Intrathecal pathogenic anti-aquaporin-4 antibodies in early neuromyelitis optica

Key roles of aquaporins in tumor biology

Aquaporins are protein channels that facilitate the flow of water across plasma cell membranes in response to osmotic gradients. This review summarizes the evidence that aquaporins play key roles in tumor biology including tumor-associated edema, tumor cell migration, tumor proliferation and tumor angiogenesis. Aquaporin inhibitors may thus be a novel class of anti-tumor agents. However, attempts to produce small molecule aquaporin inhibitors have been largely unsuccessful. Recently, monoclonal human IgG antibodies against extracellular aquaporin-4 domains have become available and could be engineered to kill aquaporin-4 over-expressing cells in the malignant brain tumor glioblastoma. We conclude this review by discussing future directions in aquaporin tumor research. This article is part of a Special Issue entitled: Membrane channels and transporters in cancers.

Potential therapeutic benefit of C1-esterase inhibitor in neuromyelitis optica evaluated in vitro and in an experimental rat model

Neuromyelitis optica (NMO) is an autoimmune demyelinating disease of the central nervous system in which binding of anti-aquaporin-4 (AQP4) autoantibodies (NMO-IgG) to astrocytes causes complement-dependent cytotoxicity (CDC) and inflammation resulting in oligodendrocyte and neuronal injury. There is compelling evidence for a central role of complement in NMO pathogenesis. Here, we evaluated the potential of C1-esterase inhibitor (C1-inh) for complement-targeted therapy of NMO. C1-inh is an anti-inflammatory plasma protein with serine protease inhibition activity that has a broad range of biological activities on the contact (kallikrein), coagulation, fibrinolytic and complement systems. C1-inh is approved for therapy of hereditary angioedema (HAE) and has been studied in a small safety trial in acute NMO relapses (NCT 01759602). In vitro assays of NMO-IgG-dependent CDC showed C1-inh inhibition of human and rat complement, but with predicted minimal complement inhibition activity at a dose of 2000 units in humans. Inhibition of complement by C1-inh was potentiated by ∼10-fold by polysulfated macromolecules including heparin and dextran sulfate. In rats, intravenous C1-inh at a dose 30-fold greater than that approved to treat HAE inhibited serum complement activity by <5%, even when supplemented with heparin. Also, high-dose C1-inh did not reduce pathology in a rat model of NMO produced by intracerebral injection of NMO-IgG. Therefore, although C1r and C1s are targets of C1-inh, our in vitro data with human serum and in vivo data in rats suggest that the complement inhibition activity of C1-inh in serum is too low to confer clinical benefit in NMO.

Experimental models of neuromyelitis optica

For a long time, the most important inflammatory demyelinating diseases of the central nervous system (CNS), for example, multiple sclerosis (MS) and neuromyelitis optica (NMO), were extremely hard to differentiate, often with severe consequences for affected patients. This changed with the discovery of NMO‐immunoglobulin G (IgG), a specific autoantibody which was detected in the vast majority of NMO patients, and with the demonstration that this autoantibody targets aquaporin 4 (AQP4), a water channel found on astrocytes in the CNS. These findings paved the way for the generation of experimental models of NMO. This chapter will discuss the contribution of experimental models to NMO research and what key questions remain to be addressed.

The pathology of an autoimmune astrocytopathy: lessons learned from neuromyelitis optica

Neuromyelitis optica (NMO) is a disabling autoimmune astrocytopathy characterized by typically severe and recurrent attacks of optic neuritis and longitudinally extensive myelitis. Until recently, NMO was considered an acute aggressive variant of multiple sclerosis (MS), despite the fact that early studies postulated that NMO and MS may be two distinct diseases with a common clinical picture. With the discovery of a highly specific serum autoantibody (NMO-IgG), Lennon and colleagues provided the first unequivocal evidence distinguishing NMO from MS and other central nervous system (CNS) inflammatory demyelinating disorders. The target antigen of NMO-IgG was confirmed to be aquaporin-4 (AQP4), the most abundant water channel protein in the CNS, mainly expressed on astrocytic foot processes at the blood-brain barrier, subpial and subependymal regions. Pathological studies demonstrated that astrocytes were selectively targeted in NMO as evidenced by the extensive loss of immunoreactivities for the astrocytic proteins, AQP4 and glial fibrillary acidic protein (GFAP), as well as perivascular deposition of immunoglobulins and activation of complement even within lesions with a relative preservation of myelin. In support of these pathological findings, GFAP levels in the cerebrospinal fluid (CSF) during acute NMO exacerbations were found to be remarkably elevated in contrast to MS where CSF-GFAP levels did not substantially differ from controls. Additionally, recent experimental studies showed that AQP4 antibody is pathogenic, resulting in selective astrocyte destruction and dysfunction in vitro, ex vivo and in vivo. These findings strongly suggest that NMO is an autoimmune astrocytopathy where damage to astrocytes exceeds both myelin and neuronal damage. This chapter will review recent neuropathological studies that have provided novel insights into the pathogenic mechanisms, cellular targets, as well as the spectrum of tissue damage in NMO.

Targeting of aquaporin 4 into lipid rafts and its biological significance

Neuromyelitis optica (NMO) is an inflammatory demyelinating disease of the central nervous system and is considered to be caused by the binding of NMO-IgG to aquaporin 4 (AQP4) on astrocytes, which initiates complement-dependent cytotoxicity. AQP4 has two isoforms, i.e., M1 and M23. AQP4 is considered to form heterotetramers containing both isoforms in vivo. Most of the previous studies were performed using either one of the isoforms expressing cell lines. In this study, we generated a fluorescent epitope-tagged AQP4 M1 and M23 co-expressing astrocyte cell line and examined the subcellular targeting of AQP4. In this cell line, AQP4 was targeted mostly to membrane lipid rafts fraction evidenced by sucrose density gradient ultracentrifugation followed by Western blotting with anti-AQP4 antibody. Cholesterol depletion with methyl-β-cyclodextrin or simvastatin resulted in the dislocation (relocation) of AQP4 from lipid rafts to non-rafts fraction of the membrane and AQP4 was not internalized intracellularly. This change in the localization of AQP4 on membrane significantly reduced complement-dependent cytotoxic effects of NMO-IgG obtained from patients with NMO without affecting AQP4 orthogonal arrays. Thus, these data strongly suggest that the targeting of AQP4 in the lipid rafts is closely related to the pathogenic effects of NMO-IgG.

Treatment of neuromyelitis optica: state-of-the-art and emerging therapies

Neuromyelitis optica (NMO) is an autoimmune disease of the CNS that is characterized by inflammatory demyelinating lesions in the spinal cord and optic nerve, potentially leading to paralysis and blindness. NMO can usually be distinguished from multiple sclerosis (MS) on the basis of seropositivity for IgG antibodies against the astrocytic water channel aquaporin-4 (AQP4). Differentiation from MS is crucial, because some MS treatments can exacerbate NMO. NMO pathogenesis involves AQP4-IgG antibody binding to astrocytic AQP4, which causes complement-dependent cytotoxicity and secondary inflammation with granulocyte and macrophage infiltration, blood-brain barrier disruption and oligodendrocyte injury. Current NMO treatments include general immunosuppressive agents, B-cell depletion, and plasma exchange. Therapeutic strategies targeting complement proteins, the IL-6 receptor, neutrophils, eosinophils and CD19--all initially developed for other indications--are under clinical evaluation for repurposing for NMO. Therapies in the preclinical phase include AQP4-blocking antibodies and AQP4-IgG enzymatic inactivation. Additional, albeit currently theoretical, treatment options include reduction of AQP4 expression, disruption of AQP4 orthogonal arrays, enhancement of complement inhibitor expression, restoration of the blood-brain barrier, and induction of immune tolerance. Despite the many therapeutic options in NMO, no controlled clinical trials in patients with this condition have been conducted to date.

