The neuropathological basis to the functional role of microglia/macrophages in gliomas

Advancements and perspectives of RBX2 as a molecular hallmark in cancer

Cancer is a challenging issue for human health. One of the key methods to address this issue is by comprehending the molecular causes of tumors and creating medications that target those causes. RBX2 (RING box protein 2), also known as ROC2 (Regulator of Cullins 2), RNF7 (RING Finger Protein 7), or SAG (Sensitive to Apoptosis Gene) is a key component of the Cullin-RING-type E3 ubiquitin ligases (CRLs) and overexpressed in various human cancers. RBX2 is a potential drug target, the expression of which correlates with tumor staging, grading, and prognosis analysis. Through a synergistically biological interaction with Kras mutation in preclinical models, RBX2 accelerated the progression of skin cancer, pancreatic cancer, and lung cancer. In accordance, the aberrant expression of RBX2 will lead to dysregulation of many signaling pathways, which is crucial for tumor initiation and growth. However, the impact of RBX2 on tumors also intriguingly demonstrates a spatial reliance manner. In this review, we summarized the current understanding of RBX2 in multiple cancer types and suggested a significant potential of RBX2 as a therapeutic target.

Deciphering immune microenvironment and cell evasion mechanisms in human gliomas

Gliomas are considered one of the most malignant cancers in the body. Despite current therapies, including surgery, chemotherapy, and radiotherapy, these tumors usually recur with more aggressive and resistant phenotypes. Indeed, the survival following these conventional therapies is very poor, which makes immunotherapy the subject of active research at present. The anti-tumor immune response could also be considered a prognostic factor since each stage of cancer development is regulated by immune cells. However, glioma microenvironment contains malignant cells that secrete numerous chemokines, cytokines and growth factors, promoting the infiltration of immunosuppressive cells into the tumor, which limit the functioning of the immune system against glioma cells. Recently, researchers have been able to reverse the immune resistance of cancer cells and thus activate the anti-tumor immune response through different immunotherapy strategies. Here, we review the general concept of glioma's immune microenvironment and report the impact of its distinct components on the anti-tumor immune response. We also discuss the mechanisms of glioma cell evasion from the immune response and pinpoint some potential therapeutic pathways, which could alleviate such resistance.

Recent advances and future challenges of tumor vaccination therapy for recurrent glioblastoma

Glioblastoma (GBM) is the most malignant CNS tumor with a highest incidence rate, and most patients would undergo a recurrence. Recurrent GBM (rGBM) shows an increasing resistance to chemotherapy and radiotherapy, leading to a significantly poorer prognosis and the urgent need for novel treatments. Immunotherapy, a rapidly developing anti-tumor therapy in recent years, has shown its potential value in rGBM. Recent studies on PD-1 immunotherapy and CAR-T therapy have shown some efficacy, but the outcome was not as expected. Tumor vaccination is the oldest approach of immunotherapies, which has returned to the research focus because of the failure of other strategies and subversive understanding of CNS. The isolation effect of blood brain barrier and the immunosuppressive cell infiltration could lead to resistance existing in all phases of the anti-tumor immune response, where novel tumor vaccines have been designed to overcome these problems through new tumor antigenic targets and regulatory of the systematic immune response. In this review, the immunological characteristics of CNS and GBM would be discussed and summarized, as well as the mechanism of each novel tumor vaccine for rGBM. And through the review of completed early-phase studies and ongoing large-scale phase III clinical trials, evaluation could be conducted for potential immune response, biosecurity and initial clinical outcome, which further draw a panorama of this vital research field and provide some deep thoughts for the prospective tendency of vaccination strategy. Video Abstract.

