Thymic myogenesis, T-lymphocytes and the pathogenesis of myasthenia gravis

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.

Reflections on the "intrathymic pathogenesis" of myasthenia gravis

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

Thymic myoid cells as a source of cells for myoblast transfer

Transplantation of disaggregated myoblasts from normal donor to the muscles of a diseased host, or reimplantation of genetically modified host myoblasts, has been suggested as a possible route to therapy for inherited myopathies such as Duchenne muscular dystrophy, or to supply missing proteins that are required systemically in diseases such as hemophilia. With two exceptions, studies of myoblast transfer in the mouse have involved transplantation of donor myoblasts isolated from adult or neonatal skeletal muscle satellite cells. In this study we present evidence that thymic myoid cells are capable of participating in the regeneration of postnatal skeletal muscle, resulting in the expression of donor-derived proteins such as dystrophin and retrovirally encoded proteins such as beta-galactosidase within host muscles. This leads us to conclude that thymic myoid cells may provide an alternative to myoblasts derived from skeletal muscle as a source of myogenic cells for myoblast transfer.

Establishment of a human thymic myoid cell line. Phenotypic and functional characteristics

The subset of myoid cells is a normal component of the thymic stroma. To characterize these cells, we immortalized stromal cells from human thymus by using a plasmid vector encoding the SV40 T oncogene. Among the eight cell lines obtained, one had myoid characteristics including desmin and troponin antigens. This new line was designated MITC (myoid immortalized thymic cells). These cells expressed both the fetal and adult forms of muscle acetylcholine receptor (AChR) at the mRNA level, as well as the myogenic transcription factor MyoD1. alpha-Subunit AChR protein expression was detected by flow cytometry and the AChR was functional in patch-clamp studies. In addition, AChR expression was down-modulated by myasthenia gravis sera or by monoclonal antibody anti-AChR on MITC line similarly to TE671 rhabdomyosarcoma cells, making the MITC line an interesting tool for AChR antigenic modulation experiments. Finally, the MITC line expressed LFA-3, produced several cytokines able to act on T cells, and protected total thymocytes from spontaneous apoptosis in vitro. These results are compatible with a role of thymic myoid cells in some steps of thymocyte development. Therefore MITC line appears to be a useful tool to investigate the physiological role of thymic myoid cells.

Normal human immunoglobulin suppresses experimental myasthenia gravis in SCID mice

Serum IgM has been shown to participate in the control of IgG autoreactivity in healthy subjects. We have recently shown that an immunoglobulin preparation of pooled normal human IgM (IVIgM) contains anti-idiotypic antibodies against disease-associated IgG autoantibodies in autoimmune patients and protects rats from experimental autoimmunity. The aim of the present study was to asses the in vitro and in vivo immunomodulatory effects of IVIgM in comparison with IgG, in SCID mice reconstituted with thymic cells from a myasthenia gravis patient. Non-leaky SCID mice were injected i.p. with 60 x 10(6) thymic cells from a patient with myasthenia gravis and 1 day later boosted with 10(6) irradiated acetylcholine receptor (AchR)-expressing TE671 cells. On days 14, 21 and 28, mice were treated with IVIgM or with equimolar amounts of human serum albumin. The level of anti-AchR antibodies in the sera of three out of four IgM-treated animals was less than 1 nM. Further, there was a significant decrease in the loss of endplate AchR on the diaphragms of IgM-treated SCID mice. These findings indicate that pooled normal IgM exerts an immunoregulatory role in experimental myasthenia gravis, and suggests that IgM may be considered as an alternative approach in the therapy of autommune diseases.

The impact of HLA on long-term outcome after thymectomy for myasthenia gravis

In a retrospective series of 86 patients with myasthenia gravis, the only factors predictive of improvement in muscular strength after transsternal thymectomy were preoperative severity of myasthenia (90% versus 54%, p = 0.0014) and HLA-B8 (79% versus 50%, p = 0.0060) in bivariable and multivariable analyses. Both factors were not interrelated (p = 0.824). The statistical effect of HLA-B8 was independent from preoperative severity of disease. Typing for HLA-B8 may thus be a valuable adjunct in predicting the benefit of thymectomy in myasthenia. The observation that an MHC class I allele is associated with clinical improvement after thymectomy suggests that the clinical course of myasthenia may be influenced by class I restricted T-cells.

Depletion of CD8+ T cells suppresses the development of experimental autoimmune myasthenia gravis in Lewis rats

To understand the role of CD8+ T cells in experimental autoimmune myasthenia gravis (EAMG), CD8+ T cells were depleted by injecting a monoclonal anti-rat CD8 antibody (OX8) into Lewis rats immunized with Torpedo acetylcholine receptor (AChR) in complete Freund's adjuvant (CFA). CD8-depleted EAMG rats showed strikingly less muscle weakness and lower anti-AChR IgG antibody levels compared to Hy2-15-injected control EAMG rats. Moreover, the numbers of AChR-specific IgG antibody-secreting cells, AChR-reactive interferon-gamma-secreting T helper type 1-like cells and lymphocyte proliferation to AChR were lower in the CD8-depleted group than in control EAMG rats. These differences were significant among mononuclear cells from inguinal and popliteal lymph nodes, mesenteric lymph nodes and spleen, but not from thymus when examined 3, 5 and 7 weeks post-immunization. We suggest that CD8+ T cells are involved in the induction and persistance of EAMG by directly or indirectly affecting AChR-reactive T cells and anti-AChR IgG antibody-secreting cells.

