Bystander mechanism for complement-initiated early oligodendrocyte injury in neuromyelitis optica

Aquaporin-4-IgG-seropositive neuromyelitis optica spectrum disorders: progress of experimental models based on disease pathogenesis

Neuromyelitis optica spectrum disorders are neuroinflammatory demyelinating disorders that lead to permanent visual loss and motor dysfunction. To date, no effective treatment exists as the exact causative mechanism remains unknown. Therefore, experimental models of neuromyelitis optica spectrum disorders are essential for exploring its pathogenesis and in screening for therapeutic targets. Since most patients with neuromyelitis optica spectrum disorders are seropositive for IgG autoantibodies against aquaporin-4, which is highly expressed on the membrane of astrocyte endfeet, most current experimental models are based on aquaporin-4-IgG that initially targets astrocytes. These experimental models have successfully simulated many pathological features of neuromyelitis optica spectrum disorders, such as aquaporin-4 loss, astrocytopathy, granulocyte and macrophage infiltration, complement activation, demyelination, and neuronal loss; however, they do not fully capture the pathological process of human neuromyelitis optica spectrum disorders. In this review, we summarize the currently known pathogenic mechanisms and the development of associated experimental models in vitro, ex vivo, and in vivo for neuromyelitis optica spectrum disorders, suggest potential pathogenic mechanisms for further investigation, and provide guidance on experimental model choices. In addition, this review summarizes the latest information on pathologies and therapies for neuromyelitis optica spectrum disorders based on experimental models of aquaporin-4-IgG-seropositive neuromyelitis optica spectrum disorders, offering further therapeutic targets and a theoretical basis for clinical trials.

Interferon-γ controls aquaporin 4-specific Th17 and B cells in neuromyelitis optica spectrum disorder

Neuromyelitis optica spectrum disorder (NMOSD) is a CNS autoimmune inflammatory disease mediated by T helper 17 (Th17) and antibody responses to the water channel protein, aquaporin 4 (AQP4), and associated with astrocytopathy, demyelination and axonal loss. Knowledge about disease pathogenesis is limited and the search for new therapies impeded by the absence of a reliable animal model. In our work, we determined that NMOSD is characterized by decreased IFN-γ receptor signalling and that IFN-γ depletion in AQP4201-220-immunized C57BL/6 mice results in severe clinical disease resembling human NMOSD. Pathologically, the disease causes autoimmune astrocytic and CNS injury secondary to cellular and humoral inflammation. Immunologically, the absence of IFN-γ allows for increased expression of IL-6 in B cells and activation of Th17 cells, and generation of a robust autoimmune inflammatory response. Consistent with NMOSD, the experimental disease is exacerbated by administration of IFN-β, whereas repletion of IFN-γ, as well as therapeutic targeting of IL-17A, IL-6R and B cells, ameliorates it. We also demonstrate that immune tolerization with AQP4201-220-coupled poly(lactic-co-glycolic acid) nanoparticles could both prevent and effectively treat the disease. Our findings enhance the understanding of NMOSD pathogenesis and provide a platform for the development of immune tolerance-based therapies, avoiding the limitations of the current immunosuppressive therapies.

Serum LDL Promotes Microglial Activation and Exacerbates Demyelinating Injury in Neuromyelitis Optica Spectrum Disorder

Neuromyelitis optica spectrum disorder (NMOSD) is an autoimmune inflammatory demyelinating disease of the central nervous system (CNS) accompanied by blood-brain barrier (BBB) disruption. Dysfunction in microglial lipid metabolism is believed to be closely associated with the neuropathology of NMOSD. However, there is limited evidence on the functional relevance of circulating lipids in CNS demyelination, cellular metabolism, and microglial function. Here, we found that serum low-density lipoprotein (LDL) was positively correlated with markers of neurological damage in NMOSD patients. In addition, we demonstrated in a mouse model of NMOSD that LDL penetrates the CNS through the leaky BBB, directly activating microglia. This activation leads to excessive phagocytosis of myelin debris, inhibition of lipid metabolism, and increased glycolysis, ultimately exacerbating myelin damage. We also found that therapeutic interventions aimed at reducing circulating LDL effectively reversed the lipid metabolic dysfunction in microglia and mitigated the demyelinating injury in NMOSD. These findings shed light on the molecular and cellular mechanisms underlying the positive correlation between serum LDL and neurological damage, highlighting the potential therapeutic target for lowering circulating lipids to alleviate the acute demyelinating injury in NMOSD.

A comprehensive review of the advances in neuromyelitis optica spectrum disorder

Neuromyelitis optica spectrum disorder (NMOSD) is a rare relapsing neuroinflammatory autoimmune astrocytopathy, with a predilection for the optic nerves and spinal cord. Most cases are characterised by aquaporin-4-antibody positivity and have a relapsing disease course, which is associated with accrual of disability. Although the prognosis in NMOSD has improved markedly over the past few years owing to advances in diagnosis and therapeutics, it remains a severe disease. In this article, we review the evolution of our understanding of NMOSD, its pathogenesis, clinical features, disease course, treatment options and associated symptoms. We also address the gaps in knowledge and areas for future research focus.

Neuromyelitis Optica spectrum disorders in Argentina: A hospital-based study

BACKGROUND

Neuromyelitis Optica spectrum disorder (NMOSD) is an antibody-mediated autoimmune disease of the CNS, which especially affects the optic nerves and spinal cord. There is little known in Latin America (LATAM) about NMOSD, and few reports have been published in the literature so far. We aimed to describe an NMOSD study in a single center from Argentina.

METHODS

A retrospective cross sectional study was carried out in a single reference center in the city of Buenos Aires, Argentina. Data were collected from January 2000 through December 2021 using medical records from patients attending Ramos Mejia Hospital in Buenos Aires, Argentina. Here we describe the clinical, laboratory, MRI, disability course, and treatment of 92 NMOSD patients.

RESULTS

Mean age at the onset of symptoms was 31 years (range 2-68) with a female/male ratio of 4.8:1. 71.7 % had an early onset before the age of 50 years old, 8.7 % had a late onset of the disease and 19.6 % had an onset at pediatric age. The first symptom of NMOSD was optic neuritis in 47.8 % of the patients, followed by transverse myelitis, 33.7 % and area postrema syndrome, 5.4 %. 96.7 % of patients relapsed at least once during the follow-up period. The mean of the expanded disability status scale (EDSS) was 4.0 (range 2-8). 34,8 % had one or more associated autoimmune diseases. 78,6 % had a positive result for AQP4-IgG. The ratio of male to female was 1:8.4 vs.1:1.2 in the seropositive group vs. the seronegative. CSF results showed OCB type 2 in 6.3 %. The brain MRI did not show brain lesions in 71,7 % of the patients. 17 % presented spinal cord lesions with less than 3 vertebral segments. All patients received treatment with immunosuppressive drugs. Rituximab and azathioprine were the most used.

CONCLUSIONS

This is the largest hospital-based study in an Argentina cross-sectional study of patients with NMOSD. Recurrent disease, early age at onset, female prevalence in AQP4-IgG+ patients, and the difficulty to assess new treatments, are the highlight features in our study of patients. Further Argentinian and LATAM studies will provide more information.

