Astrocytic TIMP-1 regulates production of Anastellin, an inhibitor of oligodendrocyte differentiation and FTY720 responses.

Astrocyte activation is associated with neuropathology and the production of tissue inhibitor of metalloproteinase-1 (TIMP1). TIMP1 is a pleiotropic extracellular protein that functions both as a protease inhibitor and as a growth factor. Astrocytes that lack expression of Timp1 do not support rat oligodendrocyte progenitor cell (rOPC) differentiation, and adult global Timp1 knockout (Timp1KO) mice do not efficiently remyelinate following a demyelinating injury. Here, we performed an unbiased proteomic analysis and identified a fibronectin-derived peptide called Anastellin (Ana) that was unique to the Timp1KO astrocyte secretome. Ana was found to block rOPC differentiation in vitro and enhanced the inhibitory influence of fibronectin on rOPC differentiation. Ana is known to act upon the sphingosine-1-phosphate receptor 1, and we determined that Ana also blocked the pro-myelinating effect of FTY720 (or fingolimod) on rOPC differentiation in vitro. Administration of FTY720 to wild-type C57BL/6 mice during MOG35-55-experimental autoimmune encephalomyelitis ameliorated clinical disability while FTY720 administered to mice lacking expression of Timp1 (Timp1KO) had no effect. Analysis of Timp1 and fibronectin (FN1) transcripts from primary human astrocytes from healthy and multiple sclerosis (MS) donors revealed lower TIMP1 expression was coincident with elevated FN1 in MS astrocytes. Last, analyses of proteomic databases of MS samples identified Ana peptides to be more abundant in the cerebrospinal fluid (CSF) of human MS patients with high disease activity. A role for Ana in MS as a consequence of a lack of astrocytic TIMP-1 production could influence both the efficacy of fingolimod responses and innate remyelination potential in the MS brain.

CD8+ T cell depletion prevents neuropathology in a mouse model of globoid cell leukodystrophy.

Globoid cell leukodystrophy (GLD) or Krabbe's disease is a fatal genetic demyelinating disease of the central nervous system caused by loss-of-function mutations in the galactosylceramidase (galc) gene. While the metabolic basis for disease is known, the understanding of how this results in neuropathology is not well understood. Herein, we report that the rapid and protracted elevation of CD8+ cytotoxic T lymphocytes occurs coincident with clinical disease in a mouse model of GLD. Administration of a function-blocking antibody against CD8α effectively prevented disease onset, reduced morbidity and mortality, and prevented CNS demyelination in mice. These data indicate that subsequent to the genetic cause of disease, neuropathology is driven by pathogenic CD8+ T cells, thus offering novel therapeutic potential for treatment of GLD.

Antigen-specific downregulation of miR-150 in CD4 T cells promotes cell survival.

MicroRNA-150 (miR-150) has been shown to play a general role in the immune system, but very little is known about its role on CD4+ T cell responses. During T cell responses against superantigen Staphylococcal Enterotoxin A, miR-150 expression was down-regulated in antigen-specific CD4+ T cells but up-regulated in CD8+ T cells. CD4+ and CD8+ T cell clonal expansion was greater in miR-150-KO mice than in WT mice, but miR-150 selectively repressed IL-2 production in CD4+ T cells. Transcriptome analysis of CD4+ T cells demonstrated that apoptosis and mTOR pathways were highly enriched in the absence of miR-150. Mechanistic studies confirmed that miR-150 promoted apoptosis specifically in antigen-specific CD4+ T cells, but not in bystander CD4+ nor in CD8+ T cells. Furthermore, inhibition of mTOR-linked mitochondrial superoxidedismutase-2 increased apoptosis in miR-150-/- antigen-specific CD4+ T. Thus, miR-150 impacts CD4+ T cell helper activity by attenuating IL-2 production along with clonal expansion, and suppresses superoxidedismutase to promote apoptosis.

Astrocytic TIMP-1 regulates production of Anastellin, a novel inhibitor of oligodendrocyte differentiation and FTY720 responses.

