Characterization of the immune microenvironment of diffuse intrinsic pontine glioma: implications for development of immunotherapy

Pediatric gliomas immunity challenges and immunotherapy advances

Pediatric gliomas, the most frequent brain tumors in children, are characterized by heterogeneity and a unique tumor immune microenvironment. They are categorized into different subtypes, including low-grade gliomas like pilocytic astrocytomas and high-grade gliomas such as diffuse midline gliomas and diffuse intrinsic pontine gliomas, each exhibiting distinct immunological profiles. The tumor immune microenvironment in pediatric gliomas is shaped by cellular and non-cellular components, including immune cells, cytokines, and the extracellular matrix, involved in tumor progression, immune evasion, and response to therapy. While pediatric low-grade gliomas often display an immunosuppressed microenvironment, high-grade gliomas are characterized by complex immune infiltrates and intricate immunosuppressive mechanisms. The blood-brain barrier further obscures immune cell recruitment and therapeutic delivery. Despite advances in understanding adult gliomas, the immunobiology of pediatric tumors is poorly investigated, with limited data on the interactions between glioma cells and immune populations such as T and natural killer cells, as well as tumor-associated macrophages. Herein, we provide an update of the current knowledge on tumor immune microenvironment interactions in pediatric gliomas, highlighting the immunosuppressive mechanisms and emerging immunotherapeutic strategies aiming at overcoming these barriers to improve clinical outcomes for affected children.

Targeting the HLA-E-NKG2A axis in combination with MS-275 enhances NK cell-based immunotherapy against DMG

BACKGROUND

Diffuse midline glioma (DMG) is an aggressive pediatric brain tumor with limited treatment options. Although natural killer (NK) cell-based immunotherapy is promising, its efficacy remains limited, necessitating strategies to enhance NK cell cytotoxicity. Histone deacetylase (HDAC) inhibition demonstrate potential to enhance NK-mediated killing. However, the combination of HDAC inhibitors and NK cell therapy for DMG remains unexplored.

METHODS

Patient-derived DMG cell lines and orthotopic mouse models were used to evaluate the effects of the class I HDAC inhibitor MS-275 on cytotoxicity. NK cell-mediated lysis was measured using both luciferase and calcein AM-based assays. The downstream signaling pathways affected by MS-275 were investigated via RNA-seq, CUT&Tag assay, RT‒qPCR, and chromatin immunoprecipitation with qPCR.

RESULTS

Based on bioinformatic analysis, class I HDACs are identified as therapeutic targets in DMG. The corresponding HDAC inhibitor, MS-275 upregulated NK cell-mediated cytotoxicity pathway through GSEA analysis. Pretreating DMG cells with MS-275 elevated NK cell ligand gene expression and enhanced NK cell-induced lysis. In addition to NK-activating ligands, MS-275 elevated the NK-inhibitory ligand HLA-E, thereby enhancing the efficacy of immunotherapies targeting the NKG2A-HLA-E axis. Mechanistically, MS-275 increased HLA-E expression by promoting STAT3 acetylation at lysine 685. Combining MS-275 with NK cell therapy and blockade of the NKG2A-HLA-E axis extended overall survival in orthotopic mouse models.

CONCLUSIONS

This study is the first to demonstrate that HDAC inhibition enhances NK cell-mediated cytotoxicity in DMG. Combining HDAC inhibition with NK cell therapy represents a promising therapeutic strategy for treating DMG by targeting NKG2A-HLA-E axis.

Dissecting the immune landscape in pediatric high-grade glioma reveals cell state changes under therapeutic pressure

Pediatric high-grade gliomas (pHGGs) are among the most lethal childhood tumors. While therapeutic approaches were largely adapted from adult treatment regime, significant biological differences between pediatric and adult gliomas exist, which influence the immune microenvironment and may contribute to the limited response to current pHGG treatment strategies. We provide a comprehensive transcriptomic analysis of the pHGG immune landscape using single-cell RNA sequencing and spatial transcriptomics. We analyze matched malignant, myeloid, and T cells from patients with pediatric diffuse high-grade glioma (HGG) or high-grade ependymoma, examining immune microenvironment distinctions after chemo-/radiotherapy, immune checkpoint inhibition treatment, and by age. Our analysis reveals differences in the proportions of pediatric myeloid subpopulations compared to adult counterparts. Additionally, we observe significant shifts toward immune-suppressive environments following cancer therapy. Our findings offer valuable insights into potential immunotherapy targets and serve as a robust resource for understanding immune microenvironmental variations across HGG age groups and treatment regimens.

Therapeutic approaches for targeting the pediatric brain tumor microenvironment

Central nervous system (CNS) tumors are the most frequent solid malignant tumors in pediatric patients and are the leading cause of tumor-related death in children. Treatment for this heterogeneous group of tumors consists of various combinations of safe maximal surgical resection, chemotherapy, and radiation therapy which offer a cure for some children but often cause debilitating adverse late effects in others. While therapies targeting the tumor microenvironment (TME) like immune checkpoint inhibition (ICI) have been successful in treating some cancers, these therapies failed to exhibit treatment efficacy in the majority of pediatric brain tumors in the clinic. Importantly, the pediatric TME is unique and distinct from adult brain tumors and designing therapies to effectively target these tumors requires understanding the unique biology of pediatric brain tumors and the use of translational models that recapitulate the TME. Here we describe the TME of medulloblastoma (MB) and diffuse midline glioma (DMG), specifically diffuse intrinsic pontine glioma (DIPG), and further present the current drug delivery approaches and clinical administration routes targeting the TME in these tumors, including preclinical and clinical studies.

Ara-C suppresses H3 K27-altered spinal cord diffuse midline glioma growth and enhances immune checkpoint blockade sensitivity

H3 K27-altered spinal cord diffuse midline glioma (H3-SCDMG) poses therapeutic challenges. Analysis of 73 clinical samples revealed heightened proliferation in H3-SCDMG versus wild-type tumors, suggesting therapeutic vulnerabilities. Drug screening identified cytarabine (Ara-C) as highly effective in inhibiting proliferation in H3 K27M cell models, recently established patient-derived cells, and patient-derived xenograft models. Mechanistically, Ara-C can suppress tumor growth through DNA damage, cell-cycle arrest, and apoptosis. An investigator-initiated clinical trial involving four patients showed benefits in three cases. In addition, a subset of cells exhibited senescence and senescence-associated secretory phenotype post-Ara-C treatment, accompanied by several immune checkpoint ligands' up-regulation and more immune cell infiltration. Combining Ara-C with dual Programmed cell death protein 1 (PD-1) and TIGIT blockade emerged as a promising strategy to disrupt immune evasion by senescent cells, enhancing antitumor responses. These findings highlight Ara-C's potential as a monotherapy and in synergy with immunotherapy for H3-SCDMG, offering potential strategies for clinical management.

Immunocytes in the tumor microenvironment: recent updates and interconnections

The tumor microenvironment (TME) is a complex, dynamic ecosystem where tumor cells interact with diverse immune and stromal cell types. This review provides an overview of the TME's evolving composition, emphasizing its transition from an early pro-inflammatory, immune-promoting state to a later immunosuppressive milieu characterized by metabolic reprogramming and hypoxia. It highlights the dual roles of key immunocytes-including T lymphocytes, natural killer cells, macrophages, dendritic cells, and myeloid-derived suppressor cells-which can either inhibit or support tumor progression based on their phenotypic polarization and local metabolic conditions. The article further elucidates mechanisms of immune cell plasticity, such as the M1/M2 macrophage switch and the balance between effector T cells and regulatory T cells, underscoring their impact on tumor growth and metastasis. Additionally, emerging therapeutic strategies, including checkpoint inhibitors and chimeric antigen receptor (CAR) T and NK cell therapies, as well as approaches targeting metabolic pathways, are discussed as promising avenues to reinvigorate antitumor immunity. By integrating recent molecular insights and clinical advancements, the review underscores the importance of deciphering the interplay between immunocytes and the TME to develop more effective cancer immunotherapies.

