Monitoring innate immune cell dynamics in the glioma microenvironment by magnetic resonance imaging and multiphoton microscopy (MR-MPM).

Rationale: Glioblastoma is the most frequent, primary brain tumor that is characterized by a highly immunosuppressive tumor microenvironment (TME). The TME plays a key role for tumor biology and the effectiveness of immunotherapies. Composition of the TME correlates with overall survival and governs therapy response. Non invasive assessment of the TME has been notoriously difficult. Methods: We have designed an in vivo imaging approach to non invasively visualize innate immune cell dynamics in the TME in a mouse glioma model by correlated MRI and multiphoton microscopy (MR-MPM) using a bimodal, fluorescently labeled iron oxide nanoparticle (NP). The introduction of Teflon cranial windows instead of conventional Titanium rings dramatically reduced susceptibility artifacts on MRI and allowed longitudinal MR-MPM imaging for innate immune cell tracking in the same animal. Results: We visualized tumor associated macrophage and microglia (TAM) dynamics in the TME and dissect the single steps of NP uptake by blood-born monocytes that give rise to tumor-associated macrophages. Next to peripheral NP-loading, we identified a second route of direct nanoparticle uptake via the disrupted blood-brain barrier to directly label tissue resident TAMs. Conclusion: Our approach allows innate immune cell tracking by MRI and multiphoton microscopy in the same animal to longitudinally investigate innate immune cell dynamics in the TME.

Dietary tryptophan links encephalogenicity of autoreactive T cells with gut microbial ecology.

The interaction between the mammalian host and its resident gut microbiota is known to license adaptive immune responses. Nutritional constituents strongly influence composition and functional properties of the intestinal microbial communities. Here, we report that omission of a single essential amino acid - tryptophan - from the diet abrogates CNS autoimmunity in a mouse model of multiple sclerosis. Dietary tryptophan restriction results in impaired encephalitogenic T cell responses and is accompanied by a mild intestinal inflammatory response and a profound phenotypic shift of gut microbiota. Protective effects of dietary tryptophan restriction are abrogated in germ-free mice, but are independent of canonical host sensors of intracellular tryptophan metabolites. We conclude that dietary tryptophan restriction alters metabolic properties of gut microbiota, which in turn have an impact on encephalitogenic T cell responses. This link between gut microbiota, dietary tryptophan and adaptive immunity may help to develop therapeutic strategies for protection from autoimmune neuroinflammation.

MRI of Iron Oxide Nanoparticles and Myeloperoxidase Activity Links Inflammation to Brain Edema in Experimental Cerebral Malaria.

Purpose To investigate the association of inflammation and brain edema in a cerebral malaria (CM) mouse model with a combination of bis-5-hydroxy-tryptamide-diethylenetriaminepentaacetate gadolinium, referred to as MPO-Gd, and cross-linked iron oxide nanoparticle (CLIO-NP) imaging. Materials and Methods Female wild-type (n = 23) and myeloperoxidase (MPO) knock-out (n = 5) mice were infected with the Plasmodium berghei ANKA strain from May 2016 to July 2018. Seven healthy mice served as control animals. At a Rapid Murine Coma and Behavioral Scale (RMCBS) score of less than 15, mice underwent MRI at 9.4 T and received gadodiamide, MPO-Gd, or CLIO-NPs. T1-weighted MRI was used to assess MPO activity, and T2*-weighted MRI was used to track CLIO-NPs. Immunofluorescent staining and flow cytometric analyses characterized CLIO-NPs, MPO, endothelial cells, and leukocytes. An unpaired, two-tailed Student t test was used to compare groups; Spearman correlation analysis was used to determine the relationship of imaging parameters to clinical severity. Results MPO-Gd enhancement occurred in inflammatory CM hotspots (olfactory bulb > rostral migratory stream > brainstem > cortex, P < .05 for all regions compared with control mice; mean olfactory bulb signal intensity ratio: 1.40 ± 0.07 vs 0.96 ± 0.01, P < .01). The enhancement was reduced in MPO knockout mice (mean signal intensity ratio at 60 minutes: 1.13 ± 0.04 vs 1.40 ± 0.07 in CM, P < .05). Blood-brain barrier compromise was suggested by parenchymal gadolinium enhancement, leukocyte recruitment, and endothelial activation. CLIO-NPs accumulated mainly intravascularly and at the vascular endothelium. CLIO-NPs were also found in the choroid plexus, indicating inflammation of the ventricular system. Blood-cerebrospinal fluid barrier breakdown showed correlation with brain swelling (r2: 0.55, P < .01) and RMCBS score (r2: 0.75, P < .001). Conclusion Iron oxide nanoparticle imaging showed strong inflammatory involvement of the microvasculature in a murine model of cerebral malaria. Furthermore, bis-5-hydroxy-tryptamide-diethylenetriaminepentaacetate gadolinium imaging depicted parenchymal and intraventricular inflammation. This combined molecular imaging approach links vascular inflammation to breakdown of the blood-brain barrier and blood-cerebrospinal fluid barrier that correlate with global brain edema and disease severity. © RSNA, 2018 Online supplemental material is available for this article. See also the editorial by Kiessling in this issue.

