Question:

What are the different known antibodies discovered to be causing Autoimmune Encephalitis?

 

Answer:

There are multiple types of antibodies in Autoimmune Encephalitis. They have different clinical presentations and syndromes and have different responses to treatment.  We know from the immunology world that antibodies come in many shapes and forms.   There is a growing list of antibodies in each category.  Autoimmune encephalitis may be divided into several groups of diseases: Those antibodies that can access the cell surface of an antibody or the synaptic receptor of an antibody that is accessible to the targeting antibody because it is exposed on the outside.  Cytoxic T-cell diseases associated with antibodies to intracellular antigens, and those associated with other autoimmune disorders.

​Extracellular antibodies target the receptor on the outside surface of the healthy neuronal cell and then binds to the receptor on the outside surface of the cell.​

As more cases appeared, and neuronal targets could not be identified, that lead researchers to discover that the unknown cell surface antigens were not cell surface antigens at all, but in fact were found to be synaptic receptors, including the N-methyl-D-aspartate receptor (NMDAR).  This discovery led investigators to distinguish a new category of autoimmune encephalitis those where the offending antibody is actually targeting the synaptic receptors in the brain.  ​

In the disorders investigated so far that occur with autoimmune encephalitis, researchers who have studied their underlying mechanisms have seen that the antibodies alter the structure or function of the antigen they have targeted and bonded to.  This attacking antibody that the immune system has created, now causes the neuron it has attached itself to, to malfunction or die. ​

These attacks can occur throughout the brain.  Symptoms wax and wane (come and go in severity) as the attacks occur.  At times, some symptoms may be more dominate and not so severe at other times.  This is due to the fact that different areas of the brain may be under attack at different times.  It is the fluctuation of symptoms and the vast array of symptoms the patient is experiencing that causes the neurologist unfamiliar with these disorders to fail in arriving at an accurate and timely diagnosis.  This is a key reason we advocate that those suffering from Autoimmune Encephalitis hire a top expert in the field of autoimmune neurology as experience bares out that this leads to an accurate and expedited diagnosis and aggressive treatment plan that leads to best outcomes.  ​

The first antibody was identified in 2005 and published about in 2007.  Since that time, many antibodies have been identified and still some are yet to be discovered. Along with the rapidly expanding spectrum of autoimmune encephalitis (AE), probable AE with no detected autoantibody (AE-NoDab) has become a major category of AE. Approximately 45-50-% of patients do not have an identified antibody. (antibody negative/seronegative/possible, probable AE). Below are the most common types of antibody-mediated AE.

 

Specific syndromes due to antibodies to cell surface antigens

NMDAR antibody

Is seen in ages throughout the life span, but most commonly younger women. Modal onset: 18–23 years; range: infancy–89 years. Well-defined association with ovarian teratoma in young adult patients; overall rate ~ 25%. Other varied tumors are rare. There is a prodromal period of Flu-like (headache, malaise, mild fever, gastrointestinal symptoms). NMDAr usually has an abrupt or subacute onset Progressive: Prodrome → psychiatric → neurological → critical illness.

Psychiatric symptoms occur in almost all adult patients, less dominant in childhood cases. Polymorphic syndrome involving multiple psychopathological domains, e.g. sleep, mood, psychosis, behavioral, and catatonia.  Neurological symptoms include: Seizures common (~ 70%) but rarely frequent
Movement disorder in > 90%. Often multiple simultaneous phenomenologies with a dominant triad of dystonia, stereotypies, and chorea. Orofacial dyskinesia is common.

In the acute phase deficits across all domains; memory, information processing, attention, executive function, language, visuospatial processing, and social cognition are all affected.  The post-acute phase episodic memory, processing speed, and executive function remain impaired with the greatest deficit in episodic and delayed verbal memory.
Dysautonomia. Outcomes: Overall: mortality up to 15%, relapse rate up to 30%. With immunotherapy, > 75% of patients with mRS < 3.

