Influence of Normal Animal Sera on Influenza Viruses in Cultures of Trypsin-Dispersed Monkey Kidney Cells

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Abstract
Summary: The normal sera of most species of animals contain varying amounts of both Francis and Chu inhibitors. The factor capable of neutralizing the cytopathogenic effects of influenza A′ strains in monkey kidney tissue cultures was heat labile, as is the Chu inhibitor. There is no factor in normal sera which inhibits the cytopathogenic effects of the B′ or mouse-pathogenic A strains tested. Presumably, these strains lack the receptors necessary for effective combination with the inhibitor. The neutralizing effect of normal serum inhibitor is due primarily to direct combination of the inhibitor with the virus. This combination takes place rapidly, and the complex formed is stable and unaffected by further incubation or dilution. If present in sufficiently high concentration, the inhibitor of cytopathogenicity can exert a secondary effect, namely, neutralization of progeny virus released from infected cells. The presence of this inhibitor in normal sera has practical implications in the use of tissue culture methods for the study of influenza viruses.