An autopsy case of limbic encephalitis with voltage-gated potassium channel antibodies

A feline model of spontaneously occurring autoimmune limbic encephalitis

Autoimmune encephalitis (AE) is an important cause of encephalitis in humans and occurs at a similar rate to infectious encephalitis. It is frequently associated with antibodies against the extracellular domain of neuronal proteins. Among human AE, that with antibodies against leucine-rich glioma-inactivated 1 (LGI1) is one of the most prevalent forms, and was recently described in cats with limbic encephalitis (LE). In this study, we describe a large cohort (n = 32) of cats with AE, tested positive for voltage gated potassium channel (VGKC)-antibodies, of which 26 (81%) harboured LGI1-antibodies. We delineate their clinical and paraclinical features as well as long-term outcomes up to 5 years. Similar to human cases, most cats with LGI1-antibodies had a history of focal seizures (83%), clustering in the majority (88%), with interictal behavioural changes (73%). Among feline AE patients, there was no seizure type or other clinical characteristic that could distinguish LGI1-antibody positive from negative cats, unlike the pathognomic faciobrachial dystonic seizures seen in humans. Although six cats were euthanased in the first year for epilepsy-associated reasons, those attaining at least 1-year survival had good seizure control and quality of life with appropriate veterinary care and medication. Acute-phase immunotherapy (prednisolone) was given to the most severely unwell cases and its effect is retrospectively evaluated in 10 cats. Our data show LGI1-antibodies are an important cause of feline encephalitis, sharing many features with human AE. Further research should examine optimal therapeutic management strategies and the cause of LE in seronegative cats, building on paradigms established in the counterpart human disease.

Cytokine dynamics and targeted immunotherapies in autoimmune encephalitis

Autoimmune encephalitides constitute a diverse group of immune-mediated central nervous system disorders mainly characterized by the presence of antibodies targeting neuronal or glial antigens. Despite the notable contribution of antibody discovery to the understanding of their physiopathology, the specific immune cells and inflammatory mediators involved in autoimmune encephalitis are still poorly defined. However, cytokines have recently emerged as crucial signalling molecules in the pathogenesis of autoimmune encephalitis. Cytokines are biologically active, soluble, low-molecular-weight proteins or glycoproteins involved in a wide variety of physiological functions, including central nervous system development and homeostasis, immune surveillance, as well as proliferation and maturation of immune cells. Since unbalanced cytokine expression is considered a hallmark of many autoimmune central nervous system disorders, their identification and quantification has become an essential element in personalized medicine applied to the field of neuroimmunology. Several studies have explored the cytokine profile of autoimmune encephalitis, but their interpretation and comparison is challenging due to their small sample sizes and extremely high heterogeneity, especially regarding the cytokines analysed, type of sample used, and associated neural antibody. Only the cytokine profile of anti-N-methyl-D-aspartate receptor encephalitis has extensively been investigated, with findings suggesting that, although humoral immunity is the main effector, T cells may also be relevant for the development of this disorder. A better understanding of cytokine dynamics governing neuroinflammation might offer the opportunity of developing new therapeutic strategies against specific immune cells, cytokines, antibodies, or intracellular signalling cascades, therefore leading to better outcomes and preventing undesired side effects of the presently used strategies. In this review, we first summarize the current knowledge about the role of cytokines in the pathogenesis of autoimmune encephalitis, combining theoretical analysis with experimental validations, to assess their suitability as clinical biomarkers. Second, we discuss the potential applicability of the novel targeted immunotherapies in autoimmune encephalitis depending on the immunobiology of the associated antibody, their limitations, as well as the main limitations that should be addressed in future studies.

Deciphering the contributions of neuroinflammation to neurodegeneration: lessons from antibody-mediated encephalitis and coronavirus disease 2019

PURPOSE OF REVIEW

Does neuroinflammation promote neurodegeneration? Does neurodegeneration promote neuroinflammation? Or, is the answer to both questions, yes? These questions have proven challenging to answer in patients with typical age-related neurodegenerative diseases in whom the onset of neuroinflammation and neurodegeneration are largely unknown. Patients recovering from diseases associated with abrupt-onset neuroinflammation, including rare forms of antibody-mediated encephalitis (AME) and common complications of novel coronavirus disease 2019 (COVID-19), provide a unique opportunity to untangle the relationship between neuroinflammation and neurodegeneration. This review explores the lessons learned from patients with AME and COVID-19.

