Innate immune memory in the brain shapes neurological disease hallmarks

LPS priming-induced immune tolerance mitigates LPS-stimulated microglial activation and social avoidance behaviors in mice

In this study, we investigated the regulatory mechanisms underlying the effects of LPS tolerance on the inflammatory homeostasis of immune cells. LPS priming-induced immune tolerance downregulated cyclooxygenase-2, and lowered the production of prostaglandin-E2 in microglial cells. In addition, LPS tolerance downregulated the expression of suppressor of cytokine signaling 3, and inducible nitric oxide synthase/nitric oxide; suppressed the LPS-mediated induction of tumor necrosis factor-α, interleukin (IL)-6, and IL-1; and reduced reactive oxygen species production in microglial cells. LPS stimulation increased the levels of the adaptive response-related proteins heme oxygenase-1 and superoxide dismutase 2, and the levels of heme oxygenase-1 (HO-1) enhanced after LPS priming. Systemic administration of low-dose LPS (0.5 mg/kg) to mice for 4 consecutive days attenuated high-dose LPS (5 mg/kg)-induced inflammatory response, microglial activation, and proinflammatory cytokine expression. Moreover, repeated exposure to low-dose LPS suppressed the recruitment of peripheral monocytes or macrophages to brain regions and downregulated the expression of proinflammatory cytokines. Notably, LPS-induced social avoidance behaviors in mice were mitigated by immune tolerance. In conclusion, immune tolerance may reduce proinflammatory cytokine expression and reactive oxygen species production. Our findings provide insights into the effects of endotoxin tolerance on innate immune cells and social behaviors.

Hypoxia inducible factor-1α regulates microglial innate immune memory and the pathology of Parkinson's disease

Neuroinflammation is one of the core pathological features of Parkinson's disease (PD). Innate immune cells play a crucial role in the progression of PD. Microglia, the major innate immune cells in the brain, exhibit innate immune memory effects and are recognized as key regulators of neuroinflammatory responses. Persistent modifications of microglia provoked by the first stimuli are pivotal for innate immune memory, resulting in an enhanced or suppressed immune response to second stimuli, which is known as innate immune training and innate immune tolerance, respectively. In this study, LPS was used to establish in vitro and in vivo models of innate immune memory. Microglia-specific Hif-1α knockout mice were further employed to elucidate the regulatory role of HIF-1α in innate immune memory and MPTP-induced PD pathology. Our results showed that different paradigms of LPS could induce innate immune training or tolerance in the nigrostriatal pathway of mice. We found that innate immune tolerance lasting for one month protected the dopaminergic system in PD mice, whereas the effect of innate immune training was limited. Deficiency of HIF-1α in microglia impeded the formation of innate immune memory and exerted protective effects in MPTP-intoxicated mice by suppressing neuroinflammation. Therefore, HIF-1α is essential for microglial innate immune memory and can promote neuroinflammation associated with PD.

Disease-associated astrocyte epigenetic memory promotes CNS pathology

Disease-associated astrocyte subsets contribute to the pathology of neurologic diseases, including multiple sclerosis and experimental autoimmune encephalomyelitis1-8 (EAE), an experimental model for multiple sclerosis. However, little is known about the stability of these astrocyte subsets and their ability to integrate past stimulation events. Here we report the identification of an epigenetically controlled memory astrocyte subset that exhibits exacerbated pro-inflammatory responses upon rechallenge. Specifically, using a combination of single-cell RNA sequencing, assay for transposase-accessible chromatin with sequencing, chromatin immunoprecipitation with sequencing, focused interrogation of cells by nucleic acid detection and sequencing, and cell-specific in vivo CRISPR-Cas9-based genetic perturbation studies we established that astrocyte memory is controlled by the metabolic enzyme ATP-citrate lyase (ACLY), which produces acetyl coenzyme A (acetyl-CoA) that is used by histone acetyltransferase p300 to control chromatin accessibility. The number of ACLY+p300+ memory astrocytes is increased in acute and chronic EAE models, and their genetic inactivation ameliorated EAE. We also detected the pro-inflammatory memory phenotype in human astrocytes in vitro; single-cell RNA sequencing and immunohistochemistry studies detected increased numbers of ACLY+p300+ astrocytes in chronic multiple sclerosis lesions. In summary, these studies define an epigenetically controlled memory astrocyte subset that promotes CNS pathology in EAE and, potentially, multiple sclerosis. These findings may guide novel therapeutic approaches for multiple sclerosis and other neurologic diseases.

Association between doxycycline use and long-term functioning in patients with schizophrenia

IMPORTANCE AND OBJECTIVE

The brain-penetrant tetracycline antibiotics, minocycline and doxycycline, have been proposed as potential candidate drugs for treatment of schizophrenia, based on preclinical studies and clinical trials. A potential long-term beneficial effect of these antibiotics for schizophrenia patients has not been investigated. This study was designed to determine if redemption of doxycycline prescription in schizophrenia is associated with decreased incidence of disability pension, a proxy for long-term functioning.

DESIGN

We performed a population-based cohort study with data from schizophrenia patients available through the Danish registers. Survival analysis models with time-varying covariates were constructed to assess incidence rate ratios (IRR) of disability pension after exposure to doxycycline or a non-brain penetrant tetracycline, defined as at least one filled prescription. The analysis was adjusted for age, sex, calendar year, parental psychiatric status and educational level.

RESULTS

We used data from 11,157 individuals with schizophrenia (4,945 female and 6,212 male; average age 22.4 years old, standard deviation (std) 4.50). 718 of these were exposed to brain-penetrant doxycycline, and 1,498 individuals redeemed a prescription of one or more of the non-brain-penetrant tetracyclines. The average years at risk per person in this cohort was 4.9, and 2,901 individuals received disability pension in the follow-up period. There was a significantly lower incidence rate of disability pension in schizophrenia patients who had redeemed doxycycline compared to patients who did not redeem a prescription of any tetracycline antibiotics (Incidence rate ratio (IRR) 0.68; 95 % CI 0.56, 0.83). There was also a significant lower rate of disability pension in schizophrenia patients who redeemed doxycycline compared to individuals who redeemed a prescription of one of the non-brain penetrant tetracycline antibiotics (IRR 0.69 95 % CI 0.55, 0.87).

CONCLUSIONS

In this observational study, doxycycline exposure is associated with a reduced incidence of disability pension. These data support further studies on the potential long term neuroprotective effects of doxycycline and level of functioning in schizophrenia patients.

Radiotherapy induces persistent innate immune reprogramming of microglia into a primed state

Over half of patients with brain tumors experience debilitating and often progressive cognitive decline after radiotherapy treatment. Microglia, the resident macrophages in the brain, have been implicated in this decline. In response to various insults, microglia can develop innate immune memory (IIM), which can either enhance (priming or training) or repress (tolerance) the response to subsequent inflammatory challenges. Here, we investigate whether radiation affects the IIM of microglia by irradiating the brains of rats and later exposing them to a secondary inflammatory stimulus. Comparative transcriptomic profiling and protein validation of microglia isolated from irradiated rats show a stronger immune response to a secondary inflammatory insult, demonstrating that radiation can lead to long-lasting molecular reprogramming of microglia. Transcriptomic analysis of postmortem normal-appearing non-tumor brain tissue of patients with glioblastoma indicates that radiation-induced microglial priming is likely conserved in humans. Targeting microglial priming or avoiding further inflammatory insults could decrease radiotherapy-induced neurotoxicity.

Immune regulation and blood-brain barrier permeability in cerebral small vessel disease: study protocol of the INflammation and Small Vessel Disease (INSVD) study - a multicentre prospective cohort study

INTRODUCTION

The INflammation and Small Vessel Disease (INSVD) study aims to investigate whether peripheral inflammation, immune (dys)regulation and blood-brain barrier (BBB) permeability relate to disease progression in cerebral small vessel disease (SVD). This research aims to pinpoint specific components of the immune response in SVD relating to disease progression. This could identify biomarkers of SVD progression, as well as potential therapeutic targets to inform the development and repurposing of drugs to reduce or prevent SVD, cognitive decline and vascular dementia.

METHODS AND ANALYSIS

INSVD is a prospective observational multicentre cohort study in individuals with symptomatic SVD. This longitudinal study combines comprehensive immunophenotyping of the peripheral blood immune compartment with advanced neuroimaging markers of SVD and BBB permeability. The main SVD marker of interest is white matter microstructure as determined by diffusion tensor imaging, a valuable marker of disease progression owing to its sensitivity to early alterations to white matter integrity. The research is being conducted in two sites-in the UK (Cambridge) and the Netherlands (Nijmegen)-with each site recruiting 100 participants (total n=200). Participants undergo clinical and cognitive assessments, blood draws, and brain MRI at baseline and 2-year follow-up.

