Comparison of the Pathogenicity of SARS-CoV-2 Delta and Omicron Variants by Analyzing the Expression Patterns of Immune Response Genes in K18-hACE2 Transgenic Mice

Abstract

Background: The recently emerged variants of the severe acute respiratory coronavirus 2 (SARS-CoV-2) pose a threat to public health. Understanding the pathogenicity of these variants is a salient factor in the development of effective SARS-CoV-2 therapeutics. This study aimed to compare the expression patterns of genes involved in immune responses in K18-hACE2 mice infected with the wild-type, Delta, and Omicron SARS-CoV-2 variants. Methods: K18-hACE2 mice were intranasally infected with either wild-type (B.1), Delta (B.1.617.2), or Omicron (B.1.1.529) variants. On day 6 post-infection, lung, brain, and kidney tissues were collected from each variant-infected group. The mRNA expression levels of 39 immune response genes in all three groups were compared by RT-qPCR. Viral titers were measured using the median tissue culture infectious dose (TCID50) assay and expressed as Log10 TCID50/0.1 g. The statistical significance of the differences in gene expression was determined by one-way analysis of variance (ANOVA) (alpha = 0.05). Results: The expression of toll-like receptors (TLRs) was upregulated in the lung and brain tissues of the wild-type- and Delta-infected groups but not in those of the Omicron-infected group. The highest expression of cytokines, including interleukin (IL)-1α, IL-1β, IL-17α, interferon, and tumor necrosis factors, was observed in the lungs of mice infected with the wild-type variant. Additionally, CCL4, CCL11, CXCL9, and CXCL10 were upregulated (>3-fold) in wild-type-infected mice, with markedly higher expressions in the brain than in the lungs. Most of the apoptotic factors were mainly expressed in the brain tissues of Omicron-infected mice (caspase 8, caspase 9, p53, Bax, Bak, BCL-2, and Bcl-XL), whereas neither the lung nor kidney showed more than 3-fold upregulation of these apoptotic factors. Conclusions: Collectively, our findings revealed that the wild-type SARS-CoV-2 variant exhibited the highest pathogenicity, followed by the Delta variant, then the Omicron variant.