IL13RA2-integrated genetically engineered mouse model allows for CAR T cells targeting pediatric high-grade gliomas

Pediatric high-grade gliomas (pHGG) and pediatric diffuse midline gliomas (pDMG) are devastating diseases without durable and curative options. Although targeted immunotherapy has shown promise, the field lacks immunocompetent animal models to study these processes in detail. To achieve this, we developed a fully immunocompetent, genetically engineered mouse model (GEMM) for pDMG and pHGG that incorporates the glioma-associated antigen, interleukin 13 receptor alpha 2 (IL13RA2). Utilizing the RCAS-Tva delivery system in Nestin-Tva mice, we induced gliomagenesis by overexpressing PDGFB and deleting p53 (p53fl/fl) or both p53 and PTEN (p53fl/fl PTENfl/fl), with or without IL13RA2 in neonatal mice. De novo tumors developed in models with and without IL13RA2, showing no statistical difference in onset (n = 33, 38 days, p = 0.62). The p53fl/fl PTENfl/fl tumors displayed more aggressive characteristics (n = 12, 31 days). Tumors exhibited features typical of high-grade glioma, including infiltration, pseudopalisading necrosis, and microvascular proliferation. They also showed a high Ki-67 index, variable IL13RA2 expression, a high frequency of CD11b + macrophages, and a low proportion of CD3 + T cells. The model proved effective for evaluating IL13RA2-targeted immunotherapies, with a significant response to CAR T-cell treatment that extended survival (46 days vs. 28 days control; p < 0.0001) and achieved 25% long-term survival in mice. This model facilitates the preclinical assessment of IL13RA2-directed therapies and holds potential for clinical application.

Differential Phagocytosis induces Diverse Macrophage Activation States in Malignant Gliomas

Diffuse midline glioma (DMG) and Glioblastoma are malignant brain tumors in pediatric and adult patients. The current standard-of-care treatment for DMG is radiotherapy (RT), whereas GBM treatment includes surgery, followed by RT and chemotherapy. Although RT is known to modulate immune responses in cancer and enhance the effectiveness of myeloid checkpoint blockade, the downstream macrophage responses to differential phagocytosis induction remain poorly understood. This study examined macrophage-mediated phagocytosis caused by either RT, anti-CD47 checkpoint blockade, or their combination. We found that RT increased the expression of several damage-associated molecular patterns on the surface of glioma cell lines. Furthermore, RT enhanced anti-CD47-mediated macrophage phagocytosis of glioma cell lines in vitro . Single-cell RNA-sequencing revealed the diverse transcriptional and functional signatures of human macrophage subsets that either promoted or inhibited phagocytosis of glioma cells pretreated with RT, anti-CD47 therapy, or both. Consistent with these results, the combination therapy significantly reduced tumor growth, prolonged survival in glioma-bearing mice, and induced distinct macrophage activation states in vivo compared to either treatment alone. These findings highlight the plasticity and heterogeneity of macrophage responses during phagocytosis and provide compelling evidence for combining RT with anti-CD47 therapy as a promising therapeutic strategy for glioma treatment.

The Complexity of Malignant Glioma Treatment

Malignant glioma is a highly aggressive, therapeutically non-responsive, and deadly disease with a unique tumor microenvironment (TME). Of the 14 currently recognized and described cancer hallmarks, five are especially implicated in malignant glioma and targetable with repurposed drugs: cancer stem-like cells, in general, and glioma stem-like cells in particular (GSCs), vascularization and hypoxia, metabolic reprogramming, tumor-promoting inflammation and sustained proliferative signaling. Each hallmark drives malignant glioma development, both individually and through interactions with other hallmarks, in which the TME plays a critical role. To combat the aggressive malignant glioma spatio-temporal heterogeneity driven by TME interactions, and to overcome its therapeutic challenges, a combined treatment strategy including anticancer therapies, repurposed drugs and multimodal immunotherapy should be the aim for future treatment approaches.

Neuroimmune-competent human brain organoid model of diffuse midline glioma

BACKGROUND

Pediatric high-grade gliomas, such as diffuse midline glioma (DMG), have a poor prognosis and lack curative treatments. Current research models of DMG primarily rely on human DMG cell lines cultured in vitro or xenografted into the brains of immunodeficient mice. However, these models are insufficient to recapitulate the complex cell-cell interactions between DMG and the tumor-immune microenvironment (TIME), therefore fall short of accurately reflecting how efficacious therapeutic agents or combinations will be in the clinical setting.

METHODS

To address these challenges, we developed a neuroimmune-competent brain/tumor fusion organoid model system consisting entirely of human cells to investigate the interactions between DMG cells and the primary innate immune cells of the brain, microglia, in the TIME at both cellular and subcellular levels. We generated microglia-containing brain organoids (MiCBOs) that carry morphologically mature, motile microglia and multiple subtypes of neurons to mimic the brain tumor microenvironment. These organoids were then fused with H3K27M mutant, TP53P27R/K132R DMG tumor spheroids to create the MiCBO-tumor fusion (MiCBO-TF) model.

RESULTS

We utilized live imaging methods to simultaneously track the mobility of microglial cell bodies and the motility of their process, as well as the behavior of tumor cells within a human brain tissue environment. Our MiCBO-TF model faithfully recapitulated the diffuse infiltration pattern of DMG into brain tissue and revealed that microglial mobility and interactions with tumor cells are highly influenced by external factors and the surrounding tissue environment.

CONCLUSIONS

The MiCBO-TF model represents a powerful platform for both mechanistic investigations and the development of precision medicine approaches for DMG.

Chimeric antigen receptor T-cell therapy in patients with malignant glioma-From neuroimmunology to clinical trial design considerations

Clinical trials evaluating chimeric antigen receptor (CAR) T-cell therapy in patients with malignant gliomas have shown some early promise in pediatric and adult patients. However, the long-term benefits and safety for patients remain to be established. The ultimate success of CAR T-cell therapy for malignant glioma will require the integration of an in-depth understanding of the immunology of the central nervous system (CNS) parenchyma with strategies to overcome the paucity and heterogeneous expression of glioma-specific antigens. We also need to address the cold (immunosuppressive) microenvironment, exhaustion of the CAR T-cells, as well as local and systemic immunosuppression. Here, we discuss the basics and scientific considerations for CAR T-cell therapies and highlight recent clinical trials. To help identify optimal CAR T-cell administration routes, we summarize our current understanding of CNS immunology and T-cell homing to the CNS. We also discuss challenges and opportunities related to clinical trial design and patient safety/monitoring. Finally, we provide our perspective on future prospects in CAR T-cell therapy for malignant gliomas by discussing combinations and novel engineering strategies to overcome immuno-regulatory mechanisms. We hope this review will serve as a basis for advancing the field in a multiple discipline-based and collaborative manner.

Radiotherapy Plus the Neurokinin-1 Receptor Antagonist Aprepitant: A Potent Therapeutic Strategy for the Treatment of Diffuse Intrinsic Pontine Glioma

Background: Diffuse intrinsic pontine glioma (DIPG) is a devastating childhood brainstem tumor. The median survival of DIPG is 16-24 months independent of the treatment received. Therefore, new therapeutic strategies against DIPG are urgently needed. Substance P (SP) peptide, through the neurokinin neurokinin-1 receptor (NK-1R), is involved in glioma progression. It induces glioma cell proliferation by activating MAPKs (p38 MAPK, ERK1/2, and JNK), c-Myc, AP-1, and NF-κB and induces antiapoptotic effects via PI3K/Akt/mTOR in glioma cells. SP favors glycogen breakdown that is essential for glycolysis. The SP/NK-1R system also regulates the migration and invasion of glioma cells, stimulates angiogenesis, and triggers inflammation which contributes to glioma progression. Moreover, all glioma cells express NK-1R, and NK-1R is essential for the viability of glioma cells and not of normal cells. In contrast, in glioma, NK-1R antagonists, such as the drug aprepitant, penetrate the brain and reach therapeutic concentrations, thereby inhibiting mitogenesis, inducing apoptosis, and inhibiting the breakdown of glycogen in glioma cells. In addition, they inhibit angiogenesis and exert antimetastatic and anti-inflammatory effects. The combination of radiotherapy with NK-1R antagonists produces radiosensitization and radioneuroprotection, reduces both peritumoral- and radiation-induced inflammation, and also provides antinausea and antivomiting effects. Objective: This review updates the involvement of the SP/NK-1R system in glioma promotion and progression and the potential clinical application of NK-1R antagonist drugs in DIPG therapy. Conclusions: NK-1R plays a crucial role in glioma progression and NK-1R antagonists such as aprepitant could be used in combination with radiotherapy as a potent therapeutic strategy for the treatment of patients with DIPG.

