The identity of cells expressing MHC class II antigens in normal and pathological human brain

Single-Cell Molecular and Cellular Architecture of the Mouse Neurohypophysis

The neurohypophysis (NH), located at the posterior lobe of the pituitary, is a major neuroendocrine tissue, which mediates osmotic balance, blood pressure, reproduction, and lactation by means of releasing the neurohormones oxytocin (OXT) and arginine-vasopressin (AVP) from the brain into the peripheral blood circulation. The major cellular components of the NH are hypothalamic axonal termini, fenestrated endothelia and pituicytes, the resident astroglia. However, despite the physiological importance of the NH, the exact molecular signature defining neurohypophyseal cell types and in particular the pituicytes, remains unclear. Using single-cell RNA sequencing (scRNA-Seq), we captured seven distinct cell types in the NH and intermediate lobe (IL) of adult male mouse. We revealed novel pituicyte markers showing higher specificity than previously reported. Bioinformatics analysis demonstrated that pituicyte is an astrocytic cell type whose transcriptome resembles that of tanycyte. Single molecule in situ hybridization revealed spatial organization of the major cell types implying intercellular communications. We present a comprehensive molecular and cellular characterization of neurohypophyseal cell types serving as a valuable resource for further functional research.

Clozapine reduces Toll-like receptor 4/NF-κB-mediated inflammatory responses through inhibition of calcium/calmodulin-dependent Akt activation in microglia

Clozapine is an atypical antipsychotic agent used in the treatment of schizophrenia and severe mood disorders. Accumulating evidence suggests that neuroinflammation is closely associated with the pathogenesis of various neurodegenerative diseases and psychiatric disorders. Clozapine exerts anti-inflammatory activity. However, the molecular mechanism underlying the anti-inflammatory activity of clozapine is poorly understood. In this study, we found that clozapine suppressed lipopolysaccharide (LPS)-induced phosphorylation of IκBα at Ser-32 and of p65/RelA at Ser-468, as well as nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB)-dependent transcriptional activity in microglial cells. Clozapine downregulated LPS-induced Akt phosphorylation at Ser-473. Pharmacological Akt inhibitors ameliorated LPS-induced NF-κB activation. Removal of extracellular Ca²⁺ by EGTA or sequestration of intracellular Ca²⁺ by BAPTA-AM attenuated LPS-induced Akt phosphorylation. Treatment with calmodulin (CaM) antagonists and the CaM kinase inhibitor, KN-93, also prevented LPS-induced Akt and NF-κB activation, suggesting that Ca²⁺/CaM-dependent Akt activation is critical in LPS-induced NF-κB activation in microglia. These results suggest that clozapine exhibits anti-inflammatory activity through the inhibition of Ca²⁺/CaM/Akt-mediated NF-κB activation.

Hypothalamic inflammation without astrogliosis in response to high sucrose intake is modulated by neonatal nutrition in male rats

Hypothalamic inflammation and gliosis are proposed to participate in the pathogenesis of high-fat diet-induced obesity. Because other factors and nutrients also induce weight gain and adiposity, we analyzed the inflammatory and glial responses to a sucrose (S)-enriched diet. Neonatal overnutrition (NON) exacerbates weight gain in response to metabolic challenges; thus, we compared the inflammatory response of male Wistar rats with NON (4 pups/litter) and controls (12 pups/litter) to increased S intake. At weaning rats received water or a 33% sucrose solution and normal chow ad libitum for 2 months. Sucrose increased serum IL-1β and -6 and hypothalamic IL-6 mRNA levels in NON and TNFα mRNA levels in control and NON rats, whereas NON alone had no effect. The astrocyte marker glial fibrillary acidic protein was increased by NON but decreased by S. This was associated with hypothalamic nuclei specific changes in glial fibrillary acidic protein-positive cell number and morphology. Sucrose increased the number of microglia and phosphorylation of inhibitor of -κB and c-Jun N-terminal kinase in control but not NON rats, with no effect on microglia activation markers. Proteins highly expressed in astrocytes (glutamate, glucose, and lactate transporters) were increased by NON but not S, with no increase in vimentin expression in astrocytes, further suggesting that S-induced adiposity is not associated with hypothalamic astrogliosis. Hence, activation of hypothalamic inflammatory processes and gliosis depend not only on weight gain but also on the diet inducing this weight gain and the early nutritional status. These diverse inflammatory processes could indicate a differential disposition to obesity-induced pathologies.

