Metastatic cancer cells facilitate recruitment of MDSCs. (A-B) EMT6 and 4T1 cells were injected into the mammary fat pads of BALB/c mice. (A) Primary tumor growth (n = 8). (B) Representative photographs of lungs 26 days after cell injection (H & E). (C) The percentages and absolute numbers of MDSCs (CD11b+Gr-1+) at 19 days (n = 4). (D) 4T1 cell-bearing mice were treated intraperitoneally with anti-Gr-1 antibodies. Primary tumor growth (n = 8). (E) Numbers of tumor nodules in the lungs from 4T1 cell-bearing mice at 26 days. MDSCs, myeloid-derived suppressor cells.