Induction of recruitment and reconstitution of MDSCs in non-metastasizing EMT6 cell-bearing mice enhanced cancer cell metastasis. (A-C) EMT6_Con and EMT6_IL-6 cells were injected into the mammary fat pads of BALB/c mice. MDSCs were analyzed at 21 days. (A) Percentages of MDSCs at 21 days (n = 6). (B) Primary tumor growth (n = 7). (C) Numbers of metastatic nodules in the lungs at 26 days (n = 7) and lung sections at 26 days (H & E). (D) EMT6 cells were injected into the mammary fat pads. Three days later, mice were intravenously injected with splenic MDSCs (5×106/mouse) from 4T1 cell-bearing mice, a total of nine times. Primary tumor growth (n = 5) and representative photographs of lungs at 26 days. MDSCs, myeloid-derived suppressor cells.