Update in Autoimmune Movement Disorders: Newly Described Antigen Targets in Autoimmune and Paraneoplastic Cerebellar Ataxia

The pathogenesis of neurological immune-related adverse events following immune checkpoint inhibitor therapy

Cancer is a leading cause of morbidity and mortality worldwide. The development of immune checkpoint inhibitors (ICI) has revolutionised cancer therapy, and patients who were previously incurable can now have excellent responses. These therapies work by blocking inhibitory immune pathways, like cytotoxic T lymphocyte-associated protein 4 (CTLA-4), programmed cell death-1 (PD-1), its ligand PD-L1, and lymphocyte activation gene 3 (LAG-3); which leads to increased anti-tumour immune responses. However, their use can lead to the development of immune-related adverse events (irAEs), which may result in severe disability, interruption of cancer therapy, and even death. Neurological autoimmune sequelae occur in 1-10 % of patients treated with ICIs and can be fatal. They encompass a broad spectrum of diseases, may affect the central and the peripheral nervous system, and include syndromes like encephalitis, cerebellitis, neuropathy, and myositis. In some cases, neurological irAEs can be associated with autoantibodies recognising neuronal or glial targets. In this review, we first describe the key targets in ICI therapy, followed by a formulation of irAEs and their clinical presentations, where we focus on neurological syndromes. We comprehensively formulate the current literature evaluating cell surface and intracellular autoantibodies, cytokines, chemokines, leukocyte patterns, other blood derived biomarkers, and immunogenetic profiles; and highlight their impact on our understanding of the pathogenesis of neurological irAEs. Finally, we describe therapeutic pathways and patient outcomes, and provide an overview on future aspects of ICI cancer therapy.

Overlapping syndrome with concomitant mGluR2-Ab and GFAP-Ab: A case report

An 11-year-old male patient presented with acute neurological deterioration, including meningitis, encephalitis, and myelitis. Initial testing detected antibody against metabotropic glutamate receptor 2 (mGluR2-Ab); retesting identified coexisting anti-glial fibrillary acidic protein antibody (GFAP-Ab). Pathogenic examinations were negative. The patient received a combination of immunotherapies, including intravenous methylprednisolone pulse therapy, human immunoglobulin pulse therapy, and plasma exchange. Five months after onset, the patient showed a favorable prognosis, though a residual neurogenic bladder remained. We present the first documented case of overlapping autoimmune syndrome with concurrent mGluR2-Ab and GFAP-Ab. The coexistence of these antibodies can lead to a wide range of clinical manifestations, including simultaneous GFAP-Ab-related meningoencephalomyelitis and mGluR2-Ab-related ataxia. This complexity complicates diagnosis and treatment, underscoring the need for a comprehensive understanding of the pathogenic mechanisms of these antibodies and timely combined immunotherapy. Additionally, this case suggests that mGluR2-Ab and GFAP-Ab may serve as the jointly responsible antibodies for this condition.

A Case Series of Anti-Metabotropic Glutamate Receptor 2 Antibody-Related Diseases with Distinct Neurological Involvement

Background

Anti-metabotropic glutamate receptor 2 (mGluR2)-related diseases are rare autoimmune disorders of the central nervous system that primarily affect the cerebellum and are occasionally associated with malignancies.

Methods

Data, including demographics, symptoms, blood and cerebrospinal fluid (CSF) tests, and brain magnetic resonance imaging (MRI), were retrospectively collected from two patients with informed consent at Xiangya Hospital from February 2024 to October 2024. Autoantibodies associated with autoimmune encephalitis were tested using cell-based assays. The literature describing anti-mGluR2 antibody-related diseases was searched for in PubMed and five cases were reviewed.

