OTME-11. Characterizing the immunologic context of pediatric brain tumors

Abstract Therapy for pediatric central nervous system (CNS) malignancies can be toxic, and outcomes are suboptimal. Immunotherapy holds promise as a therapeutic avenue, but the poorly understood microenvironment limits its application. The Children’s Brain Tumor Network (CBTN) released the Pediatric Brain Tumor Atlas, containing expression profiles of nearly 700 primary CNS tumors. To study the immune microenvironment, a classification from The Cancer Genome Atlas project is applied. High-grade lesions are predominantly lymphocyte deplete (C4, 81%) or immunologically quiet (C5, 11%). Low-grade lesions are more mixed with 46% C4, 21% C5, and a higher proportion of inflammatory subtype (C3, 31%). For survival parameters, adjusting for tumor grade and extent of resection, the hazard ratio is 2.2 (0.78 – 6.3), p = 0.13) and 2.4 (0.6 – 10.0, p = 0.24) for C4 and C5, respectively. With no events among low-grade tumors, progression-free survival will be another useful metric and released by CBTN in April. Deconvolution of immune cell gene signatures among C4 samples reveals decreased abundance of T cells (OR 0.26, 0.1 – 0.5) yet increasing T-cell abundance is associated with decreased survival time in high-grade samples (HR 3.7, 1.4 – 10.1). Additionally, there are increased macrophage and decreased microglia signatures among high-grade samples and the C4 and C5 subtypes. It is hypothesized that expression of inhibitory immunomodulators contributes to a pro-tumorigenic microenvironment and represent potential therapeutic targets. In lieu of normal tissue in the data set, differential gene expression experiments between disease states reveals upregulated immunomodulators. Conventional immunomodulators, e.g. PDL1 and CTLA4, are expressed in low-grade samples with C3 subtype, which is abundant in craniopharyngioma. Alternative inhibitory immunomodulators, e.g. KDM1A, EZH2, CD276, are significantly expressed in high-grade samples including diffuse midline glioma. Overall, our analysis contributes to the understanding of the immune microenvironment and identifies potential mechanisms of immune escape among pediatric CNS tumors.