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 recep tor-binding pocket of the hemagglutinin molecule, yet the mechanism by which this third form of influenza variation occurs (the other two be ing antigenic drift and shift) has not been clearly defined. Because e gg-mediated variation might affect influenza vaccine and surveillance programs, we have initiated studies to determine the site(s) of varian t virus selection within the embryonated egg. In this report we show t hat both the cells of the chorioallantoic membrane (CAM) and the fluid s from embryonated chicken eggs are capable of selecting variant influ enza viruses, but that these variants are distinct at the molecular le vel depending on the conditions of virus propagation. Serial passage o f viruses in cells of the chorioallantoic membrane selects one set of variants which possess specific amino acid changes near the receptor b inding pocket of the hemagglutinin molecule characteristic of viruses grown in embryonated eggs. However, passage of the same viruses in mam malian tissue culture cells supplemented with egg fluids selects a sep arate set of hemagglutinin variants also characteristic of viruses gro wn in eggs, yet at different residues from those observed following pa ssage in CAM. These results suggest that two separate mechanisms may e xist in the embryonated egg that lead to the selection of variant infl uenza viruses: one at the cellular level and another at the extracellu lar level. (C) 1995 Academic Press, Inc.