emergence of the SARS-CoV-2 virus, investigators have rapidly responded to the pandemic by evaluating the potential for already-generated (54), but not widely available, hACE2 transgenic mice in supporting SARS-CoV-2 infection and pathogenesis (22,23,41). Although useful, these models produced variable results in which some supported low level SARS-CoV-2 replication with minimal morbidity or lung pathology (22), and others exhibited signs of local and systemic disease which partially mimic symptoms of severe human COVID-19 infection, including increased mortality in male mice (23).…”

“…Evaluation of brain H&E sections determined that no significant microscopic findings were observed. However, other studies have reported histological changes in the brains of K18-hACE2 mice when higher SARS-CoV-2 challenge doses were used (41).…”

“…In agreement with published studies, no signs of neurological deficit (ie. limb paralysis, convulsions) were observed (41). In groups allocated for histology, the experiment was terminated on D6 post-challenge as the mice reached the humane endpoint and had to be euthanized.…”