Microglial immune checkpoint mechanisms

Persistent Acidic Environment Induces Impaired Phagocytosis via ERK in Microglia

Acidic environment evoked by stroke, traumatic brain injury, and Alzheimer's disease may change the functional properties of microglia. Nevertheless, the underlying mechanisms of functional changes in microglia remain unclear. In this study, we found that acidic stimuli (pH 6.8) increased rapidly interleukin (IL)-1β and IL-6 mRNA levels and subsequently reduced IL-10, transforming growth factor (TGF)-β1, Cx3cr1, and P2ry12 as the exposure time to acidic environment increase in BV2 cells. In addition, persistent acidic environment (pH 6.8 for 6 h) induced impaired phagocytic function in BV2 cells. Short-term acidic exposure (pH 6.8 for 30 min) increased cyclic AMP (cAMP) and phospho-protein kinase A (PKA) but inhibited phospho-extracellular signal-regulated kinase (p-ERK). However, under persistent acidic environment (pH 6.8 for 6 h), cyclic AMP and PKA were normalized and p-ERK was increased with TDAG8 (T cell death associated gene 8; GPR65) reduction. FR 180,204, an ERK inhibitor, rescued the persistent acidic environment-induced functional changes in BV2 cells and its effect was recapitulated in primary neonatal microglia. Thus, we propose that ERK targeting may be an alternative strategy to restore microglial dysfunction in the central nervous system (CNS) acidic environment in various neurological disorders.

Aged Microglia in Neurodegenerative Diseases: Microglia Lifespan and Culture Methods

Microglia have been recognized as macrophages of the central nervous system (CNS) that are regarded as a culprit of neuroinflammation in neurodegenerative diseases. Thus, microglia have been considered as a cell that should be suppressed for maintaining a homeostatic CNS environment. However, microglia ontogeny, fate, heterogeneity, and their function in health and disease have been defined better with advances in single-cell and imaging technologies, and how to maintain homeostatic microglial function has become an emerging issue for targeting neurodegenerative diseases. Microglia are long-lived cells of yolk sac origin and have limited repopulating capacity. So, microglial perturbation in their lifespan is associated with not only neurodevelopmental disorders but also neurodegenerative diseases with aging. Considering that microglia are long-lived cells and may lose their functional capacity as they age, we can expect that aged microglia contribute to various neurodegenerative diseases. Thus, understanding microglial development and aging may represent an opportunity for clarifying CNS disease mechanisms and developing novel therapies.

A new method for obtaining bankable and expandable adult-like microglia in mice

Background

The emerging role of microglia in neurological disorders requires a novel method for obtaining massive amounts of adult microglia. We aim to develop a new method for obtaining bankable and expandable adult-like microglia in mice.

Methods

The head neuroepithelial layer (NEL) that composed of microglial progenitor and neuroepithelial cells at mouse E13.5 was dissected and then cultured or banked. Microglia (MG) isolated from the cultured NEL by magnetic-activated cell sorting system were obtained and named NEL-MG.

Results

The NEL included microglia progenitors that proliferate and ramify over time with neuroepithelial cells as feeder. In functional analysis, NEL-MG exhibited microglial functions, such as phagocytosis (microbeads, amyloid β, synaptosome), migration, and inflammatory response following lipopolysaccharide (LPS) stimulation. NEL was passage cultured and the NEL-MG exhibited a higher expression of microglia signature genes than the neonatal microglia, a widely used in vitro surrogate. Banking or long-term passage culture of NEL did not affect NEL-MG characteristics. Transcriptome analysis revealed that NEL-MG exhibited better conservation of microglia signature genes with a closer fidelity to freshly isolated adult microglia than neonatal microglia. NEL-MG could be re-expandable when they were plated again on neuroepithelial cells.

Conclusions

This new method effectively contributes to obtaining sufficient matured form of microglia (adult-like microglia), even when only a small number of experimental animals are available, leading to a broad application in the field of neuroscience.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-021-02351-4.

Signaling inflammation across the gut-brain axis

The brain and gastrointestinal tract are critical sensory organs responsible for detecting, relaying, integrating, and responding to signals derived from the internal and external environment. At the interface of this sensory function, immune cells in the intestines and brain consistently survey environmental factors, eliciting responses that inform on the physiological state of the body. Recent research reveals that cross-talk along the gut-brain axis regulates inflammatory nociception, inflammatory responses, and immune homeostasis. Here, we discuss molecular and cellular mechanisms involved in the signaling of inflammation across the gut-brain axis. We further highlight interactions between the gut and the brain in inflammation-associated diseases.

Cortical and phase rim lesions on 7 T MRI as markers of multiple sclerosis disease progression

In multiple sclerosis, individual lesion-type patterns on magnetic resonance imaging might be valuable for predicting clinical outcome and monitoring treatment effects. Neuropathological and imaging studies consistently show that cortical lesions contribute to disease progression. The presence of chronic active white matter lesions harbouring a paramagnetic rim on susceptibility-weighted magnetic resonance imaging has also been associated with an aggressive form of multiple sclerosis. It is, however, still uncertain how these two types of lesions relate to each other, or which one plays a greater role in disability progression. In this prospective, longitudinal study in 100 multiple sclerosis patients (74 relapsing-remitting, 26 secondary progressive), we used ultra-high field 7-T susceptibility imaging to characterize cortical and rim lesion presence and evolution. Clinical evaluations were obtained over a mean period of 3.2 years in 71 patients, 46 of which had a follow-up magnetic resonance imaging. At baseline, cortical and rim lesions were identified in 96% and 63% of patients, respectively. Rim lesion prevalence was similar across disease stages. Patients with rim lesions had higher cortical and overall white matter lesion load than subjects without rim lesions (P = 0.018-0.05). Altogether, cortical lesions increased by both count and volume (P = 0.004) over time, while rim lesions expanded their volume (P = 0.023) whilst lacking new rim lesions; rimless white matter lesions increased their count but decreased their volume (P = 0.016). We used a modern machine learning algorithm based on extreme gradient boosting techniques to assess the cumulative power as well as the individual importance of cortical and rim lesion types in predicting disease stage and disability progression, alongside with more traditional imaging markers. The most influential imaging features that discriminated between multiple sclerosis stages (area under the curve±standard deviation = 0.82 ± 0.08) included, as expected, the normalized white matter and thalamic volume, white matter lesion volume, but also leukocortical lesion volume. Subarachnoid cerebrospinal fluid and leukocortical lesion volumes, along with rim lesion volume were the most important predictors of Expanded Disability Status Scale progression (area under the curve±standard deviation = 0.69 ± 0.12). Taken together, these results indicate that while cortical lesions are extremely frequent in multiple sclerosis, rim lesion development occurs only in a subset of patients. Both, however, persist over time and relate to disease progression. Their combined assessment is needed to improve the ability of identifying multiple sclerosis patients at risk of progressing disease.

Precisely Shaped Self-Adjuvanting Peptide Vaccines with Enhanced Immune Responses for HPV-Associated Cancer Therapy

Peptide vaccines exhibit great potential in cancer therapy via eliciting antigen-specific host immune response and long-term immune memory to defend cancer cells. However, the low induced immune response of many developing vaccines implies the imperatives for understanding the favorable structural features of efficient cancer vaccines. Herein, we report on the two groups of self-adjuvanting peptide vaccines with distinct morphology and investigate the relationship between the morphology of peptide vaccines and the induced immune response. Two nanofibril peptide vaccines were created via co-assembly of a pentapeptide with a central 4-aminoproline residue, with its derivative functionalized with antigen epitopes derived from human papillomavirus E7 proteins, whereas utilization of a pentapeptide with a natural proline residue led to the formation of two nanoparticle peptide vaccines. The immunological results of dendritic cell (DCs) maturation and antigen presentation induced by the peptide assemblies implied the self-adjuvanting property of the resulting peptide vaccines. In particular, cellular uptake studies revealed the enhanced internalization and elongated retention of the nanofibril peptide vaccines in DCs, leading to their advanced performance in DC maturation, accumulation at lymph nodes, infiltration of cytotoxic T lymphocytes into tumor tissues, and eventually lysis of in vivo tumor cells, compared to the nanoparticle counterparts. The antitumor immune response caused by the nanofibril peptide vaccines was further augmented when simultaneously administrated with anti-PD-1 checkpoint blockades, suggesting the opportunity of the combinatorial immunotherapy by utilizing the nanofibril peptide vaccines. Our findings strongly demonstrate a robust relationship between the immune response of peptide vaccines and their morphology, thereby elucidating the critical role of morphological control in the design of efficient peptide vaccines and providing the guidance for the design of efficient peptide vaccines in the future.

