Detection of anti-aquaporin-4 antibodies in neuromyelitis optica: comparison of tissue-based and cell-based indirect immunofluorescence assays and ELISA

Biomarkers for neuromyelitis optica: a visual analysis of emerging research trends

Neuromyelitis optica is an inflammatory demyelinating disease of the central nervous system that differs from multiple sclerosis. Over the past 20 years, the search for biomarkers for neuromyelitis optica has been ongoing. Here, we used a bibliometric approach to analyze the main research focus in the field of biomarkers for neuromyelitis optica. Research in this area is consistently increasing, with China and the United States leading the way on the number of studies conducted. The Mayo Clinic is a highly reputable institution in the United States, and was identified as the most authoritative institution in this field. Furthermore, Professor Wingerchuk from the Mayo Clinic was the most authoritative expert in this field. Keyword analysis revealed that the terms "neuromyelitis optica" (261 times), "multiple sclerosis" (220 times), "neuromyelitis optica spectrum disorder" (132 times), "aquaporin 4" (99 times), and "optical neuritis" (87 times) were the most frequently used keywords in literature related to this field. Comprehensive analysis of the classical literature showed that the majority of publications provide conclusive research evidence supporting the use of aquaporin-4-IgG and neuromyelitis optica-IgG to effectively diagnose and differentiate neuromyelitis optica from multiple sclerosis. Furthermore, aquaporin-4-IgG has emerged as a highly specific diagnostic biomarker for neuromyelitis optica spectrum disorder. Myelin oligodendrocyte glycoprotein-IgG is a diagnostic biomarker for myelin oligodendrocyte glycoprotein antibody-associated disease. Recent biomarkers for neuromyelitis optica include cerebrospinal fluid immunological biomarkers such as glial fibrillary acidic protein, serum astrocyte damage biomarkers like FAM19A5, serum albumin, and gamma-aminobutyric acid. The latest prospective clinical trials are exploring the potential of these biomarkers. Preliminary results indicate that glial fibrillary acidic protein is emerging as a promising candidate biomarker for neuromyelitis optica spectrum disorder. The ultimate goal of future research is to identify non-invasive biomarkers with high sensitivity, specificity, and safety for the accurate diagnosis of neuromyelitis optica.

Update on aquaporin-4 antibody detection: the early diagnosis of neuromyelitis optica spectrum disorders

Neuromyelitis optica spectrum disorder (NMOSD) is an autoimmune-mediated primary inflammatory myelinopathy of the central nervous system that primarily affects the optic nerve and spinal cord. The aquaporin 4 antibody (AQP4-Ab) is a specific autoantibody marker for NMOSD. Most patients with NMOSD are seropositive for AQP4-Ab, thus aiding physicians in identifying ways to treat NMOSD. AQP4-Ab has been tested in many clinical and laboratory studies, demonstrating effectiveness in diagnosing NMOSD. Recently, novel assays have been developed for the rapid and accurate detection of AQP4-Ab, providing further guidance for the diagnosis and treatment of NMOSD. This article summarizes the importance of rapid and accurate diagnosis for treating NMOSD based on a review of the latest relevant literature. We discussed current challenges and methods for improvement to offer new ideas for exploring rapid and accurate AQP4-Ab detection methods, aiming for early diagnosis of NMOSD.

Testing for Antibodies Against Aquaporin-4 and Myelin Oligodendrocyte Glycoprotein in the Diagnosis of Patients With Suspected Autoimmune Myelopathy

Autoimmune myelopathies are immune-mediated disorders of the spinal cord that can cause significant neurologic disability. Discoveries of antibodies targeting aquaporin-4 (AQP4-IgG) and myelin oligodendrocyte glycoprotein (MOG-IgG) have facilitated the diagnosis of autoimmune myelopathies that were previously considered to be atypical presentations of multiple sclerosis (MS) or idiopathic, and represent major advancements in the field of autoimmune neurology. The detection of these antibodies can substantially impact patient diagnosis and management, and increasing awareness of this has led to a dramatic increase in testing for these antibodies among patients with suspected autoimmune myelopathy. In this review we discuss test methodologies used to detect these antibodies, the role of serum vs. cerebrospinal fluid testing, and the value of antibody titers when interpreting results, with the aim of helping laboratorians and clinicians navigate this testing when ordered as part of the diagnostic evaluation for suspected autoimmune myelopathy.

