Glycosylated Ectodomain of the Human Thyrotropin Receptor Induces Antibodies Capable of Reacting with Multiple Blocking Antibody Epitopes

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Recently, we showed that the glycosylated ectodomain of the human thyrotropin receptor (hET-gp) reacts with autoantibodies from autoimmune thyroid disease (AITD) patients' sera. To better understand the effects of glycosylation of thyrotropin receptor (TSHR) in antibody induction, we immunized rabbits with hET-gp protein. The rabbits developed relatively high titers of antibodies with highly potent TSH binding inhibitory immunoglobulin (TBII) and thyroid stimulatory blocking antibody (TSBAb) activities. Both the hET-gp and a nonglycosylated ectodomain of the human TSHR (hETSHR) protein significantly reversed the TBII as well as TSBAb activity. Based on the ability of synthetic peptides to significantly reverse the functional activity of these rabbit antisera, we identified three discrete regions of the TSHR, represented by amino acids 202–221, 292–311 and 367–386, as TBII epitopes and four regions represented by amino acids 352–371, 367–386, 382–401 and 392–415 as TSBAb epitopes. These data demonstrate that rabbit antibodies that bind to amino acids 367–386 mediate their TSBAb activity by inhibiting the binding of TSH to TSHR; whereas, antibodies to regions 352–415, excluding aa 367–386, exert their TSBAb activity by affecting a step subsequent to TSH binding. Coincident with the elevation of TBII and TSBAb activity, serum total T4 levels declined and thus suggested that the antibodies exerted functional effects on thyroid in vivo. Together, these data demonstrate that glycosylated hET-gp protein is a more potent immunogen and it can induce a broader antibody response directed against multiple TBII and TSBAb epitopes.