In vitro modulation of microglia motility by glioma cells is mediated by hepatocyte growth factor/scatter factor

Autophagy in glioblastoma: A mechanistic perspective

Glioblastoma (GBM) is one of the most lethal malignancies in humans. Even after surgical resection and aggressive radio- or chemotherapies, patients with GBM can survive for less than 14 months. Extreme inter-tumor and intra-tumor heterogeneity of GBM poses a challenge for resolving recalcitrant GBM pathophysiology. GBM tumor microenvironment (TME) exhibits diverse heterogeneity in cellular composition and processes contributing to tumor progression and therapeutic resistance. Autophagy is such a cellular process; that demonstrates a cell-specific and TME context-dependent role in GBM progression, leading to either the promotion or suppression of GBM progression. Autophagy can regulate GBM cell function directly via regulation of survival, migration, and invasion, or indirectly by affecting GBM TME composition such as immune cell population, tumor metabolism, and glioma stem cells. This review comprehensively investigates the role of autophagy in GBM pathophysiology.

Association between glioma and neurodegenerative diseases risk: a two-sample bi-directional Mendelian randomization analysis

Background

Earlier observational studies have demonstrated a correlation between glioma and the risk of neurodegenerative diseases (NDs), but the causality and direction of their associations remain unclear. The objective of this study was to ascertain the causal link between glioma and NDs using Mendelian randomization (MR) methodology.

Methods

Genome-wide association study (GWAS) data were used in a two-sample bi-directional MR analysis. From the largest meta-analysis GWAS, encompassing 18,169 controls and 12,488 cases, summary statistics data on gliomas was extracted. Summarized statistics for NDs, including Alzheimer's disease (AD), multiple sclerosis (MS), amyotrophic lateral sclerosis (ALS) and Parkinson's disease (PD) were obtained from the GWAS of European ancestry. Inverse variance weighted (IVW) method was elected as the core MR approach with weighted median (WM) method and MR-Egger method as complementary methods. In addition, sensitivity analyses were performed. A Bonferroni correction was used to correct the results.

Results

Genetically predicted glioma had been related to decreased risk of AD. Specifically, for all glioma (IVW: OR = 0.93, 95% CI = 0.90-0.96, p = 4.88 × 10-6) and glioblastoma (GBM) (IVW: OR = 0.93, 95% CI = 0.91-0.95, p = 5.11 × 10-9). We also found that genetically predicted all glioma has a suggestive causative association with MS (IVW: OR = 0.90, 95% CI = 0.81-1.00, p = 0.045). There was no evidence of causal association between glioma and ALS or PD. According to the results of reverse MR analysis, no discernible causal connection of NDs was found on glioma. Sensitivity analyses validated the robustness of the above associations.

Conclusion

We report evidence in support of potential causal associations of different glioma subtypes with AD and MS. More studies are required to uncover the underlying mechanisms of these findings.

Glioma Stem Cells-Features for New Therapy Design

On a molecular level, glioma is very diverse and presents a whole spectrum of specific genetic and epigenetic alterations. The tumors are unfortunately resistant to available therapies and the survival rate is low. The explanation of significant intra- and inter-tumor heterogeneity and the infiltrative capability of gliomas, as well as its resistance to therapy, recurrence and aggressive behavior, lies in a small subset of tumor-initiating cells that behave like stem cells and are known as glioma cancer stem cells (GCSCs). They are responsible for tumor plasticity and are influenced by genetic drivers. Additionally, GCSCs also display greater migratory abilities. A great effort is under way in order to find ways to eliminate or neutralize GCSCs. Many different treatment strategies are currently being explored, including modulation of the tumor microenvironment, posttranscriptional regulation, epigenetic modulation and immunotherapy.

The Role of Microglia in Brain Metastases: Mechanisms and Strategies

Brain metastases and related complications are one of the major fatal factors in cancer. Patients with breast cancer, lung cancer, and melanoma are at a high risk of developing brain metastases. However, the mechanisms underlying the brain metastatic cascade remain poorly understood. Microglia, one of the major resident macrophages in the brain parenchyma, are involved in multiple processes associated with brain metastasis, including inflammation, angiogenesis, and immune modulation. They also closely interact with metastatic cancer cells, astrocytes, and other immune cells. Current therapeutic approaches against metastatic brain cancers, including small-molecule drugs, antibody-coupled drugs (ADCs), and immune-checkpoint inhibitors (ICIs), have compromised efficacy owing to the impermeability of the blood-brain barrier (BBB) and complex brain microenvironment. Targeting microglia is one of the strategies for treating metastatic brain cancer. In this review, we summarize the multifaceted roles of microglia in brain metastases and highlight them as potential targets for future therapeutic interventions.

Cuprotosis clusters predict prognosis and immunotherapy response in low-grade glioma

Cuprotosis, an emerging mode of cell death, has recently caught the attention of researchers worldwide. However, its impact on low-grade glioma (LGG) patients has not been fully explored. To gain a deeper insight into the relationship between cuprotosis and LGG patients' prognosis, we conducted this study in which LGG patients were divided into two clusters based on the expression of 18 cuprotosis-related genes. We found that LGG patients in cluster A had better prognosis than those in cluster B. The two clusters also differed in terms of immune cell infiltration and biological functions. Moreover, we identified differentially expressed genes (DEGs) between the two clusters and developed a cuprotosis-related prognostic signature through the least absolute shrinkage and selection operator (LASSO) analysis in the TCGA training cohort. This signature divided LGG patients into high- and low-risk groups, with the high-risk group having significantly shorter overall survival (OS) time than the low-risk group. Its predictive reliability for prognosis in LGG patients was confirmed by the TCGA internal validation cohort, CGGA325 cohort and CGGA693 cohort. Additionally, a nomogram was used to predict the 1-, 3-, and 5-year OS rates of each patient. The analysis of immune checkpoints and tumor mutation burden (TMB) has revealed that individuals belonging to high-risk groups have a greater chance of benefiting from immunotherapy. Functional experiments confirmed that interfering with the signature gene TNFRSF11B inhibited LGG cell proliferation and migration. Overall, this study shed light on the importance of cuprotosis in LGG patient prognosis. The cuprotosis-related prognostic signature is a reliable predictor for patient outcomes and immunotherapeutic response and can help to develop new therapies for LGG.

CNS and CNS diseases in relation to their immune system

The central nervous system is the most important nervous system in vertebrates, which is responsible for transmitting information to the peripheral nervous system and controlling the body's activities. It mainly consists of the brain and spinal cord, which contains rich of neurons, the precision of the neural structures susceptible to damage from the outside world and from the internal factors of inflammation infection, leading to a series of central nervous system diseases, such as traumatic brain injury, nerve inflammation, etc., these diseases may cause irreversible damage on the central nervous or lead to subsequent chronic lesions. After disease or injury, the immune system of the central nervous system will play a role, releasing cytokines to recruit immune cells to enter, and the immune cells will differentiate according to the location and degree of the lesion, and become specific immune cells with different functions, recognize and phagocytose inflammatory factors, and repair the damaged neural structure. However, if the response of these immune cells is not suppressed, the overexpression of some genes can cause further damage to the central nervous system. There is a need to understand the molecular mechanisms by which these immune cells work, and this information may lead to immunotherapies that target certain diseases and avoid over-activation of immune cells. In this review, we summarized several immune cells that mainly play a role in the central nervous system and their roles, and also explained the response process of the immune system in the process of some common neurological diseases, which may provide new insights into the central nervous system.

