Complement and demyelinating disease: no MAC needed?

Fish decay-accelerating factor (DAF) regulates intestinal complement pathway and immune response to bacterial challenge

Decay-accelerating factor (DAF) is an essential member of the complement regulatory protein family that plays an important role in immune response and host homeostasis in mammals. However, the immune function of DAF has not been well characterized in bony fish. In this study, a complement regulatory protein named CiDAF was firstly characterized from Ctenopharyngodon idella and its potential roles were investigated in intestine following bacterial infection. Similar to mammalian DAFs, CiDAF has multiple complement control protein (CCP) functional domains, suggesting the evolutionary conservation of DAFs. CiDAF was broadly expressed in all tested tissues, with a relatively high expression level detected in the spleen and kidney. In vivo immune challenge experiments revealed that CiDAF strongly responded to bacterial pathogens (Aeromonas hydrophila and Aeromonas veronii) and PAMPs (lipopolysaccharide (LPS) or muramyl dipeptide (MDP)) challenges. In vitro RNAi experiments indicated that knockdown of CiDAF could upregulate the expression of complement genes (C4b, C5 and C7) and inflammatory cytokines (TNF-α, IL-1β and IL-8). Moreover, 2000 ng/mL of CiDAF agonist progesterone effectively alleviated LPS- or MDP-induced intestinal inflammation by regulating expression of complement factors, TLR/PepT1 pathway genes and inflammatory cytokines. Overall, these findings revealed that CiDAF may act as a negative regulator of intestinal complement pathway and immune response to bacterial challenge in grass carp.

Microglial Pruning: Relevance for Synaptic Dysfunction in Multiple Sclerosis and Related Experimental Models

Microglia, besides being able to react rapidly to a wide range of environmental changes, are also involved in shaping neuronal wiring. Indeed, they actively participate in the modulation of neuronal function by regulating the elimination (or “pruning”) of weaker synapses in both physiologic and pathologic processes. Mounting evidence supports their crucial role in early synaptic loss, which is emerging as a hallmark of several neurodegenerative diseases, including multiple sclerosis (MS) and its preclinical models. MS is an inflammatory, immune-mediated pathology of the white matter in which demyelinating lesions may cause secondary neuronal death. Nevertheless, primitive grey matter (GM) damage is emerging as an important contributor to patients’ long-term disability, since it has been associated with early and progressive cognitive decline (CD), which seriously worsens the quality of life of MS patients. Widespread synapse loss even in the absence of demyelination, axon degeneration and neuronal death has been demonstrated in different GM structures, thus raising the possibility that synaptic dysfunction could be an early and possibly independent event in the neurodegenerative process associated with MS. This review provides an overview of microglial-dependent synapse elimination in the neuroinflammatory process that underlies MS and its experimental models.

The good, the bad, and the opportunities of the complement system in neurodegenerative disease

The complement cascade is a critical effector mechanism of the innate immune system that contributes to the rapid clearance of pathogens and dead or dying cells, as well as contributing to the extent and limit of the inflammatory immune response. In addition, some of the early components of this cascade have been clearly shown to play a beneficial role in synapse elimination during the development of the nervous system, although excessive complement-mediated synaptic pruning in the adult or injured brain may be detrimental in multiple neurogenerative disorders. While many of these later studies have been in mouse models, observations consistent with this notion have been reported in human postmortem examination of brain tissue. Increasing awareness of distinct roles of C1q, the initial recognition component of the classical complement pathway, that are independent of the rest of the complement cascade, as well as the relationship with other signaling pathways of inflammation (in the periphery as well as the central nervous system), highlights the need for a thorough understanding of these molecular entities and pathways to facilitate successful therapeutic design, including target identification, disease stage for treatment, and delivery in specific neurologic disorders. Here, we review the evidence for both beneficial and detrimental effects of complement components and activation products in multiple neurodegenerative disorders. Evidence for requisite co-factors for the diverse consequences are reviewed, as well as the recent studies that support the possibility of successful pharmacological approaches to suppress excessive and detrimental complement-mediated chronic inflammation, while preserving beneficial effects of complement components, to slow the progression of neurodegenerative disease.

