Specific ablation of human immunodeficiency virus Tat-expressing cells by conditionally toxic retroviruses

The identification of human immunodeficiency virus (HIV) as the etiologic agent of AIDS has led to the proposal of novel intervention strategies to block HIV infection and viral replication or eliminate HIV-infected cells. We have produced recombinant retroviruses for a molecular ablation system, whereby a toxin gene can be delivered to hematopoietic cells for the specific elimination of HIV Tat-expressing cells. For this cell-specific ablation, we have coupled the conditional toxin herpes simplex virus type 1 thymidine kinase (tk) gene to the HIV-2 promoter and Tat responsive region (TAR) in order that transcriptional activity be under the absolute control of HIV and simian immunodeficiency virus Tat trans-activator proteins. Since the HIV-2 promoter has a considerable level of basal expression in the absence of Tat, we constructed a number of modifications in the HIV-2 promoter to minimize the risk of cytotoxicity to cells not containing HIV Tat. We demonstrate that certain promoter modifications reduce basal transcription while maintaining high trans-activated levels of expression when transfected or transduced by retroviral vectors into several different cell lines. In mouse and human cells infected with HIV-2 tk retroviruses, we show that Tat-induced expression from the HIV-2 promoter results in differential ablation and a massive reduction in Tat-positive cells after ganciclovir treatment. Thus, the retroviruses produced in these studies may be applicable to HIV ablative therapy.