Sonia Berrih-Aknin

Human thymus contains IFN-α–producing CD11c–, myeloid CD11c+, and mature interdigitating dendritic cells

Three distinct dendritic cell (DC) subsets capable of stimulating allogeneic naive T cells were isolated from human thymus. The most abundant subset was represented by plasmacytoid DCs (pDCs), which secreted high amounts of IFN-alpha upon stimulation with inactivated influenza virus and thus likely correspond to the recently identified peripheral blood natural IFN-alpha/beta-producing cells (IPCs). Like those latter cells, thymic pDCs had distinctive phenotypic features (i.e., Lin(-), HLA-DR(int), IL-3R alpha(hi), CD45RA(hi), CD11c(-), CD13(-), and CD33(lo)) and developed into mature DCs upon culture in IL-3 and CD40L. Of the two other DC subsets, one displayed a phenotype of immature myeloid DCs (imDCs) (HLA-DR(int), CD11c(+), CD13(+), CD33(+)), and the other represented HLA-DR(hi) CD11c(+) mature DCs (mDCs). Since they also expressed DC-LAMP, these mDCs appear to correspond to interdigitating dendritic cells (IDCs). Thymic pDCs, but not myeloid imDCs, strongly expressed lymphoid-specific transcripts such as pre-T alpha, lambda-like, and Spi-B, thereby suggesting a possible lymphoid origin. The detection of Spi-B mRNA, not only upon in vitro maturation of pDCs, but also in freshly purified IDCs, suggests that in vivo pDCs may differentiate into IDCs.

Fewer thymic changes in MuSK antibody-positive than in MuSK antibody-negative MG

In generalized myasthenia gravis (MG) patients without detectable acetylcholine receptor (AChR) antibodies (SNMG), the thymus is often reported as “normally involuted.” We analyzed thymic compartments in 67 patients with generalized MG, with AChR antibodies (AChR+, n = 23), with muscle‐specific kinase (MuSK) antibodies (MuSK+, n = 14) or with neither (MuSK−, n = 30), and in 11 non‐MG controls. Four of 14 MuSK+ thymi had rare small germinal centers, but overall they were not different from age‐matched controls. However, approximately 75% MuSK− samples showed lymph node–type infiltrates similar to those in AChR+ patients, but with fewer germinal centers. These variations may explain some apparent differences in responses to thymectomy in SNMG. Ann Neurol 2005;57:444–448

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.

The Role of the Thymus in Myasthenia Gravis: Immunohistological and Immunological Studies in 115 Casesa

The thymus and its cellular products, the T cells, are involved in many aspects of autoimmune diseases. T cells may act as effector cells, as is suspected in chronic active hepatitis and in type I diabetes. They may also operate as helper T cells for autoantibody formation. Last, suppressor T cells are known to regulate negatively humoral and cell-mediated autoimmune responses, and their deficiency has been incriminated in the pathogenesis of several experimental and clinical autoimmune diseases. These contrasting actions explain why thymectomy may show diverse effects on the course of experimental autoimmune diseases. Neonatal thymectomy prevents the onset of lupus in MRL/l mice, of diabetes in NOD mice, and of most experimentally induced autoimmune diseases. Conversely, it aggravates the lupus of NZB (NZB X NZW) F1 and B X SB mice and the thyroiditis of obese strain chickens.’.’ Myasthenia gravis (MG) is an autoimmune disease associated with antibodies against acetylcholine receptor ( AChR). Several arguments suggest the existence of a close relationship between disease pathogenesis and thymus function. The disease is frequently associated with morphological abnormalities of the thymus gland (hyperplasia or thym~ma);~.‘ thymectomy has been reported to improve the symptoms of the disease; 5*6 thymocytes from myasthenics produce anti-AChR antibodies;’.’ and their epithelial cells synthesize increased amounts of thymic hormone^,^ one of which has been reported to depress neuromuscular conduction. lo Last, the presence of AChR in extracts of myasthenic thymuses can be demonstrated both by its function (abungarotoxin binding) and its antigenicity (cross-immunoreactivity with muscle AChR).”.’’ It is the purpose of this article to review the data in the literature and

A comprehensive analysis of the epidemiology and clinical characteristics of anti-LRP4 in myasthenia gravis.