ETHICS AND DISSEMINATION

This study received ethical approval from the local ethics boards (UK: East of England-Cambridge Central Research Ethics Committee (REC) ref: 22/EE/00141, Integrated Research Application System (IRAS) ID: 312 747. Netherlands: Medical Research Ethics Committee (MREC) Oost-Nederland, ref: 2022-13623, NL-number: NL80258.091.22). Written informed consent was obtained from all subjects before the study. Any participant-derived benefits resulting from this research, such as new insights into disease mechanisms or possible novel therapies, will be disseminated to study participants, patient groups and members of the public.

TRIAL REGISTRATION NUMBER

NCT05746221.

Changes in the Expression of Proteins Associated with Neurodegeneration in the Brains of Mice after Infection with Influenza A Virus with Wild Type and Truncated NS1

Influenza type A virus (IAV) infection is a major cause of morbidity and mortality during influenza epidemics. Recently, a specific link between IAV infection and neurodegenerative disease progression has been established. The non-structural NS1 protein of IAV regulates viral replication during infection and antagonizes host antiviral responses, contributing to influenza virulence. In the present study, we have prepared a mouse lung-to-lung adapted to the NS1-truncated virus (NS80ad). Transcriptome analysis of the gene expression in the lungs revealed that infection with wild-type A/WSN/33 (WSN), NS80, and NS80ad viruses resulted in different regulation of genes involved in signaling pathways associated with the cell proliferation, inflammatory response, and development of neurodegenerative diseases. NS1 protein did not influence the genes involved in the RIG-I-like receptor signaling pathway in the brains. Lethal infection with IAVs dysregulated expression of proteins associated with the development of neurodegenerative diseases (CX3CL1/Fractalkine, Coagulation factor III, and CD105/Endoglin, CD54/ICAM-1, insulin-like growth factor-binding protein (IGFBP)-2, IGFBP-5, IGFBP-6, chitinase 3-like 1 (CHI3L1), Myeloperoxidase (MPO), Osteopontin (OPN), cystatin C, and LDL R). Transcription of GATA3 mRNA was decreased, and expression of MPO was inhibited in the brain infected with NS80 and NS80ad viruses. In addition, the truncation of NS1 protein led to reduced expression of IGFBP-2, CHI3L1, MPO, and LDL-R proteins in the brains. Our results indicate that the influenza virus influences the expression of proteins involved in brain function, and this might occur mostly through the NS1 protein. These findings suggest that the abovementioned proteins represent a promising target for the development of potentially effective immunotherapy against neurodegeneration.

"Lipopolysaccharide-induced animal models for neuroinflammation - An overview."

Neuroinflammation is a pathological mechanism contributing to neurodegenerative diseases. For in-depth studies of neuroinflammation, several animal models reported reproducing behavioral dysfunctions and cellular pathological mechanisms induced by brain inflammation. One of the most popular models of neuroinflammation is the one generated by lipopolysaccharide exposure. Despite its importance, the reported results using this model show high heterogeneity, making it difficult to analyze and compare the outcomes between studies. Therefore, the current review aims to summarize the different experimental paradigms used to reproduce neuroinflammation by lipopolysaccharide exposure and its respective outcomes, helping to choose the model that better suits each specific research aim.

Trained Innate Immunity in Animal Models of Cardiovascular Diseases

Acquisition of immunological memory is an important evolutionary strategy that evolved to protect the host from repetitive challenges from infectious agents. It was believed for a long time that memory formation exclusively occurs in the adaptive part of the immune system with the formation of highly specific memory T cells and B cells. In the past 10-15 years, it has become clear that innate immune cells, such as monocytes, natural killer cells, or neutrophil granulocytes, also have the ability to generate some kind of memory. After the exposure of innate immune cells to certain stimuli, these cells develop an enhanced secondary response with increased cytokine secretion even after an encounter with an unrelated stimulus. This phenomenon has been termed trained innate immunity (TI) and is associated with epigenetic modifications (histone methylation, acetylation) and metabolic alterations (elevated glycolysis, lactate production). TI has been observed in tissue-resident or circulating immune cells but also in bone marrow progenitors. Risk-factors for cardiovascular diseases (CVDs) which are associated with low-grade inflammation, such as hyperglycemia, obesity, or high salt, can also induce TI with a profound impact on the development and progression of CVDs. In this review, we briefly describe basic mechanisms of TI and summarize animal studies which specifically focus on TI in the context of CVDs.

Fate mapping of Spp1 expression reveals age-dependent plasticity of disease-associated microglia-like cells after brain injury

Microglial reactivity to injury and disease is emerging as a heterogeneous, dynamic, and crucial determinant in neurological disorders. However, the plasticity and fate of disease-associated microglia (DAM) remain largely unknown. We established a lineage tracing system, leveraging the expression dynamics of secreted phosphoprotein 1（Spp1） to label and track DAM-like microglia during brain injury and recovery. Fate mapping of Spp1+ microglia during stroke in juvenile mice revealed an irreversible state of DAM-like microglia that were ultimately eliminated from the injured brain. By contrast, DAM-like microglia in the neonatal stroke models exhibited high plasticity, regaining a homeostatic signature and integrating into the microglial network after recovery. Furthermore, neonatal injury had a lasting impact on microglia, rendering them intrinsically sensitized to subsequent immune challenges. Therefore, our findings highlight the plasticity and innate immune memory of neonatal microglia, shedding light on the fate of DAM-like microglia in various neuropathological conditions.

Microglia nuclear receptor corepressor 1 deficiency alleviates neuroinflammation in mice

Given the established role of nuclear receptor corepressor 1 (NCoR1) in sensing environmental cues and the importance of inflammation in neurodegenerative diseases, elucidation of NCoR1 involvement in neuroinflammation has notable implications. Yet, its regulatory mechanism remains largely unclear. Under in vitro conditions, NCoR1 expression peaked and then decreased at 12 h after lipopolysaccharides (LPS) stimulation in BV2 cells, However, NCoR1 knockdown using si-RNA attenuated microglial inflammation, evident by reduced the levels of inducible nitric oxide synthase (iNOS), cyclooxygenase-2 (COX2), phosphorylated-JNK and high mobility group box-1 (HMGB1). Furthermore, NCoR1 suppression could counteract the decline in mitochondrial membrane potential while simultaneously enhancing the expression of peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α). Under in vivo conditions, microglia-specific NCoR1 knockout (MNKO) mice after LPS injections alleviated the symptoms of anhedonia, diminished autonomic activity and cognitive impairment. Additionally, MNKO mice showed attenuation of microglial activation, downregulated HMGB1 and COX2, and upregulated PGC-1α expression in the cortex. In conclusion, these findings suggest that NCoR1 deficiency leads to a modest reduction in neuroinflammation, possibly attributed to the increased expression of PGC-1α.

TREM2 regulates microglial phagocytosis of synapses in innate immune tolerance

Increasing evidence indicates that innate immune cells also possess immunological memory. Microglia are brain-resident innate immune cells and execute inflammatory and phagocytic functions upon environmental stimulation, during which processes triggering receptor expressed on myeloid cells 2 (TREM2) plays an important regulatory role. However, although microglia are known to exhibit innate immune memory related to inflammation when subjected to continuous inflammatory stimuli, whether microglia exhibit innate immune memory related to phagocytosis and whether TREM2 participates in innate immune memory of microglia remain unknown. Herein, we treated WT and Trem2 KO mice with peripheral injection of lipopolysaccharides (LPS) to induce microglial activation or microglial immune tolerance. We found that Tnfα and Il-1β expression levels in the hippocampi were significantly elevated after 1xLPS and then dramatically decreased after 4xLPS in both WT and Trem2 KO mice; and their level changes were indistinguishable between WT and Trem2 KO mice. Moreover, 1xLPS significantly promoted microglial phagocytosis of synapses and caused microglial morphology changes resembling activated status in both WT and Trem2 KO mice. However, 4xLPS significantly reduced synapse phagocytosis and largely reversed morphology changes in WT microglia. While 4xLPS had no effect on reducing synapse phagocytosis in Trem2 KO microglia. RNA-seq analysis revealed that TREM2 deficiency reprogrammed complement and phagosome-related transcriptional landscape during immune tolerance. Our results demonstrate that microglia also exhibit immune tolerance related to phagocytosis of synapses and that TREM2 plays a crucial role in this process possibly through regulating complement system and phagosome-related gene expressions.

