Effects of dexamethasone on microglial activation in vivo: selective downregulation of major histocompatibility complex class II expression in regenerating facial nucleus

Following axotomy of the facial nerve microglial cells in the facial nucleus become activated, proliferate, and newly express class I and class II major histocompatibility complex (MHC) antigens. Dexamethasone treatment, starting 2 days prior to axotomy at 1 mg/kg/day, selectively inhibited axotomy-induced MHC class II expression on microglial and perivascular cells. In contrast, MHC class I expression was not significantly affected, nor was the expression of other microglial activation markers and the light microscopic morphology of activated microglia. A recently suggested inducer of MHC expression in rat nervous tissue, neuronal gamma interferon-like immunoreactive material, was also unaffected, as was glial fibrillary acidic protein immunoreactivity as a marker for concomitant astroglial activation. The differential effects of the drug suggest the presence of distinct regulatory pathways for different aspects of microglial activation. Inhibition of class II expression on activated microglia might be one mechanism how glucocorticoids act in the suppression of neuroinflammatory disease.     

Spinal cord microglial cells and DRG satellite cells rapidly respond to transection of the rat sciatic nerve

Transection of the rat sciatic nerve induces retrograde changes in the dorsal root ganglia (DRG) neurons and in the motoneurons in the ventral grey matter of the lumbar L4-L6 spinal cord segments. In the ipsilateral dorsal grey matter and in the ipsilateral nucleus gracilis, transganglionic changes occur in the terminal fields of the centrally projecting axons of injured DRG neurons. As revealed by immunocytochemistry, the neuronal reactions were associated with a rapid proliferation and activation of microglial cells in the lumbar spinal cord as well as in the nucleus gracilis. Reactive microglial cells were detected as early as 24 h after sciatic axotomy. The microglial reaction had a maximum around day 7 postlesion and disappeared around 6 weeks after axotomy. In addition to light microscopy, activated, perineuronal microglia were identified by immuno-electron microscopy in the ventral grey matter. In the DRG, satellite cells constitutively expressed major histocompatibility complex (MHC) class II antigens. Sciatic axotomy led to a proliferation of satellite cells and an increased expression of MHC class II molecules in particular. This satellite cell reaction started 24 h after axotomy and continued to increase gradually until about 6 weeks after the lesion. Resident macrophages, detected in the DRG interstitial tissue by their expression of monocyte/macrophage markers, also reacted to sciatic axotomy. Our data suggest that (1) sciatic axotomy leads to a rapid microglial reaction in both the ventral and dorsal grey matter of the lumbar spinal cord and in the ipsilateral nucleus gracilis; (2) the immunophenotype of activated microglia following sciatic axotomy is comparable with that observed after axotomy of cranial nerves, e.g. the facial nerve; (3) satellite cells in DRG constitutively express MHC class II molecules; and (4) sciatic axotomy leads to a rapid activation of satellite cells and interstitial macrophages in the axotomized DRG.     

Characterization of microglial reaction after middle cerebral artery occlusion in rat brain

We have studied the microglial reaction that accompanies cortical infarction induced by middle cerebral artery occlusion (MCAO). Lectin histochemistry with the B₄-isolectin from Griffonia simplicifoliaas well as immunocytochemistry with a panel of monoclonal antibodies directed against major histocompatibility complex (MHC) and lymphocytic antigens were performed. Principal attention was focused on neocortical and thalamic regions, representative of primary and secondary ischemic damage, respectively. With the lectin procedure, activated microglial cells were abundant in the neocortex 24 hours after MCAO. In contrast, microglial activation in the thalamus was not apparent until day 2 after MCAO. On day 5, MHC class II antigen was expressed by reactive microglia in fiber tracts traversing the striatum, but was absent from activated microglia in the primary cortical infarction area. MHC class I and lymphocytic antigens were expressed differentially on microglia with class I antigens appearing early and lymphocytic antigens appearing late in the time course after focal ischemia. The findings are compatible with previous studies during global ischemia and confirm the early activation and the progressive nature of immunomolecule expression on activated microglia after an ischemic insult. In addition to neocortical and thalamic sites, our results showed an early microglial activation to be present also in forebrain regions outside of the middle cerebral artery (MCA) territory, such as the contralateral cortex and hippocampus. A unilateral microglial reaction was also detectable after long-term survival (≥4 weeks) in the pyramidal tracts, as well as in the corticospinal tracts at cervical but not lumbar spinal cord levels. Ischemia-induced neuronal damage, as evaluated by Nissl staining, was found only in cortical and thalamic regions. We conclude that the demonstration of reactive microglia indicates not only imminent ischemic neuronal damage within MCA territory but can also delineate extra-focal disturbances, possibly reflecting subtle and transitory changes in neuronal activity. © 1993 Wiley-Liss, Inc.     

