Differential immunochemical markers reveal the normal distribution of brain macrophages and microglia in the developing rat brain.

Brain macrophages and microglia play important roles in central nervous system (CNS) development, especially during regressive events in which particular neuronal and glial constituents are eliminated. The purpose of this study is to provide a complete map of brain macrophage and microglia distribution in all regions of the neuraxis from birth to sexual maturity. We have utilized morphology and immunostaining with the specific antibodies OX-42 and ED1 to distinguish between brain macrophages and microglia. Brain macrophages are large, round cells, 10-15 microns in diameter, with few or no cytoplasmic processes; these cells are ED1- and OX-42-immunopositive. Microglia have small cell bodies with numerous, ramified cytoplasmic processes. These cells are OX-42-positive, and ED1-negative. We found a specific pattern of distribution of brain macrophages, targeting specific cortical and subcortical areas transiently, including developing fiber tracts. These cells disappeared completely by the third postnatal week. In contrast, OX-42-positive microglia exhibited a gradual increase in number and were distributed uniformly throughout gray matter and within white matter tracts. These cells remain in the adult CNS, constituting the resident microglia population. We suggest that these two distinct phagocytic cell populations perform unique functions in the developing brain, including remodeling of restricted CNS areas by brain macrophages that is part of a normal morphological process.     

SCRAPIE: HOW MUCH DO WE REALLY UNDERSTAND?

Biological studies have produced convincing evidence for different scrapie strains, some of which undergo mutation. This argues strongly in favour of the infectious scrapie agent having a genome. The length of incubation period is influenced by the strain of agent but is also under strict host control. In mice, this control is exerted by a gene called Sinc which affects the overall rate of agent replication in the CNS. After peripheral infection, invasion of the CNS from lymphoreticular sites of agent replication is a key step in pathogenesis. Evidence from one scrapie model indicates spread of infection along autonomic nerves to the thoracic spinal cord and then to other parts of the CNS. Other studies have shown that infection can spread in neurons. There are close relationships between the presence of replicating agent and the development of vacuolation, and also of cerebral amyloid when it occurs. We can, therefore, begin to understand the patterns of lesion development in the brain in terms of the targeting of infection and its replication at certain sites. Structures known as SAF (Scrapie Associated Fibrils) have been discovered in extracts of scrapie brain (but not uninfected brain) and a glycoprotein (PrP 27-30: SAF protein) is a major constituent of purified SAF. The glycoprotein is coded by a single gene which is present in several species and expressed in uninfected brain. The normal protein seems to be modified in scrapie infected brain so that it accumulates as SAF. The modified protein may also be deposited as extracellular amyloid because there appear to be common epitopes between SAF and scrapie amyloid. The biochemical nature of the scrapie agent remains in doubt and the association between infectivity and purified SAF may arise fortuitously from the fact that scrapie agent is 'sticky'.     

Characterization of microglia and macrophages in the central nervous system of rats: Definition of the differential expression of molecules using standard and novel monoclonal antibodies in normal CNS and in four models of parenchymal reaction

This report describes the development of a new panel of monoclonal antibodies produced following immunization of mice with cultured rat microglial cells. Using these new reagents and previously defined antibodies that bind to microglia or macrophages, the responses of parenchymal microglia, perivascular “microglial” cells, and infiltrating macrophage/monocytes were examined in 4 divergent models of central nervous system reaction. These were brain abscess, experimental allergic encephalomyelitis, Wallerian degeneration, and stab wound. No single new anitbody was specific only for microglia; all antibodies positively staining microglial cells also labeled various subsets of macrophage/monocytic cells in normal tissues of the immune system. Moreover, the results indicate that microglia are capable of different levels and a variety of types of response, as defined by the molecules they elaborate. These findings suggest that these CNS resident cells belong to the extended monocyte/macrophage/dendritic cell family and that they do not respond in a stereotypic manner to all forms of CNS insult.     

