Biological sex influences immune response to influenza infection in juvenile outbred mice.

Children are highly susceptible to influenza virus infection and lung inflammation. The majority of studies of influenza pneumonia in mice are in adult, inbred, same-sex mice with limited variation in disease outcomes, unlike what is observed in children. In this study, juvenile outbred mice were challenged with influenza A/California/07/2009 H1N1 virus to examine the temporal effect of infection on lung inflammation in genetically dissimilar male and female mice. In this model, influenza infection induced variable disease severity characterized by viral load, airspace leak and inflammation, and lung function. This response was not only clustered by sex at 3 and 4 days postinfection but also was associated with disease severity by day 8. Influenza virus infection induced altered production of cytokines and chemokines by sex and timing. Males displayed an early cytokine and chemokine response driven by severity, whereas females showed a late response only in severe mice. This was independent of viral RNA load. Flow cytometry was then conducted to analyze lymphocytes. In male mice, neutrophil recruitment was elevated by disease severity. In contrast, female mice showed increased T cell numbers late in infection regardless of severity. Finally, unsupervised clustering of mice by immune parameters at day 8 showed similarity by sex and severity, with more variability in male mice. These data demonstrate that the immune response to influenza virus infection is impacted by sex and disease severity in pediatric equivalent outbred mice. This complexity of immune activation is important to consider when evaluating drivers of disease pathogenesis or planning therapeutic interventions targeting immune mediators.NEW & NOTEWORTHY This study defines the temporal immune response to influenza A virus infection in the juvenile lung. Lung inflammation differs by biological sex and disease severity in juveniles, which is reflected in cytokine and chemokine mediators and innate and adaptive immune cells recruited to the tissue. These data inform future generations of computational models of severe disease drivers and impact the potential use of immunomodulatory therapy in children with influenza A pneumonia.