Major histocompatibility complex class II expression by activated microglia caudal to lesions of descending tracts in the human spinal cord is not associated with a T cell response

Lesion-induced microglial/macrophage responses were investigated in post-mortem human spinal cord tissue of 20 patients who had died at a range of survival times after spinal trauma or brain infarction. Caudal to the spinal cord injury or brain infarction, a strong increase in the number of activated microglial cells was observed within the denervated intermediate grey matter and ventral horn of patients who died shortly after the insult (4–14 days). These cells were positive for the leucocyte common antigen (LCA) and for the major histocompatibility complex class II antigen (MHC II), with only a small proportion staining for the CD68 antigen. After longer survival times (1–4 months), MHC II-immunoreactivity (MHC II-IR) was clearly reduced in the grey matter but abundant in the white matter, specifically within the degenerating corticospinal tract, co-localising with CD68. In this fibre tract, elevated MHC II-IR and CD68-IR were still detectable 1 year after trauma or stroke. It is likely that the subsequent expression of CD68 on MHC II-positive microglia reflects the conversion to a macrophage phenotype, when cells are phagocytosing degenerating presynaptic terminals in grey matter target regions at early survival times and removing axonal and myelin debris in descending tracts at later survival times. No T or B cell invasion or involvement of co-stimulatory B7 molecules (CD80 and CD86) was observed. It is possible that the up-regulation of MHC II on microglia that lack the expression of B7 molecules may be responsible for the prevention of a T cell response, thus protecting the spinal cord from secondary tissue damage. Received: 12 October 1999 / Revised: 31 January 2000 / Accepted: 8 February 2000     

Granulocyte-macrophage colony stimulating factor inhibits class II major histocompatibility complex expression and antigen presentation by microglia

Granulocyte-macrophage colony stimulating factor (GM-CSF) modulates various functions of monocytes/ macrophages including antigen-presenting capacity. Recently it was found that astrocytes produce GM-CSF in the central nervous system (CNS) and that GM-CSF can induce proliferation and morphological changes of microglia. Here we show that GM-CSF can down regulate the interferon-γ-mediated induction of major histocompatibility complex (MHC) class II antigens in microglia, but not in astrocytes. GM-CSF pretreatment completely prevents myelin basic protein-specific T cell proliferation induced by microglia but not astrocytes. GM-CSF did not affect the cell surface expression on microglia of either MHC class I or cell adhesion molecules. The inhibition of microglial MHC class II expression and antigen-presenting function is specific for GM-CSF, as treatment with a different CSF (interleukin-3) did not modulate microglial phenotype or functional capacity. These data suggest that GM-CSF might be involved in the regulation of immune responses within the central nervous system.     

Expression of glucose transporter 5 by microglia in human gliomas

Our previous studies indicate that glucose transporter 5 (GLUT5) is a microglial marker in routine paraffin sections, and is rarely present in monocytes/macrophages of the peripheral organs. We examined the expression of GLUT5 in 91 cases of human gliomas to characterize the microglial phenotype in glioma tissues. Immunohistochemistry was performed on formalin-fixed, paraffin-embedded sections using such antibodies as a GLUT5 antibody, two markers for activated microglia: major histocompatibility complex (MHC) class II Ag and macrophage scavenger receptor class A (MSR-A), and MIB-1 antibody. The immunoreactivity of GLUT5 was present in three microglial phenotypes: ramified (resting), activated, and ameboid (macrophagic) microglia in most of the cases. A double-labelling study of astrocytic tumours using GLUT5 and MIB-1 antibodies demonstrated a proportion of proliferating microglia. However, no morphological difference between MIB-1-positive, microglial cells and MIB-1-negative, microglial cells was found. The number of GLUT5-positive microglia was significantly (P < 0.001) higher in astrocytic tumours than in oligodendroglial tumours. Many GLUT5-positive microglia (up to 52% in total cells) were often observed in pilocytic astrocytomas, where microglial cells were predominantly ramified, and the number of MHC class II- or MSR-A-positive microglia was less than GLUT5-positive microglia. Thus, the present study indicated that intrinsic microglia can be a source of microglia/macrophages cell populations in astrocytic tumours, and that pilocytic astrocytomas often have a high proportion of microglial cells with mild activation.     

