IgG thyrotrophin receptor antibody activity in Graves' disease; a study of TSH agonist and antagonist activities by isoelectric focusing

Reciprocal Relation of Thyroid-Stimulating Hormone and Thyroid-Stimulating Immunoglobulin in a Patient with Endogenous Subclinical Hyperthyroidism

A 72-year-old white woman with an abnormal serum thyroid-stimulating hormone (TSH) concentration was referred to our facility for a comprehensive evaluation. Circulating thyroxine (T4) and free thyroxine (FT4) concentrations were all in the normal range. Tri-iodothyronine (T3) concentrations were in the low end or slightly below the normal range. TSH was detectable but was below the limits of the normal range. The patient was clinically euthyroid and was receiving medication only for treatment of hypertension. The clinical and laboratory thyroid function status was consistent with a diagnosis of subclinical hyperthyroidism. The physical examination did not reveal thyroid enlargement, nor was there any evidence for the presence of thyroid nodules or ocular changes suggestive of Graves disease. Among thyroid autoantibodies of particular interest was the presence of a moderate thyroid-stimulating immunoglobulin (TSI) level that was stable and consistently present during a 7-month observational period. At 13 months after the initial visit, TSI antibodies were absent and TSH concentration had returned to the normal range. Based on the TSH agonist activity of TSI and the observed reciprocal relation of TSI and TSH, there was no need to suggest pituitary hypersensitivity to thyroid hormone.

The thyrotropin receptor as a model to illustrate receptor and receptor antibody diseases

The thyrotropin receptor (TSHR) has been used as an example to illustrate how disease may be the consequence of: 1. Modifications or inappropriate production of the natural ligand. 2. Production of abnormal agonists or antagonists such as autoantibodies. 3. Modifications in receptor structure resulting in constitutive activation or the absence of activation following ligand binding. 4. Changes in the cellular machinery which transduces the signal from the receptor to the cytoplasmic or nuclear endpoint target. This chapter concentrates on mechanisms (2) and (3). Since the cloning of the TSHR it has been shown that approximately 50% of cases of toxic adenoma can be explained by somatic point mutations in the nucleotide sequence of the receptor gene which causes single amino acid substitutions. The resulting modified TSHR structure constitutively activates adenylate cyclase (via Gs), intracellular cAMP levels are increased and, since cAMP controls both growth and function of the human thyrocyte clonal expansion of the mutated cell ensues. Similarly, activating mutations of the TSH receptor gene in the germline are responsible for hereditary hyperthyroidism with goitre, which is transmitted in the autosomal dominant mode. Changes in receptor primary structure, i.e. a modified autoantigen, do not seem to be responsible for the escape from tolerance which must precede production of thyroid stimulating antibodies (TSAB) which cause hyperthyroid Graves' disease and thyroid blocking antibodies (TBAB) which are responsible for some cases of hypothyroid idiopathic myxoedema. The eukaryotic expression of wild-type, experimentally mutated and chimeric TSHR has enabled some progress in delineating the residues involved in binding TSH, TSAB and TBAB. All three ligands bind numerous discontinuous residues in the extracellular domain of the receptor. The difference between the bioactivity of TSAB and TBAB cannot be explained completely by different binding sites on the receptor. Subtle differences in, for example, glycosylation and sialation of the immunoglobulins may be implicated, since bioactivity of TSH itself seems to depend on these. Attempts to define T cell epitopes have not identified a major immunogenic region. Indeed heterogeneity seems to be a hallmark of TSHR autoantibodies (TRAB). The possibility that thyroid-associated ophthalmopathy and pretibial myxoedema may be receptor antibody diseases is discussed. Further progress awaits large-scale production of TSHR able to bind TSH to facilitate X-ray crystallographic studies, the development of specific T cell clones and the cloning of TSAB autoantibodies.

Further characterization of the receptor-binding region of the thyroid-stimulating hormone alpha subunit: a comprehensive synthetic peptide study of the alpha-subunit 26-46 sequence

Previously, using a synthetic peptide strategy, we determined that the region of the common glycoprotein hormone alpha subunit between residues 26 and 46 is a site of interaction of the hormone with the thyroid membrane-bound receptor for thyroid-stimulating hormone (TSH). We have undertaken to identify further the specific residues within this 21-amino acid span that are critical in hormone receptor binding. We synthesized three nested sets of peptide, two in which we systematically truncated the amino-terminal region of the sequence and another in which we truncated the carboxyl-terminal region, and we synthesized a fourth nested set in which we systematically substituted alanine for the native residues from the region of highest activity. Each peptide was tested in a TSH radioreceptor assay for its ability to inhibit binding of 125I-labeled bovine TSH to porcine thyroid membranes. Removal either by truncation or alanine substitution, of several specific residues resulted in a significant reduction in the ability of the sequence to interact with receptor; these residues included Cys31, Cys32, Phe33, Arg35, Arg42, Lys44, and Lys45, suggesting that they are crucial for binding activity. Loss of activity also occurred with substitution for Gly30 and Ser34, but the reduction was less pronounced. Amino-terminal truncation of the sequence through Arg35 (leaving the alpha-subunit peptide 36-46) resulted in greater than 98% loss of activity of the sequence. We conclude that two distinct receptor binding regions lie within the alpha-subunit 26-46 sequence. The first lies between residues Gly30 and Arg35 and includes Cys31, Cys32, and Phe33 as important constituents, and the second region lies between residues Arg42 and Lys45 and includes Lys44 as an important residue and Ser43 as a less important component.

