Georg W. Kreutzberg

Microglia: a sensor for pathological events in the CNS.

The most characteristic feature of microglial cells is their rapid activation in response to even minor pathological changes in the CNS. Microglia activation is a key factor in the defence of the neural parenchyma against infectious diseases, inflammation, trauma, ischaemia, brain tumours and neurodegeneration. Microglia activation occurs as a graded response in vivo. The transformation of microglia into potentially cytotoxic cells is under strict control and occurs mainly in response to neuronal or terminal degeneration, or both. Activated microglia are mainly scavenger cells but also perform various other functions in tissue repair and neural regeneration. They form a network of immune alert resident macrophages with a capacity for immune surveillance and control. Activated microglia can destroy invading micro-organisms, remove potentially deleterious debris, promote tissue repair by secreting growth factors and thus facilitate the return to tissue homeostasis. An understanding of intercellular signalling pathways for microglia proliferation and activation could form a rational basis for targeted intervention on glial reactions to injuries in the CNS.

Microglia: intrinsic immuneffector cell of the brain.

Microglia form a regularly spaced network of resident glial cells throughout the central nervous system (CNS). They are morphologically, immunophenotypically and functionally related to cells of the monocyte/macrophage lineage. In the ultimate vicinity of the blood-brain barrier two specialized subsets of macrophages/microglia can be distinguished: firstly, perivascular cells which are enclosed within the basal lamina and secondly juxtavascular microglia which make direct contact with the parenchymal side of the CNS vascular basal lamina but represent true intraparenchymal resident microglia. Bone marrow chimera experiments indicates that a high percentage of the perivascular cells undergoes replacement with bone marrow-derived cells. In contrast, juxtavascular microglia like other resident microglia form a highly stable pool of CNS cells with extremely little turnover with the bone marrow compartment. Both the perivascular cells and the juxtavascular microglia play an important role in initiating and maintaining CNS autoimmune injury due to their strategic localization at a site close to the blood-brain barrier, their rapid inducibility for MHC class II antigens and their potential scavenger role as phagocytic cells. The constantly replaced pool of perivascular cells probably represents an entry route by which HIV gets access to the brain. Microglia are the first cell type to respond to several types of CNS injury. Microglial activation involves a stereotypic pattern of cellular responses, such as proliferation, increased or de-novo expression of immunomolecules, recruitment to the site of injury and functional changes, e.g., the release of cytotoxic and/or inflammatory mediators. In addition, microglia have a strong antigen presenting function and a pronounced cytotoxic function. Microglial activation is a graded response, i.e., microglia only transform into intrinsic brain phagocytes under conditions of neuronal and or synaptic/terminal degeneration. In T-cell-mediated autoimmune injury of the nervous system, microglial activation follows these lines and occurs at an early stage of disease development. In experimental autoimmune encephalomyelitis (EAE), microglia proliferate vigorously, show a strong expression of MHC class I and II antigens, cell adhesion molecules, release of reactive oxygen intermediates and inflammatory cytokines and transform into phagocytic cells. Due to their pronounced antigen presenting function in vitro, activated microglia rather than astrocytes or endothelial cells are the candidates as intrinsic antigen presenting cel of the brain. In contrast to microglia, astrocytes react with a delay, appear to encase morphologically the inflammatory lesion and may be instrumental in downregulating the T-cell-mediated immune injury by inducing T-cell apoptosis.(ABSTRACT TRUNCATED AT 400 WORDS)

Neuroglial activation repertoire in the injured brain : graded response, molecular mechanisms and cues to physiological function

Damage to the central nervous system (CNS) leads to cellular changes not only in the affected neurons but also in adjacent glial cells and endothelia, and frequently, to a recruitment of cells of the immune system. These cellular changes form a graded response which is a consistent feature in almost all forms of brain pathology. It appears to reflect an evolutionarily conserved program which plays an important role in the protection against infectious pathogens and the repair of the injured nervous system. Moreover, recent work in mice that are genetically deficient for different cytokines (MCSF, IL1, IL6, TNFalpha, TGFbeta1) has begun to shed light on the molecular signals that regulate this cellular response. Here we will review this work and the insights it provides about the biological function of the neuroglial activation in the injured brain.

Displacement of synaptic terminals from regenerating motoneurons by microglial cells

SummaryAxonal reaction of motoneurons has been shown to be usually accompanied by an early and brisk proliferation of perineuronal microgliacytes. In order to clarify the real nature of such newly formed microglial satellites and their fine structural relationships to the regenerating nerve cells, facial nuclei from bilateral preparations were examined by light and electron microscopy 4 days after cutting the right facial nerve in rats. On the transected side, microgliacytes could often be observed closely adjoining motoneuron perikarya and main dendrites over long distances, and thereby removing morphologically intact synaptic terminals from the neuronal surface membranes. This displacement of boutons by microglial cells is probably preceded by a loosening of the synaptic contacts due to some unknown membrane changes in the regenerating motoneurons. The functional significance of this considerable deafferentation process could not be entirely elucidated.