Pathogenic myelin-specific antibodies in multiple sclerosis target conformational proteolipid protein 1-anchored membrane domains

B cell clonal expansion and cerebrospinal fluid (CSF) oligoclonal IgG bands are established features of the immune response in multiple sclerosis (MS). Clone-specific recombinant monoclonal IgG1 Abs (rAbs) derived from MS patient CSF plasmablasts bound to conformational proteolipid protein 1 (PLP1) membrane complexes and, when injected into mouse brain with human complement, recapitulated histologic features of MS pathology: oligodendrocyte cell loss, complement deposition, and CD68+ phagocyte infiltration. Conformational PLP1 membrane epitopes were complex and governed by the local cholesterol and glycolipid microenvironment. Abs against conformational PLP1 membrane complexes targeted multiple surface epitopes, were enriched within the CSF compartment, and were detected in most MS patients, but not in inflammatory and noninflammatory neurologic controls. CSF PLP1 complex Abs provide a pathogenic autoantibody biomarker specific for MS.

T follicular helper cells contribute to pathophysiology in a model of neuromyelitis optica spectrum disorders

Neuromyelitis optica spectrum disorders (NMOSD) are inflammatory autoimmune disorders of the CNS. IgG autoantibodies targeting the aquaporin-4 water channel (AQP4-IgGs) are the pathogenic effector of NMOSD. Dysregulated T follicular helper (Tfh) cells have been implicated in loss of B cell tolerance in autoimmune diseases. The contribution of Tfh cells to disease activity and therapeutic potential of targeting these cells in NMOSD remain unclear. Here, we established an autoimmune model of NMOSD by immunizing mice against AQP4 via in vivo electroporation. After AQP4 immunization, mice displayed AQP4 autoantibodies in blood circulation, blood-brain barrier disruption, and IgG infiltration in spinal cord parenchyma. Moreover, AQP4 immunization induced motor impairments and NMOSD-like pathologies, including astrocytopathy, demyelination, axonal loss, and microglia activation. These were associated with increased splenic Tfh, Th1, and Th17 cells; memory B cells; and plasma cells. Aqp4-deficient mice did not display motor impairments and NMOSD-like pathologies after AQP4 immunization. Importantly, abrogating ICOS/ICOS-L signaling using anti-ICOS-L antibody depleted Tfh cells and suppressed the response of Th1 and Th17 cells, memory B cells, and plasma cells in AQP4-immunized mice. These findings were associated with ameliorated motor impairments and spinal cord pathologies. This study suggests a role of Tfh cells in the pathophysiology of NMOSD in a mouse model with AQP4 autoimmunity and provides an animal model for investigating the immunological mechanisms underlying AQP4 autoimmunity and developing therapeutic interventions targeting autoimmune reactions in NMOSD.

ACT001 Relieves NMOSD Symptoms by Reducing Astrocyte Damage with an Autoimmune Antibody

Neuromyelitis optica spectrum disorder (NMOSD) is a central nervous system inflammatory demyelinating disease, the pathogenesis of which involves autoantibodies targeting the extracellular epitopes of aquaporin-4 on astrocytes. We neutralized the AQP4-IgG from NMOSD patient sera using synthesized AQP4 extracellular epitope peptides and found that the severe cytotoxicity produced by aquaporin-4 immunoglobin (AQP4-IgG) could be blocked by AQP4 extracellular mimotope peptides of Loop A and Loop C in astrocyte protection and animal models. ACT001, a natural compound derivative, has shown anti-tumor activity in various cancers. In our study, the central nervous system anti-inflammatory effect of ACT001 was investigated. The results demonstrated the superior astrocyte protection activity of ACT001 at 10 µM. Furthermore, ACT001 decreases the behavioral score in the mouse NMOSD model, which was not inferior to Methylprednisolone Sodium Succinate, the first-line therapy of NMOSD in clinical practice. In summary, our study showed that astrocytes are protected by specific peptides, or small molecular drugs, which is a new strategy for the treatment of NMOSD. It is possible for ACT001 to be a promising therapy for NMOSD.

Remyelination in neuromyelitis optica spectrum disorder is promoted by edaravone through mTORC1 signaling activation

Neuromyelitis optica spectrum disorder (NMOSD) is a severe inflammatory autoimmune disease of the central nervous system that is manifested as secondary myelin loss. Oligodendrocyte progenitor cells (OPCs) are the principal source of myelinating oligodendrocytes (OLs) and are abundant in demyelinated regions of NMOSD patients, thus possibly representing a cellular target for pharmacological intervention. To explore the therapeutic compounds that enhance myelination due to endogenous OPCs, we screened the candidate drugs in mouse neural progenitor cell (NPC)-derived OPCs. We identified drug edaravone, which is approved by the Food and Drug Administration (FDA), as a promoter of OPC differentiation into mature OLs. Edaravone enhanced remyelination in organotypic slice cultures and in mice, even when edaravone was administered following NMO-IgG-induced demyelination, and ameliorated motor impairment in a systemic mouse model of NMOSD. The results of mechanistic studies in NMO-IgG-treated mice and the biopsy samples of the brain tissues of NMOSD patients indicated that the mTORC1 signaling pathway was significantly inhibited, and edaravone promoted OPC maturation and remyelination by activating mTORC1 signaling. Furthermore, pharmacological activation of mTORC1 signaling significantly enhanced myelin regeneration in NMOSD. Thus, edaravone is a potential therapeutic agent that promotes lesion repair in NMOSD patients by enhancing OPC maturation.

More Efficient Complement Activation by Anti–Aquaporin-4 Compared With Anti–Myelin Oligodendrocyte Glycoprotein Antibodies

Background and Objectives

The objective was to study complement-mediated cytotoxicity induced by immunoglobulin G (IgG) anti–aquaporin-4 antibodies (AQP4-IgG) and anti–myelin oligodendrocyte glycoprotein antibodies (MOG-IgG) in human serum samples from patients suffering from the rare demyelinating diseases of the CNS neuromyelitis optica spectrum disorder (NMOSD) and MOG-IgG–associated disease (MOGAD).

Methods

A cell-based assay with HEK293A cells expressing different MOG isoforms (MOGα1-3β1-3) or AQP4-M23 was used. Cells were incubated with human MOG-IgG or AQP4-IgG–positive serum samples together with active or heat-inactivated human complement, and complement-dependent cytotoxicity (CDC) was measured with a lactate dehydrogenase assay. To further quantify antibody-mediated cell damage, formation of the terminal complement complex (TCC) was analyzed by flow cytometry. In addition, immunocytochemistry of the TCC and complement component 3 (C3) was performed.

Results

AQP4-IgG–positive serum samples induced higher CDC and TCC levels than MOG-IgG–positive sera. Notably, both showed a correlation between antibody titers and CDC and also between titers and TCC levels. In addition, all 6 MOG isoforms tested (MOGα1-3β1-3) could induce at least some CDC; however, the strongest MOG-IgG–induced CDC levels were found on MOGα1, MOGα3, and MOGβ1. Different MOG-IgG binding patterns regarding recognition of different MOG isoforms were investigated, and it was found that MOG-IgG recognizing all 6 isoforms again induced highest CDC levels on MOGα1and MOGβ1. Furthermore, surface staining of TCC and C3 revealed positive staining on all 6 MOG isoforms tested, as well as on AQP4-M23.

Discussion

Both MOG-IgG and AQP4-IgG are able to induce CDC in a titer-dependent manner. However, AQP4-IgG showed markedly higher levels of CDC compared with MOG in vitro on target cells. This further highlights the role of complement in AQP4-IgG–mediated disease and diminishes the importance of complement activation in MOG-IgG–mediated autoimmune disease.

