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Mucosal-associated invariant T (MAIT) cells express semi-invariant T cell receptors (TCR) for recognizing bacterial and yeast antigens derived from riboflavin metabolites presented on the non-polymorphic MHC class I-related protein 1 (MR1). Neuroinflammation in multiple sclerosis (MS) is likely initiated by autoreactive T cells and perpetuated by infiltration of additional immune cells, but the precise role of MAIT cells in MS pathogenesis remains unknown. Here, we use experimental autoimmune encephalomyelitis (EAE), a mouse model of MS, and find an accumulation of MAIT cells in the inflamed central nervous system (CNS) enriched for MAIT17 (RORγt + ) and MAIT1/17 (T-bet + RORγt + ) subsets with inflammatory and protective features. Results from transcriptome profiling and Nur77GFP reporter mice show that these CNS MAIT cells are activated via cytokines and TCR. Blocking TCR activation with an anti-MR1 antibody exacerbates EAE, whereas enhancing TCR activation with the cognate antigen, 5-(2-oxopropylideneamino)-6-D-ribitylaminouracil, ameliorates EAE severity, potentially via the induction of amphiregulin (AREG). In summary, our findings suggest that TCR-mediated MAIT cell activation is protective in CNS inflammation, likely involving an induction of AREG., (© 2024. The Author(s).)

Inflammation-induced neurodegeneration is a defining feature of multiple sclerosis (MS), yet the underlying mechanisms remain unclear. By dissecting the neuronal inflammatory stress response, we discovered that neurons in MS and its mouse model induce the stimulator of interferon genes (STING). However, activation of neuronal STING requires its detachment from the stromal interaction molecule 1 (STIM1), a process triggered by glutamate excitotoxicity. This detachment initiates non-canonical STING signaling, which leads to autophagic degradation of glutathione peroxidase 4 (GPX4), essential for neuronal redox homeostasis and thereby inducing ferroptosis. Both genetic and pharmacological interventions that target STING in neurons protect against inflammation-induced neurodegeneration. Our findings position STING as a central regulator of the detrimental neuronal inflammatory stress response, integrating inflammation with glutamate signaling to cause neuronal cell death, and present it as a tractable target for treating neurodegeneration in MS., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2024 The Author(s). Published by Elsevier Inc. All rights reserved.)

A disturbed balance between excitation and inhibition (E/I balance) is increasingly recognized as a key driver of neurodegeneration in multiple sclerosis (MS), a chronic inflammatory disease of the central nervous system. To understand how chronic hyperexcitability contributes to neuronal loss in MS, we transcriptionally profiled neurons from mice lacking inhibitory metabotropic glutamate signaling with shifted E/I balance and increased vulnerability to inflammation-induced neurodegeneration. This revealed a prominent induction of the nuclear receptor NR4A2 in neurons. Mechanistically, NR4A2 increased susceptibility to excitotoxicity by stimulating continuous VGF secretion leading to glycolysis-dependent neuronal cell death. Extending these findings to people with MS (pwMS), we observed increased VGF levels in serum and brain biopsies. Notably, neuron-specific deletion of Vgf in a mouse model of MS ameliorated neurodegeneration. These findings underscore the detrimental effect of a persistent metabolic shift driven by excitatory activity as a fundamental mechanism in inflammation-induced neurodegeneration.

Background: Differences in immune responses between women and men are leading to a strong sex bias in the incidence of autoimmune diseases that predominantly affect women, such as multiple sclerosis (MS). MS manifests in more than twice as many women, making sex one of the most important risk factor. However, it is incompletely understood which genes contribute to sex differences in autoimmune incidence. To address that, we conducted a gene expression analysis in female and male human spleen and identified the transmembrane protein CD99 as one of the most significantly differentially expressed genes with marked increase in men. CD99 has been reported to participate in immune cell transmigration and T cell regulation, but sex-specific implications have not been comprehensively investigated., Methods: In this study, we conducted a gene expression analysis in female and male human spleen using the Genotype-Tissue Expression (GTEx) project dataset to identify differentially expressed genes between women and men. After successful validation on protein level of human immune cell subsets, we assessed hormonal regulation of CD99 as well as its implication on T cell regulation in primary human T cells and Jurkat T cells. In addition, we performed in vivo assays in wildtype mice and in Cd99-deficient mice to further analyze functional consequences of differential CD99 expression., Results: Here, we found higher CD99 gene expression in male human spleens compared to females and confirmed this expression difference on protein level on the surface of T cells and pDCs. Androgens are likely dispensable as the cause shown by in vitro assays and ex vivo analysis of trans men samples. In cerebrospinal fluid, CD99 was higher on T cells compared to blood. Of note, male MS patients had lower CD99 levels on CD4 + T cells in the CSF, unlike controls. By contrast, both sexes had similar CD99 expression in mice and Cd99-deficient mice showed equal susceptibility to experimental autoimmune encephalomyelitis compared to wildtypes. Functionally, CD99 increased upon human T cell activation and inhibited T cell proliferation after blockade. Accordingly, CD99-deficient Jurkat T cells showed decreased cell proliferation and cluster formation, rescued by CD99 reintroduction., Conclusions: Our results demonstrate that CD99 is sex-specifically regulated in healthy individuals and MS patients and that it is involved in T cell costimulation in humans but not in mice. CD99 could potentially contribute to MS incidence and susceptibility in a sex-specific manner., (© 2024. The Author(s).)