CD19 as a molecular target in CNS autoimmunity

Multiple sclerosis (MS) and neuromyelitis optica (NMO) are the most prevalent neuroinflammatory diseases of the central nervous system (CNS). The immunological cascade of these disorders is complex, and the exact spatial and temporal role of different immune cells is not fully understood. Although MS has been considered for many years to be primarily T cell driven, it is well established that B cells and the humoral immune response play an important role in its pathogenesis. This has long been evident from laboratory findings that include the presence of oligoclonal bands in the CSF. In NMO, the importance of the humoral immune system appears even more obvious as evidenced by pathogenic antibodies against aquaporin 4 (AQP4). Besides their capacity to mature into antibody-producing plasma cells, B cells are potent antigen-presenting cells to T lymphocytes and they can provide soluble factors for cell activation and differentiation to other immune-competent cells. In MS and NMO, there are substantial data from clinical trials that B cell depletion with CD20-directed agents is effective and relatively safe. Plasma cells, which produce antibodies against molecular targets expressed by the host, but which also provide humoral immune responses against pathogens, are not targeted by anti-CD20 therapies. Therefore, the depletion of CD19-expressing cells would offer potential advantages with regard to efficacy, but potentially higher risks with regard to infectious complications. This review will outline the rationale for CD19 as a molecular target in CNS autoimmunity. The current stage of drug development is illustrated. Potential safety concerns will be discussed.

Dextran as a generally applicable multivalent scaffold for improving immunoglobulin-binding affinities of peptide and peptidomimetic ligands

Molecules able to bind the antigen-binding sites of antibodies are of interest in medicine and immunology. Since most antibodies are bivalent, higher affinity recognition can be achieved through avidity effects in which a construct containing two or more copies of the ligand engages both arms of the immunoglobulin simultaneously. This can be achieved routinely by immobilizing antibody ligands at high density on solid surfaces, such as ELISA plates, but there is surprisingly little literature on scaffolds that routinely support bivalent binding of antibody ligands in solution, particularly for the important case of human IgG antibodies. Here we show that the simple strategy of linking two antigens with a polyethylene glycol (PEG) spacer long enough to span the two arms of an antibody results in higher affinity binding in some, but not all, cases. However, we found that the creation of multimeric constructs in which several antibody ligands are displayed on a dextran polymer reliably provides much higher affinity binding than is observed with the monomer in all cases tested. Since these dextran conjugates are simple to construct, they provide a general and convenient strategy to transform modest affinity antibody ligands into high affinity probes. An additional advantage is that the antibody ligands occupy only a small number of the reactive sites on the dextran, so that molecular cargo can be attached easily, creating molecules capable of delivering this cargo to cells displaying antigen-specific receptors.

Cerebrospinal fluid aquaporin-4 antibody levels in neuromyelitis optica attacks

To elucidate immunopathogenetic roles of aquaporin-4 antibodies in the cerebrospinal fluid (CSF) of neuromyelitis optica spectrum disorders (NMOSD), we analyzed aquaporin-4 antibody titers, cellular and inflammatory markers in the CSF collected from 11 aquaporin-4 antibody seropositive patients. The CSF aquaporin-4 antibody levels during attacks (but not in sera) closely correlated with pleocytosis, inflammatory cytokines including interleukin-6 that can regulate antibody-producing plasmablasts, and glial fibrillary acidic protein levels in the CSF. The amount of aquaporin-4 antibodies present in the central nervous system may have therapeutic implications, as it is associated with astrocyte injury and inflammatory responses during NMOSD attacks. Ann Neurol 2014;76:305–309

Inhibitor(s) of the classical complement pathway in mouse serum limit the utility of mice as experimental models of neuromyelitis optica

Neuromyelitis optica (NMO) is an inflammatory demyelinating disease of the central nervous system in which anti-aquaporin-4 (AQP4) autoantibodies (AQP4-IgG) cause damage to astrocytes by complement-dependent cytotoxicity (CDC). Various approaches have been attempted to produce NMO lesions in rodents, some involving genetically modified mice with altered immune cell function. Here, we found that mouse serum strongly inhibits complement from multiple species, preventing AQP4-IgG-dependent CDC. Effects of mouse serum on complement activation were tested in CDC assays in which AQP4-expressing cells were incubated with AQP4-IgG and complement from different species. Biochemical assays and mass spectrometry were used to characterize complement inhibitor(s) in mouse serum. Sera from different strains of mice produced almost no AQP4-IgG-dependent CDC compared with human, rat and guinea pig sera. Remarkably, addition of mouse serum prevented AQP4-IgG-dependent CDC caused by human, rat or guinea pig serum, with 50% inhibition at <5% mouse serum. Hemolysis assays indicated that the inhibitor(s) in mouse serum target the classical and not the alternative complement pathway. We found that the complement inhibitor(s) in mouse serum were contained in a serum fraction purified with protein-A resin; however, the inhibitor was not IgG as determined using serum from IgG-deficient mice. Mass spectrometry on the protein A-purified fraction produced several inhibitor candidates. The low intrinsic complement activity of mouse serum and the presence of complement inhibitor(s) limit the utility of mouse models to study disorders, such as NMO, involving the classical complement pathway.

Neuromyelitis optica: clinical features, immunopathogenesis and treatment

The term 'neuromyelitis optica' ('Devic's syndrome', NMO) refers to a syndrome characterized by optic neuritis and myelitis. In recent years, the condition has raised enormous interest among scientists and clinical neurologists, fuelled by the detection of a specific serum immunoglobulin (Ig)G reactivity (NMO-IgG) in up to 80% of patients with NMO. These autoantibodies were later shown to target aquaporin-4 (AQP4), the most abundant water channel in the central nervous system (CNS). Here we give an up-to-date overview of the clinical and paraclinical features, immunopathogenesis and treatment of NMO. We discuss the widening clinical spectrum of AQP4-related autoimmunity, the role of magnetic resonance imaging (MRI) and new diagnostic means such as optical coherence tomography in the diagnosis of NMO, the role of NMO-IgG, T cells and granulocytes in the pathophysiology of NMO, and outline prospects for new and emerging therapies for this rare, but often devastating condition.

Aquaporin-4 antibodies (NMO-IgG) as a serological marker of neuromyelitis optica: a critical review of the literature

Antibodies to aquaporin-4 (called NMO-IgG or AQP4-Ab) constitute a sensitive and highly specific serum marker of neuromyelitis optica (NMO) that can facilitate the differential diagnosis of NMO and classic multiple sclerosis. NMO-IgG/AQP4-Ab seropositive status has also important prognostic and therapeutic implications in patients with isolated longitudinally extensive myelitis (LETM) or optic neuritis (ON). In this article, we comprehensively review and critically appraise the existing literature on NMO-IgG/AQP4-Ab testing. All available immunoassays-including tissue-based (IHC), cell-based (ICC, FACS) and protein-based (RIPA, FIPA, ELISA, Western blotting) assays-and their differential advantages and disadvantages are discussed. Estimates for sensitivity, specificity, and positive and negative likelihood ratios are calculated for all published studies and accuracies of the various immunoassay techniques compared. Subgroup analyses are provided for NMO, LETM and ON, for relapsing vs. monophasic disease, and for various control groups (eg, MS vs. other controls). Numerous aspects of NMO-IgG/AQP4-Ab testing relevant for clinicians (eg, impact of antibody titers and longitudinal testing, indications for repeat testing, relevance of CSF testing and subclass analysis, NMO-IgG/AQP4-Ab in patients with rheumatic diseases) as well as technical aspects (eg, AQP4-M1 vs. AQP4-M23-based assays, intact AQP4 vs. peptide substrates, effect of storage conditions and freeze/thaw cycles) and pitfalls are discussed. Finally, recommendations for the clinical application of NMO-IgG/AQP4-Ab serology are given.

Biology of AQP4 and anti-AQP4 antibody: therapeutic implications for NMO

The water channel aquaporin-4 (AQP4) is the target of the immunoglobulin G autoantibody (AQP4-IgG) in neuromyelitis optica (NMO). AQP4 is expressed in foot processes of astrocytes throughout the central nervous system, as well as in skeletal muscle and epithelial cells in kidney, lung and gastrointestinal organs. Phenotype analysis of AQP4 knockout mice indicates the involvement of AQP4 in water movement into and out of the brain, astrocyte migration, glial scar formation and neuroexcitatory phenomena. AQP4 monomers form tetramers in membranes, which further aggregate to form supramolecular assemblies called orthogonal arrays of particles. AQP4-IgG is pathogenic in NMO by a mechanism involving complement- and cell-mediated astrocyte cytotoxicity, which produces an inflammatory response with oligodendrocyte injury and demyelination. AQP4 orthogonal arrays are crucial in NMO pathogenesis, as they increase AQP4-IgG binding to AQP4 and greatly enhance complement-dependent cytotoxicity. Novel NMO therapeutics are under development that target AQP4-IgG or AQP4, including aquaporumab monoclonal antibodies and small molecules that block AQP4-IgG binding to AQP4, and enzymatic inactivation strategies to neutralize AQP4-IgG pathogenicity.

