Macrophages in the central nervous system of the rat

Early postnatal microglial ablation in the Ccdc39 mouse model reveals adverse effects on brain development and in neonatal hydrocephalus

BACKGROUND

Neonatal hydrocephalus is a congenital abnormality resulting in an inflammatory response and microglial cell activation both clinically and in animal models. Previously, we reported a mutation in a motile cilia gene, Ccdc39 that develops neonatal progressive hydrocephalus (prh) with inflammatory microglia. We discovered significantly increased amoeboid-shaped activated microglia in periventricular white matter edema, reduced mature homeostatic microglia in grey matter, and reduced myelination in the prh model. Recently, the role of microglia in animal models of adult brain disorders was examined using cell type-specific ablation by colony-stimulating factor-1 receptor (CSF1R) inhibitor, however, little information exists regarding the role of microglia in neonatal brain disorders such as hydrocephalus. Therefore, we aim to see if ablating pro-inflammatory microglia, and thus suppressing the inflammatory response, in a neonatal hydrocephalic mouse line could have beneficial effects.

METHODS

In this study, Plexxikon 5622 (PLX5622), a CSF1R inhibitor, was subcutaneously administered to wild-type (WT) and prh mutant mice daily from postnatal day (P) 3 to P7. MRI-estimated brain volume was compared with untreated WT and prh mutants P7-9 and immunohistochemistry of the brain sections was performed at P8 and P18-21.

RESULTS

PLX5622 injections successfully ablated IBA1-positive microglia in both the WT and prh mutants at P8. Of the microglia that are resistant to PLX5622 treatment, there was a higher percentage of amoeboid-shaped microglia, identified by morphology with retracted processes. In PLX-treated prh mutants, there was increased ventriculomegaly and no change in the total brain volume was observed. Also, the PLX5622 treatment significantly reduced myelination in WT mice at P8, although this was recovered after full microglia repopulation by P20. Microglia repopulation in the mutants worsened hypomyelination at P20.

CONCLUSIONS

Microglia ablation in the neonatal hydrocephalic brain does not improve white matter edema, and actually worsens ventricular enlargement and hypomyelination, suggesting critical functions of homeostatic ramified microglia to better improve brain development with neonatal hydrocephalus. Future studies with detailed examination of microglial development and status may provide a clarification of the need for microglia in neonatal brain development.

Effects of Microglia on Neurogenesis

This review summarizes and organizes the literature concerning the effects of microglia on neurogenesis, particularly focusing on the subgranular zone (SGZ) of the hippocampus and subventricular zone (SVZ) of the lateral ventricles, in which the neurogenic potential is progressively restricted during the life of the organism. A comparison of microglial roles in neurogenesis in these two regions indicates that microglia regulate neurogenesis in a temporally and spatially specific manner. Microglia may also sense signals from the surrounding environment and have regulatory effects on neurogenesis. We speculate microglia function as a hub for the information obtained from the inner and outer brain regions for regulating neurogenesis.

Regulation of postnatal forebrain amoeboid microglial cell proliferation and development by the transcription factor Runx1

Microglia are the immune cells of the nervous system, where they act as resident macrophages during inflammatory events underlying many neuropathological conditions. Microglia derive from primitive myeloid precursors that colonize the nervous system during embryonic development. In the postnatal brain, microglia are initially mitotic, rounded in shape (amoeboid), and phagocytically active. As brain development proceeds, they gradually undergo a transition to a surveillant nonphagocytic state characterized by a highly branched (ramified) morphology. This ramification process is almost recapitulated in reverse during the process of microglia activation in the adult brain, when surveillant microglia undergo a ramified-to-amoeboid morphological transformation and become phagocytic in response to injury or disease. Little is known about the mechanisms controlling amoeboid microglial cell proliferation, activation, and ramification during brain development, despite the critical role of these processes in the establishment of the adult microglia pool and their relevance to microglia activation in the adult brain. Here we show that the mouse transcription factor Runx1, a key regulator of myeloid cell proliferation and differentiation, is expressed in forebrain amoeboid microglia during the first two postnatal weeks. Runx1 expression is then downregulated in ramified microglia. Runx1 inhibits mouse amoeboid microglia proliferation and promotes progression to the ramified state. We show further that Runx1 expression is upregulated in microglia following nerve injury in the adult mouse nervous system. These findings provide insight into the regulation of postnatal microglia activation and maturation to the ramified state and have implications for microglia biology in the developing and injured brain.

Challenges towards MR imaging of the peripheral inflammatory response in the subacute and chronic stages of transient focal ischemia

Intravenous administration of iron oxide nanoparticles after experimental stroke has been shown to produce focal signal intensity changes in the ischemic boundary on MRI images. These changes have been attributed to the influx of iron-laden blood-borne macrophages, although it has been suggested that this effect might not always be completely specific to inflammatory cells. The aim of the present study was to investigate this phenomenon in a subacute time frame that is more relevant to the peripheral inflammatory response. Imaging experiments (T(2) -, T(2)*- and T(1) -weighted sequences) were acquired in Wistar rats 6 days after transient middle cerebral artery occlusion (MCAO). Animals were intravenously infused with different doses of ultrasmall superparamagnetic iron oxide (USPIO) nanoparticles (300, 600 or 1000 µmol Fe/kg), or saline and gadolinium, and imaged again 24 h later. Tissue was immediately processed for immunohistochemistry with the macrophage marker ED-1, in combination with Prussian blue for iron. Ischemic tissue exhibited a large increase in T(2) values, and overall contrast enhancement was apparent in the brain and surrounding muscle. In contrast with previous reports, there were no regions of focal signal intensity changes in the ischemic territory in any of the images, although a region of interest analysis revealed a trend towards iron accumulation in the ischemic hemisphere, particularly in the cortex of T(2)* images. However, histological examination revealed that, despite extensive ED-1-positive macrophage accumulation in the entire ischemic territory, none of these cells were Prussian blue positive, except in the meninges of one animal that received a high dose of USPIO nanoparticles. These results imply that the observed trend is a result of the presence of contrast agent in the blood, or meninges, and not iron-containing inflammatory cells.

Dual mode of signalling of the axotomy reaction: retrograde electric stimulation or block of retrograde transport differently mimic the reaction of motoneurons to nerve transection in the rat brainstem

Axotomy of a peripheral nerve causes a complex central response of neuronal perikarya, astroglia and microglia. The signal initiating this axotomy reaction is currently explained either by deprivation of target-derived trophic factors after interruption of transport (trophic hypothesis) or by electrophysiological disturbances of the axotomized neurons (electric hypothesis). In 108 adult Wistar rats we have compared the time course and intensity of the axotomy reaction in the hypoglossal nucleus after (1) resection of the nerve (permanent axotomy), (2) one-time electric stimulation (intact nerve, brief transient electric disturbance), and (3) colchicine block of transport (intact nerve, prolonged transient loss of trophic factors). Nerve resection activated microglia at 2-35 days post-operation (dpo), elevated GFAP in astrocytes at 3-35 dpo and increased CGRP in motoneurons at 2-15 dpo. Fluorogold prelabeling revealed neurophagocytosis and 25% neuron loss at 25 dpo. Colchicine block similarly activated microglia at 5-35 dpo, elevated GFAP at 7-35 dpo and upregulated CGRP at 7-25 dpo. Neurophagocytosis and 15% motoneuron loss were evident at 25 dpo. Electric stimulation (15 min, 4 Hz, 0.1 msec impulse, 2 mAmp) of the intact nerve activated microglia at 1-10 dpo, elevated astroglial GFAP-expression at 7-35 dpo, and upregulated CGRP at 1-10 dpo, but no neuron death and neurophagocytosis were detected. Hence electric stimulation elicited a faster, shorter-lasting response, but transport block as well as axotomy a slower, longer-lasting response. This suggests a dual mode of signaling: Onset and early phase of the axotomy reaction are triggered by electric disturbances, late phase and neuron death by deprivation of trophic factors.