Migratory dendritic cells (migDCs) continuously patrol tissues and are activated by injury and inflammation. Extracellular adenosine triphosphate (ATP) is released by damaged cells or actively secreted during inflammation and increases migDC motility. However, the underlying molecular mechanisms by which ATP accelerates migDC migration is not understood. Here, we show that migDCs can be distinguished from other DC subsets and immune cells by their expression of the voltage-gated calcium channel subunit β3 (Cavβ3; CACNB3), which exclusively facilitates ATP-dependent migration in vitro and during tissue damage in vivo. By contrast, CACNB3 does not regulate lipopolysaccharide-dependent migration. Mechanistically, CACNB3 regulates ATP-dependent inositol 1,4,5-trisphophate receptor-controlled calcium release from the endoplasmic reticulum. This, in turn, is required for ATP-mediated suppression of adhesion molecules, their detachment, and initiation of migDC migration. Thus, Cacnb3 -deficient migDCs have an impaired migration after ATP exposure. In summary, we identified CACNB3 as a master regulator of ATP-dependent migDC migration that controls tissue-specific immunological responses during injury and inflammation.

Multiple sclerosis (MS) is a progressive inflammatory demyelinating disease of the CNS. Increasing evidence suggests that vulnerable neurons in MS exhibit fatal metabolic exhaustion over time, a phenomenon hypothesized to be caused by chronic hyperexcitability. Axonal Kv7 (outward-rectifying) and oligodendroglial Kir4.1 (inward-rectifying) potassium channels have important roles in regulating neuronal excitability at and around the nodes of Ranvier. Here, we studied the spatial and functional relationship between neuronal Kv7 and oligodendroglial Kir4.1 channels and assessed the transcriptional and functional signatures of cortical and retinal projection neurons under physiological and inflammatory demyelinating conditions. We found that both channels became dysregulated in MS and experimental autoimmune encephalomyelitis (EAE), with Kir4.1 channels being chronically downregulated and Kv7 channel subunits being transiently upregulated during inflammatory demyelination. Further, we observed that pharmacological Kv7 channel opening with retigabine reduced neuronal hyperexcitability in human and EAE neurons, improved clinical EAE signs, and rescued neuronal pathology in oligodendrocyte-Kir4.1-deficient (OL-Kir4.1-deficient) mice. In summary, our findings indicate that neuron-OL compensatory interactions promoted resilience through Kv7 and Kir4.1 channels and identify pharmacological activation of nodal Kv7 channels as a neuroprotective strategy against inflammatory demyelination.

G-protein coupled receptors (GPCR) regulate 3',5'-cyclic adenosine monophosphate (cAMP) levels in T cells. cAMP as ubiquitous second messenger is crucial for adequate physiology of T cells by mediating effector T cell (Teff) function as well as regulatory T cell (Treg)-mediated immunosuppression. Several GPCRs have been identified to be crucial for Teff and Treg function. However, the role of the orphan, constitutively active Gs-coupled GPCR GPR52 is unknown. Here we show that GPR52 regulates cAMP levels in T cells but does not affect T cell function. We found that stimulation of transfected HEK cells or primary T cells with a GPR52 agonist results in a rise of intracellular cAMP. However, neither Gpr52 deficiency nor pharmacological modulation of GPR52 by antagonists or agonists affected T cell activation, differentiation, and proliferation or Treg-mediated immunosuppression. Moreover, Gpr52 deletion did not modify the clinical disease course of experimental autoimmune encephalomyelitis (EAE). Our results demonstrate that a modulation of cAMP levels in T cells does not inevitably result in altered T cell function. While we could not identify an obvious role of GPR52 in in vitro T cell assays and in vivo CNS autoimmunity, it might regulate T cell function in a different context or affect the function of other GPR52-expressing cells., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2023 Krieg, Sonner, Kurelic, Engler, Scharenberg, Bauer, Nikolaev and Friese.)

