Quantification and Functional Characterization of Antibodies to Native Aquaporin 4 in Neuromyelitis Optica

Background: Antibodies targeting membrane proteins play an important role in various autoimmune diseases of the nervous system. So far, assays allowing proper analysis of such autoantibodies are largely missing. A serum autoantibody to aquaporin 4 (AQP4) is associated with neuromyelitis optica (NMO). Although several assays are able to detect this autoantibody, they do not allow determination of the biological activity of anti-AQP4 antibodies. Objective: To develop a bioassay for quantification and characterization of human anti-AQP4 antibodies. Design, Setting, and Participants: We developed a novel bioassay for quantification and characterization of human anti-AQP4 antibodies based on high-level expression of native AQP4 (nAQP4) protein on the surface of human astroglioma cells. The test was validated in 2 independent cohorts of patients with NMO spectrum disease. Results: We detected anti-nAQP4-IgG with a sensitivity of 57.9% and specificity of 100% in patients with NMO spectrum diseases, suggesting that our bioassay is at least as sensitive and specific as the gold-standard NMO-IgG assay. The anti-AQP4 antibodies belonged predominantly to the IgG1 isotype and bound with high affinity to the extracellular domain of nAQP4. Our data suggest that the autoantibody exerts pathological properties because nAQP4-IgG-positive sera induced cell death of nAQP4-expressing cells by antibody-dependent cellular natural killer cell cytotoxic effect and complement activation. Furthermore, nAQP4-IgG titers strongly correlated with in vitro cytotoxic effect. Conclusions: In NMO, this assay may help to unravel the biological function of anti-nAQP4-IgG. Our findings demonstrate the potential of bioassays to characterize biologically relevant antibodies in human autoimmune diseases.