Communicating systems in the body: how microbiota and microglia cooperate

Microglia are tissue macrophages of the central nervous system (CNS ). Their key tasks are immune surveillance as well as responding to infections or other pathological states such as neurological diseases or injury. In recent years it has been discovered that microglia are additionally crucial for the maintenance of brain homeostasis during development and adulthood by adjusting the neuronal network and phagocytosing neuronal debris. Microglia persist in the CNS throughout the life of the organism and self‐renew without engraftment of bone‐marrow‐derived cells. Until recently it remained unknown what controls their maturation and activation under homeostatic conditions. In this review we discuss new aspects of the interaction between host microbiota and brain function with special focus on the brain‐resident innate immune cells, the microglia.     

Microglia in antiviral immunity of the brain and spinal cord

Viral infections of the central nervous system (CNS) are a significant cause of neurological impairment and mortality worldwide. As tissue resident macrophages, microglia are critical initial responders to CNS viral infection. Microglia seem to coordinate brain-wide antiviral responses of both brain resident cells and infiltrating immune cells. This review discusses how microglia may promote this antiviral response at a molecular level, from potential mechanisms of virus recognition to downstream cytokine responses and interaction with antiviral T cells. Recent advancements in genetic tools to specifically target microglia in vivo promise to further our understanding about the precise mechanistic role of microglia in CNS infection.     

小胶质细胞在中枢神经系统和胶质瘤中的免疫学作用及其相关研究进展

血脑屏障在正常情况下能够保证脑的内环境的高度稳定性,以利于中枢神经系统的机能活动.中枢神经系统内缺乏各种常见的抗原递呈细胞,但小胶质细胞可作为潜在的抗原递呈细胞将MHC分子结合的抗原肽与T细胞受体结合介导相应的免疫反应.而在胶质瘤组织中高度浸润的小胶质细胞可通过对肿瘤微环境的免疫抑制而促进肿瘤生长.一旦小胶质细胞被激活就能成为强大的免疫效应细胞,在中枢神经系统损伤和疾病中发挥多种生物学功能.     

小胶质细胞在中枢神经系统和胶质瘤中的免疫学作用及其相关研究进展

血脑屏障在正常情况下能够保证脑的内环境的高度稳定性,以利于中枢神经系统的机能活动。中枢神经系统内缺乏各种常见的抗原递呈细胞,但小胶质细胞可作为潜在的抗原递呈细胞将MHC分子结合的抗原肽与T细胞受体结合介导相应的免疫反应。而在胶质瘤组织中高度浸润的小胶质细胞可通过对肿瘤微环境的免疫抑制而促进肿瘤生长。一旦小胶质细胞被激活就能成为强大的免疫效应细胞,在中枢神经系统损伤和疾病中发挥多种生物学功能。     

Maturation and localization of macrophages and microglia during infection with a neurotropic murine coronavirus

Macrophages and microglia are critical in the acute inflammatory response and act as final effector cells of demyelination during chronic infection with the neutrotropic MHV-JHM strain of mouse hepatitis virus (MHV-JHM). Herein, we show that "immature" F4/80(+)Ly-6C(hi) monocytes are the first cells, along with neutrophils, to enter the MHV-JHM-infected central nervous system (CNS). As the infection progresses, macrophages in the CNS down-regulate expression of Ly-6C and CD62L, consistent with maturation, and a higher frequency express CD11c, a marker for dendritic cells (DCs). Microglia also express CD11c during this phase of the infection. CD11c(+) macrophages in the infected CNS exhibit variable properties of immature antigen-presenting cells (APCs), with modestly increased CD40 and MHC expression, and equivalent potent antigen uptake when compared with CD11c(-) macrophages. Furthermore, CDllc(+) and F4/80(+) macrophages and microglia are localized to areas of demyelination, in some instances directly associated with damaged axons. These results suggest that chronic CNS infection results in the appearance of CD11c-expressing macrophages from the blood that exhibit properties of immature APCs, are closely associated with areas of demyelination, and may act as final effectors of myelin destruction.     

