Dynamic changes in glioma macrophage populations after radiotherapy reveal CSF-1R inhibition as a strategy to overcome resistance

Therapeutic Targeting of the Tumor Microenvironment

Strategies to therapeutically target the tumor microenvironment (TME) have emerged as a promising approach for cancer treatment in recent years due to the critical roles of the TME in regulating tumor progression and modulating response to standard-of-care therapies. Here, we summarize the current knowledge regarding the most advanced TME-directed therapies, which have either been clinically approved or are currently being evaluated in trials, including immunotherapies, antiangiogenic drugs, and treatments directed against cancer-associated fibroblasts and the extracellular matrix. We also discuss some of the challenges associated with TME therapies, and future perspectives in this evolving field. SIGNIFICANCE: This review provides a comprehensive analysis of the current therapies targeting the TME, combining a discussion of the underlying basic biology with clinical evaluation of different therapeutic approaches, and highlighting the challenges and future perspectives.

Multifaceted microglia - key players in primary brain tumour heterogeneity

Microglia are the resident innate immune cells of the immune-privileged CNS and, as such, represent the first line of defence against tissue injury and infection. Given their location, microglia are undoubtedly the first immune cells to encounter a developing primary brain tumour. Our knowledge of these cells is therefore important to consider in the context of such neoplasms. As the heterogeneous nature of the most aggressive primary brain tumours is thought to underlie their poor prognosis, this Review places a special emphasis on the heterogeneity of the tumour-associated microglia and macrophage populations present in primary brain tumours. Where available, specific information on microglial heterogeneity in various types and subtypes of brain tumour is included. Emerging evidence that highlights the importance of considering the heterogeneity of both the tumour and of microglial populations in providing improved treatment outcomes for patients is also discussed.

Single-cell profiling of myeloid cells in glioblastoma across species and disease stage reveals macrophage competition and specialization

Glioblastomas are aggressive primary brain cancers that recur as therapy-resistant tumors. Myeloid cells control glioblastoma malignancy, but their dynamics during disease progression remain poorly understood. Here, we employed single-cell RNA sequencing and CITE-seq to map the glioblastoma immune landscape in mouse tumors and in patients with newly diagnosed disease or recurrence. This revealed a large and diverse myeloid compartment, with dendritic cell and macrophage populations that were conserved across species and dynamic across disease stages. Tumor-associated macrophages (TAMs) consisted of microglia- or monocyte-derived populations, with both exhibiting additional heterogeneity, including subsets with conserved lipid and hypoxic signatures. Microglia- and monocyte-derived TAMs were self-renewing populations that competed for space and could be depleted via CSF1R blockade. Microglia-derived TAMs were predominant in newly diagnosed tumors, but were outnumbered by monocyte-derived TAMs following recurrence, especially in hypoxic tumor environments. Our results unravel the glioblastoma myeloid landscape and provide a framework for future therapeutic interventions.

Perspective of mesenchymal transformation in glioblastoma

Despite aggressive multimodal treatment, glioblastoma (GBM), a grade IV primary brain tumor, still portends a poor prognosis with a median overall survival of 12–16 months. The complexity of GBM treatment mainly lies in the inter- and intra-tumoral heterogeneity, which largely contributes to the treatment-refractory and recurrent nature of GBM. By paving the road towards the development of personalized medicine for GBM patients, the cancer genome atlas classification scheme of GBM into distinct transcriptional subtypes has been considered an invaluable approach to overcoming this heterogeneity. Among the identified transcriptional subtypes, the mesenchymal subtype has been found associated with more aggressive, invasive, angiogenic, hypoxic, necrotic, inflammatory, and multitherapy-resistant features than other transcriptional subtypes. Accordingly, mesenchymal GBM patients were found to exhibit worse prognosis than other subtypes when patients with high transcriptional heterogeneity were excluded. Furthermore, identification of the master mesenchymal regulators and their downstream signaling pathways has not only increased our understanding of the complex regulatory transcriptional networks of mesenchymal GBM, but also has generated a list of potent inhibitors for clinical trials. Importantly, the mesenchymal transition of GBM has been found to be tightly associated with treatment-induced phenotypic changes in recurrence. Together, these findings indicate that elucidating the governing and plastic transcriptomic natures of mesenchymal GBM is critical in order to develop novel and selective therapeutic strategies that can improve both patient care and clinical outcomes. Thus, the focus of our review will be on the recent advances in the understanding of the transcriptome of mesenchymal GBM and discuss microenvironmental, metabolic, and treatment-related factors as critical components through which the mesenchymal signature may be acquired. We also take into consideration the transcriptomic plasticity of GBM to discuss the future perspectives in employing selective therapeutic strategies against mesenchymal GBM.

