Glioblastomas (GBMs) are tumors of the central nervous system that remain recalcitrant to both standard of care chemo-radiation and immunotherapies. Emerging approaches to treat GBMs include depletion or re-education of innate immune cells including microglia (MG) and macrophages (MACs). Here we show myeloid cell restricted expression of triggering receptor expressed on myeloid cells 2 (TREM2) across low- and high-grade human gliomas. TREM2 expression did not correlate with immunosuppressive pathways, but rather showed strong positive association with phagocytosis markers such as lysozyme (LYZ) and CD163 in gliomas. In line with these observations in patient tumors, Trem2-/- mice did not exhibit improved survival compared to wildtype (WT) mice when implanted with mouse glioma cell lines, unlike observations previously seen in peripheral tumor models. Gene expression profiling revealed pathways related to inflammation, adaptive immunity, and autophagy that were significantly downregulated in tumors from Trem2-/- mice compared to WT tumors. Using ZsGreen-expressing CT-2A orthotopic implants, we found higher tumor antigen engulfment in Trem2+ MACs, MG, and dendritic cells. Our data uncover TREM2 as an important immunomodulator in gliomas and inducing TREM2 mediated phagocytosis can be a potential immunotherapeutic strategy for brain tumors.

The brain tumor immune microenvironment (TIME) continuously evolves during glioma progression, but only a limited view of a highly complex glioma associated immune contexture across isocitrate dehydrogenase mutation (IDH) classified gliomas is known. Herein, we present an unprecedentedly comprehensive view of myeloid and lymphoid cell type diversity based on our single cell RNA sequencing and spectral cytometry-based interrogation of tumor-associated leukocytes from fifty-five IDH stratified primary and recurrent human gliomas and three non-glioma brains. Our analyses revealed twenty-two myeloid and lymphoid cell types within and across glioma subtypes. Glioma severity correlated with microglial attrition concomitant with a continuum of invading monocyte-derived microglia-like and macrophages amongst other infiltrating conventional T and NK lymphocytes and unconventional mucosa associated invariant T (MAIT) cells. Specifically, certain microglial and monocyte-derived subpopulations were associated with antigen presentation gene modules, akin to cross-presenting dendritic cells (DCs). Furthermore, we identified phagocytosis and antigen presentation gene modules enriched in Triggering receptor expressed on myeloid (TREM)-2+ cells as a putative anti-glioma axis. Accelerated glioma growth was observed in Trem2 deficient mice implanted with CT2A glioma cells affirming the anti-glioma role of TREM2+ myeloid cells. In addition to providing a comprehensive landscape of glioma-specific immune contexture, our investigations discover TREM2 as a novel immunotherapy target for brain malignancies.

The tumor microenvironment in glioblastoma (GBM) contains numerous cell types, notably a rich immune compartment dominated by myeloid cells, such as brain-resident microglia and infiltrating macrophages. However, current immunotherapies target lymphoid components like T cells, which are sparse in the brain and the tumors originating there. Therefore, immune checkpoint drugs designed for systemic cancers have not yielded significant benefits for patients with glioblastoma, and the efficacy of immunotherapy for central nervous system malignancies lags behind treatments for other solid tumors. Further understanding of the myeloid compartment is necessary for developing glioblastoma immunotherapies. Our recent single cell RNA-seq (scRNA-seq) analyses have identified microglial clusters that are enriched for antigen presentation and phagocytic gene expression modules. These clusters were restricted to microglia and macrophages seen only in glioma patients, but not in myeloid cells derived from non-glioma brain controls. Triggering receptor expressed on myeloid cells 2 (TREM2), a receptor abundant on microglia and vital for the detection and phagocytosis of pathogens, was highly expressed in these clusters. Because microglia expressing TREM2 play a protective role in neurodegenerative diseases like Alzheimer's disease, we examined the function of TREM2+ myeloid cells in GBM using mice lacking TREM2. Implantation of GL261 cells in TREM2-/- mice showed significantly reduced survival relative to wild-type mice, suggesting a protective role of TREM2+ myeloid cells in glioma. Data on immunophenotyping of glioma xenografts from WT and TREM2-/- mice brains using state of the art technologies (e.g. Cytek) will be presented. We conclude that TREM2 is a phagocytic immunoregulator in glioblastoma, manifesting anti-glioma properties to TREM2+ myeloid cells.