Targeting PDGF-mediated recruitment of pericytes blocks vascular mimicry and tumor growth.

Aggressive tumor cells can adopt an endothelial cell-like phenotype and contribute to the formation of a tumor vasculature, independent of tumor angiogenesis. This adoptive mechanism is referred to as vascular mimicry and it is associated with poor survival in cancer patients. To what extent tumor cells capable of vascular mimicry phenocopy the angiogenic cascade is still poorly explored. Here, we identify pericytes as important players in vascular mimicry. We found that pericytes are recruited by vascular mimicry-positive tumor cells in order to facilitate sprouting and to provide structural support of the vascular-like networks. The pericyte recruitment is mediated through platelet-derived growth factor (PDGF)-B. Consequently, preventing PDGF-B signaling by blocking the PDGF receptors with either the small tyrosine kinase inhibitor imatinib or blocking antibodies inhibits vascular mimicry and tumor growth. Collectively, the current study identifies an important role for pericytes in the formation of vascular-like structures by tumor cells. Moreover, the mechanism that controls the pericyte recruitment provides therapeutic opportunities for patients with aggressive vascular mimicry-positive cancer types. © 2018 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

Suppression of antitumor T cell immunity by the oncometabolite (R)-2-hydroxyglutarate.

The oncometabolite (R)-2-hydroxyglutarate (R-2-HG) produced by isocitrate dehydrogenase (IDH) mutations promotes gliomagenesis via DNA and histone methylation. Here, we identify an additional activity of R-2-HG: tumor cell-derived R-2-HG is taken up by T cells where it induces a perturbation of nuclear factor of activated T cells transcriptional activity and polyamine biosynthesis, resulting in suppression of T cell activity. IDH1-mutant gliomas display reduced T cell abundance and altered calcium signaling. Antitumor immunity to experimental syngeneic IDH1-mutant tumors induced by IDH1-specific vaccine or checkpoint inhibition is improved by inhibition of the neomorphic enzymatic function of mutant IDH1. These data attribute a novel, non-tumor cell-autonomous role to an oncometabolite in shaping the tumor immune microenvironment.

K27M-mutant histone-3 as a novel target for glioma immunotherapy.

Mutation-specific vaccines have become increasingly important in glioma immunotherapy; however, shared neoepitopes are rare. For diffuse gliomas, a driver mutation in the gene for isocitrate dehydrogenase type-1 has been shown to produce an immunogenic epitope currently targeted in clinical trials. For highly aggressive midline gliomas, a recurrent point mutation in the histone-3 gene (H3F3A) causes an amino acid change from lysine to methionine at position 27 (K27M). Here, we demonstrate that a peptide vaccine against K27M-mutant histone-3 is capable of inducing effective, mutation-specific, cytotoxic T-cell- and T-helper-1-cell-mediated immune responses in a major histocompatibility complex (MHC)-humanized mouse model. By proving an immunologically effective presentation of the driver mutation H3K27M on MHC class II in human H3K27M-mutant gliomas, our data provide a basis for the further clinical development of vaccine-based or cell-based immunotherapeutic approaches targeting H3K27M.

Tweety-Homolog 1 Drives Brain Colonization of Gliomas.