Glioma cancer stem cells modulating the local tumor immune environment

Glioma stem cells (GSCs) drive the resistance mechanism in glioma tumors and mediate the suppression of innate and adaptive immune responses. Here we investigate the expression of mesenchymal-epithelial transition factor (c-Met) and Fas receptor in GSCs and their role in potentiating the tumor-mediated immune suppression through modulation of tumor infiltrating lymphocyte (TIL) population. Tumor tissues were collected from 4 patients who underwent surgery for glioblastoma. GSCs were cultured as neurospheres and evaluated for the co-expression of CD133, c-Met and FasL through flow cytometry. TILs were isolated and evaluated for the lymphocyte subset frequencies including CD3 +, CD4 +, CD8 +, regulatory T cells (FOXP3 + CD25) and microglia (CD11b + CD45) using flow cytometry. Our findings revealed that a significant population of GSCs in all four samples expressed c-Met (89–99%) and FasL (73–97%). A significantly low microglia population was found in local immune cells ranging from 3 to 5%. We did not find a statistically significant correlation between expressions of c-Met + GSC and FasL + GSC with local and systemic immune cells. This may be regarded to the small sample size. The percent c-Met + and FasL + GSC population appeared to be related to percent cytotoxic T cells, regulatory T cells and microglia populations in glioblastoma patients. Further investigation is warranted in a larger sample size.

Research Progress on the Regulation Mechanism of Key Signal Pathways Affecting the Prognosis of Glioma

As is known to all, glioma, a global difficult problem, has a high malignant degree, high recurrence rate and poor prognosis. We analyzed and summarized signal pathway of the Hippo/YAP, PI3K/AKT/mTOR, miRNA, WNT/β-catenin, Notch, Hedgehog, TGF-β, TCS/mTORC1 signal pathway, JAK/STAT signal pathway, MAPK signaling pathway, the relationship between BBB and signal pathways and the mechanism of key enzymes in glioma. It is concluded that Yap1 inhibitor may become an effective target for the treatment of glioma in the near future through efforts of generation after generation. Inhibiting PI3K/Akt/mTOR, Shh, Wnt/β-Catenin, and HIF-1α can reduce the migration ability and drug resistance of tumor cells to improve the prognosis of glioma. The analysis shows that Notch1 and Sox2 have a positive feedback regulation mechanism, and Notch4 predicts the malignant degree of glioma. In this way, notch cannot only be treated for glioma stem cells in clinic, but also be used as an evaluation index to evaluate the prognosis, and provide an exploratory attempt for the direction of glioma treatment. MiRNA plays an important role in diagnosis, and in the treatment of glioma, VPS25, KCNQ1OT1, KB-1460A1.5, and CKAP4 are promising prognostic indicators and a potential therapeutic targets for glioma, meanwhile, Rheb is also a potent activator of Signaling cross-talk etc. It is believed that these studies will help us to have a deeper understanding of glioma, so that we will find new and better treatment schemes to gradually conquer the problem of glioma.

Research Progress on the Regulation Mechanism of Key Signal Pathways Affecting the Prognosis of Glioma

As is known to all, glioma, a global difficult problem, has a high malignant degree, high recurrence rate and poor prognosis. We analyzed and summarized signal pathway of the Hippo/YAP, PI3K/AKT/mTOR, miRNA, WNT/β-catenin, Notch, Hedgehog, TGF-β, TCS/mTORC1 signal pathway, JAK/STAT signal pathway, MAPK signaling pathway, the relationship between BBB and signal pathways and the mechanism of key enzymes in glioma. It is concluded that Yap1 inhibitor may become an effective target for the treatment of glioma in the near future through efforts of generation after generation. Inhibiting PI3K/Akt/mTOR, Shh, Wnt/β-Catenin, and HIF-1α can reduce the migration ability and drug resistance of tumor cells to improve the prognosis of glioma. The analysis shows that Notch1 and Sox2 have a positive feedback regulation mechanism, and Notch4 predicts the malignant degree of glioma. In this way, notch cannot only be treated for glioma stem cells in clinic, but also be used as an evaluation index to evaluate the prognosis, and provide an exploratory attempt for the direction of glioma treatment. MiRNA plays an important role in diagnosis, and in the treatment of glioma, VPS25, KCNQ1OT1, KB-1460A1.5, and CKAP4 are promising prognostic indicators and a potential therapeutic targets for glioma, meanwhile, Rheb is also a potent activator of Signaling cross-talk etc. It is believed that these studies will help us to have a deeper understanding of glioma, so that we will find new and better treatment schemes to gradually conquer the problem of glioma.

Signal Pathways Involved in the Interaction Between Tumor-Associated Macrophages/TAMs and Glioblastoma Cells

It is commonly recognized, that glioblastoma is a large complex composed of neoplastic and non-neoplastic cells. Tumor-associated macrophages account for the majority of tumor bulk and play pivotal roles in tumor proliferation, migration, invasion, and survival. There are sophisticated interactions between malignant cells and tumor associated-macrophages. Tumor cells release a variety of chemokines, cytokines, and growth factors that subsequently lead to the recruitment of TAMs, which in return released a plethora of factors to construct an immunosuppressive and tumor-supportive microenvironment. In this article, we have reviewed the biological characteristics of glioblastoma-associated macrophages and microglia, highlighting the emerging molecular targets and related signal pathways involved in the interaction between TAMs and glioblastoma cells, as well as the potential TAMs-associated therapeutic targets for glioblastoma.

The Benedict Arnold of the Central Nervous System Tumor Microenvironment? The Role of Microglia/Macrophages in Glioma

The glioma microenvironment is heavily infiltrated by non-neoplastic myeloid cells, including bone marrow-derived macrophages and central nervous system-resident microglia. As opposed to executing the antitumor functions of immune surveillance, antigen presentation, and phagocytosis, these tumor-associated myeloid cells are co-opted to promote an immunosuppressive milieu and support tumor invasion and angiogenesis. This review explores evolving evidence and the research paradigms used to determine the interplay of tumor genetics, immune cell composition, and immune function in gliomas. Understanding these cells and how they are reprogrammed will be instrumental in finding new and effective treatments for these lethal tumors.