Mannose receptor expression specifically reveals perivascular macrophages in normal, injured, and diseased mouse brain

Perivascular macrophages are believed to have a significant role in inflammation in the central nervous system (CNS). They express a number of different receptors that point toward functions in both innate immunity, through pathogen-associated molecular pattern recognition, phagocytosis, and cytokine responsiveness, and acquired immunity, through antigen presentation and co-stimulation. We are interested in the receptors that are differentially expressed by perivascular macrophages and microglia in both the normal CNS as well as in neuroinflammation and neurodegeneration. In this article we report the use of a well-characterized monoclonal antibody, 5D3, to localize the expression of the mannose receptor to perivascular macrophages in the normal CNS and in various models of brain pathology. Mannose receptor expression was limited to perivascular, meningeal, and choroid plexus macrophages in normal, inflamed, injured, and diseased CNS. In particular, activated microglia and invading hematogenous leukocytes were mannose receptor negative while expressing the F4/80 antigen, macrosialin (CD68), FcRII (CD32), scavenger receptor (CD204), and CR3 (CD11b/CD18). Since the perivascular macrophages expressing the mannose receptor are known to be the only constitutively phagocytic cells in the normal CNS, we injected clodronate-loaded liposomes intracerebroventricularly in control mice to deplete these cells. In these mice, there was no detectable mannose receptor expression in perivascular spaces after immunocytochemistry with the 5D3 monoclonal antibody. This finding underlines the value of the monoclonal antibody 5D3 as a tool to study murine perivascular macrophages selectively. Mannose receptor expression by macrophages located at blood-brain (perivascular), brain-cerebrospinal fluid (CSF) (meningeal), and CSF-blood (choroid plexus) interfaces supports a functional role of these cells in responding to external stimuli such as infection.     

Ameboid microglia as effectors of inflammation in the central nervous system

Techniques for selective isolation, labeling, stimulation, and destruction of ameboid microglia allow study of some fundamental questions in neuroimmunology. Examination of surface morphology, proliferative capacity, and cytochemistry suggests that microglia are a class of brain mononuclear phagocytes distinct from blood monocytes, spleen macrophages, or resident peritoneal macrophages. Moreover, cultured ameboid microglia isolated from newborn brain can be induced to grow thin cytoplasmic projections several hundred microns in length; these process-bearing cells resemble a differentiated form of microglia found in adult brain. Ameboid microglia may contribute to brain inflammation by engulfing debris, by releasing cytotoxins, by killing neighboring cells, and by secreting astroglial growth factors. Importantly, ameboid microglia are closely tied to a network of immunomodulators that include colony-stimulating factors and Interleukin-1. The presence of activated microglia during normal embryogenesis and at sites of penetrating brain injury suggests that these cells serve as important effectors linking the immune system with growth and repair of the CNS.     

Brain macrophages: evaluation of microglia and their functions

There is now evidence approaching, if not having already surpassed, overwhelming in support of microglial cells as macrophages. Consistent with this cellular identity, they appear to arise from monocytes in developing brain where amoeboid microglia function in removing cell death-associated debris and in regulating gliogenesis. In normal adult tissue, ramified microglial cells with down-regulated macrophage functional properties may serve a constitutive role in cleansing the extracellular fluid. Under all conditions of brain injury, microglia appear to activate and convert into active macrophages. Activated and reactive microglia participate in inflammation, removal of cellular debris and wound-healing, the latter through regulation of gliosis in scar formation and a potential contribution to neural regeneration and neovascularization. In the activated state, microglia also express MHC's and, thus, may function in antigen presentation and lymphocyte activation for CNS immune responses. As uniquely adapted tissue resident macrophages within the CNS, microglia serve a variety of functional roles over the lifespan of this tissue. These cells may therefore be involved in or contribute to some disease states; such has been indicated in multiple sclerosis and AIDS dementia complex.     

Determination of the origin and nature of brain macrophages and microglial cells in mouse central nervous system, using non-radioactive in situ hybridization and immunoperoxidase techniques.