Diversity of glial cell components in pilocytic astrocytoma

To characterize the cellular density and proliferative activity of GFAP-negative cells in pilocytic astrocytoma (PA), surgically excised tissues of PAs (n=37) and diffuse astrocytomas (DAs) (n=11) were examined morphologically and immunohistochemically using antibodies against GFAP, Olig2, Iba1 and Ki-67 (MIB-1). In PA, Olig2 immunoreactivity was significantly expressed in protoplasmic astrocytes in microcystic, loose areas and cells in oligodendroglioma-like areas. Iba1-positive, activated microglia/macrophages were also commonly observed in microcystic areas. In compact areas, a prominent reaction for GFAP was observed, but for Olig2 and Iba1 to a lesser degree. On semiquantitative analysis, the number of Olig2-positive cells was significantly higher in PAs (mean labeling index (LI) +/- standard deviation (SD): 46.8+/-15.4%) than in DAs (13.3+/-7.8%) (P<0.001). Many Iba1-positive, microglia/macrophages were observed in PAs (19.9+/-6.5%), similarly to DAs (20.9+/-9.9%). Re-immunostaining of PA demonstrated that most Ki-67-positive, proliferating cells expressed Olig2, whereas GFAP or Iba1 expression in Ki-67-positive cells was less frequent (14.7+/-13.7%, and 8.8+/-13.6%) in a double immunostaining study. Conversely, the percentage of Olig2-positive, proliferating cells in total Olig2-positive cells (7.2+/-3.9%) was higher than that of Iba1-positive, proliferating cells in total Iba1-positive cells (0.9+/-0.6%). In conclusion, the present study found that PA consisted of numerous GFAP-negative cells, including Olig2-positive cells with high proliferation. Semiquantitative analysis of Olig2 immunohistochemistry in microcystic areas might therefore be useful for the differential diagnosis of PA and DA.

Entry and Distribution of Microglial Cells in Human Embryonic and Fetal Cerebral Cortex

Microglial cells penetrate into and scatter throughout the human cortical grey and white matter according to a specific spatiotemporal pattern during the first 2 trimesters of gestation. Routes of entry were quantitatively and qualitatively different from those identified in the diencephalon. Starting at 4.5 gestational weeks, amoeboid microglial cells, characterized by different antibodies as Iba1, CD68, CD45, and MHC-II, entered the cerebral wall from the ventricular lumen and the leptomeninges. Migration was mainly radial and tangential toward the immature white matter, subplate layer, and cortical plate, whereas pial cells populated the prospective layer I. The intraparenchymal vascular route of entry was detectable only from 12 gestational weeks. Interestingly, microglial cells accumulated in restricted laminar bands particularly at 19 to 24 gestational weeks among the corona radiata fibers rostrally, extending caudally in the immature white matter to reach the visual radiations. This accumulation of proliferating MIB1-positive microglia (as shown by MIB1-Iba1 double immunolabeling) was located at the site of white matter injury in premature neonates. The spatiotemporal organization of microglia in the immature white and grey matter suggests that these cells may play active roles in developmental processes and in injury to the developing brain.

Expression of glucose transporter 5 by microglia in human gliomas

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

Reciprocal interactions between microglia and neurons: from survival to neuropathology

Microglia represent a major cellular component of the brain, where they constitute a widely distributed network of immunoprotective cells. During the last decades, it has become clear that the functions traditionally ascribed to microglia, i.e. to dispose of dead cells and debris and to mediate brain inflammatory states, are only a fraction of a much wider repertoire of functions spanning from brain development to aging and neuropathology. The aim of the present survey is to critically discuss some of these functions, focusing in particular on the reciprocal microglia-neuron interactions and on the complex signaling systems subserving them. We consider first some of the functional interactions dealing with invasion, proliferation and migration of microglia as well as with the establishment of the initial blueprint of neural circuits in the developing brain. The signals related to the suppression of immunological properties of microglia by neurons in the healthy brain, and the derangement from this physiological equilibrium in aging and diseases, are then examined. Finally, we make a closer examination of the reciprocal signaling between damaged neurons and microglia and, on these bases, we propose that microglial activation, consequent to neuronal injury, is primarily aimed at neuroprotection. The loss of specific communication between damaged neurons and microglia is viewed as responsible for the turning of microglia to a hyperactivated state, which allows them to escape neuronal control and to give rise to persistent inflammation, resulting in exacerbation of neuropathology. The data surveyed here point at microglial-neuron interactions as the basis of a complex network of signals conveying messages with high information content and regulating the most important aspects of brain function. This network shares similar features with some fundamental principles governing the activity of brain circuits: it is provided with memory and it continuously evolves in relation to the flow of time and information.

Activation mechanism of brain microglia in patients with diffuse neurofibrillary tangles with calcification: a comparison with Alzheimer disease

Diffuse neurofibrillary tangles with calcification (DNTC) is an atypical dementia and is characterized pathologically by diffuse neurofibrillary tangles (NFTs) without senile plaques (SPs). In this study, we investigated the distribution of human leukocyte antigen (HLA)-DR-positive activated microglia in postmortem brain tissue of six patients with DNTC and six patients with Alzheimer disease (AD). HLA-DR-positive activated microglia were observed to associate with SPs in AD. In the DNTC brain, which lacks SPs, HLA-DR-positive microglia were mainly accumulated around weakly tau-positive NFTs, which were also positive for anti-amyloid-P and anti-C3d antibodies. The results of this study suggest that the complement pathway is also activated in the DNTC brain and that immune and inflammatory responses, including microglia activation, may occur around extracellular NFTs in DNTC patients.

