Identification of binding sites for anti-aquaporin 4 antibodies in patients with neuromyelitis optica

AQP4-DARPin1: A Chimeric Antigen Based on Scaffold Protein DARPin for Efficient Detection of AQP4-IgG in NMOSD

AQP4-IgG is an autoantibody associated with neuromyelitis optica spectroscopic disorder (NMOSD), a central nervous system inflammatory disease that requires early diagnosis and treatment. We designed two fusion proteins, AQP4-DARPin1 and AQP4-DARPin2, comprising the complete antigenic epitopes of aquaporin-4 (AQP4) and the constant region of the scaffold protein DARPin. These fusion proteins were expressed and purified from Escherichia coli and coated on microplates to develop an efficient method for detecting AQP4-IgG. Molecular dynamics simulation revealed that the fusion of AQP4 extracellular epitopes with DARPin did not alter the main structure of DARPin. The purified AQP4-DARPins bound recombinant antibody rAb-53 (AQP4-IgG) with affinities of 135 and 285 nM, respectively. Enzyme-linked immunosorbent assay (ELISA) and immunoprecipitation demonstrated that AQP4-DARPin1 specifically recognized AQP4-IgG in the NMOSD patient serum. AQP4-DARPin1 as a coated antigen showed higher ELISA signal and end point dilution ratio than full-length AQP4. Our AQP4-DARPin1-coated AQP4-IgG ELISA had 100% specificity and 90% sensitivity. These results indicate that AQP4-DARPin1, compared to existing detection strategies that use full-length or extracellular loop peptides of AQP4, provides a new and more effective approach to the ELISA detection of NMOSD.

Pathogenesis, Clinical Features, and Treatment of Patients with Myelin Oligodendrocyte Glycoprotein (MOG) Autoantibody-Associated Disorders Focusing on Optic Neuritis with Consideration of Autoantibody-Binding Sites: A Review

Although there is a substantial amount of data on the clinical characteristics, diagnostic criteria, and pathogenesis of myelin oligodendrocyte glycoprotein (MOG) autoantibody-associated disease (MOGAD), there is still uncertainty regarding the MOG protein function and the pathogenicity of anti-MOG autoantibodies in this disease. It is important to note that the disease characteristics, immunopathology, and treatment response of MOGAD patients differ from those of anti-aquaporin 4 antibody-positive neuromyelitis optica spectrum disorders (NMOSDs) and multiple sclerosis (MS). The clinical phenotypes of MOGAD are varied and can include acute disseminated encephalomyelitis, transverse myelitis, cerebral cortical encephalitis, brainstem or cerebellar symptoms, and optic neuritis. The frequency of optic neuritis suggests that the optic nerve is the most vulnerable lesion in MOGAD. During the acute stage, the optic nerve shows significant swelling with severe visual symptoms, and an MRI of the optic nerve and brain lesion tends to show an edematous appearance. These features can be alleviated with early extensive immune therapy, which may suggest that the initial attack of anti-MOG autoantibodies could target the structures on the blood-brain barrier or vessel membrane before reaching MOG protein on myelin or oligodendrocytes. To understand the pathogenesis of MOGAD, proper animal models are crucial. However, anti-MOG autoantibodies isolated from patients with MOGAD do not recognize mouse MOG efficiently. Several studies have identified two MOG epitopes that exhibit strong affinity with human anti-MOG autoantibodies, particularly those isolated from patients with the optic neuritis phenotype. Nonetheless, the relations between epitopes on MOG protein remain unclear and need to be identified in the future.

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.

Pathobiology, Diagnosis, and Current Biomarkers in Neuromyelitis Optica Spectrum Disorders

Background

Neuromyelitis optica spectrum disorder (NMOSD) is characterized by chronic inflammation of the central nervous system (CNS), particularly the optic nerves and spinal cord. Although it displays some clinical features similar to multiple sclerosis (MS), the etiology and treatment are distinct, and therefore accurate diagnosis is essential. Autoantibodies targeting the water channel protein aquaporin-4 (AQP4) and the myelin sheath protein myelin oligodendrocyte glycoprotein are the major antigen-specific serological biomarkers known to date, with destruction of astrocytes as the primary mode of CNS damage in AQP4-positive disease.