The origin and nature of brain macrophages and microglial cells in the mouse central nervous system (CNS) were investigated. First, the expression and localization of determinants recognized by the different monoclonal antibodies (mAbs) MOMA-1, Mac-1-alpha, and F4/80 (raised against cells of the mononuclear phagocyte system) were immunohistochemically studied in the developing and adult mouse brain. In order to clarify the origin of brain macrophages and microglial cells, we used bacteriophage lambda transgenic mice as donors for bone marrow transplantations in recipient mice of different ages. During ontogeny, numerous MOMA-1-, Mac-1-alpha-, and F4/80-positive blood monocyte-derived brain macrophages (amoeboid microglia) infiltrated the CNS parenchyma. These brain macrophages gradually disappeared from the brain parenchyma at postnatal day 7 (P7). From P17 on, Mac-1-alpha- and F4/80-positive cells were detected within the brain parenchyma with the morphology of resting microglial cells. Transitional forms between brain macrophages and "resting" microglia were not observed in the developing brain. Combined non-radioactive in situ hybridization and immunohistochemistry revealed many MOMA-1-positive bone marrow-derived brain macrophages that were located in the leptomeninges, the ventricles, and occasionally the blood vessel walls. These results show that brain macrophages are of bone marrow origin. Many "resting" microglial cells were detected in the brain, mainly in the white matter. It appeared that about 10% of these cells displayed the transgenic signal. This result indicates that the majority of "resting" microglial cells are of local, presumably neuroectodermal, origin.     

Neonatal and adult microglia cross-present exogenous antigens

Some observations have suggested that cells from the central nervous system (CNS) could present exogenous antigens on major histocompatibility complex (MHC) class I molecules to CD8(+) T cells (a process called cross-presentation). Microglia are the major myeloid immunocompetent cells of the CNS. When activated, following the injury of the nervous parenchyma, they become fully competent antigen-presenting cells (APC) that prime CD4(+) T lymphocytes. We therefore tested the cross-presentation capacity of murine microglia. We report that a microglial cell line (C8-B4), neonatal microglia, and interestingly adult microglia cross-present soluble exogenous antigen (ovalbumin) to a OVA-specific CD8(+) T-cell hybridoma and cross-prime OVA-specific naive OT-1 CD8(+) T cells. In both these cases, C8-B4 and neonatal microglia cross-present OVA as well as peritoneal macrophages. Although cross-presentation by adult microglia is less efficient, it is increased by GM-CSF and CpG oligodeoxynucleotide (ODN) stimulation. Using microglial cells either exposed to an inhibitor of proteasome, lactacystin, or purified from TAP(-/-) mice, we demonstrate that the microglia cross-present antigen in proteasome- and TAP-dependant pathways, respectively. Last, microglia purified from adult mice injected intracerebrally with OVA efficiently stimulate OVA-specific CD8(+) T cells, thereby showing that microglia take up and process exogenous antigen into MHC class I in vivo. This first demonstration of the cross-presentation property of microglia offers novel therapeutic approaches to modulate CD8 T-cell responses in the brain.     

Transformation of donor-derived bone marrow precursors into host microglia during autoimmune CNS inflammation and during the retrograde response to axotomy

Macrophages in the brain can have a triple source. They may originate from recently blood-derived precursors, from the largely resident perivascular cell population (perivascular macrophages and related cells), and from intrinsic parenchymal as well as perivascular microglia. Although continuous exchange of part of the perivascular cell population with bone marrow-derived precursors is now accepted, the turnover of adult parenchymal microglia has remained enigmatic. Using bone-marrow chimeras carrying an unexpressed marker gene and carbon labeling of peripheral monocyte/macrophages in a combined model of facial nerve axotomy and transfer experimental autoimmune encephalitis, we demonstrate for the first time that there is an easy to induce exchange between parenchymal central nervous system (CNS) microglia and the macrophage precursor cell pool of the bone marrow. Furthermore, very low level infiltration of the CNS parenchyma by recently bone marrow-derived microglia could be observed after simple peripheral nerve axotomy that is followed by neuronal regeneration. Thus, microglial cells can be considered wanderers between the peripheral immune system and the CNS where they may act as a "Trojan horse" in infections. The fact that recently bone marrow-derived parenchymal microglia fully integrate into a regenerating brain nucleus' architecture encourages entirely new approaches for delivering genes into the adult CNS.     