Results

Two cases of anti-mGluR2 antibody-related diseases were reported: one with acute cerebellitis and the other with refractory seizures. Brain MRI showed cerebellar involvement in the cerebellitis patient. Anti-mGluR2 antibodies were detected in the serum but not in the CSF of both cases, and both responded well to immunotherapy. A review of five patients (all female, aged 3-78 years) found four with cerebellar ataxia or cerebellitis and one with immune-related epilepsy. Common symptoms included dysarthria, gait instability, and gaze/nystagmus, while seizures were rare. MRI revealed cerebellar involvement in most cases. Anti-mGluR2 antibodies were present in the serum of all patients but only in the CSF of two. Three patients responded well to immunosuppressive treatment, and two had malignancies.

Conclusion

Anti-mGluR2 antibody-related diseases are autoimmune disorders primarily characterized by ataxic manifestations, though seizures may also occur. The effectiveness of immunosuppressive treatment is uncertain and screening for tumors is necessary.

Demystifying the Etiology of ILOCA in the Genomic Era: A Narrative Review

BACKGROUND

Idiopathic Late-Onset Cerebellar Ataxia (ILOCA) is a challenging and heterogeneous disorder characterized by progressive cerebellar ataxia beginning after the age of 40 without a family history of cerebellar ataxia. Despite extensive investigations, many cases remain undiagnosed. The advent Next Generation Sequencing (NGS) has significantly advanced the identification of genetic causes associated with ILOCA.

OBJECTIVE

This study aims to review the concept of ILOCA, its historical perspective, epidemiology, diagnostic criteria, and the impact of the new era of genetic diagnosis facilitated by NGS technologies.

METHODS

A comprehensive literature review was conducted, focusing on the genetic advancements in diagnosing ILOCA.

RESULTS

ILOCA accounts for a significant proportion of late-onset cerebellar ataxias. The prevalence of late-onset cerebellar ataxias ranges from 2.2 to 12.4 per 100,000 individuals, with genetic causes identified in up to 30-50% of cases using NGS. Key genetic findings include repeat expansion disorders such as Spinocerebellar Ataxia type 27 B, Cerebellar Ataxia, Neuropathy and Vestibular Areflexia Syndrome and Friedreich Ataxia. SCAs and Autosomal Recessive Cerebellar Ataxia caused by point mutations are also frequently observed in large cohorts. Advances in NGS have increased the diagnostic yield for ILOCA.

CONCLUSION

ILOCA represents a significant diagnostic challenge due to its heterogeneous nature and the overlap with other neurodegenerative and genetic conditions. The use of NGS technologies has revolutionized the diagnostic approach, uncovering genetic causes in a substantial number of previously undiagnosed cases. Routine investigation of specific genes associated with ILOCA is recommended to improve diagnostic accuracy and patient management.

Raphe and ventrolateral medulla proteomics in sudden unexplained death in childhood with febrile seizure history

Sudden unexplained death in childhood (SUDC) is death of a child ≥ 12 months old that is unexplained after autopsy and detailed analyses. Among SUDC cases, ~ 30% have febrile seizure (FS) history, versus 2-5% in the general population. SUDC cases share features with sudden unexpected death in epilepsy (SUDEP) and sudden infant death syndrome (SIDS), in which brainstem autonomic dysfunction is implicated. To understand whether brainstem protein changes are associated with FS history in SUDC, we performed label-free quantitative mass spectrometry on microdissected midbrain dorsal raphe, medullary raphe, and the ventrolateral medulla (n = 8 SUDC-noFS, n = 11 SUDC-FS). Differential expression analysis between SUDC-FS and SUDC-noFS at p < 0.05 identified 178 altered proteins in dorsal raphe, 344 in medullary raphe, and 100 in the ventrolateral medulla. These proteins were most significantly associated with increased eukaryotic translation initiation (p = 3.09 × 10-7, z = 1.00), eukaryotic translation elongation (p = 6.31 × 10-49, z = 6.01), and coagulation system (p = 1.32 × 10-5, z = 1.00). The medullary raphe had the strongest enrichment for altered signaling pathways, including with comparisons to three other brain regions previously analyzed (frontal cortex, hippocampal dentate gyrus, cornu ammonus). Immunofluorescent tissue analysis of serotonin receptors identified 2.1-fold increased 5HT2A in the medullary raphe of SUDC-FS (p = 0.025). Weighted gene correlation network analysis (WGCNA) of case history indicated that longer FS history duration significantly correlated with protein levels in the medullary raphe and ventrolateral medulla; the most significant gene ontology biological processes were decreased cellular respiration (p = 9.8 × 10-5, corr = - 0.80) in medullary raphe and decreased synaptic vesicle cycle (p = 1.60 × 10-7, corr = - 0.90) in the ventrolateral medulla. Overall, FS in SUDC was associated with more protein differences in the medullary raphe and was related with increased translation-related signaling pathways. Future studies should assess whether these changes result from FS or may in some way predispose to FS or SUDC.

