PROJECT SUMMARY Brain metastasis is a major clinical challenge associated with high morbidity and mortality. Growing evidence suggests a critical role for the immune system in the pathophysiology of brain metastasis. Our group and others have demonstrated that microbiota, i.e. the assembly of microbes residing within the human body, plays a major role in shaping immune responses and tumor immunity. However, the impact of different microbial communities on brain metastasis remains poorly understood, limiting our ability to harness the power of accessible and cost- effective microbiome modulation strategies to enhance clinical care and improve the outcome of brain metastasis patients. My postdoctoral studies demonstrated, for the first time, that the brain metastasis tumor microenvironment harbors intracellular bacterial signals, confirming the existence of a tumor microbiome in brain metastasis tumors. Importantly, this tumor microbiome was enriched in oral bacteria, suggesting that the oral microbiota plays a role in the formation of the tumor microbiome in brain metastases. Furthermore, spatial transcriptomic analysis of patient and murine samples demonstrated that the oral microbiota and the orally derived tumor microbiome can be associated with transcriptional alterations in the brain, involving both resident microglial cells at early stages and infiltrated innate immune cells at later stages. Building on these studies, this proposal pioneers the investigation of the impact of the oral microbiota in brain metastasis development and progression. In Specific Aim 1 (K99 phase), I will elucidate the mechanisms underlying the formation of the orally derived tumor microbiome in brain metastases. In Specific Aim 2 (K99/R00 phase), I will use longitudinal clinical studies and spontaneous mouse models of brain metastasis to determine the local and systemic contribution of the oral microbiota in brain metastasis development and progression. Insights gained from these studies can inform the development of novel diagnostic, preventive, and therapeutic strategies for metastatic brain tumors. The mentored phase of this project will be conducted under the supervision of Drs. Jennifer Wargo, James Allison, and Michael Davies, at the University of Texas MD Anderson Cancer Center, who are world-renowned experts in microbiome, cancer immunology, and brain metastasis research. An outstanding advisory team will also provide guidance on the various aspects of this proposal and training activities. My exceptional mentoring and advisory teams and the rich intellectual environment at the MD Anderson Cancer Center offer valuable opportunities for successful completion of the proposed research and career development plan. Completion of the mentored phase of this award will set the stage for a smooth transition to an independent career where I can integrate the emerging fields of microbiome and brain metastasis research to tackle outstanding questions in cancer research.

PROJECT NARRATIVE Brain metastasis, the spread of tumor cells from the primary tumor to the brain, is the most common type of brain tumor in adults and associated with a significantly poor outcome. Different microbial communities reside within our bodies including the oral cavity and can impact tumor development through changing the behavior of the immune cells. This proposal aims to improve our understanding of the role of oral microbes in the development and progression of brain metastasis, with the ultimate goal of providing novel opportunities to develop diagnostic, preventive, and therapeutic strategies to improve the outcome of this disease.