Severe Stiff-Person Syndrome After COVID: The First Video-Documented COVID Exacerbation and Viral Implications

OBJECTIVES

To describe a patient with mild GAD-positive stiff-leg syndrome (SLS) who developed severely disabling stiff-person syndrome (SPS) 1 week after mild COVID-19 and discuss the impact of viral implications.

METHODS

Video-documented serial clinical observations at baseline, after acute COVID-19, and after IVIG treatments.

RESULTS

A 39-year-old man with left-SLS was stable during a 2-year follow-up with low-dose antispasmodics, working fully and functioning normally, even able to run. One week after mild COVID-19, he started to experience generalized SPS symptomatology that steadily worsened the following 2-3 weeks, becoming unable to walk, requiring a walker, with significant thoracolumbar and bilateral leg stiffness and spasms. GAD ab were very high. After 3 monthly IVIg infusions he showed improvements, but his gait remains significantly stiff. All clinical changes, from baseline to post-Covid, and then post- IVIg have been video-documented.

DISCUSSION

This is the first, clearly documented, severe GAD-positive SPS after COVID-19. Although viral or postviral causation can be incidental, the temporal connection with acute COVID-19, the severe disease worsening after symptom-onset, and the subsequent steady improvement after IVIg, suggest viral-triggered autoimmunity. Because COVID-19 reportedly can trigger or worsen GAD-associated diabetes type 1 through proinflammatory mediators, and SPS has been reportedly triggered by West Nile Virus, possibly through molecular mimicry, this case of acutely converting GAD-SLS to GAD-SPS suggest the need to explore viral etiologies in patients with GAD-SPS experiencing acute, long-lasting episodic exacerbations of stiffness and spasms.

Late-onset stiff-person syndrome: challenges in diagnosis and management

Background

Stiff person syndrome (SPS) is a rare slowly progressive autoimmune neuronal hyperexcitability disease with very-high GAD-65 antibody titers that most commonly presents above the age of 20, with muscle stiffness, painful muscle spasms, slow gait, and falls leading to disability. In other autoimmune disorders, late-onset disease has different symptom-spectrum and outcomes, but there is no information regarding late-onset SPS (LOSPS).

Objective

Highlight delayed diagnosis and poor tolerance or incomplete response to therapies of patients with LOSPS and outline how best to increase disease awareness early at onset.

Design A retrospective chart reviewMethods

We reviewed GAD-positive SPS patients with symptom onset above age 60, identified among 54 SPS patients, examined, treated and followed-up by the same clinicians, focused on clinical presentation, misdiagnoses, response and tolerance to therapies, and evolved disability.

Results

Nine patients had LOSPS with symptom onset at median age of 61 years (range 60-78), and current median age of 73. The median time from symptom onset to SPS diagnosis was 3 years; prior to diagnosis, five patients were treated for lumbosacral radiculopathies (one with laminectomy), two for Parkinson's disease, one for multiple sclerosis, and another for cerebellar degeneration. Progressive decline occurred rapidly in all patients; at time of diagnosis, six patients were already using a cane or walker and two were wheelchair-bound. Tolerance and response to treatment were limited; two patients did not respond to IVIg, two discontinued IVIg despite early response due to comorbidities (cardiac disease, thrombosis), four others partially responded to IVIg and one to rituximab; several could not tolerate high doses of oral antispasmodics due to somnolence; and two patients died.

Conclusions

LOSPS is almost always misdiagnosed for other similar conditions commonly seen in the elderly. Patients with LOSPS decline quickly to clinically severe disease due to delayed treatment initiation, poor response or tolerance, other comorbidities, and possibly immunosenescence. Increased awareness that SPS can occur in the elderly mimicking other disorders is important for early diagnosis and treatment, even necessitating earlier immunotherapy initiation, compared to their younger counterparts, to prevent faster-evolving severe disability.