Advances in Nanotechnology-Based Immunotherapy for Glioblastoma

Glioblastoma (GBM) is the most aggressive type of brain tumor. Despite the multimodal therapies, the effectiveness of traditional treatments is not much satisfying. In recent years, immunotherapy has become the focus of tumor treatment. Unlike traditional treatments that directly target tumor cells, immunotherapy uses the body’s immune system to kill tumors. However, due to the severe immunosuppressive microenvironment of GBM, it generally has a poor response to immunotherapy. In addition, the existence of the blood-brain barrier (BBB) also compromises the immunotherapeutic efficacy. Therefore, effective immunotherapy of GBM requires the therapeutic agents to not only efficiently cross the BBB but also relieve the strong immunosuppression of the tumor microenvironment of GBM. In this review, we will first introduce the CNS immune system, immunosuppressive mechanism of GBM, and current GBM immunotherapy strategies. Then, we will discuss the development of nanomaterials for GBM immunotherapy based on different strategies, roughly divided into four parts: immune checkpoint therapy, targeting tumor-associated immune cells, activating immune cells through immunogenic cell death, and combination therapy, to provide new insights for future GBM immunotherapy.

Multiple Irradiation Affects Cellular and Extracellular Components of the Mouse Brain Tissue and Adhesion and Proliferation of Glioblastoma Cells in Experimental System In Vivo

Intensive adjuvant radiotherapy (RT) is a standard treatment for glioblastoma multiforme (GBM) patients; however, its effect on the normal brain tissue remains unclear. Here, we investigated the short-term effects of multiple irradiation on the cellular and extracellular glycosylated components of normal brain tissue and their functional significance. Triple irradiation (7 Gy*3 days) of C57Bl/6 mouse brain inhibited the viability, proliferation and biosynthetic activity of normal glial cells, resulting in a fast brain-zone-dependent deregulation of the expression of proteoglycans (PGs) (decorin, biglycan, versican, brevican and CD44). Complex time-point-specific (24–72 h) changes in decorin and brevican protein and chondroitin sulfate (CS) and heparan sulfate (HS) content suggested deterioration of the PGs glycosylation in irradiated brain tissue, while the transcriptional activity of HS-biosynthetic system remained unchanged. The primary glial cultures and organotypic slices from triple-irradiated brain tissue were more susceptible to GBM U87 cells’ adhesion and proliferation in co-culture systems in vitro and ex vivo. In summary, multiple irradiation affects glycosylated components of normal brain extracellular matrix (ECM) through inhibition of the functional activity of normal glial cells. The changed content and pattern of PGs and GAGs in irradiated brain tissues are accompanied by the increased adhesion and proliferation of GBM cells, suggesting a novel molecular mechanism of negative side-effects of anti-GBM radiotherapy.

Role of Inflammatory Mediators, Macrophages, and Neutrophils in Glioma Maintenance and Progression: Mechanistic Understanding and Potential Therapeutic Applications

Gliomas are the most common, highly malignant, and deadliest forms of brain tumors. These intra-cranial solid tumors are comprised of both cancerous and non-cancerous cells, which contribute to tumor development, progression, and resistance to the therapeutic regimen. A variety of soluble inflammatory mediators (e.g., cytokines, chemokines, and chemotactic factors) are secreted by these cells, which help in creating an inflammatory microenvironment and contribute to the various stages of cancer development, maintenance, and progression. The major tumor infiltrating immune cells of the tumor microenvironment include TAMs and TANs, which are either recruited peripherally or present as brain-resident macrophages (microglia) and support stroma for cancer cell expansion and invasion. These cells are highly plastic in nature and can be polarized into different phenotypes depending upon different types of stimuli. During neuroinflammation, glioma cells interact with TAMs and TANs, facilitating tumor cell proliferation, survival, and migration. Targeting inflammatory mediators along with the reprogramming of TAMs and TANs could be of great importance in glioma treatment and may delay disease progression. In addition, an inhibition of the key signaling pathways such as NF-κB, JAK/STAT, MAPK, PI3K/Akt/mTOR, and TLRs, which are activated during neuroinflammation and have an oncogenic role in glioblastoma (GBM), can exert more pronounced anti-glioma effects.

