Anti–IFN-γ, but not anti–TGF-β or anti–IL-4, neutralizing antibody reduced the ability of HMBPP-activated Vγ2Vδ2 T cells to down-regulate the IL-2–induced expansion of CD4⁺CD25⁺Foxp3⁺ T cells. (A) Flow histograms from one representative monkey (7741) show that IL-2–induced expansion of Tregs was inhibited by HMBPP + IL-2, but the inhibition was reversed significantly by adding neutralizing anti–IFN-γ mAb (1 or 3 μg/mL) to HMBPP + IL-2 in PBMC culture. No effect was seen when adding neutralizing anti–TGF-β mAb, or neutralizing anti–IL-4 mAb to the HMBPP + IL-2 culture. (B,C) Numbers of CD4⁺CD25⁺Foxp3⁺ T cells and Vγ2Vδ2 T cells in PBMC cultures treated with medium alone, IL-2, HMBPP + IL-2, HMBPP + IL-2 + anti–TGF-β mAb, HMBPP + IL-2 + anti–IL-4 mAb, and HMBPP + IL-2 + anti–IFN-γ mAb. Data were mean values with error bars of SEM from PBMCs of 3 monkeys in 2 independent experiments. (D) Concentrations of IFN-γ protein in PBMC cultures treated with medium alone, IL-2, HMBPP + IL-2, HMBPP + IL-2 + anti–TGF-β mAb, HMBPP + IL-2 + anti–IL-4 mAb, and HMBPP + IL-2 + anti–IFN-γ mAb. **P < .01; *P < .05.