The molecular profile of microglia under the influence of glioma

RNA Sequencing of Tumor-Associated Microglia Reveals Ccl5 as a Stromal Chemokine Critical for Neurofibromatosis-1 Glioma Growth

Solid cancers develop within a supportive microenvironment that promotes tumor formation and growth through the elaboration of mitogens and chemokines. Within these tumors, monocytes (macrophages and microglia) represent rich sources of these stromal factors. Leveraging a genetically engineered mouse model of neurofibromatosis type 1 (NF1) low-grade brain tumor (optic glioma), we have previously demonstrated that microglia are essential for glioma formation and maintenance. To identify potential tumor-associated microglial factors that support glioma growth (gliomagens), we initiated a comprehensive large-scale discovery effort using optimized RNA-sequencing methods focused specifically on glioma-associated microglia. Candidate microglial gliomagens were prioritized to identify potential secreted or membrane-bound proteins, which were next validated by quantitative real-time polymerase chain reaction as well as by RNA fluorescence in situ hybridization following minocycline-mediated microglial inactivation in vivo. Using these selection criteria, chemokine (C-C motif) ligand 5 (Ccl5) was identified as a chemokine highly expressed in genetically engineered Nf1 mouse optic gliomas relative to nonneoplastic optic nerves. As a candidate gliomagen, recombinant Ccl5 increased Nf1-deficient optic nerve astrocyte growth in vitro. Importantly, consistent with its critical role in maintaining tumor growth, treatment with Ccl5 neutralizing antibodies reduced Nf1 mouse optic glioma growth and improved retinal dysfunction in vivo. Collectively, these findings establish Ccl5 as an important microglial growth factor for low-grade glioma maintenance relevant to the development of future stroma-targeted brain tumor therapies.

Immunotherapy for neuro-oncology: the critical rationale for combinatorial therapy

A successful therapeutic paradigm established historically in oncology involves combining agents with potentially complementary mechanisms of antitumor activity into rationally designed regimens. For example, cocktails of cytotoxic agents, which were carefully designed based on mechanisms of action, dose, and scheduling considerations, have led to dramatic improvements in survival including cures for childhood leukemia, Hodgkin's lymphoma, and several other complex cancers. Outcome for glioblastoma, the most common primary malignant CNS cancer, has been more modest, but nonetheless our current standard of care derives from confirmation that combination therapy surpasses single modality therapy. Immunotherapy has recently come of age for medical oncology with exciting therapeutic benefits achieved by several types of agents including vaccines, adoptive T cells, and immune checkpoint inhibitors against several types of cancers. Nonetheless, most benefits are relatively short, while others are durable but are limited to a minority of treated patients. Critical factors limiting efficacy of immunotherapeutics include insufficient immunogenicity and/or inadequate ability to overcome immunosuppressive factors exploited by tumors. The paradigm of rationally designed combinatorial regimens, originally established by cytotoxic therapy for oncology, may also prove relevant for immunotherapy. Realization of the true therapeutic potential of immunotherapy for medical oncology and neuro-oncology patients may require development of combinatorial regimens that optimize immunogenicity and target tumor adaptive immunosuppressive factors.

The role of microglia and macrophages in glioma maintenance and progression

There is a growing recognition that gliomas are complex tumors composed of neoplastic and non-neoplastic cells, which each individually contribute to cancer formation, progression and response to treatment. The majority of the non-neoplastic cells are tumor-associated macrophages (TAMs), either of peripheral origin or representing brain-intrinsic microglia, that create a supportive stroma for neoplastic cell expansion and invasion. TAMs are recruited to the glioma environment, have immune functions, and can release a wide array of growth factors and cytokines in response to those factors produced by cancer cells. In this manner, TAMs facilitate tumor proliferation, survival and migration. Through such iterative interactions, a unique tumor ecosystem is established, which offers new opportunities for therapeutic targeting.

Phenotypic dynamics of microglial and monocyte-derived cells in glioblastoma-bearing mice

Inflammatory cells, an integral component of tumor evolution, are present in Glioblastomas multiforme (GBM). To address the cellular basis and dynamics of the inflammatory microenvironment in GBM, we established an orthotopic syngenic model by grafting GL261-DsRed cells in immunocompetent transgenic LysM-EGFP//CD11c-EYFP reporter mice. We combined dynamic spectral two-photon imaging with multiparametric cytometry and multicolor immunostaining to characterize spatio-temporal distribution, morphology and activity of microglia and blood-derived infiltrating myeloid cells in live mice. Early stages of tumor development were dominated by microglial EYFP⁺ cells invading the tumor, followed by massive recruitment of circulating LysM-EGFP⁺ cells. Fluorescent invading cells were conventional XCR1⁺ and monocyte-derived dendritic cells distributed in subpopulations of different maturation stages, located in different areas relative to the tumor core. The lethal stage of the disease was characterized by the progressive accumulation of EGFP⁺/EYFP⁺ monocyte-derived dendritic cells. This local phenotypic regulation of monocyte subtypes marked a transition in the immune response.

Ang-2/VEGF bispecific antibody reprograms macrophages and resident microglia to anti-tumor phenotype and prolongs glioblastoma survival

Inhibition of the vascular endothelial growth factor (VEGF) pathway has failed to improve overall survival of patients with glioblastoma (GBM). We previously showed that angiopoietin-2 (Ang-2) overexpression compromised the benefit from anti-VEGF therapy in a preclinical GBM model. Here we investigated whether dual Ang-2/VEGF inhibition could overcome resistance to anti-VEGF treatment. We treated mice bearing orthotopic syngeneic (Gl261) GBMs or human (MGG8) GBM xenografts with antibodies inhibiting VEGF (B20), or Ang-2/VEGF (CrossMab, A2V). We examined the effects of treatment on the tumor vasculature, immune cell populations, tumor growth, and survival in both the Gl261 and MGG8 tumor models. We found that in the Gl261 model, which displays a highly abnormal tumor vasculature, A2V decreased vessel density, delayed tumor growth, and prolonged survival compared with B20. In the MGG8 model, which displays a low degree of vessel abnormality, A2V induced no significant changes in the tumor vasculature but still prolonged survival. In both the Gl261 and MGG8 models A2V reprogrammed protumor M2 macrophages toward the antitumor M1 phenotype. Our findings indicate that A2V may prolong survival in mice with GBM by reprogramming the tumor immune microenvironment and delaying tumor growth.

KCa3.1 inhibition switches the phenotype of glioma-infiltrating microglia/macrophages

Among the strategies adopted by glioma to successfully invade the brain parenchyma is turning the infiltrating microglia/macrophages (M/MΦ) into allies, by shifting them toward an anti-inflammatory, pro-tumor phenotype. Both glioma and infiltrating M/MΦ cells express the Ca(2+)-activated K(+) channel (KCa3.1), and the inhibition of KCa3.1 activity on glioma cells reduces tumor infiltration in the healthy brain parenchyma. We wondered whether KCa3.1 inhibition could prevent the acquisition of a pro-tumor phenotype by M/MΦ cells, thus contributing to reduce glioma development. With this aim, we studied microglia cultured in glioma-conditioned medium or treated with IL-4, as well as M/MΦ cells acutely isolated from glioma-bearing mice and from human glioma biopsies. Under these different conditions, M/MΦ were always polarized toward an anti-inflammatory state, and preventing KCa3.1 activation by 1-[(2-Chlorophenyl)diphenylmethyl]-1H-pyrazole (TRAM-34), we observed a switch toward a pro-inflammatory, antitumor phenotype. We identified FAK and PI3K/AKT as the molecular mechanisms involved in this phenotype switch, activated in sequence after KCa3.1. Anti-inflammatory M/MΦ have higher expression levels of KCa3.1 mRNA (kcnn4) that are reduced by KCa3.1 inhibition. In line with these findings, TRAM-34 treatment, in vivo, significantly reduced the size of tumors in glioma-bearing mice. Our data indicate that KCa3.1 channels are involved in the inhibitory effects exerted by the glioma microenvironment on infiltrating M/MΦ, suggesting a possible role as therapeutic targets in glioma.

