Immunization with the glutamate receptor-derived peptide GluR3B induces neuronal death and reactive gliosis, but confers partial protection from pentylenetetrazole-induced seizures

The correlation of GluR3B antibody with T lymphocyte subsets and inflammatory factors and their role in the progression of epilepsy

OBJECTIVE

To investigate changes in proportions of peripheral blood lymphocyte subsets, the correlation between the lymphocyte subsets and cytokine levels in patients with GluR3B antibody-positive epilepsy, analyze the role of GluR3B antibodies and cytokines in the progression of epilepsy. In addition, the immunotherapeutic effect in patients with GluR3B antibody-positive epilepsy will be evaluated.

METHODS

Patients with epilepsy hospitalized in the Department of Neurology of the affiliated Hospital of Xuzhou Medical University from December 2016 to May 2023 were recruited. GluR3B antibody levels were measured by enzyme-linked immunosorbent assay (ELISA). Lymphocyte subset proportions were determined using flow cytometry, and serum concentrations of 12 cytokines were measured using cytometric beads array. Differences in T lymphocyte subsets and inflammatory factors were analysed between GluR3B antibody positive and negative patients. Structural equation modeling (SEM) was used to analyse the role of GluR3B antibodies and inflammatory factors in drug-resistant epilepsy (DRE). Finally, the therapeutic effect of immunotherapy on epilepsy patients with GluR3B antibodies was assessed.

RESULTS

In this study, sixty-four cases of DRE, sixty-six cases of drug-naïve epilepsy (DNE), and forty-one cases of drug-responsive epilepsy were recruited. (1) DRE patients with positive GluR3B antibody were characterized by a significant increase in the proportion of cluster of differentiation (CD)4+ T lymphocytes, a decrease in CD8+ T lymphocytes, and an increase of CD4+/CD8+ ratio. Similar alterations in T lymphocyte subsets were observed in GluR3B antibody-positive patients with DNE. GluR3B antibody levels correlated positively with CD4+ T lymphocytes (r = 0.23) and negatively with CD8+ T lymphocytes (r=-0.18). (2) In patients with DRE, the serum concentrations of interleukin-1β (IL-1β), IL-8, and interferon-gamma (IFN-γ) were significantly higher in those with positive GluR3B antibody compared to those with negative GluR3B antibody. Serum IL-1β levels were also higher in GluR3B antibody-positive DNE patients compared to antibody-negative DNE patients. In drug-responsive epilepsy patients with GluR3B antibody-positive, both serum IL-1β and IFN-γ levels were higher than those with GluR3B antibody-negative. Moreover, the concentrations of serum GluR3B antibody were positively correlated with the levels of IL-1β, IL-8, and IFN-γ. (3) SEM analysis indicated that GluR3B antibody may be a direct risk factor for DRE (direct effect = 4.479, 95%CI 0.409-8.503), or may be involved in DRE progression through affecting IFN-γ and IL-8 levels (total indirect effect = 5.101, 95%CI 1.756-8.818). (4) Immunotherapy significantly decreased seizure frequency and serum GluR3B antibody levels, and the seizure frequency was positively correlated with the levels of GluR3B antibody levels in patients receiving immunotherapy.

CONCLUSIONS

This study demonstrates that GluR3B antibody may influence the progression of epilepsy through altering the proportion of CD4+ and CD8+ lymphocyte subsets and increasing proinflammatory cytokines. The seizure suppression of immunotherapy is associated with the decrease of GluR3B antibody levels. Thus, the present study contributes to a better understanding of the immunoregulatory mechanisms of autoimmune-associated epilepsy and provides a potential target for DRE.

Anti-AMPA Receptor Autoantibodies Reduce Excitatory Currents in Rat Hippocampal Neurons

The GluR3 subunit of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) has been identified as a target for autoantibodies (Aabs) in autoimmune encephalopathy and other diseases. Recent studies have proposed mechanisms by which these Aabs act, but their exact role in neuronal excitability is yet to be established. Patient Aabs have been shown to bind to specific regions within the GluR3 subunit. GLUR3B peptides were designed based on described (ELISA) immunogenic epitopes for Aabs and an immunisation strategy was used to generate novel anti-AMPAR Aabs. Target-specific binding and specificity of affinity-purified anti-AMPAR Aabs was confirmed using enzyme-linked immunosorbent assay, immunocytochemistry and Western blot. Functional anti-AMPAR Aab effects were determined on excitatory postsynaptic currents (EPSCs) from primary hippocampal neurons using whole-cell patch-clamp electrophysiology. Acute (10 or 30 min) or longer-term (24 h) application of anti-AMPAR Aabs caused a significant reduction in the mean frequency of spontaneous and miniature EPSCs in hippocampal neurons. Our data demonstrate that anti-AMPAR Aabs targeting peptides linked to auto-immune diseases mediate inhibitory effects on neuronal excitability at the synaptic level, such effects may lead to disruption of the excitatory/inhibitory balance at a network level.

