Synergistic Toll-like Receptor 3/9 Signaling Affects Properties and Impairs Glioma-Promoting Activity of Microglia

Phagocytosis Checkpoints in Glioblastoma: CD47 and Beyond

Glioblastoma multiforme (GBM) is one of the deadliest human cancers with very limited treatment options available. The malignant behavior of GBM is manifested in a tumor which is highly invasive, resistant to standard cytotoxic chemotherapy, and strongly immunosuppressive. Immune checkpoint inhibitors have recently been introduced in the clinic and have yielded promising results in certain cancers. GBM, however, is largely refractory to these treatments. The immune checkpoint CD47 has recently gained attention as a potential target for intervention as it conveys a "don't eat me" signal to tumor-associated macrophages (TAMs) via the inhibitory SIRP alpha protein. In preclinical models, the administration of anti-CD47 monoclonal antibodies has shown impressive results with GBM and other tumor models. Several well-characterized oncogenic pathways have recently been shown to regulate CD47 expression in GBM cells and glioma stem cells (GSCs) including Epidermal Growth Factor Receptor (EGFR) beta catenin. Other macrophage pathways involved in regulating phagocytosis including TREM2 and glycan binding proteins are discussed as well. Finally, chimeric antigen receptor macrophages (CAR-Ms) could be leveraged for greatly enhancing the phagocytosis of GBM and repolarization of the microenvironment in general. Here, we comprehensively review the mechanisms that regulate the macrophage phagocytosis of GBM cells.

Microglia in Glioblastomas: Molecular Insight and Immunotherapeutic Potential

Glioblastoma (GBM) is one of the most aggressive and devastating primary brain tumors, with a median survival of 15 months following diagnosis. Despite the intense treatment regimen which routinely includes maximal safe neurosurgical resection followed by adjuvant radio- and chemotherapy, the disease remains uniformly fatal. The poor prognosis associated with GBM is multifactorial owing to factors such as increased proliferation, angiogenesis, and metabolic switching to glycolytic pathways. Critically, GBM-mediated local and systemic immunosuppression result in inadequate immune surveillance and ultimately, tumor-immune escape. Microglia-the resident macrophages of the central nervous system (CNS)-play crucial roles in mediating the local immune response in the brain. Depending on the specific pathological cues, microglia are activated into either a pro-inflammatory, neurotoxic phenotype, known as M1, or an anti-inflammatory, regenerative phenotype, known as M2. In either case, microglia secrete corresponding pro- or anti-inflammatory cytokines and chemokines that either promote or hinder tumor growth. Herein, we review the interplay between GBM cells and resident microglia with a focus on contemporary studies highlighting the effect of GBM on the subtypes of microglia expressed, the associated cytokines/chemokines secreted, and ultimately, their impact on tumor pathogenesis. Finally, we explore how understanding the intricacies of the tumor-immune landscape can inform novel immunotherapeutic strategies against this devastating disease.

Immunogenic cell death mediated TLR3/4-activated MSCs in U87 GBM cell line

Background and aims

Glioblastoma (GBM) is an aggressive primary brain cancer with no promising curative therapies. It has been indicated that MSCs can interact with the tumour microenvironment (TME) through the secretion of soluble mediators regulating intercellular signalling within the TME. TLRs are a multigene family of pattern recognition receptors with evolutionarily conserved regions and are widely expressed in immune and other body cells. MSCs by TLRs can recognize conserved molecular components (DAPMPs and PAPMPs) and activate signalling pathways, which regulate immune and inflammatory responses. MSCs may exert immunomodulatory functions through interaction with their expressed toll-like receptors (TLRs) and exert a protective effect against tumour antigens. As an emerging approach, we aimed to monitor the U87 cell line growth, migration and death markers following specific TLR3/4-primed-MSCs-CMs treatment.

