Rat muscle cells were grown in culture for use as an experimental model in which to study the myolytic effects of myasthenic serum in vitro. Use was made of a procedure which depends upon the selective uptake of tritium-labelled carnitine by cultured myotubes, loss of which can be monitored following cytolytic damage. The studies demonstrated that heat-inactivated myasthenic serum samples caused myotube-specific lysis in a manner that was dependent on the addition of complement. The concentration and activity of the complement source was shown to be a major factor in detecting myotoxicity. Using optimised assay conditions, a myotoxicity study was carried out using a range of normal and myasthenic serum samples. In the presence of guinea-pig complement, heat-inactivated serum samples from 9 out of 13 myasthenic patients showed clear rnyotoxicity in contrast to 0 out of 12 normal controls and 0 out of 6 polymyositis patients. Neither heat-inactivated sera alone nor guinea-pig complement alone showed myotoxicity. A further study defined new conditions under which previously 'non-toxic' myasthenic serum samples demonstrated myotoxicity. Removal of anti-AChR antibodies from a myasthenic serum sample by affinity absorption led to a loss of complement-mediated myotoxicity. Finally, studies were carried out in which IgG or IgG depleted of subclass 3, was purified from myasthenic serum samples and tested for complement-mediated myotoxicity. The IgG fractions caused myotoxicity in a similar manner to the whole serum. The studies were extended to human foetal muscle cells in culture which were shown to be less mature than the cultured rat muscle cells in this study. Attempts were made to define optimal growth conditions for the human foetal muscle cells vitro but these were inconclusive. Comparable complement-mediated myotoxicity by myasthenic serum towards human muscle cultures was not shown. However, manipulation of assay conditions resulted in clear myotoxicity by the 2 myasthenic serum samples tested, relative to normal controls. The results gained from this work support the suggestion that complement-mediated cell damage, initiated by anti-AChR antibodies, may contribute to post-synaptic membrane degeneration in myasthenia gravis.
|Date of Award||1985|