Severe influenza is characterized clinicopathologically by multiple organ failure, although the relationship amongst virus and host factors that influence this morbid outcome and the underlying mechanisms of action remain unclear. The present study identified marked upregulation of matrix metalloproteinase (MMP)-9 and pro-inflammatory cytokine tumor necrosis factor alpha (TNF-alpha) in various organs after intranasal infection of influenza A WSN virus. MMP-9 and TNF-alpha were upregulated in the lung, the site of initial infection, as well as in the brain and heart. The infection-induced MMP-9 upregulation was inhibited by anti-TNF-alpha antibodies and by anti-oxidative reagents pyrrolidine dithiocarbamate and N-acetyl-L-cysteine, which inhibit activation of nuclear factor kappa B (NF-kappaB), as well as by nordihydroguaiaretic acid, which inhibits activation of activator protein 1 (AP-1). In addition, MMP-9 upregulation via TNF-alpha was also suppressed by inhibitors of mitogen-activated protein kinases (MAPKs), such as extracellular signal-regulated kinase 1/2 and p38, and partly by a c-Jun N-terminal kinase inhibitor. These results indicated that the influenza-induced MMP-9 upregulation in various organs is mediated through MAPK-NF-kappaB- and/or AP-1-dependent mechanisms. Strategies that neutralize TNF-alpha as well as inhibitors of MAPK-NF-kappa B- and/or AP-1-dependent pathways may be useful for suppressing the MMP-9 effect and thus preventing multiple organ failure in severe influenza.