 

LGI1 antibody-associated encephalitis

Limbic encephalitis is the second most common type of AE in adults and is associated with the leucine-rich, glioma-inactivated 1 (LGI1) antibody. This affects men above 60 years, who outnumber women by a ratio of 3:1. The most common primary symptoms include subacute progressive short and long term memory impairment,  mental,  behavioral and space orientation abnormalities, myoclonus, or tonic seizures, especially faciobrachial dystonic (FBDS) seizures which are unique to LGI1 encephalitis and are a useful clinical differentiator, which denote the onset of encephalitis.  Seizures typically precede the onset of cognitive impairment, with a progressive amnesia usually developing at their crescendo. Prominent amnesia is a hallmark of limbic encephalitis associated with LGI1-antibodies; autobiographical memory is particularly impaired, often with significant confusion and disorientation.  An isolated amnestic syndrome can occur in up to 10% of cases. Spatial disorientation, hallucinations, are also common and some patients may suffer from sleeping disorders and autonomic dysfunctions, for example, sexual dysfunction, sweating, and sinus bradycardia.  A tenth has autonomic symptoms and low sodium levels occur in about 60%.  Autonomic and muscle hyperexcitability, pain only. FBDS appear earlier than other symptoms in many patients while tonic-clonic seizures often occur concurrently or immediately after a decline in the patient’s cognitive function.

Looking at long-term outcomes cognitive deficits correlate with antibody titer and are most marked for verbal memory while processing speed and executive function are relatively spared.  Most patients have a chronic cognitive impairment, with memory predominantly affected but deficits of attention and executive function also reported. Greater disease severity, delays to immunotherapy, or longer courses of immunotherapy (likely mandated by greater disease severity) are all associated with more profound cognitive dysfunction in LGI1 encephalitis. Approximately 5%–11% of cases are associated with cancer; the most common associated tumor is thymoma.

 GABA(b)-antibody-associated encephalitis

Anti-GABA-B (γ-Aminobutyric acid B) receptor encephalitis is also typical limbic encephalitis with seizures, memory loss, as well as cognitive and behavioral symptoms which are almost universally seen.  Status epilepticus or seizure activity is predominant due to the involvement of the inhibitory GABA receptors. Cognitive impairment and seizures remain the central symptoms, almost universally affecting patients in the acute phase. Memory deficits and confusion present in 47% of patients.  The nature of neuropsychological impairments has not been examined in detail.

Occurring in older men and women, with an average age of 60 years, these patients often have other antibodies such as N-type voltage-gated calcium channels or glutamic acid decarboxylase antibodies. Lung cancer (small cell) is present in 50%, making this the most common immune encephalitis in this class of patients. This recent case series identified a subset of patients with GABA_BR encephalitis presenting with a “rapidly progressive dementia” with subacute cognitive impairment in the absence of seizures. Relapse and recovery rate: Some patients are immunotherapy responsive, poor outcomes largely attributed to underlying malignancy; up to 10% may relapse, mortality in up to 40%.  Prognosis is often poor, with a median survival of 17 months, and the long term outcomes for GABA_BR encephalitis have yet to be studied.

 

CASPR2 antibody-associated encephalitis

CASPR2 antibodies associate with a wide range of neurological syndromes, which often overlap in the same patient.  Similar features of limbic encephalitis are often seen; seizures, cognitive impairment, personality change. Cognitive dysfunction is common with memory impairments but confusion and behavioral disorders are less prominent. Anterograde and episodic memory disorders are typically seen.

Anti-Caspr2 (contactin-associated protein-like 2) antibodies usually lead to slow-onset diffuse encephalitis and autonomic imbalance in many cases, as well as hyperexcitability of the peripheral nerves (in which case it is called Morvan’s syndrome) with neuropsychiatric changes, dysautonomia, cognitive decline, seizures,  sleep disturbance (insomnia, agrypnia excitata), weitht loss, paroxysmal movement disorders, limbic encephalitis, and neuromyotonia.  Neuropathic pain is seen in about 60% of cases and relapse is common following tapering of immunotherapy. Patients are typically about 60 years old although rare pediatric cases have been described.  The disorder is usually not associated with cancer. A tumor, usually thymoma, may be revealed in up to 32% of cases. Patients with tumors  are more likely to develop Morvan syndrome. Relapse rate and outcome: >80% have favorable responses to immunotherapy – especially in absence of a tumor; 10% mortality rate; up to 30% relapse rate.

Long term cognitive outcomes for CASPR2 encephalitis are not clear. Looking at the long term outcome of CASPR2 patients cognitive deficits correlate with antibody titer and are most marked for verbal memory while processing speed and executive function are relatively spared.

 

GABA(a)-antibody-associated encephalitis

Anti-GABA-A (γ-Aminobutyric acid A) receptor encephalitis occurs in younger age groups and presents as acute encephalitis with status epilepticus or epilepsy partialis continua when high levels of the antibody are present.