Microglial reaction and neuronal death in the hippocampus of rat models of epilepsy

Reaction of microglial cells as well as DNA fragmentation in pyramidal cells was investigated using immunohisto-chemistry and in situ end-labeling method (TUNEL) in the hippocampus of rats after rapid kindling or kainic acid treatment. In intact rats, no or very little DNA fragmentation was detected in the hippocampus. Resting microglia distributed evenly throughout the hippocampus. Neither major histocompatibility complex antigens class I (MHC I) nor class II (MHC II) immunoreactivity was seen in the hippocampus. In the rapid-kindling model, no DNA fragmentation, reactive microglia or MHC antigen-positive cells were present in the hippocampus. In rats given an intraperitoneal injection of kainic acid (12 mg/kg), reactive microglial cells were seen around pyramidal neurons in the CA1 and CA3 field of the hippocampus as well as in the hilus of the dentate gyrus at 3 h. At that point in time, DNA fragmentation was not detected. DNA fragmentation was clearly observed, mainly in the CA1 region of the hippocampus, from 24 h to 4 weeks after the kainic acid injection. The number of reactive microglia was quickly increased and reached a maximum at 7 days after the injection, and continued until 8 weeks thereafter. During this period, many reactive microglia expressed MHC I and MHC II. The present study indicates that epileptic seizures do not depend on microglial activation and that microglial activation is closely related to the neuronal death process induced by kainic acid.     

Lesion of the rat entorhinal cortex leads to a rapid microglial reaction in the dentate gyrus

Summary Stereotaxic lesioning of the entorhinal cortex leads to an anterograde axonal degeneration in the molecular layer of the dentate gyrus. As revealed by immunocytochemical and histochemical methods, lesion of the entorhinal cortex induced a proliferation of microglia and an increased expression of established microglial activation markers within the deafferented zone. Reactive microglial cells were detected as early as 24 h after the lesion. The microglial reaction showed a maximum around day 3 post-lesion and disappeared by day 8 post-lesion. Reactive microglia were strongly positive for the B₄-isolectin from Griffonia simplicifolia (GSI-B₄), expressed high levels of CR3 complement receptor and 5-nucleotidase, but lacked CD4 and MHC class I and II antigens. In addition, microglial cells were identified using MUC 102, a new monoclonal antibody against rat microglia. At the ultrastructural level, reactive microglial cells were consistently seen to phagocytose degenerating terminals. Our data suggest that (1) axonal degeneration represents a sufficient stimulus for inducing microglial activation and proliferation in the deafferented dentate gyrus; (2) these activated microglial cells are characterized by immunophenotypes different from those observed in other types of CNS injury; (3) the early microglial reaction precedes the well-documented astrocyte reaction in the dentate gyrus; and (4) the timed interaction of microglia and astrocytes could be important for regulating regenerative sprouting processes in the mature CNS.     

Biochemical analysis of proteasomes from mouse microglia: induction of immunoproteasomes by interferon-gamma and lipopolysaccharide

The 20S proteasome is a multicatalytic threonine protease and serves to process peptides that are subsequently presented as antigenic epitopes by MHC class I molecules. In the brain, microglial cells are the major antigen presenting cells and they respond sensitive to pathologic events. We used cultured mouse microglia and a microglial cell line, the BV-2 line, as a model to study the correlation between microglial activation parameters and structural plasticity of the 20S/26S proteasome. Lipopolysaccharide (LPS)- or interferon-gamma (IFN-gamma)-stimulated microglia or BV-2 cells exhibit properties of activated microglia such as high levels of TNFalpha and IL-6 release. In response to IFN-gamma or LPS, three constitutive beta subunits (beta1/Delta, beta2/MC14, beta5/MB1) were replaced by the immunoproteasome subunits ibeta1/LMP2, ibeta2/MECL-1, and ibeta5/LMP7, indicating that activated microglia adapts its proteasomal subunit composition to the requirements of an optimized MHC class I epitope processing. Induction of immunoproteasomes in BV-2 cells was solely provoked by IFN-gamma, but not by LPS. Moreover, LPS (but not IFN-gamma) triggered the expression of a novel protein of approximately 50 kD as part of the proteasome activator PA700, that is the substrate-recognizing and unfolding unit of the 26S proteasome. These results indicate that both the 20S core protease as well as the proteasome activator PA700 are targets of modulatory subunit replacements or transient association of regulatory components in the course of microglial activation.     

