Monoclonal antibodies to the thyrotropin receptor: the identification of blocking and stimulating antibodies

Monoclonal antibodies to the thyrotropin (TSH) receptor have been obtained from fusions of mouse myeloma cells with spleen cells immunized with solubilized thyroid membrane preparations. Two monoclonal antibodies which inhibit 125I-TSH binding and are reactive with the glycoprotein component of the bovine TSH receptor (11E8 and 13D11), are shown to inhibit basal and TSH stimulated adenylate cyclase activity in bovine thyroid membranes and human thyroid cells. Both antibodies also inhibit 125I-TSH binding in vitro, whether binding is measured at pH 6.0 in low salts and at 0-4 C or at pH 7.4 in 50 mM NaCl and at 37 C. The glycoprotein component is thus a portion of the physiologic TSH receptor in vivo and 125I-TSH binding studies apparently measure the high affinity glycoprotein component under nonphysiologic conditions and conditions more representative of the physiologic milieu. A third monoclonal antibody whose interaction with thyroid membranes is prevented by TSH is shown to stimulate adenylate cyclase activity in bovine thyroid membranes and human thyroid cells. This stimulating antibody only weakly inhibits 125I-TSH binding to thyroid membranes or to the glycoprotein component of the TSH receptor. The 22A6 antibody does, however, immunoprecipitate mixed brain gangliosides, in distinct contrast to the monoclonal antibodies to the glycoprotein receptor component, i.e., 11E8 and 13D11. The results support the speculation that autoimmune antibodies which inhibit TSH binding to thyroid membranes are not necessarily identical to antibodies which stimulate function; that antibodies directed at the high affinity initial site of TSH interaction with a cell can behave as blocking rather than stimulating antibodies and that a possible relationship exists between stimulating antibodies and the low affinity TSH binding sites (gangliosides) on thyroid membranes.