Objectivity, practicality, and significance of practice guidelines for the practicing neurologists: What we learnt from consensus criteria in CIDP, Myasthenia Gravis and Inflammatory Myopathies

The value of practice guidelines in the three most common autoimmune neuromuscular disorders, namely Chronic Inflammatory Demyelinating Polyneuropathy (CIDP), Myasthenia Gravis (MG) and Autoimmune Inflammatory Myopathies (AIM), has been extensively debated regarding their usefulness in clinical practice, objectivity and universal value considering that guidelines are also established regionally in certain countries. This commentary highlights common concerns on how guidelines are presently generated, pointing out: (a) non-sufficient diversity among Task-Force members to identify and address not only routine clinical and electrophysiology issues but also immunology, imaging, pathology, biomarkers, epidemiology or treatment economics; (b) Task-Force being often comprised by the same or seemingly like-minded members conveying the erroneous impression that experts with opposing views might have been excluded, even if this is clearly not the case; and (c) relying on web-based registries or retrospective data collections from heterogeneous sources. As a result, the existing practice guidelines in CIDP, MG and AIM remain an unfinished business but an excellent base for further enhancement. Guidelines can be extremely helpful not only for clinical trials but also in clinical practice if viewed as a living document with continuously updated versions by experts even with opposing views with precise information on diagnostics, pathomechanisms, therapeutic schemes, evolving biomarkers and economics of new therapies with validation of the post-guidelines criteria. Geographic diversity should be taken into consideration because the availability of biomarker testing, and therapies differ among countries. Patient preferences need to be also considered in therapeutic guidelines because newly marketed drugs offer more options steadily changing the therapeutic algorithms in autoimmune neuromuscular diseases generating also questions as to whether they also influence decisions on insurance coverage. Collectively, these startup considerations are aimed to make practice guidelines more objective, widely acceptable worldwide and more practical or easier to follow in clinical practice.

Spatial and Ecological Factors Modulate the Incidence of Anti-NMDAR Encephalitis-A Systematic Review

Anti-NMDAR encephalitis has been associated with multiple antigenic triggers (i.e., ovarian teratomas, prodromal viral infections) but whether geographic, climatic, and environmental factors might influence disease risk has not been explored yet. We performed a systematic review and a meta-analysis of all published papers reporting the incidence of anti-NMDAR encephalitis in a definite country or region. We performed several multivariate spatial autocorrelation analyses to analyze the spatial variations in the incidence of anti-NMDA encephalitis depending on its geographical localization and temperature. Finally, we performed seasonal analyses in two original datasets from France and Greece and assessed the impact of temperature using an exposure-lag-response model in the French dataset. The reported incidence of anti-NMDAR encephalitis varied considerably among studies and countries, being higher in Oceania and South America (0.2 and 0.16 per 100,000 persons-year, respectively) compared to Europe and North America (0.06 per 100,000 persons-year) (p < 0.01). Different regression models confirmed a strong negative correlation with latitude (Pearson's R = -0.88, p < 0.00001), with higher incidence in southern hemisphere countries far from the equator. Seasonal analyses showed a peak of cases during warm months. Exposure-lag-response models confirmed a positive correlation between extreme hot temperatures and the incidence of anti-NMDAR encephalitis in France (p = 0.03). Temperature analyses showed a significant association with higher mean temperatures and positive correlation with higher ultraviolet exposure worldwide. This study provides the first evidence that geographic and climatic factors including latitude, mean annual temperature, and ultraviolet exposure, might modify disease risk.

Stiff person spectrum disorder diagnosis, misdiagnosis, and suggested diagnostic criteria

BACKGROUND

Stiff person spectrum disorder (SPSD) is heterogeneous, and accurate diagnosis can be challenging.

METHODS

Patients referred for diagnosis/suspicion of SPSD at the Mayo Autoimmune Neurology Clinic from July 01, 2016, to June 30, 2021, were retrospectively identified. SPSD diagnosis was defined as clinical SPSD manifestations confirmed by an autoimmune neurologist and seropositivity for high-titer GAD65-IgG (>20.0 nmol/L), glycine-receptor-IgG or amphiphysin-IgG, and/or confirmatory electrodiagnostic studies (essential if seronegative). Clinical presentation, examination, and ancillary testing were compared to differentiate SPSD from non-SPSD.