Patient-Derived Tumor Xenografts Plus Ex Vivo Models Enable Drug Validation for Tenosynovial Giant Cell Tumors

INTRODUCTION

Tenosynovial giant cell tumor (TGCT) is a locally aggressive tumor with colony-stimulating factor 1 receptor (CSF1R) signal expression. However, there is a lack of better in vivo and ex vivo models for TGCT. This study aims to establish a favorable preclinical translational platform, which would enable the validation of efficient and personalized therapeutic candidates for TGCT.

PATIENTS AND METHODS

Histological analyses were performed for the included patients. Fresh TGCT tumors were collected and sliced into 1.0-3.0 mm³ sections using a sterilized razor blade. The tumor grafts were surgically implanted into subrenal capsules of athymic mice to establish patient-derived tumor xenograft (PDTX) mouse models. Histological and response patterns to CSF1R inhibitors evaluations were analyzed. In addition, ex vivo cultures of patient-derived explants (PDEs) with endpoint analysis were used to validate TGCT graft response patterns to CSF1R inhibitors.

RESULTS

The TGCT tumor grafts that were implanted into athymic mice subrenal capsules maintained their original morphological and histological features. The "take" rate of this model was 95% (19/20). Administration of CSF1R inhibitors (PLX3397, and a novel candidate, WXFL11420306) to TGCT-PDTX mice was shown to reduce tumor size while inducing intratumoral apoptosis. In addition, the CSF1R inhibitors suppressed circulating nonspecific monocyte levels and CD163-positive cells within tumors. These response patterns of engrafts to PDTX were validated by ex vivo PDE cultures.

CONCLUSIONS

Subrenal capsule supports the growth of TGCT tumor grafts, maintaining their original morphology and histology. This TGCT-PDTX model plus ex vivo explant cultures is a potential preclinical translational platform for locally aggressive tumors, such as TGCT.

Radiotherapy: An immune response modifier for immuno-oncology

The ability of radiotherapy to enhance antigenicity and adjuvanticity of an irradiated tumor has stimulated the interest for its combination with immuno-oncology agents. However, radiotherapy often generates multiple layers of host responses which likely depends on the tumor biology, the immune cell infiltration and the induction of immunosuppressive signals post radiotherapy. Consequently, translation of preclinical findings to the clinic is more convoluted than anticipated which underscore the need to decipher molecular and cellular mechanisms elicited by radiotherapy. Here we review pro-inflammatory and immunosuppressive mechanisms triggered by radiotherapy that impact the outcome of antigen specific T cell killing and discuss how radiation-induced immunostimulatory mechanisms can be exploited to reactivate the host's immune system, especially in the context of immunotherapy.

The Role of Cytokines and Chemokines in Shaping the Immune Microenvironment of Glioblastoma: Implications for Immunotherapy

Glioblastoma is the most common form of primary brain tumour in adults. For more than a decade, conventional treatment has produced a relatively modest improvement in the overall survival of glioblastoma patients. The immunosuppressive mechanisms employed by neoplastic and non-neoplastic cells within the tumour can limit treatment efficacy, and this can include the secretion of immunosuppressive cytokines and chemokines. These factors can play a significant role in immune modulation, thus disabling anti-tumour responses and contributing to tumour progression. Here, we review the complex interplay between populations of immune and tumour cells together with defined contributions by key cytokines and chemokines to these intercellular interactions. Understanding how these tumour-derived factors facilitate the crosstalk between cells may identify molecular candidates for potential immunotherapeutic targeting, which may enable better tumour control and improved patient survival.

Allogeneic CAR T Cells: An Alternative to Overcome Challenges of CAR T Cell Therapy in Glioblastoma

Chimeric antigen receptor (CAR) T cell therapy has emerged as one of the major breakthroughs in cancer immunotherapy in the last decade. Outstanding results in hematological malignancies and encouraging pre-clinical anti-tumor activity against a wide range of solid tumors have made CAR T cells one of the most promising fields for cancer therapies. CAR T cell therapy is currently being investigated in solid tumors including glioblastoma (GBM), a tumor for which survival has only modestly improved over the past decades. CAR T cells targeting EGFRvIII, Her2, or IL-13Rα2 have been tested in GBM, but the first clinical trials have shown modest results, potentially due to GBM heterogeneity and to the presence of an immunosuppressive microenvironment. Until now, the use of autologous T cells to manufacture CAR products has been the norm, but this approach has several disadvantages regarding production time, cost, manufacturing delay and dependence on functional fitness of patient T cells, often reduced by the disease or previous therapies. Universal “off-the-shelf,” or allogeneic, CAR T cells is an alternative that can potentially overcome these issues, and allow for multiple modifications and CAR combinations to target multiple tumor antigens and avoid tumor escape. Advances in genome editing tools, especially via CRISPR/Cas9, might allow overcoming the two main limitations of allogeneic CAR T cells product, i.e., graft-vs.-host disease and host allorejection. Here, we will discuss how allogeneic CAR T cells could allow for multivalent approaches and alteration of the tumor microenvironment, potentially allowing the development of next generation therapies for the treatment of patients with GBM.

