Surg Today (2010) 40:102–107 DOI 10.1007/s00595-009-4134-2
Biological Implications of Thymectomy for Myasthenia Gravis MEINOSHIN OKUMURA, MASAYOSHI INOUE, YOSHIHISA KADOTA, AKIO HAYASHI, TOSHITERU TOKUNAGA, TAKASHI KUSU, NORIYOSHI SAWABATA, and HIROYUKI SHIONO Department of General Thoracic Surgery, Osaka University Graduate School of Medicine, (L-5), 2-2 Yamada-oka, Suita, Osaka 565-0871, Japan
Abstract 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 antiAchR 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 selfreactive 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. Key words Autoimmune disease · Paraneoplastic autoimmunity · Thymus · Thymoma · Acetylcholine receptor
Reprint requests to: M. Okumura Received: January 6, 2009 / Accepted: February 10, 2009
Introduction Myasthenia gravis (MG) is a relatively rare disease characterized by fatigable muscle weakness, the essential pathogenesis of which has been shown to be autoimmunity mediated by autoantibodies to the striated muscle tissue. The nicotinic acetylcholine receptor (AchR) is now known to be the major auto-antigen in MG,1–3 and the disease is recognized as a prototype of organ-specific autoimmune diseases. In 1939, Blalock et al. reported the remission of MG after removing a thymoma,4 following which frequent associations of thymic abnormalities in MG patients suggested the involvement of the thymus in the pathogenesis of this disease.5–7 Furthermore, thymectomy has been reported to result in remission and palliation of symptoms in many MG patients.8 The oncologic implications of resecting the thymus and an associated thymoma are understood; however, the role of thymectomy for treating patients with nonthymomatous MG is still unclear, as the perceived effectiveness of that procedure is largely based on retrospective reviews of clinical experience. Since thymectomy is more frequently indicated in young patients with generalized symptoms, a reduction in MG symptoms after surgery might only reflect differences between the surgical and nonsurgical groups. Indeed, a recent meta-analysis of 28 English-based reports and 21 cohort studies did not conclusively support the benefits of thymectomy for nonthymomatous autoimmune MG because of the differences between the patients who underwent this procedure and those who did not.9 As a result, some researchers have advocated randomized clinical trials to define the implications of thymectomy, although a long-term study is needed for any definitive conclusions. This review focuses on the role of the thymus in the pathogenesis of MG, pathologically and immunologically, and attempts to define the biological implications of thymectomy performed for MG.
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Classification of MG and General Treatment Strategies Myasthenia gravis is heterogeneous in terms of the severity of symptoms, age of onset, association with thymoma, and the target of the autoantibody (Table 1). The symptoms are generally classified into two groups: ocular and general. Patients with the ocular type of MG may experience spontaneous remission, so it is generally managed with observation or, at most, treatment with a cholinesterase inhibitor. On the other hand, MG with generalized symptoms can progress to respiratory failure, and should be treated either by thymectomy or with medication such as steroids. Myasthenia gravis can manifest in all age groups, and the clinical characteristics are sometimes dependent on the age of onset. Typically, newborn infants delivered from mothers with MG have only transient symptoms until the anti-AchR antibodies transferred through the placenta spontaneously diminish. Anti-AchR Ab belongs to the IgG class of antibodies. Because the halflife of IgG is generally 2–3 weeks, these infants show MG symptoms for only a short period of time. Juvenile type MG, which typically appears in the first decade of life, often manifests as ocular symptoms. These patients should be treated actively because ocular symptoms such as diplopia can result in impaired eyesight development. It is thought that the thymus begins functioning actively in the first or second decade of life to establish the immune system; thus, the indications for thymectomy should be considered carefully. For many patients with persistent symptoms, steroid therapy is the treatment of choice. Thymoma is frequently associated with MG in adults and resection via an extended thymectomy is strongly recommended for these patients, irrespective of the severity of symptoms or age of onset. It also serves to treat the neoplasm.