The inflammatory reaction following 1-methyl-4-phenyl-1,2,3, 6-tetrahydropyridine intoxication in mouse

In degenerative disorders of the CNS an immune system involvement in the pathological process is postulated. The MPTP model of Parkinson's disease seem to be a good model for studying an inflammation following toxic neurodegeneration. In this model, microglial and astroglial reactions were previously found around impaired neurons. In the present work we showed an immune reaction, including lymphocytic infiltration of CD4+ and CD8+ T cells in the substantia nigra and striatum and elevated MHC class I and II antigens expression on microglia. Many activated lymphocytes were present, showing increased LFA-1 and CD44 antigen expression. We found also that ICAM-1 expression increased on the endothelium and appeared on microglia in the injured regions. Treatment with dexamethasone inhibited T-cell infiltration and MHC class II expression, lessened the glial reaction, and also diminished neuronal impairment. These findings suggest that an immune mechanism may contribute to the neuronal damage following MPTP administration.     

Microglial cell activation in demyelinating canine distemper lesions

Microglia cells are the principal immune effector elements of the brain responding to any pathological event. To elucidate the possible role of microglia in initial non-inflammatory demyelination in canine distemper virus (CDV) infection, microglia from experimentally CDV infected dogs were isolated ex vivo by density gradient centrifugation and characterized immunophenotypically and functionally using flow cytometry. Results from dogs with demyelinating lesions were compared to results from recovered dogs and two healthy controls. CDV antigen could be detected in microglia of dogs with histopathologically confirmed demyelination. Microglia of these dogs showed marked upregulation of the surface molecules CD18, CD11b, CD11c, CD1c, MHC class I and MHC class II and a tendency for increased expression intensity of ICAM-1 (CD54), B7-1 (CD80), B7-2 (CD86), whereas no increased expression was found for CD44 and CD45. Functionally, microglia exhibited distinctly enhanced phagocytosis and generation of reactive oxygen species (ROS). It was concluded that in CDV infection, there is a clear association between microglial activation and demyelination. This strongly suggests that microglia contribute to acute myelin destruction in distemper.     

Inhibition of hippocampal estrogen synthesis by reactive microglia leads to down-regulation of synaptic protein expression

Activation of microglia may facilitate age-related impairment in cognitive functions including hippocampal-dependent memory. Considerable evidence indicates that hippocampal-derived estrogen improves hippocampal-dependent learning and memory. We hypothesize that activated microglia may inhibit de novo hippocampal estrogen synthesis and in turn suppress hippocampal synaptic protein expression. The present study aimed to elucidate the role of lipopolysaccharide (LPS)-activated microglial HAPI cells on estrogen synthesis and expression of synaptic proteins using H19-7 hippocampal neurons with a neuron–microglia co-culture system. LPS induced expression of the microglial activation markers major histocompatibility complex II (MHC II), CD11b, and ionized calcium-binding adapter molecule 1 (Iba1). Prolonged LPS exposure also enhanced the secretion of interleukin (IL)-6 and nitric oxide (NO) from microglial HAPI cells. Exposure to either LPS-activated microglia or IL-6, significantly suppressed the expression of synaptic proteins and the secretion of de novo hippocampal estrogen in H19-7 hippocampal neurons. In addition, LPS-activated microglia also decreased the expression of estrogen receptors (ERα and ERβ) in H19-7 hippocampal neurons. Our findings demonstrate a potential mechanism of microglia activation underlying the reduction in estrogen-mediated signaling on synaptic proteins in hippocampal neurons, which may be involved in hippocampal-dependent memory formation.     

MHC class II-positive microglia in human brain: association with Alzheimer lesions.