Immunomodulatory Effect of Microglia-Released Cytokines in Gliomas

Microglia, a type of differentiated tissue macrophage, are considered to be the most plastic cell population of the central nervous system (CNS). Microglia substantially contribute to the growth and invasion of tumor mass in brain tumors including glioblastoma (GB). In response to pathological conditions, resting microglia undergo a stereotypic activation process and become capable of phagocytosis, antigen presentation, and lymphocyte activation. Considering their immune effector function, it is not surprising to see microglia accumulation in almost every CNS disease process, including malignant brain tumors. Large numbers of glioma associated microglia and macrophages (GAMs) can accumulate within the tumor where they appear to have an important role in prognosis. GAMs constitute the largest portion of tumor infiltrating cells, contributing up to 30% of the entire glioma mass and upon interaction with neoplastic cells. GAMs acquire a unique phenotype of activation, including both M1 and M2 specific markers. It has been demonstrated that microglia possess a dual role: on one hand, microglia may represent a CNS anti-tumor response, which is inactivated by local secretion of immunosuppressive factors by glioma cells. On the other hand, taking into account that microglia are capable of secreting a variety of immunomodulatory cytokines, it is possible that they are attracted by gliomas to promote tumor growth. A better understanding of microglia-glioma interaction will be helpful in designing novel immune-based therapies against these fatal tumors. Concluding, as microglia significantly may contribute to glioma biology, favoring tumor growth and invasiveness, these cells represent a valuable alternative/additional target for the development of more effective treatments for gliomas.

Macrophages and microglia: the cerberus of glioblastoma

Glioblastoma (GBM) is the most aggressive and deadliest of the primary brain tumors, characterized by malignant growth, invasion into the brain parenchyma, and resistance to therapy. GBM is a heterogeneous disease characterized by high degrees of both inter- and intra-tumor heterogeneity. Another layer of complexity arises from the unique brain microenvironment in which GBM develops and grows. The GBM microenvironment consists of neoplastic and non-neoplastic cells. The most abundant non-neoplastic cells are those of the innate immune system, called tumor-associated macrophages (TAMs). TAMs constitute up to 40% of the tumor mass and consist of both brain-resident microglia and bone marrow-derived myeloid cells from the periphery. Although genetically stable, TAMs can change their expression profiles based upon the signals that they receive from tumor cells; therefore, heterogeneity in GBM creates heterogeneity in TAMs. By interacting with tumor cells and with the other non-neoplastic cells in the tumor microenvironment, TAMs promote tumor progression. Here, we review the origin, heterogeneity, and functional roles of TAMs. In addition, we discuss the prospects of therapeutically targeting TAMs alone or in combination with standard or newly-emerging GBM targeting therapies.

Immune biology of glioma-associated macrophages and microglia: functional and therapeutic implications

CNS immune defenses are marshaled and dominated by brain resident macrophages and microglia, which are the innate immune sentinels and frontline host immune barriers against various pathogenic insults. These myeloid lineage cells are the predominant immune population in gliomas and can constitute up to 30-50% of the total cellular composition. Parenchymal microglial cells and recruited monocyte-derived macrophages from the periphery exhibit disease-specific phenotypic characteristics with spatial and temporal distinctions and are heterogeneous subpopulations based on their molecular signatures. A preponderance of myeloid over lymphoid lineage cells during CNS inflammation, including gliomas, is a contrasting feature of brain immunity relative to peripheral immunity. Herein we discuss glioma-associated macrophage and microglia immune biology in the context of their identity, molecular drivers of recruitment, nomenclature and functional paradoxes, therapeutic reprogramming and polarization strategies, relevant challenges, and our perspectives on therapeutic modulation.

Immune response in glioma's microenvironment

Objectives

Glioma is the most common tumor of the central nervous system. In this review, we outline the immunobiological factors that interact with glioma cells and tumor microenvironment (TME), providing more potential targets for clinical inhibition of glioma development and more directions for glioma treatment.

Content

Recent studies have shown that glioma cells secrete a variety of immune regulatory factors and interact with immune cells such as microglial cells, peripheral macrophages, myeloid-derived suppressor cells (MDSCs), and T lymphocytes in the TME. In particular, microglia plays a key role in promoting glioma growth. Infiltrating immune cells induce local production of cytokines, chemokines and growth factors. Further leads to immune escape of malignant gliomas.

Summary and Outlook

The complex interaction of tumor cells with the TME has largely contributed to tumor heterogeneity and poor prognosis. We review the immunobiological factors, immune cells and current immunotherapy of gliomas, provide experimental evidence for future research and treatment of gliomas.

Glioma-associated microglia/macrophages augment tumorigenicity in canine astrocytoma, a naturally occurring model of human glioma

BACKGROUND

Glioma-associated microglia/macrophages (GAMs) markedly influence glioma progression. Under the influence of transforming growth factor beta (TGFB), GAMs are polarized toward a tumor-supportive phenotype. However, neither therapeutic targeting of GAM recruitment nor TGFB signaling demonstrated efficacy in glioma patients despite efficacy in preclinical models, underscoring the need for a comprehensive understanding of the TGFB/GAM axis. Spontaneously occurring canine gliomas share many features with human glioma and provide a complementary translational animal model for further study. Given the importance of GAM and TGFB in human glioma, the aims of this study were to further define the GAM-associated molecular profile and the relevance of TGFB signaling in canine glioma that may serve as the basis for future translational studies.

METHODS

GAM morphometry, levels of GAM-associated molecules, and the canonical TGFB signaling axis were compared in archived samples of canine astrocytomas versus normal canine brain. Furthermore, the effect of TGFB on the malignant phenotype of canine astrocytoma cells was evaluated.

RESULTS

GAMs diffusely infiltrated canine astrocytomas. GAM density was increased in high-grade tumors that correlated with a pro-tumorigenic molecular signature and upregulation of the canonical TGFB signaling axis. Moreover, TGFB1 enhanced the migration of canine astrocytoma cells in vitro.

CONCLUSIONS

Canine astrocytomas share a similar GAM-associated immune landscape with human adult glioma. Our data also support a contributing role for TGFB1 signaling in the malignant phenotype of canine astrocytoma. These data further support naturally occurring canine glioma as a valid model for the investigation of GAM-associated therapeutic strategies for human malignant glioma.

Distinction of Microglia and Macrophages in Glioblastoma: Close Relatives, Different Tasks?

For decades, it has been known that the tumor microenvironment is significant for glioma progression, namely the infiltration of myeloid cells like microglia and macrophages. Hence, these cell types and their specific tasks in tumor progression are subject to ongoing research. However, the distribution of the brain resident microglia and the peripheral macrophages within the tumor tissue and their functional activity are highly debated. Results depend on the method used to discriminate between microglia and macrophages, whereby this specification is already difficult due to limited options to distinguish between these both cell populations that show mostly the same surface markers and morphology. Moreover, there are indications about various functions of microglia and macrophages but again varying on the method of discrimination. In our review, we summarize the current literature to determine which methods have been applied to differentiate the brain resident microglia from tumor-infiltrated macrophages. Furthermore, we compiled data about the proportion of microglia and macrophages in glioma tissues and ascertained if pro- or anti-tumoral effects could be allocated to one or the other myeloid cell population. Recent research made tremendous efforts to distinguish microglia from recruited macrophages. For future studies, it could be essential to verify which role these cells play in brain tumor pathology to proceed with novel immunotherapeutic strategies.