Content

This mini-review summarizes the pathobiology, clinical features, and current methods of serological testing used to assess NMOSD and differentiate this disorder from MS. A brief summary of emerging therapies is also presented.

Summary

NMOSD can be distinguished from MS through a combination of clinical findings, imaging investigations, and serological analysis. Seronegative cases are particularly difficult to diagnose and can pose a challenge to clinicians. As knowledge deepens, new therapies and biomarkers are expected to improve treatment of this rare debilitating disease.

Distribution of Aquaporins 1 and 4 in the Central Nervous System

The aquaporins (AQP), a protein family, were first discovered in the early 1990s. The primary role of aquaporins is to facilitate water transport across multiple cell types. In the spinal cord and brain responsible for most of the water diffusion are AQP4 and AQP1. In this paper, we describe the structure, localization and role of this water channel family, especially AQP4 and AQP1. AQP4 is involved in various pathologies such as: stroke, brain tumors, Neuromyelitis optica (NMO), Alzheimer’s Disease (AD), traumatic brain injury, Parkinson’s Disease, hydrocephalus, schizophrenia, epilepsy, major depressive disorder, autism. Brain edema is the most important acute complication of the hypoxic-ischemic and it has no pathogenic treatment. Imaging and histopathology studies have shown that inhibition of AQP4 reduces brain edema.

Neuromyelitis optica spectrum disorders: Features of aquaporin-4, myelin oligodendrocyte glycoprotein and double-seronegative-mediated subtypes

The new diagnostic classification of neuromyelitis optica spectrum disorder (NMOSD) in 2015 highlights the central role of biomarkers, such as antibodies against aquaporin-4 (AQP4-Ab), in diagnosis. Also, in approximately 20-25% of patients without AQP4-Ab (NMOSD^AQP4-) the presence of an antibody directed against myelin oligodendrocyte glycoprotein (MOG) characterizes a specific population of NMOSD patients (NMOSD^MOG+), according to their demographic and clinical data and prognoses. While double-seronegative cases (NMOSD^NEG) have not been fully described, they may correspond to the very first patients with opticospinal demyelination reported by Devic and Gault in 1894. The present report reviews the current knowledge of the pathophysiology and clinical features of NMOSD^AQP4+, NMOSD^MOG+ and NMOSD^NEG patients, and also discusses the relationship between the extended spectrum of MOG disease and NMOSD^MOG+. Finally, the current treatments for acute relapses and relapse prevention are described, with a focus on serological-based therapeutic responses and the promising new therapeutic targets.

Functional Epitopes for Anti-Aquaporin 5 Antibodies in Sjögren Syndrome

We recently reported the presence of anti-aquaporin 5 (AQP5) immunoglobulin G (IgG) in patients with primary Sjögren syndrome (SS) with a sensitivity of 0.73 and a specificity of 0.68. The aim of this study was to identify functional epitopes for the anti-AQP5 autoantibodies detected in control subjects and patients with SS. Recognition of epitopes by anti-AQP5 autoantibodies in sera ( n = 13 for control and n = 24 for SS) or purified IgG ( n = 1 for control and n = 3 for SS) was evaluated by indirect immunofluorescence (IIF) assay performed in the presence or absence of peptides corresponding to the second transmembrane helix and extracellular loops A, C, and E of AQP5. Functional epitopes were determined by measuring the effects of purified IgG and neutralizing peptides on transepithelial osmotic permeability (P_f^T) of MDCK cells expressing AQP5. In the IIF assay, 89% of SS samples were inhibited by at least 1 peptide, while only half of control samples were inhibited by any peptide. Overall, SS samples were inhibited by peptides corresponding to extracellular loops A, C, and E by 40% to 50%, whereas control samples were inhibited only by peptides corresponding to loop E by <20%. A cyclized peptide (E1) mimicking loop E was most frequently recognized and best differentiated between the SS and control samples. Incubation of MDCK-AQP5 cells with SS but not with control IgG, significantly decreased P_f^T, which was reversed by neutralization of IgG binding to any of the extracellular loops. In conclusion, the anti-AQP5 autoantibodies detected in control and SS groups showed differences in fine specificity to the functional epitopes of AQP5. The prevalent recognition of functional epitopes by anti-AQP5 autoantibodies from SS patients suggests that anti-AQP5 autoantibodies act as mediators of glandular hypofunction and are a potential therapeutic target in SS.

