Pathogenesis of myasthenia gravis

Generalized Myasthenia Gravis: Classification, Clinical Presentation, Natural History, and Epidemiology

Myasthenia gravis (MG) is a rare disease, but the most common disorder of the neuromuscular junction. It is the prototypic autoimmune disease most commonly caused by antibodies to the acetylcholine receptor (AChR) leading to characteristic fatigable weakness of the ocular, bulbar, respiratory, axial, and limb muscles. The majority of patients with MG first present with ocular symptoms. Most patients with MG will experience at least 1 exacerbation of symptoms throughout the course of their illness. This article will cover the epidemiology, clinical presentation, classification, and natural history of MG.

Increased frequency of thymic T follicular helper cells in myasthenia gravis patients with thymoma

BACKGROUND

To investigate the presence of T follicular helper (TFH) cells and their associated molecules in myasthenia gravis (MG) patients with thymoma.

METHODS

TFH cells are detected in thymus around the thymoma region of 50 patients and atrophic thymus in 10 patients as control. The percentage of TFH cells among CD4(+) T cells and the expression level of surface markers CXC chemokine receptor 5 (CXCR5), inducible co-stimulator (ICOS), programmed cell death 1 and the cytoplasmic marker B cell lymphoma 6 (Bcl-6) were analyzed by immunohistochemistry (IHC) staining, immunofluorescence (IF) and western blotting (WB).

RESULTS

Higher percentage of thymic TFH cells was found in MG patients with thymoma compared with both thymoma patients without MG and control group. The expression levels of the four markers in thymoma of MG patients were significantly higher than thymoma patients without MG and control group. No significant difference was found in the levels of programmed cell death 1 (PD-1) and Bcl-6 between thymoma patients without MG and the control, while the levels of CXCR5 and ICOS in thymoma patients without MG were higher than control group.

CONCLUSIONS

These results suggested thymic TFH cells might involve in the pathogenesis of MG with thymoma. However, it needs further study to test if the inhibition of the function of TFH cells could effectively alleviate the severity of MG.

Myasthenia gravis and Lambert-Eaton myasthenic syndrome

PURPOSE OF REVIEW

This article reviews the clinical presentations, diagnostic findings, and treatment options for autoimmune myasthenia gravis (MG) and Lambert-Eaton myasthenic syndrome.

RECENT FINDINGS

Immunologic research is unraveling the immunopathology of MG and identifying targets for novel immune-based therapy of this condition. MG patients with antibodies to muscle-specific tyrosine kinase (MuSK) frequently present with symptoms and clinical findings that suggest nerve or muscle disease.

SUMMARY

Early diagnosis and treatment have a marked effect on outcome in these diseases. In most cases, the diagnosis of MG or Lambert-Eaton myasthenic syndrome can be made from the history, supplemented with directed questions, and a physical examination designed to demonstrate variable weakness in affected muscle groups. Appropriate confirmatory tests almost always establish the diagnosis. Although several novel treatment modalities for MG are under investigation, currently available therapies produce substantial improvement in function and quality of life in most patients with this condition. Knowledge about the dosing, adverse effects, and costs of immunomodulatory therapies is essential for the effective management of patients with MG and Lambert-Eaton myasthenic syndrome.

Quality of life in 188 patients with myasthenia gravis in China

Myasthenia gravis (MG) is a kind of chronic autoimmune disease which can weaken patients' motor function and, furthermore, produce negative impact on the health-related quality of life (HRQoL). The primary purpose of this research was to evaluate factors that might affect the HRQoL of MG patients. A cross-sectional clinical research was carried out including 188 successive patients with MG. Myasthenia Gravis Foundation of America (MGFA) classification and Quantitative Myasthenia Gravis (QMG) score were applied to assess the severity of the disease. The Medical Outcome Survey 36-Item Short-Form Health Survey (SF-36) was used to estimate the HRQoL. Hamilton Depression Rating Scale (HDRS) and Hamilton Anxiety Rating Scale (HARS) were utilized to measure the depression and anxiety symptom. Factors may influence the HRQoL of MG patients include age, educational level, occupation, the situation of the thymus, the type of MG and generalized myasthenia gravis (GMG), the severity of the disease and the psychological disorder. Higher QMG and HARS scores were two significant factors that can prognosticate lower Physical Composite Score (PCS) and Mental Composite Score (MCS), while older age was just a significant factor which has prognostic value for lower PCS. The results of this research may have a potential guiding significance for the clinical treatment strategy and improve the quality of life in patients with MG consequently. In addition to the treatment of physical symptoms, the psychological symptoms such as anxiety and depression should be concerned as well.

