06OIn-depth examination of motor endplate pathology in AChR-Ab-positive myasthenia gravis

Since over 40 years it has been well established that complement deposition at the neuromuscular junction (NMJ) in AChR-antibody-positive myasthenia gravis (AChR-ab+ MG) contributes to the destruction of the motor endplate. However, despite these early findings, no comprehensive studies have been conducted to further investigate the underlying mechanisms or explore potential therapeutic strategies. Motivated by the rapid effects of novel therapies, which suggest that complete NMJ destruction is unlikely, we aim to re-examine the NMJ in AChR-ab+ MG using different techniques. In particular, using histology, electron microscopy (EM), transcript and proteomic analysis we investigated intercostal muscle (ICM) biopsies from 30 AChR-ab+ MG patients and compared these to non-myasthenic controls. Our histological analysis revealed clear C5b-9 deposition in all AChR-ab+ MG patients, with an average of 75% of investigated neuromuscular junctions (NMJs) showing complement positivity and an interindividual range of 33–100%, while no deposition was observed in control muscles. Moreover, electron microscopy (EM) studies showed postsynaptic simplification and shortened synaptic clefts, though not all NMJs exhibited these signs of endplate destruction. Notably, there was also considerable inter- and intraindividual variability in the extent of endplate damage, despite the overall high rate of complement deposition. Even though morphological changes in ICM tissue is low, we identified transcript changes indicating an upregulation of immune cell markers, with particularly strong expression of IL1B and IL6. Interestingly, we also observed a significant increase in the B cell chemoattractant BAFF and its receptor CXCR5, despite the absence of detectable B cells in ICM. Furthermore, proteomic analysis identified an upregulation of DNM2 (Dynamin-2), which mediates endocytosis of synaptic vesicles and mutations of the DNM2 gene were found to be associated with centronuclear myopathies, neuropathies and other neuromuscular diseases. Moreover, we detected a downregulation of TARSH (Target of Nesh-SH3), which is especially interesting, as this protein is predicted to be involved in several processes, including extracellular matrix organization and was found in CHRNE-related congenital myasthenic syndrome. In summary, our findings demonstrate variability in complement deposition and NMJ destruction in AChR-ab+ MG, indicating that not all NMJs are equally affected. Histologic and transcriptomic analyses suggest a critical role of macrophages, which could promote future therapeutic strategies. Project supported by Janssen Pharmaceutica NV, a Johnson and Johnson company.