High-Density Peptide Microarray Analysis of IgG Autoantibody Reactivities in Serum and Cerebrospinal Fluid of Multiple Sclerosis Patients

Intrathecal immunoglobulin G (IgG) synthesis and oligoclonal IgG bands in cerebrospinal fluid (CSF) are hallmarks of multiple sclerosis (MS), but the antigen specificities remain enigmatic. Our study is the first investigating the autoantibody repertoire in paired serum and CSF samples from patients with relapsing-remitting MS (RRMS), primary progressive MS (PPMS), and other neurological diseases by the use of high-density peptide microarrays. Protein sequences of 45 presumed MS autoantigens (e.g.MOG, MBP, and MAG) were represented on the microarrays by overlapping 15mer peptides. IgG reactivities were screened against a total of 3991 peptides, including also selected viral epitopes. The measured antibody reactivities were highly individual but correlated for matched serum and CSF samples. We found 54 peptides to be recognized significantly more often by serum or CSF antibodies from MS patients compared with controls (pvalues <0.05). The results for RRMS and PPMS clearly overlapped. However, PPMS patients presented a broader peptide-antibody signature. The highest signals were detected for a peptide mapping to a region of the Epstein-Barr virus protein EBNA1 (amino acids 392-411), which is homologous to the N-terminal part of human crystallin alpha-B. Our data confirmed several known MS-associated antigens and epitopes, and they delivered additional potential linear epitopes, which await further validation. The peripheral and intrathecal humoral immune response in MS is polyspecific and includes antibodies that are also found in serum of patients with other diseases. Further studies are required to assess the pathogenic relevance of autoreactive and anti-EBNA1 antibodies as well as their combinatorial value as biomarkers for MS.

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.

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.

Establishment of monoclonal antibodies against the extracellular domain that block binding of NMO-IgG to AQP4

Neuromyelitis optica is a demyelinating disease characterized by a disease-specific autoantibody designated as NMO-IgG that specifically recognizes aquaporin-4, and the binding of NMO-IgG to AQP4 causes the progress of the disease. Prevention of the binding of NMO-IgG, therefore, may alleviate the disease. Here we have developed monoclonal antibodies against AQP4 with a baculovirus display system in order to obtain high affinity monoclonal antibodies against the extracellular domains of AQP4. Our monoclonal antibodies can block the binding of NMO-IgG in spite of their heterogeneity. Taken together, we propose that our monoclonal antibodies can be applied in clinical therapy for NMO patients.

Passively transferred human NMO-IgG exacerbates demyelination in mouse experimental autoimmune encephalomyelitis

BACKGROUND

Neuromyelitis optica (NMO) is a devastating inflammatory disorder of the optic nerves and spinal cord characterized by frequently recurring exacerbations of humoral inflammation. NMO is associated with the highly specific NMO-IgG biomarker, an antibody that binds the aquaporin-4 water channel. Aquaporin-4 is present on glial endfeet in the central nervous system (CNS). In humans, the NMO-IgG portends more frequent exacerbations and a worse long-term clinical outcome.

METHODS

We tested the longer-term outcome of mice with EAE injected with NMO-IgG and followed them for 60 days. Clinical exams and pathology of the spinal cord and optic nerves were compared to mice that received control human IgG.

RESULTS

Passively transferred human NMO-IgG leads to more severe neurology disability over two months after onset of disease. Clinical worsening is associated with an increased concentration of large demyelinating lesions primarily to subpial AQP4-rich regions of the spinal cord.

