Serum AQP4-IgG antibodies of an NMO patient activate the complement cascade in the presence of active complement resulting in the deposition of the terminal C5b-9 complement complex. A. Formation of the terminal complement complex (TCC, red) on the surface of M23 AQP4-EmGFP expressing HEK-293A cells (green) following addition of a heat- inactivated AQP4-IgG positive serum sample (titer of 1:10,240). The membrane attack complex resulted in lysis of AQP4-expressing cells (DAPI, blue). B. No antibody mediated complement activation was detectable following incubation of the same heat-inactivated AQP4-IgG positive NMO serum sample supplemented with inactive complement (TCC, red). Images are representative of similar staining patterns observed in independent complement activation assays of the 27 AQP4-IgG positive patients (Table 3). Images are shown in 20 × (A-upper panel and B) and 63 × (A-lower panel) magnification.