Understanding the glioblastoma immune microenvironment as basis for the development of new immunotherapeutic strategies

1During 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 benefit from ICB. Even more troublesome, some tumor types have shown nearly complete refractoriness to ICB, for as yet not fully defined reasons.

2Glioblastoma (GBM), the highest-grade, most prevalent and most aggressive glial tumor, is one of the cancers in which ICB has met little success so far. Several underlying mechanisms could be responsible for this failure, including the inherently heterogenous nature of this tumor type within individuals and the establishment of an immunosuppressive tumor microenvironment.

3Growth of GBM tumors, but also resistance to radiotherapy and chemotherapies, is mediated by stem-like cells, whose tumor-propagating nature is fully regulated by a core set of neurodevelopmental transcription factors such as POU3F2, SOX2, SALL2, and OLIG2 (Figure 1). Various markers have been suggested for glioblastoma stem cells, 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), which tend to differentially associate with abnormalities in PDGFRA, IDH1, EGFR and NF1. 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. More recent high-resolution single-cell RNA sequencing provided even more granularity to the concept of intra-tumoral heterogeneity by identifying four cellular states for glioblastoma cells: mesenchymal-like (MES-like), astrocyte-like (AC-like), oligodendrocytic precursor cell-like (OPC-like) and neural progenitor cell-like (NPC-like). 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 EGFR overexpression driving an AC-like program. 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. 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.

4In addition, defects in anti-tumor T-cell responses are commonly observed in GBM, suggesting the active induction of immunosuppression. In this respect, intracranial tumors of various histological origins (not only GBM) cause an entrapment of T cells in the bone marrow due to a loss of surface spingosine-1-phosphate receptor 1 (S1P1). Why only intracranial tumors induce this defect and via which mechanism is currently unknown, but a reversal of this deficiency enables an enhanced migration of T cells into GBM tumors. Along the same line, thymic involution in GBM tumor-bearers results in diminished T-cell production and, hence, reduced availability of T cells for anti-GBM immunity. Moreover, once T cells do get inside the GBM tumor, they are likely to become dysfunctional via various mechanisms. Recent studies provide evidence for the enriched presence of exhausted T cells within GBM tumors, with tumor-infiltrating lymphocytes (TILs) more often co-expressing the immune checkpoint molecules PD-1, LAG3 and TIM-3 than peripheral blood cells of patients or healthy controls. Interestingly, PD-1+ LAG3+ TIM-3+ T cells showed impaired T-cell functions, while single positive cells for any of these markers did not. A second prominent mechanism of T-cell dysfunctionality is tolerance, mediated by the presence of suppressive cells such as regulatory T cells (Treg) and tumor-associated macrophages (TAM). These aspects will be discussed in detail in this review.

5Previous reviews have described the attempts to incorporate ICB in the treatment of GBM. In summary, preclinical models have provided a proof-of-concept that ICB with anti-CTLA-4 and/or anti-PD-1, either as monotherapy or in conjunction with standard-of-care treatments, improves tumor outcome. This has set the stage for multiple clinical trials in glioblastoma patients, the results of which are up to this date mostly unavailable. Here, we will focus on the most recent progress in this field. One of the most noticeable advances comes from the realization that the timing of ICB administration could majorly impact its effect. Classically, ICB is given in an adjuvant setting, improving recurrence-free survival and overall survival after surgery in cancer types such as melanoma. Neoadjuvant (pre-operative) therapy has proven to be advantageous in breast, bladder and other types of cancer, but this finding remained restricted to chemo- and radiation therapy with very little data available on ICB neoadjuvant therapy until recently. demonstrated increased anti-tumor immune responses and survival upon neoadjuvant immunotherapy, in two mouse models of triple-negative breast cancer. This finding has now been reproduced in several other preclinical models, and importantly, also in the clinical setting of resectable melanoma and glioblastoma. In resectable glioblastoma, either as primary or recurrent tumor, one neoadjuvant injection of anti-PD-1 monoclonal antibodies, followed by surgical resection of the tumor and repeated adjuvant anti-PD-1 treatment, resulted in a significant increase in overall survival and progression-free survival compared to adjuvant treatment alone. Remarkably, the same conclusions were reached by two independent research groups that used a different monoclonal antibody, either pembrolizumab or nivolumab. Both studies also demonstrated an intratumoral immunomodulatory effect of the neoadjuvant treatment, with an increased interferon response, production of chemokines, and signs of increased TIL activity. However, larger-scale randomized trials should be performed to fully prove the benefit of this neoadjuvant approach.