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Several autoantibodies associated with MG have been identified, with anti-AchR Ab considered the most important. Most MG patients have anti-AchR Ab, and all thymomatous MG patients have it. Accordingly, anti-AchR Ab-negative MG constitutes a small percentage of nonthymomatous MG patients. Recently, the antibody against muscle-specific kinase (MuSK) was identified in anti-AchR Ab-negative patients10 and a retrospective study revealed that thymectomy had no effect on patients with the anti-MuSK antibody (antiMuSK Ab).11 Notably, MG was not associated with a thymoma or thymic hyperplasia in those anti-MuSK Ab-positive patients. Although it was thought that the anti-MuSK Ab might explain the pathogenesis of all anti-AchR Ab-negative MG, a few patients have been found to be both anti-AchR Ab-negative and antiMuSK Ab-negative (so-called double-negative). Therefore, additional investigations are required to reveal another autoantigen associated with MG. Adult MG patients are divided into two groups according to the age of onset: those with an onset age of younger than 40 years (early-onset MG; EOMG) and those with an onset age of over 40 years (late-onset MG; LOMG). A number of studies have revealed the effectiveness of thymectomy for EOMG, but not for LOMG, which might be attributed to morphological and functional differences in the thymus. Accumulated clinical experience suggests the following general guidelines for the indications of surgery for MG. For patients with a thymoma, surgical treatment is absolutely indicated, to resect the neoplasm as well treat the MG. In nonthymomatous AchR Ab-positive patients, the indications for thymectomy depend on several factors, including age, duration of symptoms, and association with thymic hyperplasia. Thymectomy is not recommended for anti-AchR Ab-negative/antiMUSK Ab-positive patients, although its indication for double-negative patients remains controversial.
Table 1. Classification of myasthenia gravis (1) Symptoms Ocular General (2) Age of onset Newborn Juvenile Adult Early-onset (≤40 years old) Late-onset (>40 years old) (3) Thymoma Not associated Associated (4) Autoantibody Anti-acetylcholine receptor (AchR) antibody-positive Anti-AchR-negative Anti-muscle specific kinase (MuSK) antibody-positive Anti-MuSK antibody-negative
Thymic Pathology and Biological Interpretation in Anti-AchR Antibody-Positive MG Patients Without a Thymoma Nonthymomatous MG is classified according to the presence or absence of the anti-AchR antibody. In this section, we focus on the thymic pathology of anti-AchR positive MG. The macroscopic appearance of the thymus in MG patients varies greatly from involution to hyperplasia, although the most prominent characteristics in nonthymomatous MG are medullary hyperplasia with the abundant formation of germinal centers. The germinal center has been shown to be the site of B-cell differentiation, in which B cells recognize a specific antigen and undergo somatic hypermutation by
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interacting with T cells and antigen-presenting cells, then finally differentiating into cells producing the antibody with a higher affinity. Previous investigators found that B cells recovered from the thymus of MG patients secrete anti-AchR Ab.12,13 More importantly, the proportion of anti-AchR Ab in the entire immunoglobulin produced by these thymus-derived B cells was found to be greater than that from B cells derived from the peripheral blood or bone marrow of the same patients.14 Thus, the specific differentiation of B cells producing anti-AchR Ab in the MG thymus is suggested. Furthermore, B cells in some, but not all, of the germinal centers in MG thymi recognized AchR and somatic hypermutation. Consequently, convergent mutations in the complementarity determining region also occurred in the germinal centers of MG thymi.15 The mechanism of B-cell differentiation in the thymus, including the source of AchR and the ontogeny of T cells specific for AchR, remains unclear. Myoid cells are known to reside in the thymus and have been suspected to present AchR as an antigen; however, since myoid cells do not express HLA-DR at a high level, they are not considered to function as antigenpresenting cells. Interestingly, in EOMG thymi, myoid cells were found to be in contact with the germinal center at the site of laminin breakdown, suggesting that AchR derived from myoid cells is expressed by follicular dendritic cells in the germinal center.16 Thus, it is thought that the differentiation of B cells specific for AchR takes place in the thymus of MG patients. We also found previously that Bcl-2 protein was not fully downregulated in B cells in the germinal centers of thymi in MG patients.17 This observation might indicate the differentiation of autoreactive B cells, rather than their elimination. Considering that the thymus is the central organ of T-cell