Progressive multiple sclerosis (MS) is characterized by unrelenting neurodegeneration, which causes cumulative disability and is refractory to current treatments. Drug development to prevent disease progression is an urgent clinical need yet is constrained by an incomplete understanding of its complex pathogenesis. Using spatial transcriptomics and proteomics on fresh-frozen human MS brain tissue, we identified multicellular mechanisms of progressive MS pathogenesis and traced their origin in relation to spatially distributed stages of neurodegeneration. By resolving ligand-receptor interactions in local microenvironments, we discovered defunct trophic and anti-inflammatory intercellular communications within areas of early neuronal decline. Proteins associated with neuronal damage in patient samples showed mechanistic concordance with published in vivo knockdown and central nervous system (CNS) disease models, supporting their causal role and value as potential therapeutic targets in progressive MS. Our findings provide a new framework for drug development strategies, rooted in an understanding of the complex cellular and signaling dynamics in human diseased tissue that facilitate this debilitating disease., (© 2022. The Author(s), under exclusive licence to Springer Nature America, Inc.)

Purpose: Gliomas are intrinsic brain tumors with a high degree of constitutive and acquired resistance to standard therapeutic modalities such as radiotherapy and alkylating chemotherapy. Glioma subtypes are recognized by characteristic mutations. Some of these characteristic mutations have shown to generate immunogenic neoepitopes suitable for targeted immunotherapy., Experimental Design: Using peptide-based ELISpot assays, we screened for potential recurrent glioma neoepitopes in MHC-humanized mice. Following vaccination, droplet-based single-cell T-cell receptor (TCR) sequencing from established T-cell lines was applied for neoepitope-specific TCR discovery. Efficacy of intraventricular TCR-transgenic T-cell therapy was assessed in a newly developed glioma model in MHC-humanized mice induced by CRISPR-based delivery of tumor suppressor-targeting guide RNAs., Results: We identify recurrent capicua transcriptional repressor (CIC) inactivating hotspot mutations at position 215 CICR215W/Q as immunogenic MHC class II (MHCII)-restricted neoepitopes. Vaccination of MHC-humanized mice resulted in the generation of robust MHCII-restricted mutation-specific T-cell responses against CICR215W/Q. Adoptive intraventricular transfer of CICR215W-specific TCR-transgenic T cells exert antitumor responses against CICR215W-expressing syngeneic gliomas., Conclusions: The integration of immunocompetent MHC-humanized orthotopic glioma models in the discovery of shared immunogenic glioma neoepitopes facilitates the identification and preclinical testing of human leukocyte antigen (HLA)-restricted neoepitope-specific TCRs for locoregional TCR-transgenic T-cell adoptive therapy., (©2021 The Authors; Published by the American Association for Cancer Research.)

3',5'-cyclic adenosine monophosphate (cAMP) is well-known for its diverse immunomodulatory properties, primarily inhibitory effects during T cell activation, proliferation, and production of pro-inflammatory cytokines. A decrease in cAMP levels, due to the hydrolyzing activity of phosphodiesterases (PDE), is favoring inflammatory responses. This can be prevented by selective PDE inhibitors, which makes PDEs important therapeutic targets for autoimmune disorders. In this study, we investigated the specific roles of PDE2A and PDE3B in the regulation of intracellular cAMP levels in different mouse T cell subsets. Unexpectedly, T cell receptor (TCR) activation led to a selective upregulation of PDE2A at the protein level in conventional T cells (Tcon), whereas no changes were detected in regulatory T cells (Treg). In contrast, protein expression of PDE3B was significantly higher in both non-activated and activated Tcon subsets as compared to Treg, with no changes upon TCR engagement. Live-cell imaging of T cells expressing a highly sensitive Förster resonance energy transfer (FRET)-based biosensor, Epac1-camps, has enabled cAMP measurements in real time and revealed stronger responses to the PDE2A inhibitors in activated vs non-activated Tcon. Importantly, stimulation of intracellular cGMP levels with natriuretic peptides led to an increase of cAMP in non-activated and a decrease of cAMP in activated Tcon, suggesting that TCR activation changes the PDE3B-dependent positive to PDE2A-dependent negative cGMP/cAMP cross-talk. Functionally, this switch induced higher expression of early activation markers CD25 and CD69. This constitutes a potentially interesting feed-forward mechanism during autoimmune and inflammatory responses that may be exploited therapeutically., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The reviewer MS declared a past co-authorship with one of the authors VON to the handling editor., (Copyright © 2021 Kurelic, Krieg, Sonner, Bhaiyan, Ramos, Frantz, Friese and Nikolaev.)