6A second level of innovation comes from the assessment of novel combination therapies in preclinical models, whereby anti-PD-1 efficacy is boosted by a concurrent administration of chemotherapy, radiation therapy, other immune checkpoint blocking monoclonal antibodies, cancer cell-directed immunotoxins, and nanocarriers that improve the shuttling of ICB across the blood-brain barrier. Most of these studies show a clear benefit of combining different approaches to increase anti-GBM immunity. Indeed, monotherapies usually have little impact, highlighting the strongly immunosuppressive nature of the GBM tumor microenvironment (TME). A novel avenue to increase the immunogenicity of the GBM TME comes from virotherapy, whereby oncolytic viruses not only cause a direct killing of highly proliferative cancer cells but also trigger an (innate) immune response. For example, a recent clinical trial applied intratumoral injections of a polio-rhinovirus chimera and could achieve a remarkable 21% of longer term survivors (at least 2 to 3 years), while <4% of such patients is observed in control groups. In a phase I study with DNX-2401, a tumor-selective replication-competent oncolytic adenovirus, 20% of patients survived >3 years from treatment, and three patients had a ≥ 95% reduction of the tumor, which was probably due to direct oncolytic effects of the virus followed by elicitation of an immune-mediated antiglioma response. A passive transfer of anti-tumor T-effector cells could be an alternative strategy to attack GBM tumors, although such T cells should then be able to withstand the suppressive TME. Novel generations of CAR T-cells are being generated that should show improved functionality in the TME, including the TME from GBM. Finally, the adoptive transfer of antigen-presenting dendritic cells (i.e. DC vaccination) has yielded some efficacy in improving the median overall survival in clinical trials and further research is warranted to improve the treatment regimens and the choice of the most optimal DC type and the most optimal tumor antigens. Neoantigens are the most tumor-specific and arguably the most promising antigen candidates, that show efficacy as vaccine candidates in glioblastoma patients.

7The immune contexture of human tumors, referring to the exact composition of distinct immune cell types within tumors, changes at each tumor stage and is known to have an impact on clinical outcome. Hence, understanding the tumor (immune) microenvironment has recently been recognized as one of the future challenges to improve therapy of brain tumors. Glioblastoma tumors are known to contain a variety of immune cell types, but with a dominance of immunosuppressive cells. Recent findings further finetune this concept through the identification of distinct subpopulations within the immunoregulatory macrophage compartment of tumors, but also distinct Treg and dendritic cell subsets have been identified and will be discussed in this review.

8As mentioned earlier, innovative techniques such as extensive exome and transcriptome sequencing have revealed the existence of molecularly distinct proneural, mesenchymal and classical subtypes of glioblastoma. In addition, a diverse array of recurrent genomic mutations were found in these tumors, including TP53, IDH1, NF1, PTEN, PDGFRA, EGFR and MAPK pathway mutations, with particular mutations being enriched in particular glioblastoma subtypes.

9Of importance in the context of ICB immunotherapy, is the observation that the number of immune cells in the human glioblastoma microenvironment appears to be associated with the predominant genetic alteration (Figure 1). TILs are enriched in TCGA-MES glioblastomas and strongly associated with mutations in NF1 and RB1. Conversely, TILs are depleted in the TCGA-CL class, EGFR-amplified, and homozygous PTEN-deleted tumors and rare in histologies characterized by these alterations. In addition, the IDH mutant status in glioblastoma tumors significantly associates with TIL infiltration. The IDH-wt status is associated with significantly higher TIL infiltration and PD-L1 expression, while IDH mutations result in the opposite phenotype, that is a reduced IFNγ signature and reduced presence of CD8+ and CD4+ cells. Nevertheless, patients bearing IDH-mutated tumors have a more favourable outcome, suggesting that other cancer cell-intrinsic or -extrinsic mechanisms are at play beyond the presence of TILs.