Monte-Carlo model development for evaluation of current clinical target volume definition for heterogeneous and hypoxic glioblastoma

Clinical target volume (CTV) determination may be complex and subjective. In this work a microscopic-scale tumour model was developed to evaluate current CTV practices in glioblastoma multiforme (GBM) external radiotherapy. Previously, a Geant4 cell-based dosimetry model was developed to calculate the dose deposited in individual GBM cells. Microscopic extension probability (MEP) models were then developed using Matlab-2012a. The results of the cell-based dosimetry model and MEP models were combined to calculate survival fractions (SF) for CTV margins of 2.0 and 2.5 cm. In the current work, oxygenation and heterogeneous radiosensitivity profiles were incorporated into the GBM model. The genetic heterogeneity was modelled using a range of α/β values (linear-quadratic model parameters) associated with different GBM cell lines. These values were distributed among the cells randomly, taken from a Gaussian-weighted sample of α/β values. Cellular oxygen pressure was distributed randomly taken from a sample weighted to profiles obtained from literature. Three types of GBM models were analysed: homogeneous-normoxic, heterogeneous-normoxic, and heterogeneous-hypoxic. The SF in different regions of the tumour model and the effect of the CTV margin extension from 2.0-2.5 cm on SFs were investigated for three MEP models. The SF within the beam was increased by up to three and two orders of magnitude following incorporation of heterogeneous radiosensitivities and hypoxia, respectively, in the GBM model. However, the total SF was shown to be overdominated by the presence of tumour cells in the penumbra region and to a lesser extent by genetic heterogeneity and hypoxia. CTV extension by 0.5 cm reduced the SF by a maximum of 78.6  ±  3.3%, 78.5  ±  3.3%, and 77.7  ±  3.1% for homogeneous and heterogeneous-normoxic, and heterogeneous hypoxic GBMs, respectively. Monte-Carlo model was developed to quantitatively evaluate SF for genetically heterogeneous and hypoxic GBM with two CTV margins and three MEP distributions. The results suggest that photon therapy may not provide cure for hypoxic and genetically heterogeneous GBM. However, the extension of the CTV margin by 0.5 cm could be beneficial to delay the recurrence time for this tumour type due to significant increase in tumour cell irradiation.

Glioma Stem Cells but Not Bulk Glioma Cells Upregulate IL-6 Secretion in Microglia/Brain Macrophages via Toll-like Receptor 4 Signaling

Peripheral macrophages and resident microglia constitute the dominant glioma-infiltrating cells. The tumor induces an immunosuppressive and tumor-supportive phenotype in these glioma-associated microglia/brain macrophages (GAMs). A subpopulation of glioma cells acts as glioma stem cells (GSCs). We explored the interaction between GSCs and GAMs. Using CD133 as a marker of stemness, we enriched for or deprived the mouse glioma cell line GL261 of GSCs by fluorescence-activated cell sorting (FACS). Over the same period of time, 100 CD133(+ )GSCs had the capacity to form a tumor of comparable size to the ones formed by 10,000 CD133(-) GL261 cells. In IL-6(-/-) mice, only tumors formed by CD133(+ )cells were smaller compared with wild type. After stimulation of primary cultured microglia with medium from CD133-enriched GL261 glioma cells, we observed an selective upregulation in microglial IL-6 secretion dependent on Toll-like receptor (TLR) 4. Our results show that GSCs, but not the bulk glioma cells, initiate microglial IL-6 secretion via TLR4 signaling and that IL-6 regulates glioma growth by supporting GSCs. Using human glioma tissue, we could confirm the finding that GAMs are the major source of IL-6 in the tumor context.

Targeting CXCR4 by a selective peptide antagonist modulates tumor microenvironment and microglia reactivity in a human glioblastoma model

BACKGROUND

The CXCL12/CXCR4 pathway regulates tumor cell proliferation, metastasis, angiogenesis and the tumor-microenvironment cross-talk in several solid tumors, including glioblastoma (GBM), the most common and fatal brain cancer. In the present study, we evaluated the effects of peptide R, a new specific CXCR4 antagonist that we recently developed by a ligand-based approach, in an in vitro and in vivo model of GBM. The well-characterized CXCR4 antagonist Plerixafor was also included in the study.

METHODS

The effects of peptide R on CXCR4 expression, cell survival and migration were assessed on the human glioblastoma cell line U87MG exposed to CXCL12, by immunofluorescence and western blotting, MTT assay, flow cytometry and transwell chamber migration assay. Peptide R was then tested in vivo, by using U87MG intracranial xenografts in CD1 nude mice. Peptide R was administered for 23 days since cell implantation and tumor volume was assessed by magnetic resonance imaging (MRI) at 4.7 T. Glioma associated microglia/macrophage (GAMs) polarization (anti-tumor M1 versus pro-tumor M2 phenotypes) and expressions of vascular endothelial growth factor (VEGF) and CD31 were assessed by immunohistochemistry and immunofluorescence.

RESULTS

We found that peptide R impairs the metabolic activity and cell proliferation of human U87MG cells and stably reduces CXCR4 expression and cell migration in response to CXCL12 in vitro. In the orthotopic U87MG model, peptide R reduced tumor cellularity, promoted M1 features of GAMs and astrogliosis, and hindered intra-tumor vasculature.

CONCLUSIONS

Our findings suggest that targeting CXCR4 by peptide R might represent a novel therapeutic approach against GBM, and contribute to the rationale to further explore in more complex pre-clinical settings the therapeutic potential of peptide R, alone or in combination with standard therapies of GBM.

Microglia-blood vessel interactions: a double-edged sword in brain pathologies

Microglia are long-living resident immune cells of the brain, which secure a stable chemical and physical microenvironment necessary for the proper functioning of the central nervous system (CNS). These highly dynamic cells continuously scan their environment for pathogens and possess the ability to react to damage-induced signals in order to protect the brain. Microglia, together with endothelial cells (ECs), pericytes and astrocytes, form the functional blood-brain barrier (BBB), a specialized endothelial structure that selectively separates the sensitive brain parenchyma from blood circulation. Microglia are in bidirectional and permanent communication with ECs and their perivascular localization enables them to survey the influx of blood-borne components into the CNS. Furthermore, they may stimulate the opening of the BBB, extravasation of leukocytes and angiogenesis. However, microglia functioning requires tight control as their dysregulation is implicated in the initiation and progression of numerous neurological diseases. Disruption of the BBB, changes in blood flow, introduction of pathogens in the sensitive CNS niche, insufficient nutrient supply, and abnormal secretion of cytokines or expression of endothelial receptors are reported to prime and attract microglia. Such reactive microglia have been reported to even escalate the damage of the brain parenchyma as is the case in ischemic injuries, brain tumors, multiple sclerosis, Alzheimer's and Parkinson's disease. In this review, we present the current state of the art of the causes and mechanisms of pathological interactions between microglia and blood vessels and explore the possibilities of targeting those dysfunctional interactions for the development of future therapeutics.

[The immunosuppressive microenvironment of malignant gliomas]

The dogma of the central nervous system (CNS) as an immune-privileged site has been substantially revised in recent years. CNS is an immunocompetent organ and actively interacts with the immune system. Microglia plays a leading role in a CNS immune response. However, in malignant gliomas, there is M2-polarization of microglia acquiring immunosuppressive and tumor-supportive properties. It occurs under the influence of tumor cytokines, such as transforming growth factor-β, interleukin-10, and prostaglandin E2. M2-polarized microglia exhibits reduced phagocytic activity, changes in the expression of many cellular determinants, or inverse of their functions, STAT3 activation, and production of immunosuppressive cytokines that suppress the function of cytotoxic CD8+ T cells or CD4+ T-helper cells type I. Myeloid-derived suppressor cells and regulatory T-lymphocytes, which have been recruited from peripheral blood into tumor tissue, also have immunosuppressive properties. The development of new treatment options for malignant gliomas must consider the role of the microenvironment in maintaining tumor vitality and progression.

Evaluation of TgH(CX3CR1-EGFP) mice implanted with mCherry-GL261 cells as an in vivo model for morphometrical analysis of glioma-microglia interaction

Background

Glioblastoma multiforme is the most aggressive brain tumor. Microglia are prominent cells within glioma tissue and play important roles in tumor biology. This work presents an animal model designed for the study of microglial cell morphology in situ during gliomagenesis. It also allows a quantitative morphometrical analysis of microglial cells during their activation by glioma cells.

Methods

The animal model associates the following cell types: 1- mCherry red fluorescent GL261 glioma cells and; 2- EGFP fluorescent microglia, present in the TgH(CX3CR1-EGFP) mouse line. First, mCherry-GL261 glioma cells were implanted in the brain cortex of TgH(CX3CR1-EGFP) mice. Epifluorescence − and confocal laser-scanning microscopy were employed for analysis of fixed tissue sections, whereas two-photon laser-scanning microscopy (2P-LSM) was used to track tumor cells and microglia in the brain of living animals.