Cells of the mononuclear phagocytic system (MPS) present foreign antigen on their cell surfaces bound to major histocompatibility complex (MHC) class II molecules. Previous studies of normal human brain samples reported MHC class II expression primarily by perivascular MPS cells and white matter microglial cells. Marked increases in MHC class II-expressing microglia have been shown in many neuropathologic disorders, including Alzheimer's disease (AD). A close morphologic association between these cells and Alzheimer senile plaque beta-amyloid has been demonstrated. The present study used a mixed aldehyde fixative to enhance the localization of MHC class II-expressing MPS cells in non-AD and AD brain. Two antibodies against MHC class II (HLA-DR; LN3), as well as the lectin Ricinus communis agglutinin (which recognizes both ramified and activated microglia) were used for light and electron microscopic analyses. We now report that MHC class II-expressing ramified microglia are distributed in a uniform reticular array throughout the grey, as well as the white matter in non-AD cases. In AD cases, immunolabelled cells had the morphology of activated microglia, with darkly stained plump somata and short, thick processes. Microglia clustered around senile plaque amyloid and neurofibrillary tangles (NFT), rather than forming the uniform array characteristic of control tissue. Finally, we report that perivascular MPS cells are found in a morphologic relationship with vascular amyloid identical to that seen between microglial cells and senile plaque beta-amyloid. These data suggest that MHC class II-expressing cells may be involved in the degradation of NFT-laden neurons and the posttranslational modification of extracellular-NFT epitopes. In addition, both parenchymal and perivascular MPS cells are ideally situated to uptake and process the beta-amyloid protein precursor and deposit beta-amyloid on senile plaques, NFT, and the cerebrovasculature.     

Immunohistochemical study of microglia in the Creutzfeldt-Jakob diseased brain

Immunohistochemical techniques have been used to investigate microglial reaction in Creutzfeldt-Jakob diseased (CJD) brains. Autopsy cases of six patients with CJD and age-matched controls were studied. Formalin-fixed, paraffin-embedded brain tissue samples were stained with antibodies against major histocompatibility complex (MHC) class II antigen (Ag), leukocyte common antigen (LCA), CDw75, CD68 and glial fibrillary acidic protein. Of the patients with CJD, two with a subacute spongiform encephalopathic type and short-survival periods after onset of the disease showed an increased number of reactive microglia labeled with anti-MHC class II Ag or LCA in the affected cerebral cortex. In advanced cases of the panencephalopathic type of CJD, in which both cerebral atrophy and astrocytosis were marked, the increase of reactive microglia was small. Some vacuoles developing in the neuropil of the CJD patients were surrounded by MHC class II Ag- or LCA-immunoreactive microglial cells. The number of ramified microglia in the affected lesions was decreased, although their number in the hippocampus was not affected. These results indicate that microglia can frequently be involved in the process of CJD and may be activated at the early stage of the disease.     

Control of glial immune function by neurons

The immune status of the central nervous system (CNS) is strictly regulated. In the healthy brain, immune responses are kept to a minimum. In contrast, in a variety of inflammatory and neurodegenerative diseases, including multiple sclerosis, infections, trauma, stroke, neoplasia, and Alzheimer's disease, glial cells such as microglia gain antigen-presenting capacity through the expression of major histocompatibility complex (MHC) molecules. Further, proinflammatory cytokines, such as tumor necrosis factor-alpha (TNF), interleukin-1beta (IL-1beta), and interferon-gamma (IFN-gamma), as well as chemokines, are synthesized by resident brain cells and T lymphocytes invade the affected brain tissue. The proinflammatory cytokines stimulate microglial MHC expression in the lesioned CNS areas only. However, the induction of brain immunity is strongly counterregulated in intact CNS areas. For instance, recent work demonstrated that microglia are kept in a quiescent state in the intact CNS by local interactions between the microglia receptor CD200 and its ligand, which is expressed on neurons. Work done in our laboratory showed that neurons suppressed MHC expression in surrounding glial cells, in particular microglia and astrocytes. This control of MHC expression by neurons was dependent on their electrical activity. In brain tissue with intact neurons, the MHC class II inducibility of microglia and astrocytes by the proinflammatory cytokine IFN-gamma was reduced. Paralysis of neuronal electric activity by neurotoxins restored the induction of MHC molecules on microglia and astrocytes. Loss of neurons or their physiological activity would render the impaired CNS areas recognizable by invading T lymphocytes. Thus, immunity in the CNS is inhibited by the local microenvironment, in particular by physiologically active neurons, to prevent unwanted immune mediated damage of neurons.     

