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The blood–brain barrier, which is formed by tightly interconnected microvascular endothelial cells, separates the brain from the peripheral circulation. Together with other central nervous system-resident cell types, including pericytes and astrocytes, the blood–brain barrier forms the neurovascular unit. Upon neuroinflammation, this barrier becomes leaky, allowing molecules and cells to enter the brain and to potentially harm the tissue of the central nervous system. Despite the significance of animal models in research, they may not always adequately reflect human pathophysiology. Therefore, human models are needed. This review will provide an overview of the blood–brain barrier in terms of both health and disease. It will describe all key elements of the in vitro models and will explore how different compositions can be utilized to effectively model a variety of neuroinflammatory conditions. Furthermore, it will explore the existing types of models that are used in basic research to study the respective pathologies thus far.

Encephalitis is a severe inflammatory disorder of the brain with many possible causes and a complex differential diagnosis. Advances in autoimmune encephalitis research in the past 10 years have led to the identification of new syndromes and biomarkers that have transformed the diagnostic approach to these disorders. However, existing criteria for autoimmune encephalitis are too reliant on antibody testing and response to immunotherapy, which might delay the diagnosis. We reviewed the literature and gathered the experience of a team of experts with the aims of developing a practical, syndrome-based diagnostic approach to autoimmune encephalitis and providing guidelines to navigate through the differential diagnosis. Because autoantibody test results and response to therapy are not available at disease onset, we based the initial diagnostic approach on neurological assessment and conventional tests that are accessible to most clinicians. Through logical differential diagnosis, levels of evidence for autoimmune encephalitis (possible, probable, or definite) are achieved, which can lead to prompt immunotherapy.

In the past few years, acquired demyelinating syndromes of the central nervous system associated with antibodies against myelin oligodendrocyte glycoprotein (MOG) have evolved into a new inflammatory disease entity distinct from neuromyelitis optica spectrum disorders or multiple sclerosis. The meticulous clinical description of patients with MOG IgG antibodies (MOG-IgG) has been achieved by development and use of highly specific cell-based assays. MOG-IgG associated disorders comprise a wide spectrum of syndromes ranging from acute disseminated encephalomyelitis predominantly in children to optic neuritis or myelitis mostly in adults. In recent studies, phenotype of MOG-IgG associated disorders has further broadened with the description of cases of brainstem encephalitis, encephalitis with seizures and overlap syndromes with other types of autoimmune encephalitis. In this review, we provide an overview of current knowledge of MOG-IgG associated disorders, describe the clinical presentations identified, highlight differences from neuromyelitis optica spectrum disorders and multiple sclerosis, summarize clinical outcome and concepts of immune treatment, depict the underlying mechanisms of antibody pathogenicity and provide the methodological essentials of MOG-IgG assays.

Serum antibodies directed against myelin oligodendrocyte glycoprotein (MOG) are found in patients with acquired CNS demyelinating syndromes that are distinct from multiple sclerosis and aquaporin-4-seropositive neuromyelitis optica spectrum disorder. Based on an extensive literature review and a structured consensus process, we propose diagnostic criteria for MOG antibody-associated disease (MOGAD) in which the presence of MOG-IgG is a core criterion. According to our proposed criteria, MOGAD is typically associated with acute disseminated encephalomyelitis, optic neuritis, or transverse myelitis, and is less commonly associated with cerebral cortical encephalitis, brainstem presentations, or cerebellar presentations. MOGAD can present as either a monophasic or relapsing disease course, and MOG-IgG cell-based assays are important for diagnostic accuracy. Diagnoses such as multiple sclerosis need to be excluded, but not all patients with multiple sclerosis should undergo screening for MOG-IgG. These proposed diagnostic criteria require validation but have the potential to improve identification of individuals with MOGAD, which is essential to define long-term clinical outcomes, refine inclusion criteria for clinical trials, and identify predictors of a relapsing versus a monophasic disease course.