Results

Implanted mCherry-GL261 cells successfully developed brain tumors. They mimic the aggressive behavior found in human disease, with a rapid increase in size and the presence of secondary tumors apart from the injection site. As tumor grows, mCherry-GL261 cells progressively lost their original shape, adopting a heterogeneous and diffuse morphology at 14–18 d. Soma size increased from 10–52 μm. At this point, we focused on the kinetics of microglial access to glioma tissues. 2P-LSM revealed an intense microgliosis in brain areas already shortly after tumor implantation, i.e. at 30 min. By confocal microscopy, we found clusters of microglial cells around the tumor mass in the first 3 days. Then cells infiltrated the tumor area, where they remained during all the time points studied, from 6–18 days. Microglia in contact with glioma cells also present changes in cell morphology, from a ramified to an amoeboid shape. Cell bodies enlarged from 366 ± 0.0 μm², in quiescent microglia, to 1310 ± 146.0 μm², and the cell processes became shortened.

Conclusions

The GL261/CX3CR1 mouse model reported here is a valuable tool for imaging of microglial cells during glioma growth, either in fixed tissue sections or living animals. Remarkable advantages are the use of immunocompetent animals and the simplified imaging method without the need of immunohistochemical procedures.

Glioma Stem Cells: Signaling, Microenvironment, and Therapy

Glioblastoma remains the most common and devastating primary brain tumor despite maximal therapy with surgery, chemotherapy, and radiation. The glioma stem cell (GSC) subpopulation has been identified in glioblastoma and likely plays a key role in resistance of these tumors to conventional therapies as well as recurrent disease. GSCs are capable of self-renewal and differentiation; glioblastoma-derived GSCs are capable of de novo tumor formation when implanted in xenograft models. Further, GSCs possess unique surface markers, modulate characteristic signaling pathways to promote tumorigenesis, and play key roles in glioma vascular formation. These features, in addition to microenvironmental factors, present possible targets for specifically directing therapy against the GSC population within glioblastoma. In this review, the authors summarize the current knowledge of GSC biology and function and the role of GSCs in new vascular formation within glioblastoma and discuss potential therapeutic approaches to target GSCs.

Directly visualized glioblastoma-derived extracellular vesicles transfer RNA to microglia/macrophages in the brain

BACKGROUND

To understand the ability of gliomas to manipulate their microenvironment, we visualized the transfer of vesicles and the effects of tumor-released extracellular RNA on the phenotype of microglia in culture and in vivo.

METHODS

Extracellular vesicles (EVs) released from primary human glioblastoma (GBM) cells were isolated and microRNAs (miRNAs) were analyzed. Primary mouse microglia were exposed to GBM-EVs, and their uptake and effect on proliferation and levels of specific miRNAs, mRNAs, and proteins were analyzed. For in vivo analysis, mouse glioma cells were implanted in the brains of mice, and EV release and uptake by microglia and monocytes/macrophages were monitored by intravital 2-photon microscopy, immunohistochemistry, and fluorescence activated cell sorting analysis, as well as RNA and protein levels.

RESULTS

Microglia avidly took up GBM-EVs, leading to increased proliferation and shifting of their cytokine profile toward immune suppression. High levels of miR-451/miR-21 in GBM-EVs were transferred to microglia with a decrease in the miR-451/miR-21 target c-Myc mRNA. In in vivo analysis, we directly visualized release of EVs from glioma cells and their uptake by microglia and monocytes/macrophages in brain. Dissociated microglia and monocytes/macrophages from tumor-bearing brains revealed increased levels of miR-21 and reduced levels of c-Myc mRNA.

CONCLUSIONS

Intravital microscopy confirms the release of EVs from gliomas and their uptake into microglia and monocytes/macrophages within the brain. Our studies also support functional effects of GBM-released EVs following uptake into microglia, associated in part with increased miRNA levels, decreased target mRNAs, and encoded proteins, presumably as a means for the tumor to manipulate its environs.

Principles of immunotherapy

Gliomas are the most common primary brain tumors of the central nervous system, and carry a grim prognosis. Novel approaches utilizing the immune system as adjuvant therapy are quickly emerging as viable and effective options. Immunotherapeutic strategies being investigated to treat glioblastoma include: vaccination therapy targeted against either specific tumor antigens or whole tumor lysate, adoptive cellular therapy with cytotoxic T lymphocytes, chimeric antigen receptors and bi-specific T-cell engaging antibodies allowing circumvention of major histocompatibility complex restriction, aptamer therapy with aims for more efficient target delivery, and checkpoint blockade in order to release the tumor-mediated inhibition of the immune system. Given the heterogeneity of glioblastoma and its ability to gain mutations throughout the disease course, multifaceted treatment strategies utilizing multiple forms of immunotherapy in combination with conventional therapy will be most likely to succeed moving forward.

The Microenvironment in Gliomas: Phenotypic Expressions

The microenvironment of malignant gliomas is described according to its definition in the literature. Beside tumor cells, a series of stromal cells (microglia/macrophages, pericytes, fibroblasts, endothelial cells, normal and reactive astrocytes) represents the cell component, whereas a complex network of molecular signaling represents the functional component. Its most evident expressions are perivascular and perinecrotic niches that are believed to be the site of tumor stem cells or progenitors in the tumor. Phenotypically, both niches are not easily recognizable; here, they are described together with a critical revision of their concept. As for perinecrotic niches, an alternative interpretation is given about their origin that regards the tumor stem cells as the residue of those that populated hyperproliferating areas in which necroses develop. This is based on the concept that the stem-like is a status and not a cell type, depending on the microenvironment that regulates a conversion of tumor non-stem cells and tumor stem cells through a cell reprogramming.

Microglia and astrocytes attenuate the replication of the oncolytic vaccinia virus LIVP 1.1.1 in murine GL261 gliomas by acting as vaccinia virus traps

Background

Oncolytic virotherapy is a novel approach for the treatment of glioblastoma multiforme (GBM) which is still a fatal disease. Pathologic features of GBM are characterized by the infiltration with microglia/macrophages and a strong interaction between immune- and glioma cells. The aim of this study was to determine the role of microglia and astrocytes for oncolytic vaccinia virus (VACV) therapy of GBM.

Methods

VACV LIVP 1.1.1 replication in C57BL/6 and Foxn1^nu/nu mice with and without GL261 gliomas was analyzed. Furthermore, immunohistochemical analysis of microglia and astrocytes was investigated in non-, mock-, and LIVP 1.1.1-infected orthotopic GL261 gliomas in C57BL/6 mice. In cell culture studies virus replication and virus-mediated cell death of GL261 glioma cells was examined, as well as in BV-2 microglia and IMA2.1 astrocytes with M1 or M2 phenotypes. Co-culture experiments between BV-2 and GL261 cells and apoptosis/necrosis studies were performed. Organotypic slice cultures with implanted GL261 tumor spheres were used as additional cell culture system.

Results

We discovered that orthotopic GL261 gliomas upon intracranial virus delivery did not support replication of LIVP 1.1.1, similar to VACV-infected brains without gliomas. In addition, recruitment of Iba1⁺ microglia and GFAP⁺ astrocytes to orthotopically implanted GL261 glioma sites occurred already without virus injection. GL261 cells in culture showed high virus replication, while replication in BV-2 and IMA2.1 cells was barely detectable. The reduced viral replication in BV-2 cells might be due to rapid VACV-induced apoptotic cell death. In BV-2 and IMA 2.1 cells with M1 phenotype a further reduction of virus progeny and virus-mediated cell death was detected. Application of BV-2 microglial cells with M1 phenotype onto organotypic slice cultures with implanted GL261 gliomas resulted in reduced infection of BV-2 cells, whereas GL261 cells were well infected.

Conclusion

Our results indicate that microglia and astrocytes, dependent on their activation state, may preferentially clear viral particles by immediate uptake after delivery. By acting as VACV traps they further reduce efficient virus infection of the tumor cells. These findings demonstrate that glia cells need to be taken into account for successful GBM therapy development.

