Project Summary/Abstract Triple negative breast cancer (TNBC) is aggressive and a large percentage of patients develop metastatic disease. Disseminated tumor cells (DTCs) found in the bone marrow (BM) of TNBC patients may be the intermediaries of the metastatic process. Data from our lab as well as others suggest that the immune landscape of BM may influence DTC latency, treatment resistance, and metastatic potential. We have already defined and validated an 8 gene expression-based biomarker panel that can detect DTCs in the BM of treatment naïve TNBC patients and that predicts development of distant metastatic disease. Our recent data indicate that TNBC patients with DTC-positive BM have altered populations of immune cell precursors and this is associated with recurrent disease development. Based on these findings, we hypothesize that immune checkpoint inhibitors will facilitate the elimination of BM DTCs in TNBC patients by altering the immune microenvironment in patients with specific DTC and/or BM immune cell populations, and that cell population-specific gene expression signatures can predict which patients will benefit most from aggressive immunotherapy to prevent metastatic disease relapse. We will test this hypothesis using our extensive biorepository of BM specimens collected from TNBC patients who received conventional chemotherapy, as well as prospectively collected specimens from TNBC patients participating in an independently funded institutional phase II immune checkpoint inhibitor (ICI) trial of carboplatin/paclitaxel/nivolumab with or without cabiralizumab. Our goals are: 1. To evaluate the ability of our 8- gene DTC gene panel to predict distant disease development in TNBC patients enrolled in our ICI therapeutic trial of carboplatin/paclitaxel/nivolumab with or without cabiralizumab; 2. To understand the specific subpopulations of BM DTCs in TNBC patients treated with conventional chemotherapy and ICI therapy which are resistant to therapy, and; 3. To understand alterations in specific T cell and conventional dendritic cell (cDC) populations in the BM when DTCs are present, and how this is impacted by conventional and ICI therapy. The results of this proposal will lead to a greater understanding of immune escape and heterogeneity of BM micrometastatic disease as well as biomarkers for improving conventional and ICI therapy in TNBC patients.