Structural insights into autoreactive determinants in thyroid peroxidase composed of discontinuous and multiple key contact amino acid residues contributing to epitopes recognized by patients' autoantibodies

Iron: Not Just a Passive Bystander in AITD

Autoimmune thyroid disease (AITD) is the most prevalent autoimmune disease all over the world and the most frequent cause of hypothyroidism in areas of iodine sufficiency. The pathogenesis of AITD is multifactorial and depends on complex interactions between genetic and environmental factors, with epigenetics being the crucial link. Iron deficiency (ID) can reduce the activities of thyroid peroxidase and 5′-deiodinase, inhibit binding of triiodothyronine to its nuclear receptor, and cause slower utilization of T3 from the serum pool. Moreover, ID can disturb the functioning of the immune system, increasing the risk of autoimmune disorders. ID can be responsible for residual symptoms that may persist in patients with AITD, even if their thyrometabolic status has been controlled. The human lifestyle in the 21st century is inevitably associated with exposure to chemical compounds, pathogens, and stress, which implies an increased risk of autoimmune disorders and thyroid dysfunction. To summarize, in our paper we discuss how iron deficiency can impair the functions of the immune system, cause epigenetic changes in human DNA, and potentiate tissue damage by chemicals acting as thyroid disruptors.

Autoimmune thyroiditis (review of literature)

Autoimmune thyroiditis is a group of organ-specific autoimmune thyropathies, which are caused by a genetically determined defect in immune tolerance to thyroid antigens, as a result of which its autoimmune damage occurs. The aim of the study was to analyze literature data on the pathogenetic role of genetic and biochemical parameters in patients with autoimmune thyroiditis.

Cellular and molecular basis of thyroid autoimmunity

Autoimmune thyroid disease (AITD) is the most common human autoimmune disease. The two major clinical manifestations of AITD are Graves' disease and Hashimoto's thyroiditis (HT). AITD is characterized by lymphocytic infiltration of the thyroid gland, leading either to follicular cell damage, thyroid gland destruction, and development of hypothyroidism (in HT) or thyroid hyperplasia, induced by thyroid antibodies which activate thyrotropin receptor (TSHR) on thyrocytes, leading to hyperthyroidism. The aim of this review is to present up-to-date picture of the molecular and cellular mechanisms that underlie the pathology of AITD. Based on studies involving patients, animal AITD models, and thyroid cell lines, we discuss the key events leading to the loss of immune tolerance to thyroid autoantigens as well as the signaling cascades leading to the destruction of thyroid gland. Special focus is given on the interplay between the environmental and genetic factors, as well as ncRNAs and microbiome contributing to AITD development. In particular, we describe mechanistic models by which SNPs in genes involved in immune regulation and thyroid function, such as CD40, TSHR, FLT3, and PTPN22, underlie AITD predisposition. The clinical significance of novel diagnostic and prognostic biomarkers based on ncRNAs and microbiome composition is also underscored. Finally, we discuss the possible significance of probiotic supplementation on thyroid function in AITD.

Hashimotos' thyroiditis: Epidemiology, pathogenesis, clinic and therapy

Hashimoto's thyroiditis (HT), the most frequent autoimmune thyroid disorders (AITDs), is the leading cause of hypothyroidism in the iodine-sufficient areas of the world. About 20-30% of patients suffers from HT, whose cause is thought to be a combination of genetic susceptibility and environmental factors that causes the loss of immunological tolerance, with a consequent autoimmune attack to the thyroid tissue and appearance of the disease. The pathologic features of lymphocytic infiltration, especially of T cells, and follicular destruction are the histological hallmark of autoimmune thyroiditis (AIT), that lead to gradual atrophy and fibrosis. An important role in the immune-pathogenesis of AITDs is due to chemokines and cytokines. In about 20% of patients, AITDs are associated with other organ specific/systemic autoimmune disorders. Many studies have demonstrated the relationship between papillary thyroid cancer and AITD. The treatment of hypothyroidism, as result of AIT, consists in daily assumption of synthetic levothyroxine.

A redundant role of human thyroid peroxidase propeptide for cellular, enzymatic, and immunological activity

BACKGROUND

Thyroid peroxidase (TPO) is a dimeric membrane-bound enzyme of thyroid follicular cells, responsible for thyroid hormone biosynthesis. TPO is also a common target antigen in autoimmune thyroid disease (AITD). With two active sites, TPO is an unusual enzyme, and thus there is much interest in understanding its structure and role in AITD. Homology modeling has shown TPO to be composed of different structural modules, as well as a propeptide sequence. During the course of studies to obtain homogeneous preparations of recombinant TPO for structural studies, we investigated the role of the large propeptide sequence in TPO.

