Morphological studies of the neutralization of influenza virus by IgM

Quantitative relationships between neutralization, aggregation and attachment to monolayers of chick embryo fibroblast (CEF) cells have been studied using a constant amount of influenza A/fowl plague virus/Rostock/34 (H7N1) and varying amounts of purified mouse polyclonal IgM directed against the haemagglutinin, the major viral neutralization antigen. There are two major types of interaction. (i) At low concentrations of IgM there is aggregation of virus, but no neutralization provided that the aggregates are dispersed by vortexing and dilution. Maximum aggregation occurs at less than seven molecules of IgM per virion and the IgM is probably bound in the 'staple' or 'crab' conformation at these concentrations. (ii) At higher concentrations there is neutralization and this coincides with inhibition of attachment of virus to CEF cells. Neutralization of 50% infectivity requires about 35 molecules of IgM per virion. The maximum neutralization observed was only 87%. Quantitative data and electron microscopy observations suggest that molecules of IgM at the higher concentrations adopt a planar stance approximately perpendicular to the viral surface. It appears that IgM neutralizes fowl plague virus in vitro primarily by interfering with its attachment to cells; the fraction of neutralized virus that does attach is known not be internalized.