RECENT FINDINGS

Persistent cognitive impairment is increasingly recognized in patients recovering from AME or COVID-19, yet the drivers of impairment remain largely unknown. Clinical observations, neuroimaging and biofluid biomarkers, and pathological studies imply a link between the severity of acute neuroinflammation, subsequent neurodegeneration, and disease-associated morbidity.

SUMMARY

Data from patients with AME and COVID-19 inform key hypotheses that may be evaluated through future studies incorporating longitudinal biomarkers of neuroinflammation and neurodegeneration in larger numbers of recovering patients. The results of these studies may inform the contributors to cognitive impairment in patients with AME and COVID-19, with potential diagnostic and therapeutic applications in patients with age-related neurodegenerative diseases.

Limbic encephalitis

Limbic encephalitis (LE) is a clinical syndrome defined by subacutely evolving limbic signs and symptoms with structural and functional evidence of mediotemporal damage in the absence of a better explanation than an autoimmune (or paraneoplastic) cause. There are features common to all forms of LE. In recent years, antibody(ab)-defined subtypes have been established. They are distinct regarding underlying pathophysiologic processes, clinical and magnetic resonance imaging courses, cerebrospinal fluid signatures, treatment responsivity, and likelihood of a chronic course. With immunotherapy, LE with abs against surface antigens has a better outcome than LE with abs to intracellular antigens. Diagnostic and treatment challenges are, on the one hand, to avoid overlooking and undertreatment and, on the other hand, to avoid overdiagnoses and overtreatment. LE can be conceptualized as a model disease for the consequences of new onset mediotemporal damage by different mechanisms in adult life. It may be studied as an example of mediotemporal epileptogenesis.

Core cerebrospinal fluid biomarker profile in anti-LGI1 encephalitis

OBJECTIVE

To compare CSF biomarkers' levels in patients suffering from anti-Leucine-rich Glioma-Inactivated 1 (LGI1) encephalitis to neurodegenerative [Alzheimer's disease (AD), Creutzfeldt-Jakob's disease (CJD)] and primary psychiatric (PSY) disorders.

METHODS

Patients with LGI1 encephalitis were retrospectively selected from the French Reference Centre database between 2010 and 2019 and enrolled if CSF was available for biomarkers analysis including total tau (T-tau), phosphorylated tau (P-tau), amyloid-beta Aβ1-42, and neurofilaments light chains (Nf L). Samples sent for biomarker determination as part of routine practice, and formally diagnosed as AD, CJD, and PSY, were used as comparators.

RESULTS

Twenty-four patients with LGI1 encephalitis were compared to 39 AD, 20 CJD and 20 PSY. No significant difference was observed in T-tau, P-tau, and Aβ1-42 levels between LGI1 encephalitis and PSY patients. T-Tau and P-Tau levels were significantly lower in LGI1 encephalitis (231 and 43 ng/L) than in AD (621 and 90 ng/L, p < 0.001) and CJD patients (4327 and 55 ng/L, p < 0.001 and p < 0.01). Nf L concentrations of LGI1 encephalitis (2039 ng/L) were similar to AD (2,765 ng/L) and significantly higher compared to PSY (1223 ng/L, p < 0.005), but significantly lower than those of CJD (13,457 ng/L, p < 0.001). Higher levels of Nf L were observed in LGI1 encephalitis presenting with epilepsy (3855 ng/L) compared to LGI1 without epilepsy (1490 ng/L, p = 0.02). No correlation between CSF biomarkers' levels and clinical outcome could be drawn.

CONCLUSION

LGI encephalitis patients showed higher Nf L levels than PSY, comparable to AD, and even higher when presenting epilepsy suggesting axonal or synaptic damage linked to epileptic seizures.

