Epigenomic features related to microglia are associated with attenuated effect of APOE ε4 on Alzheimer's disease risk in humans.

Not all apolipoprotein E (APOE) ε4 carriers who survive to advanced age develop Alzheimer's disease (AD); factors attenuating the risk of ε4 on AD may exist. Guided by the top ε4-attenuating signals from methylome-wide association analyses (N = 572, ε4+ and ε4-) of neurofibrillary tangles and neuritic plaques, we conducted a meta-analysis for pathological AD within the ε4+ subgroups (N = 235) across four independent collections of brains. Cortical RNA-seq and microglial morphology measurements were used in functional analyses. Three out of the four significant CpG dinucleotides were captured by one principal component (PC1), which interacts with ε4 on AD, and is associated with expression of innate immune genes and activated microglia. In ε4 carriers, reduction in each unit of PC1 attenuated the odds of AD by 58% (odds ratio = 2.39, 95% confidence interval = [1.64,3.46], P = 7.08 × 10-6 ). An epigenomic factor associated with a reduced proportion of activated microglia (epigenomic factor of activated microglia, EFAM) appears to attenuate the risk of ε4 on AD.

EPIGENOMIC FEATURES RELATED TO MICROGLIA ARE ASSOCIATED WITH ATTENUATED EFFECT OF APOE ε4 ON ALZHEIMER'S DISEASE RISK IN HUMANS.

Not all APOE ε4 carriers who survive to advanced age develop Alzheimer's disease (AD); factors attenuating the risk of ε4 on AD may exist. Guided by the top ε4-attenuating signals from methylome-wide association analyses (N=572, ε4+ and ε4-) of neurofibrillary tangles and neuritic plaques, we conducted a meta-analysis for pathological AD within the ε4+ subgroups (N=235) across four independent collections of brains. Cortical RNA-seq and microglial morphology measurements were used in functional analyses. Three out of the four significant CpG dinucleotides were captured by one principle component (PC1), which interacts with ε4 on AD, and is associated with expression of innate immune genes and activated microglia. In ε4 carriers, reduction in each unit of PC1 attenuated the odds of AD by 58% (OR=2.39, 95%CI=[1.64,3.46], P=7.08x10-6). An epigenomic factor associated with a reduced proportion of activated microglia (microglial epigenomic factor 1) appears to attenuate the risk of ε4 on AD.

BCL6 controls Th9 cell development by repressing Il9 transcription.

The transcriptional repressor B cell lymphoma 6 (BCL6) is required for the development of Th follicular cells, and it has been shown to suppress Th2 cell differentiation. We demonstrate that BCL6 is a key regulator of Th9 cell development. BCL6 expression is transiently downregulated in polarized Th9 cells, and forced expression of BCL6 in Th9 cells impairs Th9 cell differentiation. In contrast, BCL6 knockdown upregulated IL-9 production in Th9 cells. The function of BCL6 in Th9 cells is under the control of IL-2/JAK3/STAT5 signaling pathway. Using chromatin immunoprecipitation, we show that, in Th9 cells, BCL6 and STAT5 bind to adjacent motifs in the Il9 promoter. Furthermore, we found that STAT5 binding was associated with the abundance of a permissive histone mark at the Il9 promoter, whereas under conditions in which BCL6 binding was predominant, a repressive histone mark was prevalent. The effects of STAT5 and BCL6 on IL-9 transcription were further demonstrated using an IL-9 luciferase reporter assay in which BCL6 repressed STAT5-mediated Il9 transactivation. In experimental autoimmune encephalomyelitis, forced expression of BCL6 in myelin oligodendrocyte glycoprotein35-55-specific Th9 cells resulted in decreased IL-9 production and induction of IFN-γ, causing an exacerbation of the clinical disease. Our findings demonstrate a novel role of BCL6 in the regulation of Th9 cell development and their encephalitogenicity.

Cell surface glycan engineering of neural stem cells augments neurotropism and improves recovery in a murine model of multiple sclerosis.