Advances in the Repurposing and Blood-Brain Barrier Penetrance of Drugs in Pediatric Brain Tumors

Central nervous system (CNS) tumors are the leading cause of cancer-related mortality in children, with prognosis remaining dismal for some of these malignancies. Though the past two decades have seen advancements in surgery, radiation, and targeted therapy, major unresolved hurdles continue to undermine the therapeutic efficacy. These include challenges in suboptimal drug delivery through the blood-brain barrier (BBB), marked intra-tumoral molecular heterogeneity, and the elusive tumor microenvironment. Drug repurposing or re-tasking FDA-approved drugs with evidence of penetration into the CNS, using newer methods of intracranial drug delivery facilitating optimal drug exposure, has been an area of intense research. This could be a valuable tool, as most of these agents have already gone through the lengthy process of drug development and the evaluation of safety risks and the optimal pharmacokinetic profile. They can now be used and tested in clinics with an accelerated and different approach. Conclusions: The next-generation therapeutic strategy should prioritize repurposing oncologic and non-oncologic drugs that have been used for other indication, and have demonstrated robust preclinical activity against pediatric brain tumors. In combination with novel drug delivery techniques, these drugs could hold significant therapeutic promise in pediatric neurooncology.

The Role of Chronic Inflammation in Pediatric Cancer

Inflammation plays a crucial role in wound healing and the host immune response following pathogenic invasion. However, unresolved chronic inflammation can result in tissue fibrosis and genetic alterations that contribute to the pathogenesis of human diseases such as cancer. Recent scientific advancements exploring the underlying mechanisms of malignant cellular transformations and cancer progression have exposed significant disparities between pediatric and adult-onset cancers. For instance, pediatric cancers tend to have lower mutational burdens and arise in actively developing tissues, where cell-cycle dysregulation leads to gene, chromosomal, and fusion gene development not seen in adult-onset counterparts. As such, scientific findings in adult cancers cannot be directly applied to pediatric cancers, where unique mutations and inherent etiologies remain poorly understood. Here, we review the role of chronic inflammation in processes of genetic and chromosomal instability, the tumor microenvironment, and immune response that result in pediatric tumorigenesis transformation and explore current and developing therapeutic interventions to maintain and/or restore inflammatory homeostasis.

IL13RA2-integrated genetically engineered mouse model allows for CAR T cells targeting pediatric high-grade gliomas

Pediatric high-grade gliomas (pHGG) and pediatric diffuse midline gliomas (pDMG) are devastating diseases without durable and curative options. Although targeted immunotherapy has shown promise, the field lacks immunocompetent animal models to study these processes in detail. To achieve this, we developed a fully immunocompetent, genetically engineered mouse model (GEMM) for pDMG and pHGG that incorporates the glioma-associated antigen, interleukin 13 receptor alpha 2 (IL13RA2). Utilizing the RCAS-Tva delivery system in Nestin-Tva mice, we induced gliomagenesis by overexpressing PDGFB and deleting p53 (p53fl/fl) or both p53 and PTEN (p53fl/fl PTENfl/fl), with or without IL13RA2 in neonatal mice. De novo tumors developed in models with and without IL13RA2, showing no statistical difference in onset (n = 33, 38 days, p = 0.62). The p53fl/fl PTENfl/fl tumors displayed more aggressive characteristics (n = 12, 31 days). Tumors exhibited features typical of high-grade glioma, including infiltration, pseudopalisading necrosis, and microvascular proliferation. They also showed a high Ki-67 index, variable IL13RA2 expression, a high frequency of CD11b + macrophages, and a low proportion of CD3 + T cells. The model proved effective for evaluating IL13RA2-targeted immunotherapies, with a significant response to CAR T-cell treatment that extended survival (46 days vs. 28 days control; p < 0.0001) and achieved 25% long-term survival in mice. This model facilitates the preclinical assessment of IL13RA2-directed therapies and holds potential for clinical application.

Microglia and monocyte-derived macrophages drive progression of pediatric high-grade gliomas and are transcriptionally shaped by histone mutations

Pediatric high-grade gliomas (pHGGs), including hemispheric pHGGs and diffuse midline gliomas (DMGs), harbor mutually exclusive tumor location-specific histone mutations. Using immunocompetent de novo mouse models of pHGGs, we demonstrated that myeloid cells were the predominant infiltrating non-neoplastic cell population. Single-cell RNA sequencing (scRNA-seq), flow cytometry, and immunohistochemistry illustrated the presence of heterogeneous myeloid cell populations shaped by histone mutations and tumor location. Disease-associated myeloid (DAM) cell phenotypes demonstrating immune permissive characteristics were identified in murine and human pHGG samples. H3.3K27M DMGs, the most aggressive DMG, demonstrated enrichment of DAMs. Genetic ablation of chemokines Ccl8 and Ccl12 resulted in a reduction of DAMs and an increase in lymphocyte infiltration, leading to increased survival of tumor-bearing mice. Pharmacologic inhibition of chemokine receptors CCR1 and CCR5 resulted in extended survival and decreased myeloid cell infiltration. This work establishes the tumor-promoting role of myeloid cells in DMG and the potential therapeutic opportunities for targeting them.

The emerging field of viroimmunotherapy for pediatric brain tumors

Pediatric brain tumors are the most common solid tumors in children. Even to date, with the advances in multimodality therapeutic management, survival outcomes remain dismal in some types of tumors, such as pediatric-type diffuse high-grade gliomas or central nervous system embryonal tumors. Failure to understand the complex molecular heterogeneity and the elusive tumor and microenvironment interplay continues to undermine therapeutic efficacy. Developing a strategy that would improve survival for these fatal tumors remains unmet in pediatric neuro-oncology. Oncolytic viruses (OVs) are emerging as a feasible, safe, and promising therapy for brain tumors. The new paradigm in virotherapy implies that the direct cytopathic effect is followed, under certain circumstances, by an antitumor immune response responsible for the partial or complete debulking of the tumor mass. OVs alone or combined with other therapeutic modalities have been primarily used in adult neuro-oncology. A surge in encouraging preclinical studies in pediatric brain tumor models recently led to the clinical translation of OVs with encouraging results in these tumors. In this review, we summarize the different virotherapy tested in preclinical and clinical studies in pediatric brain tumors, and we discuss the limitations and future avenues necessary to improve the response of these tumors to this type of therapy.

Understanding spinal cord astrocytoma: Molecular mechanism, therapy, and comprehensive management

Spinal cord astrocytoma is a rare and highly debilitating tumor, yet our knowledge of its clinical characteristics, molecular features, and pathogenesis remains limited compared to that of its counterparts in the brain. Current diagnostic and therapeutic approaches for spinal cord astrocytomas are primarily based on established guidelines for brain astrocytomas. However, recent studies have revealed unique clinical and pathological attributes that distinguish spinal cord astrocytomas from their corresponding brain counterparts. These findings underscore the inadequacy of directly applying the clinical guidelines developed for brain astrocytomas to spinal astrocytomas. In this review, we provided an up-to-date overview of the advancements in understanding spinal cord astrocytomas. We also discussed the challenges and future research prospects in this field with the aim of improving the precision of diagnosis and therapy for these tumors. Specifically, we emphasized the importance of enhancing our understanding of the molecular heterogeneity, immune characteristics, and clinical trials of spinal cord astrocytomas.

Transcriptomic and proteomic spatial profiling of pediatric and adult diffuse midline glioma H3 K27-Altered

Diffuse midline glioma, H3 K27-altered (DMG) are highly aggressive malignancies of the central nervous system (CNS) that primarily affect the pediatric population. Large scale spatial transcriptomic studies have implicated that tumor microenvironmental landscape plays an important role in determining the phenotypic differences in tumor presentation and clinical course, however, data connecting overall transcriptomic changes to the protein level is lacking. The NanoString GeoMx™ Digital Spatial Profiler platform was used to determine the spatial transcriptomic and proteomic landscape in a cohort of both pediatric and adult H3 K27-altered DMG biopsy samples. Three fluorescently labeled antibodies targeting immune cells (CD45), epithelial cells (PanCK), tumor cells (H3 K27M) and a nucleic acid stain (SYTO-13) were used to establish regions of interest (ROI) for genomic and proteomic analysis. We found genetic alterations within the tumor which can be delineated across patient age and spatial location. We show that the H3 K27M mutation itself has a profound impact on tumor cells transcriptomics and interestingly we found limited fidelity between overall transcriptome and proteome. Our data also validate a previously described genomic signature at the proteomic level and reveal a special shift in the signature based on the local TME composition.