Complement-dependent synapse loss and microgliosis in a mouse model of multiple sclerosis

Multiple sclerosis (MS) is an inflammatory, neurodegenerative disease of the CNS characterized by both grey and white matter injury. Microglial activation and a reduction in synaptic density are key features of grey matter pathology that can be modeled with MOG_35-55 experimental autoimmune encephalomyelitis (EAE). Complement deposition combined with microglial engulfment has been shown during normal development and in disease as a mechanism for pruning synapses. We tested whether there is excess complement production in the EAE hippocampus and whether complement-dependent synapse loss is a source of degeneration in EAE using C1qa and C3 knockout mice. We found that C1q and C3 protein and mRNA levels were elevated in EAE mice. Genetic loss of C3 protected mice from EAE-induced synapse loss, reduced microglial activation, decreased the severity of the EAE clinical score, and protected memory/freezing behavior after contextual fear conditioning. C1qa KO mice with EAE showed little to no change on these measurements compared to WT EAE mice. Thus, pathologic expression and activation of the early complement pathway, specifically at the level of C3, contributes to hippocampal grey matter pathology in the EAE.

Therapeutic inhibition of the alternative complement pathway attenuates chronic EAE

Previous studies from our laboratory using complement-mutant mice demonstrated that the alternative pathway is the dominant activation pathway responsible for complement-mediated pathology in demyelinating disease. Using a well-characterized inhibitory monoclonal antibody (mAb 1379) directed against mouse factor B, we assessed the therapeutic value of inhibiting the alternative complement pathway in experimental autoimmune encephalomyelitis (EAE), the animal model for multiple sclerosis. Administration of anti-factor B antibody to mice prior to the onset of clinical signs of active EAE had no affect on the onset or acute phase of disease, but significantly attenuated the chronic phase of disease resulting in reduced cellular infiltration, inflammation and demyelination in antibody-treated mice. Attenuation of the chronic phase of disease was long lasting even though antibody administration was terminated shortly after disease onset. Chronic disease was also attenuated in transferred EAE when anti-factor B antibody was administered before or after disease onset. Similar levels of disease attenuation were observed in transferred EAE using MOG-specific encephalitogenic T cells. These studies demonstrate the therapeutic potential for inhibition of factor B in the chronic phase of demyelinating disease, where treatment options are limited.

Gene expression in the spinal cord in female lewis rats with experimental autoimmune encephalomyelitis induced with myelin basic protein

Background

Experimental autoimmune encephalomyelitis (EAE), the best available model of multiple sclerosis, can be induced in different animal strains using immunization with central nervous system antigens. EAE is associated with inflammation and demyelination of the nervous system. Micro-array can be used to investigate gene expression and biological pathways that are altered during disease. There are few studies of the changes in gene expression in EAE, and these have mostly been done in a chronic mouse EAE model. EAE induced in the Lewis with myelin basic protein (MBP-EAE) is well characterised, making it an ideal candidate for the analysis of gene expression in this disease model.

Methodology/Principal Findings

MBP-EAE was induced in female Lewis rats by inoculation with MBP and adjuvants. Total RNA was extracted from the spinal cords and used for micro-array analysis using AffimetrixGeneChip Rat Exon 1.0 ST Arrays. Gene expression in the spinal cords was compared between healthy female rats and female rats with MBP-EAE. Gene expression in the spinal cord of rats with MBP-EAE differed from that in the spinal cord of normal rats, and there was regulation of pathways involved with immune function and nervous system function. For selected genes the change in expression was confirmed with real-time PCR.

Conclusions/Significance

EAE leads to modulation of gene expression in the spinal cord. We have identified the genes that are most significantly regulated in MBP-EAE in the Lewis rat and produced a profile of gene expression in the spinal cord at the peak of disease.

Targeted inhibition of complement using complement receptor 2-conjugated inhibitors attenuates EAE

Multiple sclerosis (MS) is the most common autoimmune demyelinating disease, affecting millions of individuals worldwide. In the last two decades, many therapeutic options for the treatment of MS have become available, however they are limited in terms of effectiveness and some remain plagued by safety issues. The currently available treatment options target relapsing remitting forms of MS and are not effective against the more progressive forms of the disease. These limitations highlight a significant unmet treatment need for MS. In experimental autoimmune encephalomyelitis (EAE) studies from our laboratory, we have previously shown, using a number of complement mutant and transgenic mice, that inhibition of the alternative complement pathway and the C3 convertase confers significant protection from disease. We report here that targeted inhibition of complement activation using complement receptor 2 (CR2)-conjugated inhibitors significantly attenuates EAE. Administration of CR2-Crry (blocks all complement pathways at C3 activation) and CR2-fH (specifically blocks the alternative pathway) just prior to and during the onset of EAE blocks progression of both acute and chronic disease. These data indicate that inhibition of complement may offer an effective therapeutic approach to treating both acute and chronic forms of demyelinating disease through blocking the alternative pathway or complement convertases.