CONCLUSIONS

NMO-IgG is pathogenic in the context of EAE in mice.

Aquaporin 4 molecular mimicry and implications for neuromyelitis optica

Neuromyelitis optica (NMO) is associated with antibodies to aquaporin 4 (AQP4). We hypothesized that antibodies to AQP4 can be triggered by exposure to environmental proteins. We compared human AQP4 to plant and bacterial proteins to investigate the occurrence of significantly similar structures and sequences. High similarity to a known epitope for NMO-IgG, AQP4(207-232), was observed for corn ZmTIP4-1. NMO and non-NMO sera were assessed for reactivity to AQP4(207-232) and the corn peptide. NMO patient serum showed reactivity to both peptides as well as to plant tissue. These findings warrant further investigation into the role of the environment in NMO etiology.

Aquaporin water channels in the nervous system

The aquaporins (AQPs) are plasma membrane water-transporting proteins. AQP4 is the principal member of this protein family in the CNS, where it is expressed in astrocytes and is involved in water movement, cell migration and neuroexcitation. AQP1 is expressed in the choroid plexus, where it facilitates cerebrospinal fluid secretion, and in dorsal root ganglion neurons, where it tunes pain perception. The AQPs are potential drug targets for several neurological conditions. Astrocytoma cells strongly express AQP4, which may facilitate their infiltration into the brain, and the neuroinflammatory disease neuromyelitis optica is caused by AQP4-specific autoantibodies that produce complement-mediated astrocytic damage.

Molecular pathogenesis of neuromyelitis optica

Neuromyelitis optica (NMO) is a rare autoimmune disorder, distinct from multiple sclerosis, causing inflammatory lesions in the optic nerves and spinal cord. An autoantibody (NMO IgG) against aquaporin-4 (AQP4), a water channel expressed on astrocytes is thought to be causative. Peripheral production of the antibody is triggered by an unknown process in genetically susceptible individuals. Anti-AQP4 antibody enters the central nervous system (CNS) when the blood brain barrier is made permeable and has high affinity for orthogonal array particles of AQP4. Like other autoimmune diseases, Th17 cells and their effector cytokines (such as interleukin 6) have been implicated in pathogenesis. AQP4 expressing peripheral organs are not affected by NMO IgG, but the antibody causes extensive astrocytic loss in specific regions of the CNS through complement mediated cytotoxicity. Demyelination occurs during the inflammatory process and is probably secondary to oligodendrocyte apoptosis subsequent to loss of trophic support from astrocytes. Ultimately, extensive axonal injury leads to severe disability. Despite rapid advances in the understanding of NMO pathogenesis, unanswered questions remain, particularly with regards to disease mechanisms in NMO IgG seronegative cases. Increasing knowledge of the molecular pathology is leading to improved treatment strategies.

Cross-immunoreactivity between bacterial aquaporin-Z and human aquaporin-4: potential relevance to neuromyelitis optica

Neuromyelitis optica (NMO) is a chronic inflammatory disease of the CNS that is mediated, in part, by a self-reactive Ab against the astrocyte aquaporin-4 protein. In the current study, we examined the possibility and the biological significance of cross-immunoreactivity between bacterial aquaporin-Z and human aquaporin-4 proteins. Sequence-alignment analysis of these proteins revealed several regions of significant structural homology. Some of the homologous regions were also found to overlap with important immune and disease-relevant epitopes. Cross-immunoreactivity between aquaporin-Z and aquaporin-4 was investigated and ascertained in multiple immune-based assays using sera from patients with neuromyelitis optica, immune mouse serum, and Abs raised against aquaporin-Z. The biological significance of this phenomenon was established in series of experiments demonstrating that induction of an immune response against aquaporin-Z or its homologous regions can also trigger an autoimmune reaction against aquaporin-4 and inflammation of the CNS. Our study indicates that the autoimmune response against aquaporin-4 in neuromyelitis optica may be triggered by infection-induced cross-immunoreactivity and presents a new perspective on the pathogenesis of this disease.