GluR3B Antibody Was a Biomarker for Drug-Resistant Epilepsy in Patients With Focal to Bilateral Tonic-Clonic Seizures

Considering the role of GluR3B antibody-mediated excitotoxicity in the progression of epilepsy, the purpose of this study was to evaluate the clinical significance of GluR3B antibody level as a novel biomarker for the prognosis of unknown etiology drug-resistant epilepsy (DRE) in patients with focal to bilateral tonic-clonic seizures. The study included 193 patients with focal to bilateral tonic-clonic seizures in the modeling cohort. Serum and CSF samples from patients were collected, and GluR3B antibody levels were detected by an ELISA kit. Serum and CSF GluR3B antibody levels in patients with DRE were significantly increased compared with those in patients with drug-responsive epilepsy. Univariate logistic regression analysis underlined that patients with high GluR3B antibody levels had a significantly increased risk of developing DRE. A logistic regression model demonstrated that increased GluR3B antibody levels were an independent factor in predicting DRE. External verification showed that the model constructed for the prediction of DRE had good adaptability. Finally, decision curve analysis highlighted the superior clinical net benefit in DRE prognosis by GluR3B antibody level. In summary, elevated levels of GluR3B antibody are an early biomarker to predict the prognosis of DRE; in addition, targeting GluR3B antibody may be a promising treatment strategy for patients with DRE.

Autoimmune Epilepsy - Novel Multidisciplinary Analysis, Discoveries and Insights

Epilepsy affects ~50 million people. In ~30% of patients the etiology is unknown, and ~30% are unresponsive to anti-epileptic drugs. Intractable epilepsy often leads to multiple seizures daily or weekly, lasting for years, and accompanied by cognitive, behavioral, and psychiatric problems. This multidisciplinary scientific (not clinical) 'Perspective' article discusses Autoimmune Epilepsy from immunological, neurological and basic-science angles. The article includes summaries and novel discoveries, ideas, insights and recommendations. We summarize the characteristic features of the respective antigens, and the pathological activity in vitro and in animal models of autoimmune antibodies to: Glutamate/AMPA-GluR3, Glutamate/NMDA-NR1, Glutamate/NMDA-NR2, GAD-65, GABA-R, GLY-R, VGKC, LGI1, CASPR2, and β2 GP1, found in subpopulations of epilepsy patients. Glutamate receptor antibodies: AMPA-GluR3B peptide antibodies, seem so far as the most exclusive and pathogenic autoimmune antibodies in Autoimmune Epilepsy. They kill neural cells by three mechanisms: excitotoxicity, Reactive-Oxygen-Species, and complement-fixation, and induce and/or facilitate brain damage, seizures, and behavioral impairments. In this article we raise and discuss many more topics and new insights related to Autoimmune Epilepsy. 1. Few autoimmune antibodies tilt the balance between excitatory Glutamate and inhibitory GABA, thereby promoting neuropathology and epilepsy; 2. Many autoantigens are synaptic, and have extracellular domains. These features increase the likelihood of autoimmunity against them, and the ease with which autoimmune antibodies can reach and harm these self-proteins. 3. Several autoantigens have 'frenetic character'- undergoing dynamic changes that can increase their antigenicity; 4. The mRNAs of the autoantigens are widely expressed in multiple organs outside the brain. If translated by default to proteins, broad spectrum detrimental autoimmunity is expected; 5. The autoimmunity can precede seizures, cause them, and be detrimental whether primary or epiphenomenon; 6. Some autoimmune antibodies induce, and associate with, cognitive, behavioral and psychiatric impairments; 7. There are evidences for epitope spreading in Autoimmune Epilepsy; 8. T cells have different 'faces' in the brain, and in Autoimmune Epilepsy: Normal T cells are needed for the healthy brain. Normal T cells are damaged by autoimmune antibodies to Glutamate/AMPA GluR3, which they express, and maybe by additional autoantibodies to: Dopamine-R, GABA-R, Ach-R, Serotonin-R, and Adrenergic-R, present in various neurological diseases (summarized herein), since T cells express all these Neurotransmitter receptors. However, autoimmune and/or cytotoxic T cells damage the brain; 9. The HLA molecules are important for normal brain function. The HLA haplotype can confer susceptibility or protection from Autoimmune Epilepsy; 10. There are several therapeutic strategies for Autoimmune Epilepsy.

Structure, Function, and Pharmacology of Glutamate Receptor Ion Channels

Many physiologic effects of l-glutamate, the major excitatory neurotransmitter in the mammalian central nervous system, are mediated via signaling by ionotropic glutamate receptors (iGluRs). These ligand-gated ion channels are critical to brain function and are centrally implicated in numerous psychiatric and neurologic disorders. There are different classes of iGluRs with a variety of receptor subtypes in each class that play distinct roles in neuronal functions. The diversity in iGluR subtypes, with their unique functional properties and physiologic roles, has motivated a large number of studies. Our understanding of receptor subtypes has advanced considerably since the first iGluR subunit gene was cloned in 1989, and the research focus has expanded to encompass facets of biology that have been recently discovered and to exploit experimental paradigms made possible by technological advances. Here, we review insights from more than 3 decades of iGluR studies with an emphasis on the progress that has occurred in the past decade. We cover structure, function, pharmacology, roles in neurophysiology, and therapeutic implications for all classes of receptors assembled from the subunits encoded by the 18 ionotropic glutamate receptor genes. SIGNIFICANCE STATEMENT: Glutamate receptors play important roles in virtually all aspects of brain function and are either involved in mediating some clinical features of neurological disease or represent a therapeutic target for treatment. Therefore, understanding the structure, function, and pharmacology of this class of receptors will advance our understanding of many aspects of brain function at molecular, cellular, and system levels and provide new opportunities to treat patients.

