Perivascular macrophages in the CNS: From health to neurovascular diseases

The role of monocytes and macrophages in the progression of Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder characterized by β-amyloid (Aβ) plaques, neurofibrillary tangles (NFTs), and neuroinflammation. Monocytes and macrophages, particularly microglia, play a dual role in AD pathogenesis. In the early stages, they delay disease progression by phagocytosing Aβ, but chronic activation leads to Aβ accumulation and exacerbated neuroinflammation. Monocyte chemoattractant protein 1 (MCP-1) is a key regulator in neuroinflammation, Aβ deposition, and tau pathology, making it a potential therapeutic target. Moreover, recent breakthroughs in fluid and imaging biomarkers and targeted immunomodulatory agents underscore the growing importance of early diagnostic and therapeutic interventions. This review explores the complex interplay between monocytes, macrophages, and AD pathology, highlighting their roles in neuroinflammation, Aβ metabolism, and tau phosphorylation. Understanding these mechanisms offers new insights into developing effective diagnostic biomarkers and therapeutic strategies for AD.

Perivascular macrophages in the central nervous system: insights into their roles in health and disease

Perivascular macrophages (PVMs) are a specialized subset of macrophages situated near blood vessels in the brain. Their strategic positioning around these vessels enables them to perform key functions in immune surveillance and response to inflammation and injury. These cells are crucial for modulating the immune response within the brain, contributing to normal central nervous system (CNS) processes. In pathological conditions, the role of PVMs becomes more complex. Depending on the specific disease or injury, they may contribute to inflammation, blood-brain barrier (BBB) dysfunction, and the clearance of abnormal materials. PVMs are implicated in degenerative diseases, cerebrovascular impairment, and microhemorrhages associated with amyloid-β immunotherapy. Despite their important roles in the CNS, research on PVMs remains limited, and the mechanisms underlying their involvement in both physiological and pathological processes within the brain are not yet fully elucidated. Therefore, this review will focus on the current advancements in PVM research, including their origin, classification, roles in neuroinflammation and neuroprotection, and their potential roles as therapeutic targets for neurodegenerative diseases.

TAMing Gliomas: Unraveling the Roles of Iba1 and CD163 in Glioblastoma

Gliomas, the most common type of primary brain tumor, are a significant cause of morbidity and mortality worldwide. Glioblastoma, a highly malignant subtype, is particularly common, aggressive, and resistant to treatment. The tumor microenvironment (TME) of gliomas, especially glioblastomas, is characterized by a distinct presence of tumor-associated macrophages (TAMs), which densely infiltrate glioblastomas, a hallmark of these tumors. This macrophage population comprises both tissue-resident microglia as well as macrophages derived from the walls of blood vessels and the blood stream. Ionized calcium-binding adapter molecule 1 (Iba1) and CD163 are established cellular markers that enable the identification and functional characterization of these cells within the TME. This review provides an in-depth examination of the roles of Iba1 and CD163 in malignant gliomas, with a focus on TAM activation, migration, and immunomodulatory functions. Additionally, we will discuss how recent advances in AI-enhanced cell identification and visualization techniques have begun to transform the analysis of TAMs, promising unprecedented precision in their characterization and providing new insights into their roles within the TME. Iba1 and CD163 appear to have both unique and shared roles in glioma pathobiology, and both have the potential to be targeted through different molecular and cellular mechanisms. We discuss the therapeutic potential of Iba1 and CD163 based on available preclinical (experimental) and clinical (human tissue-based) evidence.

