The Delta SARS-CoV-2 Variant of Concern Induces Distinct Pathogenic Patterns of Respiratory Disease in K18-hACE2 Transgenic Mice Compared to the Ancestral Strain from Wuhan

Abstract

Compared to the original ancestral strain of SARS-CoV-2, the Delta variant of concern has shown increased transmissibility and resistance toward COVID-19 vaccines and therapies. However, the pathogenesis of the disease associated with Delta is still not clear. In this study, using K18-hACE2 transgenic mice, we assessed the pathogenicity of the Delta variant by characterizing the immune response following infection. We found that Delta induced the same clinical disease manifestations as the ancestral SARS-CoV-2, but with significant dissemination to multiple tissues, such as brain, intestine, and kidney. Histopathological analysis showed that tissue pathology and cell infiltration in the lungs of Delta-infected mice were the same as in mice infected with the ancestral SARS-CoV-2. Delta infection caused perivascular inflammation in the brain and intestinal wall thinning in K18-hACE2 transgenic mice. Increased cell infiltration in the kidney was observed in both ancestral strain- and Delta-infected mice, with no clear visible tissue damage identified in either group. Interestingly, compared with mice infected with the ancestral strain, the numbers of CD45+ cells, T cells, B cells, inflammatory monocytes, and dendritic cells were all significantly lower in the lungs of the Delta-infected mice, although there was no significant difference in the levels of proinflammatory cytokines between the two groups. Our results showed distinct immune response patterns in the lungs of K18-hACE2 mice infected with either the ancestral SARS-CoV-2 or Delta variant of concern, which may help to guide therapeutic interventions for emerging SARS-CoV-2 variants. IMPORTANCE SARS-CoV-2 variants, with the threat of increased transmissibility, infectivity, and immune escape, continue to emerge as the COVID-19 pandemic progresses. Detailing the pathogenesis of disease caused by SARS-CoV-2 variants, such as Delta, is essential to better understand the clinical threat caused by emerging variants and associated disease. This study, using the K18-hACE2 mouse model of severe COVID-19, provides essential observation and analysis on the pathogenicity and immune response of Delta infection. These observations shed light on the changing disease profile associated with emerging SARS-CoV-2 variants and have potential to guide COVID-19 treatment strategies.

Keywords: Delta variant; SARS-CoV-2; coronavirus; mouse model.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

FIG 1

Disease and weight loss of the mice infected by SARS-CoV-2 ancestral strain and Delta variant. Eight-week-old female K18-hACE2 mice were intranasally inoculated with PBS (mock group) or 104 PFU of SARS-CoV-2 ancestral or Delta strain (n = 8). Mock-infected mice (n = 4) received 20 μL sterile DMEM with 2% FBS intranasally. Individual mice were monitored daily until reaching a clinical score of >3, when twice-daily monitoring was performed. (A) Weight change was monitored and compared with the initial weight on day 0. (B) Mice were given a disease score according to general health (eating habit, locomotion, and behavior), appearance, and weight loss. All values represent the means ± standard errors of the mean from one experiment. Data were analyzed using two-way analysis of variance (ANOVA) with Bonferroni post hoc test (ns, not significant).

FIG 2

Viral burden in tissues of the mice infected by SARS-CoV-2 ancestral strain and Delta variant. Eight-week-old female K18-hACE2 mice were intranasally inoculated with PBS (mock group) or 104 PFU of SARS-CoV-2 ancestral or Delta strain (n = 5). At 6 dpi, the titers of infectious virus in the lung (A), brain (B), intestine (C), and kidney (D) were determined by plaque assay. The copy numbers of viral genome in lung (E), brain (F), intestine (G), kidney (H), heart (I), and serum (J) were determined by probe-based qRT-PCR. Each symbol represents the mean titer for one mouse. Data were analyzed by Mann-Whitney test (**, P < 0.01; ns, not significant).

FIG 3

Histopathological analysis of lung, brain, kidney, and intestine from the mice infected by SARS-CoV-2 ancestral strain and Delta variant. Eight-week-old female K18-hACE2 mice were intranasally inoculated with PBS (mock group) or 104 PFU of SARS-CoV-2 ancestral or Delta strain (n = 5). At 6 dpi, lung (A and E), brain (B and F), kidney (C and G), and intestine (D, H, and I) tissues were collected and processed with H&E staining. Cell density of the whole-tissue region was quantified using ImageScope. Arrows: green arrows, perivascular infiltration (B); blue arrows, cell infiltration (C); red arrows, intestine wall (D). Each image is representative of ≥3 mice. Statistical analysis performed using one-way ANOVA with Bonferroni post hoc test (ns, not significant; *, P < 0.05; **, P < 0.01; ***, P < 0.001; ****, P < 0.0001).

FIG 4

Cytokine and chemokine profile in the lung tissue of the mice infected by SARS-CoV-2 ancestral strain and Delta variant. Eight-week old female K18-hACE2 mice were intranasally inoculated with PBS (mock group) or 104 PFU of SARS-CoV-2 ancestral or Delta strain (n = 6). Transcriptional profiles of immune mediators, IL-6 (A), TNF-α (B), IL-1β (C), CCL2 (D), CXCL10 (E), CCL5 (F), IFN-γ (G), IL-10 (H), GM-CSF (I), IL-13 (J), IFN-β (K) and IL-12p35 (L), were determined by qRT-PCR in the lung at day 6 post infection. Data were normalized to HTRP levels and are shown as fold change from the level in mock-infected mice. Each symbol represents the mean titer for one mouse. Data were analyzed by Student's t test (ns, not significant).

FIG 5

Leukocyte profile in the lung tissue of the mice infected by SARS-CoV-2 ancestral strain and Delta variant. Eight-week-old female K18-hACE2 mice were intranasally inoculated with PBS (mock group) or 104 PFU of SARS-CoV-2 ancestral or Delta strain (n = 4 for mock group; n = 6 for infection groups). At 6 dpi, lung tissues were collected and processed for flow cytometry. Gating strategy for lung leukocytes (A), the numbers of CD45+ leukocytes (B), Ly6G+ neutrophils (C), SSChi neutrophils (D), Ly6Chi CCR2+ inflammatory monocytes (IM) (E), CD11b+ CD64+ SiglecF+ alveolar macrophages (F), MHC-II+ CD11c+ dendritic cells (DCs) (G), B220+ CD11b+ plasmacytoid dendritic cells (pDCs) (H), CD3+ T cells (I), CD4+ T cells (J), CD8+ T cells (K), NK cells (L), and MHC-II+ B220+ B cells (M) were analyzed. Data were analyzed by one-way ANOVA with Bonferroni post hoc test (ns, not significant; *, P < 0.05; **, P < 0.01; ***, P < 0.001).

FIG 6

Immunofluorescence analysis of the lung tissue from K18-hACE2 mice infected by SARS-CoV-2 ancestral strain and Delta variant. Eight-week-old female K18-hACE2 mice were intranasally inoculated with PBS (mock group) or 104 PFU of SARS-CoV-2 ancestral or Delta strain (n = 4 for mock group; n = 6 for infection groups). At 6 dpi, lung tissues were collected and processed for immunofluorescence staining. Lung cryosections were labeled with CD3 (T cells), Gr-1 (neutrophils), CD169 (alveolar macrophages), and laminin (parenchymal tissue). Colocalization and distribution of CD3+ T cells, Gr-1+ neutrophils, and CD169+ alveolar macrophages are shown in the image. Images representative of n = 6 mice per group were acquired by confocal microscopy as a z-stack using a 10× (1.0-NA) objective. Scale bars in panels = 150 mm.