10One of those mechanisms could be the presence of immunoregulatory macrophages and neutrophils. In a study comparing a small number of treatment-naive IDH-mutated and IDH-wt glioblastoma tumors, IDH-mutants were shown to contain less macrophages which displayed a more pro-inflammatory M1-like activation state, possibly contributing to the longer survival of these patients. The reduced presence of macrophages in IDH-mutant, but also CDK4-amplified tumors, was also demonstrated via a computational analysis of a large number of TCGA RNAseq samples. Hence, although IDH-mutated tumors contain a lower number of T lymphocytes, these cells may be more optimally activated due to the relative paucity of immunosuppressive macrophages and neutrophils. The frequency of macrophages and neutrophils is generally increased in TCGA-MES tumors versus the other TCGA types. Remarkably, genes for both tumor-promoting M2-like as well as potentially anti-tumoral M1-like macrophages were enriched in TCGA-MES tumors, in particular in the NF1-deficient cases. The absence of NF1 appears to result in chemoattraction of macrophages via an as yet undefined mechanism. In addition, high B-cell and CD4+ T-cell components were commonly observed in TCGA-MES samples, while immune cell components tend to be low in TCGA-PN samples. In general terms, computational analysis grouped glioblastoma tumors into three immune response-related subgroups, termed negative (defined by a relative paucity of immune cells; enriched by TCGA-PN and CDK4-MARCH9 amplification), humoral (defined by a high B-cell and CD4+ T-cell compartment; enriched by TCGA-MES) and cellular-like (defined by a higher ‘negative regulation of T-cell activation’ and ‘gamma delta T-cell’ cluster; enriched by TCGA-CL and samples with a high macrophage content).

11The question remains how these findings relate to the responsiveness of patients to ICB. Surprisingly, in spite of the immunologically different tumor microenvironment in TCGA subtypes, no association was found with ICB responsiveness. Rather, non-responding patients bear significantly more PTEN mutations, which are associated with immunosuppressive gene expression signatures, while responders are enriched in MAPK pathway alterations (PTPN11, BRAF). These findings illustrate that responsiveness to ICB is of a greater complexity than the mere presence of immune cell types within the microenvironment.

12Long-standing treatment options for glioblastoma include surgery, radiotherapy and temozolomide chemotherapy, all of which may affect the immune composition of glioblastoma tumors. In addition, brain oedema is most often treated with dexamethasone, a steroid with known immunoregulatory capacities.

13Radiation therapy obviously aims to eliminate the proliferating cancer cells, but one of its side effects is the induction of hypoxia. In turn, hypoxia initiates a lethal cascade of events consisting of the increased production of CXCL12, the subsequent recruitment of CXCR4 and CXCR7-positive bone marrow-derived monocytes and hematopoietic progenitor cells that differentiate into tumor-promoting macrophages and mediate vasculogenesis and tumor recurrence in mice. Hence, blockade of CXCL12 and CXCR7 delays recurrence of tumors upon irradiation in preclinical models. Most importantly, a recent Phase I/II clinical trial with the reversible CXCR4 inhibitor plerixafor – a strategy named Macrophage Exclusion after Radiation Therapy or MERT – in conjunction with chemoirradiation improved local control of tumor recurrence in newly diagnosed glioblastoma patients. Another effect of radiation is the upregulation of PD-L1 at the surface of tumor-infiltrating myeloid cells. This effect has been exploited to target anti-PD-L1-functionalized lipid nanoparticles, carrying toxic compounds, to these myeloid cells in conjunction with irradiation, leading to an improved anti-tumor response in mice.

14Temozolomide is the most widely used chemotherapy for patients with glioblastoma (GBM), whose action not only directly affects cancer cells but also depends on its immunomodulatory properties. Temozolomide induces lymphopenia, which, counterintuitively, can be harnessed to improve immunotherapy. Indeed, lymphoablative doses of temozolomide were shown to increase tumor antigen-specific immune responses in GBM patients and GBM-bearing mice. Mechanistically, it was suggested that compensatory homeostatic cytokines after temozolomide therapy cause enhanced immune responses by reduction of the T-cell activation threshold and proliferation induction.

15In recent years, it became clear that dexamethasone treatment of patients may strongly affect outcome, with a lower overall survival in recurrent glioblastoma patients. This adverse effect is most likely due to the well-known immunomodulatory function of steroids. Indeed, glioblastoma patients treated with dexamethasone contained significantly less lymphocytes in their blood, but more myeloid cells with potentially suppressive functions.

