gp120-mediated activation of human regulatory T cells. (A) Induction of suppressive activity in human Tregs by recombinant gp120. Tregs were incubated with irradiated syngeneic, T-cell-depleted PBMC and stimulated with 1 μg/mL recombinant gp120 (strain JR-FL). After 16 hours, allogeneic CD8⁺ T cells were added to the cultures. Cultures without addition (Ø) or stimulated with 0.5 μg/mL anti-CD3 mAbs served as controls. Results represent the mean plus or minus SE (Treg:CD8 ratio 1:1, of 10 experiments; **P < .005). (B) Dose-dependent activation of Tregs by gp120. Experimental setup as under panel A. (C) No induction of suppressive activity in CD4⁺CD25⁻ T helper cells by gp120, experimental setup as under panel A but with CD4⁺CD25⁻ T helper cells instead of Tregs. One of 4 experiments (**P < .005). (A-B) : allogeneic CD8⁺ T effector cells; □: Tregs, ■: coculture of Tregs and allogeneic CD8⁺ T effector cells. (C) : allogeneic CD8⁺ T effector cells; ▤: CD4⁺CD25⁻ T helper cells; ▨: coculture of CD4⁺CD25⁻ T helper cells with allogeneic CD8⁺ T effector cells. (D) Induction of suppressive activity in human Tregs by prestimulation with gp120. Tregs were stimulated with plate-bound gp120 or anti-CD3 mAbs (OKT-3, 1 μg/mL each) or left unstimulated (Ø) for 8 hours. Thereafter, cells were washed and cocultured with T-cell-depleted syngenic PBMC and allogeneic CFSE-labeled CD8⁺ T cells. Proliferation of alloreactive CD8⁺ T cells was determined on day 4. Results are representative of 3 experiments.