Astrocyte activation is associated with neuropathology and the production of tissue inhibitor of metalloproteinase-1 (TIMP1). TIMP1 is a pleiotropic extracellular protein that functions both as a protease inhibitor and as a growth factor. We have previously demonstrated that murine astrocytes that lack expression of Timp1 do not support rat oligodendrocyte progenitor cell (rOPC) differentiation, and adult global Timp1 knockout ( Timp1 KO ) mice do not efficiently remyelinate following a demyelinating injury. To better understand the basis of this, we performed unbiased proteomic analyses and identified a fibronectin-derived peptide called anastellin that is unique to the murine Timp1 KO astrocyte secretome. Anastellin was found to block rOPC differentiation in vitro and enhanced the inhibitory influence of fibronectin on rOPC differentiation. Anastellin is known to act upon the sphingosine-1-phosphate receptor 1 (S1PR1), and we determined that anastellin also blocked the pro-myelinating effect of FTY720 (or fingolimod) on rOPC differentiation in vitro . Further, administration of FTY720 to wild-type C57BL/6 mice during MOG 35-55 -EAE ameliorated clinical disability while FTY720 administered to mice lacking expression of Timp1 in astrocytes ( Timp1 cKO ) had no effect. Analysis of human TIMP1 and fibronectin ( FN1 ) transcripts from healthy and multiple sclerosis (MS) patient brain samples revealed an inverse relationship where lower TIMP1 expression was coincident with elevated FN1 in MS astrocytes. Lastly, we analyzed proteomic databases of MS samples and identified anastellin peptides to be more abundant in the cerebrospinal fluid (CSF) of human MS patients with high versus low disease activity. The prospective role for anastellin generation in association with myelin lesions as a consequence of a lack of astrocytic TIMP-1 production could influence both the efficacy of fingolimod responses and the innate remyelination potential of the the MS brain. Significance Statement: Astrocytic production of TIMP-1 prevents the protein catabolism of fibronectin. In the absence of TIMP-1, fibronectin is further digested leading to a higher abundance of anastellin peptides that can bind to sphingosine-1-phosphate receptor 1. The binding of anastellin with the sphingosine-1-phosphate receptor 1 impairs the differentiation of oligodendrocytes progenitor cells into myelinating oligodendrocytes in vitro , and negates the astrocyte-mediated therapeutic effects of FTY720 in the EAE model of chronic CNS inflammation. These data indicate that TIMP-1 production by astrocytes is important in coordinating astrocytic functions during inflammation. In the absence of astrocyte produced TIMP-1, elevated expression of anastellin may represent a prospective biomarker for FTY720 therapeutic responsiveness.

Splice factor polypyrimidine tract-binding protein 1 (Ptbp1) primes endothelial inflammation in atherogenic disturbed flow conditions.

NF-κB-mediated endothelial activation drives leukocyte recruitment and atherosclerosis, in part through adhesion molecules Icam1 and Vcam1. The endothelium is primed for cytokine activation of NF-κB by exposure to low and disturbed blood flow (LDF)but the molecular underpinnings are not fully understood. In an experimental in vivo model of LDF, platelets were required for the increased expression of several RNA-binding splice factors, including polypyrimidine tract binding protein (Ptbp1). This was coordinated with changes in RNA splicing in the NF-κB pathway in primed cells, leading us to examine splice factors as mediators of priming. Using Icam1 and Vcam1 induction by tumor necrosis factor (TNF)-α stimulation as a readout, we performed a CRISPR Cas9 knockout screen and identified a requirement for Ptbp1 in priming. Deletion of Ptbp1 had no effect on cell growth or response to apoptotic stimuli, but reversed LDF splicing patterns and inhibited NF-κB nuclear translocation and transcriptional activation of downstream targets, including Icam1 and Vcam1. In human coronary arteries, elevated PTBP1 correlates with expression of TNF pathway genes and plaque. In vivo, endothelial-specific deletion of Ptbp1 reduced Icam1 expression and myeloid cell infiltration at regions of LDF in atherosclerotic mice, limiting atherosclerosis. This may be mediated, in part, by allowing inclusion of a conserved alternative exon in Ripk1 leading to a reduction in Ripk1 protein. Our data show that Ptbp1, which is induced in a subset of the endothelium by platelet recruitment at regions of LDF, is required for priming of the endothelium for subsequent NF-κB activation, myeloid cell recruitment and atherosclerosis.

Optimal CD8+ T cell effector function requires costimulation-induced RNA-binding proteins that reprogram the transcript isoform landscape.

