IMMU-10. PILOCYTIC ASTROCYTOMAS SHOW EVIDENCE OF INTRINSIC IMMUNE SURVEILLANCE AND ACTIVATION WITHIN THE TUMOR MICROENVIRONMENT

PURPOSE: To clarify the immunoreactivity and potential immunotherapeutic targets of pediatric low- and high-grade gliomas (HGG). METHODS: Spatial sequential immunofluorescence (seqIFTM) of 24 protein markers, NanoString analysis of 770 immune genes, and scRNA sequencing were used to profile and characterize the tumor microenvironment (TME) of 13 newly diagnosed WHO grade 1 pilocytic astrocytomas (PA), 6 pediatric HGG, and 3 normal brain (NB) samples. RESULTS: Relative to NB, HGGs were enriched with T cells, cancer-associated fibroblasts (CAF), CD11c+ antigen-presenting cells, and CD163+ macrophages. In contrast, PA showed more CD11c+P2RY12+ microglia and CD11c+CD205+ dendritic cells (DCs). TMEM119+ microglia were present in NB and brain adjacent to PA but were rare within the TME of all gliomas regardless of histology. PA uniquely contained naïve CD45RO-FoxP3-PD-1-TIM-3-LAG-3- T cells forming immunological synapses, as visualized by Lck+ expression at the membrane interfaces, during interaction with either microglia or DCs. This finding was not encountered in HGG. TIM-3 was the most frequently expressed immune modulatory target on all the myeloid populations, regardless of tumor histology. PD-L1 expression, especially on microglia, was more frequent in PA than in HGG. NanoString analysis revealed the pro-inflammatory and enabler of diapedesis IL-1 pathway genes (IL1RAP, TICAM2, and IRAK4) were upregulated in PA relative to the matched adjacent brain using unsupervised hierarchical clustering. CONCLUSIONS: Based on multiple orthogonal analysis strategies, PAs display a more pro-inflammatory phenotype, in comparison to HGG, suggesting immunological surveillance that may be driven by IL-1, as a baseline feature at presentation.