An acetylcholine receptor alpha subunit promoter confers intrathymic expression in transgenic mice. Implications for tolerance of a transgenic self-antigen and for autoreactivity in myasthenia gravis

Prospects for a T-cell receptor vaccination against myasthenia gravis

T-cell receptor (TCR) vaccination has been proposed as a specific therapy against autoimmune diseases. It is already used in clinical trials, which are supported by pharmaceutical companies for the treatment of multiple sclerosis, rheumatoid arthritis and psoriasis. Current vaccine developments are focusing on enhancement of immunogenicity as well as selecting the best route of immunization and adjuvant to favor the therapeutic effect. In the meantime, academic laboratories are tackling the regulatory mechanisms involved in the beneficial effect of the vaccines to further understand how to control the therapeutic tool. Indeed, several examples in experimental models of autoimmune diseases indicate that any specific therapy may rely on a delicate balance between the pathogenic and regulatory mechanisms. This review presents a critical analysis of the potential of such therapy in myasthenia gravis, a prototype antibody-mediated disease. Indeed, a specific pathogenic T-cell target population and a TCR-specific regulatory mechanism mediated by anti-TCR antibodies and involved in protection from the disease have recently been identified in a patient subgroup. The presence of spontaneous anti-TCR antibodies directed against the pathogenic T-cells that may be boosted by a TCR vaccine provides a rationale for such therapy in myasthenia gravis. The development of this vaccine may well benefit from experience gained in the other autoimmune diseases in which clinical trials are ongoing.

Pathogenic immune mechanisms at the neuromuscular synapse: the role of specific antibody-binding epitopes in myasthenia gravis

Autoantibodies against three different postsynaptic antigens and one presynaptic antigen at the neuromuscular junction are known to cause myasthenic syndromes. The mechanisms by which these antibodies cause muscle weakness vary from antigenic modulation and complement-mediated membrane damage to inhibition of endogenous ligand binding and blocking of essential protein-protein interactions. These mechanisms are related to the autoantibody titre, specific epitopes on the target proteins and IgG autoantibody subclass. We here review the role of specific autoantibody-binding epitopes in myasthenia gravis, their possible relevance to the pathophysiology of the disease and potential implications of epitope mapping knowledge for new therapeutic strategies.

Thymectomy for non-thymomatous myasthenia gravis

BACKGROUND

Treatments currently used for patients with myasthenia gravis (MG) include steroids, non-steroid immune suppressive agents, plasma exchange, intravenous immunoglobulin and thymectomy. Data from randomized controlled trials (RCTs) support the use of some of these therapeutic modalities and the evidence for non-surgical therapies are the subject of other Cochrane reviews. Significant uncertainty and variation persist in clinical practice regarding the potential role of thymectomy in the treatment of people with MG.

OBJECTIVES

To assess the efficacy and safety of thymectomy in the management of people with non-thymomatous MG.

SEARCH METHODS

On 31 March 2013, we searched the Cochrane Neuromuscular Disease Group Specialized Register, CENTRAL (2013, Issue 3), MEDLINE (January 1966 to March 2013), EMBASE (January 1980 to March 2013) and LILACS (January 1992 to March 2013) for RCTs. Two authors (RS and GC) read all retrieved abstracts and reviewed the full texts of potentially relevant articles. These two authors checked references of all manuscripts identified in the review to identify additional articles that were of relevance and contacted experts in the field to identify additional published and unpublished data. Where necessary, authors were contacted for further information.

SELECTION CRITERIA

Randomized or quasi-randomized controlled trials of thymectomy against no treatment or any medical treatment, and thymectomy plus medical treatment against medical treatment alone, in people with non-thymomatous MG.We did not use measured outcomes as criteria for study selection.

DATA COLLECTION AND ANALYSIS

We planned that two authors would independently extract data onto a specially designed data extraction form and assess risk of bias; however, there were no included studies in the review. We would have identified any adverse effects of thymectomy from the included trials.

MAIN RESULTS

We did not identify any RCTs testing the efficacy of thymectomy in the treatment of MG. In the absence of data from RCTs, we were unable to do any further analysis.

AUTHORS' CONCLUSIONS

There is no randomized controlled trial literature that allows meaningful conclusions about the efficacy of thymectomy on MG. Data from several class III observational studies suggest that thymectomy could be beneficial in MG. An RCT is needed to elucidate if thymectomy is useful, and to what extent, in MG.