Effects of macrophage-colony-stimulating factor deficiency on the maturation of microglia and brain macrophages and on their expression of scavenger receptor

Macrophage-colony-stimulating factor (M-CSF) regulates the survival, proliferation and differentiation of the mononuclear phagocyte lineage. Osteopetrotic (op/op) mice defective in producing functional M-CSF were used in order to investigate the role of M-CSF on the development of microglia and brain macrophages and the expression of scavenger receptor (SR). Adult op/op and littermate mice at 10-47 weeks of age were investigated by immunohistochemistry with a panel of monoclonal antibodies (F4/80, Mac-1, anti-major histocompatibility complex (MHC) class II and anti-SR), electron microscopy and reverse transcriptase-polymerase chain reaction (RT-PCR). Microglia were weakly immunolabeled with F4/80 and Mac-1 in op/op and littermate mice, but the number of microglia in op/op mice was reduced in the cerebrum, cerebellum and brainstem compared with that of normal littermates. The numbers of Mac-1-positive microglia in op/op mice was 39% (pons) and 30% (cerebellar cortex) lower than that in normal littermates (P<0.05). In addition, the microglia cell processes in op/op mice were often shorter than those in control mice. In op/op and littermate mice, both MHC class II and SR were present in perivascular cells and macrophages of the leptomeninx and choroid plexus. Ultrastructurally, perivascular cells appeared to be immature, since their cytoplasm was narrow and contained few inclusion bodies compared with those of control mice. Reverse transcriptase-polymerase chain reaction showed a weak expression for SR mRNA in the brains of op/op mice as well as littermate mice. These results indicate that microglia are partly dependent on M-CSF for their proliferation and differentiation and that M-CSF has no significant effect on the expression of SR in the physiological brain. The study also suggests that M-CSF affects the maturation of perivascular cells at the ultrastructural level.

Brain inflammatory response induced by intracerebroventricular infusion of lipopolysaccharide: an immunohistochemical study

Inflammatory processes may play a critical role in the pathogenesis of the degenerative changes associated with Alzheimer's disease (AD). In the present study, we used an animal model of brain inflammation in order to study a possible mechanism involved in AD. Lipopolysaccharide (LPS) was used to produce global microglial reactivity within the brain of young rats. Time-dependent changes in the inflammatory reaction and the participation of glial cells after acute injection of LPS (50 or 100 microg) into the lateral ventricle or the fourth ventricle were compared with the chronic infusion of LPS (0.15, 0.5, 1.5 or 5.0 microg/h) into the fourth ventricle (14 days). Several immunohistochemical markers were used to characterize the microglial response. Acute and chronic exposure to LPS induced major histocompatibility complex class II (MHC II) antigen expression, detected with OX-6 antibody, in a sub-population of microglial cells in defined brain areas. The morphological features and distribution of OX-6 positive cells observed in the proximity of the cannula track after LPS injection into the lateral ventricle suggested the recruitment of monocytes/macrophages from the periphery. The activation of the resident microglial cells was delayed and mainly concentrated within the temporal lobe regions and the limbic system. Chronic infusion to LPS into the fourth ventricle induced a comparable activation of microglial cells. Quantitative analysis of OX-6 positive cells showed a dose-dependent response to LPS exposure.

Dynamics of microglial activation in the spinal cord after cerebral infarction are revealed by expression of MHC class II antigen

Microglial reactivity associated with induction of MHC class II (HLA-DR) antigen is a sensitive indicator for pathological events in the CNS. To assess the response of glial cells after lesions of supraspinal descending tracts, HLA-DR, CD68 and GFAP were studied immunohistochemically on spinal cord tissue of 5 patients who died after unilateral infarction of the middle cerebral artery territory, and 5 control cases. In patients who died shortly after a stroke (4-14 days) increased HLA-DR-immunoreactivity (HLA-DR-IR) could be observed in the intermediate grey matter and in the ventral horn. The CD68-IR was much less intense. After longer survival times (5 weeks to 4 months). HLA-DR-IR in the grey matter was clearly lower than that observed in the spinal cord of short survival times, but very abundant in the dorsolateral funiculus, specifically within the corticospinal tract. In white matter areas, CD68-IR was almost identical to the HLA-DR-IR. Within the grey matter, CD68-IR was similar to the control tissue. A moderate increase of GFAP-positive astrocytes could be seen only in the grey matter after longer survival times. It seems probable, that the dynamics of HLA-DR-positive microglia reflect the early phagocytosis of presynaptic terminals by microglia in target regions of descending fibre tracts. In the white matter, the removal of degenerating axons by phagocytosing microglia expressing HLA-DR and CD68 antigens is a slower process which occurs over a period of months.