Microglia Activate Migration of Glioma Cells through a Pyk2 Intracellular Pathway

Glioblastoma is one of the most aggressive and fatal brain cancers due to the highly invasive nature of glioma cells. Microglia infiltrate most glioma tumors and, therefore, make up an important component of the glioma microenvironment. In the tumor environment, microglia release factors that lead to the degradation of the extracellular matrix and stimulate signaling pathways to promote glioma cell invasion. In the present study, we demonstrated that microglia can promote glioma migration through a mechanism independent of extracellular matrix degradation. Using western blot analysis, we found upregulation of proline rich tyrosine kinase 2 (Pyk2) protein phosphorylated at Tyr579/580 in glioma cells treated with microglia conditioned medium. This upregulation occurred in rodent C6 and GL261 as well as in human glioma cell lines with varying levels of invasiveness (U-87MG, A172, and HS683). siRNA knock-down of Pyk2 protein and pharmacological blockade by the Pyk2/focal-adhesion kinase (FAK) inhibitor PF-562,271 reversed the stimulatory effect of microglia on glioma migration in all cell lines. A lower concentration of PF-562,271 that selectively inhibits FAK, but not Pyk2, did not have any effect on glioma cell migration. Moreover, with the use of the CD11b-HSVTK microglia ablation mouse model we demonstrated that elimination of microglia in the implanted tumors (GL261 glioma cells were used for brain implantation) by the local in-tumor administration of Ganciclovir, significantly reduced the phosphorylation of Pyk2 at Tyr579/580 in implanted tumor cells. Taken together, these data indicate that microglial cells activate glioma cell migration/dispersal through the pro-migratory Pyk2 signaling pathway in glioma cells.

Glioblastoma cells induce differential glutamatergic gene expressions in human tumor-associated microglia/macrophages and monocyte-derived macrophages

Glioblastoma cells produce and release high amounts of glutamate into the extracellular milieu and subsequently can trigger seizure in patients. Tumor-associated microglia/macrophages (TAMs), consisting of both parenchymal microglia and monocytes-derived macrophages (MDMs) recruited from the blood, are known to populate up to 1/3 of the glioblastoma tumor environment and exhibit an alternative, tumor-promoting and supporting phenotype. However, it is unknown how TAMs respond to the excess extracellular glutamate in the glioblastoma microenvironment. We investigated the expressions of genes related to glutamate transport and metabolism in human TAMs freshly isolated from glioblastoma resections. Quantitative real-time PCR analysis showed (i) significant increases in the expressions of GRIA2 (GluA2 or AMPA receptor 2), SLC1A2 (EAAT2), SLC1A3 (EAAT1), (ii) a near-significant decrease in the expression of SLC7A11 (cystine-glutamate antiporter xCT) and (iii) a remarkable increase in GLUL expression (glutamine synthetase) in these cells compared to adult primary human microglia. TAMs co-cultured with glioblastoma cells also exhibited a similar glutamatergic profile as freshly isolated TAMs except for a slight increase in SLC7A11 expression. We next analyzed these genes expressions in cultured human MDMs derived from peripheral blood monocytes for comparison. In contrast, MDMs co-cultured with glioblastoma cells compared to MDMs co-cultured with normal astrocytes exhibited decreased expressions in the tested genes except for GLUL. This is the first study to demonstrate transcriptional changes in glutamatergic signaling of TAMs in a glioblastoma microenvironment, and the findings here suggest that TAMs and MDMs might potentially elicit different cellular responses in the presence of excess extracellular glutamate.

Identification of a 6-cytokine prognostic signature in patients with primary glioblastoma harboring M2 microglia/macrophage phenotype relevance

BACKGROUND

Glioblastomas (GBM) are comprised of a heterogeneous population of tumor cells, immune cells, and extracellular matrix. Interactions among these different cell types and pro-/anti-inflammatory cytokines may promote tumor development and progression.

AIMS

The objective of this study was to develop a cytokine-related gene signature to improve outcome prediction for patients with primary GBM.

METHODS

Here, we used Cox regression and risk-score analysis to develop a cytokine-related gene signature in primary GBMs from the whole transcriptome sequencing profile of the Chinese Glioma Genome Atlas (CGGA) database (n=105). We also examined differences in immune cell phenotype and immune factor expression between the high-risk and low-risk groups.

RESULTS

Cytokine-related genes were ranked based on their ability to predict survival in the CGGA database. The six genes showing the strongest predictive value were CXCL10, IL17R, CCR2, IL17B, IL10RB, and CCL2. Patients with a high-risk score had poor overall survival and progression-free survival. Additionally, the high-risk group was characterized by increased mRNA expression of M2 microglia/macrophage markers and elevated levels of IL10 and TGFβ1.

CONCLUSION

The six cytokine-related gene signature is sufficient to predict survival and to identify a subgroup of primary GBM exhibiting the M2 cell phenotype.

Interleukin-6/STAT3 Pathway Signaling Drives an Inflammatory Phenotype in Group A Ependymoma

Ependymoma (EPN) in childhood is a brain tumor with substantial mortality. Inflammatory response has been identified as a molecular signature of high-risk Group A EPN. To better understand the biology of this phenotype and aid therapeutic development, transcriptomic data from Group A and B EPN patient tumor samples, and additional malignant and normal brain data, were analyzed to identify the mechanism underlying EPN Group A inflammation. Enrichment of IL6 and STAT3 pathway genes were found to distinguish Group A EPN from Group B EPN and other brain tumors, implicating an IL6 activation of STAT3 mechanism. EPN tumor cell growth was shown to be dependent on STAT3 activity, as demonstrated using shRNA knockdown and pharmacologic inhibition of STAT3 that blocked proliferation and induced apoptosis. The inflammatory factors secreted by EPN tumor cells were shown to reprogram myeloid cells, and this paracrine effect was characterized by a significant increase in pSTAT3 and IL8 secretion. Myeloid polarization was shown to be dependent on tumor secretion of IL6, and these effects could be reversed using IL6-neutralizing antibody or IL6 receptor-targeted therapeutic antibody tocilizumab. Polarized myeloid cell production of IL8 drove unpolarized myeloid cells to upregulate CD163 and to produce a number of proinflammatory cytokines. Collectively, these findings indicate that constitutive IL6/STAT3 pathway activation is important in driving tumor growth and inflammatory cross-talk with myeloid cells within the Group A EPN microenvironment. Effective design of Group A-targeted therapy for children with EPN may require reversal of this potentially immunosuppressive and protumor pathway.

Enriched environment reduces glioma growth through immune and non-immune mechanisms in mice

Mice exposed to standard (SE) or enriched environment (EE) were transplanted with murine or human glioma cells and differences in tumour development were evaluated. We report that EE exposure affects: (i) tumour size, increasing mice survival; (ii) glioma establishment, proliferation and invasion; (iii) microglia/macrophage (M/Mφ) activation; (iv) natural killer (NK) cell infiltration and activation; and (v) cerebral levels of IL-15 and BDNF. Direct infusion of IL-15 or BDNF in the brain of mice transplanted with glioma significantly reduces tumour growth. We demonstrate that brain infusion of IL-15 increases the frequency of NK cell infiltrating the tumour and that NK cell depletion reduces the efficacy of EE and IL-15 on tumour size and of EE on mice survival. BDNF infusion reduces M/Mφ infiltration and CD68 immunoreactivity in tumour mass and reduces glioma migration inhibiting the small G protein RhoA through the truncated TrkB.T1 receptor. These results suggest alternative approaches for glioma treatment.

Tumor-associated macrophages in SHH subgroup of medulloblastomas

PURPOSE

Medulloblastoma in children can be categorized into at least four molecular subgroups, offering the potential for targeted therapeutic approaches to reduce treatment-related morbidities. Little is known about the role of tumor microenvironment in medulloblastoma or its contribution to these molecular subgroups. Tumor microenvironment has been shown to be an important source for therapeutic targets in both adult and pediatric neoplasms. In this study, we investigated the hypothesis that expression of genes related to tumor-associated macrophages (TAM) correlates with the medulloblastoma molecular subgroups and contributes to a diagnostic signature.

METHODS

Gene-expression profiling using human exon array (n = 168) was analyzed to identify medulloblastoma molecular subgroups and expression of inflammation-related genes. Expression of 45 tumor-related and inflammation-related genes was analyzed in 83 medulloblastoma samples to build a gene signature predictive of molecular subgroups. TAMs in medulloblastomas (n = 54) comprising the four molecular subgroups were assessed by immunohistochemistry (IHC).

RESULTS

A 31-gene medulloblastoma subgroup classification score inclusive of TAM-related genes (CD163 and CSF1R) was developed with a misclassification rate of 2%. Tumors in the Sonic Hedgehog (SHH) subgroup had increased expression of inflammation-related genes and significantly higher infiltration of TAMs than tumors in the Group 3 or Group 4 subgroups (P < 0.0001 and P < 0.0001, respectively). IHC data revealed a strong association between location of TAMs and proliferating tumor cells.

