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 the 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 the wild-type virus, Delta (B.1.617.2) and Omicron (B.1.1.529) 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 lungs, 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, brains, medulla oblongata and olfactory bulbs isolated from infected mice. Results: At 3 dpi, mice infected with wild type and Delta displayed significantly higher levels of viral RNA in the lungs than mice infected with Omicron, while in the brain, Delta and Omicron resulted in higher levels of viral RNA than with the wild type. Viral RNA was also detected in the medulla oblongata of mice infected by all these virus strains. At this time point, the mice infected with wild type and Delta displayed a marked upregulation of many inflammatory markers in the lungs. On the other hand, the upregulation of inflammatory markers was observed only in the brains of mice infected with Delta and Omicron. At the humane endpoint, we observed a significant increase in the levels of viral RNA in the lungs and brains of mice infected with wild type and Delta, which was accompanied by the elevated expression of many inflammatory markers. In contrast, mice which survived infection with the Omicron variant showed high levels of viral RNA and the upregulation of cytokine and chemokine expression only in the lungs at 8 dpi, suggesting that infection and inflammatory response by this variant is attenuated in the brain. Reduced RNA levels and the downregulation of inflammatory markers was also observed in the medulla oblongata and olfactory bulbs of mice infected with Omicron at 8 dpi as compared with mice infected with wild-type and Delta at the humane end point. Collectively, these data demonstrate that wild-type, Delta, and Omicron SARS-CoV-2 induce distinct levels of infection and inflammatory responses in K18-hACE2 mice. Notably, sustained brain infection accompanied by the upregulation of inflammatory markers is a critical outcome in mice infected with wild type and Delta but not Omicron.