HLA-DR-positive dendritic cells of the normal human choroid plexus: a potential reservoir of HIV in the central nervous system

In a previous study of choroid plexus (CPx) from patients with the acquired immunodeficiency syndrome (AIDS), we found a population of stromal cells infected with the human immunodeficiency virus (HIV). To determine whether these represented antigen-presenting dendritic cells, we examined the phenotype of normal human choroid plexus by light and electron microscopy (EM) and established the HIV-infected cell type by immunohistochemistry in AIDS cases with HIV-infected CPx. Monoclonal antibodies were used to detect class II major histocompatibility antigens (MHC), S-100 and S-100beta protein, lymphocytes, monocytes/macrophages, and HIV glycoprotein. A variable number of stromal cells had slightly elongated nuclei and long branching processes that were strongly immunoreactive for class II MHCs, rarely reactive for S-100 and S-100beta and immunonegative for monocyte/macrophage markers. Phagocytic activity was absent by EM and immunomarkers. They were numerous in the subepithelial region, and their processes occasionally extended toward the stromal capillaries or between the CPx epithelial cells. The HIV-infected cells were intensely immunoreactive for class II MHC markers and often displayed a dendritic morphology. These results document the presence of dendritic cells in the normal human CPx whose morphology and immunophenotype closely resemble those of DCs elsewhere in the body. They also show that these immunoreactive MHC class II cells are the cell type infected by HIV. We suggest that the functional activity of the CPx DCs is similar to that of antigen-presenting dendritic cells elsewhere in the body. This includes the potential to harbor HIV during the prolonged period of clinical latency, acting as a central nervous system reservoir of infection before the onset of AIDS.

Induction of major histocompatibility class II antigen on microglial cells in postnatal and adult rats following intraperitoneal injections of lipopolysaccharide

Microglial cells, notably the ramified form, were induced to express major histocompatibility complex (MHC) class II antigen in postnatal and adult rats given intraperitoneal injections of lipopolysaccharide (LPS). The immunoreactive microglia which occurred in cell colonies or clusters were detected immunohistochemically with the monoclonal antibody OX-6. Some of the widely distributed MHC II positive cells were round or amoeboidic located preferentially in the perivascular area. In view of the widespread occurrence of microglial cells showing OX-6 immunoreactivity which is negligible in normal animals, it is suggested that the effect of LPS on microglia in vivo is a widespread phenomenon and is independent of age. It is suggested that the endotoxin not only triggers off the immunological potentiality of these cells but also elicits the entry of some mononuclear cells into the brain parenchyma.

Human choroid plexus is an uniquely involved area of the brain in amyloidosis: a histochemical, immunohistochemical and ultrastructural study

To better understand the characteristics of amyloid deposition in the choroid plexus, we examined autopsied brain by routine histology, immunohistochemistry, and electron microscopy in three group of patients: primary systemic amyloidosis (n = 7), cerebral amyloid angiopathy (CAA, n = 6), and controls (n = 3). Three of the CAA patients had Alzheimer's disease. Congophilic, birefringent amyloid deposits of the choroid plexus were seen in six of the seven cases of systemic light chain amyloidosis. Immunohistochemistry revealed that the deposited amyloids had reactivity for immunoglobulin light chain and amyloid P component. Accumulation of macrophages labeled with monoclonal antibodies against CD 68 and major histocompatibility complex class II antigens were observed around the massive amyloid deposits. The presence of approximately 10 nm amyloid fibrils along the epithelial basement membrane as well as in the vascular walls was ascertained by electron microscopy. In CAA, Congo red-positive amyloid deposits were consistently present in meningeal blood vessels and were often found in senile plaques of the cerebral parenchyma; congophilic amyloid deposits were absent in the choroid plexus. Choroid plexus epithelial cells exhibited immunostaining for beta amyloid precursor protein (APP) with N-terminal- and C-terminal-specific antibodies; in particular, consistent staining was obtained for the latter antibody. Immunoreactivity for amyloid beta protein (A beta) with monoclonal antibodies (6E10, 4G8) was often found in choroid plexus epithelial cells. These findings suggest that amyloid deposition of the choroid plexus depends on the major component protein in amyloidosis, and that the choroid plexus may produce APP and A beta protein although A beta amyloidosis is not evident in the choroid plexus.