Methods and results

We investigated the phenotypic and functional outcomes of primed-CMs and glioma cell line co-culture following short-term, low-dose TLR3/4 priming. The gene expression profile of target genes, including apoptotic markers and related genes, was analyzed by qRT-PCR. MicroRNA-Seq examined the miRNA expression patterns, and flow cytometry evaluated the cell viability and cycle stages. The results showed significant changes in apoptosis and likely necroptosis-related markers following TLR3/4-primed-MSCs-CMs exposure in the glioma cell line. Notably, we observed a considerable induction of selective pro-apoptotic markers and both the early and late stages of apoptosis in treated U87 cell lines. Additionally, the migration rate of glioma cells significantly decreased following MSCs-CM treatment.

Conclusion

Our findings confirmed that the exposure of TLR3/4-activated-MSCs-CMs with glioma tumour cells possibly changes the immunogenicity of the tumour microenvironment and induces immunogenic programmed cell death. Our results can support the idea that TLR3/4-primed-MSCs can lead to innate immune-mediated cell death and modify tumour cell biology in invasive and metastatic cancers.

Neutrophils in glioma microenvironment: from immune function to immunotherapy

Glioma is a malignant tumor of the central nervous system (CNS). Currently, effective treatment options for gliomas are still lacking. Neutrophils, as an important member of the tumor microenvironment (TME), are widely distributed in circulation. Recently, the discovery of cranial-meningeal channels and intracranial lymphatic vessels has provided new insights into the origins of neutrophils in the CNS. Neutrophils in the brain may originate more from the skull and adjacent vertebral bone marrow. They cross the blood-brain barrier (BBB) under the action of chemokines and enter the brain parenchyma, subsequently migrating to the glioma TME and undergoing phenotypic changes upon contact with tumor cells. Under glycolytic metabolism model, neutrophils show complex and dual functions in different stages of cancer progression, including participation in the malignant progression, immune suppression, and anti-tumor effects of gliomas. Additionally, neutrophils in the TME interact with other immune cells, playing a crucial role in cancer immunotherapy. Targeting neutrophils may be a novel generation of immunotherapy and improve the efficacy of cancer treatments. This article reviews the molecular mechanisms of neutrophils infiltrating the central nervous system from the external environment, detailing the origin, functions, classifications, and targeted therapies of neutrophils in the context of glioma.

Harnessing innate immune pathways for therapeutic advancement in cancer

The innate immune pathway is receiving increasing attention in cancer therapy. This pathway is ubiquitous across various cell types, not only in innate immune cells but also in adaptive immune cells, tumor cells, and stromal cells. Agonists targeting the innate immune pathway have shown profound changes in the tumor microenvironment (TME) and improved tumor prognosis in preclinical studies. However, to date, the clinical success of drugs targeting the innate immune pathway remains limited. Interestingly, recent studies have shown that activation of the innate immune pathway can paradoxically promote tumor progression. The uncertainty surrounding the therapeutic effectiveness of targeted drugs for the innate immune pathway is a critical issue that needs immediate investigation. In this review, we observe that the role of the innate immune pathway demonstrates heterogeneity, linked to the tumor development stage, pathway status, and specific cell types. We propose that within the TME, the innate immune pathway exhibits multidimensional diversity. This diversity is fundamentally rooted in cellular heterogeneity and is manifested as a variety of signaling networks. The pro-tumor effect of innate immune pathway activation essentially reflects the suppression of classical pathways and the activation of potential pro-tumor alternative pathways. Refining our understanding of the tumor's innate immune pathway network and employing appropriate targeting strategies can enhance our ability to harness the anti-tumor potential of the innate immune pathway and ultimately bridge the gap from preclinical to clinical application.