Boosting T cell activation through costimulation directs defense against cancer and viral infections. Despite multiple studies targeting costimulation in clinical trials, the increased potency and reprogramming of T cells endowed by costimulation is poorly understood. Canonical dogma states that transcription mediates T cell activation. Here, we show that the spliceosome, controlling post-transcriptional alternative splicing and alternative polyadenylation, is the most enriched pathway in T cells after CD134/CD137 costimulation. Costimulation of CD8+ T cells significantly increases expression of 29 RNA-binding proteins while RNA-seq uncovers over 1000 differential alternative splicing and polyadenylation events. Using in vivo mouse and in vitro human models, we demonstrate that RNA-binding protein Tardbp is required for effector cytokine production, CD8+ T cell clonal expansion, and isoform regulation after costimulation. The prospect of immune response optimization through reprogramming of mRNA isoform production offered herein opens new avenues for experimentally and therapeutically tuning the activities of T cells.

Evaluating the glycolytic potential of mouse costimulated effector CD8+ T cells ex vivo.

Studying the metabolic fitness of T cells is fundamental to understand how immune responses are regulated. Here, we describe a step-by-step protocol optimized to efficiently generate and isolate effector antigen-specific CD8+ T cells ex vivo using costimulation. We also detail steps to evaluate their metabolic activity using Seahorse technology. This protocol can be used to measure the glycolytic potential of effector murine T cells in response to different manipulations, such as infections, adjuvant studies, gene editing, or metabolite supplementation. For complete details on the use and execution of this protocol, please refer to Agliano et al. (2022).

Nicotinamide breaks effector CD8 T cell responses by targeting mTOR signaling.

Nicotinamide (NAM) shapes T cell responses but its precise molecular mechanism of action remains elusive. Here, we show that NAM impairs naive T cell effector transition but also effector T cells themselves. Although aerobic glycolysis is a hallmark of activated T cells, CD8+ T cells exposed to NAM displayed enhanced glycolysis, yet producing significantly less IFNγ. Mechanistically, NAM reduced mTORC1 activity independently of NAD+ metabolism, decreasing IFNγ translation and regulating T cell transcriptional factors critical to effector/memory fate. Finally, the role of NAM in a biomedically relevant model of lung injury was tested. Specifically, a NAM-supplemented diet reduced systemic IL-2, antigen-specific T cell clonal expansion, and effector function after inhalation of Staphylococcus aureus enterotoxin A. These findings identify NAM as a potential therapeutic supplement that uncouples glycolysis from effector cytokine production and may be a powerful treatment for diseases associated with T cell hyperactivation.

A high OXPHOS CD8 T cell subset is predictive of immunotherapy resistance in melanoma patients.

Immune checkpoint inhibitor (ICI) therapy continues to revolutionize melanoma treatment, but only a subset of patients respond. Major efforts are underway to develop minimally invasive predictive assays of ICI response. Using single-cell transcriptomics, we discovered a unique CD8 T cell blood/tumor-shared subpopulation in melanoma patients with high levels of oxidative phosphorylation (OXPHOS), the ectonucleotidases CD38 and CD39, and both exhaustion and cytotoxicity markers. We called this population with high levels of OXPHOS "CD8+ TOXPHOS cells." We validated that higher levels of OXPHOS in tumor- and peripheral blood-derived CD8+ TOXPHOS cells correlated with ICI resistance in melanoma patients. We then developed an ICI therapy response predictive model using a transcriptomic profile of CD8+ TOXPHOS cells. This model is capable of discerning responders from nonresponders using either tumor or peripheral blood CD8 T cells with high accuracy in multiple validation cohorts. In sum, CD8+ TOXPHOS cells represent a critical immune population to assess ICI response with the potential to be a new target to improve outcomes in melanoma patients.

Intracellular immune sensing promotes inflammation via gasdermin D-driven release of a lectin alarmin.

Inflammatory caspase sensing of cytosolic lipopolysaccharide (LPS) triggers pyroptosis and the concurrent release of damage-associated molecular patterns (DAMPs). Collectively, DAMPs are key determinants that shape the aftermath of inflammatory cell death. However, the identity and function of the individual DAMPs released are poorly defined. Our proteomics study revealed that cytosolic LPS sensing triggered the release of galectin-1, a ß-galactoside-binding lectin. Galectin-1 release is a common feature of inflammatory cell death, including necroptosis. In vivo studies using galectin-1-deficient mice, recombinant galectin-1 and galectin-1-neutralizing antibody showed that galectin-1 promotes inflammation and plays a detrimental role in LPS-induced lethality. Mechanistically, galectin-1 inhibition of CD45 (Ptprc) underlies its unfavorable role in endotoxin shock. Finally, we found increased galectin-1 in sera from human patients with sepsis. Overall, we uncovered galectin-1 as a bona fide DAMP released as a consequence of cytosolic LPS sensing, identifying a new outcome of inflammatory cell death.

