Divergent macrophage responses to Influenza A virus and Streptococcus pneumoniae: co-infection drives bacterial dominance whereas superinfection favors viral priming.

INTRODUCTION: Respiratory coinfections involving Influenza viruses, including Influenza A viruses (IAV) and bacteria, significantly worsen disease severity and remain a major public health concern, particularly during seasonal and pandemic flu outbreaks. Among bacterial pathogens, Streptococcus pneumoniae (Spn) and Streptococcus suis cause secondary infections in humans and swine respectively following influenza. The immunological mechanisms driving coinfection severity, especially the differences between simultaneous and sequential infections, are incompletely defined.

METHODS: We developed an in vitro differentiated bone marrow-derived macrophages (BMDMs) model to examine transcriptional and protein-level responses during IAV-Spn coinfection or sequential infection. BMDMs were infected with IAV and Spn either simultaneously or with a 48-hour delay.

RESULTS: RNA-Seq and OLINK proteomic analyses revealed that simultaneous coinfection elicits a synergistic inflammatory response similar to that caused by Spn alone, with strong activation of NF-κB-dependent genes. In sequential superinfection, responses were shaped by viral priming, with bacterial challenge further amplifying genes linked to inflammation and fibrin clot formation, potentially contributing to disease severity. These effects were consistent across different IAV subtypes when tested in combination with porcine Streptococcus suis serotypes that impose a comparable burden in pigs during influenza coinfection. Additionally, age is a determinant of BMDM responses. This model offers an advantageous tool for studying coinfection dynamics in human and veterinary medicine.