ABSTRACT
The COVID-19 pandemic has been fueled by novel variants of concern (VOC) that have increased transmissibility, receptor binding affinity, and other properties that enhance disease. The goal of this study is to characterize unique pathogenesis of the Delta VOC strain in the K18-hACE2-mouse challenge model. Challenge studies suggested that the lethal dose of Delta was higher than Alpha or Beta strains. To characterize the differences in the Delta strains pathogenesis, a time-course experiment was performed to evaluate the overall host response to Alpha or Delta variant challenge. qRT-PCR analysis of Alpha- or Delta- challenged mice revealed no significant difference between viral RNA burden in the lung, nasal wash or brain. However, histopathological analysis revealed high lung tissue inflammation and cell infiltration following Delta- but not Alpha-challenge at day 6. Additionally, pro-inflammatory cytokines were highest at day 6 in Delta-challenged mice suggesting enhanced pneumonia. Total RNA-sequencing analysis of lungs comparing infected to uninfected mice revealed that Alpha-challenged mice have more total genes differentially activated, conversely, Delta-challenged mice have a higher magnitude of differential gene expression. Delta-challenged mice have increased interferon-dependent gene expression and IFN-{gamma} production compared to Alpha. Analysis of TCR clonotypes suggested that Delta challenged mice have increased T-cell infiltration compared to Alpha challenged. Our data suggest that Delta has evolved to engage interferon responses in a manner that may enhance pathogenesis. The in vivo and in silico observations of this study underscore the need to conduct experiments with VOC strains to best model COVID-19 when evaluating therapeutics and vaccines. ImportanceThe Delta variant of SARS-CoV-2 is known to be more transmissible and cause severe disease in human hosts due to mutations in its genome that are divergent from previous variants of concern (VOC). Our study evaluates the pathogenesis of Delta in the K18-hACE2 mouse model compared to the Alpha VOC. We observed that relative to Alpha, Delta challenge results in enhanced inflammation and tissue damage with stronger antiviral responses. These observations provide insight into Deltas unique pathogenesis.
ABSTRACT
SARS-CoV-2 variants of concern (VoCs) are impacting responses to the COVID-19 pandemic. Here we present a comparison of the SARS-CoV-2 USA-WA1/2020 (WA-1) strain with B.1.1.7 and B.1.351 VoCs and identify significant differences in viral propagation in vitro and pathogenicity in vivo using K18-hACE2 transgenic mice. Passive immunization with plasma from an early pandemic SARS-CoV-2 patient resulted in significant differences in the outcome of VoC-infected mice. WA-1-infected mice were protected by plasma, B.1.1.7-infected mice were partially protected, and B.1.351-infected mice were not protected. Serological correlates of disease were different between VoC-infected mice, with B.1.351 triggering significantly altered cytokine profiles than other strains. In this study, we defined infectivity and immune responses triggered by VoCs and observed that early 2020 SARS-CoV-2 human immune plasma was insufficient to protect against challenge with B.1.1.7 and B.1.351 in the mouse model.
