Anti-IgLON5 Disease 10 Years Later

Abstract

Anti-IgLON5 disease was identified 10 years ago, thanks to the discovery of IgLON5 antibodies and the joint effort of specialists in sleep medicine, neuroimmunology, and neuropathology. Without this collaboration, it would have been impossible to untangle fundamental aspects of this disease. After the seminal description in 2014, today there is growing evidence that most patients present a chronic progressive course with gait instability, abnormal movements, bulbar dysfunction, and a sleep disorder characterized by nonrapid eye movement and REM parasomnias, and obstructive sleep apnea with stridor. Unlike other autoimmune encephalitides, the response to immunotherapy is suboptimal. Neuropathologic studies in patients with a prolonged clinical course showed a novel 3-repeat and 4-repeat neuronal tauopathy mainly involving the hypothalamus and tegmentum of the brainstem. The absence of tau deposits in the brain of patients who died early, the demonstration that IgLON5 antibodies cause an irreversible decrease in cell-surface levels of IgLON5, and a disorganization of the neuronal cytoskeleton suggest that the disease is primarily autoimmune and the tauopathy a secondary event. After a decade, we now know the disease much better, but important issues still need to be addressed. We have to gather more information on the natural course of the disease, develop better treatments, and identify robust predictors of outcome. More basic research is needed on the physiology of IgLON5, how antibodies disrupt its function, and the downstream effects leading to neurodegeneration. Finally, better designed passive transfer and active immunization models are needed to confirm the pathogenic effect of IgLON5 antibodies.

Anti-IgLON5 Disease: The Story

Anti-IgLON5 disease was identified, thanks to the discovery of IgLON5 antibodies, which are the diagnostic hallmark of the disease.¹ Although the clinical manifestations are heterogeneous, most patients present with a combination of gait instability, abnormal movements, and bulbar dysfunction (dysphagia, dysarthria, central hypoventilation) associated with a sleep disorder characterized by nonrapid eye movements (NREMs) and REM parasomnias and obstructive sleep apnea with stridor. Unlike other autoimmune encephalitides associated with antibodies against neuronal surface antigens, anti-IgLON5 disease runs a chronic progressive course in more than 70% of patients while less than 40% have a meaningful response to immunotherapy.²

The search for IgLON5 antibodies started in 2010 when J. Santamaria, head of the Sleep Disorders Unit, presented to F. Graus a patient with a sleep disorder and mild dysautonomic features that were suggestive of Morvan syndrome and asked to check for antibodies against Caspr2.³ Although Caspr2 and other antibodies against neuronal surface antigens were not found, the immunohistochemical study on frozen rat brain sections and cultured rat hippocampal neurons identified an antibody against a neuronal cell-surface protein for which identity was unknown. Several months later, J. Santamaria sent to the laboratory the serum of 2 additional patients he selected on the basis of having a sleep disorder similar to that observed in the index patient, and both samples showed the same immunohistochemical pattern. Then, we retrieved from the laboratory database 5 additional samples with the same immunoreactivity and reviewed those patients' symptoms. One of them had been recently admitted with a subacute course of gait instability and bulbar symptoms (Patient 7 in reference 1). The clinical information of these 8 patients and the identification of IgLON5 protein as the target antigen were the key features that led to the first publication of this disease in 2014.¹ To our surprise, the brain autopsy of two of these patients revealed a neuronal 3-repeat and 4-repeat (R) tauopathy mainly involving the hypothalamus and tegmentum of the brainstem, which could not be classified among the known tauopathies. These findings led us to propose the term anti-IgLON5 tauopathy, but this was considered too preliminary by the editors of Lancet Neurology because it was based in only 2 autopsies. Instead, we used the term anti-IgLON5 syndrome until one of the reviewers of a retrospective clinical analysis of the first 22 patients reported in Neurology in 2017⁴ indicated, “a syndrome is defined as a group of symptoms which consistently occur together, or a condition characterized by a set of associated symptoms. Given the diversity of symptoms here (which the authors ended up dividing into 4 groups) and the selection process (presence of anti-IgLON5 antibodies) I wonder if anti-IgLON5 associated disorder or disease is a better term.” We thought that their comment was appropriate and have adopted the term anti-IgLON5 disease since then.⁴

Three lessons can be extracted from this short historical outline. First, in the publication of any novel antibody, the accurate description of the clinical features is as important as the immunologic findings, which together help to establish a causal inference, for example, whether the antibodies are biomarkers or the cause of a previously unrecognized disease. Second, open communication lines among specialists from different fields are crucial to tackle fundamental aspects of many diseases. Anti-IgLON5 disease is a good example on how the close collaboration of sleep medicine neurologists, neuroimmunologists, and neuropathologists was instrumental in the characterization of the disease. Third, the role of journals' editors and reviewers in improving the final version of an article is in many instances important but seldom acknowledged.

