Abstract
BackgroundAntibodies 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.MethodsHEK293 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.ResultsPatients’ 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.ConclusionsIgLON5 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.Electronic supplementary materialThe online version of this article (doi:10.1186/s12974-016-0689-1) contains supplementary material, which is available to authorized users.
Highlights
Antibodies against IgLON5, a neuronal adhesion protein of unknown function, are markers of a novel neurological disorder termed anti-IgLON5 syndrome
IgLON5 antibodies target non-glycosylated epitopes in the Ig-like domain 2 To determine the immunodominant region recognized by IgLON5 antibodies, HEK293 cells transfected with the indicated mutated clones expressing different combinations of the three immunoglobulin-like domains were tested by cell-based assay (CBA) using serum samples of 15 patients with the anti-IgLON5 syndrome
Immunoblots of protein extracts of human cerebellum treated with PNGase F showed a decrease of apparent molecular weight of the band recognized by the commercial IgLON5 antibody (Fig. 2)
Summary
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 recent identification of antibodies against IgLON5, a neuronal cell adhesion protein of unknown function, in serum and CSF of eight patients with similar symptoms was critical to discover a novel neurological disorder characterized by chronic, less often subacute development of gait instability, chorea, dysarthria, dysphagia, episodic central hypoventilation, stridor, and a previously unrecognized sleep disorder with non REM (rapid eye movement) and REM parasomnias and obstructive sleep apnea. All tested patients (12/12) had the same HLA-DQB1*0501 and HLA-DRB1*1001 alleles indicating a genetic susceptibility for this disorder [1] These findings support an underlying immune pathogenesis, patients rarely improve with immunotherapy. The postmortem study in three patients demonstrated a novel tauopathy characterized by deposits of hyperphosphorylated tau restricted to neurons and predominantly involving the hypothalamus and tegmentum of the brainstem without evidence of inflammation [1]
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