Identification of DAGLA as an autoantibody target in cerebellar ataxia

BACKGROUND

We aimed to investigate the clinical, imaging and fluid biomarker characteristics in patients with antidiacylglycerol lipase alpha (DAGLA)-autoantibody-associated cerebellitis.

METHODS

Serum and cerebrospinal fliud (CSF) samples from four index patients were subjected to comprehensive autoantibody screening by indirect immunofluorescence assay (IIFA). Immunoprecipitation, mass spectrometry and recombinant protein assays were used to identify the autoantigen. Sera from 101 patients with various neurological symptoms and a similar tissue staining pattern as the index patient samples, and 102 healthy donors were analysed in recombinant cell-based IIFA (RC-IIFA) with the identified protein. Epitope characterisation of all positive samples was performed via ELISA, immunoblot, immunoprecipitation and RC-IIFA using different DAGLA fragments.

RESULTS

All index patients were relatively young (age: 18-34) and suffered from pronounced gait ataxia, dysarthria and visual impairments. Paraclinical hallmarks in early-stage disease were inflammatory CSF changes and cerebellar cortex hyperintensity in MRI. Severe cerebellar atrophy developed in three of four patients within 6 months. All patient samples showed the same unclassified IgG reactivity with the cerebellar molecular layer. DAGLA was identified as the target antigen and confirmed by competitive inhibition experiments and DAGLA-specific RC-IIFA. In RC-IIFA, serum reactivity against DAGLA was also found in 17/101 disease controls, including patients with different clinical phenotypes than the one of the index patients, and in 1/102 healthy donors. Epitope characterisation revealed that 17/18 anti-DAGLA-positive control sera reacted with a C-terminal intracellular DAGLA 583-1042 fragment, while the CSF samples of the index patients targeted a conformational epitope between amino acid 1 and 157.

CONCLUSIONS

We propose that anti-DAGLA autoantibodies detected in CSF, with a characteristic tissue IIFA pattern, represent novel biomarkers for rapidly progressive cerebellitis.

Immune-mediated ataxias: Guide to clinicians

Immune-mediated cerebellar ataxias were initially described as a clinical entity in the 1980s, and since then, an expanding body of evidence has contributed to our understanding of this topic. These ataxias encompass various etiologies, including postinfectious cerebellar ataxia, gluten ataxia, paraneoplastic cerebellar degeneration, opsoclonus-myoclonus-ataxia syndrome and primary autoimmune cerebellar ataxia. The increased permeability of the brain-blood barrier could potentially explain the vulnerability of the cerebellum to autoimmune processes. In this manuscript, our objective is to provide a comprehensive review of the most prevalent diseases within this group, emphasizing clinical indicators, pathogenesis, and current treatment approaches.

A not so incidental 'incidentaloma' - pediatric ganglioneuroma-associated cerebellar degeneration and super-refractory status epilepticus: case report and literature review

Paraneoplastic neurological disorders are rare in children, with paraneoplastic cerebellar degeneration (PCD) considered highly atypical. We describe a 13-year-old girl with progressive neurobehavioral regression, cerebellar ataxia, and intractable epilepsy presenting in super-refractory status epilepticus. After an extensive evaluation, her clinical picture was suggestive of probable autoimmune encephalitis (AE). Further diagnostic testing revealed a molecularly undefined neural-restricted autoantibody in both serum and CSF, raising suspicion over an adrenal mass previously considered incidental. Surgical resection led to a robust clinical improvement, and pathology revealed a benign ganglioneuroma. This report widens the spectrum of paraneoplastic manifestations of ganglioneuroma, reviews the diagnostic approach to antibody-negative pediatric AE, and raises important clinical considerations regarding benign and incidentally found tumors in the setting of a suspected paraneoplastic neurologic syndrome.