Microglial distribution and apoptosis in fetal rat brain

By histochemical and immunocytochemical techniques, this study aimed to determine the possible involvement of apoptosis in regulating the microglial distribution in fetal rat brain. While microglial cells were labeled with the isolectin Griffonia simplicifolia (GSA I-B4), apoptotic cells were detected by using terminal transferase-mediated dUTP nick end-labeling (TUNEL). TUNEL-labeled cells occurred mainly in the dorsal midline along its rostral-caudal axis of the brain where lectin-labeled microglia were also observed. Occasional TUNEL-labeled cells were observed in the intermediate zone lateral to the striatum (IZS) where lectin-labeled microglia were common from embryonic day 16 (E16) onwards. Some of lectin-labeled microglia showing different morphological forms ingested TUNEL-labeled bodies. In contrast, lectin-labeled microglia showing signs of apoptosis appeared to be lacking. These results clearly demonstrated that lectin-labeled microglia were distributed in areas with and without the occurrence of a large concentration of TUNEL-labeled cells. Our studies suggest that microglia in fetal rat brain will undergo differentiation and activation rather than apoptotic death to govern their population.

Development of Neurons and Glial Cells in Cerebral Cortex, Cultured in the Presence or Absence of Bioelectric Activity: Morphological Observations

Chronic blockade of bioelectric activity (BEA) has been shown to increase neuronal cell death in tissue culture, but the effects of this treatment on non-neuronal cells have not been investigated. To determine which cell types are affected by chronic suppression of BEA, we investigated their morphological development in primary cultures of rat cerebral cortex, grown with or without the sodium channel blocker tetrodotoxin (TTX). Morphological development was monitored by phase-contrast microscopy and by immunofluorescent staining of markers specific for neurons (NSE, MAP2, B-50, and the 200 kD neurofilament protein), astrocytes (GFAP), oligodendrocytes (galactocerebroside), macrophages (ED-1) and fibroblasts (fibronectin). Neurons in control cultures steadily increased in size and elaborated a dense network of axons and dendrites during the first 3 weeks. Astrocytes proliferated strongly and formed a 'bottom-layer' on which other cells grew. Part of the astrocytes migrated into the peripheral area of the culture, but retracted to the centre after 14 days in vitro (DIV). Oligodendrocytes and macrophages also increased in number, but oligodendrocytes were completely lost by 28 DIV. After 3 weeks, axons that had grown into the periphery of the culture gradually retracted and/or degenerated, following the retracting astrocytes. Some of the neurons died after 21 DIV, but a large part persisted until 42 DIV. Upon TTX treatment from 5/6 DIV, cultures with few macrophages showed an increase in the proportion of necrotic nuclei at 14 and 21 DIV. The retraction of peripherally located fibres was accelerated by 3 - 4 days and their degeneration was augmented. Neuronal density decreased to zero between 21 and 42 DIV. Astrocytes showed a clear decrease in density from 28 DIV. Conversely, the density of macrophages was increased about two-fold from 14 DIV. These results indicate that both neurons and glia are affected by chronic TTX treatment.

Cytokines, chemokines, and cytokine receptors in human microglia

Enriched populations of human microglial cells were isolated from mixed cell cultures prepared from embryonic human telencephalon tissues. Human microglial cells exhibited cell type-specific antigens for macrophage-microglia lineage cells including CD11b (Mac-1), CD68, B7-2 (CD86), HLA-ABC, HLA-DR and ricinus communis aggulutinin lectin-1 (RCA-1), and actively phagocytosed latex beads. Gene expression and protein production of cytokines, chemokines and cytokine/chemokine receptors were investigated in the purified populations of human microglia. Normal unstimulated human microglia expressed constitutively mRNA transcripts for interleukin- 1beta (IL-1beta) -6, -8, -10, -12, -15, tumor necrosis factor-alpha (TNF-alpha), macrophage inflammatory protein-1alpha (MIP-1alpha), MIP-1beta, and monocyte chemoattractant protein-1 (MCP-1), while treatment with lipopolysaccharide (LPS) or amyloid beta peptides (Abeta) led to increased expression of mRNA levels of IL-8, IL-10, IL-12, TNF-alpha, MIP-1alpha, MIP-1beta, and MCP-1. Human microglia, in addition, expressed mRNA transcripts for IL-1RI, IL-1RII, IL-5R, IL-6R, IL-8R, IL-9R, IL-10R, IL-12R, IL-13R, and IL-15R. Enzyme-linked immunosorbent assays (ELISA) showed increased protein levels in culture media of IL-1beta, IL-8, TNF-alpha, and MIP-1alpha in human microglia following treatment with LPS or Abeta. Increased TNF-alpha release from human microglia following LPS treatment was completely inhibited with IL-10 pretreatment, but not with IL-6, IL-9, IL-12, IL-13, or transforming growth factor-beta (TGF-beta). Present results should help in understanding the basic microglial biology, but also the pathophysiology of activated microglia in neurological diseases such as Alzheimer disease, Parkinson disease, Huntington disease, amyotrophic lateral sclerosis, stroke, and neurotrauma.

Generation and characterization of immortalized human microglial cell lines: expression of cytokines and chemokines

Microglia are a major glial component of the central nervous system (CNS), play a critical role as resident immunocompetent and phagocytic cells in the CNS, and serve as scavenger cells in the event of infection, inflammation, trauma, ischemia, and neurodegeneration in the CNS. Studies of human microglia have been hampered by the difficulty of obtaining sufficient numbers of human microglia. One way to circumvent this difficulty is to establish permanent cell lines of human microglia. In the present study we report the generation of immortalized human microglial cell line, HMO6, from human embryonic telencephalon tissue using a retroviral vector encoding myc oncogene. The HMO6 cells exhibited cell type-specific antigens for microglia-macrophage lineage cells including CD11b (Mac-1), CD68, CD86 (B7-2), HLA-ABC, HLA-DR, and ricinus communis agglutinin lectin-1 (RCA), and actively phagocytosed latex beads. In addition, HMO6 cells showed ATP-induced responses similar to human primary microglia in Ca2+ influx spectroscopy. Both human primary microglia and HMO6 cells showed the similar cytokine gene expression in IL-1beta, IL-6, IL-8, IL-10, IL-12, IL-15, and TNF-alpha. Using HMO6 cells, we investigated whether activation was induced by Amyloid-beta fragments or lipopolysaccharide (LPS). Treatment of HMO6 cells with Amyloid-beta 25-35 fragment (Abeta(25-35)) or Amyloid-beta 1-42 fragment (Abeta(1-42)) led to increased expression of mRNA levels of cytokine/chemokine IL-8, IL-10, IL-12, MIP-1beta MIP-1, and MCP-1, and treatment with LPS produced same results. Expression of TNF-alpha and MIP1-alpha was not detected in unstimulated HMO6 cells, but their expression was later induced by long-term exposure to Abeta(25-35) or Abeta(1-42.) ELISA assays of spent culture media showed increased protein levels of TNF-alpha and IL-8 in HMO6 cells following treatment with Abeta(25-35) or LPS. Taken together, our results demonstrate that treatment of human primary microglia and HMO6 immortalized human microglia cell line with Abeta(25-35), Abeta(1-42) and LPS upregulate gene expression and protein production of proinflammatory cytokines and chemokines in these cells. The human microglial cell line HMO6 exhibits similar properties to those documented in human microglia and should have considerable utility as an in vitro model for the studies of human microglia in health and disease.

