The synergistic neutralization of Newcastle disease virus by two monoclonal antibodies to its haemagglutinin-neuraminidase protein.

Monoclonal antibodies (MAbs) to 2 epitopes on the haemagglutinin-neuraminidase (HN) protein of the Ulster strain of Newcastle disease virus neutralized synergistically: MAbs to HN-1 and HN-2 neutralized 1.2 and 1.7 log10 infectious units (i.u.) of virus when single as compared to 4.1 when combined. Although MAb to HN-1 but not to HN-2 inhibited haemagglutination they both neutralized more virus on desialyzed cells compared to normal cells and were considered to have interfered with viral attachments in a cooperative manner when combined. A third MAb to fusion (F) protein reduced infectivity by five log10 i.u. HN-1, HN-2 and F were not the only immunodominant epitopes because mutants, which simultaneously lacked all 3 epitopes, were indistinguishable from wild type when neutralized by polyclonal mouse, rabbit or chicken antiserum to whole virus.