CONCLUSIONS

These data show that SHH tumors have a unique tumor microenvironment among medulloblastoma subgroups. The interactions of TAMs and SHH medulloblastoma cells may contribute to tumor growth revealing TAMs as a potential therapeutic target.

Ontogeny of CX3CR1-EGFP expressing cells unveil microglia as an integral component of the postnatal subventricular zone

The full spectrum of cellular interactions within CNS neurogenic niches is still poorly understood. Only recently has the monocyte counterpart of the nervous system, the microglial cells, been described as an integral cellular component of neurogenic niches. The present study sought to characterize the microglia population in the early postnatal subventricular zone (SVZ), the major site of postnatal neurogenesis, as well as in its anterior extension, the rostral migratory stream (RMS), a pathway for neuroblasts during their transit toward the olfactory bulb (OB) layers. Here we show that microglia within the SVZ/RMS pathway are not revealed by phenotypic markers that characterize microglia in other regions. Analysis of the transgenic mice strain that has one locus of the constitutively expressed fractalkine CX3CR1 receptor replaced by the gene encoding the enhanced green fluorescent protein (EGFP) circumvented the antigenic plasticity of the microglia, thus allowing us to depict microglia within the SVZ/RMS pathway during early development. Notably, microglia within the early SVZ/RMS are not proliferative and display a protracted development, retaining a more immature morphology than their counterparts outside germinal layers. Furthermore, microglia contact and phagocyte radial glia cells (RG) processes, thereby playing a role on the astroglial transformation that putative stem cells within the SVZ niche undergo during the first postnatal days.

Glioma-associated microglia/macrophages display an expression profile different from M1 and M2 polarization and highly express Gpnmb and Spp1

Malignant glioma belong to the most aggressive neoplasms in humans with no successful treatment available. Patients suffering from glioblastoma multiforme (GBM), the highest-grade glioma, have an average survival time of only around one year after diagnosis. Both microglia and peripheral macrophages/monocytes accumulate within and around glioma, but fail to exert effective anti-tumor activity and even support tumor growth. Here we use microarray analysis to compare the expression profiles of glioma-associated microglia/macrophages and naive control cells. Samples were generated from CD11b+ MACS-isolated cells from naïve and GL261-implanted C57BL/6 mouse brains. Around 1000 genes were more than 2-fold up- or downregulated in glioma-associated microglia/macrophages when compared to control cells. A comparison with published data sets of M1, M2a,b,c-polarized macrophages revealed a gene expression pattern that has only partial overlap with any of the M1 or M2 gene expression patterns. Samples for the qRT-PCR validation of selected M1 and M2a,b,c-specific genes were generated from two different glioma mouse models and isolated by flow cytometry to distinguish between resident microglia and invading macrophages. We confirmed in both models the unique glioma-associated microglia/macrophage phenotype including a mixture of M1 and M2a,b,c-specific genes. To validate the expression of these genes in human we MACS-isolated CD11b+ microglia/macrophages from GBM, lower grade brain tumors and control specimens. Apart from the M1/M2 gene analysis, we demonstrate that the expression of Gpnmb and Spp1 is highly upregulated in both murine and human glioma-associated microglia/macrophages. High expression of these genes has been associated with poor prognosis in human GBM, as indicated by patient survival data linked to gene expression data. We also show that microglia/macrophages are the predominant source of these transcripts in murine and human GBM. Our findings provide new potential targets for future anti-glioma therapy.

Novel association between microglia and stem cells in human gliomas: A contributor to tumour proliferation?

Brain tumour stem cells and microglia both promote the growth of astrocytomas, the commonest form of primary brain tumour, with recent emerging evidence that these cell types may interact in glioma models. It is unclear whether microglia and stem cells are associated in human gliomas. To investigate this question, we used the technique of tissue microarrays to perform a correlative study of a large number of tumour samples. We quantified immunostaining of human astrocytic tumour tissue microarrays (86 patients; World Health Organisation grade II-IV) for microglia Ionized calcium binding adaptor molecule 1 (Iba1) and CD68, and stem cell nestin, SOX2 and CD133. Ki67 was used to assess proliferation and GFAP for astrocytic differentiation. Immunoreactivity for both microglial markers and stem cell markers nestin and SOX2 significantly increased with increasing tumour grade. GFAP was higher in low grade astrocytomas. There was a positive correlation between: (i) both microglial markers and nestin and CD133, (ii) nestin and tumour cell proliferation Ki67 and (iii) both microglial markers and Ki67. SOX2 was not associated with microglia or tumour proliferation. To test the clinical relevance, we investigated the putative association of these markers with clinical outcomes. High expression for nestin and Iba1 correlated with significantly shorter survival times, and high expression for nestin, Iba1, CD68 and Ki67 was associated with faster tumour progression on univariate analysis. On multivariate analysis, nestin, CD133 and Ki67 remained significant predictors of poorer survival, after adjustment for other markers. These results confirm previous in vitro findings, demonstrating their functional relevance as a therapeutic target in humans. This is the first report of a novel correlation between microglia and stem cells that may drive human astrocytic tumour development.

Ephs and Ephrins in malignant gliomas

Eph receptor tyrosine kinases and the corresponding ephrin ligands play a pivotal role in the glioma development and progression. Aberrant protein expression levels of the Eph receptors and ephrins are often associated with higher tumor grade and poor prognosis. Their function in tumorigenesis is complex due to the intricate network of possible co-occurring interactions between neighboring tumor cells and tumor microenvironment. Both Ephs and ephrins localize on the surface of tumor cells, tumor vasculature, glioma stem cells, tumor cells infiltrating brain, and immune cells infiltrating tumors. They can both promote and inhibit tumorigenicity depending on the downstream forward and reverse signalling generated. All the above-mentioned features make the Ephs/ephrins system an intriguing candidate for the development of new therapeutic strategies in glioma treatment. This review will give a general overview on the structure and the function of Ephs and ephrins, with a particular emphasis on the state of the knowledge of their role in malignant gliomas.

5-aminolevulinic acid-induced protoporphyrin IX with multi-dose ionizing irradiation enhances host antitumor response and strongly inhibits tumor growth in experimental glioma in vivo

Ionizing irradiation is a well‑established therapeutic modality for malignant gliomas. Due to its high cellular uptake, 5‑aminolevulinic acid (ALA) is used for fluorescence‑guided resection of malignant gliomas. We have previously shown that 5‑ALA sensitizes glioma cells to irradiation in vitro. The aim of the present study was to assess whether 5‑ALA acts as a radiosensitizer in experimental glioma in vivo. Rats were subcutaneously injected with 9L gliosarcoma cells and administered 5‑ALA. The accumulation of 5‑ALA‑induced protoporphyrin IX was confirmed by high‑performance liquid chromatography (HPLC) analysis. Subcutaneous (s.c.) tumors were subsequently irradiated with 2 Gy/day for five consecutive days. In the experimental glioma model, high‑performance liquid chromatography analysis revealed a high level of accumulation of 5‑ALA‑induced protoporphyrin IX in s.c. tumors 3 h after 5‑ALA administration. Multi‑dose ionizing irradiation induced greater inhibition of tumor growth in rats that were administered 5‑ALA than in the non‑5‑ALA‑treated animals. Immunohistochemical analysis of the s.c. tumors revealed that numerous ionized calcium‑binding adapter molecule 1 (Iba1)‑positive macrophages gathered at the surface of and within the s.c. tumors following multi‑dose ionizing irradiation in combination with 5‑ALA administration. By contrast, the s.c. tumors in the control group scarcely showed aggregation of Iba1‑positive macrophages. These results suggested that multi‑dose ionizing irradiation with 5‑ALA induced not only a direct cytotoxic effect but also enhanced the host antitumor immune response and thus caused high inhibition of tumor growth in experimental glioma.

