Cell surface complement regulators moderate experimental myasthenia gravis pathology

Myasthenia gravis (MG) is an autoimmune disease of neuromuscular transmission characterized by skeletal muscle weakness and fatigue upon exertion. Antibodies directed against the nicotinic acetylcholine receptor (AChR) located on the postsynaptic surface of the neuromuscular junction are the initiators of pathology in the vast majority of patients, and the complement system is a major effector mechanism in both the human disease and animal models of experimentally acquired myasthenia gravis (EAMG).1,2 Membrane attack complex (MAC) deposition was identified at the neuromuscular junction (NMJ) in both EAMG and human MG over 30 years ago.3 MAC deposition at the NMJ in EAMG has been shown to be a necessary condition to produce EAMG.4 An array of soluble and cell surface complement regulators protect “self-cells” from the deleterious effect of complement activation. Decay accelerator factor (DAF), complement receptor 1-related gene/protein y (Crry), and CD59 are three membrane-bound proteins that restrict complement at different levels of the activation cascade. They are all concentrated at the NMJ5 and appear to protect it from injury produced by EAMG induced by infusion of monoclonal antibodies directed toward the AChR. Mice deficient in either DAF or CD59a (one of the two mouse CD59 genes) exhibit more severe weakness and complement deposition than wild-type (WT) mice.6-8 Daf and CD59a double-knockout mice demonstrate a very severe EAMG phenotype, indicating that Crry does not provide a significant shield from complement attack.6,8 The advent of drugs that inhibit complement at different steps of the activation cascade has made it critical to define more precisely the involvement of complement in the pathogenesis of MG.9,10 DAF accelerates the decay of and thereby inactivates C3 (C4b2a and C3bBb) and C5 (C4b2a3b and C3bBb3b) convertases that assemble on self-cells.11,12 CD59 specifically inhibits formation of the MAC, the effector of complement-mediated cell damage, by interfering with polymerization of C9 on cell-bound C5b-8 complexes.13-15 Crry, a rodent-specific complement regulator, combines functional properties of DAF and human membrane cofactor protein (MCP), inhibiting complement at the level of the C3 convertases.16 A detailed analysis of the role of DAF and CD59 in animal models may lead to better understanding of the human disease. Evaluation of Crry is less relevant, because the protein is not present in humans and Crry deletion is lethal to the fetus.17 Previous studies from our laboratory concluded that DAF is the primary shield from complement-induced injury of the postsynaptic surface.6 The interpretation of previous studies on the role of complement regulators in EAMG conducted with mCD59a−/− mice is compromised by the lack of consideration of a second isoform of mCD59b, designated mCD59.18-20 Similar to human CD59, mCD59a is widely expressed among tissues. Initial studies of CD59b suggested that its expression is restricted to the testis, but subsequent work documented a wider expression of CD59b transcripts in tissues,21 although its expression in skeletal muscle has not been evaluated. The finding that mCD59a and mCD59b have two promoters raised the possibility of compensatory expression of the alternate gene in single CD59a or CD59b knockout mice.20 Based on evidence that the expression levels of complement components and complement regulators vary and may be differentially regulated in different tissues, and on the availability of Daf1−/− and CD59ab−/− mice, we reassessed the relative roles of CD59 and DAF in the pathogenesis of EAMG.