An Update on the Laboratory Diagnosis of Neuromyelitis Optica Spectrum Disorders

Neuromyelitis optica spectrum disorder (NMOSD) is an autoimmune disorder of the central nervous system that is specifically associated with demyelination of spinal cord and optic nerves. The discovery of specific autoantibody markers such as aquaporin-4 IgG and myelin oligodendrocyte glycoprotein IgG has led to several methodologies being developed and validated. There have been numerous investigations of the clinical and radiological presentations used in the clinical diagnosis of NMOSD. However, although various laboratory diagnostic techniques have been standardized and validated, a gold-standard test has yet to be finalized due to uncertain sensitivities and specificities of the methodologies. For this review, the literature was surveyed to compile the standardized laboratory techniques utilized for the differential diagnosis of NMOSD. Enzyme-linked immunosorbent assays enable screening of NMOSD, but they are considered less sensitive than cell-based assays (CBAs), which were found to be highly sensitive and specific. However, CBAs are laborious and prone to batch variations in their results, since the expression levels of protein need to be maintained and monitored meticulously. Standardizing point-of-care devices and peptide-based assays would make it possible to improve the turnaround time and accessibility of the test, especially in resource-poor settings.

Sera from Patients with NMOSD Reduce the Differentiation Capacity of Precursor Cells in the Central Nervous System

INTRODUCTION

AQP4 (aquaporin-4)-immunoglobulin G (IgG)-mediated neuromyelitis optica spectrum disorder (NMOSD) is an inflammatory demyelinating disease that affects the central nervous system, particularly the spinal cord and optic nerve; remyelination capacity in neuromyelitis optica is yet to be determined, as is the role of AQP4-IgG in cell differentiation.

MATERIAL AND METHODS

We included three groups-a group of patients with AQP4-IgG-positive neuromyelitis optica, a healthy group, and a sham group. We analyzed differentiation capacity in cultures of neurospheres from the subventricular zone of mice by adding serum at two different times: early and advanced stages of differentiation. We also analyzed differentiation into different cell lines.

RESULTS AND CONCLUSIONS

The effect of sera from patients with NMOSD on precursor cells differs according to the degree of differentiation, and probably affects oligodendrocyte progenitor cells from NG2 cells to a lesser extent than cells from the subventricular zone; however, the resulting oligodendrocytes may be compromised in terms of maturation and possibly limited in their ability to generate myelin. Furthermore, these cells decrease in number with age. It is very unlikely that the use of drugs favoring the migration and differentiation of oligodendrocyte progenitor cells in multiple sclerosis would be effective in the context of neuromyelitis optica, but cell therapy with oligodendrocyte progenitor cells seems to be a potential alternative.

Detection of autoantibodies in central nervous system inflammatory disorders: Clinical application of cell-based assays

The identification of autoantibodies in central nervous system (CNS) inflammatory disorders improves diagnostic accuracy and the identification of patients with a relapsing disease. Usual methods to detect autoantibodies are usually divided into 3 categories: tissue-based assays, protein-based assays and cell-based assays (CBA). Tissue-based assays are commonly used for initial identification of autoantibodies based on staining patterns and co-localization. Once the antigen is known, autoantibodies can be detected using other antigen-specific methods based on recombinant proteins and CBA using transfected cells expressing the protein in their cell membranes. Compared to traditional methods using recombinant proteins such as ELISA and western blot, the CBA have advantage of detecting conformational sensitive antibodies using natively folded proteins in the cell membrane. This article reviews the utility of CBA into the clinical practice.

AQP4 Antibody Assay Sensitivity Comparison in the Era of the 2015 Diagnostic Criteria for NMOSD

We have compared five different assays for antibodies to aquaporin-4 in 181 cases of suspected Neuromyelitis optica spectrum disorders (NMOSD) and 253 controls to assess their relative utility. As part of a clinically-based survey of NMOSD in Australia and New Zealand, cases of suspected NMOSD were referred from 23 centers. Clinical details and magnetic imaging were reviewed and used to apply the 2015 IPND diagnostic criteria. In addition, 101 age- and sex-matched patients with multiple sclerosis were referred. Other inflammatory disease (n = 49) and healthy controls (n = 103) were also recruited. Samples from all participants were tested using tissue-based indirect immunofluorescence assays and a subset were tested using four additional ELISA and cell-based assays. Antibodies to myelin oligodendrocyte glycoprotein (MOG) were also assayed. All aquaporin-4 antibody assays proved to be highly specific. Sensitivities ranged from 60 to 94%, with cell-based assays having the highest sensitivity. Antibodies to MOG were detected in 8/79 (10%) of the residual suspected cases of NMOSD. Under the 2015 IPND diagnostic criteria for NMOSD, cell-based assays for aquaporin-4 are sensitive and highly specific, performing better than tissue-based and ELISA assays. A fixed cell-based assay showed near-identical results to a live-cell based assay. Antibodies to MOG account for only a small number of suspected cases.

