Pathogenicity of Anti-GAD Antibody

Synaptic Actions of Anti-GAD Ab

GAD participates in the synaptic transmission of GABA. The binding of anti-GAD65 antibody to GAD65 can cause presynaptic inhibition of GABA energy between basket cells and Purkinje cells (PCs) and reduce the release of GABA. When recombinant GAD65 was used to bind to an anti-GAD65 antibody, the effect of the original GAD65 was no longer inhibited. This has been proved by experiments that the use of anti-GAD65 antibody in GAD65 knockout mice has no effect, in which the inhibitory transmission effect is mediated by GAD67 compensation.

The effect of anti-GAD65 antibody depends on the epitope recognized by the antibody. Some prepared human monoclonal antibodies (GAD65) recognize specific epitopes in patients with cerebellar ataxia and cause disease, but they cannot cause disease in type 1 diabetes mellitus (T1DM) because they cannot recognize the corresponding epitopes. The effects of antibodies vary among patients with different diseases, such as impaired exocytosis in cerebellar patients and reduced GABA synthesis in SPS patients. In addition, the researchers found that low titers of anti-GAD antibodies had no pathogenic effect, suggesting that low titers of anti-GAD antibodies recognize epitopes differently from high titers.

In the cerebellar-cerebellar loop, connected GABA neurons (basket cells and PCs) determine the timing of coordination and phasic coordination commands, with particular emphasis on the precise timing of GABA release. In contrast, in the spinocerebellar circuit, GABA output from PCs may regulate excitatory signals, with particular emphasis on catatonic GABA delivery.

Anti-GAD65 antibodies can be internalized by endocytosis or endocytosis, which weakens the binding of GAD65 to vesicles in cells, making it difficult for GABA to enter the vesicles and be transferred to the cell membrane for release. The researchers believe that the mechanism is that GAD65 attached to the membrane of the vesicular cell is exposed to the vesicle space during efflux, allowing the anti-GAD65 antibody in the vesicle to bind to the exposed part of GAD65. Reduced GABA release impairs the spillover effect of GABA-induced presynaptic inhibition on glutamate release from adjacent parallel fibers, causing a severe imbalance between GABA and glutamate, leading to excitotoxicity. Some patients with cerebellar atrophy responded well to immunotherapy, and the improvement in clinical symptoms was closely related to the decrease in antibody titers, suggesting that antibody titers may well reflect the dysfunction. Functional excitotoxicity induced by autoantibodies induces microglial activation, which interferes with glutamate uptake by astrocytes. This enhancement may promote cell death at a later stage.

Figure 2. Pathogenic actions of anti-GAD65 antibody
(Source: Mitoma H, et al. 2023)

Anti-GAD Epilepsy

Anti-GAD65 epilepsy can be divided into acute/subacute seizures and chronic epilepsy. The clinical manifestations can be mild non-pharmacoresistant epilepsy, limbic encephalitis (LE), and extralimbic encephalitis (ELE). Most of the patients with chronic epilepsy are focal epilepsy, located in the temporal lobe, and have no previous history of brain injury. The most common symptoms during seizures are mental symptoms, followed by somatosensory, motor, and visual symptoms. One study found that anti-GAD65 epilepsy also presents with a distinctive seizure type, with music-induced reflex epilepsy occurring in 9% to 15% of patients. 75% of patients had no epileptogenic foci detected on the first MRI scan. The most common abnormality on imaging in disease progression is the loss of brain or cerebellar volume disproportionate to age. Patients with unexplained mesio-temporal lobe epilepsy (MTLE) can be examined for insula involvement when encountered clinically, which is an important diagnostic clue for anti-GAD65 epilepsy.

The onset of LE is subacute and its course is less than 3 months. It is characterized by pathological structural abnormalities of the temporal lobe on neuroimaging, temporal lobe seizures, and some rapidly progressive memory and affective disorders. Anti-GAD65 antibodies can cause LE seizures, the main clinical symptoms are epilepsy and memory impairment, there are oligoclonal bands in cerebrospinal fluid accompanied by intrathecal synthesis and show a chronic course with persistent antibodies.

Clinical treatment includes symptomatic treatment, immunotherapy, and surgical treatment. Most patients have received different combinations of treatment and intervention, but the curative effect and prognosis are poor. At present, it is considered that the treatment methods and intervention time should take different measures according to the clinical progress of anti-GAD65 epilepsy.

The clinical course of anti-GAD65 epilepsy is divided into three stages. In the first stage, acute reversible immune activation leads to the first seizure, with no permanent brain damage and no obvious brain magnetic resonance imaging changes. At this time, emphasis should be placed on immunotherapy, and early immunotherapy can prevent it from developing into refractory epilepsy. In the second stage, there has been irreversible minor brain damage, leading to magnetic resonance imaging abnormalities and refractory epilepsy. At this time, although immunotherapy is still effective, the focus should be shifted to refractory epilepsy. In the third stage, the proliferation of astrocytes after inflammation leads to secondary hippocampal sclerosis or more diffuse brain injury, at this time the effect of immunotherapy is very poor, the focus of treatment should be focused on the control of seizures. However, the above three stages of clinical progression and treatment are not suitable for patients with non-acute non-pharmacoresistant epilepsy.