The Ins and Outs of Central Nervous System Inflammation-Lessons Learned from Multiple Sclerosis.

Multiple sclerosis (MS) is a chronic disease that is characterized by the inappropriate invasion of lymphocytes and monocytes into the central nervous system (CNS), where they orchestrate the demyelination of axons, leading to physical and cognitive disability. There are many reasons immunologists should be interested in MS. Aside from the fact that there is still significant unmet need for patients living with the progressive form of the disease, MS is a case study for how immune cells cross CNS barriers and subsequently interact with specialized tissue parenchymal cells. In this review, we describe the types of immune cells that infiltrate the CNS and then describe interactions between immune cells and glial cells in different types of lesions. Lastly, we provide evidence for CNS-compartmentalized immune cells and speculate on how this impacts disease progression for MS patients.

Recirculating Intestinal IgA-Producing Cells Regulate Neuroinflammation via IL-10.

Plasma cells (PC) are found in the CNS of multiple sclerosis (MS) patients, yet their source and role in MS remains unclear. We find that some PC in the CNS of mice with experimental autoimmune encephalomyelitis (EAE) originate in the gut and produce immunoglobulin A (IgA). Moreover, we show that IgA+ PC are dramatically reduced in the gut during EAE, and likewise, a reduction in IgA-bound fecal bacteria is seen in MS patients during disease relapse. Removal of plasmablast (PB) plus PC resulted in exacerbated EAE that was normalized by the introduction of gut-derived IgA+ PC. Furthermore, mice with an over-abundance of IgA+ PB and/or PC were specifically resistant to the effector stage of EAE, and expression of interleukin (IL)-10 by PB plus PC was necessary and sufficient to confer resistance. Our data show that IgA+ PB and/or PC mobilized from the gut play an unexpected role in suppressing neuroinflammation.

Age-related changes in multiple sclerosis and experimental autoimmune encephalomyelitis.

A better understanding of the pathological mechanisms that drive neurodegeneration in people living with multiple sclerosis (MS) is needed to design effective therapies to treat and/or prevent disease progression. We propose that CNS-intrinsic inflammation and re-modelling of the sub-arachnoid space of the leptomeninges sets the stage for neurodegeneration from the earliest stages of MS. While neurodegenerative processes are clinically silent early in disease, ageing results in neurodegenerative changes that become clinically manifest as progressive disability. Here we review pathological correlates of MS disease progression, highlight emerging mouse models that mimic key progressive changes in MS, and provide new perspectives on therapeutic approaches to protect against MS-associated neurodegeneration.

Integration of Th17- and Lymphotoxin-Derived Signals Initiates Meningeal-Resident Stromal Cell Remodeling to Propagate Neuroinflammation.

Tertiary lymphoid tissues (TLTs) have been observed in the meninges of multiple sclerosis (MS) patients, but the stromal cells and molecular signals that support TLTs remain unclear. Here, we show that T helper 17 (Th17) cells induced robust TLTs within the brain meninges that were associated with local demyelination during experimental autoimmune encephalitis (EAE). Th17-cell-induced TLTs were underpinned by a network of stromal cells producing extracellular matrix proteins and chemokines, enabling leukocytes to reside within, rather than simply transit through, the meninges. Within the CNS, interactions between lymphotoxin αß (LTαß) on Th17 cells and LTßR on meningeal radio-resistant cells were necessary for the propagation of de novo interleukin-17 responses, and activated T cells from MS patients expressed elevated levels of LTßR ligands. Therefore, input from both Th17 cells and the lymphotoxin pathway induce the formation of an immune-competent stromal cell niche in the meninges.

Stromal Cell-Mediated Coordination of Immune Cell Recruitment, Retention, and Function in Brain-Adjacent Regions.

The CNS is tightly regulated to maintain immune surveillance and efficiently respond to injury and infections. The current appreciation that specialized "brain-adjacent" regions in the CNS are in fact not immune privileged during the steady state, and that immune cells can take up residence in more immune-privileged areas of the CNS during inflammation with consequences on the adjacent brain parenchyma, beg the question of what cell types support CNS immunity. As they do in secondary lymphoid organs, we provide evidence in this review that stromal cells also underpin brain-resident immune cells. We review the organization and function of stromal cells in different anatomical compartments of the CNS and discuss their capacity to rapidly establish and elaborate an immune-competent niche that further sustains immune cells entering the CNS from the periphery. In summary, we argue that stromal cells are key cellular agents that support CNS-compartmentalized immunity.

