Human astrocytes express membrane cofactor protein (CD46), a regulator of complement activation

The complement system in neurodegenerative diseases

Complement is an important component of innate immune defence against pathogens and crucial for efficient immune complex disposal. These core protective activities are dependent in large part on properly regulated complement-mediated inflammation. Dysregulated complement activation, often driven by persistence of activating triggers, is a cause of pathological inflammation in numerous diseases, including neurological diseases. Increasingly, this has become apparent not only in well-recognized neuroinflammatory diseases like multiple sclerosis but also in neurodegenerative and neuropsychiatric diseases where inflammation was previously either ignored or dismissed as a secondary event. There is now a large and rapidly growing body of evidence implicating complement in neurological diseases that cannot be comprehensively addressed in a brief review. Here, we will focus on neurodegenerative diseases, including not only the 'classical' neurodegenerative diseases such as Alzheimer's disease and Parkinson's disease, but also two other neurological diseases where neurodegeneration is a neglected feature and complement is implicated, namely, schizophrenia, a neurodevelopmental disorder with many mechanistic features of neurodegeneration, and multiple sclerosis, a demyelinating disorder where neurodegeneration is a major cause of progressive decline. We will discuss the evidence implicating complement as a driver of pathology in these diverse diseases and address briefly the potential and pitfalls of anti-complement drug therapy for neurodegenerative diseases.

The role of the complement system in Multiple Sclerosis: A review

The complement system has been involved in the pathogenesis of multiple neuroinflammatory and neurodegenerative conditions. In this review, we evaluated the possible role of complement activation in multiple sclerosis (MS) with a focus in progressive MS, where the disease pathogenesis remains to be fully elucidated and treatment options are limited. The evidence for the involvement of the complement system in the white matter plaques and gray matter lesions of MS stems from immunohistochemical analysis of post-mortem MS brains, in vivo serum and cerebrospinal fluid biomarker studies, and animal models of Experimental Autoimmune Encephalomyelitis (EAE). Complement knock-out studies in these animal models have revealed that this system may have a “double-edge sword” effect in MS. On the one hand, complement proteins may aid in promoting the clearance of myelin degradation products and other debris through myeloid cell-mediated phagocytosis. On the other, its aberrant activation may lead to demyelination at the rim of progressive MS white matter lesions as well as synapse loss in the gray matter. The complement system may also interact with known risk factors of MS, including as Epstein Barr Virus (EBV) infection, and perpetuate the activation of CNS self-reactive B cell populations. With the mounting evidence for the involvement of complement in MS, the development of complement modulating therapies for this condition is appealing. Herein, we also reviewed the pharmacological complement inhibitors that have been tested in MS animal models as well as in clinical trials for other neurologic diseases. The potential use of these agents, such as the C5-binding antibody eculizumab in MS will require a detailed understanding of the role of the different complement effectors in this disease and the development of better CNS delivery strategies for these compounds.

Complement cascade functions during brain development and neurodegeneration

The complement system, an essential tightly regulated innate immune system, is a key regulator of normal central nervous system (CNS) development and function. However, aberrant complement component expression and activation in the brain may culminate into marked neuroinflammatory response, neurodegenerative processes and cognitive impairment. Over the years, complement-mediated neuroinflammatory responses and complement-driven neurodegeneration have been increasingly implicated in the pathogenesis of a wide spectrum of CNS disorders. This review describes how complement system contributes to normal brain development and function. We also discuss how pathologic insults such as misfolded proteins, lipid droplet/lipid droplet-associated protein or glycosaminoglycan accumulation could trigger complement-mediated neuroinflammatory responses and neurodegenerative process in neurodegenerative proteinopathies, age-related macular degeneration and neurodegenerative lysosomal storage disorders.

C5a-Preactivated Neutrophils Are Critical for Autoimmune-Induced Astrocyte Dysregulation in Neuromyelitis Optica Spectrum Disorder

Neuromyelitis optica spectrum disorder (NMOSD) is an autoimmune neuroinflammatory disease. In contrast to multiple sclerosis, autoantibodies against aquaporin-4 (AQP4) expressed on astrocytic end-feet have been exclusively detected in sera of NMOSD patients. Several lines of evidence suggested that anti-AQP4 autoantibodies are pathogenic, but the mechanism triggering inflammation, impairment of astrocyte function, and the role of neutrophils presented in NMOSD cerebrospinal fluid remains unknown. In this study, we tested how human neutrophils affect astrocytes in the presence of anti-AQP4 Ab-positive serum derived from NMOSD patients. An in vitro model of inflammation consisted of human astrocyte line, NMOSD serum, and allogenic peripheral blood neutrophils from healthy individuals. We showed evidence of pathogenicity of NMOSD serum, which by consecutive action of anti-AQP4 Abs, complement system, and neutrophils affected astrocyte function. Anti-AQP4 Ab binding astrocytes initiated two parallel complementary reactions. The first one was dependent on the complement cytotoxicity via C5b-9 complex formation, and the second one on the reverse of astrocyte glutamate pump into extracellular space by C5a-preactivated neutrophils. As a consequence, astrocytes were partially destroyed; however, a major population of astrocytes polarized into proinflammatory cells which were characterized by pathological glutamate removal from extracellular space.

