Antibody to immunoglobulin G and polyethylene glycol augment cyclic adenosine monophosphate production by TSH receptor antibody bound to porcine thyroid cells

Experimental Reproduction of Dynamic Fluctuation of TSH Receptor-Binding Antibodies Between Stimulation and Inhibition

Context

Hyperthyroidism in Graves disease (GD) is caused by autoantibody stimulation of the TSH receptor (TSHR). TSHR autoantibody (TSHR-Ab) activity is measured routinely by inhibition of labeled ligand (TSH or M22) binding to the TSHR [TSH-binding inhibitory immunoglobulins (TBIIs)] or by stimulation of cAMP production in isolated cells [TSH receptor–stimulating antibodies (TSAbs)]. Usually, measurements of TSHR-Abs by TBIIs agree reasonably well with TSAb values at least in the setting of hyperthyroidism, and both measurements tend to change in parallel during treatment with some exceptions. In this study, we describe three unusual cases, which illustrate nearly pure stimulating, blocking, or neutral properties of TSHR-Abs.

Objective

Whether patient serum TSHR-Abs can be reproduced by mixtures of human monoclonal autoantibodies to the TSHR was studied because the sera in most patients show moderate properties having both of TBII and TSAb activities.

Design

We compared the TBII and TSAb activities of serum from four unusual patients in detail with mixtures of human monoclonal TSHR-Abs (mAbs) M22 (stimulating), K1-18 (stimulating), and K1-70 (blocking).

Results

Characteristic of a patient’s serum was similar to M22 or K1-18, another was similar to K1-70, whereas another was similar to a mixture of K1-70 and M22 (or K1-18). Additionally, some patients seemed to have neutral TSHR-Abs in their sera.

Conclusions

Our studies suggest that the characteristics of TSHR-Abs in the patient serum can be mimicked by mixtures of human mAbs to the TSHR, stimulating, blocking, and neutral if any.

A novel hypothesis for the etiology of Graves' disease: TSAb may be thyroid stimulating animal IgG-like hormone and TBAb may be the precursor of TSAb

There are doubtful points about the theory that autoimmunity with auto-antibody (Ab) to TSH receptor (R) causes hyperthyroidism in Graves' disease (GD). A main doubtful point is no curative effect of corticosteroid on Graves' hyperthyroidism in spite of curative effect of corticosteroid for all autoimmune diseases. Recently we demonstrated the immunological similarity of TSAb and TBAb-IgG to animal IgGs, except for human (h)IgG, by neutralization and purification of TSAb and TBAb-IgG using (1) heterophilic Ab to animal IgG in GD sera and (2) experimentally generated anti-animal IgG Abs [such as dog (d), bovine (b), porcine (p), and rabbit (rb)]. Furthermore, greater immunological similarity of Fab- and F(ab')(2)-portion of TSAb- and TBAb-IgG to bovine Fab, compared to hFab, was demonstrated using goat anti-bovine F(ab')(2) Ab. Existence of b and p TSH-like portions in the LATS-IgG molecule (probably Fab portion) was suggested by a previous report of neutralization of LATS activity by anti-b- or anti-p-TSH Ab. We suggested the existence of a mammalian animal-TSH-like structure, excepting hTSH, in the TSAb-IgG molecule (probably Fab portion), by discovery of anti-mammalian TSH Ab (such as d, b, p, guinea-pig, rat, whale, except h) in sera of GD. Lately, similar TSHR binding of H- and L-chain of human stimulating monoclonal TSHR Ab (M22)-Fab with TSH-α and-β subunit was reported. This evidence suggests that Fab portion of TSAb has a structure like mammalian TSH, but not hTSH. IgG-λ type of d, horse, b, p, goat, ovine is 95% and IgG-κ type is 5%, while human κ and λ chain is 60:40. Previous report that LATS (TSAb)-IgG composed of predominant λ type is supporting evidence that TRAb-IgG has immunological similarity with these animal IgGs compared to hIgG. We speculate that TSAb-IgG may be referred as a mermaid consisted in face (Fab) and trunk-leg (Fc). Face may be a kind of hormone with animal TSH-like structure and trunk-leg has animal IgG-like structure (in spite of no antibody function). There are many reports for co-existence of TSAb and TBAb-IgG in sera of GD. We reported conversion from TBAb (non-thyroid stimulating type IgG) to TSAb by co-incubation of anti-hIgG Ab (containing anti-animal IgG Ab as a cross-reaction) with TBAb-bound porcine thyroid cells. Thus, we suggest that TBAb may be the precursor form of TSAb.