Cortical microinfarcts potentiate recurrent ischemic injury through NLRP3-dependent trained immunity

Microinfarcts are common among the elderly and patients with microinfarcts are more vulnerable to another stroke. However, the impact of microinfarcts on recurrent stroke has yet to be fully understood. The purpose of this study was to explore the negative effects of microinfarcts on recurrent stroke. To achieve this, two-photon laser was used to induce microinfarcts, while photothrombotic stroke was induced on the opposite side. The results showed that microinfarcts led to trained immunity in microglia, which worsened the pro-inflammatory response and ischemic injury in the secondary photothrombotic stroke. Additionally, the study clarified the role of NLRP3 in microglial nuclei, indicating that it interacts with the MLL1 complex through NACHT domain and increases H3K4 methylation, which suggests that NLRP3 is critical in the formation of innate immune memory caused by microinfarcts. Furthermore, the knockout of NLRP3 in microglia alleviated the trained immunity and reduced the harmful effects of microinfarcts on recurrent stroke. This study emphasizes the detrimental effect of trained immunity on recurrent stroke and highlights the critical role of NLRP3 in mediating the formation of this memory, which may offer a potential therapeutic target for mitigating recurrent strokes.

Disease-associated astrocyte epigenetic memory promotes CNS pathology

Astrocytes play important roles in the central nervous system (CNS) physiology and pathology. Indeed, astrocyte subsets defined by specific transcriptional activation states contribute to the pathology of neurologic diseases, including multiple sclerosis (MS) and its pre-clinical model experimental autoimmune encephalomyelitis (EAE) 1-8 . However, little is known about the stability of these disease-associated astrocyte subsets, their regulation, and whether they integrate past stimulation events to respond to subsequent challenges. Here, we describe the identification of an epigenetically controlled memory astrocyte subset which exhibits exacerbated pro-inflammatory responses upon re-challenge. Specifically, using a combination of single-cell RNA sequencing (scRNA-seq), assay for transposase-accessible chromatin with sequencing (ATAC-seq), chromatin immunoprecipitation with sequencing (ChIP-seq), focused interrogation of cells by nucleic acid detection and sequencing (FIND-seq), and cell-specific in vivo CRISPR/Cas9-based genetic perturbation studies we established that astrocyte memory is controlled by the metabolic enzyme ATP citrate lyase (ACLY), which produces acetyl coenzyme A (acetyl-CoA) used by the histone acetyltransferase p300 to control chromatin accessibility. ACLY + p300 + memory astrocytes are increased in acute and chronic EAE models; the genetic targeting of ACLY + p300 + astrocytes using CRISPR/Cas9 ameliorated EAE. We also detected responses consistent with a pro-inflammatory memory phenotype in human astrocytes in vitro ; scRNA-seq and immunohistochemistry studies detected increased ACLY + p300 + astrocytes in chronic MS lesions. In summary, these studies define an epigenetically controlled memory astrocyte subset that promotes CNS pathology in EAE and, potentially, MS. These findings may guide novel therapeutic approaches for MS and other neurologic diseases.

Genetic Overlap Between Global Cortical Brain Structure, C-Reactive Protein, and White Blood Cell Counts

BACKGROUND

For many brain disorders, a subset of patients jointly exhibit alterations in cortical brain structure and elevated levels of circulating immune markers. This may be driven in part by shared genetic architecture. Therefore, we investigated the phenotypic and genetic associations linking global cortical surface area and thickness with blood immune markers (i.e., white blood cell counts and plasma C-reactive protein levels).

METHODS

Linear regression was used to assess phenotypic associations in 30,823 UK Biobank participants. Genome-wide and local genetic correlations were assessed using linkage disequilibrium score regression and local analysis of covariance annotation. The number of shared trait-influencing genetic variants was estimated using MiXeR. Shared genetic architecture was assessed using a conjunctional false discovery rate framework, and mapped genes were included in gene-set enrichment analyses.

RESULTS

Cortical structure and blood immune markers exhibited predominantly inverse phenotypic associations. There were modest genome-wide genetic correlations, the strongest of which were for C-reactive protein levels (rg_surface_area = -0.13, false discovery rate-corrected p = 4.17 × 10-3; rg_thickness = -0.13, false discovery rate-corrected p = 4.00 × 10-2). Meanwhile, local genetic correlations showed a mosaic of positive and negative associations. White blood cells shared on average 46.24% and 38.64% of trait-influencing genetic variants with surface area and thickness, respectively. Additionally, surface area shared 55 unique loci with the blood immune markers while thickness shared 15. Overall, monocyte count exhibited the largest genetic overlap with cortical brain structure. A series of gene enrichment analyses implicated neuronal-, astrocytic-, and schizophrenia-associated genes.

CONCLUSIONS

The findings indicate shared genetic underpinnings for cortical brain structure and blood immune markers, with implications for neurodevelopment and understanding the etiology of brain-related disorders.

Macrophage memory: Types, mechanisms, and its role in health and disease

On the basis of the mechanisms of action and characteristics of immune effects, immunity is commonly categorized into innate and adaptive immunity. Adaptive immunity is associated with the response to non-self-entities and is characterized by high specificity and memory properties. In contrast, innate immunity has traditionally been considered devoid of memory characteristics. However, an increasing number of studies have sought to challenge this conventional immunological dogma and shown that innate immune cells exhibit a more robust and rapid response to secondary stimulation, thus providing evidence of the immunological memory in innate immunity. Macrophages, which are among the most important innate immune cells, can also acquire memory phenotype that facilitates the mediation of recall responses. Macrophage memory is a relatively new concept that is revolutionizing our understanding of macrophage biology and immunological memory and could lead to a new class of vaccines and immunotherapies. In this review, we describe the characteristics and mechanisms of macrophage memory, as well as its essential roles in various diseases.

Oxylipin transport by lipoprotein particles and its functional implications for cardiometabolic and neurological disorders

Lipoprotein metabolism is critical to inflammation. While the periphery and central nervous system (CNS) have separate yet connected lipoprotein systems, impaired lipoprotein metabolism is implicated in both cardiometabolic and neurological disorders. Despite the substantial investigation into the composition, structure and function of lipoproteins, the lipoprotein oxylipin profiles, their influence on lipoprotein functions, and their potential biological implications are unclear. Lipoproteins carry most of the circulating oxylipins. Importantly, lipoprotein-mediated oxylipin transport allows for endocrine signaling by these lipid mediators, long considered to have only autocrine and paracrine functions. Alterations in plasma lipoprotein oxylipin composition can directly impact inflammatory responses of lipoprotein metabolizing cells. Similar investigations of CNS lipoprotein oxylipins are non-existent to date. However, as APOE4 is associated with Alzheimer's disease-related microglia dysfunction and oxylipin dysregulation, ApoE4-dependent lipoprotein oxylipin modulation in neurological pathologies is suggested. Such investigations are crucial to bridge knowledge gaps linking oxylipin- and lipoprotein-related disorders in both periphery and CNS. Here, after providing a summary of existent literatures on lipoprotein oxylipin analysis methods, we emphasize the importance of lipoproteins in oxylipin transport and argue that understanding the compartmentalization and distribution of lipoprotein oxylipins may fundamentally alter our consideration of the roles of lipoprotein in cardiometabolic and neurological disorders.

Angiotensin-II drives changes in microglia-vascular interactions in rats with heart failure

Activation of microglia, the resident immune cells of the central nervous system, leading to the subsequent release of pro-inflammatory cytokines, has been linked to cardiac remodeling, autonomic disbalance, and cognitive deficits in heart failure (HF). While previous studies emphasized the role of hippocampal Angiotensin II (AngII) signaling in HF-induced microglial activation, unanswered mechanistic questions persist. Evidence suggests significant interactions between microglia and local microvasculature, potentially affecting blood-brain barrier integrity and cerebral blood flow regulation. Still, whether the microglial-vascular interface is affected in the brain during HF remains unknow. Using a well-established ischemic HF rat model, we demonstrate increased vessel-associated microglia (VAM) in HF rat hippocampi, which showed heightened expression of AngII AT1a receptors. Acute AngII administration to sham rats induced microglia recruitment to the perivascular space, along with increased expression of TNFa. Conversely, administering an AT1aR blocker to HF rats prevented the recruitment of microglia to the perivascular space, normalizing their levels to those in healthy rats. These results highlight the critical importance of a rather understudied phenomenon (i.e., microglia-vascular interactions in the brain) in the context of the pathophysiology of a highly prevalent cardiovascular disease, and unveil novel potential therapeutic avenues aimed at mitigating neuroinflammation in cardiovascular diseases.