Myelin oligodendrocyte glycoprotein (MOG), a member of the immunoglobulin (Ig) superfamily, is a myelin protein solely expressed at the outermost surface of myelin sheaths and oligodendrocyte membranes. This makes MOG a potential target of cellular and humoral immune responses in inflammatory demyelinating diseases. Due to its late postnatal developmental expression, MOG is an important marker for oligodendrocyte maturation. Discovered about 30 years ago, it is one of the best-studied autoantigens for experimental autoimmune models for multiple sclerosis (MS). Human studies, however, have yielded controversial results on the role of MOG, especially MOG antibodies (Abs), as a biomarker in MS. But with improved detection methods using different expression systems to detect Abs in patients' samples, this is meanwhile no longer the case. Using cell-based assays with recombinant full-length, conformationally intact MOG, several recent studies have revealed that MOG Abs can be found in a subset of predominantly pediatric patients with acute disseminated encephalomyelitis (ADEM), aquaporin-4 (AQP4) seronegative neuromyelitis optica spectrum disorders (NMOSD), monophasic or recurrent isolated optic neuritis (ON), or transverse myelitis, in atypical MS and in -methyl-d-aspartate receptor-encephalitis with overlapping demyelinating syndromes. Whereas MOG Abs are only transiently observed in monophasic diseases such as ADEM and their decline is associated with a favorable outcome, they are persistent in multiphasic ADEM, NMOSD, recurrent ON, or myelitis. Due to distinct clinical features within these diseases it is controversially disputed to classify MOG Ab-positive cases as a new disease entity. Neuropathologically, the presence of MOG Abs is characterized by MS-typical demyelination and oligodendrocyte pathology associated with Abs and complement. However, it remains unclear whether MOG Abs are a mere inflammatory bystander effect or truly pathogenetic. This article provides deeper insight into recent developments, the clinical relevance of MOG Abs and their role in the immunpathogenesis of inflammatory demyelinating disorders.

Myelin-oligodendrocyte glycoprotein antibody-associated disease (MOGAD) is a recently identified autoimmune disorder that presents in both adults and children as CNS demyelination. Although there are clinical phenotypic overlaps between MOGAD, multiple sclerosis, and aquaporin-4 antibody-associated neuromyelitis optica spectrum disorder (NMOSD) cumulative biological, clinical, and pathological evidence discriminates between these conditions. Patients should not be diagnosed with multiple sclerosis or NMOSD if they have anti-MOG antibodies in their serum. However, many questions related to the clinical characterisation of MOGAD and pathogenetic role of MOG antibodies are still unanswered. Furthermore, therapy is mainly based on standard protocols for aquaporin-4 antibody-associated NMOSD and multiple sclerosis, and more evidence is needed regarding how and when to treat patients with MOGAD.

In this Review, Reindl et al . discuss a range of CNS disorders that are known to be associated with autoantibodies against myelin oligodendrocyte glycoprotein (MOG). They examine the experimental evidence for a role for MOG autoantibodies in the pathogenesis of demyelinating CNS disorders such as multiple sclerosis and acute disseminated encephalomyelitis, and explore the potential of MOG to function as a biomarker in these 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. Key Points Myelin oligodendrocyte glycoprotein (MOG) is a target of demyelinating autoantibodies in animal models of inflammatory demyelinating diseases of the CNS Studies using cell-based immunoassays have identified MOG antibodies in human inflammatory demyelinating CNS diseases High-titre MOG antibodies are present in paediatric patients with acute disseminated encephalomyelitis, multiple sclerosis (MS), aquaporin-4-seronegative neuromyelitis optica, or isolated optic neuritis or transverse myelitis High-titre MOG antibodies are only rarely seen in adults with these demyelinating CNS disorders MOG antibodies might be associated with a broad spectrum of acquired human CNS demyelinating diseases The clinical presentation observed in anti-MOG-positive patients resembles the disease features in experimental animal models more closely than the clinical presentation of patients with MS

In this Review, Reindl and Waters provide an overview of what we currently know about anti-myelin oligodendrocyte glycoprotein antibodies and their association with demyelinating diseases, including the value of detection assays and evidence for antibody pathogenicity and its mechanism.