Early and progressive colonization of the healthy brain is one hallmark of diffuse gliomas, including glioblastomas. We recently discovered ultralong (>10 to hundreds of microns) membrane protrusions [tumor microtubes (TMs)] extended by glioma cells. TMs have been associated with the capacity of glioma cells to effectively invade the brain and proliferate. Moreover, TMs are also used by some tumor cells to interconnect to one large, resistant multicellular network. Here, we performed a correlative gene-expression microarray and in vivo imaging analysis, and identified novel molecular candidates for TM formation and function. Interestingly, these genes were previously linked to normal CNS development. One of the genes scoring highest in tests related to the outgrowth of TMs was tweety-homolog 1 (TTYH1), which was highly expressed in a fraction of TMs in mice and patients. Ttyh1 was confirmed to be a potent regulator of normal TM morphology and of TM-mediated tumor-cell invasion and proliferation. Glioma cells with one or two TMs were mainly responsible for effective brain colonization, and Ttyh1 downregulation particularly affected this cellular subtype, resulting in reduced tumor progression and prolonged survival of mice. The remaining Ttyh1-deficient tumor cells, however, had more interconnecting TMs, which were associated with increased radioresistance in those small tumors. These findings imply a cellular and molecular heterogeneity in gliomas regarding formation and function of distinct TM subtypes, with multiple parallels to neuronal development, and suggest that Ttyh1 might be a promising target to specifically reduce TM-associated brain colonization by glioma cells in patients.SIGNIFICANCE STATEMENT In this report, we identify tweety-homolog 1 (Ttyh1), a membrane protein linked to neuronal development, as a potent driver of tumor microtube (TM)-mediated brain colonization by glioma cells. Targeting of Ttyh1 effectively inhibited the formation of invasive TMs and glioma growth, but increased network formation by intercellular TMs, suggesting a functional and molecular heterogeneity of the recently discovered TMs with potential implications for future TM-targeting strategies.

In vivo nanoparticle imaging of innate immune cells can serve as a marker of disease severity in a model of multiple sclerosis.

Innate immune cells play a key role in the pathogenesis of multiple sclerosis and experimental autoimmune encephalomyelitis (EAE). Current clinical imaging is restricted to visualizing secondary effects of inflammation, such as gliosis and blood-brain barrier disruption. Advanced molecular imaging, such as iron oxide nanoparticle imaging, can allow direct imaging of cellular and molecular activity, but the exact cell types that phagocytose nanoparticles in vivo and how phagocytic activity relates to disease severity is not well understood. In this study we used MRI to map inflammatory infiltrates using high-field MRI and fluorescently labeled cross-linked iron oxide nanoparticles for cell tracking. We confirmed nanoparticle uptake and MR detectability ex vivo. Using in vivo MRI, we identified extensive nanoparticle signal in the cerebellar white matter and circumscribed cortical gray matter lesions that developed during the disease course (4.6-fold increase of nanoparticle accumulation in EAE compared with healthy controls, P < 0.001). Nanoparticles showed good cellular specificity for innate immune cells in vivo, labeling activated microglia, infiltrating macrophages, and neutrophils, whereas there was only sparse uptake by adaptive immune cells. Importantly, nanoparticle signal correlated better with clinical disease than conventional gadolinium (Gd) imaging (r, 0.83 for nanoparticles vs. 0.71 for Gd-imaging, P < 0.001). We validated our approach using the Food and Drug Administration-approved iron oxide nanoparticle ferumoxytol. Our results show that noninvasive molecular imaging of innate immune responses can serve as an imaging biomarker of disease activity in autoimmune-mediated neuroinflammation with potential clinical applications in a wide range of inflammatory diseases.

Correlated magnetic resonance imaging and ultramicroscopy (MR-UM) is a tool kit to assess the dynamics of glioma angiogenesis.

Neoangiogenesis is a pivotal therapeutic target in glioblastoma. Tumor monitoring requires imaging methods to assess treatment effects and disease progression. Until now mapping of the tumor vasculature has been difficult. We have developed a combined magnetic resonance and optical toolkit to study neoangiogenesis in glioma models. We use in vivo magnetic resonance imaging (MRI) and correlative ultramicroscopy (UM) of ex vivo cleared whole brains to track neovascularization. T2* imaging allows the identification of single vessels in glioma development and the quantification of neovessels over time. Pharmacological VEGF inhibition leads to partial vascular normalization with decreased vessel caliber, density, and permeability. To further resolve the tumor microvasculature, we performed correlated UM of fluorescently labeled microvessels in cleared brains. UM resolved typical features of neoangiogenesis and tumor cell invasion with a spatial resolution of ~5 µm. MR-UM can be used as a platform for three-dimensional mapping and high-resolution quantification of tumor angiogenesis.