Emerging and Biological Concepts in Pediatric High-Grade Gliomas

Primary central nervous system tumors are the most frequent solid tumors in children, accounting for over 40% of all childhood brain tumor deaths, specifically high-grade gliomas. Compared with pediatric low-grade gliomas (pLGGs), pediatric high-grade gliomas (pHGGs) have an abysmal survival rate. The WHO CNS classification identifies four subtypes of pHGGs, including Grade 4 Diffuse midline glioma H3K27-altered, Grade 4 Diffuse hemispheric gliomas H3-G34-mutant, Grade 4 pediatric-type high-grade glioma H3-wildtype and IDH-wildtype, and infant-type hemispheric gliomas. In recent years, we have seen promising advancements in treatment strategies for pediatric high-grade gliomas, including immunotherapy, CAR-T cell therapy, and vaccine approaches, which are currently undergoing clinical trials. These therapies are underscored by the integration of molecular features that further stratify HGG subtypes. Herein, we will discuss the molecular features of pediatric high-grade gliomas and the evolving landscape for treating these challenging tumors.

Immune landscape of oncohistone-mutant gliomas reveals diverse myeloid populations and tumor-promoting function

Histone H3-mutant gliomas are deadly brain tumors characterized by a dysregulated epigenome and stalled differentiation. In contrast to the extensive datasets available on tumor cells, limited information exists on their tumor microenvironment (TME), particularly the immune infiltrate. Here, we characterize the immune TME of H3.3K27M and G34R/V-mutant gliomas, and multiple H3.3K27M mouse models, using transcriptomic, proteomic and spatial single-cell approaches. Resolution of immune lineages indicates high infiltration of H3-mutant gliomas with diverse myeloid populations, high-level expression of immune checkpoint markers, and scarce lymphoid cells, findings uniformly reproduced in all H3.3K27M mouse models tested. We show these myeloid populations communicate with H3-mutant cells, mediating immunosuppression and sustaining tumor formation and maintenance. Dual inhibition of myeloid cells and immune checkpoint pathways show significant therapeutic benefits in pre-clinical syngeneic mouse models. Our findings provide a valuable characterization of the TME of oncohistone-mutant gliomas, and insight into the means for modulating the myeloid infiltrate for the benefit of patients.

Glioblastoma stem cell metabolism and immunity

Despite enormous efforts being invested in the development of novel therapies for brain malignancies, there remains a dire need for effective treatments, particularly for pediatric glioblastomas. Their poor prognosis has been attributed to the fact that conventional therapies target tumoral cells, but not glioblastoma stem cells (GSCs). GSCs are characterized by self-renewal, tumorigenicity, poor differentiation, and resistance to therapy. These characteristics represent the fundamental tools needed to recapitulate the tumor and result in a relapse. The mechanisms by which GSCs alter metabolic cues and escape elimination by immune cells are discussed in this article, along with potential strategies to harness effector immune cells against GSCs. As cellular immunotherapy is making significant advances in a variety of cancers, leveraging this underexplored reservoir may result in significant improvements in the treatment options for brain malignancies.

H3K27M diffuse midline glioma is homologous recombination defective and sensitized to radiotherapy and NK cell-mediated antitumor immunity by PARP inhibition

Background

Radiotherapy (RT) is the primary treatment for diffuse midline glioma (DMG), a lethal pediatric malignancy defined by histone H3 lysine 27-to-methionine (H3K27M) mutation. Based on the loss of H3K27 trimethylation producing broad epigenomic alterations, we hypothesized that H3K27M causes a functional double-strand break (DSB) repair defect that could be leveraged therapeutically with PARP inhibitor and RT for selective radiosensitization and antitumor immune responses.

Methods

H3K27M isogenic DMG cells and orthotopic brainstem DMG tumors in immune deficient and syngeneic, immune competent mice were used to evaluate the efficacy and mechanisms of PARP1/2 inhibition by olaparib or PARP1 inhibition by AZD9574 with concurrent RT.

Results

H3K27M mutation caused an HRR defect characterized by impaired RT-induced K63-linked polyubiquitination of histone H1 and inhibition of HRR protein recruitment. H3K27M DMG cells were selectively radiosensitized by olaparib in comparison to isogenic controls, and this effect translated to efficacy in H3K27M orthotopic brainstem tumors. Olaparib and RT induced an innate immune response and induction of NK cell (NKG2D) activating ligands leading to increased NK cell-mediated lysis of DMG tumor cells. In immunocompetent syngeneic orthotopic DMG tumors, either olaparib or AZD9574 in combination with RT enhanced intratumoral NK cell infiltration and activity in association with NK cell-mediated therapeutic responses and favorable activity of AZD9574.

Conclusions

The HRR deficiency in H3K27M DMG can be therapeutically leveraged with PARP inhibitors to radiosensitize and induce an NK cell-mediated antitumor immune response selectively in H3K27M DMG, supporting the clinical investigation of best-in-class PARP inhibitors with RT in DMG patients.

Key points

H3K27M DMG are HRR defective and selectively radiosensitized by PARP inhibitor.PARP inhibitor with RT enhances NKG2D ligand expression and NK cell-mediated lysis.NK cells are required for the therapeutic efficacy of PARP inhibitor and RT.

Importance of the Study

Radiotherapy is the cornerstone of H3K27M-mutant diffuse midline glioma treatment, but almost all patients succumb to tumor recurrence with poor overall survival, underscoring the need for RT-based precision combination therapy. Here, we reveal HRR deficiency as an H3K27M-mediated vulnerability and identify a novel mechanism linking impaired RT-induced histone H1 polyubiquitination and the subsequent RNF168/BRCA1/RAD51 recruitment in H3K27M DMG. This model is supported by selective radiosensitization of H3K27M DMG by PARP inhibitor. Notably, the combination treatment results in NKG2D ligand expression that confers susceptibility to NK cell killing in H3K27M DMG. We also show that the novel brain penetrant, PARP1-selective inhibitor AZD9574 compares favorably to olaparib when combined with RT, prolonging survival in a syngeneic orthotopic model of H3K27M DMG. This study highlights the ability of PARP1 inhibition to radiosensitize and induce an NK cell-mediated antitumor immunity in H3K27M DMG and supports future clinical investigation.

Pediatric Hemispheric High-Grade Gliomas and H3.3-G34 Mutation: A Review of the Literature on Biological Features and New Therapeutic Strategies

Pediatric high-grade glioma (pHGG) encompasses a wide range of gliomas with different genomic, epigenomic, and transcriptomic features. Almost 50% of pHGGs present a mutation in genes coding for histone 3, including the subtype harboring the H3.3-G34 mutation. In this context, histone mutations are frequently associated with mutations in TP53 and ATRX, along with PDGFRA and NOTCH2NL amplifications. Moreover, the H3.3-G34 histone mutation induces epigenetic changes in immune-related genes and exerts modulatory functions on the microenvironment. Also, the functionality of the blood-brain barrier (BBB) has an impact on treatment response. The prognosis remains poor with conventional treatments, thus eliciting the investigation of additional and alternative therapies. Promising molecular targets include PDGFRA amplification, BRAF mutation, EGFR amplification, NF1 loss, and IDH mutation. Considering that pHGGs harboring the H3.3-G34R mutation appear to be more susceptible to immunotherapies (ITs), different options have been recently explored, including immune checkpoint inhibitors, antibody mediated IT, and Car-T cells. This review aims to summarize the knowledge concerning cancer biology and cancer-immune cell interaction in this set of pediatric gliomas, with a focus on possible therapeutic options.

