Cytokines in the central nervous system: expression of macrophage colony stimulating factor and its receptor during development

Multifaceted microglia during brain development: Models and tools

Microglia, the brain resident macrophages, are multifaceted glial cells that belong to the central nervous and immune systems. As part of the immune system, they mediate innate immune responses, regulate brain homeostasis and protect the brain in response to inflammation or injury. At the same time, they can perform a wide array of cellular functions that relate to the normal functioning of the brain. Importantly, microglia are key actors of brain development. Indeed, these early brain invaders originate outside of the central nervous system from yolk sac myeloid progenitors, and migrate into the neural folds during early embryogenesis. Before the generation of oligodendrocytes and astrocytes, microglia thus occupy a unique position, constituting the main glial population during early development and participating in a wide array of embryonic and postnatal processes. During this developmental time window, microglia display remarkable features, being highly heterogeneous in time, space, morphology and transcriptional states. Although tremendous progress has been made in our understanding of their ontogeny and roles, there are several limitations for the investigation of specific microglial functions as well as their heterogeneity during development. This review summarizes the current murine tools and models used in the field to study the development of these peculiar cells. In particular, we focus on the methodologies used to label and deplete microglia, monitor their behavior through live-imaging and also discuss the progress currently being made by the community to unravel microglial functions in brain development and disorders.

Neural progenitor cells orchestrate microglia migration and positioning into the developing cortex

Microglia are observed in the early developing forebrain and contribute to the regulation of neurogenesis through still unravelled mechanisms. In the developing cerebral cortex, microglia cluster in the ventricular/subventricular zone (VZ/SVZ), a region containing Cxcl12-expressing basal progenitors (BPs). Here we show that the ablation of BP as well as genetic loss of Cxcl12 affect microglia recruitment into the SVZ. Ectopic Cxcl12 expression or pharmacological blockage of CxcR4 further supports that Cxcl12/CxcR4 signalling is involved in microglial recruitment during cortical development. Furthermore, we found that cell death in the developing forebrain triggers microglial proliferation and that this is mediated by the release of macrophage migration inhibitory factor (MIF). Finally, we show that the depletion of microglia in mice lacking receptor for colony-stimulating factor-1 (Csf-1R) reduces BPs into the cerebral cortex.

Newborn screening for autism: in search of candidate biomarkers

BACKGROUND

Autism spectrum disorder (ASD) represents a wide range of neurodevelopmental disorders characterized by impairments in social interaction, language, communication and range of interests. Autism is usually diagnosed in children 3-5 years of age using behavioral characteristics; thus, diagnosis shortly after birth would be beneficial for early initiation of treatment.

AIM

This retrospective study sought to identify newborns at risk for ASD utilizing bloodspot specimens in an immunoassay.

MATERIALS & METHODS

The present study utilized stored frozen specimens from ASD children already diagnosed at 15-36 months of age. The newborn specimens and controls were analyzed by immunoassay in a multiplex system that included 90 serum biomarkers and subjected to statisical analysis.

RESULTS

Three sets of five biomarkers associated with ASD were found that differed from control groups. The 15 candidate biomarkers were then discussed regarding their association with ASD.

CONCLUSION

This study determined that a statistically selected panel of 15 biomarkers successfully discriminated presumptive newborns at risk for ASD from those of nonaffected controls.

Inflammation and the pathophysiology of Alzheimer's disease

There is increasing evidence that a chronic inflammatory response in the brain in Alzheimer's disease (AD) ultimately leads to neuronal injury and cognitive decline. Microglia, the primary immune effector cells of the brain, are thought to be key to this process. This paper discusses the evidence for inflammation in AD, and describes the mechanism whereby microglia generate neurotoxic cytokines, reactive oxygen species, and nitric oxide. Evidence that the cytokine macrophage colony-stimulating factor (M-CSF) is an important cofactor in microglial activation in AD is presented. Ongoing work using organotypic hippocampal expiant cultures to model the inflammatory process in the AD brain is also discussed. Potential avenues for therapeutic intervention are outlined.

Absence of colony stimulation factor-1 receptor results in loss of microglia, disrupted brain development and olfactory deficits

The brain contains numerous mononuclear phagocytes called microglia. These cells express the transmembrane tyrosine kinase receptor for the macrophage growth factor colony stimulating factor-1 (CSF-1R). Using a CSF-1R-GFP reporter mouse strain combined with lineage defining antibody staining we show in the postnatal mouse brain that CSF-1R is expressed only in microglia and not neurons, astrocytes or glial cells. To study CSF-1R function we used mice homozygous for a null mutation in the Csflr gene. In these mice microglia are >99% depleted at embryonic day 16 and day 1 post-partum brain. At three weeks of age this microglial depletion continues in most regions of the brain although some contain clusters of rounded microglia. Despite the loss of microglia, embryonic brain development appears normal but during the post-natal period the brain architecture becomes perturbed with enlarged ventricles and regionally compressed parenchyma, phenotypes most prominent in the olfactory bulb and cortex. In the cortex there is increased neuronal density, elevated numbers of astrocytes but reduced numbers of oligodendrocytes. Csf1r nulls rarely survive to adulthood and therefore to study the role of CSF-1R in olfaction we used the viable null mutants in the Csf1 (Csf1(op)) gene that encodes one of the two known CSF-1R ligands. Food-finding experiments indicate that olfactory capacity is significantly impaired in the absence of CSF-1. CSF-1R is therefore required for the development of microglia, for a fully functional olfactory system and the maintenance of normal brain structure.

