Thymic remodeling associated with hyperplasia in myasthenia gravis

Graves' Disease with Thymic Hyperplasia: The Response of the Thyroid Function, Thyrotropin Receptor Autoantibody, and Thymic Size to Thiamazole Treatment

We treated a 22-year-old woman suffering from Graves' disease and thymic hyperplasia. She was referred to our institution for a close investigation of thyrotoxicosis and thymic mass. Thyroid tests and magnetic resonance imaging resulted in a diagnosis of Graves' disease and thymic hyperplasia. The thyroid function and thyroid-stimulating hormone receptor antibody (TRAb) were normalized one and five months after thiamazole initiation, respectively. The thymic size began to decrease after 1 month and was further decreased after 5 months; it was normalized after 12 months. The correlation between TRAb titers and the thymic size (R²=0.99) suggested that the patient's autoimmunity might have contributed to the thymic hyperplasia.

Pathogenesis of follicular thymic hyperplasia associated with rheumatoid arthritis

Lymphoproliferative disorders may occur in patients with rheumatoid arthritis (RA) who are treated with methotrexate. However, follicular thymic hyperplasia (FTH) associated with RA (FTH‐RA) is generally not considered a lymphoproliferative disorder. To investigate the pathogenesis of FTH‐RA, we examined 12 cases of FTH involving thymic enlargement, four of FTH involving RA and eight of FTH involving myasthenia gravis (MG). Increased numbers and larger germinal center (GC) size were observed in FTH‐RA group. The percentage of distorted GCs was 13.3% in FTH‐RA group and 3.25% in FTH associated with MG (FTH‐MG) group. A greater meshwork of follicular dendritic cells was observed in the GCs of FTH‐RA group. Positive indices of CD27⁺ cells and PD‐1⁺ cells per GC in FTH‐RA group were significantly higher than those in FTH‐MG group, though positive indices of CD68⁺ cells and CD163⁺ cells were similar. Myoid cell proliferation, as evaluated by α‐SMA, tenascin‐C, and l‐caldesmon expression, was significantly increased in the FTH‐RA group compared with the FTH‐MG group. These results suggest that FTH should be considered in patients with RA treated with methotrexate. The pathogenesis of FTH‐RA includes GC expansion and increased numbers of memory B cells, follicular helper T cells, and myoid cells, indicating humoral immunity activation.

[Thymic hyperplasia: A study of 46 cases]

BACKGROUND

Thymic hyperplasia presents as an anterior mediastinal mass and poses important diagnostic and therapeutic challenge. Two types of thymic hyperplasia are described: true hyperplasia and follicular hyperplasie. Literature data are peculiar concerning both entities. We aimed to describe the clinical and microscopic characteristics of thymic hyperplasia through a single institution experience during an 11-year-period.

METHODS

Thymic hyperplasia diagnosed during the period between 2009 and 2020 were included.

RESULTS

In all, 46 thymic hyperplasias were diagnosed. The 46 patients consisted in 33 women and 13 men with a mean age of 30 years. Microscopic diagnosis concluded to a follicular hyperplasia in 12 cases and a true thymic hyperplasia in 34 cases. The diagnosis of true thymic hyperplasia posed a diagnostic challenge with an involuted thymus in 1 case and a thymolipoma in 1 case. The confrontation with the clinical data allowed retaining the diagnosis.

CONCLUSION

The diagnosis of thymic hyperplasia is based on microscopic features. The confrontation with clinical data and the measurements of the thymus according to the age allow to retain the diagnosis in most challenging cases.

Viral infection in thymoma and thymic tumors with autoimmune diseases

A thymoma is a type of thymic tumor which is rarely malignant that is frequently reported in adult patients. A number of thymoma-related immune disorders are observed including autoimmune diseases, which suggests a strong connection between thymoma development and immunological mechanisms. Characterized by association with humoral and cellular immunodeficiency, thymoma patients are susceptible to opportunistic infections by environmental factors. Recent reports have suggested that viral infection may play a role in the etiological mechanisms of thymoma development associated with dysregulated immunity. In this review, we summarize the case reports and studies related to viral infection, such as CMV, EBV and HSV, that probably play a part in the pathogenesis of thymoma and related diseases. Furthermore, we demonstrate the underlying mechanisms by which viruses may induce the occurrence of thymoma with autoimmune diseases. Lastly, we discuss the potential application of antiviral therapy in the treatment of thymic diseases.

NKT and NKT-like Cells in Autoimmune Neuroinflammatory Diseases-Multiple Sclerosis, Myasthenia Gravis and Guillain-Barre Syndrome

NKT cells comprise three subsets—type I (invariant, iNKT), type II, and NKT-like cells, of which iNKT cells are the most studied subset. They are capable of rapid cytokine production after the initial stimulus, thus they may be important for polarisation of Th cells. Due to this, they may be an important cell subset in autoimmune diseases. In the current review, we are summarising results of NKT-oriented studies in major neurological autoimmune diseases—multiple sclerosis, myasthenia gravis, and Guillain-Barre syndrome and their corresponding animal models.

Thymus variants on imaging in patients with rheumatoid arthritis-Clinical and immunological significance

OBJECTIVES

We sought to clarify the presence of radiographic thymus variants using a scoring system, and their association with clinical and immunological features in RA patients.

METHODS

387 RA patients randomly selected from all patients visiting our department who underwent chest CT scanning, with exclusion of patients with thymoma or thymic cyst, or age < 30 y. Thymus size and attenuation score in axial CT images were quantitatively interpreted and assessed. Associations between immunophenotype data and clinical and serological features were analysed in a subset of patients.

