Fig.1: C57BL/6N and C3H/HeJ mice on a high fat diet develop diet-induced obesity (DIO).
Four-week-old C57BL/6N (n=22-23) and C3H/HeJ (n=14-20) were fed a HFD (60% fat) or a LFD (10% fat) for 18 weeks. (A & B) Weight gain in lean and obese C57BL/6N and C3H/HeJ mice respectively. Weights were measured throughout the feeding period and statistical analysis was carried out using mixed effects analysis with Sidak’s multiple comparisons test. (C & D) Glucose levels in lean and obese C57BL/6N and C3H/HeJ mice respectively. Two weeks before infection (16-weeks-post-diet-initiation), blood was collected by submandibular cheek bleeds and non-fasting glucose levels were measured. Statistical analysis was done using an unpaired t-test. The level of significance is represented as follows: p < 0.001 (**); p = 0.001 to 0.0001 (***); p < 0.0001 (****). The error bars indicate the standard deviation (SD) of the mean.
Obese C3H/HeJ mice exposed to MHV-1 experience more severe disease outcomes compared to C57BL/6N mice. Since its outbreak in 2019, severe COVID-19 outcomes have been strongly and independently correlated with obesity43, with severe obesity leading to 3.6 times more ICU admissions17. Obesity significantly increased the risk of mortality in young COVID-19 patients11 and has been shown in COVID-19 patients across the world44-46. To determine the impact of obesity on disease severity and identify an appropriate mouse model for studying coronavirus pathogenesis and biomarker discovery, lean and obese C57BL/6N and C3H/HeJ mice were inoculated intranasally with 3x104 PFUs of MHV-1 and monitored for weight loss, mortality, and other signs of illness until 8 days-post-infection (dpi) (Fig.2A) when mice were euthanized to collect tissues for histopathology.
Obese C57BL/6N mice inoculated with MHV-1 showed significantly greater weight loss compared to their lean counterparts at 2 and 8 dpi (Fig.2B, **p=0.0029; *p=0.0483, respectively). More strikingly, obese C3H/HeJ mice showed significantly higher weight loss at 1, 2, 6 and 7 dpi (Fig.2C, ***p=0.0004, **p=0.0018, *p=0.0.0472 and *p=0.0290, respectively). There was no difference in the survival up to 8 dpi between lean and obese C57BL/6N mice (Fig.2D); in contrast, obese C3H/HeJ mice had 100% mortality by 8 dpi (Fig.2E, ***p=0.0002) compared to 50% mortality in lean mice. The significant weight loss and mortality rates in our obese C3H/HeJ mice inoculated with MHV-1, therefore, mirror the morbidity and mortality observed in obese humans suffering from COVID-19, suggesting the relevance of our infection model to study COVID-19 in the context of obesity.