METHODS

An engineered recombinant human TPO preparation expressed in Chinese hamster ovary (CHO) cells lacking the propeptide (TPOΔpro; amino acid residues 21-108) was characterized and its properties compared to wild-type TPO. Plasma membrane localization was determined by cell surface protein biotinylation, and biochemical studies were performed to evaluate enzymatic activity and the effect of deglycosylation. Immunological investigations using autoantibodies from AITD patients and other epitope-specific antibodies that recognize conformational determinants on TPO were evaluated for binding to TPOΔpro by flow cytometry, immunocytochemistry, and capture enzyme-linked immunosorbent assay. Molecular modeling and dynamics simulation of TPOΔpro comprising a dimer of myeloperoxidase-like domains was performed in order to investigate the impact of propeptide removal and the role of glycosylation.

RESULTS

The TPOΔpro was expressed on the cell surface at comparable levels to wild-type TPO. The TPOΔpro was enzymatically active and recognized by patients' autoantibodies and a panel of epitope-specific antibodies, confirming structural integrity of the two major conformational determinants recognized by autoantibodies. Faithful intracellular trafficking and N-glycosylation of TPOΔpro was also maintained. Molecular modeling and dynamics simulations were consistent with these observations.

CONCLUSIONS

Our results point to a redundant role for the propeptide sequence in TPO. The successful expression of TPOΔpro in a membrane-anchored, enzymatically active form that is insensitive to intramolecular proteolysis, and importantly is recognized by patients' autoantibodies, is a key advance for purification of substantial quantities of homogeneous preparation of TPO for crystallization, structural, and immunological studies.

Autoimmune thyroid diseases: genetic susceptibility of thyroid-specific genes and thyroid autoantigens contributions

Autoimmune thyroid diseases are common polygenic multifactorial disorders with the environment contributing importantly to the emergence of the disease phenotype. Some of the disease manifestations, such as severe thyroid-associated ophthalmopathy, pretibial myxedema and thyroid antigen/antibody immune complex nephritis are unusual to rare. The spectrum of autoimmune thyroid diseases includes: Graves' disease (GD), Hashimoto's thyroiditis (HT), atrophic autoimmune thyroiditis, postpartum thyroiditis, painless thyroiditis unrelated to pregnancy and thyroid-associated ophthalmopathy. This spectrum present contrasts in terms of thyroid function, disease duration and spread to other anatomic location. The genetic basis of autoimmune thyroid disease (AITD) is complex and likely to be due to genes of both large and small effects. In GD the autoimmune process results in the production of thyroid-stimulating antibodies and lead to hyperthyroidism, whereas in HT the end result is destruction of thyroid cells and hypothyroidism. Recent studies in the field of autoimmune thyroid diseases have largely focused on (i) the genes involved in immune response and/or thyroid physiology with could influence susceptibility to disease, (ii) the delineation of B-cell autoepitopes recognized by the main autoantigens, thyroglobulin, thyroperoxidase and TSH receptor, to improve our understanding of how these molecules are seen by the immune system and (iii) the regulatory network controlling the synthesis of thyroid hormones and its dysfunction in AITD. The aim of the present review is to summarize the current knowledge regarding the relation existing between some susceptibility genes, autoantigens and dysfunction of thyroid function during AITD.

Thyroid peroxidase as an autoantigen

Thyroid peroxidase (TPO) evokes high-affinity, IgG-class autoantibodies [TPO autoantibodies (TPOAbs)] and TPO-specific T cells that are markers of thyroid infiltration or implicated in thyroid destruction, respectively. A diverse repertoire of human monoclonal TPOAbs, unparalleled in other autoimmune diseases, provides invaluable probes for investigating antibody epitopes. Human TPOAbs recognize an immunodominant region comprising overlapping A and B domains on conformationally intact TPO. Amino acids recognized by TPOAbs are located in the regions with homology to myeloperoxidase (MPO) and the complement control protein (CCP) but not in the epidermal growth factor (EGF)-like region. T cells recognize epitopes in the MPO-like region but not in the CCP- or EGF-like regions in humans. Monoclonal human TPOAbs modulate processing of TPO protein to provide peptides for some T cells. A human T cell clone expressed transgenically in mice induces lymphocytic infiltration and hypothyroidism. This T cell's epitope is only generated by thyrocyte processing of endogenous TPO. Further, intact TPO expressed in vivo is also required for induction of TPOAbs in mice that resemble human autoantibodies. Overall, some TPO-specific T cells and the majority of autoantibodies in humans develop in response to TPO presented by thyroid cells, rather than to TPO released by damaged thyrocytes.