The role of perivascular and meningeal macrophages in experimental allergic encephalomyelitis

Role of innate immune cells in multiple sclerosis

Multiple sclerosis (MS) is a chronic autoimmune, inflammatory and neurodegenerative disease affecting the central nervous system (CNS). MS is associated with a complex interplay between neurodegenerative and inflammatory processes, mostly attributed to pathogenic T and B cells. However, a growing body of preclinical and clinical evidence indicates that innate immunity plays a crucial role in MS promotion and progression. Accordingly, preclinical and clinical studies targeting different innate immune cells to control MS are currently under study, highlighting the importance of innate immunity in this pathology. Here, we reviewed recent findings regarding the role played by innate immune cells in the pathogenesis of MS. Additionally, we discuss potential new treatments for MS based on targets against innate immune components.

A New Perspective on Mechanisms of Neurodegeneration in Experimental Autoimmune Encephalomyelitis and Multiple Sclerosis: the Early and Critical Role of Platelets in Neuro/Axonal Loss

Multiple sclerosis (MS) is a central nervous system (CNS) autoimmune disorder, with limited treatment options. This disease is characterized by differential pathophysiology between grey matter (GM) and white matter (WM). The predominant WM hallmark is the perivascular plaque, associated with blood brain barrier (BBB) loss of function, lymphocytic infiltration, microglial reactivity, demyelination and axonal injury and is adequately addressed with immunomodulatory drugs. By contrast, mechanisms underlying GM damage remain obscure, with consequences for neuroprotective strategies. Cortical GM pathology is already significant in early MS and characterized by reduced BBB disruption and lymphocytic infiltration relative to WM, but a highly inflammatory environment, microglial reactivity, demyelination and neuro/axonal loss. There is no satisfactory explanation for the occurrence of neurodegeneration without large-scale inflammatory cell influx in cortical GM. A candidate mechanism suggests that it results from soluble factors originating from meningeal inflammatory cell aggregates, which diffuse into the underlying cortical tissue and trigger microglial activation. However, the recent literature highlights the central role of platelets in inflammation, together with the relationship between coagulation factors, particularly fibrinogen, and tissue damage in MS. Using the experimental autoimmune encephalomyelitis (EAE) model, we identified platelets as drivers of neuroinflammation and platelet-neuron associations from the pre-symptomatic stage. We propose that fibrinogen leakage across the BBB is a signal for platelet infiltration and that platelets represent a major and early participant in neurodegeneration. This concept is compatible with the new appreciation of platelets as immune cells and of neuronal damage driven by inflammatory cells sequestered in the meninges.

Depleting parenchymal border macrophages alleviates cerebral edema and neuroinflammation following status epilepticus

BACKGROUND

Status epilepticus (SE) is a common severe neurological emergency. Cerebral edema caused by SE is unavoidable and may exacerbate epilepsy. Recent studies have identified cerebrospinal fluid (CSF) as a crucial fluid source of initial cerebral edema following ischemic stroke and cardiac arrest. Moreover, synchronized neuronal firings drive CSF influx into interstitial fluid (ISF). Parenchymal border macrophages (PBMs) have been found to play a role in regulating CSF flow dynamics. However, the involvement of CSF and PBMs in cerebral edema during SE remains unclear. Here, we investigated the fluid source of cerebral edema in the initial phase of SE with the role of PBMs involved.

METHODS

Lithium chloride-pilocarpine was used to induce SE in C57BL/6 J mice. Electroencephalogram (EEG) was acquired to assess changes in relative EEG power pre- and post-seizure onset. Apparent diffusion coefficient (ADC) maps reconstructed from diffusion-weighted imaging (DWI) were utilized to evaluate cytotoxic edema. Blood-brain barrier (BBB) permeability was examined using sodium fluorescein (NaFl). CSF tracer influx into the brain was assessed by transcranial imaging and brain slices. PBMs were depleted using clodronate liposomes. Immunohistochemistry was used to evaluate PBM depletion, severity of vasogenic edema, inflammation, and neuronal damage.

RESULTS

During the initial stage of SE, relative EEG power sharply increased and ADC values significantly decreased. Concurrently, CSF tracer influx into the cortex significantly elevated, though NaFl leakage from blood to brain parenchyma did not evidently alter. Following depletion of PBM, CSF influx declined but AQP4 expression and polarization remained unaffected. Post-PBM depletion, there was no significant alteration in relative EEG power, yet CSF influx decreased substantially during the initial stage of SE. The degree of ADC decline lessened, IgG extravasation after SE decreased, activated microglia and proliferating astrocytes count fell, and neuronal damage post-SE alleviated.

CONCLUSIONS

CSF appeared to contribute to cerebral edema in SE. Depletion of PBM alleviated cytotoxic edema in the initial phase of SE, and subsequent vasogenic edema, inflammatory response and neurological damage were reduced. These findings may provide potential novel strategies for treating cerebral edema following SE.

Cerebral biomimetic nano-drug delivery systems: A frontier strategy for immunotherapy

Brain diseases are a significant threat to human health, especially in the elderly, and this problem is growing as the aging population increases. Efficient brain-targeted drug delivery has been the greatest challenge in treating brain disorders due to the unique immune environment of the brain, including the blood-brain barrier (BBB). Recently, cerebral biomimetic nano-drug delivery systems (CBNDSs) have provided a promising strategy for brain targeting by mimicking natural biological materials. Herein, this review explores the latest understanding of the immune microenvironment of the brain, emphasizing the immune mechanisms of the occurrence and progression of brain disease. Several brain targeting systems are summarized, including cell-based, exosome-based, protein-based, and microbe-based CBNDSs, and their immunological mechanisms are highlighted. Moreover, given the rise of immunotherapy, the latest applications of CBNDSs in immunotherapy are also discussed. This review provides a comprehensive understanding of CBNDSs and serves as a guideline for immunotherapy in treating brain diseases. In addition, it provides inspiration for the future of CBNDSs.

Central nervous system-associated macrophages modulate the immune response following stroke in aged mice

Age is a major nonmodifiable risk factor for ischemic stroke. Central nervous system-associated macrophages (CAMs) are resident immune cells located along the brain vasculature at the interface between the blood circulation and the parenchyma. By using a clinically relevant thromboembolic stroke model in young and aged male mice and corresponding human tissue samples, we show that during aging, CAMs acquire a central role in orchestrating immune cell trafficking after stroke through the specific modulation of adhesion molecules by endothelial cells. The absence of CAMs provokes increased leukocyte infiltration (neutrophils and CD4+ and CD8+ T lymphocytes) and neurological dysfunction after stroke exclusively in aged mice. Major histocompatibility complex class II, overexpressed by CAMs during aging, plays a significant role in the modulation of immune responses to stroke. We demonstrate that during aging, CAMs become central coordinators of the neuroimmune response that ensure a long-term fine-tuning of the immune responses triggered by stroke.