Interactions of connexin 43 and aquaporin-4 in the formation of glioma-induced brain edema

Connexin 43 (Cx43) and aquaporin-4 (AQP4) have important roles in the formation of glioma-induced brain edema; however, the association between these two factors in the development of edema has remained to be elucidated. In the present study, immunofluorescence and western blot analysis revealed that in a rat model of intracranial C6 glioma, Cx43 expression levels were low to undetectable and AQP4 expression levels were low in glioma cells. Significantly higher Cx43 and AQP4 levels were detected in the tissue surrounding the glioma. To further investigate the potential interaction between Cx43 and AQP4, normal glial cells and C6 glioma cells were cultured in hypotonic medium. Reverse transcription quantitative polymerase chain reaction indicated that AQP4 and Cx43 mRNA expression levels increased as a function of time in normal glial cells and C6 glioma cells in a hypotonic environment. However, the increase observed in normal glial cells was significantly lower than that observed in C6 glioma cells. Furthermore, AQP4 expression levels changed prior to alterations in Cx43 expression. Following AQP4 silencing in C6 cells, the increase in Cx43 expression was significantly attenuated (P<0.05). In normal cells, Cx43 silencing did not influence AQP4 expression (P>0.05). Therefore, it was hypothesized that AQP4 and Cx43 had two distinct mechanisms underlying brain edema formation within and surrounding the glioma. Cx43 may be a downstream effector of AQP4. The elucidation of this pathway may aid in the development of drugs targeting the interaction between AQP4 and Cx43, providing novel therapeutic possibilities for glioma-induced brain edema.

Thrombin-processed Ecrg4 recruits myeloid cells and induces antitumorigenic inflammation

BACKGROUND

Extensive infiltration of brain tumors by microglia and macrophages is a hallmark of tumor progression, and yet the overall tumor microenvironment is characterized by an immunosuppressive phenotype. Here we identify esophageal cancer-related gene 4 (Ecrg4) as a novel thrombin-processed monocyte chemoattractant that recruits myeloid cells, promotes their activation, and leads to a blockade of tumor progression.

METHODS

Both xenograft glioma and syngeneic glioma models were used to measure orthotopic tumor progression and overall survival. Flow cytometry and immunohistochemical analyses were performed to assess myeloid cell localization, recruitment, and activation.

RESULTS

Ecrg4 promotes monocyte recruitment and activation of microglia in a T-/B-cell-independent mechanism, which leads to a reduction in glioma tumor burden and increased survival. Mutational analysis reveals that the biological activity of Ecrg4 is dependent on a thrombin-processing site at the C-terminus, inducing monocyte invasion in vivo and in vitro. Furthermore, tumor-induced myeloid cell recruitment is impaired in Ecrg4 knockout mice, leading to increased tumor burden and decreased survival.

CONCLUSIONS

Together, these results identify Ecrg4 as a paracrine factor that activates microglia and is chemotactic for monocytes, with potential as an antitumor therapeutic.

Mechanisms of intimate and long-distance cross-talk between glioma and myeloid cells: how to break a vicious cycle

Glioma-associated microglia and macrophages (GAMs) and myeloid-derived suppressor cells (MDSCs) condition the glioma microenvironment to generate an immunosuppressed niche for tumour expansion. This immunosuppressive microenvironment is considered to be shaped through a complex multi-step interactive process between glioma cells, GAMs and MDSCs. Glioma cells recruit GAMs and MDSCs to the tumour site and block their maturation. Glioma cell-derived factors subsequently skew these cells towards an immunosuppressive, tumour-promoting phenotype. Finally, GAMs and MDSCs enhance immune suppression in the glioma microenvironment and promote glioma growth, invasiveness, and neovascularization. The local and distant cross-talk between glioma cells and GAMs and MDSCs is regulated by a plethora of soluble proteins and cell surface-bound factors, and possibly via extracellular vesicles and platelets. Importantly, GAMs and MDSCs have been reported to impair the efficacy of glioma therapy, in particular immunotherapeutic approaches. Therefore, advancing our understanding of the function of GAMs and MDSCs in brain tumours and targeted intervention of their immunosuppressive function may benefit the treatment of glioma.

Glioma-associated microglia and macrophages/monocytes display distinct electrophysiological properties and do not communicate via gap junctions

Both brain-resident microglia and peripheral macrophages/monocytes infiltrate into glioma and promote glioma growth. In the present study we analyzed coupling and membrane currents in glioma-associated microglia and macrophages/monocytes and compared this to control and stab wound-associated microglia. Using the Cx3cr1(GFP/wt)Ccr2(RFP/wt) knock-in mouse line, we distinguished membrane currents of glioma-associated microglia and macrophages/monocytes in acute brain slices prepared 14-16 days after inoculation of GL261 glioma cells. The current profile of microglia showed inward rectifying currents reminiscent of an intermediate activation state when compared to other disease models or cell culture. Macrophages/monocytes showed a higher specific outward conductance and a significantly lower capacitance indicative of a smaller membrane area than microglia. As controls, we also recorded currents from control microglia and stab wound-associated microglia. Since there are reports of microglial coupling in vitro, we injected biocytin into these cells and analyzed for cell coupling after fixing the slices and processed for biocytin labeling with Cy3-conjugated-Streptavidin. Neither control microglia nor glioma-associated microglia and macrophages/monocytes nor stab wound-associated microglia showed any sign of coupling. Moreover, performing qRT-PCR revealed that no connexin43 was detectable on isolated and sorted glioma-associated microglia and macrophages/monocytes, indicating that these cells are not part of a coupled network.

Transcriptional regulation of adrenomedullin by oncostatin M in human astroglioma cells: implications for tumor invasion and migration

Adrenomedullin (ADM), a secretory peptide with multiple functions in physiological to pathological conditions, is upregulated in several human cancers, including brain, breast, colon, prostate, and lung cancer. However, the molecular mechanisms underlying the regulation of ADM expression in cancerous cells are not fully understood. Here, we report that oncostatin M (OSM), a cytokine belonging to the interleukin-6 family, induces ADM expression in astroglioma cells through induction of signal transducer and activator of transcription-3 (STAT-3) phosphorylation, nuclear translocation, and subsequent DNA binding to the ADM promoter. STAT-3 knockdown decreased OSM-mediated expression of ADM, indicating that ADM expression is regulated by STAT-3 in astroglioma cells. Lastly, scratch wound healing assay showed that astroglioma cell migration was significantly enhanced by ADM peptides. These data suggest that aberrant activation of STAT-3, which is observed in malignant brain tumors, may function as one of the key regulators for ADM expression and glioma invasion.

Immunotherapy advances for glioblastoma

Survival for patients with glioblastoma, the most common high-grade primary CNS tumor, remains poor despite multiple therapeutic interventions including intensifying cytotoxic therapy, targeting dysregulated cell signaling pathways, and blocking angiogenesis. Exciting, durable clinical benefits have recently been demonstrated for a number of other challenging cancers using a variety of immunotherapeutic approaches. Much modern research confirms that the CNS is immunoactive rather than immunoprivileged. Preliminary results of clinical studies demonstrate that varied vaccine strategies have achieved encouraging evidence of clinical benefit for glioblastoma patients, although multiple variables will likely require systematic investigation before optimal outcomes are realized. Initial preclinical studies have also revealed promising results with other immunotherapies including cell-based approaches and immune checkpoint blockade. Clinical studies to evaluate a wide array of immune therapies for malignant glioma patients are being rapidly developed. Important considerations going forward include optimizing response assessment and identifiying correlative biomarkers for predict therapeutic benefit. Finally, the potential of complementary combinatorial immunotherapeutic regimens is highly exciting and warrants expedited investigation.

The mTOR kinase inhibitors polarize glioma-activated microglia to express a M1 phenotype

BACKGROUND

Increased activation of mammalian target of rapamycin (mTOR) is observed in numerous human cancers. Recent studies on the glioma kinome have identified several deregulated pathways that converge and activate mTOR. The available evidence on the role of microglia in CNS cancers would suggest a dual role, a tumoricidal role and -on the contrary- a role favoring tumor growth.

METHODS

In the present paper, we have compared the effects of μM concentrations of rapamycin (RAPA) and its analog, RAD001 (RAD), on activated microglia; the latter was obtained by exposing cells to conditioned medium harvested either from inflammatory activated glioma cells (LI-CM) or from glioma cells kept under basal conditions (C-CM).

RESULTS

Here we show that the inhibition of mTOR polarizes glioma-activated microglial cells towards the M1 phenotype, with cytotoxic activities, preventing the induction of the M2 status that promotes tumor growth. In fact RAPA and RAD significantly increased iNOS expression and activity, while on the same time significantly reducing IL-10 gene expression induced by C-CM, thus suggesting that the drugs prevent the acquisition of a M2 phenotype in response to glioma factors promoting a classic M1 activation. Similar results were obtained using the conditioned media obtained after glioma stimulation with LPS-IFNγ (LI-CM), which was found to induce a mixture of M1 and M2a/b polarization phenotypes. In these conditions, the inhibition of mTOR led to a significant up-regulation of iNOS, and in parallel to the down-regulation of both ARG and IL-10 gene expression.