A novel association of osmotic demyelination in Sjögren's syndrome prompts revisiting role of aquaporins in CNS demyelinating diseases: A literature review

BACKGROUND

Primary Sjögren's syndrome (SS) is a chronic systemic autoimmune disease with varied neurological manifestations. SS is associated with anti-aquaporin-4 antibody (AQP4-IgG)-positive neuromyelitis optica spectrum disorder (NMOSD), a demyelinating autoimmune disorder of the central nervous system (CNS). Intriguingly, there are reports of osmotic demyelinating syndrome (ODS), a supposedly non-inflammatory disorder, in the context of SS and renal tubular acidosis (RTA), both of which are not yet established risk factors for ODS.

METHODS

A literature search was undertaken to identify case reports of ODS in patients with SS. Details of the clinical and laboratory features of these patients were compiled. Additionally, we searched for NMOSD in patients with SS. We looked for co-existing RTA in patients with SS-ODS as well as SS-NMOSD. We also screened for reports of ODS in RTA without underlying SS.

RESULTS & DISCUSSION

We identified 15 patients (all women, median age 40 years) with ODS in SS, and all of these patients had comorbid RTA. There were only three reported cases of ODS in RTA without underlying SS. We identified a total of 67 patients with SS-NMOSD, of whom only 3 (4.5%) had RTA. Hence, unlike NMOSD, the development of ODS in SS requires a prolonged osmotic or electrolyte abnormality caused by the comorbid RTA. The 15 patients with ODS and SS -RTA, showed heterogeneous clinical manifestations and outcomes. The most common symptom was quadriparesis, seen in 14 of the 15 patients. Eleven of the 15 patients had one of the following features, either alone or in combination: worsening of the sensorium, extensor plantar response, dysphagia/dysarthria, and facial palsy. The latter four manifestations were present at the onset in 7 patients and later in the course of the illness in the remaining 4 patients. Ocular palsy was seen in only four of the 15 patients and was a late manifestation. One patient who had extensive long-segment myelitis and subsequent ODS died, but most patients recovered without significant sequelae. None had hyponatremia, while all patients had hypokalemia and/or hypernatremia. Hypokalemia causing nephrogenic diabetes insipidus (NDI) followed by rapid rise in sodium and the resultant osmotic stress could potentially explain the occurrence of ODS in SS-RTA. Aquaporin (AQP) in astrocytes is implicated in ODS, and renal AQP is downregulated in NDI. Antibodies against AQPs are present in some patients with SS. Defective AQP is therefore a common link underlying all the connected diseases, namely SS, NDI, and ODS, raising the possibility of immune-mediated AQP dysfunction in the pathogenesis.

CONCLUSION

The hitherto unreported association between SS-RTA and ODS may implicate SS and/or RTA in the development of ODS. In the setting of SS-RTA, ODS must be suspected when a patient with flaccid quadriparesis does not respond to the correction of potassium or develops additional neurological features along with a rise in sodium. Defective functions of AQPs may be a possible mechanism linking demyelinating CNS lesions, SS, and RTA. Studies evaluating AQP functions and serum antibodies against AQPs in these conditions are warranted.

Neutrophils activated by membrane attack complexes increase the permeability of melanoma blood vessels

Cancer cell dissemination is the seed for metastasis and adversely linked to patients’ benefit. Critical for hematogenous dissemination is the entrance of the cancer cell into the circulation, which is regulated by vascular permeability within the primary tumor. Here, we describe pathophysiological communication between endothelial cells, tumor infiltrating neutrophils, and the complement system, with implications for vascular barrier opening and melanoma cell dissemination. Experiments in complement-deficient animals indicate that interference with complement-mediated activation of neutrophils stabilizes blood vessel integrity and abolishes the systemic spread of melanoma cells.

The microenvironment of malignant melanomas defines the properties of tumor blood vessels and regulates infiltration and vascular dissemination of immune and cancer cells, respectively. Previous research in other cancer entities suggested the complement system as an essential part of the tumor microenvironment. Here, we confirm activation of the complement system in samples of melanoma patients and murine melanomas. We identified the tumor endothelium as the starting point of the complement cascade. Generation of complement-derived C5a promoted the recruitment of neutrophils. Upon contact with the vascular endothelium, neutrophils were further activated by complement membrane attack complexes (MACs). MAC-activated neutrophils release neutrophil extracellular traps (NETs). Close to the blood vessel wall, NETs opened the endothelial barrier as indicated by an enhanced vascular leakage. This facilitated the entrance of melanoma cells into the circulation and their systemic spread. Depletion of neutrophils or lack of MAC formation in complement component 6 (C6)–deficient animals protected the vascular endothelium and prevented vascular intravasation of melanoma cells. Our data suggest that inhibition of MAC-mediated neutrophil activation is a potent strategy to abolish hematogenous dissemination in melanoma.

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.

Sera of Neuromyelitis Optica Patients Increase BID-Mediated Apoptosis in Astrocytes

Neuromyelitis optica (NMO) is a rare disease usually presenting with bilateral or unilateral optic neuritis with simultaneous or sequential transverse myelitis. Autoantibodies directed against aquaporin-4 (AQP4-IgG) are found in most patients. They are believed to cross the blood−brain barrier, target astrocytes, activate complement, and eventually lead to astrocyte destruction, demyelination, and axonal damage. However, it is still not clear what the primary pathological event is. We hypothesize that the interaction of AQP4-IgG and astrocytes leads to DNA damage and apoptosis. We studied the effect of sera from seropositive NMO patients and healthy controls (HCs) on astrocytes’ immune gene expression and viability. We found that sera from seropositive NMO patients led to higher expression of apoptosis-related genes, including BH3-interacting domain death agonist (BID), which is the most significant differentiating gene (p < 0.0001), and triggered more apoptosis in astrocytes compared to sera from HCs. Furthermore, NMO sera increased DNA damage and led to a higher expression of immunological genes that interact with BID (TLR4 and NOD-1). Our findings suggest that sera of seropositive NMO patients might cause astrocytic DNA damage and apoptosis. It may be one of the mechanisms implicated in the primary pathological event in NMO and provide new avenues for therapeutic intervention.

A new form of axonal pathology in a spinal model of neuromyelitis optica

Neuromyelitis optica is a chronic neuroinflammatory disease, which primarily targets astrocytes and often results in severe axon injury of unknown mechanism. Neuromyelitis optica patients harbour autoantibodies against the astrocytic water channel protein, aquaporin-4 (AQP4-IgG), which induce complement-mediated astrocyte lysis and subsequent axon damage. Using spinal in vivo imaging in a mouse model of such astrocytopathic lesions, we explored the mechanism underlying neuromyelitis optica-related axon injury. Many axons showed a swift and morphologically distinct 'pearls-on-string' transformation also readily detectable in human neuromyelitis optica lesions, which especially affected small calibre axons independently of myelination. Functional imaging revealed that calcium homeostasis was initially preserved in this 'acute axonal beading' state, ruling out disruption of the axonal membrane, which sets this form of axon injury apart from previously described forms of traumatic and inflammatory axon damage. Morphological, pharmacological and genetic analyses showed that AQP4-IgG-induced axon injury involved osmotic stress and ionic overload, but does not appear to use canonical pathways of Wallerian-like degeneration. Subcellular analysis demonstrated remodelling of the axonal cytoskeleton in beaded axons, especially local loss of microtubules. Treatment with the microtubule stabilizer epothilone, a putative therapy approach for traumatic and degenerative axonopathies, prevented axonal beading, while destabilizing microtubules sensitized axons for beading. Our results reveal a distinct form of immune-mediated axon pathology in neuromyelitis optica that mechanistically differs from known cascades of post-traumatic and inflammatory axon loss, and suggest a new strategy for neuroprotection in neuromyelitis optica and related diseases.