Cerebral amyloid angiopathy and the immune system

Cerebral amyloid angiopathy (CAA) is characterized by the accumulation of amyloid protein in the walls of cerebral blood vessels. This deposition of amyloid causes damage to the cerebral vasculature, resulting in blood-brain barrier disruption, cerebral hemorrhage, cognitive decline, and dementia. The role of the immune system in CAA is complex and not fully understood. While the immune system has a clear role in the rare inflammatory variants of CAA (CAA related inflammation and Abeta related angiitis), the more common variants of CAA also have immune system involvement. In a protective role, immune cells may facilitate the clearance of beta-amyloid from the cerebral vasculature. The immune system can also contribute to CAA pathology, promoting vascular injury, blood-brain barrier breakdown, inflammation, and progression of CAA. In this review, we summarize the role of the immune system in CAA, including the potential of immune based treatment strategies to slow vascular disease in CAA and associated cognitive impairment, white matter disease progression, and reduce the risk of cerebral hemorrhage. HIGHLIGHTS: The immune system has a role in cerebral amyloid angiopathy (CAA) which is summarized in this review. There is an inflammatory response to beta-amyloid that may contribute to brain injury and cognitive impairment. Immune cells may facilitate the clearance of beta-amyloid from the cerebral vasculature. Improved understanding of the immune system in CAA may afford novel treatment to improve outcomes in patients with CAA.

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.

Brain perivascular macrophages: current understanding and future prospects

Brain perivascular macrophages are specialized populations of macrophages that reside in the space around cerebral vessels, such as penetrating arteries and venules. With the help of cutting-edge technologies, such as cell fate mapping and single-cell multi-omics, their multifaceted, pivotal roles in phagocytosis, antigen presentation, vascular integrity maintenance and metabolic regulation have more recently been further revealed under physiological conditions. Accumulating evidence also implies that perivascular macrophages are involved in the pathogenesis of neurodegenerative disease, cerebrovascular dysfunction, autoimmune disease, traumatic brain injury and epilepsy. They can act in either protective or detrimental ways depending on the disease course and stage. However, the underlying mechanisms of perivascular macrophages remain largely unknown. Therefore, we highlight potential future directions in research on perivascular macrophages, including the utilization of genetic mice and novel therapeutic strategies that target these unique immune cells for neuroprotective purposes. In conclusion, this review provides a comprehensive update on the current knowledge of brain perivascular macrophages, shedding light on their pivotal roles in central nervous system health and disease.

Brain macrophage development, diversity and dysregulation in health and disease

Brain macrophages include microglia in the parenchyma, border-associated macrophages in the meningeal-choroid plexus-perivascular space, and monocyte-derived macrophages that infiltrate the brain under various disease conditions. The vast heterogeneity of these cells has been elucidated over the last decade using revolutionary multiomics technologies. As such, we can now start to define these various macrophage populations according to their ontogeny and their diverse functional programs during brain development, homeostasis and disease pathogenesis. In this review, we first outline the critical roles played by brain macrophages during development and healthy aging. We then discuss how brain macrophages might undergo reprogramming and contribute to neurodegenerative disorders, autoimmune diseases, and glioma. Finally, we speculate about the most recent and ongoing discoveries that are prompting translational attempts to leverage brain macrophages as prognostic markers or therapeutic targets for diseases that affect the brain.

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.

Microglia and macrophages in the neuro-glia-vascular unit: From identity to functions

Although both are myeloid cells located surrounding cerebral vasculature, vessel-associated microglia (VAM) and perivascular macrophages (PVMs) can be distinguished by their distinct morphologies, signatures and microscopic location. As key component of neuro-glia-vascular unit (NGVU), they play prominent roles in neurovasculature development and pathological process of various central nervous system (CNS) diseases, including phagocytosis, angiogenesis, vessel damage/protection and blood flow regulation, therefore serving as potential targets for therapeutics of a broad array of CNS diseases. Herein, we will provide a comprehensive overview of heterogeneity of VAM/PVMs, highlight limitations of current understanding in this field, and discuss possible directions of future investigations.

Mechanisms of antigen-induced reversal of CNS inflammation in experimental demyelinating disease

Autoimmune central nervous system (CNS) demyelinating diseases are a major public health burden and poorly controlled by current immunosuppressants. More precise immunotherapies with higher efficacy and fewer side effects are sought. We investigated the effectiveness and mechanism of an injectable myelin-based antigenic polyprotein MMPt (myelin oligodendrocyte glycoprotein, myelin basic protein and proteolipid protein, truncated). We find that it suppresses mouse experimental autoimmune encephalomyelitis without major side effects. MMPt induces rapid apoptosis of the encephalitogenic T cells and suppresses inflammation in the affected CNS. Intravital microscopy shows that MMPt is taken up by perivascular F4/80⁺ cells but not conventional antigen-presenting dendritic cells, B cells, or microglia. MMPt-stimulated F4/80⁺ cells induce reactive T cell immobilization and apoptosis in situ, resulting in reduced infiltration of inflammatory cells and chemokine production. Our study reveals alternative mechanisms that explain how cognate antigen suppresses CNS inflammation and may be applicable for effectively and safely treating demyelinating diseases.

Trends in perivascular macrophages research from 1997 to 2021: A bibliometric analysis

INTRODUCTION

Perivascular macrophages (PVMs) play pivotal roles in maintaining the physiological function of the brain. Dysfunction of PVMs is emerging as an important mechanism in various disease conditions in the brain.

METHODS

In this work, we analyzed recent research advances in PVMs, especially in the brain, from the Web of Science (WoS) core database using bibliometric analysis based on the search terms "perivascular macrophages" and "perivascular macrophage" on October 27, 2021. Visualization and collaboration analysis were performed by Citespace (5.8 R3 mac).

RESULTS

We found 2384 articles published between 1997 and 2021 in the field of PVMs, which were selected for analysis. PVMs were involved in several physio-pathological fields, in which Neurosciences and Neurology, Neuroscience, Immunology, Pathology, and Cardiovascular System and Cardiology were most reported. The research focuses on PVMs mainly in the central nervous system (CNS), inflammation, macrophage or T-cell, and disease, and highlights the related basic research regarding its activation, oxidative stress, angiotensin II, and insulin resistance. Tumor-associated macrophage, obesity, myeloid cell, and inflammation were relatively recent highlight keywords that attracted increasing attention in recent years. Harvard Univ, Vrije Univ Amsterdam, occupied important positions in the research field of PVMs. Meanwhile, PVM research in China (Peking Univ, Sun Yat Sen Univ, Shanghai Jiao Tong Univ, and Shandong Univ) is on the rise. Cluster co-citation analysis revealed that the mechanisms of CNS PVMs and related brain diseases are major specialties associated with PVMs, while PVMs in perivascular adipose tissue and vascular diseases or obesity are another big category of PVMs hotspots.

CONCLUSION

In conclusion, the research on PVMs continues to deepen, and the hotspots are constantly changing. Future studies of PVMs could have multiple disciplines intersecting.