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

OBJECTIVE

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

METHODS

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

RESULTS

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

INTERPRETATION

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

Regulation of insulin gene expression by cytokines and cell-cell interactions in mouse medullary thymic epithelial cells

AIMS/HYPOTHESIS

The expression of tissue-specific self-antigens in the thymus is essential for self-tolerance. Genetic susceptibility to type 1 diabetes correlates inversely with thymic insulin expression and, in mice, lowered levels of this expression result in T cell responses against insulin. This study was undertaken to examine whether thymic insulin expression is regulated by the same metabolic stimuli as in beta cells or by different inputs, possibly of an immune nature.

METHODS

Ins2 mRNA changes in mouse thymus were evaluated in vivo, following intraperitoneal glucose injection. We also examined the effect of a high glucose concentration on Ins2 mRNA in clones of insulin-expressing medullary thymus epithelial cell lines (mTECs). The same in vitro system was used to evaluate the effect of IFN-gamma and cell-to-cell contact with thymocytes in co-culture.

RESULTS

Ins2 mRNA was significantly increased in the pancreas following a glucose load, but remained unchanged in the thymus. Furthermore, stimulation of insulin-expressing mTECs in vitro with IFN-gamma, a cytokine involved in T cell negative selection, decreased levels of insulin expression even though expression of Aire was increased. Last, co-culture of mTECs with thymocytes resulted in an upregulation of both Aire and insulin expression.

CONCLUSIONS/INTERPRETATION

We conclude that regulation of insulin transcription in the thymus is not dependent on metabolic stimuli but it may, instead, be under the control of cytokines and cell-to-cell interactions with lymphoid cells. That this regulation is not always coordinated with that of Aire, a non-specific master switch, suggests insulin-specific mechanisms.

Functional tolerance of CD8+ T cells induced by muscle-specific antigen expression

Skeletal muscles account for more than 30% of the human body, yet mechanisms of immunological tolerance to this tissue remain mainly unexplored. To investigate the mechanisms of tolerance to muscle-specific proteins, we generated transgenic mice expressing the neo-autoantigen OVA exclusively in skeletal muscle (SM-OVA mice). SM-OVA mice were bred with OT-I or OT-II mice that possess a transgenic TCR specific for OVA peptides presented by MHC class I or class II, respectively. Tolerance to OVA did not involve clonal deletion, anergy or an increased regulatory T cell compartment. Rather, CD4+ T cell tolerance resulted from a mechanism of ignorance revealed by their response following OVA immunization. In marked contrast, CD8+ T cells exhibited a loss of OVA-specific cytotoxic activity associated with up-regulation of the immunoregulatory programmed death-1 molecule. Adoptive transfer experiments further showed that OVA expression in skeletal muscle was required to maintain this functional tolerance. These results establish a novel asymmetric model of immunological tolerance to muscle autoantigens involving Ag ignorance for CD4+ T cells, whereas muscle autoantigens recognized by CD8+ T cells results in blockade of their cytotoxic function. These observations may be helpful for understanding the breakage of tolerance in autoimmune muscle diseases.

Discrete T cell populations with specificity for a neo-self-antigen bear distinct imprints of tolerance

Mice expressing the Torpedo acetylcholine receptor alpha-chain as a neo-self-Ag exhibit a reduced frequency of T cells responding to the immunodominant epitope Talpha146-162 indicating a degree of tolerance. We characterized tolerance induction in these animals by analyzing the residual Talpha146-162-responsive T cell population and comparing it to that of nontransgenic littermates. Using CD4(high) sorting, we isolated the vast majority of Ag-reactive T cells from both strains of mice. Quantitative studies of the CD4(high) populations in transgenic mice following immunization with Talpha146-162 revealed a diminished expansion of cells expressing the canonical TCRBV6 but not other TCRBV gene segments when compared with nontransgenic littermates. In addition, CD4(high) cells from transgenic mice were functionally hyporesponsive to Talpha146-162 in terms of proliferation and cytokine secretion regardless of TCRBV gene segment use. TCR sequence analysis of transgenic Vbeta6(+)CD4(high) cells revealed a reduced frequency of cells expressing a conserved motif within the TCRbeta CDR3. Thus, the canonical Talpha146-162 responsive, Vbeta6(+) population demonstrates both quantitative and qualitative deficits that correlate with an altered TCR repertoire whereas the non-Vbeta6 population in transgenic mice exhibits only a reduction in peptide responsiveness, a qualitative defect. These data demonstrate that discrete autoreactive T cell populations with identical peptide/MHC specificity in Torpedo acetylcholine receptor-alpha-transgenic animals bear distinct tolerance imprints.

