AQP4 (205-215) is a fragment of the water channel protein Aquaporin-4 (AQP4). This protein is an autoimmune antigen in optic neuromyelitis and, upon binding with CD40, is upregulated and presented in B cells. AQP4 is associated with neuromyelitis optica (NMO), an autoimmune inflammatory disease of the central nervous system (CNS).