development, where T-cell tolerance is established through positive and negative selections of a repertoire of T-cell antigen receptors, the pathogenic role of B-cell differentiation in the thymi of MG patients is hard to understand. Our previous results showed that the CD4+ T-lymphocyte (helper T-cell) subset in the thymus was correlated with the anti-AchR Ab titer.18 This observation seems compatible with the above notion, since antibody production requires interactions with helper T cells. This raises another important question: how are T cells reactive to AchR, which is an autoantigen? A recent study revealed that a specific variant of the promoter of the AchR α subunit causes abrogation of promoter activity and a bi-allelic variant of this promoter was identified in EOMG patients.19 The function of the autoimmune regulator (AIRE) protein was also implicated in regulation of the expression level of the AchR α subunit in thymic medullary
M. Okumura et al.: Thymectomy for Myasthenia Gravis
epithelial cells.19 Thus, a reduction in, or absence of AchR α subunit expression in thymic medullary epithelial cells may lead to insufficient negative selection of the T-cell repertoire and trigger the onset of MG. However, if a specific genetic abnormality is the principal cause of MG, the age of onset should be much younger. Therefore, this hypothesis alone is not sufficient to explain adult onset MG or the prevalence of the disease in females. Although the ontogeny of helper T cells that recognize AchR and the mechanisms that activate autoreactive T and B cells have not been elucidated, the notion that the differentiation of B cells producing high-affinity anti-AchR Ab occurs in the thymus of MG patients is plausible, and may provide a clue to the biological implications of thymectomy in those patients. As expected, quantitative evaluation of lymphocytes in the thymus revealed that the anti-AchR Ab titer was reduced following thymectomy in correlation with the abundance of germinal center B cells (CD19+CD38+ cells).20 This suggests that thymectomy eliminates the site of anti-AchR Ab production. It is also important to consider why it often takes months or even years after a thymectomy for the antiAchR Ab titer to be reduced to half the preoperative value, as the half-life of IgG class antibodies in serum is only 2–3 weeks. If the thymus is the exclusive site of anti-AchR Ab production, the anti-AchR Ab titer is thought to decrease to half of the preoperative value within a few weeks after thymectomy. A possible explanation is that anti-AchR Ab is produced not only in the thymus but also in other organs. Since the typical thymus abnormality in MG patients is abundant germinal centers, the thymus may function to provide anti-AchR Ab-producing cells to other organs. If this is true, then thymectomy should terminate anti-AchR Ab antibody production by the thymus, but not by other parts of the body, while also halting the provision of high-affinity anti-AchR Ab-producing B cells to peripheral organs. Thus, a thymectomy may only bring about a gradual decline in the anti-AchR Ab titer according to reduction in the total number of anti-AchR Ab-producing cells in other organs. Figure 1 presents an illustration of this hypothetical model.
Thymic Pathology in Anti-AchR Ab-Negative MG Patients Without a Thymoma Nearly all MG patients with a thymoma have anti-AchR Ab; thus, anti-AchR Ab-negative MG patients are categorized into the nonthymomatous group. This patient population is further divided into two groups depending on the presence of the anti-MuSK Ab, although interestingly, anti-MuSK Ab was shown to be unrelated to
M. Okumura et al.: Thymectomy for Myasthenia Gravis Anti-AchR antibody Germinal center
High affinity anti-AchR Ab producing cell
Production of high affinity anti-AchR antibody
(2)
bone marrow, lymph nodes, and blood
105 Autoreactive T cells are provided to the peripheral organs Thymoma
Anti-AchR Antibody production
Activation
Helper T cell specific for AchR
B cell producing anti-AchR antibody
(1) Thymectomy
Fig. 1. Hypothetical role of the thymus in myasthenia gravis patients without a thymoma. Thymectomy terminates antiacetylcholine receptor (anti-AchR) antibody (Ab) production by the thymus and the provision of high affinity anti-AchR antibody-producing cells to peripheral organs
thymic hyperplasia.21 As MG patients with the antiMuSK Ab do not benefit from thymectomy, there is consensus that thymectomy is not indicated for antiMuSK Ab-positive MG patients. On the other hand, the indications for thymectomy in double-negative MG patients remain unclear. We previously investigated the thymic pathology of patients in this group and found thymic germinal centers in some, but not all. In addition, double-negative MG patients with thymic germinal centers experienced a reduction in symptoms.22 Thus, although thymectomy might be considered as an option, the autoantigen remains to be identified.
