Administration of aerosolized SARS-CoV-2 to K18-hACE2 mice uncouples respiratory infection from fatal neuroinvasion

Abstract

The development of a tractable small animal model faithfully reproducing human coronavirus disease 2019 pathogenesis would arguably meet a pressing need in biomedical research. Thus far, most investigators have used transgenic mice expressing the human ACE2 in epithelial cells (K18-hACE2 transgenic mice) that are intranasally instilled with a liquid severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) suspension under deep anesthesia. Unfortunately, this experimental approach results in disproportionate high central nervous system infection leading to fatal encephalitis, which is rarely observed in humans and severely limits this model’s usefulness. Here, we describe the use of an inhalation tower system that allows exposure of unanesthetized mice to aerosolized virus under controlled conditions. Aerosol exposure of K18-hACE2 transgenic mice to SARS-CoV-2 resulted in robust viral replication in the respiratory tract, anosmia, and airway obstruction but did not lead to fatal viral neuroinvasion. When compared with intranasal inoculation, aerosol infection resulted in a more pronounced lung pathology including increased immune infiltration, fibrin deposition, and a transcriptional signature comparable to that observed in SARS-CoV-2–infected patients. This model may prove useful for studies of viral transmission, disease pathogenesis (including long-term consequences of SARS-CoV-2 infection), and therapeutic interventions.

The neuroinvasiveness, neurotropism, and neurovirulence of SARS-CoV-2

“…It has already been shown that vaccination partly protects against the development of long COVID [99]. Other variables that might influence the development of CNS disease include, for example, the infection dose and primary replication site of the virus [73].…”

“…Furthermore, multifocal microgliosis and astrogliosis were reported in older patients [71], although it could not be ruled out that these were associated with host factors [72]. Perivascular and parenchymal infiltrations of CD8 + cytotoxic T cells and macrophages have been reported postmortem in patients with COVID-19 and in intranasally inoculated mice at 6 days post inoculation [61,71,73]. Extensive inflammatory responses, such as astrogliosis, activation of microglia, and perivascular cuffing of T cells, were detected postmortem in both white and gray matter of patient brains regardless of their COVID-19 disease severity, a finding that appeared most pronounced in the cranial medulla oblongata and olfactory bulb [74].…”

“…Mice are not naturally sensitive to SARS-CoV-2 replication, but transgenic expression of human ACE2 or transduction of mice with adenovirus or adeno-associated viruses expressing human ACE2 sensitizes mice to SARS-CoV-2 infection. Although the tissue and cell ACE2 expression in these models differs from that in humans, these mice models have been used to study the effect of both virus replication [79][80][81][82] and inflammation-induced changes [73,83,84] in the brain. Although the observations cannot be extrapolated directly to humans, they provide important insights into the different mechanisms that could contribute to the large spectrum of neurological complications following SARS-CoV-2 infection.…”

Genetically diverse mouse models of SARS-CoV-2 infection reproduce clinical variation in type I interferon and cytokine responses in COVID-19

“…These limitations may be addressed by further modifications of the CC founder mouse models engineered to express hACE2 under more normal physiological conditions as has been achieved with humanization of the mouse Ace2 gene, H353K 61 . In addition, recent reports of limited CNS infection in K18-hACE2 mice infected with SARS-CoV-2 via aerosolized route of inoculation indicating that optimized experimental procedures may further improve the mouse model 62 . Alternatively, many of the strain backgrounds used here (including WSB) demonstrated relatively low levels of neuro-invasion and could be further evaluated as a more useful model of certain COVID-19 associated pathology, including the disorder known as long-COVID.…”

Fatal Neurodissemination and SARS-CoV-2 Tropism in K18-hACE2 Mice Is Only Partially Dependent on hACE2 Expression

“…However, the confounding impact of neuroinvasion and neurodissemination and its role in the clinical decline of SARS-CoV-2 infected K18-hACE2 mice is becoming more readily acknowledged [ 41 , 42 , 43 , 44 ]. Furthermore, administration of aerosolized SARS-CoV-2 to K18-hACE2 mice resulted solely in respiratory infection and limited clinical disease at 6 days post infection (6 dpi) in contrast with those inoculated intranasally, suggesting that severe clinical outcome in K18-hACE2 mice is attributed to CNS disease [ 45 ].…”