Cigarette smoking prolongs immune responses against influenza and delays its clearance in mice

Introduction Epidemiological studies have shown that smoking is associated with increased incidence of severe viral infections leading to hospitalisation. Moreover, studies in experimental models have identified impaired antiviral responses and altered inflammatory responses, yet it is unclear which immune cells are involved and whether this varies over the course of infection. Methods To test how cigarette smoking affects the response to influenza viral infection over time, female BALB/c mice were exposed to cigarette smoke or air twice a day for 24-28 days and infected with H3N2 influenza or mock infection on day 21. Three and seven days after infection, changes in immune cell populations, and mRNA expression/viral clearance in lung tissue were analysed. Results Smoke-exposed mice lost significantly more weight than air-exposed controls after influenza infection, indicating that smoking resulted in more severe disease. Immune cell and lung tissue transcriptome analysis revealed that neutrophil infiltration was prolonged and macrophage activation dysregulated after infection in smoke-exposed mice compared to air-exposed controls. Expression of genes in IL-6 and interferon pathways was similarly longer active. In parallel, we observed lower clearance of viral RNA in smoke-exposed mice after infection compared to air-exposed controls, indicating ineffective antiviral responses. Adaptive immune responses were unchanged in infected smoking animals compared to nonsmoking mice. Conclusion Altogether, the data from our mouse model indicate that cigarette smoke exposure prolongs innate immune responses against influenza. The results from this study help to explain the susceptibility of current smokers to severe influenza disease.