Polyoma virus-transformed rat cell lines inducible for viral capsid antigen synthesis.

A number of polyoma virus (PV)-transformed lines of rat, mouse and hamster origin established in the course of this study were investigated for virus induction by mitomycin C. Neither infectious virus nor V-antigen was detected in six mouse and four hamster lines. Out of 14 rat lines, three were found inducible for infectious virus, and one could be induced only for V-antigen synthesis. Two lines, designated as RPA and RPB differ from the others with respect to their response to mitomycin C and elevated temperature (40°). The proportion of V-antigen-containing cells increased 50- and 60-fold in the RPA and RPB lines, respectively, when the mitomycin C-treated cultures were subsequently incubated at 40° for 24 hr, as compared to the cultures kept continuously at 37°. No infectious virus was detected in the mitomycin C-treated RPA cultures incubated at either temperature. The RPB line can be induced for infectious virus production at a cell frequency of $\text{1}\text{100}$ V-antigen-containing cells when incubated at 37°. Though the proportion of V-antigen-containing cells increased in the heated cultures of this line 60-fold, the number of plaques produced by the cell extracts and the proportion of virus-producing cells were similar both in the heated and unheated cultures. Fifty percent of clones isolated from the RPA line and about 80% of RPB clones are inducible, exhibiting, except for one RPB clone, a similar pattern of response to the inducing agents as the parental lines. In heterokaryon cultures of untreated mouse embryo cells and RPA or RPB cells pretreated with mitomycin C and heat, a proportion of multinucleated cells was shown to contain V-antigen. However, virus maturation was not detected in the heterokaryon cultures with RPA cells and was not significantly enhanced in the cultures with RPB cells. We also studied the effect of elevated temperature (40°) on PV induction by mitomycin C in highly inducible PV-transformed lines. In contrast to the RPA and RPB lines, in the highly inducible lines, the elevated temperature, though exerting a relatively weak enhancing effect on induction by mitomycin C, increased the proportion of V-antigen-containing cells and the virus yields to an almost equal extent. These results would indicate that the lack of virus production in the V-antigen-containing RPA and in most RPB cells is due to a deficiency of the viral genome persisting in these cells and that a temperature-sensitive factor is involved in the process of V-antigen induction in these lines.