Immunotherapy in Glioblastoma: A Clinical Perspective

Glioblastoma is the most frequent and the most aggressive brain tumor. It is notoriously resistant to current treatments, and the prognosis remains dismal. Immunotherapies have revolutionized the treatment of numerous cancer types and generate great hope for glioblastoma, alas without success until now. In this review, the rationale underlying immune targeting of glioblastoma, as well as the challenges faced when targeting these highly immunosuppressive tumors, are discussed. Innovative immune-targeting strategies including cancer vaccines, oncolytic viruses, checkpoint blockade inhibitors, adoptive cell transfer, and CAR T cells that have been investigated in glioblastoma are reviewed. From a clinical perspective, key clinical trial findings and ongoing trials are discussed for each approach. Finally, limitations, either biological or arising from trial designs are analyzed, and strategies to overcome them are presented. Proof of efficacy for immunotherapy approaches remains to be demonstrated in glioblastoma, but our rapidly expanding understanding of its biology, its immune microenvironment, and the emergence of novel promising combinatorial approaches might allow researchers to finally fulfill the medical need for GBM patients.

Expression Profile Analysis Identifies a Novel Seven Immune-Related Gene Signature to Improve Prognosis Prediction of Glioblastoma

Glioblastoma multiform (GBM) is a malignant central nervous system cancer with dismal prognosis despite conventional therapies. Scientists have great interest in using immunotherapy for treating GBM because it has shown remarkable potential in many solid tumors, including melanoma, non-small cell lung cancer, and renal cell carcinoma. The gene expression patterns, clinical data of GBM individuals from the Cancer Genome Atlas database (TCGA), and immune-related genes (IRGs) from ImmPort were used to identify differentially expressed IRGs through the Wilcoxon rank-sum test. The association between each IRG and overall survival (OS) of patients was investigated by the univariate Cox regression analysis. LASSO Cox regression assessment was conducted to explore the prognostic potential of the IRGs of GBM and construct a risk score formula. A Kaplan–Meier curve was created to estimate the prognostic role of IRGs. The efficiency of the model was examined according to the area under the receiver operating characteristic (ROC) curve. The TCGA internal dataset and two GEO external datasets were used for model verification. We evaluated IRG expression in GBM and generated a risk model to estimate the prognosis of GBM individuals with seven optimal prognostic expressed IRGs. A landscape of 22 types of tumor-infiltrating immune cells (TIICs) in glioblastoma was identified, and we investigated the link between the seven IRGs and the immune checkpoints. Furthermore, there was a correlation between the IRGs and the infiltration level in GBM. Our data suggested that the seven IRGs identified in this study are not only significant prognostic predictors in GBM patients but can also be utilized to investigate the developmental mechanisms of GBM and in the design of personalized treatments for them.

Down-regulation of Aquaporin-1 mediates a microglial phenotype switch affecting glioma growth

Aquaporin 1 (AQP1), a transmembrane protein that forms water channels, has previously been shown to facilitate growth and progression of many types of tumors by modulating tumor cell migration, proliferation and angiogenesis. Here, we determined the impact of AQP1 expression in the tumor environment on the progression of brain tumors. Primary microglia from wild type(WT) and AQP1 knockout(KO) mice were used to test AQP1 effect on microglia function by using Western blot, quantative PCR, in an experimental in vivo mouse glioma model and organotypic brain slice culture. Deletion of AQP1 in the host tissue significantly reduced the survival of the mice implanted with GL261 glioma cells. The density of glioma-associated microglia/macrophages was almost doubled in AQP1KO mice. We found that factors secreted from GL261 cells decrease microglial AQP1 expression via the MEK/ERK pathway, and that inhibition of this pathway with Trametinib reduced tumor growth and prolonged the survival of tumor bearing mice, an effect which required the presence of microglia. Deletion of AQP1 in cultured microglia resulted in an increase in migratory activity and a decrease in TLR4-dependent innate immune responses. Our study demonstrates a functional relevance of AQP1 expression in microglia and hints to AQP1 as a potential novel target for glioma therapy.