CCR6 Expression on B Cells Is Not Required for Clinical or Pathological Presentation of MOG Protein-Induced Experimental Autoimmune Encephalomyelitis despite an Altered Germinal Center Response.

B cells have been implicated in the pathogenesis of multiple sclerosis, but the mechanisms that guide B cell activation in the periphery and subsequent migration to the CNS remain incompletely understood. We previously showed that systemic inflammation induces an accumulation of B cells in the spleen in a CCR6/CCL20-dependent manner. In this study, we evaluated the role of CCR6/CCL20 in the context of myelin oligodendrocyte glycoprotein (MOG) protein-induced (B cell-dependent) experimental autoimmune encephalomyelitis (EAE). We found that CCR6 is upregulated on murine B cells that migrate into the CNS during neuroinflammation. In addition, human B cells that migrate across CNS endothelium in vitro were found to be CCR6+, and we detected CCL20 production by activated CNS-derived human endothelial cells as well as a systemic increase in CCL20 protein during EAE. Although mice that lack CCR6 expression specifically on B cells exhibited an altered germinal center reaction in response to MOG protein immunization, CCR6-deficient B cells did not exhibit any competitive disadvantage in their migration to the CNS during EAE, and the clinical and pathological presentation of EAE induced by MOG protein was unaffected. These data, to our knowledge, provide new information on the role of B cell-intrinsic CCR6 expression in a B cell-dependent model of neuroinflammation.

The eIF2α kinase HRI triggers the autophagic clearance of cytosolic protein aggregates.

Large cytosolic protein aggregates are removed by two main cellular processes, autophagy and the ubiquitin-proteasome system, and defective clearance of these protein aggregates results in proteotoxicity and cell death. Recently, we found that the eIF2α kinase heme-regulated inhibitory (HRI) induced a cytosolic unfolded protein response to prevent aggregation of innate immune signalosomes, but whether HRI acts as a general sensor of proteotoxicity in the cytosol remains unclear. Here we show that HRI controls autophagy to clear cytosolic protein aggregates when the ubiquitin-proteasome system is inhibited. We further report that silencing the expression of HRI resulted in decreased levels of BAG3 and HSPB8, two proteins involved in chaperone-assisted selective autophagy, suggesting that HRI may control proteostasis in the cytosol at least in part through chaperone-assisted selective autophagy. Moreover, knocking down the expression of HRI resulted in cytotoxic accumulation of overexpressed α-synuclein, a protein known to aggregate in Parkinson's disease, dementia with Lewy bodies, and multiple system atrophy. In agreement with these data, protein aggregate accumulation and microglia activation were observed in the spinal cord white matter of 7-month-old Hri-/- mice as compared with Hri+/+ littermates. Moreover, aged Hri-/- mice showed accumulation of misfolded α-synuclein in the lateral collateral pathway, a region of the sacral spinal cord horn that receives visceral sensory afferents from the bladder and distal colon, a pathological feature common to α-synucleinopathies in humans. Together, these results suggest that HRI contributes to a general cytosolic unfolded protein response that could be leveraged to bolster the clearance of cytotoxic protein aggregates.

Aged hind-limb clasping experimental autoimmune encephalomyelitis models aspects of the neurodegenerative process seen in multiple sclerosis.

Experimental autoimmune encephalomyelitis (EAE) is the most common model of multiple sclerosis (MS). This model has been instrumental in understanding the events that lead to the initiation of central nervous system (CNS) autoimmunity. Though EAE has been an effective screening tool for identifying novel therapies for relapsing-remitting MS, it has proven to be less successful in identifying therapies for progressive forms of this disease. Though axon injury occurs in EAE, it is rapid and acute, making it difficult to intervene for the purpose of evaluating neuroprotective therapies. Here, we describe a variant of spontaneous EAE in the 2D2 T cell receptor transgenic mouse (2D2+ mouse) that presents with hind-limb clasping upon tail suspension and is associated with T cell-mediated inflammation in the posterior spinal cord and spinal nerve roots. Due to the mild nature of clinical signs in this model, we were able to maintain cohorts of mice into middle age. Over 9 mo, these mice exhibited a relapsing-remitting course of hind-limb clasping with the development of progressive motor deficits. Using a combined approach of ex vivo magnetic resonance (MR) imaging and histopathological analysis, we observed neurological progression to associate with spinal cord atrophy, synapse degradation, and neuron loss in the gray matter, as well as ongoing axon injury in the white matter of the spinal cord. These findings suggest that mild EAE coupled with natural aging may be a solution to better modeling the neurodegenerative processes seen in MS.