How brain 'cleaners' fail: Mechanisms and therapeutic value of microglial phagocytosis in Alzheimer's disease

Microglia are the resident phagocytes of the brain, where they primarily function in the clearance of dead cells and the removal of un- or misfolded proteins. The impaired activity of receptors or proteins involved in phagocytosis can result in enhanced inflammation and neurodegeneration. RNA-seq and genome-wide association studies have linked multiple phagocytosis-related genes to neurodegenerative diseases, while the knockout of such genes has been demonstrated to exert protective effects against neurodegeneration in animal models. The failure of microglial phagocytosis influences AD-linked pathologies, including amyloid β accumulation, tau propagation, neuroinflammation, and infection. However, a precise understanding of microglia-mediated phagocytosis in Alzheimer's disease (AD) is still lacking. In this review, we summarize current knowledge of the molecular mechanisms involved in microglial phagocytosis in AD across a wide range of pre-clinical, post-mortem, ex vivo, and clinical studies and review the current limitations regarding the detection of microglia phagocytosis in AD. Finally, we discuss the rationale of targeting microglial phagocytosis as a therapeutic strategy for preventing AD or slowing its progression.

Harnessing the innate immune system by revolutionizing macrophage-mediated cancer immunotherapy

Immunotherapy is a promising and safer alternative to conventional cancer therapies. It involves adaptive T-cell therapy, cancer vaccines, monoclonal antibodies, immune checkpoint blockade (ICB), and chimeric antigen receptor (CAR) based therapies. However, most of these modalities encounter restrictions in solid tumours owing to a dense, highly hypoxic and immune-suppressive microenvironment as well as the heterogeneity of tumour antigens. The elevated intra-tumoural pressure and mutational rates within fastgrowing solid tumours present challenges in efficient drug targeting and delivery. The tumour microenvironment is a dynamic niche infiltrated by a variety of immune cells, most of which are macrophages. Since they form a part of the innate immune system, targeting macrophages has become a plausible immunotherapeutic approach. In this review, we discuss several versatile approaches (both at pre-clinical and clinical stages) such as the direct killing of tumour-associated macrophages, reprogramming pro-tumour macrophages to anti-tumour phenotypes, inhibition of macrophage recruitment into the tumour microenvironment, novel CAR macrophages, and genetically engineered macrophages that have been devised thus far. These strategies comprise a strong and adaptable macrophage-toolkit in the ongoing fight against cancer and by understanding their significance, we may unlock the full potential of these immune cells in cancer therapy.

Microglia in Glioma

Myeloid cells are fundamental constituents of the brain tumor microenvironment. In this chapter, we describe the state-of-the-art knowledge on the role of microglial cells in the cross-talk with the most common and aggressive brain tumor, glioblastoma. We report in vitro and in vivo studies related to glioblastoma patients and glioma models to outline the symbiotic interactions that microglia develop with tumoral cells, highlighting the heterogeneity of microglial functions in shaping the brain tumor microenvironment.

TREM2 promotes glioma progression and angiogenesis mediated by microglia/brain macrophages

Triggering receptor expressed on myeloid cell 2 (TREM2), a myeloid cell-specific signaling molecule, controls essential functions of microglia and impacts on the pathogenesis of Alzheimer's disease and other neurodegenerative disorders. TREM2 is also highly expressed in tumor-associated macrophages in different types of cancer. Here, we studied whether TREM2 influences glioma progression. We found a gender-dependent effect of glioma growth in wild-type (WT) animals injected with GL261-EGFP glioma cells. Most importantly, TREM2 promotes glioma progression in male but not female animals. The accumulation of glioma-associated microglia/macrophages (GAMs) and CD31+ blood vessel density is reduced in male TREM2-deficient mice. A transcriptomic analysis of glioma tissue revealed that TREM2 deficiency suppresses immune-related genes. In an organotypic slice model devoid of functional vascularization and immune components from periphery, the tumor size was not affected by TREM2-deficiency. In human resection samples from glioblastoma, TREM2 is upregulated in GAMs. Based on the Cancer Genome Atlas Program (TCGA) and the Chinese Glioma Genome Atlas (CGGA) databases, the TREM2 expression levels were negatively correlated with survival. Thus, the TREM2-dependent crosstalk between GAMs and the vasculature formation promotes glioma growth.