RESULTS

Thymic enlargement was found in 76 (19.6%) patients, and a thymus attenuation score ≥ 2 was found in 50 (12.9%) patients. The score was significantly associated with antibodies to citrullinated peptide antigens (ACPA) positivity. Thymic enlargement was significantly associated with the proportions of CD4+ effector memory T cells.

CONCLUSION

Radiographic thymus variants were frequently observed in RA patients, and may reflect an abnormal immune response involved in the pathogenesis of RA.

Association between Epstein-Barr virus and Thymic epithelial tumors: a systematic review

The possible role of Epstein-Barr virus (EBV) in the pathogenesis of thymic epithelial tumors (TET) remains controversial. This study aimed to determine the prevalence of EBV in TET. We conducted a systematic review of relevant English-language studies published between January 1980 and December 2013. Effect size was calculated as event rates (95% confidence interval [CI]) by homogeneity testing using Cochran’s Q and I² statistics for benign TET, benign TET with myasthenia gravis (MG), and thymic carcinoma (TC). Among 136 potentially relevant studies, 22 met the inclusion criteria. Despite a considerable degree of heterogeneity, the pooled estimated incidences were 9% (95% CI, 1–23%), 20% (95% CI, 0–54%), and 6% (95% CI, 0–21%) for benign TET, benign TET with MG, and TC, respectively. There was significant heterogeneity among studies that used in situ hybridization (ISH) for both benign TET and benign TET with MG. According to the random-effects model, studies employing ISH yielded lower point estimates of EBV prevalence (5%) than those employing other methods (33%). Using the random-effects model, we found a lack of significant heterogeneity among studies from different geographic regions (p = 0.0848). Further, 12 of 23 lymphoepithelioma-like carcinoma (LELC) cases tested EBV-positive. The prevalence of EBV in benign TET with or without MG was lower than in nasopharyngeal carcinoma, suggesting that EBV plays a minor role in TET pathogenesis. Although the prevalence of EBV in TC was also low, EBV may play an important causal role in LELC. Further research is needed to clarify these associations.

Il-23/Th17 cell pathway: A promising target to alleviate thymic inflammation maintenance in myasthenia gravis

IL-23/Th17 pathway has been identified to sustain inflammatory condition in several autoimmune diseases and therefore being targeted in various therapeutic and effective approaches. Patients affected with autoimmune myasthenia gravis exhibit a disease effector tissue, the thymus, that harbors ectopic germinal centers that sustain production of auto-antibodies, targeting proteins located in the neuromuscular junction, cause of the organ-specific chronic autoimmune disease. The present study aims to investigate the IL-23/Th17 cell pathway in the thymic inflammatory and pathogenic events. We found that thymuses of MG patients displayed overexpression of Interleukin-17, signature cytokine of activated Th17 cells. This activation was sustained by a higher secretion of Interleukin-23 by TEC, in addition to the increased expression of cytokines involved in Th17 cell development. The overexpression of Interleukin-23 was due to a dysregulation of interferon type I pathway. Besides, Interleukin-17 secreted, and Th17 cells were localized around thymic ectopic germinal centers. These cells expressed podoplanin, a protein involved in B-cell maturation and antibody secretion. Finally, production of Interleukin-23 was also promoted by Interleukin-17 secreted itself by Th17 cells, highlighting a chronic loop of inflammation sustained by thymic cell interaction. Activation of the IL-23/Th17 pathway in the thymus of autoimmune myasthenia gravis patients creates an unstoppable loop of inflammation that may participate in ectopic germinal center maintenance. To alleviate the physio-pathological events in myasthenia gravis patients, this pathway may be considered as a new therapeutic target.

Assessment of pre and post-thymectomy myasthenia gravis

OBJECTIVE

To evaluate transcervical and transsternal thymectomy benefits in large myasthenia gravis (MG) cohort.

METHOD

We retrospectively evaluated MG patients (n = 184) who had undergone thymectomy between 2004 and 2015 at National Institute of Mental Health and Neurosciences, Bangalore (India). Myasthenia gravis foundation of America guidelines were followed to assess clinical outcome. Anti-acetylcholine receptors (AChR) antibodies, repetitive nerve stimulation (RNS) and Neostigmine tests were performed at pre and post-thymectomy stage.

RESULTS

Most of the patients were fell under MG grade IIA (82 of 184, 44.56%) and grade IIB (61 of 184, 33.15%). Thymoma and thymic hyperplasia was established in 64 (34.78%) and 89 (48.37%) patients respectively. Other thymic abnormalities such thymic atrophy, cysts and lipoma were established in 31 (16.85%) patients. MG patients were treated either with transcervical (n = 79) or (n = 105) transsternal thymectomy. At the pre-thymectomy stage, the majority of the patients were positive for anti-AChR antibodies (179 of 184, 97.28%), RNS (170 of 184, 92.4%), and Neostigmine (175 of 184, 95.11%). At the post-thymectomy stage, a significant reduction observed in anti-AChR antibodies positivity (p < 0.022) and RNS positivity (p < 0.015). Overall, benefits were observed in 61.41% (113 of 184) of patients. Clinical benefits (complete stable remission, pharmacological remission, minimal manifestation, and improvement) of transcervical and transsternal thymectomy observed in 69.62% (55 of 79) and 55.24% (58 of 105) of patients respectively. MG patients with thymoma showed the least improvement compared to thymic hyperplasia.

DISCUSSION

Transcervical and transsternal thymectomy showed clinical benefits, however, there was no significant difference between them.