Background A subset of patients with neuromyelitis optica spectrum disorders (NMOSD) has been shown to be seropositive for myelin oligodendrocyte glycoprotein antibodies (MOG-IgG). Objective To describe the epidemiological, clinical, radiological, cerebrospinal fluid (CSF), and electrophysiological features of a large cohort of MOG-IgG-positive patients with optic neuritis (ON) and/or myelitis ( n  = 50) as well as attack and long-term treatment outcomes. Methods Retrospective multicenter study. Results The sex ratio was 1:2.8 (m:f). Median age at onset was 31 years (range 6-70). The disease followed a multiphasic course in 80% (median time-to-first-relapse 5 months; annualized relapse rate 0.92) and resulted in significant disability in 40% (mean follow-up 75 ± 46.5 months), with severe visual impairment or functional blindness (36%) and markedly impaired ambulation due to paresis or ataxia (25%) as the most common long-term sequelae. Functional blindness in one or both eyes was noted during at least one ON attack in around 70%. Perioptic enhancement was present in several patients. Besides acute tetra-/paraparesis, dysesthesia and pain were common in acute myelitis (70%). Longitudinally extensive spinal cord lesions were frequent, but short lesions occurred at least once in 44%. Fourty-one percent had a history of simultaneous ON and myelitis. Clinical or radiological involvement of the brain, brainstem, or cerebellum was present in 50%; extra-opticospinal symptoms included intractable nausea and vomiting and respiratory insufficiency (fatal in one). CSF pleocytosis (partly neutrophilic) was present in 70%, oligoclonal bands in only 13%, and blood-CSF-barrier dysfunction in 32%. Intravenous methylprednisolone (IVMP) and long-term immunosuppression were often effective; however, treatment failure leading to rapid accumulation of disability was noted in many patients as well as flare-ups after steroid withdrawal. Full recovery was achieved by plasma exchange in some cases, including after IVMP failure. Breakthrough attacks under azathioprine were linked to the drug-specific latency period and a lack of cotreatment with oral steroids. Methotrexate was effective in 5/6 patients. Interferon-beta was associated with ongoing or increasing disease activity. Rituximab and ofatumumab were effective in some patients. However, treatment with rituximab was followed by early relapses in several cases; end-of-dose relapses occurred 9-12 months after the first infusion. Coexisting autoimmunity was rare (9%). Wingerchuk’s 2006 and 2015 criteria for NMO(SD) and Barkhof and McDonald criteria for multiple sclerosis (MS) were met by 28%, 32%, 15%, 33%, respectively; MS had been suspected in 36%. Disease onset or relapses were preceded by infection, vaccination, or pregnancy/delivery in several cases. Conclusion Our findings from a predominantly Caucasian cohort strongly argue against the concept of MOG-IgG denoting a mild and usually monophasic variant of NMOSD. The predominantly relapsing and often severe disease course and the short median time to second attack support the use of prophylactic long-term treatments in patients with MOG-IgG-positive ON and/or myelitis.

Neuromyelitis optica spectrum disorder (NMOSD) is a rare neurological autoimmune disease caused by autoantibodies targeting the astrocytic water channel aquaporin-4 (AQP4). Binding to AQP4 initiates the activation of innate immune components, especially the complement system. Both in-vivo and in-vitro models have been developed to study the molecular pathophysiology of NMOSD. Our study aimed to characterize the molecular response of four human cell lines (AQP4-ECFP expressing U-87MG glioblastoma cells, U-87MG expressing only ECFP, HEK293 cells expressing AQP4-EmGFP, and human primary astrocytes) to a treatment with AQP4 antibody E5415A and human complement. Complement-dependent cytotoxicity was induced by this treatment in AQP4-expressing cells by the terminal complement pathway. Transcriptomic profiles of the in-vitro U-87MG-AQP4-ECFP model and an in-vivo rat model shared a proinflammatory shift towards NF-κB and interleukin-6 pathways. These findings were confirmed at both the mRNA and protein levels, and treatment with serum samples from AQP4 antibody seropositive NMOSD patients resulted in a similar response. Additionally, NF-κB upregulation was shown by immunohistochemistry in medulla oblongata lesions of NMOSD patients. In conclusion, interleukin-6 and NF-κB pathways play a key role in inflammation caused by the activation of the terminal complement pathway in a human cellular model of NMOSD using U-87MG-AQP4-ECFP cells.