B cells in MS and NMO: pathogenesis and therapy

B linage cells are versatile players in multiple sclerosis (MS) and neuromyelitis optica/neuromyelitis optica spectrum disorder (NMO). New potential targets of autoantibodies have been described recently. Pathogenic mechanisms extend further to antigen presentation and cytokine production, which are increasingly recognized as therapeutic targets. In addition to pro-inflammatory effects of B cells, they may act also as anti-inflammatory via production of interleukin (IL)-10, IL-35, and other mechanisms. Definition of regulatory B cell subsets is an ongoing issue. Recent studies have provided evidence for a loss of B cell self-tolerance in MS. An immunogenetic approach demonstrated exchange of B cell clones between CSF and blood. The central nervous system (CNS) of MS patients fosters B cell survival, at least partly via BAFF and APRIL. The unexpected increase of relapses in a trial with a soluble BAFF/APRIL receptor (atacicept) suggests that this system is involved in MS, but with features that are not yet understood. In this review, we further discuss evidence for B cell and Ig contribution to human MS and NMO pathogenesis, pro-inflammatory and regulatory B cell effector functions, impaired B cell immune tolerance, the B cell-fostering microenvironment in the CNS, and B cell-targeted therapeutic interventions for MS and NMO, including CD20 depletion (rituximab, ocrelizumab, and ofatumumab), anti-IL6-R (tocilizumab), complement-blocking (eculizumab), inhibitors of AQP4-Ig binding (aquaporumab, small molecular compounds), and BAFF/BAFF-R-targeting agents.

Immunopathogenesis of neuromyelitis optica

Neuromyelitis optica (NMO, Devic's syndrome) is a clinical syndrome characterized by optic neuritis and (mostly longitudinally extensive) myelitis. If untreated, NMO usually takes a relapsing course and often results in blindness and tetra- or paraparesis. The discovery of autoantibodies to aquaporin-4, the most abundant water channel in the CNS, in 70-80% of patients with NMO (termed NMO-IgG or AQP4-Ab) and subsequent investigations into the pathogenic impact of this new reactivity have led to the recognition of NMO as an autoimmune condition and as a disease entity in its own right, distinct from classic multiple sclerosis. Here, we comprehensively review the current knowledge on the role of NMO-IgG/AQP4-Ab, B cells, T cells, and the innate immune system in the pathogenesis of NMO.

Neuromyelitis optica pathology in rats following intraperitoneal injection of NMO-IgG and intracerebral needle injury

Introduction

Animal models of neuromyelitis optica (NMO) are needed for drug testing and evaluation of NMO disease pathogenesis mechanisms.

Results

We describe a novel passive-transfer model of NMO in which rats made seropositive for human anti-aquaporin-4 (AQP4) immunoglobulin G antibody (NMO-IgG) by intraperitoneal (IP) injections were subject to intracerebral needle injury. Following a single IP injection, NMO-IgG distributed rapidly to peripheral AQP4-expressing cells (kidney collecting duct, gastric glands, airways, skeletal muscle) and area postrema in brain, but not elsewhere in the central nervous system; however, no pathology was seen in brain, spinal cord, optic nerve or peripheral tissues. After testing various maneuvers to produce NMO-IgG-dependent pathology in brain, we found that transient puncture of brain parenchyma with a 28-gauge needle in NMO-IgG seropositive rats produced robust NMO pathology around the needle track, with loss of AQP4 and glial fibrillary acidic protein, granulocyte and macrophage infiltration, centrovascular deposition of activated complement, and blood–brain barrier disruption, with demyelination by 5 days. Pathology was not seen in rats receiving control (non-NMO) human IgG or in NMO-IgG-seropositive rats made complement-deficient by cobra venom factor. Interestingly, at 1 day a reversible, multifocal astrocytopathy was seen with loss of AQP4 and GFAP (but not myelin) in areas away from the needle track.

Conclusions

NMO-IgG-seropositivity alone is not sufficient to cause NMO pathology in rats, but a single intracerebral needle insertion, without pre-existing inflammation or infusion of pro-inflammatory factors, was sufficient to produce robust NMO pathology in seropositive rats.

Early loss of oligodendrocytes in human and experimental neuromyelitis optica lesions

Neuromyelitis optica (NMO) is a chronic, mostly relapsing inflammatory demyelinating disease of the CNS characterized by serum anti-aquaporin 4 (AQP4) antibodies in the majority of patients. Anti-AQP4 antibodies derived from NMO patients target and deplete astrocytes in experimental models when co-injected with complement. However, the time course and mechanisms of oligodendrocyte loss and demyelination and the fate of oligodendrocyte precursor cells (OPC) have not been examined in detail. Also, no studies regarding astrocyte repopulation of experimental NMO lesions have been reported. We utilized two rat models using either systemic transfer or focal intracerebral injection of recombinant human anti-AQP4 antibodies to generate NMO-like lesions. Time-course experiments were performed to examine oligodendroglial and astroglial damage and repair. In addition, oligodendrocyte pathology was studied in early human NMO lesions. Apart from early complement-mediated astrocyte destruction, we observed a prominent, very early loss of oligodendrocytes and oligodendrocyte precursor cells (OPCs) as well as a delayed loss of myelin. Astrocyte repopulation of focal NMO lesions was already substantial after 1 week. Olig2-positive OPCs reappeared before NogoA-positive, mature oligodendrocytes. Thus, using two experimental models that closely mimic the human disease, our study demonstrates that oligodendrocyte and OPC loss is an extremely early feature in the formation of human and experimental NMO lesions and leads to subsequent, delayed demyelination, highlighting an important difference in the pathogenesis of MS and NMO.

Unique neuromyelitis optica pathology produced in naïve rats by intracerebral administration of NMO-IgG

Animal models of neuromyelitis optica (NMO) are needed for elucidation of disease mechanisms and for testing new therapeutics. Prior animal models of NMO involved administration of human anti-aquaporin-4 immunoglobulin G antibody (NMO-IgG) to rats with pre-existing neuroinflammation, or to naïve mice supplemented with human complement. We report here the development of NMO pathology following passive transfer of NMO-IgG to naïve rats. A single intracerebral infusion of NMO-IgG to adult Lewis rats produced robust lesions around the needle track in 100 % of rats; at 5 days there was marked loss of aquaporin-4 (AQP4), glial fibrillary acidic protein (GFAP) and myelin, granulocyte and macrophage infiltration, vasculocentric complement deposition, blood-brain barrier disruption, microglial activation and neuron death. Remarkably, a distinct 'penumbra' was seen around lesions, with loss of AQP4 but not of GFAP or myelin. No lesions or penumbra were seen in rats receiving control IgG. The size of the main lesion with loss of myelin was greatly reduced in rats made complement-deficient by cobra venom factor or administered NMO-IgG lacking complement-dependent cytotoxicity (CDC) effector function. However, the penumbra was seen under these conditions, suggesting a complement-independent pathogenesis mechanism. The penumbra was absent with NMO-IgG lacking both CDC and antibody-dependent cellular cytotoxicity (ADCC) effector functions. Finally, lesion size was significantly reduced after macrophage depletion with clodronate liposomes. These results: (i) establish a robust, passive-transfer model of NMO in rats that does not require pre-existing neuroinflammation or complement administration; (ii) implicate ADCC as responsible for a unique type of pathology also seen in human NMO; and (iii) support a pathogenic role of macrophages in NMO.

