ANALYSIS OF TAT FUNCTION IN HUMAN-IMMUNODEFICIENCY-VIRUS TYPE 1-INFECTED LOW-LEVEL-EXPRESSION CELL-LINES U1 AND ACH-2

The U1 and ACH-2 cell lines are subclones of human monocytic and T-lym phoid cells, respectively, persistently infected with human immunodefi ciency virus type 1. These cell lines harbor the viral genome but prod uce only very low levels of viral progeny, which can be increased by s timulation with agents such as phorbol ester and cytokines. As such, t hey provide an in vitro model for human immunodeficiency virus type 1 latency. In order to examine the basis for their latent state, we have analyzed the activity of endogenous Tat protein in these cells and in vestigated the effect on viral replication of the addition of exogenou s Tat protein. We find that U1 cells seem to have levels of Tat protei n that are suboptimal for long terminal repeat (LTR) transcription, be cause transcription from a transfected LTR-chloramphenicol acetyltrans ferase plasmid can be enhanced by cotransfection of a Tat expression p lasmid. Furthermore, viral replication can be stimulated in this cell line by incubation with purified Tat protein. In contrast, ACH-2 cells are not limited for LTR-chloramphenicol acetyltransferase transcripti on by endogenous levels of Tat, and virus production is not increased by the addition of exogenous Tat protein. By semiquantitative PCR anal ysis of viral RNA, we have demonstrated that Tat protein caused an inc rease in human immunodeficiency virus RNA expression in U1 cells but h ad no effect in ACH-2 cells. This suggests that a different mechanism underlies the latent state in U1 and ACH-2 cells.