Augmented interferon-gamma, interleukin-4 and transforming growth factor-beta mRNA expression in blood mononuclear cells in myasthenia gravis

The abnormal T lymphocyte-dependent production of antibodies to the nicotinic acetylcholine receptor (AChR) in myasthenia gravis (MG) suggests a role for immunoregulatory cytokines. We examined the T helper type 1 (Th1) cell-associated interferon-gamma (IFN-gamma) that promotes cell-mediated immunity, the Th2 cell-related interleukin-4 (IL-4) that augments B cell immunity, and transforming growth factor-beta (TGF-beta) that downregulates immune responses but enhances isotype switching. Blood mononuclear cells (MNC) expressing cytokine mRNA were enumerated after in situ hybridization with labelled complementary DNA oligonucleotide probes for IFN-gamma, IL-4 and TGF-beta. MG patients had elevated numbers of cells expressing IFN-gamma and IL-4 compared to patients with non-inflammatory neurological diseases and healthy controls, implying that both Th1- and Th2-like cells are involved in MG. TGF-beta-positive cells were also elevated in MG. The levels of cytokine-positive MNC were similar in MG and in control patients with other inflammatory neurological diseases. There were no associations between numbers of cytokine-positive blood MNC and clinical variables of MG, but individual patients need to be studied over the course of MG to clarify a relation between the cytokines under study and clinical or laboratory variables of MG.

Characterization of a 58- and a 78-kD monocytic membrane protein with affinity to the acetylcholine receptor in myasthenia gravis patients

The autoimmune disease myasthenia gravis (MG), caused by the effect of specific antibodies, directed towards the nicotinic acetylcholine receptor, is triggered by autoantigen-specific T cells. In order to investigate cellular parts of the immune response in MG, the authors investigated the binding of the nicotinic acetylcholine receptor (AChR) to peripheral blood mononuclear cells (PBMC) from MG patients. AChR binding cells were identified by rosetting experiments using AChR-coated fluorescein beads. Applying this technique, a significant percentage of PBMC (21.2 +/- 7.65%) from MG patients formed rosettes with AChR-coated beads. Membrane preparations of nycodenz- or percoll-separated monocytes from MG patients or T-cell depleted monocytic subpopulations were applied to SDS-PAGE under reducing conditions. Ligand-blotting studies with biotinylated AChRs revealed two cell-membrane proteins with molecular weights of 58- and 78-kD. In parallel the same results were obtained by affinity chromatography of monocytic membrane proteins using AChR-sepharose. A possible interference of anti-AChR IgG was excluded. The 58- and the 78-kD proteins are detectable under reducing conditions by ligand blotting with AChR-biotin, while under non-reducing conditions only the 58-kD protein can be detected. Furthermore, in experiments using Endoglycosidase-H, the 58-kD protein appears to be non-glycosylated, while the 78-kD protein bears carbohydrates. These findings suggest that monocytes which bind the AChR via specific membrane proteins on their surface might act as antigen-presenting cells and may lead to an induction of the T-cell response, in the early phase of the disease.

The role of the thymus in myasthenia gravis

The experimental work discussed here supports the hypothesis that in the pathogenesis of MG the initial and essential steps take place within the thymus. Most if not all thymuses of MG patients contain B cells capable of producing AChR specific autoantibody along with appropriate stroma elements. Hyperplastic thymuses characteristically contain germinal centers with cellular complexes of AChR-producing MC and surrounding interdigitating dendritic cells. In thymomas, the source of the myasthenogenic autoantigen is less obvious. There are data suggesting that thymoma epithelium expresses a protein sharing certain peptide epitopes with the AChR alpha chain, although there is no further molecular similarity. A unique type of 'molecular self-mimicry' cold be involved in the initiation of thymoma-associated MG.

T helper cell recognition of muscle acetylcholine receptor in myasthenia gravis. Epitopes on the gamma and delta subunits

We tested the response of CD4+ cells and/or total lymphocytes from the blood of 22 myasthenic patients and 10 healthy controls to overlapping synthetic peptides, 20 residues long, to screen the sequence of the gamma and delta subunits of human muscle acetylcholine receptor (AChR). The gamma subunit is part of the AChR expressed in embryonic muscle and is substituted in the AChRs of most adult muscles by an epsilon subunit. The delta subunit is present in both embryonic and adult AChRs. Adult extrinsic ocular muscles, which are preferentially and sometimes uniquely affected by myasthenic symptoms, and thymus, which has a still obscure but important role in the pathogenesis of myasthenia gravis, express the embryonic gamma subunit. Anti-AChR CD4+ responses were more easily detected after CD8+ depletion. All responders recognized epitopes on both the gamma and delta subunits and had severe symptoms. In four patients the CD4+ cell response was tested twice, when the symptoms were severe and during a period of remission. Consistently, the response was only detectable, or larger, when the patients were severely affected.

