Antibodies against IgLON5, a neuronal adhesion protein of unknown function, are markers of a novel neurological disorder termed anti-IgLON5 syndrome. The disorder shows a remarkable association with the HLA-DQB1*0501 and HLA-DRB1*1001 alleles, and postmortem studies demonstrate a novel neuronal tauopathy predominantly involving the hypothalamus and tegmentum of the brainstem. The role of IgLON5 antibodies in the pathogenesis of the disease is currently unknown. Here, we have determined the target epitopes of IgLON5 antibodies, the effects of the IgLON5 antibodies in rat hippocampal neurons, and the IgG subclass responsible for these effects. HEK293 cells expressing several deletion constructs of IgLON5 were used to determine the epitopes recognized by the serum of 15 patients with anti-IgLON5 syndrome. The role of glycosylation in immunogenicity was tested with PNGase F treatment of transfected cells. Dissociated hippocampal neuronal cultures were used to test by immunocytochemistry the effects of total IgG, IgG1, and IgG4 subclasses of IgLON5 antibodies. Patients’ antibodies reacted with the immunoglobulin-like domain 2 of IgLON5. Glycosylation was not required for immunoreactivity. The predominant subclass of IgLON5 antibodies was IgG4 but all patients also had IgG1. The mean percentage of specific IgLON5 IgG4 and IgG1 of the samples analyzed by flow cytometry was 64 and 33 %, respectively. In cultures of hippocampal neurons, patients’ antibodies caused a decrease of cell surface IgLON5 clusters that was not reversed after IgLON5 antibodies were removed from the media. The decrease of surface IgLON5 clusters correlated with the rate of antibody internalization. These effects were observed with purified IgG1 but not with the IgG4 antibodies. IgLON5 antibodies recognize the immunoglobulin-like domain 2 of the antigen, and the reactivity is not dependent on glycosylation. The effects observed on hippocampal neuronal cultures indicate an irreversible antibody-mediated internalization of surface IgLON5. These effects were mediated by specific IgLON5 IgG1 antibodies and suggest a pathogenic role of these antibodies in the disease.

Objective To report the clinical and oncologic associations of antibodies against Kelch-like protein 11 (KLHL11-ab), recently suggested as biomarkers of a paraneoplastic brainstem cerebellar syndrome associated with testicular seminoma, and to determine the value of immunohistochemistry as a screening technique. Methods Studies included 432 sera or CSF from 329 patients with paraneoplastic (157) or autoimmune neurologic syndromes (172); 63 with neurologic symptoms and benign teratomas; 28 with small-cell lung cancer, and 12 healthy subjects. KLHL11-abs were examined using a cell-based assay (CBA) with HEK293 cells transfected with a human KLHL11 clone. The CBA specificity was confirmed by immunoprecipitation. All positive samples were examined by immunohistochemistry on rat brain sections. Results KLHL11-abs were detected in 32 patients by CBA, and patients9 antibodies immunoprecipitated KLHL11. Using rat brain immunohistochemistry, only 7 samples (22%) were positive. Patients9 median age was 28 years (range 9–76 years), and 16 (50%) were women. Tumors were identified in 23/32 (72%) patients, including 14 teratomas and 7 seminomas or mixed germ cell tumors. Thirteen (41%) patients had cerebellar ataxia (7) or encephalitis with brainstem cerebellar symptoms (6), 7 (22%) anti-NMDA receptor (NMDAR) encephalitis (5 with ovarian teratoma), 5 (16%) opsoclonus-myoclonus, 3 (9%) limbic encephalitis, and 4 (12%) diverse neurologic symptoms (3 with benign teratomas). Concurrent autoantibodies occurred in 14 (44%) patients (7 anti-NMDAR, 6 Ma2, and 1 Hu). Conclusions KLHL11-abs associate with a spectrum of syndromes and tumors wider than those previously reported; 44% of patients have concurrent neuronal antibodies, some of them (anti-NMDAR) pathogenically relevant. Brain immunostaining is not useful for routine screening of KLHL11-abs.

The goal of this study is to determine whether patients with paraneoplastic cerebellar degeneration (PCD) and small-cell lung cancer (SCLC) have a specific repertoire of antibodies, if SOX1 antibodies (SOX1-ab) can predict the presence of SCLC, and if antibodies to cell surface antigens occur in this syndrome. Antibody analysis was done using immunohistochemistry on rat brain, immunoblot with recombinant antigens, screening of cDNA expression libraries, and immunolabeling of live neurons in 39 patients with PCD and SCLC. VGCC-ab were measured by RIA, and SOX1-ab, Hu-ab, and ZIC4-ab by immunoblot. Lambert-Eaton myastenic syndrome (LEMS) was present in 10 of 23 patients with electrophysiological studies. At least one antibody was detected in 72% of patients. The individual frequencies were: 49% SOX1-ab, 44% VGCC-ab, 31% Hu-ab, and 13% ZIC4-ab. SOX1-ab occurred in 76% of patients with VGCC-ab and 27% of those without VGCC-ab (p = 0.0036). SOX1-ab were not found in 39 patients with sporadic late-onset cerebellar ataxia, 23 with cerebellar ataxia and glutamic acid decarboxylase antibodies, and 73 with PCD and cancer types other than SCLC (31 without onconeural antibodies, 25 with Yo-ab , 17 with Tr-ab). Five patients (13%) had antibodies against unknown neuronal cell surface antigens but none of them improved with immunotherapy. One serum immunoreacted against the axon initial segment of neurons and another serum against ELKS1, a protein highly expressed in the cerebellum that interacts with the beta4-subunit of the VGCC. In conclusion, 72% of patients with PCD and SCLC had one or more antibodies that indicate the presence of this tumor. In these patients, VGCC-ab and SOX1-ab occur tightly associated. SOX1-ab are predictors of SCLC in ataxia patients with a specificity of 100% and sensitivity of 49%. Unlike limbic encephalitis with SCLC, antibodies to cell surface antigens other than VGCC-ab, are infrequent and do not predict response to treatment.