Single-cell map of diverse immune phenotypes in the acute myeloid leukemia microenvironment

BACKGROUND

Knowledge of immune cell phenotypes, function, and developmental trajectory in acute myeloid leukemia (AML) microenvironment is essential for understanding mechanisms of evading immune surveillance and immunotherapy response of targeting special microenvironment components.

METHODS

Using a single-cell RNA sequencing (scRNA-seq) dataset, we analyzed the immune cell phenotypes, function, and developmental trajectory of bone marrow (BM) samples from 16 AML patients and 4 healthy donors, but not AML blasts.

RESULTS

We observed a significant difference between normal and AML BM immune cells. Here, we defined the diversity of dendritic cells (DC) and macrophages in different AML patients. We also identified several unique immune cell types including T helper cell 17 (TH17)-like intermediate population, cytotoxic CD4+ T subset, T cell: erythrocyte complexes, activated regulatory T cells (Treg), and CD8+ memory-like subset. Emerging AML cells remodels the BM immune microenvironment powerfully, leads to immunosuppression by accumulating exhausted/dysfunctional immune effectors, expending immune-activated types, and promoting the formation of suppressive subsets.

CONCLUSION

Our results provide a comprehensive AML BM immune cell census, which can help to select pinpoint targeted drug and predict efficacy of immunotherapy.

IL-34 and CSF-1, deciphering similarities and differences at steady state and in diseases

Although IL-34 and CSF-1 share actions as key mediators of monocytes/macrophages survival and differentiation, they also display differences that should be identified to better define their respective roles in health and diseases. IL-34 displays low sequence homology with CSF-1 but has a similar general structure and they both bind to a common receptor CSF-1R, although binding and subsequent intracellular signaling shows differences. CSF-1R expression has been until now mainly described at a steady state in monocytes/macrophages and myeloid dendritic cells, as well as in some cancers. IL-34 has also 2 other receptors, protein-tyrosine phosphatase zeta (PTPζ) and CD138 (Syndecan-1), expressed in some epithelium, cells of the central nervous system (CNS), as well as in numerous cancers. While most, if not all, of CSF-1 actions are mediated through monocyte/macrophages, IL-34 has also other potential actions through PTPζ and CD138. Additionally, IL-34 and CSF-1 are produced by different cells in different tissues. This review describes and discusses similarities and differences between IL-34 and CSF-1 at steady state and in pathological situations and identifies possible ways to target IL-34, CSF-1, and its receptors.

The Irradiated Brain Microenvironment Supports Glioma Stemness and Survival via Astrocyte-Derived Transglutaminase 2

The tumor microenvironment plays an essential role in supporting glioma stemness and radioresistance. Following radiotherapy, recurrent gliomas form in an irradiated microenvironment. Here we report that astrocytes, when pre-irradiated, increase stemness and survival of cocultured glioma cells. Tumor-naïve brains increased reactive astrocytes in response to radiation, and mice subjected to radiation prior to implantation of glioma cells developed more aggressive tumors. Extracellular matrix derived from irradiated astrocytes were found to be a major driver of this phenotype and astrocyte-derived transglutaminase 2 (TGM2) was identified as a promoter of glioma stemness and radioresistance. TGM2 levels increased after radiation in vivo and in recurrent human glioma, and TGM2 inhibitors abrogated glioma stemness and survival. These data suggest that irradiation of the brain results in the formation of a tumor-supportive microenvironment. Therapeutic targeting of radiation-induced, astrocyte-derived extracellular matrix proteins may enhance the efficacy of standard-of-care radiotherapy by reducing stemness in glioma. SIGNIFICANCE: These findings presented here indicate that radiotherapy can result in a tumor-supportive microenvironment, the targeting of which may be necessary to overcome tumor cell therapeutic resistance and recurrence. GRAPHICAL ABSTRACT: http://cancerres.aacrjournals.org/content/canres/81/8/2101/F1.large.jpg.

Molecular Mechanisms of Treatment Resistance in Glioblastoma

Glioblastoma is the most common malignant primary brain tumor in adults and is almost invariably fatal. Despite our growing understanding of the various mechanisms underlying treatment failure, the standard-of-care therapy has not changed over the last two decades, signifying a great unmet need. The challenges of treating glioblastoma are many and include inadequate drug or agent delivery across the blood-brain barrier, abundant intra- and intertumoral heterogeneity, redundant signaling pathways, and an immunosuppressive microenvironment. Here, we review the innate and adaptive molecular mechanisms underlying glioblastoma's treatment resistance, emphasizing the intrinsic challenges therapeutic interventions must overcome-namely, the blood-brain barrier, tumoral heterogeneity, and microenvironment-and the mechanisms of resistance to conventional treatments, targeted therapy, and immunotherapy.