Neural stem cell (NSC)-based therapies offer potential for neural repair in central nervous system (CNS) inflammatory and degenerative disorders. Typically, these conditions present with multifocal CNS lesions making it impractical to inject NSCs locally, thus mandating optimization of vascular delivery of the cells to involved sites. Here, we analyzed NSCs for expression of molecular effectors of cell migration and found that these cells are natively devoid of E-selectin ligands. Using glycosyltransferase-programmed stereosubstitution (GPS), we glycan engineered the cell surface of NSCs ("GPS-NSCs") with resultant enforced expression of the potent E-selectin ligand HCELL (hematopoietic cell E-/L-selectin ligand) and of an E-selectin-binding glycoform of neural cell adhesion molecule ("NCAM-E"). Following intravenous (i.v.) injection, short-term homing studies demonstrated that, compared with buffer-treated (control) NSCs, GPS-NSCs showed greater neurotropism. Administration of GPS-NSC significantly attenuated the clinical course of experimental autoimmune encephalomyelitis (EAE), with markedly decreased inflammation and improved oligodendroglial and axonal integrity, but without evidence of long-term stem cell engraftment. Notably, this effect of NSC is not a universal property of adult stem cells, as administration of GPS-engineered mouse hematopoietic stem/progenitor cells did not improve EAE clinical course. These findings highlight the utility of cell surface glycan engineering to boost stem cell delivery in neuroinflammatory conditions and indicate that, despite the use of a neural tissue-specific progenitor cell population, neural repair in EAE results from endogenous repair and not from direct, NSC-derived cell replacement.

IL-4 and retinoic acid synergistically induce regulatory dendritic cells expressing Aldh1a2.

Although activated inflammatory monocytes (IMCs) and inflammatory dendritic cells (IDCs) are potent T cell suppressors, nonactivated IMCs and IDCs promote T cell activation and Th1/Th17 cell differentiation. In this study, we investigated how to reduce the proinflammatory properties of IMCs and IDCs and further convert them into immune regulatory dendritic cells (DCs). We found that IL-4 and retinoic acid (RA) cotreatment of GM-CSF-differentiated IDCs synergistically induced the expression of aldehyde dehydrogenase family 1, subfamily A2, a rate-limiting enzyme for RA synthesis in DCs. IL-4 plus RA-treated IDCs upregulated CD103 expression and markedly reduced the production of proinflammatory cytokines upon activation. IL-4 plus RA-treated IDCs strongly induced CD4⁺Foxp3⁺ regulatory T cell differentiation and suppressed Th1 and Th17 differentiation. Mechanistically, the transcription factors Stat6 and RA receptor ß play important roles in aldehyde dehydrogenase family 1, subfamily A2, induction. In addition, IL-4 and RA signaling pathways interact closely to enhance the regulatory function of treated DCs. Adoptive transfer of IL-4 plus RA-treated DCs significantly increased regulatory T cell frequency in vivo. Direct treatment with IL-4 and RA also markedly suppressed actively induced experimental autoimmune encephalomyelitis. Our data demonstrate the synergistic effect of IL-4 and RA in inducing a regulatory phenotype in IDCs, providing a potential treatment strategy for autoimmune diseases.

An integrated toolkit for human microglia functional genomics.

BACKGROUND: Microglia, the brain's resident immune cells, play vital roles in brain development, and disorders like Alzheimer's disease (AD). Human iPSC-derived microglia (iMG) provide a promising model to study these processes. However, existing iMG generation protocols face challenges, such as prolonged differentiation time, lack of detailed characterization, and limited gene function investigation via CRISPR-Cas9. METHODS: Our integrated toolkit for in-vitro microglia functional genomics optimizes iPSC differentiation into iMG through a streamlined two-step, 20-day process, producing iMG with a normal karyotype. We confirmed the iMG's authenticity and quality through single-cell RNA sequencing, chromatin accessibility profiles (ATAC-Seq), proteomics and functional tests. The toolkit also incorporates a drug-dependent CRISPR-ON/OFF system for temporally controlled gene expression. Further, we facilitate the use of multi-omic data by providing online searchable platform that compares new iMG profiles to human primary microglia: https://sherlab.shinyapps.io/IPSC-derived-Microglia/ . RESULTS: Our method generates iMG that closely align with human primary microglia in terms of transcriptomic, proteomic, and chromatin accessibility profiles. Functionally, these iMG exhibit Ca2 + transients, cytokine driven migration, immune responses to inflammatory signals, and active phagocytosis of CNS related substrates including synaptosomes, amyloid beta and myelin. Significantly, the toolkit facilitates repeated iMG harvesting, essential for large-scale experiments like CRISPR-Cas9 screens. The standalone ATAC-Seq profiles of our iMG closely resemble primary microglia, positioning them as ideal tools to study AD-associated single nucleotide variants (SNV) especially in the genome regulatory regions. CONCLUSIONS: Our advanced two-step protocol rapidly and efficiently produces authentic iMG. With features like the CRISPR-ON/OFF system and a comprehensive multi-omic data platform, our toolkit equips researchers for robust microglial functional genomic studies. By facilitating detailed SNV investigation and offering a sustainable cell harvest mechanism, the toolkit heralds significant progress in neurodegenerative disease drug research and therapeutic advancement.

