AQP4 and myeloid sarcoma: In MS, microglial activation is wide-spread in the meninges and subpial cortex (31), while in AQP4-IgG positive NMO it is largely confined to regions that normally are AQP4-rich, like the CSF–brain interface in the corpus callosum, hypothalamic, and periventricular areas (13, 14, 32).