Immune profiling of pediatric oncohistone gliomas reveals diverse myeloid populations and tumor-promoting behaviors

Abstract Pediatric high-grade gliomas pHGG are lethal and frequently bear missense mutations in histone H3, which drive tumorigenesis by altering the epigenome and cell fate/differentiation. While previous studies showed intrinsic contingencies associated with tumor development, limited information exists on the tumor microenvironment (TME) and immune cells for these tumors. To define the immune landscape of pediatric tumors at single-cell resolution, we profiled and compared 69 pediatric gliomas samples using the Chromium 10X technologies, H3.3 K27M (N=19) and G34R (N=16) mutant tumors, low-grade gliomas (N=11) and ependymomas (N=23). Additionally, to enable the spatial resolution of immune lineages, we performed Imaging Mass Cytometry (IMC) on H3-mutant samples (H3.3 K27M (N=7) and G34R (N=5)). We demonstrate that pediatric histone H3-mutant gliomas are highly infiltrated by myeloid cells, more specifically resident microglia, bone-marrow derived macrophages (BMDM) and monocytes, and are devoid of lymphoid infiltration. Different myeloid subsets were found to interact with H3-mutant cancer cells. Classical BMDM showed strong interactions with H3-mutant cancer cells while monocytes subsets showed tendency to interact only with G34R cells. We have generated a novel H3.3K27M glioma immunocompetent murine model, establishing a highly-penetrant and reliable tool. H3.3K27M murine tumors recapitulated the TME from human tumors, showing great myeloid infiltration. Using in vivo orthotopic serial engraftments, we observed that secondaries transplanted tumors had a significantly worse survival, possessing a less diverse immune infiltration compared to initial engraftments and a dominant presence of myeloid cells. Interestingly, in vivo myeloid depletion combined with PD-1 blockade extended overall mice survival. Our findings provide a valuable characterization of the biology of these tumors, reinforcing the several roles of myeloid cells in the context of pediatric brain tumours, providing a framework for understanding the H3.3K27M and G34R tumors, including other pediatric gliomas, and designing future immunotherapies.