Oligodendrocyte death and myelin loss in the cuprizone model: an updated overview of the intrinsic and extrinsic causes of cuprizone demyelination

The dietary consumption of cuprizone – a copper chelator – has long been known to induce demyelination of specific brain structures and is widely used as model of multiple sclerosis. Despite the extensive use of cuprizone, the mechanism by which it induces demyelination are still unknown. With this review we provide an updated understanding of this model, by showcasing two distinct yet overlapping modes of action for cuprizone-induced demyelination; 1) damage originating from within the oligodendrocyte, caused by mitochondrial dysfunction or reduced myelin protein synthesis. We term this mode of action ‘intrinsic cell damage’. And 2) damage to the oligodendrocyte exerted by inflammatory molecules, brain resident cells, such as oligodendrocytes, astrocytes, and microglia or peripheral immune cells – neutrophils or T-cells. We term this mode of action ‘extrinsic cellular damage’. Lastly, we summarize recent developments in research on different forms of cell death induced by cuprizone, which could add valuable insights into the mechanisms of cuprizone toxicity. With this review we hope to provide a modern understanding of cuprizone-induced demyelination to understand the causes behind the demyelination in MS.

Aquaporin-4 in Neuromyelitis Optica Spectrum Disorders: A Target of Autoimmunity in the Central Nervous System

Since the discovery of a specific autoantibody in patients with neuromyelitis optica spectrum disorder (NMOSD) in 2004, the water channel aquaporin-4 (AQP4) has attracted attention as a target of autoimmune diseases of the central nervous system. In NMOSD, the autoantibody (NMO-IgG) binds to the extracellular loops of AQP4 as expressed in perivascular astrocytic end-feet and disrupts astrocytes in a complement-dependent manner. NMO-IgG is an excellent marker for distinguishing the disease from other inflammatory demyelinating diseases, such as multiple sclerosis. The unique higher-order structure of AQP4—called orthogonal arrays of particles (OAPs)—as well as its subcellular localization may play a crucial role in the pathogenesis of the disease. Recent studies have also demonstrated complement-independent cytotoxic effects of NMO-IgG. Antibody-induced endocytosis of AQP4 has been suggested to be involved in this mechanism. This review focuses on the binding properties of antibodies that recognize the extracellular region of AQP4 and the characteristics of AQP4 that are implicated in the pathogenesis of NMOSD.

Mesenchymal Stem Cell-Derived Extracellular Vesicles and Their Therapeutic Use in Central Nervous System Demyelinating Disorders

Autoimmune demyelinating diseases-including multiple sclerosis, neuromyelitis optica spectrum disorder, anti-myelin oligodendrocyte glycoprotein-associated disease, acute disseminated encephalomyelitis, and glial fibrillary acidic protein (GFAP)-associated meningoencephalomyelitis-are a heterogeneous group of diseases even though their common pathology is characterized by neuroinflammation, loss of myelin, and reactive astrogliosis. The lack of safe pharmacological therapies has purported the notion that cell-based treatments could be introduced to cure these patients. Among stem cells, mesenchymal stem cells (MSCs), obtained from various sources, are considered to be the ones with more interesting features in the context of demyelinating disorders, given that their secretome is fully equipped with an array of anti-inflammatory and neuroprotective molecules, such as mRNAs, miRNAs, lipids, and proteins with multiple functions. In this review, we discuss the potential of cell-free therapeutics utilizing MSC secretome-derived extracellular vesicles-and in particular exosomes-in the treatment of autoimmune demyelinating diseases, and provide an outlook for studies of their future applications.

Cytoprotective IgG antibodies in sera from a subset of patients with AQP4-IgG seropositive neuromyelitis optica spectrum disorder

Neuromyelitis optica spectrum disorder (NMOSD) is an autoimmune inflammatory disease of the central nervous system. Most NMOSD patients are seropositive for immunoglobulin G (IgG) autoantibodies against astrocyte water channel aquaporin-4 (AQP4), called AQP4-IgG. AQP4-IgG binding to aquaporin-4 causes complement-dependent cytotoxicity (CDC), leading to inflammation and demyelination. Here, CDC was measured in AQP4-expressing cells exposed to human complement and heat-inactivated sera from 108 AQP4-IgG seropositive NMOSD subjects and 25 non-NMOSD controls. AQP4-IgG positive sera produced a wide range of CDC, with 50% maximum cytotoxicity produced by as low as 0.2% serum concentration. Unexpectedly, 58 samples produced no cytotoxicity, and of those, four sera were cytoprotective against cytotoxic AQP4-IgG. Cytoprotection was found against different cytotoxic monoclonal AQP4-IgGs and NMOSD patient sera, and in primary astrocyte cultures. Mechanistic studies revealed that the protective factor is an IgG antibody that did not inhibit complement directly, but interfered with binding of cytotoxic AQP4-IgG to AQP4 and consequent C1q binding and complement activation. Further studies suggested that non-pathogenic AQP4-IgG, perhaps with altered glycosylation, may contribute to reduced or ineffectual binding of cytotoxic AQP4-IgG, as well as reduced cell-surface AQP4. The presence of natural cytoprotective antibodies in AQP4-IgG seropositive sera reveals an added level of complexity in NMOSD disease pathogenesis, and suggests the potential therapeutic utility of ‘convalescent’ serum or engineered protective antibody to interfere with pathogenic antibody in AQP4-IgG seropositive NMOSD.

Neuromyelitis optica spectrum disorders: from pathophysiology to therapeutic strategies

Neuromyelitis optica (NMO) is a chronic inflammatory autoimmune disease of the central nervous system (CNS) characterized by acute optic neuritis (ON) and transverse myelitis (TM). NMO is caused by a pathogenic serum IgG antibody against the water channel aquoporin 4 (AQP4) in the majority of patients. AQP4-antibody (AQP4-ab) presence is highly specific, and differentiates NMO from multiple sclerosis. It binds to AQP4 channels on astrocytes, triggering activation of the classical complement cascade, causing granulocyte, eosinophil, and lymphocyte infiltration, culminating in injury first to astrocyte, then oligodendrocytes followed by demyelination and neuronal loss. NMO spectrum disorder (NMOSD) has recently been defined and stratified based on AQP4-ab serology status. Most NMOSD patients experience severe relapses leading to permanent neurologic disability, making suppression of relapse frequency and severity, the primary objective in disease management. The most common treatments used for relapses are steroids and plasma exchange.Currently, long-term NMOSD relapse prevention includes off-label use of immunosuppressants, particularly rituximab. In the last 2 years however, three pivotal clinical trials have expanded the spectrum of drugs available for NMOSD patients. Phase III studies have shown significant relapse reduction compared to placebo in AQP4-ab-positive patients treated with satralizumab, an interleukin-6 receptor (IL-6R) inhibitor, inebilizumab, an antibody against CD19⁺ B cells; and eculizumab, an antibody blocking the C5 component of complement. In light of the new evidence on NMOSD pathophysiology and of preliminary results from ongoing trials with new drugs, we present this descriptive review, highlighting promising treatment modalities as well as auspicious preclinical and clinical studies.

