Early-onset myasthenia gravis: a recurring T-cell epitope in the adult-specific acetylcholine receptor epsilon subunit presented by the susceptibility allele HLA-DR52a

Prominent T-Cell Responses against the Acetylcholine Receptor ε Subunit in Myasthenia Gravis

The human acetylcholine receptor (AChR) is well characterized as the target antigen in myasthenia gravis (MG). Pathogenic antibody responses against the AChR alpha-chain have been investigated extensively and are of diagnostic and prognostic value. However, less is known on the pathogenetic relevance of T-cell responses against epitopes of the different AChR chains (alpha, epsilon, gamma). Using an enzyme-linked immunospot (ELISPOT) assay we measured T-cell responses against recombinant fragments and synthetic peptides of the α and the ε subunits of the human AChR in MG patients (n=15) and in healthy donors (HD; n=9). In MG, highest T-cell responses were noted against recombinantly expressed Epsilon 1-221. Among the synthetic peptides Epsilon 201-215 showed the most prominent T-cell response and represented the peptide with the most remarkable difference between MG and HD. Taken together, prominent T-cell responses against the ε subunit of the human AChR indicate an important role in the pathogenesis of MG.

The prevalence of antibodies against the HLA-DRB3 protein in kidney transplantation and the correlation with HLA expression

Human leukocyte antigen (HLA)-DRB3 is a functional HLA class II gene, which has a limited allele diversity in the human population. Furthermore, the HLA-DRB3 gene is only present in a subset of individuals. Therefore, in organ transplantation, this HLA molecule is frequently mismatched between patient and graft donor and thus antibodies against this mismatched HLA molecule can develop. In this study, we aimed to evaluate the prevalence and reactivity of these antibodies and aimed to identify factors that underlie antibody formation against HLA-DRB3. We showed in our patient cohort that HLA-DRB3 antibodies are identified in about 7% of all patients that were screened with solid phase assays. In these assays, we observed multiple antibody reactivity patterns indicating that HLA-DRB3 harbours multiple epitopes. In those cases, where we succeeded at tracing back the induction of these antibodies to the molecular HLA typing of the immunogenic event, we noticed a different frequency of HLA-DRB1 allele groups in the donors as compared to a control group. To a certain extent this distribution (e.g. HLA-DRB1*11 individuals) could be linked to an altered expression level. However, it also appears that different HLA-DRB3 alleles (e.g. HLA-DRB3*01 group) vary in their immunogenicity without having an expression difference. In conclusion, our study provides information on the immunogenicity and reactivity patterns of antibodies against HLA-DRB3 in kidney transplantation, and it points towards the possibility of HLA expression as a factor underlying antibody formation.

Clinical features, pathogenesis, and treatment of myasthenia gravis: a supplement to the Guidelines of the German Neurological Society

Myasthenia gravis (MG) is an autoimmune antibody-mediated disorder of neuromuscular synaptic transmission. The clinical hallmark of MG consists of fluctuating fatigability and weakness affecting ocular, bulbar and (proximal) limb skeletal muscle groups. MG may either occur as an autoimmune disease with distinct immunogenetic characteristics or as a paraneoplastic syndrome associated with tumors of the thymus. Impairment of central thymic and peripheral self-tolerance mechanisms in both cases is thought to favor an autoimmune CD4(+) T cell-mediated B cell activation and synthesis of pathogenic high-affinity autoantibodies of either the IgG1 and 3 or IgG4 subclass. These autoantibodies bind to the nicotinic acetylcholine receptor (AchR) itself, or muscle-specific tyrosine-kinase (MuSK), lipoprotein receptor-related protein 4 (LRP4) and agrin involved in clustering of AchRs within the postsynaptic membrane and structural maintenance of the neuromuscular synapse. This results in disturbance of neuromuscular transmission and thus clinical manifestation of the disease. Emphasizing evidence from clinical trials, we provide an updated overview on immunopathogenesis, and derived current and future treatment strategies for MG divided into: (a) symptomatic treatments facilitating neuromuscular transmission, (b) antibody-depleting treatments, and

