Project Summary Breast cancer remains a major health threat and is the second leading cause of cancer-related death in women in the United States. Treatment of triple negative breast cancer (TNBC) patients is particularly challenging due to the heterogeneity of the disease and plasticity of the cancer cells. TNBC represents about 15% of all breast cancers and is a particularly aggressive subtype, with limited treatment options and very poor prognosis. The plasticity and heterogeneity of TNBCs makes them difficult to target therapeutically. Mounting evidence suggests that normal cells within the tumor microenvironment (TME) play an important role in dictating tumor cell fate. Compared to the unstable genome of cancer cells, the stable genomes of normal cells within the TME make them ideal targets for drug therapies that can be used in combination with therapies targeting cancer cells. Immune cells within the TME play an essential role in tumor progression and metastasis. Moreover, increased numbers of one subtype of immune cell, myeloid-derived suppressor cells (MDSCs), in circulating blood correlates with clinical stage, metastatic tumor burden and increased numbers of circulating tumor cells (CTCs), suggesting it may be a promising prognostic marker in breast cancer patients. However, it is currently unclear if MDSCs are recruited to the TME of TNBC and/or if they contribute to tumor progression and metastasis. Using human patient samples, we found increased numbers of MDSCs in TNBC patient tumors compared to other breast cancer subtypes. Moreover, we found increased expression of Np63 in TNBC patient samples with a high degree of MDSC infiltration, raising the intriguing possibility that MDSC recruitment and/or survival may be driven by Np63 expression in TNBC patients. In support, our published and preliminary studies demonstrate that reducing the level of Np63 in TNBCs decreases tumor growth, progression and metastasis and is associated with reduced MDSC infiltrate. Additionally, we showed reduction of chemokine and Tumor necrosis Factor (TNF) pathway genes in Np63-KD TNBC cells compared to control cells, thus identifying a potential pathways by which Np63+ TNBC cells may regulate chemokine-dependent recruitment and TNF-mediated survival of MDSCs in TNBCs. Finally, we show that MDSCs can promote cancer stem cell function of TNBC cells. Together, our data indicates a novel crosstalk between Np63+ TNBC cells and MDSCs in TNBCs, which could promote the aggressive nature of TNBC tumors, thereby increasing metastasis and mortality of patients. Based on these data, the goal of the current proposal is to delineate a novel mechanism by which Np63+ TNBC cells mediated chemokine and TNF signaling promotes MDSC infiltration and survival, which then feeds back to TNBC cells to promote cancer progression and metastasis.