The C5 convertase is not required for activation of the terminal complement pathway in murine experimental cerebral malaria

Cerebral malaria (CM) is the most severe manifestation of clinical malaria syndromes and has a high fatality rate especially in the developing world. Recent studies demonstrated that C5(-/-) mice are resistant to experimental CM (ECM) and that protection was due to the inability to form the membrane attack complex. Unexpectedly, we observed that C4(-/-) and factor B(-/-) mice were fully susceptible to disease, indicating that activation of the classical or alternative pathways is not required for ECM. C3(-/-) mice were also susceptible to ECM, indicating that the canonical C5 convertases are not required for ECM development and progression. Abrogation of ECM by treatment with anti-C9 antibody and detection of C5a in serum of C3(-/-) mice confirmed that C5 activation occurs in ECM independent of C5 convertases. Our data indicate that activation of C5 in ECM likely occurs via coagulation enzymes of the extrinsic protease pathway.

Systemic complement profiling in multiple sclerosis as a biomarker of disease state

BACKGROUND

There is increasing evidence of significant and dynamic systemic activation and upregulation of complement in multiple sclerosis (MS), which may contribute to disease pathogenesis.

OBJECTIVE

We aimed to investigate the pathological role of complement in MS and the potential role for complement profiling as a biomarker of MS disease state.

METHODS

Key components of the classical, alternative and terminal pathways of complement were measured in plasma and cerebrospinal fluid (CSF) of patients with MS in different clinical phases of disease and in matched controls.

RESULTS

Increased plasma levels of C3 (p<0.003), C4 (p<0.001), C4a (p<0.001), C1 inhibitor (p<0.001), and factor H (p<0.001), and reduced levels of C9 (p<0.001) were observed in MS patients compared with controls. Combined profiling of these analytes produced a statistical model with a predictive value of 97% for MS and 73% for clinical relapse when combined with selected demographic data. CSF-plasma correlations suggested that source of synthesis of these components was both systemic and central.

CONCLUSION

These data provide further evidence of alterations in both local and systemic expression and activation of complement in MS and suggest that complement profiling may be informative as a biomarker of MS disease, although further work is needed to determine its use in distinguishing MS from its differential.

Fusion of metabolomics and proteomics data for biomarkers discovery: case study on the experimental autoimmune encephalomyelitis

Background

Analysis of Cerebrospinal Fluid (CSF) samples holds great promise to diagnose neurological pathologies and gain insight into the molecular background of these pathologies. Proteomics and metabolomics methods provide invaluable information on the biomolecular content of CSF and thereby on the possible status of the central nervous system, including neurological pathologies. The combined information provides a more complete description of CSF content. Extracting the full combined information requires a combined analysis of different datasets i.e. fusion of the data.

Results

A novel fusion method is presented and applied to proteomics and metabolomics data from a pre-clinical model of multiple sclerosis: an Experimental Autoimmune Encephalomyelitis (EAE) model in rats. The method follows a mid-level fusion architecture. The relevant information is extracted per platform using extended canonical variates analysis. The results are subsequently merged in order to be analyzed jointly. We find that the combined proteome and metabolome data allow for the efficient and reliable discrimination between healthy, peripherally inflamed rats, and rats at the onset of the EAE. The predicted accuracy reaches 89% on a test set. The important variables (metabolites and proteins) in this model are known to be linked to EAE and/or multiple sclerosis.

Conclusions

Fusion of proteomics and metabolomics data is possible. The main issues of high-dimensionality and missing values are overcome. The outcome leads to higher accuracy in prediction and more exhaustive description of the disease profile. The biological interpretation of the involved variables validates our fusion approach.

Role and mechanism of action of complement in regulating T cell immunity

Complement is a part of the innate immune system that contributes to first-line host defense. It is also implicated in a number of human inflammatory conditions and has attracted interest as a potential therapeutic target. Understanding the basic biology of complement and its mechanism(s) of action is imperative for developing complement-based treatments for infectious and autoimmune diseases. One of the exciting new developments in this regard is the revelation that complement plays an important role in T cell immunity. In this review, we highlight recent published studies implicating complement in models of CD4+ and CD8+ T cell immune responses, and discuss its potential mechanism(s) action in these processes. We also comment on issues that may impact data interpretation and draw attention to their consideration in future studies.

