Reaction Of Microglial Cells Research Articles

Menstrual blood-derived endometrial stem cells inhibit neuroinflammation by regulating microglia through the TLR4/MyD88/NLRP3/Casp1 pathway.

Neuroinflammation is a common response in various neurological disorders. Mesenchymal stem cell-based treatment has become a promising therapy for neuroinflammation-associated diseases. However, the effects of mesenchymal stem cells are controversial, and the underlying mechanism is incompletely understood. In the present study, menstrual blood-derived endometrial stem cells were intravenously transplanted into a mouse model of neuroinflammation established by peripheral injection of lipopolysaccharide. Microglial cells challenged with lipopolysaccharide were cultured with conditioned medium from endometrial stem cells. The levels of cytokines were detected by enzyme-linked immunosorbent assay. Cell proliferation and death were detected by Cell Counting Kit 8 and flow cytometry, respectively. The expression levels of Toll-like receptor 4 (TLR4), myeloid differentiation primary response gene 88 (MyD88), NLR family pyrin domain containing 3 (NLRP3) and caspase 1 (Casp1) were evaluated by western blotting. The results showed that intravenous transplantation of endometrial stem cells downregulated proinflammatory factors and upregulated anti-inflammatory factors in the brain of mice with neuroinflammation. Conditioned medium suppressed the inflammatory reaction and hyperactivation of microglial cells and protected microglial cells from cell death induced by lipopolysaccharide in vitro. The expression of TLR4, MyD88, NLRP3 and Casp1 in the brain of mice with neuroinflammation and in lipopolysaccharide-stimulated microglial cells was downregulated by endometrial stem cells and conditioned medium, respectively. These data suggested that menstrual blood-derived endometrial stem cells may suppress neuroinflammatory reactions partially by regulating microglia through the TLR4/MyD88/NLRP3/Casp1 signalling pathway. Our findings may be very useful for the development of an alternative stem cell-based therapy for neuroinflammation-associated disorders.

Toxicologic Pathology Forum Opinion: Interpretation of Gliosis in the Brain and Spinal Cord Observed During Nonclinical Safety Studies.

Gliosis, defined as a nonneoplastic reaction (hypertrophy and/or proliferation) of astrocytes and/or microglial cells, is a frequent finding in the central nervous system (CNS [brain and/or spinal cord]) in nonclinical safety studies. Gliosis in rodents and nonrodents occurs at low incidence as a spontaneous finding and is induced by various test articles (e.g., biomolecules, cell and gene therapies, small molecules) delivered centrally (i.e., by injection or infusion into cerebrospinal fluid or neural tissue) or systemically. Several CNS gliosis patterns occur in nonclinical species. First, gliosis may accompany degeneration and/or necrosis of cells (mainly neurons) or neural parenchyma (neuron processes and myelin). Second, gliosis often follows inflammation (i.e., leukocyte accumulation causing parenchymal damage) or neoplasm formation. Third, gliosis may appear as variably sized, randomly scattered foci of reactive glial cells in the absence of visible parenchymal damage or inflammation. In interpreting test article-related CNS gliosis, adversity is indicated by parenchymal injury (e.g., degeneration, necrosis, or inflammation) and not the mere existence of a glial reaction. In the absence of clear structural damage to the parenchyma, gliosis as a standalone CNS finding should be interpreted as a nonadverse reaction to regional alterations in microenvironmental conditions rather than as evidence of a glial reaction associated with neurotoxicity.

