EXPRESSION OF HUMAN-TORPEDO HYBRID ACETYLCHOLINE-RECEPTOR (ACHR) FOR ANALYZING THE SUBUNIT SPECIFICITY OF ANTIBODIES IN SERA FROM PATIENTS WITH MYASTHENIA-GRAVIS (MG)

The nicotinic AChR, a pentamer composed of alpha(2) beta gamma(or epsi lon)delta subunits, is the autoantigen in the human autoimmune disease MG. Anti-AChR antibodies in MG sera bind mainly to conformational epi topes, therefore determination of their specificities requires the use of intact AChR. Indirect antibody competition studies have suggested that most MG antibodies are inhibited from binding to AChR by MoAb to the main immunogenic region (MLR) on the alpha-subunits. More recently , based on the knowledge that MG antibodies show little detectable cro ss-reaction with Torpedo AChR, we have shown, using mouse-Torpedo hybr id AChR, that most MG antibodies that detectably cross-react with the mouse AChR bind to the alpha-subunit. To analyse the whole anti-AChR a ntibody repertoire in MG sera, we expressed on stably transfected fibr oblasts a novel human alpha+ Torpedo beta gamma delta AChR and compare d the antibody titres against human, Torpedo, and the hybrid AChR. Dir ect information was provided for the subunit specificity of several Mo Abs and sera from 50 MG patients. On average, at least 48% of the anti -AChR antibodies in the sera were directed against the or-subunit. Int erestingly, the anti-alpha-subunit antibodies predominated in low titr e (0.6-7.4 nM) but not in high titre (10-386 nM) sera, where they comp rised on average 68% versus 23% of the antibodies, respectively. Final ly, the directly determined anti-alpha-subunit antibodies and the anti -MIR antibodies defined by antibody competition were significantly cor related, thus suggesting that at least a significant fraction of the a nti-MIR antibodies in MG sera bind to the alpha-subunit.