Lipopolysaccharide-induced maternal immune activation modulates microglial CX3CR1 protein expression and morphological phenotype in the hippocampus and dentate gyrus, resulting in cognitive inflexibility during late adolescence

Inflammation during pregnancy can disturb brain development and lead to disorders in the progeny, including autism spectrum disorder and schizophrenia. However, the mechanism by which a prenatal, short-lived increase of cytokines results in adverse neurodevelopmental outcomes remains largely unknown. Microglia-the brain's resident immune-cells-stand as fundamental cellular mediators, being highly sensitive and responsive to immune signals, which also play key roles during normal development. The fractalkine signaling axis is a neuron-microglia communication mechanism used to regulate neurogenesis and network formation. Previously, we showed hippocampal reduction of fractalkine receptor (Cx3cr1) mRNA at postnatal day (P) 15 in male offspring exposed to maternal immune activation induced with lipopolysaccharide (LPS) during late gestation, which was concomitant to an increased dendritic spine density in the dentate gyrus, a neurogenic niche. The current study sought to evaluate the origin and impact of this reduced hippocampal Cx3cr1 mRNA expression on microglia and cognition. We found that microglial total cell number and density are not affected in the dorsal hippocampus and dentate gyrus, respectively, but that the microglial CX3CR1 protein is decreased in the hippocampus of LPS-male offspring at P15. Further characterization of microglial morphology in the dentate gyrus identified a more ameboid phenotype in LPS-exposed offspring, predominantly in males, at P15. We thus explored maternal plasma and fetal brain cytokines to understand the mechanism behind microglial priming, showing a robust immune activation in the mother at 2 and 4 hrs after LPS administration, while only IL-10 tended towards upregulation at 2 hrs after LPS in fetal brains. To evaluate the functional long-term consequences, we assessed learning and cognitive flexibility behavior during late adolescence, finding that LPS affects only the latter with a male predominance on perseveration. A CX3CR1 gene variant in humans that results in disrupted fractalkine signaling has been recently associated with an increased risk for neurodevelopmental disorders. We show that an acute immune insult during late gestation can alter fractalkine signaling by reducing the microglial CX3CR1 protein expression, highlighting neuron-microglial fractalkine signaling as a relevant target underlying the outcomes of environmental risk factors on neurodevelopmental disorders.

Absence of Apolipoprotein E is associated with exacerbation of prion pathology and promotes microglial neurodegenerative phenotype

Prion diseases or prionoses are a group of rapidly progressing and invariably fatal neurodegenerative diseases. The pathogenesis of prionoses is associated with self-replication and connectomal spread of PrP^Sc, a disease specific conformer of the prion protein. Microglia undergo activation early in the course of prion pathogenesis and exert opposing roles in PrP^Sc mediated neurodegeneration. While clearance of PrP^Sc and apoptotic neurons have disease-limiting effect, microglia-driven neuroinflammation bears deleterious consequences to neuronal networks. Apolipoprotein (apo) E is a lipid transporting protein with pleiotropic functions, which include controlling of the phagocytic and inflammatory characteristics of activated microglia in neurodegenerative diseases. Despite the significance of microglia in prion pathogenesis, the role of apoE in prionoses has not been established. We showed here that infection of wild type mice with 22L mouse adapted scrapie strain is associated with significant increase in the total brain apoE protein and mRNA levels and also with a conspicuous cell-type shift in the apoE expression. There is reduced expression of apoE in activated astrocytes and marked upregulation of apoE expression by activated microglia. We also showed apoE ablation exaggerates PrP^Sc mediated neurodegeneration. Apoe^−/− mice have shorter disease incubation period, increased load of spongiform lesion, pronounced neuronal loss, and exaggerated astro and microgliosis. Astrocytes of Apoe^−/− mice display salient upregulation of transcriptomic markers defining A1 neurotoxic astrocytes while microglia show upregulation of transcriptomic markers characteristic for microglial neurodegenerative phenotype. There is impaired clearance of PrP^Sc and dying neurons by microglia in Apoe^−/− mice along with increased level of proinflammatory cytokines. Our work indicates that apoE absence renders clearance of PrP^Sc and dying neurons by microglia inefficient, while the excess of neuronal debris promotes microglial neurodegenerative phenotype aggravating the vicious cycle of neuronal death and neuroinflammation.

Specific microglial phagocytic phenotype and decrease of lipid oxidation in white matter areas during aging: Implications of different microenvironments

Physiological aging is characterized by an imbalance of pro-inflammatory and anti-inflammatory mediators leading to neuroinflammation. Microglial cells, which are highly regulated by the local microenvironment, undergo specific changes depending upon the brain area during aging. The aim of this study was to evaluate the influence of age over microglial cells along different brain areas and microenvironments. For this purpose, transgenic mice with overproduction of either the anti-inflammatory IL-10 cytokine or the pro-inflammatory IL-6 cytokine were used. Our results show that, during aging, microglial cells located in white matter (WM) areas maintain their phagocytic capacity but present a specific phagocytic phenotype with receptors involved in myelin recognition, arguing for aging-derived myelin damage. Whereas IL-10 overproduction anticipates the age-related microglial phagocytic phenotype, maintaining it over time, IL-6 overproduction exacerbates this phenotype in aging. These modifications were linked with a higher efficiency of myelin engulfment by microglia in aged transgenic animals. Moreover, we show, in a novel way, lower lipid oxidation during aging in WM areas, regardless of the genotype. The novelty of the insights presented in this study open a window to deeply investigate myelin lipid oxidation and the role of microglial cells in its regulation during physiological aging.

Microglial Calhm2 regulates neuroinflammation and contributes to Alzheimer's disease pathology

Alzheimer's disease (AD) is the most common neurodegenerative disease in the world. Neuronal calcium dysfunction and microglial-mediated neuroinflammation are closely associated with the development of AD. However, it remains unknown whether calcium dysfunction contributes to microglial activation and, in turn, AD pathology in vivo. In this study, we demonstrated that the expression of calcium homeostasis modulator family protein 2 (Calhm2) is increased in an AD mouse model. In 5×FAD mice carrying five familial AD gene mutations, both conventional knockout of Calhm2 and conditional microglial knockout of Calhm2 significantly reduced amyloid β deposition, neuroinflammation, and cognitive impairments. Mechanistically, knockout of Calhm2 inhibited microglial proinflammatory activity but increased phagocytic activity, leading to restoration of the balance between inflammation and phagocytosis. In addition, knockout of Calhm2 reduced acute LPS-induced neuroinflammation. These results highlight an important role for Calhm2 in microglial activation and provide a potential therapeutic target for diseases related to microglia-mediated neuroinflammation.

Microglia Morphological Changes in the Motor Cortex of hSOD1G93A Transgenic ALS Mice

Amyotrophic lateral sclerosis (ALS) is characterized by the progressive degeneration of spinal motor neurons as well as corticospinal (CSN) large pyramidal neurons within cortex layer V. An intense microglia immune response has been associated with both upper and lower motor neuron degeneration in ALS patients, whereas microgliosis occurrence in the motor cortex of hSOD1^G93A mice—the best characterized model of this disease—is not clear and remains under debate. Since the impact of microglia cells in the neuronal environment seems to be crucial for both the initiation and the progression of the disease, here we analyzed the motor cortex of hSOD1^G93A mice at the onset of symptoms by the immunolabeling of Iba1/TMEM119 double positive cells and confocal microscopy. By means of Sholl analysis, we were able to identify and quantify the presence of presumably activated Iba1/TMEM119-positive microglia cells with shorter and thicker processes as compared to the normal surveilling and more ramified microglia present in WT cortices. We strongly believe that being able to analyze microglia activation in the motor cortex of hSOD1^G93A mice is of great importance for defining the timing and the extent of microglia involvement in CSN degeneration and for the identification of the initiation stages of this disease.

