Immune-mediated neuropathies.

Since the discovery of an acute monophasic paralysis, later coined Guillain-Barré syndrome, almost 100 years ago, and the discovery of chronic, steroid-responsive polyneuropathy 50 years ago, the spectrum of immune-mediated polyneuropathies has broadened, with various subtypes continuing to be identified, including chronic inflammatory demyelinating polyradiculoneuropathy (CIDP) and multifocal motor neuropathy (MMN). In general, these disorders are speculated to be caused by autoimmunity to proteins located at the node of Ranvier or components of myelin of peripheral nerves, although disease-associated autoantibodies have not been identified for all disorders. Owing to the numerous subtypes of the immune-mediated neuropathies, making the right diagnosis in daily clinical practice is complicated. Moreover, treating these disorders, particularly their chronic variants, such as CIDP and MMN, poses a challenge. In general, management of these disorders includes immunotherapies, such as corticosteroids, intravenous immunoglobulin or plasma exchange. Improvements in clinical criteria and the emergence of more disease-specific immunotherapies should broaden the therapeutic options for these disabling diseases.

Anti-MAG neuropathy: Role of IgM antibodies, the paranodal junction and juxtaparanodal potassium channels.

OBJECTIVE: To improve understanding of disease pathophysiology in anti-myelin-associated glycoprotein (anti-MAG) neuropathy to guide further treatment approaches. METHODS: Anti-MAG neuropathy patients underwent clinical assessments, nerve conduction and excitability studies, and ultrasound assessment. RESULTS: Patients demonstrated a distinctive axonal excitability profile characterised by a reduction in superexcitability [MAG: -14.2 ±â€¯1.6% vs healthy controls (HC): -21.8 ±â€¯1.2%; p < 0.01] without alterations in most other excitability parameters. Mathematical modelling of nerve excitability recordings suggested that changes in axonal function could be explained by a 72.5% increase in juxtaparanodal fast potassium channel activation and an accompanying hyperpolarization of resting membrane potential (by 0.3 mV) resulting in a 94.2% reduction in discrepancy between anti-MAG data and the healthy control model. Superexcitability changes correlated strongly with clinical and neurophysiological parameters. Furthermore, structural assessments demonstrated a proximal pattern of nerve enlargement (C6 nerve root cross-sectional area: 15.9 ±â€¯8.1 mm2 vs HC: 9.1 ±â€¯2.3 mm2; p < 0.05). CONCLUSIONS: The imaging and neurophysiological results support the pathogenicity of anti-MAG IgM. Widening between adjacent loops of paranodal myelin due to antibodies would expand the pathway from the node to the juxtaparanode, increasing activation of juxtaparanodal fast potassium channels, thereby impairing saltatory conduction. SIGNIFICANCE: Potassium channel blockers may prove beneficial in restoring conduction closer to its normal state and improving nerve function in anti-MAG neuropathy.

Neurofascin-155 IGG4 Neuropathy: Pathophysiological Insights, Spectrum of Clinical Severity and Response To treatment.

INTRODUCTION: Sensorimotor neuropathy associated with IgG4 antibodies to neurofascin-155 (NF155) was recently described. The clinical phenotype is typically associated with young onset, distal weakness, and in some cases, tremor. METHODS: From a consecutive cohort of 55 patients diagnosed with chronic inflammatory demyelinating polyneuropathy, screening for anti-NF155 antibodies was undertaken. Patients underwent clinical assessment, diagnostic neurophysiology, including peripheral axonal excitability studies and nerve ultrasound. RESULTS: Three of 55 chronic inflammatory demyelinating polyneuropathy patients (5%) tested positive for anti-NF155 IgG4. Patients presenting with more severe disease had higher antibody titers. Ultrasound demonstrated diffuse nerve enlargement. Axonal excitability studies were markedly abnormal, with subsequent mathematical modeling of the results supporting disruption of the paranodal seal. DISCUSSION: A broad spectrum of disease severity and treatment response may be observed in anti-NF155 neuropathy. Excitability studies support the pathogenic role of anti-NF155 IgG4 antibodies targeting the paranodal region. Muscle Nerve 57: 848-851, 2018.

Motor unit remodelling in multifocal motor neuropathy: The importance of axonal loss.

