HIV-specific CTL clone 352-env52 does not lyse bystander cells in the presence of specific target cells, but HIV-infected CD4 T-cell blasts do. Fas-mediated cytolysis can be induced by stimulation with anti-CD3 and PMA. (A) Uninfected fas-sensitive Jurkat cells were labeled with ⁵¹Cr and used as indicator target cells for bystander lysis by CTL. Combinations of the unlabeled CTL clone, uninfected and magnetically selected HIV_IIIB-infected CD4 T-cell blasts were added to the indicator cells. Five times as many CTL were added as CD4 T cells to give an effective E:T ratio of 5:1. The fas agonist antibody CH11 was used as a positive control for fas-mediated Jurkat cell lysis. Assays were performed either without (▪) or with (⧅) fas blocking antibody pretreatment of the radiolabeled indicator cells. (B) PMA activation induces fas-mediated cytotoxicity by clone 352-env52. The clone was treated with medium or was stimulated with PMA and anti-CD3. After 2 hours of culture, the washed CTL were added to ⁵¹Cr-labeled Jurkat cells at an E:T ratio of 2:1. Cytotoxicity was assessed against both untreated (▪) or fas blocking antibody-pretreated (⧅) targets.