An unusual case of coma related to glutamate receptor 3 (GluR3) auto-antibodies after a traumatic spinal cord injury: Clinical and pathological aspects

We report the case of a 28-year-old patient with a history of post-traumatic epilepsy who, following a spinal cord injury and the onset of nosocomial infection, went into a deep coma. A brain MRI revealed non-specific findings, anti-Glutamate Receptor 3 (GluR3) auto-antibodies were detected in the liquor, and the patient's serum and immunomodulatory therapy proved ineffective. Autopsy and histological investigations led to the diagnosis of autoimmune encephalitis. This case highlights the diagnostic difficulties of a rare and still poorly researched disease and the possible role that traumatic and infectious episodes may play in the progression or acceleration of an immune response. As for patients with unexplained encephalitis, tests for autoantibodies against GluR3 in cerebrospinal fluid and serum should be considered. Forensic pathologists should be aware of encephalitis and epilepsies and that complete post-mortem investigations are required in such cases.

Dual-Targeted Autoimmune Sword in Fatal Epilepsy: Patient's glutamate receptor AMPA GluR3B peptide autoimmune antibodies bind, induce Reactive Oxygen Species (ROS) in, and kill both human neural cells and T cells

Nodding Syndrome (NS) is a fatal pediatric epilepsy of unknown etiology, accompanied by multiple neurological impairments, and associated with Onchocerca volvulus (Ov), malnutrition, war-induced trauma, and other insults. NS patients have neuroinflammation, and ~50% have cross-reactive Ov/Leiomodin-1 neurotoxic autoimmune antibodies. RESULTS: Studying 30 South Sudanese NS patients and a similar number of healthy subjects from the same geographical region, revealed autoimmune antibodies to 3 extracellular peptides of ionotropic glutamate receptors in NS patients: AMPA-GluR3B peptide antibodies (86%), NMDA-NR1 peptide antibodies (77%) and NMDA-NR2 peptide antibodies (87%) (in either 1:10, 1:100 or 1:1000 serum dilution). In contrast, NS patients did not have 26 other well-known autoantibodies that target the nervous system in several autoimmune-mediated neurological diseases. We demonstrated high expression of both AMPA-GluR3 and NMDA-NR1 in human neural cells, and also in normal human CD3⁺ T cells of both helper CD4⁺ and cytotoxic CD8⁺ types. Patient's GluR3B peptide antibodies were affinity-purified, and by themselves precipitated short 70 kDa neuronal GluR3. NS patient's affinity-purified GluR3B peptide antibodies also bound to, induced Reactive Oxygen Species (ROS) in, and killed both human neural cells and T cells within 1-2 hours only. NS patient's purified IgGs, or serum (1:10 or 1:30), induced similar effects. In vivo video EEG experiments in normal mice, revealed that when NS patient's purified IgGs were released continuously (24/7 for 1 week) in normal mouse brain, they induced all the following: 1.Seizures, 2. Cerebellar Purkinje cell loss, 3. Degeneration in the hippocampus and cerebral cortex, and 4. Elevation of CD3⁺ T cells, and of activated Mac-2⁺microglia and GFAP⁺astrocytes in both the gray and white matter of the cerebral cortex, hippocampus, corpus calossum and cerebellum of mice. NS patient's serum cytokines: IL-1β, IL-2, IL-6, IL-8, TNFα, IFNγ, are reduced by 85-99% compared to healthy subjects, suggesting severe immunodeficiency in NS patients. This suspected immunodeficiency could be caused by combined effects of the: 1. Chronic Ov infection, 2. Malnutrition, 3. Killing of NS patient's T cells by patient's own GluR3B peptide autoimmune antibodies (alike the killing of normal human T cells by the NS patient's GluR3B peptide antibodies found herein in vitro). CONCLUSIONS: Regardless of NS etiology, NS patients suffer from 'Dual-targeted Autoimmune Sword': autoimmune AMPA GluR3B peptide antibodies that bind, induce ROS in, and kill both neural cells and T cells. These neurotoxic and immunotoxic GluR3B peptide autoimmune antibodies, and also NS patient's NMDA-NR1/NR2A and Ov/Leiomodin-1 autoimmune antibodies, must be silenced or removed. Moreover, the findings of this study are relevant not only to NS, but also to many more patients with other types of epilepsy, which have GluR3B peptide antibodies in serum and/or CSF. This claim is based on the following facts: 1. The GluR3 subunit is expressed in neural cells in crucial brains regions, in motor neurons in the spinal cord, and also in other cells in the body, among them T cells of the immune system, 2. The GluR3 subunit has diverse neurophysiological role, and its deletion or abnormal function can: disrupt oscillatory networks of both sleep and breathing, impair motor coordination and exploratory activity, and increase the susceptibility to generate seizures, 3. GluR3B peptide antibodies were found so far in ~27% of >300 epilepsy patients worldwide, which suffer from various other types of severe, intractable and enigmatic epilepsy, and which turned out to be 'Autoimmune Epilepsy'. Furthermore, the findings of this study could be relevant to different neurological diseases besides epilepsy, since other neurotransmitter-receptors autoantibodies are present in other neurological and psychiatric diseases, e.g. autoimmune antibodies against other GluRs, Dopamine receptors, GABA receptors, Acetylcholine receptors and others. These neurotransmitter-receptors autoimmune autoantibodies might also act as 'Dual-targeted Autoimmune Sword' and damage both neural cells and T cells (as the AMPA-GluR3B peptide antibodies induced in the present study), since T cells, alike neural cells, express most if not all these neurotransmitter receptors, and respond functionally to the respective neurotransmitters - a scientific and clinical topic we coined 'Nerve-Driven Immunity'.

