Prüß, Harald

This Viewpoint discusses the discovery of numerous neuronal autoantibodies and how autoimmune encephalitis is diagnosed and treated.

Abstract Severe neuropathies with predominant involvement of motor fibres can resemble lower motor neuron disease (LMND) phenotypes. Given the fatal prognosis of LMND, identifying underlying autoimmune syndromes is crucial to provide treatment options to patients. We investigated a novel autoantibody binding pattern observed on murine teased sciatic nerve fibres. Target antigens were identified using immunoprecipitation combined with mass spectrometry. Target specificity of these autoantibodies was validated in cell-based assays, neutralization assays and knock-out models. A retrospective study cohort consisting of different neuropathies (chronic inflammatory demyelinating polyradiculopathy n = 86, Guillain–Barré syndrome n = 37, multifocal motor neuropathy n = 18, diabetic neuropathy n = 30, other inflammatory neuropathies n = 10), amyotrophic lateral sclerosis (n = 50), multiple sclerosis (n = 50) and healthy controls (n = 50) was negative for septin multimer autoantibodies. Histopathological analysis of skin and the sural nerve including electron microscopy was performed in one seropositive patient, and autoantibody binding was characterized in vitro. Extensive immunotherapy was initiated in one patient, with clinical and serological follow-up over 4 years. Among 3543 total samples tested, three patients (two male, one female)—diagnosed with the LMND variant of amyotrophic lateral sclerosis (aged 65, 72 and 79 years, respectively)—showed a novel and distinct autoantibody binding pattern of indirect immunofluorescence staining on peripheral nerves, targeting Schmidt–Lanterman incisures (SLIs), paranodes and the abaxonal myelin. Target identification and validation revealed septin multimers as autoantibody epitopes. Despite the primarily intracellular location of septins, autoantibody binding was evident in living myelinated dorsal root ganglia, primarily at SLIs (‘incisuropathy’). Septin multimer autoantibodies further initiated complement deposition on fixed and permeabilized cell-based assays. Sural nerve and skin biopsies showed inflammation, myelin and axonal pathology. Extensive immunotherapy in one patient was followed by disease stabilization over 3 years. The other two patients died of rapid disease progression: one of them received no immunotherapy while the other had ineffective treatments with single administrations of intravenous immunoglobulin and rituximab. Our data suggest that septin multimer autoimmunity occurs in severe motor-predominant neuropathies which can clinically resemble a neurodegenerative LMND. Screening for septin multimer autoantibodies should be considered in patients presenting with this phenotype. Follow-up studies need to determine the direct pathogenicity of septin multimer autoantibodies, their potential as a biomarker of an autoimmune syndrome and responses to immunotherapy in larger cohorts.

Abstract Anti-IgLON5 disease is an autoimmune encephalitis that presents with a heterogenous clinical phenotype, including sleep disorders, movement abnormalities and bulbar involvement. It is characterized by autoantibodies against IgLON5, 85% association with HLA-DQB1*05:∼ and a brainstem-dominant tauopathy. Cellular and murine models report pathogenic effects of the autoantibodies, and neurodegenerative factors suggest progressive atrophy as a common sequela. However, evidence from in vivo patient data and long-term follow-up is limited, and the degree of progression remains elusive. In this multicentre study, clinical and brain MRI data were collected from 127 patients across 12 countries to investigate the relationships between clinical presentations and the development of distinct brain atrophy patterns. Our data show that most patients develop a complex multisystem phenotype as the disease progresses; however, neuromuscular manifestations rarely emerge at later disease stages. By comparison to healthy controls, this disease presents with severe substructure-specific atrophy, especially affecting the hypothalamus, brainstem, accumbens and basal ganglia, which, in age-independent analyses, show significant ventricular enlargement and also suggest progression of brainstem atrophy over the disease course. Moreover, the focality of atrophy was functionally linked to specific symptoms, with more severe involvement of the basal ganglia in patients with movement disorders, and greater atrophy in the hippocampus and thalamus in patients with cognitive impairment. Taken together, our results provide evidence of distinct atrophy patterns in anti-IgLON5 disease, which closely mirror sites of pathophysiologic processes, including autoantibody binding and tau deposition. Our data emphasize the brainstem as the pathophysiological hub of the disease and provide normative data for the incorporation of atrophy measurements into routine clinical assessments and future treatment studies to monitor disease trajectory and evaluate future treatment strategies.

Abstract Anti-IgLON5 disease is a rare and likely underdiagnosed subtype of autoimmune encephalitis. The disease displays a heterogeneous phenotype that includes sleep, movement and bulbar-associated dysfunction. The presence of IgLON5-antibodies in CSF/serum, together with a strong association with HLA-DRB1*10:01∼DQB1*05:01, supports an autoimmune basis. In this study, a multicentric human leukocyte antigen (HLA) study of 87 anti-IgLON5 patients revealed a stronger association with HLA-DQ than HLA-DR. Specifically, we identified a predisposing rank-wise association with HLA-DQA1*01:05∼DQB1*05:01, HLA-DQA1*01:01∼DQB1*05:01 and HLA-DQA1*01:04∼DQB1*05:03 in 85% of patients. HLA sequences and binding cores for these three DQ heterodimers were similar, unlike those of linked DRB1 alleles, supporting a causal link to HLA-DQ. This association was further reflected in an increasingly later age of onset across each genotype group, with a delay of up to 11 years, while HLA-DQ-dosage dependent effects were also suggested by reduced risk in the presence of non-predisposing DQ1 alleles. The functional relevance of the observed HLA-DQ molecules was studied with competition binding assays. These proof-of-concept experiments revealed preferential binding of IgLON5 in a post-translationally modified, but not native, state to all three risk-associated HLA-DQ receptors. Further, a deamidated peptide from the Ig2-domain of IgLON5 activated T cells in two patients, compared with one control carrying HLA-DQA1*01:05∼DQB1*05:01. Taken together, these data support a HLA-DQ-mediated T-cell response to IgLON5 as a potentially key step in the initiation of autoimmunity in this disease.