Major histocompatibility class II molecules in the CNS: increased microglial expression at the onset of narcolepsy in canine model

Human narcolepsy is a neurological disorder known to be closely associated with HLA-DR2 and DQB1*0602. Because most autoimmune diseases are HLA-associated, a similar mechanism has been proposed for narcolepsy. However, neither systemic nor CNS evidence of an autoimmune abnormality has ever been reported. In this study, major histocompatibility (MHC) class I and class II expression was studied in the CNS of human and canine narcoleptics using immunohistochemistry and Northern analysis. Results indicated that canine narcolepsy is associated with a significant increase of MHC class II expression by the microglia. Moreover, the highest values were found between 3 and 8 months of age, strikingly concomitant to the development of narcolepsy in the canine model. In humans, class II expression was not found significantly different between control and narcoleptic subjects. This result could be explained by the old age of the subjects (69.86 +/- 5.31 and 68.36 +/- 4.74 years in narcoleptics and controls, respectively), because class II expression is significantly correlated with age in both humans and dogs. For the first time, this study demonstrated that the expression of MHC class II molecules in the CNS is age-dependent and that a consistent increase of their expression by the microglia might be critically involved in the development of narcolepsy.

Glycation and microglial reaction in lesions of Alzheimer's disease

Single, double, and triple immunostaining of cryostat sections of elderly normal and Alzheimer disease (AD) brain was performed with monoclonal and polyclonal antibodies to advanced glycation end products (AGE). The sections were counterstained with thioflavin-S or with immunocytochemistry for A beta and also stained with markers for microglia. AGE-immunoreactivity was detected in senile plaques and neurofibrillary tangles (NFT). AGE immunoreactivity was most intense in dense or reticular amyloid deposits and extracellular NFT, while intracellular NFT and diffuse amyloid had less AGE immunoreactivity. This pattern of immunoreactivity was similar to that noted in previous studies with antibodies to apolipoprotein-E (apo-E). Therefore, double labeling with antibodies to apo-E and AGE was performed. AGE immunoreactivity colocalized to a very high degree with apo-E immunoreactivity, except that relatively more intense apo-E immunoreactivity was detected in amyloid deposits and more intense AGE immunoreactivity in NFT. The lesions that were immunostained with antibodies to AGE and apo-E were often, but not always, associated with a local microglial reaction. The results raise the possibility that apo-E or a fragment of apo-E may be glycated. Biochemical studies are needed to determine the extent of possible apo-E glycation in AD. The present results raise the possibility that glycation may serve as one of the signals for activation of microglia associated with amyloid deposits and extracellular NFT.

The immunophenotype of perivascular cells in the human brain

The immunophenotype of perivascular cells (PC) in temporal lobe tissues obtained at autopsy in 48 patients (aged 41-88 years) was characterized using light and electron microscopic immunocytochemistry with a variety of antibodies. In all cases studied, PC bearing CD11c (Ki-M1P) and CD68 (KP1) were distributed throughout the temporal cortex. In addition to Ki-M1P and KP1, the monoclonal antibodies against major histocompatibility complex (MHC) class II antigen (Ag) (LN-3, CR3.43), anti-leucocyte common antigen (LCA), LN-5, Mac 387 were all found in PC with variable immunoreactivity. In contrast, LN-1 and OPD4 were not found in PC, although the former showed nearly constant staining of resting microglia. Semiquantitative analysis disclosed differences in the numbers of cells labeled with the markers in the 21 normal brains (Ki-M1P > KP1 >> LCA, LN-3, LN-5 >> Mac 387). Ultrastructurally, immunoreactivity for Ki-M1P, KP1, and LN-3 was observed in PC with cytoplasm containing dense lysosomal bodies. In brains from patients with Alzheimer's type dementia, PC were seen in the wall of beta-amyloid protein-positive small vessels. However, there was no definite alteration of antigenicity in PC from AD brains compared with those from normal brains. The immunophenotype of PC was similar to that of macrophages, which were observed in the perivascular spaces and the leptomeninges in some normal and diseased brains. In contrast with PC, however, macrophages showed high incidence of labeling for some macrophage markers LN-5 and Mac 387. These findings demonstrate that PC may be a normal constituent of the adult human brain with a variable expression of monocyte/macrophages markers and MHC class II Ag and that PC could be distinguished from resting microglia by their morphology and by their immunophenotype.

Characterization of perivascular cells in astrocytic tumours and peritumoral oedematous brain