Microglia Dysfunction Caused by the Loss of Rhoa Disrupts Neuronal Physiology and Leads to Neurodegeneration

Nervous tissue homeostasis requires the regulation of microglia activity. Using conditional gene targeting in mice, we demonstrate that genetic ablation of the small GTPase Rhoa in adult microglia is sufficient to trigger spontaneous microglia activation, producing a neurological phenotype (including synapse and neuron loss, impairment of long-term potentiation [LTP], formation of β-amyloid plaques, and memory deficits). Mechanistically, loss of Rhoa in microglia triggers Src activation and Src-mediated tumor necrosis factor (TNF) production, leading to excitotoxic glutamate secretion. Inhibiting Src in microglia Rhoa-deficient mice attenuates microglia dysregulation and the ensuing neurological phenotype. We also find that the Rhoa/Src signaling pathway is disrupted in microglia of the APP/PS1 mouse model of Alzheimer disease and that low doses of Aβ oligomers trigger microglia neurotoxic polarization through the disruption of Rhoa-to-Src signaling. Overall, our results indicate that disturbing Rho GTPase signaling in microglia can directly cause neurodegeneration.

TREM2/PLCγ2 signalling in immune cells: function, structural insight, and potential therapeutic modulation

The central role of the resident innate immune cells of the brain (microglia) in neurodegeneration has become clear over the past few years largely through genome-wide association studies (GWAS), and has rapidly become an active area of research. However, a mechanistic understanding (gene to function) has lagged behind. That is now beginning to change, as exemplified by a number of recent exciting and important reports that provide insight into the function of two key gene products – TREM2 (Triggering Receptor Expressed On Myeloid Cells 2) and PLCγ2 (Phospholipase C gamma2) – in microglia, and their role in neurodegenerative disorders. In this review we explore and discuss these recent advances and the opportunities that they may provide for the development of new therapies.

The effect of aged microglia on synaptic impairment and its relevance in neurodegenerative diseases

Microglia serve key functions in the central nervous system (CNS), participating in the establishment and regulation of synapses and the neuronal network, and regulating activity-dependent plastic changes. As the neuroimmune system, they respond to endogenous and exogenous signals to protect the CNS. In aging, one of the main changes is the establishment of inflamm-aging, a mild chronic inflammation that reduces microglial response to stressors. Neuroinflammation depends mainly on the increased activation of microglia. Microglia over-activation may result in a reduced capacity for performing normal functions related to migration, clearance, and the adoption of an anti-inflammatory state, contributing to an increased susceptibility for neurodegeneration. Oxidative stress contributes both to aging and to the progression of neurodegenerative diseases. Increased production of reactive oxygen species (ROS) and neuroinflammation associated with age- and disease-dependent mechanisms affect synaptic activity and neurotransmission, leading to cognitive dysfunction. Astrocytes prevent microglial cell cytotoxicity by mechanisms mediated by transforming growth factor β1 (TGFβ1). However, TGFβ1-Smad3 pathway is impaired in aging, and the age-related impairment of TGFβ signaling can reduce protective activation while facilitating cytotoxic activation of microglia. A critical analysis on the effect of aging microglia on neuronal function is relevant for the understanding of age-related changes on neuronal function. Here, we present evidence in the context of the "microglial dysregulation hypothesis", which leads to the reduction of the protective functions and increased cytotoxicity of microglia, to discuss the mechanisms involved in neurodegenerative changes and Alzheimer's disease.

The Survival Benefit of Postoperative Bacterial Infections in Patients With Glioblastoma Multiforme: Myth or Reality?

Glioblastoma multiforme (GBM), the most common malignant brain tumor, universally carries a poor prognosis. Despite aggressive multimodality treatment, the median survival is ~18-20 months, depending on molecular subgroups. A long history of observations suggests antitumor effects of bacterial infections against malignant tumors. The present review summarizes and critically analyzes the clinical data providing evidence for or against the survival benefit of post-operative bacterial infections in GBM patients. Furthermore, we explore the probable underlying mechanism(s) from basic science studies on the topic. There are plausible explanations from immunobiology for the mechanism of the "favorable effect" of bacterial infections in GBM patients. However, available clinical literature does not provide a definitive association between postoperative bacterial infection and prolonged survival in GBM patients. The presently available, single-/multi-center and national database retrospective case-control studies on the topic provide conflicting results. A prospective randomized study on the subject is clearly not possible. Immunobiology literature supports development of genetically modified bacteria as part of multimodal regimen against GBM.

Diet-dependent regulation of TGFβ impairs reparative innate immune responses after demyelination

Proregenerative responses are required for the restoration of nervous-system functionality in demyelinating diseases such as multiple sclerosis (MS). Yet, the limiting factors responsible for poor CNS repair are only partially understood. Here, we test the impact of a Western diet (WD) on phagocyte function in a mouse model of demyelinating injury that requires microglial innate immune function for a regenerative response to occur. We find that WD feeding triggers an ageing-related, dysfunctional metabolic response that is associated with impaired myelin-debris clearance in microglia, thereby impairing lesion recovery after demyelination. Mechanistically, we detect enhanced transforming growth factor beta (TGFβ) signalling, which suppresses the activation of the liver X receptor (LXR)-regulated genes involved in cholesterol efflux, thereby inhibiting phagocytic clearance of myelin and cholesterol. Blocking TGFβ or promoting triggering receptor expressed on myeloid cells 2 (TREM2) activity restores microglia responsiveness and myelin-debris clearance after demyelinating injury. Thus, we have identified a druggable microglial immune checkpoint mechanism regulating the microglial response to injury that promotes remyelination.

Mechanisms underlying progression in multiple sclerosis

PURPOSE OF REVIEW

In multiple sclerosis, currently approved disease-modifying treatments are effective in modulating peripheral immunity, and coherently, in reducing clinical/radiological relapses, but still, they perform poorly in preventing disease progression and overall disability accrual. This review provides an up-to-date overview of the neuropathology of progressive multiple sclerosis, including a summary of the main mechanisms of disease progression.

RECENT FINDINGS

Clinical progression in multiple sclerosis is likely related to the accumulation of neuro-axonal loss in a lifelong inflammatory CNS environment (both adaptive and innate) and relative un-balance between damage, repair and brain functional reserve. A critical driver appears to be the T-cell and B-cell-mediated compartmentalized inflammation within the leptomeninges and within the parenchyma. Recent perspective highlighted also the role of the glial response to such lifelong inflammatory injury as the critical player for both pathological and clinical outcomes.

SUMMARY

The neuropathological and biological understanding of disease progression in multiple sclerosis have progressed in the last few years. As a consequence, new therapeutic approaches are emerging outside the modulation of T-cell activity and/or the depletion of B cells.

Microglia in neurodegenerative diseases

A major feature of neurodegeneration is disruption of central nervous system homeostasis, during which microglia play diverse roles. In the central nervous system, microglia serve as the first line of immune defense and function in synapse pruning, injury repair, homeostasis maintenance, and regulation of brain development through scavenging and phagocytosis. Under pathological conditions or various stimulations, microglia proliferate, aggregate, and undergo a variety of changes in cell morphology, immunophenotype, and function. This review presents the features of microglia, especially their diversity and ability to change dynamically, and reinterprets their role as sensors for multiple stimulations and as effectors for brain aging and neurodegeneration. This review also summarizes some therapeutic approaches for neurodegenerative diseases that target microglia.