Impaired inflammatory response to glial cell death in genetically metallothionein-I- and -II-deficient mice

Metallothionein I+II (MT-I+II) are acute-phase proteins which are upregulated during pathological conditions in the brain. To elucidate the neuropathological importance of MT-I+II, we have examined MT-I+II-deficient mice following ip injection with 6-aminonicotinamide (6-AN). 6-AN is antimetabolic and toxic for bone marrow cells and grey matter astrocytes. In MT+/+ mice, injection with 6-AN resulted in breakdown of the blood-brain barrier (BBB) and absence of GFAP-positive astrocytes in specific grey matter areas of the brain stem. Reactive astrocytosis encircled the damaged grey matter areas, which were heavily infiltrated by microglia/macrophages. The recruitment of hematogenous macrophages was accompanied by leakage of the BBB. The immunoreactivity (ir) of granulocyte-macrophage-colony-stimulating factor (GM-CSF) and the receptor for GM-CSF (GM-CSFrec) was significantly upregulated in astrocytes and microglia/macrophages, respectively. MT-I+IIir was also clearly increased in astrocytes surrounding the damaged areas, while that of the CNS-specific MT isoform, MT-III, was mildly increased in both astrocytes and microglia/macrophages. In MT-/- mice injected with 6-AN, the BBB remained almost intact. The damage to specific grey matter areas was similar to that observed in MT+/+ mice, but reactive astrocytosis, microglia/macrophages infiltration, and GM-CSFir and GM-CSFrecir were clearly reduced in MT-/- mice. In contrast, MT-IIIir was dramatically increased in MT-/- mice. Total zinc decreased and histochemically detectable zinc increased in the brain stem after 6-AN similarly in MT+/+ and MT-/- mice. Bone marrow myeloid monocytes and macrophages were increased as a reaction to 6-AN only in MT+/+ mice. The results demonstrate that the capability of MT-/- mice to mount a normal inflammatory response in the brain is severely attenuated, at least in part because of 6-AN-induced bone marrow affectation, involving MT-I+II for the first time as major factors during CNS tissue damage.     

The failure of microglia in normal brain to exhibit mononuclear phagocyte markers.

The origin of brain macrophages or "reactive microglia" has been the subject of considerable controversy. The fundamental question is whether or not there is a morphologically and functionally distinct population of cells, called microglia, which are resident in normal brain and differentiate into macrophages in response to inflammatory stimuli. The present study was performed to determine if any cells in the normal brain have the common markers of mononuclear phagocytes; phagocytosis, IgGFc receptors or macrophage specific antigens. In studies of the newborn and the adult murine brain and adult human brain no cells were detected which had any of those markers, although the highly sensitive marker methods were capable of detecting mononuclear phagocytes in all other tissues where they are known to occur. The results suggest that microglia, if they exist as a distinct cell type, are unrelated to mononuclear phagocytes. Furthermore, they suggest, but do not prove, that all inflammatory macrophages are derived from hematogenous precursors.     

CNS wound healing is severely depressed in metallothionein I- and II-deficient mice.

To characterize the physiological role of metallothioneins I and II (MT-I+II) in the brain, we have examined the chronological effects of a freeze injury to the cortex in normal and MT-I+II null mice. In normal mice, microglia/macrophage activation and astrocytosis were observed in the areas surrounding the lesion site, peaking at approximately 1 and 3 d postlesion (dpl), respectively. At 20 dpl, the parenchyma had regenerated. Both brain macrophages and astrocytes surrounding the lesion increased the MT-I+II immunoreactivity, peaking at approximately 3 dpl, and at 20 dpl it was similar to that of unlesioned mice. In situ hybridization analysis indicates that MT-I+II immunoreactivity reflects changes in the messenger levels. In MT-I+II null mice, microglia/macrophages infiltrated the lesion heavily, and at 20 dpl they were still present. Reactive astrocytosis was delayed and persisted at 20 dpl. In contrast to normal mice, at 20 dpl no wound healing had occurred. The rate of apoptosis, as determined by using terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling, was drastically increased in neurons of ipsilateral cortex of the MT-I+II null mice. Our results demonstrate that MT-I+II are essential for a normal wound repair in the CNS, and that their deficiency impairs neuronal survival.     

Microglia function in brain tumors

Microglia play an important role in inflammatory diseases of the central nervous system (CNS). These cells have also been identified in brain neoplasms; however, as of yet their function largely remains unclear. More recent studies designed to characterize further tumor-associated microglia suggest that the immune effector function of these cells may be suppressed in CNS tumors. Furthermore, microglia and macrophages can secrete various cytokines and growth factors that may contribute to the successful immune evasion, growth, and invasion of brain neoplasms. A better understanding of microglia and macrophage function is essential for the development of immune-based treatment strategies against malignant brain tumors.     