Age-Related Macular Degeneration: A Connection between Human Herpes Virus-6A-Induced CD46 Downregulation and Complement Activation?

Viruses are able to interfere with the immune system by docking to receptors on host cells that are important for proper functioning of the immune system. A well-known example is the human immunodeficiency virus that uses CD4 cell surface molecules to enter host lymphocytes and thereby deleteriously destroying the helper cell population of the immune system. A more complicated mechanism is seen in multiple sclerosis (MS) where human herpes virus-6A (HHV-6A) infects astrocytes by docking to the CD46 surface receptor. Such HHV-6A infection in the brain of MS patients has recently been postulated to enable Epstein-Barr virus (EBV) to transform latently infected B-lymphocytes in brain lesions leading to the well-known phenomenon of oligoclonal immunoglobulin production that is widely used in the diagnosis of MS. The cellular immune response to HHV-6A and EBV is one part of the pathogenic mechanisms in MS. A more subtle pathogenic mechanism can be seen in the downregulation of CD46 on astrocytes by the infecting HHV-6A. Since CD46 is central in regulating the complement system, a lack of CD46 can lead to hyperactivation of the complement system. In fact, activation of the complement system in brain lesions is a well-known pathogenic mechanism in MS. In this review, it is postulated that a similar mechanism is central in the development of age-related macular degeneration (AMD). One of the earliest changes in the retina of AMD patients is the loss of CD46 expression in the retinal pigment epithelial (RPE) cells in the course of geographic atrophy. Furthermore, CD46 deficient mice spontaneously develop dry-type AMD-like changes in their retina. It is also well known that certain genetic polymorphisms in the complement-inhibiting pathways correlate with higher risks of AMD development. The tenet is that HHV-6A infection of the retina leads to downregulation of CD46 and consequently to hyperactivation of the complement system in the eyes of susceptible individuals.

Role of membrane complement regulators in neuromyelitis optica

BACKGROUND

It is unclear why AQP4-IgG primarily causes central nervous system lesions by activating complement, but generally spares peripheral AQP4-expressing organs.

OBJECTIVES

To determine whether peripheral AQP4-expressing cells are protected from complement-mediated damage by expressing complement regulators.

METHODS

Human tissue and cultured human cells were immunostained for aquaporin-4 (AQP4), CD46, CD55 and CD59. We also determined the vulnerability to AQP4-IgG and complement-mediated damage of astrocytes cultured alone or co-cultured with endothelial cells.

RESULTS

In normal brain, astrocyte end-feet express AQP4, but are devoid of CD46, CD55 and CD59. Immunoreactivity for CD46, CD55 and CD59 is not increased in or around neuromyelitis optica lesions. In kidney AQP4 is co-expressed with CD46 and CD55, in stomach AQP4 is co-expressed with CD46 and in skeletal muscle AQP4 is co-expressed with CD46. Astrocytes cultured alone co-express AQP4 and CD59 but, in astrocyte-endothelial co-cultures, AQP4 is found in cell processes devoid of CD59. Astrocytes co-cultured with endothelial cells are more vulnerable to AQP4-IgG and complement-mediated lysis than astrocytes cultured alone.

CONCLUSIONS

Complement regulators protect peripheral organs, but not the central nervous system, from AQP4-IgG and complement-mediated damage. Our findings may explain why neuromyelitis optica primarily damages the central nervous system, but spares peripheral organs.