Astrocytic autoantibody of neuromyelitis optica (NMO-IgG) binds to aquaporin-4 extracellular loops, monomers, tetramers and high order arrays

The principal central nervous system (CNS) water channel, aquaporin-4 (AQP4), is confined to astrocytic and ependymal membranes and is the target of a pathogenic autoantibody, neuromyelitis optica (NMO)-IgG. This disease-specific autoantibody unifies a spectrum of relapsing CNS autoimmune inflammatory disorders of which NMO exemplifies the classic phenotype. Multiple sclerosis and other immune-mediated demyelinating disorders of the CNS lack a distinctive biomarker. Two AQP4 isoforms, M1 and M23, exist as homotetrameric and heterotetrameric intramembranous particles (IMPs). Orthogonal arrays of predominantly M23 particles (OAPs) are an ultrastructural characteristic of astrocytic membranes. We used high-titered serum from 32 AQP4-IgG-seropositive patients and 85 controls to investigate the nature and molecular location of AQP4 epitopes that bind NMO-IgG, and the influence of supramolecular structure. NMO-IgG bound to denatured AQP4 monomers (68% of cases), to native tetramers and high order arrays (90% of cases), and to AQP4 in live cell membranes (100% of cases). Disease-specific epitopes reside in extracellular loop C more than in loops A or E. IgG binding to intracellular epitopes lacks disease specificity. These observations predict greater disease sensitivity and specificity for tissue-based and cell-based serological assays employing "native" AQP4 than assays employing denatured AQP4 and fragments. NMO-IgG binds most avidly to plasma membrane surface AQP4 epitopes formed by loop interactions within tetramers and by intermolecular interactions within high order structures. The relative abundance and localization of AQP4 high order arrays in distinct CNS regions may explain the variability in clinical phenotype of NMO spectrum disorders.

Aquaporin-4 positron emission tomography imaging of the human brain: first report

BACKGROUND AND PURPOSE

Aquaporin 4 (AQP-4) is the most abundant aquaporin isoform in the brain. Alterations in its expression and distribution have been correlated with the progression of several clinical disorders; however, the specific roles of AQP-4 in those disorders are not well understood. Visualizing AQP-4 in vivo is expected to provide fresh insights into its roles in disease pathology, as well as aiding the clinical assessment of those disorders.

METHODS

We developed a 11C-labeled analogue of the AQP-4 ligand TGN-020 (2-nicotinamido-1,3,4-thiadiazole) suitable for in vivo positron emission tomography (PET) imaging.

RESULTS

In the present study, we report the first PET images of AQP-4 in the human brain. The results unequivocally demonstrated a specific distribution pattern for AQP-4 within the brain, namely, the subpial and perivascular endfeet of astrocytes. The choroid plexus, where both AQP-4 and AQP-1 are expressed, also showed substantial uptake of the ligand.

CONCLUSIONS

Based on these initial results, we believe [11C]TGN-020 PET will be valuable in determining the role of AQP-4 in disease progression, and for the clinical assessment of water homeostasis under various settings.

Testing for antibodies to human aquaporin-4 by ELISA: sensitivity, specificity, and direct comparison with immunohistochemistry

BACKGROUND

Several assays have been developed to detect antibodies to aquaporin-4 (NMO-IgG/AQP4-Ab). However, many of these assays require sophisticated techniques and are thus only available at specialized laboratories. This is problematic since NMO-IgG/AQP4-Ab testing has important prognostic and therapeutic implications.

OBJECTIVE

To evaluate a newly developed, commercial, enzyme-linked immunosorbent assay (ELISA) for detecting NMO-IgG/AQP4-Ab.

METHODS

Serum samples from 261 patients with NMO spectrum disorders (NMOSD; n=108) and controls (n=153) were tested for AQP4-Ab by using ELISA. Of these patients, 207 were tested in parallel using a standard immunohistochemical (IHC) assay.

