Pre-activation timing determines influenza severity and viral pathogenicity via STING Inhibition

Severe influenza is frequently associated with bacterial or viral co-infections. Although immune-mediated mechanisms have been extensively investigated, the cell-intrinsic responses of respiratory epithelial cells prior to immune engagement remain poorly characterized. To model co-infection in vitro, we pre-activated B16 melanoma cells using the TLR9 agonist CpG ODN, followed by infection with influenza A virus (IAV) strain FM1. Key molecular events, including STING expression and activation, were assessed by Western blotting and RT-qPCR. The in vivo relevance was assessed in a mouse model of intranasal CpG ODN pre-treatment and IAV challenge. RNA-seq was performed as a supplementary approach to explore possible cell death pathways. Pre-activation significantly enhanced IAV-induced cell death within a restricted vulnerability window (≤ 24 h), independent of viral replication or TLR9 signaling. This was associated with inflammatory priming and initial STING activation. However, IAV infection subsequently suppressed STING phosphorylation and elevated inflammatory cell death. Mice pre-treated with CpG ODN exhibited increased mortality and aggravated lung injury following IAV challenge. RNA-seq analysis suggested the potential involvement of pyroptosis. These findings demonstrate that cellular pre-stimulation exacerbates influenza severity by promoting inflammatory cell death, a process mechanistically linked to the subsequent suppression of STING activation. This study reveals a novel pathogenic cell-intrinsic mechanism in co-infection pathogenesis.