Failure to down-regulate Bcl-2 protein in thymic germinal center B cells in myasthenia gravis

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.

Thymic Germinal Centers and Corticosteroids in Myasthenia Gravis: an Immunopathological Study in 1035 Cases and a Critical Review

The most common form of Myasthenia gravis (MG) is due to anti-acetylcholine receptor (AChR) antibodies and is frequently associated with thymic pathology. In this review, we discuss the immunopathological characteristics and molecular mechanisms of thymic follicular hyperplasia, the effects of corticosteroids on this thymic pathology, and the role of thymic epithelial cells (TEC), a key player in the inflammatory thymic mechanisms. This review is based not only on the literature data but also on thymic transcriptome results and analyses of pathological and immunological correlations in a vast cohort of 1035 MG patients without thymoma. We show that among patients presenting a thymic hyperplasia with germinal centers (GC), 80 % are females, indicating that thymic follicular hyperplasia is mainly a disease of women. The presence of anti-AChR antibodies is correlated with the degree of follicular hyperplasia, suggesting that the thymus is a source of anti-AChR antibodies. The degree of hyperplasia is not dependent upon the time from the onset, implying that either the antigen is chronically expressed and/or that the mechanisms of the resolution of the GC are not efficiently controlled. Glucocorticoids, a conventional therapy in MG, induce a significant reduction in the GC number, together with changes in the expression of chemokines and angiogenesis. These changes are likely related to the acetylation molecular process, overrepresented in corticosteroid-treated patients, and essential for gene regulation. Altogether, based on the pathological and molecular thymic abnormalities found in MG patients, this review provides some explanations for the benefit of thymectomy in early-onset MG patients.

Analysis of serum miRNA profiles of myasthenia gravis patients

Myasthenia gravis (MG) is an autoimmune disease characterized by the presence of autoantibodies, mainly against the acetylcholine receptor (AChR). The mechanisms triggering and maintaining this chronic disease are unknown. MiRNAs are regulatory molecules that play a key role in the immune system and are altered in many autoimmune diseases. The aim of this study was to evaluate miRNA profiles in serum of 61 AChR MG patients. We studied serum from patients with early onset MG (n = 22), late onset MG (n = 27) and thymoma (n = 12), to identify alterations in the specific subgroups. In a discovery cohort, we analysed 381 miRNA arrays from 5 patients from each subgroup, and 5 healthy controls. The 15 patients had not received any treatment. We found 32 miRNAs in different levels in MG and analysed 8 of these in a validation cohort that included 46 of the MG patients. MiR15b, miR122, miR-140-3p, miR185, miR192, miR20b and miR-885-5p were in lower levels in MG patients than in controls. Our study suggests that different clinical phenotypes in MG share common altered mechanisms in circulating miRNAs, with no additional contribution of the thymoma. MG treatment intervention does not modify the profile of these miRNAs. Novel insights into the pathogenesis of MG can be reached by the analysis of circulating miRNAs since some of these miRNAs have also been found low in MG peripheral mononuclear cells, and have targets with important roles in B cell survival and antibody production.

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.

Biological implications of thymectomy for myasthenia gravis

Myasthenia gravis (MG) is an autoimmune disease mediated by autoantibodies to the striated muscle tissue. It is often treated by thymectomy. We review recent studies to investigate the biological implications of thymectomy. In anti-acetylcholine receptor antibody (anti-AchR Ab)-positive patients without a thymoma, abnormal germinal center formation in the thymus seems to play an essential role in the pathogenesis of MG. Specific differentiation of B cells producing anti-AchR Ab takes place uniquely in the thymus, and thymectomy is thought to assist in terminating the provision of high-affinity anti-AchR antibody-producing cells to peripheral organs. Thymectomy is not indicated for anti-AchR Ab-negative MG patients who are antimuscle specific kinase antibody (anti-MuSK Ab)-positive, although some anti-MuSK Ab-negative patients may benefit from the procedure. A thymoma can be considered as an acquired thymus with insufficient function of negative selection. The resection of a thymoma is thought to terminate the production of self-reactive T cells. Thus, the biological implications of thymectomy for MG have been partially revealed. Nevertheless, additional studies are needed to elucidate the ontogeny of T cells that recognize AchR and the mechanism of the activation of anti-AchR antibodies producing B cells.