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.

Functional defect of regulatory CD4(+)CD25+ T cells in the thymus of patients with autoimmune myasthenia gravis

Thymus-derived CD4(+)CD25+ regulatory T (Treg) cells are essential for the maintenance of immunologic self-tolerance. Despite their critical role in the active suppression of experimental autoimmune disorders, little is known about their involvement in human autoimmune diseases. Myasthenia gravis (MG) is a CD4+ T cell-dependent autoimmune disease and the thymus is assumed to be the initiation site. To identify possible defects in the Treg cells in MG, we analyzed CD4(+)CD25+ cells in thymi from patients with MG compared to those from healthy subjects. We found a normal CD4(+)CD25+ number but a severe functional defect in their regulatory activity together with a decreased expression of the transcription factor, Foxp3, which is essential for T-cell regulatory function. The phenotypic analysis of CD4(+)CD25+ thymocytes revealed an increased number of activated effector cells with strong Fas expression in patients with MG. However, whatever their level of Fas, CD4(+)CD25+ thymocytes from patients with MG remained unable to suppress the proliferation of responding cells, indicating that the impaired Treg cell function is not due to contamination by activated effector T cells. These data are the first to demonstrate a severe functional impairment of thymic Treg cells in MG, which could contribute to the onset of this autoimmune disease.

Self-representation in the thymus: an extended view

The thymus has been viewed as the main site of tolerance induction to self-antigens that are specifically expressed by thymic cells and abundant blood-borne self-antigens, whereas tolerance to tissue-restricted self-antigens has been ascribed to extrathymic (peripheral) tolerance mechanisms. However, the phenomenon of promiscuous expression of tissue-restricted self-antigens by medullary thymic epithelial cells has led to a reassessment of the role of central T-cell tolerance in preventing organ-specific autoimmunity. Recent evidence indicates that both genetic and epigenetic mechanisms account for this unorthodox mode of gene expression. As we discuss here, these new insights have implications for our understanding of self-tolerance in humans, its breakdown in autoimmune diseases and the significance of this tolerance mode in vertebrate evolution.

Myasthenia gravis with thymus hyperplasia and pure red cell aplasia

A case of myasthenia gravis (MG) with thymus hyperplasia and pure red cell aplasia (PRCA) is reported. A 57-year-old woman was diagnosed as having MG and was treated with thymectomy 26 years ago. The histology of the resected thymus was thymic lymphoid follicular hyperplasia. She developed rapidly progressive anemia and a bone marrow examination revealed PRCA. Her hematological results improved with oral administration of cyclosporine A. Cases of MG, thymoma and PRCA have been reported in the literature. We report the first case of MG without thymoma and PRCA.

Thymic myoid cells express high levels of muscle genes

To explore the possible contribution of thymic myoid cells in tolerance induction mechanisms, we quantified by real-time RT-PCR, the expression of 12 muscle genes (the five subunits of acetylcholine receptor, Musk, rapsyn, utrophin, ErbB2, ErbB3, troponin T, and MCK) in a human thymic myoid cell line (MITC), compared to thymic epithelial cells (TEC) and thymocytes. Although expression of all the genes analyzed was detected in TEC and thymocytes, the level of expression in these cells was much lower than in MITC, except for -AChR, utrophin and ErbB3 genes. Since myoid cells express high level of most muscle genes and are consistently found in the thymic medulla, they may contribute to the mechanisms involved in the induction and maintenance of immune tolerance.

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.

Myelin/oligodendrocyte glycoprotein-deficient (MOG-deficient) mice reveal lack of immune tolerance to MOG in wild-type mice

