Abstract A023: Tumor-associated microglia mirror neurodevelopment

Abstract Long thought to be immunologically inert, brain cancers feature dynamic microenvironments, comprising complex inflammatory and immunosuppressive programs that contribute to disease progression. However, the tumor microenvironment of pediatric brain cancer—particularly its relationship to embryonic development—remains poorly understood. As resident immune cells of the brain that influence neurological development, homeostasis, and disease, microglia and other myeloid cells may drive brain tumorigenesis in children and represent powerful clinical targets. To gain an initial understanding of the pediatric brain tumor microenvironment, we first examined medulloblastoma using an integrated human single cell RNA sequencing dataset of more than 200 thousand cells. Myeloid cells, featuring granulocyte-like, macrophage-like, and microglia-like transcriptional programs, were the most prominent components of the medulloblastoma microenvironment. We subsequently interrogated myeloid populations in novel genetically engineered mouse models of several pediatric brain tumors, including WNT- and SHH-activated medulloblastoma in addition to ZFTA-RELA fusion-driven ependymoma and choroid plexus carcinoma. In all tumors, relative to wildtype adult brains, we observed pronounced infiltration of Iba1+ microglia exhibiting large, amoeboid morphologies in stark contrast to ramified forms found in surrounding normal parenchyma. Considering that these cancers are thought to derive from persistent embryonic states, we reasoned that their myeloid microenvironments may recapitulate phenotypes of normal development. Histology of embryonic and early postnatal murine brains revealed Iba1+ cells with tumor-associated morphology that disappeared in adulthood. To more directly assess the tumor microenvironment’s relationship with development and other neurological contexts, we integrated single cell RNA sequencing data of human microglia derived from brain tumors—including medulloblastoma, ependymoma, and glioblastoma—with normal embryonic and postnatal development in addition to neurodegenerative conditions such as Alzheimer’s disease. Analysis of nearly 200 thousand microglia across these contexts revealed multiple transcriptional states, including homeostatic and reactive phenotypes. Notably, in contrast to normal post-natal microglia, normal fetal microglia expressed not homeostatic but rather inflammatory and antigen presenting signatures shared with tumor-associated counterparts. In both human sequencing and murine histology, microglia enriched for disease-associated markers, including lysosomal cathepsins, were prominent in early development and malignancy though rare in normal post-natal samples. The myeloid repertoire of malignancy resembles embryonic neurodevelopment, suggesting that these cells may be essential to pediatric brain tumor origins. We are currently conducting in vivo perturbation studies to further explore the role of microglia in brain tumorigenesis, with the hope of unveiling novel treatment strategies for children with these devasting diseases. Citation Format: Katherine E. Masih, Jessica T. Taylor, Elizabeth Cooper, Oscar Baldwin, Shaoli Das, Erica Pehrsson, Jun Wei, Javed Khan, Richard J. Gilbertson, Joseph S. Toker. Tumor-associated microglia mirror neurodevelopment [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Brain Cancer; 2026 Mar 23-25; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2026;86(6_Suppl):Abstract nr A023.