Longitudinally extensive NMO spinal cord pathology produced by passive transfer of NMO-IgG in mice lacking complement inhibitor CD59

Spinal cord pathology with inflammatory, demyelinating lesions spanning three or more vertebral segments is a characteristic feature of neuromyelitis optica (NMO). NMO pathogenesis is thought to involve binding of immunoglobulin G anti-aquaporin-4 autoantibodies (NMO-IgG) to astrocytes, causing complement-dependent cytotoxicity (CDC) and secondary inflammation, demyelination and neuron loss. We investigated the involvement of CD59, a glycophosphoinositol (GPI)-anchored membrane protein on astrocytes that inhibits formation of the terminal C5b-9 membrane attack complex. CD59 inhibition by a neutralizing monoclonal antibody greatly increased NMO-IgG-dependent CDC in murine astrocyte cultures and ex vivo spinal cord slice cultures. Greatly increased NMO pathology was also found in spinal cord slice cultures from CD59 knockout mice, and in vivo following intracerebral injection of NMO-IgG and human complement. Intrathecal injection (at L5-L6) of small amounts of NMO-IgG and human complement in CD59-deficient mice produced robust, longitudinally extensive white matter lesions in lumbar spinal cord. Pathology was most severe at day 2 after injection, showing loss of AQP4 and GFAP, C5b-9 deposition, microglial activation, granulocyte infiltration, and demyelination. Hind limb motor function was remarkably impaired as well. There was partial remyelination and recovery of motor function by day 5. Our results implicate CD59 as an important modulator of the immune response in NMO, and provide a novel animal model of NMO that closely recapitulates human NMO pathology. Up-regulation of CD59 on astrocytes may have therapeutic benefit in NMO.

Neuromyelitis optica MOG-IgG causes reversible lesions in mouse brain

Introduction

Antibodies against myelin oligodendrocyte glycoprotein (MOG-IgG) are present in some neuromyelitis optica patients who lack antibodies against aquaporin-4 (AQP4-IgG). The effects of neuromyelitis optica MOG-IgG in the central nervous system have not been investigated in vivo. We microinjected MOG-IgG, obtained from patients with neuromyelitis optica, into mouse brains and compared the results with AQP4-IgG.

Results

MOG-IgG caused myelin changes and altered the expression of axonal proteins that are essential for action potential firing, but did not produce inflammation, axonal loss, neuronal or astrocyte death. These changes were independent of complement and recovered within two weeks. By contrast, AQP4-IgG produced complement-mediated myelin loss, neuronal and astrocyte death with limited recovery at two weeks.

Conclusions

These differences mirror the better outcomes for MOG-IgG compared with AQP4-IgG patients and raise the possibility that MOG-IgG contributes to pathology in some neuromyelitis optica patients.

Changes in B and T-cell subsets and NMO-IgG levels after immunoglobulins and rituximab treatment for an acute attack of neuromyelitis optica

BACKGROUND

There is increasing evidence supporting that neuromyelitis optica (NMO) is an inflammatory humoral mediated disorder associated with NMO-IgG/AQP-4 antibodies. However, little is known about the subsets of B cells and T cells that contribute to the pathogenesis or therapy response.

OBJECTIVES

To describe the clinical and immunological changes associated with intravenous immunoglobulins (IV-Igs) plus rituximab (RTX) in a patient with a severe acute attack of NMO and intrathecal synthesis of NMO-IgG/AQP-4, who previously did not respond to intravenous methylprednisolone and plasma exchange.

METHODS

We sequentially analysed the levels of NMO-IgG/AQP-4 by immunohistochemistry, and B and T cells subsets by multiparametric flow-cytometry, in the CSF and peripheral blood (PB), before and alter IV-Igs plus RTX therapy.

RESULTS

In the CSF before treatment, and compared with PB, there was a higher percentage of CD4(+) T cells and a lower percentage of CD8(+) T cells and CD19(+) B cells. After therapy, the percentage of CD4(+) T cells remained high, and that of CD8(+) T cells increased. The observed decrease in the percentage of CD19(+) B cells was lower than in the PB. When the CSF was compared, it was found that the percentage of effector-memory and effector CD8(+) T cells had increased after therapy, and that of IgM memory B cells and switched-memory B cells decreased. The observed changes paralleled the decrease of NMO-IgG/AQP-4 results to negative and the clinical improvement.

CONCLUSIONS

Our findings confirm that, besides intrathecal humoral immune response against AQP4, B and T cell subsets are involved in the modulation of inflammation within and outside the central nervous system.

Treatment of neuromyelitis optica/neuromyelitis optica spectrum disorders with methotrexate

BACKGROUND

To review our experience using methotrexate as a single long-term immunosuppressant (IS) therapy in neuromyelitis optica/neuromyelitis optica spectrum disorders (NMO/NMOSD).

METHODS

We performed a retrospective chart review of all patients with a diagnosis of NMO/NMOSD, supported by a positive NMO-IgG testing, who were treated with methotrexate. A paired sample 2 tailed t test was used to assess the annualized relapse rate during 18 months pre treatment with methotrexate to annualized relapse rate 18 months post treatment with methotrexate.

RESULTS

We followed 9 patients meeting criteria for the study for a median of 62 months. All patients were stabilized during attacks with high-dose steroids and/or plasmapheresis. Five patients (55.55%) were started on methotrexate as an initial long-term immunosuppressant strategy. Three patients (33.33%) were initially treated with pulse cyclophosphamide followed by methotrexate as a preplanned step-down strategy. One patient was started on azathioprine prior to methotrexate. No patient had side effects requiring change in methotrexate therapy. Five patients (55.55%) had stabilization of Expanded Disability Status Scale (EDSS) on methotrexate. One patient had a small increase in EDSS due to concomitant illness. Three patients (33.33%) had methotrexate treatment failure evidenced by worsening EDSS and ongoing relapses while on methotrexate, mandating a change in methotrexate therapy. Average annualized relapse rate in the entire group comparing 18 months prior versus 18 months after methotrexate treatment was reduced by an absolute value of 64% (3.11 vs 1.11). A paired t-test showed this reduction was highly significant (p = .009).

CONCLUSION

In our experience, methotrexate is safe and possibly efficacious as a single long-term IS therapy along with low dose corticosteroids that can reasonably be offered to patients with NMO/NMOSD.

Human immunoglobulin G reduces the pathogenicity of aquaporin-4 autoantibodies in neuromyelitis optica

Neuromyelitis optica (NMO) pathogenesis involves binding of anti-aquaporin-4 (AQP4) autoantibodies (NMO-IgG) present in serum to AQP4 on astrocytes, which causes complement-dependent cytotoxicity (CDC) and antibody-dependent cellular cytotoxicity (ADCC). Human immunoglobulin G (hIgG) is effective for treatment of humorally mediated neurological autoimmune diseases and has been reported to improve disease outcome in a limited number of NMO patients. Here, we investigated hIgG actions on NMO-IgG pathogenicity using an in vivo rat model of NMO and in vitro assays. In rats administered NMO-IgG by intracerebral injection, the size of neuroinflammatory demyelinating lesions was reduced by ~50% when hIgG was administered by intraperitoneal injection to reach levels of 10-25mg/mL in rat serum, comparable with human therapeutic levels. In vitro, hIgG at 10mg/mL reduced by 90% NMO-IgG-mediated CDC following addition of NMO-IgG and human complement to AQP4-expressing cells. The hIgG effect was mainly on the classical complement pathway. hIgG at 10mg/mL also reduced by up to 90% NMO-IgG-mediated ADCC as assayed with human natural killer cells as effector cells. However, hIgG at up to 40mg/mL did not affect AQP4 cell surface expression or its supramolecular assembly in orthogonal arrays of particles, nor did it affect NMO-IgG binding to AQP4. We conclude that hIgG reduces NMO-IgG pathogenicity by inhibition of CDC and ADCC, providing a mechanistic basis to support further clinical evaluation of its therapeutic efficacy in NMO.

To look for a needle in a haystack: the search for autoantibodies in multiple sclerosis

The search for autoantibodies in multiple sclerosis (MS) has been challenging for the last 3 decades. With the development of new proteomic methods and advances in expression and assay technologies, progress in the identification of MS autoantibodies has been made. A number of MS-specific autoantibodies have been proposed, most of them targeting proteins expressed in oligodendrocytes and along the myelin sheath. In this review, we summarize the status of antibody research in MS and then discuss recent developments and future strategies in defining and characterizing the potential antigenic targets of autoantibodies in MS.

