Etiology of myasthenia gravis: Innate immunity signature in pathological thymus

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"THE THYMUS IN MYASTHENIA GRAVIS: SITE OF ""INNATE AUTOIMMUNITY""?"

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Use of Toll-Like Receptor Agonists to Induce Ectopic Lymphoid Structures in Myasthenia Gravis Mouse Models

Myasthenia gravis (MG) is an autoimmune disease mediated by autoantibodies against the acetylcholine receptor (AChR) at the neuromuscular junction. MG symptoms are characterized by muscle weaknesses. The thymus of MG patients is very often abnormal and possesses all the characteristics of tertiary lymphoid organs such as neoangiogenesis processes, overexpression of inflammatory cytokines and chemokines, and infiltration of B lymphocytes leading to ectopic germinal center (GC) development. We previously demonstrated that injections of mice with polyinosinic–polycytidylic acid [Poly(I:C)], a synthetic double-stranded RNA mimicking viral infection, induce thymic changes and trigger MG symptoms. Upon Poly(I:C) injections, we observed increased thymic expressions of α-AChR, interferon-β and chemokines such as CXCL13 and CCL21 leading to B-cell recruitment. However, these changes were only transient. In order to develop an experimental MG model associated with thymic GCs, we used Poly(I:C) in the classical experimental autoimmune MG model induced by immunizations with purified AChR emulsified in complete Freund’s adjuvant. We observed that Poly(I:C) strongly favored the development of MG as almost all mice displayed MG symptoms. Nevertheless, we did not observe any ectopic GC development. We next challenged mice with Poly(I:C) together with other toll-like receptor (TLR) agonists known to be involved in GC development and that are overexpressed in MG thymuses. Imiquimod and CpG oligodeoxynucleotides that activate TLR7 and TLR9, respectively, did not induce thymic changes. In contrast, lipopolysaccharide that activates TLR4 potentiated Poly(I:C) effects and induced a significant expression of CXCL13 mRNA in the thymus associated with a higher recruitment of B cells that induced over time thymic B-lymphoid structures. Altogether, these data suggest that tertiary lymphoid genesis in MG thymus could result from a combined activation of TLR signaling pathways

Decreased expression of miR-29 family associated with autoimmune myasthenia gravis.

BACKGROUND: Myasthenia gravis (MG) is a rare autoimmune disease mainly mediated by autoantibodies against the acetylcholine receptor (AChR) at the neuromuscular junction. The thymus is the effector organ, and its removal alleviates the symptoms of the disease. In the early-onset form of MG, the thymus displays functional and morphological abnormalities such as B cell infiltration leading to follicular hyperplasia, and the production of AChR antibodies. Type-I interferon (IFN-I), especially IFN-β, is the orchestrator of thymic changes observed in MG. As Dicer and miR-29 subtypes play a role in modulating the IFN-I signalization in mouse thymus, we investigated their expression in MG thymus. METHODS: The expression of DICER and miR-29 subtypes were thoroughly investigated by RT-PCR in human control and MG thymuses, and in thymic epithelial cells (TECs). Using miR-29a/b-1-deficient mice, with lower miR-29a/b-1 expression, we investigated their susceptibility to experimental autoimmune MG (EAMG) as compared to wild-type mice. RESULTS: DICER mRNA and all miR-29 subtypes were down-regulated in the thymus of MG patients and DICER expression was correlated with the lower expression of miR-29a-3p. A decreased expression of miR-29 subtypes was similarly observed in MG TECs; a decrease also induced in TECs upon IFN-β treatment. We demonstrated that miR-29a/b-1-deficient mice were more susceptible to EAMG without higher levels of anti-AChR IgG subtypes. In the thymus, if no B cell infiltration was observed, an increased expression of Ifn-β associated with Baff expression and the differentiation of Th17 cells associated with increased expression of Il-6, Il-17a and Il-21 and decreased Tgf-β1 mRNA were demonstrated in miR-29a/b-1-deficient EAMG mice. CONCLUSIONS: It is not clear if the decreased expression of miR-29 subtypes in human MG is a consequence or a causative factor of thymic inflammation. However, our results from the EAMG mouse model indicated that a reduction in miR-29a/b1 may contribute to the pathophysiological process involved in MG by favoring the increased expression of IFN-β and the emergence of pro-inflammatory Th17 cells