RESULTS

Of 173 cases, 48 (28%) were diagnosed with SPSD and 125 (72%) with non-SPSD. Most SPSD were seropositive (41/48: GAD65-IgG 28/41, glycine-receptor-IgG 12/41, amphiphysin-IgG 2/41). Pain syndromes or functional neurologic disorder were the most common non-SPSD diagnoses (81/125, 65%). SPSD patients more commonly reported exaggerated startle (81% vs. 56%, p = 0.02), unexplained falls (76% vs. 46%, p = 0.001), and other associated autoimmunity (50% vs. 27%, p = 0.005). SPSD more often had hypertonia (60% vs. 24%, p < 0.001), hyperreflexia (71% vs. 43%, p = 0.001), and lumbar hyperlordosis (67% vs. 9%, p < 0.001) and less likely functional neurologic signs (6% vs. 33%, p = 0.001). SPSD patients more frequently had electrodiagnostic abnormalities (74% vs. 17%, p < 0.001), and at least moderate symptomatic improvement with benzodiazepines (51% vs. 16%, p < 0.001) or immunotherapy (45% vs. 13% p < 0.001). Only 4/78 non-SPSD patients who received immunotherapy had alternative neurologic autoimmunity.

INTERPRETATION

Misdiagnosis was threefold more common than confirmed SPSD. Functional or non-neurologic disorders accounted for most misdiagnoses. Clinical and ancillary testing factors can reduce misdiagnosis and exposure to unnecessary treatments. SPSD diagnostic criteria are suggested.

Therapies in Stiff-Person Syndrome: Advances and Future Prospects Based on Disease Pathophysiology

Among the glutamic acid decarboxylase (GAD)-antibody-spectrum disorders, the most common phenotypic subset is the stiff-person syndrome (SPS), caused by impaired GABAergic inhibitory neurotransmission and autoimmunity characterized by very high titers of GAD antibodies and increased GAD-IgG intrathecal synthesis. If not properly treated or untreated because of delayed diagnosis, SPS progresses leading to disability; it is therefore fundamental to apply the best therapeutic schemes from the outset. This article is focused on the rationale of specific therapeutic strategies based on the SPS pathophysiology targeting both the impaired reciprocal GABAergic inhibition to symptomatically improve the main clinical manifestations of stiffness in the truncal and proximal limb muscles, gait dysfunction, and episodic painful muscle spasms and the autoimmunity to enhance improvement and slow down disease progression. A practical, step-by-step therapeutic approach is provided, highlighting the importance of combination therapies with the preferred gamma-aminobutyric acid-enhancing antispasmodic drugs, such as baclofen, tizanidine, benzodiazepines, and gabapentin, that provide the first-line symptomatic therapy, while detailing the application of current immunotherapies with intravenous immunoglobulin (IVIg) plasmapheresis, and rituximab. The pitfalls and concerns of long-term therapies in different age groups, including children, women planning pregnancy, and especially the elderly considering their comorbidities are emphasized, also highlighting the challenges in distinguishing the conditioning effects or expectations of chronically applied therapies from objective meaningful clinical benefits. Finally, the need for future targeted immunotherapeutic options based on disease immunopathogenesis and the biologic basis of autoimmune hyperexcitability are discussed, pointing out the unique challenges in the design of future controlled clinical trials especially in quantifying the extend and severity of stiffness, episodic or startle-triggered muscle spasms, task-specific phobias, and excitability.

Repurposing MS immunotherapies for CIDP and other autoimmune neuropathies: unfulfilled promise or efficient strategy?

Despite advances in the treatment of chronic inflammatory demyelinating polyradiculoneuropathy (CIDP) and other common autoimmune neuropathies (AN), still-many patients with these diseases do not respond satisfactorily to the available treatments. Repurposing of disease-modifying therapies (DMTs) from other autoimmune conditions, particularly multiple sclerosis (MS) and neuromyelitis optica spectrum disorders (NMOSD), is a promising strategy that may accelerate the establishment of novel treatment choices for AN. This approach appears attractive due to homologies in the pathogenesis of these diseases and the extensive post-marketing experience that has been gathered from treating MS and NMOSD patients. The idea is also strengthened by a number of studies that explored the efficacy of DMTs in animal models of AN but also in some CIDP patients. We here review the available preclinical and clinical data of approved MS therapeutics in terms of their applicability to AN, especially CIDP. Promising therapeutic approaches appear to be B cell-directed and complement-targeting strategies, such as anti-CD20/anti-CD19 agents, Bruton's tyrosine kinase inhibitors and anti-C5 agents, as they exert their effects in the periphery. This is a major advantage because, in contrast to MS, their action in the periphery is sufficient to exert significant immunomodulation.