Immunological Functions of Thymoma Thymoma arises from the epithelial cells of the thymus and seems to retain thymic cortical epithelial function to induce T-cell differentiation, according not only to our review but also to that by another group.7,23 Thymomas with type AB, B1, and B2 World Health Organization histological classification have a structure similar to that of the cortex of a normal thymus, although they might lack normal mechanisms for selection of the Tcell repertoire. It stands to reason that autoreactive T cells possibly emerging in a thymoma could trigger autoimmune disorders; however, this does not explain the extraordinarily high incidence of MG in people with a thymoma. Other important questions arise when we consider where and how AchR-specific T cells activate AchR-specific B cells and induce the differentiation of high-affinity Ab-producing cells. Thus, the above speculation is just one part of the development of MG associated with a thymoma, although it could provide a biological reason for thymoma removal, which is thought to terminate the supply of autoreactive T cells. This proposed model is illustrated in Fig. 2. Our speculation is supported by the finding that Tcell receptor excision circles (TRECs), a marker of T
Fig. 2. Hypothetical role of a thymoma in myasthenia gravis. A thymoma produces helper T cells specific for AchR and provides them to the periphery. These helper T cells activate B cells to produce anti-AchR antibodies
cells newly provided from the thymus, were more frequent in patients with a thymoma than in normal controls.24 Interestingly, the frequency of TRECs decreased after thymoma resection and increased when relapse occurred. Furthermore, B cells are extremely rare in thymomas, and the amount of anti-AchR Ab produced by B cells purified from a thymoma is inconsequential (unpublished observation), suggesting that the B-cell subset inside a thymoma is not responsible for the pathogenesis of MG. In addition to the alteration of T-cell tolerance in thymoma patients, the association of the antiinterleukin-12 and anti-interferon-α antibodies in these patients is also interesting.25,26 The precise role of these autoantibodies to specific cytokines is unclear; however, a function to modify Th1/Th2 balance or stimulate some T-cell subsets, resulting in the onset of autoimmunity, is possible. On the other hand, this notion does not explain the high incidence of the association of MG with thymoma.
Role of Immunosuppressive Therapy for MG Patients and Its Relationship with Surgical Treatment Immunosuppressants such as cyclosporine A and tacrolimus (FK506) have recently been shown to be effective against MG.27,28 Thus, steroid therapy combined with immunosuppressant therapy might become the treatment of choice, which would reduce the indications for surgery. However, the long-term use of these drugs with steroids is associated with severe side effects, such as cardiovascular disease, diabetes mellitus, and renal dysfunction. Thus, the decision to use immunosuppressant therapy as primary treatment should be made with great care. On the other hand, advances in surgical techniques have enabled extended thymectomy to be performed via video-assisted thoracoscopic surgery, which is expected to achieve the consensus of a much less invasive procedure than the conventional approach through a median sternotomy.29,30
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In our experience, thymectomy has failed to relieve MG symptoms in only about 10% of patients, who have subsequently been managed with immunosuppressive therapy. The further development of minimally invasive thymectomy procedures will support the indications for surgical treatment prior to requirement for immunosuppressant therapy.
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4.
5. 6. 7.
Conclusions The biological implications of the thymus and presence of a thymoma in the initiation and development of MG have not been fully elucidated. Although the symptoms of the disease are almost identical, irrespective of the presence or absence of a thymoma, the pathogeneses seem to be different and should be investigated independently. In anti-AchR Ab-positive patients without a thymoma, abnormal germinal center formation in the thymus is thought to play an essential role in the pathogenesis of MG. Specific differentiation of B cells producing antiAchR Ab seems to take place uniquely in the thymus; thus, removal of the thymus is thought to contribute to terminating the provision of high-affinity anti-AchR Ab-producing cells to other organs, such as peripheral blood, lymph nodes, and bone marrow. Thymectomy is not indicated for anti-AchR Ab-negative MG patients who are positive for the anti-MuSK Ab. On the other hand, some anti-MUSK-negative patients have germinal centers in the thymus and are thought to be potentially benefited by the procedure. A thymoma can be considered as an acquired thymus with insufficient function of negative selection. Resection of a thymoma and the thymus should lead to termination of the production of self-reactive T cells by the thymoma, although further studies are urgently required. The ontogeny of T cells recognizing AchR and the mechanism of activating anti-AchR Abproducing B cells remain unclear; however, the biological role of the thymus, especially the germinal centers in the medulla, in MG has been partially revealed. Additional immunological studies of MG and the development of methods to evaluate the functions of the thymus will be helpful to establish solid indications for thymectomy.
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