Selective reduction in microglia density and function in the white matter of colony-stimulating factor-1-deficient mice

It is still debated whether microglia play a beneficial or harmful role in myelin disorders such as multiple sclerosis and leukodystrophies as well as in other pathological conditions of the central nervous system. The osteopetrotic (op/op) mouse has reduced numbers of cells of monocyte lineage as a result of an inactivating mutation in the colony stimulating factor-1 gene. To determine whether this mutant mouse might be used to study the role of microglia in myelin disorders, we quantified the number of microglia in the central nervous system of op/op mice and explored their ability to respond to brain injury created by a stab wound. Microglial density in the 2-month-old op/op mice was significantly decreased in the white matter tracts compared with the -ge matched wild-type controls (by 63.6% in the corpus callosum and 86.4% in the spinal dorsal column), whereas the decrease was less in the gray matter, cerebral cortex (24.0%). A similar decrease was seen at 7 months of age. Morphometric studies of spinal cord myelination showed that development of myelin was not affected in op/op mice. In response to a stab wound, the increase in the number of microglia/macrophages in op/op mice was significantly less pronounced than that in wild-type control. These findings demonstrate that this mutant is a valuable model in which to study roles of microglia/macrophages in the pathophysiology of myelin disorders.

Cytokines and CNS development

Cytokines are pleotrophic proteins that coordinate the host response to infection as well as mediate normal, ongoing signaling between cells of nonimmune tissues, including the nervous system. As a consequence of this dual role, cytokines induced in response to maternal infection or prenatal hypoxia can profoundly impact fetal neurodevelopment. The neurodevelopmental roles of individual cytokine signaling pathways are being elucidated through gain- and loss-of-function studies in cell culture and model organisms. We review this work with a particular emphasis on studies where cytokines, their receptors, or components of their signaling pathways have been altered in vivo. The extensive and diverse requirements for properly regulated cytokine signaling during normal nervous system development revealed by these studies sets the foundation for ongoing and future work aimed at understanding how cytokines induced normally and pathologically during critical stages of fetal development alter nervous system function and behavior later in life.

Homeobox gene HLX1 is a regulator of colony stimulating factor-1 dependent trophoblast cell proliferation

The cytokine colony stimulating factor-1 (CSF-1) is a key regulator of the proliferation, differentiation and activation of mononuclear phagocytes. CSF-1 also plays an important role in reproduction. CSF-1 is produced in the placenta and activates signal transduction pathways that significantly increase the proliferation of placental trophoblast cells in culture. The target genes activated by CSF-1 mediated signal transduction in the nucleus are not well understood. Here, we use placental trophoblast cells to investigate potential downstream effector genes of CSF-1. HLX1 is a homeobox gene that controls proliferation in embryonic cell types and haematopoietic cell lineages. We have shown HLX1 is expressed in placental trophoblast cells but its functional role in the placenta is unknown. Following CSF-1 stimulation, HLX1 mRNA expression was significantly increased in SGHPL-4 and HTR-8/SVNeo cultured trophoblast cells (p<0.001, n=3). siRNA-mediated reduction of HLX1 mRNA levels with four independent oligonucleotides (siRNAs) resulted in significantly decreased cell proliferation in both cell lines (p<0.001, n=4). When HLX1 mRNA levels were reduced in the presence of CSF-1 stimulation, proliferation remained significantly decreased (p<0.001, n=4) in both the cell lines. We have shown for the first time that a homeobox gene, HLX1, is a downstream effector gene of CSF-1, that HLX1 regulates placental cell proliferation and that CSF-1 acts, at least in part, through HLX1 to control cell proliferation.

Differentiation of primary adult microglia alters their response to TLR8-mediated activation but not their capacity as APC

Activated microglia are found in a variety of neuroinflammatory disorders where they have attributed roles as effector as well as antigen-presenting cells (APC). Critical determinants for the multifaceted role of microglia are the differentiation potential of microglia and their mode of activation. In this study, we have investigated the effects of M-CSF and GM-CSF-mediated differentiation of adult primate microglia on their cellular phenotype, antigen presentation, and phagocytic function as well as on Toll-like receptor (TLR)-mediated responses. We show that although cell morphology and expression levels of activation markers were markedly different, differentiation with either factor yielded microglia that phenotypically and functionally resemble macrophages. Both M-CSF and GM-CSF-differentiated microglia were responsive to TLR1/2, 2, 3, 4, 5, 6/2, and 8-mediated activation, but not to TLR7 or 9-mediated activation. Intriguingly, M-CSF-differentiated microglia expressed higher levels of TLR8-encoding mRNA and protein, and produced larger amounts of proinflammatory cytokines in response to TLR8-mediated activation as compared to GM-CSF-differentiated microglia. While differentiation of adult microglia by growth factors that can be produced endogenously in the central nervous system is thus unlikely to change their APC function, it can alter their innate responses to infectious stimuli such as ssRNA viruses. Resident primate microglia may thereby help shape rather than initiate adaptive immune responses.

Amyloid beta protein deposition in osteopetrotic (op/op) mice is reduced by injections of macrophage colony stimulating factor

The deposition of amyloid beta (Abeta) protein is a neuropathological change that characterizes Alzheimer's disease. Animals with the osteopetrosis (op/op) mutation suffer from a general skeletal sclerosis, a significantly reduced number of macrophages and osteoclasts in various tissues, and have no systemic macrophage colony stimulating factor (M-CSF). This study examined the effect that M-CSF injections had on Abeta deposition and microglial cell distribution in the brains of normal and op/op mice. Abeta-positive plaques were detected in the cerebral cortex of op/op mice, but not in normal mice. M-CSF reduced the numbers of Abeta-positive plaques in op/op mice. The microglial cell population was reduced in op/op mice compared with normal mice, and M-CSF increased the numbers to 65.8% of that observed in normal mice. Our results suggest that a clearer understanding of the role that microglial cells play in Abeta deposition may help determine the mechanisms involved in the pathogenesis of Alzheimer's disease.

