A pathogenetic role for the thymoma in myasthenia gravis. Autosensitization of IL-4- producing T cell clones recognizing extracellular acetylcholine receptor epitopes presented by minority class II isotypes

Correlation of thymic pathology with HLA in myasthenia gravis

The aim of this study was to investigate associations between thymic pathology and HLA in myasthenia gravis. HLA typing was performed in 95 of 125 Caucasian patients who underwent transsternal thymectomy for myasthenia gravis between 1976 and 1995. Multiple comparison procedures applied within each HLA locus demonstrated significant correlations between the ancestral suprahaplotype A1 B8 DRB1*0301 DRB3*0101 DQA1*0501 and thymic hyperplasia and between HLA-A24 and thymoma. A weaker association was found between A3 and thymic atrophy and thymolipoma. On logistic discriminant analysis, HLA-B8 (P = 0.001) and HLA-A3 (P = 0.028) were identified as the only significant classifiers to jointly provide a good discriminator between the thymic pathologies. When the suitability of HLA for detection of thymoma was examined in a second logistic regression analysis, both HLA-A24 (OR 9.7; 95% CI [1.6, 73.7]) and HLA-B8 (OR 0.1; 95% CI [0.0, 0.5]) were significant predictive factors. The above correlations between thymic pathology and HLA-A3, HLA-A24, and HLA-B8 (but not MHC class II alleles) suggest an involvement of MHC class I restricted T cells in myasthenic autoimmunity that may partially be reflected by thymic pathology.

Potent immunostimulatory dendritic cells can be cultured in bulk from progenitors in normal infant and adult myasthenic human thymus

Low density cells can readily be enriched from thymus tissue both of children undergoing cardiac surgery and of older patients with myasthenia gravis, and can be cryostored in bulk. When fresh or thawed cells are cultured with granulocyte-macrophage colony-stimulating factor and stem cell factor with or without tumour necrosis factor-alpha (TNF-alpha), they generate numerous cells with the characteristic ultrastructural, phenotypic and functional properties of dendritic cells. These proved to be very potent, both as stimulators of primary mixed leucocyte responses and as costimulators in oxidative mitogenesis. Especially after exposure to TNF-alpha, these dendritic cells also processed a natural epitope from a 437-residue polypeptide and presented it efficiently to an autoimmune T-cell clone (of T helper type 0 phenotype). Thus, immunostimulatory dendritic cells can be cultured in relative abundance from progenitors in infant and adult human thymus. Both are convenient sources of potent antigen-presenting cells of identifiable origins, e.g. for use in selecting human T-cell lines.

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.

Determinant spreading and immune responses to acetylcholine receptors in myasthenia gravis

In myasthenia gravis (MG), antibodies to the muscle acetylcholine receptor (AChR) cause muscle weakness. Experimental autoimmune myasthenia gravis (EAMG) can be induced by immunisation against purified AChR; the main immunogenic region (MIR) is a conformation-dependent site that includes alpha 67-76. EAMG can also occur after immunisation against extracellular AChR sequences, but this probably involves intramolecular determinant spreading. In MG patients, thymic hyperplasia and germinal centres are found in about 50%, and thymoma in 10-15%. The heterogeneous, high affinity, IgG anti-AChR antibodies appear to be end-products of germinal centre responses, and react mainly with the MIR or a site on fetal AChR; the latter contains a gamma subunit and is mainly expressed on myoid cells in the thymic medulla. T cells cloned against recombinant AChR subunits recognise principally two naturally processed epitopes: epsilon 201-219 derived from adult AChR which is expressed in muscle, and sometimes in thymic epithelium, and alpha 146-160, common to fetal and adult AChR. Since AChR is not normally co-expressed with class II, it is unclear how CD4+ responses to AChR alpha and epsilon subunits are initiated, and how and where these spread to induce antibodies against fetal AChR. Various possibilities, including upregulation of class II on muscle/myoid cells and involvement of CD8+ responses to AChR and other muscle antigens, are discussed.