Targeting the fetal acetylcholine receptor in rhabdomyosarcoma

INTRODUCTION

Rhabdomyosarcoma (RMS) is the most common soft tissue sarcoma of childhood and adolescence. Recent efforts to enhance overall survival of patients with clinically advanced RMS have failed and there is a demand for conceptually novel treatments. Immune therapeutic options targeting the fetal nicotinic acetylcholine receptor (fnAChR), which is broadly expressed on RMS, are novel approaches to overcome the therapeutic resistance of RMS. Expression of the fnAChR is restricted to developing fetal muscles, some apparently dispensable ocular muscle fibers and thymic myoid cells. Therefore, after-birth fnAChR is a tumor-associated and almost tumor-specific antigen on RMS cells.

AREAS COVERED

This review gives an overview on nAChR function and expression pattern in RMS tumor cells, and deals with the immunological significance of fnAChR-expressing cells, including the risk of anti-nAChR autoimmunity as a potential side effect of fnAChR-directed immunotherapies. The article also addresses the advantages and disadvantages of vaccination strategies, immunotoxins and chimeric T cells targeting the fnAChR.

EXPERT OPINION

Finally, we suggest technical and biological strategies to improve the available immunotherapeutic tools including increasing the in vivo expression of the target fnAChR on RMS cells.

Granzyme B cleavage of autoantigens in autoimmunity

The systemic autoimmune diseases are a complex group of disorders characterized by elaboration of high titer autoantibodies and immune-mediated damage of tissues. Two striking features of autoimmune rheumatic diseases are their self-sustaining nature and capacity for autoamplification, exemplified by disease flares. These features suggest the presence of a feed-forward cycle in disease propagation, in which immune effector pathways drive the generation/release of autoantigens, which in turn fuel the immune response. There is a growing awareness that structural modification during cytotoxic granule-induced cell death is a frequent and striking feature of autoantigens, and may be an important principle driving disease. This review focuses on granzyme B (GrB)-mediated cleavage of autoantigens including (i) features of GrB cleavage sites within autoantigens, (ii) co-location of cleavage sites with autoimmune epitopes, and (iii) GrB sensitivity of autoantigens in disease-relevant target tissue. The mechanisms whereby GrB-induced changes in autoantigen structure may contribute to the initiation and propagation of autoimmunity are reviewed and reveal that GrB has the potential to create or destroy autoimmune epitopes. As there remains no direct evidence showing a causal function for GrB cleavage of antigens in the generation of autoimmunity, this review highlights important outstanding questions about the function of GrB in autoantigen selection.

Animal Models of Graves' Hyperthyroidism

An animal model of Graves' disease (GD) will help us to clearly understand the role of thyroid-stimulating hormone receptor (TSHR)-specific T cells and TSHR-Abs during the development of GD and to develop TSHR-specific immunotherapy. This review focuses on four different recent approaches towards the development of an animal model of GD. These approaches are: (1) Immunization of AKR/N mice with fibroblasts coexpressing syngeneic major histocompatibility complex (MHC) class II and TSHR. (2) Immunization of selected strains of mice with an expression vector containing TSHR cDNA. (3) Immunization of BALB/c mice with syngeneic M12 cells or xenogenic HEK-293 cells expressing full-length or extracellular domain of TSHR (ETSHR). (4) Injection of adenovirus-expressing TSHR into BALB/c mice.

Evidence for the specificity for platelet HPA-1a alloepitope and the presenting HLA-DR52a of diverse antigen-specific helper T cell clones from alloimmunized mothers

Maternal alloantibodies against the human platelet Ag (HPA)-1a allotype of the platelet beta(3) integrin GpIIb/IIIa can cause severe fetal or neonatal hemorrhage. Almost all anti-HPA-1a-immune mothers are homozygous for HPA-1b and carry HLA-DR52a (DRB3*0101). The single Pro(33) -->Leu substitution (HPA-1b-->HPA-1a) was previously predicted to create a binding motif for HLA-DR52a that can lead to alloimmunization. We have isolated six CD4(+) T cell clones from three such mothers, which all respond to intact HPA-1a(+), but not HPA-1b(+), platelets. We used them to define the "core" and "anchor" residues of this natural T cell epitope. Molecular modeling based on a recently published crystal structure can explain the preferential presentation of the Leu(33) (but not Pro(33) variant) by HLA-DR52a rather than the linked HLA-DR3 or the allelic DR52b. The modeling also predicts efficient anchoring at position 33 by several alternative hydrophobic alpha-amino acids; indeed, a recently identified variant with Val(33) is presented well to two clones, and is therefore potentially alloimmunogenic. Finally, these HPA-1a-specific T cell clones use a variety of T cell receptors, but all have a "Th1" (IFN-gamma-producing) profile and are suitable for testing selective immunotherapies that might be applicable in vivo.

