Features of MOG required for recognition by patients with MOG antibody-associated disorders

Abstract

Antibodies (Abs) to myelin oligodendrocyte glycoprotein (MOG) define a distinct disease entity. Here we aimed to understand essential structural features of MOG required for recognition by autoantibodies from patients. We produced the N-terminal part of MOG in a conformationally correct form; this domain was insufficient to identify patients with MOG-Abs by ELISA even after site-directed binding. This was neither due to a lack of lipid embedding nor to a missing putative epitope at the C-terminus, which we confirmed to be an intracellular domain. When MOG was displayed on transfected cells, patients with MOG-Abs recognized full-length MOG much better than its N-terminal part with the first hydrophobic domain (p < 0.0001). Even antibodies affinity-purified with the extracellular part of MOG recognized full-length MOG better than the extracellular part of MOG after transfection. The second hydrophobic domain of MOG enhanced the recognition of the extracellular part of MOG by antibodies from patients as seen with truncated variants of MOG. We confirmed the pivotal role of the second hydrophobic domain by fusing the intracellular part of MOG from the evolutionary distant opossum to the human extracellular part; the chimeric construct restored the antibody-binding completely. Further, we found that in contrast to 8-18C5, MOG-Abs from patients bound preferentially as F(ab’)2 rather than Fab. It was previously found that bivalent binding of human IgG1, the prominent isotype of MOG-Abs, requires that its target antigen is displayed at a distance of 13-16 nm. We found that, upon transfection, molecules of MOG did not interact so closely to induce a Förster resonance energy transfer (FRET) signal, indicating that they are more than 6 nm apart. We propose that the intracellular part of MOG holds the monomers apart at a suitable distance for bivalent binding; this could explain why a cell-based assay is needed to identify MOG-Abs. Our finding that MOG-Abs from most patients require bivalent binding has implications for understanding the pathogenesis of MOG-antibody-associated-disorders. Since bivalently bound antibodies have been reported to only poorly bind C1q, we speculate that the pathogenicity of MOG-Abs is mostly mediated by other mechanisms than complement activation. Therefore, therapeutic inhibition of complement activation should be less efficient in MOG-Ab associated disorders than in patients with Abs to aquaporin-4.

Differential Binding of Autoantibodies to MOG Isoforms in Inflammatory Demyelinating Diseases

“… 37 This explanation is supported by a recent study demonstrating that the second hydrophobic domain of MOG (only present in α1 and β1) enhanced the recognition of the extracellular part of MOG by human MOG-IgG. 39 Moreover, MOG-IgG from most patients may require bivalent binding to MOG dimers, whereas a smaller subset of MOG-IgG shows monovalent binding to monomers such as the 8-18-C5 monoclonal antibody.…”

Pathogenesis of autoimmune demyelination: from multiple sclerosis to neuromyelitis optica spectrum disorders and myelin oligodendrocyte glycoprotein antibody‐associated disease

“…MOG Ab has been shown to contribute to CNS demyelination through CDC or ADCC‐mediated lysis of MOG‐expressing cells 31,164,165 . These findings are supported by the observation that the majority of MOG Ab are IgG1, 33,166–169 a subclass efficient in fixing complement and binding to Fc receptors for ADCC initiation although the potency of these effector mechanisms may be dependent on autoantibody affinity 170 . Further studies suggest murine and human MOG Ab induce cytoskeleton disruption and microtubule destabilisation 37,171 …”

“…31,164,165 These findings are supported by the observation that the majority of MOG Ab are IgG1, 33,[166][167][168][169] a subclass efficient in fixing complement and binding to Fc receptors for ADCC initiation although the potency of these effector mechanisms may be dependent on autoantibody affinity. 170 Further studies suggest murine and human MOG Ab induce cytoskeleton disruption and microtubule destabilisation. 37,171 Several studies have further observed CNS inflammation and complement-dependent lysis of MOG-expressing cells following passive transfer of high-affinity patient MOG Ab to rodents.…”

“…179,180 Moreover, studies on the isoform of MOG have shown that C-terminal truncation of the full-length protein decreases binding of human MOG Ab 181 and is likely because of the effect of truncation on the second hydrophobic domain of MOG which has recently been revealed to be important in the bivalent binding of human MOG Ab. 170 Although major human MOG Ab epitopes have been described in the extracellular domain of the protein, 168,175 it has been postulated that the transmembrane domains and cytoplasmic tail may play an important role in the surface expression and oligomerisation of the antigen. Further, the use of fixatives in cell-based assays has been shown to alter the native conformation of MOG and significantly decrease the sensitivity of the MOG Ab test.…”

“…Differences in assay methodologies, data analyses and cut‐offs for MOG Ab positivity largely account for these discrepancies 179,180 . Moreover, studies on the isoform of MOG have shown that C‐terminal truncation of the full‐length protein decreases binding of human MOG Ab 181 and is likely because of the effect of truncation on the second hydrophobic domain of MOG which has recently been revealed to be important in the bivalent binding of human MOG Ab 170 . Although major human MOG Ab epitopes have been described in the extracellular domain of the protein, 168,175 it has been postulated that the transmembrane domains and cytoplasmic tail may play an important role in the surface expression and oligomerisation of the antigen.…”