T-helper epitopes on human nicotinic acetylcholine receptor in myasthenia gravis

The synthesis of AChR antibodies requires intervention of AChR-specific Th cells. Because of the paucity of anti-AChR Th cells in the blood of myasthenia gravis (MG) patients, direct studies of these autoimmune cells in the blood are seldom possible. Propagation in vitro of anti-AChR T cells from MG patients by cycles of stimulation with AChR antigens selectively enriches and expands the autoimmune T-cell clones, allowing investigation of their function and epitope specificity. Torpedo electroplax AChR was initially used for propagation of anti-AChR T-cell lines. Those studies demonstrated the feasibility of in vitro propagation of AChR-specific T cells. These are bona fide CD4+ Th cells, which stimulate production in vitro of anti-AChR antibodies by B cells of myasthenic patients and recognize equally well denatured and native AChR, suggesting the usefulness of synthetic human AChR sequences as antigens for propagation of the autoimmune Th cells. We used pools of overlapping synthetic peptides, corresponding to the complete sequences of the human AChR alpha-, beta-, gamma-, and delta-subunits, to propagate AChR-specific Th cells from the blood of MG patients. The AChR sequence regions forming epitopes recognized by the autoimmune T cells were determined by challenging the lines with individual synthetic peptides, 20 residues long, screening the AChR subunit sequences. Although each line had an individual pattern of epitope recognition--as expected from their different HLA-DR haplotype--some peptides were recognized by most of all the CD4+ T-cell lines, irrespective of their DR haplotype. The existence of immunodominant regions of the AChR sequence was verified by investigating the response of unselected CD4+ cells from the blood of a relatively large number of MG patients to the individual peptides screening the human alpha-, gamma-, and delta-subunit sequences. Those studies confirmed that each patient has an individual pattern of peptide recognition. The studies also identified a large number of T epitopes of the human AChR and verified the existence of sequence regions immunodominant for T-helper sensitization, because a limited number of sequence regions, including all those immunodominant for the T-helper lines, were recognized by most patients. Anti-AChR CD4+ T lines could be propagated from some healthy controls only for a brief period of time. They recognized AChR sequences poorly, suggesting a low affinity of their T-cell receptors for the corresponding AChR epitopes.(ABSTRACT TRUNCATED AT 400 WORDS)

T-cell immunity to acetylcholine receptor and its subunits in Lewis rats over the course of experimental autoimmune myasthenia gravis

Lymph nodes, spleen and thymus obtained from Lewis rats were examined over the course of experimental autoimmune myasthenia gravis (EAMG) for the distribution and the number of antigen-reactive CD4+ T helper cells which, upon recognition of Torpedo acetylcholine receptor (AChR) or the alpha, beta, gamma or delta subunits of Torpedo AChR, responded by secretion of interferon-gamma (IFN-gamma). T cells with these specificities were detected in these three immune organs. Numbers were highest in lymph nodes. In spleen and thymus, numbers of antigen-reactive T cells did not differ. T cells reacting against the intact AChR were more frequent than T cells recognizing any of the subunits. The immunogenicity between the four subunits did not differ, with the exception that the alpha subunit induced a slightly higher T-cell response. No restriction of the T-cell repertoire to the four subunits was detected during early compared to late phases of EAMG. The AChR and subunit-reactive T cells could--via secretion of effector molecules including IFN-gamma--play an important role in the initiation and perpetuation of EAMG, and consequently also of human myasthenia gravis. T cells with the same specificities were also detected in control animals injected with adjuvant only, but at much lower numbers which were within the range of T cells recognizing the control antigen myelin basic protein. They could represent naturally occurring autoimmune T cells.

Potential role of thymoma and other mediastinal tumors in the pathogenesis of myasthenia gravis

Ten non-myasthenic thymoma patients and 12 patients with other mediastinal tumors were compared with 19 myasthenic thymoma patients with regard to an increase in circulating CD4+CD8+ cells and the presence of anti-acetylcholine receptor and anti-skeletal muscle antibodies. Although seven non-myasthenic thymoma patients showed positive results, the proportion of myasthenic thymoma patients who were positive for more than one parameter was significantly larger than that of non-myasthenic thymoma patients (89% vs. 40%). Moreover, one patient with a non-thymomatous mediastinal tumor showing a high CD4+CD8+ cell level had a recent history of seronegative myasthenia gravis. The results indicate that measurements of these parameters may predict the risk of the development of MG in patients with thymoma and other mediastinal tumors.

Myasthenia gravis. CD4+ T epitopes on the embryonic gamma subunit of human muscle acetylcholine receptor

In myasthenia gravis (MG) an autoimmune response against muscle acetylcholine receptor (AChR) occurs. Embryonic muscle AChR contains a gamma subunit, substituted in adult muscle by a homologous epsilon subunit. Antibodies and CD4+ cells specific for embryonic AChR have been demonstrated in MG patients. We identified sequence segments of the human gamma subunit forming epitopes recognized by four embryonic AChR-specific CD4+ T cell lines, propagated from MG patients' blood by stimulation with synthetic peptides corresponding to the human gamma subunit sequence. Each line had an individual epitope repertoire, but two 20-residue sequence regions were recognized by three lines of different HLA haplotype. Most T epitope sequences were highly diverged between the gamma and the other AChR subunits, confirming the specificity of the T cells for embryonic AChR. These T cells may have been sensitized against AChR expressed by a tissue other than innervated skeletal muscle, possibly the thymus, which expresses an embryonic muscle AChR-like protein, containing a gamma subunit. Several sequence segments forming T epitopes are similar to regions of microbial and/or mammalian proteins unrelated to the AChR. These findings are consistent with the possibility that T cell cross-reactivity between unrelated proteins ("molecular mimicry"), proposed as a cause of autoimmune responses, is not a rare event.