Deciphering the heterogeneity of the Lyve1+ perivascular macrophages in the mouse brain

Perivascular macrophages (pvMs) are associated with cerebral vasculature and mediate brain drainage and immune regulation. Here, using reporter mouse models, whole brain and section immunofluorescence, flow cytometry, and single cell RNA sequencing, besides the Lyve1⁺F4/80⁺CD206⁺CX3CR1⁺ pvMs, we identify a CX3CR1^- pvM population that shares phagocytic functions and location. Furthermore, the brain parenchyma vasculature mostly hosts Lyve1⁺MHCII^- pvMs with low to intermediate CD45 expression. Using the double Cx3cr1^GFP x Cx3cr1-Cre;Rosa^tdT reporter mice for finer mapping of the lineages, we establish that CD45^lowCX3CR1^- pvMs are derived from CX3CR1⁺ precursors and require PU.1 during their ontogeny. In parallel, results from the Cxcr4-CreErt2;Rosa26^tdT lineage tracing model support a bone marrow-independent replenishment of all Lyve1⁺ pvMs in the adult mouse brain. Lastly, flow cytometry and 3D immunofluorescence analysis uncover increased percentage of pvMs following photothrombotic induced stroke. Our results thus show that the parenchymal pvM population is more heterogenous than previously described, and includes a CD45^low and CX3CR1^- pvM population.

Role of P2X7 Receptors in Immune Responses During Neurodegeneration

P2X7 receptors are ion-gated channels activated by ATP. Under pathological conditions, the extensive release of ATP induces sustained P2X7 receptor activation, culminating in induction of proinflammatory pathways with inflammasome assembly and cytokine release. These inflammatory conditions, whether occurring peripherally or in the central nervous system (CNS), increase blood-brain-barrier (BBB) permeability. Besides its well-known involvement in neurodegeneration and neuroinflammation, the P2X7 receptor may induce BBB disruption and chemotaxis of peripheral immune cells to the CNS, resulting in brain parenchyma infiltration. For instance, despite common effects on cytokine release, P2X7 receptor signaling is also associated with metalloproteinase secretion and activation, as well as migration and differentiation of T lymphocytes, monocytes and dendritic cells. Here we highlight that peripheral immune cells mediate the pathogenesis of Multiple Sclerosis and Parkinson's and Alzheimer's disease, mainly through T lymphocyte, neutrophil and monocyte infiltration. We propose that P2X7 receptor activation contributes to neurodegenerative disease progression beyond its known effects on the CNS. This review discusses how P2X7 receptor activation mediates responses of peripheral immune cells within the inflamed CNS, as occurring in the aforementioned diseases.

Dwellers and Trespassers: Mononuclear Phagocytes at the Borders of the Central Nervous System

The central nervous system (CNS) parenchyma is enclosed and protected by a multilayered system of cellular and acellular barriers, functionally separating glia and neurons from peripheral circulation and blood-borne immune cells. Populating these borders as dynamic observers, CNS-resident macrophages contribute to organ homeostasis. Upon autoimmune, traumatic or neurodegenerative inflammation, these phagocytes start playing additional roles as immune regulators contributing to disease evolution. At the same time, pathological CNS conditions drive the migration and recruitment of blood-borne monocyte-derived cells across distinct local gateways. This invasion process drastically increases border complexity and can lead to parenchymal infiltration of blood-borne phagocytes playing a direct role both in damage and in tissue repair. While recent studies and technical advancements have highlighted the extreme heterogeneity of these resident and CNS-invading cells, both the compartment-specific mechanism of invasion and the functional specification of intruding and resident cells remain unclear. This review illustrates the complexity of mononuclear phagocytes at CNS interfaces, indicating how further studies of CNS border dynamics are crucially needed to shed light on local and systemic regulation of CNS functions and dysfunctions.

Depletion of brain perivascular macrophages regulates acute restraint stress-induced neuroinflammation and oxidative/nitrosative stress in rat frontal cortex

The central nervous system can respond to peripheral immune stimuli through the activation of the neurovascular unit. One of the cellular types implicated are perivascular macrophages (PVMs), hematopoietic-derived brain-resident cells located in the perivascular space. PVMs have been implicated in the immune surveillance and in the regulation of the accumulation/trafficking of macromolecules in brain-blood interfaces. Recent studies suggested that the role of PVMs could vary depending on the nature and duration of the immune challenge applied. Here, we investigate the role of PVMs in stress-induced neuroinflammation and oxidative/nitrosative consequences. The basal phagocytic activity of PVMs was exploited to selectively deplete them by ICV injection of liposomes encapsulating the pro-apoptotic drug clodronate. Acute restraint stress-induced neuroinflammation and oxidative/nitrosative stress in rat brain frontal cortex samples were assessed by western blot and RT-PCR analyses. The depletion of PVMs: (1) decreased tumor necrosis-α levels (2) prevented the Janus kinase/signal transducers and activators of transcription pathway and increased interleukin-6 receptor protein-expression in stress conditions; (3) prevented the stress-induced Toll-like receptor 4/Myeloid differentiation primary response 88 protein signaling pathway; (4) down-regulated the pro-inflammatory nuclear factor κB/cyclooxygenase-2 pathway; (5) prevented stress-induced lipid peroxidation and the concomitant increase of the endogenous antioxidant mediators nuclear factor (erythroid-derived 2)-like 2, glutathione reductase 1 and Parkinsonism-associated deglycase mRNA expression. Our results point to PVMs as regulators of stress-induced neuroinflammation and oxidative/nitrosative stress. Much more scientific effort is still needed to evaluate whether their selective manipulation is promising as a therapeutic strategy for the treatment of stress-related neuropsychopathologies.

CD163 expression defines specific, IRF8-dependent, immune-modulatory macrophages in the bone marrow

BACKGROUND

Scavenger receptor CD163 is exclusively expressed on monocytes/macrophages and is widely used as a marker for alternatively activated macrophages. However, the role of CD163 is not yet clear.

OBJECTIVES

We sought to examine the function of CD163 in steady-state as well as in sterile and infectious inflammation.

METHODS

Expression of CD163 was analyzed under normal and inflammatory conditions in mice. Functional relevance of CD163 was investigated in models of inflammation in wild-type and CD163^-/- mice.

RESULTS

We describe a subpopulation of bone marrow-resident macrophages (BMRMs) characterized by a high expression of CD163 and functionally distinct from classical bone marrow-derived macrophages. Development of CD163⁺ BMRMs is strictly dependent on IFN regulatory factor-8. CD163⁺ BMRMs show a specific transcriptome and cytokine secretion pattern demonstrating a specific immunomodulatory profile of these cells. Accordingly, CD163^-/- mice show a stronger inflammation in allergic contact dermatitis, indicating a regulatory role of CD163. However, CD163^-/- mice are highly susceptible to S aureus infections, demonstrating the relevance of CD163 for antimicrobial defense as well.

CONCLUSIONS

Our data indicate that anti-inflammatory and immunosuppressive mechanisms are not necessarily associated with a decreased antimicrobial activity. In contrast, our data define a novel macrophage population that controls overwhelming inflammation on one hand but is also necessary for an effective control of infections on the other hand.