The role of the complement system and the activation fragment C5a in the central nervous system

The complement system is a pivotal component of the innate immune system which protects the host from infection and injury. Complement proteins can be induced in all cell types within the central nervous system (CNS), where the pathway seems to play similar roles in host defense. Complement activation produces the C5 cleavage fragment C5a, a potent inflammatory mediator, which recruits and activates immune cells. The primary cellular receptor for C5a, the C5a receptor (CD88), has been reported to be on all CNS cells, including neurons and glia, suggesting a functional role for C5a in the CNS. A second receptor for C5a, the C5a-like receptor 2 (C5L2), is also expressed on these cells; however, little is currently known about its potential role in the CNS. The potent immune and inflammatory actions of complement activation are necessary for host defense. However, if over-activated, or left unchecked it promotes tissue injury and contributes to brain disease pathology. Thus, complement activation, and subsequent C5a generation, is thought to play a significant role in the progression of CNS disease. Paradoxically, complement may also exert a neuroprotective role in these diseases by aiding in the elimination of aggregated and toxic proteins and debris which are a principal hallmark of many of these diseases. This review will discuss the expression and known roles for complement in the CNS, with a particular focus on the pro-inflammatory end-product, C5a. The possible overarching role for C5a in diseases of the CNS is reviewed, and the therapeutic potential of blocking C5a/CD88 interaction is evaluated.

Innate immunity and protective neuroinflammation: new emphasis on the role of neuroimmune regulatory proteins

Brain inflammation due to infection, hemorrhage, and aging is associated with activation of the local innate immune system as expressed by infiltrating cells, resident glial cells, and neurons. The innate immune response relies on the detection of "nonself" and "danger-self" ligands behaving as "eat me signals" by a plethora of pattern recognition receptors (PRRs) expressed by professional and amateur phagocytes to promote the clearance of pathogens, toxic cell debris (amyloid fibrils, aggregated synucleins, prions), and apoptotic cells accumulating within the brain parenchyma and the cerebrospinal fluid (CSF). These PRRs (e.g., complement, TLR, CD14, scavenger receptors) are highly conserved between vertebrates and invertebrates and may represent the most ancestral innate scavenging system involved in tissue homeostasis. However, in some diseases, these protective mechanisms lead to neurodegeneration on the ground that several innate immune molecules have neurocytotoxic activities. The response is a "double-edged sword" representing a fine balance between protective and detrimental effects. Several key regulatory mechanisms have now been evidenced in the control of CNS innate immunity, and these could be harnessed to explore novel therapeutic avenues. We will herein provide new emphasis on the role of neuroimmune regulatory proteins (NIRegs), such as CD95L, TNF, CD200, CD47, sialic acids, CD55, CD46, fH, C3a, HMGB1, which are involved in silencing innate immunity at the cellular and molecular levels and suppression of inflammation. For instance, NIRegs may play an important role in controlling lymphocyte/macrophage/microglia hyperinflammatory responses, while sparing host defense and repair mechanisms. Moreover, NIRegs have direct beneficial effects on neurogenesis and contributing to brain tissue remodeling.

Modulation of the cytokine network in human adult astrocytes by human herpesvirus-6A

Human herpesvirus-6A (HHV-6A) is a common pathogen whose role in CNS disorders including multiple sclerosis remains controversial. To understand how HHV-6A could influence inflammatory pathways in the CNS, we infected cultured human adult astrocytes and examined the expression of 268 cytokines, chemokines, growth factors and their receptors by gene profiling. HHV-6 infection alone had little effect on the astrocyte gene profile but strongly altered the astrocyte response to proinflammatory cytokines. Under those conditions astrocytes express higher levels of anti-inflammatory mediators including IL-10 and IL-11, chemotactic factors, growth factors and factors controlling type I interferon production. Our data suggest that HHV-6 itself does not evoke a pro-inflammatory response in astrocytes but rather triggers immune modulatory factors in the face of inflammation.

Characterization of the rhesus cytomegalovirus US28 locus

Human cytomegalovirus (CMV) US28 (and the related open reading frame [ORF] US27) are G-protein-coupled receptor homologs believed to play a role in viral pathogenesis. In vitro, US28 has been shown to bind and internalize ligands, as well as activate intracellular signaling in response to certain chemokines, and to initiate the migration of smooth muscle cells to chemokine gradients. To assess the role of US28 in vivo, we examined the rhesus model and sequenced and characterized the rhesus CMV US28 locus. We found that rhesus CMV carries five tandem homologs of US28, all widely divergent from US28 and from each other. By reverse transcription-PCR and Northern analysis, we demonstrated expression of these ORFs in infected cells. With stable cell lines expressing these ORFs, we analyzed the homolog's binding and signaling characteristics across a wide range of chemokines and found one (RhUS28.5) to have a ligand binding profile similar to that of US28. In addition, we localized US28 and the rhesus CMV homolog RhUS28.5 to the envelope of infectious virions, suggesting a role in viral entry or cell tropism.