A benzimidazole inhibitor attenuates sterile inflammation induced in a model of systemic autoinflammation in female mice.

Sterile stimuli can trigger inflammatory responses, and in some cases can lead to a variety of acute or chronic diseases. In this study, we hypothesize that a benzimidazole inhibitor may be used as a therapeutic in the treatment of sterile inflammation. In vitro, this inhibitor blocks TLR signalling and inflammatory responses. The benzimidazole inhibitor does not prevent mouse macrophage activation after stimulation with 2,6,10,14-tetramethylpentadecane (TMPD, also known as pristane), a hydrocarbon oil that mimics features of sterile inflammation when injected in vivo. However, C57BL/6J female mice treated with the benzimidazole inhibitor exhibited a significant reduction of pristane-dependent induction of splenocyte number and weight. Conversely, no significant difference was observed in males. Using mass spectrometry, we found that the urine of pristane-injected mice contained increased levels of putative markers for several inflammatory diseases, which were reduced by the benzimidazole inhibitor. To study the mechanism, we showed that pristane-injected mice had increased cell free DNA in serum, which was not impacted by inhibitor treatment. However, chemokine release (e.g. MCP-1, RANTES and TARC) was significantly reduced in inhibitor-treated mice. Thus, the benzimidazole inhibitor might be used as a new drug to block the recruitment of immune cells during sterile inflammatory diseases in humans.

GRK2 enforces androgen receptor dependence in the prostate and prostate tumors.

Metastatic tumors that have become resistant to androgen deprivation therapy represent the major challenge in treating prostate cancer. Although these recurrent tumors typically remain dependent on the androgen receptor (AR), non-AR-driven tumors that also emerge are particularly deadly and becoming more prevalent. Here, we present a new genetically engineered mouse model for non-AR-driven prostate cancer that centers on a negative regulator of G protein-coupled receptors that is downregulated in aggressive human prostate tumors. Thus, prostate-specific expression of a dominant-negative G protein-coupled receptor kinase 2 (GRK2-DN) transgene diminishes AR and AR target gene expression in the prostate, and confers resistance to castration-induced involution. Further, the GRK2-DN transgene dramatically accelerates oncogene-initiated prostate tumorigenesis by increasing primary tumor size, potentiating visceral organ metastasis, suppressing AR, and inducing neuroendocrine marker mRNAs. In summary, GRK2 enforces AR-dependence in the prostate, and the loss of GRK2 function in prostate tumors accelerates disease progression toward the deadliest stage.

Costimulation Induces CD4 T Cell Antitumor Immunity via an Innate-like Mechanism.

Chronic exposure to tumor-associated antigens inactivates cognate T cells, restricting the repertoire of tumor-specific effector T cells. This problem was studied here by transferring TCR transgenic CD4 T cells into recipient mice that constitutively express a cognate self-antigen linked to MHC II on CD11c-bearing cells. Immunotherapeutic agonists to CD134 plus CD137, "dual costimulation," induces specific CD4 T cell expansion and expression of the receptor for the Th2-associated IL-1 family cytokine IL-33. Rather than producing IL-4, however, they express the tumoricidal Th1 cytokine IFNγ when stimulated with IL-33 or IL-36 (a related IL-1 family member) plus IL-12 or IL-2. IL-36, which is induced within B16-F10 melanomas by dual costimulation, reduces tumor growth when injected intratumorally as a monotherapy and boosts the efficacy of tumor-nonspecific dual costimulated CD4 T cells. Dual costimulation thus enables chronic antigen-exposed CD4 T cells, regardless of tumor specificity, to elaborate tumoricidal function in response to tumor-associated cytokines.

Extracellular vesicle fibrinogen induces encephalitogenic CD8+ T cells in a mouse model of multiple sclerosis.

Extracellular vesicles (EVs) are emerging as potent mediators of intercellular communication with roles in inflammation and disease. In this study, we examined the role of EVs from blood plasma (pEVs) in an experimental autoimmune encephalomyelitis mouse model of central nervous system demyelination. We determined that pEVs induced a spontaneous relapsing-remitting disease phenotype in MOG35-55-immunized C57BL/6 mice. This modified disease phenotype was found to be driven by CD8+ T cells and required fibrinogen in pEVs. Analysis of pEVs from relapsing-remitting multiple sclerosis patients also identified fibrinogen as a significant portion of pEV cargo. Together, these data suggest that fibrinogen in pEVs contributes to the perpetuation of neuroinflammation and relapses in disease.

