The U1 and ACH-2 cell lines are human monocytic and T-lymphoid cells respectively, persistently infected with HIV-1. These cell lines express viral RNA and proteins at very low levels, but can be induced to produce larger amounts virion particles. For this reason, they have been used extensively as in vitro models of HIV latency. In order to examine the basis for the latent state of HIV in these cell lines, we have investigated the roles of the essential viral regulatory factor Tat. First, we transiently transfected the HIV LTR as a reporter plasmid for Tat function. Second, purified Tat protein was added to the culture to test whether HIV production could be increased. Third, we constructed U1 and ACH-2 derivatives constitutively expressing Tat. Results obtained from these analyses indicated that the provirus in the U1 cell line is latent because of suboptimal levels of Tat, while Tat is not limiting for viral production in ACH-2 cells. A different mechanism therefore underlies the latent state of HIV in U1 and ACH-2 cells. This result also argues against a model of HIV latency based simply on suboptimal levels of the Rev protein. We are currently studying the role(s) of Rev using experimental schemes similar to those employed for the analysis of Tat.

Cell Line Gene Expression Gene Products, rev/GENETICS/PHYSIOLOGY Gene Products, tat/GENETICS/PHYSIOLOGY Genes, tat Human HIV Infections/ETIOLOGY HIV Long Terminal Repeat HIV-1/GENETICS/*PHYSIOLOGY/PATHOGENICITY Models, Biological Monocytes/MICROBIOLOGY T-Lymphocytes/MICROBIOLOGY Transfection Virus Replication/GENETICS/PHYSIOLOGY ABSTRACT

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