Microglial colonization routes and their impacts on cellular diversity

Microglia are the resident immune cells of the central nervous system. Unlike other glial cells-such as astrocytes and oligodendrocytes-which originate from neural stem cells alongside neurons, microglia derive from erythromyeloid progenitors that emerge in the yolk sac during early embryonic development. Once they reach the brain, microglia expand their population through proliferation during development. A growing body of research has revealed that microglia play diverse roles throughout life, both in physiological and pathological contexts. With recent advancements in single-cell transcriptomics, it has become increasingly evident that microglia exhibit substantial heterogeneity in their gene expression patterns. While various functions and subtypes of microglia are being uncovered, the mechanisms underlying their diversity remain largely unknown. Two key hypotheses may explain how microglial diversity arises. One possibility is that their diversity is influenced by the different colonization routes they take before settling in the brain. Alternatively, microglia may acquire distinct properties in response to their local environment. This review explores both possibilities, with a particular focus on the first hypothesis, drawing on recent findings that highlight the multiple routes microglia utilize to colonize the brain. It discusses how these processes contribute to the establishment of microglial diversity during brain development.

Enlarged perivascular spaces correlate with blood-brain barrier leakage and cognitive impairment in Alzheimer's disease

BackgroundThe clinical significance of enlarged perivascular spaces (EPVS) in Alzheimer' s disease (AD) was ambiguous.ObjectiveTo investigate whether EPVS contribute to blood-brain barrier (BBB) leakage and cognition in AD.MethodsThe study included a total of 64 participants (26 healthy controls and 38 patients with AD). The evaluation of EPVS and BBB permeability was performed in specific anatomical locations: the centrum semiovale (CSO), basal ganglia, and hippocampus. The EPVS ratings were performed according to Potter's instructions. BBB permeability was evaluated using dynamic contrast-enhanced-MRI. The relationship between EPVS and global cognition (Mini-Mental State Examination and Montreal Cognitive Assessment), cognitive subdomains, and BBB permeability were examined in both groups. Finally, the relationship between CSO BBB permeability and cognition in AD patients was investigated.ResultsHigh-grade CSO EPVS was found associated with AD (OR: 3.40, 95% CI: 1.11-11.90, p = 0.04). In the AD group, a significant correlation was observed between high-grade CSO EPVS and lower MMSE score (r = -0.36, p = 0.03) and verbal fluency (r = -0.44, p = 0.01). High-grade CSO EPVS positively correlated with BBB leakage (r = 0.58, p < 0.001). The BBB permeability of CSO negatively correlated with verbal fluency (r = -0.52, p < 0.001) and attention (r = -0.40, p = 0.01).ConclusionsHigh-grade CSO EPVS is related to BBB leakage, which contributes to cognitive impairment in AD patients, especially verbal frequency. CSO EPVS can function as a convenient AD marker for intervention and therapy.

All Three Supersystems-Nervous, Vascular, and Immune-Contribute to the Cortical Infarcts After Subarachnoid Hemorrhage

The recently published DISCHARGE-1 trial supports the observations of earlier autopsy and neuroimaging studies that almost 70% of all focal brain damage after aneurysmal subarachnoid hemorrhage are anemic infarcts of the cortex, often also affecting the white matter immediately below. The infarcts are not limited by the usual vascular territories. About two-fifths of the ischemic damage occurs within ~ 48 h; the remaining three-fifths are delayed (within ~ 3 weeks). Using neuromonitoring technology in combination with longitudinal neuroimaging, the entire sequence of both early and delayed cortical infarct development after subarachnoid hemorrhage has recently been recorded in patients. Characteristically, cortical infarcts are caused by acute severe vasospastic events, so-called spreading ischemia, triggered by spontaneously occurring spreading depolarization. In locations where a spreading depolarization passes through, cerebral blood flow can drastically drop within a few seconds and remain suppressed for minutes or even hours, often followed by high-amplitude, sustained hyperemia. In spreading depolarization, neurons lead the event, and the other cells of the neurovascular unit (endothelium, vascular smooth muscle, pericytes, astrocytes, microglia, oligodendrocytes) follow. However, dysregulation in cells of all three supersystems-nervous, vascular, and immune-is very likely involved in the dysfunction of the neurovascular unit underlying spreading ischemia. It is assumed that subarachnoid blood, which lies directly on the cortex and enters the parenchyma via glymphatic channels, triggers these dysregulations. This review discusses the neuroglial, neurovascular, and neuroimmunological dysregulations in the context of spreading depolarization and spreading ischemia as critical elements in the pathogenesis of cortical infarcts after subarachnoid hemorrhage.