Emerging Techniques in Imaging of Glioma Microenvironment

Magnetic resonance imaging (MRI) has been the cornerstone of imaging of brain tumors in the past 4 decades. Conventional MRI remains the workhorse for neuro-oncologic imaging, not only for basic information such as location, extent, and navigation but also able to provide information regarding proliferation and infiltration, angiogenesis, hemorrhage, and more. More sophisticated MRI sequences have extended the ability to assess and quantify these features; for example, permeability and perfusion acquisitions can assess blood-brain barrier disruption and angiogenesis, diffusion techniques can assess cellularity and infiltration, and spectroscopy can address metabolism. Techniques such as fMRI and diffusion fiber tracking can be helpful in diagnostic planning for resection and radiation therapy, and more sophisticated iterations of these techniques can extend our understanding of neurocognitive effects of these tumors and associated treatment responses and effects. More recently, MRI has been used to go beyond such morphological, physiological, and functional characteristics to assess the tumor microenvironment. The current review highlights multiple recent and emerging approaches in MRI to characterize the tumor microenvironment.

Identification of Aberrantly Expressed Genes in Murine Glioblastoma During Radiotherapy via Bioinformatic Data Mining

Objective

Glioblastoma (GBM) is an aggressive tumor with a fast growth rate. Radioresistance of GBM can lead to high recurrence. In general, due to the protection of the blood-brain barrier, the immune environment of the central nervous system is unique. The immune response induced by radiotherapy is weak in GBM. In the present study, aberrantly expressed genes during radiotherapy were assessed in murine models based on microarray RNA data.

Methods

The microarray data were extracted from the Intergovernmental Group on Earth Observations and differentially expressed genes (DEGs) screened out. Gene expression profiles of 115 samples in GSE56113 were analyzed and 104 genes were identified as aberrantly expressed based on GEO2R 8 d after radiotherapy. Then, the Database for Annotation, Visualization, and Integrated Discovery was used to analyze Genome Kyoto Encyclopedia of Gene pathways and Gene Ontology (GO) terms. The 20 core candidate genes were identified using protein-protein interaction network analysis and Cytoscape software with Molecular Complex Detection plug-in.

Results

Post-irradiated tumor tissues expressed significantly more immune-associated genes than contralateral brain tissues. GO and pathway analyses showed core DEGs were mainly enriched in the chemokine signaling and IL-6 signaling pathways, which could lead to immunosuppressive inflammatory monocyte infiltration and radioresistance. Chemokine signaling and IL-6 signaling pathway-associated genes were increased in the irradiated U87 cell strain.

Conclusion

Chemokine signaling and IL-6 signaling pathways were activated after radiation in murine glioma and human glioma cell lines which could lead to changes in the immune microenvironment and treatment failure. The results of the present study could provide potential therapeutic targets especially when immune therapy and radiotherapy are combined to treat GBM patients.

Fingolimod Augments Monomethylfumarate Killing of GBM Cells

Previously we demonstrated that the multiple sclerosis drug dimethyl fumarate (DMF) and its plasma breakdown product MMF could interact with chemotherapeutic agents to kill both GBM cells and activated microglia. The trial NCT02337426 demonstrated the safety of DMF in newly diagnosed GBM patients when combined with the standard of care Stupp protocol. We hypothesized that another multiple sclerosis drug, fingolimod (FTY720) would synergize with MMF to kill GBM cells. MMF and fingolimod interacted in a greater than additive fashion to kill PDX GBM isolates. MMF and fingolimod radiosensitized glioma cells and enhanced the lethality of temozolomide. Exposure to [MMF + fingolimod] activated an ATM-dependent toxic autophagy pathway, enhanced protective endoplasmic reticulum stress signaling, and inactivated protective PI3K, STAT, and YAP function. The drug combination reduced the expression of protective c-FLIP-s, MCL-1, BCL-XL, and in parallel caused cell-surface clustering of the death receptor CD95. Knock down of CD95 or over-expression of c-FLIP-s or BCL-XL suppressed killing. Fingolimod and MMF interacted in a greater than additive fashion to rapidly enhance reactive oxygen species production and over-expression of either thioredoxin or super-oxide dismutase two significantly reduced the drug-induced phosphorylation of ATM, autophagosome formation and [MMF + fingolimod] lethality. In contrast, the production of ROS was only marginally reduced in cells lacking ATM, CD95, or Beclin1. Collectively, our data demonstrate that the primary generation of ROS by [MMF + fingolimod] plays a key role, via the induction of toxic autophagy and death receptor signaling, in the killing of GBM cells.

