Passive transfer of experimental autoimmune myasthenia gravis by monoclonal antibodies to the main immunogenic region of the acetylcholine receptor (1)
Antibodies to the acetylcholine receptor (AChR) cause muscular weakness by impairing neuromuscular transmission in experimental autoimmune myasthenia gravis (EAMG) induced by immunization of animals with AChR, and in human myasthenia gravis (MG) induced by unknown factors. Anti-AChR seracan also passively transfers EAMG to normal rats by fixing complement, thereby targeting the postsynaptic membrane for phagocytosis by macrophage.
Monoclonal antibodies(mAbs) to the AChR helped to identify the main immunogenic region (MIR) of the AChR which is located on the extracellular side of the AChR a-subunit which are this region was localized between amino acid residues 6-85 or 46-127 of the a-subunit.
Rats injected with either of two mAbs to the MIR (mAb 35 or mAb 42) developed muscular weakness within 24 h and showed loss of muscle AChR at 72 h. Only 5-10% of the AChRs which remained 3 days after passive transfer had mAbs bound, suggesting that most mAb-bound AChRs were destroyed in the 3 days between injection and sacrifice. Rats injected with mAbs 155, 137, and 124 did not develop EAMG, since these mAbs bind to the cytoplasmic side of the AChR. The anti-a-subunit mAb 64 does not cause EAMG. It binds to only 1.5% of the AChRs. This mAb bound also to less than 2% of the AChRs on intact BC3H1 cells, but bound to 92% after solubilization.
The weakness of rats injected with anti-MIR mAbs was found to be most severe on day 2. The Two anti-MIR mAbs 208 and 210 were very efficient at passively transferring EAMG. All rats became moribund and lost about half of their AChRs, and most of the remaining AChRs had antibodies bound. All five anti-MIR mAbs which cross reacted with rat muscle AChR were able to passively transfer EAMG. Although these five mAbs have the same specificity, they are of different IgG subclasses (mAbs 6, 35, and 210 are IgG1; mAbs 42 and 210 are IgG2a). The anti-b-subunit mAb 73 was not able to cause EAMG or AChR loss in these experiments although 83% of the AChRs had bound the antibody. MAb 73, although unable to cause AChR loss through complement fixation, a mechanism which occurs the first few days, might be able to cause AChR loss by antigenic modulation after several days. However, none of the nine rats which received this mAb was affected). The rats which received mAb 208 were all ill after 2 days, but they recovered by 5 days, even though their AChR concentration was always low and a large percentage of the remaining AChRs had antibodies bound.