Heterogeneity of circulating TSH-receptor antibodies in thyroid disease demonstrated directly by chromatography

The heterogeneity of circulating TSH-receptor antibodies (TRAb) has been demonstrated directly by affinity chromatography on the dye Remazol Yellow-GGL. The technique resolved these antibodies into discrete peaks within the general serum immunoglobulin background. Peaks were detected initially by a binding (radioreceptor) assay and then characterized by their ability to stimulate the uptake of iodine-125 into FRTL-5 cells. Thirteen subjects with high serum levels of TRAb were studied. The two hypothyroid patients each produced a single peak of TRAb activity with no ability to stimulate uptake of iodine-125 into cultured cells. The remaining 11 patients had Graves' disease; their sera produced a range of elution profiles after chromatography with TRAb activity resolved into as many as five peaks. Considerable differences were seen in the relative abilities of the resolved TRAb components to bind to receptors and to stimulate cellular uptake of iodine-125 both between and within individual samples. No two patients produced components which chromatographed in the same position on the gradients employed for elution from the dye.

The relationships between serum T3 index, thyroid volume, and thyroid stimulating, TSH receptor binding and thyroid growth stimulating antibodies in untreated Graves' disease

This study represents an international double-blind collaborative study of abnormal immunoglobulin activity in untreated Graves' disease. Laboratories in two countries participated in a comparison of thyrotrophin binding inhibiting (TBII), thyroid stimulating (TSAb), and growth stimulating (TGI) immunoglobulins with clinical data, including ultrasonically measured thyroid size. The correlation between TGI and thyroid volume (n = 25, Rs = 0.54, P less than 0.05) and the fact that 9 of 10 patients with high range TGI values had large goitres establish the relationship between TGI and goitre, confirming that the in-vitro activity of these antibodies is related to an in-vivo action. In addition, both TBII and TSAb correlated with serum free T3 indices (TBII: n = 60, Rs = 0.46, P less than 0.001, and TSAb: n = 60, Rs = 0.64, P less than 0.001). Moreover, both TBII and TSAb correlated with thyroid volume (TBII: n = 60, Rs = 0.37, P less than 0.01, and TSAb: n = 60, Rs = 0.41, P less than 0.01) suggesting that these antibodies are also important in development of goitre in Graves' disease. Finally, some correlation between the antibodies was observed. TBII correlated with TSAb (n = 60, Rs = 0.47, P less than 0.001), and in the 16 patients with positive TGI results, this activity correlated with TBII (Rs = 0.54, P less than 0.05), but not with TSAb. Also some cases were found with corresponding high range TBII and TGI, while negative for TSAb, suggesting a close relationship between the in-vitro measurement of TSH binding and TGI.

Characterization of TSH antagonist activity in the serum of patients with thyroid disease

The ability of sera from patients with thyroid disease to block TSH stimulation of cyclic AMP release from isolated porcine thyroid cells has been assessed and the blocking activity characterized. TSH receptor binding activity was also measured. No blocking or receptor binding activity was detectable in patients with primary myxoedema (n = 23), Hashimoto's disease (n = 11), multinodular goitre (n = 6), or rheumatoid arthritis (n = 10). However, analysis of sera from 23 patients (out of an initial screen of 110 patients) with treated Graves' disease which did not stimulate cyclic AMP production in the bioassay showed that two of these sera contained powerful blocking and receptor binding activity. Both these patients had been treated with 131I. Analysis of the two sera by gel filtration on Sephadex G-200 indicated that blocking and TSH receptor binding activity were associated only with the IgG fraction. Digestion of the IgG with pepsin followed by reduction showed that both (Fab)2 and Fab fragments contained high levels of blocking and binding activity. Antibody divalency was not necessary therefore for TSH antagonist activity. However, our studies suggest that autoantibodies of this type with TSH antagonist activity do not occur frequently in patients from the Cardiff region with primary myxoedema, Hashimoto's or treated Graves' disease.

Isoelectric focusing of the human TSH receptor A subunit

The water soluble A subunit of the human TSH receptor has been shown to have an isoelectric point of 5. As both TSH and TSH receptor antibodies have isoelectric points in the region of 8-10, charge-charge interactions must be of major importance in the binding of hormone or antibody to the TSH receptor A subunit.