Thymus-derived B cell clones persist in the circulation after thymectomy in myasthenia gravis

Abstract

Myasthenia gravis (MG) is a neuromuscular, autoimmune disease caused by autoantibodies that target postsynaptic proteins, primarily the acetylcholine receptor (AChR) and inhibit signaling at the neuromuscular junction. The majority of patients under 50 y with AChR autoantibody MG have thymic lymphofollicular hyperplasia. The MG thymus is a reservoir of plasma cells that secrete disease-causing AChR autoantibodies and although thymectomy improves clinical scores, many patients fail to achieve complete stable remission without additional immunosuppressive treatments. We speculate that thymus-associated B cells and plasma cells persist in the circulation after thymectomy and that their persistence could explain incomplete responses to resection. We studied patients enrolled in a randomized clinical trial and used complementary modalities of B cell repertoire sequencing to characterize the thymus B cell repertoire and identify B cell clones that resided in the thymus and circulation before and 12 mo after thymectomy. Thymus-associated B cell clones were detected in the circulation by both mRNA-based and genomic DNA-based sequencing. These antigen-experienced B cells persisted in the circulation after thymectomy. Many circulating thymus-associated B cell clones were inferred to have originated and initially matured in the thymus before emigration from the thymus to the circulation. The persistence of thymus-associated B cells correlated with less favorable changes in clinical symptom measures, steroid dose required to manage symptoms, and marginal changes in AChR autoantibody titer. This investigation indicates that the diminished clinical response to thymectomy is related to persistent circulating thymus-associated B cell clones.

Elevated N-glycosylation of immunoglobulin G variable regions in myasthenia gravis highlights a commonality across autoantibody-associated diseases

“…Similarly, the thymus in a subset of MG patients includes germinal centers, which are thought to contribute to the generation and maturation of AChR autoantibody producing B cells (38–40). Thus, positive selection of N-linked glycosylation DNA motifs may occur in this compartment, and support for this possibility is provided by a previous study where we found that the IgG-switched BCR sequences in MG thymus were enriched N-linked glycosylation DNA motifs (41).…”

“…First, RNA was isolated from frozen peripheral blood mononuclear cells using the RNAEasy Mini kit (Qiagen) per manufacturer’s instructions. Bulk libraries were prepared from RNA using reagents from New England Biolabs as part of the NEBNext® Immune Sequencing Kit as described previously (41, 42). Briefly, cDNA was reverse-transcribed by a template-switch reaction to add a 17-nucleotide unique molecular identifier (UMI) to the 5’ end with streptavidin magnetic bead purification.…”

Human B cell lineages engaged by germinal centers following influenza vaccination are measurably evolving

“…Data from Wang et al (2014) 43 were processed in Hoehn et al (2019). 44 Data from Jiang et al (2020b) 45 used only blood samples.…”

Thymectomy and myasthenia gravis

“…Using a series of state-of-the-art molecular biologic techniques, Jiang et al (8) demonstrate shared B cell receptor repertoires between B cell and plasmablast populations from areas enriched for germinal follicular regions in thymuses of patients undergoing thymectomy (eight patients from the thymectomy trial and one other individual) and the equivalent cell populations in the blood of these same individuals. Both the thymic and blood cells had the characteristics of being antigen-driven.…”

“…Jiang et al (8) attempted to see if there were quantitative changes in the presumed thymic origin of peripheral blood cell populations that correlated with clinical response to thymectomy in these same individuals. In the relatively small number of patients in this highly sophisticated immunologic study there was a trend toward correlation between reduced thymus-derived cells in blood sampled at 1 y postthymectomy and the time to clinical improvement, often at 2 y or longer, but it is hard to come to a firm conclusion.…”