Autoimmune inflammatory myopathies

The autoimmune inflammatory myopathies constitute a heterogeneous group of acquired myopathies that have in common the presence of endomysial inflammation and moderate to severe muscle weakness. Based on currently evolved distinct clinical, histologic, immunopathologic, and autoantibody features, these disorders can be best classified as dermatomyositis, necrotizing autoimmune myositis, antisynthetase syndrome-overlap myositis, and inclusion body myositis. Although polymyositis is no longer considered a distinct subset but rather an extinct entity, it is herein described because its clinicopathologic information has provided over many years fundamental information on T-cell-mediated myocytotoxicity, especially in reference to inclusion body myositis. Each inflammatory myopathy subset has distinct immunopathogenesis, prognosis, and response to immunotherapies, necessitating the need to correctly diagnose each subtype from the outset and avoid disease mimics. The paper describes the main clinical characteristics that aid in the diagnosis of each myositis subtype, highlights the distinct features on muscle morphology and immunopathology, elaborates on the potential role of autoantibodies in pathogenesis or diagnosis , and clarifies common uncertainties in reference to putative triggering factors such as statins and viruses including the 2019-coronavirus-2 pandemic. It extensively describes the main autoimmune markers related to autoinvasive myocytotoxic T-cells, activated B-cells, complement, cytokines, and the possible role of innate immunity. The concomitant myodegenerative features seen in inclusion body myositis along with their interrelationship between inflammation and degeneration are specifically emphasized. Finally, practical guidelines on the best therapeutic approaches are summarized based on up-to-date knowledge and controlled studies, highlighting the prospects of future immunotherapies and ongoing controversies.

Intravenous immunoglobulin in chronic inflammatory demyelinating polyradiculoneuropathy (CIDP): mechanisms of action and clinical and genetic considerations

INTRODUCTION

Chronic inflammatory demyelinating polyradiculoneuropathy (CIDP) is an autoimmune peripheral nerve disorder that is characterized by subacute onset, progressive or relapsing weakness, and sensory deficits. Proven treatments include intravenous immunoglobulin (IVIg), corticosteroids, and plasma exchange. This review focuses on the mechanisms of action, pharmacodynamics, genetic variations, and disease characteristics that can affect the efficacy of IVIg.

AREAS COVERED

The proposed mechanisms of action of IVIg that can mediate its therapeutic effects are reviewed. These include anti-idiotypic interactions, inhibition of neonatal Fc receptors (FcRn), anti-complement activity, upregulation of inhibitory FcγRIIB receptors, and downregulation of macrophage activation or co-stimulatory and adhesion molecules. Clinical and genetic factors that can affect the therapeutic response include misdiagnosis, degree of axonal damage, pharmacokinetic variability, and genetic variations.

EXPERT OPINION

The mechanisms of action of IVIg in CIDP and their relative contribution to its efficacy are subject of ongoing investigation. Studies in other autoimmune neurological conditions, in addition, highlight the role of key immunopathological pathways and factors that are likely to be affected. Further investigation into the pathogenesis of CIDP and the mechanisms of action of IVIg may lead to the development of improved diagnostics, better utilization of IVIg, and more targeted and effective therapies.

Long-term Effectiveness of IVIg Maintenance Therapy in 36 Patients With GAD Antibody–Positive Stiff-Person Syndrome

Background and Objectives

IVIg has been the preferred immunotherapy in stiff-person syndrome (SPS) based on a 3-month controlled trial, but whether it is also effective in inducing long-term benefits or arresting disease progression is unknown. The information is needed because SPS is a progressively disabling disease and IVIg is liberally used as chronic therapy without efficacy data. The present study explores the long-term effects of IVIg in the largest cohort of well-characterized patients with SPS followed by the same clinicians over 10 years.

Methods

Data of 36 patients (32 glutamic acid decarboxylase [GAD] positive), diagnosed and treated with monthly maintenance IVIg by the same neurologists, were analyzed. Response was assessed by physician-observed changes, patients' reports of symptom improvement, modified Rankin Scale (mRS) scores, and dependency trials evaluating symptom recurrence after stopping IVIg, prolonging infusion frequency, decreasing monthly dose, or wearing-off effects in between doses. Clinically meaningful long-term response was defined by improved mRS scores, improvement in physician-assessed stiffness, balance and gait, and functional decline with dependency trials.

