**3. Pathogenesis**

Autoimmune thyroid diseases are the most prevalent organ- specific which includes Graves' Disease (GD) and Hashimoto's Thyroiditis (HT). The hyperactivity of the thyroid gland is due to the production of thyroid stimulating antibodies (autoantibodies) and are known to recognize and activate thyroid stimulating hormone receptor. The autoantibodies produced by TSHR increase the growth and functioning to thyroid follicular cells resulting over production of T3 and T4. It has been suggested by the studies that a genetic clonal lack of suppressor T cells may be responsible for the inappropriate and unregulated production of TSH receptor antibody [13].

#### **3.1 Autoimmunity mechanism**

The pathological process involved in Graves' Disease is similar to other autoimmune disease but a unique aspect found in the majority of patients is TSHR antibodies which cannot be found in normal individuals. The characterization of the autoimmunity process includes a lymphocytic infiltrate seen at the target organ and the presence of antigen-reactive T and B cells against thyroid antigens. As in all autoimmune diseases it is observed when self-tolerance is broken; T cells identify self-antigens, and B cells produce antibodies targeting host cells. Many cell *Autoimmune Mechanism and Recurrence Risk in Graves' Disease DOI: http://dx.doi.org/10.5772/intechopen.97641*

self-specific T cells escape thymic deletion; however, additional mechanisms like clonal anergy and peripheral suppression normally prevent reactions to auto antigen. B cells which recognize a specific cell auto antigen in the lymphoid organs are taken into T lymphocyte areas; B lymphocytes normally kill by apoptosis if they are not activated by T available cells while B lymphocytes, binding soluble self-antigen, are anergised; they down- regulate expression of membrane IgM and live for a short period. B cells are inactivated by T-cells available for support. An additional B cell tolerance mechanism includes allelic exclusion and clonal ignorance in receptor and autoreactive B cell receptor (BCRs).

#### **3.2 The thyroid antigens**

#### *3.2.1 Tg and TPO*

Thyroglobulin (Tg) is 660 kDa, a dimeric glycoprotein secreted by the follicular cells of the thyroid gland and acts as a substrate for the synthesis of T3 and T4. Besides, it stores an inactive form of thyroid hormone and iodine in the lumen of the thyroid follicle. The antibodies produced in response to Tg induce massive destruction of the thyroid gland but few studies propose to show high levels of Tg do not *perse* induce antibody production. Thyroid hormones are synthesized on the Tg with the help of thyroid peroxidase (TPO). TPO is an enzyme Antithyroglobulin antibody (ATA) and TPO- specific T cells are often found in GD patients. Tg antibodies recognize the confirmation of a large fragment of Tg [14]. These are directed against the same epitopes which are mainly observed in GD. Antibodies against TPO are involved in complement/antibody-mediated cell toxicity and target toward conformational epitopes.

#### *3.2.2 GD auto antigen and TSH receptor antibodies*

The auto antigen which is mainly expressed in Graves' Disease is thyroid stimulating hormone receptor (TSHR). It is a G protein coupled receptor with 7 trans membrane spanning domains expressed in thyroid gland but also in adipose cells, fibroblasts, bone cells including heart cells [15]. Binding to circulating TSH, a G-Protein signal activates adenylcyclase cascade events and intracellular levels of c AMP increases. Rise in c AMP activates all the functional activities of thyroid cell which includes iodine pumping; synthesis of thyroglobulin, iodination, endocytosis, and proteolysis; activity involving thyroid peroxidases; and hormone release. Literatures suggest that there may be a shedding of the TSHR ectodomain [16–18] even though it has not been confirmed in vivo.

The presence of TSHR-Antibodies (TSHR-Abs) [19] is the most unusual feature of most GD-patients. The early analysis of TSHR-Abs [20] represents the most accurate definition of their features, which resulted by analysis of monoclonal antibodies to the TSHR from various sources, including human, mouse and hamster: mouse and hamster antibodies are secondary to TSHR [21–23]. Three types of TSHR-Ab are identified among autoimmune-thyroid patients and similar diseases are observed in immunized rodents; stimulating, blocking, and so-called "neutral." TSHR-Abs that has proved to be beyond neutral in their biological activity has been characteristic of the TSHR ectodomain's cleavage. Stimulating Antibodies (Stimulating Abs) induce cAMP production and inhibit any simultaneous activation of the thyroid function that bind to naturally conformed TSHR. TSHR blocking antibodies, its main bioactivity of this is to prevent TSH receptor binding in a manner that may cause thyroid problems, and also it act as weak TSH agonists. Such blocking antibodies depend on conformation, and others are very similar to the

decreased TSHR antigen and/or linear peptides. At the end, neutral TSHR antibodies may not prevent or stimulate cAMP production or TSH adhering. Neutral TSHRantibodies bind only to linear epitopes and are targeted against the "unique region" located in receptor among specific amino acids 316–366 [24]. In GD patients, the presence of different ratios with high-affinity TSHR-Abs contributes to the clinical phenotype. Therefore, a function-based classification of these antibodies appears more relevant than their ability or failure to influence the binding of TSHs or cAMPs.

#### **3.3 Different epitopes of TSHR Abs**

### *3.3.1 Monoclonal antibodies against TSHR*

Any monoclonal antibody raised to TSHR protein or antigen includes synthesized peptides and recombinant ones, that have been shown to be neutralizing in mechanism. Only natural intact TSH receptors or genetic immune are used to stimulate thyroid-stimulating antibodies and establish an animal hyperthyroidism model [25–28]. Monoclonal antibodies, such as MS-1, have been raised in hamsters utilizing rare B cells that secrete TSHR-enhancing antibodies [29]. Rodents and humans have been used to identify blocking and neutral monoclonal antibodies (mAbs).

#### *3.3.2 Stimulating TSHR-Ab epitopes*

Part of the TSHR ectodomain has also been crystallized with the support of appropriate stimulating monoclonal fragment TSHR Fab bound in situ [30]. Many amino acids have a large section of the concave surface of the TSHR ectodomain that has been identified as important for antibody binding, in the leucin-rich repeat region (LRR). Reviewing recent studies particularly at conformational epitopes using mass spectrometry [29] have shown that epitopes exist outside the LRR for blocking and stimulating monoclonal antibodies. The prominent region exist at N terminal region of the extracellular domain (ECD) has been well demonstrated and also at the residues in the "hinge" region [31].

#### *3.3.3 Blocking TSHR-Ab epitopes*

TSHR antibodies block epitopes are widely distributed compared to stimulating antibodies. Thus, blocking TSHR monoclonal antibodies (TSHR-mAbs) have been shown to have binding affinities to independent or confirmational epitopes [32]. In patients with GD or HT, TSHR autoantibodies showed themselves to compete in N-terminal TSHR beta subunit (aa382–415), with a blocking TSHRmAb. Blocking hypothyroid antibodies therefore is heterogeneous in nature and this repertoire of anti-bodies involves multiple epitopes. Crystallization and modeling of human and mouse blocking TSHR Abs attempting to block and these TSHR-Abs strongly proposed that the N-terminal and the leucine-rich binding are linked [33, 34].

#### *3.3.4 Cleavage TSHR-Ab epitopes*

Peptide binding (ELISA) and the monoclonal competition of antibodies in patients with Graves' Disease is shown throughout cleavage (aaa 316–366) (competitive inhibition assay by FACS). The tissues of the TSHR are strongly related. The main linear epitopes in animal GD models are known in the area of cleavage [35].

Such antibodies are not competitive to TSH-borne binding in the cleaved area and are therefore often called "neutral."