A new M23-based ELISA assay for anti-aquaporin 4 autoantibodies: diagnostic accuracy and clinical correlation

Purpose

Although many assays have been developed to detect anti-aquaporin-4 (AQP4) antibodies, most of these assays require sophisticated techniques and are thus only available at specialized laboratories. The aim of this study was to evaluate the analytical and clinical performance of a new commercial enzyme-linked immunosorbent assay (ELISA RSR, AQP4 Ab Version 2) to detect anti-AQP4 antibodies performed on a fully automated system (SkyLAB 752).

Methods

Serum samples from 64 patients with neuromyelitis optica spectrum disorders (NMOSD) (including NMO, longitudinally extensive myelitis-LETM, optical neuritis and myelitis) and 27 controls were tested for anti-AQP4 antibodies. All sera were previously tested using an indirect immunofluorescence (IIF) method on primate tissue, as the reference method. Commercial control sera were used to determine within-run, between-day and within-laboratory precision (CLSI guidelines).

Results

At a cut-off value of 2.1 U/mL as determined by ROC curves, sensitivity and specificity for NMO were 83.3% and 100%, respectively. The ELISA assay provided 100% concordant results with the reference IIF method. The median concentration of anti-AQP4 antibodies was statistically higher in patients with NMO than in patients with LETM (p = 0.0006) or with other NMOSD and in controls (p < 0.0001). At the concentration of 12.4 and 28.1 U/mL, the within-run, between-day and within-laboratory coefficients of variation (CV) were 3.2% and 3%, 7.6% and 7.4%, and 8.2% and 8.0%, respectively.

Conclusions

This new ELISA method performed on a fully automated system, showed high sensitivity and absolute specificity, good CV in precision tests, and provided observer-independent quantitative results.

Host-Cell Type Dependent Features of Recombinant Human Aquaporin-4 Orthogonal Arrays of Particles-New Insights for Structural and Functional Studies

The CNS plasma-membrane water channel aquaporin-4 (AQP4) is expressed as two major isoforms able to aggregate into supramolecular assemblies known as ‘orthogonal arrays of particles’ (OAPs). OAP subnanometric features are largely unknown mainly because a method for the expression, isolation, and crystallization of integral human OAPs has not been developed. Here, the human OAP-forming isoform M23-AQP4 was expressed in insect and mammalian cell lines and AQP4 and OAP features evaluated. Native size exclusion chromatography was employed to isolate and analyze authentically folded OAPs, and neuromyelitis optica (NMO)-specific sandwich ELISA was developed to test OAP-integrity. The results demonstrate that in insect cells most AQP4 remains intracellular and unfolded and that OAPs are largely disassembled after the detergent extraction step. In mammalian cells, AQP4 showed regular plasma membrane targeting and OAPs exhibited strong post-extraction stability. Starting from the mammalian cell expression system, we isolated authentically folded OAPs. Together these data suggest a new strategy for expressing and isolating integral recombinant human OAPs and providing new insights into the cell-type dependent OAP-assembly and post-extraction stability, potentially useful to design new approaches for structural and functional studies of OAP and for other plasma membrane proteins organized into supramolecular structures.

Development of an Aquaporin-4 Orthogonal Array of Particle-Based ELISA for Neuromyelitis Optica Autoantibodies Detection

Serological markers of Nuromyelitis Optica (NMO), an autoimmune disorder of the central nervous system, are autoantibodies targeting the astrocytic water channel aquaporin-4 (AQP4). We have previously demonstrated that the main epitopes for these autoantibodies (AQP4-IgG) are generated by the supramolecular arrangement of AQP4 tetramers into an Orthogonal Array of Particles (OAPs). Many tests have been developed to detect AQP4-IgG in patient sera but several procedural issues affect OAP assembly and consequently test sensitivity. To date, the protein based ELISA test shows the lowest sensitivity while representing a valid alternative to the more sensitive cell based assay (CBA), which, however, shows economic, technical and interpretation problems. Here we have developed a high perfomance ELISA in which native OAPs are used as the molecular target. To this aim a native size exclusion chromatography method has been developed to isolate integral, highly pure and AQP4-IgG-recognized OAPs from rat brain. These OAPs were immobilized and oriented on a plastic plate by a sandwich approach and 139 human sera were tested, including 67 sera from NMO patients. The OAP-ELISA showed a 99% specificity and a higher sensitivity (91%) compared to the CBA test. A comparative analysis revealed an end-point titer three orders of magnitude higher than the commercial ELISA and six times higher than our in-house CBA test. We show that CNS-extracted OAPs are crucial elements in order to perform an efficient AQP4-IgG test and the OAP-ELISA developed represents a valid alternative to the CBA currently used.