Demographic and Clinical Features

There are no population-based studies on the incidence and prevalence of anti-IgLON5 disease, which probably remains underdiagnosed. The experience between 2017 and 2021 in our Institution, which is a referral center for the diagnosis and treatment of autoimmune and paraneoplastic encephalitis, showed that IgLON5 antibodies were the third most commonly detected (8.5% of all antibody-positive samples) after NMDAR (30%) and LGI1 (32%) antibodies.⁵

Anti-IgLON5 disease usually affects older people, and the onset in individuals younger than 50 years is unusual. In our cohort of 98 patients identified until now, the median age at diagnosis is 69 years (range: 46–91 years) with no gender predominance (54% male). The frequency of concurrent autoimmune disorders or cancer is similar to that seen in the normal population. Eleven patients (11%) had different types of cancer (10 diagnosed after the onset of anti-IgLON5 disease) and 12 (12%) concurrent systemic autoimmune diseases. Previous studies in anti-IgLON5 disease have focused on the symptoms at diagnosis, but the natural history of the disease, which is critical for the evaluation of treatments, is much less known. Presently, we know that the clinical course is slowly progressive in 70%–75% of the patients with exacerbations in 20%–50% of them.² These exacerbations can include worsening of pre-existing symptoms or, less frequently, onset of new clinical manifestations. The remaining patients (25%–30%) present with a rapid clinical course resulting in substantial neurologic dysfunction in ≤3 months. In some patients, symptoms remain stable for months or years between relapses, with or without treatment maintenance. However, the clinical course cannot be considered benign. In our cohort, 29 (40%) of 73 patients with adequate follow-up developed severe disability and 32 (32%) of the 98 died after a median follow-up of 12 months since the diagnosis of the disease. Twelve (37%) of the 32 patients died suddenly, but the exact cause was unclear. Determining the cause of death is an important task for future investigations; for example, if upper airway obstruction due to vocal cord palsy, which is frequent in patients with anti-IgLON5 disease, is confirmed to be a main cause of death, early continuous positive airway pressure (CPAP) therapy and tracheostomy should be considered.

Patients may present with a wide array of clinical manifestations, which make the diagnosis difficult. Two tips are useful to suspect the diagnosis: (1) In the anamnesis, patients almost always disclose multiple alterations even when the reason for consultation is an apparently isolated symptom (Table 1). Some of the symptoms can be mild and overlooked if they are not specifically considered. For example, careful inquiry of sleep problems by direct questioning of the patient and bed partner is crucial to identify the sleep disorder that is characteristic of the disease. Truly isolated clinical manifestations at diagnosis are very unusual. In our cohort, only 2 patients (2%) had an isolated symptom: rapidly progressive dementia and abdominal dyskinesias. (2) Anti-IgLON5 disease can mimic several neurodegenerative diseases (mentioned further); thus, it has to be suspected when patients do not fulfil established clinical criteria for those diseases or present symptoms that are common in anti-IgLON5 disease. There is no need to test for anti-IgLON5 antibodies in patients with a well-established diagnosis of a neurodegenerative disease. For example, IgLON5 antibodies were not found in patients with idiopathic REM sleep behavior disorder, obstructive sleep apnea, multiple system atrophy (MSA), and progressive supranuclear palsy (PSP).^1,6-9

Table 1.

Frequency of Additional Symptoms Depending on the Predominant Clinical Involvement at Diagnosis in 98 Patients With Anti-IgLON5 Disease

Predominant manifestation (N)	Bulbar (27)	Abnormal movements (34)	Sleep (14)	Cognitive (15)	Neuromuscular (8)
Additional symptoms (%)
Bulbar	—	68	100	47	25
Abnormal movements	67	—	79	73	37
Gait disorder	70	97	71	60	37
Oculomotor symptoms	74	71	43	47	37
Sleep	91	82	—	87	50
Cognitive	19	56	57	—	25
Neuromuscular	6	15	14	7	—

Most patients present at diagnosis a combination of bulbar symptoms (mainly dysphagia), abnormal gait, movement disorders (mainly chorea and facial dyskinesias), and sleep-related symptoms.⁴ When we inquired the main reason that prompted patients to seek medical attention, the most common was dysphagia and other bulbar symptoms in 23 (23%) of 98 patients, followed by gait problems in 20 (20%), abnormal movements (chorea and facial/abdominal dyskinesias, sometimes with criteria of myorhythmia) in 18 (18%), sleep concerns (sleep-related vocalizations and movements, sleep apneas, excessive daytime sleepiness) in 15 (15%), and cognitive impairment in 13 (13%). A few patients (7%) presented with predominant neuromuscular symptoms, mainly fasciculations, muscle stiffness, or peripheral neuropathy (Figure 1).

Figure 1. Reason for Consultation in 98 Patients With Anti-IgLON5 Disease.

