Banati RB, Graeber MB (1994) Surveillance, intervention and cytotoxicity: is there a protective role of microglia? Dev Neurosci 16:114_127

Summary of Article

The study of microglial cell biology has become the key to understanding the brain's fundamental tissue reactions as well as the cellular mechanisms underlying CNS disease. This article focuses on glial_neuronal interactions with special reference to human pathology. Three important areas of brain pathology are critically reviewed: multiple sclerosis and CNS inflammation, the brain in AIDS and opportunistic infections, and neurodegenerative disorders. Although microglial cytotoxicity may cause bystander damage, e.g. in ischemia, there is little evidence to support the view that microglial activation per se is pathogenic. Results suggesting that one important normal function of microglia is to protect the integrity of the central nervous system are discussed. The concept is proposed that microglia function as a highly developed guardian to the CNS

Graeber MB, Bise K, Mehraein P (1994) CR3/43, a marker for activated human microglia: application to diagnostic neuropathology. Neuropathol Appl Neurobiol 20:406_408

Graeber MB (1994) Development of the microglia literature. Neuropathol Appl Neurobiol 20:215_216

Graeber MB, Mehraein P (1994) Microglial rod cells. Neuropathol Appl Neurobiol 20:178_180

Graeber MB (1993) Microglia, macrophages and the blood_brain barrier. Clin Neuropathol 12:296_297

Graeber MB, Bise K, Mehraein P (1993) Synaptic stripping in the human facial nucleus. Acta Neuropathol Berl 86:179_181

Summary of Article

An autopsy case of severe peripheral facial nerve paresis with disconnection of synapses from facial motor neurons is reported. A 77_year_old man presented with left_sided otitis media and subsequent development of facial nerve paresis. Three months later, the patient died of an acute gastrointestinal bleeding from a chronic duodenal ulcer. Gross inspection of the brain revealed non_stenosing arteriosclerotic vascular changes and a single small cystic lesion in the right putamen. Microscopically, marked chromatolytic changes were observed in the left facial nucleus. Immunocytochemistry for synaptophysin revealed a marked loss of afferent synaptic contacts from somatic and stem dendritic surface membranes of all chromatolytic motor neurons. Wrapping of a number of neurons by newly formed glial fibrillary acidic protein_positive astrocytic cell processes could be detected in the regenerating facial motor nucleus. In addition, expression of HLA_DR was increased on a small number of microglia and perivascular cells. These changes were absent from the contralateral, normal_appearing facial nucleus. To our knowledge, this case provides the first evidence for disconnection of synapses following peripheral nerve lesioning in humans. Occurrence of synaptic stripping is likely to explain nuclear hyperexcitability and failure of recovery of complex fine motor movements that are commonly observed following peripheral injury to the facial nerve

Streit WJ, Graeber MB (1993) Heterogeneity of microglial and perivascular cell populations: insights gained from the facial nucleus paradigm. Glia 7:68_74

Summary of Article

We reflect here on the development of a neuroimmunological concept which has been formulated over the past 5 years through studying microglial cell responses in the facial nerve system. A simple axotomy of the adult rat facial nerve which causes regeneration of facial motor neurons and little, if any, cell death can activate microglial cells just as easily as a full_ blown degeneration of the entire nucleus induced by toxic ricin. In both instances, the prompt microglial reaction is characterized by a series of structural and phenotypic changes which are in many ways similar to an immune response, e.g., there is cell proliferation and upregulation of MHC antigens. However, since white blood cells do not participate in the retrograde response of facial motor neurons, we have adopted a notion which views microglia as a CNS_wide network of immunocompetent cells whose morphological dissimilarities from leukocytes are a result of their unique adaptation to the CNS architecture. We have continued our in vivo investigations of the phagocytic and immunophenotypic properties of microglial and perivascular cells during the retrograde reaction of facial motor neurons by using intra_neural injections of fluorogold (FG) and ricin followed by lectin and immunostaining for microglia. Two new findings can be added to the microglial neuroimmune network: (1) Microglia take up FG only after motor neuron degeneration, whereas perivascular cells may take up FG under nondegenerating conditions. (2) Immunologically important molecules, such as MHC class II, CD4, and leukocyte common antigens, are expressed by different microglial subpopulations. Thus there is functional and phenotypic heterogeneity among immunocompetent cells of the CNS

Graeber MB, Streit WJ, Buringer D, Sparks DL, Kreutzberg GW (1992) Ultrastructural location of major histocompatibility complex (MHC) class II positive perivascular cells in histologically normal human brain. J Neuropathol Exp Neurol 51:303_311