RESULTS

Fifty of 66 (75.8%) patients with NMO, 17/25 (68%) with LETM, 3/14 (21.4%) with ON, 2/3 (66.7%) with ON and non-extensive transverse myelitis, and 2/153 (1.3%) controls tested positive in the ELISA. Of those NMOSD patients tested by both ELISA and IHC, 10 were positive only in the ELISA and 3 exclusively in the IHC assay, suggesting that the overall sensitivity of the ELISA was higher than that of the standard IHC assay. The ELISA yielded very good intra- and inter-run reproducibility with regard to AQP4-Ab detection and good intrarun, but only moderate inter-run reproducibility with regard to AQP4-Ab quantification. Anti-AQP4 serum concentrations correlated with disease activity (p<0.00001), but did not differ between patients with NMO and patients with isolated LETM or ON.

CONCLUSION

The ELISA evaluated here provides a relatively sensitive and easy-to-use diagnostic tool for detecting antibodies to AQP4 and could make AQP4-Ab testing, which is of high clinical relevance, more widely available.

Quantitative measurement of anti-aquaporin-4 antibodies by enzyme-linked immunosorbent assay using purified recombinant human aquaporin-4

BACKGROUND

Antibodies to aquaporin-4 (AQP4-Ab), known as NMO-IgG, are a sensitive and specific marker for neuromyelitis optica (NMO).

METHODS

To develop an enzyme-linked immunosorbent assay (ELISA) for AQP4-Ab, we expressed M23 isoform of human AQP4 in a baculovirus system, and used it as an antigen. We measured AQP4-Ab in the sera of 300 individuals: 64 with definite NMO, 31 with high-risk NMO, 105 with multiple sclerosis (MS), 57 with other neurological diseases (ONDs), and 43 healthy controls. We also performed longitudinal measurements of AQP4-Ab in 787 samples collected from 51 patients with definite or high-risk NMO.

RESULTS

AQP4-Abs were positive in 72% with definite NMO, 55% with high-risk NMO, and 4% with MS, but none of the OND patients and the healthy individuals. The longitudinal measurement showed AQP4-Ab levels correlating with disease activity. Out of 38 initially seropositive patients, 21 became seronegative under effective immunosuppressive therapy. During most relapses, the serum AQP4-Ab levels were either high or rising compared with the previous value, although rising AQP4-Ab levels did not always lead to acute exacerbation. Two of the 13 initially seronegative patients converted to seropositive following acute exacerbations.

CONCLUSIONS

We established an AQP4-Ab ELISA, which could be a potential monitoring tool of disease activity.

Early stages of mesothelioma, screening and biomarkers

The early diagnosis of mesothelioma is notoriously difficult, both from a clinical and pathological perspective. Patients often undergo several medical investigations without definitive diagnosis. The discovery of biomarkers that can be assessed in pleural effusions, histological samples, and serum may assist with the difficult early diagnosis of mesothelioma. In this chapter we focus on those markers that have been examined in the setting of either early diagnosis of mesothelioma in symptomatic individuals or that have been proposed as suitable for screening of asbestos-exposed individuals, with an emphasis on cytology and histology.

Fine specificity of antibodies against AQP4: epitope mapping reveals intracellular epitopes