Thymoma related myasthenia gravis in humans and potential animal models

Thymoma-associated Myasthenia gravis (TAMG) is one of the anti-acetylcholine receptor MG (AChR-MG) subtypes. The clinico-pathological features of TAMG and its pathogenesis are described here in comparison with pathogenetic models suggested for the more common non-thymoma AChR-MG subtypes, early onset MG and late onset MG. Emphasis is put on the role of abnormal intratumorous T cell selection and activation, lack of intratumorous myoid cells and regulatory T cells as well as deficient expression of the autoimmune regulator (AIRE) by neoplastic thymic epithelial cells. We review spontaneous and genetically engineered thymoma models in a spectrum of animals and the extensive clinical and immunological overlap between canine, feline and human TAMG. Finally, limitations and perspectives of the transplantation of human and murine thymoma tissue into nude mice, as potential models for TAMG, are addressed.

Thymic neoplasms: a clinical update

Thymoma is a unique neoplasm of the anterior mediastinum that is frequently associated with indolent growth and a variety of paraneoplastic syndromes. One third of cases are detected during the evaluation of myasthenia gravis. Classification systems of thymoma have limited ability in accurately predicting prognosis and course of disease. Thus, staging is the only way to predict clinical behavior. Encapsulated tumors that are surgically resected carry the best prognosis. Adjuvant radiotherapy is recommended for incompletely excised and most invasive thymomas. Chemotherapy in anthracycline-based chemotherapy remains the most effective chemotherapy for neoadjuvant, adjuvant or palliative treatment.

The predictive value of the presence of different antibodies and thymus pathology to the clinical outcome in patients with generalized myasthenia gravis

OBJECTIVES

To analyze the predictive value of anti-acetylcholine receptor antibodies (anti-AChR Ab) and anti-muscle specific kinase antibodies (anti-MuSK Ab), as well as the thymus pathology to the clinical outcome in patients with generalized myasthenia gravis (MG).

METHODS

We analyzed 138 patients with generalized MG, who were thymectomized and assayed for anti-AChR Ab and anti-MuSK Ab.

RESULTS

Anti-AChR Ab were detected in 84% of patients, while anti-MuSK Ab were present in 36% of the AChR Ab negative patients. Severe forms of the disease were more frequent in MuSK Ab positive, compared to the AChR Ab positive and complete seronegative patients. Thymic lymphoid follicular hyperplasia (LFH) was present in 60%, thymoma in 23%, atrophic thymus in 9% and the normal thymus in 8% of patients. LFH was more frequent among women, while thymoma and atrophic thymus were more frequent in men. The younger patients mainly had LFH and normal thymus, while thymoma and atrophic thymus were more frequent in older patients. The mildest clinical presentation was present in patients with normal thymus, while severe forms of the disease were registered in the patients with thymoma. The AChR Ab positive patients had more often LFH and thymoma, while within MuSK Ab positive patients atrophic thymus was most common.

CONCLUSION

The best disease outcome was observed in patients with normal thymus or LFH with anti-AChR Ab or without both types of antibodies.

Inflammation and epstein-barr virus infection are common features of myasthenia gravis thymus: possible roles in pathogenesis

The thymus plays a major role in myasthenia gravis (MG). Our recent finding of a persistent Epstein-Barr (EBV) virus infection in some MG thymuses, combined with data showing that the thymus is in a proinflammatory state in most patients, supports a viral contribution to the pathogenesis of MG. Aim of this study was to gain further evidence for intrathymic chronic inflammation and EBV infection in MG patients. Transcriptional profiling by low density array and real-time PCR showed overexpression of genes involved in inflammatory and immune response in MG thymuses. Real-time PCR for EBV genome, latent (EBER1, EBNA1, LMP1) and lytic (BZLF1) transcripts, and immunohistochemistry for LMP1 and BZLF1 proteins confirmed an active intrathymic EBV infection, further supporting the hypothesis that EBV might contribute to onset or perpetuation of the autoimmune response in MG. Altogether, our results support a role of inflammation and EBV infection as pathogenic features of MG thymus.

Thymoma and paraneoplastic myasthenia gravis

Paraneoplastic autoimmune diseases associate occasionally with small cell lung cancers and gynecologic tumors. However, myasthenia gravis (MG) occurs in at least 30% of all patients with thymomas (usually present at MG diagnosis). These epithelial neoplasms almost always have numerous admixed maturing polyclonal T cells (thymocytes). This thymopoiesis-and export of mature CD4(+)T cells-particularly associates with MG, though there are rare/puzzling exceptions in apparently pure epithelial WHO type A thymomas. Other features potentially leading to inefficient self-tolerance induction include defective epithelial expression of the autoimmune regulator (AIRE) gene and/or of major histocompatibility complex class II molecules in thymomas, absence of myoid cells, failure to generate FOXP3(+) regulatory T cells, and genetic polymorphisms affecting T-cell signaling. However, the strong focus on MG/neuromuscular targets remains unexplained and suggests some biased autoantigen expression, T-cell selection, or autoimmunization within thymomas. There must be further clues in the intriguing serological and cellular parallels in some patients with late-onset MG but without thymomas-and in others with AIRE mutations-and in the contrasts with early-onset MG, as discussed here.