Protein kinase C and phosphoinositol-3-kinase mediate differentiation or proliferation of slice-derived rat microglia

Granulocyte-macrophage colony-stimulating factor plays an important role in the activation of microglia in the central nervous system. We have recently shown (see text) that granulocyte-macrophage colony-stimulating factor activates the proliferation and subsequent migration of microglia from organotypic cortex brain slices. The aim of this study was to investigate whether this activation is modulated by different putative intracellular pathway inhibitors. Our data show that the protein kinase C inhibitor staurosporine enhanced the proliferation as well as the differentiation of slice-derived microglia, while the phosphoinositol-3-kinase inhibitor LY294002 markedly suppressed the proliferative activity. In conclusion, proliferation, migration, as well as differentiation of rat microglia are highly regulated by intracellular signaling cascades.

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.

Expression of macrophage colony-stimulating factor and its receptor in microglia activation is linked to teratogen-induced neuronal damage

Prenatal exposure to teratogen agents is linked to the pathogenesis of neurodevelopment disorders, but the mechanisms leading to the neurodevelopmental disturbance are poorly understood. To elucidate this, an in vitro model of microglial activation induced by neuronal injury has been characterized. In this connection, exposure of primary microglial cells to the conditioned medium from the neuronal damage induced by teratogen, cyclophosphamide, is accompanied by a reactive microgliosis as assessed by reverse transcription-polymerase chain reaction, enzyme-linked immunosorbent assay, lectin histochemistry, double labeling immunohistochemistry and in situ hybridization. Our results showed that reactive microglia were capable of releasing various cytokines such as tumor necrosis factor-alpha, interleukin-1, interleukin-6, transforming growth factor-beta and nitric oxide. Also, we have shown that macrophage colony-stimulating factor (M-CSF) was in fact produced by the reactive microglia. Concomitant to this was the increased expression of M-CSF receptor in these cells following the teratogen-induced neuronal injury. The up-regulation of M-CSF receptor suggests that the cells are capable of responding to self-derived M-CSF in an autocrine fashion. Results with antibody neutralization further suggest that microglial proinflammatory response, as manifested by cytokine expression in culture, is mediated by M-CSF, which acts as a molecular signal that initiates a microglial reaction. We therefore suggest that microglial activation following cyclophosphamide treatment is not only a response to the neuronal damage, but is also a cause of the damage during pathogenesis of neurodevelopment disorders. To this end, the increased expression of M-CSF and its receptor on microglia would be directly linked to the active cell proliferation and proinflammatory response in the teratogen-induced injury.

Granulocyte macrophage-colony stimulating factor activates microglia in rat cortex organotypic brain slices

Cytokines play an important role in the regulation of proliferation and migration in the central nervous system. The aim of this study was to determine if granulocyte macrophage-colony stimulating factor (GM-CSF) activates cells in the cortex of organotypic brain slice cultures. Our data show that murine GM-CSF markedly stimulated the proliferation and migration of small round microglia from a cortex slice. These round cells were strongly positive for integrin CD11b (OX-42), isolectin B4-lectin-binding, the monocytic marker ED1 and partly expressed major histocompatibility complex (MHC) class II antigen (OX-6). Only some differentiated microglia were visible which expressed the integrin CD11c and MHCII. GM-CSF enhanced the proliferation as analyzed by bromodeoxyuridine incorporation. The number of migrated cells decreased during culturing and enhanced terminal dUTP nick-end labelling positive nuclei were found. Taken together, our data conclude that GM-CSF is an important cytokine, which regulates the proliferation and migration of cortical microglia.

G-CSF therapy of ongoing experimental allergic encephalomyelitis via chemokine- and cytokine-based immune deviation

Converging evidence that G-CSF, the hemopoietic growth factor of the myeloid lineage, also exerts anti-inflammatory and pro-Th2 effects, prompted us to evaluate its direct therapeutic potential in autoimmune diseases. Here we report a novel activity of G-CSF in experimental allergic encephalomyelitis, a murine model for multiple sclerosis, driven by Th1-oriented autoaggressive cells. A short 7-day treatment with G-CSF, initiated at the onset of clinical signs, provided durable protection from experimental autoimmune encephalomyelitis. G-CSF-treated mice displayed limited demyelination, reduced recruitment of T cells to the CNS, and very discrete autoimmune inflammation, as well as barely detectable CNS mRNA levels of cytokines and chemokines. In the periphery, G-CSF treatment triggered an imbalance in the production by macrophages as well as autoreactive splenocytes of macrophage inflammatory protein-1alpha and monocyte chemoattractant protein-1, the prototypical pro-Th1 and pro-Th2 CC chemokines, respectively. This chemokine imbalance was associated with an immune deviation of the autoreactive response, with reduced IFN-gamma and increased IL-4 and TGF-beta1 levels. Moreover, G-CSF limited the production of TNF-alpha, a cytokine also associated with early CNS infiltration and neurological deficit. These findings support the potential application of G-CSF in the treatment of human autoimmune diseases such as multiple sclerosis, taking advantage of the wide clinical favorable experience with this molecule.

Astrogliosis in the adult and developing CNS: is there a role for proinflammatory cytokines?