Transcriptomic characterization of microglia activation in a rat model of ischemic stroke

Microglia are key regulators of inflammatory response after stroke and brain injury. To better understand activation of microglia as well as their phenotypic diversity after ischemic stroke, we profiled the transcriptome of microglia after 75 min transient focal cerebral ischemia in 3-month- and 12-month-old male spontaneously hypertensive rats. Microglia were isolated from the brains by FACS sorting on days 3 and 14 after cerebral ischemia. GeneChip Rat 1.0ST microarray was used to profile the whole transcriptome of sorted microglia. We identified an evolving and complex pattern of activation from 3 to 14 days after stroke onset. M2-like patterns were extensively and persistently upregulated over time. M1-like patterns were only mildly upregulated, mostly at day 14. Younger 3-month-old brains showed a larger microglial response in both pro- and anti-inflammatory pathways, compared to older 12-month-old brains. Importantly, our data revealed that after stroke, most microglia are activated towards a wide spectrum of novel polarization states beyond the standard M1/M2 dichotomy, especially in pathways related to TLR2 and dietary fatty acid signaling. Finally, classes of transcription factors that might potentially regulate microglial activation were identified. These findings should provide a comprehensive database for dissecting microglial mechanisms and pursuing neuroinflammation targets for acute ischemic stroke.

CNS-Native Myeloid Cells Drive Immune Suppression in the Brain Metastatic Niche through Cxcl10

Brain metastasis (br-met) develops in an immunologically unique br-met niche. Central nervous system-native myeloid cells (CNS-myeloids) and bone-marrow-derived myeloid cells (BMDMs) cooperatively regulate brain immunity. The phenotypic heterogeneity and specific roles of these myeloid subsets in shaping the br-met niche to regulate br-met outgrowth have not been fully revealed. Applying multimodal single-cell analyses, we elucidated a heterogeneous but spatially defined CNS-myeloid response during br-met outgrowth. We found Ccr2+ BMDMs minimally influenced br-met while CNS-myeloid promoted br-met outgrowth. Additionally, br-met-associated CNS-myeloid exhibited downregulation of Cx3cr1. Cx3cr1 knockout in CNS-myeloid increased br-met incidence, leading to an enriched interferon response signature and Cxcl10 upregulation. Significantly, neutralization of Cxcl10 reduced br-met, while rCxcl10 increased br-met and recruited VISTAHi PD-L1+ CNS-myeloid to br-met lesions. Inhibiting VISTA- and PD-L1-signaling relieved immune suppression and reduced br-met burden. Our results demonstrate that loss of Cx3cr1 in CNS-myeloid triggers a Cxcl10-mediated vicious cycle, cultivating a br-met-promoting, immune-suppressive niche.

Dynamic Neuroimmune Profile during Mid-life Aging in the Female Brain and Implications for Alzheimer Risk

Aging and endocrine transition states can significantly impact inflammation across organ systems. Neuroinflammation is well documented in Alzheimer disease (AD). Herein, we investigated neuroinflammation that emerges during mid-life aging, chronological and endocrinological, in the female brain as an early initiating mechanism driving AD risk later in life. Analyses were conducted in a translational rodent model of mid-life chronological and endocrinological aging followed by validation in transcriptomic profiles from women versus age-matched men. In the translational model, the neuroinflammatory profile of mid-life aging in females was endocrine and chronological state specific, dynamic, anatomically distributed, and persistent. Microarray dataset analyses of aging human hippocampus indicated a sex difference in neuroinflammatory profile in which women exhibited a profile comparable to the pattern discovered in our translational rodent model, whereas age-matched men exhibited a profile consistent with low neuroimmune activation. Translationally, these findings have implications for therapeutic interventions during mid-life to decrease late-onset AD risk.

How Repair-or-Dispose Decisions Under Stress Can Initiate Disease Progression

Glia, the helper cells of the brain, are essential in maintaining neural resilience across time and varying challenges: By reacting to changes in neuronal health glia carefully balance repair or disposal of injured neurons. Malfunction of these interactions is implicated in many neurodegenerative diseases. We present a reductionist model that mimics repair-or-dispose decisions to generate a hypothesis for the cause of disease onset. The model assumes four tissue states: healthy and challenged tissue, primed tissue at risk of acute damage propagation, and chronic neurodegeneration. We discuss analogies to progression stages observed in the most common neurodegenerative conditions and to experimental observations of cellular signaling pathways of glia-neuron crosstalk. The model suggests that the onset of neurodegeneration can result as a compromise between two conflicting goals: short-term resilience to stressors versus long-term prevention of tissue damage.

Association of Chronic Active Multiple Sclerosis Lesions With Disability In Vivo

Importance

In multiple sclerosis (MS), chronic active lesions, which previously could only be detected at autopsy, can now be identified on susceptibility-based magnetic resonance imaging (MRI) in vivo as non-gadolinium-enhancing lesions with paramagnetic rims. Pathologically, they feature smoldering inflammatory demyelination at the edge, remyelination failure, and axonal degeneration. To our knowledge, the prospect of long-term in vivo monitoring makes it possible for the first time to determine their contribution to disability and value as a treatment target.

Objective

To assess whether rim lesions are associated with patient disability and long-term lesion outcomes.

Design, Setting, Participants

We performed 3 studies at the National Institutes of Health Clinical Center: (1) a prospective clinical/radiological cohort of 209 patients with MS (diagnosis according to the 2010 McDonald revised MS criteria, age ≥18 years, with 7-T or 3-T susceptibility-based brain MRI results) who were enrolled from January 2012 to March 2018 (of 209, 17 patients [8%] were excluded because of uninterpretable MRI scans); (2) a radiological/pathological analysis of expanding lesions featuring rims; and (3) a retrospective longitudinal radiological study assessing long-term lesion evolution in 23 patients with MS with yearly MRI scans for 10 years or more (earliest scan, 1992).

Main Outcomes and Measures

(1) Identification of chronic rim lesions on 7-T or 3-T susceptibility-based brain MRI in 192 patients with MS and the association of rim counts with clinical disability (primary analysis) and brain volume changes (exploratory analysis). (2) Pathological characterization of 10 expanding lesions from an adult with progressive MS who came to autopsy after 7 years of receiving serial in vivo MRI scans. (3) Evaluation of annual lesion volume change (primary analysis) and T1 times (exploratory analysis) in 27 rim lesions vs 27 rimless lesions.

Results

Of 209 participants, 104 (50%) were women and 32 (15%) were African American. One hundred seventeen patients (56%) had at least 1 rim lesion regardless of prior or ongoing treatment. Further, 84 patients (40%) had no rims (mean [SD] age, 47 [14] years), 66 (32%) had 1 to 3 rims (mean [SD] age, 47 [11] years), and 42 (20%) had 4 rims or more (mean [SD] age, 44 [11] years). Individuals with 4 rim lesions or more reached motor and cognitive disability at an earlier age. Normalized volumes of brain, white matter, and basal ganglia were lower in those with rim lesions. Whereas rimless lesions shrank over time (-3.6%/year), rim lesions were stable in size or expanded (2.2%/year; P < .001). Rim lesions had longer T1 times, suggesting more tissue destruction, than rimless lesions. On histopathological analysis, all 10 rim lesions that expanded in vivo had chronic active inflammation.

Conclusions and Relevance

Chronic active lesions are common, are associated with more aggressive disease, exert ongoing tissue damage, and occur even in individuals treated with effective disease-modifying therapies. These results prompt the planning of MRI-based clinical trials aimed at treating perilesional chronic inflammation in MS.

Microglia and Inflammatory Responses in Diabetic Retinopathy

Diabetic retinopathy is a vision-threatening disease affecting neurons and microvasculature of the retina. The development of this disease is associated with the action of inflammatory factors that are connected to the activation of microglial cells, the resident tissue macrophages of the CNS. In the quiescent state, microglial cells help maintain tissue homeostasis in the retina through phagocytosis and control of low-grade inflammation. However, prolonged tissue stress due to hyperglycemia primes microglia to become overly reactive with the concomitant production of pro-inflammatory cytokines and chemokines causing chronic inflammation. In this review, we provide evidence of microglial cell activation and pro-inflammatory molecules associated with the development and progression of diabetic retinopathy. We further highlight innovative animal models that can mimic the disease in humans and discuss strategies in modulating microglial-mediated inflammation as potential therapeutic approaches in managing the disease.