Heterogeneity of immunopathological features of AChR/MuSK autoantibody-negative myasthenia gravis

We compared B cells and germinal centers in thymus from myasthenia gravis (MG) patients either with anti-acetylcholine receptor (AChR) autoantibodies or with neither anti-muscle-specific tyrosine kinase (MuSK) nor anti-AChR (seronegative MG: SN-MG). The numbers and frequencies of total and germinal center B cells varied in the SN-MG thymi, some of which were normal/atrophic. Others were clearly hyperplastic, their B cell parameters overlapping with those in AChR-positive MG, which implicates the thymus in pathogenesis. Indeed, some SN-MG patients apparently benefited from thymectomy, which should be considered a management option.

The thymus is a source of B-cell-survival factors-APRIL and BAFF-in myasthenia gravis

The accumulation of B cells in the thymus is a common feature of myasthenia gravis (MG). To understand whether factors enhancing B-cell survival are increased in MG, we studied the expression of APRIL, BAFF and three of their receptors in the thymus. In hyperplastic thymi, macrophages expressed APRIL and BAFF, and germinal-center B cells, BAFF-R. CD138-positive plasma cells were abundant in MG thymi. By contrast, BCMA-positive plasma cells were scarce. The expression of APRIL and BAFF in MG thymi may reflect the establishment of an environment favorable to B-cell survival.

Inclusion of the transcervical approach in video-assisted thoracoscopic extended thymectomy (VATET) for myasthenia gravis: a prospective trial

BACKGROUND

Because evidence-based data regarding the quality of video-assisted thoracoscopic thymectomy for the treatment of myasthenia gravis are lacking, a prospective trial comparing three different operative approaches was conducted to evaluate their efficacy.

METHODS

This prospective study enrolled 20 consecutive patients with nonthymomatous myasthenia gravis. A series of three approaches for bilateral video-assisted thoracoscopic extended thymectomy (VATET) using the anterior chest wall-lifting method (original), the original method with a flexed-neck position (modified), and the original method with a transcervical approach (final) were prospectively performed in each patient for quantitative and pathologic evaluation of the residual thymus after each approach.

RESULTS

Complete VATET required 242 +/- 48 min, with the transcervical procedure requiring 23 +/- 12 min. After the modified method, the residual thymus in the cervical region was 1.5 cm in size and weighed 0.8 g (0.8% of the entire thymus), as compared with a size of 2.2 cm and a weight of 1.3 g (3.2%) after the original method. Each value is the result of comparison with the final method. Histopathologic studies showed residual tissue in the germinal center as well as Hassall's corpuscles in more than 70% of cases.

CONCLUSION

The findings show that VATET without the transcervical approach could be an immunologically incomplete treatment for myasthenia gravis. Therefore, the transcervical approach should be included in VATET procedures to ensure radicality.

Lymphoid neogenesis in chronic inflammatory diseases

The frequent observation of organized lymphoid structures that resemble secondary lymphoid organs in tissues that are targeted by chronic inflammatory processes, such as autoimmunity and infection, has indicated that lymphoid neogenesis might have a role in maintaining immune responses against persistent antigens. In this Review, we discuss recent progress in several aspects of lymphoid neogenesis, focusing on the similarities with lymphoid tissue development, the mechanisms of induction, functional competence and pathophysiological significance. As more information on these issues becomes available, a better understanding of the role of lymphoid neogenesis in promoting chronic inflammation might eventually lead to new strategies to target immunopathological processes.

High frequency of allergic conjunctivitis in myasthenia gravis without thymoma

OBJECTIVES

To investigate the frequency of allergic disorders in myasthenia gravis (MG) patients and characterize the features of MG associated with allergic disorders.

METHODS

Frequencies of past and present common allergic disorders in 160 MG patients who visited the Department of Neurology, Kyushu University Hospital from April 2000 to July 2003 and in 81 neurological normal controls were studied.