Direct CD137 costimulation of CD8 T cells promotes retention and innate-like function within nascent atherogenic foci.

Effector CD8 T cells infiltrate atherosclerotic lesions and are correlated with cardiovascular events, but the mechanisms regulating their recruitment and retention are not well understood. CD137 (4-1BB) is a costimulatory receptor induced on immune cells and expressed at sites of human atherosclerotic plaque. Genetic variants associated with decreased CD137 expression correlate with carotid-intimal thickness and its deficiency in animal models attenuates atherosclerosis. These effects have been attributed in part to endothelial responses to low and disturbed flow (LDF), but CD137 also generates robust effector CD8 T cells as a costimulatory signal. Thus, we asked whether CD8 T cell-specific CD137 stimulation contributes to their infiltration, retention, and IFNγ production in early atherogenesis. We tested this through adoptive transfer of CD8 T cells into recipient C57BL/6J mice that were then antigen primed and CD137 costimulated. We analyzed atherogenic LDF vessels in normolipidemic and PCSK9-mediated hyperlipidemic models and utilized a digestion protocol that allowed for lesional T-cell characterization via flow cytometry and in vitro stimulation. We found that CD137 activation, specifically of effector CD8 T cells, triggers their intimal infiltration into LDF vessels and promotes a persistent innate-like proinflammatory program. Residence of CD137+ effector CD8 T cells further promoted infiltration of endogenous CD8 T cells with IFNγ-producing potential, whereas CD137-deficient CD8 T cells exhibited impaired vessel infiltration, minimal IFNγ production, and reduced infiltration of endogenous CD8 T cells. Our studies thus provide novel insight into how CD137 costimulation of effector T cells, independent of plaque-antigen recognition, instigates their retention and promotes innate-like responses from immune infiltrates within atherogenic foci. NEW & NOTEWORTHY Our studies identify CD137 costimulation as a stimulus for effector CD8 T-cell infiltration and persistence within atherogenic foci, regardless of atherosclerotic-antigen recognition. These costimulated effector cells, which are generated in pathological states such as viral infection and autoimmunity, have innate-like proinflammatory programs in circulation and within the atherosclerotic microenvironment, providing mechanistic context for clinical correlations of cardiovascular morbidity with increased CD8 T-cell infiltration and markers of activation in the absence of established antigen specificity.

Single cell transcriptomics based-MacSpectrum reveals novel macrophage activation signatures in diseases.

Adipose tissue macrophages (ATM) are crucial for maintaining adipose tissue homeostasis and mediating obesity-induced metabolic abnormalities, including prediabetic conditions and type 2 diabetes mellitus. Despite their key functions in regulating adipose tissue metabolic and immunologic homeostasis under normal and obese conditions, a high-resolution transcriptome annotation system that can capture ATM multifaceted activation profiles has not yet been developed. This is primarily attributed to the complexity of their differentiation/activation process in adipose tissue and their diverse activation profiles in response to microenvironmental cues. Although the concept of multifaceted macrophage action is well-accepted, no current model precisely depicts their dynamically regulated in vivo features. To address this knowledge gap, we generated single-cell transcriptome data from primary bone marrow-derived macrophages under polarizing and non-polarizing conditions to develop new high-resolution algorithms. The outcome was creation of a two-index platform, MacSpectrum (https://macspectrum.uconn.edu), that enables comprehensive high-resolution mapping of macrophage activation states from diverse mixed cell populations. MacSpectrum captured dynamic transitions of macrophage subpopulations under both in vitro and in vivo conditions. Importantly, MacSpectrum revealed unique "signature" gene sets in ATMs and circulating monocytes that displayed significant correlation with BMI and homeostasis model assessment of insulin resistance (HOMA-IR) in obese human patients. Thus, MacSpectrum provides unprecedented resolution to decode macrophage heterogeneity and will open new areas of clinical translation.

Gasdermin D Restrains Type I Interferon Response to Cytosolic DNA by Disrupting Ionic Homeostasis.