Brain perivascular macrophages: Recent advances and implications in health and diseases

Brain perivascular macrophages (PVMs) belong to a distinct population of brain‐resident myeloid cells located within the perivascular space surrounding arterioles and venules. Their characterization depends on the combination of anatomical localization, phagocytic capacity, and molecular markers. Under physiological status, they provide structural and functional support for maintaining brain homeostasis, including facilitation of blood‐brain barrier integrity and lymphatic drainage, and exertion of immune functions such as phagocytosis and antigen presentation. Increasing evidence also implicates their specific roles in diseased brain, ranging from cerebrovascular diseases, Aβ pathologies, infections, and autoimmunity. Collectively, PVMs are key components of the brain‐resident immune system, actively participate in a broad‐spectrum of processes in normal and diseased status. Details of the processes are largely underexplored. Targeting PVMs would lead to new insights and be a promising strategy for a broad array of human diseases.

Perivascular macrophages in health and disease

Macrophages are a heterogeneous group of cells that are capable of carrying out distinct functions in different tissues, as well as in different locations within a given tissue. Some of these tissue macrophages lie on, or close to, the outer (abluminal) surface of blood vessels and perform several crucial activities at this interface between the tissue and the blood. In steady-state tissues, these perivascular macrophages maintain tight junctions between endothelial cells and limit vessel permeability, phagocytose potential pathogens before they enter tissues from the blood and restrict inappropriate inflammation. They also have a multifaceted role in diseases such as cancer, Alzheimer disease, multiple sclerosis and type 1 diabetes. Here, we examine the important functions of perivascular macrophages in various adult tissues and describe how these functions are perturbed in a broad array of pathological conditions.

Inflammation in ALS/FTD pathogenesis

Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) are neurodegenerative diseases that overlap in their clinical presentation, pathology and genetics, and likely represent a spectrum of one underlying disease. In ALS/FTD patients, neuroinflammation characterized by innate immune responses of tissue-resident glial cells is uniformly present on end-stage pathology, and human imaging studies and rodent models support that neuroinflammation begins early in disease pathogenesis. Additionally, changes in circulating immune cell populations and cytokines are found in ALS/FTD patients, and there is evidence for an autoinflammatory state. However, despite the prominent role of neuro- and systemic inflammation in ALS/FTD, and experimental evidence in rodents that altering microglial function can mitigate pathology, therapeutic approaches to decrease inflammation have thus far failed to alter disease course in humans. Here, we review the characteristics of inflammation in ALS/FTD in both the nervous and peripheral immune systems. We further discuss evidence for direct influence on immune cell function by mutations in ALS/FTD genes including C9orf72, TBK1 and OPTN, and how this could lead to the altered innate immune system “tone” observed in these patients.

Targeting Microglia and Macrophages: A Potential Treatment Strategy for Multiple Sclerosis

Multiple sclerosis (MS) is a chronic inflammatory neurodegenerative disease of the central nervous system (CNS). The early stage is characterized by relapses and the later stage, by progressive disability. Results from experimental and clinical investigations have demonstrated that microglia and macrophages play a key part in the disease course. These cells actively initiate immune infiltration and the demyelination cascade during the early phase of the disease; however, they promote remyelination and alleviate disease in later stages. This review aims to provide a comprehensive overview of the existing knowledge regarding the neuromodulatory function of macrophages and microglia in the healthy and injured CNS, and it discusses the feasibility of harnessing microglia and macrophage physiology to treat MS. The review encourages further investigations into macrophage-targeted therapy, as well as macrophage-based drug delivery, for realizing efficient treatment strategies for MS.

Antigen-presenting cell diversity for T cell reactivation in central nervous system autoimmunity

Autoreactive T cells are considered the major culprits in the pathogenesis of many autoimmune diseases like multiple sclerosis (MS). Upon activation in the lymphoid organs, autoreactive T cells migrate towards the central nervous system (CNS) and target the myelin sheath-forming oligodendrocytes, resulting in detrimental neurological symptoms. Despite the availability of extensively studied systems like the experimental autoimmune encephalomyelitis (EAE) model, our understanding of this disease and the underlying pathogenesis is still elusive. One vividly discussed subject represents the T cell reactivation in the CNS. In order to exert their effector functions in the CNS, autoreactive T cells must encounter antigen-presenting cells (APCs). This interaction provides an antigen-restricted stimulus in the context of major histocompatibility complex class II (MHC-II) and other co-stimulatory molecules. Peripherally derived dendritic cells (DCs), B cells, border-associated macrophages (BAM), CNS-resident microglia, and astrocytes have the capacity to express molecules required for antigen presentation under inflammatory conditions. Also, endothelial cells can fulfill these prerequisites in certain situations. Which of these cells in fact act as APCs for T cell reactivation and to which extent they can exert this function has been studied intensively, but unfortunately with no firm conclusion. In this review, we will summarize the findings that support or question the antigen presenting capacities of the mentioned cell types of CNS-localized T cell reactivation.

CNS-border associated macrophages respond to acute ischemic stroke attracting granulocytes and promoting vascular leakage

The central nervous system (CNS) contains several types of immune cells located in specific anatomic compartments. Macrophages reside at the CNS borders surrounding the brain vessels, in leptomeningeal spaces and the choroid plexus, where they interact with the vasculature and play immunological surveillance and scavenging functions. We investigated the phenotypic changes and role of these macrophages in response to acute ischemic stroke. Given that CD163 expression is a hallmark of perivascular and meningeal macrophages in the rat and human brain, we isolated CD163⁺ brain macrophages by fluorescence activated cell sorting. We obtained CD163⁺ cells from control rats and 16 h following transient middle cerebral artery occlusion, after verifying that infiltration of CD163⁺ peripheral myeloid cells is negligible at this acute time point. Transcriptome analysis of the sorted CD163⁺ cells identified ischemia-induced upregulation of the hypoxia inducible factor-1 pathway and induction of genes encoding for extracellular matrix components and leukocyte chemoattractants, amongst others. Using a cell depletion strategy, we found that CNS border-associated macrophages participate in granulocyte recruitment, promote the expression of vascular endothelial growth factor (VEGF), increase the permeability of pial and cortical blood vessels, and contribute to neurological dysfunction in the acute phase of ischemia/reperfusion. We detected VEGF expression surrounding blood vessels and in some CD163⁺ perivascular macrophages in the brain tissue of ischemic stroke patients deceased one day after stroke onset. These findings show ischemia-induced reprogramming of the gene expression profile of CD163⁺ macrophages that has a rapid impact on leukocyte chemotaxis and blood-brain barrier integrity, and promotes neurological impairment in the acute phase of stroke.

Developmental Exposure to a Mixture of 23 Chemicals Associated With Unconventional Oil and Gas Operations Alters the Immune System of Mice

Chemicals used in unconventional oil and gas (UOG) operations have the potential to cause adverse biological effects, but this has not been thoroughly evaluated. A notable knowledge gap is their impact on development and function of the immune system. Herein, we report an investigation of whether developmental exposure to a mixture of chemicals associated with UOG operations affects the development and function of the immune system. We used a previously characterized mixture of 23 chemicals associated with UOG, and which was demonstrated to affect reproductive and developmental endpoints in mice. C57Bl/6 mice were maintained throughout pregnancy and during lactation on water containing two concentrations of this 23-chemical mixture, and the immune system of male and female adult offspring was assessed. We comprehensively examined the cellularity of primary and secondary immune organs, and used three different disease models to probe potential immune effects: house dust mite-induced allergic airway disease, influenza A virus infection, and experimental autoimmune encephalomyelitis (EAE). In all three disease models, developmental exposure altered frequencies of certain T cell sub-populations in female, but not male, offspring. Additionally, in the EAE model disease onset occurred earlier and was more severe in females. Our findings indicate that developmental exposure to this mixture had persistent immunological effects that differed by sex, and exacerbated responses in an experimental model of autoimmune encephalitis. These observations suggest that developmental exposure to complex mixtures of water contaminants, such as those derived from UOG operations, could contribute to immune dysregulation and disease later in life.