Structural and Ultrastructural Localization of NGF and NGF Receptors in the Thymus of Subjects Affected by Myasthenia Gravis

We have previously reported that the thymus of patients affected by myasthenia gravis (MG) is characterized by an elevated level of nerve growth factor (NGF), an endogenous polypeptide which plays a marked role in the cell biology of nervous and immune system. A consistent number of studies has shown altered expression of NGF in diseases associated with inflammatory and/or autoimmune responses. To evaluate the biochemical and molecular mechanisms implicated in NGF action in human myasthenic thymus, it is important to identify the cellular and structural organization of NGF receptors. To address this question, we investigated, both at light and electron microscopic levels, the cellular distribution of immunoreactivity for NGF and its low-affinity receptors, (p75) and its high-affinity receptor (TrkA) in the thymus of patients with MG. The present investigation shows that NGF and NGF receptors are overexpressed in the thymic cells of patients with MG compared to control subjects.

Preferential expression of AChR epsilon-subunit in thymomas from patients with myasthenia gravis

The role of antigen expression by thymomas in myasthenia gravis (MG) is not clear. Previous reports of acetylcholine receptor (AChR) mRNA expression by the highly sensitive reverse transcription-polymerase chain reactions (RT-PCR) produced varying results. To try to clarify this issue, we first used RT-PCR but then turned to the more accurate and quantitative RNase protection assays (RPA) to assess AChR subunit mRNA expression in thymomas from 25 patients (22 with MG). By RT-PCR, all five AChR subunits could be detected in many thymomas. However, by RPA, the mRNA for the adult-specific AChR epsilon-subunit was found in 13/25 (52%) thymomas, but not mRNA for the other subunits. AChR epsilon-subunit was more frequently detected in thymomas of A or AB histology (WHO classification) than those with B1-B3 histology. Overall, 6/6 with thymomas of A or AB histology were positive compared with only 8/19 with B histology (p=0.02). Autoantibodies in the two patients with the highest levels of epsilon-subunit mRNA bound better to adult (alpha(2)betadeltaepsilon) AChR than to fetal (alpha(2)betadeltagamma) AChR, whereas the other sera bound better to fetal AChR. The greater abundance of mRNA for AChR epsilon-subunit than for other subunits suggests that the AChR epsilon-subunit may play a distinctive role in autosensitization in MG-associated thymomas, particularly those of type A or AB.

Reflections on the "intrathymic pathogenesis" of myasthenia gravis

The beneficial effects of thymectomy argue for a causal role of the thymus in myasthenia gravis (MG). The MG thymus contains acetylcholine receptor (AChR), which is expressed by myoid cells (whole AChR), and by medullary thymic epithelial cells (AChR subunits). The myoid cells are closely associated with antigen-presenting dendritic cells, helper T cells, and antibody-producing B cells in lymphoid follicles ("lymphofollicular hyperplasia"). Thus, all the cellular components required to initiate and maintain an autoimmune response to AChR are present in the MG thymus. It is unlikely that the cellular alterations in the thymus are secondary to an ongoing peripheral immune response, because they are absent in experimental autoimmune myasthenia gravis.

Myasthenia gravis: a long term follow-up study of Swedish patients with specific reference to thymic histology

BACKGROUND

Myasthenia gravis (MG) is an autoimmune disorder of neuromuscular transmission. The majority of patients show abnormal thymic histology.

SETTING

The study was performed at the Myasthenia Gravis Centre, Karolinska University Hospital, and at the Immunological Research Laboratory, Centre for Molecular Medicine, Karolinska Institutet, Stockholm, Sweden.

PATIENTS AND METHODS

Information was collected retrospectively from 1956 and prospectively from 1975 on clinical data, concomitant diseases, concentration of serum acetylcholine receptor antibodies (AChR-abs), immunosuppressive treatment (IS) and response to it, in 537 patients of whom 326 were thymectomised. Follow-up time was 1.5-50 years.

RESULTS

Age at onset of MG increased from a median age of 24 years before 1975 to a median age of 61 years after 2000. Thymoma was found in 65, hyperplasia (HPL) in 185 and a normal thymus in 76 patients. The trans-sternal surgical approach for thymectomy was used in 255 patients (78%). In five patients with thymoma, MG appeared after thymectomy. Of 537 patients, 466 (87%) had circulating AChR-abs. IS was given to 300 (56%) patients, mostly those with thymoma (85%). In total, 441 patients (82%) showed an improvement. One-third of patients with HPL, a quarter of those with thymoma, one-fifth of those with a normal thymus and one-seventh of those not operated on went into remission.

