IL-4 protects mice against lethal influenza and Streptococcus pneumoniae co-infected pneumonia.

Streptococcus pneumoniae (S. pneumoniae) co-infection post influenza is a major cause of mortality characterized by uncontrolled bacteria burden and excessive immune response during influenza pandemics. IL-4 is a canonical type 2 immune cytokine known for its wide range of biological activities on different cell types. It displayed protective roles in numerous infectious diseases and immune-related diseases. But its role in influenza and S. pneumoniae (influenza/S. pneumoniae) co-infected pneumonia has not been reported. In our study, we used C57BL/6 wild-type (WT) and IL-4-deficient (IL-4-/- ) mice to establish co-infection model with S. pneumoniae after influenza virus infection. Co-infected IL-4-/- mice showed increased mortality and weight loss compared with WT mice. IL-4 deficiency led to increased bacterial loads in lungs without altering influenza virus replication, suggesting a role of IL-4 in decreasing post-influenza susceptibility to S. pneumoniae co-infection. Loss of IL-4 also resulted in aggravated lung damage along with massive pro-inflammatory cytokine production and immune cell infiltration during co-infection. Administration of recombinant IL-4 rescued the survival and weight loss of IL-4-/- mice in lethal co-infection. Additionally, IL-4 deficiency led to more immune cell death in co-infection. Gasdermin D (GSDMD) during co-infection was induced in IL-4-/- mice that subsequently activated cell pyroptosis. Treatment of recombinant IL-4 or inhibition of GSDMD activity by disulfiram decreased immune cell death and bacterial loads in lungs of IL-4-/- co-infected mice. These results suggest that IL-4 decreases post-influenza susceptibility to S. pneumoniae co-infection via suppressing GSDMD induced pyroptosis. Collectively, this study demonstrates the protective role of IL-4 in influenza/S. pneumoniae co-infected pneumonia.