Muscle carnosine in experimental autoimmune encephalomyelitis and multiple sclerosis

BACKGROUND

Muscle carnosine is related to contractile function (Ca⁺⁺ handling) and buffering of exercise-induced acidosis. As these muscular functions are altered in Multiple Sclerosis (MS) it is relevant to understand muscle carnosine levels in MS.

METHODS

Tibialis anterior muscle carnosine was measured in an animal MS model (EAE, experimental autoimmune encephalomyelitis, n = 40) and controls (CON, n = 40) before and after exercise training (EAE_EX, CON_EX, 10d, 1 h/d, 24 m/min treadmill running) or sedentary conditions (EAE_SED, CON_SED). Human m. vastus lateralis carnosine of healthy controls (HC, n = 22) and MS patients (n = 24) was measured.

RESULTS

EAE muscle carnosine levels were decreased (p < .0001) by ~ 40% to ~ 64% at 10d and 17d following EAE induction (respectively) regardless of exercise (p = .823). Similarly, human MS muscle carnosine levels were decreased (- 25%, p = .03).

CONCLUSION

Muscle carnosine concentrations in an animal MS model and MS patients are substantially reduced. In EAE exercise therapy does not restore this.

Brain perivascular macrophages: characterization and functional roles in health and disease

Perivascular macrophages (PVM) are a distinct population of resident brain macrophages characterized by a close association with the cerebral vasculature. PVM migrate from the yolk sac into the brain early in development and, like microglia, are likely to be a self-renewing cell population that, in the normal state, is not replenished by circulating monocytes. Increasing evidence implicates PVM in several disease processes, ranging from brain infections and immune activation to regulation of the hypothalamic-adrenal axis and neurovascular-neurocognitive dysfunction in the setting of hypertension, Alzheimer disease pathology, or obesity. These effects involve crosstalk between PVM and cerebral endothelial cells, interaction with circulating immune cells, and/or production of reactive oxygen species. Overall, the available evidence supports the idea that PVM are a key component of the brain-resident immune system with broad implications for the pathogenesis of major brain diseases. A better understanding of the biology and pathobiology of PVM may lead to new insights and therapeutic strategies for a wide variety of brain diseases.

Rotarod motor performance and advanced spinal cord lesion image analysis refine assessment of neurodegeneration in experimental autoimmune encephalomyelitis

BACKGROUND

Experimental autoimmune encephalomyelitis (EAE) is a commonly used experimental model for multiple sclerosis (MS). Experience with this model mainly comes from the field of immunology, while data on its use in studying the neurodegenerative aspects of MS is scarce.

NEW METHOD

The aim of this study is to improve and refine methods to assess neurodegeneration and function in EAE. Using the rotarod, a tool used in neuroscience to monitor motor performance, we evaluated the correlation between motor performance, disease severity as measured using a clinical scale and area covered by inflammatory lesions.

RESULTS

The included parameters are highly correlated in a non-linear manner, with motor performance rapidly decreasing in the intermediate values of the clinical scale. The relation between motor performance and histopathological damage is exclusively determined by lesions in the ventral and lateral columns, based on a new method of analysis of the entire spinal cord. Using a set of definitions for distinct disease milestones, we quantified disease duration as well as severity.

COMPARISON WITH EXISTING METHODS

The rotarod measures motor performance in a more objective and quantitative manner compared to using a clinical score. The outcome shows a strong correlation to the surface area of inflammatory lesions in the motor systems of the spinal cord.

CONCLUSIONS

These results provide an improved workflow for interpreting the outcome of EAE from a neurological point of view, with the eventual goal of dissecting neurodegeneration and evaluating neuroprotective drugs in EAE for application in MS.

Perivascular Arrest of CD8+ T Cells Is a Signature of Experimental Cerebral Malaria

There is significant evidence that brain-infiltrating CD8⁺ T cells play a central role in the development of experimental cerebral malaria (ECM) during Plasmodium berghei ANKA infection of C57BL/6 mice. However, the mechanisms through which they mediate their pathogenic activity during malaria infection remain poorly understood. Utilizing intravital two-photon microscopy combined with detailed ex vivo flow cytometric analysis, we show that brain-infiltrating T cells accumulate within the perivascular spaces of brains of mice infected with both ECM-inducing (P. berghei ANKA) and non-inducing (P. berghei NK65) infections. However, perivascular T cells displayed an arrested behavior specifically during P. berghei ANKA infection, despite the brain-accumulating CD8⁺ T cells exhibiting comparable activation phenotypes during both infections. We observed T cells forming long-term cognate interactions with CX₃CR1-bearing antigen presenting cells within the brains during P. berghei ANKA infection, but abrogation of this interaction by targeted depletion of the APC cells failed to prevent ECM development. Pathogenic CD8⁺ T cells were found to colocalize with rare apoptotic cells expressing CD31, a marker of endothelial cells, within the brain during ECM. However, cellular apoptosis was a rare event and did not result in loss of cerebral vasculature or correspond with the extensive disruption to its integrity observed during ECM. In summary, our data show that the arrest of T cells in the perivascular compartments of the brain is a unique signature of ECM-inducing malaria infection and implies an important role for this event in the development of the ECM-syndrome.

Cerebral malaria is the most severe complication of Plasmodium falciparum infection. Utilizing the murine experimental model of cerebral malaria (ECM), it has been found that CD8⁺ T cells are a key immune cell type responsible for development of cerebral pathology during malaria infection. To identify how CD8⁺ T cells cause cerebral pathology during malaria infection, in this study we have performed detailed in vivo analysis (two photon imaging) of CD8⁺ T cells within the brains of mice infected with strains of malaria parasites that cause or do not cause ECM. We found that CD8⁺ T cells appear to accumulate in similar numbers and in comparable locations within the brains of mice infected with parasites that do or do not cause ECM. Importantly, however, brain accumulating CD8⁺ T cells displayed significantly different movement characteristics during the different infections. CD8⁺ T cells interacted with myeloid cells within the brain during infection with parasites causing ECM, but this association was not required for development of cerebral complications. Furthermore, our results suggest that CD8⁺ T cells do not cause ECM through the widespread killing of brain microvessel cells. The results in this study significantly improve our understanding of the ways through which CD8⁺ T cells can mediate cerebral pathology during malaria infection.