The Psychopharmacology of Obsessive-Compulsive Disorder: A Preclinical Roadmap

This review evaluates current knowledge about obsessive-compulsive disorder (OCD), with the goal of providing a roadmap for future directions in research on the psychopharmacology of the disorder. It first addresses issues in the description and diagnosis of OCD, including the structure, measurement, and appropriate description of the disorder and issues of differential diagnosis. Current pharmacotherapies for OCD are then reviewed, including monotherapy with serotonin reuptake inhibitors and augmentation with antipsychotic medication and with psychologic treatment. Neuromodulatory therapies for OCD are also described, including psychosurgery, deep brain stimulation, and noninvasive brain stimulation. Psychotherapies for OCD are then reviewed, focusing on behavior therapy, including exposure and response prevention and cognitive therapy, and the efficacy of these interventions is discussed, touching on issues such as the timing of sessions, the adjunctive role of pharmacotherapy, and the underlying mechanisms. Next, current research on the neurobiology of OCD is examined, including work probing the role of various neurotransmitters and other endogenous processes and etiology as clues to the neurobiological fault that may underlie OCD. A new perspective on preclinical research is advanced, using the Research Domain Criteria to propose an adaptationist viewpoint that regards OCD as the dysfunction of a normal motivational system. A systems-design approach introduces the security motivation system (SMS) theory of OCD as a framework for research. Finally, a new perspective on psychopharmacological research for OCD is advanced, exploring three approaches: boosting infrastructure facilities of the brain, facilitating psychotherapeutic relearning, and targeting specific pathways of the SMS network to fix deficient SMS shut-down processes. SIGNIFICANCE STATEMENT: A significant proportion of patients with obsessive-compulsive disorder (OCD) do not achieve remission with current treatments, indicating the need for innovations in psychopharmacology for the disorder. OCD may be conceptualized as the dysfunction of a normal, special motivation system that evolved to manage the prospect of potential danger. This perspective, together with a wide-ranging review of the literature, suggests novel directions for psychopharmacological research, including boosting support systems of the brain, facilitating relearning that occurs in psychotherapy, and targeting specific pathways in the brain that provide deficient stopping processes in OCD.

[The association of aromatic microbial metabolites, inflammatory and autoimmune biomarkers with clinical dynamics in severe diseases of the central nervous system]

OBJECTIVE

To identify the association between the changes in the profile of microbial metabolites, inflammatory and autoimmune markers and the dynamics of neurological status in chronic critically ill patients with diseases of the central nervous system (CNS).

MATERIAL AND METHODS

Sixty serum samples from 37 patients, aged 19-77 years, with severe CNS diseases were studied. The changes in clinical condition were assessed with NIHSS, the Rankin scale, the Glasgow Coma Scale, the FOUR Coma Scale and the Rivermead Mobility Index. The levels of aromatic microbial metabolites (AMM) and several inflammatory and autoimmune biomarkers, including the contents of procalcitonin (PCT) and S100, the activity of leukocyte elastase (LE) and a1-proteinase inhibitor a1-PI, the levels of autoantibodies to S100b and MBP were measured. Serum from 60 age- and sex-matched healthy people with no signs of neurological and somatic pathology was used as a control.

RESULTS

All patients were divided into groups depending on the neurological dynamics: A - positive (n=16), B - without dynamics (n=15), C - negative (n=6). The study revealed a profile of AMM, as well as inflammatory and autoimmune biomarkers associated with the severity of neurological disorders. A significant increase in acute phase proteins, S-100 level and a decrease in the functional activity of neutrophils (via LE activity) were observed in the serum of patients. The different dynamics of neurological status was associated with the multidirectional changes in the microbial metabolites profile and biomarkers. The correlations between the clinical and biological parameters indicate that AMM might modulate immune reactions in patients with different dynamics of neurological status.

CONCLUSION

The results suggest the involvement of AMM and the level of immune activation via biomarkers in the pathogenesis of neurological dysfunction. Dynamic changes in the profile of microbial metabolites and the level of activation of the immune system may be a promising tool for prediction of neurological recovery.

Microglia Gone Awry: Linking Immunometabolism to Neurodegeneration

Age-related chronic inflammatory activation of microglia and their dysfunction are observed in many neurodegenerative diseases, and the potential contributions of these dysfunctional cells to neurodegeneration have been demonstrated recently. The housekeeping and defensive functions of microglia, such as surveying the brain parenchyma and phagocytosis of neuronal debris after injury, are important for brain homeostasis and immunity. During neurodegenerative diseases, loss of these functions can promote disease pathology by producing proinflammatory cytokines and increasing oxidative stress, which can exaggerate the ongoing neuroinflammation. A recent surge in microglial research has unraveled myriads of microglial phenotypes associated with aging and neurodegenerative diseases, in addition to the conventional M1/M2 paradigm. Each of these phenotypes can be characterized by distinct transcriptional profiles as well as altered metabolism, migration, and phagocytosis characteristics. Mutations in triggering receptor expressed on myeloid cells 2 (Trem2) and granulin (GRN) are associated with various neurodegenerative diseases, and these genes are dysregulated in the majority of recently identified microglial phenotypes. These genes act as checkpoint regulators and maintain microglial inflammatory fitness, principally through metabolic modulation. Dysfunctional microglia typically show mitochondrial deficits, glycolysis elevation, and lipid droplet accumulation, which results in reduced migration and phagocytosis and increased proinflammatory cytokine secretion and reactive oxygen species release. In this mini-review article, we discuss the existing data regarding metabolic perturbations in dysfunctional microglia and their documented associations with neurodegeneration, highlighting how aging-induced chronic microglial activation alters microglial bioenergetics, leading to impaired homeostatic and housekeeping functions. Dysfunctional microglia initiate or exacerbate neurodegeneration, and key pathways involved in the dysfunctional processes, including metabolism, may represent potential intervention targets for correcting imbalances.

Immunity in amyotrophic lateral sclerosis: blurred lines between excessive inflammation and inefficient immune responses

Despite wide genetic, environmental and clinical heterogeneity in amyotrophic lateral sclerosis, a rapidly fatal neurodegenerative disease targeting motoneurons, neuroinflammation is a common finding. It is marked by local glial activation, T cell infiltration and systemic immune system activation. The immune system has a prominent role in the pathogenesis of various chronic diseases, hence some of them, including some types of cancer, are successfully targeted by immunotherapeutic approaches. However, various anti-inflammatory or immunosuppressive therapies in amyotrophic lateral sclerosis have failed. This prompted increased scrutiny over the immune-mediated processes underlying amyotrophic lateral sclerosis. Perhaps the biggest conundrum is that amyotrophic lateral sclerosis pathogenesis exhibits features of three otherwise distinct immune dysfunctions-excessive inflammation, autoimmunity and inefficient immune responses. Epidemiological and genome-wide association studies show only minimal overlap between amyotrophic lateral sclerosis and autoimmune diseases, so excessive inflammation is usually thought to be secondary to protein aggregation, mitochondrial damage or other stresses. In contrast, several recently characterized amyotrophic lateral sclerosis-linked mutations, including those in TBK1, OPTN, CYLD and C9orf72, could lead to inefficient immune responses and/or damage pile-up, suggesting that an innate immunodeficiency may also be a trigger and/or modifier of this disease. In such cases, non-selective immunosuppression would further restrict neuroprotective immune responses. Here we discuss multiple layers of immune-mediated neuroprotection and neurotoxicity in amyotrophic lateral sclerosis. Particular focus is placed on individual patient mutations that directly or indirectly affect the immune system, and the mechanisms by which these mutations influence disease progression. The topic of immunity in amyotrophic lateral sclerosis is timely and relevant, because it is one of the few common and potentially malleable denominators in this heterogenous disease. Importantly, amyotrophic lateral sclerosis progression has recently been intricately linked to patient T cell and monocyte profiles, as well as polymorphisms in cytokine and chemokine receptors. For this reason, precise patient stratification based on immunophenotyping will be crucial for efficient therapies.