Siponimod Modulates the Reaction of Microglial Cells to Pro-Inflammatory Stimulation

Siponimod (Mayzent®), a sphingosine 1-phosphate receptor (S1PR) modulator which prevents lymphocyte egress from lymphoid tissues, is approved for the treatment of relapsing-remitting and active secondary progressive multiple sclerosis. It can cross the blood-brain barrier (BBB) and selectively binds to S1PR1 and S1PR5 expressed by several cell populations of the central nervous system (CNS) including microglia. In multiple sclerosis, microglia are a key CNS cell population moving back and forth in a continuum of beneficial and deleterious states. On the one hand, they can contribute to neurorepair by clearing myelin debris, which is a prerequisite for remyelination and neuroprotection. On the other hand, they also participate in autoimmune inflammation and axonal degeneration by producing pro-inflammatory cytokines and molecules. In this study, we demonstrate that siponimod can modulate the microglial reaction to lipopolysaccharide-induced pro-inflammatory activation.

Glial Cell Activation and Oxidative Stress in Retinal Degeneration Induced by β-Alanine Caused Taurine Depletion and Light Exposure.

We investigate glial cell activation and oxidative stress induced by taurine deficiency secondary to β-alanine administration and light exposure. Two months old Sprague-Dawley rats were divided into a control group and three experimental groups that were treated with 3% β-alanine in drinking water (taurine depleted) for two months, light exposed or both. Retinal and external thickness were measured in vivo at baseline and pre-processing with Spectral-Domain Optical Coherence Tomography (SD-OCT). Retinal cryostat cross sections were immunodetected with antibodies against various antigens to investigate microglial and macroglial cell reaction, photoreceptor outer segments, synaptic connections and oxidative stress. Taurine depletion caused a decrease in retinal thickness, shortening of photoreceptor outer segments, microglial cell activation, oxidative stress in the outer and inner nuclear layers and the ganglion cell layer and synaptic loss. These events were also observed in light exposed animals, which in addition showed photoreceptor death and macroglial cell reactivity. Light exposure under taurine depletion further increased glial cell reaction and oxidative stress. Finally, the retinal pigment epithelial cells were Fluorogold labeled and whole mounted, and we document that taurine depletion impairs their phagocytic capacity. We conclude that taurine depletion causes cell damage to various retinal layers including retinal pigment epithelial cells, photoreceptors and retinal ganglion cells, and increases the susceptibility of the photoreceptor outer segments to light damage. Thus, beta-alanine supplements should be used with caution.

Contributive Role of TNF-α to L-DOPA-Induced Dyskinesia in a Unilateral 6-OHDA Lesion Model of Parkinson's Disease.

Our present objective was to better characterize the mechanisms that regulate striatal neuroinflammation in mice developing L-DOPA-induced dyskinesia (LID). For that, we used 6-hydroxydopamine (6-OHDA)-lesioned mice rendered dyskinetic by repeated intraperitoneal injections of 3,4-dihydroxyphenyl-L-alanine (L-DOPA) and quantified ensuing neuroinflammatory changes in the dopamine-denervated dorsal striatum. LID development was associated with a prominent astrocytic response, and a more moderate microglial cell reaction restricted to this striatal area. The glial response was associated with elevations in two pro-inflammatory cytokines, tumor necrosis factor-α (TNF-α) and interleukin-1β. Treatment with the phytocannabinoid cannabidiol and the transient receptor potential vanilloid-1 (TRPV-1) channel antagonist capsazepine diminished LID intensity and decreased TNF-α levels without impacting other inflammation markers. To possibly reproduce the neuroinflammatory component of LID, we exposed astrocyte and microglial cells in culture to candidate molecules that might operate as inflammatory cues during LID development, i.e., L-DOPA, dopamine, or glutamate. Neither L-DOPA nor dopamine produced an inflammatory response in glial cell cultures. However, glutamate enhanced TNF-α secretion and GFAP expression in astrocyte cultures and promoted Iba-1 expression in microglial cultures. Of interest, the antidyskinetic treatment with cannabidiol + capsazepine reduced TNF-α release in glutamate-activated astrocytes. TNF-α, on its own, promoted the synaptic release of glutamate in cortical neuronal cultures, whereas cannabidiol + capsazepine prevented this effect. Therefore, we may assume that the release of TNF-α by glutamate-activated astrocytes may contribute to LID by exacerbating corticostriatal glutamatergic inputs excitability and maintaining astrocytes in an activated state through a self-reinforcing mechanism.