Distinct regional ontogeny and activation of tumor associated macrophages in human glioblastoma

Tumor-associated macrophages (TAMs) constitute up to 50% of tumor bulk in glioblastoma (GBM) and play an important role in tumor maintenance and progression. The recently discovered differences between invading tumour periphery and hypoxic tumor core implies that macrophage biology is also distinct by location. This may provide further insight into the observed treatment resistance to immune modulation. We hypothesize that macrophage activation occurs through processes that are distinct in tumor periphery versus core. We therefore investigated regional differences in TAM recruitment and evolution in GBM by combining open source single cell and bulk gene expression data. We used single cell gene expression data from 4 glioblastomas (total of 3589 cells) and 122 total bulk samples obtained from 10 different patients. Cell identity, ontogeny (bone-marrow derived macrophages-BMDM vs microglia), and macrophage activation state were inferred using verified gene expression signatures. We captured the spectrum of immune states using cell trajectory analysis with pseudotime ordering. In keeping with previous studies, TAMs carrying BMDM identity were more abundant in tumor bulk while microglia-derived TAMs dominated the tumor periphery across all macrophage activation states including pre-activation. We note that core TAMs evolve towards a pro-inflammatory state and identify a subpopulation of cells based on a gene program exhibiting strong, opposing correlation with Programmed cell Death-1 (PD-1) signaling, which may correlate to their response to PD-1 inhibition. By contrast, peripheral TAMs evolve towards anti-inflammatory phenotype and contains a population of cells strongly associated with NFkB signaling. Our preliminary analysis suggests important regional differences in TAMs with regard to recruitment and evolution. We identify regionally distinct and potentially actionable cell subpopulations and advocate the need for a multi-targeted approach to GBM therapeutics.

CCR2 of Tumor Microenvironmental Cells Is a Relevant Modulator of Glioma Biology

Glioblastoma multiforme (GBM) shows a high influx of tumor-associated macrophages (TAMs). The CCR2/CCL2 pathway is considered a relevant signal for the recruitment of TAMs and has been suggested as a therapeutic target in malignant gliomas. We found that TAMs of human GBM specimens and of a syngeneic glioma model express CCR2 to varying extents. Using a Ccr2-deficient strain for glioma inoculation revealed a 30% reduction of TAMs intratumorally. This diminished immune cell infiltration occurred with augmented tumor volumes likely based on increased cell proliferation. Remaining TAMs in Ccr2^-/- mice showed comparable surface marker expression patterns in comparison to wildtype mice, but expression levels of inflammatory transcription factors (Stat3, Irf7, Cox2) and cytokines (Ifnβ, Il1β, Il12α) were considerably affected. Furthermore, we demonstrated an impact on blood vessel integrity, while vascularization of tumors appeared similar between mouse strains. The higher stability and attenuated leakiness of the tumor vasculature imply improved sustenance of glioma tissue in Ccr2^-/- mice. Additionally, despite TAMs residing in the perivascular niche in Ccr2^-/- mice, their pro-angiogenic activity was reduced by the downregulation of Vegf. In conclusion, lacking CCR2 solely on tumor microenvironmental cells leads to enhanced tumor progression, whereby high numbers of TAMs infiltrate gliomas independently of the CCR2/CCL2 signal.

TREM-1 and TREM-2 Expression on Blood Monocytes Could Help Predict Survival in High-Grade Glioma Patients

Objective

In recent years, the role of the modern inflammatory markers TREM-1 (triggering receptors expressed on myeloid cells) and HMGB1 (high mobility group box 1 protein) in tumorigenesis has begun to be studied. Their role in gliomas is not clear. The aim of our study was to find the role of inflammation in gliomas. Patients and Methods. In 63 adult patients with gliomas and 31 healthy controls, the expressions of TREM-1 and TREM-2 on CD14⁺ blood cells (method: flow cytometry) and the levels of soluble sTREM-1, HMGB1, IL-6, and IL-10 (Elisa tests) were analyzed.

Results

Cox proportional hazard analysis showed that a TREM-1/TREM-2 ratio was associated with reduced overall survival (HR = 1.001, P = 0.023). Patients with a TREM-1/TREM-2 ratio above 125 survived significantly shorter than patients with a TREM-1/TREM-2 ratio below 125. The percentage of CD14⁺ TREM-1⁺ cells was strongly associated with a plasma IL-6/IL-10 ratio (positively) and with IL-10 (negatively). Conversely, we found a higher percentage of CD14⁺ TREM-2⁺ monocytes in better surviving patients; these cells could downregulate the exaggerated inflammation and potentiate the phagocytosis in the tumor. The serum levels of HMGB1 negatively correlated with the percentage of CD14⁺ TREM-1⁺ cells and with the TREM-1/TREM-2 ratio. The positive correlation between the serum levels of a late proinflammatory cytokine HMGB1 with the percentage of TREM2⁺ CD14⁺ monocytes can be explained as an effort for suppression of systemic inflammation by anti-inflammatory acting CD14⁺ TREM-2⁺ cells.

Conclusion

We showed that the TREM-1/TREM-2 ratio (expression on the surface of blood monocytes) could help predict prognosis in patients with gliomas, especially in high-grade gliomas, and that systemic inflammation has an impact on the patient's overall survival. This is the first study that showed that TREM expression on monocytes in peripheral blood could help predict prognosis in patients with gliomas.

The CXCR2/CXCL2 signalling pathway - An alternative therapeutic approach in high-grade glioma

OBJECTIVE

Besides VEGF, alternative signalling via CXCR2 and its ligands CXCL2/CXCL8 is a crucial part of angiogenesis in glioblastoma. Our aim was to understand the role of CXCR2 for glioma biology and elucidate the therapeutic potential of its specific inhibition.

METHODS

GL261 glioma cells were implanted intracranially in syngeneic mice. The 14 or 7 days of local or systemic treatment with CXCR2-antagonist (SB225002) was initiated early on the day of tumour cell implantation or delayed after 14 days of tumour growth. Glioma volume was verified using MRI before and after treatment. Immunofluorescence staining was used to investigate tumour progression, angiogenesis and microglial behaviour. Furthermore, in vitro assays and gene expression analyses of glioma and endothelial cells were performed to validate inhibitor activity.