Inflammation, ictogenesis, and epileptogenesis: An exploration through human disease

Epilepsy is seen historically as a disease of aberrant neuronal signaling manifesting as seizures. With the discovery of numerous auto-antibodies and the subsequent growth in understanding of autoimmune encephalitis, there has been an increasing emphasis on the contribution of the innate and adaptive immune system to ictogenesis and epileptogenesis. Pathogenic antibodies, complement activation, CD8+ cytotoxic T cells, and microglial activation are seen, to various degrees, in different seizure-associated neuroinflammatory and autoimmune conditions. These aberrant immune responses are thought to cause disruptions in neuronal signaling, generation of acute symptomatic seizures, and, in some cases, the development of long-term autoimmune epilepsy. Although early treatment with immunomodulatory therapies improves outcomes in autoimmune encephalitides and autoimmune epilepsies, patient identification and treatment selection are not always clear-cut. This review examines the role of the different components of the immune system in various forms of seizure disorders including autoimmune encephalitis, autoimmune epilepsy, Rasmussen encephalitis, febrile infection-related epilepsy syndrome (FIRES), and new-onset refractory status epilepticus (NORSE). In particular, the pathophysiology and unique cytokine profiles seen in these disorders and their links with diagnosis, prognosis, and treatment decision-making are discussed.

Network-wide abnormalities explain memory variability in hippocampal amnesia

Patients with hippocampal amnesia play a central role in memory neuroscience but the neural underpinnings of amnesia are hotly debated. We hypothesized that focal hippocampal damage is associated with changes across the extended hippocampal system and that these, rather than hippocampal atrophy per se, would explain variability in memory between patients. We assessed this hypothesis in a uniquely large cohort of patients (n = 38) after autoimmune limbic encephalitis, a syndrome associated with focal structural hippocampal pathology. These patients showed impaired recall, recognition and maintenance of new information, and remote autobiographical amnesia. Besides hippocampal atrophy, we observed correlatively reduced thalamic and entorhinal cortical volume, resting-state inter-hippocampal connectivity and activity in posteromedial cortex. Associations of hippocampal volume with recall, recognition, and remote memory were fully mediated by wider network abnormalities, and were only direct in forgetting. Network abnormalities may explain the variability across studies of amnesia and speak to debates in memory neuroscience.

Autoimmune Encephalitides

Autoimmune encephalitis is a severe inflammatory disorder of the brain with diverse causes and a complex differential diagnosis. Recent advances in the past decade have led to the identification of new syndromes and biological markers of limbic encephalitis, the commonest presentation of autoimmune encephalitis. The successful use of serum and intrathecal antibodies to diagnose affected patients has resulted in few biopsy and postmortem examinations. In those available, there can be variable infiltrating inflammatory T cells with cytotoxic granules in close apposition to neurons, consistent with an inflammatory autoimmune basis, but true vasculitis is rarely seen. The exception is Hashimoto encephalopathy.

Paraneoplastic autoimmune encephalitis associated with pleomorphic lung carcinoma: An autopsy case report

A 64-year-old man was admitted with acute onset disturbed consciousness. Cerebrospinal fluid analysis revealed pleocytosis and elevated protein, with negative cultures and PCR. Serum antibodies for autoimmune encephalitis were also negative. Brain magnetic resonance imaging (MRI) was unremarkable, but whole-body CT scan showed a tumor in the left lower lung lobe. Bronchial brush cytology demonstrated clusters of malignant cells, and 18 F-fluorodeoxyglucose positron emission tomography showed multiple lesions and increased uptake in the lung tumor. Clinically the patient had a stage IV lung carcinoma, graded as T3N3M1b (OSS). Steroid therapy had limited efficacy, but chemotherapy dramatically improved his neurological symptoms. Therefore, he was diagnosed with paraneoplastic autoimmune encephalitis based on the diagnostic criteria for paraneoplastic neurological syndromes. He died due to disease progression 14 months later. Subsequent postmortem examination revealed white ill-defined nodules in the left lung, with similar nodules in other organs. The brain weighed 1500 g before fixation, and a nodule was observed in the right precentral gyrus. Microscopically, the lung tumor was a pleomorphic carcinoma with an adenocarcinoma component. Multiple areas of micro-softening (≤500 μm) were identified in the cerebral cortex, gray-white matter junction and basal ganglia, and were distributed diffusely in both the limbic and non-limbic systems. Mild lymphocytic infiltrates were observed involving few intraparenchymal vessels. Few tumor metastases were observed in the right precentral gyrus. The multiple micro-softenings may reflect a chronic neuropathologic change of paraneoplastic autoimmune encephalitis. They were too small to be detected by brain MRI. However, these lesions may have the potential to cause the neurological symptoms in the acute phase because they were observed in many anatomical regions. We should pay attention to subtle findings such as micro-softenings when estimating the neuropathology of autoimmune encephalitis. Further investigations are needed to understand the characteristic neuropathology of this condition.

