Unknown Immunoregulatory Effects of FcRn Inhibition by Efgartigimod in Myasthenia Gravis

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BACKGROUND AND OBJECTIVES: Efgartigimod (EFG), a biological drug targeting the IgG recycling neonatal Fc receptor (FcRn), leads to clinical improvements in patients affected by myasthenia gravis (MG), a prototypic autoantibody (Ab)-mediated autoimmune disease affecting neuromuscular junction. Because FcRn is a multifunctional protein expressed in different immune system cells, including B cells, we investigated whether FcRn blockade by EFG may have further immunologic effects, other than IgG reduction, in patients with MG. METHODS: Anti-acetylcholine receptor Ab-positive (AChR-MG) patients were treated with EFG according to the GENERATIVE protocol. Clinical evaluation, IgG and autoAb quantification, and circulating T-cell and B-cell subpopulation analyses by flow cytometry were performed at different time points. The expression of regulatory plasma cell-related candidate genes (CD38, lymphocyte-activation gene 3 [LAG3], IL-12a, Ebi3) was assessed by real-time PCR in peripheral blood mononuclear cells (PBMCs) from patients on treatment and in PBMCs either untreated or in vitro treated with an EFG-mimicking anti-FcRn monoclonal Ab (mAb) or with EFG (Vyvgart). RESULTS: A significantly increased percentage of CD19+/CD27+ memory B cells and CD27+/CD138+ plasma cells was observed at the end of EFG treatment cycle 1 and cycle 2 in patients with AChR-MG. Plasma cell increase, maintained up to cycle 3, significantly correlated with Quantitative Myasthenia Gravis score improvement. Moreover, PBMCs from EFG-treated patients showed overexpression of CD38, LAG3, and IL-12a genes, suggesting EFG's ability to induce non-pathogenic regulatory plasma cells. This ability was confirmed in vitro, because anti-FcRn mAb-treated and EFG-treated PBMCs displayed an up-regulation of CD38 and LAG3 compared with untreated cells. DISCUSSION: Our findings indicate an unknown immunoregulatory action of EFG in patients with AChR-MG, by unraveling a drug effect on B-cell differentiation, and suggest the induction of regulatory plasma cells as a further mechanism, beyond IgG reduction, associated with clinical improvement. A deep understanding of the immunologic effects of EFG can help to optimize its usage over time in individual patients and disclose biomarkers suitable for monitoring the long-term patient-specific response.

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