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

Recent advances in diagnosis of immune-mediated cerebellar ataxias: novel concepts and fundamental questions on autoimmune mechanisms

Immune-mediated cerebellar ataxias (IMCAs) represent a group of disorders in which the immune system targets mainly the cerebellum and related structures. We address fundamental questions on the diagnosis and immunological pathogenesis of IMCAs, as illuminated by recent advances in the field. Various types of IMCAs have been identified, including post-infectious cerebellitis, Miller Fisher syndrome, gluten ataxia, paraneoplastic cerebellar degeneration (PCD), opsoclonus and myoclonus syndrome, and anti-GAD ataxia. In some cases, identification of several well-characterized autoantibodies points to a specific etiology in IMCAs and leads to a firm diagnosis. In other cases, various autoantibodies have been reported, but their interpretation requires a careful consideration. Indeed, some autoantibodies have only been documented in a limited number of cases and the causal relationship is not established. In order to facilitate an early treatment and prevent irreversible lesions, new entities have been defined in recent years, such as primary autoimmune cerebellar ataxia (PACA) and latent autoimmune cerebellar ataxia (LACA). PACA is characterized by autoimmune features which do not align with traditional etiologies, while LACA corresponds to a prodromal stage. LACA does not imply the initiation of an immunotherapy but requires a close follow-up. Concurrently, accumulation of clinical data has led to intriguing hypotheses regarding the mechanisms of autoimmunity, such as a pathogenesis of autoimmunity against synapses (synaptopathies), and the vulnerability of the entire nervous system when the immunity targets ion channels and astrocytes. The development of PCD in patients treated with immune-checkpoint inhibitors suggests that molecular mimicry specifically determines the direction of autoimmunity, and that the strength of this response is modulated by co-signaling molecules that either enhance or dampen signals from the antigen-specific T cell receptor.

Neuronal antibodies in nonparaneoplastic autoimmune cerebellar ataxias

PURPOSE OF REVIEW

To describe relevant advances in nonparaneoplastic autoimmune cerebellar ataxias (ACA) with neuronal antibodies.

RECENT FINDINGS

Apart from metabotropic glutamate receptor 1(mGluR1) antibodies, in recent years, the number of neuronal antibodies against surface antigens in ACA has increased with the description of glutamate kainate receptor subunit 2 (GluK2) antibodies in young patients with cerebellitis. Around 20% of patients with contactin-associated protein-like 2 (CASPR2) encephalitis also present prominent cerebellar ataxia. However, isolate cerebellar ataxia is unusual (<4%). Outcome in patients with neuronal antibodies against surface antigens remains suboptimal despite the cerebellar ataxia probably is antibody-mediated.Concerning neuronal antibodies against intracellular antigens, up to 25% of patients with glutamic acid decarboxylase (GAD) antibodies present transient episodes of vertigo or diplopia that antedate the development of the ACA. There is in-vitro evidence that septin-5 is partially exposed to the membrane and the antibodies may interfere with septin-5 function. The clinical significance of the remaining antibodies against intracellular antigens remains unclear.

SUMMARY

The number of antibodies against surface antigens is increasing in ACA, but the response to the immunotherapy remains suboptimal. More studies are needed to clarify the role of most of the antibodies against intracellular antigens described in these patients.

Recent Advances in Immune-Mediated Cerebellar Ataxias: Pathogenesis, Diagnostic Approaches, Therapies, and Future Challenges-Editorial

The clinical category of immune-mediated cerebellar ataxias (IMCAs) has been established after 3 decades of clinical and experimental research. The cerebellum is particularly enriched in antigens (ion channels and related proteins, synaptic adhesion/organizing proteins, transmitter receptors, glial cells) and is vulnerable to immune attacks. IMCAs include various disorders, including gluten ataxia (GA), post-infectious cerebellitis (PIC), Miller Fisher syndrome (MFS), paraneoplastic cerebellar degeneration (PCD), opsoclonus myoclonus syndrome (OMS), and anti-GAD ataxia. Other disorders such as multiple sclerosis (MS), acute disseminated encephalomyelitis (ADEM), Behçet disease, and collagen vascular disorders may also present with cerebellar symptoms when lesions are localized to cerebellar pathways. The triggers of autoimmunity are established in GA (gluten sensitivity), PIC and MFS (infections), PCD (malignancy), and OMS (infections or malignant tumors). Patients whose clinical profiles do not match those of classic types of IMCAs are now included in the spectrum of primary autoimmune cerebellar ataxia (PACA). Recent remarkable progress has clarified various characteristics of these etiologies and therapeutic strategies in terms of immunotherapies. However, it still remains to be elucidated as to how immune tolerance is broken, leading to autoimmune insults of the cerebellum, and the consecutive sequence of events occurring during cerebellar damage caused by antibody- or cell-mediated mechanisms. Antibodies may specifically target the cerebellar circuitry and impair synaptic mechanisms (synaptopathies). The present Special Issue aims to illuminate what is solved and what is unsolved in clinical practice and the pathophysiology of IMCAs. Immune ataxias now represent a genuine category of immune insults to the central nervous system (CNS).