Complement-associated loss of CA2 inhibitory synapses in the demyelinated hippocampus impairs memory.

The complement system is implicated in synapse loss in the MS hippocampus, but the functional consequences of synapse loss remain poorly understood. Here, in post-mortem MS hippocampi with demyelination we find that deposits of the complement component C1q are enriched in the CA2 subfield, are linked to loss of inhibitory synapses and are significantly higher in MS patients with cognitive impairments compared to those with preserved cognitive functions. Using the cuprizone mouse model of demyelination, we corroborated that C1q deposits are highest within the demyelinated dorsal hippocampal CA2 pyramidal layer and co-localized with inhibitory synapses engulfed by microglia/macrophages. In agreement with the loss of inhibitory perisomatic synapses, we found that Schaffer collateral feedforward inhibition but not excitation was impaired in CA2 pyramidal neurons and accompanied by intrinsic changes and a reduced spike output. Finally, consistent with excitability deficits, we show that cuprizone-treated mice exhibit impaired encoding of social memories. Together, our findings identify CA2 as a critical circuit in demyelinated intrahippocampal lesions and memory dysfunctions in MS.

Isotype-Switched Autoantibodies Are Necessary To Facilitate Central Nervous System Autoimmune Disease in Aicda-/- and Ung-/- Mice.

B cell-depleting therapies have been shown to ameliorate symptoms in multiple sclerosis (MS) patients; however, the mechanism of action remains unclear. Following priming with Ag, B cells undergo secondary diversification of their BCR, including BCR class-switch recombination (CSR) and somatic hypermutation (SHM), with both processes requiring the enzyme activation-induced (cytidine) deaminase. We previously reported that activation-induced (cytidine) deaminase is required for full clinical manifestation of disease in an animal model of MS (experimental autoimmune encephalomyelitis; EAE) provoked by immunization with the extracellular domain of recombinant human myelin oligodendrocyte glycoprotein (hMOG). In this study, we investigated the role of CSR versus SHM in the pathogenesis of EAE. We found that passive transfer of class-switched anti-MOG IgG1 Abs into hMOG-primed Aicda-/- mice is sufficient to fully rescue EAE disease. In addition, we found that the nature of the Ag is an important determinant of EAE severity in Aicda-/- mice because the lack of a diversified BCR does not affect the induction of EAE when immunized with the extracellular domain of rat MOG. To discriminate the effect of either CSR or SHM, we induced EAE in uracil DNA glycosylase-deficient mice (Ung-/-) that exhibit a defect primarily in CSR. We observed that Ung-/- mice exhibit milder clinical disease compared with control mice, concomitant with a reduced amount of anti-MOG IgG1 class-switched Abs that preserved normal affinity. Collectively, these results indicate that CSR plays an important role in governing the incidence and severity of EAE induced with hMOG but not rat MOG.

Progressive multiple sclerosis patients show substantial lesion activity that correlates with clinical disease severity and sex: a retrospective autopsy cohort analysis.

Multiple sclerosis (MS) is a highly heterogeneous disease with large inter-individual differences in disease course. MS lesion pathology shows considerable heterogeneity in localization, cellular content and degree of demyelination between patients. In this study, we investigated pathological correlates of disease course in MS using the autopsy cohort of the Netherlands Brain Bank (NBB), containing 182 MS brain donors. Using a standardized autopsy procedure including systematic dissection from standard locations, 3188 tissue blocks containing 7562 MS lesions were dissected. Unbiased measurements of lesion load were made using the tissue from standard locations. Lesion demyelinating and innate inflammatory activity were visualized by immunohistochemistry for proteolipid protein and human leukocyte antigen. Lesions were classified into active, mixed active/inactive (also known as chronic active), inactive or remyelinated, while microglia/macrophage morphology was classified as ramified, amoeboid or foamy. The severity score was calculated from the time from first symptoms to EDSS-6. Lesion type prevalence and microglia/macrophage morphology were analyzed in relation to clinical course, disease severity, lesion load and sex, and in relation to each other. This analysis shows for the first time that (1) in progressive MS, with a mean disease duration of 28.6 ± 13.3 years (mean ± SD), there is substantial inflammatory lesion activity at time to death. 57% of all lesions were either active or mixed active/inactive and 78% of all patients had a mixed active/inactive lesion present; (2) patients that had a more severe disease course show a higher proportion of mixed active/inactive lesions (p = 6e-06) and a higher lesion load (p = 2e-04) at the time of death, (3) patients with a progressive disease course show a higher lesion load (p = 0.001), and a lower proportion of remyelinated lesions (p = 0.03) compared to patients with a relapsing disease course, (4) males have a higher incidence of cortical grey matter lesions (p = 0.027) and a higher proportion of mixed active/inactive lesions compared to females across the whole cohort (p = 0.007). We confirm that there is a higher proportion of mixed active/inactive lesions (p = 0.006) in progressive MS compared to relapsing disease. Identification of mixed active/inactive lesions on MRI is necessary to determine whether they can be used as a prognostic tool in living MS patients.

