MDSCs undergo apoptosis by the caspase3 pathway when cocultured with activated FasL⁺ T cells. Purified splenic T cells from FasL^+/+ or FasL^−/− mice were incubated in anti-CD3–coated T flasks at 37°C (“activated”) or 4°C (“resting”). Blood MDSCs (> 90% Gr1⁺CD11b⁺ cells) were added 6 hours later, and the flasks were incubated for an additional 16 hours. (A) Cells were then stained with mAbs to CD11b, Gr1, and cleaved caspase3. (Top) Gating of Gr1⁺CD11b⁺ cells from individual mice. (Bottom) Average percentage of CD11b⁺Gr1⁺ cells expressing cleaved caspase3. Data are average + SD of 2 experiments; n = 6 mice/group. (B) At the end of the 16-hour culture, cells were stained with trypan blue, and the absolute number of live CD11b⁺Gr1⁺ cells was determined. Cells were pooled from 3 mice/group, data are average + SD of triplicates. *P < .01. Percentage of live MDSCs = (total trypan blue negative cells) × (percentage of Gr1⁺CD11b⁺ cells as assessed by flow cytometry).