Influenza-induced oxidative stress sensitizes lung cells to bacterial toxin-mediated necroptosis

ABSTRACT Rationale Pneumonia caused by Influenza A virus (IAV) co- and secondary bacterial infections are characterized by their severity. Previously we have shown that pore-forming toxin (PFT)-mediated necroptosis is a key driver of acute lung injury during bacterial pneumonia. Here, we evaluate the impact of IAV on PFT-induced acute lung injury during co- and secondary Streptococcus pneumoniae (Spn) infection. Objectives Determine the impact of IAV infection on bacterial PFT-mediated lung epithelial cell (LEC) necroptosis. Determine the molecular basis for increased sensitivity and if inhibition of necroptosis or oxidative stress blocks IAV sensitization of LEC to PFT. Methods Mice and cells were challenged with IAV followed by Spn. Necroptosis was monitored by measuring cell death at fixed time points post-infection and immunofluorescent detection of necroptosis. Wildtype mice and LEC were treated with necroptosis inhibitors. Necroptosis effector molecule MLKL deficiency was tested for infection synergy. Oxidative damage to DNA and lipids as result of infection was measured in vitro and in vivo. Necroptosis and anti-oxidant therapy efficacy to reduce disease severity was tested in vivo. Measurements and Main Results IAV synergistically sensitized LEC for PFT-mediated necroptosis in vitro and in murine models of Spn co-infection and secondary infection. Pharmacological induction of oxidative stress sans virus sensitized cells for PFT-mediated necroptosis. Necroptosis inhibition reduced disease severity during secondary bacterial infection. Conclusions IAV-induced oxidative stress sensitizes LEC for PFT-mediated necroptosis. This is a new molecular explanation for severe influenza-associated bacterial infections. Necroptosis inhibitors are potential therapeutic strategies to reduce IAV-primed bacterial pneumonia severity. Summary Here we demonstrate that Influenza A virus (IAV) infection synergistically sensitizes lung cells to bacterial pore-forming toxin (PFT)-mediated necroptosis. Moreover, this contributes to the severity of lung injury that is observed during co- and secondary infection with Streptococcus pneumoniae. IAV-induced oxidative stress was identified as a key factor contributing to cell sensitization and induction of oxidative stress sans virus was sufficient to synergistically enhance susceptibility to PFT-mediated killing. Our results advance our understanding on the molecular basis of co- and secondary bacterial infection to influenza and identifies necroptosis inhibition and antioxidant therapy as potential intervention strategies.