An anticomplement agent that homes to the damaged brain and promotes recovery after traumatic brain injury in mice.

Activation of complement is a key determinant of neuropathology and disability after traumatic brain injury (TBI), and inhibition is neuroprotective. However, systemic complement is essential to fight infections, a critical complication of TBI. We describe a targeted complement inhibitor, comprising complement receptor of the Ig superfamily (CRIg) fused with complement regulator CD59a, designed to inhibit membrane attack complex (MAC) assembly at sites of C3b/iC3b deposition. CRIg and CD59a were linked via the IgG2a hinge, yielding CD59-2a-CRIg dimer with increased iC3b/C3b binding avidity and MAC inhibitory activity. CD59-2a-CRIg inhibited MAC formation and prevented complement-mediated lysis in vitro. CD59-2a-CRIg dimer bound C3b-coated surfaces with submicromolar affinity (KD). In experimental TBI, CD59-2a-CRIg administered posttrauma homed to sites of injury and significantly reduced MAC deposition, microglial accumulation, mitochondrial stress, and axonal damage and enhanced neurologic recovery compared with placebo controls. CD59-2a-CRIg inhibited MAC-induced inflammasome activation and IL-1ß production in microglia. Given the important anti-infection roles of complement opsonization, site-targeted inhibition of MAC should be considered to promote recovery postneurotrauma.

Complement C3 on microglial clusters in multiple sclerosis occur in chronic but not acute disease: Implication for disease pathogenesis.

Microglial clusters with C3d deposits are observed in the periplaque of multiple sclerosis (MS) brains and were proposed as early stage of lesion formation. As such they should appear in the brain of MS donors with acute disease but thus far this has not been shown. Using postmortem brain tissue from acute (n = 10) and chronic (n = 15) MS cases we investigated whether C3d+ microglial clusters are part of an acute attack against myelinated axons, which could have implications for disease pathogenesis. The specificity of our findings to MS was tested in ischemic stroke cases (n = 8) with initial or advanced lesions and further analyzed in experimental traumatic brain injury (TBI, n = 26), as both conditions are primarily nondemyelinating but share essential features of neurodegeneration with MS lesions. C3d+ microglial clusters were found in chronic but not acute MS. They were not associated with antibody deposits or terminal complement activation. They were linked to slowly expanding lesions, localized on axons with impaired transport and associated with neuronal C3 production. C3d+ microglial clusters were not specific to MS as they were also found in stroke and experimental TBI. We conclude that C3d+ microglial clusters in MS are not part of an acute attack against myelinated axons. As such it is unlikely that they drive formation of new lesions but could represent a physiological mechanism to remove irreversibly damaged axons in chronic disease. GLIA 2017;65:264-277.

Complement activation at the motor end-plates in amyotrophic lateral sclerosis.