The dynamics and phenotypes of intratumoral myeloid cells during tumor progression are poorly understood. Here we define myeloid cellular states in gliomas by longitudinal single-cell profiling and demonstrate their strict control by the tumor genotype: in isocitrate dehydrogenase (IDH)-mutant tumors, differentiation of infiltrating myeloid cells is blocked, resulting in an immature phenotype. In late-stage gliomas, monocyte-derived macrophages drive tolerogenic alignment of the microenvironment, thus preventing T cell response. We define the IDH-dependent tumor education of infiltrating macrophages to be causally related to a complex re-orchestration of tryptophan metabolism, resulting in activation of the aryl hydrocarbon receptor. We further show that the altered metabolism of IDH-mutant gliomas maintains this axis in bystander cells and that pharmacological inhibition of tryptophan metabolism can reverse immunosuppression. In conclusion, we provide evidence of a glioma genotype-dependent intratumoral network of resident and recruited myeloid cells and identify tryptophan metabolism as a target for immunotherapy of IDH-mutant tumors., (© 2021. The Author(s).)

Dendritic cell (DC) vaccination has proven to be an effective and safe adjuvant for cancer immunotherapies. As the presence of DCs within the tumor microenvironment promotes adaptive antitumor immunity, enhancement of DC migration toward the tumor microenvironment following DC vaccination might represent one possible approach to increase its therapeutic efficacy. While recent findings suggest the activity-regulated cytoskeleton-associated protein/activity-regulated gene 3.1 (Arc/Arg3.1) as critical regulator of DC migration in the context of autoimmune diseases, we aimed to investigate the impact of Arc/Arg3.1 expression for DC-based cancer vaccines. To this end, DC migration capacity as well as the induction of T cell-mediated antitumor immunity was assessed in an experimental B16 melanoma model with Arc/Arg3.1 -/- and Arc/Arg3.1 -expressing BMDCs applied as a subcutaneous vaccine. While antigen presentation on DCs was critical for unleashing effective T cell mediated antitumor immune responses, Arc/Arg3.1 expression enhanced DC migration toward the tumor and secondary lymphoid organs. Moreover, Arc/Arg3.1-expressing BMDCs shape the tumor immune microenvironment by facilitating tumor recruitment of antigen-specific effector T cells. Thus, Arc/Arg3.1 may represent a novel therapeutic target in DCs in order to increase the therapeutic efficacy of DC vaccination., (© 2021 The Author(s). Published with license by Taylor & Francis Group, LLC.)

Multiple sclerosis (MS) is a chronic inflammatory disease of the central nervous system with continuous neuronal loss. Treatment of clinical progression remains challenging due to lack of insights into inflammation-induced neurodegenerative pathways. Here, we show that an imbalance in the neuronal receptor interactome is driving glutamate excitotoxicity in neurons of MS patients and identify the MS risk-associated metabotropic glutamate receptor 8 (GRM8) as a decisive modulator. Mechanistically, GRM8 activation counteracted neuronal cAMP accumulation, thereby directly desensitizing the inositol 1,4,5-trisphosphate receptor (IP3R). This profoundly limited glutamate-induced calcium release from the endoplasmic reticulum and subsequent cell death. Notably, we found Grm8-deficient neurons to be more prone to glutamate excitotoxicity, whereas pharmacological activation of GRM8 augmented neuroprotection in mouse and human neurons as well as in a preclinical mouse model of MS. Thus, we demonstrate that GRM8 conveys neuronal resilience to CNS inflammation and is a promising neuroprotective target with broad therapeutic implications., Competing Interests: Disclosures:   M.A. Friese reported personal fees from Novartis, personal fees from Roche, personal fees from Merck EMD, and personal fees from Biogen outside the submitted work. No other disclosures were reported., (© 2021 Woo et al.)