The T-cell repertoire in myasthenia gravis involves multiple cholinergic receptor epitopes

Antibodies against the alpha-subunit of the acetylcholine receptor (AChR) are found in most patients with myasthenia gravis and are considered to contribute to the receptor damage which leads to the characteristic signs and symptoms of the disease. This B-cell response is T-cell driven. Elevated T-cell reactivities to AChR and its alpha-subunit have been described in myasthenia gravis, and AChR alpha-subunit peptide reactive T-cell lines and clones preferentially recognizing certain defined sequence segments have been reported, thereby disclosing the possibility of specific immunotherapy. We have defined the T-cell repertoire to AChR, its alpha-subunit and the synthetic peptide sequences 100-117, 113-130, 143-163, 161-179, 207-225, 221-240, and 235-255 of the alpha-subunit in an immunospot assay which is based on secretion of interferon-gamma (IFN-gamma) by individual memory T cells upon stimulation with specific antigen in short-term cultures. Most patients with myasthenia gravis displayed T-cell reactivities to 1 to 6 different peptides. The mean numbers of T cells recognizing individual peptides varied in the myasthenia gravis patients between 1 per 77,000 and 1 per 167,000 peripheral blood mononuclear cells. None of the seven peptides evaluated could be identified as an immunodominant T-cell epitope, and any of them was found to dominate in individual patients. The numbers of T cells reacting with AChR and recombinant human AChR alpha-subunit were slightly higher (mean numbers 1 per 26,000 and 1 per 50,000 mononuclear cells, respectively). Such cells, as well as AChR alpha-subunit peptide reactive T cells, were also found in patients with other neurological diseases and in healthy subjects, but at lower frequencies and numbers. In myasthenia gravis, the elevated numbers of memory T cells recognizing multiple AChR alpha-subunit peptides may be crucial for the development of the disease, and the IFN-gamma released by such T cells might be important for its perpetuation.

Transplantation of thymic autoimmune microenvironment to severe combined immunodeficiency mice. A new model of myasthenia gravis

To study the role of the thymus in the cellular pathogenesis of myasthenia gravis (MG) we transplanted thymus tissue fragments from MG thymuses beneath the kidney capsule of severe combined immunodeficiency (SCID) mice. Immunocytochemical studies documented that the human thymus tissues are accepted as long-term grafts in the host SCID mice, with human lymphocytes, thymic stroma, and thymic myoid cells demonstrable in transplanted thymus for at least 15 weeks after transplantation. Human anti-acetylcholine receptor antibodies became detectable 1 to 2 weeks after transplantation, and in most chimeras the titers increased over at least 11 weeks to reach levels typically found in severe human MG. Human Ig deposits were detected at skeletal muscle end-plates, demonstrating that the human (auto)antibodies bound to murine acetylcholine receptor. In contrast, transfers of dissociated thymus cells only lead to a transient increase of anti-acetylcholine receptor antibodies. Our data prove that myasthenia gravis thymus is able to induce and maintain autoantibody production in immunodeprived host animals, and that this tissue contains all cellular components required for autoantibody production. Transplantation of solid thymus tissue seems to transfer an autoimmune microenvironment, which will allow direct studies of the mechanism of autosensitization inside the thymus.

T cell responses to human recombinant acetylcholine receptor-alpha subunit in myasthenia gravis and controls

Antibodies against the nicotinic acetylcholine receptor (AChR) of the neuromuscular junction are detectable in most patients with myasthenia gravis (MG) and assumed to participate in the destruction of the AChR, thereby, causing the characteristics signs and symptoms of the disease. The extent and importance of T cell responses to AChR and its subunits in MG are still unsettled. We have now examined T cell reactivities using human recombinant AChR-alpha subunit as antigen. Upon recognition of appropriate antigen in an MHC-class II-restricted fashion, memory T cells secrete interferon-gamma (IFN-gamma). Adopting this principle in an immunospot assay we found that 73% of MG patients had recombinant human AChR-alpha subunit-reactive T cells at a median value of 1 per 56,000 blood mononuclear cells, while only 27% of the MG patients responded to the alpha subunit in a conventional lymphocyte proliferation assay. This compares with even lower numbers of AChR-reactive T cells and 14% positivity in the proliferation assay among control subjects. The T cell responses to the control antigens purified protein derivative and myelin basic protein did not differ between MG and controls, underlining the specificity of an augmented T cell reactivity to AChR-alpha subunit in MG. Alpha Subunit-specific T cell lines and clones propagated from patients with MG and healthy controls yielded a high proportion of alpha subunit-reactive T cells in the IFN-gamma immunospot assay. Their appearance was inhibited by the addition of monoclonal anti-MHC class II antibodies, demonstrating that an MHC-restricted T cell response was measured. Our data underline that the AChR-alpha subunit is a major T cell autoantigen in MG.