Comparison of matrix metalloproteinase expression during Wallerian degeneration in the central and peripheral nervous systems

The matrix metalloproteinases (MMPs) are a large family of zinc-dependent enzymes which are able to degrade the protein components of the extracellular matrix. They can be placed into subgroups based on structural similarities and substrate specificity. Aberrant expression of these destructive enzymes has been implicated in the pathogenesis of immune-mediated neuroinflammatory disorders. In this study we investigate the involvement of MMPs, from each subgroup, in Wallerian degeneration in both the central and peripheral nervous systems. Wallerian degeneration describes the process initiated by transection of a nerve fibre and entails the degradation and removal of the axon and myelin from the distal stump. A similar degenerative process occurs as the final shared pathway contributing to most common neuropathies. MMP expression and localisation in the peripheral nervous system are compared with events in the CNS during Wallerian degeneration. Within 3 days after axotomy in the peripheral nervous system, MMP-9, MMP-7 and MMP-12 are elevated. These MMPs are produced by Schwann cells, endothelial cells and macrophages. The temporospatial expression of activated MMP-9 correlates with breakdown of the blood-nerve barrier. In the CNS, 1 week after optic nerve crush, four MMPs are induced and primarily localised to astrocytes, not microglia or oligodendrocytes. In the degenerating optic nerve, examined at later time points (4, 8, 12 and 18 weeks), MMP expression was down-regulated. The absence of MMPs in oligodendrocytes and mononuclear phagocytes during Wallerian degeneration may contribute to the slower removal of myelin debris observed in the CNS. The low level of the inactive pro-form of MMP-9 in the degenerating optic nerve may explain why the blood-brain barrier remains intact, while the blood-nerve barrier is rapidly broken down. We conclude that the difference in the level of expression, activation state and cellular distribution of MMPs may contribute to the different sequence of events observed during Wallerian degeneration in the peripheral compared to the CNS.     

Human brain parenchymal microglia express CD14 and CD45 and are productively infected by HIV-1 in HIV-1 encephalitis

Microglia are endogenous brain macrophages that show distinct phenotypes such as expression of myeloid antigens, ramified morphology, and presence within the neural parenchyma. They play significant roles in a number of human CNS diseases including AIDS dementia. Together with monocyte-derived (perivascular) macrophages, microglia represent a major target of HIV-1 infection. However, a recent report challenged this notion based on findings in SIV encephalitis. This study concluded that perivascular macrophages can be distinguished from parenchymal microglial cells by their expression of CD14 and CD45, and that macrophages, but not microglia, are productively infected in SIV and HIV encephalitis. To address whether parenchymal microglia are productively infected in HIV encephalitis, we analyzed expression of CD14, CD45 and HIV-1 p24 in human brain. Microglia were identified based on their characteristic ramified morphology and location in the neural parenchyma. We found that parenchymal microglia are CD14+ (activated), CD45+ (resting and activated), and constitute approximately two thirds of the p24+ cells in HIV encephalitis cases. These results demonstrate that microglia are major targets of infection by HIV-1, and delineate possible differences between HIVE and SIVE. Because productively infected tissue macrophages serve as the major viral reservoir, these findings have important implications for AIDS.     

CC chemokine receptor 8 in the central nervous system is associated with phagocytic macrophages

CC chemokine receptor 8 (CCR8) has been detected in vitro on type 2 helper and regulatory lymphocytes, which might exert beneficial functions in multiple sclerosis (MS) and on macrophages and microglia, possibly promoting tissue injury in MS lesions. To discriminate the relevant expression pattern in vivo, we defined the cell types that expressed CCR8 in MS lesions and determined the relationship of CCR8 expression and demyelinating activity. CCR8 was not expressed on T cells but was associated with phagocytic macrophages and activated microglia in MS lesions and directly correlated with demyelinating activity. To identify factors associated with CCR8 expression, the study was extended to other central nervous system (CNS) pathologies. CCR8 was consistently expressed on phagocytic macrophages and activated microglia in stroke and progressive multifocal leukoencephalopathy, but not expressed on microglia in pathologies that lacked phagocytic macrophages such as senile change of the Alzheimer's type. CCR8 was up-regulated by macrophage differentiation and activating stimuli in vitro. In summary CNS CCR8 expression was associated with phagocytic macrophages and activated microglial cells in human CNS diseases, suggesting that CCR8 may be a feasible target for therapeutic intervention in MS. CCR8 expression may also indicate a selective program of mononuclear phagocyte gene expression.     