CONCLUSION

The prognosis for the majority of patients with MG is favourable, irrespective of thymic histology. The cause may be the use of immunomodulating therapy.

Antigen-specific T-cell activation in hyperplastic thymus in myasthenia gravis

In order to determine whether antigen-specific T-cell activation by dendritic cells (DCs) is accelerated in thymuses exhibiting lymphofollicular hyperplasia (TLFH) among patients with early-onset myasthenia gravis (EOMG), we investigated the expression levels of phosphorylated protein kinase C (PKC)theta and the local relationship between the presence of phosphorylated PKCtheta and the homing receptor CD44 or CD83, a marker for mature DCs, in samples taken from EOMG patients with early improvement following thymectomy, in remnant thymuses from late-onset MG patients, and in non-MG control thymuses. Antigen-specific T-cell activation was markedly accelerated in TLFH from EOMG patients. Activated T cells and adjacent DCs appeared to be components of a CD44(high) cell population circulating from the blood to the thymus. Although there is no convincing evidence that thymectomy is of benefit in MG, in some EOMG patients with early improvement following thymectomy, blockade of CD44-associated circulation mechanisms is probably the cause for the early benefits of thymectomy and is a potential alternative to thymectomy.

Autoimmune Channelopathies and Related Neurological Disorders

Ion channels are crucial elements in neuronal signaling and synaptic transmission, and defects in their function are known to underlie rare genetic disorders, including some forms of epilepsy. A second class of channelopathies, characterized by autoantibodies against ligand- and voltage-gated ion channels, cause a variety of defects in peripheral neuromuscular and ganglionic transmission. There is also emerging evidence for autoantibody-mediated mechanisms in subgroups of patients with central nervous system disorders, particularly those involving defects in cognition or sleep and often associated with epilepsy. In all autoimmune channelopathies, the relationship between autoantibody specificity and clinical phenotype is complex. But with this new information, autoimmune channelopathies are detected and treated with increasing success, and future research promises new insights into the mechanisms of dysfunction at neuronal synapses and the determinants of clinical phenotype.

Successful treatment of MuSK antibody-positive myasthenia gravis with rituximab

We report on a 56-year-old woman with muscle-specific receptor tyrosine kinase (MuSK) antibody-positive myasthenia with predominant bulbar symptoms and respiratory insufficiency. Conventional immunosuppression (prednisolone, azathioprine, mycophenolate mofetil) could not maintain the clinical improvement initially achieved by repeated plasma exchanges. Therefore, treatment with rituximab was initiated. After 2 months of rituximab treatment, remarkable clinical improvement correlating with a reduction of MuSK serum antibodies was seen. The patient continued to remain stable 12 months after initiation of therapy. This case report demonstrates that rituximab may be an effective and tolerable treatment in MuSK antibody-positive myasthenia gravis.

[Physiological and pathological autoimmunity in neurological disease]

INTRODUCTION

Over the last years, the functions of the immune system have radically been revised. It has been illustrated how the different responses of the innate defense and the adaptative system were intimately intricated.

STATE OF KNOWLEDGE

Innate immunity is an elaborate strategy, switched on by an invariant receptor-based response that is able to develop a specific defense against some pathogens. Moreover, this innate immunity governs T- and B-cell-dependent adaptive immune response, mediated via dendritic cells whose maturation is controlled by immune specificity. The concepts of autoreactivity have also strongly progressed and the functions of the physiological autoimmune response have been highlighted. The notion of T- and B-cell self-antigens not only shapes the immune repertoire, but also the self-recognition process which is a tool for the control of the immune response itself.

PERSPECTIVE

New concepts of the pathophysiology of autoimmune diseases have emerged from a better understanding of the immune response, balancing between an intrinsic deregulation of the immune system and system deficiency to mount an effective response against an initial injury.

CONCLUSION

Of course, therapeutic strategies are challenged by these data. We should expect that control of several autoimmune processes will be achieved in a few years, e.g. in rheumatoid arthritis or Crohn's disease controlled with biotherapies.