Perivascular cells (PVCs) form an immunophenotypically defined population that plays an important scavenging role in the perivascular fluid drainage pathways in the rat brain; such cells may also act as antigen-presenting cells. The present study tests the hypotheses that (a) PVCs in human brain are distinct from microglia and haematogenous macrophages, and (b) PVCs within astrocytic tumours and peritumoral oedematous brain tissue react in a similar way to PVCs in the rat brain. Paraffin sections of formalin-fixed tissue from 10 astrocytomas, 10 anaplastic astrocytomas, 10 glioblastoma multiforme, peritumoral oedematous brain and from normal human brain were examined immunocytochemically using antibodies HLA-DR beta-chain for MHC class II antigen, PGM1 and MAC 387 directed against macrophage components, MT1 for T lymphocytes and GFAP for astrocytes. No PVCs, microglia or macrophages were labelled by these techniques in paraffin sections of normal brain. Microglia, macrophages recently derived from haematogenous monocytes and PVCs were labelled by immunocytochemistry in all tumours but were more numerous in glioblastomas than in astrocytomas or anaplastic astrocytomas. Perivascular cells were distinguished by their perivascular position, their expression of MHC class II antigen and were labelled by PGM1 antibody but not by MAC 387 antibody. Microglia and monocyte/macrophages, remote from blood vessels, on the other hand, were strongly labelled by MAC 387, moderately by PGM1 and showed weak expression of MHC class II antigen. A similar pattern of staining was seen in peritumoral oedematous tissue. These findings suggest that PVCs form a defined population of resident cells in the human brain and that they are distinct from microglia, monocytes and macrophages.(ABSTRACT TRUNCATED AT 250 WORDS)

HLA-DR expression in peripheral neuropathies: the role of Schwann cells, resident and hematogenous macrophages, and endoneurial fibroblasts

The expression of HLA-DR and the macrophage marker CD 68 was studied in 44 sural nerve biopsies from patients with inflammatory and non-inflammatory neuropathies and controls using immunohistochemistry on non-osmicated semithin sections, a technique that has not been used before in such a biopsy study. Most HLA-DR-immunoreactive (ir) cells were fibroblasts, macrophages or perineural cells, some were perivascular and endothelial cells, and only few were Schwann cells. Counts of immunoreactive cells revealed (a) increased HLA-DR expression in severe as compared to less severe neuropathies and to controls, (b) no correlation between the numbers of HLA-DR-ir cells and CD 68-ir macrophages, and (c) no close correlation between diagnostic groups and the number of HLA-DR-ir cells, but higher numbers in inflammatory neuropathies. We conclude that endoneurial fibroblasts and macrophages as antigen-presenting cells may be mediators in various peripheral nerve diseases, not only in inflammatory disorders.

Lipopolysaccharide intracerebral administration induces minimal inflammatory reaction in rat brain

An inflammatory reaction, essential for defence against infection and for wound repair, may also induce irreversible tissue damage. It appears that the central nervous system has developed its own immunosuppressive strategy in order to limit the destructive effects of inflammation. To clarify this point, we have characterized in one unique model of inflammation induced in the rat by intracerebral lipopolysaccharide injection the kinetics of the inflammatory reaction, the participation of immunitary and glial cells and of three growth factors. Among these molecules, brain-derived neurotrophic factor mRNA expression was found decreased following LPS injection. No striking differences were observed in the brain parenchyma after stab lesion or inflammatory lesion apart from an increase in the number of monocytes/macrophages recruited early to the lesion area. Macrophages were later accumulated around the lesion when astroglia and microglia reactions occurred. Some of the macrophages and microglia expressed major histocompatibility complex class II antigens on their surface whereas no T or B lymphocytes were observed in the brain parenchyma. However, a subpopulation of CD3- and CD4-negative CD8-positive cells, likely natural killer cells, was observed around the lesion site; this recruitment was inhibited by the highest dose of LPS. This study therefore supports the hypothesis of a suppression of some aspects of cell-mediated immunity in the brain, mechanisms which need to be further characterized.

Vascular basement membrane components and the lesions of Alzheimer's disease: light and electron microscopic analyses

Alzheimer's disease (AD) is one of several systemic and cerebral diseases that involve the abnormal deposition of fibrillar proteins called amyloids. All amyloids share conformational and staining characteristics, as well as an association with resident tissue macrophages and two extracellular matrix components [heparan sulfate proteoglycan (HSPG) and amyloid P component]. Vascular, glomerular, and Schwann cell basement membrane pathologies have been documented in many forms of amyloidosis, and often amyloid fibrils fuse to and project from the basement membrane in these diseases. The present report demonstrates the vascular basement membrane (VBM) alterations in AD autopsy samples, and details the methodologies used. Electron microscopy reveals the fusion of amyloid fibrils with the VBM and the alteration of the VBM in the absence of amyloid accumulation. Double-labelling and pre-embed immuno-electron microscopy techniques demonstrate the colocalization of amyloid P component and VBM components with amyloid, and also reveal that amyloid P component is not localized to the cerebral VBM. Finally, a novel correlative light/electron microscopy technique demonstrates the association between amyloid P component and cerebral resident tissue macrophages, the microglia. Taken together, these data suggest that the physicochemical processes of amyloid formation, rather than amyloid deposition, may be responsible for VBM pathology.