Results

Twenty-four of 36 (67%) patients had clinically meaningful response over a median 40-month period. Patients with improved mRS scores by 1–2 points manifested improved gait, posture, balance and decreased stiffness, spasms, and startle response; some patients using a wheelchair and those ambulating with devices walked unassisted. In 25% of responders, treatment benefit was sustained for a 40-month median period, but in 29.1%, it declined over a 39-month period; 12.5% exhibited a conditioning effect. Three of 5 patients with cerebellar GAD-SPS variant also improved over time. The 12 patients who did not respond the first 3 months remained unresponsive even if IVIg continued for several months.

Discussion

This is a large study in 36 patients with SPS demonstrating that monthly maintenance IVIg therapy offers long-term benefits in 67% of patients for a median 3.3-year period. Because 29.1% experienced diminishing benefit over time due to disease progression, the study highlights the need for more effective therapies.

The role of complement and complement therapeutics in neuromyelitis optica spectrum disorders

INTRODUCTION

Neuromyelitis optica spectrum disorders (NMOSD) are characterized in the majority of cases by the presence of IgG1 autoantibodies against aquaporin 4 (AQP4) and myelin-oligodendrocyte glycoprotein (MOG), both capable of activating complement.

AREAS COVERED

We review evidence of complement involvement in NMOSD pathophysiology from pathological, in vitro, in vivo, human studies, and clinical trials.

EXPERT OPINION

In AQP4 NMOSD, complement deposition is a prominent pathological feature, while in vitro and in vivo studies have demonstrated complement-dependent pathogenicity of AQP4 antibodies. Consistent with these studies, the anti-C5 monoclonal antibody eculizumab was remarkably effective and safe in a phase 2/3 trial of AQP4-NMOSD patents leading to FDA-approved indication. Several other anti-complement agents, either approved or in trials for other neuro-autoimmunities, like myasthenia, CIDP, and GBS, are also relevant to NMOSD generating an exciting group of evolving immunotherapies. Limited but compelling in vivo and in vitro data suggest that anti-complement therapeutics may be also applicable to a subset of MOG NMOSD patients with severe disease. Overall, anticomplement agents, along with the already approved anti-IL6 and anti-CD19 monoclonal antibodies sartralizumab and inebilizumab, are rapidly changing the therapeutic algorithm in NMOSD, a previously difficult-to-treat autoimmune neurological disorder.

Role of complement, anti-complement therapeutics, and other targeted immunotherapies in myasthenia gravis

INTRODUCTION

Several patients with myasthenia gravis (MG) do not adequately respond to available drugs or exhibit poor tolerance, necessitating the need for new therapies.

AREAS COVERED

The paper discusses the rapidly evolving target-specific immunotherapies that promise long-standing remissions in the management of MG. It is specifically focused on the role of complement, anti-complement therapeutics, and the anti-FcRn and B cell monoclonals.

EXPERT OPINION

Anti-AChR antibodies cause internalization of the receptors and activate complement leading to in situ MAC formation that damages the post-synaptic membrane of the neuromuscular junction. Inhibiting MAC formation by antibodies targeting key complements subcomponents is a reasonable therapeutic goal. Indeed, the anti-C5 monoclonal antibodies, Eculizumab, Ravulizumab, and Zilucoplan, have been successfully tested in MG with Eculizumab first and now Ravulizumab FDA-approved for refractory MG based on sustained long-term benefits. Among the biologics that inhibit FcRn, Efgartigimod caused rapid reduction of the circulating IgG in the lysosomes, and induced sustained clinical remission with good safety profile leading to FDA-approved indication. Anti-B cell agents, like Rituximab, can induce sustained long-term remissions, especially in IgG4 antibody-mediated Musk-MG, by targeting short-lived antibody-secreting plasmablasts. These biologics offer effective targeted immunotherapies with good tolerance promising to change the therapeutic algorithm in the chronic MG management.

Peripheral Neuropathy Evaluations of Patients With Prolonged Long COVID

BACKGROUND AND OBJECTIVES

Recovery from severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection appears exponential, leaving a tail of patients reporting various long COVID symptoms including unexplained fatigue/exertional intolerance and dysautonomic and sensory concerns. Indirect evidence links long COVID to incident polyneuropathy affecting the small-fiber (sensory/autonomic) axons.