MicroRNA-93/STAT3 signalling pathway mediates retinal microglial activation and protects retinal ganglion cells in an acute ocular hypertension model

Glaucoma is a common neurodegenerative disease and a leading cause of irreversible blindness worldwide. Retinal microglia-mediated neuroinflammation is involved in the process of optic nerve damage, but the mechanisms driving this microglial activation remain mostly elusive. Previous investigations reported that microRNAs are associated with the retinal microglial reaction and neural apoptosis. In the present study, we found that microRNA-93-5p (miR-93) played a key role in the reaction of retinal microglial cells in vivo and in vitro. The miR-93 level was significantly reduced in the retinae of rat acute ocular hypertension (AOH) models, which were accompanied by retinal microglial activation, overproduction of inflammatory cytokines, and subsequent retinal ganglion cells (RGCs) death, versus the retinae of controls. The induction of miR-93 overexpression significantly reduced microglial proliferation, migration and cytokine release, inhibited the expression of the target gene signal transducer and activator of transcription 3 (STAT3) and p-STAT3, and was associated with a reduced loss of RGCs. Treatment with a STAT3 inhibitor also decreased retinal microglial activation after AOH injury. Taken together, these results suggest that the miR-93/STAT3 pathway is directly related to the downregulation of retinal microglia-mediated neuro-inflammation and showed a neuroprotective effect. Regulating microglial activation through miR-93 may serve as a target for neuroprotective therapy in pathological ocular hypertension.

Noni Inhibits Neuronal Damage Caused by the Immune Reaction of Microglial Cells Activated by Doxorubicin

Noni Inhibits Neuronal Damage Caused by the Immune Reaction of Microglial Cells Activated by Doxorubicin

Microglia in neuropathology caused by protozoan parasites.

Involvement of the central nervous system (CNS) is the most severe consequence of some parasitic infections. Protozoal infections comprise a group of diseases that together affect billions of people worldwide and, according to the World Health Organization, are responsible for more than 500000 deaths annually. They include African and American trypanosomiasis, leishmaniasis, malaria, toxoplasmosis, and amoebiasis. Mechanisms underlying invasion of the brain parenchyma by protozoa are not well understood and may depend on parasite nature: a vascular invasion route is most common. Immunosuppression favors parasite invasion into the CNS and therefore the host immune response plays a pivotal role in the development of a neuropathology in these infectious diseases. In the brain, microglia are the resident immune cells active in defense against pathogens that target the CNS. Beside their direct role in innate immunity, they also play a principal role in coordinating the trafficking and recruitment of other immune cells from the periphery to the CNS. Despite their evident involvement in the neuropathology of protozoan infections, little attention has given to microglia-parasite interactions. This review describes the most prominent features of microglial cells and protozoan parasites and summarizes the most recent information regarding the reaction of microglial cells to parasitic infections. We highlight the involvement of the periphery-brain axis and emphasize possible scenarios for microglia-parasite interactions.

Lipofuscin-dependent stimulation of microglial cells.

To examine the reaction of microglial cells (MG) when incubated with lipofuscin (LP) in vitro with emphasis on the immunological reaction of the MG toward LP and the suppression of this reaction by immunomodulatory agents. MG are involved in the pathogenesis of degenerative eye disorders such as age-related macular degeneration (AMD). LP is a heterogeneous waste material that accumulates in the retinal pigment epithelium (RPE) cells with advancing age. LP is known to have toxic effects on RPE cells and therefore an elevated LP-derived fundus autofluorescence is a risk factor for AMD development. MG in the subretinal space have been reported in eyes affected by AMD. Moreover, in senescent mice, subretinal MG were found, which display an autofluorescence that may be derived from LP uptake. In this study, we incubated MG (BV-2 cell line and primary cells from murine brain) in vitro with LP isolated from the human RPE. We observed phagocytosis, studied cell morphologies, and analyzed the cell culture supernatants. We also investigated the effect of the immunomodulatory agents hydrocortisone (HC), minocycline, and the tripeptide TKP. The MG phagocytosed the LP quickly and completely. We detected highly elevated levels of pro-inflammatory cytokines (especially of IL-6, IL-23p19, TNF-α, KC, RANTES, and IL-1α) in the cell culture supernatants. Furthermore, levels of vascular endothelial growth factor (VEGF) were raised in BV-2 cells. Anti-inflammatory agents added to the cell cultures inhibited the inflammatory reaction, in particular hydrocortisone (HC). Minocycline and TKP had less impact on the cytokine release. The interaction of MG and LP could play a role in the development of retinal degeneration by triggering an inflammatory reaction and angiogenesis.

