Restricted HEL-12 virus infections in de novo infected human and canine cells

Model systems to study restricted primate retrovirus expression were established by de novo infection of canine foetal thymus cells (CF-2Th) and superinfection of HEL-12 cells with HEL-12 virus. In the resulting CF-2Th/HEL-12V cells and HEL-12/HEL-12V cells, four sequential stages of virus infection were defined by the production of reverse transcriptase (RT)-containing particles and expression of virus antigens as detected by radioimmunoassays. Stage 1 cells did not synthesize virus antigens or produce RT-containing particles. Stage 2 cells synthesized virus antigen but not RT-containing particles. Stage 3 cells synthesized antigen and produced RT-containing particles, and stage 4 cells synthesized virus antigens but no longer produced RT-containing particles. The duration of the four stage infection is 2 to 3 weeks in both cell types. Monospecific competition radioimmunoassays to detect HEL-12V p30 or gp70 antigen showed high levels of virus antigen throughout stages 2 to 4 of infection. Analysis of immunoprecipitates formed under conditions to detect either p30- or or gp70-containing proteins in cells pulsed and pulsed--chased with [3H]leucine showed the same spectrum of virus precursor polyproteins, intermediates and mature virion components in stage 2 to 4 cells in canine and human infections. Spent culture fluids collected from stage 3 and stage 4 CF-2Th/HEL-12V cells failed to reveal inhibitors of RT activity. Stage 4 CF-2Th/HEL-12V or HEL-12/HEL-12V cells labelled with [3H]uridine produced virions which incorporated [3H]uridine but did not have RT activity, suggesting that restricted infection is characterized by production of HEL-12V defective in RT activity.