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.

Recombinant thyroid antigens: preparation and application in the diagnosis and treatment of autoimmune diseases

New opportunities are opening up in the study of the relationship between the antigenic structure of thyroid autoantigens and autoantibodies in the pathogenesis of such autoimmune diseases as Graves’ disease (GD) and autoimmune thyroiditis (AIT) using synthesized recombinant proteins of the thyroid-stimulating hormone receptor (TSHR) and human thyroid peroxidase (TPO). In the present work, the results of cloning of the fragments of the TPO extracellular domain and fragments of the RNA sequence of the α-subunit of TSHR, which do not contain nucleotide substitutions, are demonstrated. Recombinant vectors for the expression of proteins of the α-subunit of thyroid-stimulating hormone receptor and fragments of the TPO extracellular domain have been obtained.

An update on the pathogenesis of Hashimoto’s thyroiditis

It is 70 years since Noel Rose embarked on his pioneering studies that lead to the discovery of autoimmune thyroiditis and the elucidation of Hashimoto’s thyroiditis. This short review to honour his passing focuses on the developments in our understanding of the causes and pathogenesis of HT over the last five years. Recent genetic studies have reported heritability estimates for HT and associated diseases for the first time, and emphasised the complexity of the genetic factors involved, including monogenic forms of HT. Environmental factors continue to be elucidated, especially as a side effect of drugs which modulate the immune system therapeutically. Regarding pathogenetic mechanisms, multiple cytokine networks have been identified which involve the thyroid cells in a circuit of escalating proinflammatory effects, such as the expression of inflammasome components, and an array of different defects in T regulatory cells may underlie the loss of self-tolerance to thyroid autoantigens. Finally, a number of studies have revealed fresh insights into disease associations with HT which may have both pathological and clinical significance, the most intriguing of which is a possible direct role of the autoimmune process itself in causing some of the persistent symptoms reported by a minority of patients with levothyroxine-treated HT.

Immunological Mechanisms of Autoimmune Thyroid Diseases: A Shift in The Traditional TH1/TH2 Paradigm

Autoimmune thyroid diseases (AITD) mainly include Hashimoto’s thyroiditis (HT) and Graves’ disease (GD), which are characterised by the presence of circulating antibodies against various thyroid autoantigens and infiltration of the thyroid gland by autoreactive lymphocytes. Despite the significant advancement in the knowledge of AITD pathogenesis in the last decade, the specific immunological mechanisms responsible for development of the disease are not thoroughly understood. Classically, HT has long been considered as a T helper (Th)1-mediated disease, while a Th2-driven autoimmune response is dominant for GD development. However, this classification has changed due to the description of Th17 lymphocytes, which suggested participation of these cells in AITD, particularly HT pathogenesis. Moreover, a shift in the balance between Th17 and T regulatory (Treg) cells has been observed in thyroid autoimmunity. We have observed overexpression of IL-17, the prominent effector cytokine of Th17, within thyroid tissues from HT and GD patients in our studies. The present review will focus on recent data regarding the role of Treg and Th17 lymphocytes in AITD pathogenesis. In addition, the impact and proposed mechanisms of the predominant environmental factors triggering the autoimmune response to the thyroid will be discussed.

Reflections on Thyroid Autoimmunity: A Personal Overview from the Past into the Future

After investigating thyroid autoimmunity for more than 40 years, we present a personal perspective on the field. Despite effective therapies for Graves’ hyperthyroidism and Hashimoto’s thyroiditis, cures are elusive. Novel forms of therapy are being developed, such as small molecule inhibitors of the TSH receptor (TSHR), but cure will require immunotherapy. This goal requires advances in understanding the pathogenesis of thyroid autoimmunity, the ‘keys’ for which are the thyroid antigens themselves. Presently, however, greater investigative focus is on non-thyroid specific immune cell types and molecules. Thyroid autoantigens are the drivers of the autoimmune response, a prime example being the TSHR. In our view, the TSHR is the culprit as well as the victim in Graves’ disease because of its unique structure. Unlike the closely related gonadotropin receptors, the TSHR cleaves into subunits and there is strong evidence that its shed extracellular A-subunit, not the holoreceptor, is the major antigen driving pathogenic thyroid stimulating autoantibodies (TSAb) development. There is no Graves’ disease of the gonads. Studies of potential antigen-specific immunotherapies require an animal model. Such models have been developed in which TSAb can be induced or, more importantly, arise spontaneously. Not appreciated until recently by thyroid investigators is that B cell surface autoantibodies are highly efficient ‘antigen receptors’ and the epitope to which an autoantibody binds influences antigen processing and which peptide is presented to T cells. These animal models and recombinant human autoantibodies cloned from Graves’ and Hashimoto’s B cells (plasma cells) are available for study by future generations.

Evolution of the views on pathogenesis of autoimmune thyroid diseases and prospects for their target therapy

Modern scientific literature contains few reports concerning the influence of target therapy on pathogenetic factors of autoimmune thyroid diseases (AITD). Despite a large number of hypotheses of AITD pathogenesis, the only well established fact is the starting stage of Graves disease (GD) and autoimmune thyroiditis (AIT) is the loss of tolerance to thyroid autoantigens and the final stage is production of autoantibodies to them. Up to 75-80% of the patients with GD have antibodies against thyroid peroxidase and only few of them have anti-thyroglobulin antibodies more characteristic of AIT. Thyrotropin releasing hormone (TRH) is known to stimulate T-lymphocyte production via local effect on insulin-like growth factor (IGF). Modern studies confirm the important role of cytokines in immunopathogenesis of GD and AIT. Moreover, excess activation of this system in AITD provides a basis for the development of specific therapeutic approaches to personified pharmacotherapy. The effectiveness of anti-cytokine therapy of GD and AIT was demonstrated in animal experiments. Studies of therapy targeted on orbital and thyroid autoantigens in AITD are currently underway. The existence of specific receptors and the ability of immunocompetent cells to produce neuropeptides create prerequisites for their participation in intercellular cooperative processes. It can be supposed, by analogy with the influence of hormones and neuromediators on immunocytes, that neurohormones act on them via specific receptors with the involvement of cyclic nucleotides. It opens up opportunity for targeted correction of these relationships. Further studies of immunopathogenetic mechanisms of GD and AIT for better understanding the role of interaction between inborn and acquired immunity, its regulation, and intersystem transmission of signals in the development of these diseases are needed to realize modern strategies of their target therapy.