Complement-dependent bystander injury to neurons in AQP4-IgG seropositive neuromyelitis optica

Tianjiao Duan,Alan S Verkman,Alex J Smith

doi:10.1186/s12974-018-1333-z

Abstract

BackgroundAquaporin-4-immunoglobulin G (AQP4-IgG) seropositive neuromyelitis optica spectrum disorder (herein called NMO) is an autoimmune disease of the central nervous system in which AQP4-IgG binding to AQP4 on astrocytes results in complement-dependent astrocyte injury and secondary inflammation, demyelination, and neuron loss. We previously reported evidence for a complement bystander mechanism for early oligodendrocyte injury in NMO. Herein, we tested the hypothesis that complement bystander injury, which involves diffusion to nearby cells of activated soluble complement components from complement-injured astrocytes, is a general phenomenon that may contribute to neuronal injury in NMO.MethodsPrimary cocultures of rat astrocytes and cortical neurons were established to study complement-dependent cell death after exposure to AQP4-IgG and complement. In animal experiments, AQP4-IgG was delivered to adult rats by intracerebral injection. Cell cultures and rat brain were studied by immunofluorescence.ResultsIn primary astrocyte-neuron cocultures, addition of AQP4-IgG and complement resulted in death of neurons nearby astrocytes. Deposition of complement membrane attack complex C5b-9 was seen on neurons nearby astrocytes, whereas C1q, the initiating protein in the complement pathway, was seen only on astrocytes. Neuron death was not seen with a complement inhibitor, with C1q- or C6-depleted complement, in pure neuron cultures exposed to AQP4-IgG and complement or in cocultures exposed to an astrocyte toxin. Intracerebral injection in rats of AQP4-IgG and a fixable dead cell fluorescent marker produced death of neurons near astrocytes, with C5b-9 deposition. Neuron death was not seen in rats receiving a complement inhibitor or in AQP4-IgG-injected AQP4 knockout rats.ConclusionThese results support a novel mechanism for early neuron injury in NMO and provide evidence that complement bystander injury may be a general phenomenon for brain cell injury following AQP4-IgG-targeted astrocyte death.

Highlights

Aquaporin-4-immunoglobulin G (AQP4-IgG) seropositive neuromyelitis optica spectrum disorder is an autoimmune disease of the central nervous system in which AQP4-IgG binding to AQP4 on astrocytes results in complement-dependent astrocyte injury and secondary inflammation, demyelination, and neuron loss
CD59 was expressed on astrocytes, with little expression seen on neurons
In astrocyte-neuron cocultures, CD59 expression was seen on astrocytes and not on neurons (Fig. 1c)

Summary

Introduction

Aquaporin-4-immunoglobulin G (AQP4-IgG) seropositive neuromyelitis optica spectrum disorder ( called NMO) is an autoimmune disease of the central nervous system in which AQP4-IgG binding to AQP4 on astrocytes results in complement-dependent astrocyte injury and secondary inflammation, demyelination, and neuron loss. Aquaporin-4-immunoglobulin G (AQP4-IgG) seropositive neuromyelitis optica spectrum disorder ( called NMO) is an autoimmune demyelinating disease of the central nervous system. NMO pathogenesis involves binding of AQP4-IgG autoantibodies to water channel AQP4 on astrocytes, resulting in complement- and cell-mediated astrocyte injury, inflammation, demyelination, and neuron. We recently reported evidence for complement bystander injury to oligodendrocytes, in which complement activation following AQP4-IgG binding to AQP4 on astrocytes results in killing of nearby oligodendrocytes by a bystander mechanism involving local diffusion of activated, soluble complement components, leading to formation of the complement membrane attack complex (MAC) on oligodendrocytes [20]. Neurons have been reported to express low levels of endogenous complement inhibitors, including CD59 [26,27,28], and are potential targets for bystander damage

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