In vitro modelling of the neurovascular unit for ischemic stroke research: Emphasis on human cell applications and 3D model design

Ischemic stroke has garnered global medical attention as one of the most serious cerebrovascular diseases. The mechanisms involved in both the development and recovery phases of ischemic stroke are complex, involving intricate interactions among different types of cells, each with its own unique functions. To better understand the possible pathogenesis, neurovascular unit (NVU), a concept comprising neurons, endothelial cells, mural cells, glial cells, and extracellular matrix components, has been used in analysing various brain diseases, particularly in ischemic stroke, aiming to depict the interactions between cerebral vasculature and neural cells. While in vivo models often face limitations in terms of reproducibility and the ability to precisely mimic human pathophysiology, it is now important to establish in vitro NVU models for ischemic stroke research. In order to accurately portray the pathological processes occurring within the brain, a diverse array of NVU 2D and 3D in vitro models, each possessing unique characteristics and advantages, have been meticulously developed. This review presents a comprehensive overview of recent advancements in in vitro models specifically tailored for investigating ischemic stroke. Through a systematic categorization of these developments, we elucidate the intricate links between NVU components and the pathogenesis of ischemic stroke. Furthermore, we explore the distinct advantages offered by innovative NVU models, notably 3D models, which closely emulate in vivo conditions. Additionally, an examination of current therapeutic modalities for ischemic stroke developed utilizing in vitro NVU models is provided. Serving as a valuable reference, this review aids in the design and implementation of effective in vitro models for ischemic stroke research.

PM2.5 potentiates oxygen glucose deprivation-induced neurovascular unit damage via inhibition of the Akt/β-catenin pathway and autophagy dysregulation

Air pollution has recently emerged as a significant risk factor for ischemic stroke. Although there is a robust association between higher concentrations of ambient particulate matter (PM2.5) and increased incidence and mortality rates of ischemic stroke, the precise mechanisms underlying PM2.5-induced ischemic stroke remain to be fully elucidated. The purpose of this study was to examine the synergistic effect of PM2.5 and hypoxic stress using in vivo and in vitro ischemic stroke models. Intravenously administered PM2.5 exacerbated the ischemic brain damage induced by middle cerebral artery occlusion (MCAo) in Sprague Dawley rats. Alterations in autophagy flux and decreased levels of tight junction proteins were observed in the brain of PM2.5-administered rats after MCAo. The underlying mechanism of PM2.5-induced potentiation of ischemic brain damage was investigated in neurons, perivascular macrophages, and brain endothelial cells, which are the major components of the integrated neurovascular unit. Co-treatment with PM2.5 and oxygen-glucose deprivation (OGD) amplified the effects of OGD on the reduction of viability in primary neurons, immortalized murine hippocampal neuron (HT-22), and brain endothelial cells (bEND.3). After co-treatment with PM2.5 and OGD, the Akt/β-catenin and autophagy flux were significantly inhibited in HT-22 cells. Notably, the protein levels of metalloproteinase-9 and cystatin C were elevated in the conditioned media of murine macrophages (RAW264.7) exposed to PM2.5, and tight junction protein expression was significantly decreased after OGD exposure in bEND.3 cells pretreated with the conditioned media. Our findings suggest that perivascular macrophages may mediate PM2.5-induced brain endothelial dysfunction following ischemia and that PM2.5 can exacerbate ischemia-induced neurovascular damage.