Increased toll-like receptor 4 expression in thymus of myasthenic patients with thymitis and thymic involution

Thymic abnormalities are present in approximately 80% of myasthenia gravis (MG) patients, and the thymus seems to be the main site of autosensitization to the acetylcholine receptor. In view of findings that the innate immune system can generate an autoimmune response, we studied the expression of Toll-like receptors (TLRs) 2 to 5, key components of innate immunity signaling pathways, in 37 thymuses from patients with autoimmune MG. TLR4 mRNA levels were significantly greater in thymitis (hyperplasia with diffuse B-cell infiltration) and involuted thymus than in germinal center hyperplasia and thymoma. By immunohistochemistry and confocal microscopy, cells positive for TLR4 protein were rarely detected in thymoma. However, in thymitis TLR4 protein was mostly found on epitheliomorphic (cytokeratin-positive) cells located in close association with clusters of acetylcholine receptor-positive myoid cells in thymic medulla and also at the borders between cortical and medullary areas. B cells were never TLR4-positive. TLR4 protein was also present in remnant tissue of involuted thymus. This is the first finding of a possible link between innate immunity and MG. We speculate that in a subgroup of MG patients, an exogenous or endogenous danger signal may activate the innate immune system and give rise to TLR4-mediated mechanisms contributing to autoimmunity.

Molecular mimicry may contribute to pathogenesis of ulcerative colitis

Ulcerative colitis (UC) is a chronic inflammatory bowel disease with mucosal inflammation and ulceration of the colon. There seems to be no single etiological factor responsible for the onset of the disease. Autoimmunity has been emphasized in the pathogenesis of UC. Perinuclear anti-neutrophil cytoplasmic antibodies (pANCA) are common in UC, and recently two major species of proteins immunoreactive to pANCA were detected in bacteria from the anaerobic libraries. This implicates colonic bacterial protein as a possible trigger for the disease-associated immune response. Autoantibodies and T-cell response against human tropomyosin isoform 5 (hTM5), an isoform predominantly expressed in colon epithelial cells, were demonstrated in patients with UC but not in Crohn's colitis. We identified two bacterial protein sequences in NCBI database that have regions of significant sequence homology with hTM5. Our hypothesis is that molecular mimicry may be responsible for the pathogenesis of UC.

Myasthenia gravis: clinical, immunological, and therapeutic advances

We give an update on clinical, immunological, and therapeutic advances in the field of myasthenia gravis, including a summary of suggested therapeutic recommendations.

Thymic myoid cells as a myasthenogenic antigen and antigen-presenting cells

We investigated immune property of a myoid cell line, established from Fisher rat thymus. Immunization of syngeneic rats with the myoid cells induced anti-rat acetylcholine receptor (AChR). Implantation of them into the thymus failed to induce typical thymic pathology of human myasthenia gravis (MG) or anti-AChR responses. We also demonstrated that the myoid cells were able to present exogenous antigens to T cells and induce antigen-specific T cell proliferation. These results suggest that myoid cells have the potential antigenicity to induce anti-AChR and the functions of antigen-presenting cells, but their expansion in the thymus may not directly cause MG.

Incidence of myasthenia gravis in the province of Ferrara: a community-based study

BACKGROUND

The reported annual incidence of myasthenia gravis (MG) ranges from 0.25 to 15 per million. The sex- and age-related pattern of disease incidence is still debated.

METHODS

An intensive descriptive study was performed in the province of Ferrara (mean population 360,950 people) over the period 1985 through 2000.

RESULTS

The average crude annual incidence rate was 2 per 100,000. We confirm a female preponderance in the total population, particularly in the youngest age groups.

CONCLUSIONS

We observed an early increase in incidence in females, partly due to thymoma-associated MG, while MG without thymoma showed increasing incidence with age nonsignificantly different in the two sexes.

Clinical and pathologic predictors of outcome in Thymoma-Associated myasthenia gravis

BACKGROUND

Myasthenia gravis is by far the most common paraneoplastic syndrome of thymomas. There is little information regarding the influence of clinical variables and thymoma-associated factors on biologic development of myasthenia gravis. The aim of the study was to determine independent predictors of clinical outcome in thymoma with myasthenia gravis.

METHODS

We studied 108 patients with thymoma-associated myasthenia gravis undergoing removal of the mediastinal mass between 1967 and 2000. Clinical and pathologic variables associated with clinical outcome of myasthenia were assessed by multivariate Cox regression analysis.

RESULTS

Patients were followed for a mean period of 10 years (9 months to 33 years). A total of 38 patients died (35.2%), in 14 cases (37%) because of myasthenia gravis and in 6 (16%) because of recurrence of thymoma. With respect to clinical outcome of myasthenia gravis, at the end of the follow-up period, the rate of remission was 16% (n = 17). Of the 91 patients in whom remission was not achieved, 55 had no symptoms with immunosuppressive medication and 36 had symptoms with medication.

CONCLUSIONS

In patients with thymoma-associated myasthenia gravis, well-differentiated thymic carcinoma (Müller-Hermelink system), age more than 55 years, and interval from the onset of symptoms to thymectomy of less than 1 year were found to be independent predictors of nonremission of myasthenia gravis after thymectomy.

