N-Glycan–Dependent Proinflammatory Effects of IgM in Anti-MAG Neuropathy

Anti-myelin-associated glycoprotein (anti-MAG) neuropathy is a chronic demyelinating neuropathy with deposits of IgM and sural nerve fiber demyelination. While growing evidence supports the critical role of IgG glycosylation in autoimmune diseases, IgM glycosylation profiles markedly differ from those of IgG but are largely neglected. The aim of this study was to characterize IgM N-glycosylation in patients with anti-MAG neuropathy and its involvement in anti-MAG pathogenicity. IgM antibodies were isolated from patients with anti-MAG neuropathy (n = 17), asymptomatic patients with monoclonal gammopathy of undetermined significance (n = 8), and healthy donors (HDs, n = 6). N-glycan analysis was performed using mass spectrometry. Binding to myelin-associated glycoprotein (MAG), complement (C1q), and IgM Fc receptors (Fcα/μR and DC-SIGN) was compared using ELISA between anti-MAG and MGUS IgM, before and after N-deglycosylation/desialylation. Finally, we assessed how IgM N-glycosylation influences cytokine production by monocyte-derived macrophages by measuring cytokine levels in culture supernatants. Anti-MAG IgM exhibited a unique glycosylation pattern, dominated by fucosylated, monosialylated N-glycan with a bisecting N-acetyl glucosamine (N-glycan 12), representing 48.5% of the total N-glycan pool, compared with 27.3% in MGUS IgM and 35.6% in HD IgM. We showed that deglycosylation and desialylation significantly reduced anti-MAG activity and C1q binding (average % of decrease 58.3 ± 18.8, p < 0.01, and 40.0 ± 19.9%, p < 0.05, respectively). Furthermore, anti-MAG IgM binding to C1q was significantly higher than that of MGUS IgM and HD IgM (p < 0.0001 and p < 0.001, respectively). Compared with MGUS IgM, anti-MAG IgM also significantly increased the production of proinflammatory cytokines IL-1, IL-6, IL-8, TNF-α, and IFN-Υ by macrophages, in a glycan-dependent manner (p < 0.01 to p < 0.001), with IL-8 being particularly elevated. Finally, we found that anti-MAG IgM bound more strongly Fcα/μ receptor and DC-SIGN compared with MGUS IgM (p < 0.05 and p = 0.06). This study uncovered a unique N-glycosylation pattern of anti-MAG IgM, crucial for its interaction with MAG and binding to C1q. Moreover, anti-MAG IgM increased the macrophage cytokine production, driven by their glycosylation. The increased IL-8 expression and anti-MAG IgM binding to C1q might open 2 potential therapeutic avenues: inhibiting IL-8 activity or targeting the complement pathway. In addition, the glycosylation and C1q binding of anti-MAG IgM could serve as biomarkers for monitoring this neuropathy.