Microglial cell reaction after optic nerve lesions

AbstractMicroglial cells may be involved in the degeneration and regeneration processes of Retinal Ganglion Cells (RGCs). Optic nerve (ON) lesions induce microglial cell activation both in goldfish (an animal with important regeneration capabilities; Salvador‐Silva et al., 2000) and rodents (animals with limited regenerating capabilities; Sobrado‐Calvo et al., 2007; Galindo‐Romero et al., 2013). The microglial cell reaction after an ON lesion is common to all the animal models studied and typically includes: increased enzyme activities, morphologic changes and cell hyperplasia. Because these reactions are found in regenerating and non‐regenerating animal models, they may represent a general response of the microglia that is directed towards helping the regeneration process. However, the location, severity and timing of the reaction varies depending on the animal model, amount of RGC death and type of lesion used and on other factors such as whether other eye lesions, trophic or other factors are administered concomitantly. Therefore the microglial cell reaction after an ON lesion can be modified externally and may subsequently influence the degeneration and regeneration processes.

Locus ceruleus controls Alzheimer's disease pathology by modulating microglial functions through norepinephrine

Locus ceruleus (LC)-supplied norepinephrine (NE) suppresses neuroinflammation in the brain. To elucidate the effect of LC degeneration and subsequent NE deficiency on Alzheimer's disease pathology, we evaluated NE effects on microglial key functions. NE stimulation of mouse microglia suppressed Abeta-induced cytokine and chemokine production and increased microglial migration and phagocytosis of Abeta. Induced degeneration of the locus ceruleus increased expression of inflammatory mediators in APP-transgenic mice and resulted in elevated Abeta deposition. In vivo laser microscopy confirmed a reduced recruitment of microglia to Abeta plaque sites and impaired microglial Abeta phagocytosis in NE-depleted APP-transgenic mice. Supplying the mice the norepinephrine precursor L-threo-DOPS restored microglial functions in NE-depleted mice. This indicates that decrease of NE in locus ceruleus projection areas facilitates the inflammatory reaction of microglial cells in AD and impairs microglial migration and phagocytosis, thereby contributing to reduced Abeta clearance. Consequently, therapies targeting microglial phagocytosis should be tested under NE depletion.

Brain damage in experimental neonatal hydrocephalus: correlations between diffusion tensor imaging and cytopathology

