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Background: A major contributing factor to glioblastoma (GBM) development and progression is its ability to evade the immune system by creating an immune-suppressive environment, where GBM-associated myeloid cells, including resident microglia and peripheral monocyte-derived macrophages, play critical pro-tumoral roles. However, it is unclear whether recruited myeloid cells are phenotypically and functionally identical in GBM patients and whether this heterogeneity is recapitulated in patient-derived orthotopic xenografts (PDOXs). A thorough understanding of the GBM ecosystem and its recapitulation in preclinical models is currently missing, leading to inaccurate results and failures of clinical trials., Methods: Here, we report systematic characterization of the tumor microenvironment (TME) in GBM PDOXs and patient tumors at the single-cell and spatial levels. We applied single-cell RNA sequencing, spatial transcriptomics, multicolor flow cytometry, immunohistochemistry, and functional studies to examine the heterogeneous TME instructed by GBM cells. GBM PDOXs representing different tumor phenotypes were compared to glioma mouse GL261 syngeneic model and patient tumors., Results: We show that GBM tumor cells reciprocally interact with host cells to create a GBM patient-specific TME in PDOXs. We detected the most prominent transcriptomic adaptations in myeloid cells, with brain-resident microglia representing the main population in the cellular tumor, while peripheral-derived myeloid cells infiltrated the brain at sites of blood-brain barrier disruption. More specifically, we show that GBM-educated microglia undergo transition to diverse phenotypic states across distinct GBM landscapes and tumor niches. GBM-educated microglia subsets display phagocytic and dendritic cell-like gene expression programs. Additionally, we found novel microglial states expressing cell cycle programs, astrocytic or endothelial markers. Lastly, we show that temozolomide treatment leads to transcriptomic plasticity and altered crosstalk between GBM tumor cells and adjacent TME components., Conclusions: Our data provide novel insights into the phenotypic adaptation of the heterogeneous TME instructed by GBM tumors. We show the key role of microglial phenotypic states in supporting GBM tumor growth and response to treatment. Our data place PDOXs as relevant models to assess the functionality of the TME and changes in the GBM ecosystem upon treatment., (© 2024. The Author(s).)

The immunosuppressive nature of the tumor microenvironment poses a significant challenge to effective immunotherapies against glioblastoma (GB). Boosting the immune response is critical for successful therapy. Here, we adopted a cancer gene therapy approach to induce T‐cell‐mediated killing of the tumor through increased activation of the immune system. Patient‐based three‐dimensional (3D) GB models were infected with a replication‐deficient adenovirus (AdV) armed with the class II major histocompatibility complex (MHC‐II) transactivator (CIITA) gene (Ad‐CIITA). Successful induction of surface MHC‐II was achieved in infected GB cell lines and primary human GB organoids. Infection with an AdV carrying a mutant form of CIITA with a single amino acid substitution resulted in cytoplasmic accumulation of CIITA without subsequent MHC‐II expression. Co‐culture of infected tumor cells with either peripheral blood mononuclear cells (PBMCs) or isolated T‐cells led to dramatic breakdown of GB organoids. Intriguingly, both wild‐type and mutant Ad‐CIITA, but not unarmed AdV, triggered immune‐mediated tumor cell death in the co‐culture system, suggesting an at least partially MHC‐II‐independent process. We further show that the observed cancer cell killing requires the presence of either CD8+ or CD4+ T‐cells and direct contact between GB and immune cells. We did not, however, detect evidence of activation of canonical T‐cell‐mediated cell death pathways. Although the precise mechanism remains to be determined, these findings highlight the potential of AdV‐mediated CIITA delivery to enhance T‐cell‐mediated immunity against GB. [ABSTRACT FROM AUTHOR]