Dysregulation of inflammasome activation in glioma

Gliomas are the most common brain tumors characterized by complicated heterogeneity. The genetic, molecular, and histological pathology of gliomas is characterized by high neuro-inflammation. The inflammatory microenvironment in the central nervous system (CNS) has been closely linked with inflammasomes that control the inflammatory response and coordinate innate host defenses. Dysregulation of the inflammasome causes an abnormal inflammatory response, leading to carcinogenesis in glioma. Because of the clinical importance of the various physiological properties of the inflammasome in glioma, the inflammasome has been suggested as a promising treatment target for glioma management. Here, we summarize the current knowledge on the contribution of the inflammasomes in glioma and therapeutic insights. Video Abstract.

Interactions between microglia and glioma in tumor microenvironment

Gliomas, the most prevalent primary tumors in the central nervous system, are marked by their immunosuppressive properties and consequent poor patient prognosis. Current evidence emphasizes the pivotal role of the tumor microenvironment in the progression of gliomas, largely attributed to tumor-associated macrophages (brain-resident microglia and bone marrow-derived macrophages) that create a tumor microenvironment conducive to the growth and invasion of tumor cells. Yet, distinguishing between these two cell subgroups remains a challenge. Thus, our review starts by analyzing the heterogeneity between these two cell subsets, then places emphasis on elucidating the complex interactions between microglia and glioma cells. Finally, we conclude with a summary of current attempts at immunotherapy that target microglia. However, given that independent research on microglia is still in its initial stages and has many shortcomings at the present time, we express our related concerns and hope that further research will be carried out to address these issues in the future.

Emerging non-proinflammatory roles of microglia in healthy and diseased brains

Microglia, the resident myeloid cells of the central nervous system, are the first line of defense against foreign pathogens, thereby confining the extent of brain injury. However, the role of microglia is not limited to macrophage-like functions. In addition to proinflammatory response mediation, microglia are involved in neurodevelopmental remodeling and homeostatic maintenance in the absence of disease. An increasing number of studies have also elucidated microglia-mediated regulation of tumor growth and neural repair in diseased brains. Here, we review the non-proinflammatory roles of microglia, with the aim of promoting a deeper understanding of the functions of microglia in healthy and diseased brains and contributing to the development of novel therapeutics that target microglia in neurological disorders.

Astrocyte-derived exosomal lncRNA 4933431K23Rik modulates microglial phenotype and improves post-traumatic recovery via SMAD7 regulation

Astrocyte-microglial interaction plays a crucial role in brain injury-associated neuroinflammation. Our previous data illustrated that astrocytes secrete microRNA, leading to anti-inflammatory effects on microglia. Long non-coding RNAs participate in neuroinflammation regulation after traumatic brain injury. However, the effect of astrocytes on microglial phenotype via long non-coding RNAs and the underlying molecular mechanisms remain elusive. We used long non-coding RNA sequencing on murine astrocytes and found that exosomal long non-coding RNA 4933431K23Rik attenuated traumatic brain injury-induced microglial activation in vitro and in vivo and ameliorated cognitive function deficiency. Furthermore, microRNA and messenger RNA sequencing together with binding prediction illustrated that exosomal long non-coding RNA 4933431K23Rik up-regulates E2F7 and TFAP2C expression by sponging miR-10a-5p. Additionally, E2F7 and TFAP2C, as transcription factors, regulated microglial Smad7 expression. Using Cx3cr1-Smad7 overexpression of adeno-associated virus, microglia specifically overexpressed Smad7 in the attenuation of neuroinflammation, resulting in less cognitive deficiency after traumatic brain injury. Mechanically, overexpressed Smad7 physically binds to IκBα and inhibits its ubiquitination, preventing NF-κB signaling activation. The Smad7 activator asiaticoside alleviates neuroinflammation and protects neuronal function in traumatic brain injury mice. This study revealed that an exosomal long non-coding RNA from astrocytes attenuates microglial activation after traumatic brain injury by up-regulating Smad7, providing a potential therapeutic target.

TLR9 and Glioma: Friends or Foes?