Cultured Human Thymic-Derived Cells Display Medullary Thymic Epithelial Cell Phenotype and Functionality

Thymic epithelial cells are one of the main components of the thymic microenvironment required for T-cell development. In this work, we describe an efficient method free of enzymatic and Facs-sorted methods to culture human medullary thymic epithelial cells without affecting the cell phenotypic, physiologic and functional features. Human medulla thymic epithelial cells (mTECs) are obtained by culturing thymic biopsies explants. After 7 days of primo-culture, mTECs keep their ability to express key molecules involved in immune tolerance processes such as autoimmune regulator, tissue-specific antigens, chemokines, and cytokines. In addition, the cells sensor their cultured environment and consequently adjust their gene expression network. Therefore, we describe and provide a human mTEC model that may be used to test the effect of various molecules on thymic epithelial cell homeostasis and physiology. This method should allow the investigations of the specificities and the knowledge of human mTECs in normal or pathological conditions and therefore discontinue the extrapolations done on the murine models.

Advances in autoimmune myasthenia gravis management

INTRODUCTION

Myasthenia gravis (MG) is an autoimmune neuromuscular disorder with no cure and conventional treatments limited by significant adverse effects and variable benefit. In the last decade, therapeutic development has expanded based on improved understanding of autoimmunity and financial incentives for drug development in rare disease. Clinical subtypes exist based on age, gender, thymic pathology, autoantibody profile, and other poorly defined factors, such as genetics, complicate development of specific therapies. Areas covered: Clinical presentation and pathology vary considerably among patients with some having weakness limited to the ocular muscles and others having profound generalized weakness leading to respiratory insufficiency. MG is an antibody-mediated disorder dependent on autoreactive B cells which require T-cell support. Treatments focus on elimination of circulating autoantibodies or inhibition of effector mechanisms by a broad spectrum of approaches from plasmapheresis to B-cell elimination to complement inhibition. Expert commentary: Standard therapies and those under development are disease modifying and not curative. As a rare disease, clinical trials are challenged in patient recruitment. The great interest in development of treatments specific for MG is welcome, but decisions will need to be made to focus on those that offer significant benefits to patients.

Pathophysiological mechanisms of autoimmunity

Autoimmune diseases (AIDs) are chronic disorders characterized by inflammatory reactions against self-antigens that can be either systemic or organ specific. AIDs can differ in their epidemiologic features and clinical presentations, yet all share a remarkable complexity. AIDs result from an interplay of genetic and epigenetic factors with environmental components that are associated with imbalances in the immune system. Many of the pathogenic mechanisms of AIDs are also implicated in myasthenia gravis (MG), an AID in which inflammation of the thymus leads to a neuromuscular disorder. Our goal here is to highlight the similarities and differences between MG and other AIDs by reviewing the common transcriptome signatures and the development of germinal centers and by discussing some unresolved questions about autoimmune mechanisms. This review will propose hypotheses to explain the origin of regulatory T (T_reg ) cell defects and the causes of chronicity and specificity of AIDs.

An imbalance between regulatory T cells and T helper 17 cells in acetylcholine receptor-positive myasthenia gravis patients

A chronic autoimmune disease, myasthenia gravis (MG) is characterized in 85% of patients by antibodies directed against the acetylcholine receptor (AChR) located at the neuromuscular junction. The functional and effective balance between regulatory T cells (T_reg cells) and effector T cells (T_eff cells) is lost in the hyperplastic thymus of MG patients with antibodies specific for the AChR (AChR⁺ MG patients). The objective of this review is to describe how T_reg cells and inflammatory T cells participate in this imbalance and contribute to induce a chronic inflammatory state in the MG thymus. We discuss the origins and characteristics of T_reg cells and their reported dysfunctions in AChR⁺ MG patients. We also review the inflammatory condition observed in MG thymus, including overexpression of interleukin (IL)-1β, IL-6, and IL-23, cytokines that promote the differentiation of T helper 17 (T_H 17) cells and the expression of IL-17. We summarize the preclinical models used to determine the implication of expression of cytokines, such as IL-6, IL-12 (IL-23 subunit), IL-17, and interferon γ to the development of experimental autoimmune MG. Finally, we suggest that biological agents, such as humanized monoclonal antibodies that target the IL-23/T_H 17 pathway, should be investigated in the context of MG, as they have proven efficiency in other autoimmune diseases.

Toll-like receptor 9 antagonist suppresses humoral immunity in experimental autoimmune myasthenia gravis

Recent studies have demonstrated the important role of toll-like receptor 9 (TLR9) signalling in autoimmune diseases, but its role in myasthenia gravis (MG) has not been fully established. We show herein that blocking TLR9 signalling via the suppressive oligodeoxynucleotide (ODN) H154 alleviated the symptoms of experimental autoimmune myasthenia gravis (EAMG). With the downregulation of dendritic cells (DCs), TLR9 interruption reduced follicular helper T cells (Tfh) and germinal centre (GC) B cells, leading to decreased antibody production. In addition, TLR9⁺ B cells as well as total B cells in the spleen were inhibited by H154. These findings highlight the critical role of TLR9 in EAMG and suggest that the inhibition of the TLR9 pathway might be a potential pharmacological strategy for the treatment of myasthenia gravis.