CONCLUSIONS

These data suggest that mTOR inhibition may prevent glioma induced M2 polarization of microglial cells and increase their cytotoxic potential, possibly resulting in antitumor actions.

Increased expression of stress inducible protein 1 in glioma-associated microglia/macrophages

Factors released by glioma-associated microglia/macrophages (GAMs) play an important role in the growth and infiltration of tumors. We have previously demonstrated that the co-chaperone stress-inducible protein 1 (STI1) secreted by microglia promotes proliferation and migration of human glioblastoma (GBM) cell lines in vitro. In the present study, in order to investigate the role of STI1 in a physiological context, we used a glioma model to evaluate STI1 expression in vivo. Here, we demonstrate that STI1 expression in both the tumor and in the infiltrating GAMs and lymphocytes significantly increased with tumor progression. Interestingly, high expression of STI1 was observed in macrophages and lymphocytes that infiltrated brain tumors, whereas STI1 expression in the circulating blood monocytes and lymphocytes remained unchanged. Our results correlate, for the first time, the expression of STI1 and glioma progression, and suggest that STI1 expression in GAMs and infiltrating lymphocytes is modulated by the brain tumor microenvironment.

Prognostic value of the preoperative immunological profile in patients with glioblastoma

Background:

Aim of our study was to determine the predictive impact of certain serum immunological markers on overall survival (OS) in patients with glioblastoma multiforme (GBM).

Methods:

We assayed prospectively values of interleukin 2 (IL-2), immunoglobulin G (IgG), C4, CD3+, CD4+ and CD8+ cells via flow cytometry, enzyme-linked immunosorbent assay (ELISA) and radial immunodiffusion in preoperative sera of adult patients with de novo histologically confirmed supratentorial GBM. Kaplan-Meier method and Cox proportional hazards models were used to assess clinical, laboratory, and treatment prognostic factors for OS.

Results:

Twenty-six consecutive patients were identified with a mean age of 59.6 years. Median follow up was 12 months. Lower IL-2 values (<7.97 pg/ml vs. ≥7.97 pg/ml, P = 0.029) und CD4+ counts (<200 cells/μl vs. ≥200 cells/μl, P < 0.001) correlated significantly with a shorter OS. The independent prognostic relevance of CD4 + counts was confirmed by the multivariate analysis (HR = 0.010, 95% CI 0.001-0.226, P = 0.011). Further independent prognostic factors for OS were type of resection (resection vs. biopsy) and administration of radiotherapy (yes/no).

Conclusion:

Preoperative values IL-2 and CD4+ cells in sera may carry a prognostic impact. Novel diagnostic models prior to histopathological confirmation may be used to predict prognosis of patients with GBM. Future studies should investigate whether targeting immune factors, such as CD4+ and IL-2, may improve the prognosis of patients with GBM.

Chemokine expression profile of freshly isolated human glioblastoma-associated macrophages/microglia

Several studies have substantiated the hypothesis that tumor progression is not only driven by the tumor cells themselves but also by their interaction with intrinsic and surrounding stromal cells. Tumor-associated macrophages and microglial cells (TAMs) represent one major stromal cell component of glioblastomas. Additionally, in many gliomas, chemokines are highly expressed and some chemokines were already linked to settlement of TAMs in tumors. However, although chemoattraction mechanisms mediated by chemokines and their receptors are well documented, information on their expression and role in TAMs, particularly in patients, is limited. Therefore, we investigated the transcription of the chemokine-receptor combinations CXCL12-CXCR4-CXCR7, CXCL16-CXCR6 and CX3CL1-CX3CR1 in freshly isolated TAMs from 20 human glioblastomas in relation to in vitro polarized M1- and M2-macrophages. We demonstrated that TAMs express both M1- and M2-markers. Compared to in vitro polarized macrophages, the M1-marker interleukin (IL)-6 was similarly expressed, whereas IL-1β and tumor necrosis factor (TNF)-α were found at lower levels. The M2-marker IL-10 was comparably expressed, while CD163 and transforming growth factor (TGF)-β were detected with one tenth lower intensities in TAMs. All investigated chemokines/receptors were transcribed at moderate to high levels in TAMs as well as in vitro polarized macrophages. However, CX3CR1 was markedly higher and CXCR7 was somewhat higher expressed in TAMs, whereas M2-macrophages were characterized by the highest CXCL12 and a moderate CX3CL1 expression. Collectively, TAMs share properties of M1- and M2-macrophages and show a considerably higher expression of the chemokine receptors CXCR7 and CX3CR1.

Loss of the tyrosine phosphatase PTPRD leads to aberrant STAT3 activation and promotes gliomagenesis

PTPRD, which encodes the protein tyrosine phosphatase receptor-δ, is one of the most frequently inactivated genes across human cancers, including glioblastoma multiforme (GBM). PTPRD undergoes both deletion and mutation in cancers, with copy number loss comprising the primary mode of inactivation in GBM. However, it is unknown whether loss of PTPRD promotes tumorigenesis in vivo, and the mechanistic basis of PTPRD function in tumors is unclear. Here, using genomic analysis and a glioma mouse model, we demonstrate that loss of Ptprd accelerates tumor formation and define the oncogenic context in which Ptprd loss acts. Specifically, we show that in human GBMs, heterozygous loss of PTPRD is the predominant type of lesion and that loss of PTPRD and the CDKN2A/p16(INK4A) tumor suppressor frequently co-occur. Accordingly, heterozygous loss of Ptprd cooperates with p16 deletion to drive gliomagenesis in mice. Moreover, loss of the Ptprd phosphatase resulted in phospho-Stat3 accumulation and constitutive activation of Stat3-driven genetic programs. Surprisingly, the consequences of Ptprd loss are maximal in the heterozygous state, demonstrating a tight dependence on gene dosage. Ptprd loss did not increase cell proliferation but rather altered pathways governing the macrophage response. In total, we reveal that PTPRD is a bona fide tumor suppressor, pinpoint PTPRD loss as a cause of aberrant STAT3 activation in gliomas, and establish PTPRD loss, in the setting of CDKN2A/p16(INK4A) deletion, as a driver of glioma progression.

Semapimod sensitizes glioblastoma tumors to ionizing radiation by targeting microglia

Glioblastoma is the most malignant and lethal form of astrocytoma, with patients having a median survival time of approximately 15 months with current therapeutic modalities. It is therefore important to identify novel therapeutics. There is mounting evidence that microglia (specialized brain-resident macrophages) play a significant role in the development and progression of glioblastoma tumors. In this paper we show that microglia, in addition to stimulating glioblastoma cell invasion, also promote glioblastoma cell proliferation and resistance to ionizing radiation in vitro. We found that semapimod, a drug that selectively interferes with the function of macrophages and microglia, potently inhibits microglia-stimulated GL261 invasion, without affecting serum-stimulated glioblastoma cell invasion. Semapimod also inhibits microglia-stimulated resistance of glioblastoma cells to radiation, but has no significant effect on microglia-stimulated glioblastoma cell proliferation. We also found that intracranially administered semapimod strongly increases the survival of GL261 tumor-bearing animals in combination with radiation, but has no significant benefit in the absence of radiation. In conclusion, our observations indicate that semapimod sensitizes glioblastoma tumors to ionizing radiation by targeting microglia and/or infiltrating macrophages.

Proinflammatory-activated glioma cells induce a switch in microglial polarization and activation status, from a predominant M2b phenotype to a mixture of M1 and M2a/B polarized cells

Malignant gliomas are primary brain tumors characterized by morphological and genetic complexities, as well as diffuse infiltration into normal brain parenchyma. Within gliomas, microglia/macrophages represent the largest tumor-infiltrating cell population, contributing by at least one-third to the total tumor mass. Bi-directional interactions between glioma cells and microglia may therefore play an important role on tumor growth and biology. In the present study, we have characterized the influence of glioma-soluble factors on microglial function, comparing the effects of media harvested under basal conditions with those of media obtained after inducing a pro-inflammatory activation state in glioma cells. We found that microglial cells undergo a different pattern of activation depending on the stimulus; in the presence of activated glioma-derived factors, i.e. a condition mimicking the late stage of pathology, microglia presents as a mixture of polarization phenotypes (M1 and M2a/b), with up-regulation of iNOS (inducible nitric oxide synthase), ARG (arginase) and IL (interleukine)-10. At variance, microglia exposed to basal glioma-derived factors, i.e. a condition resembling the early stage of pathology, shows a more specific pattern of activation, with increased M2b polarization status and up-regulation of IL-10 only. As far as viability and cell proliferation are concerned, both LI-CM [LPS (lipopolysaccharide)–IFNγ (interferon γ) conditioned media] and C-CM (control-conditioned media) induce similar effects on microglial morphology. Finally, in human glioma tissue obtained from surgical resection of patients with IV grade glioblastoma, we detected a significant amount of CD68 positive cells, which is a marker of macrophage/microglial phagocytic activity, suggesting that in vitro findings presented here might have a relevance in the human pathology as well.