Astrogliosis, characterized by the enhanced expression of GFAP, represents a remarkably homotypic response of astrocytes to all types of injuries of the CNS, including injuries of the developing CNS. As such, astrocytes serve as microsensors of the injured microenvironment regardless of their location in the CNS. The diversity of insults that engender astrogliosis and the brain-wide nature of the astrocytic response suggest that common injury factors serve as the trigger of this cellular reaction. One prominent theme that has emerged in recent years is that proinflammatory cytokines and chemokines serve as a stimulus for induction of astrogliosis. Here we present a brief critique of this hypothesis based on a review of literature and some of our own recentfindings. Studies of astrocytes, in vitro, clearly indicate that these cell types are responsive to a variety of growth factors, including cytokines and chemokines. A somewhat different picture, however, can be seen from data obtained in vivo. It is true that trauma and diseases of the nervous system, as well as some exposures to neurotoxic chemicals, can be associated with the expression in brain of large varieties of cytokines and chemokines. That these same conditions result in astrogliosis has fostered the circumstantial link between cytokine/chemokine expression and the induction of astrogliosis. Several lines of evidence argue against this view, including (a) suppression of cytokine expression does not suppress gliosis, (b) gliosis can occur in the absence of enhanced expression of cytokines, (c) elevations in brain cytokines can occur in the absence of gliosis and (d) the patterns of cytokine expression in the adult and developing CNS are more consistent with a trophic role for these chemical messengers rather than a role in the induction of inflammation. Enhanced expression of cytokines and chemokines after brain injury appear to be signal transduction events unrelated to the induction of astrogliosis.

Overexpression of macrophage colony-stimulating factor receptor on microglial cells induces an inflammatory response

Microglia are important in the inflammatory response in Alzheimer's disease (AD). We showed previously that macrophage colony-stimulating factor receptor (M-CSFR), encoded by the c-fms protooncogene, is overexpressed on microglia surrounding amyloid beta (Abeta) deposits in the APP(V717F) mouse model for AD. The M-CSFR is also increased on microglia after experimental brain injury and in AD. To determine the relevance of these findings, we transiently expressed M-CSFR on murine BV-2 and human SV-A3 microglial cell lines using an SV40-promoted c-fms construct. M-CSFR overexpression resulted in microglial proliferation and increased expression of inducible nitric-oxide synthase, the proinflammatory cytokines interleukin-1alpha, macrophage inflammatory protein 1-alpha, and interleukin-6 and of macrophage colony-stimulating factor (M-CSF) itself. Antibody neutralization of M-CSF showed that the M-CSFR-induced proinflammatory response was dependent on M-CSF in the culture media. By using a co-culture of c-fms-transfected murine microglia and rat organotypic hippocampal slices and a species-specific real time reverse transcriptase-polymerase chain reaction assay and enzyme-linked immunosorbent assay, we showed that M-CSFR overexpression on exogenous microglia induced expression of interleukin-1alpha by the organotypic culture. These results show that increased M-CSFR expression induces microglial proliferation, cytokine expression, and a paracrine inflammatory response, suggesting that in APP(V717F) mice increased M-CSFR on microglia could be an important factor in Abeta-induced inflammatory response.

Macrophage colony-stimulating factor is expressed in neuron and microglia after focal brain injury

In a previous study, we have demonstrated that damaged neurons within a boundary area around necrosis fall into delayed neuronal death owing to the cytotoxic effect of microglial nitric oxide (NO), and these neurons are finally eliminated by activated microglia. In this process, microglia are activated to release NO, increase in number, and accumulate toward the damaged area. In this study, we investigated the expression of macrophage colony-stimulating factor (M-CSF, also called colony stimulating factor-1; CSF-1) and other cytokines, which are reported to relate to activation, proliferation, or migration of microglia. The mRNA of M-CSF arose biphasically from 30 min to 1 hr and from 6 to 72 hr after the injury, as demonstrated by semiquantitative RT-PCR. However, another cytokine of granulocyte-macrophage CSF (GM-CSF) or interleukin-3 (IL-3), which causes proliferation of microglia in vitro, was not detected. From immunohistochemical studies, positive staining of M-CSF was observed mainly in neuron-specific enolase (NSE)-positive cells from 1 to 12 hr after the injury, and after that M-CSF became positive in Griffonia simplicifolia isolectin-B4 (GSA-I-B4)-positive cells from 24 to 72 hr in the boundary area around necrosis. These results suggest that neurons around the damaged area express M-CSF in the early phase after injury, which may initially activate microglia, and these activated microglia also express M-CSF later, causing further proliferation or migration of microglia themselves to eliminate damaged neurons or necrotic brain tissue.

Response of amoeboid microglia/brain macrophages in fetal rat brain exposed to a teratogen

This study examined the time course response of amoeboid microglia/brain macrophages in the rat fetus induced by a single intraperitoneal injection of cyclophosphamide, a teratogen, into the mother rat at 13 days of gestation. Compared to the normal fetal brain, a marked increase in amoeboid microglia was observed in the telencephalon and diencephalon of experimental fetuses, especially in those killed at embryonic day 15. Conglomerations of microglia occurred in the dorsal and superior neuroepithelium of diencephalon, basal telencephalon, cortical neuroepithelium, and hippocampal formation as identified with OX-42, OX-18, and ED-1 by immunohistochemistry. Rhodamine isothiocynate (RhIc) as a tracer was injected via the tail vein into the pregnant rat to assess the phagocytic capability of these cells. Following the tracer injection, none of microglial cells in normal fetal brain was detectable. RhIc, however, was readily taken up by amoeboid microglia in fetal brain with injury insult. Double labeling has shown that the RhIc-labeled cells were immunoreactive with ED-1, OX-42, OX-18, and OX-6, confirming their microglial nature. Microglial proliferation was assessed by immunohistochemistry using bromodeoxyuridine, which showed a marked increase in mitotic activity. Confocal microscopic analysis revealed that a varying number of microglia coexpressed iNOS, macrophage colony-stimulating factor (M-CSF), and ICAM-1. RT-PCR analysis showed increased expression of M-CSF mRNA. Furthermore, colony-stimulating factor-1 receptor mRNA was localized in microglia by in situ hybridization. The present results suggest that NO along with M-CSF and ICAM-1 is involved in microglial proliferation in prenatal brain injury.