High frequencies of circulating Tfh-Th17 cells in myasthenia gravis patients

Recent studies show that the frequencies of circulating follicullar helper T (cTfh) cells are significantly higher in myasthenia gravis (MG) patients compared with healthy controls (HC). And, they are positively correlated with levels of serum anti-acetylcholine receptor antibody (anti-AchR Ab). It is unclear whether cTfh cell subset frequencies are altered and what role they play in MG patients. In order to clarify this, we examined the frequencies of cTfh cell counterparts, their subsets, and circulating plasmablasts in MG patients by flow cytometry. We determined the concentrations of serum anti-AChR Ab by enzyme-linked immunosorbent assay (ELISA). We assayed the function of cTfh cell subsets by flow cytometry and real-time polymerase chain reaction (RT-PCR). We found higher frequencies of cTfh cell counterparts, cTfh-Th17 cells, and plasmablasts in MG patients compared with HC. The frequencies of cTfh cell counterparts and cTfh-Th17 cells were positively correlated with the frequencies of plasmablasts and the concentrations of anti-AChR Ab in MG patients. Functional assays showed that activated cTfh-Th17 cells highly expressed key molecular features of Tfh cells including ICOS, PD-1, and IL-21. Results indicate that, just like cTfh cell counterparts, cTfh-Th17 cells may play a role in the immunopathogenesis and the production of anti-AChR Ab of MG.

Review on Toll-Like Receptor Activation in Myasthenia Gravis: Application to the Development of New Experimental Models

Abnormal toll-like receptor (TLR) activation and uncontrolled resolution of inflammation are suspected to play a key role in the development of autoimmune diseases. Acquired myasthenia gravis (MG) is an invalidating neuromuscular disease leading to muscle weaknesses. MG is mainly mediated by anti-acetylcholine receptor (AChR) autoantibodies, and thymic hyperplasia characterized by ectopic germinal centers is a common feature in MG. An abnormal expression of certain TLRs is observed in the thymus of MG patients associated with the overexpression of interferon (IFN)-β, the orchestrator of thymic changes in MG. Experimental models have been developed for numerous autoimmune diseases. These models are induced by animal immunization with a purified antigen solubilized in complete Freund's adjuvant (CFA) containing heat-inactivated mycobacterium tuberculosis (MTB). Sensitization against the antigen is mainly due to the activation of TLR signaling pathways by the pathogen motifs displayed by MTB, and attempts have been made to substitute the use of CFA by TLR agonists. AChR emulsified in CFA is used to induce the classical experimental autoimmune MG model (EAMG). However, the TLR4 activator lipopolysaccharide (LPS) has proved to be efficient to replace MTB and induce a sensitization against purified AChR. Poly(I:C), the well-known TLR3 agonist, is also able by itself to induce MG symptoms in mice associated with early thymic changes as observed in human MG. In this review, we discuss the abnormal expression of TLRs in MG patients and we describe the use of TLR agonists to induce EAMG in comparison with other autoimmune experimental models.

Novel CXCL13 transgenic mouse: inflammation drives pathogenic effect of CXCL13 in experimental myasthenia gravis

Abnormal overexpression of CXCL13 is observed in many inflamed tissues and in particular in autoimmune diseases. Myasthenia gravis (MG) is a neuromuscular disease mainly mediated by anti-acetylcholine receptor autoantibodies. Thymic hyperplasia characterized by ectopic germinal centers (GCs) is a common feature in MG and is correlated with high levels of anti-AChR antibodies. We previously showed that the B-cell chemoattractant, CXCL13 is overexpressed by thymic epithelial cells in MG patients. We hypothesized that abnormal CXCL13 expression by the thymic epithelium triggered B-cell recruitment in MG. We therefore created a novel transgenic (Tg) mouse with a keratin 5 driven CXCL13 expression. The thymus of Tg mice overexpressed CXCL13 but did not trigger B-cell recruitment. However, in inflammatory conditions, induced by Poly(I:C), B cells strongly migrated to the thymus. Tg mice were also more susceptible to experimental autoimmune MG (EAMG) with stronger clinical signs, higher titers of anti-AChR antibodies, increased thymic B cells, and the development of germinal center-like structures. Consequently, this mouse model finally mimics the thymic pathology observed in human MG. Our data also demonstrated that inflammation is mandatory to reveal CXCL13 ability to recruit B cells and to induce tertiary lymphoid organ development.

Aberrant decrease of microRNA19b regulates TSLP expression and contributes to Th17 cells development in myasthenia gravis related thymomas

Myasthenia gravis (MG) is an organ-specific autoimmune disease. The imbalance of T helper type 17 cells (Th17) plays a key role in the pathogenesis of thymomatous MG. But the regulatory mechanism for Th17 cell development in MG-related thymoma remains undefined. Here we demonstrated that thymic stromal lymphopoietin (TSLP) is significantly decreased in thymomas. We also proved that TSLP was post-trancriptionally regulated by microRNA-19b. The expression of microRNA-19b was negatively correlated with the expression of TSLP mRNA and protein in thymomas. This study indicated that the elevation of microRNA-19b suppressed TSLP expression and then influenced T cell development in thymomatous MG.

Proteasome inhibition with bortezomib depletes plasma cells and specific autoantibody production in primary thymic cell cultures from early-onset myasthenia gravis patients

Bortezomib is a potent inhibitor of proteasomes currently used to eliminate malignant plasma cells in multiple myeloma patients. It is also effective in depleting both alloreactive plasma cells in acute Ab-mediated transplant rejection and their autoreactive counterparts in animal models of lupus and myasthenia gravis (MG). In this study, we demonstrate that bortezomib at 10 nM or higher concentrations killed long-lived plasma cells in cultured thymus cells from nine early-onset MG patients and consistently halted their spontaneous production not only of autoantibodies against the acetylcholine receptor but also of total IgG. Surprisingly, lenalidomide and dexamethasone had little effect on plasma cells. After bortezomib treatment, they showed ultrastructural changes characteristic of endoplasmic reticulum stress after 8 h and were no longer detectable at 24 h. Bortezomib therefore appears promising for treating MG and possibly other Ab-mediated autoimmune or allergic disorders, especially when given in short courses at modest doses before the standard immunosuppressive drugs have taken effect.