Spontaneous classical pathway activation and deficiency of membrane regulators render human neurons susceptible to complement lysis

This study investigated the capacity of neurons and astrocytes to spontaneously activate the complement system and control activation by expressing complement regulators. Human fetal neurons spontaneously activated complement through the classical pathway in normal and immunoglobulin-deficient serum and C1q binding was noted on neurons but not on astrocytes. A strong staining for C4, C3b, iC3b neoepitope and C9 neoepitope was also found on neurons. More than 40% of human fetal neurons were lysed when exposed to normal human serum in the presence of a CD59-blocking antibody, whereas astrocytes were unaffected. Significant reduction in neuronal cell lysis was observed after the addition of soluble complement receptor 1 at 10 microg/ml. Fetal neurons were stained for CD59 and CD46 and were negative for CD55 and CD35. In contrast, fetal astrocytes were strongly stained for CD59, CD46, CD55, and were negative for CD35. This study demonstrates that human fetal neurons activate spontaneously the classical pathway of complement in an antibody-independent manner to assemble the cytolytic membrane attack complex on their membranes, whereas astrocytes are unaffected. One reason for the susceptibility of neurons to complement-mediated damage in vivo may reside in their poor capacity to control complement activation.

Complement components of the innate immune system in health and disease in the CNS

The innate immune system and notably the complement (C) system play important roles in host defense to recognise and kill deleterious invaders or toxic entities, but activation at inappropriate sites or to an excessive degree can cause severe tissue damage. C has been implicated as a factor in the exacerbation and propagation of tissue injury in numerous diseases including neurodegenerative disorders. In this article, we review the evidence indicating that brain cells can synthesise a full lytic C system and also express specific C inhibitors (to protect from C activation and C lysis) and C receptors (involved in cell activation, chemotaxis and phagocytosis). We also summarise the mechanisms involved in the antibody-independent activation of the classical pathway of C in Alzheimer's disease, Huntington's disease and Pick's disease. Although the primary role of C activation on a target cell is to induce cell lysis (particularly of neurons), we present evidence indicating that C (C3a, C5a, sublytic level of C5b-9) may also be involved in pro- as well as anti-inflammatory activities. Moreover, we discuss evidence suggesting that local C activation may contribute to tissue remodelling activities during repair in the CNS.

Distribution of membrane cofactor protein (MCP/CD46) on pig tissues. Relevance To xenotransplantation

Membrane cofactor protein (MCP; CD46) is a 50-60 000 MW glycoprotein, expressed on a wide variety of cells and tissues in man, which plays an important role in regulating complement activation. Human MCP has also been shown to be the receptor for measles virus. We have recently identified the pig analogue of MCP and demonstrated that pig MCP has cofactor activity for factor I-mediated cleavage of C3b when these components are derived either from pig or human. As a consequence, pig MCP is an efficient regulator of the classic and alternative pathways of human and pig complement. In order to define the potential importance of MCP in protecting against complement activation in the pig, we have conducted a comprehensive survey of its distribution in pig cells and organs. As in humans, MCP in the pig is broadly and abundantly distributed. Pig MCP is highly expressed on all circulating cells, including erythrocytes, in contrast to its absence on human erythrocytes. Multiple isoforms of MCP are found on cells and in tissues, probably representing products of alternative splicing analogous to those found in man. MCP is abundantly expressed throughout all tissues examined with particularly strong staining on the vascular endothelium. Connective tissue elements within liver and testis are also strongly stained by anti-pig MCP antibodies. Pig MCP is expressed only weakly on skeletal muscle cells and expression is absent from smooth muscle cells in the lung and vessel walls, sites at which human MCP is expressed. Of particular note, MCP is not expressed in B-cell areas of the germinal centres of lymph nodes.

Characterization of rat C5a anaphylatoxin receptor (C5aR): cloning of rat C5aR cDNA and study of C5aR expression by rat astrocytes

Complement system activation within the central nervous system (CNS) is involved in demyelinating and neurodegenerative disorders, but the role of complement in the pathogenic process or in the repair remains unclear. Besides the direct lytic effects of complement on target cells (oligodendrocytes or neurons), complement can exert other functions through interaction of complement fragments with specific receptors. The C5a anaphylatoxin, an inflammatory peptide which is formed during complement activation, might play a role in the CNS pathogenesis, and activation and recruitment of glial cells by binding to its receptor (C5aR) on CNS cells. Using degenerate primers corresponding to homologous regions between human and mouse C5aR cDNAs, we have cloned a rat C5aR cDNA probe from rat monocytes RNAs after RT-PCR experiment. The rat C5aR probe isolated by this procedure allowed us to clone the rat C5aR cDNA-coding sequence using a library screening cloning strategy. This probe was also used to study the expression of the C5aR mRNA in the rat CNS. Northern blotting and RT-PCR experiments demonstrated the constitutive expression of C5aR mRNA in brain, spleen, kidney and lung. This transcript was also observed in primary culture of rat astrocytes. Microfluorimetry experiments demonstrated that C5aR expressed by astrocytes in culture is functional since the addition of C5a induced a dose-dependent increase of intracellular calcium concentration. The expression of the C5aR by astrocytes suggests new roles for the C5a anaphylatoxin in reactive astrogliosis to CNS injuries.