Immune Response following FLASH and Conventional Radiation in Diffuse Midline Glioma.

PURPOSE: Diffuse midline glioma (DMG) is a fatal tumor traditionally treated with radiation therapy (RT) and previously characterized as having a noninflammatory tumor immune microenvironment (TIME). FLASH is a novel RT technique using ultra-high dose rate that is associated with decreased toxicity and effective tumor control. However, the effect of FLASH and conventional (CONV) RT on the DMG TIME has not yet been explored. METHODS AND MATERIALS: Here, we performed single-cell RNA sequencing (scRNA-seq) and flow cytometry on immune cells isolated from an orthotopic syngeneic murine model of brainstem DMG after the use of FLASH (90 Gy/sec) or CONV (2 Gy/min) dose-rate RT and compared to unirradiated tumor (SHAM). RESULTS: At day 4 post-RT, FLASH exerted similar effects as CONV in the predominant microglial (MG) population, including the presence of two activated subtypes. However, at day 10 post-RT, we observed a significant increase in the type 1 interferon α/ß receptor (IFNAR+) in MG in CONV and SHAM compared to FLASH. In the non-resident myeloid clusters of macrophages (MACs) and dendritic cells (DCs), we found increased type 1 interferon (IFN1) pathway enrichment for CONV compared to FLASH and SHAM by scRNA-seq. We observed this trend by flow cytometry at day 4 post-RT in IFNAR+ MACs and DCs, which equalized by day 10 post-RT. DMG control and murine survival were equivalent between RT dose rates. CONCLUSIONS: Our work is the first to map CONV and FLASH immune alterations of the DMG TIME with single-cell resolution. Although DMG tumor control and survival were similar between CONV and FLASH, we found that changes in immune compartments differed over time. Importantly, although both RT modalities increased IFN1, we found that the timing of this response was cell-type and dose-rate dependent. These temporal differences, particularly in the context of tumor control, warrant further study.

Stat1 is an inducible transcriptional repressor of neural stem cells self-renewal program during neuroinflammation.

A central issue in regenerative medicine is understanding the mechanisms that regulate the self-renewal of endogenous stem cells in response to injury and disease. Interferons increase hematopoietic stem cells during infection by activating STAT1, but the mechanisms by which STAT1 regulates intrinsic programs in neural stem cells (NSCs) during neuroinflammation is less known. Here we explored the role of STAT1 on NSC self-renewal. We show that overexpressing Stat1 in NSCs derived from the subventricular zone (SVZ) decreases NSC self-renewal capacity while Stat1 deletion increases NSC self-renewal, neurogenesis, and oligodendrogenesis in isolated NSCs. Importantly, we find upregulation of STAT1 in NSCs in a mouse model of multiple sclerosis (MS) and an increase in pathological T cells expressing IFN-γ rather than interleukin 17 (IL-17) in the cerebrospinal fluid of affected mice. We find IFN-γ is superior to IL-17 in reducing proliferation and precipitating an abnormal NSC phenotype featuring increased STAT1 phosphorylation and Stat1 and p16ink4a gene expression. Notably, Stat1-/- NSCs were resistant to the effect of IFN-γ. Lastly, we identified a Stat1-dependent gene expression profile associated with an increase in the Sox9 transcription factor, a regulator of self-renewal. Stat1 binds and transcriptionally represses Sox9 in a transcriptional luciferase assay. We conclude that Stat1 serves as an inducible checkpoint for NSC self-renewal that is upregulated during chronic brain inflammation leading to decreased self-renewal. As such, Stat1 may be a potential target to modulate for next generation therapies to prevent progression and loss of repair function in NSCs/neural progenitors in MS.

Identification of a microglia phenotype supportive of remyelination.