The transcription of MyoD1 and myogenin genes in thymic cells in vivo

The skeletal muscle specific genes MyoD1 and myogenin are closely associated with commitment of cells to the myogenic lineage and differentiation of skeletal muscle precursor cells. The transcription of these genes was studied in the thymus where mononuclear cells termed myoid cells appear to closely resemble skeletal muscle precursors. In thymus from adult SJL/J and BALB/c mice, in situ hybridization with either MyoD1 or myogenin riboprobes showed probe-positive cells concentrated in the medullary region. In neonatal thymus, mRNA for these genes was not detected. These data are the first demonstration in a higher vertebrate of MyoD1 and myogenin expression in a tissue other than skeletal muscle. The sustained expression of MyoD1 and myogenin genes in thymi of adult mice shows that myoid cells are not equivalent to quiescent stem cells of mature skeletal muscle. In addition, studies with antistriational antibodies indicate that myoid cells do not continue to differentiate within the normal murine thymic environment. This arrested differentiation process presents an unusual model for investigating conditions regulating myogenesis in vivo.

Actuarial analysis of the occurrence of remissions following thymectomy for myasthenia gravis in 400 patients

The role of thymectomy in the treatment of myasthenia gravis (MG) was analysed in 400 patients affected with generalised MG operated on between 1974-83, and prospectively followed up for five years after surgery. The occurrence of stable remission (SR) (that is, complete clinical drug-free remission that remains stable for all the subsequent follow up) was the endpoint of survival analyses and the distribution of SR time (SRT, that is, the interval from thymectomy to the occurrence of SR) was assessed by actuarial and Cox multivariate analyses. SRT distribution after surgery showed a slow progressive increase of cumulative SR rate that could both be ascribed to a delayed effect of thymectomy as well as reflect the natural history of MG, itself characterised by an increasing probability of spontaneous remission with time. SRT distribution was similar after stratification for all variables studied except when patients without thymoma were stratified for the need for immunosuppressive treatment in addition to thymectomy. Patients without thymoma who did not require additional immunosuppressive therapy (n = 130) had the highest SR rate occurring in the two years after thymectomy, and differed from patients treated with immunosuppressive drugs who showed the highest SR rate five years after surgery. Actuarial analysis has therefore identified a subgroup of patients where SR, occurring in the first years after surgery, is more likely to be ascribed to thymectomy than merely reflect the natural course of the disease.

Anti-acetylcholine receptor antibodies decrease after thymectomy in patients with myasthenia gravis. Clinical correlations

Clinical course and changes in serum levels of antibodies to the acetylcholine receptor (a-AChR) were followed in 82 patients with myasthenia gravis during a period of 1-8 years after thymectomy. Decrease in a-AChR immediately after thymectomy was influenced by changes in a total IgG. Immunosuppressive medication affected serum a-AChR at all points of time. In a subgroup of 41 patients without thymoma who had no immunosuppressive drugs, there was a steady decrease in a-AChR concomitant with clinical improvement from 6 weeks after thymectomy.

Expression of mRNAs in human thymus coding for the alpha 3 subunit of a neuronal acetylcholine receptor

We report the isolation of clones from a human thymus cDNA library that code for the alpha 3 subunit of a neuronal acetylcholine receptor (AcChR). The clones hybridize to one major 3.0-kb mRNA thymic species and four minor ones of approximately 2.3, 4.0, 5.0, and 6.5 kb, but they do not hybridize to human muscle mRNA. These clones may be of value in defining the cholinergic thymic makeup and the putative role that a thymic AcChR may have in the triggering and/or maintenance of an anti-AcChR response in the autoimmune condition myasthenia gravis.

Circulating CD4+CD8+ cells in myasthenia gravis: supplementary immunological parameter for long-term prognosis

Twenty patients with myasthenia gravis (MG) were studied prospectively for up to 5 years after thymectomy, in order to clarify the relationships between disease severity, anti-acetylcholine receptor antibody (anti-AChR) titres, proportions of circulating CD4+CD8+ cells (CD4+CD8+ cell level) and major lymphocyte subsets. The CD4+CD8+ cell levels were closely related to the clinical change within 1 year after surgery in 8 patients who showed a preoperative elevation in the cell levels. This group of patients consisted of six thymomatous and two non-thymomatous patients; the latter were both negative for anti-AChR. The anti-AChR titres generally changed in parallel with the clinical state in 9 of the 16 patients who were followed up for more than a year after thymectomy, and the CD4+CD8+ cell levels were useful in predicting the clinical course in 6 of the above 9 patients and 3 other patients, including antibody-negative cases. The present study suggests that the CD4+CD8+ cell levels may serve as an indicator for long-term prognosis of MG.