Higher Levels of C-reactive Protein Are Associated With Higher Cortical Surface Area and Lower Cortical Thickness in Youth With Bipolar Disorder

BACKGROUND

Inflammation is implicated in the neuropathology of bipolar disorder (BD). The association of C-reactive protein (CRP) with brain structure has been examined in relation to BD among adults but not youth.

METHODS

Participants included 101 youth (BD, n = 55; control group [CG], n = 46; aged 13-20 years). Blood samples were assayed for levels of CRP. T1-weighted brain images were acquired to obtain cortical surface area (SA), volume, and thickness for 3 regions of interest (ROI; whole-brain cortical gray matter, prefrontal cortex, orbitofrontal cortex [OFC]) and for vertex-wise analyses. Analyses included CRP main effects and interaction effects controlling for age, sex, and intracranial volume.

RESULTS

In ROI analyses, higher CRP was associated with higher whole-brain SA (β = 0.16; P = .03) and lower whole-brain (β = -0.31; P = .03) and OFC cortical thickness (β = -0.29; P = .04) within the BD group and was associated with higher OFC SA (β = 0.17; P = .03) within the CG. In vertex-wise analyses, higher CRP was associated with higher SA and lower cortical thickness in frontal and parietal regions within BD. A significant CRP-by-diagnosis interaction was found in frontal and temporal regions, whereby higher CRP was associated with lower neurostructural metrics in the BD group but higher neurostructural metrics in CG.

CONCLUSIONS

This study found that higher CRP among youth with BD is associated with higher SA but lower cortical thickness in ROI and vertex-wise analyses. The study identified 2 regions in which the association of CRP with brain structure differs between youth with BD and the CG. Future longitudinal, repeated-measures studies incorporating additional inflammatory markers are warranted.

Severity of Peripheral Infection Differentially Affects Brain Functions in Mice via Microglia-Dependent and -Independent Mechanisms

Peripheral infection induces inflammation in peripheral tissues and the brain, impacting brain function. Glial cells are key players in this process. However, the effects of peripheral infection on glial activation and brain function remain unknown. Here, we showed that varying degrees of peripheral infection had different effects on the regulation of brain functions by microglia-dependent and -independent mechanisms. Acute mild infection (one-day LPS challenge: 1LPS) exacerbated middle cerebral artery occlusion (MCAO) injury, and severe infection (four-day LPS challenge: 4LPS) for one week suppressed it. MCAO injury was assessed by triphenyltetrazolium chloride staining. We observed early activation of microglia in the 1LPS and 4LPS groups. Depleting microglia with a colony-stimulating factor-1 receptor (CSF1R) antagonist had no effect on 1LPS-induced brain injury exacerbation but abolished 4LPS-induced protection, indicating microglial independence and dependence, respectively. Microglia-independent exacerbation caused by 1LPS involved peripheral immune cells including macrophages. RNA sequencing analysis of 4LPS-treated microglia revealed increased factors related to anti-inflammatory and neuronal tissue repair, suggesting their association with the protective effect. In conclusion, varying degrees of peripheral inflammation had contradictory effects (exacerbation vs. protection) on MCAO, which may be attributed to microglial dependence. Our findings highlight the significant impact of peripheral infection on brain function, particularly in relation to glial cells.

Sex-dimorphic neuroprotective effect of CD163 in an α-synuclein mouse model of Parkinson's disease

Alpha-synuclein (α-syn) aggregation and immune activation represent hallmark pathological events in Parkinson's disease (PD). The PD-associated immune response encompasses both brain and peripheral immune cells, although little is known about the immune proteins relevant for such a response. We propose that the upregulation of CD163 observed in blood monocytes and in the responsive microglia in PD patients is a protective mechanism in the disease. To investigate this, we used the PD model based on intrastriatal injections of murine α-syn pre-formed fibrils in CD163 knockout (KO) mice and wild-type littermates. CD163KO females revealed an impaired and differential early immune response to α-syn pathology as revealed by immunohistochemical and transcriptomic analysis. After 6 months, CD163KO females showed an exacerbated immune response and α-syn pathology, which ultimately led to dopaminergic neurodegeneration of greater magnitude. These findings support a sex-dimorphic neuroprotective role for CD163 during α-syn-induced neurodegeneration.

Trained immunity induced by high-salt diet impedes stroke recovery

A high-salt diet (HSD) elicits sustained sterile inflammation and worsens tissue injury. However, how this occurs after stroke, a leading cause of morbidity and mortality, remains unknown. Here, we report that HSD impairs long-term brain recovery after intracerebral hemorrhage, a severe form of stroke, despite salt withdrawal prior to the injury. Mechanistically, HSD induces innate immune priming and training in hematopoietic stem and progenitor cells (HSPCs) by downregulation of NR4a family and mitochondrial oxidative phosphorylation. This training compromises alternative activation of monocyte-derived macrophages (MDMs) without altering the initial inflammatory responses of the stroke brain. Healthy mice transplanted with bone marrow from HSD-fed mice retain signatures of reduced MDM reparative functions, further confirming a persistent form of innate immune memory that originates in the bone marrow. Loss of NR4a1 in macrophages recapitulates HSD-induced negative impacts on stroke outcomes while gain of NR4a1 enables stroke recovery in HSD animals. Together, we provide the first evidence that links HSD-induced innate immune memory to the acquisition of persistent dysregulated inflammatory responses and unveils NR4a1 as a potential therapeutic target.

Role of trigger receptor 2 expressed on myeloid cells in neuroinflammation－neglected multidimensional regulation of microglia

Neuroinflammation is an inflammatory cascade involved in various neurological disorders, including Alzheimer's disease, multiple sclerosis, and other relevant diseases. The triggering receptor expressed on myeloid cells 2 (TREM2) is a transmembrane immune receptor that is primarily expressed by microglia in the central nervous system (CNS). While TREM2 is initially believed to be an anti-inflammatory factor in the CNS, increasing evidence suggests that TREM2 plays a more complex role in balancing neuroinflammation. However, the exact mechanism remains unclear. Notably, TREM2 directly regulates microglia inflammation through various signaling pathways. Additionally, studies have suggested that TREM2 mediates microglial phagocytosis, autophagy, metabolism, and microglia phenotypes, which may be involved in the modulation of neuroinflammation. In this review, we aim to discuss the critical role of TREM2 in several microglia functions and the underlying molecular mechanism the modulatory which further mediate neuroinflammation, and elaborate. Finally, we discuss the potential of TREM2 as a therapeutic target in neuroinflammatory disorders.

Long-term LPS systemic administration leads to memory impairment and disturbance in astrocytic homeostasis

Dementia is the most prevalent neurodegenerative disorder, characterized by progressive loss of memory and cognitive function. Inflammation is a major aspect in the progression of brain disorders, and inflammatory events have been associated with accelerated deterioration of cognitive function. In the present work, we investigated the impact of low-grade repeated inflammation stimuli induced by lipopolysaccharide (LPS) in hippocampal function and spatial memory. Adult male Wistar rats received a weekly injection of LPS (500 ug/kg) for sixteen weeks, eliciting systemic inflammation. Animals submitted to LPS presented impaired spatial memory and neuroinflammation. While neuronal synaptic markers such as synaptophysin and PSD-95 were unaltered, critical aspects of astrocyte homeostatic functions, such as glutamate uptake and glutathione content, were reduced. Also, glucose uptake and astrocyte lactate transporters were altered, suggesting a disturbance in the astrocyte-neuron coupling. Our present work demonstrates that long-term repeated systemic inflammation can lead to memory impairment and hippocampal metabolic disorders, especially regarding astrocyte function.