RESULTS

CXCR2-blocking led to significantly reduced glioma volumes of around 50% after early and delayed local treatments. The treated tumours were comparable with controls regarding invasiveness, proliferation and apoptotic cell activity. Furthermore, no differences in CXCR2/CXCL2 expression were observed. However, immunostaining revealed reduction in vessel density and accumulation of microglia/macrophages, whereas interaction of these myeloid cells with tumour vessels was enhanced. In vitro analyses of the CXCR2-antagonist showed its direct impact on proliferation of glioma and endothelial cells if used at higher concentrations. In addition, expression of CXCR2/CXCL2 signalling genes was increased in both cell types by SB225002, but VEGF-relevant genes were unaffected.

CONCLUSION

The CXCR2-antagonist inhibited glioma growth during tumour initiation and progression, whereas treatment was well-tolerated by the recipients. Thus, the CXCR2/CXCL2 signalling represents a promising therapeutic target in glioma.

Role of Infiltrating Microglia/Macrophages in Glioma

In this chapter we describe the state of the art knowledge of the role played by myeloid cells in promoting and supporting the growth and the invasive properties of a deadly brain tumor, glioblastoma. We provide a review of the works describing the intercellular communication among glioma and associated microglia/macrophage cells (GAMs) using in vitro cellular models derived from mice, rats and human patients and in vivo animal models using syngeneic or xenogeneic experimental systems. Special emphasis will be given to 1) the timing alteration of brain microenvironment under the influence of glioma, 2) the bidirectional communication among tumor and GAMs, 3) possible approaches to interfere with or to guide these interactions, with the aim to identify molecular and cellular targets which could revert or delay the vicious cycle that favors tumor biology.

MET in glioma: signaling pathways and targeted therapies

Gliomas represent the most common type of malignant brain tumor, among which, glioblastoma remains a clinical challenge with limited treatment options and dismal prognosis. It has been shown that the dysregulated receptor tyrosine kinase (RTK, including EGFR, MET, PDGFRα, ect.) signaling pathways have pivotal roles in the progression of gliomas, especially glioblastoma. Increasing evidence suggests that expression levels of the RTK MET and its specific stimulatory factors are significantly increased in glioblastomas compared to those in normal brain tissues, whereas some negative regulators are found to be downregulated. Mutations in MET, as well as the dysregulation of other regulators of cross-talk with MET signaling pathways, have also been identified. MET and its ligand hepatocyte growth factor (HGF) play a critical role in the proliferation, survival, migration, invasion, angiogenesis, stem cell characteristics, and therapeutic resistance and recurrence of glioblastomas. Therefore, combined targeted therapy for this pathway and associated molecules could be a novel and attractive strategy for the treatment of human glioblastoma. In this review, we highlight progress made in the understanding of MET signaling in glioma and advances in therapies targeting HGF/MET molecules for glioma patients in recent years, in addition to studies on the expression and mutation status of MET.

Interactions between tumor-associated macrophages and tumor cells in glioblastoma: unraveling promising targeted therapies

INTRODUCTION

Glioblastoma (GBM) is the deadliest primary malignant central nervous system (CNS) tumor with a median overall survival of 15 months despite a very intensive therapeutic regimen including maximal safe surgery, radiotherapy, and chemotherapy. Therefore, GBM treatment still raises major biological and therapeutic challenges. Areas covered: One of the hallmarks of the GBM is its tumor microenvironment including tumor-associated macrophages (TAM). TAM, accounting for approximately 30% of the GBM bulk cell population, may explain, at least in part, the immunosuppressive features of GBMs. The TAM are active and highly plastic immune cells and include two major ontogenetically different cell populations: (i) microglia and, (ii) monocytes-derived macrophages (MDM). TAM recruited to the tumor bulk can be reprogramed by GBM cells resulting in an ineffective anti-tumor response. Interestingly, interactions between TAM and GBM cells promote tumor oncogenesis (i.e. tumor cells proliferation and migration/invasion). This review aims to explore TAM targeting in GBM as a promising therapeutic option in the near future. Expert Commentary: A better understanding of TAM-GBM interactions and dynamics will certainly uncover new anti-GBM therapeutic avenues.

S100B suppression alters polarization of infiltrating myeloid-derived cells in gliomas and inhibits tumor growth

S100B, a member of the multigene family of Ca²⁺-binding proteins, is overexpressed by most malignant gliomas but its biological role in gliomagenesis is unclear. Recently, we demonstrated that low concentrations of S100B attenuated microglia activation through the induction of STAT3. Furthermore, S100B downregulation in a murine glioma model inhibited macrophage trafficking and tumor growth. Based on these observations, we hypothesized that S100B inhibitors may have antiglioma properties through modulation of tumor microenvironment.

To discover novel S100B inhibitors, we developed a high-throughput screening cell-based S100B promoter-driven luciferase reporter assay. Initial screening of 768 compounds in the NIH library identified 36 hits with >85% S100B inhibitory activity. Duloxetine (Dul, an SNRI) was selected for the initial proof-of-concept studies. At low concentrations (1–5 μM) Dul inhibited S100B and CCL2 production in mouse GL261 glioma cells, but had minimal cytotoxic activity in vitro. In vivo, however, Dul (30 mg/kg/14 days) inhibited S100B production, altered the polarization and trafficking of tumor-associated myeloid-derived cells, and inhibited the growth of intracranial GL261 gliomas. Dul therapeutic efficacy, however, was not observed in the K-Luc glioma model that expresses low levels of S100B. These findings affirm the role of S100B in gliomagenesis and justify the development of more potent S100B inhibitors for glioma therapy.

Impact of pathological features of brain metastases in prognosis

AIM

To evaluate the prognostic value of tumor-infiltrating lymphocytes (TILs) and Ki67 in brain metastasis lesions, and the effect of adding them to variables of graded prognostic assessment score.

PATIENTS & METHODS

Clinicopathological information from 111 medical charts of brain metastasis patients was obtained, and TIL distribution (n = 84), Ki67 index (n = 79) and CD3 TIL (n = 64) were prospectively evaluated.

RESULTS

Most frequent TIL pattern was perivascular (67.8%), and median Ki67 and CD3 TIL percents were 30 and 4.8%, respectively. Ki67 ≥15 was associated with shorter survival (p = 0.018) but CD3 TIL was not (p = 0.870). The highest graded prognostic assessment score was not associated with survival (p = 0.648), however, those with low Ki67 and high score was associated with better outcome (p = 0.007).

CONCLUSION

High Ki67 index in brain metastasis carries a worse prognosis.

Developmentally regulated signaling pathways in glioma invasion

Malignant gliomas are the most common, infiltrative, and lethal primary brain tumors affecting the adult population. The grim prognosis for this disease is due to a combination of the presence of highly invasive tumor cells that escape surgical resection and the presence of a population of therapy-resistant cancer stem cells found within these tumors. Several studies suggest that glioma cells have cleverly hijacked the normal developmental program of neural progenitor cells, including their transcriptional programs, to enhance gliomagenesis. In this review, we summarize the role of developmentally regulated signaling pathways that have been found to facilitate glioma growth and invasion. Furthermore, we discuss how the microenvironment and treatment-induced perturbations of these highly interconnected signaling networks can trigger a shift in cellular phenotype and tumor subtype.