Pathogenesis of autoimmune demyelination: from multiple sclerosis to neuromyelitis optica spectrum disorders and myelin oligodendrocyte glycoprotein antibody-associated disease

Autoimmunity plays a significant role in the pathogenesis of demyelination. Multiple sclerosis (MS), neuromyelitis optica spectrum disorders (NMOSD) and myelin oligodendrocyte glycoprotein antibody‐associated disease (MOGAD) are now recognised as separate disease entities under the amalgam of human central nervous system demyelinating disorders. While these disorders share inherent similarities, investigations into their distinct clinical presentations and lesion pathologies have aided in differential diagnoses and understanding of disease pathogenesis. An interplay of various genetic and environmental factors contributes to each disease, many of which implicate an autoimmune response. The pivotal role of the adaptive immune system has been highlighted by the diagnostic autoantibodies in NMOSD and MOGAD, and the presence of autoreactive lymphocytes in MS lesions. While a number of autoantigens have been proposed in MS, recent emphasis on the contribution of B cells has shed new light on the well‐established understanding of T cell involvement in pathogenesis. This review aims to synthesise the clinical characteristics and pathological findings, discuss existing and emerging hypotheses regarding the aetiology of demyelination and evaluate recent pathogenicity studies involving T cells, B cells, and autoantibodies and their implications in human demyelination.

Targeting the complement system in neuromyelitis optica spectrum disorder

INTRODUCTION

Neuromyelitis optica spectrum disorder (NMOSD) is characterized by central nervous system inflammation and demyelination. In AQP4-IgG seropositive NMOSD, circulating immunoglobulin G (IgG) autoantibodies against astrocyte water channel aquaporin-4 (AQP4) cause tissue injury. Compelling evidence supports a pathogenic role for complement activation following AQP4-IgG binding to AQP4. Clinical studies supported the approval of eculizumab, an inhibitor of C5 cleavage, in AQP4-IgG seropositive NMOSD.

AREAS COVERED

This review covers in vitro, animal models, and human evidence for complement-dependent and complement-independent tissue injury in AQP4-IgG seropositive NMOSD. Complement targets are discussed, including complement proteins, regulators and anaphylatoxin receptors, and corresponding drug candidates.

EXPERT OPINION

Though preclinical data support a central pathogenic role of complement activation in AQP4-IgG seropositive NMOSD, they do not resolve the relative contributions of complement-dependent vs. complement-independent disease mechanisms such as antibody-dependent cellular cytotoxicity, T cell effector mechanisms, and direct AQP4-IgG-induced cellular injury. The best evidence that complement-dependent mechanisms predominate in AQP4-IgG seropositive NMOSD comes from eculizumab clinical data. Various drug candidates targeting distinct complement effector mechanisms may offer improved safety and efficacy. However, notwithstanding the demonstrated efficacy of complement inhibition in AQP4-IgG seropositive NMOSD, the ultimate niche for complement inhibition is not clear given multiple drug options with alternative mechanisms of action.Abbreviations: AAV2, Adeno-associated virus 2; ADCC, antibody-dependent cellular cytotoxicity; ANCA, antineutrophilic cytoplasmic autoantibody; AQP4, aquaporin-4; AQP4-IgG, AQP4-immunoglobulin G; C1-INH, C1-esterase inhibitor; C3aR, C3a receptor; C4BP, C4 binding protein; C5aR, C5a receptor; CDC, complement-dependent cytotoxicity; CFHR1, complement factor H related 1; CNS, central nervous system; EAE, experimental autoimmune encephalomyelitis; EndoS, endoglycosidase S; FHL-1, factor-H-like protein 1; GFAP, glial fibrillary acidic protein; Iba-1, ionized calcium-binding adaptor protein-1; IgG, immunoglobulin G; IVIG, intravenous human immunoglobulin G; MAC, membrane attack complex; MBL, maltose-binding lectin; MBP, myelin basic protein; MOG, myelin oligodendrocyte glycoprotein; NK cell, natural killer cell; NMOSD, neuromyelitis optica spectrum disorder; OAP, orthogonal arrays of particles; PNH, paroxysmal nocturnal hemoglobinuria.

Effect of Low Complement C4 on Clinical Characteristics of Patients with First-Episode Neuromyelitis Optica Spectrum Disorder

PURPOSE

To describe and compare the clinical features of patients with first-episode neuromyelitis optica spectrum disorder (NMOSD) in a normal complement C4 group and a low complement C4 group, and explore the mechanism by which low complement C4 affects the clinical features of patients with NMOSD.

PATIENTS AND METHODS

We retrospectively analyzed clinical data of 169 aquaporin-4 (AQP4) antibody positive patients with NMOSD from the First Affiliated Hospital of Zhengzhou University from December 2013 to March 2021. Prior to treatment, the blood was drawn for detection, and the patients underwent a 3.0 Tesla MRI examination. A low complement C4 level was defined as a serum complement C4 level <0.14 g/L. Depending on whether the complement C4 level was reduced, it was divided into the normal complement C4 group and low complement C4 group. The basic demographics, clinical manifestations, laboratory examinations, and imaging findings of the two groups were compared.

RESULTS

Among the 169 AQP4 antibody positive patients, 54 were low-complement C4 patients and 115 were normal. There were no significant differences in the demographics, clinical manifestations, treatment options, or admission Expanded Disability Status Scale (EDSS) score between two groups (P > 0.05). The median of discharged EDSS was the same (4 vs 4), but the difference between the two was statistically significant (P = 0.019). Compared with the normal complement C4 group, the blood uric acid level (225 vs 179; P = 0.001) and the C3 level (1.06 vs 0.87, P = 0.000) of the low complement C4 group were significantly lower. The incidence of brainstem lesions in patients with low complement C4 was higher (53.7% vs 33%, P = 0.01).

CONCLUSION

The treatment effect of the first-episode AQP4 antibody positive NMOSD low complement C4 group was poor, the blood-brain barrier was more severely damaged, and the disease changes were likely to involve the brainstem.

Eculizumab: A Review in Neuromyelitis Optica Spectrum Disorder

The terminal complement protein (C5) inhibitor eculizumab (Soliris^®) is the first agent to be specifically approved in the EU, USA, Canada and Japan for the treatment of neuromyelitis optica spectrum disorder (NMOSD) in adults who are aquaporin-4 water channel autoantibody (AQP4-IgG) seropositive and (in the EU only) for those with a relapsing course of disease. In the phase III PREVENT trial, eculizumab significantly reduced the risk of adjudicated relapse relative to placebo in patients with AQP4-IgG-seropositive NMOSD, approximately a quarter of whom did not receive concomitant immunosuppressive therapies. The beneficial effect of eculizumab was seen across all patient subgroups analysed and was accompanied by improvements in neurological and functional disability assessments, as well as generic health-related quality of life measures; it was sustained through 4 years of treatment, according to combined data from the PREVENT trial and an interim analysis of its ongoing open-label extension study. The safety profile of eculizumab in AQP4-IgG-seropositive NMOSD was consistent with that seen for the drug in other approved indications. Thus, eculizumab provides an effective, generally well tolerated and approved treatment option for this rare, disabling and potentially life-threatening condition.

Neuromyelitis optica

Neuromyelitis optica (NMO; also known as Devic syndrome) is a clinical syndrome characterized by attacks of acute optic neuritis and transverse myelitis. In most patients, NMO is caused by pathogenetic serum IgG autoantibodies to aquaporin 4 (AQP4), the most abundant water-channel protein in the central nervous system. In a subset of patients negative for AQP4-IgG, pathogenetic serum IgG antibodies to myelin oligodendrocyte glycoprotein, an antigen in the outer myelin sheath of central nervous system neurons, are present. Other causes of NMO (such as paraneoplastic disorders and neurosarcoidosis) are rare. NMO was previously associated with a poor prognosis; however, treatment with steroids and plasma exchange for acute attacks and with immunosuppressants (in particular, B cell-depleting agents) for attack prevention has greatly improved the long-term outcomes. Recently, a number of randomized controlled trials have been completed and the first drugs, all therapeutic monoclonal antibodies, have been approved for the treatment of AQP4-IgG-positive NMO and its formes frustes.