Peptides as regulators of the immune system: emphasis on somatostatin

Study of the communication between nervous and immune systems culminated in the understanding that cytokines, formerly considered exclusively as immune system-derived peptides, are endogenous to the brain and display central actions. More recently, immune cells have been recognized as a peripheral source of "brain-specific" peptides with immunomodulatory actions. This article reviews studies concerning reciprocal effects of selected cytokines and neuropeptides in the nervous and immune systems, respectively. The functional equivalence of these two categories of communicators is discussed with reference to the example of the actions of neuropeptide somatostatin in the immune system.

The distribution of granulocyte-macrophage colony-stimulating factor and its receptor in the developing human fetus

The purpose of the present study was to determine the distribution of granulocyte-macrophage colony-stimulating factor (GM-CSF) and its receptor (GM-CSF-R) in the human fetus. We used reverse transcription PCR to detect GM-CSF and GM-CSF-R mRNA in human fetal organs at 8 and 16 wk postconception, and cell-specific protein expression was localized in tissues by immunohistochemistry. GM-CSF was also measured by ELISA in paired samples of spinal fluid and plasma. GM-CSF mRNA and/or protein were detected in lung macrophages, spleen, adrenal cortex, placenta, and CNS including neurons and astrocytes. GM-CSF was detected by ELISA in 10 of the 39 cerebrospinal fluid samples tested. GM-CSF-R mRNA expression was present in all organs tested. Immunoreactivity for GM-CSF-R in most organs was limited to macrophages, but, brain, neurons and glial cells showed immunoreactivity. We conclude that GM-CSF is produced in lung, spleen, adrenal, placenta, and neural tissues during human fetal development and that GM-CSF-responsive cells include macrophages, neurons, and glial cells.

Molecular signals for glial activation: pro- and anti-inflammatory cytokines in the injured brain

Injury to the central nervous system leads to cellular changes not only in the affected neurons but also in adjacent glial cells. This neuroglial activation is a consistent feature in almost all forms of brain pathology and appears to reflect an evolutionarily-conserved program which plays an important role for the repair of the injured nervous system. Recent work in mice that are genetically-deficient for different cytokines (M-CSF, IL-6, TNF-alpha, TGF-beta 1) has begun to shed light on the molecular signals that regulate this cellular response. Here, the availability of cytokine-deficient animals with reduced or abolished neuroglial activation provides a direct approach to determine the function of the different components of the cellular response leading to repair and regeneration following neural trauma.

Antigen presentation function of brain-derived dendriform cells depends on astrocyte help

In mouse brain primary culture, supplementation with granulocyte macrophage colony-stimulating factor (GM-CSF) induces development of dendriform cells emerging on the astroglia monolayer. As revealed by flow cytofluorimetric analysis, >70% of isolated cells are CD11c(+) and express the dendritic cell (DC) marker 33D1. Additional expression of F4/80 and CD11b suggests a myeloid origin of these cells. The lymphoid DC marker CD8alpha is lacking while DEC-205 has been detected on approximately 10% of the cells. When freshly isolated, such brain-derived DC-like cells are excellent antigen-presenting cells (APC) but their functional capability is lost during subculture with GM-CSF. In contrast, their antigen presentation function remains stable in the presence of GM-CSF plus astrocytes or astrocyte-conditioned medium. The responsible astrocytic activity co-fractionates with macrophage colony-stimulating factor (M-CSF). Neutralization of the activity with anti-M-CSF antibody and substitution with recombinant M-CSF provide evidence that, in addition to GM-CSF, M-CSF is required to preserve the functional capability of these brain-derived APC. Responsiveness of the isolated cells to M-CSF is substantiated by the expression of c-fms/M-CSF receptor gene. Consistently, GM-CSF proves stimulatory for astrocytes by up-regulating their secretion of M-CSF. Furthermore, depletion or blocking of endogenous M-CSF in primary brain cell culture prevents the development of functionally active APC regardless of exogenous GM-CSF. In sum, these findings ascribe an immature DC phenotype to GM-CSF-grown myeloid brain cells and indicate a role for astrocytic M-CSF in maintaining their antigen presentation function.

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.

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.

Augmented expression of macrophage migration inhibitory factor (MIF) in the telencephalon of the developing rat brain

Macrophage migration inhibitory factor (MIF) has been characterized as a proinflammatory cytokine, hormone, and immunomodulator. We previously demonstrated the distribution of MIF in the adult rat brain and revealed its expression in neurons as well as glial cells. In this study, we investigated the localization of MIF in the developing rat brain from embryonic day (ED) 13 to ED19, and after birth from postnatal day (PD) 1 to PD28 using both immunohistochemistry and in situ hybridization. On ED16, the signals of MIF mRNA were high in the ventricular, subventricular, and intermediate zones of the telencephalon, in which 'neuropoietic' progenitor cells proliferate and migrate from the ventricular zone to the superficial layer in the cerebral cortex. The mRNA expression was detected throughout the postnatal period, and intense signals of the transcript were seen in the ventricle and in layers I, II and III of the cerebral cortex at PD5. Similarly, positive staining of MIF protein was seen in the ventricular zone by immunohistochemical analysis, although the positively stained area appeared to be smaller than the mRNA expression. Taken together, these results suggest that the increase in MIF mRNA in the developing brain reflects the growth and maturation of neurons and glial cells.