We studied the immunological basis for the very potent encephalitogenicity of myelin/oligodendrocyte glycoprotein (MOG), a minor component of myelin in the CNS that is widely used to induce experimental autoimmune encephalomyelitis (EAE). For this purpose, we generated a mutant mouse lacking a functional mog gene. This MOG-deficient mouse presents no clinical or histological abnormalities, permitting us to directly assess the role of MOG as a target autoantigen in EAE. In contrast to WT mice, which developed severe EAE following immunization with whole myelin, MOG-deficient mice had a mild phenotype, demonstrating that the anti-MOG response is a major pathogenic component of the autoimmune response directed against myelin. Moreover, while MOG transcripts are expressed in lymphoid organs in minute amounts, both MOG-deficient and WT mice show similar T and B cell responses against the extracellular domain of MOG, including the immunodominant MOG 35-55 T cell epitope. Furthermore, no differences in the fine specificity of the T cell responses to overlapping peptides covering the complete mouse MOG sequence were observed between MOG+/+ and MOG-/- mice. In addition, upon adoptive transfer, MOG-specific T cells from WT mice and those from MOG-deficient mice are equally pathogenic. This total lack of immune tolerance to MOG in WT C57BL/6 mice may be responsible for the high pathogenicity of the anti-MOG immune response as well as the high susceptibility of most animal strains to MOG-induced EAE.

Circulating regulatory anti-T cell receptor antibodies in patients with myasthenia gravis

Serum anti-T cell receptor (TCR) Ab's are involved in immune regulation directed against pathogenic T cells in experimental models of autoimmune diseases. Our identification of a dominant T cell population expressing the Vbeta5.1 TCR gene (TCRBV5-1), which is responsible for the production of pathogenic anti-acetylcholine receptor (AChR) autoantibodies in HLA-DR3 patients with early-onset myasthenia gravis (EOMG), prompted us to explore the occurrence, reactivity, and regulatory role of anti-TCR Ab's in EOMG patients and disease controls with clearly defined other autoantibodies. In the absence of prior vaccination against the TCR, EOMG patients had elevated anti-Vbeta5.1 Ab's of the IgG class. This increase was restricted largely to EOMG cases with HLA-DR3 and with less severe disease, and it predicted clinical improvement in follow-up studies. EOMG patient sera containing anti-TCR Ab's bound specifically the native TCR on intact Vbeta5.1-expressing cells and specifically inhibited the proliferation and IFN-gamma production of purified Vbeta5.1-expressing cells to alloantigens in mixed lymphocyte reaction and the proliferation of a Vbeta5.1-expressing T cell clone to an AChR peptide, indicating a regulatory function for these Ab's. This evidence of spontaneously active anti-Vbeta5.1 Ab's in EOMG patients suggests dynamic protective immune regulation directed against the excess of pathogenic Vbeta5.1-expressing T cells. Though not sufficient to prevent a chronic, exacerbated autoimmune process, it might be boosted using a TCR peptide as vaccine.

Split tolerance in a novel transgenic model of autoimmune myasthenia gravis

Because it is one of the few autoimmune disorders in which the target autoantigen has been definitively identified, myasthenia gravis (MG) provides a unique opportunity for testing basic concepts of immune tolerance. In most MG patients, Abs against the acetylcholine receptors (AChR) at the neuromuscular junction can be readily identified and have been directly shown to cause muscle weakness. T cells have also been implicated and appear to play a role in regulating the pathogenic B cells. A murine MG model, generated by immunizing mice with heterologous AChR from the electric fish Torpedo californica, has been used extensively. In these animals, Abs cross-react with murine AChR; however, the T cells do not. Thus, to study tolerance to AChR, a transgenic mouse model was generated in which the immunodominant Torpedo AChR (T-AChR) alpha subunit is expressed in appropriate tissues. Upon immunization, these mice showed greatly reduced T cell responses to T-AChR and the immunodominant alpha-chain peptide. Limiting dilution assays suggest the likely mechanism of tolerance is deletion or anergy. Despite this tolerance, immunization with intact T-AChR induced anti-AChR Abs, including Abs against the alpha subunit, and the incidence of MG-like symptoms was similar to that of wild-type animals. Furthermore, evidence suggests that this B cell response to the alpha-chain receives help from T cells directed against the other AChR polypeptides (beta, gamma, or delta). This model offers a novel opportunity to elucidate mechanisms of tolerance regulation to muscle AChR and to clarify the role of T cells in MG.

