Encephalitis with mGluR5 antibodies: Symptoms and antibody effects

Abstract

Objective: To report the clinical features of 11 patients with metabotropic glutamate receptor 5 (mGluR5) antibody-associated encephalitis, immunoglobulin G (IgG) subclass, and effects of the antibodies on neuronal mGluR5 clusters.

Methods: Clinical information was retrospectively obtained from referring physicians. Antibodies to mGluR5 and IgG subclasses were determined with brain immunohistochemistry and cell-based assays. The effects of the antibodies were examined on rat hippocampal neurons with reported techniques.

Results: From January 2005 to May 2017, 11 patients (median age 29 years, range 6-75 years, 5 female) were identified. The main clinical features were psychiatric (10), cognitive (10), movement disorders (7), sleep dysfunction (7), and seizures (6). Median modified Rankin Scale score at the peak of the disease was 4; 4 patients required intensive care. Five patients had Hodgkin lymphoma, and 1 had small cell lung cancer. CSF showed pleocytosis (median white blood cell count 22 mm³) in all patients; brain MRI was abnormal in 5, involving limbic (1) or extralimbic (4) regions. Treatments included immunotherapy and/or oncologic therapy; at the last follow-up (median 48 months), 6 patients had complete and 5 had partial recovery. Neurologic relapse occurred in 2 patients. Antibodies were IgG1 alone (4 of 9) or in combination with IgG2 (1 of 9), IgG3 (3 of 9), or both (1). Patients' IgG caused a significant and specific decrease of cell-surface synaptic and extrasynaptic mGluR5 without altering the levels of postsynaptic density protein 95.

Conclusions: Anti-mGluR5 encephalitis associates with a complex neuropsychiatric syndrome, not restricted to limbic encephalitis, and can occur without tumor. Patients respond to treatment, but relapses can occur. The antibodies have pathogenic effects altering the levels of cell-surface mGluR5.

Figure 1. IgG subclass of patient's mGluR5 antibodies

Serum of a representative patient (patient 10) showing reactivity (green; A) with human embryonic kidney cells expressing metabotropic glutamate receptor 5 (mGluR5); the colocalization of reactivity with a commercial antibody against mGluR5 (red; B) is shown in the merged images (yellow; C). This patient had mGluR5 antibodies (Abs) of immunoglobulin G1 (IgG1), IgG2, and IgG3, but not IgG4, subclasses. Scale bar 10 μm.

Figure 2. Abrogation of patient's serum reactivity with rat brain after immunoabsorption with HEK cells expressing mGluR5

The reactivity of patient's serum with rat brain, shown in (A), and with human embryonic kidney (HEK) cells expressing metabotropic glutamate receptor 5 (mGluR5), shown in (B), is abolished (E and F) after the serum has been immunoabsorbed with HEK cells expressing mGluR5. Panels (C and G) correspond to HEK cells incubated with a commercial mGluR5 antibody, and panels (D and H) correspond to the colocalization of the reactivities (patient's and commercial antibody). Scale bar: rat brain = 2 mm; HEK cells = 10 μm.

Figure 3. Patient's antibodies cause a specific decrease of density of cell-surface mGluR5 clusters in cultured neurons

Representative confocal images (from 40 dendrites per condition) showing a decrease of density of cell-surface metabotropic glutamate receptor 5 (mGluR5) clusters in neurons treated for 24 hours with patient's immunoglobulin G (IgG) compared with neurons treated with control IgG (A). Quantification analysis of total mGluR5 clusters after 24 hours of treatment is shown in panel (C). Specific decrease of mGluR5 protein, but not α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) protein, is also demonstrated using immunoblot of biotinylated neuronal membrane fractions from neurons treated for 24 hours with patient's IgG compared to control IgG (B). These effects were reversible, and baseline levels of total (A, right column, 7 days recovery) and synaptic (D) neuronal cell-surface mGluR5 clusters (from analysis of 20 dendrites per condition per time point) were progressively restored over 96 hours to 7 days. Statistical analyses by 2-way analysis of variance; mean and SEM are plotted. Scale bar = 10 μm. **p < 0.05, ****p < 0.0001. PSD95 = postsynaptic density protein 95; R = recovery; T = treatment.