Neuropathology of autoimmune encephalitides

In recent years a large number of antibody-associated or antibody-defined encephalitides have been discovered. These conditions are often referred to as autoimmune encephalitides. The clinical features include prominent epileptic seizures, cognitive and psychiatric disturbance. These encephalitides can be divided in those with antibodies against intracellular antigens and those with antibodies against surface antigens. The discovery of new antibodies against targets on the surface of neurons is especially interesting since patients with such antibodies can be successfully treated immunologically. This chapter focuses on the pathology and the pathogenetic mechanisms involved in these encephalitides and discusses some of the questions that are raised in this exciting new field. It is important to realise, however, that because of the use of antibodies to diagnose the patients, and their improvement with treatment, there are relatively few biopsy or postmortem reports, limiting the neuropathological data and conclusions that can be drawn. For this reason we especially focus on the most frequent autoimmune encephalitides, those with antibodies to the NMDA receptor and with antibodies to the known protein components of the VGKC complex. Analysis of these encephalitides show completely different pathogenic mechanisms. In VGKC complex encephalitis, antibodies seem to bind to their target and activate complement, leading to destruction and loss of neurons. On the other hand, in NMDAR encephalitis, complement activation and neuronal degeneration seems to be largely absent. Instead, binding of antibodies leads to a decrease of NMDA receptors resulting in a hypofunction. This hypofunction offers an explanation for some of the clinical features such as psychosis and episodic memory impairment, but not for the frequent seizures. Thus, additional analysis of the few human brain specimens present and the use of specific animal models are needed to further understand the effects of these antibodies in autoimmune encephalitides.

Remembering Preservation in Hippocampal Amnesia

The lesion-deficit model dominates neuropsychology. This is unsurprising given powerful demonstrations that focal brain lesions can affect specific aspects of cognition. Nowhere is this more evident than in patients with bilateral hippocampal damage. In the past 60 years, the amnesia and other impairments exhibited by these patients have helped to delineate the functions of the hippocampus and shape the field of memory. We do not question the value of this approach. However, less prominent are the cognitive processes that remain intact following hippocampal lesions. Here, we collate the piecemeal reports of preservation of function following focal bilateral hippocampal damage, highlighting a wealth of information often veiled by the field's focus on deficits. We consider how a systematic understanding of what is preserved as well as what is lost could add an important layer of precision to models of memory and the hippocampus.

Scenes, Spaces, and Memory Traces: What Does the Hippocampus Do?

The hippocampus is one of the most closely scrutinized brain structures in neuroscience. While traditionally associated with memory and spatial cognition, in more recent years it has also been linked with other functions, including aspects of perception and imagining fictitious and future scenes. Efforts continue apace to understand how the hippocampus plays such an apparently wide-ranging role. Here we consider recent developments in the field and in particular studies of patients with bilateral hippocampal damage. We outline some key findings, how they have subsequently been challenged, and consider how to reconcile the disparities that are at the heart of current lively debates in the hippocampal literature.

IgG and complement deposition and neuronal loss in cats and humans with epilepsy and voltage-gated potassium channel complex antibodies