OBJECTIVE: To estimate the degree of axonal loss in patients diagnosed with multifocal motor neuropathy (MMN) using a novel assessment of motor unit numbers and size. METHODS: Automated motor unit number estimation using a compound muscle action potential (CMAP) scan was undertaken in median nerves with conduction block. Results were compared with 30 age-matched healthy controls. RESULTS: Compared with healthy controls, MMN patients had fewer motor units (MMN: 33±11vs HC: 93±36 [mean±SD]; p<0.0001) and larger 'size of the largest unit' (MMN: 1.2±0.5mVvs HC: 0.4±0.1mV; p<0.0001), despite having normal distal CMAP amplitudes (MMN: 7.6±1.8mVvs HC: 8.7±2.5mV; p=0.24). CONCLUSIONS: MMN is associated with marked axonal loss which may be masked by striking re-innervation resulting in preservation of distal CMAP amplitudes. SIGNIFICANCE: Assessment of motor unit properties should be incorporated into assessment of disease progression in MMN, given that nerve conduction studies are insensitive to motor unit remodelling.

Chronic inflammatory demyelinating polyradiculoneuropathy: from pathology to phenotype.

Chronic inflammatory demyelinating polyradiculoneuropathy (CIDP) is an inflammatory neuropathy, classically characterised by a slowly progressive onset and symmetrical, sensorimotor involvement. However, there are many phenotypic variants, suggesting that CIDP may not be a discrete disease entity but rather a spectrum of related conditions. While the abiding theory of CIDP pathogenesis is that cell-mediated and humoral mechanisms act together in an aberrant immune response to cause damage to peripheral nerves, the relative contributions of T cell and autoantibody responses remain largely undefined. In animal models of spontaneous inflammatory neuropathy, T cell responses to defined myelin antigens are responsible. In other human inflammatory neuropathies, there is evidence of antibody responses to Schwann cell, compact myelin or nodal antigens. In this review, the roles of the cellular and humoral immune systems in the pathogenesis of CIDP will be discussed. In time, it is anticipated that delineation of clinical phenotypes and the underlying disease mechanisms might help guide diagnostic and individualised treatment strategies for CIDP.

Antibodies to MOG have a demyelination phenotype and affect oligodendrocyte cytoskeleton.

OBJECTIVE: To examine the clinical features of pediatric CNS demyelination associated with positive myelin oligodendrocyte glycoprotein (MOG) antibodies and to examine the functional effects of MOG antibody on oligodendrocyte cytoskeleton. METHODS: We measured MOG antibody using a fluorescence-activated cell sorting live cell-based assay in acute sera of 73 children with CNS demyelination (DEM) (median age 8 years, range 1.3-15.3) followed for a median of 4 years. We used MO3.13 cells to examine immunoglobulin (Ig) G effects on oligodendrocyte cytoskeleton using 3D deconvolution imaging. RESULTS: MOG antibodies were found in 31/73 patients with DEM (42%) but in 0/24 controls. At first presentation, MOG antibody-positive patients were more likely to have bilateral than unilateral optic neuritis (ON) (9/10 vs 1/5, respectively, p = 0.03), less likely to have brainstem findings (2/31 vs 16/42, p = 0.005), more likely to have a raised erythrocyte sedimentation rate >20 mm/h (9/19 vs 3/21, p = 0.05), less likely to have intrathecal oligoclonal bands (0/16 vs 5/27, p = 0.18), and less likely to be homozygous or heterozygous for human leukocyte antigen DRB1*1501 (3/18 vs 7/22, p = 0.46). MOG antibody positivity varied according to clinical phenotype, with ON and relapsing ON most likely to be seropositive. Two relapsing MOG antibody-positive patients treated with mycophenolate mofetil remain in remission and have become MOG antibody seronegative. Oligodendrocytes incubated with purified IgG from MOG antibody-positive patients showed a striking loss of organization of the thin filaments and the microtubule cytoskeleton, as evidenced by F-actin and ß-tubulin immunolabelings. CONCLUSIONS: MOG antibody may define a separate demyelination syndrome, which has therapeutic implications. MOG antibody has functional effects on oligodendrocyte cytoskeleton.

Antibodies to neurofascin exacerbate adoptive transfer experimental autoimmune neuritis.