In Vivo Quantification of Inflammation in Experimental Autoimmune Encephalomyelitis Rats Using Fluorine-19 Magnetic Resonance Imaging Reveals Immune Cell Recruitment outside the Nervous System

Progress in identifying new therapies for multiple sclerosis (MS) can be accelerated by using imaging biomarkers of disease progression or abatement in model systems. In this study, we evaluate the ability to noninvasively image and quantitate disease pathology using emerging “hot-spot” ¹⁹F MRI methods in an experimental autoimmune encephalomyelitis (EAE) rat, a model of MS. Rats with clinical symptoms of EAE were compared to control rats without EAE, as well as to EAE rats that received daily prophylactic treatments with cyclophosphamide. Perfluorocarbon (PFC) nanoemulsion was injected intravenously, which labels predominately monocytes and macrophages in situ. Analysis of the spin-density weighted ¹⁹F MRI data enabled quantification of the apparent macrophage burden in the central nervous system and other tissues. The in vivo MRI results were confirmed by extremely high-resolution ¹⁹F/¹H magnetic resonance microscopy in excised tissue samples and histopathologic analyses. Additionally, ¹⁹F nuclear magnetic resonance spectroscopy of intact tissue samples was used to assay the PFC biodistribution in EAE and control rats. In vivo hot-spot ¹⁹F signals were detected predominantly in the EAE spinal cord, consistent with the presence of inflammatory infiltrates. Surprising, prominent ¹⁹F hot-spots were observed in bone-marrow cavities adjacent to spinal cord lesions; these were not observed in control animals. Quantitative evaluation of cohorts receiving cyclophosphamide treatment displayed significant reduction in ¹⁹F signal within the spinal cord and bone marrow of EAE rats. Overall, ¹⁹F MRI can be used to quantitatively monitored EAE disease burden, discover unexpected sites of inflammatory activity, and may serve as a sensitive biomarker for the discovery and preclinical assessment of novel MS therapeutic interventions.

Splicing Regulation of Pro-Inflammatory Cytokines and Chemokines: At the Interface of the Neuroendocrine and Immune Systems

Alternative splicing plays a key role in posttranscriptional regulation of gene expression, allowing a single gene to encode multiple protein isoforms. As such, alternative splicing amplifies the coding capacity of the genome enormously, generates protein diversity, and alters protein function. More than 90% of human genes undergo alternative splicing, and alternative splicing is especially prevalent in the nervous and immune systems, tissues where cells need to react swiftly and adapt to changes in the environment through carefully regulated mechanisms of cell differentiation, migration, targeting, and activation. Given its prevalence and complexity, this highly regulated mode of gene expression is prone to be affected by disease. In the following review, we look at how alternative splicing of signaling molecules—cytokines and their receptors—changes in different pathological conditions, from chronic inflammation to neurologic disorders, providing means of functional interaction between the immune and neuroendocrine systems. Switches in alternative splicing patterns can be very dynamic and can produce signaling molecules with distinct or antagonistic functions and localization to different subcellular compartments. This newly discovered link expands our understanding of the biology of immune and neuroendocrine cells, and has the potential to open new windows of opportunity for treatment of neurodegenerative disorders.

Impact of high intensity exercise on muscle morphology in EAE rats

The impact of high-intensity exercise on disease progression and muscle contractile properties in experimental autoimmune encephalomyelitis (EAE) remains unclear. Control (CON) and EAE rats were divided into sedentary and exercise groups. Before onset (experiment 1, n=40) and after hindquarter paralysis (experiment 2, n=40), isokinetic foot extensor strength, cross sectional area (CSA) of tibialis anterior (TA), extensor digitorum longus (EDL) and soleus (SOL) and brain-derived neurotrophic factor (BDNF) levels were assessed. EAE reduced muscle fiber CSA of TA, EDL and SOL. In general, exercise was not able to affect CSA, whereas it delayed hindquarter paralysis peak. CON muscle work peaked and declined, while it remained stable in EAE. BDNF-responses were not affected by EAE or exercise. In conclusion, EAE affected CSA-properties of TA, EDL and SOL, which could, partly, explain the absence of peak work during isokinetic muscle performance in EAE-animals. However, exercise was not able to prevent muscle fiber atrophy.

New Directions in Multiple Sclerosis Therapy: Matching Therapy with Pathogenesis

All currently approved therapies for multiple sclerosis (MS) modulate systemic immune components prior to their entry into the central nervous system (CNS). Available data indicate they lack impact on the progressive phases of disease; the more potent systemic immune-directed agents predispose to development of infectious or neoplastic disorders. Development of new agents that enhance disease stage related efficacy and limit systemic toxicity will need to consider the underlying mechanisms related to each phase of the clinical disorder, namely relapses, remission, and progression. This report focuses on disease related mechanisms ongoing within the CNS that contribute to the different phases of MS and how these may serve as potential therapeutic targets. Such mechanisms include CNS compartment specific immunologic properties especially as related to the innate immune system and neural cell-related properties that are determinants of the extent of actual tissue injury and repair (or lack thereof).

Expression of the mannose receptor CD206 in HIV and SIV encephalitis: a phenotypic switch of brain perivascular macrophages with virus infection

We examined the expression of the mannose receptor CD206 by perivascular macrophages (PVM) in normal human and monkey brains and in brains of HIV-infected humans and of monkeys infected with simian immunodeficiency virus (SIV). Depletion of brain PVM in SIV-infected monkeys by intrathecal injection of liposome-encapsulated bisphosphonates eliminated CD206-expressing cells in the brain, confirming their perivascular location and phagocytic capacity. In vivo labeling with bromodeoxyuridine in normal uninfected and SIV-infected macaques in combination with CD206 immunostaining revealed a CD206+-to-CD206- shift within pre-existing PVM during SIV brain infection and neuroinflammation. These findings identify CD206 as a unique marker of human and macaque PVM, and underscore the utility of this marker in studying the origin, turnover and functions of these cells in AIDS.

Macrophage subsets and microglia in multiple sclerosis

Along with microglia and monocyte-derived macrophages, macrophages in the perivascular space, choroid plexus, and meninges are the principal effector cells in neuroinflammatory and neurodegenerative disorders. These phagocytes are highly heterogeneous cells displaying spatial- and temporal-dependent identities in the healthy, injured, and inflamed CNS. In the last decade, researchers have debated on whether phagocytes subtypes and phenotypes are pathogenic or protective in CNS pathologies. In the context of this dichotomy, we summarize and discuss the current knowledge on the spatiotemporal physiology of macrophage subsets and microglia in the healthy and diseased CNS, and elaborate on factors regulating their behavior. In addition, the impact of macrophages present in lymphoid organs on CNS pathologies is defined. The prime focus of this review is on multiple sclerosis (MS), which is characterized by inflammation, demyelination, neurodegeneration, and CNS repair, and in which microglia and macrophages have been extensively scrutinized. On one hand, microglia and macrophages promote neuroinflammatory and neurodegenerative events in MS by releasing inflammatory mediators and stimulating leukocyte activity and infiltration into the CNS. On the other hand, microglia and macrophages assist in CNS repair through the production of neurotrophic factors and clearance of inhibitory myelin debris. Finally, we define how microglia and macrophage physiology can be harnessed for new therapeutics aimed at suppressing neuroinflammatory and cytodegenerative events, as well as promoting CNS repair. We conclude that microglia and macrophages are highly dynamic cells displaying disease stage and location-specific fates in neurological disorders. Changing the physiology of divergent phagocyte subsets at particular disease stages holds promise for future therapeutics for CNS pathologies.