16Macrophages are a large component of many tumor types and have been an attractive target for glioblastoma therapy. Originally, TAMs were often thought to exclusively originate from tumor-infiltrating monocytes, but both in CNS and peripheral tumor types a fraction can be derived from the tissue-resident macrophage pool. Recent work that has relied on single-cell RNA sequencing combined with fate mapping approaches has highlighted the rich diversity of murine tissue-resident brain macrophages. (Figure 1). In this respect, parenchymal microglia differ from macrophages located in the brain's border regions, such as the meninges and the choroid plexus. Whether border-associated macrophages contribute to glioblastoma or metastatic brain tumors is not known. However, there is now firm evidence that TAMs in GBM tumors are partly derived from embryonic microglia (Figure 1). Tumor-associated microglia and monocyte-derived macrophages can be distinguished via genetic lineage tracing, which was used to sort these populations from transplantable and spontaneous mouse models of glioblastoma followed by RNA-sequencing. These data revealed that the glioblastoma microenvironment instructs the transcriptional landscape of these cells, with prominent differences between microglia and monocyte-derived TAMs at multiple levels, including differential expression of genes involved in IL-1 signaling, chemokine and inflammatory cytokine production and antigen presentation. Whether this transcriptional divergence is reflected at the functional level and whether these subsets differentially affect glioblastoma progression is not known, but seems likely. Indeed, tissue-resident interstitial macrophages and their monocyte-derived counterparts are also found in mouse lung tumors, where resident TAMs are suggested to support cancer cell expansion, while monocyte-derived cells might contribute to cancer cell dissemination. Also in mouse mammary carcinoma, a distinction was made between monocyte-derived TAMs and resident mammary tissue macrophages, with only the former contributing to a suppression of anti-tumor cytotoxic T-cell responses.

17In glioblastomas, monocyte-derived macrophages tend to be more enriched in the tumor core, while microglia-derived TAM are typically found at the tumor periphery. Live in vivo 2-photon microscopy confirmed the distinction between the two TAM populations in the mouse and demonstrated that monocyte-derived cells are small and motile, while microglia are large sessile cells whose processes are continuously extending and retracting within tumors. Importantly, the existence of microglia and monocyte-derived TAM populations has now also been shown in human GBM. However, it will be interesting to evaluate whether microglia and monocyte-derived TAM further consist of specialized subpopulations, for example instructed by their precise intratumoral localization, as was demonstrated for TAM in other tumor models. For this reason, future single-cell RNA-sequencing efforts will provide crucial new insights, and will undoubtedly allow the identification of multiple TAM subsets, as was already shown in mouse and human lung tumors. An interesting possibility is that microglia- and monocyte-derived TAMs may exhibit a differential ability to infiltrate or colonize specific tumor microenvironments or tumor macrophage niches.

18Interesting novel findings in preclinical models provide further insights in the molecular mechanisms that govern the attraction of monocyte-derived macrophages to the glioblastoma microenvironment. Cancer cell- and host cell-derived osteopontin mediates macrophage infiltration through the interaction with integrin αvβ5 and instructs a pro-tumoral phenotype in these cells. Also the Aryl Hydrocarbon Receptor (AHR) is indirectly implicated in recruiting monocytes to the tumor environment by driving CCR2 expression, the receptor for the major monocyte chemoattractants CCL2 and CCL7. In PTEN-deficient tumor models, enhanced LOX production is responsible for the enhanced attraction of macrophages via the β1 integrin-PYK2 pathway, which subsequently promote tumor growth via SPP1. However, how this finding correlates with the enhanced responsiveness of PTEN-mutant tumors to ICB therapy in patients is not clear and requires further investigation, including the potential differences between the PTEN mutations found in patients and the PTEN-null phenotype in mouse models.

