Thyroid stimulating monoclonal antibodies: overcoming the road blocks and the way forward

Mechanisms of Autoantibody-Induced Pathology

Autoantibodies are frequently observed in healthy individuals. In a minority of these individuals, they lead to manifestation of autoimmune diseases, such as rheumatoid arthritis or Graves' disease. Overall, more than 2.5% of the population is affected by autoantibody-driven autoimmune disease. Pathways leading to autoantibody-induced pathology greatly differ among different diseases, and autoantibodies directed against the same antigen, depending on the targeted epitope, can have diverse effects. To foster knowledge in autoantibody-induced pathology and to encourage development of urgently needed novel therapeutic strategies, we here categorized autoantibodies according to their effects. According to our algorithm, autoantibodies can be classified into the following categories: (1) mimic receptor stimulation, (2) blocking of neural transmission, (3) induction of altered signaling, triggering uncontrolled (4) microthrombosis, (5) cell lysis, (6) neutrophil activation, and (7) induction of inflammation. These mechanisms in relation to disease, as well as principles of autoantibody generation and detection, are reviewed herein.

Graves' disease TSHR-stimulating antibodies (TSAbs) induce the activation of immature thymocytes: a clue to the riddle of TSAbs generation?

Graves' disease (GD) is an autoimmune thyroid disease defined by the production of stimulating autoantibodies to the thyroid-stimulating hormone receptor (TSHR) (TSAbs) that induce a sustained state of hyperthyroidism in patients. We previously demonstrated that TSHR, the target of this autoimmune response, is also a key susceptibility gene for GD, probably acting through thymic-dependent central tolerance. We also showed that TSHR is, unexpectedly, expressed in thymocytes. In this report, we confirm the expression of TSHR in thymocytes by protein immunoblotting and quantitative PCR, and show that expression is confined to maturing thymocytes. Using functional assays, we show that thymic TSHR is functional and that TSAbs can stimulate thymocytes through this receptor. This new activity of TSAbs on thymocytes may: 1) explain GD-associated thymic enlargement (hyperplasia), and 2) suggest the provocative hypothesis that the continuous stimulation of thymocytes by TSAbs could lead to a vicious cycle of iterative improvement of the affinity and stimulating capability of initially low-affinity antibacterial (e.g., Yersinia) Abs cross-reactive with TSHR, eventually leading to TSAbs. This may help to fill one of the gaps in our present understanding of unusual characteristics of TSAbs.

Implications of a vasodilatory human monoclonal autoantibody in postural hypotension

Functional autoantibodies to the autonomic receptors are increasingly recognized in the pathophysiology of cardiovascular diseases. To date, no human activating monoclonal autoantibodies to these receptors have been available. In this study, we describe for the first time a β2-adrenergic receptor (β2AR)-activating monoclonal autoantibody (C5F2) produced from the lymphocytes of a patient with idiopathic postural hypotension. C5F2, an IgG3 isotype, recognizes an epitope in the N terminus of the second extracellular loop (ECL2) of β2AR. Surface plasmon resonance analysis revealed high binding affinity for the β2AR ECL2 peptide. Immunoblotting and immunofluorescence demonstrated specific binding to β2AR in H9c2 cardiomyocytes, CHO cells expressing human β2AR, and rat aorta. C5F2 stimulated cyclic AMP production in β2AR-transfected CHO cells and induced potent dilation of isolated rat cremaster arterioles, both of which were specifically blocked by the β2AR-selective antagonist ICI-118551 and by the β2AR ECL2 peptide. This monoclonal antibody demonstrated sufficient activity to produce postural hypotension in its host. Its availability provides a unique opportunity to identify previously unrecognized causes and new pharmacological management of postural hypotension and other cardiovascular diseases.