Novel approaches for glioblastoma treatment: Focus on tumor heterogeneity, treatment resistance, and computational tools

BACKGROUND

Glioblastoma (GBM) is a highly aggressive primary brain tumor. Currently, the suggested line of action is the surgical resection followed by radiotherapy and treatment with the adjuvant temozolomide, a DNA alkylating agent. However, the ability of tumor cells to deeply infiltrate the surrounding tissue makes complete resection quite impossible, and, in consequence, the probability of tumor recurrence is high, and the prognosis is not positive. GBM is highly heterogeneous and adapts to treatment in most individuals. Nevertheless, these mechanisms of adaption are unknown.

RECENT FINDINGS

In this review, we will discuss the recent discoveries in molecular and cellular heterogeneity, mechanisms of therapeutic resistance, and new technological approaches to identify new treatments for GBM. The combination of biology and computer resources allow the use of algorithms to apply artificial intelligence and machine learning approaches to identify potential therapeutic pathways and to identify new drug candidates.

CONCLUSION

These new approaches will generate a better understanding of GBM pathogenesis and will result in novel treatments to reduce or block the devastating consequences of brain cancers.

SEL1L plays a major role in human malignant gliomas

Suppressor of Lin-12-like (C. elegans) (SEL1L) participates in the endoplasmic reticulum-associated protein degradation pathway, malignant transformation and stem cell biology. We explored the role of SEL1L in 110 adult gliomas, of different molecular subtype and grade, in relation to cell proliferation, stemness, glioma-associated microglia/macrophages (GAMs), prognostic markers and clinical outcome. SEL1L protein expression was assessed by immunohistochemistry and Western blotting. Genetic and epigenetic alterations were detected by molecular genetics techniques. SEL1L was overexpressed in anaplastic gliomas (World Health Organization [WHO] grade III) and in glioblastoma (GB, WHO grade IV) with the highest labelling index (LI) in the latter. Immunoreactivity was significantly associated with histological grade (p = 0.002) and cell proliferation index Ki-67/MIB-1 (p = 0.0001). In GB, SEL1L co-localised with stemness markers Nestin and Sox2. Endothelial cells and vascular pericytes of proliferative tumour blood vessels expressed SEL1L suggesting a role in tumour neo-vasculature. GAMs consistently expressed SEL1L. SEL1L overexpression was significantly associated with TERT promoter mutations (p = 0.0001), EGFR gene amplification (p = 0.0013), LOH on 10q (p = 0.0012) but was mutually exclusive with IDH1/2 mutations (p = 0.0001). SEL1L immunoreactivity correlated with tumour progression and cell proliferation, conditioning poor patient survival and response to therapy. This study emphasises SEL1L as a potential biomarker for the most common subgroup of TERT mutant/EGFR amplified/IDH-WT GBs.

Neddylation inhibition upregulates PD-L1 expression and enhances the efficacy of immune checkpoint blockade in glioblastoma