Neurological autoimmune disease and the trimolecular complex of T-lymphocytes

T-lymphocytes recognize antigen in a trimolecular complex: The T-cell receptor binds to a processed fragment of antigen that itself is bound to a major histocompatibility complex (MHC) molecule on the surface of an antigen-presenting cell. The trimolecular complex controls antigen-specific T-cell activation in normal and abnormal immune reactions. Recent progress in myasthenia gravis (MG) and experimental autoimmune encephalomyelitis (EAE) exemplifies this, leading to the following conclusions: (1) Autoimmune T cells may act by interfering with immunoregulation (as in MG) or by directly mediating autoimmune damage (as in EAE), or both. (2) In both diseases, the autoimmune T cells are clonally heterogeneous but recognize only a limited number of epitopes on the autoantigen (acetylcholine receptor in MG; myelin basic protein in EAE). Many of these epitopes can be defined as short peptide fragments of antigen, bound to a particular type of MHC molecule. (3) The MHC determines which peptides are recognized by autoimmune T cells in a given patient or inbred animal strain. (4) The discovery of the limited repertoire of autoimmune T cells has allowed considerable progress in the immunotherapy of EAE, using either monoclonal antibodies or cytotoxic T cells directed against clonotypic determinants on the autoaggressive T cells. (5) One obstacle to this approach in human disease is the polymorphism of the MHC in the species and the commensurate heterogeneity of autoimmune T cells.

Analysis of immunoglobulin and T cell receptor gene rearrangements in the thymus of myasthenia gravis patients

The thymus is an important site of sensitization of autoreactive B and T lymphocytes in myasthenia gravis (MG). We have investigated clonal diversity of B or T cells in the thymus of patients with MG by Southern blot experiments using probes specific for immunoglobulin heavy (IgH) and light chain (IgL) genes and for T cell receptor (TCR) beta- and gamma-chain genes. This method allows to detect individual clones of B or T cells if they represent at least 1% of the total cell population. We investigated thymus glands from 14 patients who underwent thymectomy. Single rearranged fragments could be demonstrated with a TCR gamma-specific probe in DNAs from both normal donors and MG patients. TCR beta gene rearrangements occurred mainly in the C beta 1 region. However, single rearranged bands could not be detected with either TCR beta, JH or with J kappa specific probes. Thus any single autoimmune B and T cell clone present in the myasthenic thymus represents presumably less than 1% of all thymocytes.

Thymus in myasthenia gravis. Isolation of T-lymphocyte lines specific for the nicotinic acetylcholine receptor from thymuses of myasthenic patients

The thymus is believed to play a central role in the pathogenesis of Myasthenia gravis (MG). According to a previous hypothesis, MG is initiated within the thymus by immunogenic presentation of locally produced nicotinic acetylcholine receptor (AChR) to potentially autoimmune T cells. Data of 10 consecutive MG patients demonstrate two critical features of MG thymuses that support the concept of intrathymic activation of autoreactive, AChR-specific lymphocytes. Morphologically, the thymuses showed lympho-follicular hyperplasia in nine cases and benign thymoma in one case. The paramount feature revealed by immunohistological double marker analyses was the intimate association of myoid cells (antigen producing) with interdigitating reticulum cells (potentially antigen presenting cells), both of which were surrounded by T3+ lymphocytes in thymus medulla. All 10 thymuses contained T lymphocytes reactive with AChR. This was in contrast to the peripheral immune compartment (blood) where in only 3 of 10 patients, significant T cell responses to AChR were observed. AChR-specific T cell lines could be established from 8 of 10 thymuses, all members of the helper/inducer subset as indicated by the expression of markers T3 and T4.

Microenvironment of thymic myoid cells in myasthenia gravis

The microenvironment of myoid cells (MyCs) was studied in myasthenia gravis (MG) thymitis with lymphoid follicular hyperplasia (LFH) (nine cases) and with diffuse B cell infiltration (one case), and compared with findings in the thymuses of non-myasthenic control subjects (ten cases). Double immunostaining was used to demonstrate MyCs labelled by anti-desmin together with other thymic components such as keratin-positive epithelial cells, Ki-M 1-positive interdigitating reticulum cells (IDCs), Ki-M 4-positive follicular dendritic reticulum cells, Ki-M 6-positive macrophages, CD22-positive B-cells, CD1-positive cells, CD3-positive T-cells or HLA-DR-positive cells. Round or elongated MyCs were confined to the thymic medulla and were surrounded by CD3-positive T-cells and CD22-positive B-cells. In MG thymitis MyCs were localized in the vicinity of, but not inside germinal centres (GCs). MyCs were always HLA-DR-negative, but were invariably embedded in a cellular micromilieu with strong HLA-DR expression. A remarkable feature of MG thymitis was that the great majority of MyCs were in intimate contact with intramedullary IDCs. Morphometric studies confirmed that such contacts were significantly less frequent in thymuses from non-myasthenic subjects. This indicates that an IDC-dependent antigen-presenting process for T-cells may actively involve MyCs in MG thymitis.

Fc gamma receptors and HLA-DR antigens on thymus cells in myasthenia gravis

Receptors for the Fc part of IgG (Fc gamma R) and HLA-DR antigens were detected in thymus tissue from patients with myasthenia gravis (MG) using monoclonal antibodies (B1D6 and OKIa1) in indirect immunofluorescence. The amount of Fc gamma R and HLA-DR antigens was increased on epithelial reticular cells and on interdigitating reticular cells in hyperplastic thymus as compared to normal thymus. In thymomas from MG patients the neoplastic epithelial cells expressed Fc gamma R, whereas only a few cells had HLA-DR antigens. Neither thymocytes nor B lymphocytes in the hyperplastic thymuses and in the thymomas were stained by B1D6.