Objective To report the clinical, neuroimaging, and antibody associations in patients with autoimmune encephalitis (AE) and thymoma. Methods A retrospective cohort study of 43 patients was conducted. Antibody determination and immunoprecipitation to characterize novel antigens were performed using reported techniques. Results Patients9 median age was 52 years (range: 23–88 years). Forty (93%) had neuronal surface antibodies: gamma-aminobutyric acid receptor A (GABA_AR) (15), amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) (13), contactin-associated protein-like 2 (CASPR2) (4), leucine-rich, glioma inactivated 1 (LGI1) (3), glycine receptor (GlyR) (3), and unknown antigens (2). Concurrent antibodies against intracellular antigens occurred in 13 (30%; 9 anti–collapsin response mediator protein 5 [CRMP5]) and were more frequent in anti-AMPAR encephalitis (54% vs 20%; p = 0.037). The most common clinical presentation was encephalitis with multiple T2/fluid-attenuated inversion recovery hyperintense lesions in 23 (53%) patients (15 GABA_AR, 5 AMPAR, and 1 unknown neuropil antibody), followed by encephalitis with peripheral nerve hyperexcitability in 7 (16%; 4 CASPR2, 2 LGI1, and 1 unknown antibody), limbic encephalitis in 6 (14%; 4 AMPAR, 1 LGI1, and 1 antibody negative), progressive encephalomyelitis with rigidity and myoclonus in 4 (9%; 3 GlyR and 1 AMPAR antibodies), and encephalitis with normal MRI in 3 (7%; AMPAR antibodies). Anti-GABA_AR encephalitis was more prevalent in Japanese patients compared with Caucasians and other ethnicities (61% vs 16%; p = 0.003). In anti-AMPAR encephalitis, 3/4 patients with poor and 0/6 with good outcome had concurrent CRMP5 antibodies (p = 0.033). Immunoprecipitation studies identified metabotropic glutamate receptor 3 antibodies that were additionally found in 5 patients (3 with and 2 without encephalitis). Conclusions AE in patients with thymoma include several clinical-radiologic syndromes that vary according to the associated antibodies. Anti-GABA_AR encephalitis was the most frequent AE and occurred more frequently in Japanese patients.

Objective: The objective of this study was to report the identification of antibodies against the glutamate kainate receptor subunit 2 (GluK2‐abs) in patients with autoimmune encephalitis, and describe the clinical‐immunological features and antibody effects.Methods: Two sera from 8 patients with similar rat brain immunostaining were used to precipitate the antigen from neuronal cultures. A cell‐based assay (CBA) with GluK2‐expressing HEK293 cells was used to assess 596 patients with different neurological disorders, and 23 healthy controls. GluK2‐ab effects were determined by confocal microscopy in cultured neurons and electrophysiology in GluK2‐expressing HEK293 cells.Results: Patients’ antibodies precipitated GluK2. GluK2 antibody‐specificity was confirmed by CBA, immunoprecipitation, GluK2‐immunoabsorption, and GluK2 knockout brain immunohistochemistry. In 2 of 8 samples, antibodies reacted with additional GluK2 epitopes present in GluK1 or GluK3; in both, the reactivity was abrogated after GluK2 immuno‐absorption. Six of 8 patients developed acute encephalitis and clinical or magnetic resonance imaging (MRI) features of predominant cerebellar involvement (4 presenting as cerebellitis, which in 2 patients caused obstructive hydrocephalus), and 2 patients had other syndromes (1 with cerebellar symptoms). One of the samples showed mild reactivity with non‐kainate receptors (alpha‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionic acid receptors [AMPAR] and N‐methyl‐D‐aspartate receptors [NMDAR]) leading to identify 6 additional cases with GluK2‐abs among patients with anti‐AMPAR (5/71) or anti‐NMDAR encephalitis (1/73). GluK2‐abs internalized GluK2 in HEK293 cells and neurons; these antibody‐effects were reversible in neurons. A significant reduction of GluK2‐mediated currents was observed in cells treated with patients’ GluK2 serum following the time frame of antibody‐mediated GluK2 internalization.Interpretation: GluK2‐abs associate with an encephalitis with prominent clinicoradiological cerebellar involvement. The antibody effects are predominantly mediated by internalization of GluK2. ANN NEUROL 2021;90:107–123