Low expression of insulin in the thymus of non-obese diabetic mice

Insulin is a predominant autoantigen in IDDM in man and the NOD mouse. Failure of negative selection of diabetogenic T cells in thymus may be an important pre-disposing cause of the disease. To obtain insight into negative selection against such T-cell clones the thymic expression of insulin was studied in NOD and Balb/c mice by quantitative competitive RT-PCR. We detected RNA for insulin in the thymus of 3-week-old Balb/c mice as well as in NOD mice. However, the NOD mice expressed only half as many insulin transcripts as the Balb/c mice. Also, insulin protein was detected in the thymic medulla of both Balb/c and NOD mice. Furthermore, thymic RNA preparations were investigated for the presence of insulin transcription factors. None of the known pancreatic transcription factors for insulin; Pdx-1, Pax6 or Nkx6.1 were detectable in the thymus of Balb/c mice. These results support the idea that low insulin expression in the thymus may be a predisposing cause for development of diabetes in NOD mice analogous with what has been found in humans with the disease-disposing IDDM2 allele. Furthermore, our results suggest that insulin expression in the thymus may be regulated by different principles from those in the pancreas.

Unravelling the pathogenesis of myasthenia gravis

Myasthenia gravis is a relatively rare neurological disease that is associated with loss of the acetylcholine receptors that initiate muscle contraction. This results in muscle weakness, which can be life-threatening. The story of how both the physiological basis of the disease and the role of acetylcholine-receptor-specific antibodies were determined is a classic example of the application of basic science to clinical medicine, and it has provided a model for defining other antibody-mediated disorders of the peripheral and central nervous systems.

lacZ transgenic rats tolerant for beta-galactosidase: recipients for gene transfer studies using lacZ as a reporter gene

Gene transfer of reporter genes may trigger immune responses against the heterologous protein resulting in shortening of gene expression and inflammation. We generated transgenic rats expressing the lacZ gene under the control of the human immunodeficiency virus type 1 (HIV-1) long-terminal repeat (LTR) (HIV-lacZ) to obtain rats with undetectable transgene expression using histologic methods, thus avoiding interference with beta-galactosidase (beta-gal) expression from gene transfer, and displaying immune tolerance toward beta-gal. LacZ transgenic mice with tolerance toward beta-gal have already been used for gene transfer but rats constitute unique animal models with several advantages compared to mice. Two transgenic lines displayed low levels of beta-gal mRNA in most organs tested, as detected only by reverse transcription-polymerase chain reaction (RT-PCR). The protein was undetectable by immunohistology and was only detected in the thymus and spleen using a sensitive enzyme-linked immunosorbent assay (ELISA). HIV-lacZ transgenic rats displayed immune tolerance to beta-gal because immunization with beta-gal resulted in markedly lower cellular and antibody responses compared to wild-type controls, whereas immunization with a nonrelated antigen, keyhole limpet hemocyanin (KLH), resulted in comparable immune responses. The usefulness of this model in gene transfer was tested using a retroviral vector, which does not elicit destructive immune responses against transduced cells. Retroviral-mediated nlslacZ gene transfer in the liver resulted in nuclear beta-gal expression for longer than 12 months in HIV-lacZ transgenic rats, whereas wild-type controls showed nuclear beta-gal expression for less than 1 month. After gene transfer of nlslacZ to the liver, antibodies, cytotoxic T lymphocytes (CTLs), and proliferation against beta-gal were detected in wild-type controls but not in HIV-lacZ transgenic rats. In conclusion, HIV-lacZ transgenic rats displaying low beta-gal expression and immune tolerance toward beta-gal are a useful tool to analyze the spatial and temporal expression of the beta-gal protein in gene transfer experiments using lacZ as a reporter gene.

Peripheral antigen-expressing cells and autoimmunity

There is growing evidence that self-molecules with tissue-restricted expression are also expressed at low levels in the thymus, where such expression may affect the development of self-tolerance. Genetic factors may modulate such expression and, in turn, influence susceptibility to autoimmune responses against specific molecules. Contrasting reports exist regarding the phenotype of the cells that express self-molecules in the thymus. Indeed, both bone marrow derived antigen-presenting cells and thymic epithelial cells were reported to express self-molecules with tissue-restricted expression. Further studies of these cells and the mechanisms by which they exert their putative tolerogenic effects clearly are necessary.

The insulin gene in diabetes

Lack of insulin production or abnormalities affecting insulin secretion are key to the development of almost all forms of diabetes, including the common type 1 (insulin-dependent) and type 2 (non-insulin-dependent) diabetes and the more rare forms of maturity-onset diabetes of the young (MODY). Because insulin has such a central role in the pathogenesis of both forms of diabetes, the insulin gene (INS) has always been considered a candidate susceptibility gene. A number of studies have shown that the allelic variation and parent-of-origin effects affect the transmission and expression of the insulin gene in pancreatic beta-cells and extra-pancreatic tissues. These observations have led to the formulation of new hypotheses to explain the biological mechanisms by which functional differences in the expression of the insulin gene may contribute to diabetes susceptibility.