BACKGROUND: Amyotrophic lateral sclerosis (ALS) is a fatal progressive neurodegenerative disease with no available therapy. Components of the innate immune system are activated in the spinal cord and central nervous system of ALS patients. Studies in the SOD1(G93A) mouse show deposition of C1q and C3/C3b at the motor end-plate before neurological symptoms are apparent, suggesting that complement activation precedes neurodegeneration in this model. To obtain a better understanding of the role of complement at the motor end-plates in human ALS pathology, we analyzed post-mortem tissue of ALS donors for complement activation and its regulators. METHODS: Post-mortem intercostal muscle biopsies were collected at autopsy from ALS (n = 11) and control (n = 6) donors. The samples were analyzed for C1q, membrane attack complex (MAC), CD55, and CD59 on the motor end-plates, using immunofluorescence or immunohistochemistry. RESULTS: Here, we show that complement activation products and regulators are deposited on the motor end-plates of ALS patients. C1q co-localized with neurofilament in the intercostal muscle of ALS donors and was absent in controls (P = 0.001). In addition, C1q was found deposited on the motor end-plates in the intercostal muscle. MAC was also found deposited on motor end-plates that were innervated by nerves in the intercostal muscle of ALS donors but not in controls (P = 0.001). High levels of the regulators CD55 and CD59 were detected at the motor end-plates of ALS donors but not in controls, suggesting an attempt to counteract complement activation and prevent MAC deposition on the end-plates before they are lost. CONCLUSIONS: This study provides evidence that complement activation products are deposited on innervated motor end-plates in the intercostal muscle of ALS donors, indicating that complement activation may precede end-plate denervation in human ALS. This study adds to the understanding of ALS pathology in man and identifies complement as a potential modifier of the disease process.

Complement is activated in progressive multiple sclerosis cortical grey matter lesions.

BACKGROUND: The symptoms of multiple sclerosis (MS) are caused by damage to myelin and nerve cells in the brain and spinal cord. Inflammation is tightly linked with neurodegeneration, and it is the accumulation of neurodegeneration that underlies increasing neurological disability in progressive MS. Determining pathological mechanisms at play in MS grey matter is therefore a key to our understanding of disease progression. METHODS: We analysed complement expression and activation by immunocytochemistry and in situ hybridisation in frozen or formalin-fixed paraffin-embedded post-mortem tissue blocks from 22 progressive MS cases and made comparisons to inflammatory central nervous system disease and non-neurological disease controls. RESULTS: Expression of the transcript for C1qA was noted in neurons and the activation fragment and opsonin C3b-labelled neurons and glia in the MS cortical and deep grey matter. The density of immunostained cells positive for the classical complement pathway protein C1q and the alternative complement pathway activation fragment Bb was significantly increased in cortical grey matter lesions in comparison to control grey matter. The number of cells immunostained for the membrane attack complex was elevated in cortical lesions, indicating complement activation to completion. The numbers of classical (C1-inhibitor) and alternative (factor H) pathway regulator-positive cells were unchanged between MS and controls, whilst complement anaphylatoxin receptor-bearing microglia in the MS cortex were found closely apposed to cortical neurons. Complement immunopositive neurons displayed an altered nuclear morphology, indicative of cell stress/damage, supporting our finding of significant neurodegeneration in cortical grey matter lesions. CONCLUSIONS: Complement is activated in the MS cortical grey matter lesions in areas of elevated numbers of complement receptor-positive microglia and suggests that complement over-activation may contribute to the worsening pathology that underlies the irreversible progression of MS.

Complement C1q-C3-associated synaptic changes in multiple sclerosis hippocampus.

OBJECTIVE: Multiple sclerosis (MS) is a demyelinating disease of the central nervous system, leading to memory impairment in up to 65% of patients. Memory dysfunction in MS has been associated with loss of synapses in the hippocampus, but its molecular basis is unknown. Accumulating evidence suggests that components of the complement system, C1q and C3, can mediate elimination of synapses. METHODS: To investigate the involvement of complement in synaptic changes in MS, gene and protein expression and localization of C1q and C3 were analyzed in relation to neuropathological changes in myelinated and demyelinated hippocampi from postmortem MS brains. Findings were compared to hippocampi of Alzheimer disease (AD) and non-neurological controls. RESULTS: C1q expression and C3 activation were increased in myelinated and demyelinated MS hippocampi, mainly in the CA3/2 and CA1 subfields, which also showed a marked decrease in synaptic density and increased neuronal staining for the mitochondrial heat shock protein 70 (mtHSP70) stress marker. Neurons were the major source of C1q mRNA. C1q protein and activated C3 localized at synapses within human leukocyte antigen-positive cell processes and lysosomes, suggesting engulfment of complement-tagged synapses by microglia. A significant association (p < 0.0001) between the density of C1q and synaptophysin-positive synapses or mtHSP70 was seen in myelinated MS hippocampi, further pointing toward a link between the complement pathway and synaptic changes. In contrast to AD, MS hippocampi were consistently negative for the terminal complement activation complex C5b9. INTERPRETATION: These data support a role for the C1q-C3 complement axis in synaptic alterations in the MS hippocampus.