Blocking type TSH receptor antibodies

TSH receptor (TSHR) autoantibodies (TRAbs) play a key role in the pathogenesis of Graves' disease. In the majority of patients, TRAbs stimulate thyroid hormone synthesis via activation of the TSHR (stimulating TRAbs, TSHR agonists). In some patients, TRAbs bind to the receptor but do not cause activation (blocking TRAbs, TSHR antagonists). Isolation of human TSHR monoclonal antibodies (MAbs) with either stimulating (M22 and K1-18) or blocking activities (5C9 and K1-70) has been a major advance in studies on the TSHR. The binding characteristics of the blocking MAbs, their interaction with the TSHR and their effect on TSHR constitutive activity are summarised in this review. In addition, the binding arrangement in the crystal structures of the TSHR in complex with the blocking MAb K1-70 and with the stimulating MAb M22 (2.55 Å and 1.9 Å resolution, respectively) are compared. The stimulating effect of M22 and the inhibiting effect of K1-70 on thyroid hormone secretion in vivo is discussed. Furthermore the ability of K1-70 to inhibit the thyroid stimulating activity of M22 in vivo is shown. Human MAbs which act as TSHR antagonists are potentially important new therapeutics. For example, in Graves' disease, K1-70 may well be effective in controlling hyperthyroidism and the eye signs caused by stimulating TRAb. In addition, hyperthyroidism caused by autonomous TSH secretion should be treatable by K1-70, and 5C9 has the potential to control hyperthyroidism associated with TSHR activating mutations. Furthermore, K1-70 has potential applications in thyroid imaging as well as targeted drug delivery to TSHR expressing tissues.

Thyrotrophin receptor antibody characteristics in a woman with long-standing Hashimoto's who developed Graves' disease and pretibial myxoedema

CONTEXT

Sequential conversion of Hashimoto's thyroiditis (HT) to Graves' disease (GD) is uncommon. Distinct immune paradigms, paucity of functioning tissue in long-standing HT, and infrequent conversion of blocking (TBAb) to stimulating (TSAb) thyrotrophin receptor antibody (TRAb) may account for this. Molecular and crystal structure analysis helps delineate TSH receptor (TSHR)/TRAb interactions in detail. Such 'fingerprinting' helps determine the behaviour and characteristics of TRAb in longitudinal studies.

PATIENT

An 80-year-old woman taking thyroxine for long-standing HT became hyperthyroid. This persisted despite thyroxine withdrawal - free T3 was 7·3 pmol/l (2·6-5·7) and TSH < 0·01 mU/l (0·2-4·5) and TRAb highly positive. She had a goitre (ultrasound - HT), pretibial myxoedema, with mild inactive Graves' orbitopathy. She had RAI treatment and is on thyroxine replacement.

MEASUREMENTS AND RESULTS

Blood samples at presentation (A) and 1 year (B) showed high TSAb and TPOAb activity but no TBAb. Experiments involving TSHR mutations confirmed that (i) TRAb had stable characteristics over 1 year; (ii) TSHR mutation R255D caused complete inhibition and (iii) R109A caused marked reduction of cAMP production by M22 (TSHR-stimulating human monoclonal antibody) and A and B; (iv) mutations R80A, E107A and K129A while affecting M22 had little effect on A and B.

CONCLUSIONS

The reasons for an immunological paradigm shift in this elderly woman remain speculative. We believe that de-novo TSAb synthesis occurred converting her long-standing HT to GD although the mechanisms responsible remain unexplained. TRAb analysis confirmed stable autoantibody characteristics over 1 year and variable effects of TSHR mutations on TRAb and M22 function.

Monoclonal autoantibodies to the TSH receptor, one with stimulating activity and one with blocking activity, obtained from the same blood sample

OBJECTIVE

Patients who appear to have both stimulating and blocking TSHR autoantibodies in their sera have been described, but the two activities have not been separated and analysed. We now describe the isolation and detailed characterization of a blocking type TSHR monoclonal autoantibody and a stimulating type TSHR monoclonal autoantibody from a single sample of peripheral blood lymphocytes.

DESIGN, PATIENTS AND MEASUREMENTS

Two heterohybridoma cell lines secreting TSHR autoantibodies were isolated using standard techniques from the lymphocytes of a patient with hypothyroidism and high levels of TSHR autoantibodies (160 units/l by inhibition of TSH binding). The ability of the two new monoclonal antibodies (MAbs; K1-18 and K1-70) to bind to the TSHR and compete with TSH or TSHR antibody binding was analysed. Furthermore, the effects of K1-18 and K1-70 on cyclic AMP production in Chinese hamster ovary cells (CHO) cells expressing the TSHR were investigated.

RESULTS

One MAb (K1-18) was a strong stimulator of cyclic AMP production in TSHR-transfected CHO cells and the other (K1-70) blocked stimulation of the TSHR by TSH, K1-18, other thyroid-stimulating MAbs and patient serum stimulating type TSHR autoantibodies. Both K1-18 (IgG1 kappa) and K1-70 (IgG1 lambda) bound to the TSHR with high affinity (0.7 x 10(10) l/mol and 4 x 10(10) l/mol, respectively), and this binding was inhibited by unlabelled K1-18 and K1-70, other thyroid-stimulating MAbs and patient serum TSHR autoantibodies with stimulating or blocking activities. V region gene analysis indicated that K1-18 and K1-70 heavy chains used the same V region germline gene but different D and J germline genes as well as having different light chains. Consequently, the two antibodies have evolved separately from different B cell clones.

