Restricted HEL-12 Virus Infection in de novo Infected Human and Canine Cells

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Abstract
Model systems to study restricted primate retrovirus expression were established by de novo infection of canine fetal thymus cells (CF-2Th) and superinfection of [human embryonic lung] HEL-12 cells with HEL-12 virus. In the resulting CF-2Th/HEL-12V cells and HEL-12/HEL-12V cells, 4 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 4 stage infection is 2-3 wk 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 infectin. Analysis of immunoprecipitates formed under conditions to detect either p30- or gp70-containing proteins in cells pulsed and pulsed-chased with [3H]leucine showed the same spectrum of virus precursor polypepteins, 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 labeled 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.