RESULTS

Among various allergic disorders, the frequency of allergic conjunctivitis (AC) was significantly higher in MG patients (39/160, 24.4%, p(corr)=0.0112), especially with MG without thymoma (36/123, 29.3%, p(corr)=0.0016), in comparison to the controls (6/81, 7.4%). MG patients with AC showed a significantly higher rate of seronegative MG (43.6% vs. 17.4%, p=0.008) and a higher tendency of ocular MG (43.6% vs. 28.1%, p=0.071). Moreover, MG with AC had significantly lower anti-acetylcholine receptor antibody titers (median 6.8 nmol/l vs. median 23.6 nmol/l, p=0.0359) as well as a lower rate of coexisting thymoma (7.7% vs. 17.4%, p=0.016). The incidence of myasthenic crisis was also lower in MG with AC than without AC, yet the difference was not significant (7.7% vs. 15.7%).

CONCLUSION

There was a significant association of AC with MG especially for ocular or seronegative MG in cases without thymoma.

Differences in thyroid-infiltrating B lymphocytes in patients with Graves' disease: relationship to autoantibody detection

OBJECTIVE

Thyroid-infiltrating B (Thyr-B) lymphocytes are thought to play an important role in the pathogenic mechanisms underlying Graves' disease.

DESIGN AND METHODS

In this study, a broad phenotypic analysis of these cells has been performed in 15 consecutive patients who underwent thyroidectomy.

RESULTS

Data reveal the occurrence of two distinct types of Thyr-B cell infiltrates. Type 1 was present in most of the cases (10/15) and consisted of a combination of IgM+ IgD(low to-) B lymphocytes showing features of marginal zone B cells, and IgG+ classic memory B cells. In contrast, in 5 of the 15 cases, a second type of Thyr-B cell infiltrate occurred, exhibiting the profile IgM- IgD- CD44(low to-) CD38++ CD71+ CD95+. This phenotype is highly suggestive of germinal center (GC) B cells, a finding not always anticipated from routine histologic examination. The presence of these ectopic GC was closely associated with the elevated serum level of anti-thyroid peroxidase (TPO), but not with anti-thyrotropin receptor (TSHR), autoantibodies. Moreover, local active anti-thyroglobulin (Tg) antibody secretion was only detected in cultures of type 2 Thyr-B cells.

CONCLUSION

These findings indicate that high titers of anti-TPO, but not anti-TSHR antibody, might be associated with intrathyroidal GC development.

The immunologic role of thymectomy in the treatment of myasthenia gravis: implication of thymus-associated B-lymphocyte subset in reduction of the anti-acetylcholine receptor antibody titer

BACKGROUND AND PURPOSE

Thymectomy is generally accepted as the major option of treatment for myasthenia gravis. To elucidate the biological role of thymectomy in the treatment of myasthenia gravis, the immunologic characteristics of the thymus was studied in association with the postoperative kinetics of the anti-acetylcholine receptor antibody titer.

MATERIALS AND METHODS

Thirty-four patients with nonthymomatous myasthenia gravis who had positive anti-acetylcholine receptor antibody titer and undergoing extended thymectomy were subjected to the study. Reduction of anti-acetylcholine receptor antibody titer was evaluated in terms of the proportion of anti-acetylcholine receptor antibody titer at 1 year after thymectomy to that before the operation. The numbers of B lymphocytes (CD19(+) cells) and the germinal center B lymphocytes (CD19(+)CD38(high) cells) present in 1 g of the thymic tissue were calculated by flow cytometry.

RESULTS

The proportion of anti-acetylcholine receptor antibody titer at 1 year after thymectomy ranged from 27.5% to 150%. The numbers of B lymphocytes and the germinal center B lymphocytes in 1 g of the thymic tissue ranged from 0.19 x 10(6)/g to 162.8 x 10(6)/g and from 0.09 x 10(6)/g to 33.4 x 10(6)/g, respectively. The proportion of anti-acetylcholine receptor antibody titer at 1 year after thymectomy had a significant inverted correlation with the number of B lymphocytes (P =.002) as well as that of the germinal center B lymphocytes (P =.007).