Materials and methods Obstructive hydrocephalus was induced by intracisternal injection of kaolin on postnatal day 1 (P1); age-matched control animals were either intact or received intracisternal injections of saline. Six animals (3 hydrocephalics and 3 intact controls) were imaged in vivo at 8-9 days of age (P8-9) and then sacrificed by cardiac perfusion of paraformaldehyde. Four animals (2 hydrocephalics and 2 saline controls) were sacrificed at postnatal day 21 (P21) and paraformaldehyde-fixed brains were imaged ex vivo with a Bruker 7 T MRI scanner. DTI was acquired in 6 directions to obtain measurements of the directionality of water diffusion through tissue (Fractional Anisotropy, FA; 0 = isotropic where water can move freely in any direction; 1 = anisotropic where water can move in only one direction) and the magnitude of diffusivity of water in tissue (Mean Diffusivity, MD). Values were computed bilaterally in the genu of the corpus callosum (gCC), external capsule (EC), internal capsule (IC), and cortical gray matter (Ctx). Results At P8-9 ventriculomegaly was severe and by P21 had progressed to where the periventricular white matter and internal capsule were so thin that DTI could only be reliably performed ex vivo. At both times FA values were significantly reduced in the gCC (p < 0.001) but not the EC, IC or Ctx. There was a trend toward higher FA values at P21 than at P8/9 for both hydrocephalic rats and normal controls. In contrast, MD values increased only in the EC. Conspicuous gliosis was prevalent in all structures examined but differed depending on cell type. Younger animals showed a more robust reaction of microglial cells compared to 21-day old hydrocephalics. Astrocytes exhibited the opposite pattern with a more robust reaction in the older animals.

Down‐regulation of microglia and NG2‐positive cells reaction in trimethyltin‐injured hippocampus of rats treated with myelin basic protein‐reactive T cells: Possible contribution to the neuroprotective effect of T cells

In our previous investigations, we demonstrated that CD4(+) antimyelin basic protein (MBP) T cells protect hippocampal neurons against trimethyltin-induced damage. We hypothesized involvement of T cells, interacting with the various glial populations activated during the neurodegeneration process. In this study, we employ immunocytochemical methods to investigate the influence of administration of T cells on the response of microglia and of NG2(+) cells to trimethyltin (TMT)-induced damage. Female Lewis rats were treated with anti-MBP CD4(+) T cells (4 million per animal, i.v) 24 hr after TMT (8 mg/kg, i.p) intoxication. TMT caused degeneration of CA4 hipppocampal neurons and evoked an abundant reaction of microglial and NG2(+) cells in the injured region. The cells changed morphology into the activated state, and the number of OX42(+) and NG2(+) cells increased about 4.5-fold and 3-fold, respectively, relative to controls as assessed on day 21 after TMT treatment. Additionally, the cells of ameboid morphology, which expressed NG2 or microglial antigens, appeared in the zone of neurodegeneration. Furthermore, certain cells of ameboid phenotype shared both antigens. In rats treated with T cells, down-regulation of the activation of both glial classes and reduction of formation of their ameboid forms was observed. The number of the total OX42(+) and NG2(+) cells decreased by 21% and 54%, respectively, and the number of their ameboid forms decreased by 46% and 73%, respectively. Our data suggest that the diminished activation of microglia and NG2(+) cells, particularly the reduced number of their ameboid forms, may contribute to the neuroprotective effect of T cells.

Norepinephrine induces a samaritan phenotype of microglia in an Alzheimer disease mouse model

Loss of locus ceruleus (LC) neurons, degeneration of noradrenergic projections and decrease of norepinephrine (NE) in projection areas represent an early feature of Alzheimer's disease (AD). To elucidate the effect of NE loss on AD pathology, we evaluated NE effects on microglial functions including cytokine generation, migration and phagocytosis in vitro and in vivo. NE stimulation abolished amyloid beta (Aß) induced microglia chemokine and cytokine secretion, but increased microglial migration and phagocytosis of fibrillar Aß in a concentration dependent manner. Similary, DSP4 induced LC degeneration and NE depletion increased mRNA and protein levels of cytokines and inflammatory enzyme systems in APP transgenic mice. NE depleted APPV717I transgenic mice showed more Aß 1–40 and 1–42 deposits within the hippocampus and frontal cortex compared to APP transgenic mice with an intact NE innervation. Confocal analysis of microglial cells and Aß revealed that the number of colabelled cells was higher in control compared to NE depleted APPV717I transgenic mice, suggesting that NE levels modulate microglial phagocytosis in vivo. These data indicate that early LC degeneration and concomitant decrease of NE concentration in LC projection areas such as the neocortex and hippocampus facilitate the inflammatory reaction of microglial cells in the AD brain. At the same time, loss of NE innervation impairs key microglial functions such as migration and phagocytosis and thus contribute to reduced Aß clearance in AD. Therefore, therapies targeting microglial phagocytosis should be tested under NE depletion.

