Summary Osteoblasts are emerging as a key component of tumor microenvironment in breast cancer bone metastasis. It has been demonstrated recently that disseminated breast cancer cells interact with the osteoblastic niche to survive and colonize the bone. However, the detailed mechanism for the interaction between tumor cells and osteoblastic niche in bone colonization remains mostly uncharacterized. Furthermore, almost no studies have been done to elucidate the functions of osteoblasts in the resistance of bone metastasis to traditional therapies (i.e., chemotherapy and radiation therapy). In our preliminary studies, we discovered that chemotherapy significantly increases Jagged1 expression level in osteoblasts and in mesenchymal stem/stromal cells (MSCs), the precursors of osteoblasts. Transgenic expression of Jagged1 in osteoblasts in our newly created Col1a1-Jagged1 transgenic mouse model significantly increases the formation of bone metastasis. Importantly, we have developed a humanized monoclonal antibody against Jagged1 (15D11), which demonstrated promising therapeutic effects against bone metastasis in preliminary studies. We hypothesize that the increased expression of Jagged1 in MSCs and osteoblasts promote bone colonization of breast cancer and contribute to chemoresistance and radioresistance by providing survival signaling to metastatic tumor cells. Neutralizing antibodies against Jagged1 may block bone metastasis formation and outgrowth, and sensitize them to traditional therapy. Col1a1-Jagged1 and a series of well establish mouse models will be used for bone metastasis allograft and xenograft studies to evaluate the functional importance of osteoblast Jagged1 in bone metastasis formation and treatment resistance. We will use Jagged1 neutralizing antibody 15D11 in vivo to test its synergistic effect with chemotherapy, radiation therapy and osteoclast-targeting therapy. Furthermore, we will perform experiments to understand the exact molecular pathways leading to elevated expression of Jagged1 in osteoblast in response to chemotherapy, as well as Jagged1-dependent osteoblast-tumor interaction in promoting metastatic colonization and treatment resistance in bone. Our proposed studies will confirm the importance of osteoblast Jagged1 in bone metastasis colonization and treatment resistance. We will also reveal molecular signaling/pathways responsible for the Jagged1-dependent function of the osteoblast niche in bone colonization and treatment resistance. Successful pre-clinical testing of Jagged1 neutralizing antibody will also pave the way toward their application in human patients in the near future. Therefore, we believe our study will likely have a significant impact on improving the treatment of metastatic breast cancer.