The central nervous system environment controls effector CD4+ T cell cytokine profile in experimental allergic encephalomyelitis

In experimental allergic encephalomyelitis (EAE), CD4⁺ T cells infiltrate the central nervous system (CNS). We derived CD4⁺ T cell lines from SJL/J mice that were specific for encephalitogenic myelin basic protein (MBP) peptides and produced both Th1 and Th2 cytokines. These lines transferred EAE to naive mice. Peptide-specific cells re-isolated from the CNS only produced Th1 cytokines, whereas T cells in the lymph nodes produced both Th1 and Th2 cytokines. Mononuclear cells isolated from the CNS, the majority of which were microglia, presented antigen to and stimulated MBP-specific T cell lines in vitro. Although CNS antigen-presenting cells (APC) supported increased production of interferon (IFN)-γ mRNA by these T cells, there was no increase in the interleukin (IL)-4 signal, whereas splenic APC induced increases in both IFN-γ and IL-4. mRNA for IL-12 (p40 subunit) was up-regulated in both infiltrating macrophages and resident microglia from mice with EAE. We have thus shown that a Th1 cytokine bias within the CNS can be induced by CNS APC, and that IL-12 is up-regulated in microglial cells within the CNS of mice with EAE. Microglia may therefore control Th1 cytokine responses within the CNS.     

凝血酶在HIV-1感染小胶质细胞中作用的研究进展

凝血酶是凝血连锁反应的主要效应蛋白酶.除了能够切割纤维蛋白原外,还发现凝血酶能够特异性切割HIV-1 V3环保守的冠部并促进HIV-1介导的细胞融合;此外,凝血酶及细胞表面凝血酶受体表达的上调可能与HIV-1相关脑病的发生相关.小胶质细胞是中枢神经系统固有的免疫细胞,在脑损害时,这些细胞迅速被激活并发挥巨噬细胞一样的功...     

From bone marrow to microglia: barriers and avenues

Microglia form a unique population of brain-resident macrophages. Although microglia have been involved in multiple disorders of the central nervous system (CNS), the issue of microglial renewal, under normal or pathological conditions, has been controversial. In mice, results from bone marrow chimera studies indicated that microglia are slowly but continuously replenished by bone marrow-derived cells. Moreover, such a microglial turnover was found to be greatly accelerated under multiple neurological conditions. However, recent works questioned the use of irradiation/reconstitution experiments to assess microglial turnover. Based on these different studies, we propose here a re-evaluation of microglia origin(s) in the inflamed CNS. We also discuss the therapeutic perspectives offered by the demonstration of an adult microglial lineage, from bone marrow to brain.     

Effective and selective immune surveillance of the brain by MHC class I-restricted cytotoxic T lymphocytes

Cytotoxic CD8(+) T cells are abundantly present in human virus-induced or putative autoimmune diseases of the central nervous system (CNS). Their direct role in the induction of inflammatory brain damage is, however, poorly understood. We have studied CD8(+) T cell-mediated brain inflammation by transferring MHC class I-restricted hemagglutinin (HA)-reactive T cells from a TCR transgenic mouse line into transgenic mice, which express HA in astrocytes. We show that activated CD8(+) T cells alone can induce monophasic brain inflammation in immunocompetent recipient animals. Similar to previous studies, involving transfer of CD4(+) cells, brain inflammation peaks after 5-7 days and then declines. The pathology of brain inflammation, however, differs fundamentally from that induced by CD4(+) cells. The inflammatory reaction is dominated by T cells and activated microglia in the virtual absence of hematogenous macrophages. This is associated with exquisitely specific destruction of antigen-containing astrocytes in the absence of any bystander damage of myelin, oligodendrocytes or neurons. Furthermore, in contrast to CD4(+) T cells, some CD8(+) cells accumulate in the brain and activate microglia in recipient animals, even in the absence of the specific antigen in the CNS. These data indicate that CD8(+) T cells are prime candidates for immune surveillance of the CNS.     

Animal Models of Demyelination

Demyelination is a pathological feature that is characteristic of many diseases of the central nervous system (CNS) including multiple sclerosis (MS), sub-acute sclerosing panencephalomyelitis (SSPE), metachromatic leukodystrophy and Pelizaeus-Merzbacher disease. While demyelination is a pathological end-point that is common to all of these diseases, the cellular and molecular mechanisms responsible for this pathology are very different. These range from genetic defects that affect lipid metabolism in the leukodystrophies, cytopathic effects of viral infection in SSPE to the action of immunological effector mechanisms in MS and the viral encephalopathies. Irrespective of the initial cause of myelin degradation, many of these disorders are associated with some degree of CNS inflammation, as indicated by the local activation of microglia, recruitment of macrophages or the intrathecal synthesis of immunoglobulin. Many of these phenomena are now being duplicated in animal models, providing not only new insights into the pathogenesis of human demyelinating diseases, but also unexpected interrelationships between the immune response in the CNS and the pathogenesis of diseases such as Alzheimers disease and HIV encephalopathy. Autoimmune mediated models of inflammatory demyelinating CNS disease have proved particularly valuable in this respect as they allow the effects of defined immune effector mechanisms to be studied in the absence of CNS infection.     