Aggregation state determines the localization and function of M1- and M23-aquaporin-4 in astrocytes

An aggregation state–dependent mechanism for segregation of plasma membrane protein complexes confers specific functional roles to the M1 and M23 isoforms of the water channel AQP4.

The astrocyte water channel aquaporin-4 (AQP4) is expressed as heterotetramers of M1 and M23 isoforms in which the presence of M23–AQP4 promotes formation of large macromolecular aggregates termed orthogonal arrays. Here, we demonstrate that the AQP4 aggregation state determines its subcellular localization and cellular functions. Individually expressed M1–AQP4 was freely mobile in the plasma membrane and could diffuse into rapidly extending lamellipodial regions to support cell migration. In contrast, M23–AQP4 formed large arrays that did not diffuse rapidly enough to enter lamellipodia and instead stably bound adhesion complexes and polarized to astrocyte end-feet in vivo. Co-expressed M1– and M23–AQP4 formed aggregates of variable size that segregated due to diffusional sieving of small, mobile M1–AQP4-enriched arrays into lamellipodia and preferential interaction of large, M23–AQP4-enriched arrays with the extracellular matrix. Our results therefore demonstrate an aggregation state–dependent mechanism for segregation of plasma membrane protein complexes that confers specific functional roles to M1– and M23–AQP4.

Experimental mouse model of optic neuritis with inflammatory demyelination produced by passive transfer of neuromyelitis optica-immunoglobulin G

Background

Although optic neuritis (ON) is a defining feature of neuromyelitis optica (NMO), appropriate animal models of NMO ON are lacking. Most NMO patients are seropositive for immunoglobulin G autoantibodies (NMO-IgG) against the astrocyte water channel aquaporin-4 (AQP4).

Methods

Several approaches were tested to develop a robust, passive-transfer mouse model of NMO ON, including NMO-IgG and complement delivery by: (i) retrobulbar infusion; (ii) intravitreal injection; (iii) a single intracranial injection near the optic chiasm; and (iv) 3-days continuous intracranial infusion near the optic chiasm.

Results

Little ON or retinal pathology was seen using approaches (i) to (iii). Using approach (iv), however, optic nerves showed characteristic NMO pathology, with loss of AQP4 and glial fibrillary acidic protein immunoreactivity, granulocyte and macrophage infiltration, deposition of activated complement, demyelination and axonal injury. Even more extensive pathology was created in mice lacking complement inhibitor protein CD59, or using a genetically modified NMO-IgG with enhanced complement effector function, including significant loss of retinal ganglion cells. In control studies, optic nerve pathology was absent in treated AQP4-deficient mice, or in wild-type mice receiving control (non-NMO) IgG and complement.

Conclusion

Passive transfer of NMO-IgG and complement by continuous infusion near the optic chiasm in mice is sufficient to produce ON with characteristic NMO pathology. The mouse model of NMO ON should be useful in further studies of NMO pathogenesis mechanisms and therapeutics.

Biomarkers of treatment response in multiple sclerosis

Multiple sclerosis (MS) is considered a heterogeneous disease with respect to disease progression and treatment response, which have both remained highly unpredictable. With an increasing number of available disease modifying therapies, strategies for treatment allocation in the individual patient or subgroup of patients has become more important. Therefore biomarkers, which will identify subgroups of MS patients and predict treatment response early in the course of the disease, are urgently needed. Here we review current and emerging biomarkers, as well as study concepts for identification of new biomarkers in MS.

Multiple sclerosis: diagnosis, differential diagnosis, and clinical presentation

The diagnosis of multiple sclerosis (MS) is based on demonstrating evidence of inflammatory-demyelinating injury within the central nervous system that is disseminated in both time and space. Diagnosis is made through a combination of the clinical history, neurologic examination, magnetic resonance imaging and the exclusion of other diagnostic possibilities. Other so-called "paraclinical" tests, including the examination of the cerebrospinal fluid, the recording of evoked potentials, urodynamic studies of bladder function, and ocular coherence tomography, may be helpful in establishing the diagnosis for individual patients, but are often unnecessary. Differential diagnosis in MS must be guided by clinical presentation and neurologic localization. While the list of conditions that can mimic MS clinically or radiologically is long, in clinical practice there are few conditions that truly mimic MS on both fronts. A positive test for a putative MS "mimic" does not unto itself exclude the diagnosis of MS. Typical symptoms of MS include discrete episodes ("attacks" or "relapses") of numbness, tingling, weakness, vision loss, gait impairment, incoordination, imbalance, and bladder dysfunction. In between attacks, patients tend to be stable, but may experience fatigue and heat sensitivity. Some MS patients go on to experience, or only experience, an insidious worsening of neurologic function and accumulation of disability ("progression") that is not associated with discrete relapse activity. Progression accounts for most of the long-term disability in MS. Diagnostic criteria for MS have evolved over the past several decades, with each revision impacting the apparent prevalence and prognosis of the disorder - the result has been to encourage earlier diagnosis without compromising accuracy.

Urea transport mediated by aquaporin water channel proteins

Aquaporins (AQPs) are a family of membrane water channels that basically function as regulators of intracellular and intercellular water flow. To date, thirteen aquaporins have been characterized. They are distributed wildly in specific cell types in multiple organs and tissues. Each AQP channel consists of six membrane-spanning alpha-helices that have a central water-transporting pore. Four AQP monomers assemble to form tetramers, which are the functional units in the membrane. Some of AQPs also transport urea, glycerol, ammonia, hydrogen peroxide, and gas molecules. AQP-mediated osmotic water transport across epithelial plasma membranes facilitates transcellular fluid transport and thus water reabsorption. AQP-mediated urea and glycerol transport is involved in energy metabolism and epidermal hydration. AQP-mediated CO2 and NH3 transport across membrane maintains intracellular acid-base homeostasis. AQPs are also involved in the pathophysiology of a wide range of human diseases (including water disbalance in kidney and brain, neuroinflammatory disease, obesity, and cancer). Further work is required to determine whether aquaporins are viable therapeutic targets or reliable diagnostic and prognostic biomarkers.

T cell-activation in neuromyelitis optica lesions plays a role in their formation

Background

Neuromyelitis optica (NMO) is an inflammatory demyelinating disease of the central nervous system (CNS), which is characterized by the presence of pathogenic serum autoantibodies against aquaporin 4 (AQP4) in the vast majority of patients. The contribution of T cells to the formation of astrocyte destructive lesions is currently unclear. However, active human NMO lesions contain CD4⁺ T-lymphocytes expressing the activation marker Ox40, and the expression is more profound compared to that seen in MS lesions of comparable activity. Therefore, we analyzed the role of T-cell activation within the CNS in the initiation of NMO lesions in an experimental model of co-transfer of different encephalitogenic T-cells and human AQP4 antibody containing NMO immunoglobulin (NMO IgG). We further studied the expression of the T-cell activation marker Ox40 in NMO and multiple sclerosis lesions in different stages of activity.

Results

All encephalitogenic T-cell lines used in our experiments induced brain inflammation with a comparable extent of blood brain barrier damage, allowing human NMO IgG to penetrate into the brain and spinal cord tissue. However, astrocyte destructive NMO lesions were only seen with T-cells, which showed signs of activation in the lesions. T-cell activation was reflected by the expression of the activation marker Ox40 and pronounced production of γ-IFN, which was able to increase the production of complement proteins and of the Fc gamma III receptor (Fcgr3) and decreased production of complement inhibitory protein Factor H in microglia.

Conclusions

Our data indicate that local activation of T-cells provide an inflammatory environment in the CNS, which allows AQP4 auto-antibodies to induce astrocyte destructive NMO-like lesions.

