[Neuromyelitis optica and anti-aquaporin 4 antibody--distinct from multiple sclerosis]

Neuromyelitis optica (NMO) is an inflammatory disease mainly affecting the optic nerve and spinal cord. There has been prolonged controversy over whether NMO is a subtype of multiple sclerosis (MS) or a distinct disease. Recently, a highly specific serum autoantibody marker, NMO-IgG, was exclusively found in the sera of NMO, and the target antigen was identified as the water channel protein aquaporin (AQP) 4. In our immunocytochemical assay using HEK293 cells transfected with human AQP4, we found that the sensitivity and specificity of AQP4 antibody in NMO were as high as 91 and 100%, respectively. Our histopathological studies of NMO revealed that AQP4 and GFAP, an astrocytic marker protein, were completely lost in the acute inflammatory lesions surrounding immunoglobulin and complement-deposited dilated vessels, but the staining of myelin basic protein was relatively preserved. These results suggest that astrocytic damage associated with autoimmunity to AQP4 is involved in the pathogenesis of NMO, which is distinct from MS, a primary demyelinating disease. Growing evidence for the in vitro cytotoxic or functional effects of AQP4 antibody on astrocytes support its pathogenic implications in NMO. Regarding the treatment of NMO, there are refractory patients which can be treated with interferon beta, so immunosuppressive agents such as low-dose oral prednisolone and/or azathioprine reduce the rate of relapse, while, unlike in MS, the therapeutic efficacy of interferon beta is unclear in NMO. Therefore, AQP4 antibody is indispensable in the diagnosis and treatment of NMO.