Detection of antibodies directed against the cytoplasmic region of the human acetylcholine receptor in sera from myasthenia gravis patients

The nicotinic acetylcholine receptor (AChR) is the autoantigen in the human autoimmune disease myasthenia gravis (MG). Anti-AChR antibodies in MG sera bind mainly to conformational epitopes, therefore the determination of the ir specificities requires the use of native AChR. Antibody competition stud ies suggest that most MG antibodies are directed against the extracellular part of the molecule, whereas antibodies directed against the cytoplasmic r egion of the AChR have not been detected. To determine whether even small q uantities of such antibodies exist in MG sera, we performed competition exp eriments based on the inhibition by MG sera of the binding of MoAbs to the human AChR, rather than inhibition by MoAbs of the binding of MG sera perfo rmed earlier. When MoAbs directed against cytoplasmic epitopes on the alpha or beta subunits (alpha 373-380 and beta 354-360) were used as test MoAbs, 17% or 9% of MG sera inhibited the binding of the anti-alpha or anti-beta subunit MoAbs, respectively, by greater than or equal to 50%. Non-specific inhibition was excluded. These results suggest the presence, in several MG sera, of antibodies directed against cytoplasmic regions of the AChR; yet t hese antibodies seemed to represent a relatively small proportion of the to tal anti-AChR antibodies. The corresponding epitopes may be involved in the inducing mechanisms in certain MG cases, and knowledge of the presence of such antibodies may be useful in understanding the autoimmune mechanism inv olved in MG.