Myasthenia Gravis: paradox versus paradigm in autoimmunity

Myasthenia Gravis (MG) is a paradigm of organ-specific autoimmune disease (AID). It is mediated by antibodies that target the neuromuscular junction. The purpose of this review is to place MG in the general context of autoimmunity, to summarize the common mechanisms between MG and other AIDs, and to describe the specific mechanisms of MG. We have chosen the most common organ-specific AIDs to compare with MG: type 1 diabetes mellitus (T1DM), autoimmune thyroid diseases (AITD), multiple sclerosis (MS), some systemic AIDs (systemic lupus erythematous (SLE), rheumatoid arthritis (RA), Sjogren's syndrome (SS)), as well as inflammatory diseases of the gut and liver (celiac disease (CeD), Crohn's disease (CD), and primary biliary cirrhosis (PBC)). Several features are similar between all AIDs, suggesting that common pathogenic mechanisms lead to their development. In this review, we address the predisposing factors (genetic, epigenetic, hormones, vitamin D, microbiota), the triggering components (infections, drugs) and their interactions with the immune system [1,2]. The dysregulation of the immune system is detailed and includes the role of B cells, Treg cells, Th17 and cytokines. We particularly focused on the role of TNF-α and interferon type I whose role in MG is very analogous to that in several other AIDS. The implication of AIRE, a key factor in central tolerance is also discussed. Finally, if MG is a prototype of AIDS, it has a clear specificity compared to the other AIDS, by the fact that the target organ, the muscle, is not the site of immune infiltration and B cell expansion, but exclusively that of antibody-mediated pathogenic mechanisms. By contrast, the thymus in the early onset subtype frequently undergoes tissue remodeling, resulting in the development of ectopic germinal centers surrounded by high endothelial venules (HEV), as observed in the target organs of many other AIDs.

Diagnostic and clinical classification of autoimmune myasthenia gravis

Myasthenia gravis is characterized by muscle weakness and abnormal fatigability. It is an autoimmune disease caused by the presence of antibodies against components of the muscle membrane localized at the neuromuscular junction. In most cases, the autoantibodies are against the acetylcholine receptor (AChR). Recently, other targets have been described such as the MuSK protein (muscle-specific kinase) or the LRP4 (lipoprotein related protein 4). Myasthenia gravis can be classified according to the profile of the autoantibodies, the location of the affected muscles (ocular versus generalized), the age of onset of symptoms and thymic abnormalities. The disease generally begins with ocular symptoms (ptosis and/or diplopia) and extends to other muscles in 80% of cases. Other features that characterize MG include the following: variability, effort induced worsening, successive periods of exacerbation during the course of the disease, severity dependent on respiratory and swallowing impairment (if rapid worsening occurs, a myasthenic crisis is suspected), and an association with thymoma in 20% of patients and with other autoimmune diseases such as hyperthyroidism and Hashimoto's disease. The diagnosis is based on the clinical features, the benefit of the cholinesterase inhibitors, the detection of specific autoantibodies (anti-AChR, anti-MuSK or anti-LRP4), and significant decrement evidenced by electrophysiological tests. In this review, we briefly describe the history and epidemiology of the disease and the diagnostic and clinical classification. The neonatal form of myasthenia is explained, and finally we discuss the main difficulties of diagnosis.

Thymic epithelial cell development and its dysfunction in human diseases

Thymic epithelial cells (TECs) are the key components in thymic microenvironment for T cells development. TECs, composed of cortical and medullary TECs, are derived from a common bipotent progenitor and undergo a stepwise development controlled by multiple levels of signals to be functionally mature for supporting thymocyte development. Tumor necrosis factor receptor (TNFR) family members including the receptor activator for NFκB (RANK), CD40, and lymphotoxin β receptor (LTβR) cooperatively control the thymic medullary microenvironment and self-tolerance establishment. In addition, fibroblast growth factors (FGFs), Wnt, and Notch signals are essential for establishment of functional thymic microenvironment. Transcription factors Foxn1 and autoimmune regulator (Aire) are powerful modulators of TEC development, differentiation, and self-tolerance. Dysfunction in thymic microenvironment including defects of TEC and thymocyte development would cause physiological disorders such as tumor, infectious diseases, and autoimmune diseases. In the present review, we will summarize our current understanding on TEC development and the underlying molecular signals pathways and the involvement of thymus dysfunction in human diseases.

Both Treg cells and Tconv cells are defective in the Myasthenia gravis thymus: roles of IL-17 and TNF-α

Myasthenia gravis (MG) is an autoimmune disease in which the thymus frequently presents follicular hyperplasia and signs of inflammation and T cells display a defect in suppressive regulation. Defects in a suppressive assay can indicate either the defective function of Treg cells or the resistance of Tconv cells to suppression by Treg cells. The aim of this study was to determine which cells were responsible for this defect and to address the mechanisms involved. We first performed cross-experiment studies using purified thymic Treg cells and Tconv cells from controls (CTRL) and MG patients. We confirmed that MG Treg cells were defective in suppressing CTRL Tconv proliferation, and we demonstrated for the first time that MG Tconv cells were resistant to Treg cell suppression. The activation of MG Tconv cells triggered a lower upregulation of FoxP3 and a higher upregulation of CD4 and CD25 than CTRL cells. To investigate the factors that could explain these differences, we analyzed the transcriptomes of purified thymic Treg and Tconv cells from MG patients in comparison to CTRL cells. Many of the pathways revealed by this analysis are involved in other autoimmune diseases, and T cells from MG patients exhibit a Th1/Th17/Tfh signature. An increase in IL-17-related genes was only observed in Treg cells, while increases in IFN-γ, IL-21, and TNF-α were observed in both Treg and Tconv cells. These results were confirmed by PCR studies. In addition, the role of TNF-α in the defect in Tconv cells from MG patients was further confirmed by functional studies. Altogether, our results indicate that the immunoregulatory defects observed in MG patients are caused by both Treg cell and Tconv cell impairment and involve several pro-inflammatory cytokines, with TNF-α playing a key role in this process. The chronic inflammation present in the thymus of MG patients could provide an explanation for the escape of thymic T cells from regulation in the MG thymus.