In multiple sclerosis, endogenous oligodendrocyte precursor cells (OPCs) attempt to remyelinate areas of myelin damage. During disease progression, however, these attempts fail. It has been suggested that modulating the inflammatory environment of the lesion might provide a promising therapeutic approach to promote endogenous remyelination. Microglia are known to play a central role in neuroinflammatory processes. To investigate the microglia phenotype that supports remyelination, we performed genome-wide gene expression analysis of microglia from the corpus callosum during demyelination and remyelination in the mouse cuprizone model, in which remyelination spontaneously occurs after an episode of toxin-induced primary demyelination. We provide evidence for the existence of a microglia phenotype that supports remyelination already at the onset of demyelination and persists throughout the remyelination process. Our data show that microglia are involved in the phagocytosis of myelin debris and apoptotic cells during demyelination. Furthermore, they express a cytokine and chemokine repertoire enabling them to activate and recruit endogenous OPCs to the lesion site and deliver trophic support during remyelination. This study not only provides a detailed transcriptomic analysis of the remyelination-supportive microglia phenotype but also reinforces the notion that the primary function of microglia is the maintenance of tissue homeostasis and the support of regeneration already at the earliest stages in the development of demyelinating lesions.

An optimized protocol for the acute isolation of human microglia from autopsy brain samples.

Microglia are increasingly recognized to be crucially involved in the maintenance of tissue homeostasis of the brain and spinal cord. Not surprisingly is therefore the growing scientific interest in the microglia phenotypes associated with various physiological and pathological processes of the central nervous system. Until recently the investigation of these phenotypes was hindered by the lack of an isolation protocol that (without an extended culturing period) would offer a microglia population of high purity and yield. Thus, our objective was to establish a rapid and efficient method for the isolation of human microglia from postmortem brain samples. We tested multiple elements of already existing protocols (e.g., density separation, immunomagnetic bead separation) and combined them to minimize preparation time and maximize yield and purity. The procedure presented in this article enables acute isolation of human microglia from autopsy (and biopsy) samples with a purity and yield that is suitable for downstream applications, such as protein and gene expression analysis and functional assays. Moreover, the present protocol is appropriate for the isolation of microglia from autopsy samples irrespective of the neurological state of the brain or specific brain regions and (with minor modification) could be even used for the isolation of microglia from human glioma tissue.

A cortical immune network map identifies distinct microglial transcriptional programs associated with ß-amyloid and Tau pathologies.

Microglial dysfunction has been proposed as one of the many cellular mechanisms that can contribute to the development of Alzheimer's disease (AD). Here, using a transcriptional network map of the human frontal cortex, we identify five modules of co-expressed genes related to microglia and assess their role in the neuropathologic features of AD in 540 subjects from two cohort studies of brain aging. Two of these transcriptional programs-modules 113 and 114-relate to the accumulation of ß-amyloid, while module 5 relates to tau pathology. We replicate these associations in brain epigenomic data and in two independent datasets. In terms of tau, we propose that module 5, a marker of activated microglia, may lead to tau accumulation and subsequent cognitive decline. We validate our model further by showing that three representative module 5 genes (ACADVL, TRABD, and VASP) encode proteins that are upregulated in activated microglia in AD.

BIN1 protein isoforms are differentially expressed in astrocytes, neurons, and microglia: neuronal and astrocyte BIN1 are implicated in tau pathology.

BACKGROUND: Identified as an Alzheimer's disease (AD) susceptibility gene by genome wide-association studies, BIN1 has 10 isoforms that are expressed in the Central Nervous System (CNS). The distribution of these isoforms in different cell types, as well as their role in AD pathology still remains unclear. METHODS: Utilizing antibodies targeting specific BIN1 epitopes in human post-mortem tissue and analyzing mRNA expression data from purified microglia, we identified three isoforms expressed in neurons and astrocytes (isoforms 1, 2 and 3) and four isoforms expressed in microglia (isoforms 6, 9, 10 and 12). The abundance of selected peptides, which correspond to groups of BIN1 protein isoforms, was measured in dorsolateral prefrontal cortex, and their relation to neuropathological features of AD was assessed. RESULTS: Peptides contained in exon 7 of BIN1's N-BAR domain were found to be significantly associated with AD-related traits and, particularly, tau tangles. Decreased expression of BIN1 isoforms containing exon 7 is associated with greater accumulation of tangles and subsequent cognitive decline, with astrocytic rather than neuronal BIN1 being the more likely culprit. These effects are independent of the BIN1 AD risk variant. CONCLUSIONS: Exploring the molecular mechanisms of specific BIN1 isoforms expressed by astrocytes may open new avenues for modulating the accumulation of Tau pathology in AD.