METHODS

We analyzed cross-sectional and longitudinal data from patients with World Health Organization (WHO)-defined long COVID without prior neuropathy history or risks who were referred for peripheral neuropathy evaluations. We captured standardized symptoms, examinations, objective neurodiagnostic test results, and outcomes, tracking participants for 1.4 years on average.

RESULTS

Among 17 patients (mean age 43.3 years, 69% female, 94% Caucasian, and 19% Latino), 59% had ≥1 test interpretation confirming neuropathy. These included 63% (10/16) of skin biopsies, 17% (2/12) of electrodiagnostic tests and 50% (4/8) of autonomic function tests. One patient was diagnosed with critical illness axonal neuropathy and another with multifocal demyelinating neuropathy 3 weeks after mild COVID, and ≥10 received small-fiber neuropathy diagnoses. Longitudinal improvement averaged 52%, although none reported complete resolution. For treatment, 65% (11/17) received immunotherapies (corticosteroids and/or IV immunoglobulins).

DISCUSSION

Among evaluated patients with long COVID, prolonged, often disabling, small-fiber neuropathy after mild SARS-CoV-2 was most common, beginning within 1 month of COVID-19 onset. Various evidence suggested infection-triggered immune dysregulation as a common mechanism.

Evolution of Anti-B Cell Therapeutics in Autoimmune Neurological Diseases

B cells have an ever-increasing role in the etiopathology of a number of autoimmune neurological disorders, acting as antigen-presenting cells facilitating antibody production but also as sensors, coordinators, and regulators of the immune response. In particular, B cells can regulate the T cell activation process through their participation in antigen presentation, production of proinflammatory cytokines (bystander activation or suppression), and contribution to ectopic lymphoid aggregates. Such an important interplay between B and T cells makes therapeutic depletion of B cells an attractive treatment strategy. The last decade, anti-B cell therapies using monoclonal antibodies against B cell surface molecules have evolved into a rational approach for successfully treating autoimmune neurological disorders, even when T cells seem to be the main effector cells. The paper summarizes basic aspects of B cell biology, discusses the roles of B cells in neurological autoimmunities, and highlights how the currently available or under development anti-B cell therapeutics exert their action in the wide spectrum and immunologically diverse neurological disorders. The efficacy of the various anti-B cell therapies and practical issues on induction and maintenance therapy is specifically detailed for the treatment of patients with multiple sclerosis, neuromyelitis-spectrum disorders, autoimmune encephalitis and hyperexcitability CNS disorders, autoimmune neuropathies, myasthenia gravis, and inflammatory myopathies. The success of anti-B cell therapies in inducing long-term remission in IgG4 neuroautoimmunities is also highlighted pointing out potential biomarkers for follow-up infusions.

Autoimmune Neurological Disorders with IgG4 Antibodies: a Distinct Disease Spectrum with Unique IgG4 Functions Responding to Anti-B Cell Therapies