Innate immunity in myasthenia gravis thymus: pathogenic effects of Toll-like receptor 4 signaling on autoimmunity

The thymus is the main site of immune sensitization to AChR in myasthenia gravis (MG). In our previous studies we demonstrated that Toll-like receptor (TLR) 4 is over-expressed in MG thymuses, suggesting its involvement in altering the thymic microenvironment and favoring autosensitization and autoimmunity maintenance processes, via an effect on local chemokine/cytokine network. Here, we investigated whether TLR4 signaling may favor abnormal cell recruitment in MG thymus via CCL17 and CCL22, two chemokines known to dictate immune cell trafficking in inflamed organs by binding CCR4. We also investigated whether TLR4 activation may contribute to immunodysregulation, via the production of Th17-related cytokines, known to alter effector T cell (Teff)/regulatory T cell (Treg) balance. We found that CCL17, CCL22 and CCR4 were expressed at higher levels in MG compared to normal thymuses. The two chemokines were mainly detected around medullary Hassall's corpuscles (HCs), co-localizing with TLR4(+) thymic epithelial cells (TECs) and CCR4(+) dendritic cells (DCs), that were present in higher number in MG thymuses compared to controls. TLR4 stimulation in MG TECs increased CCL17 and CCL22 expression and induced the production of Th17-related cytokines. Then, to study the effect of TLR4-stimulated TECs on immune cell interactions and Teff activation, we generated an in-vitro imaging model by co-culturing CD4(+) Th1/Th17 AChR-specific T cells, naïve CD4(+)CD25(+) Tregs, DCs and TECs from Lewis rats. We observed that TLR4 stimulation led to a more pronounced Teff activatory status, suggesting that TLR4 signaling in MG thymic milieu may affect cell-to-cell interactions, favoring autoreactive T-cell activation. Altogether our findings suggest a role for TLR4 signaling in driving DC recruitment in MG thymus via CCL17 and CCL22, and in generating an inflammatory response that might compromise Treg function, favoring autoreactive T-cell pathogenic responses.

Central role of interferon-beta in thymic events leading to myasthenia gravis

The thymus plays a primary role in early-onset Myasthenia Gravis (MG) mediated by anti-acetylcholine receptor (AChR) antibodies. As we recently showed an inflammatory and anti-viral signature in MG thymuses, we investigated in detail the contribution of interferon (IFN)-I and IFN-III subtypes in thymic changes associated with MG. We showed that IFN-I and IFN-III subtypes, but especially IFN-β, induced specifically α-AChR expression in thymic epithelial cells (TECs). We also demonstrated that IFN-β increased TEC death and the uptake of TEC proteins by dendritic cells. In parallel, we showed that IFN-β increased the expression of the chemokines CXCL13 and CCL21 by TECs and lymphatic endothelial cells, respectively. These two chemokines are involved in germinal center (GC) development and overexpressed in MG thymus with follicular hyperplasia. We also demonstrated that the B-cell activating factor (BAFF), which favors autoreactive B-cells, was overexpressed by TECs in MG thymus and was also induced by IFN-β in TEC cultures. Some of IFN-β effects were down-regulated when cell cultures were treated with glucocorticoids, a treatment widely used in MG patients that decreases the number of thymic GCs. Similar changes were observed in vivo. The injections of Poly(I:C) to C57BL/6 mice triggered a thymic overexpression of IFN-β and IFN-α2 associated with increased expressions of CXCL13, CCL21, BAFF, and favored the recruitment of B cells. These changes were not observed in the thymus of IFN-I receptor KO mice injected with Poly(I:C), even if IFN-β and IFN-α2 were overexpressed. Altogether, these results demonstrate that IFN-β could play a central role in thymic events leading to MG by triggering the overexpression of α-AChR probably leading to thymic DC autosensitization, the abnormal recruitment of peripheral cells and GC formation.

Myasthenia gravis: a comprehensive review of immune dysregulation and etiological mechanisms

Autoimmune myasthenia gravis (MG) is characterized by muscle weakness caused by antibodies directed against proteins of the neuromuscular junction. The main antigenic target is the acetylcholine receptor (AChR), but the muscle Specific Kinase (MuSK) and the low-density lipoprotein receptor-related protein (LRP4) are also targets. This review summarizes the clinical and biological data available for different subgroups of patients, who are classified according to antigenic target, age of onset, and observed thymic abnormalities, such as follicular hyperplasia or thymoma. Here, we analyze in detail the role of the thymus in the physiopathology of MG and propose an explanation for the development of the thymic follicular hyperplasia that is commonly observed in young female patients with anti-AChR antibodies. The influence of the pro-inflammatory environment is discussed, particularly the role of TNF-α and Th17-related cytokines, which could explain the escape of thymic T cells from regulation and the chronic inflammation in the MG thymus. Together with this immune dysregulation, active angiogenic processes and the upregulation of chemokines could promote thymic follicular hyperplasia. MG is a multifactorial disease, and we review the etiological mechanisms that could lead to its onset. Recent global genetic analyses have highlighted potential susceptibility genes. In addition, miRNAs, which play a crucial role in immune function, have been implicated in MG by recent studies. We also discuss the role of sex hormones and the influence of environmental factors, such as the viral hypothesis. This hypothesis is supported by reports that type I interferon and molecules mimicking viral infection can induce thymic changes similar to those observed in MG patients with anti-AChR antibodies.