Immuno-oncologic profiling of pediatric brain tumors reveals major clinical significance of the tumor immune microenvironment

With the success of immunotherapy in cancer, understanding the tumor immune microenvironment (TIME) has become increasingly important; however in pediatric brain tumors this remains poorly characterized. Accordingly, we developed a clinical immune-oncology gene expression assay and used it to profile a diverse range of 1382 samples with detailed clinical and molecular annotation. In low-grade gliomas we identify distinct patterns of immune activation with prognostic significance in BRAF V600E-mutant tumors. In high-grade gliomas, we observe immune activation and T-cell infiltrates in tumors that have historically been considered immune cold, as well as genomic correlates of inflammation levels. In mismatch repair deficient high-grade gliomas, we find that high tumor inflammation signature is a significant predictor of response to immune checkpoint inhibition, and demonstrate the potential for multimodal biomarkers to improve treatment stratification. Importantly, while overall patterns of immune activation are observed for histologically and genetically defined tumor types, there is significant variability within each entity, indicating that the TIME must be evaluated as an independent feature from diagnosis. In sum, in addition to the histology and molecular profile, this work underscores the importance of reporting on the TIME as an essential axis of cancer diagnosis in the era of personalized medicine.

Immune Microenvironment in Childhood Cancers: Characteristics and Therapeutic Challenges

The tumor immune microenvironment is pivotal in cancer initiation, advancement, and regulation. Its molecular and cellular composition is critical throughout the disease, as it can influence the balance between suppressive and cytotoxic immune responses within the tumor's vicinity. Studies on the tumor immune microenvironment have enriched our understanding of the intricate interplay between tumors and their immunological surroundings in various human cancers. These studies illuminate the role of significant components of the immune microenvironment, which have not been extensively explored in pediatric tumors before and may influence the responsiveness or resistance to therapeutic agents. Our deepening understanding of the pediatric tumor immune microenvironment is helping to overcome challenges related to the effectiveness of existing therapeutic strategies, including immunotherapies. Although in the early stages, targeted therapies that modulate the tumor immune microenvironment of pediatric solid tumors hold promise for improved outcomes. Focusing on various aspects of tumor immune biology in pediatric patients presents a therapeutic opportunity that could improve treatment outcomes. This review offers a comprehensive examination of recent literature concerning profiling the immune microenvironment in various pediatric tumors. It seeks to condense research findings on characterizing the immune microenvironment in pediatric tumors and its impact on tumor development, metastasis, and response to therapeutic modalities. It covers the immune microenvironment's role in tumor development, interactions with tumor cells, and its impact on the tumor's response to immunotherapy. The review also discusses challenges targeting the immune microenvironment for pediatric cancer therapies.

Current status and advances to improving drug delivery in diffuse intrinsic pontine glioma

Diffuse midline glioma (DMG), including tumors diagnosed in the brainstem (diffuse intrinsic pontine glioma - DIPG), is the primary cause of brain tumor-related death in pediatric patients. DIPG is characterized by a median survival of <12 months from diagnosis, harboring the worst 5-year survival rate of any cancer. Corticosteroids and radiation are the mainstay of therapy; however, they only provide transient relief from the devastating neurological symptoms. Numerous therapies have been investigated for DIPG, but the majority have been unsuccessful in demonstrating a survival benefit beyond radiation alone. Although many barriers hinder brain drug delivery in DIPG, one of the most significant challenges is the blood-brain barrier (BBB). Therapeutic compounds must possess specific properties to enable efficient passage across the BBB. In brain cancer, the BBB is referred to as the blood-brain tumor barrier (BBTB), where tumors disrupt the structure and function of the BBB, which may provide opportunities for drug delivery. However, the biological characteristics of the brainstem's BBB/BBTB, both under normal physiological conditions and in response to DIPG, are poorly understood, which further complicates treatment. Better characterization of the changes that occur in the BBB/BBTB of DIPG patients is essential, as this informs future treatment strategies. Many novel drug delivery technologies have been investigated to bypass or disrupt the BBB/BBTB, including convection enhanced delivery, focused ultrasound, nanoparticle-mediated delivery, and intranasal delivery, all of which are yet to be clinically established for the treatment of DIPG. Herein, we review what is known about the BBB/BBTB and discuss the current status, limitations, and advances of conventional and novel treatments to improving brain drug delivery in DIPG.

Current immunotherapeutic approaches to diffuse intrinsic pontine glioma

Diffuse intrinsic pontine glioma (DIPG) is an aggressive brain tumour that occurs in the pons of the brainstem and accounts for over 80% of all brainstem gliomas. The median age at diagnosis is 6-7 years old, with less than 10% overall survival 2 years after diagnosis and less than 1% after 5 years. DIPGs are surgically inaccessible, and radiation therapy provides only transient benefit, with death ensuing from relentless local tumour infiltration. DIPGs are now the leading cause of brain tumour deaths in children, with a societal cancer burden in years of life lost (YLL) of more than 67 per individual, versus approximately 14 and 16 YLL for lung and breast cancer respectively. More than 95 clinical drug trials have been conducted on children with DIPGs, and all have failed to improve survival. No single or combination chemotherapeutic strategy has been successful to date because of our inability to identify targeted drugs for this disease and to deliver these drugs across an intact blood-brain barrier (BBB). Accordingly, there has been an increased focus on immunotherapy research in DIPG, with explorations into treatments such as chimeric antigen receptor T (CAR-T) cells, immune checkpoint blockades, cancer vaccines, and autologous cell transfer therapy. Here, we review the most recent advances in identifying genetic factors influencing the development of immunotherapy for DIPG. Additionally, we explore emerging technologies such as Magnetic Resonance-guided Focused Ultrasound (MRgFUS) in potential combinatorial approaches to treat DIPG.

OV Modulators of the Paediatric Brain TIME: Current Status, Combination Strategies, Limitations and Future Directions

Oncolytic viruses (OVs) are characterised by their preference for infecting and replicating in tumour cells either naturally or after genetic modification, resulting in oncolysis. Furthermore, OVs can elicit both local and systemic anticancer immune responses while specifically infecting and lysing tumour cells. These characteristics render them a promising therapeutic approach for paediatric brain tumours (PBTs). PBTs are frequently marked by a cold tumour immune microenvironment (TIME), which suppresses immunotherapies. Recent preclinical and clinical studies have demonstrated the capability of OVs to induce a proinflammatory immune response, thereby modifying the TIME. In-depth insights into the effect of OVs on different cell types in the TIME may therefore provide a compelling basis for using OVs in combination with other immunotherapy modalities. However, certain limitations persist in our understanding of oncolytic viruses' ability to regulate the TIME to enhance anti-tumour activity. These limitations primarily stem from the translational limitations of model systems, the difficulties associated with tracking reliable markers of efficacy throughout the course of treatment and the role of pre-existing viral immunity. In this review, we describe the different alterations observed in the TIME in PBTs due to OV treatment, combination therapies of OVs with different immunotherapies and the hurdles limiting the development of effective OV therapies while suggesting future directions based on existing evidence.

A review of current therapeutics targeting the mitochondrial protease ClpP in diffuse midline glioma, H3 K27-altered

Diffuse midline gliomas (DMGs) are devastating pediatric brain tumors recognized as the leading cause of cancer-related death in children. DMGs are high-grade gliomas (HGGs) diagnosed along the brain's midline. Euchromatin is the hallmark feature of DMG, caused by global hypomethylation of H3K27 either through point mutations in histone H3 genes (H3K27M), or by overexpression of the enhancer of zeste homolog inhibitory protein. In a clinical trial for adults with progressive HGGs, a 22-year-old patient with a thalamic DMG, H3 K27-altered, showed a remarkable clinical and radiological response to dordaviprone (ONC201). This response in an H3 K27-altered HGG patient, coupled with the lack of response of patients harboring wildtype-H3 tumors, has increased the clinical interest in dordaviprone for the treatment of DMG. Additional reports of clinical benefit have emerged, but research defining mechanisms of action (MOA) fall behind dordaviprone's clinical use, with biomarkers of response unresolved. Here, we summarize dordaviprone's safety, interrogate its preclinical MOA identifying the mitochondrial protease "ClpP" as a biomarker of response, and discuss other ClpP agonists, expanding the arsenal of potential weapons in the fight against DMG. Finally, we discuss combination strategies including ClpP agonists, and their immunomodulatory effects suggestive of a role for the tumor microenvironment in DMG patient response.