Sampling of complementing self-antigen pools by thymic stromal cells maximizes the scope of central T cell tolerance

Expression of peripheral antigens in the thymus has been implicated in T cell tolerance and autoimmunity, yet the identity of cells involved remains elusive. Here we show that antigen expression in a minor fraction of medullary thymic epithelial cells leads to deletion of specific CD4 T cells. Strikingly, this deletion is not dependent on cross-presentation by hemopoietic antigen-presenting cells, which have been ascribed a predominant role in negative selection. By contrast, when the same antigen enters the thymus via the blood stream, negative selection is strictly dependent on antigen presentation by hemopoietic cells. These findings imply that the (re)-presentation of "self" by thymic stromal cells is non-redundant, and that different thymic antigen-presenting cells instead cover complementing sets of self-antigens, thus maximizing the scope of central tolerance

Self-antigen-presenting cells expressing diabetes-associated autoantigens exist in both thymus and peripheral lymphoid organs

Recent reports indicate that genes with tissue-restricted expression, including those encoding the type 1 diabetes autoantigens insulin, glutamic acid decarboxylase (GAD), and the tyrosine-phosphatase-like protein IA-2 (or ICA512), are transcribed in the thymus. The reported modulation of diabetes susceptibility by genetically determined differences in thymic insulin levels and studies in transgenic mice provide correlative and functional evidence that thymic expression of peripheral proteins is crucial for immunological self-tolerance. However, there are no specific data about the existence, tissue distribution, phenotype, and function of those cells that express insulin and other self-antigens in the human thymus. We find that the human thymus harbors specialized cells synthesizing (pro)insulin, GAD, and IA-2, mainly localized in the medulla, and we demonstrate such cells also in peripheral lymphoid organs (spleen and lymph nodes). Phenotypic analysis qualifies these cells as antigen-presenting cells (APCs), including both dendritic cells and macrophages. These cells often appear surrounded by apoptotic lymphocytes, both in thymus and spleen, and may therefore be involved in the deletion of autoreactive lymphocytes. Our findings demonstrate the existence of, and define the tissue distribution and phenotype of, a novel subset of APCs expressing self-antigens in human lymphoid organs that appear to be involved in the regulation of self-tolerance throughout life.

Germline incorporation of a replication-defective adenoviral vector in mice does not alter immune responses to adenoviral vectors

The utility of adenoviral vectors is limited by immune responses to adenoviral antigens. We sought to develop immune-competent mice in which the immune response to adenoviral antigens was selectively absent. To do so, we generated mice that were transgenic for a replication-defective vector. Adenoviral antigens might be seen as self-antigens by these mice, and the mice could exhibit immunologic tolerance after postnatal exposure to adenoviral vectors. In addition, characterization of these mice could reveal potential consequences of germline transmission of an adenoviral vector, as might occur in a gene therapy trial. Injection of a "null" (not containing a transgene) E1, E3-deleted vector genome into mouse zygotes yielded five founders that were capable of transmitting the vector genome. Among offspring of these mice, transgenic pups were significantly underrepresented: 108 of 255 pups (42%) were transgenic (P<0.02 versus expected frequency of 50%). Postnatal transgenic mice, however, had no apparent abnormalities. Persistence of an adenoviral vector after intravenous injection was equivalent in livers of transgenic mice and their nontransgenic littermates. Transgenic and nontransgenic mice also had equivalent humoral and cellular immune responses to adenoviral vector injection. Mice that are transgenic for an E1, E3-deleted adenoviral genome can be easily generated; however, they are not tolerant of adenovirus. Moreover, germline transmission of an adenoviral vector genome does not prevent generation of a robust immune response after exposure to adenoviral antigens.