We have characterized the influence of glioma-soluble factors on microglial, comparing the effects of media harvested under-basal conditions to those of media obtained after inducing a pro-inflammatory activation in glioma cells. Our data suggest that microglia might exert different effects on glioma depending on the stage of disease.

The potential for genetically altered microglia to influence glioma treatment

Diffuse and unstoppable infiltration of brain and spinal cord tissue by neoplastic glial cells is the single most important therapeutic problem posed by the common glioma group of tumors: astrocytoma, oligoastrocytoma, oligodendroglioma, their malignant variants and glioblastoma. These neoplasms account for more than two thirds of all malignant central nervous system tumors. However, most glioma research focuses on an examination of the tumor cells rather than on host-specific, tumor micro-environmental cells and factors. This can explain why existing diffuse glioma therapies fail and why these tumors have remained incurable. Thus, there is a great need for innovation. We describe a novel strategy for the development of a more effective treatment of diffuse glioma. Our approach centers on gaining control over the behavior of the microglia, the defense cells of the CNS, which are manipulated by malignant glioma and support its growth. Armoring microglia against the influences from glioma is one of our research goals. We further discuss how microglia precursors may be genetically enhanced to track down infiltrating glioma cells.

The battle for the brain: Brain tumor-initiating cells vs. microglia/macrophages

Brain tumor-initiating cells (BTICs) become less tumorigenic when co-cultured with microglia/macrophages (M/Ms) isolated from subjects not affected by glioma, but not when exposed to the M/Ms of glioma patients. Microglial cells and macrophages from glioma patients, however, can be reactivated by non-toxic doses of amphotericin B to curb the growth of BTICs in vitro and in vivo.

For the next trick: new discoveries in radiobiology applied to glioblastoma

Glioblastoma (GBM) is the most common malignant brain tumor. Radiotherapy post surgical resection remained the mainstay of the management of GBM for decades until the addition of temozolomide was shown to prolong the median overall survival (OS) by 2.5 months to 14.6 months in 2005. Infiltrative growth to surrounding normal brain tissue and cooption of vascular niches, peripheral microvasuclar hyperplasia, and central hypoxic regions with pseudopalisading necrosis are characteristics of GBM and are causally linked to their exceptional radio- and chemo-resistant phenotype. An intratumoral hierarchy is postulated consisting of tumor stem cells in the apex with high DNA-repair proficiency resisting radiotherapy. It is conceivable that the stem cell property is more dynamic than originally anticipated. Niche effects such as exposure to hypoxia and intercellular communication in proximities to endothelial or bone marrow-derived cells (BMDC), for example, may activate such "stem cell" programs. GBM are exceptionally stroma-rich tumors and may consist of more than 70% stroma components, such as microglia and BMDC. It becomes increasingly apparent that treatment of GBM needs to integrate therapies targeting all above-mentioned distinct pathophysiological features. Accordingly, recent approaches in GBM therapy include inhibition of invasion (e.g., integrin, EGFR, CD95, and mTOR inhibition), antiangiogenesis and stroma modulators (TGFbeta, VEGF, angiopoetin, cMET inhibitors) and activation of immune response (vaccination and blockage of negative co-stimulatory signals). In addition, high LET-radiotherapy, for example with carbon ions, is postulated to ablate tumor stem cell and hypoxic cells more efficiently as compared with conventional low-LET photon irradiation. We discuss current key concepts, their limitations, and potentials to improve the outcome in this rapidly progressive and devastating disease.

An update on vaccine therapy and other immunotherapeutic approaches for glioblastoma

Outcome for glioblastoma (GBM), the most common primary CNS malignancy, remains poor. The overall survival benefit recently achieved with immunotherapeutics for melanoma and prostate cancer support evaluation of immunotherapies for other challenging cancers, including GBM. Much historical dogma depicting the CNS as immunoprivileged has been replaced by data demonstrating CNS immunocompetence and active interaction with the peripheral immune system. Several glioma antigens have been identified for potential immunotherapeutic exploitation. Active immunotherapy studies for GBM, supported by preclinical data, have focused on tumor lysate and synthetic antigen vaccination strategies. Results to date confirm consistent safety, including a lack of autoimmune reactivity; however, modest efficacy and variable immunogenicity have been observed. These findings underscore the need to optimize vaccination variables and to address challenges posed by systemic and local immunosuppression inherent to GBM tumors. Additional immunotherapy strategies are also in development for GBM. Future studies may consider combinatorial immunotherapy strategies with complimentary actions.

NF-κB-induced IL-6 ensures STAT3 activation and tumor aggressiveness in glioblastoma

Glioblastoma (GBM) is the most aggressive, neurologically destructive and deadly tumor of the central nervous system (CNS). In GBM, the transcription factors NF-κB and STAT3 are aberrantly activated and associated with tumor cell proliferation, survival, invasion and chemoresistance. In addition, common activators of NF-κB and STAT3, including TNF-α and IL-6, respectively, are abundantly expressed in GBM tumors. Herein, we sought to elucidate the signaling crosstalk that occurs between the NF-κB and STAT3 pathways in GBM tumors. Using cultured GBM cell lines as well as primary human GBM xenografts, we elucidated the signaling crosstalk between the NF-κB and STAT3 pathways utilizing approaches that either a) reduce NF-κB p65 expression, b) inhibit NF-κB activation, c) interfere with IL-6 signaling, or d) inhibit STAT3 activation. Using the clinically relevant human GBM xenograft model, we assessed the efficacy of inhibiting NF-κB and/or STAT3 alone or in combination in mice bearing intracranial xenograft tumors in vivo. We demonstrate that TNF-α-induced activation of NF-κB is sufficient to induce IL-6 expression, activate STAT3, and elevate STAT3 target gene expression in GBM cell lines and human GBM xenografts in vitro. Moreover, the combined inhibition of NF-κB and STAT3 signaling significantly increases survival of mice bearing intracranial tumors. We propose that in GBM, the activation of NF-κB ensures subsequent STAT3 activation through the expression of IL-6. These data verify that pharmacological interventions to effectively inhibit the activity of both NF-κB and STAT3 transcription factors must be used in order to reduce glioma size and aggressiveness.

Review: molecular mechanism of microglia stimulated glioblastoma invasion

Glioblastoma multiforme is one of the deadliest human cancers and is characterized by a high degree of microglia and macrophage infiltration. The role of these glioma infiltrating macrophages (GIMs) in disease progression has been the subject of recent investigation. While initially thought to reflect an immune response to the tumor, the balance of evidence clearly suggests GIMs can have potent tumor-tropic functions and assist in glioma cell growth and infiltration into normal brain. In this review, we focus on the evidence for GIMs aiding mediating glioblastoma motility and invasion. We survey the literature for molecular pathways that are involved in paracrine interaction between glioma cells and GIMs and assess which of these might serve as attractive targets for therapeutic intervention.

Mesenchymal differentiation mediated by NF-κB promotes radiation resistance in glioblastoma

Despite extensive study, few therapeutic targets have been identified for glioblastoma (GBM). Here we show that patient-derived glioma sphere cultures (GSCs) that resemble either the proneural (PN) or mesenchymal (MES) transcriptomal subtypes differ significantly in their biological characteristics. Moreover, we found that a subset of the PN GSCs undergoes differentiation to a MES state in a TNF-α/NF-κB-dependent manner with an associated enrichment of CD44 subpopulations and radioresistant phenotypes. We present data to suggest that the tumor microenvironment cell types such as macrophages/microglia may play an integral role in this process. We further show that the MES signature, CD44 expression, and NF-κB activation correlate with poor radiation response and shorter survival in patients with GBM.