Toll-like receptor 9 (TLR9) is an intracellular innate immunity receptor that plays a vital role in chronic inflammation and in recognizing pathogenic and self-DNA in immune complexes. This activation of intracellular signaling leads to the transcription of either immune-related or malignancy genes through specific transcription factors. Thus, it has been hypothesized that TLR9 may cause glioma. This article reviews the roles of TLR9 in the pathogenesis of glioma and its related signaling molecules in either defending or promoting glioma. TLR9 mediates the invasion-induced hypoxia of brain cancer cells by the activation of matrix metalloproteinases (2, 9, and 13) in brain tissues. In contrast, the combination of the TLR9 agonist CpG ODN to radiotherapy boosts the role of T cells in antitumor effects. The TLR9 agonist CpG ODN 107 also enhances the radiosensitivity of human glioma U87 cells by blocking tumor angiogenesis. CpG enhances apoptosis in vitro and in vivo. Furthermore, it can enhance the antigen-presenting capacity of microglia, switch immune response toward CD8 T cells, and reduce the number of CD4CD25 Treg cells. CpG ODN shows promise as a potent immunotherapeutic drug against cancer, but specific cautions should be taken when activating TLR9, especially in the case of glioblastoma.

System analysis based on the pyroptosis-related genes identifies GSDMC as a novel therapy target for pancreatic adenocarcinoma

Background

Pancreatic adenocarcinoma (PAAD) is one of the most common malignant tumors of the digestive tract. Pyroptosis is a newly discovered programmed cell death that highly correlated with the prognosis of tumors. However, the prognostic value of pyroptosis in PAAD remains unclear.

Methods

A total of 178 pancreatic cancer PAAD samples and 167 normal samples were obtained from The Cancer Genome Atlas (TCGA) and Genotype-Tissue Expression (GTEx) databases. The “DESeq2” R package was used to identify differntially expressed pyroptosis-related genes between normal pancreatic samples and PAAD samples. The prognostic model was established in TCGA cohort based on univariate Cox and the least absolute shrinkage and selection operator (LASSO) Cox regression analyses, which was validated in test set from Gene Expression Omnibus (GEO) cohort. Univariate independent prognostic analysis and multivariate independent prognostic analysis were used to determine whether the risk score can be used as an independent prognostic factor to predict the clinicopathological features of PAAD patients. A nomogram was used to predict the survival probability of PAAD patients, which could help in clinical decision-making. The R package "pRRophetic" was applied to calculate the drug sensitivity of each samples from high- and low-risk group. Tumor immune infiltration was investigated using an ESTIMATE algorithm. Finally, the pro‐tumor phenotype of GSDMC was explored in PANC-1 and CFPAC-1 cells.

Result

On the basis of univariate Cox and LASSO regression analyses, we constructed a risk model with identified five pyroptosis-related genes (IL18, CASP4, NLRP1, GSDMC, and NLRP2), which was validated in the test set. The PAAD samples were divided into high-risk and low-risk groups on the basis of the risk score's median. According to Kaplan Meier curve analysis, samples from high-risk groups had worse outcomes than those from low-risk groups. The time-dependent receiver operating characteristics (ROC) analysis revealed that the risk model could predict the prognosis of PAAD accurately. A nomogram accompanied by calibration curves was presented for predicting 1-, 2-, and 3-year survival in PAAD patients. More importantly, 4 small molecular compounds (A.443654, PD.173074, Epothilone. B, Lapatinib) were identified, which might be potential drugs for the treatment of PAAD patients. Finally, the depletion of GSDMC inhibits the proliferation, invasion, and migration of pancreatic adenocarcinoma cells.

Conclusion

In this study, we developed a pyroptosis-related prognostic model based on IL18, CASP4, NLRP1, NLRP2, and GSDMC , which may be helpful for clinicians to make clinical decisions for PAAD patients and provide valuable insights for individualized treatment. Our result suggest that GSDMC may promote the proliferation and migration of PAAD cell lines. These findings may provide new insights into the roles of pyroptosis-related genes in PAAD, and offer  new therapeutic targets for the treatment of PAAD.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12967-022-03632-z.