Toxic exposures and the senses of taste and smell

This review addresses the adverse influences of neurotoxic exposures on the ability to smell and taste. These chemical senses largely determine the flavor of foods and beverages, impact food intake, and ultimately nutrition, and provide a warning for spoiled or poisonous food, leaking natural gas, smoke, airborne pollutants, and other hazards. Hence, toxicants that damage these senses have a significant impact on everyday function. As noted in detail, a large number of toxicants encountered in urban and industrial air pollution, including smoke, solvents, metals, and particulate matter can alter the ability to smell. Their influence on taste, i.e., sweet, sour, bitter, salty, and savory (umami) sensations, is not well documented. Given the rather direct exposure of olfactory receptors to the outside environment, olfaction is particularly vulnerable to damage from toxicants. Some toxicants, such as nanoparticles, have the potential to damage not only the olfactory receptor cells, but also the central nervous system structures by their entrance into the brain through the olfactory mucosa.

Cognitive Impairment and Celiac Disease: Is Transcranial Magnetic Stimulation a Trait d'Union between Gut and Brain?

Celiac disease is a systemic disorder with multifactorial pathogenesis and multifaceted symptomatology. In response to gluten exposure, a significant part of the general population produces antibodies that have been hypothesized to be deleterious to the brain. Among the well-known neurological manifestations, adult celiac patients often complain cognitive symptoms, ranging from the so-called "brain fog" till an overt dementia. Transcranial magnetic stimulation (TMS) is a non-invasive brain stimulation technique that can contribute to the assessment and monitoring of celiac patients, even in those without a clear neurological involvement. The studies here reviewed seem to converge on an impaired central motor conductivity and a "hyperexcitable celiac brain" to TMS, which partially reverts back after a long-term gluten restriction. Notably, a clear hyperexcitability is a stably reported feature of both degenerative and vascular dementia. Therefore, given its potential neuroprotective effect, the gluten-free diet should be introduced as early as possible, although the overall response of neurological symptoms (and cognition in particular) is still controversial. Identifying new and possibly modifiable risk factors may be of crucial importance for patients, clinicians, and researchers.

Cognitive Impairment and Celiac Disease: Is Transcranial Magnetic Stimulation a Trait d'Union between Gut and Brain?

Celiac disease is a systemic disorder with multifactorial pathogenesis and multifaceted symptomatology. In response to gluten exposure, a significant part of the general population produces antibodies that have been hypothesized to be deleterious to the brain. Among the well-known neurological manifestations, adult celiac patients often complain cognitive symptoms, ranging from the so-called "brain fog" till an overt dementia. Transcranial magnetic stimulation (TMS) is a non-invasive brain stimulation technique that can contribute to the assessment and monitoring of celiac patients, even in those without a clear neurological involvement. The studies here reviewed seem to converge on an impaired central motor conductivity and a "hyperexcitable celiac brain" to TMS, which partially reverts back after a long-term gluten restriction. Notably, a clear hyperexcitability is a stably reported feature of both degenerative and vascular dementia. Therefore, given its potential neuroprotective effect, the gluten-free diet should be introduced as early as possible, although the overall response of neurological symptoms (and cognition in particular) is still controversial. Identifying new and possibly modifiable risk factors may be of crucial importance for patients, clinicians, and researchers.

Effect of a Gluten-Free Diet on Cortical Excitability in Adults with Celiac Disease

INTRODUCTION

An imbalance between excitatory and inhibitory synaptic excitability was observed in de novo patients with celiac disease (CD) in a previous study with Transcranial Magnetic Stimulation (TMS), suggesting a subclinical involvement of GABAergic and glutamatergic neurotransmission in asymptomatic patients. The aim of this investigation was to monitor the eventual changes in the same cohort of patients, evaluated after a period of gluten-free diet.

METHODS

Patients were re-evaluated after a median period of 16 months during which an adequate gluten-free diet was maintained. Clinical, cognitive and neuropsychiatric assessment was repeated, as well as cortical excitability by means of single- and paired-pulse TMS from the first dorsal interosseous muscle of the dominant hand.

RESULTS

Compared to baseline, patients showed a significant decrease of the median resting motor threshold (from 35% to 33%, p<0.01). The other single-pulse (cortical silent period, motor evoked potentials latency and amplitude, central motor conduction time) and paired-pulse TMS measures (intracortical inhibition and intracortical facilitation) did not change significantly after the follow-up period. Antibodies were still present in 7 subjects.

