PROJECT SUMMARY/ABSTRACT (30 lines of text): Medulloblastomas are one of the most common childhood brain tumors and are comprised of several histological and molecularly distinct subtypes based on genetic, clinical, and prognostic factors. Transcriptional profiling has subsequently identified four subgroups which include wingless/integrated (WNT)-activated, sonic hedgehog (SHH)-activated, Group 3, and Group 4. Each subtype differs in clinical prognoses, in which WNT tumors have best outcomes, SHH and Group 4 have intermediate prognoses, and Group 3 have the worst outcomes. Standard therapeutic treatment of these tumors includes surgery, radiation and chemotherapy. However, long- term side-effects often result in negative side-effects for patients, as noted through global reductions in physiological, psychological, and economic welfare of patients. Immunotherapy, given the success in adult tumors, is an attractive option as it may impart similar therapeutic success with potentially diminished adverse side-effects. Immunotherapies hinge on the relative abundance and cytolytic effector capacity of CD8+ T cells. Activation of these cells is dependent on both the immunogenicity of tumors and on priming by competent migratory antigen presenting cells. Once migrated to tumors, relative antitumor success of T cells is dictated by the capacity of cells to overcome the immunosuppressive tumor environment. T cells must also resist T cell exhaustion which results from the persistent antigenic environment of tumors. Establishment and propagation of this exhaustive cell fate decision has been demonstrated to be intimately tied to epigenetics. Our preliminary data shows striking differences in PD1hiCD8+ T cells between Group 3 and SHH preclinical tumor models, in which there is high infiltration of these T cells into Group 3 tumors and low infiltration into SHH tumors. Conversely, we have observed significantly higher infiltration of PD1hiCD8+ T cells in the tumor-draining lymph nodes of SHH tumors expressing N-Myc as compared to Group 3 tumor-draining lymph nodes. Taken together, this dichotomy may underscore differential therapeutic responsiveness, particularly in the presence of checkpoint blockade. In this proposal, we aim to investigate both tumor immunogenicity and T cell dysfunction as potential mechanisms underscoring these dichotomous T cell responses between medulloblastoma subgroups. In particular, we will examine the inherent immunogenicity of tumors in vivo and determine if established epigenetic mechanisms of T cell exhaustion impacts the abundance and therapeutic responsiveness of T cells in Group 3 and SHH medulloblastomas. The completion of these aims will add to a deeper understanding of the underlying biological differences between the clinical outcomes of medulloblastoma subgroups, as well as inform studies aimed at enhancing the efficacy of T cell-based therapeutics in medulloblastoma. Furthermore, the techniques outlined in this proposal w...