Unique expression of HRF20 (CD59) in human nervous tissue

Damage to autologous tissue by complement is limited by several widely distributed membrane-associated glycoproteins which restrict the action of the complement in homologous species. These include decay accelerating factor (DAF), membrane cofactor protein (MCP) and 20 kDa homologous restriction factor (HRF20,CD59). Using immunohistochemical techniques, we examined the localization of these proteins in the central nervous system (CNS) and peripheral nervous system (PNS) using non-neurological human nervous tissue since some complement components have been demonstrated to be synthesized in the CNS. There was no evidence of parenchymal staining by anti-DAF or anti-MCP antibodies in either type of tissue except for the staining of the endothelium in capillaries. On the other hand, anti-HRF20 antibody clearly stained myelinated axons in the CNS as well as Schwann cells in the PNS. In addition, we detected positive staining by anti-DAF antibody in the PNS of a Paroxysmal nocturnal hemoglobinuria (PNH) patient who is genetically deficient in HRF20.

Complement expression on astrocytes and astrocytoma cell lines: failure of complement regulation at the C3 level correlates with very low CD55 expression

Primary fetal human astrocytes and an astrocytoma cell line, U373-MG, expressed membrane cofactor protein (CD46), CD59, and low levels of decay-accelerating factor (CD55). Astrocyte CD55 was capable of regulating C3 deposition on the cell surface; albeit at a lower level than primary human fibroblasts. Negligible complement-mediated lysis of primary astrocytes and the U373-MG cell line was observed, even when large amount of astrocyte-specific, complement-activating antibodies were bound to the cells. Blocking the function of CD59 on astrocytes resulted in a > 90% cell lysis, while equivalent lysis of fibroblasts could only be achieved with additional blocking of CD55.

The role of complement and activated microglia in the pathogenesis of Alzheimer's disease

A variety of inflammatory mediators including complement activation products, protease inhibitors, and cytokines are colocalized with beta-amyloid (A beta) deposits in the Alzeimer's disease (AD) brain. Activation products of the early complement components C1, C4, and C3 are always found in neuritic plaques and to a lesser extent in varying numbers of diffuse plaques. In contrast to these findings, no immunohistochemical evidence was obtained for the presence of the late complement components C7 and C9 and the complement membrane attack complex in the neuropathological lesions in AD brains. The mRNA encoding the late complement components C7 and C9 appears to be hardly or not detectable. These findings indicate that in AD the complement system does not act as an inflammatory mediator through membrane attack complex formation, but through the actions of the early complement products. In this review we focus on the role of complement in the pathological amyloid cascade in AD. In our opinion, the early complement activation products play a crucial role as mediators between the A beta deposits and the inflammatory responses leading to neurotoxicity.

Molecular aspects of inflammatory and immune responses in Alzheimer's disease

Recent advances indicate numerous molecular and cellular elements of the immune system are involved in the pathogenesis of Alzheimer's disease. Amyloid beta protein deposition induces many molecules associated with a predominantly local inflammatory response within the brain parenchyma. These responses also provoke the release of immune system mediators including cytokines, which all seem largely to be produced by reactive cells such as astrocytes and microglia. Classical acute phase proteins of the pentraxin and serine protease inhibitor (serpin) families as well as a host of complement proteins and some coagulation factor seem the most intrinsically involved. These secreted molecules display variable binding with the amyloidotic lesions. Although our understanding of the molecular specificity and significance of the interaction of these proteins within the lesions is not replete, the development of unique inhibitors of the inflammatory reactions could provide therapeutic strategies to impede the pathogenetic process. Currently, this appears a more viable option than to inhibit amyloid beta production or modify amyloid beta precursor protein processing, an approach which seems more complex.

Activation of the gene encoding decay accelerating factor following nerve growth factor treatment of sensory neurons is mediated by promoter sequences within 206 bases of the transcriptional start site

Using two independent differential screening procedures designed to identify novel mRNAs induced by nerve growth factor (NGF) treatment of adult dorsal root ganglion (DRG) neurons, we have isolated cDNA clones derived from the gene encoding decay accelerating factor (DAF). Hybridization analysis and semi-quantitative polymerase chain reaction confirmed that the DAF mRNA was indeed induced in NGF-treated adult DRG neurons. Moreover, the DAF gene promoter is NGF inducible (approximately two- to threefold) when transfected into DRG neurons, and this effect is primarily dependent on sequences between -206 and -77 relative to the transcriptional start site. Hence, the DAF gene constitutes a novel NGF-inducible gene whose mRNA is elevated in response to NGF treatment of DRG neurons. The potential significance of this effect is discussed in terms of the role of NGF in modulating the transcriptional activity and function of adult DRG neurons.