Functions and mechanisms of the GPCR adaptor protein Norbin

Norbin (Neurochondrin, NCDN) is a highly conserved 79 kDa adaptor protein that was first identified more than a quarter of a century ago as a gene up-regulated in rat hippocampus upon induction of long-term potentiation. Most research has focussed on the role of Norbin in the nervous system, where the protein is highly expressed. Norbin regulates neuronal morphology and synaptic plasticity, and is essential for normal brain development and homeostasis. Dysregulation of Norbin is linked to a variety of neurological conditions. Recently, Norbin was shown to be expressed in myeloid cells as well as neurons. Myeloid-cell specific deletion revealed an important role of Norbin as a suppressor of neutrophil-derived innate immunity. Norbin limits the ability of neutrophils to clear bacterial infections by curbing the responsiveness of these cells to inflammatory and infectious stimuli. Mechanistically, Norbin regulates cell responses through binding to its interactors, in particular to a wide range of G protein-coupled receptors (GPCRs). Norbin association with GPCRs controls GPCR trafficking and signalling. Other important Norbin interactors are the Rac guanine-nucleotide exchange factor P-Rex1 and protein kinase A. Downstream signalling pathways regulated by Norbin include ERK, Ca2+ and the small GTPase Rac. Here, we review the current understanding of Norbin structure, expression and its roles in health and disease. We also explore Norbin signalling through its interactors, with a particular focus on GPCR trafficking and signalling. Finally, we discuss avenues that could be pursued in the future to increase our understanding of Norbin biology.

Case report: Anti septin-5-encephalitis as a treatable cause of cerebellar ataxia and psychiatric symptoms

Objectives

Anti-septin-5 encephalitis is a rare disease with only few published cases, mainly based on retrospective CSF and serum analyses. Predominant symptoms are cerebellar ataxia and oculomotor abnormalities. Due to the rareness of the disease, treatment recommendations are scarce. Herein, we prospectively describe the clinical course of a female patient with anti-septin-5 encephalitis.

Methods

We describe diagnostic workup, treatment and follow-up of a 54-year-old patient presenting with vertigo, unsteady gait, lack of drive and behavioral changes.

Results

Clinical examination revealed severe cerebellar ataxia, saccadic smooth pursuit, upbeat-nystagmus, and dysarthria. Additionally, the patient presented with a depressive syndrome. MRI of the brain and spinal cord were normal. CSF analysis showed lymphocytic pleocytosis (11 cells/μl). Extensive antibody testing revealed anti septin-5 IgG in both CSF and serum without coexisting anti-neuronal antibodies. PET/CT detected no signs of malignancy. Corticosteroids, plasma exchange, and rituximab led to transient clinical improvement followed by relapse. Re-applied treatment with plasma exchange followed by bortezomib resulted in moderate but sustained clinical improvement.

Discussion

Anti septin-5 encephalitis represents a rare but treatable and therefore relevant differential diagnosis in patients with cerebellar ataxia. Psychiatric symptoms can be observed in anti septin-5 encephalitis. Immunosuppressive treatment including bortezomib is moderately effective.

Anti-neurochondrin antibody as a biomarker in primary autoimmune cerebellar ataxia-a case report and review of the literature

BACKGROUND AND PURPOSE

Neuronal autoantibodies can support the diagnosis of primary autoimmune cerebellar ataxia (PACA). Knowledge of PACA is still sparce. This article aims to highlight the relevance of anti-neurochondrin antibodies and possible therapeutical consequences in people with PACA.

METHODS

This is a case presentation and literature review of PACA associated with anti-neurochondrin antibodies.

