Cleavage of influenza A virus hemagglutinin by avian intestinal bacterial proteases
Proteolytic cleavage of hemagglutinin is required for cell entry by receptor-mediated endocytosis and plays a key role in pathogenicity of the influenza virus. Despite a number of studies describing relationships between bacterial proteases and influenza A viral activation in mammals, very little is known about the role of the normal bacterial flora of birds on hemagglutinin activation. This dissertation is a compilation of original research addressing the following objectives: (1) demonstrate the presence of protease-secreting bacteria in the avian gastrointestinal tract; (2) assess the capability of secreted proteases to cleave hemagglutinin in vitro; and (3) assess the effect of secreted proteases on influenza A virus infectivity. Numerous species of bacteria have been shown to exhibit proteolytic activity and cleavage fragmentation patterns similar to trypsin mediated HA1/HA2 hydrolysis; however, no bacterial secreted protease demonstrated viral activation. Surprisingly, all isolates exhibited a negative effect upon the virus' ability to activate the virus and thus cause disease suggesting either inappropriate HA cleavage, lipolytic enzyme viral envelope alteration, and/or a yet-to-be described enzymatic activity. Collectively, these results suggest that proteolytic bacteria found in the avian gastrointestinal tract do not give rise to direct cleavage activation of influenza virus HA.