4T1, TS/A, and AT3 mammary tumors induce Gr1⁺CD11b⁺ immune-suppressive MDSCs in BALB/c and C57BL/6 mice, respectively. (A) Gr1⁺CD11b⁺ cells were obtained from the blood of tumor-free BALB/c mice or mice with 4T1, TS/A, or AT3 tumors and incubated at the indicated ratios with splenocytes from DO11.10 transgenic mice and OVA_323-339 peptide. T-cell activation was measured by incorporation of ³H-thymidine. MDSCs were pooled from 3-4 mice/group and were > 90% Gr1⁺CD11b⁺ cells. (B) MDSCs and splenocytes from panel A were incubated at a 2:1 ratio in the presence of the arginase inhibitor nor-NOHA or the iNOS inhibitor L-NMMA. Average +SD of triplicates. One of 2 independent experiments is shown. (C) MDSCs were purified and pooled from primary tumors (TIMDSCs) of 12 mice or peripheral blood of 3 mice and were incubated at the indicated ratios with splenocytes from Clone4 transgenic mice and HA_518-526 peptide.