Differential Pathogenesis of SARS-CoV-2 Variants of Concern in Human ACE2-Expressing Mice

Abstract

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has caused the current pandemic, resulting in millions of deaths worldwide. Increasingly contagious variants of concern (VoC) have fueled recurring global infection waves. A major question is the relative severity of the disease caused by previous and currently circulating variants of SARS-CoV-2. In this study, we evaluated the pathogenesis of SARS-CoV-2 variants in human ACE-2-expressing (K18-hACE2) mice. Eight-week-old K18-hACE2 mice were inoculated intranasally with a representative virus from the original B.1 lineage or from the emerging B.1.1.7 (alpha), B.1.351 (beta), B.1.617.2 (delta), or B.1.1.529 (omicron) lineages. We also infected a group of mice with the mouse-adapted SARS-CoV-2 (MA10). Our results demonstrate that B.1.1.7, B.1.351 and B.1.617.2 viruses are significantly more lethal than the B.1 strain in K18-hACE2 mice. Infection with the B.1.1.7, B.1.351, and B.1.617.2 variants resulted in significantly higher virus titers in the lungs and brain of mice compared with the B.1 virus. Interestingly, mice infected with the B.1.1.529 variant exhibited less severe clinical signs and a high survival rate. We found that B.1.1.529 replication was significantly lower in the lungs and brain of infected mice in comparison with other VoC. The transcription levels of cytokines and chemokines in the lungs of B.1- and B.1.1.529-infected mice were significantly less when compared with those challenged with other VoC. Together, our data provide insights into the pathogenesis of previous and circulating SARS-CoV-2 VoC in mice.

Keywords: ACE2-expressing mice; COVID-19; SARS-CoV-2 variants; inflammation; omicron.

Figure 1

Analysis of survival and body weight in K18-hACE2 mice following infection with SARS-CoV-2 VoC. Eight-week-old K18-hACE2 mice were inoculated intranasally with PBS (mock) or 10⁴ PFU of B.1 and individual variants (n = 10–15 mice per group). (A) Survival curve. (B) Body weight change in percentage. Values are the mean ± SEM.

Figure 2

Replication of SARS-CoV-2 VoC in the lungs and brain. Eight-week-old K18-hACE2 mice were inoculated intranasally with 10⁴ PFU of SARS-CoV-2 or variants. Groups of 3–7 mice were euthanized on day 3 and days 5–7 after infection, and tissues were collected. Virus titers were analyzed in the lungs and brain by plaque assay. The data are expressed as PFU/g of tissue. (A) Virus titer in day 3 lungs, (B) virus titer in day 5–7 lungs, (C) virus titer in day 3 brain, and (D) virus titer in day 5–7 brain. Each data point represents an individual mouse. *, p < 0.05.

Figure 3

Analysis of inflammatory response in the lungs. Eight-week-old K18-hACE2 mice were inoculated intranasally with PBS (mock) or 10⁴ PFU of SARS-CoV-2 or variants (n = 5–7 mice per group). Lungs and brain were harvested after extensive perfusion with PBS and RNA was extracted. The mRNA levels of (A) IL-6 and (B) CCL-2 were determined by qRT-PCR. Values are the mean ± SD. *, p < 0.05, **, p < 0.001.