Targeting gene-modified hematopoietic cells to the central nervous system: Use of green fluorescent protein uncovers microglial engraftment

Gene therapy in the central nervous system (CNS) is hindered by the presence of the blood–brain barrier, which restricts access of serum constituents and peripheral cells to the brain parenchyma. Expression of exogenously administered genes in the CNS has been achieved in vivo using highly invasive routes, or ex vivo relying on the direct implantation of genetically modified cells into the brain. Here we provide evidence for a novel, noninvasive approach for targeting potential therapeutic factors to the CNS. Genetically-modified hematopoietic cells enter the CNS and differentiate into microglia after bone-marrow transplantation. Up to a quarter of the regional microglial population is donor-derived by four months after transplantation. Microglial engraftment is enhanced by neuropathology, and gene-modified myeloid cells are specifically attracted to the sites of neuronal damage. Thus, microglia may serve as vehicles for gene delivery to the nervous system.

Lectin binding by resting and reactive microglia

SummaryConjugates of the B4 isolectin fromGriffonia simplicifolia seeds and horseradish peroxidase were used as a histochemical reagent for the specific visualization of microglial cells in the rat CNS. Resident microglia bearing galactose-containing glycoconjugates were stained throughout the brainstem and cerebellum. In the first week following axotomy of the facial nerve, a profound and rapid accumulation of reactive microglia, as evidenced by increasing lectin reactivity, was seen to take place in the facial nucleus. Light microscopy of paraffin sections demonstrated binding of lectin-horeseradish peroxidase conjugates to microglial cytoplasmic processes. When ultrastructural cytochemistry was performed, reaction product was found localized on microglial plasma membranes, as well as on intracytoplasmic membranes. The glial reaction to axotomy was studied further with double labelling of microglia and astrocytes by lectin histochemistry and immunostaining for glial fibrillary acidic protein, respectively. Our results demonstrate the presence of membrane-associated glycoconjugates containing terminal α-D-galactose residues on microglia, but not on other glial cell types. The possible nature and function of these glycoconjugates are discussed.

Microglial cells but not astrocytes undergo mitosis following rat facial nerve axotomy

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.

Expression of Ia antigen on perivascular and microglial cells after sublethal and lethal motor neuron injury

Abstract 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.

Immunocytochemical Study of an Early Microglial Activation in Ischemia

Transient arrest of the cerebral blood circulation results in neuronal cell death in selectively vulnerable regions of the rat brain. To elucidate further the involvement of glial cells in this pathology, we have studied the temporal and spatial distribution pattern of activated microglial cells in several regions of the ischemic rat brain. Transient global ischemia was produced in rats by 30 min of a four-vessel occlusion. Survival times were 1, 3, and 7 days after the ischemic injury. The microglial reaction was studied immunocytochemically using several monoclonal antibodies, e.g., against CR3 complement receptor and major histocompatibility complex (MHC) antigens. Two recently produced monoclonal antibodies against rat microglial cells, designated MUC 101 and 102, were also used to identify microglial cells. Following ischemia, the microglial reaction was correlated with the development of neuronal damage. The earliest presence of activated microglial cells was observed in the dorsolateral striatum, the CA1 area, and the dentate hilus of the dorsal hippocampus. However, the microglial reaction was not confined to areas showing selective neuronal damage, but also occurred in regions that are rather resistant to ischemia, such as the CA3 area. Particularly in the frontoparietal cortex, the appearance of MHC class II–positive microglial cells provided an early indication of the subsequent distribution pattern of neuronal damage. The microglial reaction would thus seem to be an early, sensitive, and reliable marker for the occurrence of neuronal damage in ischemia.

Neogenesis of cerebellar Purkinje neurons from gene-marked bone marrow cells in vivo

The versatility of stem cells has only recently been fully recognized. There is evidence that upon adoptive bone marrow (BM) transplantation (BMT), donor-derived cells can give rise to neuronal phenotypes in the brains of recipient mice. Yet only few cells with the characteristic shape of neurons were detected 1–6 mo post-BMT using transgenic or newborn mutant mice. To evaluate the potential of BM to generate mature neurons in adult C57BL/6 mice, we transferred the enhanced green fluorescent protein (GFP) gene into BM cells using a murine stem cell virus-based retroviral vector. Stable and high level long-term GFP expression was observed in mice transplanted with the transduced BM. Engraftment of GFP-expressing cells in the brain was monitored by intravital microscopy. In a long-term follow up of 15 mo post-BMT, fully developed Purkinje neurons were found to express GFP in both cerebellar hemispheres and in all chimeric mice. GFP-positive Purkinje cells were also detected in BM chimeras from transgenic mice that ubiquitously express GFP. Based on morphologic criteria and the expression of glutamic acid decarboxylase, the newly generated Purkinje cells were functional.