Both MDSC fractions suppress antigen-driven T-cell responses via an IFN-γ–regulated mechanism, although using distinct signaling pathways and effector molecules. (A) OT-1 splenocytes were stimulated with OVA in the presence of EG7 MO- or PMN-MDSCs (1:1 ratio), with or without 10 μg/mL blocking anti–IFN-γ mAb. One representative experiment of 3 is shown. (B) OT-1 splenocytes were stimulated with OVA in the presence of EG7 MO- or PMN-MDSCs (1:1 ratio) with or without the indicated inhibitors. Values represent the mean plus or minus SD of the relative percentage suppression taken over 4 individual experiments (relative percentage suppression = suppression relative to the suppression exerted by MDSCs without the addition of inhibitors, which is arbitrarily set to 100%; “Methods”). (C) OT-1 splenocytes were stimulated with OVA in the presence of EG7 MO- or PMN-MDSCs (1:1 ratio), and at the end of the culture (42 hours) supernatant was collected and the nitrite concentration was measured. (D,E) MO- and PMN-MDSCs were purified from WT, iNOS^−/− (D), or STAT1^−/− (E) C57BL/6 mice, bearing EG7 tumors with similar diameters. Percentage suppression of OT-1 proliferation, stimulated with OVA in the presence of various amounts of MDSCs was calculated. Values of percentage suppression represent the mean of 4 (iNOS^−/−) and 6 (STAT1^−/−) individual experiments. *P < .05; **P < .01; ns indicates not significant.