RESULTS

A 33-year-old man noticed reduced control of the right leg in May 2020. During his first clinic appointment at our institution in September 2021, he complained about gait imbalance, fine motor disorders, tremor, intermittent diplopia and slurred speech. He presented a pancerebellar syndrome with stance, gait and limb ataxia, scanning speech and oculomotor dysfunction. Within 3 months the symptoms progressed. An initial cerebral magnetic resonance imaging, June 2020, was normal, but follow-up imaging in October 2021 and July 2022 revealed marked cerebellar atrophy (29% volume loss). Cerebrospinal fluid analysis showed lymphocytic pleocytosis of 11 x 10³ /L (normal range 0-4) and oligoclonal bands type II. Anti-neurochondrin antibodies (immunoglobulin G) were detected in serum (1:10,000) and cerebrospinal fluid (1:320, by cell-based indirect immunofluorescence assay and immunoblot, analysed by the EUROIMMUN laboratory). After ruling out alternative causes and neoplasia, diagnosis of PACA was given and immunotherapy (steroids and cyclophosphamide) was started in January 2022. In March 2022 a stabilization of disease was observed.

CONCLUSION

Cerebellar ataxia associated with anti-neurochondrin antibodies has only been described in 19 cases; however, the number of unrecognized PACAs may be higher. As anti-neurochondrin antibodies target an intracellular antigen and exhibit a mainly cytotoxic T-cell-mediated pathogenesis, important therapeutic implications may result. Because of the severe and rapid clinical progression, aggressive immunotherapy was warranted. This case highlights the need for rapid diagnosis and therapy in PACA, as stabilization and even improvement of symptoms are attainable.

Challenging Cases in Neuroimmunology

Neuroimmunology is rapidly evolving field extending from well-known, but incompletely understood conditions like multiple sclerosis, to novel antibody-mediated disorders, of which dozens have been described in the past 10 years. The ongoing expansion in knowledge needed to effectively diagnose and treat these patients presents myriad challenges for clinicians. Here, we discuss six informative cases from our institution. By highlighting these challenging cases, we hope to instill fundamental points on the nuances of diagnosis and management for conditions including tumefactive multiple sclerosis, antibody-mediated encephalitis, antiphospholipid antibody syndrome, neuromyelitis optica, and myelin oligodendrocyte glycoprotein IgG-associated disease.

Treatable Ataxias: How to Find the Needle in the Haystack?

Treatable ataxias are a group of ataxic disorders with specific treatments. These disorders include genetic and metabolic disorders, immune-mediated ataxic disorders, and ataxic disorders associated with infectious and parainfectious etiology, vascular causes, toxins and chemicals, and endocrinopathies. This review provides a comprehensive overview of different treatable ataxias. The major metabolic and genetic treatable ataxic disorders include ataxia with vitamin E deficiency, abetalipoproteinemia, cerebrotendinous xanthomatosis, Niemann-Pick disease type C, autosomal recessive cerebellar ataxia due to coenzyme Q10 deficiency, glucose transporter type 1 deficiency, and episodic ataxia type 2. The treatment of these disorders includes the replacement of deficient cofactors and vitamins, dietary modifications, and other specific treatments. Treatable ataxias with immune-mediated etiologies include gluten ataxia, anti-glutamic acid decarboxylase antibody-associated ataxia, steroid-responsive encephalopathy associated with autoimmune thyroiditis, Miller-Fisher syndrome, multiple sclerosis, and paraneoplastic cerebellar degeneration. Although dietary modification with a gluten-free diet is adequate in gluten ataxia, other autoimmune ataxias are managed by short-course steroids, plasma exchange, or immunomodulation. For autoimmune ataxias secondary to malignancy, treatment of tumor can reduce ataxic symptoms. Chronic alcohol consumption, antiepileptics, anticancer drugs, exposure to insecticides, heavy metals, and recreational drugs are potentially avoidable and treatable causes of ataxia. Infective and parainfectious causes of cerebellar ataxias include acute cerebellitis, postinfectious ataxia, Whipple's disease, meningoencephalitis, and progressive multifocal leukoencephalopathy. These disorders are treated with steroids and antibiotics. Recognizing treatable disorders is of paramount importance when dealing with ataxias given that early treatment can prevent permanent neurological sequelae.