Although not recognized as the main or initial symptom, sleep symptoms were reported by 82 (84%) of 98 patients or bed partners. Sixty-two patients (63%) had sleep breathing problems (apneas, severe snoring, or stridor) and 49 (50%) sleep-related vocalizations and abnormal behaviors mimicking activities of daily life. Insomnia and excessive daytime sleepiness were reported by 53 (54%) and 46 (47%) patients, respectively. The characteristic sleep pattern (mentioned further) of anti-IgLON5 disease was found in 32 (68%) of 47 patients who underwent video-polysomnographic (v-PSG) studies.¹⁰ However, this number may not reflect the true frequency because v-PSG is not routinely performed in all centers where patients were diagnosed.

Bulbar symptoms were present at diagnosis in 72 (73%) of 98 patients including dysphagia, dysarthria (which is usually mild), laryngeal stridor during sleep due to vocal cord paresis, and central hypoventilation. Moderate/severe dysphagia was the most frequent symptom (58% of patients), with up to a third of these patients requiring dietary modifications and less frequently a feeding tube or percutaneous endoscopic gastrostomy. Twenty-six (26%) of our patients developed episodes of acute respiratory failure secondary to vocal cord paresis or central hypoventilation.¹¹ Permanent tracheotomy is frequently required to prevent these acute recurrent episodes.¹²

Gait impairment is the most common movement disorder. It was observed in 72 (73%) of 98 patients of our cohort. The most common cause of gait impairment is postural instability accompanied by altered postural reflexes, resulting in frequent falls. Less frequently, the gait is affected by cerebellar ataxia, stiffness, bradykinesia, or chorea. Generalized chorea and craniofacial dyskinesias, sometimes with criteria of myorhythmia,¹³ occurred in 30% of patients. Other abnormal movements include bradykinesia, abnormal body postures (antecollis and lateral bending of the trunk), and myoclonus or myorhythmia (described as periodic twitching or contractions) of the abdominal muscles.

Cognitive impairment was found in 49 (50%) of 98 patients. Of these, 18% fulfilled the criteria of dementia, and some of them also had confusional episodes, anxiety, depression, apathy, delusions, or hallucinations. The cognitive deficits of anti-IgLON5 disease have not been studied in detail to determine whether they show a distinctive profile. Preliminary studies indicate that the pattern of CSF biomarkers (amyloid beta peptides [Aβ42, Aβ40]; total and phosphorylated tau) is not compatible with Alzheimer disease.

Neuromuscular alterations, mainly fasciculations, muscle weakness, or stiffness, were found in 22 (22%) of 98 patients and in 8 (8%) were the predominant manifestation. This clinical phenotype suggests that involvement of spinal cord motor neurons may be more common than previously considered, and EMG should be considered in patients with anti-IgLON5 disease.

There are recent reports of 3 patients who presented with bilateral optic neuropathy with papilledema, two of them accompanied by other features suggestive of anti-IgLON5 disease.^14,15 This preliminary observation suggests that screening for IgLON5 antibodies should be considered in older patients with bilateral papillitis of unknown cause.

Anti-IgLON5 Disease Mimics

Independent of the symptoms prompting the initial clinical visit, 5 neurologic profiles of anti-IgLON5 disease have been identified: bulbar dysfunction, abnormal movements including those in patients with PSP-like symptoms, sleep alterations, cognitive impairment, and neuromuscular symptoms.^4,16 Despite frequent overlapping symptoms, each profile should raise the possibility of anti-IgLON5 disease, particularly after excluding other more common diseases. These 5 neurologic profiles and the corresponding mimics are summarized in Table 2. Patients with a bulbar phenotype, mainly dysphagia, are frequently considered to have motor neuron disease or myasthenia gravis.¹⁷ The latter is further suspected when there is palpebral ptosis, a symptom recorded in 23% of our patients with anti-IgLON5 disease. The combination of gait impairment and supranuclear gaze palsy that occur in some patients with anti-IgLON5 disease may be reminiscent of PSP.¹⁸ An important diagnostic tip is that the predominant downgaze involvement, typical of PSP, is rare in anti-IgLON5 disease. MSA is another potential mimic, particularly in patients with anti-IgLON5 disease with gait impairment suggesting cerebellar ataxia along with sleep parasomnias, stridor, or dysautonomia (e.g., 26% of patients with anti-IgLON5 had urinary incontinence and 6% orthostatic hypotension). However, severe orthostatic hypotension, which is common in MSA, is not characteristic of anti-IgLON5 disease.⁹ Patients with anti-IgLON5 disease with prominent parasomnias may be initially misdiagnosed with Morvan syndrome.³ Although both disorders associate with parasomnias, the v-PSG findings of patients with anti-IgLON5 disease are substantially different from those of Morvan syndrome.¹⁹ Patients who present with predominant or isolated cognitive alterations are usually investigated for a neurodegenerative dementia or Creutzfeldt-Jakob disease depending on the clinical course. If cognitive deficits are associated with generalized chorea, Huntington disease is frequently considered.²⁰ Finally, in the few patients who present with prominent fasciculations, muscle weakness, and dysphagia with EMG recordings showing active denervation, the diagnosis of amyotrophic lateral sclerosis is usually considered.^17,21 However, the absence of primary motor neuron dysfunction and the presence of nonmotor symptoms should prompt evaluation for IgLON5 antibodies.²²

Table 2.

