Selective loss of myelin-associated glycoprotein from myelin correlates with anti-MAG antibody titre in demyelinating paraproteinaemic polyneuropathy

Abstract

The IgM monoclonal autoantibodies of patients with demyelinating paraproteinaemic polyneuropathy recognize a carbohydrate structure present on both myelin-associated glycoprotein (MAG) and protein zero (P0). These autoantibodies are sufficient to cause the disease but the mechanism of demyelination remains unclear. We have analysed nerve biopsies from eight patients with polyneuropathy and anti-MAG antibodies by quantitative immunohistochemistry and find a concordant pattern of reduced expression of myelin markers with the loss of myelinated fibres. We report here novel features of this disease, in particular a selective lack of detectable MAG in a large proportion of myelinated-fibres containing P0, myelin basic protein (MBP) and periaxin. There is also an inverse correlation of the distribution of MAG in peripheral nerve myelin with the serum anti-MAG antibody titres but no correlation of these titres with the loss of myelinated fibres. Double immunofluorescence staining of paraproteinaemic polyneuropathy (PPN) nerves shows anti-MAG IgM deposited on the periphery of myelinated fibres associated with or lacking MAG staining. These data suggest that the binding of anti-MAG antibodies to MAG and/or other myelin component(s) results in MAG downregulation and may have an essential role in the molecular mechanisms leading to demyelination and partial regeneration in this disease.

Selective in vivo removal of pathogenic anti-MAG autoantibodies, an antigen-specific treatment option for anti-MAG neuropathy

“…Tokuda et al (36) reported binding of IgM antibodies from patients with anti-MAG neuropathy and also mouse monoclonal HNK-1 antibody (20) to derivatives of the glycolipid SGPG containing only the terminal disaccharide SO 3 -3-GlcA(β1-3)Gal. Therefore, we synthesized three different inhibitors, the disaccharides 2, 3, and 4 ( Fig.…”

“…The assay was performed according to the manufacturer's protocol. Briefly, test compound, sera from patients or mouse monoclonal anti-HNK-1 IgM antibody (20), and incubation buffer (provided with the kit) were added to a final volume of 50 μL per well. The assay was run in triplicate.…”

“…For detection of human anti-MAG IgM or IgG, anti-human IgM or IgG secondary antibodies conjugated to HRP were used. The mouse HNK-1 IgM antibody (20) and anti-MAG IgM from plasma of immunized mice were detected with goat anti-mouse IgM HRP conjugate (A8786; Sigma Aldrich) diluted 1:10,000. Anti-MAG IgG in mouse plasma was detected with goat anti-mouse IgG HRP conjugate (A4416; Sigma Aldrich) diluted 1:10,000.…”

“…The inhibitory activities of these polymers were determined in a MAG-binding inhibition assay with the mouse monoclonal anti-HNK-1 IgM antibody (20) as competitor ( Fig. 2 A and B and Table 2).…”

“…(4). Several observations suggest that MAG is the main target for the IgM antibodies: (i) deposits of patients' antibodies to PNS sites are colocalized with MAG (19); (ii) MAG is selectively lost from PNS myelin in patients with high antibody titers (20); (iii) the pathology of MAG-knockout mice (21,22) shows similarities to the human nerve pathology (23,24); and (iv) human MAG has a higher HNK-1 epitope density (25) than any other myelin glycoconjugate, leading to strong IgM antibody binding (26). Based on in vitro studies, it was postulated that patients' IgM autoantibodies increase the permeability of the microvascular endothelial structure, presumably through the binding of endothelial sulfoglucuronyl glycosphingolipids, and thereby gain access to the PNS parenchyma via a leaky blood-nerve barrier (27).…”