Voltage-gated potassium channel complex (VGKC-complex) antibody (Ab) encephalitis is a well-recognized form of limbic encephalitis in humans, usually occurring in the absence of an underlying tumor. The patients have a subacute onset of seizures, magnetic resonance imaging findings suggestive of hippocampal inflammation, and high serum titers of Abs against proteins of the VGKC-complex, particularly leucine-rich, glioma-inactivated 1 (LGI1). Most patients are diagnosed promptly and recover substantially with immunotherapies; consequently, neuropathological data are limited. We have recently shown that feline complex partial cluster seizures with orofacial involvement (FEPSO) in cats can also be associated with Abs against VGKC-complexes/LGI1. Here we examined the brains of cats with FEPSO and compared the neuropathological findings with those in a human with VGKC-complex-Ab limbic encephalitis. Similar to humans, cats with VGKC-complex-Ab and FEPSO have hippocampal lesions with only moderate T-cell infiltrates but with marked IgG infiltration and complement C9neo deposition on hippocampal neurons, associated with neuronal loss. These findings provide further evidence that FEPSO is a feline form of VGKC-complex-Ab limbic encephalitis and provide a model for increasing understanding of the human disease.

Autoimmune epilepsies

In patients with immune-associated disorders of the gray central nervous system matter (including recurrent seizures), antibodies against intracellular antigens have been discovered since the 1980s/1990s. In recent years, new antibodies against surface antigens have also been discovered. In two respects, these antibodies are even more interesting than the ones to intracellular antigens as, first, they promise a better response to immunotherapy; and, second, these antibodies contribute greatly to the understanding of the disease mechanisms. Whereas in encephalitides with antibodies against intracellular antigens, a cytotoxic T-cell-mediated response seems to be responsible for neuronal cell loss, in encephalitides with autoantibodies against surface antigens these antibodies are probably the relevant pathogenic agents in the associated disease conditions. On the one hand, antibodies to the NR1 subunit of N-methyl-D-aspartate receptors have been suggested to cause internalization and loss of these receptors without any cell destruction. This mechanism can explain the reversible functional effects caused by these antibodies. On the other hand, antibody- and complement-mediated destructive, and the irreversible effects of antibodies against the voltage-gated potassium channel antigens have been noted. These emerging findings make it plausible that immunological therapies, preferably early after characterization of the antibodies, offer opportunities to restore the health of affected patients.

Molecular and cellular mechanisms underlying anti-neuronal antibody mediated disorders of the central nervous system

Over the last decade multiple autoantigens located on the plasma membrane of neurons have been identified. Neuronal surface antigens include molecules directly involved in neurotransmission and excitability. Binding of the antibody to the antigen may directly alter the target protein's function, resulting in neurological disorders. The often striking reversibility of symptoms following early aggressive immunotherapy supports a pathogenic role for autoantibodies to neuronal surface antigens. In order to better understand and treat these neurologic disorders it is important to gain insight in the underlying mechanisms of antibody pathogenicity. In this review we discuss the clinical, circumstantial, in vitro and in vivo evidence for neuronal surface antibody pathogenicity and the possible underlying cellular and molecular mechanisms. This review shows that antibodies to neuronal surface antigens are often directed at conformational epitopes located in the extracellular domain of the antigen. The conformation of the epitope can be affected by specific posttranslational modifications. This may explain the distinct clinical phenotypes that are seen in patients with antibodies to antigens that are expressed throughout the brain. Furthermore, it is likely that there is a heterogeneous antibody population, consisting of different IgG subtypes and directed at multiple epitopes located in an immunogenic region. Binding of these antibodies may result in different pathophysiological mechanisms occurring in the same patient, together contributing to the clinical syndrome. Unraveling the predominant mechanism in each distinct antigen could provide clues for therapeutic interventions.

[Pathophysiology of antibody-associated diseases of the central nervous system]

Antibodies against intracellular antigens have been known since the 1980s and 1990s but in recent years antibodies against surface antigens have also been discovered. These are "more interesting" than those to intracellular targets in two respects: they result in a better response to immunotherapy and are probably directly pathogenic, which helps to understand the disease mechanisms. There are the destructive and irreversible effects of the antibodies to antigens that are complexed with voltage-gated potassium channels (VGKC complex antibodies), especially antibodies to leucine-rich glioma inactivated 1 (LGI1) on the one hand. On the other hand, antibodies may have reversible functional effects, such as antibodies against the N-methyl-D-aspartate receptor, which cause an internalization of these receptors but do not lead to cell destruction: LGI1 antibodies also seem to have functional, in this case epileptogenic effects. These emerging findings make plausible why antibody-reducing therapies provide opportunities for the restoration of health in affected patients.