Microglia-T cell conversations in brain cancer progression

The highly immunosuppressive and heterogeneous milieu of brain malignancies contributes to their dismal prognosis. Regardless of their cellular origin, brain tumors grow in an environment with various specialized organ-resident cells. Although homeostatic microglia contribute to a healthy brain, conversations between disease-associated microglia and T cells compromise their individual and collective capacity to curb malignant growth. We review the mechanisms of T cell-microglia interactions and discuss how their collaboration fosters heterogeneity and immunosuppression in brain cancers. Because of the importance of microglia and T cells in the brain tumor microenvironment, it is crucial to understand their interactions to derive innovative therapeutics.

Innate and Adaptive Responses of Intratumoral Immunotherapy with Endosomal Toll-Like Receptor Agonists

Toll-like receptors (TLRs) are natural initial triggers of innate and adaptive immune responses. With the advent of cancer immunotherapy, nucleic acids engineered as ligands of endosomal TLRs have been investigated for the treatment of solid tumors. Despite promising results, their systemic administration, similarly to other immunotherapies, raises safety issues. To overcome these problems, recent studies have applied the direct injection of endosomal TLR agonists in the tumor and/or draining lymph nodes, achieving high local drug exposure and strong antitumor response. Importantly, intratumoral delivery of TLR agonists showed powerful effects not only against the injected tumors but also often against uninjected lesions (abscopal effects), resulting in some cases in cure and antitumoral immunological memory. Herein, we describe the structure and function of TLRs and their role in the tumor microenvironment. Then, we provide our vision on the potential of intratumor versus systemic delivery or vaccination approaches using TLR agonists, also considering the use of nanoparticles to improve their targeting properties. Finally, we collect the preclinical and clinical studies applying intratumoral injection of TLR agonists as monotherapies or in combination with: (a) other TLR or STING agonists; (b) other immunotherapies; (c) radiotherapy or chemotherapy; (d) targeted therapies.

Dialogue among Lymphocytes and Microglia in Glioblastoma Microenvironment

Simple Summary

In this review, we summarize in vitro and in vivo studies related to glioblastoma models and human patients to outline the symbiotic bidirectional interaction between microglia, lymphocytes, and tumor cells that develops during tumor progression. Particularly, we highlight the current experimental therapeutic approaches that aim to shape these interplays, such as adeno-associated virus (AAV) delivery and CAR-T and -NK cell infusion, and to modulate the tumor microenvironment in an anti-tumoral way, thus counteracting glioblastoma growth.

Abstract

Microglia and lymphocytes are fundamental constituents of the glioblastoma microenvironment. In this review, we summarize the current state-of-the-art knowledge of the microglial role played in promoting the development and aggressive hallmarks of this deadly brain tumor. Particularly, we report in vitro and in vivo studies related to glioblastoma models and human patients to outline the symbiotic bidirectional interaction between microglia, lymphocytes, and tumor cells that develops during tumor progression. Furthermore, we highlight the current experimental therapeutic approaches that aim to shape these interplays, such as adeno-associated virus (AAV) delivery and CAR-T and -NK cell infusion, and to modulate the tumor microenvironment in an anti-tumoral way, thus counteracting glioblastoma growth.

Microglia/macrophage-derived human CCL18 promotes glioma progression via CCR8-ACP5 axis analyzed in humanized slice model

Factors released from glioma-associated microglia/macrophages (GAMs) play a crucial role in glioblastoma multiforme (GBM) progression. Here, we study the importance of CCL18, a cytokine expressed in human but not in rodent GAMs, as a modulator of glioma growth. Since CCL18 signaling could not be studied in classical mouse glioma models, we developed an approach by transplanting induced pluripotent stem cell-derived human microglia and human glioma cells into mouse brain slices depleted of their intrinsic microglia. We observe that CCL18 promotes glioma cell growth and invasion. Chemokine (C-C motif) receptor 8 (CCR8) is identified as a functional receptor for CCL18 on glioma cells, and ACP5 (acid phosphatase 5) is revealed as an important part of the downstream signaling cascade for mediating glioma growth. We conclude, based on the results from an in vitro, ex vivo humanized glioma model and an in vivo GBM model that microglia/macrophage-derived CCL18 promotes glioma growth.