Perivascular macrophages in cerebrovascular diseases

Cerebrovascular diseases are a major cause of stroke and dementia, both requiring long-term care. These diseases involve multiple pathophysiologies, with mitochondrial dysfunction being a crucial contributor to the initiation of inflammation, apoptosis, and oxidative stress, resulting in injuries to neurovascular units that include neuronal cell death, endothelial cell death, glial activation, and blood-brain barrier disruption. To maintain brain homeostasis against these pathogenic conditions, brain immune cells, including border-associated macrophages and microglia, play significant roles as brain innate immunity cells in the pathophysiology of cerebrovascular injury. Although microglia have long been recognized as significant contributors to neuroinflammation, attention has recently shifted to border-associated macrophages, such as perivascular macrophages (PVMs), which have been studied based on their crucial roles in the brain. These cells are strategically positioned around the walls of brain vessels, where they mainly perform critical functions, such as perivascular drainage, cerebrovascular flexibility, phagocytic activity, antigen presentation, activation of inflammatory responses, and preservation of blood-brain barrier integrity. Although PVMs act as scavenger and surveillant cells under normal conditions, these cells exert harmful effects under pathological conditions. PVMs detect mitochondrial dysfunction in injured cells and implement pathological changes to regulate brain homeostasis. Therefore, PVMs are promising as they play a significant role in mitochondrial dysfunction and, in turn, disrupt the homeostatic condition. Herein, we summarize the significant roles of PVMs in cerebrovascular diseases, especially ischemic and hemorrhagic stroke and dementia, mainly in correlation with inflammation. A better understanding of the biology and pathobiology of PVMs may lead to new insights on and therapeutic strategies for cerebrovascular diseases.

The Association between Glymphatic System and Perivascular Macrophages in Brain Waste Clearance

The glymphatic system suggests the convective bulk flow of cerebrospinal fluid (CSF) through perivascular spaces and the interstitial spaces of the brain parenchyma for the rapid removal of toxic waste solutes from the brain. However, the presence of convective bulk flow within the brain interstitial spaces is still under debate. We first addressed this argument to determine the involvement of the glymphatic system in brain waste clearance utilizing contrast-enhanced 3D T1-weighted imaging (T1WI), diffusion tensor imaging (DTI), and confocal microscopy imaging. Furthermore, perivascular macrophages (PVMs), which are immune cells located within perivascular spaces, have not been thoroughly explored for their association with the glymphatic system. Therefore, we investigated tracer uptake by PVMs in the perivascular spaces of both the arteries/arterioles and veins/venules and the potential association of PVMs in assisting the glymphatic system for interstitial waste clearance. Our findings demonstrated that both convective bulk flow and diffusion are responsible for the clearance of interstitial waste solutes from the brain parenchyma. Furthermore, our results suggested that PVMs may play an important function in glymphatic system-mediated interstitial waste clearance. The glymphatic system and PVMs could be targeted to enhance interstitial waste clearance in patients with waste-associated neurological conditions and aging.

Roles of Macrophages and Endothelial Cells and Their Crosstalk in Acute Lung Injury

Acute lung injury (ALI) and its severe form, acute respiratory distress syndrome (ARDS), present life-threatening conditions characterized by inflammation and endothelial injury, leading to increased vascular permeability and lung edema. Key players in the pathogenesis and resolution of ALI are macrophages (Mφs) and endothelial cells (ECs). The crosstalk between these two cell types has emerged as a significant focus for potential therapeutic interventions in ALI. This review provides a brief overview of the roles of Mφs and ECs and their interplay in ALI/ARDS. Moreover, it highlights the significance of investigating perivascular macrophages (PVMs) and immunomodulatory endothelial cells (IMECs) as crucial participants in the Mφ-EC crosstalk. This sheds light on the pathogenesis of ALI and paves the way for innovative treatment approaches.

Myeloid cells in vascular dementia and Alzheimer's disease: Possible therapeutic targets?

