Hemolysis and fusion by influenza viruses with heat-inactivated neuraminidase activity.

Influenza virus hemolytic activity was found to be more heat-resistant than neuraminidase activity, and to be heat-inactivated similarly to hemagglutination activity, in sharp contrast to the case with paramyxoviruses, where the hemolytic activity is the most heat-labile, and hemagglutination and neuraminidase activities are inactivated similarly by heat. Influenza viruses with heat-inactivated neuraminidase activity, which still showed hemagglutination and hemolytic activities, were found to be able to induce cell fusion and envelope fusion. This finding suggested that the hemolytic and fusion activities are not dependent on neuraminidase activity. The hemolytic activity was largely inhibited by anti-hemagglutinin serum of the same subtype. The pH range, heat stability, and antiserum susceptibility of the hemolytic activity were found to be independent of the cells in which the virus was grown.