LGG-22. CHARACTERIZATION OF THE TUMOR MICROENVIRONMENT IN PEDIATRIC LOW-GRADE GLIOMA

BACKGROUND: Pediatric low-grade gliomas (pLGGs) are the most common brain tumors in children. Although pLGGs represent a significant number of patients with slow-growing, often curable tumors, they are also affected by neurologic injuries, multiple relapses and requiring repeated courses of therapy, including surgery, chemotherapy, and radiotherapy. Novel, more effective therapies are urgently needed. METHODS: We used a multipronged approach to better understand the complexity of the tumor microenvironment (TME) within pLGG. We utilized scRNA sequencing, spatial transcriptomics, and cytokine analyses on 23 patient-derived pLGG samples including different tumor types (pilocytic astrocytoma (PA), ganglioglioma (GG), and undefined low-grade glioma (LGG)) and mutation profiles (BRAF V600E, BRAF fusions, and BRAF wild-type) to identify cell types, their spatial arrangement within a tissue, and their underlying functional status. RESULTS: scRNA seq revealed both tumor and immune cells within the TME. Analyses of tumor cell subsets revealed a developmental hierarchy identifying progenitor and mature cell populations. Immune cells comprised both myeloid and T lineage cells, with myeloid cells comprising > 50% of the entire population. Interestingly, there was a significant difference between the prevalence of two major myeloid subclusters between two pLGG subtypes (PA and GG). To better understand the underlying difference, we performed bulk and single-cell cytokine analyses to identify immune signals via which these cells communicate. Cytokine analyses revealed distinct immune phenotypes among tumor samples. Notably, BRAF fusion tumors appeared more immunogenic, especially by secreting higher levels of immune cell activators, chemokines (including CCL3 and CCL4), compared to BRAF V600E tumors. Additionally, spatial transcriptomics revealed the differential expression of these chemokines between PA and GG tumors. CONCLUSIONS: Characterization of pLGG demonstrated the complexity of tumor microenvironment among different pLGG subtypes. Further investigation of the immune cells and tumor-immune cell interactions has the potential to unravel novel therapeutic targets against pLGG.