Cytokines in the neuroendocrine system

Cytokines are important partners in the bidirectional network interrelating the immune and the neuroendocrine systems. These substances and their specific receptors, initially thought to be exclusively present in the immune system, have recently been shown to be also expressed in the neuroendocrine system. Cytokines can modulate the responses of all endocrine axes by acting at both the central and the peripheral levels. To explain how systemic cytokines may gain access to the brain, several mechanisms have been proposed, including an active transport through the blood-brain barrier, a passage at the circumventricular organ level, as well as a neuronal pathway through the vagal nerve. The immune-neuroendocrine interactions are involved in numerous physiological and pathophysiological conditions and seem to play an important role to maintain homeostasis.

Regulation of the hypothalamic-pituitary-adrenal axis by cytokines: actions and mechanisms of action

Glucocorticoids are hormone products of the adrenal gland, which have long been recognized to have a profound impact on immunologic processes. The communication between immune and neuroendocrine systems is, however, bidirectional. The endocrine and immune systems share a common "chemical language," with both systems possessing ligands and receptors of "classical" hormones and immunoregulatory mediators. Studies in the early to mid 1980s demonstrated that monocyte-derived or recombinant interleukin-1 (IL-1) causes secretion of hormones of the hypothalamic-pituitary-adrenal (HPA) axis, establishing that immunoregulators, known as cytokines, play a pivotal role in this bidirectional communication between the immune and neuroendocrine systems. The subsequent 10-15 years have witnessed demonstrations that numerous members of several cytokine families increase the secretory activity of the HPA axis. Because this neuroendocrine action of cytokines is mediated primarily at the level of the central nervous system, studies investigating the mechanisms of HPA activation produced by cytokines take on a more broad significance, with findings relevant to the more fundamental question of how cytokines signal the brain. This article reviews published findings that have documented which cytokines have been shown to influence hormone secretion from the HPA axis, determined under what physiological/pathophysiological circumstances endogenous cytokines regulate HPA axis activity, established the possible sites of cytokine action on HPA axis hormone secretion, and identified the potential neuroanatomic and pharmacological mechanisms by which cytokines signal the neuroendocrine hypothalamus.

Correction of murine galactosialidosis by bone marrow-derived macrophages overexpressing human protective protein/cathepsin A under control of the colony-stimulating factor-1 receptor promoter

Galactosialidosis (GS) is a human neurodegenerative disease caused by a deficiency of lysosomal protective protein/cathepsin A (PPCA). The GS mouse model resembles the severe human condition, resulting in nephropathy, ataxia, and premature death. To rescue the disease phenotype, GS mice were transplanted with bone marrow from transgenic mice overexpressing human PPCA specifically in monocytes/macrophages under the control of the colony stimulating factor-1 receptor promoter. Transgenic macrophages infiltrated and resided in all organs and expressed PPCA at high levels. Correction occurred in hematopoietic tissues and nonhematopoietic organs, including the central nervous system. PPCA-expressing perivascular and leptomeningeal macrophages were detected throughout the brain of recipient mice, although some neuronal cells, such as Purkinje cells, continued to show storage and died. GS mice crossed into the transgenic background reflected the outcome of bone marrow-transplanted mice, but the course of neuronal degeneration was delayed in this model. These studies present definite evidence that macrophages alone can provide a source of corrective enzyme for visceral organs and may be beneficial for neuronal correction if expression levels are sufficient.

The expression of cytokines and their receptors in normal and mildly reactive human brain

There are many inflammatory diseases of the brain such as AIDS, other viral encephalitides and multiple sclerosis all of which are probably influenced by both systemic and focal CNS cytokine release. We have studied the expression of a wide range of pro- and anti-inflammatory cytokines and their receptors, beta2-microglobulin, and MHC Class II, using immunocytochemistry on cryostat sections of normal and mildly reactive human brain. The aim was to try to determine the cytokine 'baseline' expression in normal human brain and the results obtained indicated very low expression of various cytokines and their receptors, mainly by microglia and macrophages with some endothelial expression.

Reduced number and altered morphology of microglial cells in colony stimulating factor-1-deficient osteopetrotic op/op mice

The numerical density of microglial cells is reduced by 47% in the corpus callosum, by 37% in the parietal cortex and by 34% in the frontal cortex of mice mutant at the op locus which are totally devoid of colony stimulating factor-1 (CSF-1), the major growth factor for macrophages. Moreover, microglia in the frontal cortex of the op/op mice are smaller and have shorter cytoplasmic processes compared to control mice. Study suggests that CSF-1 plays a role in vivo in the formation and maturation of microglia and has little or no effect on perivascular cells.

Macrophage colony-stimulating factor augments beta-amyloid-induced interleukin-1, interleukin-6, and nitric oxide production by microglial cells

In Alzheimer's disease (AD), a chronic cerebral inflammatory state is thought to lead to neuronal injury. Microglia, intrinsic cerebral immune effector cells, are likely to be key in the pathophysiology of this inflammatory state. We showed that macrophage colony-stimulating factor, a microglial activator found at increased levels in the central nervous system in AD, dramatically augments beta-amyloid peptide (betaAP)-induced microglial production of interleukin-1, interleukin-6, and nitric oxide. In contrast, granulocyte macrophage colony-stimulating factor, another hematopoietic cytokine found in the AD brain, did not augment betaAP-induced microglial secretory activity. These results indicate that increased macrophage colony-stimulating factor levels in AD could magnify betaAP-induced microglial inflammatory cytokine and nitric oxide production, which in turn could intensify the cerebral inflammatory state by activating astrocytes and additional microglia, as well as directly injuring neurons.