Effect of Proinflammatory S100A9 Protein on Migration and Proliferation of Microglial Cells

Alzheimer's disease (AD) is a multifactorial disease affecting aging population worldwide. Neuroinflammation became a focus of research as one of the major pathologic processes relating to the disease onset and progression. Proinflammatory S100A9 is the central culprit in the amyloid-neuroinflammatory cascade implicated in AD and other neurodegenerative diseases. We studied the effect of S100A9 on microglial BV-2 cell proliferation and migration. The responses of BV-2 cells to S100A9 stimulation were monitored in real-time using live cell microscopy, transcriptome sequencing, immunofluorescence staining, western blot analysis, and ELISA. We observed that a low dose of S100A9 promotes migration and proliferation of BV-2 cells. However, acute inflammatory condition (i.e., high S100A9 doses) causes diminished cell viability; it is uncovered that S100A9 activates TLR-4 and TLR-7 signaling pathways, leading to TNF-α and IL-6 expression, which affect BV-2 cell migration and proliferation in a concentration-dependent manner. Interestingly, the effects of S100A9 are not only inhibited by TNF-α and IL-6 antibodies. The addition of amyloid-β (Aβ) 1-40 peptide resumes the capacities of BV-2 cells to the level of low S100A9 concentrations. Based on these results, we conclude that in contrast to the beneficial effects of low S100A9 dose, high S100A9 concentration leads to impaired mobility and proliferation of immune cells, reflecting neurotoxicity at acute inflammatory conditions. However, the formation of Aβ plaques may be a natural mechanism that rescues cells from the proinflammatory and cytotoxic effects of S100A9, especially considering that inflammation is one of the primary causes of AD.

Naïve Huntington's disease microglia mount a normal response to inflammatory stimuli but display a partially impaired development of innate immune tolerance that can be counteracted by ganglioside GM1

Chronic activation and dysfunction of microglia have been implicated in the pathogenesis and progression of many neurodegenerative disorders, including Huntington's disease (HD). HD is a genetic condition caused by a mutation that affects the folding and function of huntingtin (HTT). Signs of microglia activation have been observed in HD patients even before the onset of symptoms. It is unclear, however, whether pro-inflammatory microglia activation in HD results from cell-autonomous expression of mutant HTT, is the response of microglia to a diseased brain environment, or both. In this study, we used primary microglia isolated from HD knock-in (Q140) and wild-type (Q7) mice to investigate their response to inflammatory conditions in vitro in the absence of confounding effects arising from brain pathology. We show that naïve Q140 microglia do not undergo spontaneous pro-inflammatory activation and respond to inflammatory triggers, including stimulation of TLR4 and TLR2 and exposure to necrotic cells, with similar kinetics of pro-inflammatory gene expression as wild-type microglia. Upon termination of the inflammatory insult, the transcription of pro-inflammatory cytokines is tapered off in Q140 and wild-type microglia with similar kinetics. However, the ability of Q140 microglia to develop tolerance in response to repeated inflammatory stimulations is partially impaired in vitro and in vivo, potentially contributing to the establishment of chronic neuroinflammation in HD. We further show that ganglioside GM1, a glycosphingolipid with anti-inflammatory effects on wild-type microglia, not only decreases the production of pro-inflammatory cytokines and nitric oxide in activated Q140 microglia, but also dramatically dampen microglia response to re-stimulation with LPS in an experimental model of tolerance. These effects are independent from the expression of interleukin 1 receptor associated kinase 3 (Irak-3), a strong modulator of LPS signaling involved in the development of innate immune tolerance and previously shown to be upregulated by immune cell treatment with gangliosides. Altogether, our data suggest that external triggers are required for HD microglia activation, but a cell-autonomous dysfunction that affects the ability of HD microglia to acquire tolerance might contribute to the establishment of neuroinflammation in HD. Administration of GM1 might be beneficial to attenuate chronic microglia activation and neuroinflammation.

Trained immunity: Target for prophylaxis and therapy

Trained immunity is a de facto memory for innate immune responses, leading to long-term functional reprogramming of innate immune cells. In physiological conditions, trained immunity leads to adaptive states that enhance resistance against pathogens and contributes to immunosurveillance. Dysregulated trained immunity can however lead either to defective innate immune responses in severe infections or cancer or to inflammatory and autoimmune diseases if trained immunity is inappropriately activated. Here, we review the immunological and molecular mechanisms that mediate trained immunity induction and propose that trained immunity represents an important target for prophylactic and therapeutic approaches in human diseases. On the one hand, we argue that novel approaches that induce trained immunity may enhance vaccine efficacy. On the other hand, induction of trained immunity in cancer, and inhibition of exaggerated induction of trained immunity in inflammatory disorders, are viable targets amenable for new therapeutic approaches.

Outer Membrane Vesicle-Based Nanohybrids Target Tumor-Associated Macrophages to Enhance Trained Immunity-Related Vaccine-Generated Antitumor Activity

Trained immunity refers to the innate immune system building memory-like features in response to subsequent infections and vaccinations. Compared with classical tumor vaccines, trained immunity-related vaccines (TIrV) are independent of tumor-specific antigens. Bacterial outer membrane vesicles (OMVs) contain an abundance of PAMPs and have the potential to act as TIrV-inducer, but face challenges in endotoxin tolerance, systemic delivery, long-term training, and trained tumor-associated macrophage (TAM)-mediated antitumor phagocytosis. Here, an OMV-based TIrV is developed, OMV nanohybrids (OMV-SIRPα@CaP/GM-CSF) for exerting vaccine-enhanced antitumor activity. In the bone marrow, GM-CSF-assisted OMVs train bone marrow progenitor cells and monocytes, which are inherited by TAMs. In tumor tissues, SIRPα-Fc-assisted OMVs trigger TAM-mediated phagocytosis. This TIrV can be identified by metabolic and epigenetic rewiring using transposase-accessible chromatin (ATAC) and transcriptome sequencing. Furthermore, it is found that the TIrV-mediated antitumor mechanism in the MC38 tumor model (TAM-hot and T cell-cold) is trained immunity and activated T cell response, whereas in the B16-F10 tumor model (T cell-hot and TAM-cold) is primarily mediated by trained immunity. This study not only develops and identifies OMV-based TIrV, but also investigates the trained immunity signatures and therapeutic mechanisms, providing a basis for further vaccination strategies.

TIAM2S-positive microglia enhance inflammation and neurotoxicity through soluble ICAM-1-mediated immune priming

TIAM Rac1-associated GEF 2 short form (TIAM2S) as an oncoprotein alters the immunity of peripheral immune cells to construct an inflammatory tumor microenvironment. However, its role in the activation of microglia, the primary innate immune cells of the brain, and neuroinflammation remains unknown. This study investigated the mechanism underlying TIAM2S shapes immune properties of microglia to facilitate neuron damage. Human microglial clone 3 cell line (HMC3) and human brain samples were applied to determine the presence of TIAM2S in microglia by western blots and double immunostaining. Furthermore, TIAM2S transgenic mice combined with multiple reconstituted primary neuron-glial culture systems and a cytokine array were performed to explore how TIAM2S shaped immune priming of microglia and participated in lipopolysaccharide (LPS)-induced neuron damage. TIAM2S protein was detectable in HMC3 cells and presented in a small portion (~11.1%) of microglia in human brains referred to as TIAM2S-positive microglia. With the property of secreted soluble factor-mediated immune priming, TIAM2S-positive microglia enhanced LPS-induced neuroinflammation and neural damage in vivo and in vitro. The gain- and loss-of-function experiments showed soluble intercellular adhesion molecule-1 (sICAM-1) participated in neurotoxic immune priming of TIAM2S+ microglia. Together, this study demonstrated a novel TIAM2S-positive microglia subpopulation enhances inflammation and neurotoxicity through sICAM-1-mediated immune priming.

Impact of Immunity on Coronary Artery Disease: An Updated Pathogenic Interplay and Potential Therapeutic Strategies

Coronary artery disease (CAD) is the leading cause of death worldwide. It is a result of the buildup of atherosclerosis within the coronary arteries. The role of the immune system in CAD is complex and multifaceted. The immune system responds to damage or injury to the arterial walls by initiating an inflammatory response. However, this inflammatory response can become chronic and lead to plaque formation. Neutrophiles, macrophages, B lymphocytes, T lymphocytes, and NKT cells play a key role in immunity response, both with proatherogenic and antiatherogenic signaling pathways. Recent findings provide new roles and activities referring to endothelial cells and vascular smooth muscle cells, which help to clarify the intricate signaling crosstalk between the involved actors. Research is ongoing to explore immunomodulatory therapies that target the immune system to reduce inflammation and its contribution to atherosclerosis. This review aims to summarize the pathogenic interplay between immunity and CAD and the potential therapeutic strategies, and explore immunomodulatory therapies that target the immune system to reduce inflammation and its contribution to atherosclerosis.

