Stat3 phosphorylation in MDSC-activated cancer cells contributed to tumor invasiveness and distant lung metastasis. (A-B) 4T1_Con and 4T1_shStat3 cells were allowed to invade through Matrigel for 18 hours in the presence or absence of 4T1/MDSC-CM. (A) Invaded 4T1 cells (crystal violet). (B) Invaded cells were counted using ImageJ software. (C-D) 4T1_Con and 4T1_shStat3 cells were injected into the mammary fat pads. (C) Primary tumor growth. (D) Numbers of metastatic masses in the lungs at 26 days. Values are the means ± SEM of each group (n = 5). CM, conditioned media; MDSC, myeloid-derived suppressor cells.