Complement Activation and Anti-Aquaporin-4 Antibodies Drive Pathology in a Preclinical Model of Neuromyelitis Optica Spectrum Disorder 2261227

Introduction Neuromyelitis optica spectrum disorder (NMOSD) is a severe autoimmune disease of the central nervous system (CNS), characterized by an autoantibody response against the water channel aquaporin-4 (AQP4) on astrocyte endfeet. The disease primarily affects the spinal cord and optic nerves, leading to paralysis and loss of sight. Onset is associated with a breakdown of immune tolerance to AQP4 and the appearance of anti-AQP4 IgG in serum. These antibodies, along with complement deposition, are consistently found in CNS lesions and contribute to astrocyte damage, demyelination, and neurological deficits. The clinical efficacy of plasma exchange and anti-C5 therapies supports the pathogenic role of antibodies and complement in NMOSD. Methods We examined the contribution of anti-AQP4 antibodies and complement in a robust AQP4201-220-induced animal model. Wild-type (WT) and secretory immunoglobulin-deficient (AID⁻/⁻μS⁻/⁻) mice were immunized with AQP4201-220 and treated with pertussis toxin on days −2 and 0. Suboptimal anti-IFN-γ was used to enhance disease; IFN-γ receptor knockout (IFNGRKO) mice were similarly immunized. On day 21, spinal cord and optic nerve were collected for histology, while flow cytometry and transcriptomics assessed immune activation. Results WT + anti-IFN-γ mice showed robust IgG and C5/C5b-9 deposition with CNS upregulation of complement genes (contrasting downregulation in spleen). AID⁻/⁻μS⁻/⁻ mice failed to develop disease, confirming antibody dependence. C3⁻/⁻ mice showed reduced incidence, milder clinical scores, and decreased C5/C5b-9, indicating C3 is required for C5 activation. Anti-C5 treatment delayed onset, reduced severity, and modulated immune responses similarly to genetic C3 loss. Conclusion Our data establish anti-AQP4 antibodies and the C3-C5 (MAC) complement axis as central drivers of NMOSD pathology, reinforcing their value as biomarkers and therapeutic targets.