SPECIFIC ABLATION OF HUMAN-IMMUNODEFICIENCY-VIRUS TAT-EXPRESSING CELLS BY CONDITIONALLY TOXIC RETROVIRUSES

The identification of human immunodeficiency virus (HIV) as the etiolo gic agent of AIDS has led to the proposal of novel intervention strate gies to block HIV infection and viral replication or eliminate HIV-inf ected cells. We have produced recombinant retroviruses for a molecular ablation system, whereby a toxin gene can be delivered to hematopoiet ic cells for the specific elimination of HIV Tat-expressing cells. For this cell-specific ablation, we have coupled the conditional toxin he rpes simplex virus type 1 thymidine kinase (tk) gene to the HIV-2 prom oter and Tat responsive region (TAR) in order that transcriptional act ivity be under the absolute control of HIV and simian immunodeficiency virus Tat trans-activator proteins. Since the HIV-2 promoter has a co nsiderable level of basal expression in the absence of Tat, we constru cted a number of modifications in the HIV-2 promoter to minimize the r isk of cytotoxicity to cells not containing HIV Tat. We demonstrate th at certain promoter modifications reduce basal transcription while mai ntaining high trans-activated levels of expression when transfected or transduced by retroviral vectors into several different cell lines. I n mouse and human cells infected with HIV-2 tk retroviruses, we show t hat Tat-induced expression from the HIV-2 promoter results in differen tial ablation and a massive reduction in Tat-positive cells after ganc iclovir treatment. Thus, the retroviruses produced in these studies ma y be applicable to HIV ablative therapy.