The influenza virus-cytokine-protease cycle is one of the key mechanisms of vascular hyperpermeability in severe influenza.
Influenza A virus (IAV) is one of the most common infectious pathogens in humans. Since IVA genome does not have the processing protease for the viral membrane fusion glycoprotein precursors, entry of this virus into cells is determined primarily by host cellular, trypsin-type, processing proteases that proteolytically activate the fusion glycoprotein precursors of IAV. At least five different processing proteases have been identified in the airways of animals and humans. These proteases determine the infectious organ tropism of IAV infection as well as the efficiency of viral multiplication in the airway, and sometimes in the brain. Proteases in the upper respiratory tract are suppressed by secretory leukoprotease inhibitor, and those in the lower respiratory tract are suppressed by pulmonary surfactant which, by adsorption, inhibits the interaction between the proteases and viral membrane proteins. Since protease activities predominate over those of endogenous inhibitory compounds under normal airway conditions, administration of protease inhibitors in the early-stage of infection significantly suppresses viral entry and viral multiplication. Several viral neuraminidase inhibitors are used clinically as anti-influenza virus agents, based on their inhibitory action on viral release from infected cells. Furthermore, protease inhibitors of viral entry could be potentially useful against influenza virus as well as neuraminidase inhibitor-resistant viruses. We also found that ambroxol, a mucolytic and anti-oxidant agent, up-regulates the levels of endogenous protease inhibitory compounds in the airway fluids in early-phase infection, and that clarithromycin, a macrolide antibiotic, increases IgA levels and mucosal immunity through augmentation of interleukin-12 levels in the airway. The combination of neuraminidase inhibitors and protease inhibitors, clarithromycin or ambroxol, could be potentially used as a potent anti-influenza therapy to minimize the emergence of drug-resistant mutant viruses.
Influenza A virus (IAV) is one of the most common infectious pathogens in humans. Since IVA genome does not have the processing protease for the viral membrane fusion glycoprotein precursors, entry of this virus into cells is determined primarily by host cellular, trypsin-type, processing proteases that proteolytically activate the fusion glycoprotein precursors of IAV. At least five different processing proteases have been identified in the airways of animals and humans. These proteases determine the infectious organ tropism of IAV infection as well as the efficiency of viral multiplication in the airway, and sometimes in the brain. Proteases in the upper respiratory tract are suppressed by secretory leukoprotease inhibitor, and those in the lower respiratory tract are suppressed by pulmonary surfactant which, by adsorption, inhibits the interaction between the proteases and viral membrane proteins. Since protease activities predominate over those of endogenous inhibitory compounds under normal airway conditions, administration of protease inhibitors in the early-stage of infection significantly suppresses viral entry and viral multiplication. Several viral neuraminidase inhibitors are used clinically as anti-influenza virus agents, based on their inhibitory action on viral release from infected cells. Furthermore, protease inhibitors of viral entry could be potentially useful against influenza virus as well as neuraminidase inhibitor-resistant viruses. We also found that ambroxol, a mucolytic and anti-oxidant agent, up-regulates the levels of endogenous protease inhibitory compounds in the airway fluids in early-phase infection, and that clarithromycin, a macrolide antibiotic, increases IgA levels and mucosal immunity through augmentation of interleukin-12 levels in the airway. The combination of neuraminidase inhibitors and protease inhibitors, clarithromycin or ambroxol, could be potentially used as a potent anti-influenza therapy to minimize the emergence of drug-resistant mutant viruses.
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.
Extracellular cleavage of virus envelope fusion glycoprotein hemagglutinin (HA0) by host trypsin-like proteases is a prerequisite for the infectivity and pathogenicity of human influenza A viruses and Sendai virus. The common epidemic influenza A viruses are pneumotropic, but occasionally cause encephalopathy or encephalitis, although the HA0 processing enzyme in the brain has not been identified. In searching for the brain processing proteases, we identified a processing enzyme in rat brain that was inducible by infection with these viruses. The purified enzyme exhibited an apparent molecular mass of approximately 22 kDa on SDS-PAGE and the N-terminal amino acid sequence was consistent with that of rat pancreatic trypsin I. Its substrate specificities and inhibition profiles were the same as those of pancreatic trypsin I. In situ hybridization and immunohistochemical studies on trypsin I distribution revealed heavy deposits in the brain capillaries, particularly in the allocortex, as well as in clustered neuronal cells of the hippocampus. The purified enzyme efficiently processed the HA0 of human influenza A virus and the fusion glycoprotein precursor of Sendai virus. Our results suggest that trypsin I in the brain potentiates virus multiplication in the pathogenesis and progression of influenza-associated encephalopathy or encephalitis.
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