Colonisation of the developing human brain and spinal cord by microglia: a review

Microglia are the immune effector cells of the nervous system. The prevailing view is that microglia are derived from circulating precursors in the blood, which originate from the bone-marrow. Colonisation of the central nervous system (CNS) by microglia is an orchestrated response during human fetal development related to the maturation of the nervous system. It coincides with vascularisation, formation of radial glia, neuronal migration and myelination primarily in the 4th-5th months and beyond. Microglial influx generally conforms to a route following white matter tracts to gray areas. We have observed that colonisation of the spinal cord begins around 9 weeks, with the major influx and distribution of microglia commencing around 16 weeks. In the cerebrum, colonisation is in progress during the second trimester, and ramified microglial forms are widely distributed within the intermediate zone by the first half of intra-uterine life (20-22 weeks). A distinct pattern of migration occurs along radial glia, white matter tracts and vasculature. The distribution of these cells is likely to be co-ordinated by spatially and temporally regulated, anatomical expression of chemokines including RANTES and MCP-1 in the cortex; by ICAM-2 and PECAM on radiating cerebral vessels and on capillaries within the germinal layer, and apoptotic cell death overlying this region. The phenotype and functional characteristics of fetal microglia are also outlined in this review. The need for specific cellular interactions and targeting is greater within the central nervous system than in other tissues. In this respect, microglia may additionally contribute towards CNS histogenesis.

Microglia in the human fetal spinal cord--patterns of distribution, morphology and phenotype

Microglia, the intrinsic macrophages of the nervous system, colonise the cerebrum around the second trimester in man. In order to determine the extent of microglial influx into the nervous system, we have examined their distribution within the human fetal spinal cord in relation to astrocytic and vascular development between 9 and 16 weeks of gestation, using conventional immunohistochemistry [CD11b; CD45; CD64; CD68; ICAM-1; ICAM-2; VCAM-1; PECAM; GFAP; vimentin] and lectin histochemistry [RCA-1]. Microglia are identifiable by 9 weeks, within the ventricular/sub-ventricular zones. Human fetal microglia display heterogeneity in phenotype and are more readily identified by CD68 in the spinal cord. There is a marked influx of cells dorsal and ventral to the neural cavity, from the marginal layer [meninges/connective tissue] with advancing gestational age, with greatest cell densities towards the end of the time period in this study. This inward migration is associated with progressive vascularisation, ICAM-2 expression and co-localises with GFAP and vimentin positive radial glia. The patterns of microglial migration in human fetal cord differ from that within the cerebrum, but generally conform to a route following white to gray matter.

In vitro differentiation of neural progenitor cells from prenatal rat brain: common cell surface glycoprotein on three glial cell subsets

Glial progenitor cell differentiation and cell lineage relationships during brain development are complex hierarchical processes depending on genetic programming, cell-cell interactions, and microenvironmental factors. The identification of precursor cell-specific antigens provides a tool for the study of both normal development and deviations from lineage-specific differentiation associated with malignant transformation. Monoclonal antibody (mAb) RB13-6 recognizes a 130-kDa cell surface glycoprotein (gp130RB13-6) expressed by a subset of 9OAcGD3-positive glial precursor cells scattered in the rat neuroepithelium on prenatal day (PRD) 13. During prenatal development the fraction of gp130RB13-6-positive fetal brain cells (FBC) decreased from about 18% (PRD 14) to about 1.5% (PRD 22), coinciding with increasing fractions of more mature cell types, as indicated by the elevated expression of p24RB21-15, another cell surface determinant specified by mAb RB21-15 (Kindler-Röhrborn et al.; Differentiation 30:53-60, 1985) and other neural cell type-specific markers. Accordingly, gp130RB13-6 positive precursor cells were localized in the ventricular zones throughout brain development. Concomitant with their formation and in the adult rat brain, ependymal layers lining the ventricular surface, choroid plexus, and the leptomeninges were intensely labeled by anti-gp130RB13-6 mAb. As visualized by confocal laser scanning microscopy of FBC cultures from PRD 13, gp130RB13-6 was coexpressed with the RC1 antigen by progenitor cells morphologically resembling radial glia cells. In addition, a very small subpopulation of astrocytes coexpressing gp130RB13-6 and glial fibrillary acidic protein (GFAP; < 5%) occurred 3 days after seeding. Primary FBC cultures from PRD 18 contained an increased subset of astrocytes coexpressing gp130RB13-6 and GFAP (approximately 25% of all gp130RB13-6 expressing cells), apparently generated from gp130RB13-6-positive precursors. Corresponding to in vivo conditions, ciliated ependymal cells but also microglial cells/macrophages and leptomeningeal cells showed strong expression of gp130RB13-6 in culture. We thus present a new glycoprotein on the cell surfaces of a glial progenitor cell subset for further studies of cell lineage relationships between radial glia cells, astrocytes, and ependymal cells.

Activation of microglial and endothelial cells in the rat brain after treatment with interferon-gamma in vivo

Activation of microglial cells occurs during the pathogenesis of various neurologic diseases. However, the mechanisms of activation in vivo are still elusive. We infused adult rats intravenously with interferon-gamma and demonstrated microglial cell activation after three days of treatment. We show for the first time that microglial cells proliferate inside the brain in response to a circulating cytokine. Microglial cells were induced to express major histocompatibility (MHC) class I and class II antigens and small amounts of leukocyte function-associated molecule 1 (LFA-1, CD11a). On endothelial cells of the brain, MHC class I and class II antigens and intercellular adhesion molecule 1 (ICAM-1, CD54) were enhanced.

