Transcriptomic insights into the immune dynamics of wild-type mice challenged with SARS-CoV-2 Beta variant

Mice are useful small animal models to study the pathogenesis of SARS-CoV-2 infection. As the ancestral SARS-CoV-2 strains did not utilize murine Ace2 as a receptor, wild-type mice were not susceptible to the SARS-CoV-2 infection. Infection of human ACE2-expressing transgenic mice with SARS-CoV-2 induces fatal encephalitis, which is not commonly observed in humans. We and others have previously demonstrated the ability of the SARS-CoV-2 Beta variant to productively infect wild-type mice. Herein, we employed RNA-seq to investigate the transcriptomic landscapes in the lungs after the infection of wild-type mice with SARS-CoV-2 Beta variant.

We intranasally infected 6-week-old wild-type C57BL/6J mice with the SARS-CoV-2 (B.1.351 strain) and collected lungs at 3- and 6-days post-infection for RNA-sequencing. We used the Limma-Voom package to identify differentially expressed genes (DEGs) and the fgsea package for pathway enrichment analysis. We used Cytoscape to identify hub genes and gene networks. Lastly, we employed RT-qPCR and multiplex assay to validate the RNA-seq data.

Using a cutoff of an adjusted p-value below 0.05 and an absolute log2 fold change value greater than 0.75, we identified 285 DEGs on day 3 and 46 DEGs on day 6. The canonical pathways analysis showed that several key pathways such as apoptosis and cytokine response were upregulated in the infected lungs. Protein-protein interaction analyses identified innovative target genes such as Kif11, Ccna2, and Aurkb. We also identified the top 10 hub genes that included Prc1, Ube2c, Ccnb2, Ncapg, Aurkb, Cep55, Mki67, Dlgap5, Ccna2, and Kif11. RT-qPCR analysis for Tnfa, Il6, Ccl2, and Ccl3 further validated the RNA-seq analysis. Consistent with gene expression results, we detected significantly increased protein levels of various inflammatory mediators such as IL-6, CCL2, CXCL2, and CXCL10 in the infected lungs.

This is the first transcriptomic analysis of the lungs of wild-type mice infected with a clinical isolate of SARS-CoV-2. Our findings provide a further understanding of the pathogenic events that occur in this mouse model of SARS-CoV-2 infection.