Confusion, disorientation, hallucinations, and other psychiatric features, cognitive dysfunction/behavioral symptoms are seen in two-thirds of patients, movement disorders; lower titers associated with stiff person syndrome, opsoclonus-myoclonus. Memory deficits and confusion are less consistently reported in GABA_AR encephalitis. Relapse rate and outcomes: Over 80% may respond to immunotherapy ± tumor removal; but full recovery in only 30%; up to 20% mortality rate (especially in the context of status epilepticus); relapses in 10%. No published neuropsychological long-term outcomes have been reported as of 2020.

 

MOG (Myelin oligodendrocyte glycoprotein)

Mog antibodies are associated with relapsing syndromes involving brainstem or cortical encephalitis, sometimes with optic neuritis and transverse myelitis, which particularly involve children and young adults. Seizures may present as the index event and the syndrome can evolve to a more diffuse encephalitis, including one which radiologically mimics classical acute disseminated encephalomyelitis. Patients typically respond well to corticosteroid therapies, although the duration of their administration remains controversial as relapses are common.

 

GlyR-antibody-associated encephalitis

Anti-GlyR (Glycine receptor) antibodies occur in stiff-person syndrome (SPS), which may occur along a spectrum characterized by progressive encephalomyelitis with rigidity and myoclonus (PERM) at one end, and hyperekplexia at the other (pathologic startle response, or painful spasms brought on by touch, sound or emotional stimuli).

SPS is not specific for glycine-receptor antibodies, as GAD or amphiphysin antibodies are found in other clinical situations. Relapse rate and outcome: Generally, respond well to immunotherapy, with GlyR antibody stiff-person syndrome being more immunotherapy responsive than seronegative cases; may relapse in 15%; 10% mortality rate

 

AMPA antibody-associated encephalitis

Due to its rarity, the clinical course of AMPAR encephalitis is not yet well characterized. Anti-AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptor encephalitis affects mostly middle-aged women (90%) and presents with limbic or psychiatric features. May present with prominent memory impairment, confusion, seizures, or severe and sudden encephalitis onset. May also present with isolated amnesia, or a type of memory loss involving the inability to remember new information (anterograde) amnesia, cognitive impairment, disordered sleep, movement disorders. Immunotherapy brings about remission, relapse is common in 60%.

The common memory loss in patients can complicate treatment, and often becomes irreversible after a few relapses have occurred, being accompanied by cognitive impairment which remains a universally prominent feature and isolated amnesic syndromes have also been observed with a focal impairment to anterograde memory.  Underlying cancers of breast, lung, and thymus are common. Relapse rate and recovery: Most patients show a partial response to oncological management and immunotherapy responsiveness; relapses appear common.

There are limited data available on the neuropsychological outcomes of AMPAR encephalitis patients. Case reports have described considerable neurocognitive improvement at follow-up. Although in general outcomes appear favorable, psychiatric symptoms or severe or sudden onset of encephalopathy at onset are associated with a poor prognosis.

 

DPPX-antibody-associated encephalitis

Anti-DPPX (dipeptidyl-peptidase-like protein-6) antibodies Dipeptidyl-peptidase-like protein-6 antibodies are directed against a part of a type of potassium channel and has a prodrome of severe diarrhea and weight loss as a primary symptom. Subacute onset of cognitive impairment, agitation, confusion, hallucinations, seizures, sleep dysfunction; tremor, hyperekplexia, myoclonus; bulbar dysfunction, and autonomic dysfunction along with limbic encephalitis. Hyperexcitability is also prominent, in addition to memory loss. Relapse rate and outcome: May have multiple relapses in close to 25%; 60% can respond partially or significantly to (often intensive) immunotherapy, mortality 17%

 

IgLON5

Progressive dyssomnia, movement disorders and behavior, gait abnormalities, bulbar and respiratory dysfunction, and cognitive impairment; disease onset often more insidious compared to other autoimmune encephalitis syndromes. Relapse rate and outcome: Severe and progressive, with early reports stating > 70% mortality and minimal response to immunotherapy; later series identify broader phenotype and show immunotherapy may result in improvement and stabilization.

 

Dopamine-D2-receptor antibodies

These are associated with basal ganglia inflammation in children and with Sydenham’s chorea. Parkinsonism, dystonia, hypersomnolence, neuropsychiatric features (obsessive-compulsive disorder, psychosis, emotional lability), ‘basal ganglia encephalitis’. Relapse rate and outcome: Immunotherapy responsive, 25% may relapse.

 

α3-AChR

The nicotinic ganglionic acetylcholine receptor autoantibody (α3-AChR Ab) causes autoimmune dysautonomia that is either subacute or insidious in onset. There is a direct relationship between antibody titer and severity of dysautonomia with patients. Patients generally develop profound pandysautonomia (a rare disease with an acute widespread and severe failure of the muscles your body uses for movement to function properly) with high α3-AChR Ab titers. There is limited dysautonomia with lower α3-AChR Ab titers. Pandysautonomia is generally severe. Limited dysautonomia is confined to 1 or 2 domains and is often mild.

