Immune complexes (IgG and C3) at the motor end plate in myasthenia gravis. Ultrastructural and light microscopic localization and electrophysiologic correlations

Although there is strong evidence that myasthenia gravis (MG) is caused by an autoimmune reaction to the nicotinic postsynaptic acetylcholine receptor (AChR) protein, immune complexes have never been demonstrated at the end plate by immunocytochemistry or immunoelectron microscopy. Staphylococcal protein A (which binds to the F(c) region of human IgG subclasses 1, 2, and 4) and rabbit anti human C3 conjugated with peroxidase were used for the ultrastructural (5 patients) and light microscopic (12 patients) localization of IgG and C3, respectively, at MG end plates. Both IgG and C3 were localized on segments of the postsynaptic membrane and fragments of degenerating junctional folds in the synaptic space. In nonmyasthenic control patients no immune complexes were evident at the end plate. As judged by morphometric analysis of electron micrographs, the immune complexes were more abundant in the less severely affected MG patients than in the more severely affected ones. A linear correlation was demonstrated between the length of the postsynaptic membrane binding immune complexes and the amplitude of the miniature end plate potential. The less intense reaction for immune complexes in the more severely affected MG patients can be attributed to the smaller quantity of AChR remaining at their end plates. The findings provide unambiguous evidence for a destructive autoimmune reaction involving the postsynaptic membrane in MG. Immunopharmacologic blockade of AChR and IgG induced modulation of AChR may also contribute to the AChR deficiency at the MG end plates.