Neutralization and purification of thyroid stimulating antibody (TSAb) and thyroid blocking antibody (TBAb) by heterophilic antibody to animal IgG in Graves' disease

There are several reports that sera from Graves' patients contain heterophilic antibody (Ab) to animal IgG such as human anti-mouse antibody (HAMA). We examined the binding of TSAb and TBAb with heterophilic Ab. The binding of animal IgG with patient's IgG was examined by the inhibition of animal IgG on the binding of labeled bovine (b) IgG with patient's IgG. The binding to labeled bIgG was detected in the serum of 5 patients (2.7 %) among 185 patients with Graves' disease. The binding of the labeled bIgG with patient's IgG was inhibited by animal serum or the crude IgG (45% ammonium sulfate fraction of serum)(such as dog, horse, bovine, porcine, goat, ovine, rabbit, guinea-pig, rat, mouse) except human, monkey and chick. This heterophilic Ab which had cross-reaction with mammalian IgG (except human, monkey) was used as human anti-animal IgG Ab. TBII and TSAb activity of TSAb-positive serum, and TBII activity of TBAb-positive serum were neutralized by incubation with this Ab-bound column. Partial purification of TSAb- or TBAb- IgG from Protein A-purified TSAb- or TBAb-IgG was possible using this Ab-bound column. TBII and TSAb activity of TSAb-IgG and TBII activity of TBAb-IgG were neutralized by incubation with rabbit anti-human (h) IgG Ab (having cross-reaction with animal IgG). Further purification of Protein A-purified TSAb-IgG or TBAb-IgG by rabbit anti-hIgG Ab-bound column was impossible. The binding of TSAb and TBAb with heterophlic Ab means that TSAb-and TBAb-specific IgG have immunological similarity with mammalian species IgG compared to human IgG.

Inhibitory effect of thyroid blocking antibody (TBAb) on the thyroid stimulatory effect of human chorionic gonadotropin (HCG) and equine chorionic gonadotropin (ECG)

We examined the inhibitory effect of thyroid blocking antibody (TBAb) on the thyroid stimulating activity of human chorionic gonadotropin (HCG) and equine CG (ECG). Five TBAb positive sera obtained from patients who had been hypothyroid but were currently on T4 treatment. The TSH binding inhibitory immunoglobulin (TBII) activities of the sera were 60-160 IU/L. Inhibition of TSH binding to the TSH receptor (TSHR) [TSH binding inhibition (TBI) activity] of HCG or ECG, and inhibition of TBAb on HCG or ECG-stimulated cAMP production were examined. Both HCG and ECG preparations showed weak TBI activity in the presence of small amounts of protein [bovine serum albumin (BSA)] but were negative in the presence of large amounts of protein [normal human serum (NHS) or BSA]. Four thousand IU/mL of HCG and ECG preparation caused cAMP production similar to 100 microU/mL of bovine (b) TSH. The inhibitory effect of TBAb on cAMP production by this amount of HCG or ECG was then examined. The inhibitory effect of TBAb on cAMP production by HCG and ECG was similar to bTSH, and TBAb positive sera with more than 40 IU/L TBII activity completely blocked cAMP production by HCG, ECG and bTSH. This suggests that common alpha -subunit of both HCG and TSH are involved in the inhibitory effect of TBAb. Previous reports demonstrated that the thyroid stimulating activity of thyroid stimulating antibody (TSAb) was blocked by deglycosylated HCG (competitive antagonist of TSH binding to TSHR). The fact and our present study suggest that TSH, HCG ECG, TSAb and TBAb have a similar binding site (alpha-subunit-mimicking binding site) on the TSH receptor.