Myasthenogenicity of the main immunogenic region and endogenous muscle nicotinic acetylcholine receptors

Abstract

In myasthenia gravis (MG) and experimental autoimmune MG (EAMG) many pathologically significant autoantibodies are directed at the main immunogenic region (MIR), a conformation-dependent region at the extracellular tip of α1 subunits of muscle nicotinic acetylcholine receptors (AChRs). Human muscle AChR α1 MIR sequences were integrated into Aplysia ACh binding protein (AChBP). The chimera was potent at inducing both acute and chronic EAMG, though less potent than Torpedo electric organ AChR. Wild-type AChBP also induced EAMG but was less potent, and weakness developed slowly without an acute phase. AChBP is more closely related in sequence to neuronal α7 AChRs which are also homomeric, however autoimmune responses were induced to muscle AChR, but not to neuronal AChR subtypes. The greater accessibility of muscle AChRs to antibodies, compared to neuronal AChRs, may allow muscle AChRs to induce self-sustaining autoimmune responses. The human α1 subunit MIR is a potent immunogen for producing pathologically significant autoantibodies. Additional epitopes in this region or other parts of the AChR extracellular domain contribute significantly to myasthenogenicity. We show that an AChR-related protein can induce EAMG. Thus, in principle, an AChR-related protein could induce MG. AChBP is a water soluble protein resembling the extracellular domain of AChRs, yet rats which developed EAMG had autoantibodies to AChR cytoplasmic domains. We propose that an initial autoimmune response, directed at the MIR on the extracellular surface of muscle AChRs, leads to an autoimmune response sustained by muscle AChRs. Autoimmune stimulation sustained by endogenous muscle AChR may be a target for specific immunosuppression.

AChR-specific immunosuppressive therapy of myasthenia gravis

“…A MIR/AChBP chimera, human AChR a1(1-30, 60-81)/AChBP, is bound with high affinity by mAbs to the MIR (including a MG patient-derived mAb to the MIR) and reacts with autoantibodies from cat, dog and human MG patients [52]. This chimera is also potent at inducing EAMG [34,56]. This proves that the chimera contains pathologically significant epitopes that are sufficient to initiate a pathological autoimmune response.…”

“…A critical pathological event is complement-mediated attack on the postsynaptic membrane that causes shedding of AChR-rich membrane fragments into the synaptic cleft [57,58,60,100]. These shed muscle AChRs trigger an immune response because immunization with the MIR/AChBP chimera that lacks cytoplasmic domains produces EAMG and many autoantibodies to muscle AChR cytoplasmic domains [34,56]. We think that these endogenous AChRs drive a feed-forward cycle of autoimmune stimulation to muscle AChRs.…”

“…Rats with EAMG induced by the chimera have concentrations of antibodies to muscle AChR cytoplasmic domains as high as those developed after immunization with Torpedo AChR [34,56]. Antibodies to the MIR on the extracellular surface of rat muscle AChRs trigger complement-mediated focal lysis of the postsynaptic membrane, resulting in shedding of membrane fragments into the synaptic cleft [57][58][59][60].…”

“…Experimental autoimmune myasthenia gravis (EAMG), an animal model for MG, can be induced in rats, mice, monkeys, and other susceptible species by immunization with AChRs from fish electric organs, mammalian muscle, or recombinant proteins that preserve the native conformation of the MIR [32][33][34][35][36]. EAMG shares many clinical and immunopathological features with those of human MG.…”

Antigen-Specific Immunotherapeutic Vaccine for Experimental Autoimmune Myasthenia Gravis

“…What causes the autoimmune response to AChRs in MG is not known. EAMG can be induced by immunization with AChRs from fish electric organs, mammalian muscle, or by the MIR sequences in a chimera with ACh binding protein that preserves the native conformation of the MIR (7–9). There is no cure for MG. MG is treated with acetylcholinesterase inhibitors (with modest efficacy in improving neurotransmission) and nonspecific immunosuppressants (whose beneficial effects may be delayed for months and can cause severe side effects) (2, 10).…”