Tumor immunotherapy by immune checkpoint blockade has proven its great potential by saving the lives of a proportion of late stage patients with immunogenic tumor types. the complex cellular and molecular interplay that determines the immune refractory state in glioblastoma. This knowledge may also yield next generation molecular targets for therapeutic intervention. Introduction During the past decade, immunotherapy of cancer has reached the status of being one of the most effective cancer therapies for defined tumor types. The main progress came from immune checkpoint blockers (ICB), monoclonal antibodies that inhibit the function of molecules involved in downregulating T-cell activation such as CTLA-4 or PD-1. ICB has shown the spectacular potential of curing late stage metastatic patients with highly immunogenic tumors such as melanoma, Merkel cell carcinoma or microsatellite instability (MSI)-high cancers, largely explaining its success. However, the majority of patients, even in responsive tumor types such as melanoma, do not reap the benefits of ICB. More troublesome Even, some tumor types show full refractoriness to ICB almost, for up to now not defined factors fully. Glioblastoma (GBM), the highest-grade, most common and most intense glial tumor, is among the cancers where ICB has fulfilled little success up to now. Several underlying systems could be in charge of this failure, like the inherently heterogenous character of the tumor type within people as well as the establishment of the immunosuppressive tumor microenvironment. Development of GBM tumors, but level of resistance to radiotherapy and chemotherapies also, can be FM-381 mediated by stem-like cells, whose CCM2 tumor-propagating character can be fully regulated with a core group of neurodevelopmental FM-381 transcription factors such as POU3F2, SOX2, SALL2, and OLIG2 (Suv et al., 2014) (Figure 1). Various markers have been suggested for glioblastoma stem cells (Lathia et al., 2015), but it is unclear at present whether different subpopulations of GBM stem cells exist and whether these give rise to tumors with a different cellular composition. In any case, expression profiling of GBM tumors identified at least three GBM subtypes: proneural (TCGA-PN), classical (TCGA-CL) and mesenchymal (TCGA-MES) (Verhaak et al., 2010; Wang et al., 2017), which tend to differentially associate with abnormalities in PDGFRA, IDH1, EGFR and NF1 (Verhaak et al., 2010). This level of heterogeneity is dramatically increased by the notion that different GBM subtypes can be found within the same tumor and are dynamic in function of time or in response to therapy (Sottoriva et al., 2013; Patel et al., 2014; Wang et al., 2017). More recent high-resolution single-cell RNA sequencing provided even more granularity to the concept of intra-tumoral heterogeneity by FM-381 identifying four cellular states for glioblastoma cells: mesenchymal-like (MES-like), astrocyte-like (AC-like), oligodendrocytic FM-381 precursor cell-like (OPC-like) and neural progenitor cell-like (NPC-like) (Neftel et al., 2019). There is a preponderance of particular states in each TCGA tumor type, with TCGA-CL and TCGA-MES being enriched in AC-like and MES-like states, respectively, and TCGA-PN encompassing both OPC-like and NPC-like states. Notably, some genetic alterations favor specific cellular states, with for example overexpression driving an AC-like program (Neftel et al., 2019). Finally, non-genetic heterogeneity within GBM tumors is determined by the relative proximity of cancer cells to blood vessels, with mTOR activity being upregulated in the few cell layers closest to the vessels (Kumar et al., 2019). In these cells, mTOR conveys superior invasive and migratory capabilities and resistance to therapy. Together, this highly heterogeneous nature of GBM strongly undermines the efficacy of therapy, considering the likely presence of cancer cell clones which are able to escape. Open in a separate window Figure 1. Heterogeneity of the glioblastoma immune microenvironment and potential therapeutic targets.Within glioblastoma tumors reside FM-381 ontogenically distinct, immunoregulatory macrophages (Sall1+ tumor microglia, Sall1- monocyte-derived macrophages), immunosuppressive Treg (eg CCR8+) and dysfunctional T-cell populations (CTLA-4/PD-1hi). Not much is known about intratumoral DC subsets, although distinct DC populations are found in other brain regions, such as the dura mater (Van Hove et al., 2019). Glioblastoma also affects the phenotype of classical monocytes (Cl. Monocyte) in the periphery, which acquire an immunosuppressive (MDSC-like?) phenotype. Notably, the genetic make-up of the cancer cells (blue rectangle) and potentially also of the glioblastoma stem cells, affect the immune composition of the tumor, with for example a higher presence of lymphocytes in TCGA-MES tumors. Several potential therapeutic targets (CSF1R, SIRPa, CCR8, PD-1, CTLA-4), either examined in the center or guaranteeing for future years currently, are highlighted. Furthermore, problems in anti-tumor T-cell reactions are found in GBM frequently,.