Detection of autoantibody against extracellular epitopes of N-methyl-d-aspartate receptor by cell-based assay

The concept of anti-N-methyl-d-aspartate receptor (NMDAR) encephalitis, a severe, potentially lethal, treatment-responsive disorder, mediated by autoantibodies against NMDAR was proposed. Because paraneoplastic anti-NMDAR encephalitis has a better prognosis after tumor resection and immunotherapy, rapid quantitative systems for detecting functional autoantibodies against extracellular epitopes of NMDAR are necessary. To detect autoantibodies recognizing extracellular epitopes of NMDAR, we stably expressed mutant NMDAR that decreases Ca²⁺ permeability on a heterologous cell surface without any antagonist. Serum and CSF samples from patients were analysed using the cells expressing mutant NMDAR subunits by immunocytochemistry and on-cell Western analysis using live cells stably expressing mutant NMDAR. Furthermore, we were able to express mutant GluRζ1(NR1, GluN1) subunit of NMDAR alone on the cell surface and obtained direct evidence of the presence of autoantibodies recognizing extracellular epitopes of GluRζ1 and the induction of internalization by autoantibodies in serum and CSF from patients. The specificity of on-cell Western analysis was improved at 37°C. The combination of this rapid quantitative assay using our on-cell Western analysis, detailed analysis of extracellular epitopes of NMDAR, and internalization assay of NMDAR will be valuable for the diagnosis, evaluation of clinical treatments, and follow-up of anti-NMDAR encephalitis.