PROJECT SUMMARY With age and exposure to genotoxic stress, tissues throughout the body acquire somatic mutations. In the blood and bone marrow, somatic mutations are frequently found in leukemia-associated genes in subsets of hematopoietic cells even in the absence of hematologic cancer. This phenomenon has been termed Clonal Hematopoiesis of Indeterminate Potential (CHIP) because while many people with such mutations will have no known clinical impact, the presence of these clonal populations is associated with increased risk or poor outcomes in diseases ranging from hematologic malignancy and cardiovascular disease to osteoporosis and COVID-19. While CHIP affects at least 10% of people over age 70, it impacts as many as 25% of patients with solid tumors, likely driven by the genotoxic stress of chemotherapy and radiation. Recent studies have shown that CHIP may result in aberrant inflammatory programming, particularly in myeloid lineages, and that immune cells with CHIP mutations can infiltrate the tumor microenvironment. Moreover, research suggests that CHIP may be associated with worse overall survival among solid tumor patients. Given the importance of the tumor immune microenvironment and the increased burden of CHIP in these populations, there is a fundamental need to examine the interplay between CHIP and solid malignancies and to explore how to clinically manage solid tumor patients with CHIP. This proposal seeks to use clinical sequencing data, patient samples, mouse models, and ex vivo experiments to determine the impact of CHIP on solid tumor outcomes and immune microenvironment using triple negative breast cancer (TNBC) as a model system. TNBC is an aggressive subtype of breast cancer that lacks expression of estrogen receptor, progesterone receptor, and HER2. Importantly, TNBC often has a prominent immune cell infiltrate that is prognostic: the presence of lymphocytes is associated with favorable outcomes, while the presence of tumor-associated macrophages is a negative prognostic indicator. Aim 1 will leverage multiple large biobanks, prospectively collected patient specimens, and a novel mouse model to define CHIP as a prognostic biomarker for TNBC outcomes, including overall survival and response to therapy. Aim 2 will harness both patient-derived immune cells and mouse models to determine how myeloid cells with CHIP mutations interact with TNBC cells. Techniques to be utilized include histopathology, flow cytometry and immunophenotyping, gene expression analysis, and functional immune cell assays. Completion of this work will provide a comprehensive training vehicle for this fellowship and will simultaneously yield new insights into clinical management of solid tumor patients with CHIP, suggesting novel therapeutic approaches for these patients. These studies will also provide a greater understanding of how dysregulated myeloid cell signaling in CHIP clones leads to differential interaction in the tumor microenvironment.