Tumor-Associated Macrophages in Gliomas—Basic Insights and Treatment Opportunities

Simple Summary

Macrophages are a specialized immune cell type found in both invertebrates and vertebrates. Versatile in functionality, macrophages carry out important tasks such as cleaning cellular debris in healthy tissues and mounting immune responses during infection. In many cancer types, macrophages make up a significant portion of tumor tissue, and these are aptly called tumor-associated macrophages. In gliomas, a group of primary brain tumors, these macrophages are found in very high frequency. Tumor-associated macrophages can promote glioma development and influence the outcome of various therapeutic regimens. At the same time, these cells provide various potential points of intervention for therapeutic approaches in glioma patients. The significance of tumor-associated macrophages in the glioma microenvironment and potential therapeutic targets are the focus of this review.

Abstract

Glioma refers to a group of primary brain tumors which includes glioblastoma (GBM), astrocytoma and oligodendroglioma as major entities. Among these, GBM is the most frequent and most malignant one. The highly infiltrative nature of gliomas, and their intrinsic intra- and intertumoral heterogeneity, pose challenges towards developing effective treatments. The glioma microenvironment, in addition, is also thought to play a critical role during tumor development and treatment course. Unlike most other solid tumors, the glioma microenvironment is dominated by macrophages and microglia—collectively known as tumor-associated macrophages (TAMs). TAMs, like their homeostatic counterparts, are plastic in nature and can polarize to either pro-inflammatory or immunosuppressive states. Many lines of evidence suggest that immunosuppressive TAMs dominate the glioma microenvironment, which fosters tumor development, contributes to tumor aggressiveness and recurrence and, very importantly, impedes the therapeutic effect of various treatment regimens. However, through the development of new therapeutic strategies, TAMs can potentially be shifted towards a proinflammatory state which is of great therapeutic interest. In this review, we will discuss various aspects of TAMs in the context of glioma. The focus will be on the basic biology of TAMs in the central nervous system (CNS), potential biomarkers, critical evaluation of model systems for studying TAMs and finally, special attention will be given to the potential targeted therapeutic options that involve the TAM compartment in gliomas.

Comprehensive Analysis of Pyroptosis-Associated in Molecular Classification, Immunity and Prognostic of Glioma

Integrative analysis was performed in the Chinese Glioma Genome Atlas and The Cancer Genome Atlas to describe the pyroptosis-associated molecular classification and prognostic signature in glioma. Pyroptosis-related genes were used for consensus clustering and to develop a prognostic signature. The immune statuses, molecular alterations, and clinical features of differentially expressed genes were analyzed among different subclasses and risk groups. A lncRNA-miRNA-mRNA network was built, and drug sensitivity analysis was used to identify small molecular drugs for the identified genes. Glioma can be divided into two subclasses using 30 pyroptosis-related genes. Cluster 1 displayed high immune signatures and poor prognosis as well as high immune-related function scores. A prognostic signature based on 15 pyroptosis-related genes of the CGGA cohort can predict the overall survival of glioma and was well validated in the TCGA cohort. Cluster 1 had higher risk scores. The high-risk group had high immune cell and function scores and low DNA methylation of pyroptosis-related genes. The differences in pyroptosis-related gene mutations and somatic copy numbers were significant between the high-risk and low-risk groups. The ceRNA regulatory network uncovered the regulatory patterns of different risk groups in glioma. Nine pairs of target genes and drugs were identified. In vitro, CASP8 promotes the progression of glioma cells. Pyroptosis-related genes can reflect the molecular biological and clinical features of glioma subclasses. The established prognostic signature can predict prognosis and distinguish molecular alterations in glioma patients. Our comprehensive analyses provide valuable guidelines for improving glioma patient management and individualized therapy.