Complement in neurological disorders and emerging complement-targeted therapeutics

The complement system consists of a network of plasma and membrane proteins that modulate tissue homeostasis and contribute to immune surveillance by interacting with the innate and adaptive immune systems. Dysregulation, impairment or inadvertent activation of complement components contribute to the pathogenesis of some autoimmune neurological disorders and could even contribute to neurodegenerative diseases. In this Review, we summarize current knowledge about the main functions of the complement pathways and the involvement of complement in neurological disorders. We describe the complex network of complement proteins that target muscle, the neuromuscular junction, peripheral nerves, the spinal cord or the brain and discuss the autoimmune mechanisms of complement-mediated myopathies, myasthenia, peripheral neuropathies, neuromyelitis and other CNS disorders. We also consider the emerging role of complement in some neurodegenerative diseases, such as Alzheimer disease, amyotrophic lateral sclerosis and even schizophrenia. Finally, we provide an overview of the latest complement-targeted immunotherapies including monoclonal antibodies, fusion proteins and peptidomimetics that have been approved, that are undergoing phase I–III clinical trials or that show promise for the treatment of neurological conditions that respond poorly to existing immunotherapies.

In this Review, Dalakas et al. discuss the complement system, the role it plays in autoimmune neurological disease and neurodegenerative disease, and provide an overview of the latest therapeutics that target complement and that can be used for or have potential in neurological disorders.

Complement has an important physiological role in host immune defences and tissue remodelling.

The physiological role of complement extends to the regulation of synaptic development.

Complement has a key pathophysiological role in autoimmune neurological diseases and mediates the actions of pathogenic autoantibodies, such as acetylcholine receptor antibodies and aquaporin 4 antibodies.

For some autoimmune neurological diseases, such as myasthenia gravis and neuromyelitis optica spectrum disorders, approved complement-targeted treatments are now available.

Complement also seems to be of pathogenic relevance in neurodegenerative diseases such as Alzheimer disease, in which innate immune-driven inflammation is receiving increasing attention.

The field of complement-targeted therapeutics is rapidly expanding, with several FDA-approved agents and others currently in phase II and phase III clinical trials.

NMOSD acute attack: Understanding, treatment and innovative treatment prospect

Neuromyelitis optica spectrum disorder (NMOSD) is a group of severe inflammatory demyelinating disorders of the central nervous system that involves the optic nerve and spinal cord. Currently the therapeutic options for an acute attack in NMOSD are limited and rarely characterized in clinical studies. This review discussed the overall characteristics of acute attack of NMOSD, related risk factor, prognosis and management. Considering the huge unmet needs and the emergence of new therapeutic targets, we also reviewed innovative treatments that might alleviate attack damage, along with the challenges to evaluate new drug for acute attack in NMOSD.

Interleukin-6 in neuromyelitis optica spectrum disorder pathophysiology

Neuromyelitis optica spectrum disorder (NMOSD) is a rare autoimmune disorder that preferentially affects the spinal cord and optic nerve. Most patients with NMOSD experience severe relapses that lead to permanent neurologic disability; therefore, limiting frequency and severity of these attacks is the primary goal of disease management. Currently, patients are treated with immunosuppressants. Interleukin-6 (IL-6) is a pleiotropic cytokine that is significantly elevated in the serum and the CSF of patients with NMOSD. IL-6 may have multiple roles in NMOSD pathophysiology by promoting plasmablast survival, stimulating the production of antibodies against aquaporin-4, disrupting blood-brain barrier integrity and functionality, and enhancing proinflammatory T-lymphocyte differentiation and activation. Case series have shown decreased relapse rates following IL-6 receptor (IL-6R) blockade in patients with NMOSD, and 2 recent phase 3 randomized controlled trials confirmed that IL-6R inhibition reduces the risk of relapses in NMOSD. As such, inhibition of IL-6 activity represents a promising emerging therapy for the management of NMOSD manifestations. In this review, we summarize the role of IL-6 in the context of NMOSD.

Emerging therapeutic targets for neuromyelitis optica spectrum disorder

Introduction: Neuromyelitis optica spectrum disorder (NMOSD) is an inflammatory demyelinating disease of the central nervous system affecting primarily the spinal cord and optic nerves. Most NMOSD patients are seropositive for immunoglobulin G autoantibodies against astrocyte water channel aquaporin-4, called AQP4-IgG, which cause astrocyte injury leading to demyelination and neurological impairment. Current therapy for AQP4-IgG seropositive NMOSD includes immunosuppression, B cell depletion, and plasma exchange. Newer therapies target complement, CD19 and IL-6 receptors.Areas covered: This review covers early-stage pre-clinical therapeutic approaches for seropositive NMOSD. Targets include pathogenic AQP4-IgG autoantibodies and their binding to AQP4, complement-dependent and cell-mediated cytotoxicity, blood-brain barrier, remyelination and immune effector and regulatory cells, with treatment modalities including small molecules, biologics, and cells.Expert opinion: Though newer NMOSD therapies appear to have increased efficacy in reducing relapse rate and neurological deficit, increasingly targeted therapies could benefit NMOSD patients with ongoing relapses and could potentially be superior in efficacy and safety. Of the various early-stage therapeutic approaches, IgG inactivating enzymes, aquaporumab blocking antibodies, drugs targeting early components of the classical complement system, complement regulator-targeted drugs, and Fc-based multimers are of interest. Curative strategies, perhaps involving AQP4 tolerization, remain intriguing future possibilities.

Noninvasive, Targeted Creation of Neuromyelitis Optica Pathology in AQP4-IgG Seropositive Rats by Pulsed Focused Ultrasound

Neuromyelitis optica spectrum disorders (herein called NMO) is an autoimmune disease of the CNS characterized by astrocyte injury, inflammation, and demyelination. In seropositive NMO, immunoglobulin G autoantibodies against aquaporin-4 (AQP4-IgG) cause primary astrocyte injury. A passive transfer model of NMO was developed in which spatially targeted access of AQP4-IgG into the CNS of seropositive rats was accomplished by pulsed focused ultrasound through intact skin. Following intravenous administration of microbubbles, pulsed ultrasound at 0.5 MPa peak acoustic pressure was applied using a 1 MHz transducer with 6-cm focal length. In brain, the transient opening of the blood-brain barrier (BBB) in an approximately prolate ellipsoidal volume of diameter ∼3.5 mm and length ∼44 mm allowed entry of IgG-size molecules for up to 3-6 hours. The ultrasound treatment did not cause erythrocyte extravasation or inflammation. Ultrasound treatment in AQP4-IgG seropositive rats produced localized NMO pathology in brain, with characteristic astrocyte injury, inflammation, and demyelination after 5 days. Pathology was not seen when complement was inhibited, when non-NMO human IgG was administered instead of AQP4-IgG, or in AQP4-IgG seropositive AQP4 knockout rats. NMO pathology was similarly created in cervical spinal cord in seropositive rats. These results establish a noninvasive, spatially targeted model of NMO in rats, and demonstrate that BBB permeabilization, without underlying injury or inflammation, is sufficient to create NMO pathology in AQP4-IgG seropositive rats.

