Marked Peripheral Blood Plasmablast Expansion in MOG Antibody‐Associated Disease and Anti‐ NMDA Receptor Encephalitis Overlap Syndrome

ABSTRACT Following upper respiratory symptoms, a 30‐year‐old man developed right eye pain, psychiatric symptoms, cognitive decline, and status epilepticus. Brain MRI showed unilateral cortical FLAIR hyperintensity consistent with FLAMES (FLAIR‐hyperintense lesions in anti‐MOG‐associated encephalitis with seizures). Serum anti‐MOG antibodies and cerebrospinal fluid (CSF) anti‐NMDAR antibodies established MOG antibody‐associated disease (MOGAD) and anti‐NMDAR encephalitis overlapping syndrome (MNOS). Flow cytometry revealed marked expansion of peripheral blood plasmablasts (IgD − CD19 + CD20 − CD24 − CD27 + CD38 + CD138 − ), comprising 13.6% of peripheral blood B cells, whereas CSF B cells were predominantly naïve B cells with virtually no detectable plasmablasts and plasma cells. Following immunotherapy, peripheral blood plasmablasts decreased to 2.0% in parallel with clinical recovery, while CSF B cells shifted toward memory predominance. This case demonstrates a dissociation between peripheral blood plasmablast expansion and absent intrathecal plasmablast accumulation in MNOS, suggesting that peripheral blood B‐cell profiling may warrant further evaluation in MNOS.