The Role of Thymomas in the Development of Myasthenia Gravis

Thymic pathology occurs in 80-90% of myasthenia gravis patients. Significant associations between different thymic alterations and clinical findings are discussed. To highlight peculiarities in thymoma-associated myasthenia gravis, we briefly review myasthenia gravis associated with thymic lymphofollicular hyperplasia (TFH) and thymic atrophy.

The role of antibodies in myasthenia gravis

Myasthenia gravis is an autoimmune disease associated with antibodies directed to the postsynaptic acetylcholine receptor. These antibodies reduce the number of receptors. Autoantibodies against AChR and other muscle antigens can be used for the diagnosis of myasthenia gravis and related disorders. The origin and the role of these antibodies in the disease are discussed. Experimental autoimmune myasthenia gravis, an experimental model closely mimicking the disease, has provided answers to many questions about the role of antibodies, complement macrophages and AChR anchor proteins. Genetically modified anti-AChR antibodies may also be used in the future to treat myasthenia.

Correlation between tumor angiogenesis and invasiveness in thymic epithelial tumors

OBJECTIVE

Because it is difficult to predict the behavior of thymomas on the basis of morphology alone, other methods for determining tumor aggressiveness must be explored. This study investigated the correlation between angiogenic grade and invasiveness in thymic epithelial tumors.

METHODS

Immunohistochemical studies of 46 surgically resected thymic epithelial tumors (18 noninvasive thymomas, 20 invasive thymomas, and 8 thymic carcinomas) were conducted. To highlight the microvessels, we used a specific monoclonal antibody against factor VIII. Expression of vascular endothelial growth factor and basic fibroblast growth factor was determined by using polyclonal antibodies.

RESULTS

Mean microvessel density readings for noninvasive thymomas, invasive thymomas, and thymic carcinomas were 4.6 +/- 3.2, 12.4 +/- 7.5, and 34.4 +/- 16.7, respectively. Stages I, II, III, and IV of thymoma had microvessel density readings of 4.6 +/- 3.2, 8.5 +/- 4.3, 13.8 +/- 7.7, and 22.0 +/- 6.8, respectively. These findings suggest a significant correlation between microvessel density and tumor invasiveness. Vascular endothelial growth factor expression in noninvasive thymomas, invasive thymomas, and thymic carcinomas was present in 1 (5.6%) of 18, 11 (55.0%) of 20, and 5 (62.5%) of 8 patients, respectively, thereby indicating a distinct association between vascular endothelial growth factor expression and increased microvessel density. Basic fibroblast growth factor expression was present in only 8 patients.

CONCLUSIONS

In patients with thymic epithelial tumors, there appears to be a significant correlation between tumor angiogenesis and invasiveness. Furthermore, our data suggests that this angiogenesis in thymic epithelial tumors might be, at least in part, dependent on vascular endothelial growth factor expression.

Titin and ryanodine receptor epitopes are expressed in cortical thymoma along with costimulatory molecules

Cortical-type thymomas are associated with myasthenia gravis (MG) in 50% of the cases. MG is caused by antibodies against the acetylcholine receptors (AChR), but additional non-AChR muscle autoantibodies such as those against titin and ryanodine receptor (RyR) are found in up to 95% of MG patients with thymoma. To elucidate the induction of non-AChR autoantibodies in thymoma-associated MG, we studied cortical-type thymomas from seven thymoma MG patients, and sera from six of them. All six had titin antibodies, and four had RyR antibodies. Titin and RyR epitopes were co-expressed along with LFA3 and B7 (BB1) costimulatory molecules on thymoma antigen-presenting cells (APC) in all thymomas. In normal thymus, the staining by anti-titin, anti-RyR, anti-LFA3, and anti-BB1 antibodies was weak and occurred exclusively in the medulla and perivascularly. Our results indicate a primary autosensitization against titin and RyR antigens inside the thymoma. In MG-associated thymoma, the mechanisms involved in the initial autosensitization against titin and RyR are probably similar to those implicated in the autosensitization against AChR. In all cases, there is an overexpression of muscle-like epitopes and costimulatory molecules indicating that the T-cell autoimmunization is actively promoted by the pathogenic microenvironment inside the thymoma.

Thymic lesions in patients with myasthenia gravis: characterization with thallium 201 scintigraphy

PURPOSE

To assess thallium 201 ((201)Tl) single photon emission computed tomography (SPECT) for evaluation of thymic lesions associated with myasthenia gravis (MG), including lymphoid follicular hyperplasia (LFH) and thymoma.

MATERIALS AND METHODS

(201)Tl SPECT and computed tomography (CT) were performed preoperatively in 46 patients with MG who had undergone thymectomy. SPECT was conducted 15 (early image) and 180 (delayed image) minutes after (201)Tl injection. Results were visually assessed, and (201)Tl uptake ratios (thymic lesion count density/lung count density) were measured for quantitative analysis. Uptake was analyzed among the normal thymus, LFH, and thymoma patient groups.

