Measurement of human immunodeficiency virus type 1 preintegration transcription by using Rev-dependent Rev-CEM cells reveals a sizable transcribing DNA population comparable to that from proviral templates.

Preintegration transcription is an early process in human immunodeficiency virus type 1 infection and has been suggested to occur at a low level. The templates have also been suggested to represent a small population of nonintegrated viral DNA, particularly the two-long-terminal-repeat (2-LTR) circles. However, these determinations were made by either using PCR amplification of viral transcripts in bulk cell populations or utilizing the LTR-driving reporter cells that measure the synthesis of Tat. The intrinsic leakiness of LTR often makes the measurement of low-level viral transcription inaccurate. Since preintegration transcription also generates Rev, to eliminate the nonspecificity associated with the use of LTR alone we have developed a novel Rev-dependent indicator cell, Rev-CEM, to measure preintegration transcription based on the amount of Rev generated. In this report, using Rev-CEM cells, we demonstrate that preintegration transcription occurs on a much larger scale than expected. The transcribing population derived from nonintegrated viral DNA was comparable (at approximately 70%) to that derived from provirus in a productive viral replication cycle. Nevertheless, each nonintegrated viral DNA template exhibited a significant reduction in the level of transcriptional activity in the absence of integration. We also performed flow cytometry sorting of infected cells to identify viral templates. Surprisingly, our results suggest that the majority of 2-LTR circles are not active in directing transcription. It is likely that the nonintegrated templates are from the predominant DNA species, such as the full-length, linear DNA. Our results also suggest that a nonintegrating lentiviral vector can be as effective as an integrating vector in directing gene expression in nondividing cells, with the proper choice of an internal promoter.