Genetic Evidence for Involvement of Classical Complement Pathway in Induction of Experimental Autoimmune Myasthenia Gravis

Abstract

Abs to acetylcholine receptor (AChR) and complement are the major constituents of pathogenic events causing neuromuscular junction destruction in both myasthenia gravis (MG) and experimental autoimmune MG (EAMG). To analyze the differential roles of the classical vs alternative complement pathways in EAMG induction, we immunized C3−/−, C4−/−, C3+/−, and C4+/− mice and their control littermates (C3+/+ and C4+/+ mice) with AChR in CFA. C3−/− and C4−/− mice were resistant to disease, whereas mice heterozygous for C3 or C4 displayed intermediate susceptibility. Although C3−/− and C4−/− mice had anti-AChR Abs in their sera, anti-AChR IgG production by C3−/− mice was significantly suppressed. Both C3−/− and C4−/− mice had reduced levels of B cells and increased expression of apoptotis inducers (Fas ligand, CD69) and apoptotic cells in lymph nodes. Immunofluorescence studies showed that the neuromuscular junction of C3−/− and C4−/− mice lacked C3 or membrane attack complex deposits, despite having IgG deposits, thus providing in vivo evidence for the incapacity of anti-AChR IgGs to induce full-blown EAMG without the aid of complements. The data provide the first direct genetic evidence for the classical complement pathway in the induction of EAMG induced by AChR immunization. Accordingly, severe MG and other Ab- and complement-mediated diseases could be effectively treated by inhibiting C4, thus leaving the alternative complement pathway intact.

Effect of complement and its regulation on myasthenia gravis pathogenesis

“…Mice deficient in C3 and C4 are resistant to the disease [66]. Both mouse strains produce AChR antibodies, but their production is considerably reduced in the C3-deficient strain.…”

Genetic control of autoantibody expression in autoimmune myasthenia gravis: role of the self-antigen and of HLA-linked loci

“…4 In the majority (85%) of the patients, it is targeted by highly specific autoantibodies. 5,6 Although other factors, notably the complement pathway, are necessary to produce the clinical symptoms, 7,8 anti-AChR autoantibodies are a primary effector in MG, in addition to being an essential diagnostic tool, and understanding the mechanisms of their production and of their regulation is of substantial importance. 9 The thymus of MG patients is often abnormal and this plays an essential role in autoantibody production.…”

C5a is not involved in experimental autoimmune myasthenia gravis pathogenesis

“…MAC has been observed in the postmortem frozen muscle specimens of EAMG mice (Tuzun et al, 2003) and the number of MAC deposits has been reduced in EAMG resistant mice (Tuzun et al, 2003;Tuzun et al, 2006). Further, administration of an antibody against the C6 component of MAC before the passive transfer of anti-AChR antibody has prevented EAMG induction in rats and has inhibited accumulation of MAC components C6 and C9 (Biesecker and Gomez, 1989).…”

“…Recent years have brought growing awareness of the involvement of the complement system in anti-AChR mediated NMJ destruction in MG and EAMG (Tuzun et al, 2003;Tsujihata et al, 1989). In EAMG, activation of the complement cascade occurs by the classical pathway leading to C3 activation and generation of C3b.…”

“…Subsequent cleavage of C5 leads to release of C5a and generation of C5b. C5b initiates formation of C5b-9 complex or membrane attack complex (MAC), which is involved in lysis of the postsynaptic NMJ membrane (Tuzun et al, 2003;Tsujihata et al, 1989).…”