Reemergence of pathogenic, autoantibody-producing B-cell clones in myasthenia gravis following B-cell depletion therapy.

Myasthenia gravis (MG) is an autoantibody-mediated autoimmune disorder of the neuromuscular junction. Some patients with MG have pathogenic mAbs that recognize muscle-specific tyrosine kinase (MuSK). Patients respond well to CD20-mediated B-cell depletion therapy (BCDT); most achieve complete stable remission. However, relapse often occurs. To further understand the immunomechanisms underlying relapse we sought to study autoantibody-producing B-cells over the course of BCDT. We developed a monomeric fluorescently labelled antigen to enrich for MuSK-specific B-cells, which was validated with a novel Nalm6 cell line engineered to express a human MuSK autoantibody as B-cell receptor. We found that MuSK-specific B-cells are rare within the circulation (<3 per 10 million cells). From two MuSK-specific B-cells isolated from two distinct patients, we generated human recombinant MuSK mAbs. The autoantibodies originated from plasmablasts, used the IgG4 subclass, recognized the Ig1-like domain of MuSK and showed pathogenic capacity using an in vitro AChR clustering assay. Persistent clones of the mAbs were identified through B-cell repertoire tracing. Clonal variants of 2E6 were detected at several timepoints spanning more than five years months and reemerged after BCDT-mediated remission, appearing several months prior to relapse. These results indicate that a reservoir of rare pathogenic MuSK autoantibody-expressing B cell clones survive BCDT, then reemerge prior to manifestation of clinical relapse. This study provides both a mechanistic understanding of MuSK MG relapse and a valuable candidate biomarker for relapse prediction. Samples from eight subjects were studied. Four subjects had the MuSK subtype of myastenia gravis, four were healthy cotrols. Samples underwent bulk BCR sequencing, as well as scRNA-seq and scBCR sequencing.