AbstractObjectivesNMO‐IgG/anti‐aquaporin 4 antibody (AQP4‐Ab) is a specific diagnostic marker for neuromyelitis optica (NMO). Several clinical phenotypes exist in AQP4‐Ab‐positive patients, such as typical neuromyelitis optica, limited symptoms of myelitis or optic neuritis. The mechanisms responsible for these differences remain poorly understood. We investigated the difference of AQP4‐ab binding reactivity among clinical subtypes of AQP4‐Ab‐posive NMO cases using 3 kinds of AQP4 proteins.MethodsIndirect immunofluorescence staining was utilized to measure the binding intensities of AQP4‐Ab to each of the AQP4‐transfected HEK 293 cells, including human, mouse, and rat AQP4 (hAQP4, mAQP4, and rAQP4), each has one amino acid substitution at the extracellular domain of AQP4 protein and is speculated to show some different characters as the antigen epitope. Serum samples from 24 AQP4‐Ab positive patients, including six patients each from three adult onset groups and child‐onset group (group C), were collected. The adult‐onset patients were assigned to groups: NMO – typical neuromyelitis optica; ON – relapsing optic neuritis; M – relapsing myelitis.ResultsAll adult onset groups exhibited decreased staining intensities to mAQP4 expressing cells. All groups reacted extensively with rAQP4 expressing cells. The patients with longer duration in group ON exhibited greater staining intensity with rAQP4 expressing cells, and the reverse phenomenon was observed in group M.ConclusionsAQP4‐Abs from each adult‐onset group were considered to produce the antibodies recognizing different epitopes at least at the initial stage, and that could be a determinant of the various clinical subtypes of AQP4‐related NMO spectrum disorders (NMOSDs).