Innate immunity in the nervous system

The complement (C) system plays a central role in innate immunity and bridges innate and adaptive immune responses. A fine balance of C activation and regulation mediates the elimination of invading pathogens and the protection of the host from excessive C deposition on healthy tissues. If this delicate balance is disrupted, the C system may cause injury and contribute to the pathogenesis of various diseases, including neurodegenerative disorders and neuropathies. Here we review evidence indicating that C factors and regulators are locally synthesized in the nervous system and we discuss the evidence supporting the protective or detrimental role of C activation in health, injury, and disease of the nerve.

Deletion of both the C3a and C5a receptors fails to protect against experimental autoimmune encephalomyelitis

Multiple sclerosis (MS) is an autoimmune disease in which inflammation, leukocyte infiltration, and ultimately, demyelination occur as a result of innate and adaptive immune-mediated mechanisms. The pathophysiological role of the complement system, a major component of innate immunity, in the development and progression of experimental autoimmune encephalomyelitis (EAE), the animal model for MS has been extensively examined. Previous studies from our lab have shown that the complement receptor for the anaphylatoxin C3a, but not for C5a plays an important role in EAE. Based on the important contributions of the complement anaphylatoxin receptors to other inflammatory conditions in the CNS, we reasoned that deletion of both receptors may reveal underlying interactions between them that are important to EAE pathology. We performed EAE in C3aR/C5aR double knockout mice (C3aR/C5aR(-/-)) and observed delayed onset of disease but no attenuation of disease severity compared to wild type mice. Interestingly there was trend toward greater infiltration of CD4(+), but not CD8(+) T cells, in C3aR/C5aR(-/-) mice with EAE, suggesting altered trafficking of these cells. Antigen-specific T cells isolated from C3aR/C5aR(-/-) mice during acute EAE produced elevated levels of TNF-alpha, but markedly reduced levels of IFN-gamma and IL-12 compared to wild type mice. It remains unclear how the changes in these disease parameters contribute to the loss of the protective effect seen in C3aR(-/-) mice, however our data indicate a level of cross-modulation between the C3aR and C5aR during EAE.

Anti-aquaporin-4 auto-antibodies orchestrate the pathogenesis in neuromyelitis optica

NMO-IgG, the auto-antibody specific to the aquaporin-4 (AQP4) water channel associated with the autoimmune inflammatory disease neuromyelitis optica (NMO), is considered to be an accurate serum biomarker and is thought to be an important contributor to NMO pathology. In this review, we summarize recent evidences from our group and others indicating that NMO-IgG can be implicated at several levels in the immuno-pathology of NMO. NMO-IgG/anti-AQP4 antibodies may compromise the integrity of the blood-brain barrier and consequently facilitate and enhance the perivascular inflammation characteristic of NMO. Lastly, NMO-IgG can induce astrocyte injury which may lead to the accumulation of excitatory/toxic molecules and accordingly damage oligodendrocytes and compromise myelin integrity.

Complement factor H, a marker of self protects against experimental autoimmune encephalomyelitis

The CNS innate immune response is a "double-edged sword" representing a fine balance between protective antipathogen responses and detrimental neurocytotoxic effects. Hence, it is important to identify the key regulatory mechanisms involved in the control of CNS innate immunity and which could be harnessed to explore novel therapeutic avenues. In analogy to the newly described neuroimmune regulatory proteins also known as "don't eat me" signals (CD200, CD47, CD22, fractalkine, semaphorins), we herein identify the key role of complement regulator factor H (fH) in controlling neuroinflammation initiated in an acute mouse model of Ab-dependent experimental autoimmune encephalomyelitis. Mouse fH was found to be abundantly expressed by primary cultured neurons and neuronal cell lines (N1E115 and Neuro2a) at a level comparable to BV2 microglia and CLTT astrocytes. Mouse neurons expressed other complement regulators crry and low levels of CD55. In the brain, the expression of fH was localized to neuronal bodies and axons, endothelial cells, microglia but not oligodendrocytes and myelin sheaths and was dramatically reduced in inflammatory experimental autoimmune encephalomyelitis settings. When exogenous human fH was administered to disease Ab-dependent experimental autoimmune encephalomyelitis animals, there was a significant decrease in clinical score, inflammation, and demyelination, as compared with PBS-injected animals. We found that the accumulation of human fH in the brain parenchyma protected neurons from complement opsonization, axonal injury, and leukocyte infiltration. Our data argue for a key regulatory activity of fH in neuroprotection and provide novel therapeutic avenues for CNS chronic inflammatory diseases.