DISCUSSION

In patients under a gluten-free diet, a global increase of cortical excitability was observed, suggesting a glutamate-mediated functional reorganization compensating for disease progression. We hypothesize that glutamate receptor activation, probably triggered by CD-related immune system dysregulation, might result in a long-lasting motor cortex hyperexcitability with increased excitatory post-synaptic potentials, probably related to phenomena of long-term plasticity. The impact of the gluten-free diet on subclinical neurological abnormalities needs to be further explored.

Effect of a Gluten-Free Diet on Cortical Excitability in Adults with Celiac Disease

INTRODUCTION

An imbalance between excitatory and inhibitory synaptic excitability was observed in de novo patients with celiac disease (CD) in a previous study with Transcranial Magnetic Stimulation (TMS), suggesting a subclinical involvement of GABAergic and glutamatergic neurotransmission in asymptomatic patients. The aim of this investigation was to monitor the eventual changes in the same cohort of patients, evaluated after a period of gluten-free diet.

METHODS

Patients were re-evaluated after a median period of 16 months during which an adequate gluten-free diet was maintained. Clinical, cognitive and neuropsychiatric assessment was repeated, as well as cortical excitability by means of single- and paired-pulse TMS from the first dorsal interosseous muscle of the dominant hand.

RESULTS

Compared to baseline, patients showed a significant decrease of the median resting motor threshold (from 35% to 33%, p<0.01). The other single-pulse (cortical silent period, motor evoked potentials latency and amplitude, central motor conduction time) and paired-pulse TMS measures (intracortical inhibition and intracortical facilitation) did not change significantly after the follow-up period. Antibodies were still present in 7 subjects.

DISCUSSION

In patients under a gluten-free diet, a global increase of cortical excitability was observed, suggesting a glutamate-mediated functional reorganization compensating for disease progression. We hypothesize that glutamate receptor activation, probably triggered by CD-related immune system dysregulation, might result in a long-lasting motor cortex hyperexcitability with increased excitatory post-synaptic potentials, probably related to phenomena of long-term plasticity. The impact of the gluten-free diet on subclinical neurological abnormalities needs to be further explored.

Glutamate receptor antibodies in neurological diseases: anti-AMPA-GluR3 antibodies, anti-NMDA-NR1 antibodies, anti-NMDA-NR2A/B antibodies, anti-mGluR1 antibodies or anti-mGluR5 antibodies are present in subpopulations of patients with either: epilepsy, encephalitis, cerebellar ataxia, systemic lupus erythematosus (SLE) and neuropsychiatric SLE, Sjogren's syndrome, schizophrenia, mania or stroke. These autoimmune anti-glutamate receptor antibodies can bind neurons in few brain regions, activate glutamate receptors, decrease glutamate receptor's expression, impair glutamate-induced signaling and function, activate blood brain barrier endothelial cells, kill neurons, damage the brain, induce behavioral/psychiatric/cognitive abnormalities and ataxia in animal models, and can be removed or silenced in some patients by immunotherapy

