Cell-virus interactions with the shope fibroma virus on cultures of rabbit and rat cells

Abstract

The cell-virus interactions in vitro of the Shope fibroma virus (SFV) were studied on rabbit and rat tissue cultures. With both rabbit and rat primary or secondary monolayers, SFV produced a cytopathic effect (CPE) in cultures infected at virus: cell ratios of 1 or above, and pocks in cultures infected with aboft 300 pock-forming units (= plaque forming units) (PFU) per plate. The pocks on rabbit cultures lysed to form plaques, while those on rat cultures remained intact with only a partial cell lysis at the periphery of the pock. An inhibitor in the agar overlay, which cofld be precipitated by protamine sulfate, caused a delay in the time of appearance of pocks in rat cultures, and affected the transition of pocks to plaques in rabbit cultures. The one-step growth curves in rabbit and rat cells were similar, but rat cultures generally had a lower percentage of virus-yielding cells and a lower virus yield in such cells. The virus yields per virus-yielding cell were always low, the figures ranging from 2 to 36 PFU for rabbit and 1 to 13 PFU for rat cells. The average virus yield per pock was 5.5 × 10 3 PFU in rabbit and 1.8 × 10 2 PFU in rat cultures. The nature of the pocks produced in vitro was studied in rabbit cultures. In contrast to the results obtained after infection of primary or secondary monolayers, pocks were not produced after infection of sparse cultures, or after cloning of infected cells. Monolayers derived from 28 single cell rabbit embryo clones, also did not form typical pocks on virus infection, but exhibited CPE and plaque formation. In secondary cultures that formed pocks, a higher mitotic frequency than in noninfected cultures, coinciding with the time of pock appearance, was observed after infection of rabbit testes cells but not after infection of rabbit or rat embryo cells. The data are discussed in relation to the possibilities that pock formation by SFV in vitro might result (1) from cell aggregation in either mixed cell populations or special types of cells, or (2) from a stimulation of cell division.