Pre-existing allergic inflammation alters both innate and adaptive immune responses in mice co-infected with influenza virus
Introduction: Asthma, a chronic airway disease, is marked by allergic inflammation, hyperresponsiveness, and tissue remodeling. Influenza infections in asthma patients can cause severe exacerbations, though the underlying mechanisms remain unclear. This study investigated how pre-existing allergic inflammation affects immune responses to influenza infection in mice exposed to house dust mite (HDM). Methods: Mice were repeatedly exposed to HDM, followed by infection with influenza A virus, and were sacrificed three days post-infection. Plasma was analyzed for HDM-specific immunoglobulins, while lung tissue was used for immune cell flow cytometry and RNA sequencing analysis. Results: HDM exposure induced allergic inflammation, evidenced by more HDM-specific IgE, IgG1, IgG2, eosinophils, neutrophils, Th1, and Th17 cells compared to controls. Upon influenza infection, the effects of HDM and influenza co-infection interacted, showing fewer Th1 cells and regulatory T cells and more Th2 cells compared to mice exposed to influenza virus alone. Interestingly, RNA-seq analysis revealed less upregulation of Th1-related genes and antiviral pathways in co-exposed mice, suggesting impaired Th1 immunity and antiviral responses. Conclusion: Pre-existing allergic inflammation significantly altered immune responses in mice coinfected with influenza, revealing underdeveloped antiviral responses as early as three days post-infection. These findings may explain the increased susceptibility of patients with asthma to severe viral diseases. Overall design: The data  are the result of 3 independent experiments with each 3-6 mice per experimental group. Mice were anesthetized with isoflurane and intranasally exposed to either saline or 62.5 µg of whole culture house dust mite (HDM) extract (Dermatophagoides pteronyssinus, 15G10, Citeq, Groningen, The Netherlands, containing an endotoxin level of 1.65 × 10^7 EU/gram) in saline. The animals were exposed to saline or HDM three times per week for 24 days and a single, nasal administration of 20 units of 50% tissue culture infectious dose of H3N2 influenza A virus Hong Kong (HK) X31 or saline on day 21 and were separated into 4 groups: saline/saline, HDM/saline, saline/influenza, HDM/influenza, with 11-16 mice in each group when combining the three independent experiments. On day 24, which was one day after the final HDM exposure and three days after the X31 challenge, mice were sacrificed by cardiac exsanguination.