Glutamate is the major excitatory neurotransmitter of the Central Nervous System (CNS), and it is crucially needed for numerous key neuronal functions. Yet, excess glutamate causes massive neuronal death and brain damage by excitotoxicity--detrimental over activation of glutamate receptors. Glutamate-mediated excitotoxicity is the main pathological process taking place in many types of acute and chronic CNS diseases and injuries. In recent years, it became clear that not only excess glutamate can cause massive brain damage, but that several types of anti-glutamate receptor antibodies, that are present in the serum and CSF of subpopulations of patients with a kaleidoscope of human neurological diseases, can undoubtedly do so too, by inducing several very potent pathological effects in the CNS. Collectively, the family of anti-glutamate receptor autoimmune antibodies seem to be the most widespread, potent, dangerous and interesting anti-brain autoimmune antibodies discovered up to now. This impression stems from taking together the presence of various types of anti-glutamate receptor antibodies in a kaleidoscope of human neurological and autoimmune diseases, their high levels in the CNS due to intrathecal production, their multiple pathological effects in the brain, and the unique and diverse mechanisms of action by which they can affect glutamate receptors, signaling and effects, and subsequently impair neuronal signaling and induce brain damage. The two main families of autoimmune anti-glutamate receptor antibodies that were already found in patients with neurological and/or autoimmune diseases, and that were already shown to be detrimental to the CNS, include the antibodies directed against ionotorpic glutamate receptors: the anti-AMPA-GluR3 antibodies, anti-NMDA-NR1 antibodies and anti-NMDA-NR2 antibodies, and the antibodies directed against Metabotropic glutamate receptors: the anti-mGluR1 antibodies and the anti-mGluR5 antibodies. Each type of these anti-glutamate receptor antibodies is discussed separately in this very comprehensive review, with regards to: the human diseases in which these anti-glutamate receptor antibodies were found thus far, their presence and production in the nervous system, their association with various psychiatric/behavioral/cognitive/motor impairments, their possible association with certain infectious organisms, their detrimental effects in vitro as well as in vivo in animal models in mice, rats or rabbits, and their diverse and unique mechanisms of action. The review also covers the very encouraging positive responses to immunotherapy of some patients that have either of the above-mentioned anti-glutamate receptor antibodies, and that suffer from various neurological diseases/problems. All the above are also summarized in the review's five schematic and useful figures, for each type of anti-glutamate receptor antibodies separately. The review ends with a summary of all the main findings, and with recommended guidelines for diagnosis, therapy, drug design and future investigations. In the nut shell, the human studies, the in vitro studies, as well as the in vivo studies in animal models in mice, rats and rabbit revealed the following findings regarding the five different types of anti-glutamate receptor antibodies: (1) Anti-AMPA-GluR3B antibodies are present in ~25-30% of patients with different types of Epilepsy. When these anti-glutamate receptor antibodies (or other types of autoimmune antibodies) are found in Epilepsy patients, and when these autoimmune antibodies are suspected to induce or aggravate the seizures and/or the cognitive/psychiatric/behavioral impairments that sometimes accompany the seizures, the Epilepsy is called 'Autoimmune Epilepsy'. In some patients with 'Autoimmune Epilepsy' the anti-AMPA-GluR3B antibodies associate significantly with psychiatric/cognitive/behavior abnormalities. In vitro and/or in animal models, the anti-AMPA-GluR3B antibodies by themselves induce many pathological effects: they activate glutamate/AMPA receptors, kill neurons by 'Excitotoxicity', and/or by complement activation modulated by complement regulatory proteins, cause multiple brain damage, aggravate chemoconvulsant-induced seizures, and also induce behavioral/motor impairments. Some patients with 'Autoimmune Epilepsy' that have anti-AMPA-GluR3B antibodies respond well (although sometimes transiently) to immunotherapy, and thanks to that have reduced seizures and overall improved neurological functions. (2) Anti-NMDA-NR1 antibodies are present in patients with autoimmune 'Anti-NMDA-receptor Encephalitis'. In humans, in animal models and in vitro the anti-NMDA-NR1 antibodies can be very pathogenic since they can cause a pronounced decrease of surface NMDA receptors expressed in hippocampal neurons, and also decrease the cluster density and synaptic localization of the NMDA receptors. The anti-NMDA-NR1 antibodies induce these effects by crosslinking and internalization of the NMDA receptors. Such changes can impair glutamate signaling via the NMDA receptors and lead to various neuronal/behavior/cognitive/psychiatric abnormalities. Anti-NMDA-NR1 antibodies are frequently present in high levels in the CSF of the patients with 'Anti-NMDA-receptor encephalitis' due to their intrathecal production. Many patients with 'Anti-NMDA receptor Encephalitis' respond well to several modes of immunotherapy. (3) Anti-NMDA-NR2A/B antibodies are present in a substantial number of patients with Systemic Lupus Erythematosus (SLE) with or without neuropsychiatric problems. The exact percentage of SLE patients having anti-NMDA-NR2A/B antibodies varies in different studies from 14 to 35%, and in one study such antibodies were found in 81% of patients with diffuse 'Neuropshychiatric SLE', and in 44% of patients with focal 'Neuropshychiatric SLE'. Anti-NMDA-NR2A/B antibodies are also present in subpopulations of patients with Epilepsy of several types, Encephalitis of several types (e.g., chronic progressive limbic Encephalitis, Paraneoplastic Encephalitis or Herpes Simplex Virus Encephalitis), Schizophrenia, Mania, Stroke, or Sjorgen syndrome. In some patients, the anti-NMDA-NR2A/B antibodies are present in both the serum and the CSF. Some of the anti-NMDA-NR2A/B antibodies cross-react with dsDNA, while others do not. Some of the anti-NMDA-NR2A/B antibodies associate with neuropsychiatric/cognitive/behavior/mood impairments in SLE patients, while others do not. The anti-NMDA-NR2A/B antibodies can undoubtedly be very pathogenic, since they can kill neurons by activating NMDA receptors and inducing 'Excitotoxicity', damage the brain, cause dramatic decrease of membranal NMDA receptors expressed in hippocampal neurons, and also induce behavioral cognitive impairments in animal models. Yet, the concentration of the anti-NMDA-NR2A/B antibodies seems to determine if they have positive or negative effects on the activity of glutamate receptors and on the survival of neurons. Thus, at low concentration, the anti-NMDA-NR2A/B antibodies were found to be positive modulators of receptor function and increase the size of NMDA receptor-mediated excitatory postsynaptic potentials, whereas at high concentration they are pathogenic as they promote 'Excitotoxcity' through enhanced mitochondrial permeability transition. (4) Anti-mGluR1 antibodies were found thus far in very few patients with Paraneoplastic Cerebellar Ataxia, and in these patients they are produced intrathecally and therefore present in much higher levels in the CSF than in the serum. The anti-mGluR1 antibodies can be very pathogenic in the brain since they can reduce the basal neuronal activity, block the induction of long-term depression of Purkinje cells, and altogether cause cerebellar motor coordination deficits by a combination of rapid effects on both the acute and the plastic responses of Purkinje cells, and by chronic degenerative effects. Strikingly, within 30 min after injection of anti-mGluR1 antibodies into the brain of mice, the mice became ataxic. Anti-mGluR1 antibodies derived from patients with Ataxia also caused disturbance of eye movements in animal models. Immunotherapy can be very effective for some Cerebellar Ataxia patients that have anti-mGluR1 antibodies. (5) Anti-mGluR5 antibodies were found thus far in the serum and CSF of very few patients with Hodgkin lymphoma and Limbic Encephalopathy (Ophelia syndrome). The sera of these patients that contained anti-GluR5 antibodies reacted with the neuropil of the hippocampus and cell surface of live rat hippocampal neurons, and immunoprecipitation from cultured neurons and mass spectrometry demonstrated that the antigen was indeed mGluR5. Taken together, all these evidences show that anti-glutamate receptor antibodies are much more frequent among various neurological diseases than ever realized before, and that they are very detrimental to the nervous system. As such, they call for diagnosis, therapeutic removal or silencing and future studies. What we have learned by now about the broad family of anti-glutamate receptor antibodies is so exciting, novel, unique and important, that it makes all future efforts worthy and essential.