Single cell RNA sequencing of human microglia uncovers a subset associated with Alzheimer's disease.

The extent of microglial heterogeneity in humans remains a central yet poorly explored question in light of the development of therapies targeting this cell type. Here, we investigate the population structure of live microglia purified from human cerebral cortex samples obtained at autopsy and during neurosurgical procedures. Using single cell RNA sequencing, we find that some subsets are enriched for disease-related genes and RNA signatures. We confirm the presence of four of these microglial subpopulations histologically and illustrate the utility of our data by characterizing further microglial cluster 7, enriched for genes depleted in the cortex of individuals with Alzheimer's disease (AD). Histologically, these cluster 7 microglia are reduced in frequency in AD tissue, and we validate this observation in an independent set of single nucleus data. Thus, our live human microglia identify a range of subtypes, and we prioritize one of these as being altered in AD.

Enhanced hippocampal neurogenesis in the absence of microglia T cell interaction and microglia activation in the murine running wheel model.

Recently, activated microglia have been shown to be involved in the regulation of several aspects of neurogenesis under certain experimental conditions both in vitro and in vivo. A neurogenesis supportive microglia phenotype has been suggested to arise from the interaction of microglia with homing encephalitogenic T cells. However, a unified hypothesis regarding the exact nature of microglia activity that is supportive of neurogenesis is yet missing from the field. Our aim was to investigate the connection between microglia activity and adult hippocampal neurogenesis under physiological conditions. To address this question we compared the level of microglia activation in the hippocampus of mice, which had access to a running wheel for 10 days and that of sedentary controls. Surprisingly, despite elevated levels of proliferation of neural precursors and survival of newborn neurons in the dentate gyrus microglia remained in a "resting" state morphologically, antigenically, and at the transcriptional level. Moreover, neither T cells nor MHCII expressing microglia were present in the hippocampal brain parenchyma. Though microglia in the dentate gyrus of the runners proliferated at a higher level than in the sedentary controls, this difference was also present in non-neurogenic sites. Therefore, our findings suggest that classical signs of microglia activation and microglia activation arising from interaction with T cells in particular are not a prerequisite for the activity-induced increase in adult hippocampal neurogenesis in C57Bl/6 mice. Thus, our results draw attention on the species and model differences that might exist regarding the regulation of adult hippocampal neurogenesis.

Sleep fragmentation, microglial aging, and cognitive impairment in adults with and without Alzheimer's dementia.

Sleep disruption is associated with cognitive decline and dementia in older adults; however, the underlying mechanisms are unclear. In rodents, sleep disruption causes microglial activation, inhibition of which improves cognition. However, data from humans are lacking. We studied participants in two cohort studies of older persons-the Rush Memory and Aging Project and the Religious Orders Study. We assessed sleep fragmentation by actigraphy and related this to cognitive function, to neocortical microglial marker gene expression measured by RNA sequencing, and to the neocortical density of microglia assessed by immunohistochemistry. Greater sleep fragmentation was associated with higher neocortical expression of genes characteristic of aged microglia, and a higher proportion of morphologically activated microglia, independent of chronological age- and dementia-related neuropathologies. Furthermore, these were, in turn, associated with worse cognition. This suggests that sleep fragmentation is accompanied by accelerated microglial aging and activation, which may partially underlie its association with cognitive impairment.

Chi3l3 induces oligodendrogenesis in an experimental model of autoimmune neuroinflammation.

In demyelinating diseases including multiple sclerosis (MS), neural stem cells (NSCs) can replace damaged oligodendrocytes if the local microenvironment supports the required differentiation process. Although chitinase-like proteins (CLPs) form part of this microenvironment, their function in this differentiation process is unknown. Here, we demonstrate that murine Chitinase 3-like-3 (Chi3l3/Ym1), human Chi3L1 and Chit1 induce oligodendrogenesis. In mice, Chi3l3 is highly expressed in the subventricular zone, a stem cell niche of the adult brain, and in inflammatory brain lesions during experimental autoimmune encephalomyelitis (EAE). We find that silencing Chi3l3 increases severity of EAE. We present evidence that in NSCs Chi3l3 activates the epidermal growth factor receptor (EGFR), thereby inducing Pyk2-and Erk1/2- dependent expression of a pro-oligodendrogenic transcription factor signature. Our results implicate CLP-EGFR-Pyk2-MEK-ERK as a key intrinsic pathway controlling oligodendrogenesis.