Clinical Profiles at Diagnosis and Mimics in 98 Patients With Anti-IgLON5 Disease

Clinical profile (%)	Mimics
Bulbar (27)	Myasthenia gravis, amyotrophic lateral sclerosis
Dysphagia, dysarthria + ptosis
Abnormal movements (36)
Gait instability + supranuclear gaze palsies	Progressive supranuclear palsy
Ataxic gait + sleep symptoms + dysautonomia	Multiple systemic atrophy
Chorea + cognitive impairment	Huntington disease
Facial myorhythmia	Whipple disease
Sleep (14)
Parasomnias + insomnia + dysautonomia + cognitive deficits	Morvan syndrome
Cognitive (15)
Subacute onset	Creutzfeldt-Jakob disease
Chronic progression	Neurodegenerative dementias
Neuromuscular (8)	Motor neuron disease

Paraclinical Studies

Routine CSF examination and brain MRI studies are usually normal or show nonspecific findings. Mild CSF pleocytosis (mean 10 white blood cells/μL) was detected in 21 (21%) of 98 patients of our cohort and 33% in another series of 55 patients.² The most common CSF finding, encountered in 41% of our patients, was an elevated protein concentration. By contrast, CSF oligoclonal bands were identified in less than 15% of patients.² Brain MRI was normal or showed nonspecific changes in 84 (86%) of 98 patients, and 13% had mild brainstem/cerebellar atrophy. Only 1 patient had hyperintense fluid attenuated inversion recovery (FLAIR)/T2 lesions suggestive of inflammatory changes. The few patients previously reported with FLAIR/T2 lesions in the MRI are summarized in eTable 1. These patients show several clinical differences compared with those with normal brain MRI: (1) the clinical course was almost always subacute; (2) some patients had seizures, an unusual manifestation of anti-IgLON5 disease²³; and (3) the response to immunotherapy was better than expected.²⁴ Taken together, these observations suggest that inflammatory changes are more commonly detected when the disease runs a subacute course. This concept is supported by the findings in a retrospective series of 53 patients with anti-IgLON5 disease in whom the presence of CSF pleocytosis had a negative correlation with the time interval between symptom onset and the spinal tap.² However, inflammatory changes, despite being more common at disease onset, are not always present. In our cohort, 28 (29%) of 98 patients had a subacute onset, but only 10 (36%) had CSF pleocytosis or inflammatory lesions in brain MRI.

v-PSG studies are important to identify the characteristic sleep disorder of anti-IgLON5 disease. It should be performed even if patients do not refer sleep problems. The most common features include the following: (1) abnormal initiation of sleep with undifferentiated NREM sleep stages associated with stereotyped and finalistic behaviors and progressive normalization of sleep architecture by the end of the night, when sleep stages can be readily identified; (2) presence of REM sleep behavior disorder; and (3) detection of episodes of stridor and obstructive sleep apnea.¹⁰

Detection of the HLA-DRB1*10:01- DQB1*05:01 haplotype, which occurs in <3% of the normal population, supports the diagnosis of anti-IgLON5 disease. However, only 59% of patients have this haplotype, which seems more common in those presenting with sleep and bulbar manifestations.²⁵ By contrast, a positive IgLON5 antibody test almost always confirms the diagnosis. IgLON5 antibodies are usually detected in serum and CSF, except for 10% of patients who show antibodies only in serum. Patients without CSF antibodies are more likely to develop a PSP-like phenotype and less frequently have the HLA-DRB1*10:01- DQB1*05:01 haplotype.²⁵ Antibodies are tested with indirect immunofluorescence of HEK293 cells expressing IgLON5 (i.e., cell-based assay [CBA]). This test may show false-positive results particularly when low dilutions (1:10) of serum are used. To prevent this error, we recommend confirmation of results with rat brain immunohistochemistry. This approach substantially increases the predictive value of antibodies detected by CBA.²⁶ Despite this potential caveat, detection of IgLON5 antibodies almost always confirms the diagnosis of anti-IgLON5 disease. An important consideration is that in rare instances, IgLON5 antibodies have been detected overlapping with autoantibodies that occur in other autoimmune encephalitides or other nonautoimmune neurologic disorders (eTable 2). This possibility, albeit unusual, emphasizes the need to reasonably exclude alternative etiologies before making the firm diagnosis of anti-IgLON5 disease.²⁷