The Emerging Key Role of the mGluR1-PKCγ Signaling Pathway in the Pathogenesis of Spinocerebellar Ataxias: A Neurodevelopmental Viewpoint

Spinocerebellar ataxias (SCAs) are a heterogeneous group of autosomal dominantly inherited progressive disorders with degeneration and dysfunction of the cerebellum. Although different subtypes of SCAs are classified according to the disease-associated causative genes, the clinical syndrome of the ataxia is shared, pointing towards a possible convergent pathogenic pathway among SCAs. In this review, we summarize the role of SCA-associated gene function during cerebellar Purkinje cell development and discuss the relationship between SCA pathogenesis and neurodevelopment. We will summarize recent studies on molecules involved in SCA pathogenesis and will focus on the mGluR1-PKCγ signaling pathway evaluating the possibility that this might be a common pathway which contributes to these diseases.

Case Report: Alzheimer's Dementia Associated With Cerebrospinal Fluid Neurochondrin Autoantibodies

Background

Neurochondrin autoimmunity is a rare disorder mainly associated with cerebellar and vestibular syndromes. Our report aims to enlarge its phenotypic spectrum to encompass major cognitive disorder with very late onset never before reported in conjunction with neurochondrin antibodies.

Methods

We describe the case of an 85-year-old woman who presented in our memory clinic. Retrospective analysis of patient records included cerebrospinal fluid (CSF) analysis, magnetic resonance imaging (MRI), and neuropsychological testing using the CERAD-plus.

Results

Because of her unknown onset of progressive cognitive dysfunction in conjunction with speech and language problems, we decided to take an extensive differential diagnostic approach including a search for neural autoantibodies potentially involved in cognitive impairment. Our patient presented serum and CSF neurochondrin autoantibodies. Further CSF analysis revealed elevated tau and ptau 181 protein as well as a reduced Aß42/40 ratio in CSF, thus matching a biomarker profile of Alzheimer's disease (AD). Neuropsychological tests revealed predominant and severe deficits in verbal and visual memory. Her MRI showed reduced parietal and cerebellar brain volume.

Discussion

Taken together, this case reveals the novelty of a patient with a CSF-based and typical clinical and imaging profile of AD. She is also likely to have neurochondrin autoimmunity, as we detected neurochondrin autoantibodies in her CSF; we therefore diagnosed AD dementia associated with neurochondrin antibodies. Our case expands the spectrum of neurochondrin autoimmunity to disorders involving major cognitive disorder such as AD dementia. Furthermore, we speculate that neurochondrin autoimmunity might have triggered an acceleration of AD symptoms as its onset was reported only after a short 6-month interval via a synergistic or negatively additive hybrid mechanism of action between neurodegeneration and autoimmunity.

Anti-homer-3 Antibody Encephalitis in a 10-Year-Old Child: Case Report and Review of the Literature

OBJECTIVE

We present a rare case with anti-Homer-3 antibodies positive encephalitis in the youngest patient ever identified and reviewed the literature.

CASE REPORT

A 10-year-old, Chinese boy came for evaluation of a 2-week history of cognitive impairment, irritability, dysarthria, and cautious gait. The neurological examination was consistent with the pan-cerebellar syndrome and encephalopathy. Cerebrospinal fluid (CSF) was inflammatory with increased leukocytes. Magnetic resonance imaging of the brain showed hyperintensities in both cerebellar hemispheres and vermis in Fluid-attenuated inversion recovery (FLAIR) and T2- weighted sequences. Infectious disorders were ruled out, but positivity for anti-Homer-3 antibodies was detected in the CSF, but not in the serum. Additionally, low titers of voltage-gated calcium channel (VGCC) antibodies were found in the serum. Treatment with intravenous (IV) corticosteroids did not provide meaningful clinical improvement; however, the patient achieved almost complete recovery (modified Ranking Scale score: 1) following IV immunoglobulin.