Inhibition of the membrane attack complex of the complement system reduces secondary neuroaxonal loss and promotes neurologic recovery after traumatic brain injury in mice.

Traumatic brain injury (TBI) is the leading cause of disability and death in young adults. The secondary neuroinflammation and neuronal damage that follows the primary mechanical injury is an important cause of disability in affected people. The membrane attack complex (MAC) of the complement system is detected in the traumatized brain early after TBI; however, its role in the pathology and neurologic outcome of TBI has not yet been investigated. We generated a C6 antisense oligonucleotide that blocks MAC formation by inhibiting C6, and we compared its therapeutic effect to that of Ornithodoros moubata complement inhibitor (OmCI), a known inhibitor of C5 activation that blocks generation of the anaphylatoxin C5a and C5b, an essential component of MAC. Severe closed head injury in mice induced abundant MAC deposition in the brain. Treatment with C6 antisense reduced C6 synthesis (85%) and serum levels (90%), and inhibited MAC deposition in the injured brain (91-96%). Treatment also reduced accumulation of microglia/macrophages (50-88%), neuronal apoptosis, axonal loss and weight loss (54-93%), and enhanced neurologic performance (84-92%) compared with placebo-treated controls after injury. These data provide the first evidence, to our knowledge, that inhibition of MAC formation in otherwise complement-sufficient animals reduces neuropathology and promotes neurologic recovery after TBI. Given the importance of maintaining a functional complement opsonization system to fight infections, a critical complication in TBI patients, inhibition of the MAC should be considered to reduce posttraumatic neurologic damage. This work identifies a novel therapeutic target for TBI and will guide the development of new therapy for patients.

M. leprae components induce nerve damage by complement activation: identification of lipoarabinomannan as the dominant complement activator.

Peripheral nerve damage is the hallmark of leprosy pathology but its etiology is unclear. We previously identified the membrane attack complex (MAC) of the complement system as a key determinant of post-traumatic nerve damage and demonstrated that its inhibition is neuroprotective. Here, we determined the contribution of the MAC to nerve damage caused by Mycobacterium leprae and its components in mouse. Furthermore, we studied the association between MAC and the key M. leprae component lipoarabinomannan (LAM) in nerve biopsies of leprosy patients. Intraneural injections of M. leprae sonicate induced MAC deposition and pathological changes in the mouse nerve, whereas MAC inhibition preserved myelin and axons. Complement activation occurred mainly via the lectin pathway and the principal activator was LAM. In leprosy nerves, the extent of LAM and MAC immunoreactivity was robust and significantly higher in multibacillary compared to paucibacillary donors (p = 0.01 and p = 0.001, respectively), with a highly significant association between LAM and MAC in the diseased samples (r = 0.9601, p = 0.0001). Further, MAC co-localized with LAM on axons, pointing to a role for this M. leprae antigen in complement activation and nerve damage in leprosy. Our findings demonstrate that MAC contributes to nerve damage in a model of M. leprae-induced nerve injury and its inhibition is neuroprotective. In addition, our data identified LAM as the key pathogen associated molecule that activates complement and causes nerve damage. Taken together our data imply an important role of complement in nerve damage in leprosy and may inform the development of novel therapeutics for patients.

C3-dependent mechanism of microglial priming relevant to multiple sclerosis.

Microglial priming predisposes the brain to neurodegeneration and affects disease progression. The signal to switch from the quiescent to the primed state is unknown. We show that deleting the C3 convertase regulator complement receptor 1-related protein y (Crry) induces microglial priming. Mice that were double-knockout for Crry and either C3 or factor B did not show priming, demonstrating dependence on alternative pathway activation. Colocalization of C3b/iC3b and CR3 implicated the CR3/iC3b interaction in priming. Systemic lipopolysaccharide challenge overactivated primed microglia with florid expression of proinflammatory molecules, which were blocked by complement inhibition. Relevance for neurodegenerative disease is exemplified by human multiple sclerosis (MS) and by experimental autoimmune encephalomyelitis (EAE), a model of MS. In human MS, microglial priming was evident in perilesional white matter, in close proximity to C3b/iC3b deposits. EAE was accelerated and exacerbated in Crry-deficient mice, and was dependent on C activation. In summary, C3-dependent microglial priming confers susceptibility to other challenges. Our observations are relevant to progression in MS and other neurological diseases exacerbated by acute insults.