Triggering receptor expressed on myeloid cells 2 (TREM2) plays important roles in brain microglial function in neurodegenerative diseases, but the role of TREM2 in the GBM TME has not been examined. Here, we found that TREM2 is highly expressed in myeloid subsets, including macrophages and microglia in human and mouse GBM tumors and that high TREM2 expression correlates with poor prognosis in patients with GBM. TREM2 loss of function in human macrophages and mouse myeloid cells increased interferon-γ–induced immunoactivation, proinflammatory polarization, and tumoricidal capacity. In orthotopic mouse GBM models, mice with chronic and acute Trem2 loss of function exhibited decreased tumor growth and increased survival. Trem2 inhibition reprogrammed myeloid phenotypes and increased programmed cell death protein 1 (PD-1) + CD8 + T cells in the TME. Last, Trem2 deficiency enhanced the effectiveness of anti–PD-1 treatment, which may represent a therapeutic strategy for patients with GBM.
Tumor-associated myeloid cell populations occupy a major part of the glioblastoma (GBM) tumor microenvironment (TME). The prevailing view is that myeloid cells in the TME are immunosuppressive and promote GBM tumor progression. However, myeloid cells have the functional plasticity to either restrict or support tumor cell growth. TREM2 has been shown to alter the myeloid cell landscape in cancers arising in the body and plays important roles in brain microglial function in neurodegenerative diseases. But the role of TREM2 in the GBM TME and specifically in myeloid cell function has not been examined. Here we found that TREM2 is highly expressed in myeloid subsets, including macrophages and microglia in human and mouse GBM tumors, and that high TREM2 expression is associated with poor prognosis in GBM patients. TREM2 loss of function in human macrophages and mouse myeloid cells increased tumoricidal capacity in vitro using patient-derived glioblastoma stem cells and mouse glioblastoma cells. Accordingly, we found TREM2 in myeloid cells restricts proinflammatory polarization in both LPS-induced innate and IFNγ-induced adaptive immunity in vitro, mainly through the inhibition of NFκB and MAPK p38 signaling pathways. Orthotopic injection of mouse glioblastoma cells into TREM2 knockout mice increased animal survival compared to littermate wildtype mice. In addition, co-implantation of TREM2 knockdown macrophages or microglia with mouse glioblastoma cells in brains of immunocompetent mice increased animal survival compared to co-implantation of control myeloid cells or injection of tumor cells alone, suggesting an important role for TREM2 in myeloid cells in GBM growth in vivo. Together, these data indicate that TREM2 operates by restricting the anti-tumor and proinflammatory function of myeloid cells in GBM and that inhibition of the TREM2 pathway may represent a potential therapeutic strategy for these patients.
Glioblastoma (GBM) is the most common primary malignant neoplasm with poor survival despite treatment. Developing effective therapies remains challenging due to intratumoral heterogeneity, which drives therapeutic resistance and recurrence. To understand how genetic events alter the epigenome to enhance clonal fitness, we developed an isogenic human neural progenitor cell (NPC)-based model of the proneural (PRO) GBM subtype. We introduced TERT promoter (TERTp) C228T and gain-of-function TP53 R248Q mutations in H1 human embryonic stem cells. Wildtype (WT) cells, single TERTp, and double TERTp/TP53 mutants underwent differentiation to NPCs. Lentiviral transduction of double mutants with PDGFRA D842V resulted in triple mutant PRO NPCs. Bulk and single cell transcriptomics of our model system revealed hundreds of gene expression changes with increased mesoderm and human GBM mesenchymal (MES) subtype signatures in single TERTp and double TERTp/TP53 mutants, versus WT cells. TERTp mutation increased telomerase expression and activity, conferring proliferative advantage and immortalization in NPCs and astrocytes. Additionally, TERT expression was further increased in TERTp/TP53 mutants. Surprisingly, triple mutant PRO NPCs, versus WT, displayed < 100 differentially expressed genes, associated with neurodevelopmental and dynamic cytoskeletal processes. Evolution analyses using gene counts signature and splicing dynamics revealed a developmental trajectory model from WT to additive mutants to PRO NPCs. Only triple mutant PRO NPCs formed tumors after intracranial injection in athymic nude mice, with mean survival of 100 days. Tumors presented histopathological features of GBM, and single cell transcriptomic analyses revealed evolution from immune-interacting to both neural progenitor- and neuronal-like subpopulations, with similar cell cycling signatures. Transcription factor genes related to WNT signaling and lineage commitment as well as glial and neuronal cytoskeletal genes exhibited epigenetic selection in vivo, signatures also observed in PRO NPCs in vitro. Our model thus provides opportunity for dissection of epigenetic and functional mechanisms underlying serial mutations during PRO tumor evolution.
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