Prioritizing genes of potential relevance to diseases affected by sex hormones: an example of myasthenia gravis

Background

About 5% of western populations are afflicted by autoimmune diseases many of which are affected by sex hormones. Autoimmune diseases are complex and involve many genes. Identifying these disease-associated genes contributes to development of more effective therapies. Also, association studies frequently imply genomic regions that contain disease-associated genes but fall short of pinpointing these genes. The identification of disease-associated genes has always been challenging and to date there is no universal and effective method developed.

Results

We have developed a method to prioritize disease-associated genes for diseases affected strongly by sex hormones. Our method uses various types of information available for the genes, but no information that directly links genes with the disease. It generates a score for each of the considered genes and ranks genes based on that score. We illustrate our method on early-onset myasthenia gravis (MG) using genes potentially controlled by estrogen and localized in a genomic segment (which contains the MHC and surrounding region) strongly associated with MG. Based on the considered genomic segment 283 genes are ranked for their relevance to MG and responsiveness to estrogen. The top three ranked genes, HLA-G, TAP2 and HLA-DRB1, are implicated in autoimmune diseases, while TAP2 is associated with SNPs characteristic for MG. Within the top 35 prioritized genes our method identifies 90% of the 10 already known MG-associated genes from the considered region without using any information that directly links genes to MG. Among the top eight genes we identified HLA-G and TUBB as new candidates. We show that our ab-initio approach outperforms the other methods for prioritizing disease-associated genes.

Conclusion

We have developed a method to prioritize disease-associated genes under the potential control of sex hormones. We demonstrate the success of this method by prioritizing the genes localized in the MHC and surrounding region and evaluating the role of these genes as potential candidates for estrogen control as well as MG. We show that our method outperforms the other methods. The method has a potential to be adapted to prioritize genes relevant to other diseases.

Granzyme B: evidence for a role in the origin of myasthenia gravis

PURPOSE OF RESEARCH

Although the pathogenesis of myasthenia gravis (MG) as an antibody mediated disorder of acetylcholine receptors (AChRs) at neuromuscular junctions is well understood, the origin of the autoimmune response is unclear. The thymus is intimately involved in initiation of the autoimmune response; the antigen, AChR, is present in the thymus, but how the autoimmune response is triggered is not known. Granzyme B (GrB), a proteolytic enzyme present in cytolytic T cells and natural killer (NK) cells, selectively cleaves many potential autoantigens (but few non-autoantigens), generating novel fragments that trigger autoreactive responses. This protease has been strongly implicated in the pathogenesis of several autoimmune diseases including lupus, rheumatoid arthritis, dermatomyositis, and others. In the studies described in this manuscript, we examined the ability of GrB to cleave the AChR subunits, and performed biochemical, immunohistochemical and molecular studies on thymus glands from myasthenic patients and controls to assess GrB expression.

MAIN RESULTS

GrB efficiently and specifically cleaves subunits of AChR, especially the epsilon subunit. GrB is present in thymus glands from myasthenia patients, but is absent in control thymuses.

CONCLUSIONS

Our results provide evidence supporting a potential role for GrB in the process of initiation of MG, and are consistent with the concept of an immunodominant epsilon epitope.