The positive antibody test of α3-AChR autoantibody aids the diagnosis of neurological autoimmunity and cancer.  It is a marker for several disorders most notably the syndrome Myasthenia gravis (MG), a neuromuscular disorder that causes weakness in the skeletal muscles, which are the muscles your body uses for movement. It occurs when communication between nerve cells and muscles becomes impaired.

Encephalopathy associated with neuronal ganglionic acetylcholine receptor antibodies has been rarely reported.

 

GFAP

(GFAP) autoimmune astrocytopathy with glial fibrillary acidic protein, affected patients present with symptoms of one or more of meningitis (headache and neck ache), encephalitis (delirium, tremor, seizures, or psychiatric symptoms), and myelitis (sensory symptoms and weakness). Optic disc papillitis (blurred vision) is common. CSF is more reliable than serum (blood) for GFAP-immunoglobulin G  antibody (IgG) testing.  It can co-exist with anti-NMDAr or aquaporin-4 (AQP4).  Ovarian teratoma commonly exists within anti-NMDAr cases. Corticosteroid-responsiveness is a hallmark of the disease. Relapses occur in approximately 20% of patients, necessitating the transition to a steroid-sparing drug such as Cellcept (mycophenolate mofetil), or azathioprine (Imuran). Reported outcomes vary, though early and sustained intervention usually portends recovery.

 

Neurexin-3α

Prodromal fever, headache, gastrointestinal symptoms; subsequent encephalopathy with agitation, seizures, orofacial dyskinesias, and central hypoventilation (marked overlap with NDMAR encephalitis); may have a rapid course.

 

GAD-antibody-associated encephalitis

Glutamic acid decarboxylase is a crucial enzyme in the synthesis of the major inhibitory neurotransmitter, GABA. GAD antibodies have been found in SPS (stiff person syndrome), juvenile diabetes, limbic encephalitis, and temporal lobe epilepsy. Young women are most often affected and show high levels of antibodies. It may often co-occur with GABA(b)-receptor antibodies. Dysautonomia with and without pain, myelopathy, and spasticity (stiff person syndrome), cerebellar disease are pain associations.

The role of GAD antibodies is not yet established, however, as they occur even in the normal population. Older patients with limbic encephalitis associated with GAD antibodies, or who also have GABA(b)-receptor antibodies, are at increased risk for small cell carcinoma of the lung or thymoma. In general, immunotherapy has limited effects on the outcomes of neurological syndromes associated with GAD antibodies.

GluK2

Glutamate kainate receptor subunit 2 is associate with an encephalitis with prominent cerebellar involvement as seen clinically and on MRI.  Cerebellar symptoms, cerebellitis, May Co-exists with anti-AMPAR and anti-NMDAR encephalitis.  The antibody effects are predominantly mediated by internalization of GluK2.

 

GluRD2

Pediatric onset 12-36 months; M:F 1:1.4 Opsoclonus, myoclonus, ataxia, cognitive and behavioral impairment associated with low-titer antibodies. Neuroblastoma is seen in about half of the children. The relapse rate and the outcome are not yet known.

 

Ca_V α2δ (voltage-gated calcium channel alpha-2/delta subunit)

This new antibody was identified in January 2021. Two patients were identified. The patients were associated with preceding meningitis or neuroendocrine carcinoma and responded to immunotherapy.

 

DNER-antibody-associated encephalitis

Anti-DNER (delta/notch-like epidermal growth factor-related receptor) is directed against a Purkinje neuron transmembrane protein and presents with cerebellar degeneration, which is often irreversible. Over 90% of these patients have Hodgkin lymphoma.

 

Anti-mGluR1-antibody-associated encephalitis

Anti-mGluR1 (metabotropic glutamate receptor 1) is rare, associated with paraneoplastic cerebellar involvement, and Hodgkin’s lymphoma is common.

 

Anti-mGluR5-antibody-associated encephalitis

Anti-mGluR5 (metabotropic glutamate receptor 5) have been reported in a few patients with Ophelia syndrome, which refers to mGluR5- antibody-associated limbic encephalitis with Hodgkin’s lymphoma. Cognitive impairment, memory deficits, confusion, and disorientation to time is common, psychiatric symptoms but psychosis is rare. Generally responsive to treatment of Hodgkin lymphoma and immunotherapy.