Noteworthy perspectives on microglia in neuropsychiatric disorders

Microglia are so versatile that they not only provide immune surveillance for central nervous system, but participate in neural circuitry development, brain blood vessels formation, blood-brain barrier architecture, and intriguingly, the regulation of emotions and behaviors. Microglia have a profound impact on neuronal survival, brain wiring and synaptic plasticity. As professional phagocytic cells in the brain, they remove dead cell debris and neurotoxic agents via an elaborate mechanism. The functional profile of microglia varies considerately depending on age, gender, disease context and other internal or external environmental factors. Numerous studies have demonstrated a pivotal involvement of microglia in neuropsychiatric disorders, including negative affection, social deficit, compulsive behavior, fear memory, pain and other symptoms associated with major depression disorder, anxiety disorder, autism spectrum disorder and schizophrenia. In this review, we summarized the latest discoveries regarding microglial ontogeny, cell subtypes or state spectrum, biological functions and mechanistic underpinnings of emotional and behavioral disorders. Furthermore, we highlight the potential of microglia-targeted therapies of neuropsychiatric disorders, and propose outstanding questions to be addressed in future research of human microglia.

Microglia and Astrocytes in Amyotrophic Lateral Sclerosis: Disease-Associated States, Pathological Roles, and Therapeutic Potential

Microglial and astrocytic reactivity is a prominent feature of amyotrophic lateral sclerosis (ALS). Microglia and astrocytes have been increasingly appreciated to play pivotal roles in disease pathogenesis. These cells can adopt distinct states characterized by a specific molecular profile or function depending on the different contexts of development, health, aging, and disease. Accumulating evidence from ALS rodent and cell models has demonstrated neuroprotective and neurotoxic functions from microglia and astrocytes. In this review, we focused on the recent advancements of knowledge in microglial and astrocytic states and nomenclature, the landmark discoveries demonstrating a clear contribution of microglia and astrocytes to ALS pathogenesis, and novel therapeutic candidates leveraging these cells that are currently undergoing clinical trials.

VEGF-A in serum protects against memory impairment in APP/PS1 transgenic mice by blocking neutrophil infiltration

Activation of innate immunity in the brain is a prominent feature of Alzheimer's disease (AD). The present study investigated the regulation of innate immunity by wild-type serum injection in a transgenic AD mouse model. We found that treatment with wild-type mouse serum significantly reduced the number of neutrophils and microglial reactivity in the brains of APP/PS1 mice. Mimicking this effect, neutrophil depletion via Ly6G neutralizing antibodies resulted in improvements in AD brain functions. Serum proteomic analysis identified vascular endothelial growth factor-A (VEGF-A) and chemokine (C-X-C motif) ligand 1 (CXCL1) as factors enriched in serum samples, which are crucial for neutrophil migration and chemotaxis, leukocyte migration, and cell chemotaxis. Exogenous VEGF-A reversed amyloid β (Aβ)-induced decreases in cyclin-dependent kinase 5 (Cdk5) and increases in CXCL1 in vitro and blocked neutrophil infiltration into the AD brain. Endothelial Cdk5 overexpression conferred an inhibitory effect on CXCL1 and neutrophil infiltration, thereby restoring memory abilities in APP/PS1 mice. Our findings uncover a previously unknown link between blood-derived VEGF signaling and neutrophil infiltration and support targeting endothelial Cdk5 signaling as a potential therapeutic strategy for AD.

Reinventing the Penumbra - the Emerging Clockwork of a Multi-modal Mechanistic Paradigm

The concept of the ischemic penumbra was originally defined as the area around a necrotic stroke core and seen as the tissue at imminent risk of further damage. Today, the penumbra is generally considered as time-sensitive hypoperfused brain tissue with decreased oxygen and glucose availability, salvageable tissue as treated by intervention, and the potential target for neuroprotection in focal stroke. The original concept entailed electrical failure and potassium release but one short of neuronal cell death and was based on experimental stroke models, later confirmed in clinical imaging studies. However, even though the basic mechanisms have translated well, conferring brain protection, and improving neurological outcome after stroke based on the pathophysiological mechanisms in the penumbra has yet to be achieved. Recent findings shape the modern understanding of the penumbra revealing a plethora of molecular and cellular pathophysiological mechanisms. We now propose a new model of the penumbra, one which we hope will lay the foundation for future translational success. We focus on the availability of glucose, the brain's central source of energy, and bioenergetic failure as core pathophysiological concepts. We discuss the relation of mitochondrial function in different cell types to bioenergetics and apoptotic cell death mechanisms, autophagy, and neuroinflammation, to glucose metabolism in what is a dynamic ischemic penumbra.

Myeloid Cells As a Promising Target for Brain-Bone Degenerative Diseases from a Metabolic Point of View

Brain and bone degenerative diseases such as Alzheimer's disease and osteoporosis are common in the aging population and lack efficient pharmacotherapies. Myeloid cells are a diverse group of mononuclear cells that plays important roles in development, immune defense, and tissue homeostasis. Aging drastically alters the expansion and function of myeloid cells, which might be a common pathogenesis of the brain-bone degenerative diseases. From this perspective, the role of myeloid cells in brain-bone degenerative diseases is discussed, with a particular focus on metabolic alterations in myeloid cells. Furthermore, targeting myeloid cells through metabolic regulation via drugs such as metformin and melatonin is proposed as a potential therapy for the clinical treatment of brain-bone diseases.

LPS priming before plaque deposition impedes microglial activation and restrains Aβ pathology in the 5xFAD mouse model of Alzheimer's disease

Microglia have an innate immunity memory (IIM) with divergent functions in different animal models of neurodegenerative diseases, including Alzheimer's disease (AD). AD is characterized by chronic neuroinflammation, neurodegeneration, tau tangles and β-amyloid (Aβ) deposition. Systemic inflammation has been implicated in contributing to the progression of AD. Multiple reports have demonstrated unique microglial signatures in AD mouse models and patients. However, the proteomic profiles of microglia modified by IIM have not been well-documented in an AD model. Therefore, in the present study, we investigate whether lipopolysaccharide (LPS)-induced IIM in the pre-clinical stage of AD alters the microglial responses and shapes the neuropathology. We accomplished this by priming 5xFAD and wild-type (WT) mice with an LPS injection at 6 weeks (before the robust development of plaques). 140 days later, we evaluated microglial morphology, activation, the microglial barrier around Aβ, and Aβ deposition in both 5xFAD primed and unprimed mice. Priming induced decreased soma size of microglia and reduced colocalization of PSD95 and Synaptophysin in the retrosplenial cortex. Priming appeared to increase phagocytosis of Aβ, resulting in fewer Thioflavin S+ Aβ fibrils in the dentate gyrus. RIPA-soluble Aβ 40 and 42 were significantly reduced in Primed-5xFAD mice leading to a smaller size of MOAB2+ Aβ plaques in the prefrontal cortex. We also found that Aβ-associated microglia in the Primed-5xFAD mice were less activated and fewer in number. After priming, we also observed improved memory performance in 5xFAD. To further elucidate the molecular mechanism underlying these changes, we performed quantitative proteomic analysis of microglia and bone marrow monocytes. A specific pattern in the microglial proteome was revealed in primed 5xFAD mice. These results suggest that the imprint signatures of primed microglia display a distinctive phenotype and highlight the potential for a beneficial adaption of microglia when intervention occurs in the pre-clinical stage of AD.

Transcriptional and epigenetic decoding of the microglial aging process

As important immune cells, microglia undergo a series of alterations during aging that increase the susceptibility to brain dysfunctions. However, the longitudinal characteristics of microglia remain poorly understood. In this study, we mapped the transcriptional and epigenetic profiles of microglia from 3- to 24-month-old mice. We first discovered unexpected sex differences and identified age-dependent microglia (ADEM) genes during the aging process. We then compared the features of aging and reactivity in female microglia at single-cell resolution and epigenetic level. To dissect functions of aged microglia excluding the influence from other aged brain cells, we established an accelerated microglial turnover model without directly affecting other brain cells. By this model, we achieved aged-like microglia in non-aged brains and confirmed that aged-like microglia per se contribute to cognitive decline. Collectively, our work provides a comprehensive resource for decoding the aging process of microglia, shedding light on how microglia maintain brain functions.