Altered T cell development in human thymoma is related to impairment of MHC class II transactivator expression induced by interferon-gamma (IFN-gamma)

Thymoma is known to contain CD4+CD8+ T cells, indicating that neoplastic epithelial cells of thymoma have a function as thymic cortical epithelium. However, it has been shown that there is an impairment of CD4+ T cell development in thymoma and that IFN-gamma-induced HLA-DR expression on cultured thymic epithelial cells (TEC) derived from thymoma is decreased when compared with the normal thymus. MHC class II transactivator (CIITA) is known to play a critical role in IFN-gamma-induced MHC II expression. In this study, we attempted to elucidate whether CIITA is responsible for the impaired up-regulation of MHC II molecules in response to IFN-gamma in thymoma TEC. A quantitative reverse transriptase-polymerase chain reaction examination revealed that the induced level of CIITA was significantly lower in thymoma TEC than in normal TEC. The induced levels of invariant chain (Ii) and HLA-DR in thymoma TEC were correlated with CIITA expression. The proportion of CD3+ cells in the CD4+CD8- subset in thymoma was also correlated with CIITA expression. A gel mobility shift assay however, revealed translocation of STAT1 to the nucleus in thymoma as well as normal TEC. Intercellular adhesion molecule-1 was up-regulated in the thymoma TEC to a level similar to normal TEC in response to IFN-gamma. These results indicate that impaired up-regulation of HLA-DR in response to IFN-gamma results from insufficient induction of CIITA, but not from the signal from IFN-gamma receptor to the nucleus. The abnormal regulation of HLA-DR expression caused by impaired induction of CIITA may affect CD4+ T cell development in thymoma.

Self-antigen presentation by thymic stromal cells: a subtle division of labor

Self-antigen-MHC complexes expressed by thymic stromal cells serve as ligands for TCR-mediated positive and negative selection, resulting in a self-MHC-restricted, self-tolerant T cell repertoire. It has recently become apparent that thymic stromal cells differ in their accessibility to antigen as well as their ability to process and present antigen. These differences result in the sampling by thymic stromal cells of largely nonoverlapping self-antigen pools and the display of self-peptide profiles specific for each cell type. In conjunction with single or serial cell-cell interactions between thymocytes and stromal cells, such differences in self-antigen display allow for maximal (re)presentation of 'self' in the thymus and optimize the efficacy of positive and negative selection.

Immunology of the neuromuscular junction and presynaptic nerve terminal

The prevalence and incidence of myasthenia gravis is higher than previously thought. A potentially immunodominant T cell has been defined. The specific voltage-gated calcium channel subtype that is targeted by antibodies in the Lambert-Eaton myasthenic syndrome has been identified, and there is further evidence for the pathogenic role of autoantibodies in some cases of fetal arthrogryposis and in acquired neuromyotonia, Morvan's syndrome and Miller-Fisher syndrome.

Association of acetylcholine receptor alpha-subunit gene expression in mixed thymoma with myasthenia gravis

OBJECTIVE

To investigate the association of MG with the transcription of muscular or neuronal acetylcholine receptor (AChR) subunit genes in thymomas.

BACKGROUND

Many steps in the pathogenesis of MG have been elucidated but, with rare exceptions, its etiology is unknown. In patients with MG with thymoma, the tumor probably elicits autoimmunity to AChR, but it is enigmatic why MG develops in some patients but not in others.

METHODS

Reverse transcriptase (RT)-PCR, immunohistochemistry, and immunofluorescence studies were carried out to investigate AChR expression in 35 patients with thymoma. Statistical analysis was used to specify significant differences between thymoma subtypes.

RESULTS

Considering all thymomas (n = 35), no correlation was found between MG status and AChR gene expression as detected by RT-PCR. However, when histologically defined thymoma subtypes were studied separately, transcription of the muscular AChR P3A- alpha-subunit gene was significantly associated (alpha < 0.01) with the occurrence of MG in mixed thymomas (n = 17), but not in thymomas of the cortical type. For the other muscular AChR subunits (P3A+ alpha isoform, beta, gamma, delta, and epsilon) and the alpha2 and beta4 neuronal AChR subunits, no such correlation was detected.

CONCLUSIONS

Expression of the P3A AChR alpha-subunit gene might be important for the pathogenesis of MG in mixed thymomas, suggesting etiologic heterogeneity of paraneoplastic MG among patients with histologically different thymoma subtypes.

Peripheral-antigen-expressing cells in thymic medulla: factors in self-tolerance and autoimmunity

The thymus expresses many genes previously thought to be specific for cell types in other organs. Thus, insulin genes are expressed in rare cells of the thymic medulla. Thymus transplantation demonstrates a functional capability of such expression for self-tolerance induction. Correlative studies suggest that impaired thymic expression confers susceptibility to autoimmune disease.