Neuropathology

Early neuropathologic studies showed unexpectedly neurodegenerative changes, without obvious inflammatory infiltrates. They were associated with a prominent neuronal accumulation of hyperphosphorylated tau, a microtubule-associated protein, in the form of fibrillary tangles and pretangles composed of a mixture of 3R and 4R tau isoforms. The accumulation of these tau isoforms had been previously reported as the main components of the neurofibrillary pathology in Alzheimer disease and chronic traumatic encephalopathy, among others.²⁸ However, in patients with anti-IgLON5 disease, the particular neuroanatomical distribution of the neuronal inclusions was different from those diseases and could not be classified among any of the previously known tauopathies, making it unique. The anterior hypothalamic region and the tegmentum of the brainstem are the areas preferentially involved by tau accumulation with its maximum at the level of the medulla oblongata (Figure 2).²⁹ The distribution of pathology in the brainstem is somewhat similar to PSP, but PSP has an astroglial tau accumulation that is not observed in anti-IgLON5 disease. Hyperphosphorylated tau deposits in the hippocampus also occur in some patients, but it is unclear whether the pathology in the hippocampus is related to anti-IgLON5 disease or represents an associated primary age-related tauopathy (PART)–like pathology that may occur in the brain of older patients.³⁰ In a patient with anti-IgLON5 disease, a separate immunoblot analysis of tau obtained from the hippocampus and brainstem showed a different pattern of protein bands, suggesting that the hippocampal tau corresponded to a comorbidity rather than to the anti-IgLON5 pathology.³¹ Further studies must be performed to confirm or refute this hypothesis.

Figure 2. Neuropathologic Findings.

(A) Representative overview section through the medulla oblongata stained with antibodies against phosphorylated tau (AT8) (brown signal, counterstained with hematoxylin). There is abundant deposition of tau in the tegmental region involving multiple nuclei such as the motor nucleus of the hypoglossus (XII), the dorsal motor nucleus of the vagal nerve (DMNV), the solitary nucleus (S), the nucleus ambiguus (A), the reticular formation (RF), and the dorsal raphe, among others. The inferior olivary (IO) nucleus shows only minimal pathology. (B and C). At higher magnification, the accumulation is mainly observed within the neuronal bodies (neurofibrillary tangles and pretangles) and neuronal processes (neuropil threads and neurites). IV = fourth ventricle; PC = choroid plexus; MLF = medial longitudinal fasciculus. Scale bars: A: 4 mm, B: 160 μm, and C: 50 μm.

Subsequent neuropathologic studies showed that the predominant brainstem tauopathy was not a constant finding, and their presence could be related to patient's age, clinical phenotype, and disease duration. In a series of 9 patients with anti-IgLON5 disease with autopsy studies, 6 had the indicated tauopathy and 3 did not have tau accumulation. Five of the patients with tau pathology showed the previously reported neuronal brainstem tauopathy while 1 patient had a neuronal and glial 4R tauopathy consistent with a PSP-like phenotype, although the clinical manifestation and the distribution of tau pathology severity were more characteristic of anti-IgLON5 disease. The median disease duration of these 6 patients was 9 years compared with 1.25 years of the 3 patients without brainstem tauopathy. In two of these 3 patients (brain samples of the third patient were not available for study), there were deposits of IgG4 in the neuropil and cell membrane of neurons in the brainstem tegmentum, olivary nucleus, cerebellar cortex, and basis of the ponts.³² Taken together, these findings suggested that IgLON5 autoimmunity precedes tau pathology.

Presently, the neuropathologic studies do not explain well enough the cognitive deficits, or some of the movement disorders, particularly generalized chorea, which are prominent in around 10% and 25% of patients with anti-IgLON5 disease, respectively.¹⁸ However, the clinical manifestations of autopsied cases were bulbar symptoms, gait instability, and sleep disorder. Future neuropathologic studies of patients with predominant cognitive deficits or chorea are necessary to determine the clinical-pathological correlations of the disease. One mechanism that could explain a potential lack of clinical-pathological correlations is the induction by the brainstem/cerebellar tau pathology of remote neuronal dysfunction in connected cortical regions.³³ For example, using an 18F-PI-2620 tau-positron emission tomography (PET) protocol to simultaneously evaluate cerebral perfusion and tau deposition, patients with 4R tauopathies (PSP and corticobasal syndrome) showed hypoperfusion in the cerebral cortex beyond the primary sites of 4R tau deposition.³⁴ Another mechanism to explain symptoms in patients with anti-IgLON5disease beyond the sites of tau deposition could be related to antibody-induced functional changes in cortical and basal ganglia neurons. The recent identification of IgG4 deposits in the brainstem of patients without tauopathy would be in line with this possibility. For example, in one of the patients, IgG4 deposits were also identified in the hippocampus.³² However, the autopsy finding of IgG deposits must be assessed with caution because we cannot rule out that the antibodies diffused to the brain postmortem.³⁵ Future studies should aim to demonstrate an association between the IgG deposits and the reduction in local IgLON5 levels or interference with its function.