GM-CSF induces noninflammatory proliferation of microglia and disturbs electrical neuronal network rhythms in situ

Background

The granulocyte-macrophage colony-stimulating factor (GM-CSF) (or CSF-2) is involved in myeloid cell growth and differentiation, and, possibly, a major mediator of inflammation in body tissues. The role of GM-CSF in the activation of microglia (CNS resident macrophages) and the consequent impacts on neuronal survival, excitability, and synaptic transmission are widely unknown, however. Here, we focused on electrical neuronal network rhythms in the gamma frequency band (30–70 Hz). Gamma oscillations are fundamental to higher brain functions, such as perception, attention, and memory, and they are exquisitely sensitive to metabolic and oxidative stress.

Methods

We explored the effects of chronic GM-CSF exposure (72 h) on microglia in male rat organotypic hippocampal slice cultures (in situ), i.e., postnatal cortex tissue lacking leukocyte invasion (adaptive immunity). We applied extracellular electrophysiological recordings of local field potential, immunohistochemistry, design-based stereology, biochemical analysis, and pharmacological ablation of microglia.

Results

GM-CSF triggered substantial proliferation of microglia (microgliosis). By contrast, the release of proinflammatory cytokines (IL-6, TNF-α) and nitric oxide, the hippocampal cytoarchitecture as well as the morphology of parvalbumin-positive inhibitory interneurons were unaffected. Notably, GM-CSF induced concentration-dependent, long-lasting disturbances of gamma oscillations, such as slowing (beta frequency band) and neural burst firing (hyperexcitability), which were not mimicked by the T lymphocyte cytokine IL-17. These disturbances were attenuated by depletion of the microglial cell population with liposome-encapsulated clodronate. In contrast to priming with the cytokine IFN-γ (type II interferon), GM-CSF did not cause inflammatory neurodegeneration when paired with the TLR4 ligand LPS.

Conclusions

GM-CSF has a unique role in the activation of microglia, including the potential to induce neuronal network dysfunction. These immunomodulatory properties might contribute to cognitive impairment and/or epileptic seizure development in disease featuring elevated GM-CSF levels, blood-brain barrier leakage, and/or T cell infiltration.

Selective inhibition of mitochondrial respiratory complexes controls the transition of microglia into a neurotoxic phenotype in situ

Microglia are tissue resident macrophages (innate immunity) and universal sensors of alterations in CNS physiology. In response to pathogen or damage signals, microglia feature rapid activation and can acquire different phenotypes exerting neuroprotection or neurotoxicity. Although transcriptional aspects of microglial phenotypic transitions have been described, the underlying metabolic reprogramming is widely unknown. Employing postnatal organotypic hippocampal slice cultures, we describe that microglia transformed into a mild reactive phenotype by single TLR4 stimulation with lipopolysaccharide (LPS), which was boosted into a severe neurotoxic phenotype by IFN-γ (LPS + INF-γ). The two reactive phenotypes associated with reduction of microglial homeostatic "surveillance" markers, increase of cytokine release (IL-6, TNF-α) as well as enhancement of tissue energy demand and lactate production. These reactive phenotypes differed in the pattern of inhibition of the respiratory chain in mitochondria, however. TLR4 stimulation induced succinate dehydrogenase (complex II) inhibition by the metabolite itaconate. By contrast, TLR4 + IFN-γ receptor stimulation mainly resulted in complex IV inhibition by nitric oxide (NO) that also associated with severe oxidative stress, neuronal dysfunction and death. Notably, pharmacological depletion of microglia or treatment with itaconate resulted in effective neuroprotection reflected by well-preserved cytoarchitecture and electrical network activity, i.e., neuronal gamma oscillations (30-70 Hz) that underlie higher cognitive functions in vivo. Our findings provide in situ evidence that (i) proinflammatory microglia can substantially alter brain energy metabolism and (ii) fine-tuning of itaconate and NO metabolism determines microglial reactivity, impairment of neural network function and neurodegeneration. These data add mechanistic insights into microglial activation, with relevance to disorders featuring neuroinflammation and to drug discovery.

A 20-Year Journey from Axonal Injury to Neurodegenerative Diseases and the Prospect of Immunotherapy for Combating Alzheimer's Disease

The understanding of the dialogue between the brain and the immune system has undergone dramatic changes over the last two decades, with immense impact on the perception of neurodegenerative diseases, mental dysfunction, and many other brain pathologic conditions. Accumulated results have suggested that optimal function of the brain is dependent on support from the immune system, provided that this immune response is tightly controlled. Moreover, in contrast to the previous prevailing dogma, it is now widely accepted that circulating immune cells are needed for coping with brain pathologies and that their optimal effect is dependent on their type, location, and activity. In this perspective, we describe our own scientific journey, reviewing the milestones in attaining this understanding of the brain-immune axis integrated with numerous related studies by others. We then explain their significance in demonstrating the possibility of harnessing the immune system in a well-controlled manner for the treatment of neurodegenerative diseases.

Neuronal gamma oscillations and activity-dependent potassium transients remain regular after depletion of microglia in postnatal cortex tissue

Microglial cells (resident macrophages) feature rapid activation in CNS disease and can acquire multiple phenotypes exerting neuroprotection or neurotoxicity. The functional impact of surveying ("resting") microglia on neural excitability and neurotransmission in physiology is widely unknown, however. We addressed this issue in male rat hippocampal slice cultures (in situ) by pharmacological microglial ablation within days and by characterizing neuronal gamma-band oscillations (30-70 Hz) that are highly sensitive to neuromodulators and disturbances in ion and energy regulation. Gamma oscillations support action potential timing and synaptic plasticity, associate with higher brain functions like perception and memory, and require precise communication between excitatory pyramidal cells and inhibitory (GABAergic) interneurons. The slice cultures featured well-preserved hippocampal cytoarchitecture and parvalbumin-positive interneuron networks, microglia with ramified morphology, and low basal levels of IL-6, TNF-α, and nitric oxide (NO). Stimulation of slice cultures with the pro-inflammatory cytokine IFN-γ or bacterial LPS serving as positive controls for microglial reactivity induced MHC-II expression and increased cytokine and NO release. Chronic exposure of slice cultures to liposome-encapsulated clodronate reduced the microglial cell population by about 96%, whereas neuronal structures, astrocyte GFAP expression, and basal levels of cytokines and NO were unchanged. Notably, the properties of gamma oscillations reflecting frequency, number and synchronization of synapse activity were regular after microglial depletion. Also, electrical stimulus-induced transients of the extracellular potassium concentration ([K⁺ ]_o ) reflecting cellular K⁺ efflux, clearance and buffering were unchanged. This suggests that nonreactive microglia are dispensable for neuronal homeostasis and neuromodulation underlying network signaling and rhythm generation in cortical tissue.

Maternal Type-I interferon signaling adversely affects the microglia and the behavior of the offspring accompanied by increased sensitivity to stress

Viral infection during pregnancy is often associated with neuropsychiatric conditions. In mice, exposure of pregnant dams to the viral mimetic poly(I:C), serves as a model that simulates such pathology in the offspring, through a process known as Maternal Immune Activation (MIA). To investigate the mechanism of such effect, we hypothesized that maternal upregulation of Type-I interferon (IFN-I), as part of the dam's antiviral response, might contribute to the damage imposed on the offspring. Using mRNA sequencing and flow cytometry analyses we found that poly(I:C) treatment during pregnancy caused reduced expression of genes related to proliferation and cell cycle in the offspring's microglia relative to controls. This was found to be associated with an IFN-I signature in the embryonic yolk sac, the origin of microglia in development. Neutralizing IFN-I signaling in dams attenuated the effect of MIA on the newborn's microglia, while systemic maternal administration of IFNβ was sufficient to mimic the effect of poly(I:C), and led to increased vulnerability of offspring's microglia to subsequent stress. Furthermore, maternal elevation of IFNβ resulted in behavioral manifestations reminiscent of neuropsychiatric disorders. In addition, by adopting a "two-hit" experimental paradigm, we show a higher sensitivity of the offspring to postnatal stress subsequent to the maternal IFNβ elevation, demonstrated by behavioral irregularities. Our results suggest that maternal upregulation of IFN-I, in response to MIA, interferes with the offspring's programmed microglial developmental cascade, increases their susceptibility to postnatal stress, and leads to behavioral abnormalities.