RESULTS

Histopathologic results indicated a normal thymus, LFH, and thymoma in 19, 16, and 11 patients, respectively. Mean uptake ratios in the normal thymus, LFH, and thymoma were 0.96 (95% CI: 0.90, 1.03), 1.14 (95% CI: 1.04, 1.25), and 1.87 (95% CI: 1.56, 2.25), respectively, on early images and 1.09 (95% CI: 1.00, 1.18), 1.65 (95% CI: 1.48, 1.85), and 2.03 (95% CI: 1.65, 2.50), respectively, on delayed images. Thymoma showed more intense (201)Tl accumulation than did the normal thymus (P <.001) and LFH (P <.001) on early images. Both thymoma (P <.001) and LFH (P <.001) displayed more intense uptake than did the normal thymus on delayed images.

CONCLUSION

(201)Tl SPECT can enable differentiation between normal thymus, LFH, and thymoma in patients with MG.

Thymic malignancies

Thymomas and thymic carcinomas are unique tumors of the anterior mediastinum. The association of a variety of different paraneoplastic syndromes with such lesions has fascinated physicians and researchers for years. Most recently, it has been demonstrated by numerous authors that thymomas are chemosensitive tumors. Their indolent nature and relative rarity have made evaluation through prospective randomized clinical trials extremely difficult. Further information regarding the molecular nature of these neoplasms and immunologic aspects is needed in future investigation.

EGF regulates a complex pattern of gene expression and represses smooth muscle differentiation during the neurotypic conversion of the neural-crest-derived TC-1S cell line

EGF, known to sustain CNS neuronal progenitors, also promotes a neurotypic response in the thymic neural-crest-derived TC-1S cell line. We report here the use of TC-1S cells as a model to identify the genetic programs regulated during the neurotypic response induced by EGF and to isolate 23 EGF-responsive genes. Among them 5 represent novel cDNAs, while 18 are known genes, whose regulation by EGF is associated with the mitogenic or differentiating effects of the growth factor. The repression of smooth muscle alpha-actin and SM22alpha genes by EGF and their increase by TGFbeta suggest that the TC-1S line includes neural crest multipotent cells whose smooth muscle differentiation is repressed upon EGF treatment and stimulated by TGFbeta. Therefore, we identified a complex pattern of EGF-target genes and propose EGF as a novel signal able to recruit postmigratory neural-crest-derived cells along proliferation and cell lineage choice pathways.

T cell recognition of muscle acetylcholine receptor in ocular myasthenia gravis

We examined the proliferative response of blood CD4(+) cells to muscle acetylcholine receptor (AChR) subunits and the epitope repertoire of the epsilon and gamma subunits, in ocular myasthenia gravis (oMG) patients and healthy subjects. oMG patients seldom recognized all subunits. The frequency and intensity of recognition was the same for all subunits, irrespective of the disease duration. The responses in oMG were lower than in generalized myasthenia gravis. Healthy subjects had frequent, low responses to one or more subunits. oMG patients recognized several epitopes on the gamma and epsilon subunits, that partially overlapped those recognized in gMG. The subunits and epitopes recognized by individual oMG patients changed over time. Thus, oMG patients have minimal and unstable sensitization of anti-AChR CD4(+) cells, in agreement with their low and inconsistent synthesis of anti-AChR antibody.

Recurrent genetic aberrations in thymoma and thymic carcinoma

Apart from single reported aberrant karyotypes, genetic alterations in thymic epithelial neoplasms have not been investigated so far. In this study, 12 World Health Organization classification type A thymomas (medullary thymomas), 16 type B3 thymomas (well-differentiated thymic carcinomas), and nine type C thymomas, all of them primary thymic squamous cell carcinomas, were analyzed by comparative genomic hybridization and fluorescence in situ hybridization. With the exception of one single case, type A thymomas did not reveal chromosomal gains or losses in comparative genomic hybridization. In contrast, all type B3 thymomas showed chromosomal imbalances, with gain of 1q, loss of chromosome 6, and loss of 13q occurring in 11 (69%), six (38%), and five (31%) of 16 cases, respectively. In primary thymic squamous cell carcinoma, the most frequent chromosomal losses were observed for 16q (six of nine cases, 67%), 6 (4 of 9, 44%), and 3p and 17p (three of nine each, 33%), whereas recurrent gains of chromosomal material were gains of 1q (5 of 9, 56%), 17q, and 18 (three of nine each, 33%). This study shows that the distinct histological thymoma types A and B3 exhibit distinct genetic phenotypes, whereas type B3 thymoma and primary thymic squamous cell carcinoma partially share genetic aberrations. In addition to the possible tumorigenic role, the deletion in type B3 thymoma of chromosome 6, harboring the HLA locus, might play a role in the pathogenesis of paraneoplastic autoimmunity characteristic of thymoma.