Autoimmunizing mechanisms in thymoma and thymus

Autoimmunizing mechanisms are very hard to study in humans, so we have focused on vital clues in thymomas and hyperplastic thymuses in myasthenia gravis (MG). According to our multi-step hypothesis: thymic epithelial cells (TEC) present epitopes from the isolated acetylcholine receptor (AChR) subunits they express, and autoimmunize helper T cells; subsequently, these evoke "early antibodies" that then attack rare thymic myoid cells expressing intact AChR; in the resulting germinal centers, autoantibodies diversify to recognize native AChR. We have studied: 1) thymomas, to identify autoimmunizing cell types, focusing on IFN-alpha, against which many patients have high titer autoantibodies, as in another highly informative autoimmune syndrome. Although IFN-alpha is much easier to label than the sparse and delicate AChR subunits, we have not yet located obviously autoimmunizing micro-environments; 2) hyperplastic MG thymuses, where we find (a) upregulation of complement receptors and regulators on hyperplastic TEC and deposition of activated C3b complement component on them, (b) absence of complement regulators from almost all myoid cells, indicating vulnerability to attack, and (c) deposition of C3b, and even of the terminal membrane attack complex, especially on the myoid cells close to the infiltrating germinal centers. The changes are very similar in over 50% of the so-called seronegative patients with generalized MG (SNMG) but without detectable autoantibodies against AChR or MuSK, consistently with other evidence that they belong to the spectrum of AChR-seropositive MG. Together, moreover, our findings implicate both myoid cells and TEC in autoimmunization, and thus strongly support our hypothesis.

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.

Crystallographic structure of the human leukocyte antigen DRA, DRB3*0101: models of a directional alloimmune response and autoimmunity

We describe structural studies of the human leukocyte antigen DR52a, HLA-DRA/DRB3*0101, in complex with an N-terminal human platelet integrin alphaII(B)betaIII glycoprotein peptide which contains a Leu/Pro dimorphism. The 33:Leu dimorphism is the epitope for the T cell directed response in neonatal alloimmune thrombocytopenia and post-transfusion purpura in individuals with the alphaII(B)betaIII 33:Pro allele, and defines the unidirectional alloimmune response. This condition is always associated with DR52a. The crystallographic structure has been refined to 2.25 A. There are two alphabeta heterodimers to the asymmetric unit in space group P4(1)2(1)2. The molecule is characterized by two prominent hydrophobic pockets at either end of the peptide binding cleft and a deep, narrower and highly charged P4 opening underneath the beta 1 chain. Further, the peptide in the second molecule displays a sharp upward turn after pocket P9. The structure reveals the role of pockets and the distinctive basic P4 pocket, shared by DR52a and DR3, in selecting their respective binding peptide repertoire. We observe an interesting switch in a residue from the canonically assigned pocket 6 seen in prior class II structures to pocket 4. This occludes the P6 pocket helping to explain the distinctive "1-4-9" peptide binding motif. A beta57 Asp-->Val substitution abrogates the salt-bridge to alpha76 Arg and along with a hydrophobic beta37 is important in shaping the P9 pocket. DRB3*0101 and DRB1*0301 belong to an ancestral haplotype and are associated with many autoimmune diseases linked to antigen presentation, but whereas DR3 is susceptible to type 1 diabetes DR52a is not. This dichotomy is explored for clues to the disease.

Clonal expansions of CD4+ B helper T cells in autoimmune myasthenia gravis

The weakness in myasthenia gravis (MG) is mediated by T helper cell (Th)-dependent autoantibodies against neuromuscular epitopes. So far, analyzing Th phenotypes or antigen specificities has yielded very few clues to pathogenesis. Here we adopt an alternative antigen-independent approach, analyzing T cell receptor (TCR) Vbeta usage/expansions in blood from 118 MG patients. We found major expansions (>or= five standard deviations above the mean of 118 healthy, individually age- and sex-matched controls) in diverse Vbeta in 21 patients (17.6%, p<0.001) among CD4+ T cells, and in 45 patients (38.1%, p<0.001) among CD8+ T cells. In informative probands, the expanded CD4+ cells consistently showed a Th cell phenotype (CD57+CXCR5+) and expressed Th1 cytokines. Furthermore, their expression of markers for activation, lymphocyte trafficking and B cell-activating ability persisted for >or=3 years. Surprisingly, we noted a selective decline in the expansions/their CD57 positivity while the probands' MG was improving. CDR3 spectratyping suggested mono- or oligoclonal origins, which were confirmed by the prevalent TCR Vbeta CDR3 sequences of Th cells cloned from repeat bleeds. Thus, our data provide evidence for persistent clonally expanded CD4+ B helper T cell populations in the blood of MG patients. These unexpected CD4+ expansions might hold valuable clues to MG immunopathogenesis.