Pevonedistat (MLN4924), a specific NEDD8-activating enzyme inhibitor, has been considered as a promising treatment for glioblastoma, which is currently in Phase I/II clinical trials. On the other hand, inhibition of neddylation pathway substantially upregulates the expression of T cell negative regulator programmed death-ligand 1 (PD-L1), which might account for the potential resistance via evasion of immune surveillance checkpoints. Whether administration of anti-PD-L1 enhances the efficacy of pevonedistat through a cytotoxic T cell-dependent mechanism in glioblastoma needs to be investigated. Here, we report that depletion of neddylation pathway key enzymes markedly elevates PD-L1 expression in glioblastoma cancer cells. Consistently, neddylation inhibitor pevonedistat significantly enhances PD-L1 expression in both glioblastoma cancer cell lines and animal models. Mechanistically, pevonedistat increases PD-L1 mRNA levels mainly through inhibiting Cullin1-F-box and WD repeat domain-containing 7 E3 ligase activity and accumulating c-MYC proteins, a direct transcriptional activator of PD-L1 gene expression. In addition, inhibition of Cullin3 activity by pevonedistat also blocks PD-L1 protein degradation. Importantly, pevonedistat attenuates T cell killing through PD-L1 induction, and blockade of PD-L1 restores the sensitivity of pevonedistat-treated glioblastoma cancer cells to T cell killing. The combination of pevonedistat and anti-PD-L1 therapy compared to each agent alone significantly increased the therapeutic efficacy in vivo. Our study demonstrates inhibition of neddylation pathway suppresses cancer-associated immunity and provides solid evidence to support the combination of pevonedistat and PD-L1/programmed cell death protein 1 immune checkpoint blockade as a potential therapeutic strategy to treat glioblastoma.

Glioblastoma: Microenvironment and Niche Concept

The niche concept was originally developed to describe the location of normal neural stem cells (NSCs) in the subependymal layer of the sub-ventricular zone. In this paper, its significance has been extended to the location of tumor stem cells in glioblastoma (GB) to discuss the relationship between GB stem cells (GSCs) and endothelial cells (ECs). Their interaction is basically conceived as responsible for tumor growth, invasion and recurrence. Niches are described as the points of utmost expression of the tumor microenvironment (TME), therefore including everything in the tumor except for tumor cells: NSCs, reactive astrocytes, ECs, glioma-associated microglia/macrophages (GAMs), myeloid cells, pericytes, fibroblasts, etc. and all intrinsic and extrinsic signaling pathways. Perivascular (PVNs), perinecrotic (PNNs) and invasive niches were described from the pathological point of view, highlighting the basic significance of the EC/tumor stem cell couple. PNN development was reinterpreted based on the concept that hyperproliferative areas of GB are composed of GSCs/progenitors. TME was depicted in its function as the main regulator of everything that happens in the tumor. A particular emphasis was given to GAMs, pericytes and reactive astrocytes as important elements affecting proliferation, growth, invasion and resistance to therapies of tumor cells.

Enforced microglial depletion and repopulation as a promising strategy for the treatment of neurological disorders

Microglia are prominent immune cells in the central nervous system (CNS) and are critical players in both neurological development and homeostasis, and in neurological diseases when dysfunctional. Our previous understanding of the phenotypes and functions of microglia has been greatly extended by a dearth of recent investigations. Distinct genetically defined subsets of microglia are now recognized to perform their own independent functions in specific conditions. The molecular profiling of single microglial cells indicates extensively heterogeneous reactions in different neurological disorders, resulting in multiple potentials for crosstalk with other kinds of CNS cells such as astrocytes and neurons. In settings of neurological diseases it could thus be prudent to establish effective cell‐based therapies by targeting entire microglial networks. Notably, activated microglial depletion through genetic targeting or pharmacological therapies within a suitable time window can stimulate replenishment of the CNS niche with new microglia. Additionally, enforced repopulation through provision of replacement cells also represents a potential means of exchanging dysfunctional with functional microglia. In each setting the newly repopulated microglia might have the potential to resolve ongoing neuroinflammation. In this review, we aim to summarize the most recent knowledge of microglia and to highlight microglial depletion and subsequent repopulation as a promising cell replacement therapy. Although glial cell replacement therapy is still in its infancy and future translational studies are still required, the approach is scientifically sound and provides new optimism for managing the neurotoxicity and neuroinflammation induced by activated microglia.