Phagocytosis of microglia in the central nervous system diseases

Microglia, the resident macrophages of the central nervous system, rapidly activate in nearly all kinds of neurological diseases. These activated microglia become highly motile, secreting inflammatory cytokines, migrating to the lesion area, and phagocytosing cell debris or damaged neurons. During the past decades, the secretory property and chemotaxis of microglia have been well-studied, while relatively less attention has been paid to microglial phagocytosis. So far there is no obvious concordance with whether it is beneficial or detrimental in tissue repair. This review focuses on phagocytic phenotype of microglia in neurological diseases such as Alzheimer's disease, multiple sclerosis, Parkinson's disease, traumatic brain injury, ischemic and other brain diseases. Microglial morphological characteristics, involved receptors and signaling pathways, distribution variation along with time and space changes, and environmental factors that affecting phagocytic function in each disease are reviewed. Moreover, a comparison of contributions between macrophages from peripheral circulation and the resident microglia to these pathogenic processes will also be discussed.

Treatment for experimental autoimmune neuritis with clodronate (Bonefos)

Experimental autoimmune neuritis (EAN) serves as an animal model for human Gullain-Barre syndrome (GBS), an autoimmune disease causing demyelination and inflammation of peripheral nerves. Macrophages, which play a major role in this autoimmune inflammatory process, can be selectively targeted by high doses of bisphophonates. The goal of this study was to examine the effect of the bisphosphonate, clodronate, on the severity of the EAN model. EAN was induced in female adult rats by immunization with bovine peripheral myelin. A number of treatment protocols with clodronate were used based on the common dosage regimen of 20 mg/kg in humans starting with the appearance of clinical signs on day 10 post-immunization. The clinical parameters measured included a clinical score, a motor performance test performed on a Rotarod and body weight. The expression of the matrix metaloprotease (MMP-9) in the sciatic nerves was measured as a marker of inflammatory macrophages. Treatment with clodronate, 20 mg/kg daily and 40 mg/kg every 2 days, significantly reduced the disease severity (a 75% decrease in severity, p < 0.01 by ANOVA) as measured by the clinical score compared to controls. Performance on the Rotarod test and body weight confirmed the clinical score findings. MMP-9 expression levels were significantly lower in the sciatic nerves of clodronate-treated rats. The present findings support the efficiency of clodronate in inflammatory diseases of the peripheral nervous system. The mechanism of action includes inhibition of inflammatory macrophages. The results suggest the use of bisphosphonates be considered in humans with GBS.

Role of peripheral immune response in microglia activation and regulation of brain chemokine and proinflammatory cytokine responses induced during VSV encephalitis

We report herein that neuroinvasion by vesicular stomatitis virus (VSV) activates microglia and induces a peripheral dendritic cell (DC)-dependent inflammatory response in the central nervous system (CNS). VSV neuroinvasion rapidly induces multiple brain chemokine and proinflammatory cytokine mRNAs that display bimodal kinetics. Peripheral DC ablation or T cell depletion suppresses the second wave of this response demonstrating that infiltrating T cells are primarily responsible for the bimodal characteristics of this response. The robust infiltrate associated with VSV encephalitis likely depends on sustained production of brain CCL19 and CCR7 expression on infiltrating inflammatory cells.

Laser capture microdissection assessment of virus compartmentalization in the central nervous systems of macaques infected with neurovirulent simian immunodeficiency virus

Nonhuman primate-simian immunodeficiency virus (SIV) models are powerful tools for studying the pathogenesis of human immunodeficiency virus type 1 (HIV-1) in the brain. Our laboratory recently isolated a neuropathogenic viral swarm, SIVsmH804E, a derivative of SIVsmE543-3, which was the result of sequential intravenous passages of viruses isolated from the brains of rhesus macaques with SIV encephalitis. Animals infected with SIVsmH804E or its precursor (SIVsmH783Br) developed SIV meningitis and/or encephalitis at high frequencies. Since we observed macaques with a combination of meningitis and encephalitis, as well as animals in which meningitis or encephalitis was the dominant component, we hypothesized that distinct mechanisms could be driving the two pathological states. Therefore, we assessed viral populations in the meninges and the brain parenchyma by laser capture microdissection. Viral RNAs were isolated from representative areas of the meninges, brain parenchyma, terminal plasma, and cerebrospinal fluid (CSF) and from the inoculum, and the SIV envelope fragment was amplified by PCR. Phylogenetic analysis of envelope sequences from the conventional progressors revealed compartmentalization of viral populations between the meninges and the parenchyma. In one of these animals, viral populations in meninges were closely related to those from CSF and shared signature truncations in the cytoplasmic domain of gp41, consistent with a common origin. Apart from magnetic resonance imaging (MRI) and positron-emission tomography (PET) imaging, CSF is the most accessible assess to the central nervous system for HIV-1-infected patients. However, our results suggest that the virus in the CSF may not always be representative of viral populations in the brain and that caution should be applied in extrapolating between the properties of viruses in these two compartments.

Polarization of macrophages and microglia in inflammatory demyelination

Multiple sclerosis (MS) is an autoimmune demyelinating disease of the central nervous system, and microglia and macrophages play important roles in its pathogenesis. The activation of microglia and macrophages accompanies disease development, whereas depletion of these cells significantly decreases disease severity. Microglia and macrophages usually have diverse and plastic phenotypes. Both pro-inflammatory and antiinflammatory microglia and macrophages exist in MS and its animal model, experimental autoimmune encephalomyelitis. The polarization of microglia and macrophages may underlie the differing functional properties that have been reported. in this review, we discuss the responses and polarization of microglia and macrophages in MS, and their effects on its pathogenesis and repair. Harnessing their beneficial effects by modulating their polarization states holds great promise for the treatment of inflammatory demyelinating diseases.

Meningeal and choroid plexus cells--novel drug targets for CNS disorders

The meninges and choroid plexus perform many functions in the developing and adult human central nervous system (CNS) and are composed of a number of different cell types. In this article I focus on meningeal and choroid plexus cells as targets for the development of drugs to treat a range of traumatic, ischemic and chronic brain disorders. Meningeal cells are involved in cortical development (and their dysfunction may be involved in cortical dysplasia), fibrotic scar formation after traumatic brain injuries (TBI), brain inflammation following infections, and neurodegenerative disorders such as Multiple Sclerosis (MS) and Alzheimer's disease (AD) and other brain disorders. The choroid plexus regulates the composition of the cerebrospinal fluid (CSF) as well as brain entry of inflammatory cells under basal conditions and after injuries. The meninges and choroid plexus also link peripheral inflammation (occurring in the metabolic syndrome and after infections) to CNS inflammation which may contribute to the development and progression of a range of CNS neurological and psychiatric disorders. They respond to cytokines generated systemically and secrete cytokines and chemokines that have powerful effects on the brain. The meninges may also provide a stem cell niche in the adult brain which could be harnessed for brain repair. Targeting meningeal and choroid plexus cells with therapeutic agents may provide novel therapies for a range of human brain disorders.