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.

Myasthenia gravis patients, but not healthy subjects, recognize epitopes that are unique to the epsilon-subunit of the acetylcholine receptor

Myasthenia gravis (MG) is an autoimmune disease characterized by deficits in neuromuscular transmission due to antibody-mediated damage of the acetylcholine receptor (AChR). We examined the in vitro immune response of peripheral blood mononuclear cells isolated from MG patients (n=38) and healthy nonmyasthenic subjects (n=31) to epitopes on the alpha-, epsilon-, and gamma-chains of the AChR. The epsilon- and gamma-epitopes tested represent regions with little sequence homology to the alpha-chain, and little sequence homology between the epsilon- and gamma-chains. No differences were observed in the immune response of MG patients and healthy subjects to any of the alpha-chain epitopes tested. Serial studies of the immune response to the alpha-peptides suggest that epitope spread does occur over time. Cells from MG patients were stimulated by the epsilon- and gamma-chain peptides, although the response was weaker than that to the alpha-peptides. Cells from healthy subjects showed reactivity to gamma-chain peptides only; none of the healthy subjects responded to the epsilon-chain peptides tested. Differences between the epsilon- and gamma-chains may be important in the development of MG, because only MG patients respond to epitopes that are unique to the epsilon-subunit.

Lack of pathogenicity of immunodominant T and B cell determinants of the nicotinic acetylcholine receptor ε-chain

The nicotinic acetylcholine receptor (nAChR) is the autoantigen in seropositive myasthenia gravis (MG) that is a T cell-dependent B cell-mediated autoimmune disorder. We tested the immunogenicity and myasthenogenicity of the extracellular and first transmembrane domain of the epsilon-chain(1-221) of the nAChR in inbred and MHC congenic rat strains. Immunodominant T and B cell determinants did not induce experimental autoimmune myasthenia gravis (EAMG), although immunization resulted in strong Th1 and B cell responses, which could be mapped with overlapping peptides of the nAChR epsilon-subunit in eight different rat strains. Our data underscores the concept that immunodominant autoantigen-specific T and B cell responses can lack pathogenicity in autoimmune disease and might be of relevance for the physiological integrity of the organism.

Scenarios for autoimmunization of T and B cells in myasthenia gravis

We have studied responses in thymoma patients to interferon-alpha and to the acetylcholine receptor (AChR) in early-onset myasthenia gravis (EOMG), seeking clues to autoimmunizing mechanisms. Our new evidence implicates a two-step process: (step 1) professional antigen-presenting cells and thymic epithelial cells prime AChR-specific T cells; then (step 2) thymic myoid cells subsequently provoke germinal center formation in EOMG. Our unifying hypothesis proposes that AChR epitopes expressed by neoplastic or hyperplastic thymic epithelial cells aberrantly prime helper T cells, whether generated locally or infiltrating from the circulation. These helper T cells then induce antibody responses against linear epitopes that cross-react with whole AChR and attack myoid cells in the EOMG thymus. The resulting antigen-antibody complexes and the recruitment of professional antigen-presenting cells increase the exposure of thymic cells to the infiltrates and provoke local germinal center formation and determinant spreading. Both these and the consequently enhanced heterogeneity and pathogenicity of the autoantibodies should be minimized by early thymectomy.

