Fusion of influenza virus with the endosomal membrane is inhibited by monoclonal antibodies to defined epitopes on the hemagglutinin

Epitopes on the hemagglutinin (HA) of A/seal/Massachusetts/1/80 (H7N7) influenza virus were mapped by genetic analysis of variants selected with monoclonal antibodies (MAbs). Electron microscopic studies demonstrated that the sites and the directions to which hemagglutination-inhibiting (HI) MAbs and non-HI MAbs bound were different on the HA molecule. Morphological analysis revealed that HI MAbs blocked attachment of the virus to the cells, while non-HI MAbs did not. Virus particles bound with non-HI MAbs were then found in the intracellular vacuoles. Together with the electron microscopic findings, a fluorescence dequenching assay indicated that non-HI MAbs inhibited the fusion of virus with the intracellular vacuolar membrane. It was thus shown that non-HI neutralizing MAbs did not inhibit attachment of the virus to the host cell receptor, but inhibited the fusion step in intracellular vacuoles. The results support the hypothesis that anti-HA MAbs which lack HI activity neutralize viral infectivity by interfering with the low pH-induced conformational change in the HA molecule, resulting in inhibition of the fusion step in the viral replication process (Kida, H., Yoden, S., Kuwabara, M., Yanagawa, R., 1985. Interference with a conformational change in the HA molecule of influenza virus by antibodies as a possible neutralization mechanism. Vaccine 3, 219–222).