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Cells infected with herpes simplex virus type 1 (HSV-1) DNA by the calcium phosphate precipitation technique produce virus which leads to the formation of plaques (Graham, Veldhuisen & Wilkie, 1973). In the study reported here we show that treatment of cell monolayers with dimethyl sulphoxide (DMSO) solutions after infection with DNA-calcium phosphate complexes leads to a considerable increase in the number of plaques obtained. The conditions for this enhancement of infectivity have been optimized for baby hamster kidney (BHK) cells, and increases in plaque numbers of over 100-fold have been obtained. The treatment appears to increase the proportion of cells which respond to DNA infection by initiating plaque formation, and results in a large increase in the measured specific infectivity of HSV-1 DNA. DMSO causes similar (but quantitatively different) responses in various other cell lines infected with HSV-1 DNA. BHK cells infected with either virus particles, or virus DNA by the DEAE-dextran technique (Laithier & Sheldrick, 1975), do not exhibit this massive enhancement following exposure to DMSO.
Stow et al. (Wed,) studied this question.