Tryptophan-2,3-dioxygenase is regulated by prostaglandin E2 in malignant glioma via a positive signaling loop involving prostaglandin E receptor-4.

Malignant gliomas and other types of tumors generate a local immunosuppressive microenvironment, which prohibits an effective anti-tumor immune response and promotes tumor growth. Along with others, we have recently demonstrated that catabolism of the essential amino acid tryptophan via tryptophan-2,3-dioxygenase (TDO) is an important mechanism mediating tumor-associated immunosuppression particularly in gliomas. The pathways regulating TDO in tumors, however, are poorly understood. Here, we show that prostaglandins enhance TDO expression and enzymatic activity in malignant gliomas via activation of prostaglandin E receptor-4 (EP4). Stimulation with prostaglandin E2 (PGE2 ) up-regulated TDO-mediated kynurenine release in human glioma cell lines, whereas knockdown of the PGE2 receptor EP4 inhibited TDO expression and activity. In human malignant glioma tissue expression of the PGE2 -producing enzyme cyclooxygenase-2 (COX2) and its receptor EP4 were associated with TDO expression both on transcript and protein level. High expression of EP4 correlated with poor survival in malignant glioma patients WHO III-IV. Importantly, treatment of glioma cells with an EP4 inhibitor decreased TDO expression and activity. Moreover, TDO-over-expressing murine gliomas showed increased COX2 and EP4 expression suggesting a positive feedback mechanism in vivo. In summary, targeting EP4 may inhibit - in addition to immunosuppressive COX2 signaling - tryptophan degradation as another important immunosuppressive pathway and thus, could provide a dual clinically relevant immunotherapeutic avenue for the treatment of malignant gliomas. We proposed that in malignant gliomas prostaglandin E2 (PGE2 ) produced by cyclooxygenases (COX) up-regulates tryptophan-2,3-dioxygenase (TDO) expression and enzyme activity through binding to its Gs-coupled receptor EP4 and therefore may mediate tumor immune escape in part through aryl hydrocarbon receptor (AHR) activation. Moreover, TDO activity itself seems to induce intratumoral PGE2 metabolism suggesting an immunosuppressive loop involving COX/EP4/TDO.

The stress kinase GCN2 does not mediate suppression of antitumor T cell responses by tryptophan catabolism in experimental melanomas.

Tryptophan metabolism is a key process that shapes the immunosuppressive tumor microenvironment. The two rate-limiting enzymes that mediate tryptophan depletion, indoleamine-2,3-dioxygenase (IDO) and tryptophan-2,3-dioxygenase (TDO), have moved into the focus of research and inhibitors targeting IDO and TDO have entered clinical trials. Local tryptophan depletion is generally viewed as the crucial immunosuppressive mechanism. In T cells, the kinase general control non-derepressible 2 (GCN2) has been identified as a molecular sensor of tryptophan deprivation. GCN2 activation by tryptophan depletion induces apoptosis and mitigates T cell proliferation. Here, we investigated whether GCN2 attenuates tumor rejection in experimental B16 melanoma using T cell-specific Gcn2 knockout mice. Our data demonstrate that GCN2 in T cells did not affect immunity to B16 tumors even when animals were treated with antibodies targeting cytotoxic T lymphocyte antigen-4 (CTLA4). GCN2-deficient gp100 TCR-transgenic T cells were equally effective as wild-type pmel T cells against gp100-expressing B16 melanomas after adoptive transfer and gp100 peptide vaccination. Even augmentation of tumoral tryptophan metabolism in B16 tumors by lentiviral overexpression of Tdo did not differentially affect GCN2-proficient vs. GCN2-deficient T cells in vivo. Importantly, GCN2 target genes were not upregulated in tumor-infiltrating T cells. MALDI-TOF MS imaging of B16 melanomas demonstrated maintenance of intratumoral tryptophan levels despite high tryptophan turnover, which prohibits a drop in tryptophan sufficient to activate GCN2 in tumor-infiltrating T cells. In conclusion, our results do not suggest that suppression of antitumor immune responses by tryptophan metabolism is driven by local tryptophan depletion and subsequent GCN2-mediated T cell anergy.

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

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