Microglia versus Monocytes: Distinct Roles in Degenerative Diseases of the Retina

Unlike in the healthy mammalian retina, macrophages in retinal degenerative states are not solely comprised of microglia but may include monocyte-derived recruits. Recent studies have applied transgenics, lineage-tracing, and transcriptomics to help decipher the distinct roles of these two cell types in the diseasesettings of inherited retinal degenerations and age-related macular degeneration.Literature discussed here focuses on the ectopic presence of both macrophage types in the extracellular site surrounding the outer aspect ofphotoreceptor cells (i.e.,the subretinal space), which is crucially involved in the pathobiology. From these studies we propose a working model in which perturbed photoreceptor states cause microglial dominant migration to the subretinal space as a protective response, whereas the abundant presence ofmonocyte-derived cells there instead drives and accelerates pathology. The latter, we propose, is underpinned by specific genetic and nongenetic determinants that lead to a maladaptive macrophage state.

Leveraging preclinical models for the development of Alzheimer disease therapeutics

A large number of mouse models have been engineered, characterized and used to advance biomedical research in Alzheimer disease (AD). Early models simply damaged the rodent brain through toxins or lesions. Later, the spread of genetic engineering technology enabled investigators to develop models of familial AD by overexpressing human genes such as those encoding amyloid precursor protein (APP) or presenilins (PSEN1 or PSEN2) carrying mutations linked to early-onset AD. Recently, more complex models have sought to explore the impact of multiple genetic risk factors in the context of different biological challenges. Although none of these models has proven to be a fully faithful reproduction of the human disease, models remain essential as tools to improve our understanding of AD biology, conduct thorough pharmacokinetic and pharmacodynamic analyses, discover translatable biomarkers and evaluate specific therapeutic approaches. To realize the full potential of animal models as new technologies and knowledge become available, it is critical to define an optimal strategy for their use. Here, we review progress and challenges in the use of AD mouse models, highlight emerging scientific innovations in model development, and introduce a conceptual framework for use of preclinical models for therapeutic development.

Expression profiling of single cells and patient cohorts identifies multiple immunosuppressive pathways and an altered NK cell phenotype in glioblastoma

Glioblastoma (GBM) is an aggressive cancer with a very poor prognosis. Generally viewed as weakly immunogenic, GBM responds poorly to current immunotherapies. To understand this problem more clearly we used a combination of natural killer (NK) cell functional assays together with gene and protein expression profiling to define the NK cell response to GBM and explore immunosuppression in the GBM microenvironment. In addition, we used transcriptome data from patient cohorts to classify GBM according to immunological profiles. We show that glioma stem-like cells, a source of post-treatment tumour recurrence, express multiple immunomodulatory cell surface molecules and are targeted in preference to normal neural progenitor cells by natural killer (NK) cells ex vivo. In contrast, GBM-infiltrating NK cells express reduced levels of activation receptors within the tumour microenvironment, with hallmarks of transforming growth factor (TGF)-β-mediated inhibition. This NK cell inhibition is accompanied by expression of multiple immune checkpoint molecules on T cells. Single-cell transcriptomics demonstrated that both tumour and haematopoietic-derived cells in GBM express multiple, diverse mediators of immune evasion. Despite this, immunome analysis across a patient cohort identifies a spectrum of immunological activity in GBM, with active immunity marked by co-expression of immune effector molecules and feedback inhibitory mechanisms. Our data show that GBM is recognized by the immune system but that anti-tumour immunity is restrained by multiple immunosuppressive pathways, some of which operate in the healthy brain. The presence of immune activity in a subset of patients suggests that these patients will more probably benefit from combination immunotherapies directed against multiple immunosuppressive pathways.

CD300f immunoreceptor is associated with major depressive disorder and decreased microglial metabolic fitness

A role for microglia in neuropsychiatric diseases, including major depressive disorder (MDD), has been postulated. Regulation of microglial phenotype by immune receptors has become a central topic in many neurological conditions. We explored preclinical and clinical evidence for the role of the CD300f immune receptor in the fine regulation of microglial phenotype and its contribution to MDD. We found that a prevalent nonsynonymous single-nucleotide polymorphism (C/T, rs2034310) of the human CD300f receptor cytoplasmic tail inhibits the protein kinase C phosphorylation of a threonine and is associated with protection against MDD, mainly in women. Interestingly, CD300f-/- mice displayed several characteristic MDD traits such as augmented microglial numbers, increased interleukin 6 and interleukin 1 receptor antagonist messenger RNA, alterations in synaptic strength, and noradrenaline-dependent and persistent depressive-like and anhedonic behaviors in females. This behavioral phenotype could be potentiated inducing the lipopolysaccharide depression model. RNA sequencing and biochemical studies revealed an association with impaired microglial metabolic fitness. In conclusion, we report a clear association that links the function of the CD300f immune receptor with MDD in humans, depressive-like and anhedonic behaviors in female mice, and altered microglial metabolic reprogramming.

Progressive long-term spatial memory loss following repeat concussive and subconcussive brain injury in mice, associated with dorsal hippocampal neuron loss, microglial phenotype shift, and vascular abnormalities

There is considerable concern about the long‐term deleterious effects of repeat head trauma on cognition, but little is known about underlying mechanisms and pathology. To examine this, we delivered four air blasts to the left side of the mouse cranium, a week apart, with an intensity that causes deficits when delivered singly and considered “concussive,” or an intensity that does not yield significant deficits when delivered singly and considered “subconcussive.” Neither repeat concussive nor subconcussive blast produced spatial memory deficits at 4 months, but both yielded deficits at 14 months, and dorsal hippocampal neuron loss. Hierarchical cluster analysis of dorsal hippocampal microglia across the three groups based on morphology and expression of MHCII, CX3CR1, CD68 and IBA1 revealed five distinct phenotypes. Types 1A and 1B microglia were more common in sham mice, linked to better neuron survival and memory, and appeared mildly activated. By contrast, 2B and 2C microglia were more common in repeat concussive and subconcussive mice, linked to poorer neuron survival and memory, and characterized by low expression levels and attenuated processes, suggesting they were de‐activated and dysfunctional. In addition, endothelial cells in repeat concussive mice exhibited reduced CD31 and eNOS expression, which was correlated with the prevalence of type 2B and 2C microglia. Our findings suggest that both repeat concussive and subconcussive head injury engender progressive pathogenic processes, possibly through sustained effects on microglia that over time lead to increased prevalence of dysfunctional microglia, adversely affecting neurons and blood vessels, and thereby driving neurodegeneration and memory decline.

Disease Progression-Dependent Expression of CD200R1 and CX3CR1 in Mouse Models of Parkinson's Disease

Microglial activation is an important contributor to the pathogenesis of Parkinson’s disease (PD). Microglia are tightly and efficiently regulated by immune checkpoints, including CD200-CD200R1 and CX3CL1-CX3CR1. Understanding the involvement of these checkpoints in disease progression provides important insights into how microglial activation contributes to PD pathology. However, so far, studies have produced seemingly conflicting results. In this study, we demonstrate that CD200R1 expression is down-regulated at both early and late stage of PD model, and CX3CR1 expression is down-regulated in early stage and recovered in late stage. In primary cultured microglia, CD200R1 and CX3CR1 expressions are both directly regulated by LPS or α-synuclein, and CD200R1 expression is more sensitively regulated than CX3CR1. In addition, CD200 knockout causes an increase in proinflammatory cytokine production and microglial activation in the midbrain. Remarkably, DA neurons in the substantial nigra are degenerated in CD200^-/- mice. Finally, activation of the CD200R with CD200Fc alleviates the neuroinflammation in microglia. Together, these results suggest that immune checkpoints play distinct functional roles in different stage of PD pathology, and the CD200-CD200R1 axis plays a significant role in nigrostriatal neuron viability and function.