Double-seronegative myasthenia gravis (dSN-MG, without detectable AChR and MuSK antibodies) presents a serious gap in MG diagnosis and understanding. Recently, autoantibodies against the low-density lipoprotein receptor-related protein 4 (LRP4) have been identified in several dSN-MG sera, but with dramatic frequency variation (∼2-50%). We have developed a cell based assay (CBA) based on human LRP4 expressing HEK293 cells, for the reliable and efficient detection of LRP4 antibodies. We have screened about 800 MG patient sera from 10 countries for LRP4 antibodies. The overall frequency of LRP4-MG in the dSN-MG group (635 patients) was 18.7% but with variations among different populations (range 7-32.7%). Interestingly, we also identified double positive sera: 8/107 anti-AChR positive and 10/67 anti-MuSK positive sera also had detectable LRP4 antibodies, predominantly originating from only two of the participating groups. No LRP4 antibodies were identified in sera from 56 healthy controls tested, while 4/110 from patients with other neuroimmune diseases were positive. The clinical data, when available, for the LRP4-MG patients were then studied. At disease onset symptoms were mild (81% had MGFA grade I or II), with some identified thymic changes (32% hyperplasia, none with thymoma). On the other hand, double positive patients (AChR/LRP4-MG and MuSK/LRP4-MG) had more severe symptoms at onset compared with any single positive MG subgroup. Contrary to MuSK-MG, 27% of ocular dSN-MG patients were LRP4 antibody positive. Similarly, contrary to MuSK antibodies, which are predominantly of the IgG4 subtype, LRP4 antibodies were predominantly of the IgG1 and IgG2 subtypes. The prevalence was higher in women than in men (female/male ratio 2.5/1), with an average disease onset at ages 33.4 for females and 41.9 for males. Overall, the response of LRP4-MG patients to treatment was similar to published responses of AChR-MG rather than to MuSK-MG patients.

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 Hashimotos 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.

Mesenchymal stem cells as an immunomodulatory therapeutic strategy for autoimmune diseases

Mesenchymal stem cells (MSCs) are non-hematopoietic, multipotent progenitor cells which can be isolated from various human adult tissues. In recent years, MSCs have been shown to possess broad immunoregulatory capabilities, modulating both adaptive and innate immunity. This review discusses the documented immunomodulatory capabilities of the MSCs, the possible mechanisms underlying these functions and presents the potential of using this stem cell-based approach as an immunomodulatory tool for the treatment of autoimmune diseases.

Effects of cytokines on acetylcholine receptor expression: implications for myasthenia gravis.

Myasthenia gravis is an autoimmune disease associated with thymic pathologies, including hyperplasia. In this study, we investigated the processes that may lead to thymic overexpression of the triggering Ag, the acetylcholine receptor (AChR). Using microarray technology, we found that IFN-regulated genes are more highly expressed in these pathological thymic tissues compared with age- and sex-matched normal thymus controls. Therefore, we investigated whether proinflammatory cytokines could locally modify AChR expression in myoid and thymic epithelial cells. We found that AChR transcripts are up-regulated by IFN-γ, and even more so by IFN-γ and TNF-α, as assessed by real-time RT-PCR, with the α-AChR subunit being the most sensitive to this regulation. The expression of AChR protein was increased at the cytoplasmic level in thymic epithelial cells and at the membrane in myoid cells. To examine whether IFN-γ could influence AChR expression in vivo, we analyzed AChR transcripts in IFN-γ gene knock-out mice, and found a significant decrease in AChR transcript levels in the thymus but not in the muscle, compared with wild-type mice. However, up-regulation of AChR protein expression was found in the muscles of animals with myasthenic symptoms treated with TNF-α. Altogether, these results indicate that proinflammatory cytokines influence the expression of AChR in vitro and in vivo. Because proinflammatory cytokine activity is evidenced in the thymus of myasthenia gravis patients, it could influence AChR expression and thereby contribute to the initiation of the autoimmune anti-AChR response.

Epstein‐Barr virus persistence and reactivation in myasthenia gravis thymus

Increasing evidence supports a link between Epstein‐Barr virus (EBV), a ubiquitous B‐lymphotropic human herpesvirus, and common B‐cell–related autoimmune diseases. We sought evidence of EBV infection in thymuses from patients with myasthenia gravis (MG), an autoimmune disease characterized by intrathymic B‐cell activation.

Overexpression of IFN-Induced Protein 10 and Its Receptor CXCR3 in Myasthenia Gravis

Myasthenia gravis (MG) and its animal model, experimental autoimmune MG (EAMG), are autoimmune disorders in which the acetylcholine receptor (AChR) is the major autoantigen. Microarray technology was used to identify new potential drug targets for treatment of myasthenia that would reduce the need for the currently used nonspecific immunosuppression. The chemokine IFN-γ-inducible protein 10 (IP-10; CXCL10), a CXC chemokine, and its receptor, CXCR3, were found to be overexpressed in lymph node cells of EAMG rats. Quantitative real-time PCR confirmed these findings and revealed up-regulated mRNA levels of another chemoattractant that activates CXCR3, monokine induced by IFN-γ (Mig; CXCL9). TNF-α and IL-1β, which act synergistically with IFN-γ to induce IP-10, were also up-regulated. These up-regulations were observed in immune response effector cells, namely, lymph node cells, and in the target organ of the autoimmune attack, the muscle of myasthenic rats, and were significantly reduced after suppression of EAMG by mucosal tolerance induction with an AChR fragment. The relevance of IP-10/CXCR3 signaling in myasthenia was validated by similar observations in MG patients. A significant increase in IP-10 and CXCR3 mRNA levels in both thymus and muscle was observed in myasthenic patients compared with age-matched controls. CXCR3 expression in PBMC of MG patients was markedly increased in CD4+, but not in CD8+, T cells or in CD19+ B cells. Our results demonstrate a positive association of IP-10/CXCR3 signaling with the pathogenesis of EAMG in rats as well as in human MG patients.