Circulating miRNAs in myasthenia gravis: miR-150-5p as a new potential biomarker

Objective

Myasthenia gravis (MG) is a chronic autoimmune disorder where autoantibodies target the nicotinic acetylcholine receptors (AChR+) in about 85% of cases, in which the thymus is considered to play a pathogenic role. As there are no reliable biomarkers to monitor disease status in MG, we analyzed circulating miRNAs in sera of MG patients to find disease-specific miRNAs.

Methods

Overall, 168 miRNAs were analyzed in serum samples from four AChR+ MG patients and four healthy controls using Exiqon Focus miRNA polymerase chain reaction (PCR) Panel I + II. Specific accumulation pattern of 13 miRNAs from the discovery set was subsequently investigated in the sera of 16 AChR+ MG patients and 16 healthy controls. All patients were without immunosuppressive treatment. Selected specific miRNAs were further analyzed in the serum of nine MG patients before and after thymectomy to assess the effect of thymus removal on the accumulation of the candidate miRNAs in patient sera.

Results

Three miRNAs were specifically dysregulated in AChR+ MG patient sera samples. Hsa-miR150-5p, which induces T-cell differentiation, as well as hsa-miR21-5p, a regulator of Th1 versus Th2 cell responses, were specifically elevated in MG sera. Additionally, hsa-miR27a-3p, involved in natural killer (NK) cell cytotoxicity, was decreased in MG. Hsa-miR150-5p levels had the highest association with MG and were significantly reduced after thymus removal in correlation with disease improvement.

Interpretation

We propose that the validated miRNAs: hsa-miR150-5p, hsa-miR21-5p, and hsa-miR27a-3p can serve as novel serum biomarkers in AChR+ MG. Hsa-miR-150-5p could be a helpful marker to monitor disease severity.

Lack of Epstein-Barr virus infection in Chinese myasthenia gravis patients

OBJECTIVE

There are three recent contradictory reports on the incidence of Epstein-Barr virus in the pathogenesis of myasthenia gravis, with all studies carried out in Caucasian patients. The current study evaluated whether Epstein-Barr virus infection had a role in the pathogenesis of myasthenia gravis in a cohort of 30 Chinese patients.

MATERIALS AND METHODS

Serial paraffin sections of thymic hyperplasia obtained from myasthenia gravis patients were analyzed for the presence of Epstein-Barr virus-encoded small RNA -1 and Epstein-Barr virus latent membrane protein 1 by in situ hybridization and immunohistochemistry, respectively. Epstein-Barr virus(+) cervical lymph nodes from lymphoma patients and Epstein-Barr virus(-) thymus specimens obtained during cardiac surgery served as the positive and negative control groups, respectively.

RESULTS

All the 30 myasthenia gravis specimens were negative for both Epstein-Barr virus-encoded small RNA -1 and Epstein-Barr virus latent membrane protein 1 tests. However, we obtained well-characterized membrane and cytoplasmic immunohistochemical and in situ hybridization staining for both Epstein-Barr virus latent membrane protein 1 and Epstein-Barr virus-encoded small RNA -1, respectively, in the positive control samples.

CONCLUSIONS

Our results therefore do not support a role of thymic Epstein-Barr virus infection in myasthenia gravis pathogenesis and calls for an integration of methodological and interpretation issues in detecting Epstein-Barr virus incidence in myasthenia gravis patients.

Correlation between acetylcholine receptor antibody levels and thymic pathology in myasthenia gravis: a review

Myasthenia gravis is the most common chronic autoimmune neuromuscular disease. Anti-acetylcholine receptor (AChR) antibodies are found in at least 80% of patients with generalized myasthenia and have been implicated in disease pathogenesis. Thymic abnormalities are frequently found in seropositive patients, and the thymus is thought to be involved in generation of autoimmunity. This article reviews existing literature on the role of AChR antibodies in the pathogenesis of myasthenia gravis, and the correlation between AChR antibody titers and thymic pathology. Most studies found that highest titers are seen in thymic hyperplasia, followed by intermediate titers in thymoma, and lowest titers in atrophic or normal thymus. One publication found no difference between titers in thymoma and normal thymus.

Diagnosis and therapy of myasthenia gravis with antibodies to muscle-specific kinase

Myasthenia gravis (MG) with antibodies to the muscle-specific receptor tyrosine kinase (MuSK-MG) is a rare disease which covers 5-8% of all MG patients. Symptoms are nearly always generalized, though more focal than in MG with anti-acetylcholine receptor antibodies, with predominant involvement of cranial, bulbar and axial muscles; early respiratory crises are frequent. Focal atrophy, mostly of facial, masseter and tongue muscles, occurs in a proportion of patients. Diagnosis is often challenging on account of atypical presentation with little or no symptom fluctuations, lack of response to acetylcholinesterase inhibitors in a high proportion of patients and negative results of electrodiagnostic studies when performed on limb muscles. Immunosuppression is the mainstay of treatment, since the response to acetylcholinesterase inhibitors is generally unsatisfactory and thymectomy does not appear to improve the course of the disease. Although corticosteroids result in marked improvement, disease flares are frequent during prednisone dosage tapering and most patients remain dependent on treatment. Since treatment with rituximab, in uncontrolled studies, induced sustained benefit in patients with refractory disease, B cell depletion is an attractive option for MuSK-MG patients unresponsive to conventional immunosuppressants.