Immunopathogenesis

IgLON5 is the most recently identified member of the IgLON family of neuronal cell adhesion molecules, which form part of the immunoglobulin superfamily (eTable 3). The IgLONs contain 3 immunoglobulin-like domains attached to the membrane by a glycosylphosphatidylinositol (GPI) anchor protein without a transmembrane domain.³⁶ The IgLONs seem to play key neurodevelopmental roles including synaptogenesis and neurite outgrowth through homodimeric and heterodimeric IgLON complexes, and the metalloproteinase-dependent ectodomain (soluble extracellular part of the proteins) shedding from the neuronal surface.³⁷ Although IgLON5 also establishes homomeric and heteromeric interactions with other IgLONs and undergoes ectodomain shedding, the function of IgLON5 is largely unknown.³⁸ IgLON5 knockout mice only develop subtle motor and behavioral changes but none of the cardinal clinical features of anti-IgLON5 disease.^39,40 That IgLON5-KO mice only develop mild clinical features suggest that other members of the IgLON family may compensate the IgLON5 deficit.

IgLON5 is also expressed outside the nervous system, mainly in muscle and testis.⁴¹ Patients with anti-IgLON5 disease do not show clinical evidence of myopathy, but a systematic analysis of muscle involvement in anti-IgLON5 disease has not been reported yet. The expression of IgLON5 in muscle does not necessarily mean that patients should have signs of myopathy. For example, patients with neuromyelitis optica spectrum disorders and aquaporin 4 antibodies rarely have kidney dysfunction despite high expression of aquaporin 4 in the collecting ducts of the kidney.⁴²

The trigger of IgLON5 autoimmunity remains unknown. The genetic background of patients is probably important because up to 60% have the HLA-DRB1*10:01- DQB1*05:01 haplotype. A recent multicentric HLA study including 87 patients with anti-IgLON5 disease revealed a stronger association with HLA-DQ than HLA-DR. The study 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. Previously, an algorithm using artificial neural networks showed that 2 IgLON5-derived peptides were strong binders to HLA-DRB1 molecules: DRB1*01:01, DRB1*10:01, and DRB1*09:01.²⁵ However, in vitro studies did not confirm the theoretical binding. By contrast, a deamidated peptide from the Ig2 domain of IgLON5 was able to activate T cells in 2 patients with the HLA-DQA1*01:05∼DQB1*05:01 haplotype supporting an HLA-DQ–mediated T-cell response to IgLON5 as a potential trigger of the disease.⁴³

Since the initial description of anti-IgLON5 disease, there has been a debate on whether the disease is primarily autoimmune or the IgLON5 antibodies are simple accompaniments of a primary tauopathy.⁴⁴ Currently, the autopsy findings showing that the tauopathy is associated with a longer duration of the disease, together with the observation of antibody-mediated cytoskeletal changes in cultured neurons, support the hypothesis of a primary autoimmune disease with secondary neurodegeneration.^32,38

In an initial study in which cultured rat hippocampal neurons were exposed for 24–72 hours to patients' IgLON5 antibodies, we observed a reduction of surface IgLON5 clusters that persisted after removing the antibodies from the media.⁴⁵ In a subsequent study in which cultured neurons had a longer (3 weeks) exposure to patients' antibodies, there was an expected decrease of surface IgLON5 clusters in association with a disorganization of the neurofilament architecture, including dystrophic neurites, axonal swelling, and early termination of dendritic processes (Figure 3).⁴⁶ The cytoskeleton changes did not demonstrate hyperphosphorylated tau accumulation, but similar studies using cultures of induced pluripotent stem cell-derived human neurons, exposed to patients' IgLON5 antibodies, showed presence of hyperphosphorylated tau.⁴⁷ We have not reproduced those findings, and further studies are needed to demonstrate that IgLON5 antibodies are responsible for the hyperphosphorylated tau deposits seen in autopsy studies.

Figure 3. Effects of IgLON5 Antibodies on Rat Hippocampal Neurons in Culture.

(A). Immunofluorescence on live hippocampal neurons treated with control IgG (B) or IgLON5 antibodies (C). IgLON5 antibodies cause a decrease in IgLON5 clusters (C) that do not recover after removing the antibodies from the culture media (E). This effect is different from that observed with other antibodies against surface antigens, such as NMDAR, in which the reduction of antigen clusters is reversible (D and G). The intracellular staining of neurofilaments (H) revealed a disorganization of the cytoskeleton when neurons were treated with IgLON5 antibodies (J) but not with NMDAR antibodies (K). Figure generated from data of experiments reported in reference.⁴⁶ DIV = days in vitro. Scale bars: G: 5 µm, H: 20 µm, and K: 3 µm.