Bidirectional Microglia-Neuron Communication in Health and Disease

Microglia are ramified cells that exhibit highly motile processes, which continuously survey the brain parenchyma and react to any insult to the CNS homeostasis. Although microglia have long been recognized as a crucial player in generating and maintaining inflammatory responses in the CNS, now it has become clear, that their function are much more diverse, particularly in the healthy brain. The innate immune response and phagocytosis represent only a little segment of microglia functional repertoire that also includes maintenance of biochemical homeostasis, neuronal circuit maturation during development and experience-dependent remodeling of neuronal circuits in the adult brain. Being equipped by numerous receptors and cell surface molecules microglia can perform bidirectional interactions with other cell types in the CNS. There is accumulating evidence showing that neurons inform microglia about their status and thus are capable of controlling microglial activation and motility while microglia also modulate neuronal activities. This review addresses the topic: how microglia communicate with other cell types in the brain, including fractalkine signaling, secreted soluble factors and extracellular vesicles. We summarize the current state of knowledge of physiological role and function of microglia during brain development and in the mature brain and further highlight microglial contribution to brain pathologies such as Alzheimer’s and Parkinson’s disease, brain ischemia, traumatic brain injury, brain tumor as well as neuropsychiatric diseases (depression, bipolar disorder, and schizophrenia).

Glioblastoma niches: from the concept to the phenotypical reality

Recently, the concept of niches as sites of tumor progression, invasion, and angiogenesis in glioblastoma (GB) has been extensively debated. Niches, considered the sites in which glioblastoma stem cells (GSCs) reside, have been classified as perivascular, perinecrotic, and invasive. However, from a neuropathological point of view, it is not easy to establish when a tumor structure can be considered a niche. The relevant literature has been reviewed in the light of our recent experience on the subject. As for perinecrotic niches, the occurrence of GSCs around necrosis is interpreted as triggered by hypoxia through HIF-1α. Our alternative hypothesis is that, together with progenitors, they are the cell constituents of hyper-proliferative areas of GB, where perinecrotic niches have developed, and they would, therefore, represent the remnants of GSCs/progenitors spared by the developing necrosis. Perivascular structures originate from both transport vessels and exchange vessels, i.e., venules, arterioles, or the undefinable neo-formed small vessels, but only those in which a direct contact between GSCs/progenitors and endothelial cells occurs can be called niches. Both pericytes and microglia/macrophages play a role in niche function: Macrophages of blood origin invade GB only after the appearance of "mother vessels" with consequent blood-brain barrier disruption. Not all vessel/tumor cell structures can be considered niches, that is, crucial sites of tumor progression, invasion, and angiogenesis.

Microglia immunophenotyping in gliomas

Microglia, once assimilated to peripheral macrophages, in gliomas has long been discussed and currently it is hypothesized to play a pro-tumor role in tumor progression. Uncertain between M1 and M2 polarization, it exchanges signals with glioma cells to create an immunosuppressive microenvironment and stimulates cell proliferation and migration. Four antibodies are currently used for microglia/macrophage identification in tissues that exhibit different cell forms and cell localization. The aim of the present work was to describe the distribution of the different cell forms and to deduce their significance on the basis of what is known on their function from the literature. Normal resting microglia, reactive microglia, intermediate and bumpy forms and macrophage-like cells can be distinguished by Iba1, CD68, CD16 and CD163 and further categorized by CD11b, CD45, c-MAF and CD98. The number of microglia/macrophages strongly increased from normal cortex and white matter to infiltrating and solid tumors. The ramified microglia accumulated in infiltration areas of both high- and low-grade gliomas, when hypertrophy and hyperplasia occur. In solid tumors, intermediate and bumpy forms prevailed and there is a large increase of macrophage-like cells in glioblastoma. The total number of microglia cells did not vary among the three grades of malignancy, but macrophage-like cells definitely prevailed in high-grade gliomas and frequently expressed CD45 and c-MAF. CD98+ cells were present. Microglia favors tumor progression, but many aspects suggest that the phagocytosing function is maintained. CD98+ cells can be the product of fusion, but also of phagocytosis. Microglia correlated with poorer survival in glioblastoma, when considering CD163+ cells, whereas it did not change prognosis in isocitrate dehydrogenase-mutant low grade gliomas.