Glutamate receptor antibodies directed against AMPA receptors subunit 3 peptide B (GluR3B) can be produced in DBA/2J mice, lower seizure threshold and induce abnormal behavior

OBJECTIVE

Anti-GluR3B antibodies (GluR3B Ab's), directed against peptide B/aa372-395 of GluR3 subunit of glutamate/AMPA receptors, are found in ∼35% of epilepsy patients, activate glutamate/AMPA receptors, evoke ion currents, kill neurons and damage the brain. We recently found that GluR3B Ab's also associate with neurological/psychiatric/behavioral abnormalities in epilepsy patients. Here we asked if GluR3B Ab's could be produced in DBA/2J mice, and also modulate seizure threshold and/or cause behavioral/motor impairments in these mice.

METHODS

DBA/2J mice were immunized with the GluR3B peptide in Complete Freund's Adjuvant (CFA), or with controls: ovalbumin (OVA), CFA, or phosphate-buffer saline (PBS). GluR3B Ab's and OVA Ab's were tested. Seizures were induced in all mice by the chemoconvulsant pentylenetetrazole (PTZ) at three time points, each time with less PTZ to avoid non-specific death. Behavior was examined in Open-Field, RotaRod and Grip tests.

RESULTS

GluR3B Ab's were produced only in GluR3B-immunized mice, while OVA Ab's were produced only in OVA-immunized mice, showing high Ab's specificity. In GluR3B Ab's negative mice, seizure severity scores and percentages of animals developing generalized seizures declined in response to decreasing PTZ doses. In contrast, both parameters remained unchanged/high in the GluR3B Ab's positive mice, showing that these mice were more susceptible to seizures. The seizure scores associated significantly with the GluR3B Ab's levels. GluR3B Ab's positive mice were also more anxious in Open-Field test, fell faster in RotaRod test, and fell more in Grip test, compared to all the control mice.

CONCLUSIONS

GluR3B Ab's are produced in DBA/2J mice, facilitate seizures and induce behavioral/motor impairments. This animal model can therefore serve for studying autoimmune epilepsy and abnormal behavior mediated by pathogenic anti-GluR3B Ab's.

Antibodies to glutamate receptor subtype 3 (GluR3) are found in some patients suffering from epilepsy as the main disease, but not in patients whose epilepsy accompanies antiphospholipid syndrome or Sneddon's syndrome

Autoantibodies (Ab's) to the "B" peptide (amino acids 372-395) of glutamate/AMPA receptor subtype 3 (GluR3) are found in serum and cerebrospinal fluid of some patients with different types of epilepsy. Since such anti-GluR3B Ab's can activate and/or kill neurons in vitro and in vivo, they may contribute to epilepsy. To investigate whether anti-GluR3B Ab's may also be relevant to epilepsy when it accompanies some autoimmune-diseases, we tested for these Ab's in patients suffering from epilepsy that accompanies anti-phospholipid syndrome (APS) or Sneddon's syndrome (SNS), both being autoimmune-diseases with frequent neurological complications. We tested 77 pediatric patients whose epilepsy is their main disease; 31 adult patients whose epilepsy accompanies APS (primary or SLE-associated) or SNS; 45 epilepsy-free APS and SNS patients; and 90 healthy controls. Compared to the controls, significantly elevated anti-GluR3B Ab's were found in 22/77 (29%) patients whose epilepsy is their main disease, but in none of the patients whose seizures accompany APS or SNS. Yet, all the APS and SNS patients harbored the characteristic anti-phospholipid Ab's (aPL), directed against cardiolipin and beta2-glycoprotein I, and had lupus anti-coagulant. Thus, anti-GluR3B Ab's are not crossreactive with aPL, and not produced as a non-specific consequence of seizures on the one hand, or autoimmune-diseases on the other. Taken together with new findings accumulated recently in our lab, we suggest that anti-GluR3B Ab's are produced primarily in the periphery due to specific/non-specific "irritation" of the immune system, and that once they reach the brain via a leaky blood-brain barrier they may cause neuronal/glial damage and facilitate the outburst of epilepsy and additional neurological abnormalities. In contrast, the presence of anti-GluR3B Ab's does not seem to increase the probability of developing APS, SNS or the seizures that often accompany these autoimmune-diseases. These findings may have important diagnostic and therapeutic implications.

Diagnostic and pathogenic significance of glutamate receptor autoantibodies

Autoantibodies against glutamate receptors, first reported in Rasmussen encephalitis, have been observed in other focal epilepsies, central nervous system ischemic infarcts, transient ischemic attacks, sporadic olivopontocerebellar atrophy, systemic lupus erythematosus, and paraneoplastic encephalopathies. The detection of glutamate receptor autoantibodies is not useful in the evaluation of Rasmussen encephalitis but may be a biomarker for brain ischemia, and it is helpful in diagnosing certain paraneoplastic encephalopathies. Passive transfer of glutamate receptor autoantibodies from patients with systemic lupus erythematosus or paraneoplastic encephalopathy suggests that glutamate receptor autoantibodies can actively contribute to neurologic dysfunction.