Production of monocyte chemotactic protein-1 by rat brain macrophages

In the present study, we show that cultured rat brain macrophages release a soluble factor that stimulates the migration of bone marrow-derived macrophages, as determined by an in vitro chemotaxis assay. A checkerboard analysis indicated that most of this effect resulted from a polarized migration of the cells (chemotactic phenomenon), rather than in an increase in cell motility (chemokinesis). This activity was significantly decreased by an immune serum directed against the rat monocyte chemoattractant protein-1 (chemokine MCP-1). Northern blot analysis demonstrated expression of the MCP-1 gene in cultured brain macrophages, but its absence in unstimulated bone marrow-derived macrophages. Up-regulation of MCP-1 expression was observed when lipopolysaccharide was added to cultured brain macrophages, a peak occurring after a 6 h period of stimulation. Also, inflammatory cytokines such as interleukin (IL)-1 beta, colony stimulating factor-1, tumour necrosis factor-alpha and IL-6 individually increased the basal level of MCP-1 mRNA. Subsequently, we demonstrated the in vivo production of MCP-1 in the adult rat brain following injury induced by a local injection of kainic acid. MCP-1 synthesis was localized in both astrocytes and brain macrophages. These results suggest that the activation of resting microglial cells into brain macrophages and their subsequent secretion of chemokines could contribute to the mechanism(s), leading to the infiltration of the CNS by blood-derived monocytes, as observed in several pathologies.

Microglia in the nerve fiber layer of the cat retina: detection of postnatal changes by a new monoclonal antibody

This paper describes changes in the appearance and distribution of microglia in postnatal cat retina as demonstrated by a new antibody, H386F. This fractionated IgM antibody was created via an intrasplenic immunization of a single BALB/C mouse with about 2-3 x 10(5) large, whole cells isolated from 46 minced cat retinae. To confirm that the labeled cells are microglia, the staining properties of H386F were compared with those of four commercially available antibodies, OX-33, OX-41, OX-42, and ED-1, that have been used by others to distinguish between microglia and other cells in rat brain. These experiments show that H386F is the only antibody of the five to label only microglia in both the cat retina and hippocampus.

Differential immune responses to fetal intracameral spinal cord and cortex cerebri grafts

While the central nervous system (CNS) has been characterized as an immunologically privileged site, there are also several reports describing immunological reactions within the CNS. A certain degree of immunological privilege has also been ascribed to the anterior chamber of the eye. We have used the intraocular transplantation model to study immunological reactions in transplants of embryonic neural tissue. Outbred Sprague-Dawley rats and inbred Fisher rats were used. Pieces of rat parietal cortex or the cervical spinal cord were prepared from embryonic day 14 and implanted into the eye chambers of adult rats of the same strain. Following intraocular maturation, grafts were analysed using antibodies against: major histocompatibility complex (MHC) class I, MHC class II; rat antigens CD4, CD8, CD11b; T-cell receptor; rat antigen ED1; and glial fibrillary acidic protein. Using this set of markers for immunological reactions, transplants were scored on a blind basis. We found no significant differences in immunological scores between transplants obtained from different litters of fetuses of the outbred animals. Grafting in the outbred strain led to increased numbers of immunologically reactive cells in the grafts. This was not seen in grafts in the inbred strain. Spinal cord transplants led to a significantly higher degree of cytotoxic immunity-related cells expressing MHC class II as well as CD4-positive cells. There was a positive correlation between ED1 negativity and well-developed ramified microglia. From these results we conclude also that well-developed intraocular CNS tissue grafts do contain cellular evidence of immunological events and that different areas of the CNS may provoke different degrees of response. Reactive microglial proliferation appears to be one of the most sensitive ways to monitor the immunological condition of grafted CNS tissue.

Interactions of neurotrophic factors GDNF and NT-3, but not BDNF, with the immune system following fetal spinal cord transplantation

Glial cell line-derived neurotrophic factor (GDNF) is known to stimulate survival of dopaminergic and spinal cord motor neurons. However, little is known of the possible immune sequelae of GDNF exposure, or that of other putative trophic factors. To address these questions we utilized in oculo grafts of spinal cord, wherein we could induce different levels of immune responses via allogeneic vs. syngeneic combinations. Adult female Sprague-Dawley and Fisher rats were used as hosts for allogeneic and syngeneic grafts, respectively. Embryonic age 14-15-day-old fetuses were taken from pregnant dams of each strain, and cervical spinal cords were removed and dissected. Pieces of the spinal cord were transplanted into the anterior chamber of the eye within each strain. At 5-day intervals, 0.5 microgram of GDNF, brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3) or cytochrome c (CC) was injected into the anterior chamber of the eye and the sizes of the transplants were measured for the Sprague-Dawley rats. The same injections and measurements, but only for GDNF and CC, were carried out using Fisher rats. As expected, GDNF increased transplant survival and growth in both the Sprague-Dawley and Fisher animals. At day 41-42, all rats were sacrificed. Cameral graft appearance was evaluated by cresyl violet and immunohistochemically using antibodies against neurofilament (NF), calcitonin gene-related peptide (CGRP) and glial fibrillary acidic protein (GFAP). To monitor immune responses, the following monoclonal antibodies were used: OX38 against CD4, OX18 against MHC class I (MHCI), OX8 against CD8, OX6 against MHC class II (MHCII), OX42 against CD11b, R73 against alpha and beta T cell receptor (TcR), and ED1. In the Sprague-Dawley grafts, significantly higher amounts of CD8+, T lymphocyte+, MHCI+ and MHCII+ antigen-presenting cells (APC) were observed in GDNF-treated transplants. These markers were also increased in NT-3-treated groups. There were two types of OX-42+ cells, one was the ordinary ramified microglial cell, the other appeared to be a phagocytic cell, looking like the interstitial proliferating variety. Interestingly, the phagocytic OX-42+ cells had the same distribution as ED1+ and MHCII+ cells. In contrast, there were few immunoreactive cells after GDNF treatment in the inbred Fisher animals, similar to the CC control group. These results suggest that GDNF and to some extent NT-3, can activate the immune system in allogeneic graft combinations, but that these trophic factors do not produce overt rejection, and do not per se induce immune responses.

Dexamethasone and colchicine reduce inflammation and delayed oedema following experimental brain contusion

The effect of anti-inflammatory treatment on monocyte/macrophage infiltration, major histocompatibility complex molecules (MHC) class II expression and delayed oedema following experimental brain contusion was studied by immunohistochemistry and tissue-specific gravity measurement in 44 rats. Colchicine, chloroquine and dexamethasone administered once daily for five days after the trauma reduced inflammation and oedema. The difference was statistically significant with colchicine and dexamethasone. The findings comprise further evidence of a pathogenetically important inflammation after experimental contusion. It is probable that anti-inflammatory agents may prevent secondary neurological damage due to elevated intracranial pressure and cell to cell- or cytokine-mediated neuronal degeneration and demyelination.

Reactive glia express cytosolic phospholipase A2 after transient global forebrain ischemia in the rat

BACKGROUND AND PURPOSE

Phospholipid breakdown has been reported to be an early event in the brain after global cerebral ischemia. Our earlier observations showing the localization of cytosolic phospholipase A2 (cPLA2) to astrocytes in aged human brains and the intense glial activation observed after global forebrain ischemia prompted us to investigate the cellular localization of cPLA2 in the rat brain subjected to global ischemia.

METHODS

Immunohistochemistry was performed in sections through the dorsal hippocampus in rats subjected to 30 minutes of four- vessel occlusion. PLA2 was localized with the use of a highly selective antiserum. Double immunofluorescent localization was performed to colocalize cPLA2 with various glial cell types. cPLA2 levels were also measured by enzymatic assay and Western blot analysis.