Negative control of the HGF/c-MET pathway by TGF-β: a new look at the regulation of stemness in glioblastoma

Multiple target inhibition has gained considerable interest in combating drug resistance in glioblastoma, however, understanding the molecular mechanisms of crosstalk between signaling pathways and predicting responses of cancer cells to targeted interventions has remained challenging. Despite the significant role attributed to transforming growth factor (TGF)-β family and hepatocyte growth factor (HGF)/c-MET signaling in glioblastoma pathogenesis, their functional interactions have not been well characterized. Using genetic and pharmacological approaches to stimulate or antagonize the TGF-β pathway in human glioma-initiating cells (GIC), we observed that TGF-β exerts an inhibitory effect on c-MET phosphorylation. Inhibition of either mitogen-activated protein kinase (MAPK)/ extracellular signal-regulated kinase (ERK) or phosphatidylinositol 3-kinase (PI3K)/protein kinase B (PKB/AKT) signaling pathway attenuated this effect. A comparison of c-MET-driven and c-MET independent GIC models revealed that TGF-β inhibits stemness in GIC at least in part via its negative regulation of c-MET activity, suggesting that stem cell (SC) maintenance may be controlled by the balance between these two oncogenic pathways. Importantly, immunohistochemical analyses of human glioblastoma and ex vivo single-cell gene expression profiling of TGF-β and HGF confirm the negative interaction between both pathways. These novel insights into the crosstalk of two major pathogenic pathways in glioblastoma may explain some of the disappointing results when targeting either pathway alone in human glioblastoma patients and inform on potential future designs on targeted pharmacological or genetic intervention.

TREM2 deficiency impairs chemotaxis and microglial responses to neuronal injury

Sequence variations in the triggering receptor expressed on myeloid cells 2 (TREM2) have been linked to an increased risk for neurodegenerative disorders such as Alzheimer's disease and frontotemporal lobar degeneration. In the brain, TREM2 is predominantly expressed in microglia. Several disease-associated TREM2 variants result in a loss of function by reducing microglial phagocytosis, impairing lipid sensing, preventing binding of lipoproteins and affecting shielding of amyloid plaques. We here investigate the consequences of TREM2 loss of function on the microglia transcriptome. Among the differentially expressed messenger RNAs in wild-type and Trem2^-/- microglia, gene clusters are identified which represent gene functions in chemotaxis, migration and mobility. Functional analyses confirm that loss of TREM2 impairs appropriate microglial responses to injury and signals that normally evoke chemotaxis on multiple levels. In an ex vivo organotypic brain slice assay, absence of TREM2 reduces the distance migrated by microglia. Moreover, migration towards defined chemo-attractants is reduced upon ablation of TREM2 and can be rescued by TREM2 re-expression. In vivo, microglia lacking TREM2 migrate less towards injected apoptotic neurons, and outgrowth of microglial processes towards sites of laser-induced focal CNS damage in the somatosensory cortex is slowed. The apparent lack of chemotactic stimulation upon depletion of TREM2 is consistent with a stable expression profile of genes characterizing the homoeostatic signature of microglia.

Immune microenvironment of gliomas

High-grade gliomas are rapidly progressing tumors of the central nervous system (CNS) with a very poor prognosis despite extensive resection combined with radiation and/or chemotherapy. Histopathological and flow cytometry analyses of human and rodent experimental gliomas revealed heterogeneity of a tumor and its niche, composed of reactive astrocytes, endothelial cells, and numerous immune cells. Infiltrating immune cells consist of CNS resident (microglia) and peripheral macrophages, granulocytes, myeloid-derived suppressor cells (MDSCs), and T lymphocytes. Intratumoral density of glioma-associated microglia/macrophages (GAMs) and MDSCs is the highest in malignant gliomas and inversely correlates with patient survival. Although GAMs have a few innate immune functions intact, their ability to be stimulated via TLRs, secrete cytokines, and upregulate co-stimulatory molecules is not sufficient to initiate antitumor immune responses. Moreover, tumor-reprogrammed GAMs release immunosuppressive cytokines and chemokines shaping antitumor responses. Both GAMs and MDSCs have ability to attract T regulatory lymphocytes to the tumor, but MDSCs inhibit cytotoxic responses mediated by natural killer cells, and block the activation of tumor-reactive CD4⁺ T helper cells and cytotoxic CD8⁺ T cells. The presence of regulatory T cells may further contribute to the lack of effective immune activation against malignant gliomas. We review the immunological aspects of glioma microenvironment, in particular composition and various roles of the immune cells infiltrating malignant human gliomas and experimental rodent gliomas. We describe tumor-derived signals and mechanisms driving myeloid cell accumulation and reprogramming. Although, understanding the complexity of cell-cell interactions in glioma microenvironment is far from being achieved, recent studies demonstrated several glioma-derived factors that trigger migration, accumulation, and reprogramming of immune cells. Identification of these factors may facilitate development of immunotherapy for gliomas as immunomodulatory and immune evasion mechanisms employed by malignant gliomas pose an appalling challenge to brain tumor immunotherapy.

Insights into CCL21's roles in immunosurveillance and immunotherapy for gliomas

Chemokine (C-C) motif ligand 21 (CCL21) is involved in immunosurveillance and has recently garnered the attention of neuro-oncologists and neuroscientists. CCL21 contains an extended C-terminus, which increases binding to lymphatic glycosaminoglycans and provides a mechanism for cell trafficking by forming a stationary chemokine concentration gradient that allows cell migration via haptotaxis. CCL21 is expressed by endothelial cells of the blood-brain barrier in physiologic and pathologic conditions. CCL21 has also been implicated in leukocyte extravasation into the central nervous system. In this review, we summarize the role of CCL21 in immunosurveillance and explore its potential as an immunotherapeutic agent for the treatment of gliomas.

CCL2/CCR2 signaling pathway in glioblastoma multiforme

Glioblastoma multiform (GBM) is described as one of the most frequent primary brain tumors. These types of malignancies constitute only 15% of all primary brain tumors. Despite, extensive developments on effective therapeutic methods during the 20th century as well as the first decade of the present century (21st), the median survival rate for patients suffering from GBM is only approximately 15 months, even in response to multi-modal therapy. numerous types of reticuloendothelial system cells such as macrophages and microglial cells occupied within both GBM and also normal surrounding tissues. These immune cells acquire an otherwise activated phenotype with potent tumor-tropic functions that contribute to the glioma growth and invasion. The CC chemokine, CCL2 (previously named MCP-1) is of the most important CC chemokines family member involving in regulation of oriented migration and penetrative infiltration of mainly reticuloendothelial system cells specifically monocyte/macrophage phenotypes. Fundamental parts are played by CCL2 and its related receptor (the CCR2) in brain tumors and obviously in migration of monocytes from the bloodstream through the vascular endothelium. Therefore, CCL2/CCR2 axis is required for the routine immunological surveillance of tissues, in accordance with response to inflammation. Briefly, in this review, we have tried our best to collect the latest, straightened and summarize literature reports exist within data base regarding the interaction between microglia/macrophages and CCL2/CCR2 axis in GBM. We aimed to discuss potential application of this chemokine/receptor interaction axis for the expansion of future anti-glioma therapies as well.