Discrimination between different types of neuroglial cells in rat central nervous system using combined immuno- and enzyme-histochemical methods

The central nervous system (CNS) contains several types of neuroglial cells. In the present study, we characterized different types of glial cells in rat CNS by using single and combined immuno- and enzyme-histochemical methods, and immunofluorescence techniques. Two recently developed monoclonal antibodies (mAbs) against rat macrophages-associated antigens appeared to recognize a subpopulation of glial cells in the CNS of normal adult rats. These ED4- and ED8-positive glial cells were predominantly located in the white matter of adult rat CNS and shared morphological features with microglia. ED4 and ED8 were applied in a double staining combined with mAbs and an antiserum raised against galactocerebroside (GalC) to identify oligodendrocytes, or with anti-glial fibrillary acidic protein antiserum (GFA) to identify astrocytes. We also used a mAb against myelin basic protein (MBP) to identify oligodendrocytes. It appeared that ED4 and ED8 recognized a subpopulation of oligodendrocytes. MAbs against GalC and MBP recognized cells in an immunoperoxidase staining with a morphology identical to that of the ED8-positive cells and part of the ED4-positive cells. Frozen sections of Lewis rats CNS with acute experimental allergic encephalomyelitis (EAE) were investigated, where infiltrating brain macrophages could be found which stained positively with ED4 and ED8 as well as with the monocyte/macrophage mAbs ED1 and ED2. These brain macrophages did not stain when GalC, MBP and GFA markers were applied. Furthermore, ED4+GalC+ and ED8+GalC+ oligodendrocytes were present in the CNS white matter of EAE animals with similar appearance as in normal adult rats. With the currently used markers, we could not detect a third type of neuroglial cell, besides the astrocytes and oligodendrocytes. Thus, none of our anti-macrophage monoclonals recognized the presumptive microglia. Only under pathological conditions, e.g., in inflammatory infiltrates in the course of EAE, could brain macrophages be detected in the CNS parenchyma and only in the direct vicinity of blood vessels, indicating their hematogenous origin.     

Macrophages in the central nervous system of the rat

In an immunohistochemical study using monoclonal antibodies, which exclusively recognize cells of the monocyte-macrophage lineage, and monoclonal antibodies against the Ia-antigen, we describe the occurrence of macrophages in the developing and adult central nervous system (CNS). In normal adult brain, no macrophages could be detected in the CNS parenchyma; only in the meninges and the choroid plexes were a few macrophages found. During ontogeny, numerous phagocytic cells infiltrated the CNS parenchyma; these cells which did not express Ia are blood-borne. About three weeks after birth, all macrophages had disappeared from the CNS. As microglia in adult and developing brain do not stain with the anti-macrophage antibodies, we suggest that microglial cells are not related to the mononuclear phagocyte system and do not have a hematogenous origin.     

Paraquat induces microglial cause early neuronal synaptic deficits through activation of the classical complement cascade response

Synapse loss is considered to be an early event in the dysfunction of the central nervous system (CNS), precedes neuronal decline, which is the main pathological change in mild cognitive impairment (MCI). Accumulating evidence has shown that neuronal synapse loss is associated with hyperactivity of microglial phagocytosis. In the CNS, microglia act as macrophages to clear cellular debris and weakened synapses, but the mechanism by which microglia activation leads to phagocytosis disorder remains unclear. Therefore, we treated mice with paraquat (PQ) in intraperitoneal injection to explore the mechanism by which microglia exert immunotoxicity in the CNS and cause neuronal synapse loss. Immunofluorescence results exposed synapses decreased with PQ exposure time, but the staining HE and Nissl showed that neuronal cell bodies were hardly affected. Fluorescence co-localization found that C1q (classical complement cascade initiation factor) was gradually deposited in the postsynaptic membrane of neurons to trigger the cascade reaction, thereby causing the excessive deposition of C3, a key factor of the classical complement cascade, and further induces hyperactivation of microglia, leads to phagocytosis disorder. IHC results demonstrated that the parallel relationship. Therefore, all our preliminary results throw light on the mechanism by which PQ abnormally triggers the immune system to produce immunotoxicity leading to microglial phagocytic dysfunction.     