Affinity-matured 'aquaporumab' anti-aquaporin-4 antibody for therapy of seropositive neuromyelitis optica spectrum disorders

Pathogenesis in seropositive neuromyelitis optica spectrum disorders (herein called NMO) involves binding of IgG1 autoantibodies to aquaporin-4 (AQP4) on astrocytes in the central nervous system, which initiates complement and cellular injury. We previously developed an antibody blocking approach for potential therapy of NMO in which an engineered, monoclonal, anti-AQP4 antibody lacking cytotoxicity effector functions (called aquaporumab) blocked binding of NMO autoantibodies to astrocyte AQP4 (Tradtrantip et al. Ann. Neurol. 71, 314-322, 2012). Here, a high-affinity aquaporumab, which was generated by affinity maturation using saturation mutagenesis, was shown to block cellular injury caused by NMO patient sera. Anti-AQP4 antibody rAb-53, a fully human antibody with effector function neutralizing Fc mutations L234A/L235A and affinity-enhancing Fab mutations Y50R/S56R, called AQmab^AM, bound to AQP4 in cell cultures with K_d ~ 18 ng/ml (~0.12 nM), ~8-fold greater affinity than the original antibody. AQmab^AM, but without L234A/L235A Fc mutations, produced complement-dependent cytotoxicity (CDC) with EC₅₀ ~ 82 ng/ml. AQmab^AM prevented CDC produced by sera from eight NMO patients with IC₅₀ ranging from 40 to 80 ng/ml, and similarly prevented antibody-dependent cellular cytotoxicity (ADCC). Mechanistic studies demonstrated that AQmab^AM blocked binding of serum NMO autoantibodies to AQP4. AQmab^AM offers a targeted, non-immunosuppressive approach for therapy of seropositive NMO. Autoantibody blocking may be a useful therapeutic strategy for other autoimmune diseases as well.

Eculizumab and Beyond: The Past, Present, and Future of Complement Therapeutics

Dysregulation of the complement system underlies the pathophysiology of many diseases. Renewed interest in complement occurred with the recognition that its therapeutic inhibition was possible. Terminal complement blockade with the anti-C5 monoclonal antibody eculizumab significantly changed management and clinical outcomes of patients with paroxysmal nocturnal hemoglobinuria, and served as a proof of concept for other complement-mediated diseases. Eculizumab is also approved for atypical hemolytic uremic syndrome and myasthenia gravis. Multiple new disease indications have been identified, and novel complement inhibitors are in various stages of development, with several currently in human trials. Beyond C5, these new drugs block proximal complement, pathway-specific targets, convertase activity, and anaphylatoxin function. Though monoclonal antibodies are still common, peptides, RNAi, and small molecule inhibitors provide the opportunity for different administration routes and schedules. Several challenges still exist or will soon present themselves, including mitigation of infection risk, effective monitoring strategies, and how to choose between therapeutics when more than one is available. In this review, we will describe the lessons learned from the "eculizumab era," present many of the novel therapeutics currently or soon to be in trials, and highlight some of the challenges that will require attention as the field progresses.

Experimental animal models of aquaporin-4-IgG-seropositive neuromyelitis optica spectrum disorders: progress and shortcomings

Neuromyelitis optica spectrum disorders (NMOSD) is a heterogeneous group of neuroinflammatory conditions associated with demyelination primarily in spinal cord and optic nerve, and to a lesser extent in brain. Most NMOSD patients are seropositive for IgG autoantibodies against aquaporin-4 (AQP4-IgG), the principal water channel in astrocytes. There has been interest in establishing experimental animal models of seropositive NMOSD (herein referred to as NMO) in order to elucidate NMO pathogenesis mechanisms and to evaluate drug candidates. An important outcome of early NMO animal models was evidence for a pathogenic role of AQP4-IgG. However, available animal models of NMO, based largely on passive transfer to rodents of AQP4-IgG or transfer of AQP4-sensitized T cells, often together with pro-inflammatory maneuvers, only partially recapitulate the clinical and pathological features of human NMO, and are inherently biased toward humoral or cellular immune mechanisms. This review summarizes current progress and shortcomings in experimental animal models of seropositive NMOSD, and opines on the import of advancing animal models.

Myelin in the Central Nervous System: Structure, Function, and Pathology

Oligodendrocytes generate multiple layers of myelin membrane around axons of the central nervous system to enable fast and efficient nerve conduction. Until recently, saltatory nerve conduction was considered the only purpose of myelin, but it is now clear that myelin has more functions. In fact, myelinating oligodendrocytes are embedded in a vast network of interconnected glial and neuronal cells, and increasing evidence supports an active role of oligodendrocytes within this assembly, for example, by providing metabolic support to neurons, by regulating ion and water homeostasis, and by adapting to activity-dependent neuronal signals. The molecular complexity governing these interactions requires an in-depth molecular understanding of how oligodendrocytes and axons interact and how they generate, maintain, and remodel their myelin sheaths. This review deals with the biology of myelin, the expanded relationship of myelin with its underlying axons and the neighboring cells, and its disturbances in various diseases such as multiple sclerosis, acute disseminated encephalomyelitis, and neuromyelitis optica spectrum disorders. Furthermore, we will highlight how specific interactions between astrocytes, oligodendrocytes, and microglia contribute to demyelination in hereditary white matter pathologies.

Complement-independent bystander injury in AQP4-IgG seropositive neuromyelitis optica produced by antibody-dependent cellular cytotoxicity

Cellular injury in AQP4-IgG seropositive neuromyelitis spectrum disorder (herein called NMO) involves AQP4-IgG binding to astrocytes, resulting in astrocyte injury by complement-dependent cytotoxicity (CDC) and antibody-dependent cellular cytotoxicity (ADCC) mechanisms. The rapid disease progression, severe tissue damage, and abundant leukocyte infiltration seen in some NMO patients suggest a more direct mechanism for demyelination and neurologic deficit than secondary injury from astrocyte loss. Here, we report evidence for an ‘ADCC bystander mechanism’ in NMO involving injury to nearby cells by leukocytes following their activation by AQP4-bound AQP4-IgG on astrocytes. In model cocultures containing AQP4-expressing and null CHO cells, AQP4-IgG and complement killed bystander null cells to ~ 100 μm away from AQP4-expressing cells; AQP4-IgG and NK cells produced bystander killing to ~ 300 μm, with perforin deposition seen on injured null cells. Bystander cytotoxicity was also seen with neutrophil-mediated ADCC and in astrocyte-neuron cocultures. Mechanistic studies, including real-time imaging, suggested that leukocytes activated by an AQP4-dependent ADCC mechanism injure bystander cells by direct targeted exocytosis on neighboring cells and not by diffusion of soluble granule contents. In support of this conclusion, ADCC bystander injury was preferentially reduced by an RGDS peptide that inhibits integrin adhesion. Evidence for ADCC bystander injury to oligodendrocytes and neurons was also found in mice following intracerebral injection of AQP4-IgG and NK cells, which was inhibited by RGDS peptide. These results establish a novel cellular pathogenesis mechanism in AQP4-IgG seropositive NMO and provide evidence that inflammatory mechanisms can cause widespread tissue damage in NMO independently of the secondary effects from astrocyte loss.

Imaging the execution phase of neuroinflammatory disease models

In vivo imaging of the rodent spinal cord has advanced our understanding of how resident cells of the central nervous system (CNS) respond to neuroinflammation. By combining two-photon imaging and experimental autoimmune encephalomyelitis (EAE), the most widely used rodent model of multiple sclerosis (MS), it has been possible, for example, to study how axons degenerate when confronted with inflammatory cells, how oligodendrocytes get damaged in inflammatory lesions, and how immune cells themselves adapt their phenotype and functionality to the changing lesion environment. Similar approaches are now increasingly used to study other forms of neuroinflammation, such as antibody/complement-mediated neuromyelitis optica spectrum disease (NMOSD). To tackle the most pressing open questions in the field, new biosensors and indicator mice that report the metabolic state and interaction of cells in neuroinflammatory lesions are being developed. Moreover, the field is moving towards new anatomical sites of inflammation, such as the cortical gray matter, but also towards longer observation intervals to reveal the chronic perturbations and adaptations that characterize advanced stages of MS.