Complement in multiple sclerosis: its role in disease and potential as a biomarker

Multiple sclerosis (MS) is a common inflammatory disease of the central nervous system with a poorly defined and complex immunopathogenesis. Although initiated by reactive T cells, persistent inflammation is evident throughout the disease course. A contribution from complement has long been suspected, based on the results of pathological and functional studies which have demonstrated complement activation products in MS brain and biological fluids. However, the extent and nature of complement activation and its contribution to disease phenotype and long-term outcome remain unclear. Furthermore, functional polymorphisms in components and regulators of the complement system which cause dysregulation, and are known to contribute to other autoimmune inflammatory disorders, have not been investigated to date in MS in any detail. In this paper we review evidence from pathological, animal model and human functional and genetic studies, implicating activation of complement in MS. We also evaluate the potential of complement components and regulators and their polymorphic variants as biomarkers of disease, and suggest appropriate directions for future research.

The complement cascade: Yin-Yang in neuroinflammation--neuro-protection and -degeneration

The complement cascade has long been recognized to play a key role in inflammatory and degenerative diseases. It is a 'double edged' sword as it is necessary to maintain health, yet can have adverse effects when unregulated, often exacerbating disease. The contrasting effects of complement, depending on whether in a setting of health or disease, is the price paid to achieve flexibility in scope and degree of a protective response for the host from infection and injury. Loss or even decreased efficiency of critical regulatory control mechanisms can result in aggravated inflammation and destruction of self-tissue. The role of the complement cascade is poorly understood in the nervous system and neurological disorders. Novel studies have demonstrated that the expression of complement proteins in brain varies in different cell types and the effects of complement activation in various disease settings appear to differ. Understanding the functioning of this cascade is essential, as it has therapeutic implications. In this review, we will attempt to provide insight into how this complex cascade functions and to identify potential strategic targets for therapeutic intervention in chronic diseases as well as acute injury in the CNS.

Deletion of both ICAM-1 and C3 enhances severity of experimental autoimmune encephalomyelitis compared to C3-deficient mice

Multiple sclerosis (MS) is an autoimmune disease characterized by central nervous system (CNS) inflammation and leukocyte infiltration, demyelination of neurons, and blood-brain barrier breakdown. The development of experimental autoimmune encephalomyelitis (EAE), the animal model for MS is dependent on a number of components of the immune system including complement and adhesion molecules. Previous studies in our lab have examined the role of C3, the central complement component, and intercellular adhesion molecule-1 (ICAM-1) a key cell adhesion molecule involved in leukocyte trafficking to sites of inflammation including the CNS. In these studies we demonstrated that myelin oligodendrocyte glycoprotein (MOG)-induced EAE is markedly attenuated in both ICAM-1(-/-) and C3(-/-) mice. Given the pivotal role that these proteins play in EAE, we hypothesized that EAE in ICAM-1(-/-) and C3(-/-) double mutant mice would likely fail to develop. Unexpectedly, EAE in ICAM-1(-/-)xC3(-/-) mice was only modestly attenuated compared to wild type mice and significantly worse than C3(-/-) mice. Leukocyte infiltration was commensurate with disease severity between the three groups of mice. Spinal cord T cells from ICAM-1(-/-)xC3(-/-) mice produced the highest levels of IFN-gamma and TNF-alpha, despite reduced disease severity compared to wild type mice. The mechanisms behind the elevated EAE severity in ICAM-1(-/-)xC3(-/-) mice may relate to altered homing of leukocytes or processing of self-antigens in the double mutant background.

Pathogenesis of acute and chronic central nervous system infection with variants of mouse hepatitis virus, strain JHM

Infection of mice with variants of mouse hepatitis virus, strain JHM (MHV-JHM), provide models of acute and chronic viral infection of the central nervous system (CNS). Through targeted recombination and reverse genetic manipulation, studies of infection with MHV-JHM variants have identified phenotypic differences and examined the effects of these differences on viral pathogenesis and anti-viral host immune responses. Studies employing recombinant viruses with a modified spike (S) glycoprotein of MHV-JHM have identified the S gene as a major determinant of neurovirulence. However, the association of S gene variation and neurovirulence with host ability to generate anti-viral CD8 T cell responses is not completely clear. Partially protective anti-viral immune responses may result in persistent infection and chronic demyelinating disease characterized by myelin removal from axons of the CNS and associated with dense macrophage/microglial infiltration. Demyelinating disease during MHV-JHM infection is immune-mediated, as mice that lack T lymphocytes fail to develop disease despite succumbing to encephalitis with high levels of infectious virus in the CNS. However, the presence of T lymphocytes or anti-viral antibody can induce disease in infected immunodeficient mice. The mechanisms by which these immune effectors induce demyelination share an ability to activate and recruit macrophages and microglia, thus increasing the putative role of these cells in myelin destruction.

