Monoclonal antibodies to the thyrotropin receptor bind to a 56-kDa subunit of the thyrotropin receptor and show heterogeneous bioactivities

Four monoclonal antibodies to the thyrotropin (TSH) receptor were established by fusing human peripheral lymphocytes of patients from Graves' disease with a human myeloma cell line. Of two antibodies with TSH-binding inhibitor immunoglobulin activity (TBII), one inhibited TSH stimulation of adenylate cyclase and another stimulated adenylate cyclase. These antibodies showed competitive and noncompetitive modes of binding inhibition, respectively. Of the other two antibodies without TBII activity, one stimulated adenylate cyclase and the other inhibited TSH stimulation of adenylate cyclase. Of the two antibodies, which inhibited TSH stimulation of adenylate cyclase, one with TBII activity inhibited stimulation of adenylate cyclase by stimulating antibody with TBII activity, but another without TBII activity inhibited stimulation by both stimulating antibodies with or without TBII activity. These inhibitor antibodies did not influence the stimulation of adenylate cyclase by Forskolin and guanosine 5'-(β,γ-imido)triphosphate compounds which are known to affect other parts of the receptor-adenylate cyclase system than the receptor unit. Four antibodies with heterogeneous potencies to the TSH receptor reacted with glycoproteins extracted from thyroid membranes. One stimulating antibody without TBII activity also interacted with the glycolipid fraction of the membrane preparation, and the binding decreased after desialylation or deglycosylation of the membrane components. In order to identify the binding sites of these monoclonal antibodies, receptor proteins interacting with antibodies were visualized by Western blot analysis and by the label transfer cross-linking method. All of these antibodies with different characteristics reacted with a 56-kDa molecule.