[Potentiation of infectivity and pathogenesis of influenza virus by host and bacterial proteases].

Infectivity and pathogenicity of influenza viruses are based on the interplay between the viral glycoprotein hemagglutinin (HA) and host proteases. HA receives its full biological activities by proteolytic cleavage of a precursor molecule at a definite cleavage site. Proteases selected by the Clara cells in the bronchial epithelia and many kind of bacteria, are virus activate proteases responsible for the cleavage activation and pathogenicity of influenza viruses. Although influenza in normal individuals is usually confined to the upper respiratory tract, the infection often develops into fatal pneumonia in aged patients, where bacterial infections often occur. Synergistic effects of bacterial infections on the pathogenesis of influenza viruses are described in regard to the cleavage activation of HA.