The main IgG4 antibody-mediated neurological disorders (IgG4-ND) include MuSK myasthenia; CIDP with nodal/paranodal antibodies to Neurofascin-155, contactin-1/caspr-1, or pan-neurofascins; anti-LGI1 and CASPR2-associated limbic encephalitis, Morvan syndrome, or neuromyotonia; and several cases of the anti-IgLON5 and anti-DPPX-spectrum CNS diseases. The paper is centered on the clinical spectrum of IgG4-ND and their immunopathogenesis highlighting the unique functional effects of the IgG4 subclass compared to IgG1-3 antibody subclasses. The IgG4 antibodies exert pathogenic effects on their targeted antigens by blocking enzymatic activity or disrupting protein-protein interactions affecting signal transduction pathways, but not by activating complement, binding to inhibitory FcγRIIb receptor or engaging in cross-linking of the targeted antigen with immune complex formation as the IgG1-IgG3 antibody subclasses do. IgG4 can even inhibit the classical complement pathway by affecting the affinity of IgG1-2 subclasses to C1q binding. Because the IgG4 antibodies do not trigger inflammatory processes or complement-mediated immune responses, the conventional anti-inflammatory therapies, especially with IVIg, immunosuppressants, and plasmapheresis, are ineffective or not sufficiently effective in inducing long-term remissions. In contrast, aiming at the activated plasmablasts connected with IgG4 antibody production is a meaningful therapeutic target in IgG4-ND. Indeed, data from large series of patients with MuSK myasthenia, CIDP with nodal/paranodal antibodies, and anti-LGI1 and CASPR2-associated syndromes indicate that B cell depletion therapy with rituximab exerts long-lasting clinical remissions by targeting memory B cells and IgG4-producing CD20-positive short-lived plasma cells. Because IgG4 antibody titers seem reduced in remissions and increased in exacerbation, they may serve as potential biomarkers of treatment response supporting further the pathogenic role of self-reacting B cells. Controlled trials are needed in IgG4-ND not only with rituximab but also with the other anti-B cell agents that target CD19/20, especially those like obexelimab and obinutuzumab, that concurrently activate the inhibitory FcγRIIb receptors which have low binding affinity to IgG4, exerting a more prolonged anti-B cell action affecting also antigen presentation and cytotoxic T cells. Antibody therapies targeting FcRn, testing those anti-FcRn inhibitors that effectively catabolize the IgG4 antibody subclass, may be especially promising.

The Role of the Complement System in Chronic Inflammatory Demyelinating Polyneuropathy: Implications for Complement-Targeted Therapies

Chronic inflammatory demyelinating polyneuropathy (CIDP) is the most common, heterogeneous, immune-mediated neuropathy, characterized by predominant demyelination of motor and sensory nerves. CIDP follows a relapsing-remitting or a progressive course and causes substantial disability. The pathogenesis of CIDP involves a complex interplay of multiple aberrant immune responses, creating a pro-inflammatory environment, subsequently inflicting damage on the myelin sheath. Though the exact triggers are unclear, diverse immune mechanisms encompassing cellular and humoral pathways are implicated. The complement system appears to play a role in promoting macrophage-mediated demyelination. Complement deposition in sural nerve biopsies, as well as signs of increased complement activation in serum and CSF of patients with CIDP, suggest complement involvement in CIDP pathogenesis. Here, we present a comprehensive overview of the preclinical and clinical evidence supporting the potential role of the complement system in CIDP. This understanding furnishes a strong rationale for targeting the complement system to develop new therapies that could serve the unmet needs of patients affected by CIDP, particularly in those refractory to standard therapies.

Stiff-person Syndrome and GAD Antibody-spectrum Disorders: GABAergic Neuronal Excitability, Immunopathogenesis and Update on Antibody Therapies

Although antibodies against Glutamic Acid Decarboxylase (GAD) were originally associated with Stiff Person Syndrome (SPS), they now denote the "GAD antibody-spectrum disorders (GAD-SD)" that include Cerebellar Ataxia, Autoimmune Epilepsy, Limbic Encephalitis, PERM and eye movement disorder. In spite of the unique clinical phenotype that each of these disorders has, there is significant overlapping symptomatology characterized by autoimmune neuronal excitability. In addition to GAD, three other autoantibodies, against glycine receptors, amphiphysin and gephyrin, are less frequently or rarely associated with SPS-SD. Very high serum anti-GAD antibody titers are a key diagnostic feature for all GAD-SD, commonly associated with the presence of GAD antibodies in the CSF, a reduced CSF GABA level and increased anti-GAD-specific IgG intrathecal synthesis denoting stimulation of B-cell clones in the CNS. Because anti-GAD antibodies from the various hyperexcitability syndromes recognize the same dominant GAD epitope, the clinical heterogeneity among GAD-SD patients remains unexplained. The paper highlights the biologic basis of autoimmune hyperexcitability connected with the phenomenon of reciprocal inhibition as the fundamental mechanism of the patients' muscle stiffness and spasms; addresses the importance of high-GAD antibody titers in diagnosis, pinpointing the diagnostic challenges in patients with low-GAD titers or their distinction from functional disorders; and discusses whether high GAD-antibodies are disease markers or pathogenic in the context of their association with reduced GABA level in the brain and CSF. Finally, it focuses on therapies providing details on symptomatic GABA-enhancing drugs and the currently available immunotherapies in a step-by-step approach. The prospects of future immunotherapeutic options with antibody therapies are also summarized.