Investigating microglia-brain tumor cell interactions in vivo in the larval zebrafish brain

Glioblastoma is the most frequent and aggressive primary malignant brain tumor. Gliomas exhibit high genetic diversity in addition to complex and variable clinical features. Glioblastoma tumors are highly resistant to multimodal therapies and there is significant patient mortality within the first two years after prognosis. At present clinical treatments are palliative, not curative. Glioblastomas contain a high number of microglia and infiltrating macrophages, which are positively correlated with glioma grade and invasiveness. Microglia are the resident macrophages of the central nervous system. These cells constantly scan the brain and react promptly to any abnormality, removing detrimental factors and safeguarding the central nervous system against further damage. Microglia and macrophages that have colonized the glioblastoma display protumoral functions and promote tumor growth. The optically transparent zebrafish larva facilitates imaging of fluorescently labeled cells at high spatial and temporal resolution in vivo. It is therefore an excellent model to investigate microglia-glioma cell interactions at the early stages of tumor development. Here we provide several methods that can be used to study the early stages of microglia-glioma cell interactions in the zebrafish. We present a technique for the xenotransplantation of mammalian oncogenic cells into the zebrafish brain and provide advice for image capture and analysis.

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.

Immune Evasion Strategies of Glioblastoma

Glioblastoma (GBM) is the most devastating brain tumor, with associated poor prognosis. Despite advances in surgery and chemoradiation, the survival of afflicted patients has not improved significantly in the past three decades. Immunotherapy has been heralded as a promising approach in treatment of various cancers; however, the immune privileged environment of the brain usually curbs the optimal expected response in central nervous system malignancies. In addition, GBM cells create an immunosuppressive microenvironment and employ various methods to escape immune surveillance. The purpose of this review is to highlight the strategies by which GBM cells evade the host immune system. Further understanding of these strategies and the biology of this tumor will pave the way for developing novel immunotherapeutic approaches for treatment of GBM.

[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.

Immunosuppressive Mechanisms of Malignant Gliomas: Parallels at Non-CNS Sites

The central nervous system (CNS) possesses powerful local and global immunosuppressive capabilities that modulate unwanted inflammatory reactions in nervous tissue. These same immune-modulatory mechanisms are also co-opted by malignant brain tumors and pose a formidable challenge to brain tumor immunotherapy. Routes by which malignant gliomas coordinate immunosuppression include the mechanical and functional barriers of the CNS; immunosuppressive cytokines and catabolites; immune checkpoint molecules; tumor-infiltrating immune cells; and suppressor immune cells. The challenges to overcoming tumor-induced immunosuppression, however, are not unique to the brain, and several analogous immunosuppressive mechanisms also exist for primary tumors outside of the CNS. Ultimately, the immune responses in the CNS are linked and complementary to immune processes in the periphery, and advances in tumor immunotherapy in peripheral sites may therefore illuminate novel approaches to brain tumor immunotherapy, and vice versa.

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.

CNS macrophages and peripheral myeloid cells in brain tumours

Primary brain tumours (gliomas) initiate a strong host response and can contain large amounts of immune cells (myeloid cells) such as microglia and tumour-infiltrating macrophages. In gliomas the course of pathology is not only controlled by the genetic make-up of the tumour cells, but also depends on the interplay with myeloid cells in the tumour microenvironment. Especially malignant gliomas such as glioblastoma multiforme (GBM) are notoriously immune-suppressive and it is now evident that GBM cells manipulate myeloid cells to support tumour expansion. The protumorigenic effects of glioma-associated myeloid cells comprise a support for angiogenesis as well as tumour cell invasion, proliferation and survival. Different strategies for inhibiting the pathological functions of myeloid cells in gliomas are explored, and blocking the tropism of microglia/macrophages to gliomas or manipulating the signal transduction pathways for immune cell activation has been successful in pre-clinical models. Hence, myeloid cells are now emerging as a promising target for new adjuvant therapies for gliomas. However, it is also becoming evident that some myeloid-directed glioma therapies may only be beneficial for distinct subclasses of gliomas and that a more cell-type-specific manipulation of either microglia or macrophages may improve therapeutic outcomes.

The brain tumor microenvironment

High-grade brain tumors are heterogeneous with respect to the composition of bona fide tumor cells and with respect to a range of intermingling parenchymal cells. Glioblastomas harbor multiple cell types, some with increased tumorigenicity and stem cell-like capacity. The stem-like cells maybe the cells of origin for tumor relapse. However, the tumor-associated parenchymal cells such as vascular cells,microglia, peripheral immune cells, and neural precursor cells also play a vital role in controlling the course of pathology.In this review, we describe the multiple interactions of bulk glioma cells and glioma stem cells with parenchymal cell populations and highlight the pathological impact as well as signaling pathways known for these types of cell-cell communication. The tumor-vasculature not only nourishes glioblastomas, but also provides a specialized niche for these stem-like cells. In addition, microglial cells,which can contribute up to 30% of a brain tumor mass,play a role in glioblastoma cell invasion. Moreover, non-neoplastic astrocytes can be converted into a reactive phenotype by the glioma microenvironment and can then secrete a number of factors which influences tumor biology. The young brain may have the capacity to inhibit gliomagenesis by the endogenous neural precursor cells, which secrete tumor suppressive factors. The factors, pathways, and interactions described in this review provide a new prospective on the cell biology of primary brain tumors, which may ultimately generate new treatment modalities. However, our picture of the multiple interactions between parenchymal and tumor cells is still incomplete.

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.

The controversial role of microglia in malignant gliomas

Malignant gliomas contain stroma and a variety of immune cells including abundant activated microglia/macrophages. Mounting evidence indicates that the glioma microenvironment converts the glioma-associated microglia/macrophages (GAMs) into glioma-supportive, immunosuppressive cells; however, GAMs can retain intrinsic anti-tumor properties. Here, we review and discuss this duality and the potential therapeutic strategies that may inhibit their glioma-supportive and propagating functions.

Microglia and macrophages in malignant gliomas: recent discoveries and implications for promising therapies

Malignant gliomas are the most common primary brain tumors. Their deadliest manifestation, glioblastoma multiforme (GBM), accounts for 15% of all primary brain tumors and is associated with a median survival of only 15 months even after multimodal therapy. There is substantial presence of microglia and macrophages within and surrounding brain tumors. These immune cells acquire an alternatively activated phenotype with potent tumor-tropic functions that contribute to glioma growth and invasion. In this review, we briefly summarize recent data that has been reported on the interaction of microglia/macrophages with brain tumors and discuss potential application of these findings to the development of future antiglioma therapies.