Myeloid Cells in Glioblastoma Microenvironment

Glioblastoma (GBM) is the most aggressive, malignant primary brain tumor in adults. GBM is notoriously resistant to immunotherapy mainly due to its unique immune microenvironment. High dimensional data analysis reveals the extensive heterogeneity of immune components making up the GBM microenvironment. Myeloid cells are the most predominant contributors to the GBM microenvironment; these cells are critical regulators of immune and therapeutic responses to GBM. Here, we will review the most recent advances on the characteristics and functions of different populations of myeloid cells in GBM, including bone marrow-derived macrophages, microglia, myeloid-derived suppressor cells, dendritic cells, and neutrophils. Epigenetic, metabolic, and phenotypic peculiarities of microglia and bone marrow-derived macrophages will also be assessed. The final goal of this review will be to provide new insights into novel therapeutic approaches for specific targeting of myeloid cells to improve the efficacy of current treatments in GBM patients.

Environmentally Self-Adaptative Nanocarriers Suppress Glioma Proliferation and Stemness via Codelivery of shCD163 and Doxorubicin

Gliomas-devastating intracranial tumors with a dismal outcome-are in dire need of innovative treatment. Although nanodrugs have been utilized as a target therapy for certain types of solid tumors, their therapeutic effects in gliomas are limited due to the complications of the systemic circulation, blood-brain barrier (BBB), and specific glioma environment. Thus, we aimed to establish a nanoliposome adaptable to different environments by codelivery of shCD163 and doxorubicin (DOX) to treat gliomas. In this study, we first synthesized pH-sensitive DSPE-cRGD-Hz-PEG₂₀₀₀ to form an environmentally self-adaptative nanoliposome (cRGD-DDD Lip) via a thin film method. We used in vitro BBB models, in vitro cell uptake experiments, and in vivo biodistribution assays to confirm the long circulation time and low cell uptake of the cRGD-DDD Lip as a result of the poly(ethylene glycol) (PEG) shell of cRGD-DDD Lip in the neutral pH systemic circulation. Moreover, the cRGD-DDD Lip bypassed the BBB and attached to the intracranial glioma following the removal of the PEG shell and the exposure of cRGD to the weakly acidic tumor microenvironment. We further assembled the shCD163/DOX@cRGD-DDD Lip through cRGD-DDD Lip loading of shCD163 and DOX. In vitro, cell proliferation and self-renewal of glioma cells were inhibited by the shCD163/DOX@cRGD-DDD Lip due to the toxicity of DOX and the suppression of shCD163 via the CD163 pathway. In vivo, the shCD163/DOX@cRGD-DDD Lip disturbed the progression of in situ gliomas by inhibiting the growth and stemness of glioma cells and prevented the recurrence of gliomas after resection. In conclusion, the cRGD-DDD Lip may be a promising nanodrug-loading platform to cope with different environments and the shCD163/DOX@cRGD-DDD Lip may potentially be a novel nanodrug for glioma therapy.

Interactions between Tumor Cells, Neurons, and Microglia in the Glioma Microenvironment

Despite significant strides made in understanding the pathophysiology of high-grade gliomas over the past two decades, most patients succumb to these neoplasias within two years of diagnosis. Furthermore, there are various co-morbidities associated with glioma and standard of care treatments. Emerging evidence suggests that aberrant glutamate secretion in the glioma microenvironment promotes tumor progression and contributes to the development of co-morbidities, such as cognitive defects, epilepsy, and widespread neurodegeneration. Recent data clearly illustrate that neurons directly synapse onto glioma cells and drive their proliferation and spread via glutamatergic action. Microglia are central nervous system-resident myeloid cells, modulate glioma growth, and possess the capacity to prune synapses and encourage synapse formation. However, current literature has yet to investigate the potential role of microglia in shaping synapse formation between neurons and glioma cells. Herein, we present the literature concerning glutamate's role in glioma progression, involving hyperexcitability and excitotoxic cell death of peritumoral neurons and stimulation of glioma proliferation and invasion. Furthermore, we discuss instances in which microglia are more likely to sculpt or encourage synapse formation during glioma treatment and propose studies to delineate the role of microglia in synapse formation between neurons and glioma cells. The sex-dependent oncogenic or oncolytic actions of microglia and myeloid cells, in general, are considered in addition to the functional differences between microglia and macrophages in tumor progression. We also put forth tractable methods to safely perturb aberrant glutamatergic action in the tumor microenvironment without significantly increasing the toxicities of the standard of care therapies for glioma therapy.