CONCLUSION

Effectiveness of thymectomy was dependent on predominance of B lymphocytes and the germinal center B lymphocytes in the thymus, suggesting that one of the biological roles of thymectomy in the treatment of myasthenia gravis is removing the thymus-associated germinal centers.

Chemokines determine local lymphoneogenesis and a reduction of circulating CXCR4+ T and CCR7 B and T lymphocytes in thyroid autoimmune diseases

Chemokines and their corresponding receptors are crucial for the recruitment of lymphocytes into the lymphoid organs and for its organization acting in a multistep process. Tissues affected by autoimmune disease often contain ectopic lymphoid follicles which, in the case of autoimmune thyroid disorders, are highly active and specific for thyroid Ags although its pathogenic role remains unclear. To understand the genesis of these lymphoid follicles, the expression of relevant cytokines and chemokines was assessed by real time PCR, immunohistochemistry and by in vitro assays in autoimmune and nonautoimmune thyroid glands. Lymphotoxin alpha, lymphotoxin beta, C-C chemokine ligand (CCL) 21, CXC chemokine ligand (CXCL) 12, CXCL13, and CCL22 were increased in thyroids from autoimmune patients, whereas CXCL12, CXCL13, and CCL22 levels were significantly higher in autoimmune glands with ectopic secondary lymphoid follicles than in those without follicles. Interestingly, thyroid epithelium produced CXCL12 in response to proinflammatory cytokines providing a possible clue for the understanding of how tissue stress may lead to ectopic follicle formation. The finding of a correlation between chemokines and thyroid autoantibodies further suggests that intrathyroidal germinal centers play a significant role in the autoimmune response. Unexpectedly, the percentage of circulating CXCR4(+) T cells and CCR7(+) B and T cells (but not of CXCR5) was significantly reduced in PBMCs of patients with autoimmune thyroid disease when they were compared with their intrathyroidal lymphocytes. This systemic effect of active intrathyroidal lymphoid tissue emerges as a possible new marker of thyroid autoimmune disease activity.

Tumor necrosis factor receptor p55 and p75 deficiency protects mice from developing experimental autoimmune myasthenia gravis

The precise pathogenic role of proinflammatory cytokines belonging to the tumor necrosis factor (TNF) family has not been investigated yet in antibody-mediated myasthenia gravis (MG) and experimental autoimmune myasthenia gravis (EAMG). In this study we report that TNF receptor p55(-/-) p75(-/-) mice were resistant to the development of clinical EAMG induced by acetylcholine receptor (AChR) immunizations. The resistance was associated with reduced serum levels of IgG, IgG(1), IgG(2a), and IgG(2b) anti-AChR antibody isotypes. However, IgM anti-AChR antibodies were not reduced, suggesting defective anti-AChR IgG class switching in TNF receptor p55(-/-) p75(-/-) mice. We thus demonstrate the genetic evidence for the role of TNF receptor p55 and p75 in EAMG pathogenesis, and their requirement for the generation of anti-AChR IgG antibodies.

Thyroid autoimmune disease: demonstration of thyroid antigen-specific B cells and recombination-activating gene expression in chemokine-containing active intrathyroidal germinal centers

Autoimmune thyroid disease--Hashimoto thyroiditis and Graves' disease--patients produce high levels of thyroid autoantibodies and contain lymphoid tissue that resembles secondary lymphoid follicles (LFs). We compared the specificity, structure, and function of tonsil and lymph node LFs with those of the intrathyroidal LFs to assess the latter's capability to contribute to autoimmune response. Thyroglobulin and thyroperoxidase binding to LFs indicated that most intrathyroidal LFs were committed to response to thyroid self-antigens and were associated to higher levels of antibodies to thyroglobulin, thyroperoxidase, and thyroid-stimulating hormone receptor. Intrathyroidal LFs were microanatomically very similar to canonical LFs, ie, they had well-developed germinal centers with mantle, light, and dark zones and each of these zones contained B and T lymphocytes, follicular dendritic and interdigitating dendritic cells with typical phenotypes. Careful assessment of proliferation (Ki67) and apoptosis (terminal dUTP nick-end labeling) indicators and of the occurrence of secondary immunoglobulin gene rearrangements (RAG1 and RAG2) confirmed the parallelism. Unexpected high levels of RAG expression suggested that receptor revision occurs in intrathyroidal LFs and may contribute to generate high-affinity thyroid autoantibodies. Well-formed high endothelial venules and a congruent pattern of adhesion molecules and chemokine expression in intrathyroidal LFs were also detected. These data suggest that ectopic intrathyroidal LFs contain all of the elements needed to drive the autoimmune response and also that their microenvironment may favor the expansion and perpetuation of autoimmune response.