Regulation of MCSF receptors on microglia in the normal and injured mouse central nervous system: a quantitative immunofluorescence study using confocal laser microscopy

The macrophage colony-stimulating factor (MCSF) is a 40-76-kD glycoprotein that plays an important role in the activation and proliferation of microglia both in vitro and in injured neural tissue. Here, we examined the regulation of MCSF receptor (MCSFR) and MCSF in the normal and injured mouse central nervous system (CNS) by using confocal laser microscopy, quantitative immunofluorescence, and reverse transcriptase-polymerase chain reaction (RT-PCR) techniques. Immunohistochemistry on fixed, floating tissue sections demonstrated low to moderate MCSFR immunoreactivity (MCSFR-IR) on microglia in the gray and white matter throughout the mouse CNS in the forebrain, brainstem, cerebellum, and spinal cord. High levels of MCSFR-IR were restricted to the superficial layer of the spinal cord dorsal horn, substantia nigra, and area postrema, a CNS region that lacks the blood-brain barrier. CNS injury led to a strong and specific increase in MCSFR-IR in the directly injured dorsal forebrain, in the cervical spinal cord (C2) after transection of the sensory, minor occipital nerve, and in the axotomized facial motor nucleus. Further investigation at the mRNA level in the facial nucleus model showed that this increase was accompanied by a rapid induction of the transcript for MCSFR, with a peak 1-2 days after injury, but only a constitutive expression of MCSF-mRNA. In summary, although normal levels of MCSF receptor in most microglia are low, microglial activation is accompanied by a rapid and massive increase. In view of the constitutive expression of MCSF, the early upregulation of the MCSF receptor may play a central role in preparing these macrophage-related cells to take part in the cellular response to CNS injury.

GM-CSF affects hypothalamic neurotransmitter levels in mice: involvement of interleukin-1

We studied the effects of granulocyte-macrophage colony stimulating factor (GM-CSF) on the brain levels of several neurotransmitters in mice. Administration of GM-CSF (5.0 and 10 microg, i.p.) significantly reduced the hypothalamic levels of glutamine, glutamic acid, GABA and aspartic acid. GM-CSF (5.0 microg, i.p.) also induced a significant reduction of norepinephrine and serotonin levels in the hypothalamus, without affecting dopamine levels. The hippocampal levels of neurotransmitters were not modified by GM-CSF administration. The peripheral administration of a specific interleukin-1 receptor antagonist (IL-1ra, 50 microg, i.p.) blocked the effects of GM-CSF. These results confirm our previous behavioural data suggesting that GM-CSF is able to exert neuromodulatory actions.

Neurons promote macrophage proliferation by producing transforming growth factor-beta2

The infiltration of bone marrow-derived macrophages into the CNS contributes to growth and reactions of microglia during development or after brain injury. The proliferation of microglial cells is stimulated by colony-stimulating factor 1 (CSF-1), an astrocyte-produced growth factor that acts on mononuclear phagocytes. In the present study, we have shown, using an in vitro model system, that rodent neurons obtained from the developing cerebral cortex produce a soluble factor that strongly enhances the proliferation of macrophages cultured in the presence of CSF-1. Both macrophages isolated from the developing brain and those from the adult bone marrow were stimulated. Kinetic analyses of [3H]thymidine incorporation into macrophages indicated that their response to the neuron-derived factor involved a shortening of the cycle of proliferating cells. The effect of neurons on macrophages was blocked in the presence of antibodies neutralizing transforming growth factor-beta2 (TGF-beta2), whereas recombinant TGF-beta2 stimulated macrophage proliferation in the presence of CSF-1. Neuronal secretion of TGF-beta2 was confirmed by reverse transcription-PCR detection of TGF-beta2 transcripts and immunodetection of the protein within neurons and in their culture medium. In situ hybridization and immunohistochemical experiments showed neuronal expression of TGF-beta2 in sections of cerebral cortex obtained from 6-d-old rats, an age at which extensive developmental recruitment of macrophages occurs in this cerebral region. Altogether, our results provide direct evidence that neurons have the capacity to promote brain macrophage proliferation and demonstrate the role of TGF-beta2 in this neuronal function.

The promoter of macrophage colony-stimulating factor receptor is active in astrocytes

Macrophage colony-stimulating factor (M-CSF) is a hematopoietin whose actions are essential for growth and survival of macrophages, placental development, ramification of microglia and tumor progression. The expression of the receptor for macrophage colony-stimulating factor (c-fms) is regulated by two distinct promoters: distal and proximal. The distal promoter is active in trophoblasts during embryogenesis and the proximal promoter directs expression to the cells of myeloid lineage. Here we report the generation of transgenic mice expressing beta-galactosidase under the control of the human proximal c-fms promoter and demonstrate the promoter activity in astrocytes, cells of neurological origin that partially take over the role of the macrophages in the central nervous system. Enzymatic activity of beta-galactosidase was detected in homogenated spleen, bone marrow and brain and in the cell extracts from peritoneal macrophages of transgenic mice. Immunohistochemical staining of brain showed the presence of beta-galactosidase in astrocytes. We hypothesize that M-CSF released by astrocytes, upon stimulation by lipopolysaccharide (LPS), tumor necrosis factor alpha (TNF alpha) or interleukin-1 (IL-1), regulates the expression of its own receptor.