H3K27M mutant glioma: Disease definition and biological underpinnings

High-grade glioma (HGG) is the most common cause of cancer death in children and the most common primary central nervous system tumor in adults. While pediatric HGG was once thought to be biologically similar to the adult form of disease, research has shown these malignancies to be significantly molecularly distinct, necessitating distinct approaches to their clinical management. However, emerging data have shown shared molecular events in pediatric and adult HGG including the histone H3K27M mutation. This somatic missense mutation occurs in genes encoding one of two isoforms of the Histone H3 protein, H3F3A (H3.3), or HIST1H3B (H3.1), and is detected in up to 80% of pediatric diffuse midline gliomas and in up to 60% of adult diffuse gliomas. Importantly, the H3K27M mutation is associated with poorer overall survival and response to therapy compared to patients with H3 wild-type tumors. Here, we review the clinical features and biological underpinnings of pediatric and adult H3K27M mutant glioma, offering a groundwork for understanding current research and clinical approaches for the care of patients suffering with this challenging disease.

Transcriptomic and Proteomic Spatial Profiling of Pediatric and Adult Diffuse Midline Glioma H3 K27-Altered, Reveals Region Specific Differences and Limited Overlap between mRNA and Protein

Diffuse midline glioma, H3 K27-altered (DMG-Alt) are highly aggressive malignancies of the central nervous system (CNS) that primarily affect the pediatric population. Large scale spatial transcriptomic studies have implicated that tumor microenvironmental landscape plays an important role in determining the phenotypic differences in tumor presentation and clinical course, however, data connecting overall transcriptomic changes to the protein level is lacking. The NanoString GeoMx™ Digital Spatial Profiler platform was used to determine the spatial transcriptomic and proteomic landscape in a cohort of both pediatric and adult H3 K27-altered DMG biopsy samples. Three fluorescently labeled antibodies targeting immune cells (CD45), epithelial cells (PanCK), tumor cells (H3 K27M) and a nucleic acid stain (SYTO-13) were used to establish regions of interest (ROI) for genomic and proteomic analysis. We found genetic alterations within the tumor which can be delineated across patient age and spatial location. We show that the H3 K27M mutation itself has a profound impact on tumor cells transcriptomics and interestingly we found limited fidelity between overall transcriptome and proteome. Our data also validate the previously described OPC like precursor signature at the proteomic level and reveal a special shift in the signature based on the local TME composition.

Pediatric Solid Cancers: Dissecting the Tumor Microenvironment to Improve the Results of Clinical Immunotherapy

Despite advances in their diagnosis and treatment, pediatric cancers remain among the leading causes of death in childhood. The development of immunotherapies and other forms of targeted therapies has significantly changed the prognosis of some previously incurable cancers in the adult population. However, so far, the results in pediatric cohorts are disappointing, which is mainly due to differences in tumor biology, including extreme heterogeneity and a generally low tumor mutational burden. A central role in the limited efficacy of immunotherapeutic approaches is played by the peculiar characteristics of the tumor microenvironment (TME) in pediatric cancer, with the scarcity of tumor infiltration by T cells and the abundance of stromal cells endowed with lymphocyte suppressor and tumor-growth-promoting activity. Thus, progress in the treatment of pediatric solid tumors will likely be influenced by the ability to modify the TME while delivering novel, more effective therapeutic agents. In this review, we will describe the TME composition in pediatric solid tumors and illustrate recent advances in treatment for the modulation of immune cells belonging to the TME.

Myeloid cells as potential targets for immunotherapy in pediatric gliomas

Pediatric high-grade glioma (pHGG) including pediatric glioblastoma (pGBM) are highly aggressive pediatric central nervous system (CNS) malignancies. pGBM comprises approximately 3% of all pediatric CNS malignancies and has a 5-year survival rate of approximately 20%. Surgical resection and chemoradiation are often the standard of care for pGBM and pHGG, however, even with these interventions, survival for children diagnosed with pGBM and pHGG remains poor. Due to shortcomings associated with the standard of care, many efforts have been made to create novel immunotherapeutic approaches targeted to these malignancies. These efforts include the use of vaccines, cell-based therapies, and immune-checkpoint inhibitors. However, it is believed that in many pediatric glioma patients an immunosuppressive tumor microenvironment (TME) possess barriers that limit the efficacy of immune-based therapies. One of these barriers includes the presence of immunosuppressive myeloid cells. In this review we will discuss the various types of myeloid cells present in the glioma TME, including macrophages and microglia, myeloid-derived suppressor cells, and dendritic cells, as well as the specific mechanisms these cells can employ to enable immunosuppression. Finally, we will highlight therapeutic strategies targeted to these cells that are aimed at impeding myeloid-cell derived immunosuppression.

Preclinical pediatric brain tumor models for immunotherapy: Hurdles and a way forward

Brain tumors are the most common solid tumor in children and the leading cause of cancer-related deaths. Over the last few years, improvements have been made in the diagnosis and treatment of children with Central Nervous System tumors. Unfortunately, for many patients with high-grade tumors, the overall prognosis remains poor. Lower survival rates are partly attributed to the lack of efficacious therapies. The advent and success of immune checkpoint inhibitors (ICIs) in adults have sparked interest in investigating the utility of these therapies alone or in combination with other drug treatments in pediatric patients. However, to achieve improved clinical outcomes, the establishment and selection of relevant and robust preclinical pediatric high-grade brain tumor models is imperative. Here, we review the information that influenced our model selection as we embarked on an international collaborative study to test ICIs in combination with epigenetic modifying agents to enhance adaptive immunity to treat pediatric brain tumors. We also share challenges that we faced and potential solutions.

Transforming Growth Factor Beta 2 (TGFB2) and Interferon Gamma Receptor 2 (IFNGR2) mRNA Levels in the Brainstem Tumor Microenvironment (TME) Significantly Impact Overall Survival in Pediatric DMG Patients

This hypothesis-generating study characterized the mRNA expression profiles and prognostic impacts of antigen-presenting cell (APC) markers (CD14, CD163, CD86, and ITGAX/CD11c) in pediatric brainstem diffuse midline glioma (pbDMG) tumors. We also assessed the mRNA levels of two therapeutic targets, transforming growth factor beta 2 (TGFB2) and interferon gamma receptor 2 (IFNGR2), for their biomarker potentials in these highly aggressive pbDMG tumors. The expressions of CD14, CD163, and ITGAX/CD11c mRNAs exhibited significant decreases of 1.64-fold (p = 0.037), 1.75-fold (p = 0.019), and 3.33-fold (p < 0.0001), respectively, in pbDMG tumors relative to those in normal brainstem/pons samples. The pbDMG samples with high levels of TGFB2 in combination with low levels of APC markers, reflecting the cold immune state of pbDMG tumors, exhibited significantly worse overall survival outcomes at low expression levels of CD14, CD163, and CD86. The expression levels of IFNGR2 and TGFB2 (1.51-fold increase (p = 0.002) and 1.58-fold increase (p = 5.5 × 10-4), respectively) were significantly upregulated in pbDMG tumors compared with normal brainstem/pons samples. We performed multivariate Cox proportional hazards modelling that showed TGFB2 was a prognostic indicator (HR for patients in the TGFB2high group of pbDMG patients = 2.88 (1.12-7.39); p = 0.028) for poor overall survival (OS) and was independent of IFNGR2 levels, the age of the patient, and the significant interaction effect observed between IFNGR2 and TGFB2 (p = 0.015). Worse survival outcomes in pbDMG patients when comparing high versus low TGFB2 levels in the context of low IFNGR2 levels suggest that the abrogation of the TGFB2 mRNA expression in the immunologically cold tumor microenvironment can be used to treat pbDMG patients. Furthermore, pbDMG patients with low levels of JAK1 or STAT1 mRNA expression in combination with high levels of TGFB2 also exhibited poor OS outcomes, suggesting that the inclusion of (interferon-gamma) IFN-γ to stimulate and activate JAK1 and STAT1 in anti-tumor APC cells present the brainstem TME can enhance the effect of the TGFB2 blockade.