RESULTS

A marked induction of cPLA2 was observed in activated microglia and astrocytes in the CA1 hippocampal region at 72 hours after ischemia. Only a subset of astrocytes and microglia were immunoreactive for cPLA2. Twenty-four hours after ischemia, numerous cPLA2 immunoreactive astrocytes were observed. Western blot analysis of hippocampal homogenates at 72 hours after ischemia showed induction of a 100-kD band that comigrated with purified human cPLA2, and a threefold induction in cPLA2 activity was demonstrated by enzymatic assay.

CONCLUSIONS

These results indicate that both reactive astrocytes and microglia contain elevated levels of cPLA2. Induction of cPLA2 was confined to areas of neurodegeneration and likely precedes its onset. The results suggest that reactive glia may play a role in the pathophysiology of delayed neuronal death after transient global forebrain ischemia.

ED2-positive perivascular cells act as neuronophages during delayed neuronal loss in the facial nucleus of the rat

Injection of Fluoro-Gold (FG) into the whisker pad of rats yields a stable retrograde labeling of facial motoneurons. After removal of 10 mm from the facial nerve the microglia phagocytose the FG-prelabeled dead neurons and assume the label. A subsequent brightfield immunostaining of the sections with HRP-DAB as end-product fully quenches the fluorescence of FG from all specifically stained structures (immunoquenching). Combining FG-labeling of neuronophages with immunoquenching, we recently described a population of enigmatic fluorescent cells, found in immediate vicinity to the motoneurons after the general neuronofugal migration of microglia. As the fluorescence of these cells was not quenched after a triple immunostaining with anti neuron-specific enolase, anti-GFAP, and OX-42 (quenching all fluorescence from neurons and glia), they seemed to represent a new, immunologically not identified neuronophage. Now we have further characterized this cell type. Following triple immunostaining, we tested a broad panel of mabs (OX-33, OX-19, OX-18, OX-6, R73, ED1, and ED2) to stain, quench fluorescence, and thus immunotype the unknown phagocytes. Only the mab ED2, the classical marker for perivascular cells, specifically stained the small round neuronophages. This surprising migration of perivascular cells toward decaying neurons was additionally tested and confirmed by intracerebroventricular application of FG prior to resection of the facial nerve Providing evidence for neuronophagia by ED2-positive cells, our results strongly support the hypothesis that the latter are the APC (antigen presenting cells) of the CNS.

NADPH-diaphorase activity in brain macrophages during postnatal development in the rat

NADPH-diaphorase histochemistry, that allows the visualization of cells producing the gaseous intercellular messenger nitric oxide, was used in the study of the forebrain during the first three postnatal weeks in the rat. Subpopulations of NADPH-diaphorase positive neurons were observed at all ages studied. In addition, non-neuronal NADPH-diaphorase-stained cells were detected in the subcortical white matter, and were very numerous in the supraventricular portion of the corpus callosum, and in the internal and external capsules. These cells were present during the first two postnatal weeks, and were especially prominent at the end of the first postnatal week. They were round-shaped and morphologically similar to the brain macrophages, whose phagocytic activity has been shown in previous studies to play a role in naturally occurring cell death and elimination of exhuberant axons. Series of sections adjacent to those stained with NADPH-diaphorase were processed with immunohistochemistry, using two different antibodies (OX-42 and ED-1) that detect macrophagic and microglial markers, and antibodies that recognize the neuronal form of nitric oxide synthase. Furthermore, brain sections from rats at postnatal day 7 were sequentially processed for either OX-42 or nitric oxide synthase immunohistochemistry followed by NADPH-diaphorase histochemistry. The morphological features and distribution of the non-neuronal NADPH-diaphorase-positive cells were superimposable to those obtained with OX-42 and ED-1 immunohistochemistry. In addition, these cells did not display nitric oxide synthase immunoreactivity. Double-labelled NADPH-diaphorase-positive and OX-42-immunoreactive cells were detected at postnatal day 7. The present results show that brain macrophages express NADPH-diaphorase activity during the early stages of the normal postnatal maturation and suggest that nitric oxide produced by brain macrophages could be involved in the development reshaping of the central nervous system.

Effects of chronic intrahippocampal infusion of lipopolysaccharide in the rat

Astrogliosis and microglial activation are associated with many neurodegenerative disorders including multiple sclerosis, its animal model experimental allergic encephalomyelitis, and Alzheimer's disease. To address the hypothesis that chronic astroglial or microglial activation could be contributing factors to neuronal death or injury, the immunostimulant lipopolysaccharide was infused into the hippocampus for 16 days using Alzet mini-osmotic pumps attached to a cannula. Placement of the cannula and infusion of vehicle for 16 days caused a hippocampal lesion with a volume of 0.5 +/- 0.1 mm3. Infusion of lipopolysaccharide at the dose of 2.0 micrograms/day produced a lesion of 4.9 +/- 1.3 mm3 (P < 0.01, Newman-Keuls), whereas, a lower dose of 0.2 microgram/day caused a lesion of 1.3 +/- 0.3 mm3 (P < 0.05). The lesion was defined as a focal necrotic reaction with fibrin deposits outlining an area at an early stage of encapsulation. No apparent neuronal loss was observed by Cresyl Violet staining outside the encapsulated necrotic area. There was a pronounced astrogliosis and an increase in activated macrophages throughout the lipopolysaccharide-infused hippocampus as determined by glial fibrillary acidic protein and ED-1 immunohistochemistry, respectively. Choline acetyltransferase and glutamic acid decarboxylase enzyme activities, used as functional measures of neuronal viability for cholinergic and GABAergic neurons, respectively, were unaffected in the hippocampus following a 16 day infusion of lipopolysaccharide at the doses of 0.2, 0.6 and 2.0 micrograms/day. In addition, unilateral infusion of lipopolysaccharide into the hippocampus did not affect 24 h locomotion when tested on day 13, body temperature or weight gain. Under the experimental conditions employed in the present study, chronic infusion of lipopolysaccharide into the hippocampus resulted in a dose-dependent focal necrotic lesion at the site of infusion. In tissue surrounding the encapsulated lesion, neurons were present among the reactive astrocytes and increased number of macrophages suggesting that astrocytes and macrophages can be activated without causing neuronal loss.

Immunohistochemical detection of thrombospondin in microglia in the developing rat brain

The development of microglia involves the expression of a phenotype displaying phagocytic behaviour termed brain macrophage or amoeboid microglial cell. We have previously shown that rat brain macrophages purified in vitro secrete thrombospondin, an extracellular matrix protein, which acts on cultured neuronal cells by promoting neurite growth. In the present study, the expression of thrombospondin was investigated in tissue sections of the developing rat forebrain in relation to the distribution of microglia. These cells were identified using anti-macrophage antibodies and the isolectin B4 from Bandeiraea simplicifolia. Immunocytochemical detection of thrombospondin clearly outlined a cell population displaying the morphologies and distribution of brain macrophages, from the 17th day of embryonic life up to the end of the second postnatal week. These cells were most numerous in cortical and subcortical regions of developing fibre tracts such as the corpus callosum or the internal capsule. The localization of thrombospondin in brain macrophages was confirmed by double immunostaining using ED1 monoclonal anti-macrophage antibodies. Ramified microglial cells were also labelled transiently by anti-thrombospondin antibodies during early postnatal life. These results provide in situ evidence supporting the notion that microglial cells could favour axonal growth by producing thrombospondin during development.