Sera from patients with seropositive neuromyelitis optica spectral disorders caused the degeneration of rodent optic nerve

Neuromyelitis optica (NMO) is an autoimmune inflammatory, neurodestructive disease primarily targeting the optic nerve and spinal cord. An autoantibody against water channel protein aquaporin-4 (AQP4), which is expressed at endofeet of astrocytes has been implicated in the pathogenesis of NMO. We evaluated the impact of sera of seropositive patients with NMO spectrum disorders (NMOSDs) on the rodent optic nerve and retina. Serum was obtained either from patients with seropositive NMOSD (AQP4+), seronegative patient with idiopathic optic neuritis (AQP4-), and healthy volunteers (control). Anti-AQP4 antibody in a serum was measured by a previously established cell-based assay. The patients' sera were applied on the optic nerve after de-sheathed. Immunohistochemistry showed that at 7 days after the treatment, the area of the optic nerve exposed to the AQP4+ sera lost expression of both AQP4 and glial fibrillary acidic protein. Also, Human-IgG immunoreactivity and marked invasion of inflammation cells were observed in the optic nerve treated with AQP4+ serum. Immnoreactivity of neurofilament was reduced at 14 days after the treatment, not 7 days. Real-time polymerase chain reaction revealed the reduced gene expression of neurofilament in retina from the eye that was exposed to the AQP4+ sera at 14 days. Retrograde fluorogold-labeling on the retinal flatmount disclosed the significantly reduced number of retinal ganglion cells when the AQP4+ sera were applied. The present model has demonstrated that the sera from patients with seropositive NMOSDs led to the regional astrocytic degeneration and inflammatory cell invasion in the optic nerve, resulting in the ultimate loss of RGCs and their axons at areas beyond the injury site.

Plasmablasts as migratory IgG-producing cells in the pathogenesis of neuromyelitis optica

Neuromyelitis optica (NMO) is an inflammatory disease characterized by recurrent attacks of optic neuritis and myelitis. It is generally accepted that autoantibodies against aquaporin 4 water channel protein play a pathogenic role in neuromyelitis optica. We have recently reported that plasmablasts are increased in the peripheral blood of this autoimmune disease, and are capable of producing autoantibodies against aquaporin 4. Here, we demonstrate that CD138⁺HLA-DR⁺ plasmablasts, a subset of IgG-producing cells, are increased in the peripheral blood and are enriched among the cerebrospinal fluid (CSF) lymphocytes during the relapse of neuromyelitis optica. Notably, these CD138⁺HLA-DR⁺ plasmablasts overexpress CXCR3, whose ligands are present in the cerebrospinal fluid during the relapse of neuromyelitis optica. These results led us to speculate that plasmablasts producing anti-aquaporin 4 autoantibodies might traffic toward the central nervous system (CNS). Furthermore, we performed single-cell sorting of plasmablasts from peripheral blood and CSF samples from NMO and sequenced the complementarity-determining regions (CDRs) of the IgG heavy chain expressed by the sorted plasmablast clones. There were high frequencies of mutations in the CDRs compared with framework regions, indicating that these plasmablast clones would represent a post-germinal center B-cell lineage. Consistent with the preceding results, the plasmablast clones from the peripheral blood shared the same CDR sequences with the clones from the CSF. These results indicate that IgG-producing plasmablasts, which are guided by helper T-cells, may migrate from the peripheral blood preferentially to the CSF. Since migratory plasmablasts could be involved in the inflammatory pathology of NMO, the B-cell subset and their migration might be an attractive therapeutic target.

Update on the diagnosis and treatment of neuromyelitis optica: recommendations of the Neuromyelitis Optica Study Group (NEMOS)

Neuromyelitis optica (NMO, Devic’s syndrome), long considered a clinical variant of multiple sclerosis, is now regarded as a distinct disease entity. Major progress has been made in the diagnosis and treatment of NMO since aquaporin-4 antibodies (AQP4-Ab; also termed NMO-IgG) were first described in 2004. In this review, the Neuromyelitis Optica Study Group (NEMOS) summarizes recently obtained knowledge on NMO and highlights new developments in its diagnosis and treatment, based on current guidelines, the published literature and expert discussion at regular NEMOS meetings. Testing of AQP4-Ab is essential and is the most important test in the diagnostic work-up of suspected NMO, and helps to distinguish NMO from other autoimmune diseases. Furthermore, AQP4-Ab testing has expanded our knowledge of the clinical presentation of NMO spectrum disorders (NMOSD). In addition, imaging techniques, particularly magnetic resonance imaging of the brain and spinal cord, are obligatory in the diagnostic workup. It is important to note that brain lesions in NMO and NMOSD are not uncommon, do not rule out the diagnosis, and show characteristic patterns. Other imaging modalities such as optical coherence tomography are proposed as useful tools in the assessment of retinal damage. Therapy of NMO should be initiated early. Azathioprine and rituximab are suggested as first-line treatments, the latter being increasingly regarded as an established therapy with long-term efficacy and an acceptable safety profile in NMO patients. Other immunosuppressive drugs, such as methotrexate, mycophenolate mofetil and mitoxantrone, are recommended as second-line treatments. Promising new therapies are emerging in the form of anti-IL6 receptor, anti-complement or anti-AQP4-Ab biologicals.

Involvement of antibody-dependent cell-mediated cytotoxicity in inflammatory demyelination in a mouse model of neuromyelitis optica

Neuromyelitis optica (NMO) is an inflammatory demyelinating disease of the central nervous system that can cause paralysis and blindness. The pathogenesis of NMO involves binding of immunoglobulin G autoantibodies to aquaporin-4 (AQP4) on astrocytes, which is thought to cause complement-dependent cytotoxicity (CDC) and a secondary inflammatory response leading to oligodendrocyte and neuronal damage. Here, we investigate in vivo the role of antibody-dependent cell-mediated cytotoxicity (ADCC) triggered by AQP4 autoantibodies (AQP4-IgG) in the development of NMO pathology. A high-affinity, human recombinant monoclonal AQP4-IgG was mutated in its Fc region to produce 'NMO superantibodies' with enhanced CDC and/or ADCC effector functions, without altered AQP4 binding. Pathological effects of these antibodies were studied in a mouse model of NMO produced by intracerebral injection of AQP4-IgG and human complement. The original (non-mutated) antibody produced large NMO lesions in this model, with loss of AQP4 and GFAP immunoreactivity, inflammation and demyelination, as did a mutated antibody with enhanced CDC and ADCC effector functions. As anticipated, a mutated AQP4-IgG lacking CDC, but having tenfold enhanced ADCC, produced little pathology. However, unexpectedly, a mutated antibody with ninefold enhanced CDC, but lacking ADCC, produced much less pathology than the original AQP4-IgG. Also, pathology was greatly reduced following administration of AQP4-IgG and complement to mice lacking the FcγIII receptor involved in effector cell activation during ADCC, and to normal mice injected with an Fcγ receptor blocking antibody. Our results provide evidence for the central involvement of ADCC in NMO pathology and suggest ADCC as a new therapeutic target in NMO.

Astrocytes differentially respond to inflammatory autoimmune insults and imbalances of neural activity

BACKGROUND

Neuronal activity intimately communicates with blood flow through the blood-brain barrier (BBB) in the central nervous system (CNS). Astrocyte endfeet cover more than 90% of brain capillaries and interact with synapses and nodes of Ranvier. The roles of astrocytes in neurovascular coupling in the CNS remain poorly understood.

RESULTS

Here we show that astrocytes that are intrinsically different are activated by inflammatory autoimmune insults and alterations of neuronal activity. In the progression of experimental autoimmune encephalomyelitis (EAE), both fibrous and protoplasmic astrocytes were broadly and reversibly activated in the brain and spinal cord, indicated by marked upregulation of glial fibrillary acidic protein (GFAP) and other astrocytic proteins. In early and remitting EAE, upregulated GFAP and astrocytic endfoot water channel aquaporin 4 (AQP4) enclosed white matter lesions in spinal cord, whereas they markedly increased and formed bundles in exacerbated lesions in late EAE. In cerebellar cortex, upregulation of astrocytic proteins correlated with EAE severity. On the other hand, protoplasmic astrocytes were also markedly activated in the brains of ankyrin-G (AnkG) and Kv3.1 KO mice, where neuronal activities are altered. Massive astrocytes replaced degenerated Purkinje neurons in AnkG KO mice. In Kv3.1 KO mice, GFAP staining significantly increased in cerebellar cortex, where Kv3.1 is normally highly expressed, but displayed in a patchy pattern in parts of the hippocampus.

CONCLUSIONS

Thus, astrocytes can detect changes in both blood and neurons, which supports their central role in neurovascular coupling. These studies contribute to the development of new strategies of neuroprotection and repair for various diseases, through activity-dependent regulation of neurovascular coupling.