Epileptic encephalitis: the role of the innate and adaptive immune system

Seizures are a prominent clinical feature of encephalitis. Recent data suggest the adaptive as well as innate immune system to be involved directly in the pathomechanism of epileptogenesis. Cytotoxic T-cells and antibody-mediated complement activation are major components of the adaptive immune system, which can induce neurodegeneration, thereby probably contributing to epileptic encephalitis. The innate immune system operates via interleukin-1 and toll-like receptor-associated mechanisms and was shown to play a direct role in epileptogenesis. Here, we review neuropathology hallmarks of various encephalitis conditions such as Rasmussen encephalitis (RE) but also introduce the more recently discovered antibody-associated voltage-gated potassium channel complex (VGKC), N-methyl-D-aspartate receptor (NMDAR) or glutamic acid decarboxylase (GAD) 65 encephalitides. Neuropathological investigations are used to determine specific cellular components and molecular mechanisms used by the immune system to provoke neurodegeneration and to promote epileptogenesis. Based on recent findings, we propose concepts for the stratification of epileptic encephalitis. Knowledge of the role of the innate immunity has already translated into clinical treatment strategies and may help to discover novel drug targets for these epileptic disorders.

Paraneoplastic encephalomyelopathies: pathology and mechanisms

The last three decades have seen major advances in the understanding of paraneoplastic and idiopathic autoimmune disorders affecting the central nervous system (CNS). Neural-specific autoantibodies and their target antigens have been discovered, immunopathology and neuroimaging patterns recognized and pathogenic mechanisms elucidated. Disorders accompanied by autoantibody markers of neural peptide-specific cytotoxic effector T cells [such as anti-neuronal nuclear antibody type 1 (ANNA-1, aka anti-Hu), Purkinje cell antibody type 1 (PCA-1, aka anti-Yo) and CRMP-5 IgG] are generally poorly responsive to immunotherapy. Disorders accompanied by neural plasma membrane-reactive autoantibodies [the effectors of synaptic disorders, which include antibodies targeting voltage-gated potassium channel (VGKC) complex proteins, NMDA and GABA-B receptors] generally respond well to early immunotherapy. Here we describe in detail the neuropathological findings and pathophysiology of paraneoplastic CNS disorders with reference to antigen-specific serology and neurological and oncological contexts.

Faciobrachial dystonic seizures precede Lgi1 antibody limbic encephalitis

OBJECTIVE

To describe a distinctive seizure semiology that closely associates with voltage-gated potassium channel (VGKC)-complex/Lgi1 antibodies and commonly precedes the onset of limbic encephalitis (LE).

METHODS

Twenty-nine patients were identified by the authors (n = 15) or referring clinicians (n = 14). The temporal progression of clinical features and serum sodium, brain magnetic resonance imaging (MRI), positron emission tomography/single photon emission computed tomography, and VGKC-complex antibodies was studied.

RESULTS

Videos and still images showed a distinctive adult-onset, frequent, brief dystonic seizure semiology that predominantly affected the arm and ipsilateral face. We have termed these faciobrachial dystonic seizures (FBDS). All patients tested during their illness had antibodies to VGKC complexes; the specific antigenic target was Lgi1 in 89%. Whereas 3 patients never developed LE, 20 of the remaining 26 (77%) experienced FBDS prior to the development of the amnesia and confusion that characterize LE. During the prodrome of FBDS alone, patients had normal sodium and brain MRIs, but electroencephalography demonstrated ictal epileptiform activity in 7 patients (24%). Following development of LE, the patients often developed other seizure semiologies, including typical mesial temporal lobe seizures. At this stage, investigations commonly showed hyponatremia and MRI hippocampal high T2 signal; functional brain imaging showed evidence of basal ganglia involvement in 5/8. Antiepileptic drugs (AEDs) were generally ineffective and in 41% were associated with cutaneous reactions that were often severe. By contrast, immunotherapies produced a clear, and often dramatic, reduction in FBDS frequency.

INTERPRETATION

Recognition of FBDS should prompt testing for VGKC-complex/Lgi1 antibodies. AEDs often produce adverse effects; treatment with immunotherapies may prevent the development of LE with its potential for cerebral atrophy and cognitive impairment.