Expression and function of membrane regulators of complement on rat astrocytes in culture

Human astrocytes express CD59, decay accelerating factor and membrane cofactor protein to restrict the damaging effect of complement (C) activation on their cell surface. 5I2 antigen (5I2 Ag) is the functional analogue of the latter two proteins in rats. We here demonstrate the surface expression on rat astrocytes of CD59 and 5I2 Ag and use sodium dodecyl sulphate-polyacrylamide gel electrophoresis and Western blotting to confirm their identity and to quantify expression. Rat CD59 (MW 20,000) was expressed at 720 x 10(3) molecules per cell and 5I2 Ag (MW 58,000 and 64,000) at 625 x 10(3) molecules per cell. Reverse transcription-polymerase chain reaction using specific oligonucleotide primers demonstrated expression of mRNA for each protein. Twenty-four-hour stimulation with inflammatory cytokines (interferon-gamma, tumour necrosis factor-alpha, interleukins-1 beta, -2 and -6) or phorbol myristate acetate had no significant effect on the level of expression of either protein as determined by Western blotting. Lysis caused by classical pathway activation of C in human or rat serum was enhanced by blocking the function of CD59 and 5I2 Ag on rat astrocytes with monoclonal antibodies.

Control of the complement system

The complement system has developed a remarkably simple but elegant manner of regulating itself. It has faced and successfully dealt with how to facilitate activation on a microbe while preventing the same on host tissue. It solved this problem primarily by creating a series of secreted and membrane-regulatory proteins that prevent two highly undesirable events: activation in the fluid phase (no target) and on host tissue (inappropriate target). Also, if not checked, even on an appropriate target, the system would go to exhaustion and have nothing left for the next microbe. Therefore, the complement enzymes have an intrinsic instability and the fluid-phase control proteins play a major role in limiting activation in time. The symmetry of the regulatory process between fluid phase and membrane inhibitors at the C4/C3 step of amplification and convertase formation as well as at the MAC steps are particularly striking features of the self/nonself discrimination system. The use of glycolipid anchored proteins on membranes to decay enzymes and block membrane insertion events is unlikely to be by chance. Finally, it is economical for the cofactor regulatory activity to produce derivatives of C3b that now specifically engage additional receptors. Likewise, C1-Inh leads to C1q remaining on the immune complex to interact with the C1q receptor. Thus the complement system is designed to allow rapid, efficient, unimpeded activation on an appropriate foreign target while regulatory proteins intervene to prevent three undesirable consequences of complement activation: excessive activation on a single target, fluid phase activation, and activation on self.

The inflammatory response system of brain: implications for therapy of Alzheimer and other neurodegenerative diseases

Cultured brain cells are capable of generating many molecules associated with inflammatory and immune functions. They constitute the endogenous immune response system of brain. They include complement proteins and their regulators, inflammatory cytokines, acute phase reactants and many proteases and protease inhibitors. Most of the proteins are made by microglia and astrocytes, but even neurons are producers. Many appear in association with Alzheimer disease lesions, indicating a state of chronic inflammation in Alzheimer disease brain. Such a state can apparently exist without stimulation by peripheral inflammatory mediators or the peripheral immune system. A strong inflammatory response may be autotoxic to neurons, exacerbating the fundamental pathology in Alzheimer disease and perhaps other neurological disorders. Autotoxic processes may contribute to cellular death in chronic inflammatory diseases affecting other parts of the body, suggesting the general therapeutic value of anti-inflammatory agents. With respect to Alzheimer disease, multiple epidemiological studies indicate that patients taking anti-inflammatory drugs or suffering from conditions in which such drugs are routinely used, have a decreased risk of developing Alzheimer disease. In one very preliminary clinical trial, the anti-inflammatory drug indomethacin arrested progress of the disease. New agents directed against the inflammatory processes revealed in studies of Alzheimer disease lesions may have broad therapeutic applications.

