Chronological brain lesions after SARS-CoV-2 infection in hACE2-transgenic mice

Abstract

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) causes respiratory disease, but it can also affect other organs including the central nervous system. Several animal models have been developed to address different key questions related to Coronavirus Disease 2019 (COVID-19). Wild-type mice are minimally susceptible to certain SARS-CoV-2 lineages (beta and gamma variants), whereas hACE2-transgenic mice succumb to SARS-CoV-2 and develop a fatal neurological disease. In this article, we aimed to chronologically characterize SARS-CoV-2 neuroinvasion and neuropathology. Necropsies were performed at different time points, and the brain and olfactory mucosa were processed for histopathological analysis. SARS-CoV-2 virological assays including immunohistochemistry were performed along with a panel of antibodies to assess neuroinflammation. At 6 to 7 days post inoculation (dpi), brain lesions were characterized by nonsuppurative meningoencephalitis and diffuse astrogliosis and microgliosis. Vasculitis and thrombosis were also present and associated with occasional microhemorrhages and spongiosis. Moreover, there was vacuolar degeneration of virus-infected neurons. At 2 dpi, SARS-CoV-2 immunolabeling was only found in the olfactory mucosa, but at 4 dpi intraneuronal virus immunolabeling had already reached most of the brain areas. Maximal distribution of the virus was observed throughout the brain at 6 to 7 dpi except for the cerebellum, which was mostly spared. Our results suggest an early entry of the virus through the olfactory mucosa and a rapid interneuronal spread of the virus leading to acute encephalitis and neuronal damage in this mouse model.

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

“…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.…”

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

“…Early reports communicated lethality to be associated primarily with severe lung inflammation and impaired respiratory function, suggesting that the K18-hACE2 model recapitulates features of the respiratory disease observed in severe cases of COVID-19 diagnosed with ARDS [ 28 , 36 ]. 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 ].…”

Neuroinvasion and neurotropism by SARS-CoV-2 variants in the K18-hACE2 mouse

“…Neuronal death via apoptosis was suspected [44]. Other studies reported perivascular cuffs or vasculitis, neuronal degeneration and necrosis, satellitosis, parenchymal edema, and occasional mi-crothrombi [42,45,54–56], all at day 7 pi. The present study confirms that SARS-CoV-2 variants readily infect neurons in the K18-hACE2 mice, with viral protein accumulation in the entire cytoplasm including the cell processes but does not provide evidence of neu-ronal cell death in association with infection, indicating that SARS-CoV-2 has no direct cytopathic effect on neurons.…”

“…Activated microglia may itself undergo a phenotypic shift and display exaggerated release of proinflammatory mediators and aberrant phagocytic activity, inducing neurodegeneration [64]. This could explain the diffuse astrogliosis reported from other mouse and human studies [9,42,45] as well as neuronophagia by microglial cells which was seen in some human COVID-19 patients [23] and one mouse study [42].…”

“…Infection of the brain can be associated with nonsuppurative (meningo)encephalitis [26,[42][43][44]. With high infectious doses (105 PFU), very high viral loads were found in the brain at a time when lung burdens had already decreased, in association with upregulation of IFN-α as well as proinflammatory cytokine and chemokine transcription; the authors proposed that clinical symptoms or lethal outcome of infection in these mice was a consequence of neuroinvasion [44][45]. Another study using an even higher viral inoculum (106 PFU) elicited CNS signs (tremors, proprioceptive defects, abnormal gait, imbalance) by day 6/7 after infection [42].…”