Characterization of Prognostic Apoptosis-Related Gene Signature to Evaluate Glioma Immune Microenvironment and Experimental Verification

Purpose: Recently, apoptosis-related genes were shown to modulate cancer immunity. However, the role of apoptosis-related genes in the glioma immune microenvironment (GIME) remains unknown. This study aimed to explore the prognostic value of apoptosis-related genes in glioma. Methods: Doxorubicin was used to induce glioma cell apoptosis, and four differentially expressed apoptosis-related genes were identified: CREM, TNFSF12, PEA15, and PRKCD. Kaplan-Meier analyses, receiver operating characteristic curve analyses, and nomograms were established to determine the relationship between risk markers and the prognosis of patients with glioma. Results: Risk biomarkers were significantly associated with overall survival, immune cell infiltration, and immune checkpoints in patients with glioma. Somatic mutations and anti-PD-1/L1 immunotherapy were associated with worse prognosis in the high-risk group receiving anti-PD-1/L1 therapy. The expression of these four apoptosis-related genes was verified using quantitative polymerase chain reaction and immunohistochemistry, and the relationship between these four genes and apoptosis was examined using flow cytometry. Conclusions: This study suggests that apoptosis-related genes play a critical role in shaping the GIME. Assessing the apoptotic patterns of individual tumors will enhance our understanding of GIME infiltration features and lead to improved strategies for immunotherapy.

The unique immune ecosystems in pediatric brain tumors: integrating single-cell and bulk RNA-sequencing

Background

The significant progress of immune therapy in non-central nervous system tumors has sparked interest in employing the same strategy for adult brain tumors. However, the advancement of immunotherapy in pediatric central nervous system (CNS) tumors is not yet on par. Currently, there is a lack of comprehensive comparative studies investigating the immune ecosystem in pediatric and adult CNS tumors at a high-resolution single-cell level.

Methods

In this study, we comprehensively analyzed over 0.3 million cells from 171 samples, encompassing adult gliomas (IDH wild type and IDH mutation) as well as four major types of pediatric brain tumors (medulloblastoma (MB), ependymoma (EPN), H3K27M-mutation (DIPG), and pediatric IDH-mutation glioma (P-IDH-M)). Our approach involved integrating publicly available and newly generated single-cell datasets. We compared the immune landscapes in different brain tumors, as well as the detailed functional phenotypes of T-cell and myeloid subpopulations. Through single-cell analysis, we identified gene sets associated with major cell types in the tumor microenvironment (gene features from single-cell data, scFes) and compared them with existing gene sets such as GSEA and xCell. The CBTTC and external GEO cohort was used to analyze and validate the immune-stromal-tumor patterns in pediatric brain tumors which might potentially respond to the immunotherapy.

Results

From the perspective of single-cell analysis, it was observed that major pediatric brain tumors (MB, EPN, P-IDH-M, DIPG) exhibited lower immune contents compared with adult gliomas. Additionally, these pediatric brain tumors displayed diverse immunophenotypes, particularly in regard to myeloid cells. Notably, the presence of HLA-enriched myeloid cells in MB was found to be independently associated with prognosis. Moreover, the scFes, when compared with commonly used gene features, demonstrated superior performance in independent single-cell datasets across various tumor types. Furthermore, our study revealed the existence of heterogeneous immune ecosystems at the bulk-RNA sequencing level among different brain tumor types. In addition, we identified several immune-stromal-tumor patterns that could potentially exhibit significant responses to conventional immune checkpoint inhibitors.

Conclusion

The single-cell technique provides a rational path to deeply understand the unique immune ecosystem of pediatric brain tumors. In spite of the traditional attitudes of "cold" tumor towards pediatric brain tumor, the immune-stroma-tumor patterns identified in this study suggest the feasibility of immune checkpoint inhibitors and pave the way for the upcoming tide of immunotherapy in pediatric brain tumors.

Microglia in pediatric brain tumors: The missing link to successful immunotherapy

Brain tumors are the leading cause of cancer-related mortality in children. Despite the development of immunotherapeutic strategies for adult brain tumors, progress in pediatric neuro-oncology has been hindered by the complex and poorly understood nature of the brain's immune system during early development, a phase that is critical for the onset of many pediatric brain tumors. A defining characteristic of these tumors is the abundance of microglia, the resident immune cells of the central nervous system. In this review, we explore the concept of microglial diversity across brain regions and throughout development and discuss how their maturation stage may contribute to tumor growth in children. We also summarize the current knowledge on the roles of microglia in common pediatric brain tumor entities and provide examples of myeloid-based immunotherapeutic strategies. Our review underscores the importance of microglial plasticity in pediatric brain tumors and its significance for developing effective immunotherapeutic strategies.

GRP78-CAR T cell effector function against solid and brain tumors is controlled by GRP78 expression on T cells

Lack of targetable antigens is a key limitation for developing successful T cell-based immunotherapies. Members of the unfolded protein response (UPR) represent ideal immunotherapy targets because the UPR regulates the ability of cancer cells to resist cell death, sustain proliferation, and metastasize. Glucose-regulated protein 78 (GRP78) is a key UPR regulator that is overexpressed and translocated to the cell surface of a wide variety of cancers in response to elevated endoplasmic reticulum (ER) stress. We show that GRP78 is highly expressed on the cell surface of multiple solid and brain tumors, making cell surface GRP78 a promising chimeric antigen receptor (CAR) T cell target. We demonstrate that GRP78-CAR T cells can recognize and kill GRP78+ brain and solid tumors in vitro and in vivo. Additionally, our findings demonstrate that GRP78 is upregulated on CAR T cells upon T cell activation; however, this expression is tumor-cell-line specific and results in heterogeneous GRP78-CAR T cell therapeutic response.

TIM-3 blockade in diffuse intrinsic pontine glioma models promotes tumor regression and antitumor immune memory

Diffuse intrinsic pontine glioma (DIPG) is an aggressive brain stem tumor and the leading cause of pediatric cancer-related death. To date, these tumors remain incurable, underscoring the need for efficacious therapies. In this study, we demonstrate that the immune checkpoint TIM-3 (HAVCR2) is highly expressed in both tumor cells and microenvironmental cells, mainly microglia and macrophages, in DIPG. We show that inhibition of TIM-3 in syngeneic models of DIPG prolongs survival and produces long-term survivors free of disease that harbor immune memory. This antitumor effect is driven by the direct effect of TIM-3 inhibition in tumor cells, the coordinated action of several immune cell populations, and the secretion of chemokines/cytokines that create a proinflammatory tumor microenvironment favoring a potent antitumor immune response. This work uncovers TIM-3 as a bona fide target in DIPG and supports its clinical translation.

The tumor micro-environment in pediatric glioma: friend or foe?

Brain tumors are the leading cause of morbidity and mortality related to cancer in children, where high-grade glioma harbor the worst prognosis. It has become obvious that pediatric glioma differs significantly from their adult counterparts, rendering extrapolations difficult. Curative options for several types of glioma are lacking, albeit ongoing research efforts and clinical trials. As already proven in the past, inter- and intratumoral heterogeneity plays an important role in the resistance to therapy and thus implicates morbidity and mortality for these patients. However, while less studied, the tumor micro-environment (TME) adds another level of heterogeneity. Knowledge gaps exist on how the TME interacts with the tumor cells and how the location of the various cell types in the TME influences tumor growth and the response to treatment. Some studies identified the presence of several (immune) cell types as prognostic factors, but often lack a deeper understanding of the underlying mechanisms, possibly leading to contradictory findings. Although the TME in pediatric glioma is regarded as "cold", several treatment options are emerging, with the TME being the primary target of treatment. Therefore, it is crucial to study the TME of pediatric glioma, so that the interactions between TME, tumoral cells and therapeutics can be better understood before, during and after treatment. In this review, we provide an overview of the available insights into the composition and role of the TME across different types of pediatric glioma. Moreover, where possible, we provide a framework on how a particular TME may influence responses to conventional- and/or immunotherapy.