CD200 maintains the region-specific phenotype of microglia in the midbrain and its role in Parkinson's disease

Microglia are a specialized population of tissue macrophages in the mammalian brain. Microglial phenotype is tightly regulated by local environmental factors, although little is known about these factors and their region-preferred roles in regulating local neuroinflammatory responses. We hypothesized that microglia in different brain regions respond differently to neuroinflammatory stimulation and that CD200, an anti-inflammatory protein mainly originated from neurons, acts as a local cue inhibiting microglia activation in the midbrain. We utilized a CD200-deficient mouse line to analyze the phenotypic role of CD200 in the regulation of normal neuron-microglia homeostasis in the midbrain and in the dopaminergic degeneration in an α-synuclein overexpression model of PD. We found that systemic administration of an endotoxin lipopolysaccharide induced a region-preferred change in CD200 expression in the midbrain. Similarly, CD200^-/- mice showed a regional preference in an enhancement of microglia activation and baseline inflammatory levels in the midbrain and dopamine neuron loss in the substantia nigra (SN). In a mouse model of Parkinson's disease (PD) induced by rAAV-hSYN injection into the SN, CD200^-/- mice showed more dopamine neuron loss in the SN than wild type mice. Activation of CD200 receptors with a CD200 fusion protein alleviated the neuroinflammation and neuronal death in the SN of PD mice. These findings demonstrate that CD200 is essential for the midbrain homeostasis and acts as a critical local regulator in controlling microglial properties related to the PD pathogenesis.

Dendritic polyglycerols are modulators of microglia-astrocyte crosstalk

Aim: To determine the ability of sulfated dendritic polyglycerols (dPGS) to modulate neuroglia activation challenged with lipopolysaccharide (LPS). Materials & methods: Microglia/astrocyte activation in vivo was determined in transgenic animals expressing TLR2-/GFAP-luciferase reporter. Mechanisms implicated in microglia-astrocyte crosstalk were studied in primary mouse brain cultures. Results & discussion: dPGS significantly reduced microglia activation in vivo, and decreased astrocytic LCN2 production. Activated microglia are necessary for astrocyte stimulation and increase in LCN2 abundance. LCN2 production in astrocytes involves signaling via toll-like receptor 4, activation of NF-κB, IL6 and enhancement of reactive oxygen species. Conclusion: dPGS are powerful modulators of microglia-astrocyte crosstalk and LCN2 abundance; dPGS are promising anti-inflammatory dendritic nanostructures.

Retinal Phenotype in the rd9 Mutant Mouse, a Model of X-Linked RP

Retinal degeneration 9 (rd9) mice carry a mutation in the retina specific “Retinitis Pigmentosa GTPase Regulator (RPGR)” Open Reading Frame (ORF) 15 gene, located on the X chromosome and represent a rare model of X-linked Retinitis Pigmentosa (XLRP), a common and severe form of retinal degeneration (Wright et al., 2010; Tsang and Sharma, 2018). The rd9 RPGR-ORF15 mutation in mice causes lack of the protein in photoreceptors and a slow degeneration of these cells with consequent decrease in Outer Nuclear Layer (ONL) thickness and amplitude of ERG responses, as previously described (Thompson et al., 2012). However, relative rates of rod and cone photoreceptor loss, as well as secondary alterations occurring in neuronal and non-neuronal retinal cell types of rd9 mutants remain to be assessed. Aim of this study is to extend phenotype analysis of the rd9 mouse retina focusing on changes occurring in cells directly interacting with photoreceptors. To this purpose, first we estimated rod and cone survival and its degree of intraretinal variation over time; then, we studied the morphology of horizontal and bipolar cells and of the retinal pigment epithelium (RPE), extending our observations to glial cell reactivity. We found that in rd9 retinas rod (but not cone) death is the main cause of decrease in ONL thickness and that degeneration shows a high degree of intraretinal variation. Rod loss drives remodeling in the outer retina, with sprouting of second-order neurons of the rod-pathway and relative sparing of cone pathway elements. Remarkably, despite cone survival, functional defects can be clearly detected in ERG recordings in both scotopic and photopic conditions. Moderate levels of Muller cells and microglial reactivity are sided by striking attenuation of staining for RPE tight junctions, suggesting altered integrity of the outer Blood Retina Barrier (BRB). Because of many features resembling slowly progressing photoreceptor degeneration paradigms or early stages of more aggressive forms of RP, the rd9 mouse model can be considered a rare and useful tool to investigate retinal changes associated to a process of photoreceptor death sustained throughout life and to reveal disease biomarkers (e.g., BRB alterations) of human XLRP.

Glucocorticoids mediate stress induction of the alarmin HMGB1 and reduction of the microglia checkpoint receptor CD200R1 in limbic brain structures

Exposure to stressors primes neuroinflammatory responses to subsequent immune challenges and stress-induced glucocorticoids (GCs) play a mediating role in this phenomenon of neuroinflammatory priming. Recent evidence also suggests that the alarmin high-mobility group box-1 (HMGB1) and the microglial checkpoint receptor CD200R1 serve as proximal mechanisms of stress-induced neuroinflammatory priming. However, it is unclear whether stress-induced GCs play a causal role in these proximal mechanisms of neuroinflammatory priming; this forms the focus of the present investigation. Here, we found that exposure to a severe acute stressor (inescapable tailshock) induced HMGB1 and reduced CD200R1 expression in limbic brain regions and pharmacological blockade of GC signaling (RU486) mitigated these effects of stress. To confirm these effects of RU486, adrenalectomy (ADX) with basal corticosterone (CORT) replacement was used to block the stress-induced increase in GCs as well as effects on HMGB1 and CD200R1. As with RU486, ADX mitigated the effects of stress on HMGB1 and CD200R1. Subsequently, exogenous CORT was administered to determine whether GCs are sufficient to recapitulate the effects of stress. Indeed, exogenous CORT induced expression of HMGB1 and reduced expression of CD200R1. In addition, exposure of primary microglia to CORT also recapitulated the effects of stress on CD200R1 suggesting that CORT acts directly on microglia to reduce expression of CD200R1. Taken together, these findings suggest that GCs mediate the effects of stress on these proximal mechanisms of neuroinflammatory priming.

Open chromatin landscape of rat microglia upon proinvasive or inflammatory polarization

Microglia are brain-resident, myeloid cells that play important roles in health and brain pathologies. Herein, we report a comprehensive, replicated, false discovery rate-controlled dataset of DNase-hypersensitive (DHS) open chromatin regions for rat microglia. We compared the open chromatin landscapes in untreated primary microglial cultures and cultures stimulated for 6 hr with either glioma-conditioned medium (GCM) or lipopolysaccharide (LPS). Glioma-secreted factors induce proinvasive and immunosuppressive activation of microglia, and these cells then promote tumor growth. The open chromatin landscape of the rat microglia consisted of 126,640 reproducible DHS regions, among which 2,303 and 12,357 showed a significant change in openness following stimulation with GCM or LPS, respectively. Active genes exhibited constitutively open promoters, but there was no direct dependence between the aggregated openness of DHS regions near a gene and its expression. Individual regions mapped to the same gene often presented different patterns of openness changes. GCM-regulated DHS regions were more frequent in areas away from gene bodies, while LPS-regulated regions were more frequent in introns. GCM and LPS differentially affected the openness of regions mapped to immune checkpoint genes. The two treatments differentially affected the aggregated openness of regions mapped to genes in the Toll-like receptor signaling and axon guidance pathways, suggesting that the molecular machinery used by migrating microglia is similar to that of growing axons and that modulation of these pathways is instrumental in the induction of proinvasive polarization of microglia by glioma. Our dataset of open chromatin regions paves the way for studies of gene regulation in rat microglia.

Glia in amyotrophic lateral sclerosis and spinal cord injury: common therapeutic targets

The toolkit for repairing damaged neurons in amyotrophic lateral sclerosis (ALS) and spinal cord injury (SCI) is extremely limited. Here, we reviewed the in vitro and in vivo studies and clinical trials on nonneuronal cells in the neurodegenerative processes common to both these conditions. Special focus was directed to microglia and astrocytes, because their activation and proliferation, also known as neuroinflammation, is a key driver of neurodegeneration. Neuroinflammation is a multifaceted process that evolves during the disease course, and can be either beneficial or toxic to neurons. Given the fundamental regulatory functions of glia, pathogenic mechanisms in neuroinflammation represent promising therapeutic targets. We also discussed neuroprotective, immunosuppressive, and stem-cell based approaches applicable to both ALS and SCI.