Summary of Article

The expression of major histocompatibility complex (MHC) class I and II antigens was studied in surgical and postmortem brain biopsy tissue using light and electron microscopic immunocytochemistry. In addition, monoclonal antibodies directed against human macrophages (EBM11) and alpha_smooth muscle actin were applied. It is shown that blood vessel_associated MHC class II immunoreactivity in histologically normal human brain can be localized to a distinct class of cells, termed perivascular cells, which share macrophage but not smooth muscle cell antigen. This immunophenotype, the location in the perivascular space as well as the morphology, frequency and tissue distribution distinguish perivascular cells from pericytes and intraparenchymal microglia. It is suggested that MHC class II positive perivascular cells are a normal constituent of the human cerebral microvasculature. The potential role of these cells in immunological reactions occurring at the blood_brain interface is discussed

Schoen SW, Graeber MB, Kreutzberg GW (1992) 5'_Nucleotidase immunoreactivity of perineuronal microglia responding to rat facial nerve axotomy. Glia 6:314_317

Summary of Article

The ecto_enzyme 5'_nucleotidase was localized immunocytochemically in the axotomized rat facial nucleus. As revealed by the monoclonal antibody 5N4_2,5'_nucleotidase immunoreactivity markedly increased on perineuronal microglia during the first week following axotomy, and gradually disappeared from these cells by the end of the third post_ operative week. Interestingly, parenchymal microglia were not or only weakly stained. These findings indicate that 5'_nucleotidase 5N4_2_immunoreactivity may serve as a marker for perineuronal microglia, a population of satellite glial cells that appear to be actively engaged in lesion_induced synaptic changes during regeneration

Streit WJ, Graeber MB (1991) Perivascular location and phenotypic heterogeneity of microglial cells in the rat brain. J Neuroimmunol 33:87

Graeber MB, Streit WJ, Kreutzberg GW (1990) The third glial cell type, the microglia: cellular markers of activation in situ. Acta Histochem Suppl 38:157_160

Graeber MB, Streit WJ (1990) Perivascular microglia defined. Trends Neurosci 13:366

Graeber MB, Streit WJ, Kiefer R, Schoen SW, Kreutzberg GW (1990) New expression of myelomonocytic antigens by microglia and perivascular cells following lethal motor neuron injury. J Neuroimmunol 27:121_132

Summary of Article

The results of the present study demonstrate that following lethal motor neuron injury microglia and perivascular cells, as well as brain macrophages derived from the latter two cell types, newly express antigens of the myelomonocytic lineage as recognized by the monoclonal antibodies ED1 and ED3. It is suggested that differences in the immunophenotype of resident brain macrophage precursor cells, i.e. microglia and perivascular cells, and macrophages occurring outside the central nervous system (CNS) may be explained by differences in local macrophage antigen expression rather than by a different embryological lineage. The new appearance of antigens common to peripheral macrophages on neural phagocytes in CNS lesions may therefore not necessarily imply that most or all of these cells are of recent blood origin

Graeber MB, Streit WJ (1990) Microglia: immune network in the CNS. Brain Pathol 1:2_5

Summary of Article

In recent years much progress has been made toward a better understanding of the nature and function of microglial cells. This review summarizes new developments and attempts to provide a perspective for future avenues to take in microglial research. Microglia are considered to play an active role in a variety of neurological diseases. Their function in forming a network of immune competent cells within the CNS is discussed

Graeber MB, Streit WJ, Kreutzberg GW (1989) Formation of microglia_derived brain macrophages is blocked by adriamycin. Acta Neuropathol Berl 78:348_358

Summary of Article

Injection of ricin, the toxic lectin from Ricinus communis, into the rat facial nerve leads to rapid degeneration of motor neurons and concomitant proliferation and transformation of endogenous microglia into brain macrophages. Using [3H]_thymidine autoradiography, immunocytochemistry for microglial markers and electron microscopy, we could show that when ricin was administered together with the cytostatic drug adriamycin, the retrogradely transported adriamycin inhibits the macrophage response induced by toxic ricin. It is concluded that under conditions of neuronal degeneration, e.g., following ricin intoxication, brain macrophages are predominantly, if not exclusively, derived from endogenous microglia

Graeber MB, Streit WJ, Kreutzberg GW (1989) Identity of ED2_positive perivascular cells in rat brain. J Neurosci Res 22:103_106

Summary of Article

A controversial, though fundamental, issue in neurobiology concerns the nature, origin, and function of brain macrophages. By immunocytochemical analysis using monoclonal antibodies directed against rat macrophage antigens, i.e., ED1_3, Ox_41, Ox_ 42, and Ki_M2R, we show that a group of perivascular cells located within the basal membrane of CNS blood vessels are immunoreactive. These cells, which resemble pericytes in terms of their anatomical distribution, are distinct from resting parenchymal microglia immunologically as well as morphologically. Our results demonstrate considerable heterogeneity in the immunophenotype of resident brain macrophages, which may be part of the immune_nervous system interface

