In vitro activation, infectivity, and production of endogenous type-C virus from OM rats

T24C, a continuous cell line derived from the pooled thymic tissue of normal inbred OM rats, spontaneously produced type-C virus. The virus genome was expressed cyclically. The amount of RNA-dependent DNA polymerase (RDP) and the number of 1.14 g dense particles/ml fluctuated simultaneously with cultivation. The released virus, RPT24C, did not infect cell lines from the rat, mouse, dog, or human. T31, also a rat thymus line, during its 2.5 years of cultivation did not produce type-C virus. Cocultivation with potentially permissive lines did not rescue any virus. 5-lodo-2'-deoxyuridine treatments at earlier passages yielded negative results. Chemical treatment at passages 111, 116, 123, and 128 yielded varying amounts of 3H-uridine incorporation at a sucrose density of 1.14 g/ml. Enzyme assays on chemically treated T31 cultures tested at passage 111 showed a small but transient burst of RDP activity. T31-B, a subline of T31, which was frozen and thawed once, released rat type-C virus spontaneously at passage 56. Two additional sublines of T31 (NI-T31 and NII-T31) were maintained for 2.5 years in culture without any cell-dispersing treatment. NI-T31, but not NII-T31, spontaneously released type-C virus. Once induced, the type-C viruses from T31-B and NI-T31 were continuously produced.