Aquaporin-4 IgG autoantibodies from neuromyelitis optica spectrum disorder patients cause complement-independent spinal cord pathologies and motor deficits in mice

Abstract

Background and aims: Neuromyelitis optica spectrum disorders (NMOSD) are CNS inflammatory disorders. Autoantibodies against aquaporin-4 (AQP4-IgG) are pathogenic in NMOSD. Neuroinflammation is initiated upon binding of AQP4-IgG to astrocytic AQP4. The role of complement-independent pathophysiologies is uncertain. We aim to study the complement-independent pathological effects of AQP4-IgG in mice. Methods: Mice were pretreated with complete Freund's adjuvant and pertussis toxin to disrupt blood-brain barrier, then received daily intraperitoneal injection of IgG purified from AQP4-IgG-seropositive NMOSD patients (IgG(AQP4+)) or healthy individuals (IgG(Healthy)) for 8 days. Motor function was tested by walking across narrow beams. Cervical cord was collected for immunofluorescent analysis. Results: Human IgG infiltrated into spinal cord parenchyma. There was no deposition of complement activation product (C5b9). Mice received IgG(AQPd+) showed astrocytic injuries/loss compared to mice received IgG(Healthy) indicated by significant loss of AQP4 and glial fibrillary acidic protein immunoreactivities. These mice displayed decrease in the glutamate transporter, excitatory amino acid transporter 2, on immunostaining. There were extensive microglial/macrophage activation on ionized calcium-binding adapter molecule 1 (Iba1) and cluster of CD68 immunostaining, respectively. Spinal cord of mice received IgG(AQP4+) had patchy demyelination and axonal injuries/loss on myelin basic protein and neurofilament immunostaining. Mice received IgG(AQP4+) required longer time with more paw slips to walk across narrow beams compared to mice received IgG(Healthy). Treatment with NMDA receptor antagonist, MK-801, significantly improved motor function of IgG(AQP4+) mice. Conclusion: AQP4-IgG mediated complement-independent pathologies including AQP4 and astrocytic loss, neuroinflammation, demyelination and axonal injuries/loss may involve glutamate excitotoxicity and microglia/macrophage activation; these pathologies may be improved by NMDA receptor antagonist.