Besides the direct interaction of the antibodies with IgLON5, the interplay of IgLON5 antibodies with the innate immune system may be important in the pathogenesis of the disease. We should investigate the crosstalk between the fragment crystallizable domain of IgLON5 antibodies and the innate immune system, which may potentially activate a broad array of immune functions that could contribute to the neuronal dysfunction.⁴⁸ This crosstalk between antibodies and the innate immune system has been found important in the immune responses occurring in the context of infectious diseases and in autoimmune diseases. For example, in myelin oligodendrocyte glycoprotein antibody-associated disease, post-translational changes in the Fc region, which can vary with patients' age or disease stage, associate with distinct interactions with Fcγ receptors present in innate immune cells.⁴⁹

Despite the clear effects of IgLON5 antibodies in cultured neurons, we are far away in demonstrating that the disease is antibody-mediated. The reported models of passive transfer of patients' antibodies to animals did not fulfil all the criteria of antibody pathogenicity (eAppendix 1). The main findings of the published passive transfer models are summarized in Table 3. Two of the studies demonstrated that direct injection of IgLON5 antibodies in the hippocampus or substantia nigra pars compacta caused symptoms related to neuronal loss in the area injected. However, none of the studies demonstrated that these neuronal changes were caused by antibody-mediated alterations in the structure (e.g., reduction in surface levels) or function of IgLON5.^50,51

Table 3.

Animal Models Using Transfer of IgLON5 Antibodies

Author ref	Alvente56	Ni50	Gao51
Approach	Cerebroventricular infusion of IgLON5 antibodies × 14 d to wild-type and humanized transgenic tau mice	1. IgLON5 antibodies injected to both hippocampus × 7 d in mice 2. Cerebroventricular infusion of IgLON5 antibodies × 10 d in mice	IgLON5 antibodies injected into the substantia nigra pars compacta × 7 d in mice
Induced symptoms	Increased ventilatory period during sleep and abnormal licking behavior in women. No symptoms in men	Long-term and short-term memory deficit and anxiety-like behavior	Decline in locomotor activity
Neuropathology	Mild increased phosphorylated tau protein deposition in women	Microglial activation and gliosis. Neuronal reduction at CA1 30 d after the antibody injection	Loss of dopaminergic neurons. Increase in phosphorylated tau in the substantia nigra 3 mo after the injection
Symptoms similar to those of anti-IgLON5 disease?	No	No	Marginal
Demonstration that the infused antibodies specifically react with IgLON5?	No	Brains injected with IgLON5 antibodies had human IgG deposits. Colocalization with IgLON5 was not demonstrated	No
Demonstration that the infused antibodies alter the structure or function of IgLON5?	No	No	No
Demonstration that the induced symptoms correlate with the effects of the antibodies on IgLON5?	No	No	No
Comments	The study failed to demonstrate the main objective that was whether IgLON5 antibodies produced sleep changes in the treated mice	Effects as the consequence of the interaction with IgLON5 not demonstrated	Effects as the consequence of the interaction with IgLON5 not demonstrated

Treatment

In most patients, the effect of immunotherapy in anti-IgLON5 disease is suboptimal compared with antibody-mediated encephalitis.⁵² The poor results observed in early series were justified by a delay in treatment initiation when the tau deposits were already present in the brain.^4,53 In a more recent series of 53 patients, twenty-seven of them were treated because of the rapid progression of symptoms at either disease onset or relapse.² Patients usually received IV methylprednisolone or immunoglobulins (IVIGs), less frequently plasma exchange, and 36 (70%) had long-term immunotherapy, mainly rituximab, IVIGs, or corticosteroids. At the last visit, the modified Rankin score (mRS) was better than that of nontreated patients. Multivariate analysis showed that onset of treatment during the first year of symptoms and low pretreatment neurofilament light chain levels were independent predictors of treatment response.² In our cohort, 68 (69%) of 98 patients received immunotherapy, including IVIGs, steroids, plasma exchange, rituximab, or cyclophosphamide. A positive response to immunotherapy was observed in 37 (54%) of the patients. However, the clinical response was only partial in 35 (95%) of them, and 21 (57%) had a relapse in the ensuing months.

Several factors are important to keep in mind at the time of initiation of immunotherapy: (1) Some patients have clinical manifestations that cause minor disability and may remain stable for many months. In this setting, immunotherapy might be postponed until there is progression of symptoms.⁵⁴ Until we demonstrate that early onset of immunotherapy changes the course of the disease, holding treatment in those patients seems a logical approach to avoid potential side effects of treatments. (2) Worsening of dysphagia, dyspnea, or stridor may be life-threatening and require immediate interventions, such as CPAP therapy or tracheostomy, rather than immunotherapy.¹² (3) It is presently unclear which is the best immunotherapy or combination of treatments to induce remissions or modify the progressive course of the disease. (4) We do not know whether all symptoms have the same probability to improve with immunotherapy. The mRS, the most common tool used to evaluate treatment response, cannot address this issue in patients with anti-IgLON5 disease. For this reason, we proposed a composite score for the clinical assessment of anti-IgLON5 disease (eTable 4). This composite score was designed to assess 17 common symptoms divided in 5 clinical domains (bulbar, sleep, movement disorders, cognition, others). Each symptom was scored from 0 (absent/normal) to 3 or 6 (severe) depending on the contribution of the symptom to neurologic disability. In a series of 86 patients, the application of the composite score suggested that it is a useful tool to assess the extent and severity of anti-IgLON5 disease.⁵⁵ Future validation studies will be important to identify the symptoms that are more responsive to immunotherapy, which will help in making decisions about when to start and maintain treatments.

