Activation of the classical complement pathway in myasthenia gravis with acetylcholine receptor antibodies

Abstract Introduction/Aims Myasthenia gravis (MG) is an autoimmune disease affecting the neuromuscular junction (NMJ) of skeletal muscle. Complement activation is one of the mechanisms by which anti–acetylcholine receptor (anti‐AChR) autoantibodies reduce synaptic transmission at the NMJ. In this study, we aimed to examine the activation of the complement pathways, including the classical pathway, as potential contributors to the pathogenesis of MG with anti‐AChR antibodies. Methods In this single‐center, observational study of 45 patients with anti‐AChR–antibody‐positive generalized MG, serum concentrations of major components of the complement pathways, including C1q, C5, C5a, soluble C5b‐9 (sC5b‐9), Ba, and complement factor H, were measured using an enzyme‐linked immunosorbent assay. A total of 25 patients with a non‐inflammatory neurological disorder served as controls. In addition, the relationships of complement activation with clinical characteristics were examined. Results The patients with MG exhibited lower serum levels of C5 ( p = .0001) and higher serum levels of sC5b‐9 ( p = .004) compared with the control group. At about 6 months (range, 172–209 days) after the start of immunotherapy, serum levels of Ba were significantly higher than baseline levels ( p = .002) and were associated with improvement in MG clinical scores. Discussion Herein, we provide evidence for the activation of the classical complement pathway and its association with disease activity in anti‐AChR–antibody‐positive generalized MG.