Inflammasome-activated caspase-1 cleaves gasdermin D to unmask its pore-forming activity, the predominant consequence of which is pyroptosis. Here, we report an additional biological role for gasdermin D in limiting cytosolic DNA surveillance. Cytosolic DNA is sensed by Aim2 and cyclic GMP-AMP synthase (cGAS) leading to inflammasome and type I interferon responses, respectively. We found that gasdermin D activated by the Aim2 inflammasome suppressed cGAS-driven type I interferon response to cytosolic DNA and Francisella novicida in macrophages. Similarly, interferon-ß (IFN-ß) response to F. novicida infection was elevated in gasdermin D-deficient mice. Gasdermin D-mediated negative regulation of IFN-ß occurred in a pyroptosis-, interleukin-1 (IL-1)-, and IL-18-independent manner. Mechanistically, gasdermin D depleted intracellular potassium (K+) via membrane pores, and this K+ efflux was necessary and sufficient to inhibit cGAS-dependent IFN-ß response. Thus, our findings have uncovered an additional interferon regulatory module involving gasdermin D and K+ efflux.

T cell-directed IL-17 production by lung granular γδ T cells is coordinated by a novel IL-2 and IL-1ß circuit.

Immune-mediated lung is considered the result of an exacerbated innate injury immune response, although a role for adaptive lymphocytes is emerging. αß T cells specific for S. aureus enterotoxin A orchestrate a Tγδ17 response during lung injury. However, the mechanism driving IL-17 production is unclear. Here, we show a role for IL-2 triggering IL-17 production by lung granular γδ T cells as IL-17 synthesis and neutrophil recruitment was reduced by IL-2 blocking mAbs in vitro and in vivo. Mass cytometry analysis revealed that lung γδ T cells responded directly to IL-2 as evident from STAT5 phosphorylation and RoRγt expression. IL-2 receptor blocking mAbs and JAK inhibition impaired STAT5 phosphorylation and IL-17 release. Moreover, inhalation of S. aureus enterotoxin A induced IL-2 secretion and caspase-1-dependent IL-1ß activation to drive IL-17 production. This T-cell-mediated inflammasome-dependent IL-17 response is maximum when lung Tγδ17 cells were sequentially stimulated first with IL-2 then IL-1ß. Interestingly, when IL-2 is given therapeutically to cancer patients it carries a known risk of lung injury that is largely indistinguishable from that seen in sepsis. Hence, this novel mechanism reveals therapeutic targets treating both acute lung injury and high-dose IL-2 toxicity in cancer.

A novel biologic platform elicits profound T cell costimulatory activity and antitumor immunity in mice.

Combination immunotherapies utilizing complementary modalities that target distinct tumor attributes or immunosuppressive mechanisms, or engage different arms of the antitumor immune response, can elicit greater therapeutic efficacy than the component monotherapies. Increasing the number of agents included in a therapeutic cocktail can further increase efficacy, however, this approach poses numerous challenges for clinical translation. Here, a novel platform to simplify combination immunotherapy by covalently linking immunotherapeutic agonists to the costimulatory receptors CD134 and CD137 into a single heterodimeric drug, "OrthomAb", is shown. This reagent not only retains costimulatory T cell activity, but also elicits unique T cell functions that are not programmed by either individual agonist, and preferentially expands effector T cells over Tregs. Finally, in an aggressive melanoma model OrthomAb elicits better therapeutic efficacy compared to the unlinked agonists. This demonstration that two drugs can be combined into one provides a framework for distilling complex combination drug cocktails into simpler delivery platforms.

A Refined Bead-Free Method to Identify Astrocytic Exosomes in Primary Glial Cultures and Blood Plasma.

Astrocytes are the most abundant glial cell type in the central nervous system (CNS) and are known to fulfill critical homeostatic functions. Dysfunction of activated astrocytes is also known to participate in the development of several neurological diseases. Astrocytes can be uniquely identified by expression of the intermediate filament protein glial acidic fibrillary protein (GFAP). Herein, we report on the development of a rigorous and sensitive methodology to identify GFAP+ exosomes in primary culture using flow cytometry. We then demonstrate that activated astrocytes release increased amounts of exosomes in response to treatment with interleukin-1ß. Using this methodology, we report the identification of GFAP+ exosomes in blood and then use a mouse model of inflammatory demyelination, experimental autoimmune encephalomyelitis (EAE), to examine whether the abundance of GFAP+ exosomes in blood circulation changes during clinical illness. We find a detectable increase in the presence of GFAP+ exosomes in EAE mice when compared with non-EAE, control mice. Our data provide a novel perspective on the presence of GFAP in blood as it identifies exosomes as potential astrocyte-derived signals within blood. These data are complementary to previous clinical studies that reported elevated GFAP protein in blood samples from multiple sclerosis (MS) patients during a clinical relapse. These data also reveal the existence of a potential systemic role for astrocyte-derived exosomes in CNS conditions involving inflammation such as multiple sclerosis.