Prevention of autoantibody-mediated Graves'-like hyperthyroidism in mice with IL-4, a Th2 cytokine

Graves' hyperthyroidism has long been considered to be a Th2-type autoimmune disease because it is directly mediated by autoantibodies against the thyrotropin receptor (TSHR). However, several lines of evidence have recently challenged this concept. The present study evaluated the Th1/Th2 paradigm in Graves' disease using a recently established murine model involving injection of adenovirus expressing the TSHR (AdCMVTSHR). Coinjection with adenovirus expressing IL-4 (AdRGDCMVIL-4) decreased the ratio of Th1/Th2-type anti-TSHR Ab subclasses (IgG2a/IgG1) and suppressed the production of IFN-gamma by splenocytes in response to TSHR Ag. Importantly, immune deviation toward Th2 was accompanied by significant inhibition of thyroid-stimulating Ab production and reduction in hyperthyroidism. However, in a therapeutic setting, injection of AdRGDCMVIL-4 alone or in combination with AdCMVTSHR into hyperthyroid mice had no beneficial effect. In contrast, coinjection of adenoviruses expressing IL-12 and the TSHR promoted the differentiation of Th1-type anti-TSHR immune responses as demonstrated by augmented Ag-specific IFN-gamma secretion from splenocytes without changing disease incidence. Coinjection of adenoviral vectors expressing IL-4 or IL-12 had no effect on the titers of anti-TSHR Abs determined by ELISA or thyroid-stimulating hormone-binding inhibiting Ig assays, suggesting that Ab quality, not quantity, is responsible for disease induction. Our observations demonstrate the critical role of Th1 immune responses in a murine model of Graves' hyperthyroidism. These data may raise a cautionary note for therapeutic strategies aimed at reversing Th2-mediated autoimmune responses in Graves' disease in humans.

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.

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.

Mapping myasthenia gravis-associated T cell epitopes on human acetylcholine receptors in HLA transgenic mice

Susceptibility to myasthenia gravis (MG) is positively linked to expression of HLA-DQ8 and DR3 molecules and negatively linked to expression of the DQ6 molecule. To elucidate the molecular basis of this association, we have induced experimental autoimmune MG (EAMG) in mice transgenic for HLA-DQ8, DQ6, and DR3, and in DQ8xDQ6 and DQ8xDR3 F(1) transgenic mice, by immunization with human acetylcholine receptor (H-AChR) in CFA. Mice expressing transgenes for one or both of the HLA class II molecules positively associated with MG (DQ8 and DR3) developed EAMG. T cells from DQ8 transgenic mice responded well to three cytoplasmic peptide sequences of H-AChR (alpha320-337, alpha304-322, and alpha419-437), of which the response to alpha320-337 was the most intense. DR3 transgenic mice also responded to this sequence very strongly. H-AChR- and alpha320-337 peptide-specific lymphocyte responses were restricted by HLA class II molecules. Disease resistance in DQ6 transgenic mice was associated with reduced synthesis of anti-AChR IgG, IgG(2b), and IgG(2c) Ab's and reduced IL-2 and IFN-gamma secretion by H-AChR- and peptide alpha320-337-specific lymphocytes. Finally, we show that DQ8 imparts susceptibility to EAMG and responsiveness to an epitope within the sequence alpha320-337 as a dominant trait.

True epithelial hyperplasia in the thymus of early-onset myasthenia gravis patients: implications for immunopathogenesis

The early-onset myasthenia gravis (EOMG) thymus shows characteristic medullary epithelial bands (MEB), greatly expanded perivascular infiltrates and fenestrations of the intervening basement membranes. We now compare epithelial expression of epidermal growth factor receptor (EGFR) and many integrins in EOMG and control samples. The main differences are striking/consistent thickening (in MEB) of what is normally a monolayer of perivascular epithelium, with focal protrusion into the infiltrates. This evidently hyperplastic epithelial subpopulation also strongly expresses EGFR and certain integrins. We suggest that its enhanced interactions with the locally increased extracellular matrix protein deposits may play an important role in autosensitization.

Helper T cells in antibody-mediated, organ-specific autoimmunity

The production of pathogenic autoantibodies in organ-specific autoimmune diseases is largely T cell dependent. For many of these diseases, the precise specificities and cytokine profiles of the T cells that respond to the corresponding autoantigens have now been identified. This knowledge has been exploited to treat some models of antibody-mediated autoimmunity using peptides corresponding to the dominant helper epitopes, giving impetus to the development of a similar approach in the equivalent human diseases.