Growing evidence supports the suggestion that the peripheral immune system plays a role in different pathologies associated with cognitive impairment, such as vascular dementia (VD) or Alzheimer's disease (AD). The aim of this review is to summarize, within the peripheral immune system, the implications of different types of myeloid cells in AD and VD, with a special focus on post-stroke cognitive impairment and dementia (PSCID). We will review the contributions of the myeloid lineage, from peripheral cells (neutrophils, platelets, monocytes and monocyte-derived macrophages) to central nervous system (CNS)-associated cells (perivascular macrophages and microglia). Finally, we will evaluate different potential strategies for pharmacological modulation of pathological processes mediated by myeloid cell subsets, with an emphasis on neutrophils, their interaction with platelets and the process of immunothrombosis that triggers neutrophil-dependent capillary stall and hypoperfusion, as possible effector mechanisms that may pave the way to novel therapeutic avenues to stop dementia, the epidemic of our time. LINKED ARTICLES: This article is part of a themed issue From Alzheimer's Disease to Vascular Dementia: Different Roads Leading to Cognitive Decline. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v181.6/issuetoc.

The Association between Glymphatic System and Perivascular Macrophages in Brain Waste Clearance

The glymphatic system suggests the convective bulk flow of cerebrospinal fluid (CSF) through perivascular spaces and the interstitial spaces of the brain parenchyma for the rapid removal of toxic waste solutes from the brain. However, the presence of convective bulk flow within the brain interstitial spaces is still under debate. We first addressed this argument to determine the involvement of the glymphatic system in brain waste clearance utilizing contrast-enhanced 3D T1-weighted imaging (T1WI), diffusion tensor imaging (DTI), and confocal microscopy imaging. Furthermore, perivascular macrophages (PVMs), which are immune cells located within perivascular spaces, have not been thoroughly explored for their association with the glymphatic system. Therefore, we investigated tracer uptake by PVMs in the perivascular spaces of both the arteries/arterioles and veins/venules and the potential association of PVMs in assisting the glymphatic system for interstitial waste clearance. Our findings demonstrated that both convective bulk flow and diffusion are responsible for the clearance of interstitial waste solutes from the brain parenchyma. Furthermore, our results suggested that PVMs play an important function in glymphatic system-mediated interstitial waste clearance. The glymphatic system and PVMs could be targeted to enhance interstitial waste clearance in patients with waste-associated neurological conditions and aging.

Pretreatment with Clodronate Improved Neurological Function by Preventing Reduction of Posthemorrhagic Cerebral Blood Flow in Experimental Subarachnoid Hemorrhage

BACKGROUND

Brain perivascular macrophages (PVMs) are potential treatment targets for subarachnoid hemorrhage (SAH), and previous studies revealed that their depletion by clodronate (CLD) improved outcomes after experimental SAH. However, the underlying mechanisms are not well understood. Therefore, we investigated whether reducing PVMs by CLD pretreatment improves SAH prognosis by inhibiting posthemorrhagic impairment of cerebral blood flow (CBF).

METHODS

In total, 80 male Sprague-Dawley rats received an intracerebroventricular injection of the vehicle (liposomes) or CLD. Subsequently, the rats were categorized into the prechiasmatic saline injection (sham) and blood injection (SAH) groups after 72 h. We assessed its effects on weak and severe SAH, which were induced by 200- and 300-µL arterial blood injections, respectively. In addition, neurological function at 72 h and CBF changes from before the intervention to 5 min after were assessed in rats after sham/SAH induction as the primary and secondary end points, respectively.

RESULTS

CLD significantly reduced PVMs before SAH induction. Although pretreatment with CLD in the weak SAH group provided no additive effects on the primary end point, rats in the severe SAH group showed significant improvement in the rotarod test. In the severe SAH group, CLD inhibited acute reduction of CBF and tended to decrease hypoxia-inducible factor 1α expression. Furthermore, CLD reduced the number of PVMs in rats subjected to sham and SAH surgery, although no effects were observed in oxidative stress and inflammation.

CONCLUSIONS

Our study proposes that pretreatment with CLD-targeting PVMs can improve the prognosis of severe SAH through a candidate mechanism of inhibition of posthemorrhagic CBF reduction.