Decoding Diffuse Midline Gliomas: A Comprehensive Review of Pathogenesis, Diagnosis and Treatment

Diffuse midline gliomas (DMGs) are a group of aggressive CNS tumors, primarily affecting children and young adults, which have historically been associated with dismal outcomes. As the name implies, they arise in midline structures in the CNS, primarily in the thalamus, brainstem, and spinal cord. In more recent years, significant advances have been made in our understanding of DMGs, including molecular features, with the identification of potential therapeutic targets. We aim to provide an overview of the most recent updates in the field of DMGs, including classification, molecular subtypes, diagnostic techniques, and emerging therapeutic strategies including a review of the ongoing clinical trials, thus providing the treating multidisciplinary team with a comprehensive understanding of the current landscape and potential therapeutic strategies for this devastating group of tumors.

CAR T cell therapies for diffuse midline glioma

Diffuse midline glioma (DMG) is a fatal pediatric cancer of the central nervous system (CNS). The location and infiltrative nature of DMG prevents surgical resection and the benefits of palliative radiotherapy are temporary; median overall survival (OS) is 9-11 months. The tumor immune microenvironment (TIME) is 'cold', and has a dominant immunosuppressive myeloid compartment with low levels of infiltrating lymphocytes and proinflammatory molecules. Because survival statistics have been stagnant for many decades, and therapies targeting the unique biology of DMG are urgently needed, this has prompted the clinical assessment of chimeric antigen receptor (CAR) T cell therapies in this setting. We highlight the current landscape of CAR T cell therapy for DMG, the role the TIME may play in the response, and strategies to overcome treatment obstacles.

ELTD1 Review: New Regulator of Angiogenesis in Glioma

Glioblastoma (GBM) is a severe brain cancer in which angiogenesis is controlled by G protein-coupled receptors (GPCRs), such as Epidermal Growth Factor Latrophilin and seven transmembrane domain-containing protein 1 (ELTD1), which are crucial for tumor progression. ELTD1 is an understudied GPCR with a broad expression profile in various tissues, including the human brain, especially in the cerebral cortex. It plays a significant role in angiogenesis and tumorigenesis and is regulated by interconnected VEGF and DLL4/Notch pathways. ELTD1 also modulates the JAK/STAT3/HIF-1α signaling axis, affecting the response of cells to low-oxygen conditions and promoting cell proliferation. However, their specific ligands and functional mechanisms remain unclear. ELTD1 expression is associated with different outcomes in various cancers. For example, in GBM, higher ELTD1 levels are linked to more mature and less leaky blood vessels, potentially enhancing drug delivery and therapeutic success. It also has divergent prognostic implications in renal, ovarian, and colorectal cancer. Additionally, ELTD1 overexpression in central nervous system endothelial cells suggests that it is a potential biomarker for multiple sclerosis. Therapeutically, blocking ELTD1 inhibits vessel formation, possibly slowing tumor growth. Initial therapies used polyclonal antibodies, but the shift has been towards more targeted monoclonal antibodies, particularly in preclinical glioma models. This review aimed to translate these insights into effective clinical treatments. However, several gaps remain in our knowledge regarding ELTD1 ligands and their potential involvement in other physiological or pathological processes that future research can address to elucidate the role of ELTD1 in cancer, through angiogenesis and other intracellular pathways.

In vitro vascular differentiation system efficiently produces natural killer cells for cancer immunotherapies

Background

Immunotherapeutic innovation is crucial for limited operability tumors. CAR T-cell therapy displayed reduced efficiency against glioblastoma (GBM), likely due to mutations underlying disease progression. Natural Killer cells (NKs) detect cancer cells despite said mutations - demonstrating increased tumor elimination potential. We developed an NK differentiation system using human pluripotent stem cells (hPSCs). Via this system, genetic modifications targeting cancer treatment challenges can be introduced during pluripotency - enabling unlimited production of modified "off-the-shelf" hPSC-NKs.

Methods

hPSCs were differentiated into hematopoietic progenitor cells (HPCs) and NKs using our novel organoid system. These cells were characterized using flow cytometric and bioinformatic analyses. HPC engraftment potential was assessed using NSG mice. NK cytotoxicity was validated using in vitro and in vitro K562 assays and further corroborated on lymphoma, diffuse intrinsic pontine glioma (DIPG), and GBM cell lines in vitro.

Results

HPCs demonstrated engraftment in peripheral blood samples, and hPSC-NKs showcased morphology and functionality akin to same donor peripheral blood NKs (PB-NKs). The hPSC-NKs also displayed potential advantages regarding checkpoint inhibitor and metabolic gene expression, and demonstrated in vitro and in vivo cytotoxicity against various cancers.

Conclusions

Our organoid system, designed to replicate in vivo cellular organization (including signaling gradients and shear stress conditions), offers a suitable environment for HPC and NK generation. The engraftable nature of HPCs and potent NK cytotoxicity against leukemia, lymphoma, DIPG, and GBM highlight the potential of this innovative system to serve as a valuable tool that will benefit cancer treatment and research - improving patient survival and quality of life.

Hallmarks of the Tumour Microenvironment of Gliomas and Its Interaction with Emerging Immunotherapy Modalities

Gliomas are aggressive, primary central nervous system tumours arising from glial cells. Glioblastomas are the most malignant. They are known for their poor prognosis or median overall survival. The current standard of care is overwhelmed by the heterogeneous, immunosuppressive tumour microenvironment promoting immune evasion and tumour proliferation. The advent of immunotherapy with its various modalities-immune checkpoint inhibitors, cancer vaccines, oncolytic viruses and chimeric antigen receptor T cells and NK cells-has shown promise. Clinical trials incorporating combination immunotherapies have overcome the microenvironment resistance and yielded promising survival and prognostic benefits. Rolling these new therapies out in the real-world scenario in a low-cost, high-throughput manner is the unmet need of the hour. These will have practice-changing implications to the glioma treatment landscape. Here, we review the immunobiological hallmarks of the TME of gliomas, how the TME evades immunotherapies and the work that is being conducted to overcome this interplay.

Classification of Brainstem Gliomas Based on Tumor Microenvironment Status

The inter-tumor heterogeneity of the tumor microenvironment (TME) and how it correlates with clinical profiles and biological characteristics in brainstem gliomas (BSGs) remain unknown, dampening the development of novel therapeutics against BSGs. The TME status was determined with a list of pan-cancer conserved gene expression signatures using a single-sample gene set enrichment analysis (ssGSEA) and was subsequently clustered via consensus clustering. BSGs exhibited a high inter-tumor TME heterogeneity and were classified into four clusters: "immune-enriched, fibrotic", "immune-enriched, non-fibrotic", "fibrotic", and "depleted". The "fibrotic" cluster had a higher proportion of diffuse intrinsic pontine gliomas (p = 0.041), and "PA-like" tumors were more likely to be "immune-enriched, fibrotic" (p = 0.044). The four TME clusters exhibited distinct overall survival (p < 0.001) and independently impacted BSG outcomes. A four-gene panel as well as a radiomics approach were constructed to identify the TME clusters and achieved high accuracy for determining the classification. Together, BSGs exhibited high inter-tumor heterogeneity in the TME and were classified into four clusters with distinct clinical outcomes and tumor biological properties. The TME classification was accurately identified using a four-gene panel that can potentially be examined with the immunohistochemical method and a non-invasive radiomics method, facilitating its clinical application.

Therapeutic avenues for targeting treatment challenges of diffuse midline gliomas

Diffuse midline glioma (DMG) is the leading cause of brain tumor-related deaths in children. DMG typically presents with variable neurologic symptoms between ages 3 and 10. Currently, radiation remains the standard therapy for DMG to halt progression and reduce tumor bulk to minimize symptoms. However, tumors recur in almost 100% of patients and thus, DMG is still considered an incurable cancer with a median survival of 9-12 months. Surgery is generally contraindicated due to the delicate organization of the brainstem, where DMG is located. Despite extensive research efforts, no chemotherapeutic agents, immune therapies, or molecularly targeted therapies have been approved to provide survival benefit. Furthermore, the efficacy of therapies is limited by poor blood-brain barrier penetration and inherent resistance mechanisms of the tumor. However, novel drug delivery approaches, along with recent advances in molecularly targeted therapies and immunotherapies, have advanced to clinical trials and may provide viable future treatment options for DMG patients. This review seeks to evaluate current therapeutics at the preclinical stage and those that have advanced to clinical trials and to discuss the challenges of drug delivery and inherent resistance to these therapies.