Dietary cholesterol causes inflammatory imbalance and exacerbates morbidity in mice infected with influenza A virus

Influenza is a common cause of pneumonia-induced hospitalization and death, but how host factors function to influence disease susceptibility or severity has not been fully elucidated. Cellular cholesterol levels may affect the pathogenesis of influenza infection as cholesterol is crucial for viral entry and replication, as well as immune cell proliferation and function. But there is still conflicting evidence on the extent to which dietary cholesterol influences cholesterol metabolism. Here, we examined the effects of a high cholesterol diet in modulating the immune response to influenza A virus (IAV) infection in mice. Mice were fed a standard or a high cholesterol diet for five weeks before inoculation with mouse-adapted human IAV (Puerto Rico/8/1934), and tissues were collected at days 0, 4, 8, and 16 post-infection. Cholesterol-fed mice exhibited dyslipidemia characterized by increased levels of total serum cholesterol prior to infection and decreased triglycerides after infection. Cholesterol-fed mice also displayed increased morbidity compared to mice fed a control diet, which was neither a result of immunosuppression nor changes in viral load. Instead, transcriptomic analysis of the lungs revealed that dietary cholesterol caused upregulation of genes involved in viral-response pathways and leukocyte trafficking, which coincided with increased numbers of cytokine-producing CD4+ and CD8+ T cells and infiltrating dendritic cells. Morbidity as determined by percent weight loss was highly correlated with numbers of cytokine-producing CD4+ and CD8+ T cells as well as granulocytes. Taken together, hypercholesterolemia promoted IAV morbidity via exaggerated cellular immune responses that were independent of viral load.