Cell surface changes associated with mutation of visna virus in antibody-treated cell cultures

Sheep cells inoculated with a portion of a plaque suspension of visna virus developed acute cytopathic changes and produced progeny virus which was antigenically identical to the parental strain. In contrast, cultures inoculated with other portions of the same suspension and maintained in medium containing specific viral antiserum developed cytopathic changes more slowly and yielded genetically stable mutant strains of virus which were poorly neutralized by the same antiserum. Scanning and transmission electron microscopy showed that, in the absence of antibody, cell surfaces of infected cells were covered diffusely with viral buds, whereas in the presence of antibody, viral buds were often grouped in caps and progressive aggregation of released virus occurred. Viral aggregates were strongly anchored to viral buds or long villi and sometimes became engulfed by infected cells. In addition, single maturing virions were continuously detected close to large aggregates. These morphological events suggest that selective replication of mutant virus in antibody-treated cells is favored by (1) elimination of parental virus through aggregation and endocytosis and (2) patching and capping of viral buds, which leave free cell surface areas for maturation of mutant virus.