Complement biosynthesis in the central nervous system

Complement is an important effector arm of the human immune response. Binding of proteolytic fragments derived from activation of complement by specific receptors leads to responses as diverse as inflammation, opsonization, and B-cell activation. The importance of characterizing the expression and regulation of complement in the CNS is highlighted by growing evidence that complement plays a significant role in the pathogenesis of a variety of neurological diseases, such as multiple sclerosis and Alzheimer's disease. In vitro studies have demonstrated that astrocytes, the predominant glial cell type in the brain, are capable of expressing or producing a majority of the components of the complement system. Expression of many complement proteins synthesized by astrocytes is regulated by both pro- and anti-inflammatory cytokines, many of which are also produced by several cell types in the CNS. In addition to astrocytes, ependymal cells, endothelial cells, microglia, and neurons have recently been shown to synthesize various complement proteins or express complement receptors on their cell surfaces. Together, these studies demonstrate that several cell types throughout the brain have the potential to express complement and, in many cases, increase expression in response to mediators of the acute phase response. These studies suggest that complement may play a greater role in CNS immune responses than previously thought, and pave the way for better understanding of the dynamics of complement expression and regulation in vivo. Such understanding may lead to therapeutic manipulation of complement host defense functions in a variety of inflammatory and degenerative diseases in the CNS.

Morphological study in the early stages of complement C5a fragment-induced experimental meningitis: activation of macrophages and astrocytes

Subarachnoidal application of the complement C5a fragment was used to induce acute experimental meningitis in rabbits and rats within 30-60 min. The early stages of the cellular inflammatory response were studied by means of flow cytometry, transmission electron microscopy and immunofluorescence microscopy. Infiltration of polymorphonuclear leukocytes (PMN) into the subarachnoidal space was the earliest event of the inflammatory reaction. By morphological criteria we found that PMN interacted with cells of the mononuclear-macrophage lineage (MML) and the marginal astrocytes via pseudopodia, whereas the pial cells were not involved in early stages of the inflammatory response. The number of invaded MML that were positive with the ED2 marker increased, indicating the hematogenous origin of the immigrating cell population. PMN were found to infiltrate the perivascular space of the marginal arterial vessel segments. This perivascular infiltration was assumed to be the first manifestation of cerebral vasculitis. The intimate association of resident cerebral cells (astrocytes) with invading PMN and MML is suggestive of a transient interaction of these cell types.

Regulation of C3 deposition on gp120 coated CD4 positive cells by decay accelerating factor and factor H

We investigated complement activation by recombinant gp120 (rgp120) treated CD4 cells and the role host complement regulatory proteins play in controlling C3 deposition. Complement activation was determined by detection of C3 on rgp120 coated cells in the presence and absence of HIV seropositive sera using flow cytometry. Treatment of rgp120 coated cells with complement resulted in C3 deposition only if HIV positive sera was included. Examination of C3 fragments on these cells demonstrated rapid cleavage of C3b to iC3b. The role of the regulatory proteins was examined by pretreating cells with mAb to block decay accelerating factor (DAF) or membrane cofactor protein (MCP) or by using factor H depleted sera as a complement source. Inhibition of DAF or use of factor H depleted sera significantly increased C3 deposition on rgp120 coated cells. In contrast, C3 deposition on rgp120 coated cells was not increased after blocking MCP. The sensitivity of rgp120 coated cells to complement lysis was unchanged after inhibition of the regulatory proteins, despite the increase in C3 deposited. These results indicate that in a model of virus infected cells, C3 deposition is regulated by DAF and factor H but MCP appears to have no role.

The immunopathology of Alzheimer's disease and some related disorders

Current evidence clearly indicates that elements of the immune system are involved in the pathogenesis of the principal lesions characterizing Alzheimer's disease (AD). Findings are in accord with features associated with both the innate and adaptive immune mechanisms involved in a predominantly local inflammatory response within the parenchyma. Many of the features are unique to AD, presumably related to the unusual properties of beta amyloid protein. Remarkably, the brain holds the capacity to produce almost all the immune system mediators which largely seem to be generated by glia comprising both astrocytes and microglia. While a variety of humoral mediators including classical acute phase proteins (and serpins) are increased and released, the complement seems most intrinsically involved. The cellular response is elaborated by microglia which seem the main immunocompetent cells partaking in the response. These appear to function as pluripotent macrophages expressing both classes of MHC antigens. Further characterization of this interesting response to cerebral amyloidosis will be challenging.