19Another point of interest is the systemic influence of brain tumors on peripheral myeloid cells. GBM-secreted IL-6 induces immunosuppressive myeloid cells in the periphery that express high levels of PD-L1 in orthotopic mouse models. This finding is in line with CyTOF fingerprinting of patients’ peripheral blood, indicating a significant elevation of myeloid-derived suppressor cells (MDSC), but not Treg in the circulation (Figure 1). Hence, the myeloid cells that infiltrate GBM tumors may have already been primed into a tumor-promoting role. The importance of MDSC, both the monocytic and granulocytic subtype, is further confirmed by the finding that their presence in mouse GBM tumors is associated with a reduction in the number of tumor-infiltrating lymphocytes. Moreover, within the murine glioblastoma environment, MDSC were shown to induce regulatory B cells. Interestingly, CCL2 produced by TAM mediates the recruitment of suppressive CCR2+ monocytic MDSC and CCR4+ Treg, explaining the clinical observation that elevated intratumoral CCL2 expression levels correlate with reduced overall survival. Of note, recruited monocytic MDSC may in turn differentiate into TAM. In a preclinical setting, this notion has now been taken further by showing that a small molecule CCR2 antagonist sensitizes otherwise resistant murine gliomas to ICB therapy.

20The presence of Treg has been amply described in multiple cancer types, but their value as predictors of disease outcome is debatable in glioblastoma. Independent researchers described an increased presence of FOXP3+ Treg in higher tumor grades of various brain tumor types, including glioblastoma. However, the correlation of Treg with decreased survival, which was anticipated based on the T-cell suppressive capacity of these cells, has been very moderate at best. Nevertheless, Treg-directed therapies, such as agonist anti-GITR or anti-LAP (Latency-associated Peptide) mAb treatment, have shown some promise in mouse glioblastoma models.

21A potential reason for the unclear association of Treg with disease outcome could be the existence of distinct Treg subsets which may differ in their suppressive capacity and whose presence in tumors may vary between individual patients. Early preclinical work suggested that the majority of Tregs infiltrating glioblastoma tumors are thymus-derived, although it seems likely that the local de novo induction of Treg in the tumor microenvironment is a possibility as well. As a matter of fact, indirect evidence based on correlative studies proposed glioblastoma-induced PD-L1 expression as a mechanism of Treg induction and maintenance in patients. The attraction of these Treg to the tumor microenvironment may be mediated by various mechanisms, including IDO activity and CCL2 production, which interacts with CCR4 at the surface of Treg. Interestingly, another chemokine receptor, CCR8, has recently been identified as a marker that is specifically expressed in at least a subset of tumor-infiltrating Treg, but not Treg in the periphery, as part of a tumor-infiltrating Treg transcriptional signature that is conserved across species and tumor types (Figure 1). CCR8+ Treg were reported as very potent suppressors, suggesting that this Treg subset is likely important in the creation of an immunosuppressive tumor microenvironment. However, to what extent the tumor-infiltrating Treg signature and CCR8+ Treg are present within glioblastoma tumors is currently unknown.

22Another marker that is functionally important on Treg within tumors is Neuropilin-1 (Nrp1). The interaction of Nrp1 with its ligand Semaphorin 4a was shown to stabilize the Treg phenotype in the mouse, which turns out to be especially important for the suppression of anti-tumor immune responses. The absence of Nrp1 in Treg leads to their loss of suppressive potential but also to their production of IFNγ, which subsequently annihilates the suppressive capacity of neighbouring Nrp1+ Treg in murine tumors. This mechanism may be stimulated by enhanced HIF-1α expression in Nrp1-deficient Treg. Interestingly, HIF-1α expression in hypoxic tumor-infiltrating Treg was indeed shown to decrease the intrinsic suppressive activity of these Treg, but was at the same time required for the migration of Treg into mouse glioblastoma tumors. Consequently, the absence of HIF1α in Treg leads to the reduced growth of glioblastoma tumors.

23Altogether, suppressive Treg subsets are induced in the microenvironment of multiple tumor types. As these cells could be prime targets for therapeutic intervention, it will be important to assess their presence and relevance in glioblastoma.

24Recent findings, in other tumor types than glioblastoma, have uncovered the existence of distinct DC populations in solid tumors. Indeed, at least three types of conventional DCs (cDCs), in addition to plasmacytoid DCs (pDCs), are present in the tumor microenvironment of multiple mouse and human tumors. The functionality of these cells is being explored in mouse tumor models. cDC1 are rare cells within tumors that were repeatedly reported to migrate to the tumor-draining lymph nodes where they cross-present tumor antigens to CD8+ T cells. Interestingly, NK cells attract cDC1 to the tumor environment via the secretion of various growth factors and chemokines, such as Flt3L, CCL5 and XCL1. cDC2, which are typically more abundant within tumors than cDC1, also have the capacity to drive T-cell responses, but are more directed towards the activation of CD4+ T cells. As such, these cells were shown to initiate a Th17 response, but their full T-cell activating potential may only be unleashed in the absence of Treg. Both mouse and human lung tumors also harbor an additional DC subset that exhibits a migratory gene signature, although this cannot be taken as proof for actual migration.

