Differential outcomes of infection by wild-type SARS-CoV-2 and the B.1.617.2 and B.1.1.529 variants of concern in K18-hACE2 transgenic mice

Background SARS-CoV-2 is a respiratory virus with neurological complications including loss of smell and taste, headache, and confusion that can persist for months or longer. Severe neuronal cell damage has also been reported in some cases. The objective of this study was to compare the infectivity of Wild-type virus, Delta and Omicron variants in transgenic mice that express the human angiotensin-converting enzyme 2 (hACE2) receptor under the control of the keratin 18 promoter (K18) and characterize the progression of infection and inflammatory response in the lung, brain medulla oblongata and olfactory bulbs of these animals. We hypothesized that Wild-type, Delta and Omicron differentially infect K18-hACE2 mice, thereby inducing distinct cellular responses.

Methods K18-hACE2 female mice were intranasally infected with Wild-type, Delta, or Omicron variants and euthanized either at 3 days post-infection (dpi) or at the humane endpoint. None of the animals infected with the Omicron variant reached the humane endpoint and were euthanized at day 8 dpi. Virological and immunological analyses were performed in the lungs, olfactory bulbs, medulla oblongata, and brains.

Results Mice infected with Wild-type and Delta display higher levels of viral RNA in the lungs than mice infected with Omicron at 3dpi. Viral RNA levels in the brains of mice infected with the Wild-type virus were however significantly lower than those observed in mice infected with either Delta or Omicron at 3dpi. Viral RNA was also detected in the medulla oblongata of mice infected by all these virus strains at 3dpi. At this time point, mice infected with the Delta virus display a marked upregulation of inflammatory makers both in the lungs and brains. Upregulation of inflammatory markers was also observed in the brains of mice infected with Omicron but not in mice infected with the Wild-type virus, suggesting that during the initial phase of the infection only the Delta and Omicron variants induce strong inflammatory response in the brain. At the humane endpoint/8dpi, mice infected by any of these strains display elevated levels of viral RNA and upregulation of a subset of inflammatory markers in the lungs. There was also a significant increase in viral RNA in the brains of mice infected with Wild-type and Delta, as compared to 3dpi. This was accompanied by an increase in the expression of most cytokines and chemokines. In contrast, mice infected with the Omicron variant showed low levels of viral RNA and downregulation of cytokines and chemokines expression at 8dpi, suggesting that brain inflammation by this variant is attenuated. Reduced RNA levels and downregulation of inflammatory markers was also observed in the medulla oblongata and olfactory bulbs of mice infected with Omicron, while infection by Wild-type and Delta resulted in high levels of viral RNA and increased expression of inflammatory makers in these organs.