Evaluation of aquaporin-4 antibody assays

Aquaporin-4 (AQP4) is a water channel protein that is most highly, but not exclusively, expressed in the central nervous system. In 2005 AQP4 was shown to be the antigenic target of neuromyelitis optica-immunoglobulin G (NMO-IgG, or AQP4-IgG), an antibody found specifically in patients with NMO and in formes frustes of NMO, such as longitudinally extensive transverse myelitis (LETM) or optic neuritis (ON). This discovery facilitated the clinical, pathological, and radiological distinction of NMO and the spectrum of NMO-related disorders from classical multiple sclerosis. In addition to its use as a diagnostic tool, AQP4-IgG predicts a high risk of relapse in patients with a clinically isolated syndrome of either LETM or ON. As disability in NMO is attack-related, early diagnosis and treatment are predicted to have a major effect on long-term disability. Thus, the importance of sensitive and specific assays to detect AQP4-IgG cannot be overstated. Both academic institutions and commercial companies have developed assays to identify AQP4-IgG in patients' sera or cerebrospinal fluid. Both AQP4 isoforms from different species have been used as the antigenic target in the form of frozen tissue sections in indirect immunofluorescence assays, partially purified protein for fluorescence immunoprecipitation assay, radioimmunoprecipita-tion assay or enzyme-linked immunosorbent assay, or transfected into cells for cell based assays or flow cytometry. We carried out a systematic review of the literature reporting different methodologies used to identify AQP4-IgG, examine whether longitudinal AQP4-IgG titers predict relapses in seropositive patients, and attempt to establish a reasonable timeframe for retesting negative serum samples.

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.

Evaluation of aquaporin-4 antibody assays

Aquaporin-4 (AQP4) is a water channel protein that is most highly, but not exclusively, expressed in the central nervous system. In 2005 AQP4 was shown to be the antigenic target of neuromyelitis optica-immunoglobulin G (NMO-IgG, or AQP4-IgG), an antibody found specifically in patients with NMO and in formes frustes of NMO, such as longitudinally extensive transverse myelitis (LETM) or optic neuritis (ON). This discovery facilitated the clinical, pathological, and radiological distinction of NMO and the spectrum of NMO-related disorders from classical multiple sclerosis. In addition to its use as a diagnostic tool, AQP4-IgG predicts a high risk of relapse in patients with a clinically isolated syndrome of either LETM or ON. As disability in NMO is attack-related, early diagnosis and treatment are predicted to have a major effect on long-term disability. Thus, the importance of sensitive and specific assays to detect AQP4-IgG cannot be overstated. Both academic institutions and commercial companies have developed assays to identify AQP4-IgG in patients' sera or cerebrospinal fluid. Both AQP4 isoforms from different species have been used as the antigenic target in the form of frozen tissue sections in indirect immunofluorescence assays, partially purified protein for fluorescence immunoprecipitation assay, radioimmunoprecipita-tion assay or enzyme-linked immunosorbent assay, or transfected into cells for cell based assays or flow cytometry. We carried out a systematic review of the literature reporting different methodologies used to identify AQP4-IgG, examine whether longitudinal AQP4-IgG titers predict relapses in seropositive patients, and attempt to establish a reasonable timeframe for retesting negative serum samples.

Aquaporin-4 autoantibodies in Neuromyelitis Optica: AQP4 isoform-dependent sensitivity and specificity

Neuromyelitis Optica (NMO) is an autoimmune demyelinating disease, characterized by the presence of autoantibody (NMO-IgG) to Aquaporin-4 (AQP4). NMO-IgG identification supports NMO diagnosis and several diagnostic tests have been developed, but their sensitivity is too variable, and some assay show low sensitivity. This impairs correct diagnosis of NMO. By cell based assay (CBA) we here evaluate the efficacy of different strategies to express AQP4 in mammalian cells in terms of: a) AQP4 translation initiation signals; b) AQP4 isoforms (M1 and M23) and fluorescent tag position; c) NMO serum concentration and AQP4 degradation. Our results demonstrate that when using AQP4-M1, the nucleotide in position -3 of the AUG greatly affects the AQP4-M1/M23 protein ratio, NMO-IgG binding, and consequently test sensitivity. Test sensitivity was highest with M23 expressing cells (97.5%) and only 27.5% with AQP4-M1. The fluorescent tag added to the N-terminus of AQP4-M23 considerably affected the NMO-IgG binding, and test sensitivity, due to disruption of AQP4 suprastructures. Furthermore, sera used at high concentration resulted in AQP4 degradation which affected test sensitivity. To further evaluate the reliability of the M23 based CBA test, samples of one NMO patient collected during about 2 years clinical follow-up were tested. The results of serum titer correlated with disease activity and treatment response. In conclusion, we provide a molecular explanation for the contrasting CBA test data reported and suggest the use of M23 with a C-terminus fluorescent tag as the proper test for NMO diagnosis.

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.