Aquaporin-4 antibodies (NMO-IgG) as a serological marker of neuromyelitis optica: a critical review of the literature

Antibodies to aquaporin-4 (called NMO-IgG or AQP4-Ab) constitute a sensitive and highly specific serum marker of neuromyelitis optica (NMO) that can facilitate the differential diagnosis of NMO and classic multiple sclerosis. NMO-IgG/AQP4-Ab seropositive status has also important prognostic and therapeutic implications in patients with isolated longitudinally extensive myelitis (LETM) or optic neuritis (ON). In this article, we comprehensively review and critically appraise the existing literature on NMO-IgG/AQP4-Ab testing. All available immunoassays-including tissue-based (IHC), cell-based (ICC, FACS) and protein-based (RIPA, FIPA, ELISA, Western blotting) assays-and their differential advantages and disadvantages are discussed. Estimates for sensitivity, specificity, and positive and negative likelihood ratios are calculated for all published studies and accuracies of the various immunoassay techniques compared. Subgroup analyses are provided for NMO, LETM and ON, for relapsing vs. monophasic disease, and for various control groups (eg, MS vs. other controls). Numerous aspects of NMO-IgG/AQP4-Ab testing relevant for clinicians (eg, impact of antibody titers and longitudinal testing, indications for repeat testing, relevance of CSF testing and subclass analysis, NMO-IgG/AQP4-Ab in patients with rheumatic diseases) as well as technical aspects (eg, AQP4-M1 vs. AQP4-M23-based assays, intact AQP4 vs. peptide substrates, effect of storage conditions and freeze/thaw cycles) and pitfalls are discussed. Finally, recommendations for the clinical application of NMO-IgG/AQP4-Ab serology are given.

Distinction and temporal stability of conformational epitopes on myelin oligodendrocyte glycoprotein recognized by patients with different inflammatory central nervous system diseases

Autoantibodies targeting conformationally intact myelin oligodendrocyte glycoprotein (MOG) are found in different inflammatory diseases of the CNS, but their antigenic epitopes have not been mapped. We expressed mutants of MOG on human HeLa cells and analyzed sera from 111 patients (104 children, 7 adults) who recognized cell-bound human MOG, but had different diseases, including acute disseminated encephalomyelitis (ADEM), one episode of transverse myelitis or optic neuritis, multiple sclerosis (MS), anti-aquaporin-4 (AQP4)-negative neuromyelitis optica (NMO), and chronic relapsing inflammatory optic neuritis (CRION). We obtained insight into the recognition of epitopes in 98 patients. All epitopes identified were located at loops connecting the β strands of MOG. The most frequently recognized MOG epitope was revealed by the P42S mutation positioned in the CC'-loop. Overall, we distinguished seven epitope patterns, including the one mainly recognized by mouse mAbs. In half of the patients, the anti-MOG response was directed to a single epitope. The epitope specificity was not linked to certain disease entities. Longitudinal analysis of 11 patients for up to 5 y indicated constant epitope recognition without evidence for intramolecular epitope spreading. Patients who rapidly lost their anti-MOG IgG still generated a long-lasting IgG response to vaccines, indicating that their loss of anti-MOG reactivity did not reflect a general lack of capacity for long-standing IgG responses. The majority of human anti-MOG Abs did not recognize rodent MOG, which has implications for animal studies. Our findings might assist in future detection of potential mimotopes and pave the way to Ag-specific depletion.

Optic neuritis in neuromyelitis optica

Neuromyelitis optica (NMO) is an autoimmune demyelinating disease associated with recurrent episodes of optic neuritis and transverse myelitis, often resulting in permanent blindness and/or paralysis. The discovery of autoantibodies (AQP4-IgG) that target aquaporin-4 (AQP4) has accelerated our understanding of the cellular mechanisms driving NMO pathogenesis. AQP4 is a bidirectional water channel expressed on the plasma membranes of astrocytes, retinal Müller cells, skeletal muscle, and some epithelial cells in kidney, lung and the gastrointestinal tract. AQP4 tetramers form regular supramolecular assemblies at the cell plasma membrane called orthogonal arrays of particles. The pathological features of NMO include perivascular deposition of immunoglobulin and activated complement, loss of astrocytic AQP4, inflammatory infiltration with granulocyte and macrophage accumulation, and demyelination with axon loss. Current evidence supports a causative role of AQP4-IgG in NMO, in which binding of AQP4-IgG to AQP4 orthogonal arrays on astrocytes initiates complement-dependent and antibody-dependent cell-mediated cytotoxicity and inflammation. Immunosuppression and plasma exchange are the mainstays of therapy for NMO optic neuritis. Novel therapeutics targeting specific steps in NMO pathogenesis are entering the development pipeline, including blockers of AQP4-IgG binding to AQP4 and inhibitors of granulocyte function. However, much work remains in understanding the unique susceptibility of the optic nerves in NMO, in developing animal models of NMO optic neuritis, and in improving therapies to preserve vision.

Connexin 43 astrocytopathy linked to rapidly progressive multiple sclerosis and neuromyelitis optica

BACKGROUND

Multiple sclerosis (MS) and neuromyelitis optica (NMO) occasionally have an extremely aggressive and debilitating disease course; however, its molecular basis is unknown. This study aimed to determine a relationship between connexin (Cx) pathology and disease aggressiveness in Asian patients with MS and NMO.

METHODS/PRINCIPAL FINDINGS

Samples included 11 autopsied cases with NMO and NMO spectrum disorder (NMOSD), six with MS, and 20 with other neurological diseases (OND). Methods of analysis included immunohistochemical expression of astrocytic Cx43/Cx30, oligodendrocytic Cx47/Cx32 relative to AQP4 and other astrocytic and oligodendrocytic proteins, extent of demyelination, the vasculocentric deposition of complement and immunoglobulin, and lesion staging by CD68 staining for macrophages. Lesions were classified as actively demyelinating (n=59), chronic active (n=58) and chronic inactive (n=23). Sera from 120 subjects including 30 MS, 30 NMO, 40 OND and 20 healthy controls were examined for anti-Cx43 antibody by cell-based assay. Six NMO/NMOSD and three MS cases showed preferential loss of astrocytic Cx43 beyond the demyelinated areas in actively demyelinating and chronic active lesions, where heterotypic Cx43/Cx47 astrocyte oligodendrocyte gap junctions were extensively lost. Cx43 loss was significantly associated with a rapidly progressive disease course as six of nine cases with Cx43 loss, but none of eight cases without Cx43 loss regardless of disease phenotype, died within two years after disease onset (66.7% vs. 0%, P=0.0090). Overall, five of nine cases with Cx43 loss and none of eight cases without Cx43 loss had distal oligodendrogliopathy characterized by selective myelin associated glycoprotein loss (55.6% vs. 0.0%, P=0.0296). Loss of oligodendrocytic Cx32 and Cx47 expression was observed in most active and chronic lesions from all MS and NMO/NMOSD cases. Cx43-specific antibodies were absent in NMO/NMOSD and MS patients.

CONCLUSIONS

These findings suggest that autoantibody-independent astrocytic Cx43 loss may relate to disease aggressiveness and distal oligodendrogliopathy in both MS and NMO.