Susceptibility of perivenous macrophages to PRRSV-1 subtype 1 LV and PRRSV-1 subtype 3 Lena using a new vein explant model

Vessel pathology such as increased permeability and blue discoloration is frequently observed with highly pathogenic PRRSV strains. However, data concerning the viral replication in the environment of blood vessels are absent. In the present study, ex vivo models with swine ear and hind leg vein explants were established to study the interaction of PRRSV-1 subtype 1 reference strain LV and highly pathogenic subtype 3 strain Lena with perivenous macrophages. The replication characteristics of these two strains were compared in vein explants by immunofluorescence analysis. The explants maintained a good viability during 48 hours of in vitro culture. We found that CD163-positive macrophages were mainly present around the veins and their number gradually decreased with increasing distance from the veins and longer incubation time. More CD163+Sn- cells than CD163+Sn+ cells (6.6 times more) were observed in the vein explants. The Lena strain demonstrated a higher replication level than the LV strain, with approximately 1.4-fold more infected cells in the surrounding areas of the ear vein and 1.1-fold more infected cells in the leg vein explants at 48 hours post inoculation. In both LV and Lena inoculated vein explants, most infected cells were identified as CD163+Sn+ (> 94%). In this study, an ex vivo vein model was successfully established, and our findings will contribute to a better understanding of the vein pathology during viral infections (e.g., PRRS, classical and African swine fever).

CRTC1 is a potential target to delay aging-induced cognitive deficit by protecting the integrity of the blood-brain barrier via inhibiting inflammation

Aging can cause attenuation in the functioning of multiple organs, and blood-brain barrier (BBB) breakdown could promote the occurrence of disorders of the central nervous system during aging. Since inflammation is considered to be an important factor underlying BBB injury during aging, vascular endothelial cell senescence serves as a critical pathological basis for the destruction of BBB integrity. In the current review, we have first introduced the concepts related to aging-induced cognitive deficit and BBB integrity damage. Thereafter, we reviewed the potential relationship between disruption of BBB integrity and cognition deficit and the role of inflammation, vascular endothelial cell senescence, and BBB injury. We have also briefly introduced the function of CREB-regulated transcription co-activator 1 (CRTC1) in cognition and aging-induced CRTC1 changes as well as the critical roles of CRTC1/cyclooxygenase-2 (COX-2) in regulating inflammation, endothelial cell senescence, and BBB injury. Finally, the underlying mechanisms have been summarized and we propose that CRTC1 could be a promising target to delay aging-induced cognitive deficit by protecting the integrity of BBB through promoting inhibition of inflammation-mediated endothelial cell senescence.

Endothelial Dysfunction in Neurodegenerative Diseases

The cerebral vascular system stringently regulates cerebral blood flow (CBF). The components of the blood-brain barrier (BBB) protect the brain from pathogenic infections and harmful substances, efflux waste, and exchange substances; however, diseases develop in cases of blood vessel injuries and BBB dysregulation. Vascular pathology is concurrent with the mechanisms underlying aging, Alzheimer's disease (AD), and vascular dementia (VaD), which suggests its involvement in these mechanisms. Therefore, in the present study, we reviewed the role of vascular dysfunction in aging and neurodegenerative diseases, particularly AD and VaD. During the development of the aforementioned diseases, changes occur in the cerebral blood vessel morphology and local cells, which, in turn, alter CBF, fluid dynamics, and vascular integrity. Chronic vascular inflammation and blood vessel dysregulation further exacerbate vascular dysfunction. Multitudinous pathogenic processes affect the cerebrovascular system, whose dysfunction causes cognitive impairment. Knowledge regarding the pathophysiology of vascular dysfunction in neurodegenerative diseases and the underlying molecular mechanisms may lead to the discovery of clinically relevant vascular biomarkers, which may facilitate vascular imaging for disease prevention and treatment.