[Cerebrospinal fluid analysis and the pathogenesis of central nervous system infection by HTLV-I]

The immunopathogenesis of the HTLV-I associated myelopathy (HAM) may be studied by the CSF evaluation. The mechanism of this myelopathy remains unknown. The disturbs of the cellular and humoral immune response observed in HAM patients suggest that the immunological derangement may contribute to the disease mechanisms. For hypothesis, the migration of infected lymphocytes through the blood-brain barrier could have a main role at the pathogenesis of HAM. An increase of the production of cytokines as tumor necrosis factor alpha (TNF alpha) contributes to the migration of lymphocytes through the expression of the intercellular adhesion molecule on the surface of the endothelial cells. On the other side, new knowledges suggest that the imbalance between the production of TNF alpha and its soluble receptor (sTNF-R) could result in the lesive effects of this cytokine in the central nervous system.

Role of colony-stimulating factor-1 in reproduction and development

The CSF-1 null mouse, osteopetrotic, has provided a powerful model in which to study the biological functions of CSF-1. In this review, I will describe our studies that have used this mouse model to determine the impact of a lack of CSF-1 on developmental processes and in reproduction. A role for CSF-1 in reproduction was originally suggested by the sex steroid hormone-regulated uterine epithelial synthesis of CSF-1 and the expression of its receptor in trophoblast and decidual cells. Studies on the fertility of CSF-1 deficient osteopetrotic mice (csfmop/csfmop) mice confirmed this suggestion and in addition revealed an unexpected function for CSF-1 in male fertility. In both sexes, CSF-1 appears to regulate gonadal steroidogenesis, probably through its action on macrophages that are abundant throughout the ovary and testis. In the female, CSF-1 affects ovulation in vivo and in vitro, and impacts the preimplantation embryo, increasing both its rate of development and the number of trophectodermal cells in the blastocyst. CSF-1 also has a role in mammary gland development during pregnancy, since at mid-gestation in csfmop/csfmop mice, ductal branching is impaired, and after partiturition, there is a failure to switch to lactation. The relative failure of csfmop/csfmop mice to respond to external stimuli also suggested a role for CSF-1 in the brain. CSF-1 mRNA is expressed in a regional specific manner in the brain through development whilst the CSF-1 receptor is expressed throughout the brain in microglia. CSF-1 is neurotrophic in embryonic neuronal cultures and its absence in csfmop/csfmop mice results in severe electro-physiological abnormalities in the cortex. This suggests that CSF-1 is a neurotrophic factor acting through the microglia. The pleiotropic roles for CSF-1 in reproduction and in the brain suggest that CSF-1 exerts many of its action through the trophic activities of cells of the mononuclear phagocytic lineage.

CSF-1 deficiency in mice results in abnormal brain development

Colony stimulating factor-1 (CSF-1) was initially identified as a growth factor for mononuclear phagocytes. This study examines the role of CSF-1 in the development of the central nervous system (CNS). CSF-1 treatment of neurons cultured from embryonic brain promoted survival and process outgrowth in a dose-dependent manner. By contrast, CSF-1 treatment of neurons cultured from the osteopetrotic (op/op) mouse, a null mutant for CSF-1, promoted significantly less process outgrowth, suggesting that there are neural abnormalities in op/op animals. Nuclease protection assays were used to determine whether CSF-1 and its receptor are expressed at times appropriate to regulate neural development. Both CSF-1 and its receptor are expressed in developing mouse brain, with a unique pattern of CSF-1 mRNA splice variant expression encoding secreted, and not membrane-bound, growth factor. To determine whether brain function is altered by null mutation of CSF-1, op/op mice were examined using electrophysiologic assays. Brainstem auditory and visual evoked potentials were both abnormal in op/op mice. Further, intracortical recordings revealed aberrant neuronal function within visual cortex and alterations in the cortical circuitry that balances excitation and inhibition. Daily CSF-1 injection of postnatal op/op mice largely rescued the abnormal neural phenotype, confirming that the absence of CSF-1 during development is responsible for the abnormalities. The effects of CSF-1 on cultured embryonic neural cells, the developmentally appropriate expression of CSF-1 and its receptor, and the neurological abnormalities in op/op mice suggest a role for CSF-1 in brain development.

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.

Apoptosis of cultured microglia by the deprivation of macrophage colony-stimulating factor

Promotion of microglial proliferation and differentiation by colony-stimulating factors (CSFs) and disappearances of microglia at the late neonatal stage by decreasing of CSFs have been reported. In this study, the effects of the deprivation of macrophage CSF (M-CSF) on enriched microglia in cultures were examined by cytochemical methods including in situ nick-end labeling for DNA fragmentation, and Carrazi's hematoxylin nuclear staining. When M-CSF was deprived from the culture medium: (1) at least 40% of the cells were weakly labeled by nick-end within 3 h and more than 70% of the cells were clearly labeled by 16 h; and (2) nuclear condensation or fragmentation, and formation of apoptotic bodies were observed within 48 h. LeY-positive immunoreactivity, identified as a characteristic of cells undergoing apoptosis, was observed on cells positively labeled by nick-end and condensed nucleus, and ones budding apoptotic bodies. From these results, it is conceivable that microglia undergo apoptosis when M-CSF is deprived from the culture medium and, therefore, require CSFs for their survival. This in vitro phenomenon suggests that one of the mechanisms of microglial disappearance in vivo after synaptogenesis may be due to apoptosis by decreasing level of CSFs.