INITIATION OF REVERSE TRANSCRIPTION DURING CELL-TO-CELL TRANSMISSION OF HUMAN-IMMUNODEFICIENCY-VIRUS INFECTION USES PREEXISTING REVERSE-TRANSCRIPTASE

H3B cells, a laboratory clone of H9 cells persistently infected with t he HTLV-IIIB strain of human immunodeficiency virus (HIV), contained s ignificant levels of cell-associated reverse transcriptase (RT) activi ty measured by in vitro assays using either exogenous or endogenous te mplates. The cell-associated RT activity detected using exogenous temp late was almost wholly in a soluble (non-sedimentable) form whereas en dogenous activity sedimented as a particulate structure associated wit h viral RNA. Despite this, H3B cells did not contain episomal HIV DNA detectable by Southern blot, indicating that in vivo reverse transcrip tion was not occurring to any significant extent in these cells. Howev er, when susceptible HUT 78 cells were infected by co-cultivation with H3B cells, dramatic synthesis of episomal HIV DNA occurred. Concurren tly with this de novo initiation of reverse transcription, however, we found no detectable change in intracellular levels or cleavage profil es of immunoprecipitable RT polypeptides. Finally, actinomycin D pre-t reatment of H3B cells to prevent de novo transcription from donor cell proviral DNA after co-cultivation did not affect the initiation of in vivo reverse transcription following cell-to-cell HIV infection. Thes e results demonstrated that cells persistently infected with HIV conta ined significant fully cleaved cell-associated RT in a form that was a ctive in vitro but not in vivo and that following cell-to-cell transmi ssion of HIV infection to susceptible cells, de novo reverse transcrip tion was initiated without detectable evidence of further synthesis or proteolytic processing of HIV RT. The nature of this initiation proce ss requires further study.