ABSTRACT The current standard for treatment of regional breast tumors involves surgery, radiotherapy or chemotherapy. While effective at reducing or even eliminating the primary tumor burden, chemotherapy can paradoxically promote cancer dissemination and metastasis. Understanding the molecular mechanisms that link chemotherapeutic treatment to metastasis in breast cancer is paramount to developing more effective treatments and a durable response. Exposure to genotoxic stress, as elicited upon chemotherapy or radiotherapy, can result in the engagement of a senescence program in both tumor cells and non-transformed neighboring cells. The presence of senescent cells has recently been shown to promote aggressive traits in cancer tumor models, including increased proliferation, enhanced angiogenesis and activation of the epithelial- to-mesenchymal transition (EMT) program. EMT confers migratory and invasive features to cancer cells, which facilitate their mobilization out of primary tumor sites and into circulation, favoring the metastatic process. The secretion by senescent cells of a specific set of proinflammatory cytokines and chemokines, collectively referred to as the SASP, is believed to mediate the detrimental effects of senescence on tumor cells. The overarching goal of this study is to leverage our understanding of the SASP to blunt the emergence of aggressive tumors following genotoxic treatment. Specifically, we have identified the flagellin receptor TLR5 as a potent regulator of senescence-driven IL-6 secretion in an siRNA screen. We independently confirmed these results, and demonstrated that TLR5 depletion blunts senescence-induced expression of diverse SASP factors. Overall, TLR5 represents an ideal potential therapeutic target to prevent the detrimental effects of the SASP in chemotherapy-treated breast cancer patients. We propose here to determine the contribution of TLR5 signaling to genotoxic stress-induced SASP production in breast cancer cells (aim 1), and to test the hypothesis that blocking the TLR5 signaling pathway prevents the emergence of aggressive phenotypes in chemotherapy-treated breast cancer (aim 2). The long-term goal of this project is to uncover the therapeutic potential of targeting TLR5 as an adjuvant strategy to prevent the resurgence of aggressive breast tumor cells following chemotherapy treatment.