Thymoma

Thymomas are fascinating tumors because of their multifaceted clinical presentation, including an unrivaled frequency of associated paraneoplastic autoimmune diseases and an astounding histologic heterogeneity. Thus, thymomas challenge the clinician, pathologist, and immunologist alike with complex diagnostic and therapeutic problems in almost all fields of medicine, radiology, nuclear medicine, surgery, radiooncology, and pathology. This review highlights recent progress in these fields, including the new World Health Organization's classification of thymomas, and reports on the beginning discovery of the genetic basis of thymoma oncogenesis and the cellular and molecular immunopathology of thymoma-associated paraneoplastic autoimmunity.

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.

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.

A susceptibility region for myasthenia gravis extending into the HLA-class I sector telomeric to HLA-C

We have analyzed a series of HLA region markers in 207 UK Caucasoids with early-onset myasthenia gravis (EOMG, onset before age 40), where there is a strong female bias. The well known associations with HLA-DR3 and -B8 have now proved to be significantly stronger in the 165 females than in the 42 males. In patients (of either sex) lacking -DR3, there was also a significant increase in HLA-DR2. Although the muscle weakness in EOMG is clearly mediated by autoantibodies, the associations are consistently stronger with HLA-B8 (in class I) than with HLADR3 (in class II), as confirmed here. We therefore typed 87-137 cases for polymorphisms at four loci in the intervening class III region, and also at three in the adjacent stretch of class I. At each locus, one allele tended to co-occur with HLA-B8 and showed strong and highly significant associations in the patients. There appeared to be a region of maximal susceptibility extending from HSP70 (in class III) past HLA-B and HLA-C at least 600 kb telomerically into the class I region, which is now being mapped in detail. Any candidate genes here that act shortly after puberty may allow more precise localization of susceptibility.

T cell responses to D-penicillamine in drug-induced myasthenia gravis: recognition of modified DR1:peptide complexes

The anti-rheumatoid drug D-penicillamine (D-pen) has a reactive sulfhydryl group capable of modifying self antigens, and can provoke typical autoantibody-mediated myasthenia gravis (MG), especially in DR1+ individuals. We have selected T cell clones from one such patient that were highly specific for D-pen but not its L-isomer or D-cysteine. Moreover, they were restricted to HLA-DR1, had a Th1 phenotype and used TCR V alpha4.1, V beta6.1. They responded well to blood mononuclear cells prepulsed with D-pen either in the absence of serum or after chloroquine treatment, but not to autologous D-pen-pulsed B cell lines. Thus, D-pen may directly couple to distinctive peptides resident in surface DR1 molecules on circulating macrophages or dendritic cells.

Scanning a DRB3*0101 (DR52a)-Restricted Epitope Cross-Presented by DR3: Overlapping Natural and Artificial Determinants in the Human Acetylcholine Receptor

A recurring epitope in the human acetylcholine receptor (AChR) α subunit (α146–160) is presented to specific T cells from myasthenia gravis patients by HLA-DRB3*0101—“DR52a”—or by DR4. Here we first map residues critical for DR52a in this epitope by serial Ala substitution. For two somewhat similar T cells, this confirms the recently deduced importance of hydrophobic “anchor” residues at peptide p1 and p9; also of Asp at p4, which complements this allele’s distinctive Arg74 in DRβ. Surprisingly, despite the 9 sequence differences in DRβ between DR52a and DR3, merely reducing the bulk of the peptide’s p1 anchor residue (Trp149→Phe) allowed maximal cross-presentation to both T cells by DR3 (which has Val86 instead of Gly). The shared K71G73R74N77 motif in the α helices of DR52a and DR3 thus outweighs the five differences in the floor of the peptide-binding groove. A second issue is that T cells selected in vitro with synthetic AChR peptides rarely respond to longer Ag preparations, whereas those raised with recombinant subunits consistently recognize epitopes processed naturally even from whole AChR. Here we compared one T cell of each kind, which both respond to many overlapping α140–160 region peptides (in proliferation assays). Even though both use Vβ2 to recognize peptides bound to the same HLA-DR52a in the same register, the peptide-selected line nevertheless proved to depend on a recurring synthetic artifact—a widely underestimated problem. Unlike these contaminant-responsive T cells, those that are truly specific for natural AChR epitopes appear less heterogeneous and therefore more suitable targets for selective immunotherapy.