Biological Sex Differences after High and Low Doses of Influenza A Virus Infection in a Diet-induced Obesity Mouse Model

Obesity is an independent risk factor for influenza A virus (IAV) pathogenesis. In non-obese hosts, following IAV infection, adult females develop more severe disease than males. Limited research on biological sex differences following IAV infection in individuals with obesity forms the basis for this study. Male and female C57BL/6J mice were treated with high-fat or low-fat diets to obtain mice with or without obesity. They were inoculated intranasally with a high (10 ³ ) or low dose (10 ^1.5 TCID ₅₀ ) of mouse-adapted A/California/04/2009 H1N1 IAV and followed for 21 days post-infection (dpi) for morbidity. Subsets of mice were euthanized at different dpi to measure lung virus titers, histopathological changes, cellular infiltration, and cytokine induction. After a high-dose infection, females with obesity demonstrated shorter median survival time, greater parenchymal inflammation, and heightened induction of inflammatory markers than males with obesity. After a low-dose infection, females than males, and mice with obesity than the non-obese mice, exhibited severe morbidity. Female mice with obesity exhibited the greatest disease severity, where 25% (2/8) reached humane endpoint. Delayed but persistent inflammatory changes were observed in females with obesity, characterized by relatively lower pathological changes, lesser induction of cytokines and chemokines, and reduced myeloid and lymphoid infiltration in the lungs at 3 dpi followed by sustained pathological changes and cytokine and chemokine induction at 21 dpi. Our study suggests that the biological sex difference following IAV infection persists during obesity. Females with obesity experience greater influenza disease severity than males, possibly driven by more severe dysregulation of inflammatory responses.