Intrinsic malignant brain tumors, such as glioblastomas are frequently resistant to immune checkpoint blockade (ICB) with few hypermutated glioblastomas showing response. Modeling patient-individual resistance is challenging due to the lack of predictive biomarkers and limited accessibility of tissue for serial biopsies. Here, we investigate resistance mechanisms to anti-PD-1 and anti-CTLA-4 therapy in syngeneic hypermutated experimental gliomas and show a clear dichotomy and acquired immune heterogeneity in ICB-responder and non-responder tumors. We made use of this dichotomy to establish a radiomic signature predicting tumor regression after pseudoprogression induced by ICB therapy based on serial magnetic resonance imaging. We provide evidence that macrophage-driven ICB resistance is established by CD4 T cell suppression and T reg expansion in the tumor microenvironment via the PD-L1/PD-1/CD80 axis. These findings uncover an unexpected heterogeneity of response to ICB in strictly syngeneic tumors and provide a rationale for targeting PD-L1-expressing tumor-associated macrophages to overcome resistance to ICB.

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.

Tryptophan (Trp) metabolism is an important target in immuno-oncology as it represents a powerful immunosuppressive mechanism hijacked by tumors for protection against immune destruction. However, it remains unclear how tumor cells can proliferate while degrading the essential amino acid Trp. Trp is incorporated into proteins after it is attached to its tRNA by tryptophanyl-tRNA synthestases. As the tryptophanyl-tRNA synthestases compete for Trp with the Trp-catabolizing enzymes, the balance between these enzymes will determine whether Trp is used for protein synthesis or is degraded. In human cancers expression of the Trp-degrading enzymes indoleamine-2,3-dioxygenase-1 (IDO1) and tryptophan-2,3-dioxygenase (TDO2) was positively associated with the expression of the tryptophanyl-tRNA synthestase WARS. One mechanism underlying the association between IDO1 and WARS identified in this study is their joint induction by IFNγ released from tumor-infiltrating T cells. Moreover, we show here that IDO1- and TDO2-mediated Trp deprivation upregulates WARS expression by activating the general control non-derepressible-2 (GCN2) kinase, leading to phosphorylation of the eukaryotic translation initiation factor 2α (eIF2α) and induction of activating transcription factor 4 (ATF4). Trp deprivation induced cytoplasmic WARS expression but did not increase nuclear or extracellular WARS levels. GCN2 protected the cells against the effects of Trp starvation and enabled them to quickly make use of Trp for proliferation once it was replenished. Computational modeling of Trp metabolism revealed that Trp deficiency shifted Trp flux towards WARS and protein synthesis. Our data therefore suggest that the upregulation of WARS via IFNγ and/or GCN2-peIF2α-ATF4 signaling protects Trp-degrading cancer cells from excessive intracellular Trp depletion.

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.

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.

The catabolism of tryptophan to immunosuppressive and neuroactive kynurenines is a key metabolic pathway regulating immune responses and neurotoxicity. The rate-limiting step is controlled by indoleamine-2,3-dioxygenase (IDO) and tryptophan-2,3-dioxygenase (TDO). IDO is expressed in antigen presenting cells during immune reactions, hepatic TDO regulates blood homeostasis of tryptophan and neuronal TDO influences neurogenesis. While the role of IDO has been described in multiple immunological settings, little is known about TDO's effects on the immune system. TDO-deficiency is neuroprotective in C. elegans and Drosophila by increasing tryptophan and specific kynurenines. Here we have determined the role of TDO in autoimmunity and neurodegeneration in experimental autoimmune encephalomyelitis (EAE), a model of multiple sclerosis. We created reporter-TDO mice for in vivo imaging to show that hepatic but not CNS TDO expression is activated during EAE. TDO deficiency did not influence myelin-specific T cells, leukocyte infiltration into the CNS, demyelination and disease activity. TDO-deficiency protected from neuronal loss in the spinal cord but not in the optic nerves. While this protection did not translate to an improved overt clinical outcome, our data suggest that spatially distinct neuroprotection is conserved in mammals and support TDO as a potential target for treatment of diseases associated with neurodegeneration.

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.

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., Competing Interests: The authors declare no conflict of interest.

Relapsing-remitting multiple sclerosis (MS)(2) is characterized by phases of acute neuroinflammation followed by spontaneous remission. Termination of inflammation is accompanied by an influx of regulatory T cells (Tregs).(3) The molecular mechanisms responsible for directing Tregs into the inflamed CNS tissue, however, are incompletely understood. In an MS mouse model we show that the stress kinase general control non-derepressible 2 (GCN2),(4) expressed in T cells, contributes to the resolution of autoimmune neuroinflammation. Failure to recover from acute inflammation was associated with reduced frequencies of CNS-infiltrating Tregs. GCN2 deficient Tregs displayed impaired migration to a CCL2 gradient. These data suggest an important contribution of the T cell stress response to the resolution of autoimmune neuroinflammation., (Copyright © 2016. Published by Elsevier B.V.)