Myasthenia gravis: long-term prognostic value of thymus lactate dehydrogenase isoenzyme pattern of hyperplastic thymus and thymoma

Lactate dehydrogenase (LDH) isoenzyme pattern and the percent of H-subunit content were determined in the thymus of 62 patients (55 with hyperplasia, 7 with tumours) after thymectomy. An increase in LDH1 relative activity indicates that in the thymus of patients with myasthenia gravis the ratio of mature differentiated thymocytes was higher than in the thymus of control subjects. LDH isoenzyme profiles of thymus tumours were similar to those described in other neoplasms, except that thymomas with apparent predominance of epithelial cells and with minimal lymphocytic reaction exhibited a marked elevation only in LDH2 relative activity, presumably associated with the specific (secretory) function of epithelial cells. The elevation of H-subunit content, a parameter characteristic of both thymic components (lymphoid and epithelial), correlated closely with a poor clinical condition in patients several years after surgery.

Strategies for the modulation of neuroimmunological disease at the level of autoreactive T-lymphocytes

Cell lines of autoimmune T-lymphocytes have been established in several neuroimmunological model diseases and also in a human neurological autoimmune disease, myasthenia gravis. These cell lines generally have the T helper/inducer phenotype and recognize autoantigen in the context of class II histocompatibility antigens. Autoreactive helper T cell lines may become useful tools for the evaluation of new immunotherapeutic strategies. (1) Treatment with anti-Ia monoclonal antibodies presumably interferes with the interaction between Ia on the surface of antigen-presenting cells and the autoreactive T cell receptor; (2) Therapy with unmodified or modified autoantigen may be used to tolerize or delete the autoimmune T cells; (3) Monoclonal antibodies against the 'T cell domains' of autoantigen may prevent its recognition by the autoreactive T cells; (4) Treatment with monoclonal antibodies against T cell clonotypic or differentiation antigens may effectively delete or inactivate the autoreactive T cells. Furthermore, autoreactive helper T cells may be used to induce and establish anti-idiotypic suppressor T cell lines, or the autoimmune helper T cells may themselves display suppressive effects in an allogeneic system.

The cellular immunology of myasthenia gravis

The possible role of the thymus in autosensitization to the acetylcholine receptor (AChR) in myasthenia gravis is briefly discussed, and work on autoantibody synthesis in vitro is reviewed. Whereas blood lymphocytes sometimes make anti-AChR in response to mitogens, spontaneous production is more regularly observed in cells from lymph nodes, bone marrow and, above all, thymus, where it is selectively activated. Cell separation studies imply a dominant contribution by the germinal centre cells and plasma cells there, many of which are autonomous (i.e. no longer require T cell help). The need for germinal centre-directed immunosuppressive regimes is discussed.

Thymectomy in myasthenia with pure ocular symptoms

Eighteen patients with exclusively ocular symptoms of myasthenia were thymectomised. Suspected thymoma, resistance to pyridostigmine therapy or relapse following immunosuppressive therapy were taken as indications for surgery. The mean preoperative observation period before operation was 40 months, and after operation was 26 months. There was no operative or postoperative morbidity or mortality. Histological thymic abnormalities were found in all patients (in one case, thymoma; in four, persistent thymus; in 13, thymic hyperplasia). The histological abnormalities were identical to those found in generalised myasthenia. This included the distribution of T-cell subtypes as identified by use of monoclonal antibodies. The severity of ocular symptoms was rated using a score developed for this purpose. The score progressively declined after surgery to an average of 70% of its initial amount in 80% of patients. Full remission occurred in three cases. No patient developed generalized myasthenia. Antibody titres against acetylcholine receptors if elevated preoperatively also dropped following surgery, with one exception. Clear criteria for the expected therapeutic success of thymectomy could not be identified. Based on our results, and on the assumed significance of the thymus gland for pathogenesis, thymectomy should be considered in patients with pure ocular symptoms.

Autoantibodies to the basal cells of squamous epithelium react with thymic epithelial cells

Sera from carriers of hepatitis B surface antigen (17 out of 21), which reacted in immunofluorescence with the basal cell layer (BCL) of squamous epithelium, were also shown to react with a thymic stellate epithelial cell (SEC) characterized by long, dendritic-like cytoplasmic processes. Absorption of the autoantibodies against BCL of squamous epithelium (BCL-Ab) with a thymic homogenate abolished the reactivity with BCL and SEC, demonstrating that the same antigenic determinant was recognized in both cells. In the human thymus, SEC were present both in the cortex and in the medulla. In the outer cortex SEC delineated the septal spaces. SEC were also stained by anti-HLA-DR (Ia) but not by antiactin monoclonal antibodies. The morphology and distribution of SEC were similar to those of the previously described thymic epithelial cells containing alpha-1 thymosin (K. Hirokawa, J. E. McClure, and A. L. Goldstein, Thymus 4, 19, 1982). BCL-Ab were also found to react with five human epithelial thymomas. BCL-Ab seemed to be useful for further characterization of the thymic epithelial cells and for the immunodiagnosis of thymoma.