Resident microglia from adult mice are refractory to nitric oxide-inducing stimuli due to impaired NOS2 gene expression

Microglia are the immunoregulatory cells of the central nervous system (CNS) and share many characteristics with resident macrophages in extracerebral tissues. Nitric oxide (NO) is secreted by macrophages following induction of the NO synthase gene NOS2 by stimuli elicited during a T-cell response and/or by microbial products. NO regulates both innate and adaptive immune responses, such as killing intracellular pathogens and inhibiting T-cell proliferation. Regulation of NO production by microglia, however, is poorly understood. We find that microglia from healthy adult mice produce negligible amounts of NO compared with resident macrophages during restimulation of peptide-specific CD8 T cells, and therefore cannot block T-cell proliferation. The impaired NO response extends to exogenous NOS2-inducing stimuli, including cytokines, CD40 ligation, and lipopolysaccharide. In contrast, microglia produce proinflammatory cytokines in response to these same stimuli, and therefore possess a relatively selective block in NO production. We go on to show that resident microglia fail to produce detectable levels of either the NOS2 enzyme or NOS2 RNA in response to NO-inducing stimuli. We therefore propose that microglia in the healthy adult brain exist in an "NO-incompetent" state in which NO production is blocked at the level of NOS2 RNA. The inability of resident microglia in the healthy CNS to produce NO may allow these immunoregulatory cells to modulate immune processes temporally, and may serve to protect the CNS from irreparable damage at the onset of infection or injury.     

The characterization of Ia expression during Sindbis virus encephalitis in normal and athymic nude mice

To characterize the expression of Ia systemically and locally on mononuclear cells during acute viral encephalitis, weanling mice were inoculated intracerebrally with Sindbis virus (SV), an alphavirus. Peripheral blood mononuclear cells, splenocytes and perivascular inflammatory cells in frozen brain sections were examined immunocytochemically for the presence of Ia. Ia expression increased in the spleen, blood and brain during SV encephalitis. The majority of the cells in the central nervous system (CNS) expressing Ia were perivascular mononuclear cells but Ia was also found on stellate parenchymal cells. Using one micrometer cryopreserved serial sections we identified these parenchymal cells as macrophages and microglia but not astrocytes. We also identified rare Ia-positive cells resembling endothelial cells. Frozen brain sections of SV-infected T cell-deficient nu/nu mice were also examined for Ia expression. The number and percentage of Ia-positive cells in perivascular inflammatory cells were markedly decreased compared to normal mice and Ia-positive stellate parenchymal cells were less numerous. This suggests that immunocompetent T cells are necessary for "normal" infiltration of inflammatory cells and for Ia expression in the CNS during SV encephalitis.     

T cells contribute to lysophosphatidylcholine-induced macrophage activation and demyelination in the CNS

We have previously shown that intraspinal microinjection of lysophosphatidylcholine (LPC), a potent demyelinating agent, results in a rapid but brief influx of T cells (between 6 and 12 h). This is accompanied by a robust activation of macrophages/microglia that leads to demyelination by 48 h. In the present study, we examined whether this brief influx of T cells contributes to the activation of macrophages/microglia and demyelination by injecting LPC into the dorsal column white matter of athymic Nude mice that lack T cells. We show that there is a significant reduction in macrophage/microglial activation and myelin clearance after LPC injection in Nude mice as compared with wildtype controls. We also show that there is no difference in the recruitment of hematogenous macrophages into the spinal cord after LPC injection in the two mouse strains. Of the T cell cytokines assessed, there was a marked reduction in the mRNA expression of interleukin-2 (IL-2) in Nude mice compared with wildtype animals. Neutralizing IL-2 with function-blocking antibodies in wildtype animals resulted in a significant decrease in the number of phagocytic macrophages/microglia and a reduction in demyelination induced by LPC. While there may be other defects in Nude mice that might contribute to the effects shown here, these data suggest that the brief influx of T cells in this model of chemically-induced demyelination could play a role in macrophage/microglial activation and demyelination. These results may also have implications for remyelination in this and other types of CNS damage.     

