Apparent lack of neutralizing antibodies in Aleutian disease is due to masking of antigenic sites by phospholipids.

It is generally accepted that Aleutian disease virus (ADV) cannot be neutralized by antibodies either in vivo or in vitro. We found several ways to demonstrate neutralization of ADV by specific antibodies from mink. It was essential to make ADV monodisperse by treatment with sodium lauroyl sarkosyl or n-butanol or by filtration through 0.05-micron membranes before neutralization tests. In kinetic experiments, there was a 95% loss of virus infectivity within the first 5 min of reaction, but a resistant fraction of about 1% remained after 1.5 hr of incubation. Neutralization titers between 1:160 and 1:640 were found in sera from naturally and experimentally infected mink. A positive relation was consistently found between neutralization and ELISA titers. Furthermore, separation of phospholipids from ADV was shown by thin-layer chromatography of butanol-extracted virions. By reconstitution of monodispersed ADV with various lipids, phospholipids were found to interfere with virus neutralization by attachment to the virus surface.