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PDTM-11. GAINING INSIGHTS INTO THE INFLAMMATORY MICROENVIRONMENT OF PEDIATRIC HIGH-GRADE GLIOMAS USING GEMMs AND PATIENT SAMPLES


 Pediatric high-grade gliomas (pHGG) account for the most cancer-related deaths in children, as there are no effective therapies available. It is known tumor associated macrophages (TAM) can make up 30–40% of the total tumor cell mass in adult high-grade gliomas, promoting tumor growth and immune evasion. This raises the question of whether pHGGs possess a distinct constituency of TAMs due to their unique genetic and epigenetic landscapes. To uncover the composition and behavior of TAMs in pHGG we utilize RCAS/tva, a somatic cell-type specific gene transfer system which allows us to recapitulate all major subtypes of pHGG in newborn immunocompetent mice, including histone wild-type and histone-mutant tumors. We combine RCAS-H3.3K27M, RCAS-H3.3G34R/V, or RCAS-H3.3WT along with their driver mutations such as RCAS-shp53 and RCAS-PDGFA or RCAS-PDGFB. These tumors are induced in Nestin-positive cells, each in their respective locations found in the human population. Tumors driven by PDGFB have a significantly lower median survival compared to PDGFA-driven tumors and have increased infiltration of lymphocytes and TAMs, specifically inflammatory monocytes. In vitro bone marrow derived monocyte and microglial cultures demonstrate the BMDM population is most responsible for the production of inflammatory chemokines and angiogenic factors in the tumor microenvironment. We performed histological analyses on over 40 human patient samples to determine the role of the stromal population in TAM infiltration. Matched human samples were also utilized for pan-cancer immune profiling with NanoString to further characterize the innate and adaptive immune microenvironments. These analyses indicate DIPG/K27M tumors have a higher immune cell infiltrate compared to G34R/V and histone wildtype tumors. Further, we observe higher infiltration of T-cell populations in pHGGs compared to adult HGGs, suggesting these tumors may be amenable to immunotherapy despite being considered “immune cold.” These studies provide the critical foundation needed for the development of novel therapeutics targeting these tumors.