[Correlation of acetylcholine receptor antibodies and clinical severity of myasthenia gravis in combined immunosuppressive therapy]

The correlation between the severity of symptoms in myasthenia gravis (score) and the level of acetylcholine receptor-antibodies (ACHR-Ab) was studied in 24 patients treated with immunosuppressive therapy. This correlation was significant in every case regardless of whether or not thymectomy was performed in addition. It was demonstrated by means of a long-term follow up study, that a change in ACHR-Ab preceeds the corresponding change in score by 2-4 months in 80% of our patients. Measurements of ACHR-Ab ease the management of the therapy with immunosuppressants and make it safer. This is particularly true prior to and after termination of the treatment. A change in ACHR-Ab level by more than 20% indicates a change in score.

Rabbit antiserum to a citric acid extract of human skeletal muscle staining thymomas from myasthenia gravis patients

Rabbit antiserum to a citric acid extract of human skeletal muscle (CA) stained both the cell membrane and the cross-striational bands of skeletal muscle cells. The rabbit antiserum also stained the cell membrane of epithelial thymoma cells from myasthenia gravis (MG) patients. Normal and hyperplastic thymus tissue were not stained, apart from scattered myoid thymic cells. Absorption of the antiserum with CA abolished staining of the thymoma, indicating that human skeletal muscle and epithelial thymoma cells possess common antigens.

Clonal heterogeneity of thymic muscle-cell precursors

Three myoid-cell clones were established from the thymuses of two Wistar rats; one thymus yielded two clones, R615A and R615B2, and the other yielded one clone, R613Ad. The three clones were divided into two subtypes. Both subtypes were able to form myofibrils, expressed AChR on their cell-surface membrane, and contained myofibrillar ATPase characteristic of undifferentiated type-2C fibers, but they differed from each other in morphology, expression of Thy 1 antigen, spontaneous contractility, and qualitative accumulation of AChR. Immunolocalization studies using antisera against the respective cell subtypes also indicated regional differences in their cellular origin. These results show that the thymus contains heterogenous myoid-cell precursors.

Autoimmune human T lymphocytes specific for acetylcholine receptor

Myasthenia gravis is one of the best characterized human autoimmune disorders. Circulating autoantibodies to the nicotinic acetylcholine receptor (AChR) at the neuromuscular junction play a prominent part in the effector phase, that is, the immunoregulation. Indirect evidence, such as thymic abnormalities and the association with certain histocompatibility antigens (for example HLA-B8,-DR3) suggests a defect of immunoregulation at the level of thymus-dependent (T) lymphocytes. We report here on the isolation of autoreactive T cells from six patients with myasthenia gravis. From one of these patients, who is homozygous for HLA-DR3, we established a long-term T-cell line. The line cells are specific for purified fish and human AChR, display the surface phenotype of inducer/helper T cells and are genetically restricted to HLA-DR3. AChR-induced proliferation could be inhibited with two monoclonal antibodies against monomorphic DR determinants and also with DR3-specific alloantiserum.

Myasthenia gravis: the specificities of skeletal muscle and thymus antibodies

Antibodies against skeletal muscle antigens and against thymic myoid cells were examined in sera from 40 patients with myasthenia gravis (MG). Using an indirect immunofluorescence technique, antibodies against the surface of muscle cells were found in 23 sera, and antibodies against muscle cell cross-striations in 28 sera. Antibodies against thymic myoid cells were found in 27 sera, stained cells also occurring in fetal thymus from 14 weeks of gestation and in hyperplastic thymus from MG patients. Sera which stained myoid cells also stained muscle cell cross-striations. Sera from all the 20 patients with thymoma contained antibodies to a citric acid extract of skeletal muscle (CAE) as detected by indirect haemagglutination, whereas sera from 20 comparable patients without thymoma did not contain CAE antibodies. The myoid cell antibodies could be absorbed by homogenized muscle, but not by CAE-coated sheep erythrocytes. Different antibody patterns are seen among different MG patients; close associations were found between cross-striational and myoid cell antibodies, and between CAE-antibodies and a thymoma.

Myasthenia gravis: overlap with 'polyendocrine' autoimmunity

81 patients with spontaneously acquired myasthenia gravis (MG) were investigated for the presence of autoimmune (AI) diseases and their sera were tested for a range of organ-specific autoantibodies. 77 of the patients were HLA-phenotyped. Antibody titres to acetylcholine receptors (AChR) were higher in non-thymomatous patients who possessed HLA-B8 (p less than 0.05) and/or -DR3 (p less than 0.05) as compared to patients lacking these HLA antigens. 3 out of 20 (15%) patients with ocular MG, 7/23 (30%) with generalized MG of early onset, 11/23 (48%) generalized MG of late onset and 5/14 (35%) patients with thymoma had either overt AI diseases or significant titres of organ-specific autoantibodies suggesting subclinical AI disease. In ocular MG, low titres and an infrequent finding of antibodies to AChR (32%) as well as the low prevalence of associated autoantibodies and AI diseases indicate that this subgroup of MG consists of patients with restricted AI reactivity. HLA-B8 and -DR3 were present in all the patients with associated AI disorders in the young onset group but in none of the patients with old age of onset. In the young group, 6 out of 7 patients with associated AI conditions were women whereas the sex ratio was about equal in the older cases in both, patients with and without associated AI diseases or autoantibodies. We conclude from these observations that ageing provides conditions that allow the breakdown of self tolerance. The simultaneous presence of HLA B8, DR3 and female sex provide important additional factors for early expression of MG.