Mechanisms of neutralization of a nairovirus (Dugbe virus) by polyclonal IgG and IgM

Dugbe virus is a member of the nairovirus genus of the Bunyaviridae. Purified polyclonal anti-Dugbe virus IgG, which neutralized > 99.5% of virus, reduced attachment of virus to BSC-1 cell monolayers by only 36%. A 100-fold lower concentration neutralized virus by 88%, and had no effect upon attachment. Neutralizing IgG did not affect the ability of Dugbe virus to be internalized by or to fuse with BSC-1 cells. This suggests that IgG neutralization occurs largely at a stage subsequent to primary uncoating. Purified polyclonal anti-Dugbe virus IgM neutralized infectivity and had no effect on the attachment of virus to cells, but inhibited internalization of virus by about 50%. Thus IgM neutralizes partly by interfering with entry of virus and partly by a post-entry event. Neutralization by intermediate concentrations of IgM was enhanced 20-fold in the presence of complement. At high concentrations of IgM, complement-dependent neutralization declined. This is probably due to IgM binding in a planar rather than crab conformation, which does not expose the complement binding sites. Aggregation occurred only at relatively low concentrations of immunoglobulin. Electron microscopy and reactivation of infectivity by vortexing suggested that aggregation makes only a minor contribution to neutralization by IgG or IgM.