Calpain inhibition attenuated morphological and molecular changes in skeletal muscle of experimental allergic encephalomyelitis rats

Muscle weakness and atrophy are important manifestations of multiple sclerosis (MS). To investigate the pathophysiological mechanisms of skeletal muscle change in MS, we induced experimental autoimmune encephalomyelitis (EAE) in Lewis male rats and examined morphological and molecular changes in skeletal muscle. We also treated EAE rats with calpepetin, a calpain inhibitor, to examine its beneficial effects on skeletal muscle damage. Morphological changes in muscle tissue of EAE rats included smaller and irregularly shaped muscle fibers and fibrosis. Western blot analysis demonstrated increased calpain:calpastatin ratio, inflammation-related transcription factors (nuclear factor-κB:inhibitor of κB α ratio), and proinflammatory enzymes (cyclooxygenase-2). TUNEL-positive myonuclei in skeletal muscle cells of EAE rats indicated cell death. In addition, markers of apoptotic cell death (Bax:Bcl-2 ratio and caspase-12 protein levels) were elevated. Expression of muscle-specific ubiquitin ligases (muscle atrophy F-box and muscle ring finger protein 1), was upregulated in muscle tissue of EAE-vehicle animals. Both prophylactic and therapeutic treatment with calpeptin partially attenuated muscle changes noted in EAE animals. These results indicate that morphological and molecular changes including apoptotic cell death and protein breakdown develop in skeletal muscle of EAE animals and that these changes can be reversed by calpain inhibition.

Mechanism of experimental autoimmune encephalomyelitis in Lewis rats: recent insights from macrophages

Experimental autoimmune encephalomyelitis (EAE) in Lewis rats is an acute monophasic paralytic central nervous system disease, in which most rats spontaneously recover from paralysis. EAE in Lewis rats is induced by encephalitogenic antigens, including myelin basic protein. EAE is mediated by CD4⁺ Th1 cells, which secrete pro-inflammatory mediators, and spontaneous recovery is mediated by regulatory T cells. Recently, it was established that classically activated macrophages (M1 phenotype) play an important role in the initiation of EAE, while alternatively activated macrophages (M2 phenotype) contribute to spontaneous recovery from rat EAE. This review will summarize the neuroimmunological aspects of active monophasic EAE, which manifests as neuroinflammation followed by neuroimmunomodulation and/or neuroprotection, with a focus on the role of alternatively activated macrophages.

Upregulation of vascular endothelial growth factor receptor-3 in the spinal cord of Lewis rats with experimental autoimmune encephalomyelitis

We investigated the spatiotemporal expression of vascular endothelial growth factor receptor-3 (VEGFR-3) in the spinal cord of Lewis rats with experimental autoimmune encephalomyelitis (EAE), an animal model for multiple sclerosis. VEGFR-3 mRNA and protein were constitutively expressed in gray matter neurons and in a few white matter astrocytes. Induction of VEGFR-3 occurred predominantly in perivascular infiltrated macrophages in the spinal cord white matter during the inductive phase of EAE. VEGFR-3 expression was also induced in activated microglial cells in the gray and white matter, mainly in the peak phase. In addition, reactive astrocytes in the white matter, but not in the gray matter, expressed VEGFR-3 as disease severity increased. These data suggest that VEGFR-3 is involved in the recruitment of monocytic macrophages and in glial reactions during EAE.

miR-15a and 16-1 are downregulated in CD4+ T cells of multiple sclerosis relapsing patients

The pathology of relapsing-remitting multiple sclerosis (RR-MS) is largely attributed to activated autoreactive effector T lymphocytes. The influence of microRNAs on the immune response has been shown to occur in different pathways of lymphocyte differentiation and function. Here, the expression of the miRNAs miR-15a/16-1 in PBMC, CD4(+), and CD8(+) from RR-MS patients has been investigated. BCL2, a known miR-15a/16-1 target, has also been analyzed. The results have shown that miR-15a/16-1 is downregulated in CD4(+) T cells, whereas BCL2 is highly expressed in RR-MS patients only. Our data suggest that miR-15a/16-1 can also modulate the BCL2 gene expression in CD4(+) T cells from RR-MS patients, thereby affecting apoptosis processes.

ICV-transplanted human glial precursor cells are short-lived yet exert immunomodulatory effects in mice with EAE

Human glial precursor cells (hGPs) have potential for remyelinating lesions and are an attractive cell source for cell therapy of multiple sclerosis (MS). To investigate whether transplanted hGPs can affect the pathogenesis of experimental autoimmune encephalomyelitis (EAE), an animal model of MS, we evaluated the therapeutic effects of transplanted hGPs together with the in vivo fate of these cells using magnetic resonance imaging (MRI) and bioluminescence imaging (BLI). At 14 days post-EAE induction, mice (n = 19) were intracerebroventricularly (ICV) injected with 5 × 10(5) hGPs that were magnetically labeled with superparamagnetic iron oxide (SPIO) particles as MR contrast agent and transduced with firefly luciferase for BLI of cell survival. Control mice (n = 18) received phosphate buffered saline (PBS) vehicle only. The severity of EAE clinical disability in the hGP-transplanted group was significantly suppressed (P < 0.05) with concomitant inhibition of ConA and MOG-specific T cell proliferation in the spleen. Astrogliosis was reduced and a lower activity of macrophages and/or microglia was observed in the spinal cord (P < 0.05). On MRI, SPIO signal was detected within the lateral ventricle from 1 day post-transplantation and remained there for up to 34 days. BLI indicated that most cells did not survive beyond 5-10 days, consistent with the lack of detectable migration into the brain parenchyma and the histological presence of an abundance of apoptotic cells. Transplanted hGPs could not be detected in the spleen. We conclude that ICV transplantation of short-lived hGPs can have a remote therapeutic effect through immunomodulation from within the ventricle, without cells directly participating in remyelination.

Differential detection and distribution of microglial and hematogenous macrophage populations in the injured spinal cord of lys-EGFP-ki transgenic mice

The acute inflammatory response that follows spinal cord injury (SCI) contributes to secondary injury that results in the expansion of the lesion and further loss of neurologic function. A cascade of receptor-mediated signaling events after SCI leads to activation of innate immune responses including the migration of microglia and active recruitment of circulating leukocytes. Because conventional techniques do not always distinguish macrophages derived from CNS-resident microglia from blood-derived monocytes, the role that each macrophage type performs cannot be assessed unambiguously in these processes. We demonstrate that, in the normal and spinal cord-injured lys-EGFP-ki transgenic mouse, enhanced green fluorescent protein (EGFP) is expressed only in mature hematopoietic granulomyelomonocytic cells and not in microglia. This allowed us to assess the temporal and spatial relationships between microglia-derived and hematogenous macrophages as well as neutrophils during a period of 6 weeks after clip compression SCI. Within the lesion, EGFP-positive monocyte-derived macrophages were found at the epicenter surrounded by EGFP-negative-activated microglia and microglia-derived macrophages. Neutrophils were not present when EGFP-positive monocyte-derived macrophages were depleted, indicating that neutrophil persistence in the lesion depended on the presence of these monocytes. Thus, these 2 distinct macrophage populations can be independently identified and tracked, thereby allowing their roles in acute and chronic stages of SCI-associated inflammation to be defined.