Microglial recruitment, activation, and proliferation in response to primary demyelination

We have characterized the cellular response to demyelination/remyelination in the central nervous system using the toxin cuprizone, which causes reproducible demyelination in the corpus callosum. Microglia were distinguished from macrophages by relative CD45 expression (CD45(dim)) using flow cytometry. Their expansion occurred rapidly and substantially outnumbered infiltrating macrophages and T cells throughout the course of cuprizone treatment. We used bromodeoxyuridine incorporation and bone marrow chimeras to show that both proliferation and immigration from blood accounted for increased microglial numbers. Microglia adopted an activated phenotype during demyelination, up-regulating major histocompatibility class I and B7.2/CD86. A subpopulation of CD45(dim-high) microglia that expressed reduced levels of CD11b emerged during demyelination. These microglia expressed CD11c and were potent antigen-presenting cells in vitro. T cells were recruited to the demyelinated corpus callosum but did not appear to be activated. Our study highlights the role of microglia as a heterogeneous population of cells in primary demyelination, with the capacity to present antigen, proliferate, and migrate into demyelinated areas.     

CX3CL1 (fractalkine) and CX3CR1 expression in myelin oligodendrocyte glycoprotein-induced experimental autoimmune encephalomyelitis: kinetics and cellular origin

Background  

Multiple sclerosis (MS) is a chronic inflammatory disease of the central nervous system (CNS). It is associated with local activation of microglia and astroglia, infiltration of activated macrophages and T cells, active degradation of myelin and damage to axons and neurons. The proposed role for CX₃CL1 (fractalkine) in the control of microglia activation and leukocyte infiltration places this chemokine and its receptor CX₃CR1 in a potentially strategic position to control key aspects in the pathological events that are associated with development of brain lesions in MS. In this study, we examine this hypothesis by analyzing the distribution, kinetics, regulation and cellular origin of CX₃CL1 and CX₃CR1 mRNA expression in the CNS of rats with an experimentally induced MS-like disease, myelin oligodendrocyte glycoprotein (MOG)-induced autoimmune encephalomyelitis (EAE).     

Crosstalk Between Components of the Blood Brain Barrier and Cells of the CNS in Microglial Activation in AIDS

During the progression of AIDS, a majority of patients develop cognitive disorders such as HIV encephalitis (HIVE) and AIDS dementia complex (ADC), which correlate closely with macrophage infiltration into the brain and microglial activation. Microglial activation occurs in response to infection, inflammation and neurological disorders including HIVE, Alzheimer's disease, Parkinson's disease and multiple sclerosis. Microglia can be activated by immunoreactive cells independent of, but enhanced by HIV infection, from at least two routes. Activation may occur from signals originating from activated monocytes and lymphocytes in the blood stream, which initiate a cascade of stimuli that ultimately reach microglia in the brain or from activated macrophages/microglia/astrocytes within the brain. Effects of microglial activation stemming from both systemic and CNS HIV infection act together to commence signaling feedback, leading to HIVE and increased neurodegeneration. Most recent data indicate that in AIDS patients, microglial activation in the brain with subsequent release of excitotoxins, cytokines and chemokines leads to neurodegeneration and cognitive impairment. Since the presence of HIV in the brain results from migration of infected monocytes and lymphocytes across the vascular boundary, the development of novel therapies aimed at protecting the integrity of the blood brain barrier (BBB) upon systemic HIV infection is critical for controlling CNS infection.     

The microglial "activation" continuum: from innate to adaptive responses

Microglia are innate immune cells of myeloid origin that take up residence in the central nervous system (CNS) during embryogenesis. While classically regarded as macrophage-like cells, it is becoming increasingly clear that reactive microglia play more diverse roles in the CNS. Microglial "activation" is often used to refer to a single phenotype; however, in this review we consider that a continuum of microglial activation exists, with phagocytic response (innate activation) at one end and antigen presenting cell function (adaptive activation) at the other. Where activated microglia fall in this spectrum seems to be highly dependent on the type of stimulation provided. We begin by addressing the classical roles of peripheral innate immune cells including macrophages and dendritic cells, which seem to define the edges of this continuum. We then discuss various types of microglial stimulation, including Toll-like receptor engagement by pathogen-associated molecular patterns, microglial challenge with myelin epitopes or Alzheimer's β-amyloid in the presence or absence of CD40L co-stimulation, and Alzheimer disease "immunotherapy". Based on the wide spectrum of stimulus-specific microglial responses, we interpret these cells as immune cells that demonstrate remarkable plasticity following activation. This interpretation has relevance for neurodegenerative/neuroinflammatory diseases where reactive microglia play an etiological role; in particular viral/bacterial encephalitis, multiple sclerosis and Alzheimer disease.