Limbic Encephalitis: Potential Impact of Adaptive Autoimmune Inflammation on Neuronal Circuits of the Amygdala

Cerebrospinal fluid proteomics indicates immune dysregulation and neuronal dysfunction in antibody associated autoimmune encephalitis

Autoimmune Encephalitis (AE) spans a group of non-infectious inflammatory conditions of the central nervous system due to an imbalanced immune response. Aiming to elucidate the pathophysiological mechanisms of AE, we applied an unsupervised proteomic approach to analyze the cerebrospinal fluid (CSF) protein profile of AE patients with autoantibodies against N-methyl-d-aspartate receptor (NMDAR) (n = 9), leucine-rich glioma-inactivated protein 1 (LGI1) (n = 9), or glutamate decarboxylase 65 (GAD65) (n = 8) compared to 9 patients with relapsing-remitting multiple sclerosis as inflammatory controls, and 10 patients with somatic symptom disorder as non-inflammatory controls. We found a dysregulation of the complement system, a disbalance between pro-inflammatory and anti-inflammatory proteins on the one hand, and dysregulation of proteins involved in synaptic transmission, synaptogenesis, brain connectivity, and neurodegeneration on the other hand to a different extent in all AE subtypes compared to non-inflammatory controls. Furthermore, elevated levels of several proteases and reduction in protease inhibitors could be detected in all AE subtypes compared to non-inflammatory controls. Moreover, the different AE subtypes showed distinct protein profiles compared to each other and inflammatory controls which may facilitate future identification of disease-specific biomarkers. Overall, CSF proteomics provides insights into the complex pathophysiological mechanisms of AE, including immune dysregulation, neuronal dysfunction, neurodegeneration, and altered protease function.

Antiepileptic Drugs Elevate Astrocytic Kir4.1 Expression in the Rat Limbic Region

Inwardly rectifying potassium (Kir) channel subunits Kir4.1 are specifically expressed in astrocytes and regulate neuronal excitability by mediating spatial potassium buffering. In addition, it is now known that astrocytic Kir4.1 channels are closely involved in the pathogenesis of epilepsy. Here, to explore the role of Kir4.1 channels in the treatment of epilepsy, we evaluated the effects of the antiepileptic drugs, valproate, phenytoin, phenobarbital and ethosuximide, on Kir4.1 expression in astrocytes using immunohistochemical techniques. Repeated treatment of rats with valproate (30–300 mg/kg, i.p., for 1–10 days) significantly elevated the Kir4.1 expression levels in the cerebral cortex, amygdala and hippocampus. Up-regulation of Kir4.1 expression by valproate occurred in a dose- and treatment period-related manner, and did not accompany an increase in the number of astrocytes probed by glial fibrillary acidic protein (GFAP). In addition, repeated treatment with phenytoin (30 mg/kg, i.p., for 10 days) or phenobarbital (30 mg/kg, i.p., for 10 days) also elevated Kir4.1 expression region-specifically in the amygdala. However, ethosuximide (100 mg/kg, i.p., for 10 days), which can alleviate absence but not convulsive seizures, showed no effects on the astrocytic Kir4.1 expression. The present results demonstrated for the first time that the antiepileptic drugs effective for convulsive seizures (valproate, phenytoin, and phenobarbital) commonly elevate the astrocytic Kir4.1 channel expression in the limbic regions, which may be related to their antiepileptic actions.

Amygdala enlargement and emotional responses in (autoimmune) temporal lobe epilepsy

Temporal lobe epilepsy with amygdala enlargement (TLE-AE) is increasingly recognized as a distinct adult electroclinical syndrome. However, functional consequences of morphological alterations of the amygdala in TLE-AE are poorly understood. Here, two emotional stimulation designs were employed to investigate subjective emotional rating and skin conductance responses in a sample of treatment-naïve patients with suspected or confirmed autoimmune TLE-AE (n = 12) in comparison to a healthy control group (n = 16). A subgroup of patients completed follow-up measurements after treatment. As compared to healthy controls, patients with suspected or confirmed autoimmune TLE-AE showed markedly attenuated skin conductance responses and arousal ratings, especially pronounced for anxiety-inducing stimuli. The degree of right amygdala enlargement was significantly correlated with the degree of autonomic arousal attenuation. Furthermore, a decline of amygdala enlargement following prompt aggressive immunotherapy in one patient suffering from severe confirmed autoimmune TLE-AE with a very recent clinical onset was accompanied by a significant improvement of autonomic responses. Findings suggest dual impairments of autonomic and cognitive discrimination of stimulus arousal as hallmarks of emotional processing in TLE-AE. Emotional responses might, at least partially, recover after successful treatment, as implied by first single case data.