Myelin Oligodendrocyte Glycoprotein-IgG Contributes to Oligodendrocytopathy in the Presence of Complement, Distinct from Astrocytopathy Induced by AQP4-IgG

Immunoglobulin G against myelin oligodendrocyte glycoprotein (MOG-IgG) is detectable in neuromyelitis optica spectrum disorder (NMOSD) without aquaporin-4 IgG (AQP4-IgG), but its pathogenicity remains unclear. In this study, we explored the pathogenic mechanisms of MOG-IgG in vitro and in vivo and compared them with those of AQP4-IgG. MOG-IgG-positive serum induced complement activation and cell death in human embryonic kidney (HEK)-293T cells transfected with human MOG. In C57BL/6 mice and Sprague-Dawley rats, MOG-IgG only caused lesions in the presence of complement. Interestingly, AQP4-IgG induced astroglial damage, while MOG-IgG mainly caused myelin loss. MOG-IgG also induced astrocyte damage in mouse brains in the presence of complement. Importantly, we also observed ultrastructural changes induced by MOG-IgG and AQP4-IgG. These findings suggest that MOG-IgG directly mediates cell death by activating complement in vitro and producing NMOSD-like lesions in vivo. AQP4-IgG directly targets astrocytes, while MOG-IgG mainly damages oligodendrocytes.

Comprehensive review of neuromyelitis optica and clinical characteristics of neuromyelitis optica patients in Puerto Rico

Neuromyelitis optica (NMO) is an immune-mediated inflammatory disorder of the central nervous system. It is characterized by concurrent inflammation and demyelination of the optic nerve (optic neuritis [ON]) and the spinal cord (myelitis). Multiple studies show variations in prevalence, clinical, and demographic features of NMO among different populations. In addition, ethnicity and race are known as important factors on disease phenotype and clinical outcomes. There are little data on information about NMO patients in underserved groups, including Puerto Rico (PR). In this research, we will provide a comprehensive overview of all aspects of NMO, including epidemiology, environmental risk factors, genetic factors, molecular mechanism, symptoms, comorbidities and clinical differentiation, diagnosis, treatment, its management, and prognosis. We will also evaluate the demographic features and clinical phenotype of NMO patients in PR. This will provide a better understanding of NMO and establish a basis of knowledge that can be used to improve care. Furthermore, this type of population-based study can distinguish the clinical features variation among NMO patients and will provide insight into the potential mechanisms that cause these variations.

Complement-dependent bystander injury to neurons in AQP4-IgG seropositive neuromyelitis optica

Background

Aquaporin-4-immunoglobulin G (AQP4-IgG) seropositive neuromyelitis optica spectrum disorder (herein called NMO) is an autoimmune disease of the central nervous system in which AQP4-IgG binding to AQP4 on astrocytes results in complement-dependent astrocyte injury and secondary inflammation, demyelination, and neuron loss. We previously reported evidence for a complement bystander mechanism for early oligodendrocyte injury in NMO. Herein, we tested the hypothesis that complement bystander injury, which involves diffusion to nearby cells of activated soluble complement components from complement-injured astrocytes, is a general phenomenon that may contribute to neuronal injury in NMO.

Methods

Primary cocultures of rat astrocytes and cortical neurons were established to study complement-dependent cell death after exposure to AQP4-IgG and complement. In animal experiments, AQP4-IgG was delivered to adult rats by intracerebral injection. Cell cultures and rat brain were studied by immunofluorescence.

Results

In primary astrocyte-neuron cocultures, addition of AQP4-IgG and complement resulted in death of neurons nearby astrocytes. Deposition of complement membrane attack complex C5b-9 was seen on neurons nearby astrocytes, whereas C1q, the initiating protein in the complement pathway, was seen only on astrocytes. Neuron death was not seen with a complement inhibitor, with C1q- or C6-depleted complement, in pure neuron cultures exposed to AQP4-IgG and complement or in cocultures exposed to an astrocyte toxin. Intracerebral injection in rats of AQP4-IgG and a fixable dead cell fluorescent marker produced death of neurons near astrocytes, with C5b-9 deposition. Neuron death was not seen in rats receiving a complement inhibitor or in AQP4-IgG-injected AQP4 knockout rats.

Conclusion

These results support a novel mechanism for early neuron injury in NMO and provide evidence that complement bystander injury may be a general phenomenon for brain cell injury following AQP4-IgG-targeted astrocyte death.

Electronic supplementary material

The online version of this article (10.1186/s12974-018-1333-z) contains supplementary material, which is available to authorized users.

The complement system as a biomarker of disease activity and response to treatment in multiple sclerosis

Multiple sclerosis (MS) is an inflammatory, demyelinating, and neurodegenerative disease of the central nervous system. The complement system has an established role in the pathogenesis of MS, and evidence suggests that its components can be used as biomarkers of disease-state activity and response to treatment in MS. Plasma C4a levels have been found to be significantly elevated in patients with active relapsing-remitting MS (RRMS), as compared to both controls and patients with stable RRMS. C3 levels are also significantly elevated in the cerebrospinal fluid (CSF) of patients with RRMS, and C3 levels are correlated with clinical disability. Furthermore, increased levels of factor H can predict the transition from relapsing to progressive disease, since factor H levels have been found to increase progressively with disease progression over a 2-year period in patients transitioning from RRMS to secondary progressive (SP) MS. In addition, elevations in C3 are seen in primary progressive (PP) MS. Complement components can also differentiate RRMS from neuromyelitis optica. Response gene to complement (RGC)-32, a novel molecule induced by complement activation, is a possible biomarker of relapse and response to glatiramer acetate (GA) therapy, since RGC-32 mRNA expression is significantly decreased during relapse and increased in responders to GA treatment. The predictive accuracy of RGC-32 as a potential biomarker (by ROC analysis) is 90% for detecting relapses and 85% for detecting a response to GA treatment. Thus, complement components can serve as biomarkers of disease activity to differentiate MS subtypes and to measure response to therapy.

Complement regulator CD59 prevents peripheral organ injury in rats made seropositive for neuromyelitis optica immunoglobulin G

Pathogenesis in aquaporin-4 immunoglobulin G (AQP4-IgG) seropositive neuromyelitis optica spectrum disorders (herein called NMO) involves complement-dependent cytotoxicity initiated by AQP4-IgG binding to astrocyte AQP4. We recently reported that rats lacking complement inhibitor protein CD59 were highly susceptible to development of NMO pathology in brain and spinal cord following direct AQP4-IgG administration (Yao and Verkman, Acta Neuropath Commun 2017, 5:15). Here, we report evidence that CD59 is responsible for protection of peripheral, AQP4-expressing tissues in seropositive NMO. Rats made seropositive by intraperitoneal injection of AQP4-IgG developed marked weakness by 24 h and died soon thereafter. Serum creatine phosphokinase at 24 h was >900-fold greater in seropositive CD59^-/- rats than in seropositive CD59^+/+ (or control) rats. AQP4-expressing cells in skeletal muscle and kidney, but not in stomach, of seropositive CD59^-/- rats showed injury with deposition of AQP4-IgG and activated complement C5b-9, and inflammation. Organ injury in seropositive CD59^-/- rats was prevented by a complement inhibitor. Significant pathological changes in seropositive CD59^-/- rats were not seen in optic nerve, spinal cord or brain, including circumventricular tissue. These results implicate a major protective role of CD59 outside of the central nervous system in seropositive NMO, and hence offer an explanation as to why peripheral, AQP4-expressing cells are largely unaffected in NMO.