Investigation of LGI1 as the antigen in limbic encephalitis previously attributed to potassium channels: a case series

Summary

Background

Voltage-gated potassium channels are thought to be the target of antibodies associated with limbic encephalitis. However, antibody testing using cells expressing voltage-gated potassium channels is negative; hence, we aimed to identify the real autoantigen associated with limbic encephalitis.

Methods

We analysed sera and CSF of 57 patients with limbic encephalitis and antibodies attributed to voltage-gated potassium channels and 148 control individuals who had other disorders with or without antibodies against voltage-gated potassium channels. Immunohistochemistry, immunoprecipitation, and mass spectrometry were used to characterise the antigen. An assay with HEK293 cells transfected with leucine-rich, glioma-inactivated 1 (LGI1) and disintegrin and metalloproteinase domain-containing protein 22 (ADAM22) or ADAM23 was used as a serological test. The identity of the autoantigen was confirmed by immunoabsorption studies and immunostaining of Lgi1-null mice.

Findings

Immunoprecipitation and mass spectrometry analyses showed that antibodies from patients with limbic encephalitis previously attributed to voltage-gated potassium channels recognise LGI1, a neuronal secreted protein that interacts with presynaptic ADAM23 and postsynaptic ADAM22. Immunostaining of HEK293 cells transfected with LGI1 showed that sera or CSF from patients, but not those from control individuals, recognised LGI1. Co-transfection of LGI1 with its receptors, ADAM22 or ADAM23, changed the pattern of reactivity and improved detection. LGI1 was confirmed as the autoantigen by specific abrogation of reactivity of sera and CSF from patients after immunoabsorption with LGI1-expressing cells and by comparative immunostaining of wild-type and Lgi1-null mice, which showed selective lack of reactivity in brains of Lgi1-null mice. One patient with limbic encephalitis and antibodies against LGI1 also had antibodies against CASPR2, an autoantigen we identified in some patients with encephalitis and seizures, Morvan's syndrome, and neuromyotonia.

Interpretation

LGI1 is the autoantigen associated with limbic encephalitis previously attributed to voltage-gated potassium channels. The term limbic encephalitis associated with antibodies against voltage-gated potassium channels should be changed to limbic encephalitis associated with LGI1 antibodies, and this disorder should be classed as an autoimmune synaptic encephalopathy.

Funding

National Institutes of Health, National Cancer Institute, and Euroimmun.

Introduction

Autoimmune synaptic encephalopathies are disorders in which patients develop antibodies against synaptic proteins, including the excitatory glutamate NMDA¹ and AMPA receptors,² and the inhibitory GABA_B receptor.³ Because these receptors have crucial functions in synaptic transmission and plasticity, the autoimmunity usually causes seizures and neuropsychiatric symptoms, ranging from alterations in memory, behaviour, and cognition, to psychosis. Several features characterise these disorders: the target epitopes are extracellular, the antibody–receptor binding is detectable in transfected cells expressing the receptors, the antibodies alter the function or structure of the receptors,2, 4 the resulting syndromes are severe but treatable, and the clinical presentation is similar to symptoms seen in animal models of pharmacological or genetic dysfunction of the related receptor.

Some of these immune responses define distinct disorders and could be classified as a single clinical-immunological entity (eg, anti-NMDA receptor encephalitis).5, 6, 7 Other immune responses, caused by antibodies against AMPA and GABA_B receptors, result in psychiatric or seizure disorders that can develop alone or as part of limbic encephalitis.3, 8 The classical clinical features of these two types of synaptic disorder are similar to those of limbic encephalitis attributed to antibodies against voltage-gated potassium channels.9, 10

Synaptic autoantigens can be isolated by use of immunoprecipitation with antibodies from patients with the suspected autoimmune synaptic encephalopathy.1, 2, 3 However, antibodies against voltage-gated potassium channels from the sera or CSF of patients with limbic encephalitis were initially characterised by use of immunohistochemistry in rodent tissue and immunoprecipitation of nervous tissue lysates containing voltage-gated potassium channels labelled with ¹²⁵-I-α-dendrotoxin (¹²⁵-I-α-dendrotoxin radioimmunoassay).11, 12 Cells transfected with combinations of several Kv subunits of voltage-gated potassium channels showed reactivity with sera from 17 of 17 patients with neuromyotonia or limbic encephalitis, although only about 20–38% of successfully transfected cells were recognised by patients' antibodies.¹³ In another study, the authors suggested that voltage-gated potassium channels were not the target antigen and that some patients with neuromyotonia and Morvan's syndrome had antibodies against contactin-associated protein-like 2 (CASPR2), but the target antigen of antibodies from patients with limbic encephalitis was not studied.¹⁴ We have been unsuccessful in reproducing the reactivity of antibodies from patients with neuromyotonia, Morvan's syndrome, and limbic encephalitis in cells ectopically expressing voltage-gated potassium channels (unpublished). On the basis of these negative findings, we postulated that the antibodies of these patients might be directed against other neuronal cell-surface proteins and we aimed to identify the real autoantigen associated with limbic encephalitis.