Selective Limbic Blood-Brain Barrier Breakdown in a Feline Model of Limbic Encephalitis with LGI1 Antibodies

Human leucine-rich glioma-inactivated protein 1 encephalitis (LGI1) is an autoimmune limbic encephalitis in which serum and cerebrospinal fluid contain antibodies targeting LGI1, a protein of the voltage gated potassium channel (VGKC) complex. Recently, we showed that a feline model of limbic encephalitis with LGI1 antibodies, called feline complex partial seizures with orofacial involvement (FEPSO), is highly comparable to human LGI1 encephalitis. In human LGI1 encephalitis, neuropathological investigations are difficult because very little material is available. Taking advantage of this natural animal model to study pathological mechanisms will, therefore, contribute to a better understanding of its human counterpart. Here, we present a brain-wide histopathological analysis of FEPSO. We discovered that blood–brain barrier (BBB) leakage was present not only in all regions of the hippocampus but also in other limbic structures such as the subiculum, amygdale, and piriform lobe. However, in other regions, such as the cerebellum, no leakage was observed. In addition, this brain-region-specific immunoglobulin leakage was associated with the breakdown of endothelial tight junctions. Brain areas affected by BBB dysfunction also revealed immunoglobulin and complement deposition as well as neuronal cell death. These neuropathological findings were supported by magnetic resonance imaging showing signal and volume increase in the amygdala and the piriform lobe. Importantly, we could show that BBB disturbance in LGI1 encephalitis does not depend on T cell infiltrates, which were present brain-wide. This finding points toward another, so far unknown, mechanism of opening the BBB. The limbic predilection sites of immunoglobulin antibody leakage into the brain may explain why most patients with LGI1 antibodies have a limbic phenotype even though LGI1, the target protein, is ubiquitously distributed across the central nervous system.

Does B lymphocyte-mediated autoimmunity contribute to post-stroke dementia?

Post-stroke cognitive decline and dementia pose a significant public health problem, with 30% of stroke survivors suffering from dementia. The reason for this high prevalence is not well understood. Pathogenic B cell responses to the damaged CNS are one possible contributing factor. B-lymphocytes and antibodies are present in and around the stroke core of some human subjects who die with stroke and dementia, and mice that develop delayed cognitive dysfunction after stroke have clusters of B-lymphocytes in the stroke lesion, and antibody infiltration in the stroked hemisphere. The ablation of B-lymphocytes prevents post-stroke cognitive impairment in mice. Multiple drugs that target B cells are FDA approved, and so if pathogenic B cell responses are occurring in a subset of stroke patients, this is potentially treatable. However, it has also been demonstrated that regulatory B cells can be beneficial in mouse models of stroke. Consequently, it is important to understand the relative contribution of B-lymphocytes to recovery versus pathogenicity, and if this balance is heterogeneous in different individuals. Therefore, the purpose of this review is to summarize the current state of knowledge with regard to the role of B-lymphocytes in the etiology of post-stroke dementia.

Hashimoto's encephalitis associated with AMPAR2 antibodies: a case report

BACKGROUND

Hashimoto's encephalitis (HE) is a rare neurological complication of Hashimoto's thyroiditis (HT), while limbic encephalitis (LE) is an autoimmune inflammatory disorder frequently associated with anti-neuronal antibodies. The glutamate receptor α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptor (AMPAR) is important for synaptic transmission, memory, and learning. The etiology of HE remains unclear. We present a case of HE with antibodies to AMPAR2 both in the serum and cerebrospinal fluid.

CASE PRESENTATION

The patient presented with progressive memory loss and subsequently went into a coma. Magnetic resonance imaging revealed temporal lobe and hippocampal lesions, while the electrocardiogram showed paroxysmal delta waves. Elevated serum levels of antibodies against thyroid globulin, thyroid peroxidase, and thyroid stimulating receptor were also noted. Ultrasonography showed enlargement of the thyroid gland. Therefore, the diagnosis was established as HE. Both the CSF and serum samples of the patient tested positive for antibodies to the cell-surface antigen AMPAR2. Intravenous injection of immunoglobulin followed by dexamethasone treatment resulted in recovery from the coma. Follow-up examination three months later showed some improvement of memory. To our knowledge, this is the first report on the detection of AMPAR2 antibodies in HE.

CONCLUSIONS

Our findings suggest that antibodies to AMPAR2 may be involved in the pathogenesis of HE. Elevated levels of thyroid antibodies possibly cause immune dysfunction, leading to the production of anti-AMPAR2 antibodies that are detrimental to the neurons. We believe that encephalitis patients with thyroid abnormalities should undergo screening for anti-neuronal antibodies, and early immune therapy may improve prognosis.

Impaired Autonomic Responses to Emotional Stimuli in Autoimmune Limbic Encephalitis

Limbic encephalitis (LE) is an autoimmune-mediated disorder that affects structures of the limbic system, in particular, the amygdala. The amygdala constitutes a brain area substantial for processing of emotional, especially fear-related signals. The amygdala is also involved in neuroendocrine and autonomic functions, including skin conductance responses (SCRs) to emotionally arousing stimuli. This study investigates behavioral and autonomic responses to discrete emotion evoking and neutral film clips in a patient suffering from LE associated with contactin-associated protein-2 (CASPR2) antibodies as compared to a healthy control group. Results show a lack of SCRs in the patient while watching the film clips, with significant differences compared to healthy controls in the case of fear-inducing videos. There was no comparable impairment in behavioral data (emotion report, valence, and arousal ratings). The results point to a defective modulation of sympathetic responses during emotional stimulation in patients with LE, probably due to impaired functioning of the amygdala.