Observations on the plaque assay of visna virus.

Abstract

Visna virus is the cause of a slowly evolving neurological illness of sheep (Sigurdsson, Pálsson & Grimsson, 1957; Sigurdsson & Pálsson, 1958). The virus has been propagated in cell cultures derived from sheep choroid plexus and quantified by 50% endpoint determinations of cytopathic effects in sheep choroid plexus cells 14 to 21 days after inoculation (Sigurdsson, Thormar & Pálsson, 1960; Harter & Choppin, 1967b). Attempts to obtain a plaque assay for visna virus have been hampered by the poor affinity which sheep choroid plexus cells have for neutral red and other vital dyes. To avoid this difficulty, an assay involving the development of plaques in a secondary cellular overlay, which could be stained with neutral red, was developed (Harter & Choppin, 1967a). An infected sheep choroid plexus monolayer of cells was maintained for 12 to 14 days under a semisolid overlay containing carboxymethylcellulose; the overlay was then removed and the sheep choroid plexus cells were covered with a sufficient number of baby hamster kidney cells (BHK 21-F) to establish a second confluent monolayer.