The autoantibody to aquaporin-4 (AQP4) is a marker and a pathogenetic factor in Neuromyelitis Optica (NMO) (Devic's syndrome). Our aim was to identify B-cell antigenic linear epitopes of the AQP4 protein and investigate similarities with other molecules. To this end, we screened sera from 21 patients positive for anti-AQP4 antibodies (study group), from 23 SLE and 23 pSS patients without neurologic involvement (disease controls) and from 28 healthy individuals (normal controls). Eleven peptides, spanning the entire intracellular and extracellular domains of the AQP4 molecule, were synthesized, and all sera were screened for anti-peptide antibodies by ELISA. Specificity was evaluated by homologous inhibition assays. NMO positive sera exhibited reactivity against 3 different peptides spanning the sequences aa1-22 (AQPpep1) (42.9% of patients), aa88-113 (AQPpep4) (33%) and aa252-275 (AQPpep8) (23.8%). All epitopes were localized in the intracellular domains of AQP4. Homologous inhibition rates were ranging from 71.1% to 84.3%. A 73% sequence homology was observed between AQPpep8' aa257-271, a 15-mer peptide part of the AQPpep8 aa252-275, and the aa219-233 domain of the Tax1-HTLV-1 binding protein (TAX1BP1), a host protein associated with replication of the Human T-Lymphotropic Virus 1 (HTLV-1). Antibodies against the AQP4 and the TAX1BP1 15-mer peptides were detected in 26.3% (N = 5) and 31.6% (N = 6) of NMO positive sera (r(s) = 0.81, P < 0.0001). Healthy controls did not react with these peptides, while homologous and cross-inhibition assays confirmed binding specificity. This first epitope mapping for AQP4 reveals that a significant proportion of anti-AQP4 antibodies target linear epitopes localized in the intracellular domains of the channel. One of the epitopes displays high similarity with a portion of TAX1BP1 protein.

Identification of two major conformational aquaporin-4 epitopes for neuromyelitis optica autoantibody binding

Neuromyelitis optica (NMO) is an autoimmune demyelinating disease characterized by the presence of anti-aquaporin-4 (AQP4) antibodies in the patient sera. We recently reported that these autoantibodies are able to bind AQP4 when organized in the supramolecular structure called the orthogonal array of particles (OAP). To map the antigenic determinants, we produced a series of AQP4 mutants based on multiple alignment sequence analysis between AQP4 and other OAP-forming AQPs. Mutations were introduced in the three extracellular loops (A, C, and E), and the binding capacity of NMO sera was tested on AQP4 mutants. Results indicate that one group of sera was able to recognize a limited portion of loop C containing the amino acid sequence (146)GVT(T/M)V(150). A second group of sera was characterized by a predominant role of loop A. Deletion of four AQP4-specific amino acids ((61)G(S/T)E(N/K)(64)) in loop A substantially affected the binding of this group of sera. However, the binding capacity was further reduced when amino acids in loop A were mutated together with those in loop E or when those in loop C were mutated in combination with loop E. Finally, a series of AQP0 mutants were produced in which the extracellular loops were progressively changed to make them identical to AQP4. Results showed that none of the mutants was able to reproduce in AQP0 the NMO-IgG epitopes, indicating that the extracellular loop sequence by itself was not sufficient to determine the rearrangement required to create the epitopes. Although our data highlight the complexity of the disease, this study identifies key immunodominant epitopes and provides direct evidence that the transition from AQP4 tetramers to AQP4-OAPs involves conformational changes of the extracellular loops.

Immunodominant T cell determinants of aquaporin-4, the autoantigen associated with neuromyelitis optica

Autoantibodies that target the water channel aquaporin-4 (AQP4) in neuromyelitis optica (NMO) are IgG1, a T cell-dependent Ig subclass. However, a role for AQP4-specific T cells in this CNS inflammatory disease is not known. To evaluate their potential role in CNS autoimmunity, we have identified and characterized T cells that respond to AQP4 in C57BL/6 and SJL/J mice, two strains that are commonly studied in models of CNS inflammatory diseases. Mice were immunized with either overlapping peptides or intact hAQP4 protein encompassing the entire 323 amino acid sequence. T cell determinants identified from examination of the AQP4 peptide (p) library were located within AQP4 p21-40, p91-110, p101-120, p166-180, p231-250 and p261-280 in C57BL/6 mice, and within p11-30, p21-40, p101-120, p126-140 and p261-280 in SJL/J mice. AQP4-specific T cells were CD4⁺ and MHC II-restricted. In recall responses to immunization with intact AQP4, T cells responded primarily to p21-40, indicating this region contains the immunodominant T cell epitope(s) for both strains. AQP4 p21-40-primed T cells secreted both IFN-γ and IL-17. The core immunodominant AQP4 21-40 T cell determinant was mapped to residues 24-35 in C57BL/6 mice and 23-35 in SJL/J mice. Our identification of the AQP4 T cell determinants and characterization of its immunodominant determinant should permit investigators to evaluate the role of AQP4-specific T cells in vivo and to develop AQP4-targeted murine NMO models.