The major brain endocannabinoid 2-AG controls neuropathic pain and mechanical hyperalgesia in patients with neuromyelitis optica

Recurrent myelitis is one of the predominant characteristics in patients with neuromyelitis optica (NMO). While paresis, visual loss, sensory deficits, and bladder dysfunction are well known symptoms in NMO patients, pain has been recognized only recently as another key symptom of the disease. Although spinal cord inflammation is a defining aspect of neuromyelitis, there is an almost complete lack of data on altered somatosensory function, including pain. Therefore, eleven consecutive patients with NMO were investigated regarding the presence and clinical characteristics of pain. All patients were examined clinically as well as by Quantitative Sensory Testing (QST) following the protocol of the German Research Network on Neuropathic Pain (DFNS). Additionally, plasma endocannabinoid levels and signs of chronic stress and depression were determined. Almost all patients (10/11) suffered from NMO-associated neuropathic pain for the last three months, and 8 out of 11 patients indicated relevant pain at the time of examination. Symptoms of neuropathic pain were reported in the vast majority of patients with NMO. Psychological testing revealed signs of marked depression. Compared to age and gender-matched healthy controls, QST revealed pronounced mechanical and thermal sensory loss, strongly correlated to ongoing pain suggesting the presence of deafferentation-induced neuropathic pain. Thermal hyperalgesia correlated to MRI-verified signs of spinal cord lesion. Heat hyperalgesia was highly correlated to the time since last relapse of NMO. Patients with NMO exhibited significant mechanical and thermal dysesthesia, namely dynamic mechanical allodynia and paradoxical heat sensation. Moreover, they presented frequently with either abnormal mechanical hypoalgesia or hyperalgesia, which depended significantly on plasma levels of the endogenous cannabinoid 2-arachidonoylglycerole (2-AG). These data emphasize the high prevalence of neuropathic pain and hyperalgesia in patients with NMO. The degree of mechanical hyperalgesia reflecting central sensitization of nociceptive pathways seems to be controlled by the major brain endocannabinoid 2-AG.

Neuromyelitis optica IgG causes placental inflammation and fetal death

Neuromyelitis optica (NMO) is an inflammatory demyelinating disease of the CNS and affects women of childbearing age. Most patients with NMO have circulating Abs, termed NMO-IgG, against the astrocytic water channel protein aquaporin-4. In the CNS, NMO-IgG causes complement-mediated astrocyte damage, inflammatory cell infiltration, and myelin loss. In this study, we show that aquaporin-4 is expressed in the syncytiotrophoblast of human and mouse placenta. Placental aquaporin-4 expression is high during mid-gestation and progressively decreases with advancing pregnancy. Intraperitoneally injected NMO-IgG binds mouse placental aquaporin-4, activates coinjected human complement, and causes inflammatory cell infiltration into the placenta and placental necrosis. There was no damage to maternal organs that express aquaporin-4, including the brain, spinal cord, kidneys, and skeletal muscle. In control experiments, no placentitis was found in mice injected with NMO-IgG without complement, non-NMO-IgG with human complement, or in aquaporin-4 null mice injected with NMO-IgG and human complement. The infiltrating cells were primarily neutrophils with a few scattered eosinophils and macrophages. NMO-IgG and human complement-induced placentitis caused fetal death, but some fetuses were born normal when lower amounts of NMO-IgG and human complement were injected. Sivelestat, a neutrophil elastase inhibitor, and aquaporumab, a nonpathogenic IgG that competes with NMO-IgG for aquaporin-4 binding, significantly reduced NMO-IgG and human complement induced placentitis and fetal death. Our data suggest that NMO-IgG can cause miscarriage, thus challenging the concept that NMO affects only the CNS. These findings have implications for the management of NMO during pregnancy.

Current concept of neuromyelitis optica (NMO) and NMO spectrum disorders

Neuromyelitis optica (NMO) has been described as a disease clinically characterised by severe optic neuritis (ON) and transverse myelitis (TM). Other features of NMO include female preponderance, longitudinally extensive spinal cord lesions (>3 vertebral segments), and absence of oligoclonal IgG bands . In spite of these differences from multiple sclerosis (MS), the relationship between NMO and MS has long been controversial. However, since the discovery of NMO-IgG or aquaporin-4 (AQP4) antibody (AQP4-antibody), an NMO-specific autoantibody to AQP4, the dominant water channel in the central nervous system densely expressed on end-feet of astrocytes, unique clinical features, MRI and other laboratory findings in NMO have been clarified further. AQP4-antibody is now the most important laboratory finding for the diagnosis of NMO. Apart from NMO, some patients with recurrent ON or recurrent longitudinally extensive myelitis alone are also often positive for AQP4-antibody. Moreover, studies of AQP4-antibody-positive patients have revealed that brain lesions are not uncommon in NMO, and some patterns appear to be unique to NMO. Thus, the spectrum of NMO is wider than mere ON and TM. Pathological analyses of autopsied cases strongly suggest that unlike MS, astrocytic damage is the primary pathology in NMO, and experimental studies confirm the pathogenicity of AQP4-antibody. Importantly, therapeutic outcomes of some immunological treatments are different between NMO and MS, making early differential diagnosis of these two disorders crucial. We provide an overview of the epidemiology, clinical and neuroimaging features, immunopathology and therapy of NMO and NMO spectrum disorders.

Plasma osteopontin levels are associated with disease activity in the patients with multiple sclerosis and neuromyelitis optica

Osteopontin (OPN) up-regulates pro-inflammatory cytokines from both T helper type 1 and T helper type 17 cell pathways. We measured plasma OPN levels in Japanese multiple sclerosis (MS) and neuromyelitis optica (NMO) patients to investigate its value as a potential biomarker of disease activity. In NMO patients, plasma OPN levels were significantly higher than those in healthy individuals, being equivalent to those in MS patients. In both NMO and MS patients, OPN levels were significantly higher during relapse compared with remission. There was also a significant positive correlation between Expanded Disability Status Scale of Kurzke scores and plasma OPN levels in both NMO and MS patients, and plasma OPN levels were significantly higher in patients with secondary progressive MS compared with those with relapsing-remitting MS. Diagnostic sensitivity and specificity of plasma OPN for MS and NMO during the relapse phase were 100% and 50%, respectively (cut-off point: 31.3ng/ml). Thus, elevated plasma OPN levels could be a potential biomarker for not only MS but also NMO. These are the first results to suggest that plasma OPN in NMO patients may be a useful marker, playing an important role in inflammation, disease activity, and disease progression, as well as MS.

Immunology of neuromyelitis optica: a T cell-B cell collaboration

Neuromyelitis optica (NMO) is a debilitating autoimmune inflammatory disease of the central nervous system (CNS) that is distinct from multiple sclerosis (MS). The discovery of NMO-immunoglobulin G (IgG) in the serum of NMO-but not MS-patients was a breakthrough in defining diagnostic criteria for NMO. NMO-IgG is an antibody directed against the astrocytic water channel protein aquaporin-4 (AQP4). While there is evidence that NMO-IgG is also involved in mediating tissue damage in the CNS, many aspects of the pathogenic cascade in NMO remain to be determined. It is clear that antigen-specific T cells contribute to the generation of NMO-IgG in the peripheral immune compartment, as well as to the development of NMO lesions in the CNS. T helper 17 (Th17) cells, equipped both in providing B cell help and inducing tissue inflammation, may be involved in NMO development and pathogenesis. Here, we review immunologic aspects of NMO, placing recent findings in the biology of T-B cell cooperation into perspective with autoimmunity of the CNS.

Brain-reactive antibodies and disease

Autoimmune diseases currently affect 5-7% of the world's population; in most diseases there are circulating autoantibodies. Brain-reactive antibodies are present in approximately 2-3% of the general population but do not usually contribute to brain pathology. These antibodies penetrate brain tissue only early in development or under pathologic conditions. This restriction on their pathogenicity and the lack of correlation between serum titers and brain pathology have, no doubt, contributed to a delayed appreciation of the contribution of autoantibodies in diseases of the central nervous system. Nonetheless, it is increasingly clear that antibodies can cause damage in the brain and likely initiate or aggravate multiple neurologic conditions; brain-reactive antibodies contribute to symptomatology in autoimmune disease, infectious disease, and malignancy.

The spectrum of MOG autoantibody-associated demyelinating diseases

Myelin oligodendrocyte glycoprotein (MOG) has been identified as a target of demyelinating autoantibodies in animal models of inflammatory demyelinating diseases of the CNS, such as multiple sclerosis (MS). Numerous studies have aimed to establish a role for MOG antibodies in patients with MS, although the results have been controversial. Cell-based immunoassays using MOG expressed in mammalian cells have demonstrated the presence of high-titre MOG antibodies in paediatric patients with acute disseminated encephalomyelitis, MS, aquaporin-4-seronegative neuromyelitis optica, or isolated optic neuritis or transverse myelitis, but only rarely in adults with these disorders. These studies indicate that MOG antibodies could be associated with a broad spectrum of acquired human CNS demyelinating diseases. This Review article discusses the current literature on MOG antibodies, their potential clinical relevance, and their role in the pathogenesis of MOG antibody-associated demyelinating disorders.