Myasthenia gravis: the specificities of skeletal muscle and thymus antibodies

Muscle autoantibodies in myasthenia gravis: beyond diagnosis?

Myasthenia gravis is an autoimmune disorder of the neuromuscular junction. A number of molecules, including ion channels and other proteins at the neuromuscular junction, may be targeted by autoantibodies leading to abnormal neuromuscular transmission. In approximately 85% of patients, autoantibodies, directed against the postsynaptic nicotinic acetylcholine receptor can be detected in the serum and confirm the diagnosis, but in general, do not precisely predict the degree of weakness or response to therapy. Antibodies to the muscle-specific tyrosine kinase are detected in approximately 50% of generalized myasthenia gravis patients who are seronegative for anti-acetylcholine receptor antibodies, and levels of anti-muscle-specific tyrosine kinase antibodies do appear to correlate with disease severity and treatment response. Antibodies to other muscle antigens may be found in the subsets of myasthenia gravis patients, potentially providing clinically useful diagnostic information, but their utility as relevant biomarkers (measures of disease state or response to treatment) is currently unclear.

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.

Effect of intraventricular injection of an anti-Purkinje cell antibody (anti-Yo) in a guinea pig model

Female guinea pigs had intraventricular injections of either IgG from a patient with paraneoplastic cerebellar degeneration (PCD) and anti-Purkinje cell antibodies (anti-Yo IgG) or control IgG. In animals that received a single injection of control or anti-Yo IgG and were killed at different time intervals, IgG immunoreactivity was present in the cytoplasm of Purkinje cells at 2 h and persisted at 24 h. In guinea pigs injected for 15 days with control or anti-Yo IgG and sacrificed 24 h after the last injection, IgG was detected into the Purkinje cells in both groups, whereas animals killed 7 and 30 days after the last injection had no staining for IgG in the Purkinje cells. Clinical or pathologic evidence of cerebellar involvement was not seen in any of the animals. This study suggests that anti-Yo antibody alone may not be the cause of the Purkinje cell loss in PCD.

Thymic lymphoepitheliomas and skeletal muscle expressing common antigen(s)

Rabbit antiserum to a citric acid extract of human skeletal muscle (CA) stained epithelial thymoma cells as well as skeletal muscle. Thymomas from two myasthenia gravis (MG) patients showing no circulating anti-CA antibodies prior to thymectomy were also stained by the antiserum. Thus, in these patients as well, the thymoma and skeletal muscle possess common antigens. The rabbit and the human antibodies most probably reacted with different antigens, apparently located close to each other in the cell membrane. The reason why anti-CA antibodies cannot be detected in serum from a few MG patients with a thymoma may be that the thymoma-associated antigen is not present in vivo in these cases, or that an inhibiting factor blocks the antibody synthesis. Both patients developed anti-CA antibodies post-operatively, which favours the latter explanation.

Complement-dependent toxicity of serum from myasthenic patients to muscle cells in culture

The toxicity of myasthenic sera to rat myotubes in monolayer culture was examined by measuring the release of [Me-3H]carnitine from pre-loaded cells. In the presence of guinea pig complement, heat-inactivated serum samples from 9 out of 13 myasthenic patients showed clear myotoxicity, in contrast to 0 out of 11 normal controls and 0 out of 6 polymyositis patients. Neither heat-inactivated sera alone nor guinea pig complement sera alone showed myotoxicity. Removal of anti-acetylcholine receptor (anti-AChR) antibodies from a myasthenic serum sample by affinity absorption led to loss of myotoxicity. Myotoxicity of myasthenic sera could, in most cases, be confirmed by light microscopy. These results support the idea that complement-mediated cell damage, initiated by anti-AChR antibodies, contributes to post-synaptic membrane degeneration in myasthenia gravis.

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.