Differential activation of astrocytes and microglia during post-natal development of dopaminergic neuronal death in the weaver mouse

In order to understand the relationship between astrocytes, microglia and injured neurons, we studied the weaver mutant mouse. One of the main characteristics of this mutant is the progressive degeneration of the dopaminergic (DA) nigrostriatal pathway that starts around postnatal day 15 (P15), in the substantia nigra pars compacta (SNpc) and progresses until adult age (P60). In the present paper, we analysed the relationship between astroglial and microglial cells within DA neurons in the nigrostriatal system of homozygous weaver mice, at different postnatal ages corresponding to specific stages of the DA neuronal loss. The activation of astrocytes was found to be an early event in weaver DA denervation, appearing massively at the onset of DA neuronal loss in the SNpc at P15. Astrocytes remained activated in the adult brain even after the slowing down of the neuronal death process. Interestingly, in the ventral tegmental area, where no DA neuronal death could be detected, a profound, permanent astrogliosis was also observed in adult animals. In contrast, an activation of microglial cells was transiently observed in the SNpc but only at the postnatal age when maximal neuronal death was observed (P30). Lastly, in the striatum, where there was a massive loss of DA nerve terminals, neither astrogliosis nor microglial activation was detected. Hence, the reaction of astrocytes and microglial cells to progressive and spontaneous DA neuronal death showed different temporal kinetics, suggesting a different role for these two cell types in the DA neurodegenerative process in the weaver mouse.

Immobilized amyloid precursor protein constructs: a tool for the in vitro screening of glial cell reactivity.

Astrocytes and microglia are closely associated with amyloid plaques in Alzheimer's disease (AD). Microglia constitute the first barrier surrounding plaques, although they seem to be unable to remove them efficiently. We evaluated the reaction of microglial cells from neonatal rats and mice to plaque mimetics. The C-terminal part of the amyloid precursor protein (APP) or amyloid peptide (A beta) was immobilized to either 60-microm or 2.8-microm beads and incubated with microglial cells. Beads of 60 microm, having approximately the size of senile plaques, were not phagocytosed, in contrast to 2.8-microm beads, which were phagocytosed by microglia but not by astrocytes. Once taken up by the cells, proteins immobilized to the beads were degraded rapidly, as confirmed by mass spectrometry and immunofluorescence with an antibody against beta-amyloid. On the other hand, no protein degradation was observed with 60-microm beads. Also, probably as a reaction to its incapability to phagocytose the beads, glia organized around the beads and started to proliferate. Cell proliferation was more pronounced when the beads contained the A beta epitope compared with the beads with an inert surface. This in vitro effect could be exploited to set up a screening assay for compounds that ameliorate the adverse reaction of microglia supposed to contribute to the pathogenesis of AD.

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.

A quantitative analysis of the glial cell reaction in primary sensory termination areas following sciatic nerve injury and treatment with nerve growth factor in the adult rat

The time course of the astroglial cell reaction in the nucleus gracilis and the spinal cord dorsal horn was examined following sciatic nerve transection in the adult rat with qualitative and quantitative analysis of glial fibrillary acidic protein immunoreactivity and in situ hybridization for its mRNA. In addition, the potential effect of exogenous nerve growth factor (NGF) was examined on the astroglial and microglial cells in the spinal cord dorsal horn at certain time points following sciatic nerve transection. An increase in glial fibrillary acidic protein immunoreactivity as well as mRNA labelling was observed from 1 day after lesioning, with a peak at about 1 week and 2 days after lesioning, respectively, followed by a decline. However, NGF application during 1, 2 and 4 weeks following nerve transection did not result in any significantly reduced astroglial or microglial activity. Our results show that the astroglial cell response in the nucleus gracilis and the spinal cord dorsal horn is rapid in comparison with previously described central degenerative changes following peripheral nerve lesions (transganglionic degeneration), that the astroglial cell reaction develops concomitantly with the microglial cell reaction previously described and that the "signal" from the axotomized neurons which induces these reactions can not be prevented by exogenous NGF applied to the peripheral nerve.