Upregulation and induction of major histocompatibility complex class I and II antigens on microglial cells in early postnatal rat brain following intraperitoneal injections of recombinant interferon-gamma

Interferon-gamma when given intraperitoneally by single daily injection into one-day-old rats upregulated the expression of major histocompatibility complex class I antigen on ramified microglial cells in cerebral cortex and induced the expression of major histocompatibility complex class II antigen both on amoeboid and ramified microglial cells present in corpus callosum and cerebral cortex, respectively. In rats receiving single daily injections of interferon-gamma over a period of three consecutive days and killed at the age of seven days, the endothelium exhibited a moderate to weak immunoreaction for major histocompatibility complex class I antigen as detected with the monoclonal antibody OX-18. The immunoreactivity of major histocompatibility complex class I antigen on amoeboid microglial cells was comparable to that of control rats. On the other hand, it was remarkably enhanced on ramified microglial cells and was further intensified with four or six injections of interferon-gamma. In the latter, the endothelial cells also showed a stronger immunoreactivity with OX-18. In rats given three successive injections of interferon-gamma and killed at the age of seven days, 7.5% of amoeboid microglial cells in corpus callosum were induced to exhibit major histocompatibility complex class II antigen as detected with OX-6. An upsurge of the amoeboid microglial cells with major histocompatibility complex class II antigen amounting to about 40% was observed following four and six injections of interferon-gamma. A minimum of four successive injections of interferon-gamma were needed to elicit the expression of major histocompatibility complex class II antigen on ramified microglial cells in cerebral cortex.(ABSTRACT TRUNCATED AT 250 WORDS)

Perivascular edema fluid pathway in astrocytic tumors

Perivascular spaces are anatomical routes for the bulk flow drainage of fluid from the gray matter to the subarachnoid space in normal rat brain. Perivascular cells are the resident scavengers in perivascular spaces. Following focal brain damage, perivascular cells upregulate MHC Class II antigens associated with uptake of edema fluid. Similar cells can be defined in damaged human brain. In the present investigation, the distribution of MHC Class II upregulated perivascular cells was measured in 30 astrocytic tumors and adjacent edematous tissues by immunocytochemistry using the following antibodies: HLA-DR (MHC Class II), PGM1 and MAC387 (macrophages). Perivascular cells were PGM1+/MAC387- and were located in perivascular spaces along blood vessels of all sizes. MHC Class II+ perivascular cells were distributed mainly in the tumors but in some cases (4 of 10 in astrocytomas, 4 of 10 in anaplastic astrocytomas, and 7 of 10 in glioblastomas) they were also found in adjacent edematous brain. The extensive MHC Class II expression on perivascular cells suggests that perivascular cells play a scavenging role in the perivascular spaces in human brain. The results of the present study indicate the similarity between perivascular spaces in human and rat brains and emphasize the significance of perivascular spaces as anatomical routes for edema fluid drainage from human brain tissue.

CSF drains directly from the subarachnoid space into nasal lymphatics in the rat. Anatomy, histology and immunological significance

Cerebrospinal fluid (CSF) drainage pathways from the rat brain were investigated by the injection of 50 microliters Indian ink into the cisterna magna. The distribution of the ink, as it escaped from the cranial CSF space, was documented in 2 mm thick slices of brain and skull cleared in cedar wood oil and in decalcified paraffin sections. Following injection of the ink, deep cervical lymph nodes were selectively blackened within 30 min and lumbar para-aortic nodes within 6 h. Within the cranial cavity, carbon particles accumulated in the basal cisterns but were also distributed in the paravascular spaces around the middle cerebral arteries and the nasal-olfactory artery. Carbon particles in the subarachnoid space beneath the olfactory bulbs drained directly into discrete channels which passed through the cribriform plate and into lymphatics in the nasal submucosa. Although ink was distributed along the subarachnoid space of the optic nerves and entered the cochlea, the nasal route was the only direct connection between cranial CSF and lymphatics. Arachnoid villi associated with superior and inferior sagittal sinuses were identified and a minor amount of drainage of ink into dural lymphatics was also observed. This study demonstrates the direct drainage of cerebrospinal fluid through the cribriform plate in anatomically defined channels which connect with the nasal lymphatics.(ABSTRACT TRUNCATED AT 250 WORDS)

An immunohistochemical study of human pituicytes demonstrating frequent expression of MHC class II antigens and macrophage markers

Using a panel of antibodies (Abs) and a lectin, normal human adult pituicytes were studied in neurohypophyses obtained from 29 patients at autopsy. The pituicytes reacted frequently with Abs against major histocompatibility complex (MHC) class II antigens (Ags), macrophage markers (KP.1, PG.M1, LN-5), an anti-vimentin Ab and a biotinylated lectin Ricinus communis agglutinin (RCA-1). The number of pituicytes immunostained for these reagents varied, with the notable exception of vimentin. MHC class II Abs (LN-3, CR3.43)-positive pituicytes were numerous in approximately half. Microscopically, MHC class II Ag was found in pituicytes of various shapes, and were identified in macrophage-typed pituicytes by electron microscopic immunohistochemistry. Glial fibrillary acidic protein was found in only a small number of pituicytes and was absent in cells labeled with MHC class II Abs or macrophage markers. The results indicate that the immunophenotype of human pituicytes is distinct from other glial cells of the central nervous system, with a considerable number of cells expressing MHC class II Ags and macrophage markers.

