Abstract
Background: Seventy to ninety percent of seasonal influenza deaths occur in those aged 65 or older. Methods: To determine the innate immune responses to influenza A virus (IAV) infection, young (12-week) and old (70-week) C57BL/6J mice were infected intranasally (i.n.) with IAV PR8. Immune responses and viral replication were determined by qRT-PCR. Lung inflammation and injury were evaluated by histology and bronchoalveolar lavage (BAL) and lung to body weight ratio respectively. Results: Comparing young and old mice at the times indicated there were no differences in weight loss percentages, lung-to-body weight ratios and total immune cells in BALF after IAV infection. However, aged mice had significantly higher viral loads and lower type I interferon (IFN) expression in the lung at day 3 p.i. In contrast, at this time point, aged mice had significantly higher amounts of type III IFN expression, which correlated with higher viral loads. Histopathology revealed that IAV infection in aged mice resulted in lower pathological scores early (day 5) and higher lung pathological scores later (day 7) of disease than in young mice. Conclusion: Our models demonstrated that aging dysregulates early IFN responses to influenza infection resulting in enhanced viral replication. These altered IFN responses in aged mice also result in enhanced lung inflammation late after infection and may also be a cause of the increased incidence of secondary bacterial infections seen in the aged.