Neutralization of Egtved virus pathogenicity to cell cultures and fish by monoclonal antibodies to the viral G protein

National Veterinary Laboratory, Hangøvej 2, 8200 Århus N, Denmark Egtved virus, the rhabdovirus causing viral haemorrhagic septicaemia in rainbow trout, was analysed at the antigen level with a future subunit vaccine in mind. Three monoclonal antibodies to the viral G protein were characterized with respect to neutralizing activity at the cell culture level, as well as their ability to protect rainbow trout fingerlings against virus infection following passive immunization. Two antibodies showed strong protective activity in fish. Only one of these antibodies was able to neutralize viral infectivity in vitro . Reduction of disulphide bonds in the G protein abolished reactivity of this antibody in immunoblotting, whereas antigen deglycosylation did not influence the binding ability of any of the antibodies. These data suggest that the G protein contains linear as well as non-linear, carbohydrate-free epitopes, which are involved in the protection against Egtved virus. However, an indirect influence of oligosaccharide side chains on epitope formation could not be excluded, since in situ inhibition of glycosylation prevented the binding of the protecting antibodies in immunofluorescence. Received 1 September 1989; accepted 23 October 1989.