25Translating these findings to glioblastoma is not trivial, as a systematic analysis of DC subpopulations in these tumors is currently lacking. Recent work has shown that various dendritic cell subsets are present in the murine homeostatic brain, where they are thought be critical for initiating neuroinflammation. In the context of glioblastoma, one study reported that CCR2+ Lin- hematopoietic stem and progenitor cells (HSC) can differentiate into cross-presenting DC within mouse glioblastoma tumors. These HSC-derived DC can be isolated from glioblastoma tumors to successfully vaccinate mice and induce anti-tumor immunity in the vaccine recipients. In addition, their presence potentiates the effect of immune checkpoint blockade immunotherapy. Along the same line, increasing the number of intratumoral CD103+ cDC1 through the administration of Flt3L, or activating them via TLR3 agonists, magnifies the response of mouse glioblastomas to immune checkpoint blockade, suggesting the importance of this DC type to augment immunity against glioblastoma, similar to other tumor types. However, Fibrinogen-like Protein 2 (FGL2), that is predominantly secreted by the cancer cells, may interfere with the induction of these immunostimulatory DCs. Indeed, FGL2 interferes with GM-CSF signalling, blunting the differentiation of CD103+ cDC1 and consequently lowering the CD8+ T-cell response. Overall, it can be concluded that interventions which increase the number of CD103+ cDC1 in glioblastoma tumors or augment their activation state may be beneficial. Whether cDC2 can also contribute to anti-glioblastoma immunity remains to be determined.

26Finally, how can the recently acquired knowledge on the heterogeneity of the glioblastoma immune microenvironment be turned into novel therapeutic avenues? An important caveat in this respect is the capacity of drugs to cross the blood-brain barrier (BBB) and to reach sufficiently high concentrations in the brain. A number of very recent studies have highlighted the potential of nanoparticles to deliver therapeutic cargo to the mouse brain. Solid lipid nanoparticles can deliver small interfering RNAs to the mouse glioblastoma microenvironment, following a low-dose irradiation to prime brain uptake. A novel tumor penetrating peptide that targets cell surface p32, LinTT1 (AKRGARSTA), has also been reported as a GBM targeting ligand for systemically-administered nanoparticles. Moreover, nanoscale immunoconjugates (NICs) on a natural biopolymer scaffold, poly(β-L-malic acid), were produced with covalently attached anti-CTLA-4 or anti-PD-1 for systemic delivery across the BBB and activation of local brain anti-tumor immune responses. Notably, nanoparticles often end up being phagocytosed by tumor-associated macrophages and are, hence, interesting tools to modulate the activity of these cells. The fact that modulating the macrophage compartment is indeed a promising approach was shown a couple of years ago by the administration of a brain penetrating CSF1R inhibitor in mice, which repolarized TAM into a more anti-tumoral phenotype resulting in a reduced GBM progression. Likewise, a blocking anti-CD47 antibody mobilizes the phagocytic capacity of both monocyte-derived and microglial TAMs in mouse glioblastoma models. As a matter of fact, this type of approaches could now be combined with strategies to increase the presence of antigen-presenting cells, by the targeted delivery of growth factors or chemokines for cDCs.

27Targeting tumor-infiltrating Treg, without affecting their peripheral counterparts to avoid auto-immune complications, could be another interesting approach. The recent identification of molecules, such as CCR8, that are specifically upregulated on these cells within tumors may provide novel therapeutic anchor points in multiple tumor types. It needs to be evaluated whether these tiTreg-specific molecules are implicated in Treg functionality as this will determine the type of compound we need for therapy (agonists, antagonists,...).

28Overall, increasing knowledge about subpopulations of immune cells that either promote or inhibit glioblastoma tumor progression will allow more specific therapeutic approaches against this aggressive type of brain tumor.