Reaction of microglial cells and macrophages after cortical incision in rats: effect of a synthesized free radical scavenger, (+/-)-N,N'-propylenedinicotinamide (AVS)

Reactive microglial cells and macrophages appear after trauma to the brain. To investigate the accumulation patterns of reactive microglial cells and macrophages after cortical incision, these cells were stained immunohistochemically with anti-ED1 antibody in the brain sections before and 1, 3, 5, and 7 days after incision. And to ascertain the participation of oxygen free radicals in these cellular reactions, a synthesized free radical scavenger, (+/-)-N,N'-propylenedinicotinamide (AVS) was administered in this model. Rats were administered AVS (300 mg/kg, i.p.) 30 min before, 2.5 h and every 24 h after incision (AVS group), while only saline was administered in the same manner as a control (saline group). In the saline group, both reactive microglial cells and macrophages had already appeared on day 1 post-incision. The former continued to increase in number during the following days, whereas the latter increased in number up to day 3 and thereafter decreased. Both the numbers of reactive microglial cells and macrophages were significantly decreased (P < 0.05) in the AVS group on days 5 and 7. The results suggest the participation of oxygen free radicals in the reaction of microglial cells and macrophages in traumatic brain injury.

MHC expression in nonlymphoid tissues of the developing embryo: strongest class I or class II expression in separate populations of potential antigen-presenting cells in the skin, lung, gut, and inter-organ connective tissue

We define expression of major histocompatibility complex (MHC) antigens in the nonlymphoid tissues of the developing rat. Antibodies to class I heavy and light chains (b2-m), and to class II MHC proteins were used. Strongest MHC expression was by individual cells in the skin, lung, gut, and inter-organ connective tissue. The class I+ and class II+ cells were distinct populations, differing in morphology, distribution, and expression of macrophage-associated antigens. A nonimmunologic role for MHC proteins in development has been proposed. Yet the distributions and antigenic profiles lead us to emphasize immunologic functions that may be served by the early presence of MHC+ cells outside the forming lymphoid organs. Potential contributions to establishment of extrathymic or maternal/fetal tolerance are discussed. Localization of strongest MHC expression to individual connective tissue cells of the developing organs, rather than parenchymal cells, is of clinical relevance to transplantation of fetal tissue.

CNTF regulation of astrogliosis and the activation of microglia in the developing rat central nervous system

In response to physical or chemical brain injury, the mammalian central nervous system (CNS) often reacts by evoking astrogliosis. The most prominent feature describing this state is an upregulation of glial fibrillary acidic protein (GFAP). The agent(s) responsible for inducing astrogliosis remains unclear; however, recent observations have shown cytokines may play a pivotal role. During CNS trauma, macrophages and lymphocytes infiltrate the CNS where they are thought to synthesize and secrete cytokines; moreover, activated microglia and reactive astrocytes are known to be capable of cytokine production. We are the first to report that an intracerebral injection of the pleiotropic cytokine, ciliary neurotrophic factor (CNTF), increases astrogliosis and the appearance of activated microglia in the neonatal rat. This response to CNTF was comparable to the response observed in animals receiving a well known pro-inflammatory cytokine, tumor necrosis factor-alpha (TNF-alpha). Only a moderate increase was observed in the proliferative index of cytokine-injected animals; therefore, we conclude that GFAP is largely upregulated in a pre-existing GFAP negative cell population. Interestingly, coinjections of CNTF and TNF-alpha appeared to act synergistically. Coinjected animals displayed a wave of hypertrophied astrocytes reaching far into the contralateral hemisphere. No contralateral spreading of microglia was observed. This article clearly provides interesting information regarding the regulatory mechanisms that govern astrogliosis and discusses the probable relationship of reactive astrocytes to microglia.

Amoeboid microglia expressing GD3 ganglioside are concentrated in regions of oligodendrogenesis during development of the rat corpus callosum

In recent study we demonstrated expression of the platelet-derived growth factor alpha receptor (PDGFR alpha) in cells of the early oligodendrocyte lineage that were identified as either GD3 ganglioside + oligodendrocyte progenitors or O4 sulfatide+ preoligodendrocytes. We also identified a subpopulation of GD3 immunoreactive cells that did not express mRNA for the PDGF receptor. The distinct large amoeboid morphology of these cells was characteristic of cells in the macrophage lineage rather than in the oligodendrocyte lineage. To determine if the GD3-positive but PDGFR alpha mRNA-negative cells were in the macrophage lineage, we compared the spatial and temporal expression patterns of GD3 ganglioside and ED1, a macrophage-specific antigen. Analysis prenatally indicated that at embryonic day 15, ED1+ and GD3+ cell populations resided in the subpial connective tissue. At embryonic day 21, these two populations were seen in a region extending from the lateral ventricle through the subventricular and intermediate zones. In this study we report that these large, round, GD3 immunoreactive cells have the same cell morphology and anatomical distribution as the ED1 immunoreactive cells. Both cell populations contained pyknotic nuclei within their cytoplasm. Furthermore, the GD3+/PDGFR alpha- cells appear to be involved in clearing cellular debris in regions of gliogenesis. These data suggest that this subpopulation of GD3 immunoreactive cells belongs to the microglia/macrophage lineage.

Preparation of a monoclonal antibody to a glycidic epitope of the epidermal growth factor receptor that recognizes inhibitors of astrocyte proliferation and reactive microglia

A mouse monoclonal antibody (5B9), directed against a carbohydrate epitope of human epidermal growth factor receptor (EGFR), recognized an 81-kDalton glycoprotein in buffer-soluble and detergent-solubilized rat brain extracts (BE). The glycoprotein was more abundant in extracts prepared from injured brain than in those from normal tissue. Removal from BE of the antigens recognized by 5B9 increased their astrocyte mitogenic activity. Sections of injured rat brain and cultures derived from damaged brain, enriched in microglia, showed 5B9 immunoreactivity in ED1-positive cells. The abundance of the glycoprotein recognized by 5B9 in injured, relative to normal, tissue, suggested that molecules with EGFR immunoreactivity may be expressed in reactive microglial cells and released after injury.

Intracerebral inflammatory response to experimental brain contusion

The inflammatory reaction following experimental brain contusion was studied by immunohistochemistry in 22 rats during the first 16 days after trauma. An inflammatory mononuclear cell response was evident on day 2, with a maximum on days 5-6 and signs remained still 16 days after the trauma. The time course of the cellular infiltration adjacent to the lesion correlated with blood brain barrier dysfunction in the contralateral side of the traumatized hemisphere. The cellular infiltrate comprised NK cells, T-helper cells and T-cytotoxic/suppressor cells as well as monocytes/macrophages. Most of the macrophages appeared to be activated by T-cells. Surprisingly, polymorphonuclear cells appeared less engaged than mononuclear cells in the inflammation. The demonstration of immunocompetent cells and the induction of MHC-1 and MHC-II antigen provides a substrate for inflammatory reactions similar to those that cause neurological damage in inflammatory diseases such as viral infections, multiple sclerosis and experimental allergic encephalitis. Our observations indicate that the role of the inflammatory reactions may have a role, hitherto neglected, in the pathogenesis of secondary traumatic brain injury.