Expression of decay-accelerating factor (CD55), membrane cofactor protein (CD46) and CD59 in the human astroglioma cell line, D54-MG, and primary rat astrocytes

In this report, we have shown the expression of the complement regulatory proteins decay-accelerating factor (DAF, CD55), membrane cofactor protein (MCP, CD46) and CD59 on human D54-MG astroglioma cells by several methods, including immunofluorescence, flow cytometry and Western blotting and Northern blot analysis. These studies demonstrate that all three proteins are structurally and antigenically similar to their counterparts expressed on HepG2 and SW480 cells (hepatocyte and epithelial cell lines, respectively). D54-MG cells express mRNA for all three proteins of the appropriate size(s). The phosphatidylinositol-specific enzyme, PIPLC, cleaved DAF from the surface of D54-MG cells, demonstrating that DAF is linked by a glycophospholipid anchor as has been shown for other cell types. Flow cytometry demonstrates that primary rat astrocytes also constitutively express all three regulatory proteins. These data are the first to demonstrate the expression of CD59 on astrocytes, and the presence of all three regulatory proteins on astrocytes suggests that regulation of complement activation in the central nervous system is important in neural host defense mechanisms.

Detection of mosaic protein mRNA in human astrocytes

Mosaic proteins consist of a group of proteins that may be comprised of one or more types of a variety of different structural modules and have a diverse range of functions. We have examined primary human astrocyte cultures for the presence of three mosaic proteins, B2I and the complement proteins factor H and properdin. Using the polymerase chain reaction and an enhanced chemiluminescence detection technique, we were able to show that mRNA transcripts for each of these proteins are expressed in human astrocytes.

Expression of CD59, a regulator of the membrane attack complex of complement, on human astrocytes

The present study demonstrates that human astrocytes synthesize and express CD59, a regulator of the membrane attack complex of complement. This was shown by flow cytometry following staining of astrocytes with MAb to CD59, and Western blotting of astrocyte lysates, which revealed the characteristic 18-23,000 M(r) band of CD59. Synthesis of CD59 by astrocytes was confirmed by detection of CD59 specific mRNA by polymerase chain reaction. A low level of C3 deposition occurred on astrocytes following exposure to autologous serum. CD59 may prevent subsequent damage from C5b-9 and protect astrocytes during inflammatory and infectious disorders of the nervous system.

Molecular profile of reactive astrocytes--implications for their role in neurologic disease

The central nervous system responds to diverse neurologic injuries with a vigorous activation of astrocytes. While this phenomenon is found in many different species, its function is obscure. Understanding the molecular profile characteristic of reactive astrocytes should help define their function. The purpose of this review is to provide a summary of molecules whose levels of expression differentiate activated from resting astrocytes and to use the molecular profile of reactive astrocytes as the basis for speculations on the functions of these cells. At present, reactive astrocytosis is defined primarily as an increase in the number and size of cells expressing glial fibrillary acidic protein. In vivo, this increase in glial fibrillary acidic protein-positive cells reflects predominantly phenotypic changes of resident astroglia rather than migration or proliferation of such cells. Upon activation, astrocytes upmodulate the expression of a large number of molecules. From this molecular profile it becomes apparent that reactive astrocytes may benefit the injured nervous system by participating in diverse biological processes. For example, upregulation of proteases and protease inhibitors could help remodel the extracellular matrix, regulate the concentration of different proteins in the neuropil and clear up debris from degenerating cells. Cytokines are key mediators of immunity and inflammation and could play a critical role in the regulation of the blood-central nervous system interface. Neurotrophic factors, transporter molecules and enzymes involved in the metabolism of excitotoxic amino acids or in the antioxidant pathway may help protect neurons and other brain cells by controlling neurotoxin levels and contributing to homeostasis within the central nervous system. Therefore, an impairment of astroglial performance has the potential to exacerbate neuronal dysfunction. Based on the synopsis of studies presented, a number of issues become apparent that deserve a more extensive analysis. Among them are the relative contribution of microglia and astrocytes to early wound repair, the characterization of astroglial subpopulations, the specificity of the astroglial response in different diseases as well as the analysis of reactive astrocytes with techniques that can resolve fast physiologic processes. Differences between reactive astrocytes in vivo and primary astrocytes in culture are discussed and underline the need for the development and exploitation of models that will allow the analysis of reactive astrocytes in the intact organism.

Detection of complement protein mRNA in human astrocytes by the polymerase chain reaction

Primer sets specific for complement proteins, C3, factor B and factor I, were designed and used to amplify cDNA from cultured human astrocyte mRNA by the polymerase chain reaction. Appropriately sized PCR products of 506 bp, 885 bp and 146 bp, respectively, were generated and specificity was confirmed with Southern blotting using an enhanced chemiluminescence detection system. The sensitivity of detection was high, with amplified product from cDNA of approximately 6250 cells readily visualized. C3 and factor B have previously been reported to be produced by murine astrocytes; however, this is the first report indicating synthesis of C3, factor B and factor I by human astrocytes. These results indicate that PCR is a simple and sensitive technique to detect mRNA transcripts for proteins of the alternative pathway of complement in human astrocytes.