Causes and Consequences of miR-150-5p Dysregulation in Myasthenia Gravis

Autoimmune Myasthenia gravis (MG) is a chronic neuromuscular disease mainly due to antibodies against the acetylcholine receptor (AChR) at the neuromuscular junction that induce invalidating muscle weaknesses. In early-onset MG, the thymus is the effector organ and is often characterized by B-cell infiltrations leading to ectopic germinal center (GC) development. The microRNA miR-150-5p has been previously characterized as a biomarker in MG due to its increase in the serum of patients and its decrease after thymectomy, correlated with an improvement of symptoms. Here, we investigated the causes and consequences of the miR-150 increase in the serum of early-onset MG patients. We observed that miR-150 expression was upregulated in MG thymuses in correlation with the presence of thymic B cells and showed by in situ hybridization experiments, that miR-150 was mainly expressed by cells of the mantle zone of GCs. However, we did not observe any correlation between the degree of thymic hyperplasia and the serum levels in MG patients. In parallel, we also investigated the expression of miR-150 in peripheral blood mononuclear cells (PBMCs) from MG patients. We observed that miR-150 was down-regulated, especially in CD4+ T cells compared to controls. These results suggest that the increased serum levels of miR-150 could result from a release from activated peripheral CD4+ T cells. Next, we demonstrated that the in vitro treatment of PBMCs with miR-150 or antimiR-150 oligonucleotides, respectively, decreased or increased the expression of one of its major target gene: the proto-oncogene MYB, a well-known actor of hematopoiesis. These results revealed that increased serum levels of miR-150 in MG patients could have a functional effect on PBMCs. We also showed that antimiR-150 caused increased cellular death of CD4+ and CD8+ T cells, along with the overexpression of pro-apoptotic genes targeted by miR-150 suggesting that miR-150 controlled the survival of these cells. Altogether, these results showed that miR-150 could play a role in MG both at the thymic level and in periphery by modulating the expression of target genes and peripheral cell survival

VAV1 and BAFF, via NFκB pathway, are genetic risk factors for myasthenia gravis

Objective To identify novel genetic loci that predispose to early‐onset myasthenia gravis (EOMG) applying a two‐stage association study, exploration, and replication strategy. Methods Thirty‐four loci and one confirmation loci, human leukocyte antigen (HLA)‐DRA, were selected as candidate genes by team members of groups involved in different research aspects of MG. In the exploration step, these candidate genes were genotyped in 384 EOMG and 384 matched controls and significant difference in allele frequency were found in eight genes. In the replication step, eight candidate genes and one confirmation loci were genotyped in 1177 EOMG patients and 814 controls, from nine European centres. Results Allele frequency differences were found in four novel loci: CD86, AKAP12, VAV1, B‐cell activating factor (BAFF), and tumor necrosis factor‐alpha (TNF‐α), and these differences were consistent in all nine cohorts. Haplotype trend test supported the differences in allele frequencies between cases and controls. In addition, allele frequency difference in female versus male patients at HLA‐DRA and TNF‐α loci were observed. Interpretation The genetic associations to EOMG outside the HLA complex are novel and of interest as VAV1 is a key signal transducer essential for T‐ and B‐cell activation, and BAFF is a cytokine that plays important roles in the proliferation and differentiation of B‐cells. Moreover, we noted striking epistasis between the predisposing VAV1 and BAFF haplotypes; they conferred a greater risk in combination than alone. These, and CD86, share the same signaling pathway, namely nuclear factor‐kappaB (NFκB), thus implicating dysregulation of proinflammatory signaling in predisposition to EOMG

New Pathways and Therapeutic Targets in Autoimmune Myasthenia Gravis

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Thymectomy in myasthenia gravis: when, why, and how?

International audienc

Prédisposition aux pathologies auto-immmunes

Les maladies auto-immunes sont un groupe d’environ 80 pathologies différentes affectant 5 à 8 % de la population. Elles sont dues à un dysfonctionnement du système immunitaire au niveau de la tolérance thymique. Le thymus est le site d’éducation des lymphocytes T. Cet apprentissage repose sur des interactions entre les lymphocytes T et les cellules épithéliales thymiques (CET) qui expriment les protéines spécifiques de tissus périphériques. Ce mécanisme complexe est appelé tolérance centrale. Les maladies auto-immunes présentent, pour la majorité d’entre elles, une forte prévalence féminine. La comparaison des transcriptomes thymiques de femmes et d’hommes met en évidence des différences d’expression des protéines spécifiques de tissus périphériques. Dans le thymus, l’expression de ces protéines est contrôlée par le modulateur de transcription AIRE (autoimmune regulator). Une analyse translationnelle de la relation entre le sexe, les hormones et AIRE, dans des modèles cellulaires humains et murins, montre qu’après la puberté, ce facteur est moins exprimé chez la femme que chez l’homme. Les estrogènes induisent une augmentation du nombre de sites de méthylation au niveau du promoteur de Aire réprimant ainsi son expression. À partir de la puberté, les femmes auraient une efficacité diminuée du processus de tolérance centrale, ce qui entraînerait un accroissement des cellules autoréactives et de leur susceptibilité aux maladies auto-immunes. Ces observations nous amènent à nous interroger sur les effets d’une exposition croissante aux perturbateurs endocriniens, molécules estrogen-like qui, en diminuant l’expression de AIRE, conduiraient à une augmentation de la fréquence des maladies auto-immunes