Egg fluids and cells of the chorioallantoic membrane of embryonated chicken eggs can select different variants of influenza A (H3N2) viruses.

Growth of influenza viruses in embryonated eggs frequently results in the selection of virus variants with amino acid changes near the receptor-binding pocket of the hemagglutinin molecule, yet the mechanism by which this third form of influenza variation occurs (the other two being antigenic drift and shift) has not been clearly defined. Because egg-mediated variation might affect influenza vaccine and surveillance programs, we have initiated studies to determine the site(s) of variant virus selection within the embryonated egg. In this report we show that both the cells of the chorioallantoic membrane (CAM) and the fluids from embryonated chicken eggs are capable of selecting variant influenza viruses, but that these variants are distinct at the molecular level depending on the conditions of virus propagation. Serial passage of viruses in cells of the chorioallantoic membrane selects one set of variants which possess specific amino acid changes near the receptor binding pocket of the hemagglutinin molecule characteristic of viruses grown in embryonated eggs. However, passage of the same viruses in mammalian tissue culture cells supplemented with egg fluids selects a separate set of hemagglutinin variants also characteristic of viruses grown in eggs, yet at different residues from those observed following passage in CAM. These results suggest that two separate mechanisms may exist in the embryonated egg that lead to the selection of variant influenza viruses: one at the cellular level and another at the extracellular level.