Conclusions

Ten years after the identification of anti-IgLON5 disease, we know many important aspects of the disease (Table 4). We have improved our skills to make an earlier diagnosis. The question of what is first, autoimmunity or neurodegeneration, has been reasonably answered by autopsy studies, showing short-duration disease cases without tauopathy, and neuronal cell studies demonstrating that patients' antibodies cause a decrease in surface IgLON5 levels and induce changes in the cytoskeleton. However, many important questions need to be addressed (Table 4): as in many autoimmune diseases, we do not know the cause and triggers of the disease. The pathogenic role of IgLON5 antibodies has to be demonstrated using robust in vivo models. Moreover, the potential implication of cell-dependent mechanisms (some could be antibody-mediated through the Fc interaction with innate immune cells) in the pathophysiology of the disease cannot be overlooked.

Table 4.

Summary of What We Know and What We Do Not Know of Anti-IgLON5 Disease

What we know	What we do not know
Disease onset after the fifth decade	Triggers and predisposing factors to develop anti-IgLON5 disease
Progressive course in >70% of patients	Prevalence/incidence of the disease
Most common presentation is a combination of gait impairment, bulbar symptoms, and a characteristic sleep disorder	Natural history/cause of death
Abnormal movements common—mainly chorea and facial dyskinesias	Profile of cognitive deficits
Characteristic sleep signature of polysomnographic studies	Frequency of motor neuron involvement
Absent signs of inflammation in MRI and CSF in >70% of patients	Variables associated with the presence of inflammation in brain MRI and CSF
Association with the HLA-DRB1*10:01- DQB1*05:01 haplotype	Significance of hippocampal tau deposits detected in some patients
Presence of a novel 3R and 4R neuronal brainstem tauopathy in most patients	Neuropathologic substrate of cognitive deficits and movement disorders such as chorea
IgLON5 antibodies induce irreversible reduction of membrane IgLON5 clusters and cytoskeletal changes in neurons in culture	The function of IgLON5
Immunotherapy is indicated particularly in subacute deterioration at onset or relapses	The interactions between IgLON5 antibodies and the innate immune system
	Whether all patients must be treated with immunotherapy
	The best treatment
	The response of different symptoms to treatment

The knowledge of what we do not know of anti-IgLON5 disease (Table 4) is important to guide the direction of the research. Once the path is marked, the plan is simple: work, work, and keep working as reflected in the lyrics of this song of the great blues singers Sonny Terry and Brownie McGhee.

Walk on, walk on, walk on, I walk on

I'm gonna keep on walkin', till I find my way back home

Glossary

CBA

cell-based assay

CPAP

continuous positive airway pressure

FLAIR

fluid attenuated inversion recovery

HLA

human leucocyte antigen

IVIG

IV immunoglobulin

mRS

modified Rankin Score

MSA

multiple system atrophy

NMDA

N-methyl-D-aspartate

NREM

nonrapid eye movement

PSP

progressive supranuclear palsy

v-PSG

video-polysomnographic

Author Contributions

F. Graus: drafting/revision of the manuscript for content, including medical writing for content; major role in the acquisition of data; study concept or design; analysis or interpretation of data. L. Sabater: drafting/revision of the manuscript for content, including medical writing for content. C. Gaig: drafting/revision of the manuscript for content, including medical writing for content; major role in the acquisition of data; study concept or design; analysis or interpretation of data. E. Gelpi: drafting/revision of the manuscript for content, including medical writing for content; major role in the acquisition of data. A. Iranzo: drafting/revision of the manuscript for content, including medical writing for content; major role in the acquisition of data. J.O. Dalmau: drafting/revision of the manuscript for content, including medical writing for content. J. Santamaria: drafting/revision of the manuscript for content, including medical writing for content; major role in the acquisition of data; study concept or design; analysis or interpretation of data.

Study Funding

This study has been funded in part by Instituto de Salud Carlos III through the project PI21/00165 (cofunded by the European Regional Development Fund “Investing in your future”).

Disclosure

F. Graus receives royalties from Euroimmun for the use of IgLON5 as an autoantibody test and honoraria for Associate Editor of MedLink Neurology. J.O. Dalmau receives royalties from Athena Diagnostic for use of Ma2 as an autoantibody test and from Euroimmun for use of NMDA receptor, GABAB receptor, GABAA receptor, DPPX, and IgLON5 as autoantibody tests. All other authors report no relevant disclosures. Go to Neurology.org/NN for full disclosures.