Microglia in the immune surveillance of the brain: human microglia constitutively express HLA-DR molecules

The degree of MHC class II expression in histologically normal human brain biopsy or autopsy tissue is still controversial. According to the generally held view MHC class II expression is rather low in the normal brain with the exception of the white matter. In the present study, HLA-DR expression was examined immunocytochemically in different brain areas obtained from three autopsy cases with short post-mortem times (i.e. 6 h). Based on standard histological evaluation, the brain areas studied appeared as histologically normal tissue. In all brain areas there was a strong constitutive HLA-DR expression on ramified microglia. The number of HLA-DR-immunoreactive microglia was strongest in the white matter (the corpus callosum and the capsula interna for example). The border zone between white matter and grey matter, however, revealed a sharp contrast between a high density of HLA-DR-immunoreactive microglia in the white matter and a rather low number in the grey matter. In the grey matter, HLA-DR-immunoreactive microglia were much less frequent than in the white matter and more pronounced on perivascular cells. The staining and distribution pattern of HLA-DR-immunoreactive microglia was confirmed by immunocytochemistry with a panel of different anti-HLA-DR monoclonal antibodies as well as by quantitative analysis of the immunostaining. Unlike the HLA-DR immunoreactivity, HLA-ABC immunoreactivity (detecting MHC class I antigens) was confined to endothelia and not observed on microglia. In the choroid plexus stromal macrophages expressed both class I and II antigens (i.e. at a location which could provide the peripheral immune system access to CNS antigens). Constitutive HLA-DR expression by microglia qualifies them as the main resident antigen-presenting cell of the brain. The pronounced overall HLA-DR expression by resting microglia questions a central dogma of the brain as an immune-privileged site and further points to the key role of the microglia in brain immune surveillance.

Immunohistochemical study of microglia in the Creutzfeldt-Jakob diseased brain

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

Perivascular cells act as scavengers in the cerebral perivascular spaces and remain distinct from pericytes, microglia and macrophages

Perivascular cells in the rat brain are an immunophenotypically defined group of cells which can be identified by their expression of the ED2 antigen. The present study investigates the role of perivascular cells as scavengers in the perivascular spaces of the rat brain and the relationship of these cells to microglia, macrophages, pericytes and smooth muscle cells. Particulate matter (Indian ink) was injected selectively into the perivascular spaces of the left caudoputamen of 59 rats. Animals were killed by cardiac perfusion of formalin or glutaraldehyde 2 h-2 years after ink injection. Cerebral hemispheres were examined histologically and immunocytochemically using the ED2 antibody for perivascular cells, ED1 for microglia and macrophages and OX-6 directed against Ia antigen [major histocompatibility complex (MHC) class II]. ED2+ perivascular cells ingested Indian ink in the perivascular spaces and expressed MHC class II antigen. Reactive microglia and macrophages in the perivascular parenchyma expressed ED1, but no ED2+ cells were seen outside the perivascular spaces. Transmission electron microscopy distinguished perivascular cells, which ingested carbon particles, from pericytes, which did not. The results of this study suggest that perivascular cells remain distinct from pericytes, microglia and macrophages and that they play a major role as scavengers in the perivascular spaces of the rat brain. This role reflects the importance of perivascular spaces as drainage pathways for soluble and insoluble material from the brain.

Microglia in degenerative neurological disease

Microglia express many leukocyte surface antigens which are upregulated in such chronic degenerative neurological diseases as Alzheimer's disease (AD) and amyotrophic lateral sclerosis (ALS). These surface antigens include leukocyte common antigen, immunoglobulin Fc receptors, MHC class I and class II glycoproteins, beta 2-integrins, and the vitronectin receptor. Ligands for these receptors are also found. They include immunoglobulins, complement proteins of the classical pathway, T lymphocytes of the cytotoxic/suppressor and helper/inducer classes, and vitronectin. T lymphocytes marginate along capillary venules, with some penetrating into the tissue matrix. Immunoglobulins and complement proteins are synthesized locally in brain, although they may also come from the bloodstream if the blood-brain barrier is compromised. The membrane attack complex, which is formed from C5b-9, the terminal components of complement, has been identified in AD and multiple sclerosis brain tissue. In addition, proteins designed to defend against bystander lysis caused by the membrane attack complex, including protectin, C8 binding protein, clusterin, and vitronectin, are associated with damaged neuronal processes in AD. Autodestruction may play a prominent part in these 2 diseases.

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

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