Impairment of regulatory T cells in myasthenia gravis: studies in an experimental model

Myasthenia gravis (MG) is an antibody mediated, T cell dependent autoimmune disease characterized by muscle fatigability in which autoantibodies directed to the acetylcholine receptor (AChR) impair neuromuscular transmission. The identification of CD4⁺CD25⁺Foxp3⁺Treg cells as important regulators of tolerance opened a major area of investigation raising the possibility that a dysfunction in the Treg compartment is involved in the etiology and pathogenesis of autoimmune diseases, including MG. In this paper we summarize shortly Treg abnormalities that were reported in MG patients and report on our studies of Treg in experimental autoimmune MG (EAMG). Hopefully these studies would pave the way towards the development of novel Treg-based treatment modalities that will restore self-tolerance in MG and other autoimmune diseases. In our previous studies in EAMG we have shown that Treg cells transferred from healthy rat donors to myasthenic rats suppress EAMG. However, Treg cells from sick animals do not have the same in vivo suppressive activity as those from healthy donors. The objective of the present study was to further characterize quantitative and qualitative alterations in Treg cells of rats with EAMG. We found that the frequency of CD4⁺CD25⁺Foxp3⁺Treg cells within the spleen and PBL was decreased in EAMG rats as compared to naïve and CFA-immunized healthy controls. Treg cells from myasthenic rats were less effective than Treg cells from controls in suppressing the proliferation of CD4⁺T effector cells in response to ConA and of B cells in response to LPS. Moreover, CD4⁺CD25⁺ cells from EAMG rats exhibited an elevated extent of apoptosis and expressed upregulated levels of FAS and of Th17-associated cytokines. Since EAMG is an induced disease, these quantitative and qualitative alterations in Treg cells do not reflect predisposing impairments and seem to be associated with the specific autoimmune response resulting from AChR immunization.

Thymus pathology observed in the MGTX trial

The MGTX trial is the first prospective, randomized clinical trial that aims to evaluate the impact of extended transsternal thymectomy on myasthenic symptoms, prednisone requirements, and quality of life in patients with nonthymomatous, anti-acetylcholine receptor autoantibody-positive myasthenia gravis (MG). Here, we give an overview of the rationale of thymectomy and the standardized macroscopic and histopathological work-up of thymectomy specimens as fixed in MGTX standard operating procedures, including the grading of thymic lymphofollicular hyperplasia and the morphometric strategy to assess thymic involution.

Expansion of circulating counterparts of follicular helper T cells in patients with myasthenia gravis

Growing evidence has demonstrated that dysfunction of follicular helper T (TFH) cells results in an abnormal positive selection of autoreactive B cells, which contributes to the development of autoimmune diseases. This study reveals that the frequency of circulating counterparts of TFH cells in myasthenia gravis (MG) patients is significantly higher compared to healthy controls. Interestingly, the frequencies of circulating TFH cells were positively correlated with the levels of serum anti-AChR Ab in MG patients. Our data suggest the presence of overactivation and expansion of circulating counterparts of TFH cells in MG patients, which may contribute to the immunopathogenesis of MG.

Implication of double-stranded RNA signaling in the etiology of autoimmune myasthenia gravis

OBJECTIVE

Myasthenia gravis (MG) is an autoimmune disease mediated mainly by anti-acetylcholine receptor (AChR) antibodies. The thymus plays a primary role in MG pathogenesis. As we recently showed an inflammatory and antiviral signature in MG thymuses, we investigated whether pathogen-sensing molecules could contribute to an anti-AChR response.

METHODS

We studied the effects of toll-like receptor agonists on the expression of α-AChR and various tissue-specific antigens (TSAs) in human thymic epithelial cell (TEC) cultures. As polyinosinic-polycytidylic acid (poly[I:C]), which mimics double-stranded RNA (dsRNA), stimulated specifically α-AChR expression, the signaling pathways involved were investigated. In parallel, we analyzed the expression of dsRNA-signaling components in the thymus of MG patients, and the relevance of our data was investigated in vivo in poly(I:C)-injected mice.

RESULTS

We demonstrate that dsRNA signaling induced by poly(I:C) specifically triggers the overexpression of α-AChR in TECs and not of other TSAs. A poly(I:C) effect was also observed on MG TECs. This induction is mediated through toll-like receptor 3 (TLR3) and protein kinase R (PKR), and by the release of interferon (IFN)-β. In parallel, human MG thymuses also display an overexpression of TLR3, PKR, and IFN-β. In addition, poly(I:C) injections specifically increase thymic expression of α-AChR in wild-type mice, but not in IFN-I receptor knockout mice. These injections also lead to an anti-AChR autoimmune response characterized by a significant production of serum anti-AChR antibodies and a specific proliferation of B cells.

INTERPRETATION

Because anti-AChR antibodies are highly specific for MG and are pathogenic, dsRNA-signaling activation could contribute to the etiology of MG.

B-cell-activating factor and autoimmune myasthenia gravis

BAFF is a potent B-cell survival factor, and it plays an essential role in B-cell homeostasis and B-cell function in the periphery. Both normal and autoreactive B cells are BAFF dependent; however, excess BAFF promotes the survival, growth, and maturation of autoreactive B cells. When overexpressed, BAFF protects B cells from apoptosis, thereby contributing to autoimmunity. Three independent studies have shown higher BAFF levels in the circulation of MG patients. BAFF may play an important role in the pathogenesis of MG. BAFF antagonists may well provide new treatment options for MG patients, particularly those patients with thymic lymphoid follicular hyperplasia.