Thyroid stimulation does not require antibodies with identical epitopes but does involve recognition of a critical conformation at the N terminus of the thyrotropin receptor A-subunit.
Whether monoclonal antibodies with thyroid-stimulating activity [thyroid-stimulating antibody/antibodies (TSAb)] from immunized animals are identical to human autoantibodies in Graves' disease is unknown. Here, we compared properties of a monoclonal hamster TSAb (MS-1) with human autoantibodies. The epitopes of neither MS-1 nor human autoantibodies can be determined by peptide scanning, indicating their conformational nature. A property of human TSAb is that their epitope is partially obscured on the TSH holoreceptor on the cell surface relative to the TSH receptor (TSHR) ectodomain tethered to the membrane by a glycosylphosphatidyl inositol anchor. On flow cytometry, as for human autoantibodies, MS-1 preferentially recognized the glycosylphosphatidyl inositol-anchored ectodomain vs. the TSH holoreceptor on Chinese hamster ovary cells. Also, as with human autoantibodies, only A-subunits with the active (but not the inactive) conformation adsorbed MS-1 binding activity. This difference localizes antibody binding to a cysteine-rich region at the TSHR N terminus. Remarkably, active TSHR A-subunit more effectively ( approximately 40-fold) neutralized human autoantibodies than it did MS-1. Therefore, MS-1 interacts less well than autoantibodies with the free A-subunit. In summary, we provide evidence that TSAb need not have identical epitopes. However, the TSAb epitope does appear to require involvement of the highly conformational N terminus of the A-subunit.
Chazenbalk
Gregorio D
GD
Autoimmune Disease Unit, Cedars-Sinai Research Institute and School of Medicine, University of California, Los Angeles, Los Angeles, California 90048, USA.
Latrofa
Francesco
F
McLachlan
Sandra M
SM
Rapoport
Basil
B
D000818:Animals; D000911:Antibodies, Monoclonal / Q000276:immunology; D016466:CHO Cells; D002462:Cell Membrane / Q000378:metabolism; D006224:Cricetinae; D003412:Cricetulus; D000939:Epitopes; D006801:Humans; D018828:Immunoglobulins, Thyroid-Stimulating / Q000276:immunology*; D008968:Molecular Conformation; D010446:Peptide Fragments / Q000737:chemistry / Q000276:immunology; D020033:Protein Isoforms / Q000737:chemistry / Q000276:immunology; D017434:Protein Structure, Tertiary / Q000502:physiology; D011989:Receptors, Thyrotropin / Q000737:chemistry / Q000276:immunology* / Q000378:metabolism
D000911:Antibodies, Monoclonal; D000939:Epitopes; D018828:Immunoglobulins, Thyroid-Stimulating; D010446:Peptide Fragments; D020033:Protein Isoforms; D011989:Receptors, Thyrotropin