AQP4 antibodies in neuromyelitis optica: diagnostic and pathogenetic relevance

Antibodies to aquaporin-4 (also known as AQP4-Ab or NMO-IgG) are sensitive and highly specific serum markers of autoimmune neuromyelitis optica (NMO). Second-generation recombinant diagnostic assays can detect AQP4-Ab in >or=80% of patients with NMO, and a role for AQP4-Ab in the pathophysiology of this condition was corroborated by a series of in vitro studies that demonstrated disruption of the blood-brain barrier, impairment of glutamate homeostasis and induction of necrotic cell death by AQP4-Ab-positive serum. Additional evidence for such a role has emerged from clinical observations, including the demonstration of a correlation between serum levels of AQP4-Ab and disease activity. The finding of NMO-like CNS lesions and clinical disease following passive transfer of AQP4-Ab-positive serum in several independent animal studies provided definitive proof for a pathogenic role of AQP4-Ab in vivo. Together, these findings provide a strong rationale for the use of therapies targeted against B cells or antibodies in the treatment of NMO. In this Review, we summarize the latest evidence in support of a direct involvement of AQP4-Ab in the immunopathogenesis of NMO, and critically appraise the diagnostic tests currently available for the detection of this serum reactivity.

EFNS guidelines on diagnosis and management of neuromyelitis optica

BACKGROUND AND PURPOSE

Neuromyelitis optica (NMO) or Devic's disease is a rare inflammatory and demyelinating autoimmune disorder of the central nervous system (CNS) characterized by recurrent attacks of optic neuritis (ON) and longitudinally extensive transverse myelitis (LETM), which is distinct from multiple sclerosis (MS). The guidelines are designed to provide guidance for best clinical practice based on the current state of clinical and scientific knowledge.

SEARCH STRATEGY

Evidence for this guideline was collected by searches for original articles, case reports and meta-analyses in the MEDLINE and Cochrane databases. In addition, clinical practice guidelines of professional neurological and rheumatological organizations were studied.

RESULTS

Different diagnostic criteria for NMO diagnosis [Wingerchuk et al. Revised NMO criteria, 2006 and Miller et al. National Multiple Sclerosis Society (NMSS) task force criteria, 2008] and features potentially indicative of NMO facilitate the diagnosis. In addition, guidance for the work-up and diagnosis of spatially limited NMO spectrum disorders is provided by the task force. Due to lack of studies fulfilling requirement for the highest levels of evidence, the task force suggests concepts for treatment of acute exacerbations and attack prevention based on expert opinion.

CONCLUSIONS

Studies on diagnosis and management of NMO fulfilling requirements for the highest levels of evidence (class I-III rating) are limited, and diagnostic and therapeutic concepts based on expert opinion and consensus of the task force members were assembled for this guideline.

Neuromyelitis optica: Passive transfer to rats by human immunoglobulin

Recurrent attacks of optic neuritis and myelitis are the hallmarks of both neuromyelitis optica (NMO) and multiple sclerosis (MS). NMO immunoglobulin G (NMO-IgG), which recognizes astrocytic aquaporin-4 (AQP4) water channels, is a specific serum autoantibody that distinguishes NMO from MS. The pathogenic role of the anti-AQP4 antibody (AQP4-Ab, NMO-IgG) in NMO has been speculated based on several studies in vitro. The aim of this study was to demonstrate the pathogenicity of AQP4-Ab in vivo. We obtained IgG from patients who underwent therapeutic plasmapheresis, and developed an animal model by passive transfer of IgG to rats. The active lesions of the rats exhibited pathological characteristics strikingly similar to those of NMO, marked by astrocytic loss and perivascular deposition of immunoglobulin and complements. These findings provide the first evidence of the pathogenicity of AQP4-Ab in vivo and support the therapeutic efficacy of eliminating the antibodies by plasmapheresis.