ARTICLE ABSTRACTMatrix metalloproteinase 9 (MMP-9/Gelatinase B) is overexpressed in pancreatic ductal adenocarcinoma (PDAC) and plays a central role in tumor cell invasion and metastasis. Here we complemented mechanistic insights in the cancer biology of MMP-9 and investigated the effects of specific long-term loss-of-function, by genetic ablation, of MMP-9 on PDAC initiation and progression in the well-established KPC mouse model of spontaneous PDAC. Tumor growth and progression were analyzed by histopathology and IHC. Invasive growth of PDAC cells was analyzed by both in vitro (proliferation, survival, migration, invasion assays) and in vivo (experimental metastasis assays) methods. Retroviral shRNAi was used to knockdown target genes (MMP-9, IL6R). Gene expression was analyzed by qRT-PCR, immunoblot, ELISA, in situ hybridization, and zymography. PDAC tumors from MMP-9–deficient mice were dramatically larger, more invasive, and contained more stroma. Yet, ablation of MMP-9 in PDAC cells did not directly promote invasive growth. Interestingly, systemic ablation of MMP-9 led to increased IL6 levels resulting from abrogation of MMP-9–dependent SCF signaling in the bone marrow. IL6 levels in MMP-9−/− mice were sufficient to induce invasive growth and STAT3 activation in PDAC cells via IL6 receptor (IL6R). Interference with IL6R blocked the increased invasion and metastasis of PDAC cells in MMP-9–deficient hosts. In conclusion, ablation of systemic MMP-9 initiated fatal communication between maintenance of physiological functions of MMP-9 in the bone marrow and invasive growth of PDAC via the IL6/IL6R/STAT3 axis.Implications: Thus, the beneficial effects of host MMP-9 on PDAC are an important caveat for the use of systemic MMP-9 inhibitors in cancer. Mol Cancer Res; 14(11); 1147–58. ©2016 AACR.