Strongly GD3+ cells in the developing and adult rat cerebellum belong to the microglial lineage rather than to the oligodendrocyte lineage

A recent study has shown that ramified microglia in the adult rat optic nerve express the ganglioside GD3 [Wolswijk Glia 10:244-249, 1994], thereby raising the possibility that some GD3+ in the developing rat central nervous system (CNS) belong to the microglial lineage rather than to the oligodendrocyte lineage, as previously thought. To examine this possibility, sections of postnatal and adult cerebellum were double-labelled with markers for rat microglia [the B4 isolectin derived from Griffonia simplicifolia (GSI-B4), the ED1 monoclonal antibody (mAb), and the OX-42 mAb] and anti-GD3 mAbs (the mAbs R24 and LB1). These immunolabellings showed that ramified microglia as well as amoeboid microglia are strongly GD3+ in vivo. Moreover, most, if not all, cells that express high levels of GD3 in sections of developing cerebellum appear to belong to the microglial lineage. These observations contradict previous suggestions that the strongly GD3+ cells in the putative white matter regions of the developing brain are oligodendrocyte-type-2 astrocyte (O-2A) progenitor cells; the cells that give rise to oligodendrocytes in the CNS. The present study did, however, confirm that some O-2A progenitor cells in sections of postnatal cerebellum are weakly GD3+ in vivo. Amoeboid microglia are present in areas of the developing cerebellum where newly generated oligodendrocytes are found, suggesting that these cells play a role in the phagocytosis of the large numbers of oligodendrocytes that die as part of CNS development.

The pathophysiology of pneumococcal meningitis

The interactions between pneumococcal surface components and host defence systems that initiate pneumococcal meningitis have been studied in considerable molecular detail over the past decade. In this sense, the pneumococcus has served as a prototype for the unravelling of the genesis of inflammation caused by gram-positive bacteria. This review outlines the progression of these early events involving the cytokine cascade, the coagulation cascade, and leukocyte migration, and relates these processes to the production of blood-brain barrier permeability, the hallmark of injury in meningitis. This new understanding has radically altered the therapy of disease with the promise of greatly improved outcome.

Rat microglial interleukin-3

Interleukin-3 (IL-3, multi-CSF) is a growth factor for a variety of hematopoietic progenitor cells. Recently, microglial cells, the resident macrophages of the central nervous system (CNS) have been shown to proliferate in the presence of IL-3 both in vivo and in culture. Data obtained from cultured astrocytes gave rise to the hypothesis that astrocytes synthesize the microglial growth factor. This is the first report identifying rat microglial cells themselves as a source of IL-3. Culture media conditioned by isolated microglia enhanced microglial proliferation above fresh media controls. IL-3 polypeptide was detected in both conditioned media (CM) and in microglial cells by Western blotting and immunoprecipitation. Furthermore, anti-IL-3 antibodies were able to inhibit microglial proliferation induced by conditioned media. mRNAIL-3 was present in single microglial cells as revealed by in situ hybridization. Total RNA prepared from purified microglia yielded a single PCR amplification product. Identity of the PCR product was confirmed by Southern blot hybridization using a cDNAIL-3 probe and by DNA sequencing. Expression of mRNAIL-3 was observed in both absence and presence of lipopolysaccharide, a bacterial endotoxin, that commonly induces expression of inflammatory cytokines and inhibits microglial proliferation. It is concluded that IL-3 expression in ensuring the recruitment of enhanced numbers of immunocompetent cells at sites of lesion. In the light of weak immune reactions in the brain, it is hypothesized that the expression of a characteristic T cell feature in monocyte-derived microglia may be a partial compensation of T cell functions in brain lesions.

Grafts of fetal central nervous system tissue rescue axotomized Clarke's nucleus neurons in adult and neonatal operates

Many conditions are thought to contribute to neuron death after axotomy, including immaturity of the cell at the time of injury, inability to reestablish or maintain target contact, and dependence on trophic factors produced by targets. Exogenous application of neurotrophic factors and transplants of peripheral nerve and embryonic central nervous system (CNS) tissue temporarily rescue axotomized CNS neurons, but permanent rescue may require transplants that are normal targets of the injured neurons. We examined the requirements for survival of axotomized Clarke's nucleus (CN) neurons. Two months after hemisection of the spinal cord at the T8 segment, there was an ipsilateral 30% loss of neurons at the L1 segment in adult operates and a 40% loss in neonates. Transplants of embryonic spinal cord, cerebellum, and neocortex inserted into the T8 segment at the time of hemisection prevented virtually all of the cell death in both adults and neonates, but transplants of embryonic striatum were ineffective. None of the grafts prevented the somal atrophy of CN neurons caused by axotomy. Retrograde transport of fluoro-gold from the cerebellum demonstrated that 33% of all CN neurons at L1 project to the cerebellum, 50% of these died following a T8 hemisection, but all these projection neurons were rescued by a transplant of embryonic spinal cord. These results suggest that the rescue of axotomized CN neurons is relatively specific for the normal target areas of these neurons, but this specificity is not absolute and may depend on the distribution and synthesis of particular neurotrophic agents.

Origin of macrophages in central nervous tissue. A study using intraperitoneal transplants contained in Millipore diffusion chambers

In order to determine the origin of brain phagocytes brain slices and optic nerve segments from adult Lewis rats were transplanted into the peritoneal cavity of syngenic recipients. The specimens were contained in Millipore diffusion chambers fitted with membranes of either 0.22 or 5.0 microns pore size. The either blocked or allowed the access of non-resident cells. Each recipient rat received both a 0.22 and 5.0 microns pore chamber. Later (3-16 days), the specimens were recovered and analyzed by monoclonal antibody techniques and electron-microscopy. Endothelia, GFAP+ astrocytes, ED1-/ED2+/RCA-1+/OX-6-perivascular cells and ED1-/ED2-/RCA-1+/lysozyme--microglia were found to have survived the procedure. Cells of the macrophage phenotype (ED1+/ED2+/RCA-1+/lysozyme+/vimentin+ with phagocytic vacuoles), however, were only found in large numbers in specimens kept within 5.0 microns pore size chambers, giving access to non-resident cells, and were exceedingly rare in specimens from 0.22-micron pore chambers. It has been concluded that the majority of brain phagocytes found after lesions do not originate from microglia or perivascular monocytic cells, but rather from invading cells.

HIV-1 Tat alters normal organization of neurons and astrocytes in primary rodent brain cell cultures: RGD sequence dependence

The HIV-1 trans-activator protein Tat has been implicated as a mediator of neuronal dysfunction in several model systems. To explore the possibility that Tat can affect primary brain cells, we examined the effect of recombinant Tat protein on rat cortical brain cell cultures. Tat induced marked aggregation of neurons and astrocytes in developing cultures and caused the neuritic processes to coalesce into fascicles. Cell death was not seen and brain macrophages were not affected. These effects mapped to a different region from the trans-activation domain of Tat, as mutating the RGD (arginine-glycine-aspartic acid) sequence within the second exon abrogated aggregation and fascicle formation without affecting trans-activation capacity. Such effects on primary neurons and astrocytes may reflect specific interactions of Tat with uninfected cells within the CNS in vivo.