Decreased expression of c-myc family genes in thymuses from myasthenia gravis patients

The thymus is a critical organ for the elimination of autoreactive T cells by apoptosis. We studied the expression of apoptosis-associated genes, bcl-xL, bad, caspase-3, and c-myc family genes in myasthenia gravis (MG) thymuses. We observed that the mRNA levels of myc family genes, c-myc and max, were markedly reduced in MG thymuses. These results indicate that c-myc-mediated signaling is abnormal in MG thymuses. The levels of molecules whose expressions are associated with myc, such as STAM, prothymosin-alpha, and NFkappaB, were also analyzed.

Estrogen up-regulates Bcl-2 and blocks tolerance induction of naive B cells

Sex hormones are presumed to contribute to sexual dimorphism in the immune system. Estrogen, in particular, has been suggested to predispose women to systemic lupus erythematosus. We report here that estradiol (E(2)) can break B cell tolerance and induce a lupus-like phenotype in nonautoimmune mice transgenic for the heavy chain of a pathogenic anti-DNA antibody. E(2) treatment resulted in a rise in anti-DNA serum titers and in Ig deposition in renal glomeruli. ELISPOT analysis confirmed a significant increase in the number of high-affinity anti-DNA antibody-secreting B cells in the spleens of E(2)-treated mice. Hybridomas generated from E(2)-treated mice express high-affinity, unmutated anti-DNA antibodies, indicating that naive B cells that are normally deleted or anergized are rescued from tolerance induction. Finally, immunohistochemical studies revealed increased Bcl-2 expression in splenic B cells of E(2)-treated mice. These data demonstrate that estrogen interferes with tolerance induction of naive autoreactive B cells and that the presence of these B cells in the periphery is associated with up-regulation of Bcl-2.

Inhibition of B Cell Receptor-Mediated Apoptosis by IFN

IFNs are a family of cytokines that are involved in the regulation of immune and inflammatory responses. Clinical use of IFN-α/β encompasses treatment for a variety of diseases; however, prolonged exposure to IFN-α/β results in elevated levels of autoreactive Abs. In this study, we investigated the potential of IFNs to modulate apoptotic signals in B cells. We demonstrate that IFN-α or IFN-β inhibit Ag receptor-mediated apoptosis in a dose-dependent manner. Inhibition of phosphatidylinositol 3′ (PI3)-kinase did not abolish the effect of IFN, indicating that the antiapoptotic mechanism is PI3-kinase- and protein kinase B/Akt-independent. Instead, IFN-α and IFN-β, but not IFN-γ, significantly increase the levels of the survival protein Bcl-2, and to a lesser extent, Bcl-xL expression. Thus, IFN-α/β-mediated inhibition of B cell Ag receptor-triggered apoptosis may offer a model for the process that leads to the escape of self-reactive B cells from negative selection and consequently results in autoantibody production.

Germinal centre cell kinetics

The germinal centre is a fundamental component of the humoral immune response, representing a unique microenvironment where antigen-activated B lymphocytes undergo clonal expansion, mutate their immunoglobulin, and are subject to a stringent selection process based on their antigen affinity. This review highlights recent advances in the understanding of the cell kinetic process of activation, proliferation, differentiation, and death of germinal centre cells, which are beginning to provide important insights into the regulation of this highly complex reaction. Their definition may have considerable pathological import given the involvement of the germinal centre in non-Hodgkin's lymphomas and recent evidence suggesting that abnormal germinal centre reactions may be involved in the pathogenesis of Hodgkin's disease and some autoimmune diseases.