S100B promotes glioma growth through chemoattraction of myeloid-derived macrophages

PURPOSE

S100B is member of a multigenic family of Ca(2+)-binding proteins, which is overexpressed by gliomas. Recently, we showed that low concentrations of S100B attenuated microglia activation through the induction of Stat3. We hypothesized that overexpression of S100B in gliomas could promote tumor growth by modulating the activity of tumor-associated macrophages (TAM).

EXPERIMENTAL DESIGN

We stably transfected GL261 glioma cell lines with constructs that overexpressed (S100B(high)) or underexpressed (S100B(low)) S100B and compared their growth characteristics to intracranial wild-type (S100B(wt)) tumors.

RESULTS

Downregulation of S100B in gliomas had no impact on cell division in vitro but abrogated tumor growth in vivo. Interestingly, compared to S100B(low) tumors, S100B(wt) and S100B(high) intracranial gliomas exhibited higher infiltration of TAMs, stronger inflammatory cytokine expression, and increased vascularity. To identify the potential mechanisms involved, the expression of the S100B receptor, receptor for advanced glycation end products (RAGE), was evaluated in gliomas. Although S100B expression induced RAGE in vivo, RAGE ablation in mice did not significantly inhibit TAM infiltration into gliomas, suggesting that other pathways were involved in this process. To evaluate other mechanisms responsible for TAM chemoattraction, we then examined chemokine pathways and found that C-C motif ligand 2 (CCL2) was upregulated in S100B(high) tumors. Furthermore, analysis of The Cancer Genome Atlas's glioma data bank showed a positive correlation between S100B and CCL2 expression in human proneural and neural glioma subtypes, supporting our finding.

CONCLUSIONS

These observations suggest that S100B promotes glioma growth by TAM chemoattraction through upregulation of CCL2 and introduces the potential utility of S100B inhibitors for glioma therapy.

Tumor-associated macrophages in glioma: friend or foe?

Tumor-associated macrophages (TAMs) contribute substantially to the tumor mass of gliomas and have been shown to play a major role in the creation of a tumor microenvironment that promotes tumor progression. Shortcomings of attempts at antiglioma immunotherapy may result from a failure to adequately address these effects. Emerging evidence supports an independent categorization of glioma TAMs as alternatively activated M2-type macrophages, in contrast to classically activated proinflammatory M1-type macrophages. These M2-type macrophages exert glioma-supportive effects through reduced anti-tumor functions, increased expression of immunosuppressive mediators, and nonimmune tumor promotion through expression of trophic and invasion-facilitating substances. Much of our work has demonstrated these features of glioma TAMs, and together with the supporting literature will be reviewed here. Additionally, the dynamics of glioma cell-TAM interaction over the course of tumor development remain poorly understood; our efforts to elucidate glioma cell-TAM dynamics are summarized. Finally, the molecular pathways which underlie M2-type TAM polarization and gene expression similarly require further investigation, and may present the most potent targets for immunotherapeutic intervention. Highlighting recent evidence implicating the transcription factor STAT3 in immunosuppressive tumorigenic glioma TAMs, we advocate for gene array-based approaches to identify yet unappreciated expression regulators and effector molecules important to M2-type glioma TAMs polarization and function within the glioma tumor microenvironment.

GDNF mediates glioblastoma-induced microglia attraction but not astrogliosis

High-grade gliomas are the most common primary brain tumors. Their malignancy is promoted by the complex crosstalk between different cell types in the central nervous system. Microglia/brain macrophages infiltrate high-grade gliomas and contribute to their progression. To identify factors that mediate the attraction of microglia/macrophages to malignant brain tumors, we established a glioma cell encapsulation model that was applied in vivo. Mouse GL261 glioma cell line and human high-grade glioma cells were seeded into hollow fibers (HF) that allow the passage of soluble molecules but not cells. The glioma cell containing HF were implanted into one brain hemisphere and simultaneously HF with non-transformed fibroblasts (controls) were introduced into the contralateral hemisphere. Implanted mouse and human glioma- but not fibroblast-containing HF attracted microglia and up-regulated immunoreactivity for GFAP, which is a marker of astrogliosis. In this study, we identified GDNF as an important factor for microglial attraction: (1) GL261 and human glioma cells secret GDNF, (2) reduced GDNF production by siRNA in GL261 in mouse glioma cells diminished attraction of microglia, (3) over-expression of GDNF in fibroblasts promoted microglia attraction in our HF assay. In vitro migration assays also showed that GDNF is a strong chemoattractant for microglia. While GDNF release from human or mouse glioma had a profound effect on microglial attraction, the glioma-induced astrogliosis was not affected. Finally, we could show that injection of GL261 mouse glioma cells with GDNF knockdown by shRNA into mouse brains resulted in reduced tumor expansion and improved survival as compared to injection of control cells.

Inflammation and Gliomagenesis: Bi-Directional Communication at Early and Late Stages of Tumor Progression

Inflammation has been closely linked to various forms of cancer. Less is known about the role of inflammation in glioma, especially at the initiation stage. In this review, we first describe the unique features of the immune system in the brain. We then discuss the current understanding of the mechanisms by which glioma cells modulate the immune system, especially how bi-directional communications between immune cells and glioma cells create an immunosuppressed microenvironment that promotes tumor survival and growth. We also address the potential tumor-initiating roles of inflammation in glioma. Finally, we describe several immunotherapy approaches currently being developed to reverse these interactions and stimulate the immune system to eliminate glioma cells.

Migration of mesenchymal stem cells towards glioblastoma cells depends on hepatocyte-growth factor and is enhanced by aminolaevulinic acid-mediated photodynamic treatment

Hepatocyte-growth factor (HGF) is expressed by glioblastomas and contributes to their growth, migration and invasion. HGF also mediates migration of mesenchymal stem cells (MSC) to sites of apoptotic cell death. Moreover, MSC show tropism for glioblastomas, which is exploited in gene therapy to deliver the therapeutics to the tumor cells. Here, we have studied whether HGF contributes to the recruitment of MSC by glioblastoma cells and whether aminolaevulinic acid-mediated photodynamic therapy (ALA/PDT), a novel therapeutic approach that induces apoptosis in glioblastoma cells, affects HGF release and this migratory response. MSC expressed the HGF receptor MET and migrated towards U87 and U251 glioblastoma spheroids. Migration increased significantly when spheroids were subjected to ALA/PDT, which was associated with induction of apoptosis and up-regulation of HGF. Neutralizing HGF resulted in significant inhibition of MSC migration towards untreated as well as ALA/PDT-treated spheroids. Thus, glioblastoma cells express HGF, which contributes to the attraction of MSC. ALA/PDT induces apoptosis and augments HGF release causing enhanced MSC migration towards the tumor cells. ALA/PDT may therefore be exploited to improve targeting of MSC delivered gene therapy, but it may also constitute a risk in terms of beneficial effects for the tumor.