Tolerized Microglia Protect Neurons Against Endotoxin-Induced TNF-α Production via an LBP-Dependent Intracellular p38 MAPK Signaling Pathway

The development of microglial endotoxin tolerance (ET) is a critical event in protecting neurons against excessive immune responses when microglia are administered two consecutive lipopolysaccharide (LPS) challenges. However, the intrinsic mechanisms of microglia shape ET programs and protect neurons remain unclear. This study aimed to determine whether extracellular autocrine cascades or intracellular signaling pathways are involved in ET microglia-mediated tumor necrosis factor-alpha (TNF-α) reduction and neuroprotection. Neuron-glia cultures composed of astroglia, neurons, and microglia were performed in different conditions: with or without serum or LPS-binding proteins (LBP), along with an induction approach of ET. Enzyme-linked immunosorbent assay results revealed that LPS induced TNF-α tolerance of microglia in an LBP-dependent manner. Furthermore, we determined whether the early pro-inflammatory cytokines induced by LPS might contribute to the development of microglial ET. Our data showed that the neutralization of TNF-α using an anti-TNF-α antibody had no change in the TNF-α tolerance of microglia during the ET challenge. Furthermore, pre-incubation of TNF-α, interleukin-1 beta, and prostaglandin E2 failed to induce any TNF-α tolerance in microglia after LPS treatment. Moreover, using three specific chemical inhibitors that respectively blocked the activities of the mitogen-activated protein kinases (MAPKs) namely p38, c-Jun N-terminal kinase and extracellular signal-related kinases revealed that inhibition of p38 MAPK by SB203580 disrupted the tolerated microglia-mediated TNF-α reduction and neuroprotection. In summary, our findings demonstrated that the LPS pre-treatment immediately programmed the microglial ET to prevent endotoxin-induced TNF-α production and neuronal damage through the intracellular p38 MAPK signaling pathway.

CXCR6 orchestrates brain CD8+ T cell residency and limits mouse Alzheimer's disease pathology

Neurodegenerative diseases, including Alzheimer's disease (AD), are characterized by innate immune-mediated inflammation, but functional and mechanistic effects of the adaptive immune system remain unclear. Here we identify brain-resident CD8+ T cells that coexpress CXCR6 and PD-1 and are in proximity to plaque-associated microglia in human and mouse AD brains. We also establish that CD8+ T cells restrict AD pathologies, including β-amyloid deposition and cognitive decline. Ligand-receptor interaction analysis identifies CXCL16-CXCR6 intercellular communication between microglia and CD8+ T cells. Further, Cxcr6 deficiency impairs accumulation, tissue residency programming and clonal expansion of brain PD-1+CD8+ T cells. Ablation of Cxcr6 or CD8+ T cells ultimately increases proinflammatory cytokine production from microglia, with CXCR6 orchestrating brain CD8+ T cell-microglia colocalization. Collectively, our study reveals protective roles for brain CD8+ T cells and CXCR6 in mouse AD pathogenesis and highlights that microenvironment-specific, intercellular communication orchestrates tissue homeostasis and protection from neuroinflammation.

Water extract of goji berries improves neuroinflammation induced by a high-fat and high-fructose diet based on the bile acid-mediated gut-brain axis pathway

The high-fat and high-fructose diet (HFFD) is a common diet in westernized societies, which worsens disturbances in gut microbiota and bile acid (BA) metabolism. Herein, the present study aimed to investigate the effects of the water extract of Lycium barbarum fruits (LBE) on gut microbiota and BA metabolism in mice with HFFD-induced neuroinflammation. The results showed that supplementation of LBE for 14 weeks remarkably ameliorated weight gain and insulin resistance and suppressed microglial activation and neural neuroinflammation induced by HFFD. The results of Morris water maze and Y-maze tests demonstrated that LBE attenuated HFFD-induced cognitive impairment. Moreover, LBE elevated hepatic BA biosynthesis and excretion of BAs and increased elimination of BAs via the feces. Notably, LBE supplementation resulted in the enrichment of tauroursodeoxycholic acid in the cortex and hippocampus. Furthermore, the 16S rDNA sequencing results showed that LBE could modulate the structure of gut microbiota, and in the meantime decrease the relative abundance of Clostridium_XlVa, which is associated with BA homeostasis. Additionally, LBE exerted neuroprotective effects involving the increment of Lactococcus, known as a potentially beneficial bacterium. These results demonstrated that LBE could ameliorate neuroinflammation and cognitive impairment in HFFD-induced mice through the gut-liver-brain axis, which might be due to the regulation of BA homeostasis and gut microbiota in mice.

Live imaging of microglia during sleeping sickness reveals early and heterogeneous inflammatory responses

Introduction

Invasion of the central nervous system (CNS) is the most serious consequence of Trypanosoma brucei infection, which causes sleeping sickness. Recent experimental data have revealed some more insights into the disease during the meningoencephalitic stage. However, detailed cellular processes befalling the CNS during the disease are poorly understood.

Methods

To further address this issue, we implanted a cranial window on the cortex of B6.129P2(Cg)-Cx3cr1tm1Litt/J mice, infected them with Trypanosoma brucei expressing RFP via intraperitoneal injection, and monitored microglial cells and parasites longitudinally over 30 days using in vivo 2-photon imaging. We correlated the observed changes with histological analyses to evaluate the recruitment of peripheral immune cells.

Results and discussion

We uncovered an early involvement of microglia that precedes invasion of the CNS by the parasite. We accomplished a detailed characterization of the progressive sequence of events that correlates with microglial morphological changes and microgliosis. Our findings unveiled a heterogeneous microglial response in places of initial homeostatic disruption near brain barriers and pointed out an exceptional capability of microglia to hamper parasite proliferation inside the brain. We also found early signs of inflammation in the meninges, which synchronize with the microglial response. Moreover, we observed a massive infiltration of peripheral immune cells into the parenchyma as a signature in the final disease stage. Overall, our study provides new insights into the host-pathogen immune interactions in the meningeal and parenchymal compartments of the neocortex.

Injury-experienced satellite cells retain long-term enhanced regenerative capacity

BACKGROUND

Inflammatory memory or trained immunity is a recently described process in immune and non-immune tissue resident cells, whereby previous exposure to inflammation mediators leads to a faster and stronger responses upon secondary challenge. Whether previous muscle injury is associated with altered responses to subsequent injury by satellite cells (SCs), the muscle stem cells, is not known.

METHODS

We used a mouse model of repeated muscle injury, in which intramuscular cardiotoxin (CTX) injections were administered 50 days apart in order to allow for full recovery of the injured muscle before the second injury. The effect of prior injury on the phenotype, proliferation and regenerative potential of satellite cells following a second injury was examined in vitro and in vivo by immunohistochemistry, RT-qPCR and histological analysis.

RESULTS

We show that SCs isolated from muscle at 50 days post-injury (injury-experienced SCs (ieSCs)) enter the cell cycle faster and form bigger myotubes when cultured in vitro, compared to control SCs isolated from uninjured contralateral muscle. Injury-experienced SCs were characterized by the activation of the mTORC 1 signaling pathway, suggesting they are poised to activate sooner following a second injury. Consequently, upon second injury, SCs accumulate in greater numbers in muscle at 3 and 10 days after injury. These changes in SC phenotype and behavior were associated with accelerated muscle regeneration, as evidenced by an earlier appearance of bigger fibers and increased number of myonuclei per fiber at day 10 after the second injury.

CONCLUSIONS

Overall, we show that skeletal muscle injury has a lasting effect on SC function priming them to respond faster to a subsequent injury. The ieSCs have long-term enhanced regenerative properties that contribute to accelerated regeneration following a secondary challenge.

Yamogenin Exhibits Antidepressant-like Effects via Inhibition of ER Stress and Microglial Activation in LPS-Induced Mice

Depression is a multifaceted psychiatric disorder that affects a significant number of individuals worldwide, and its pathophysiology encompasses a variety of mechanisms, including the induction of endoplasmic reticulum (ER) stress, which has been correlated with depressive-like behaviors in animal models. Yamogenin, a bioactive compound derived from traditional Chinese medicine Dioscorea species, possesses diverse pharmacological properties. This investigation aimed to explore the antidepressant-like effects of yamogenin and the underlying mechanisms involved. By utilizing a murine model of lipopolysaccharide (LPS)-induced depressive-like behavior, we demonstrated that yamogenin enhanced sucrose preference and reduced immobility time in the forced swimming test. These effects were observed alongside the attenuation of ER stress through modulation of the PERK/eIF2α/ATF4/CHOP signaling pathway in the prefrontal cortex. Moreover, yamogenin augmented the expression of the antiapoptotic protein Bcl-2 while diminishing the expression of the proapoptotic protein caspase-3. Additionally, yamogenin exhibited inhibitory effects on microglial activation but did not elicit the promotion of brain-derived neurotrophic factor (BDNF) signaling. Collectively, our findings propose that yamogenin exerts antidepressant-like effects in LPS-induced mice by inhibiting ER stress and microglial activation. This study contributes novel insights into the potential utilization of yamogenin as a natural antidepressant agent.