Astrocyte-specific expression of a soluble form of the murine complement control protein Crry confers demyelination protection in the cuprizone model

Complement has been implicated as a potential effector mechanism in neurodegeneration; yet the precise role of complement in this process remains elusive. In this report, we have utilized the cuprizone model of demyelination-remyelination to examine the contribution of complement to disease. C1q deposition was observed in the corpus callosum of C57BL/6 mice during demyelination, suggesting complement activation by apoptotic oligodendrocyte debris. Simultaneously, these mice lost expression of the rodent complement regulatory protein, Crry. A soluble CNS-specific form of the Crry protein (sCrry) expressed in a transgenic mouse under the control of an astrocyte-specific promoter was induced in the corpus callosum during cuprizone treatment. Expression of this protein completely protected the mice from demyelination. Interestingly, sCrry mice had low levels of demyelination at later times when control mice were remyelinating. Although the sCrry transgenic mice had lower levels of demyelination, there was no decrease in overall cellularity, however there were decreased numbers of microglia in the sCrry mice relative to controls. Strikingly, sCrry mice had early recovery of mature oligodendrocytes, although they later disappeared. TUNEL staining suggested that production of the sCrry protein in the transgenic mice protected from a late apoptosis event at 3 weeks of cuprizone treatment. Our data suggest complement provides some protection of mature oligodendrocytes during cuprizone treatment but may be critical for subsequent remyelination events. These data suggest that temporal restriction of complement inhibition may be required in some disease settings.

The membrane attack complex of the complement system is essential for rapid Wallerian degeneration

The complement (C) system plays an important role in myelin breakdown during Wallerian degeneration (WD). The pathway and mechanism involved are, however, not clear. In a crush injury model of the sciatic nerve, we show that C6, necessary for the assembly of the membrane attack complex (MAC), is essential for rapid WD. At 3 d after injury, pronounced WD occurred in wild-type animals, whereas the axons and myelin of C6-deficient animals appeared intact. Macrophage recruitment and activation was inhibited in C6-deficient rats. However, 7 d after injury, the distal part of the C6-deficient nerves appeared degraded. As a consequence of a delayed WD, more myelin breakdown products were present than in wild-type nerves. Reconstitution of the C6-deficient animals with C6 restored the wild-type phenotype. Treatment with rhC1INH (recombinant human complement 1 inhibitor) blocked deposition of activated C-cleaved products after injury. These experiments demonstrate that the classical pathway of the complement system is activated after acute nerve trauma and that the entire complement cascade, including MAC deposition, is essential for rapid WD and efficient clearance of myelin after acute peripheral nerve trauma.

Complement in experimental autoimmune encephalomyelitis revisited: C3 is required for development of maximal disease

Complement per se has been shown to play an important role in demyelinating disease but controversy remains regarding the role of C3 in the development and progression of experimental autoimmune encephalomyelitis (EAE), the animal model for multiple sclerosis. In this study, we used C3(-/-) mice to confirm previous findings that C3 is required for full development of EAE. Furthermore, C3(+/-) mice (with serum C3 levels 50% that of wild-type mice) developed EAE with a severity intermediate between wild-type and C3(-/-) mice. Importantly transfer of wild-type encephalitogenic T cells to C3(-/-) mice resulted in attenuated EAE. C3(-/-) mice with EAE had fewer CD4(+) and CD8(+) T cells in the CNS and 50% fewer of these cells produced IFN-gamma compared to wild-type mice. When treated with anti-CD3 antibody, CD4(+) T cells from wild-type and C3(-/-) mice had similar activation profiles as judged by IFN-gamma production and CD25 and CD69 expression, indicating there is no gross or intrinsic defect in T cells from C3(-/-) mice. T cells from primed C3(-/-) mice proliferated comparably to that of control T cells on re-stimulation with MOG peptide. Our results confirm a requirement for C3 for maximal development of EAE and suggest that receptors for C3-derived activation fragments might be a viable therapeutic target for prevention and treatment demyelinating disease.