Resident microglia and hematogenous macrophages as phagocytes in adoptively transferred experimental autoimmune transferred experimental autoimmune encephalomyelitis: An investigation using rat radiation bone marrow chimeras

Hematogenous macrophages are known to be involved in the induction of tissue damage in the central nervous system (CNS) as well as of clinical symptoms in experimental autoimmune encephalomyelitis (EAE). Although resident microglia can become phagocytic under certain circumstances, little is known about the role of these cells in brain inflammation in vivo. We thus studied EAE in the model of radiation bone marrow chimeras that allows us to distinguish donor-derived hematogenous cells from resident effector cells. Inflammation in the CNS was qualitatively and quantitatively similar in chimeras compared to fully histocompatible Lewis rats. Although activated resident microglial cells were outnumbered four-to sevenfold in EAE lesions by hematogenous macrophages, the number of resident microglia with ingested myelin was equal to that of macrophages containing myelin debris. Phagocytic resident microglia, expressing the macrophage activation marker ED1, showed ramified as well as amoeboid morphology. From our studies the following conclusions can be drawn. First, a considerable proportion of resident microglia upregulated ED1. Second, resident microglia provide a small but substantial source of brain macrophages in EAE as compared to the large influx of macrophages. Third, our results suggest that macroglia, due to their strategic position within the CNS, are more effective in removal of myelin debris compared to hematogenous macrophages. © 1995 Wiley-Liss, Inc.     

Brain microglia and blood-derived macrophages: molecular profiles and functional roles in multiple sclerosis and animal models of autoimmune demyelinating disease

Microglia and macrophages, one a brain-resident, the other a mostly hematogenous cell type, represent two related cell types involved in the brain pathology in multiple sclerosis and its autoimmune animal model, the experimental allergic encephalomyelitis. Together, they perform a variety of different functions: they are the primary sensors of brain pathology, they are rapidly recruited to sites of infection, trauma or autoimmune inflammation in experimental allergic encephalomyelitis and multiple sclerosis and they are competent presenters of antigen and interact with T cells recruited to the inflamed CNS. They also synthesise a variety of molecules, such as cytokines (TNF, interleukins), chemokines, accessory molecules (B7, CD40), complement, cell adhesion glycoproteins (integrins, selectins), reactive oxygen radicals and neurotrophins, that could exert a damaging or a protective effect on adjacent axons, myelin and oligodendrocytes.The current review will give a detailed summary on their cellular response, describe the different classes of molecules expressed and their attribution to the blood derived or brain-resident macrophages and then discuss how these molecules contribute to the neuropathology. Recent advances using chimaeric and genetically modified mice have been particularly telling about the specific, overlapping and nonoverlapping roles of macrophages and microglia in the demyelinating disease. Interestingly, they point to a crucial role of hematogenous macrophages in initiating inflammation and myelin removal, and that of microglia in checking excessive response and in the induction and maintenance of remission.     

Migration of Bone Marrow‐Derived Cells Into the Central Nervous System in Models of Neurodegeneration

Microglia are the brain‐resident macrophages tasked with the defense and maintenance of the central nervous system (CNS). The hematopoietic origin of microglia has warranted a therapeutic potential for the hematopoietic system in treating diseases of the CNS. However, migration of bone marrow‐derived cells (BMDC) into the CNS is a marginal event under normal, healthy conditions. A busulfan‐based chemotherapy regimen was used for bone marrow transplantation in wild‐type mice before subjecting them to a hypoxic–ischemic brain injury or in APP/PS1 mice prior to the formation of amyloid plaques. The cells were tracked and analyzed throughout the development of the pathology. The efficacy of a preventive macrophage colony‐stimulating factor (M‐CSF) treatment was also studied to highlight the effects of circulating monocytes in hypoxic–ischemic brain injury. Such an injury induces a strong migration of BMDC into the CNS, without the need for irradiation. These migrating cells do not replace the entire microglial pool but rather are confined to the sites of injury for several weeks, suggesting that they could perform specific functions. M‐CSF showed neuroprotective effects as a preventive treatment. In APP/PS1 mice, the formation of amyloid plaques was sufficient to induce the entry of cells into the parenchyma, though in low numbers. This study confirms that BMDC infiltrate the CNS in animal models for stroke and Alzheimer's disease and that peripheral cells can be targeted to treat affected regions of the CNS. J. Comp. Neurol. 521:3863–3876, 2013. © 2013 Wiley Periodicals, Inc.     

New expression of myelomonocytic antigens by microglia and perivascular cells following lethal motor neuron injury

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.