Adhesion molecule expression on phagocytic microglial cells following anterograde degeneration of perforant path axons.

Entorhinal cortex lesion (ECL) leads to anterograde degeneration of perforant path axons and is known to induce a rapid and intense reaction of astrocytes and microglial cells in the deafferented dentate gyrus. Phagocytosis of degenerating axons involves the establishment and maintenance of cell-matrix and cell-cell interactions by activated glial cells. It was thus our aim to investigate whether the process of axon phagocytosis is accompanied by the expression of adhesion molecules on activated microglial cells or reactive astrocytes, as such molecules mediate bot cell-matrix and cell-cell interactions. We found that the integrin adhesion molecules leukocyte function antigen-1 (LFA-1), very late antigen-4 (VLA-4), and the ligand for LFA-1, intercellular adhesion molecule-1 (ICAM-1), were expressed on microglial cells accumulating in the outer molecular layer of the deafferented dentate gyrus. This upregulation of adhesion molecule expression on microglial cells showing morphological criteria of activation occurred rapidly following ECL, reached its peak at 3 days post lesion (dpl), and gradually returned to control levels after 9 dpl. Astrocytes were never labeled by antibodies directed against these adhesion molecules. Prelabeling of the perforant path with a fluorescent tracer and subsequent ECL led to phagocytosis of fluorescent-labeled axonal debris by cells that were located in the outer molecular layer and showed typical microglial morphology. Double-fluorescence labeling demonstrated that microglial cells engaged in the phagocytosis of axonal debris expressed LFA-1, VLA-4, and the LFA-1-ligand ICAM-1. In conclusion, our results demonstrate that anterograde degeneration of perforant path axons results in adhesion molecule expression on activated microglial cells engaged in axon phagocytosis. The expression of such molecules could represent a mechanism that retains activated microglia in areas of axonal degeneration and perhaps enables the interaction of microglial cells with each other or with other immunocompetent cells.

PERIPHERAL ADMINISTRATION OF LIPOPOLYSACCHARIDE INDUCES ACTIVATION OF MICROGLIAL CELLS IN RAT BRAIN

Using immunocytochemistry with monoclonal antibodies against surface immunomolecules and Griffonia simplicifolia lectin histochemistry, the microglial cell reaction in rat brain was studied after intravenous injection of lipopolysaccharide (LPS). Activation of microglial cells throughout the brain became apparent within hours and peaked at 8–24 h following administration of 1, 2.5 and 5 mg/kg LPS. High doses of LPS (2.5 and 5 mg/kg) induced a morphological transition of resting ramified microglia to round, macrophage-like cells in the anterior hypothalamus, thalamus and the brainstem. After injection of 1 mg/kg LPS, this morphological transition was only detected in the brainstem. Microglial cell reactivity gradually returned to control levels within 7 days after LPS administration. Furthermore, LPS induced enhanced expression of MHC class II by microglial cells. Maximal up-regulation of MHC class II Ia-antigen was found 3 days following injection of LPS, and only a few highly Ia immunoreactive cells were detectable 7 days following injection of LPS. Despite the presence of highly activated microglial cells in the rat brain, no signs of tissue damage were observed at any time point after injection of LPS examined. In addition to the activation of microglial cells, intravenous injection of LPS induced accumulations of macrophages in blood vessels of the choroid plexus and the brain, but no disruption of vessels with subsequent invasion of parenchyma by blood macrophages was detected. Our data demonstrate that a peripheral immune challenge leads to a high and transitory activation of microglial cells in the brain which could possibly contribute to the pathology of infections and septic shock. Copyright © 1996 Elsevier Science Ltd.