Early changes of macrophage-like immunoreactivity in the rat inferior olive after intraperitoneal 3-acetylpyridine injection

Early changes of macrophage-like immunoreactivity were observed in the inferior olive after intraperitoneal injection of 3-acetylpyridine (3AP) using seven monoclonal antibodies recognizing macrophage subpopulations (OX-42, OX-6, ED-1, RM-1, TRPM-1, TRPM-2, and TRPM-3). Both resting and activated forms of microglia were stained with OX-42 and TRPM-2. Some of activated microglia reacted to OX-6 and/or ED-1. Neither resting nor activated microglia reacted to any of RM-1, TRPM-1, and TRPM-3. Four h after 3AP injection, the processes of OX-42-positive microglia had increased in number and became thicker than resting microglia. Between 24 h and the 7th day after 3AP injection (day 7), OX-42-positive microglia gradually increased in number. At 24 h after 3AP injection, round cells appeared that stained with all seven antibodies. These disappeared by day 3. Double staining indicated that OX-42-positive activated microglia on day 7 were divided into subpopulations by their immunoreactivity to ED-1. We suggest that the round cells derived from blood monocytes and entered the brain only transiently while OX-42-positive activated microglia originated from parenchymal resting microglia and continued to increase in number after the disappearance of the round cells.

Altered antigen expression of microglia in the aged rodent CNS

Microglia, the resident macrophages of the central nervous system, are characterised by a highly specialized morphology and unusual antigenic phenotype. Microglia appear to be downregulated by their microenvironment when compared to other tissue macrophages. We have studied the microglia in brains of healthy, aged rats with a panel of monoclonal antibodies. We have found that microglia in the brains of these aged rats express antigens that are downregulated or absent from microglia of juvenile rats. The stimuli which give rise to this upregulated phenotype are not known. Age related changes in the phenotype of microglia should be taken into account when considering the possible role of microglia in neuropathological conditions.

Bone marrow derived elements and resident microglia in brain inflammation

Infection of the central nervous (CNS) system by the human immunodeficiency virus (HIV) depends on the migration of infected hematogenous cells into the brain. We thus used quantitative light and electron microscopic immunocytochemistry to study the homing and turnover of bone marrow derived cells in the CNS in radiation bone marrow chimeras under normal conditions and in experimental autoimmune encephalomyelitis (EAE) as an experimental model of brain inflammation. Our studies suggest the following conclusions. First, the central nervous system is continuously patrolled by a small number of T-lymphocytes and monocytes. Meningeal and perivascular monocytes are slowly replaced by hematogenous cells under normal conditions, and this turnover is accelerated in the course of inflammation. In contrast, resident microglia represent a very stable cell pool, which in adult animals is only exceptionally replaced by hematogenous cells, even after recovery from severe brain inflammation. Second, although in bone-marrow-chimeric animals resident microglia, astrocytes, and ependymal cells are not able to present antigen to Lewis T-lymphocytes, the inflammatory reaction in EAE is qualitatively and quantitatively similar in these animals compared to fully histocompatible Lewis rats. Finally, resident microglia express the macrophage activation antigen ED1. Thus, microglia cells appear to function as effector cells in EAE lesions.

Inflammatory stimuli induce a new K+ outward current in cultured rat microglia

Membrane currents of cultured rat microglia were recorded with the whole-cell patch clamp technique. Undifferentiated microglia express only inwardly rectifying K+ channels. However, treatment of the cells with bacterial lipopolysaccharide, interferon-gamma, or their incubation in hydrophobic teflon bags, procedures that promote microglial differentiation, induced the expression of an additional outward current. Cycloheximide prevented the development of this conductance indicating the synthesis of a new channel protein. The reversal potential of the outward current was near to the K+ equilibrium potential; the current was abolished by intracellular Cs+ or extracellular 4-aminopyridine, and was depressed by extracellular tetraethylammonium. Hence, the channels involved appear to be highly selective for K+; their possible function is a rapid termination of depolarizing shifts of the membrane potential.

Phenotypes and alloantigen-presenting activity of individual clones of microglia derived from the mouse brain

To clarify the origin and function of the microglia residing in the central nervous system, we cloned brain cells from newborn and adult mice in soft agar containing the macrophage-specific growth factor, colony-stimulating factor-1 and expanded the cells from individual colonies in liquid culture medium. The results of molecular, immunophenotypic and functional analyses showed that the clones consisted of microglia derived from the macrophage family of cells. For instance, the microglia contain mRNA transcripts for the receptor for colony-stimulating factor-1 and truncated CD4 transcripts similar to those found in mouse macrophages but not T helper cells. About a third of the microglial progenitors gave rise to progeny that constitutively induced the selective proliferation of naive allogeneic CD8+ T cells in a CD4+ T cell-independent manner, a response that was inhibited by monoclonal antibodies to major histocompatibility complex (MHC) class I molecules on the microglia. Since all microglia expressed similar levels of MHC class I molecules, the basis for the alloantigen presentation likely resides in the ability of some clones of microglia to synthesize co-stimulator molecules that are required for CD8+ T cell proliferation. Thus, at least some microglia in mouse brain arise from endogenous progenitors and appear capable of specialized functions.

Nuclear receptor sites for vitamin D-soltriol in midbrain and hindbrain of Siberian hamster (Phodopus sungorus) assessed by autoradiography

Autoradiograms were prepared from midbrains and hindbrains of male and female Siberian hamsters (Phodopus sungorus), kept under short-day or long-day illumination, after injection of tritium-labeled 1,25-dihydroxycholecalciferol (vitamin D, soltriol). Concentration and retention of radioactivity was noted in nuclei of certain neurons, glial cells, and ependymal cells, and in choroid epithelium. Labeled neurons of varying intensity were found throughout the brainstem in distinct populations at characteristic topographical sites, which include cranial nerve motor nuclei, the nucleus (n.) reticularis tegmenti pontis, the caudoventral region of the n. raphe dorsalis, the n. trapezoides, the n. vestibularis lateralis and n. vestibularis superior, neurons in the various nuclei of the sensory trigeminus, accessory optic nuclei, scattered neurons in nuclei of the reticular formation, the n. ambiguus, certain cells in the area postrema, and many others. Glial cells with nuclear labeling, probably microglia, were scattered predominantly in or near myelinated nerve fascicles. The choroid epithelium showed strong nuclear labeling throughout the ventricle. Nuclear labeling of ependyma was variable and weak, mainly at ventral and lateral extensions (recesses) of the ventricle. The extensive presence of nuclear binding in select neural structures indicates that vitamin D exerts specific genomic effects on cell populations that are known to be involved in the regulation of motor, sensory, autonomic, neuroendocrine, metabolic, and immune functions. The results of these studies, in conjunction with those from other brain and peripheral tissues, recognize vitamin D-soltriol as a steroid hormone with a wide scope of hormone-specific target cells, similar to estrogen, androgen, and adrenal steroids, and which are topographically distinct and characteristic for its functions as the steroid hormone of sunlight.