Graeber MB, Banati RB, Streit WJ, Kreutzberg GW (1989) Immunophenotypic characterization of rat brain macrophages in culture. Neurosci Lett 103:241_246

Summary of Article

Five monoclonal antibodies specific for rat monocytes/macrophages were used to characterize macrophages/microglia bulk isolated from neonatal and adult rat brain. The majority of brain macrophages was positive for all antibodies tested with minor differences between cultures derived from developing and mature central nervous tissue. These results contrast in vivo findings indicating that most antigens of peripheral macrophages are absent from resting, activated and phagocytic microglia in situ. We conclude that brain macrophages/microglia newly express antigens of the myelomonocytic lineage when in culture and that cultured brain macrophages may be derived from different types of precursor cells normally present within the CNS

Kreutzberg GW, Graeber MB, Streit WJ (1989) Neuron_glial relationship during regeneration of motorneurons. Metab Brain Dis 4:81_85

Summary of Article

Following axonal interruption, structural, metabolic and physiological parameters change in motorneurons. Also, glial cells are involved in this process. Microglia proliferate and express new proteins such as vimentin or MHC antigens. Astrocytes show hypertrophy, increased GFAP synthesis, and formation of lamellae. Both glial cell types participate in deafferentation and insulation of regenerating neurons, a process with significance for post_lesioning functional impairment

Rieske E, Graeber MB, Tetzlaff W, Czlonkowska A, Streit WJ, Kreutzberg GW (1989) Microglia and microglia_derived brain macrophages in culture: generation from axotomized rat facial nuclei, identification and characterization in vitro. Brain Res 492:1_14

Summary of Article

In order to study microglial cells and microglia_derived brain macrophages in vitro, a method has been developed which allows the transfer of mitotic microglial cells from adult rat brain into tissue culture. The studies were performed on facial motor nuclei which were explanted after axotomy of the facial nerve. Outgrowing cells were identified and characterized by (i) morphological criteria using light and electron microscopy, (ii) in vivo [3H]thymidine labeling combined with subsequent in vitro autoradiography, (iii) immunocytochemistry for vimentin, GFAP, Fc and complement receptors, MHC antigens, laminin, fibronectin, factor VIII related_ and 04 antigen as well as lectin histochemistry, and (iv) functional in vitro tests. In addition, a microglial cell line was established from proliferating cells. The results indicate that perineuronal microglia rather than astrocytes, perivascular cells, oligodendrocytes or endothelial cells may become phagocytic after having been activated by axotomy in situ

Streit WJ, Graeber MB, Kreutzberg GW (1989) Expression of Ia antigen on perivascular and microglial cells after sublethal and lethal motor neuron injury. Exp Neurol 105:115_126

Summary of Article

The expression of immune_associated (MHC class II) antigen was studied immunohistochemically over several months in the rat facial nucleus after nerve transection and after intraneural injection of toxic ricin. Cells expressing Ia antigen were of a perivascular type and parenchymal ramified microglia. In the first few weeks after nerve lesions we observed a gradual increase in the number of Ia_immunoreactive cells starting with an initial appearance of Ia_positive perivascular cells which were succeeded by increasing numbers of Ia_positive ramified microglia. In long_term animals Ia expression was almost exclusively found in microglia. We propose (a) the existence of a population of immunocompetent perivascular cells normally present in adult rat brain that can be stimulated to express Ia antigen, and (b) the existence of a subpopulation of ramified microglia that arises through transformation of Ia_positive perivascular cells in the adult under pathological conditions

Streit WJ, Graeber MB, Kreutzberg GW (1989) Peripheral nerve lesion produces increased levels of major histocompatibility complex antigens in the central nervous system. J Neuroimmunol 21:117_123

Summary of Article

Proliferation of central nervous system (CNS) glia in response to peripheral nerve injury occurs without apparent participation of cells of the immune system. It is shown here that following transection of the rat facial nerve there is strongly elevated expression of class I, and to a lesser extent, class II antigens of the major histocompatibility complex (MHC) in the facial nucleus. It is demonstrated by double_immunofluorescence studies that the cells responsible for increased levels of MHC class I antigens are endogenous brain microglia. These findings emphasize the thought that microglia are immunocompetent cells, but, at the same time, raise the possibility for a non_immunological function of MHC antigens under conditions of neural regeneration

Graeber MB, Streit WJ, Kreutzberg GW (1988) Axotomy of the rat facial nerve leads to increased CR3 complement receptor expression by activated microglial cells. J Neurosci Res 21:18_24