Distinct types of microglial activation in white and grey matter of rat lumbosacral cord after mid-thoracic spinal transection

The inflammatory response has been characterized in the lumbosacral segments (L4-S1) of rats after spinal transection at T8. Immune cells were identified immunohistochemically using antibodies to complement type 3 receptor, CD11b (OX-42), the macrophage lysosomal antigen, CD68 (ED1), major histocompatibility complex class II (MHC II), and CD163 (ED2), a marker of perivascular cells. One week after cord transection, OX-42+ microglial density had nearly doubled. In the white matter, microglia became enlarged, often with retracted processes. In contrast, microglia in the grey matter remained ramified although nearly half of those lying medially contained clusters of ED1+ granules. After 8 weeks, ED1+ (+/-MHC II) macrophages were prominent in regions of Wallerian degeneration extending from dorsolateral to ventral funiculi. Microglial density remained raised in grey matter, particularly in the ventral horns of L4/5. Ramified microglia expressing MHC II+ (+/-ED1) extended from deep in the dorsal columns and around the central canal to the ventral columns. More ED2+ (+/-MHC II) perivascular and meningeal cells were recruited and expressed ED1. Thus, distinct from their conversion into macrophages in the white matter, the activation of ramified microglia after degeneration in the grey matter involves expression of ED1 without morphologic transformation.     

Differential expression of MHC class II molecules by microglia and neoplastic astroglia: relevance for the escape of astrocytoma cells from immune surveillance

There is increasing evidence that microglia serve as antigen presenters in the human CNS. Although the occurrence of MHC class II immunoreactive cells has been reported in astrocytic gliomas, the relative contribution of microglia to this cell population has not been studied in detail. Using computer-assisted image analysis, we have investigated the expression of MHC class II molecules and of the microglia/macrophage markers Ki-M1P, RCA-1, KP1 and iba1 , in 97 astrocytic gliomas comprising all WHO grades to answer the question whether there is a correlation between tumour grade and the number of MHC class II positive microglia/macrophage profiles. Microglia expressing MHC class II were common in astrocytomas and anaplastic astrocytomas but rare in pilocytic tumours although there was significant variation within each group. MHC class II immunoreactivity was reduced in highly cellular areas of glioblastomas where large numbers of cells expressing macrophage markers were still present. Thus, there was no simple relationship between tumour grade and microglial/macrophage MHC class II expression. In addition, up to 55% of astrocytic gliomas contained MHC class II immunoreactive tumour cells. Microglia but not tumour cells were found to express the BB1/B7 costimulator. We conclude that microglia in astrocytic gliomas are well equipped to function as antigen presenting cells. Yet, neoplastic astroglia appear to acquire the capacity to downregulate microglial MHC class II expression and, at the same time, may induce T-cell clonal anergy through aberrant expression of MHC class II molecules.     

Morphological analysis of active microglia--rod and ramified microglia in human brains affected by some neurological diseases (SSPE,Alzheimer's disease and Wilson's disease)

The activation of microglial cells in pathological conditions is manifested primarily by their proliferation, as well as by the occurrence of a new morphological form--rod microglia. In the present study immunohistochemical identification of rod microglial phenotype against ramified microglia was performed on segments of 17 brains derived from 7 cases of encephalitis of viral aetiology (including 5 SSPE cases), 6 cases of Wilson's disease and 4 cases of Alzheimer's disease. Segments from frontal, temporal and occipital lobes, cerebellum and brainstem were subjected to histological, histochemical and immunohistochemical reactions. The presence of activated rod and ramified microglia was observed in sections derived from all structures of the brains under study. Both morphological forms of activated microglia reacted to antibodies: HLA II, CD68, HAM56 and lectin RCA-1. Expression of HLA II molecules was less intensive on the surface of microglial rod cells. A positive reaction to PCNA antibody was mainly observed in rod/elongated/cylinder-shaped nuclei, which is a characteristic feature of rod microglia. In the study material, the localisation of microglial processes seemed to depend rather on the structural topography of the cell in the brain than on the nuclear shape of the activated microglial cell. Our observations revealed a strong similarity between immunohistochemical phenotypes of both morphological forms of microglia with the indication that rod microglia is a first developmental form of activated microglia.     

