Activated MDSCs contributed to tumor invasiveness through IL-6 trans-signaling. (A-B) 4T1 cells were treated with recombinant IL-6 plus soluble IL-6Rα (A) or 4T1/MDSC-CM (B) for 30 minutes in the presence of anti-IL-6R blocking antibody or gp130-Fc. (C) 4T1 cells were allowed to invade through Matrigel for 18 hours in the presence or absence of 4T1/MDSC-CM and/or gp130-Fc (crystal violet). (D-E) 4T1 cells were injected into the mammary fat pads. Some mice underwent continuous administration using osmotic mini-pumps (5 or 10 μg for 14 days). (D) Primary tumor growth and (E) numbers of metastatic masses in the lungs at 26 days. (F) 4T1 cells were injected intravenously into BALB/c mice (n = 5 mice per group). Some mice received gp130-Fc (2.5 μg) 4 days after cancer cell injection. Numbers of metastatic masses in the lungs at day 12 were determined. Values are the means ± SEM of each group. *P < 0.05, **P < 0.01, ***P < 0.001. CM, conditioned media; MDSC, myeloid-derived suppressor cells.