Summary of Article

Axotomy of the rat facial nerve leads to mitotic divisions of microglial cells without developing into phagocytes. In order to study the functional characteristics of those activated, i.e., proliferating but nonphagocytic, microglia we investigated the expression of monocyte/macrophage antigens by these cells. Our results show that activated microglia lack monocyte/macrophage antigens recognized by the monoclonal antibodies Ox_41, ED1, ED2, and Ki_M2R but express high levels of CR3 complement receptors in situ

Graeber MB, Kreutzberg GW (1988) Delayed astrocyte reaction following facial nerve axotomy. J Neurocytol 17:209_220

Summary of Article

Transection of the facial nerve causes a rapid increase of glial fibrillary acidic protein in reactive astrocytes and a proliferation of local microglial cells. The latter is associated with a detachment of synaptic terminals from the regenerating motor neurons. About 3 weeks following axotomy the reactive astrocytes begin to form thin, sheet_like lamellar processes which cover virtually all neuronal surfaces. A high 5'_ nucleotidase enzymic activity can be demonstrated in the plasma membrane of these thin cell processes. Subsequently, the lamellar processes become arranged in stacks which persist for several months and thus isolate the regenerating motor neurons from their afferent synaptic input. It is speculated that the process may protect the motor neurons during regeneration

Graeber MB, Streit WJ, Kreutzberg GW (1988) The microglial cytoskeleton: vimentin is localized within activated cells in situ. J Neurocytol 17:573_580

Summary of Article

Unlike astrocytes and oligodendrocytes, microglia are extremely plastic making them the chameleon among the glial cells in the CNS. This great mutability of the microglial cell shape suggests the presence of an elaborate cytoskeleton which is demonstrated here by applying a new ultrastructural method. Electron microscopic immunocytochemistry shows the presence of vimentin at intermediate filament sites in reactive microglia stimulated by rat facial nerve axotomy. It is suggested that vimentin_ expression may serve as a marker for activated states of microglia, including brain macrophages

Graeber MB, Tetzlaff W, Streit WJ, Kreutzberg GW (1988) Microglial cells but not astrocytes undergo mitosis following rat facial nerve axotomy. Neurosci Lett 85:317_321

Summary of Article

Transection of the facial nerve leads to a glial response within its central nucleus of origin. Concomitant with a proliferation of satellite microglial cells an astrocytic reaction is also seen. In the present study light and electron microscopic autoradiography were performed in order to clarify whether only microglial cells undergo mitosis following facial nerve axotomy or if astrocytes also divide. Our results provide the first electron microscopical autoradiographic evidence for the labelling of endogenous microglial cells. We suggest that microglial cells are the only proliferating element during this process in the rat facial nucleus

Streit WJ, Graeber MB, Kreutzberg GW (1988) Functional plasticity of microglia: a review. Glia 1:301_307

Summary of Article

The present review summarizes recently acquired data in vivo, which support a role of CNS microglia as a source of defense cells in the CNS capable of carrying out certain immune functions autonomously. We have kept the following discussion restricted to microglial cells and have not included work on the immunological functions of astrocytes, which has been recently reviewed elsewhere (Fontana et al.: Immunological Reviews 137:3521_3527, 1987). Resting microglia are scattered uniformly throughout the CNS forming a network of potential immunoeffector cells, which can be activated by stimuli ranging from peripheral nerve injury over viral infections to direct mechanical brain trauma. The term "activated microglia" is used here to describe proliferating cells that demonstrate changes in their immunophenotype but have not undergone transformation into brain macrophages. Such a transformation can be stimulated by neuronal death but not by sublethal neuronal injury. Microglia may function as antigen_ presenting cells and may thus represent the effector cell responsible for the recruitment of lymphocytes to the brain resulting in an inflammatory reaction. The recent developments in the understanding of microglial cell function may lead to a redefinition of the often cited "immune privilege" of the brain

Graeber MB, Kreutzberg GW (1986) Astrocytes increase in glial fibrillary acidic protein during retrograde changes of facial motor neurons. J Neurocytol 15:363_373

Summary of Article

Concomitant with the proliferation of satellite microglial cells occurring in the process of motor neuron regeneration, an astrocytic hypertrophy is also seen. A remarkable increase of glial fibrillary acidic protein (GFAP) immunoreactivity is demonstrated in astrocytes of the facial nucleus within a few days following nerve transection. The increase of GFAP antigenicity is associated with an increased appearance of glial filaments and astrocytic processes. We suggest that resident protoplasmic astrocytes become involved in retrograde changes in facial motor neurons and transform into reactive astrocytes. They are of the fibrous type and highly positive for GFAP