Peripheral lipopolysaccharide (LPS) challenge promotes microglial hyperactivity in aged mice that is associated with exaggerated induction of both pro-inflammatory IL-1beta and anti-inflammatory IL-10 cytokines

In the elderly, systemic infection is associated with an increased frequency of behavioral and cognitive complications. We have reported that peripheral stimulation of the innate immune system with lipopolysaccharide (LPS) causes an exaggerated neuroinflammatory response and prolonged sickness/depressive-like behaviors in aged BALB/c mice. Therefore, the purpose of this study was to determine the degree to which LPS-induced neuroinflammation was associated with microglia-specific induction of neuroinflammatory mediators. Here, we show that peripheral LPS challenge caused a hyperactive microglial response in the aged brain associated with higher induction of inflammatory IL-1beta and anti-inflammatory IL-10. LPS injection caused a marked induction of mRNA expression of both IL-1beta and IL-10 in the cortex of aged mice compared to adults. In the next set of studies, microglia (CD11b(+)/CD45(low)) were isolated from the brain of adult and aged mice following experimental treatments. An age-dependent increase in major histocompatibility complex (MHC) class II mRNA and protein expression was detected in microglia. Moreover, peripheral LPS injection caused a more pronounced increase in IL-1beta, IL-10, Toll-like receptor (TLR)-2, and indoleamine 2,3-dioxygenase (IDO) mRNA levels in microglia isolated from aged mice than adults. Intracellular cytokine protein detection confirmed that peripheral LPS caused the highest increase in IL-1beta and IL-10 levels in microglia of aged mice. Finally, the most prominent induction of IL-1beta was detected in MHC II(+) microglia from aged mice. Taken together, these findings provide novel evidence that age-associated priming of microglia plays a central role in exaggerated neuroinflammation induced by activation of the peripheral innate immune system.     

Microglial major histocompatibility complex glycoprotein-1 in the axotomized facial motor nucleus: regulation and role of tumor necrosis factor receptors 1 and 2

Presentation of antigen is key to the development of the immune response, mediated by association of antigen with major histocompatibility complex glycoproteins abbreviated as MHC1 and MHC2. In the current study, we examined the regulation of MHC1 in the brain after facial axotomy. The normal facial motor nucleus showed no immunoreactivity for MHC1 (MHC1-IR). Transection of the facial nerve led to a strong and selective up-regulation of MHC1-IR on the microglia in the affected nucleus, beginning at day 2 and reaching a maximum 14 days after axotomy, coinciding with a peak influx of the T lymphocytes that express CD8, the lymphocyte coreceptor for MHC1. Specificity of the MHC1 staining was confirmed in beta2-microglobulin-deficient mice, which lack normal cell surface MHC1-IR. MHC1-IR was particularly strong on phagocytic microglia, induced by delayed neuronal cell death, and correlated with the induction of mRNA for tumor necrosis factor (TNF)-alpha, interleukin (IL)-1beta, and interferon-gamma and the influx of T lymphocytes. Mice with severe combined immunodeficiency (scid), lacking T and B cells, showed an increase in the number of MHC1-positive nodules but no significant effect on overall MHC1-IR. Transgenic deletion of the IL1 receptor type I, or the interferon-gamma receptor type 1 subunit, did not affect the microglial MHC1-IR. However, a combined deletion of TNF receptors 1 and 2 (TNFR1&2-KO) led to a decrease in microglial MHC1-IR and to a striking absence of the phagocytic microglial nodules. Deletion of TNFR2 (p75) did not have an effect; deletion of TNFR1 (p55) reduced the diffuse microglial staining for MHC1-IR but did not abolish the MHC1(+) microglial nodules. In summary, neural injury leads to the induction of MHC1-IR on the activated, phagocytic microglia. This induction of MHC1 precedes the interaction with the immune system, at least in the facial motor nucleus model. Finally, the impaired induction of these molecules, up to now, only in the TNFR-deficient mice underscores the central role of TNF in the immune activation of the injured nervous system.