Functional consequences of retinopetal fibers originating in the dorsal raphe nucleus.

The existence of centrifugal fibers projecting into the mammalian retina is well known. However, their precise physiological role is poorly understood. Here we report that stimulation of the dorsal raphe nucleus (DRN) in freely moving rats produces profound effects on the electroretinogram (ERG). Most notably, activation of the dorsal raphe-retinal pathway causes a significant decrease in the latency of the b-wave and accompanying oscillatory potentials. In addition, dorsal raphe stimulation leads to a significant increase in the amplitude of oscillatory potentials. These results, therefore, provide the first demonstration of a functional role for the retinopetal fiber system originating in the and suggest that this structure can exert a powerful influence over the temporal sharpness and efficacy of retinal responsiveness.

A transcriptomic atlas of aged human microglia.

With a rapidly aging global human population, finding a cure for late onset neurodegenerative diseases has become an urgent enterprise. However, these efforts are hindered by the lack of understanding of what constitutes the phenotype of aged human microglia-the cell type that has been strongly implicated by genetic studies in the pathogenesis of age-related neurodegenerative disease. Here, we establish the set of genes that is preferentially expressed by microglia in the aged human brain. This HuMi_Aged gene set captures a unique phenotype, which we confirm at the protein level. Furthermore, we find this gene set to be enriched in susceptibility genes for Alzheimer's disease and multiple sclerosis, to be increased with advancing age, and to be reduced by the protective APOEε2 haplotype. APOEε4 has no effect. These findings confirm the existence of an aging-related microglial phenotype in the aged human brain and its involvement in the pathological processes associated with brain aging.

Propagation of spike and wave activity to the medial prefrontal cortex and dorsal raphe nucleus of WAG/Rij rats.

Although there is pharmacological evidence for the involvement of the serotonergic system in the expression of spike and wave discharges (SWDs) in experimental absence epilepsy, no direct investigation of this paroxysm in the dorsal raphe nucleus (DRN), one of the main serotonergic nuclei, has been carried out. We have now recorded the EEG simultaneously with local field potentials and unit activity in DRN from WAG/Rij rats, one of the best established models of absence epilepsy during spontaneous SWDs. We have also compared this activity to that in the thalamocortical networks, where SWDs are generated, and in the medial prefrontal cortex (mPFC), as this brain area is reciprocally connected to the DRN. We have found that SWDs propagate to the DRN with a short delay, and that the firing rate of its neurons changes during this type of paroxysm. These results provide the first direct evidence for clear alterations in the firing properties of mPFC and DRN neurons during spontaneous SWDs.

Increased White Matter Inflammation in Aging- and Alzheimer's Disease Brain.

Chronic neuroinflammation, which is primarily mediated by microglia, plays an essential role in aging and neurodegeneration. It is still unclear whether this microglia-induced neuroinflammation occurs globally or is confined to distinct brain regions. In this study, we investigated microglia activity in various brain regions upon healthy aging and Alzheimer's disease (AD)-related pathology in both human and mouse samples. In purified microglia isolated from aging mouse brains, we found a profound gene expression pattern related to pro-inflammatory processes, phagocytosis, and lipid homeostasis. Particularly in white matter microglia of 24-month-old mice, abundant expression of phagocytic markers including Mac-2, Axl, CD16/32, Dectin1, CD11c, and CD36 was detected. Interestingly, in white matter of human brain tissue the first signs of inflammatory activity were already detected during middle age. Thus quantification of microglial proteins, such as CD68 (commonly associated with phagocytosis) and HLA-DR (associated with antigen presentation), in postmortem human white matter brain tissue showed an age-dependent increase in immunoreactivity already in middle-aged people (53.2 ± 2.0 years). This early inflammation was also detectable by non-invasive positron emission tomography imaging using [11C]-(R)-PK11195, a ligand that binds to activated microglia. Increased microglia activity was also prominently present in the white matter of human postmortem early-onset AD (EOAD) brain tissue. Interestingly, microglia activity in the white matter of late-onset AD (LOAD) CNS was similar to that of the aged clinically silent AD cases. These data indicate that microglia-induced neuroinflammation is predominant in the white matter of aging mice and humans as well as in EOAD brains. This white matter inflammation may contribute to the progression of neurodegeneration, and have prognostic value for detecting the onset and progression of aging and neurodegeneration.