Complement inhibition by C3-siRNA treatment prevents AChR loss and reduces complement activation in the rat Passive Transfer Myasthenia Gravis (PTMG)

Myasthenia gravis (MG), a well understood antibody-mediated autoimmune disease, is characterized by skeletal muscle weakness and fatigue. The autoantibodies are directed against neuromuscular junction (NMJ) proteins such as the acetylcholine receptor (AChR), essential for neuromuscular signal transmission. Anti-AChR antibodies are mainly of the immunoglobulin (Ig) types IgG1 and IgG3, which have a high capacity to activate the classical complement pathway. In AChR-MG, complement activation and the subsequent formation of the membrane attack complex (MAC) at the NMJ is one of the key effector mechanisms, causing substantial damage to the entire postsynaptic membrane and resulting in the typical clinical manifestations of muscle weakness and fatigue observed in both patients and animal models.

Since classical treatment strategies, which focus on general immunosuppression and enhancing the effect of ACh binding to the AChR, do not lead to full remission of symptoms in many cases, new treatment approaches are focusing more specifically on targeting effector mechanisms, such as complement activation. The first complement inhibitor approved for MG was eculizumab, a monoclonal antibody targeting C5. However, clinical trials showed that up to 28% of patients treated with eculizumab did not experience an improvement in myasthenic symptoms, emphasizing the urgent need for alternative therapies. Intervening earlier in the complement cascade might be more beneficial, as it could prevent the release of anaphylatoxins, such as C3a, that contribute to further inflammation at the NMJ.

In this study, the passive transfer myasthenia gravis (PTMG) rat model was used to investigate whether silencing hepatic C3 expression with a C3-targeted, small interfering (si)RNA, could ameliorate disease symptoms in the acute phase of MG. Female Lewis rats were injected subcutaneously with different dosages of C3-siRNA prior to PTMG induction with mAb35. C3-siRNA, administered weekly at a dose of 30 mg/kg, significantly prevented weight and AChR loss, and accordingly improved muscle function, as measured by muscle strength tests and electromyography. Consistently, the treatment reduced MAC deposition at the NMJ. Overall, these results provide insight into the efficacy of complement inhibitors in the acute phase of MG and suggest potential strategies for advanced treatment options in AChR-MG crisis.