CONCLUSIONS

This study provides proof that a patient can produce a mixture of blocking and stimulating TSHR autoantibodies at the same time.

Monoclonal pathogenic antibodies to the thyroid-stimulating hormone receptor in Graves' disease with potent thyroid-stimulating activity but differential blocking activity activate multiple signaling pathways

The thyroid target Ag for disease-inducing autoantibodies in Graves' disease is the receptor for thyroid-stimulating hormone (TSH), but little is known about the molecular basis of this pathogenic Ab response. We describe the characteristics of two high- affinity mAbs developed from an experimental murine model of hyperthyroid Graves' disease that exhibit potent thyroid-stimulating activity. Nanogram concentrations of the IgG mAbs KSAb1 and KSAb2 and their Fab induce full stimulation of the TSH receptor that is matched by the ligand TSH and, thus, act as full agonists for the receptor. However, KSAb1 and KSAb2 display differential activities in their ability to block TSH-mediated stimulation of the receptor, indicating subtle differences in their biological properties. In displacement studies, IgG and Fabs of KSAb1 and KSAb2 compete with Graves' disease autoantibodies as well as thyroid-blocking Abs present in some hypothyroid patients, indicating a close relationship between these autoimmune determinants on the receptor. In passive transfer studies, single injections of microgram quantities of KSAb1 or KSAb2 IgG led to rapid elevation of serum thyroxine and a hyperthyroid state that was maintained for a number of days. The thyroid glands showed evidence of cell necrosis, but there was no accompanying mononuclear cell infiltrate. In studying their receptor activation pathways, both KSAb1 and KSAb2 provoked phosphorylation of the intracellular ERK1/2 pathway in primary thyrocytes, indicating that multiple signaling pathways may participate in the pathogenesis of Graves' disease. In summary, our findings emphasize the similarities of the experimental mouse model in reproducing the human disorder and provide improved means for characterizing the molecular basis of this pathogenic response.

New insights into antibody-mediated hyperthyroidism

The most common cause of hyperthyroidism is Graves' disease, which represents a typical example of an organ-specific autoimmune condition. The exact triggers for the disease remain unknown, but are likely to involve a complex interaction between multiple environmental factors in a genetically predisposed individual. The main feature of the condition is the presence of thyroid-stimulating antibodies, which activate the thyroid- stimulating hormone receptor, resulting in hyperthyroidism. These antibodies may also be involved in the extrathyroidal complications of the disease. The recent generation of thyroid-stimulating antibodies in animal models and the isolation of monoclonal thyroid-stimulating antibodies from a patient with Graves' disease should allow the detailed study of thyroid-stimulating antibodies-thyroid-stimulating hormone receptor interactions. This will help to shed more light on disease pathogenesis and may offer new treatment strategies in difficult cases, particularly in patients with extrathyroidal complications.

Insight into Graves' hyperthyroidism from animal models

Graves' hyperthyroidism can be induced in mice or hamsters by novel approaches, namely injecting cells expressing the TSH receptor (TSHR) or vaccination with TSHR-DNA in plasmid or adenoviral vectors. These models provide unique insight into several aspects of Graves' disease: 1) manipulating immunity toward Th1 or Th2 cytokines enhances or suppresses hyperthyroidism in different models, perhaps reflecting human disease heterogeneity; 2) the role of TSHR cleavage and A subunit shedding in immunity leading to thyroid-stimulating antibodies (TSAbs); and 3) epitope spreading away from TSAbs and toward TSH-blocking antibodies in association with increased TSHR antibody titers (as in rare hypothyroid patients). Major developments from the models include the isolation of high-affinity monoclonal TSAbs and analysis of antigen presentation, T cells, and immune tolerance to the TSHR. Studies of inbred mouse strains emphasize the contribution of non-MHC vs. MHC genes, as in humans, supporting the relevance of the models to human disease. Moreover, other findings suggest that the development of Graves' disease is affected by environmental factors, including infectious pathogens, regardless of modifications in the Th1/Th2 balance. Finally, developing immunospecific forms of therapy for Graves' disease will require painstaking dissection of immune recognition and responses to the TSHR.