TCR Activation Eliminates Glutamate Receptor GluR3 from the Cell Surface of Normal Human T Cells, via an Autocrine/Paracrine Granzyme B-Mediated Proteolytic Cleavage

The majority of resting normal human T cells, like neuronal cells, express functional receptors for glutamate (the major excitatory neurotransmitter in the CNS) of the ionotropic alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-receptor subtype 3 (GluR3). Glutamate by itself ( approximately 10 nM) activates key T cell functions, including adhesion to fibronectin and laminin and chemotactic migration toward CXCL12/stromal cell-derived factor 1. In this study, we found by GluR3-specific immunostaining, flow cytometry, and Western blots that GluR3 cell surface expression decreases dramatically following TCR activation of human T cells. CXCR4, VLA-4, and VLA-6 also decrease substantially, whereas CD147 increases as expected, after TCR activation. Media of TCR-activated cells "eliminates" intact GluR3 (but not CXCR4 and VLA-6) from the cell surface of resting T cells, suggesting GluR3 cleavage by a soluble factor. We found that this factor is granzyme B (GB), a serine protease released by TCR-activated cells, because the extent of GluR3 elimination correlated with the active GB levels, and because three highly specific GB inhibitors blocked GluR3 down-regulation. Media of TCR-activated cells, presumably containing cleaved GluR3B peptide (GluR3 aa 372-388), inhibited the specific binding of anti-GluR3B mAb to synthetic GluR3B peptide. In parallel to losing intact GluR3, TCR-activated cells lost glutamate-induced adhesion to laminin. Taken together, our study shows that "classical immunological" TCR activation, via autocrine/paracrine GB, down-regulates substantially the expression of specific neurotransmitter receptors. Accordingly, glutamate T cell neuroimmune interactions are influenced by the T cell activation state, and glutamate, via AMPA-GluR3, may activate only resting, but not TCR-activated, T cells. Finally, the cleavage and release to the extracellular milieu of the GluR3B peptide may in principle increase its antigenicity, and thus the production, of anti-self GluR3B autoantibodies, which activate and kill neurons, found in patients with various types of epilepsy.

Acute intermittent porphyria, Rasmussen encephalitis, or both?

A case of a young woman who suffers from refractory epilepsy in the form of Rasmussen encephalitis and acute intermittent porphyria is presented. The patient developed refractory partial seizures with progressive hemispheric atrophy in the first decade. Both her serum and cerebrospinal fluid contained significantly elevated levels of anti-GluR3B antibodies. Her serum also contained anti-NR2A antibodies (directed against the N-methyl-D-aspartate receptor). Seven years later, acute intermittent porphyria was diagnosed as she developed an acute episode of abdominal pain, dark urine, and hyponatremia. For several years, all attempts to discontinue porphyrinogenic antiepileptic drugs such as phenobarbital and valproate resulted in seizure worsening. During a major acute intermittent porphyria crisis, brain edema and coma developed, allowing the discontinuation of phenobarbital. On recovery, atrophy of the right hemisphere ensued. Several etiologic hypotheses are presented. Double insults, porphyria, and an autoimmune process are suggested for the development of Rasmussen encephalitis in this patient. The authors recommend testing for porphyria in cases of Rasmussen encephalitis and other intractable seizures.

Refractory occipital lobe epilepsy treated by surgery in a child with chronic encephalitis

An 11-year-old boy developed catastrophic occipital lobe epilepsy with progressive neurocognitive decline which led to surgical treatment. Pathological examination of the resected epileptic area showed chronic inflammatory changes. Surgery was followed by a subacute and regressive encephalopathy although the patient did not suffer further seizures. We suggest that this occipital lobe epilepsy was related to chronic nonprogressive encephalitis consistent with a variant of Rasmussen's syndrome, and that the immune and/or inflammatory process was reactivated by surgery. This case indicates that there is no direct link between the epileptic and the encephalitic processes.

Autoimmune epilepsy: Distinct subpopulations of epilepsy patients harbor serum autoantibodies to either glutamate/AMPA receptor GluR3, glutamate/NMDA receptor subunit NR2A or double-stranded DNA

We studied 82 patients with different types of epilepsy and 49 neurologically intact non-epileptic controls, and identified three different subpopulations of epilepsy patients bearing significantly elevated levels of autoantibodies to either GluR3B-peptide of glutamate/AMPA receptor subtype 3 (17/82; 21% of patients), or to a peptide of NR2A subunit of glutamate/NMDA receptors (15/82; 18%), or to double-stranded (ds) DNA, the hallmark of systemic lupus erythematosus (13/80; 16%). Most patients had only one antibody type, arguing against cross-reactivity. Nearly all anti-dsDNA Ab-positive patients did not harbor anti-nuclear autoantibodies. Most patients had no history of brain damage, febrile convulsions, early onset epilepsy, acute epilepsy or intractable seizures. We suggest to measure the 'autoimmune-fingerprints' of epilepsy patients for diagnostic and therapeutic purposes.