Guillain-Barré syndrome and chronic inflammatory demyelinating polyneuropathy are autoimmune disorders of the peripheral nervous system in which autoantibodies are implicated in the disease pathogenesis. Recent work has focused on the nodal regions of the myelinated axon as potential autoantibody targets. Here we screened patient sera for autoantibodies to neurofascin and assessed the pathophysiological relevance of anti-neurofascin antibodies in vivo. Levels of anti-neurofascin antibodies were higher in sera from patients with Guillain-Barré syndrome and chronic inflammatory demyelinating polyneuropathy when compared with those of controls. Anti-neurofascin antibodies exacerbated and prolonged adoptive transfer experimental autoimmune neuritis and caused conduction defects when injected intraneurally.

Clinical implications of Schwann cell biology.

The neuroglia of the peripheral nervous system (PNS) are derived from the neural crest and are a diverse family of cells. They consist of myelinating Schwann cells, non-myelinating Schwann cells, satellite cells, and perisynaptic Schwann cells. Due to their prominent role in the formation of myelin, myelinating Schwann cells are the best recognised of these cells. However, Schwann cells and the other neuroglia of the PNS have many functions that are independent of myelination and contribute significantly to the functioning of the peripheral nerve in both health and disease. Here we discuss the contribution of PNS neuroglial cells to clinical deficit in neurodegenerative disease, peripheral neuropathy, and pain.

An update on Schwann cell biology--immunomodulation, neural regulation and other surprises.

Schwann cells are primarily discussed in the context of their ability to form myelin. However there are many subtypes of these neural crest derived cells including satellite cells of the dorsal root ganglia and autonomic ganglia, the perisynaptic Schwann cells of the neuromuscular junction and the non-myelin forming Schwann cells which ensheathe the unmyelinated fibres of the peripheral nervous system which are about 80% of peripheral nerves. This review discusses the many functions of these Schwann cell subsets including their seminal role in axonal ensheathment, perineuronal organisation, maintenance of normal neural function, synapse formation, response to damage and repair and an increasingly recognised active role in pain syndromes.

Chronic inflammatory demyelinating polyneuropathy.

Chronic inflammatory demyelinating polyneuropathy (CIDP) is the commonest treatable neuropathy in the western world. Untreated it may result in severe disability but if diagnosed and treated early there is effective treatment for the majority of patients. Typical CIDP is readily recognised but the diagnosis of other subgroups can be more challenging. The pathology of polyradiculoneuropathies such as CIDP characteristically affects the most proximal regions of the peripheral nervous system, nerve roots and major plexuses. It is important to test these regions with electrodiagnostic studies since routine neurophysiology may not encounter regions of pathology. Although accepted as an autoimmune disorder with an underlying immunopathology involving T cell and B cell responses, there is no agreement on major target antigens; however recent studies have highlighted a role for molecules in non compact myelin which play an essential role in the formation and maintenance of the nodal structures and hence in the function of ion channels central to saltatory conduction. Controlled trials have proven the efficacy of corticosteroid, intravenous immunoglobulin and plasma exchange in the short term and intravenous immunoglobulin also in the long term. Immunosuppressive agents are widely used but their efficacy has not been proven in controlled trials. Recent trials have shown the importance of attempting treatment withdrawal in patients apparently in remission to conserve treatments that are very expensive and in short supply, since a significant proportion of patients may enter long lasting remission following short term therapy. For the relatively small group of patients who do not respond to these first line therapies new agents including monoclonal antibodies may have a role.

Neurofascin as a novel target for autoantibody-mediated axonal injury.

Axonal injury is considered the major cause of disability in patients with multiple sclerosis (MS), but the underlying effector mechanisms are poorly understood. Starting with a proteomics-based approach, we identified neurofascin-specific autoantibodies in patients with MS. These autoantibodies recognize the native form of the extracellular domains of both neurofascin 186 (NF186), a neuronal protein concentrated in myelinated fibers at nodes of Ranvier, and NF155, the oligodendrocyte-specific isoform of neurofascin. Our in vitro studies with hippocampal slice cultures indicate that neurofascin antibodies inhibit axonal conduction in a complement-dependent manner. To evaluate whether circulating antineurofascin antibodies mediate a pathogenic effect in vivo, we cotransferred these antibodies with myelin oligodendrocyte glycoprotein-specific encephalitogenic T cells to mimic the inflammatory pathology of MS and breach the blood-brain barrier. In this animal model, antibodies to neurofascin selectively targeted nodes of Ranvier, resulting in deposition of complement, axonal injury, and disease exacerbation. Collectively, these results identify a novel mechanism of immune-mediated axonal injury that can contribute to axonal pathology in MS.