Oral administration of an immunodominant T-cell epitope downregulates Th1/Th2 cytokines and prevents experimental myasthenia gravis

The mucosal administration of the native antigen or peptide fragments corresponding to immunodominant regions is effective in preventing or treating several T cell-dependent models of autoimmune disease. No data are yet available on oral tolerance with immunodominant T-cell peptides in experimental autoimmune myasthenia gravis (EAMG), an animal model of B cell-dependent disease. We report that oral administration of the T-cell epitope alpha146-162 of the Torpedo californica acetylcholine receptor (TAChR) alpha-subunit suppressed T-cell responses to AChR and ameliorated the disease in C57Bl/6 (B6) mice. Protection from EAMG was associated with reduced serum Ab's to mouse AChR and reduced AChR loss in muscle. The effect of Talpha146-162 feeding was specific; treatment with a control peptide did not affect EAMG manifestations. The protective effect induced by peptide Talpha146-162 was mediated by reduced production of IFN-gamma, IL-2, and IL-10 by TAChR-reactive cells, suggesting T-cell anergy. TGF-beta-secreting Th3 cells did not seem to be involved in tolerance induction. We therefore demonstrate that feeding a single immunodominant epitope can prevent an Ab-mediated experimental model of autoimmune disease.

From basic immunobiology to the upcoming WHO-classification of tumors of the thymus. The Second Conference on Biological and Clinical Aspects of Thymic Epithelial Tumors and related recent developments

The Second Conference on Biological and Clinical Aspects of Thymic Epithelial Tumors in Leiden, The Netherlands, 1998, set the stage for an interdisciplinary meeting of immunologists, pathologists and members of various clinical disciplines to exchange their recent findings in the field of thymus-related biology, pathology, and medicine. The contributions covered such diverse subjects as the role of transcription factors and cytokines in the development of the thymic microenvironment, thymic T, B and NK cell development, the pathogenesis of myasthenia gravis and other thymoma-associated autoimmunities, the pathology of thymic epithelial tumors and germ cell neoplasms, and new approaches to their diagnosis and treatment. This editorial will briefly sum up the data presented at the Conference and will comment on related novel findings that have been reported since then. Because it was also at the Leiden Conference, that the proposal of the WHO committee for the classification of thymic tumors was discussed for the first time, a description of the upcoming WHO Classification of Tumors of the Thymus is given with emphasis on the diagnostic criteria of thymic epithelial tumors, that should now be termed as type A, AB, B1-3 and type C thymomas, to make pathological and clinical studies comparable in the future.

Neurofilament is an autoantigenic determinant in myasthenia gravis

Intratumorous expression of a 153-kd protein (p153), which contains an acetylcholine receptor-like epitope, is the only tumor marker described to date that significantly associates with thymoma in paraneoplastic myasthenia gravis (MG). Here, we report that p153 is identical to the midsize neurofilament, as verified by immunohistochemistry, immunofluorescence, and western blot analysis. Furthermore, the acetylcholine receptor-like epitope of the midsize neurofilament (NF-M) was identified by peptide epitope mapping. We also show, using T-cell proliferation assays, a significantly increased response of intratumorous T cells to a recombinant midsize neurofilament fragment in thymoma patients with MG compared with MG patients with thymic follicular hyperplasia or thymoma patients without MG. The T cells of thymic follicular hyperplasia and thymoma patients without MG seem to be unresponsive to NF-M. In contrast, we found increased T-cell responses to recombinant acetylcholine receptor fragments in MG patients in general compared with non-MG patients. Increased T-cell responses to NF-M in patients with paraneoplastic MG might be the result of an abnormal positive selection of immature T cells within thymomas, caused by the expression of NF-M in neoplastic thymic epithelial cells. Our results offer further evidence that NF-M expression in thymomas is an autoantigenic determinant in MG.

Thymoma: state of the art

Thymoma is the most common tumor of the anterior mediastinum. This tumor is associated with unique paraneoplastic syndromes, such as myasthenia gravis, hypogammaglobulinemia, and pure red cell aplasia. The rarity of this tumor, however, has somewhat obscured the optimal treatment for this disease. For the majority of patients who present with localized tumor, surgical extirpation remains the standard of choice. Adjuvant radiotherapy seems to improve local control and survival. In more advanced disease, systemic therapy has been demonstrated to produce a 50% to 80% objective response rate. These observations have led to the development of multimodality therapy for the treatment of patients with advanced thymoma. In this article, we will review the current perspectives on the management of early stage and advanced thymoma.