Clinical and pathogenic significance of IgG, IgA, and IgM antibodies against the NMDA receptor

Abstract

Objective: To determine the frequency and clinical relevance of immunoglobulin (Ig)G, IgA, and IgM N-methyl-d-aspartate receptor (NMDAR) antibodies in several diseases, and whether the IgG antibodies occur in disorders other than anti-NMDAR encephalitis.

Methods: Evaluation of IgG, IgA, and IgM NMDAR antibodies in serum of 300 patients with anti-NMDAR encephalitis, stroke, dementia, schizophrenia, or seronegative autoimmune encephalitis. Antibodies and their effect on cultured neurons were examined with cell-based assays and brain and live neuronal immunostaining. Retrospective analysis of the clinical diagnoses of a cohort of 1,147 patients with IgG NMDAR antibodies identified since 2005.

Results: Among the 300 patients studied, IgG NMDAR antibodies were only identified in those with anti-NMDAR encephalitis and all reacted with brain and live neurons. By cell-based assay, IgA or IgM antibodies were detected in 22 of 300 patients (7%) with different diseases, but only 10 (3%) reacted with brain and 7 (2%) with live neurons. In cultured neurons, IgG but not IgA or IgM antibodies caused a decrease of synaptic and extrasynaptic NMDAR. Among the cohort of 1,147 patients with IgG NMDAR antibodies, 1,015 (88.5%) had anti-NMDAR encephalitis, 45 (3.9%) a limited form of the disease, 41 (3.6%) autoimmune post-herpes simplex encephalitis, 37 (3.2%) overlapping syndromes (anti-NMDAR encephalitis and demyelinating disease), and 9 (0.8%) atypical encephalitic syndromes; none had schizophrenia.

Conclusions: IgG NMDAR antibodies are highly specific for anti-NMDAR encephalitis and cause a decrease of the levels of NMDAR. In contrast, IgA or IgM antibodies occur infrequently and nonspecifically in other diseases and do not alter the receptor levels.

© 2018 American Academy of Neurology.

Figures

Figure 1. Algorithm of antibody testing using confirmatory studies

AE = autoimmune encephalitis; CBA = cell-based assay; Ig = immunoglobulin; NMDAR = N-methyl-d-aspartate receptor; NMDARE =NMDAR encephalitis.

Figure 2. IgG, IgA, and IgM antibodies against the NMDAR demonstrated by CBA and brain tissue immunostaining

CBA for NMDAR antibodies showing the reactivity of serum of 3 different patients with IgG (A), IgA (B), and IgM (C) antibodies. Patients' antibodies are shown with green fluorescence and a commercial antibody against a noncompeting NMDAR epitope with red fluorescence. (D–F) Lack of reactivity of a control sample (without NMDAR antibodies) using a similar CBA. (G–I) Brain reactivity of the 3 patients' samples (same as in A–C) using the corresponding secondary antibodies against IgG (G), IgA (H), and IgM (I). The lack of reactivity of the control sample using the indicated secondary antibodies is shown in panels J–L. In panels A–F, the cell nuclei are shown with DAPI; scale bar = 10 μm. In panels G–L, the slides have been mildly counterstained with hematoxylin; scale bar = 500 μm. CBA = cell-based assay; DAPI = 4′,6-diamidino-2-phenylindole; Ig = immunoglobulin; NMDAR = N-methyl-d-aspartate receptor.

Figure 3. Effects of patients' antibodies on the levels of cell surface NMDAR

(A) Dendrites from live neurons exposed to serum IgG antibodies (NMDAR IgG+ S) show a decrease of total cell surface clusters of NMDAR (green fluorescence in top row) compared to neurons exposed to control serum (NC S), and 4 different sera with IgM (NMDAR IgM+ S1; NMDAR IgM+ S2) or IgA (NMDAR IgA+ S3; NMDAR IgA+ S4) antibodies. None of the patients' samples affected the levels of PSD95 (red fluorescence in middle row). The serum with IgG antibodies (NMDAR IgG+ S), but none of the 4 sera with IgA or IgM antibodies, also caused a decrease of synaptic clusters of NMDAR (yellow fluorescence in lower row). Synaptic NMDARs are defined by the colocalization of green (NMDAR) and red (PSD95) fluorescence. Scale bars in all panels = 5 μm. (B) Quantification of the above studies demonstrating that only the serum with IgG NMDAR antibodies caused a significant decrease of total surface and synaptic NMDAR. Data obtained from 3 independent experiments (16–18 dendrites per experiment, for each condition) showing the median with interquartile range. Statistical significance was analyzed with the Kruskal-Wallis test followed by the Dunn post hoc test for nonnormally distributed data. A value of p < 0.05 was considered significant in post hoc testing after correction for multiple comparisons (Dunn test); ***p < 0.001. Ig = immunoglobulin; NC = normal control; NMDAR = N-methyl-d-aspartate receptor; PSD95 = postsynaptic density protein-95.