Innate (learned) memory

With the growing body of evidence, it is now clear that not only adaptive immune cells but also innate immune cells can mount a more rapid and potent nonspecific immune response to subsequent exposures. This process is known as trained immunity or innate (learned) immune memory. This review discusses the different immune and nonimmune cell types of the central and peripheral immune systems that can develop trained immunity. This review highlights the intracellular signaling and metabolic and epigenetic mechanisms underlying the formation of innate immune memory. Finally, this review explores the health implications together with the potential therapeutic interventions harnessing trained immunity.

Chronic inflammation, neuroglial dysfunction, and plasmalogen deficiency as a new pathobiological hypothesis addressing the overlap between post-COVID-19 symptoms and myalgic encephalomyelitis/chronic fatigue syndrome

After five waves of coronavirus disease 2019 (COVID-19) outbreaks, it has been recognized that a significant portion of the affected individuals developed long-term debilitating symptoms marked by chronic fatigue, cognitive difficulties ("brain fog"), post-exertional malaise, and autonomic dysfunction. The onset, progression, and clinical presentation of this condition, generically named post-COVID-19 syndrome, overlap significantly with another enigmatic condition, referred to as myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS). Several pathobiological mechanisms have been proposed for ME/CFS, including redox imbalance, systemic and central nervous system inflammation, and mitochondrial dysfunction. Chronic inflammation and glial pathological reactivity are common hallmarks of several neurodegenerative and neuropsychiatric disorders and have been consistently associated with reduced central and peripheral levels of plasmalogens, one of the major phospholipid components of cell membranes with several homeostatic functions. Of great interest, recent evidence revealed a significant reduction of plasmalogen contents, biosynthesis, and metabolism in ME/CFS and acute COVID-19, with a strong association to symptom severity and other relevant clinical outcomes. These bioactive lipids have increasingly attracted attention due to their reduced levels representing a common pathophysiological manifestation between several disorders associated with aging and chronic inflammation. However, alterations in plasmalogen levels or their lipidic metabolism have not yet been examined in individuals suffering from post-COVID-19 symptoms. Here, we proposed a pathobiological model for post-COVID-19 and ME/CFS based on their common inflammation and dysfunctional glial reactivity, and highlighted the emerging implications of plasmalogen deficiency in the underlying mechanisms. Along with the promising outcomes of plasmalogen replacement therapy (PRT) for various neurodegenerative/neuropsychiatric disorders, we sought to propose PRT as a simple, effective, and safe strategy for the potential relief of the debilitating symptoms associated with ME/CFS and post-COVID-19 syndrome.

Ontogeny, functions and reprogramming of Kupffer cells upon infectious disease

The liver is a vital metabolic organ that also performs important immune-regulatory functions. In the context of infections, the liver represents a target site for various pathogens, while also having an outstanding capacity to filter the blood from pathogens and to contain infections. Pathogen scavenging by the liver is primarily performed by its large and heterogeneous macrophage population. The major liver-resident macrophage population is located within the hepatic microcirculation and is known as Kupffer cells (KCs). Although other minor macrophages reside in the liver as well, KCs remain the best characterized and are the best well-known hepatic macrophage population to be functionally involved in the clearance of infections. The response of KCs to pathogenic insults often governs the overall severity and outcome of infections on the host. Moreover, infections also impart long-lasting, and rarely studied changes to the KC pool. In this review, we discuss current knowledge on the biology and the various roles of liver macrophages during infections. In addition, we reflect on the potential of infection history to imprint long-lasting effects on macrophages, in particular liver macrophages.

Investigating the Synergistic Potential of Low-Dose HDAC3 Inhibition and Radiotherapy in Alzheimer's Disease Models

We have previously shown that histone deacetylase (HDAC) inhibition and cranial radiotherapy (RT) independently improve molecular and behavioral Alzheimer's disease (AD)-like phenotypes. In the present study, we investigate the synergistic potential of using both RT and HDACi as a low-dose combination therapy (LDCT) to maximize disease modification (reduce neuroinflammation and amyloidogenic APP processing, increase neurotrophic gene expression) while minimizing the potential for treatment-associated side effects.LDCT consisted of daily administration of the HDAC3 inhibitor RGFP966 and/or bi-weekly cranial x-irradiation. Amyloid-beta precursor protein (APP) processing and innate immune response to LDCT were assessed in vitro and in vivo using human and murine cell models and 3xTg-AD mice. After 2 months of LDCT in mice, behavioral analyses as well as expression and modification of key AD-related targets (Aβ, tau, Csf1r, Bdnf, etc.) were assessed in the hippocampus (HIP) and prefrontal cortex (PFC).LDCT induced a tolerant, anti-inflammatory innate immune response in microglia and increased non-amyloidogenic APP processing in vitro. Both RT and LDCT improved the rate of learning and spatial memory in the Barnes maze test. LDCT induced a unique anti-AD HIP gene expression profile that included upregulation of neurotrophic genes and downregulation of inflammation-related genes. RT lowered HIP Aβ42/40 ratio and Bace1 protein, while LDCT lowered PFC p-tau181 and HIP Bace1 levels.Our study supports the rationale for combining complementary therapeutic approaches at low doses to target multifactorial AD pathology synergistically. Namely, LDCT with RGFP966 and cranial RT shows disease-modifying potential against a wide range of AD-related hallmarks.

A brain cytokine-independent switch in cortical activity marks the onset of sickness behavior triggered by acute peripheral inflammation

Systemic inflammation triggers protective as well as pro-inflammatory responses in the brain based on neuronal and/or cytokine signaling, and it associates with acutely and protractedly disrupted cognition. However, the multiple mechanisms underlying the peripheral-central inflammatory signaling are still not fully characterized. We used intraperitoneal (i.p.) injection of lipopolysaccharide (LPS) in freely moving mice with chronically implanted electrodes for recording of local field potentials (LFP) and electrocorticography (ECoG) in the hippocampus and neocortex, respectively. We show here that a sudden switch in the mode of network activity occurred in both areas starting at 10-15 min after the LPS injection, simultaneously with a robust change from exploration to sickness behavior. This switch in cortical mode commenced before any elevations in pro-inflammatory cytokines IL-1β, TNFα, CCL2 or IL-6 were detected in brain tissue. Thereafter, this mode dominated cortical activity for the recording period of 3 h, except for a partial and transient recovery around 40 min post-LPS. These effects were closely paralleled by changes in ECoG spectral entropy. Continuous recordings for up to 72 h showed a protracted attenuation in hippocampal activity, while neocortical activity recovered after 48 h. The acute sickness behavior recovered by 72 h post-LPS. Notably, urethane (1.3 mg/kg) administered prior to LPS blocked the early effect of LPS on cortical activity. However, experiments under urethane anesthesia which were started 24 h post-LPS (with neuroinflammation fully developed before application of urethane) showed that both theta-supratheta and fast gamma CA1 activity were reduced, DG delta activity was increased, and sharp-wave ripples were abolished. Finally, we observed that experimental compensation of inflammation-induced hypothermia 24-48 h post-LPS promoted seizures and status epilepticus; and that LPS decreased the threshold of kainate-provoked seizures beyond the duration of acute sickness behavior indicating post-acute inflammatory hyperexcitability. Taken together, the strikingly fast development and initial independence of brain cytokines of the LPS-induced cortical mode, its spectral characteristics and simultaneity in hippocampus and neocortex, as well as inhibition by pre-applied urethane, strongly suggest that the underlying mechanisms are based on activation of the afferent vagus nerve and its mainly cholinergic ascending projections to higher brain areas.

Multi-transcriptomics reveals brain cellular responses to peripheral infection in Alzheimer's disease model mice

Peripheral inflammation has been linked to various neurodegenerative disorders, including Alzheimer's disease (AD). Here we perform bulk, single-cell, and spatial transcriptomics in APP/PS1 mice intranasally exposed to Staphylococcus aureus to determine how low-grade peripheral infection affects brain transcriptomics and AD-like pathology. Chronic exposure led to increased amyloid plaque burden and plaque-associated microglia, significantly affecting the transcription of brain barrier-associated cells, which resulted in barrier leakage. We reveal cell-type- and spatial-specific transcriptional changes related to brain barrier function and neuroinflammation during the acute infection. Both acute and chronic exposure led to brain macrophage-associated responses and detrimental effects in neuronal transcriptomics. Finally, we identify unique transcriptional responses at the amyloid plaque niches following acute infection characterized by higher disease-associated microglia gene expression and a larger effect on astrocytic or macrophage-associated genes, which could facilitate amyloid and related pathologies. Our findings provide important insights into the mechanisms linking peripheral inflammation to AD pathology.