Cytoprotective IgG antibodies in sera from a subset of patients with AQP4-IgG seropositive neuromyelitis optica spectrum disorder

Lukmanee Tradtrantip,A S Verkman,Michael R Yeaman

doi:10.1038/s41598-021-01294-3

Abstract

Neuromyelitis optica spectrum disorder (NMOSD) is an autoimmune inflammatory disease of the central nervous system. Most NMOSD patients are seropositive for immunoglobulin G (IgG) autoantibodies against astrocyte water channel aquaporin-4 (AQP4), called AQP4-IgG. AQP4-IgG binding to aquaporin-4 causes complement-dependent cytotoxicity (CDC), leading to inflammation and demyelination. Here, CDC was measured in AQP4-expressing cells exposed to human complement and heat-inactivated sera from 108 AQP4-IgG seropositive NMOSD subjects and 25 non-NMOSD controls. AQP4-IgG positive sera produced a wide range of CDC, with 50% maximum cytotoxicity produced by as low as 0.2% serum concentration. Unexpectedly, 58 samples produced no cytotoxicity, and of those, four sera were cytoprotective against cytotoxic AQP4-IgG. Cytoprotection was found against different cytotoxic monoclonal AQP4-IgGs and NMOSD patient sera, and in primary astrocyte cultures. Mechanistic studies revealed that the protective factor is an IgG antibody that did not inhibit complement directly, but interfered with binding of cytotoxic AQP4-IgG to AQP4 and consequent C1q binding and complement activation. Further studies suggested that non-pathogenic AQP4-IgG, perhaps with altered glycosylation, may contribute to reduced or ineffectual binding of cytotoxic AQP4-IgG, as well as reduced cell-surface AQP4. The presence of natural cytoprotective antibodies in AQP4-IgG seropositive sera reveals an added level of complexity in NMOSD disease pathogenesis, and suggests the potential therapeutic utility of ‘convalescent’ serum or engineered protective antibody to interfere with pathogenic antibody in AQP4-IgG seropositive NMOSD.

Highlights

Neuromyelitis optica spectrum disorder (NMOSD) is an autoimmune inflammatory disease of the central nervous system
We found a broad range of complement-dependent cytotoxicity (CDC) produced by AQP4-immunoglobulin G (IgG) seropositive NMOSD patient sera, including many sera that did not produce CDC, and made the unanticipated discovery that a subset of non-pathogenic sera confer cytoprotection in CDC assays in AQP4-expressing cells when added together with cytotoxic AQP4-IgG
The diversity in CDC produced by NMOSD patient sera is likely the consequence of the highly heterogeneous, polyclonal mixture of AQP4-IgGs in NMOSD sera that differ in their AQP4 binding affinity and specificity, epitope recognition, and ability to cluster[10,15,26,31], and in their ability to bind C1q and activate complement

Summary

Introduction

Neuromyelitis optica spectrum disorder (NMOSD) is an autoimmune inflammatory disease of the central nervous system. Additional evidence for a major role of complement in NMOSD pathogenesis includes deposition of activated complement in affected human tissues[7,18,19] and data in experimental animal models showing NMOSD pathology following exposure to AQP4-IgG and c omplement[20,21] which is increased in rodents deficient in complement regulator protein C D5922,23 Consistent with these findings, an engineered, high-affinity, anti-AQP4 antibody lacking effector function, called aquaporumab, blocks the binding of pathogenic AQP4-IgG to AQP4, and prevents complement activation and consequent cellular injury and pathological changes[24,25]

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