CONCLUSION

Anti-Homer-3 cerebellar ataxia with encephalopathy should be considered within the differential diagnosis of acute inflammatory cerebellar disease in children and it may coexist with VGCC antibodies.

Case Report: Hematologic Recovery Following Stereotactic Ablative Radiotherapy in a Patient With Early-Stage Non-Small Cell Lung Cancer and Paraneoplastic Myelofibrosis

Herein, we report the first case presentation of paraneoplastic myelofibrosis associated with cancer. Paraneoplastic syndromes occur in some patients with thoracic malignancies; however, myelofibrosis is not commonly seen in non-small cell lung cancer (NSCLC). We report a case of myelofibrosis in a patient with a new diagnosis of NSCLC that resolved after stereotactic ablative radiotherapy (SABR). In conclusion, NSCLC may evoke unexpected systemic effects that resolve with treatment.

Acute Cerebellar Inflammation and Related Ataxia: Mechanisms and Pathophysiology

The cerebellum governs motor coordination and motor learning. Infection with external microorganisms, such as viruses, bacteria, and fungi, induces the release and production of inflammatory mediators, which drive acute cerebellar inflammation. The clinical observation of acute cerebellitis is associated with the emergence of cerebellar ataxia. In our animal model of the acute inflammation of the cerebellar cortex, animals did not show any ataxia but hyperexcitability in the cerebellar cortex and depression-like behaviors. In contrast, animal models with neurodegeneration of the cerebellar Purkinje cells and hypoexcitability of the neurons show cerebellar ataxia. The suppression of the Ca2+-activated K+ channels in vivo is associated with a type of ataxia. Therefore, there is a gap in our interpretation between the very early phase of cerebellar inflammation and the emergence of cerebellar ataxia. In this review, we discuss the hypothesized scenario concerning the emergence of cerebellar ataxia. First, compared with genetically induced cerebellar ataxias, we introduce infection and inflammation in the cerebellum via aberrant immunity and glial responses. Especially, we focus on infections with cytomegalovirus, influenza virus, dengue virus, and SARS-CoV-2, potential relevance to mitochondrial DNA, and autoimmunity in infection. Second, we review neurophysiological modulation (intrinsic excitability, excitatory, and inhibitory synaptic transmission) by inflammatory mediators and aberrant immunity. Next, we discuss the cerebellar circuit dysfunction (presumably, via maintaining the homeostatic property). Lastly, we propose the mechanism of the cerebellar ataxia and possible treatments for the ataxia in the cerebellar inflammation.

A Breakdown of Immune Tolerance in the Cerebellum

Cerebellar dysfunction can be associated with ataxia, dysarthria, dysmetria, nystagmus and cognitive deficits. While cerebellar dysfunction can be caused by vascular, traumatic, metabolic, genetic, inflammatory, infectious, and neoplastic events, the cerebellum is also a frequent target of autoimmune attacks. The underlying cause for this vulnerability is unclear, but it may be a result of region-specific differences in blood–brain barrier permeability, the high concentration of neurons in the cerebellum and the presence of autoantigens on Purkinje cells. An autoimmune response targeting the cerebellum—or any structure in the CNS—is typically accompanied by an influx of peripheral immune cells to the brain. Under healthy conditions, the brain is protected from the periphery by the blood–brain barrier, blood–CSF barrier, and blood–leptomeningeal barrier. Entry of immune cells to the brain for immune surveillance occurs only at the blood-CSF barrier and is strictly controlled. A breakdown in the barrier permeability allows peripheral immune cells uncontrolled access to the CNS. Often—particularly in infectious diseases—the autoimmune response develops because of molecular mimicry between the trigger and a host protein. In this review, we discuss the immune surveillance of the CNS in health and disease and also discuss specific examples of autoimmunity affecting the cerebellum.