Glial and myeloid heterogeneity in the brain tumour microenvironment

Brain cancers carry bleak prognoses, with therapeutic advances helping only a minority of patients over the past decade. The brain tumour microenvironment (TME) is highly immunosuppressive and differs from that of other malignancies as a result of the glial, neural and immune cell populations that constitute it. Until recently, the study of the brain TME was limited by the lack of methods to de-convolute this complex system at the single-cell level. However, novel technical approaches have begun to reveal the immunosuppressive and tumour-promoting properties of distinct glial and myeloid cell populations in the TME, identifying new therapeutic opportunities. Here, we discuss the immune modulatory functions of microglia, monocyte-derived macrophages and astrocytes in brain metastases and glioma, highlighting their disease-associated heterogeneity and drawing from the insights gained by studying these malignancies and other neurological disorders. Lastly, we consider potential approaches for the therapeutic modulation of the brain TME.

Toll-like receptors and toll-like receptor-targeted immunotherapy against glioma

Glioma represents a fast proliferating and highly invasive brain tumor which is resistant to current therapies and invariably recurs. Despite some advancements in anti-glioma therapies, patients’ prognosis remains poor. Toll-like receptors (TLRs) act as the first line of defense in the immune system being the detectors of those associated with bacteria, viruses, and danger signals. In the glioma microenvironment, TLRs are expressed on both immune and tumor cells, playing dual roles eliciting antitumoral (innate and adaptive immunity) and protumoral (cell proliferation, migration, invasion, and glioma stem cell maintenance) responses. Up to date, several TLR-targeting therapies have been developed aiming at glioma bulk and stem cells, infiltrating immune cells, the immune checkpoint axis, among others. While some TLR agonists exhibited survival benefit in clinical trials, it attracts more attention when they are involved in combinatorial treatment with radiation, chemotherapy, immune vaccination, and immune checkpoint inhibition in glioma treatment. TLR agonists can be used as immune modulators to enhance the efficacy of other treatment, to avoid dose accumulation, and what brings more interests is that they can potentiate immune checkpoint delayed resistance to PD-1/PD-L1 blockade by upregulating PD-1/PD-L1 overexpression, thus unleash powerful antitumor responses when combined with immune checkpoint inhibitors. Herein, we focus on recent developments and clinical trials exploring TLR-based treatment to provide a picture of the relationship between TLR and glioma and their implications for immunotherapy.

Breaking Bad: Autophagy Tweaks the Interplay Between Glioma and the Tumor Immune Microenvironment

Though significant strides in tumorigenic comprehension and therapy modality have been witnessed over the past decades, glioma remains one of the most common and malignant brain tumors characterized by recurrence, dismal prognosis, and therapy resistance. Immunotherapy advance holds promise in glioma recently. However, the efficacy of immunotherapy varies among individuals with glioma, which drives researchers to consider the modest levels of immunity in the central nervous system, as well as the immunosuppressive tumor immune microenvironment (TIME). Considering the highly conserved property for sustaining energy homeostasis in mammalian cells and repeatedly reported links in malignancy and drug resistance, autophagy is determined as a cutting angle to elucidate the relations between glioma and the TIME. In this review, heterogeneity of TIME in glioma is outlined along with the reciprocal impacts between them. In addition, controversies on whether autophagy behaves cytoprotectively or cytotoxically in cancers are covered. How autophagy collapses from its homeostasis and aids glioma malignancy, which may depend on the cell type and the cellular context such as reactive oxygen species (ROS) and adenosine triphosphate (ATP) level, are briefly discussed. The consecutive application of autophagy inducers and inhibitors may improve the drug resistance in glioma after overtreatments. It also highlights that autophagy plays a pivotal part in modulating glioma and the TIME, respectively, and the intricate interactions among them. Specifically, autophagy is manipulated by either glioma or tumor-associated macrophages to conform one side to the other through exosomal microRNAs and thereby adjust the interactions. Given that some of the crosstalk between glioma and the TIME highly depend on the autophagy process or autophagic components, there are interconnections influenced by the status and well-being of cells presumably associated with autophagic flux. By updating the most recent knowledge concerning glioma and the TIME from an autophagic perspective enhances comprehension and inspires more applicable and effective strategies targeting TIME while harnessing autophagy collaboratively against cancer.

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

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