Thyroid Autoimmunity: Exploring the Role of Th17-associated Cytokines and Pathomorphological Mechanisms Involved in the Pathogenesis of Hashimoto’s Thyroiditis and Graves’ Disease

The prevalence and incidence of autoimmune thyroid diseases (AITD), presenting as Graves’ disease (GD) or Hashimoto’s thyroiditis (HT), has increased significantly in recent decades. It is crucial to identify immunological and pathomorphological factors involved in thyroid autoimmunity. Classically, HT has long been considered as a T helper (Th)1-mediated disease, while a Th2-driven autoimmune response is dominant for GD. Recently, Th17 cells have been established to play a role in the pathogenesis of AITD, however, their contribution to the initiation and progression of AITD remains unclear. Furthermore, selenium deficiency can impair the differentiation of Th cells, leading to dysfunction of cellular and humoral response. The aim of this thesis was to explore the role of Th17 cells in the pathogenesis of HT and GD by the use of different morphology methods and xMAP technology, and correlating these data with the selenium status. The initial study included 29 adult patients with AITD who underwent thyroidectomy, whereas subsequent clinical research project involved 52 patients with newly diagnosed, treatment-naïve AITD, as well as 26 healthy subjects served as controls. The plasma levels of Th17-associated cytokines – interleukin (IL)-17, IL-22, IL-23, IL-6, and IL-10 and the distribution and levels of immunoexpression IL-17, IL-23, and IL-1β within thyroid tissue were measured to characterize Th17 immune response in AITD. The integrity of the thyroid follicle by studying immunoexpression of cellular tight junctions – zonula occludens-1 and claudin-1 proteins, coupled to IL-17 and CD68, was explored. In addition, the selenium status was assessed. No significant differences in the plasma levels of Th17-associated cytokines were found among the patients with AITD and control subjects. However, the expression level of IL-17 in the thyrocytes was significantly higher in the HT and GD patients than in controls, simultaneously correlating with IL-23 and IL-1β immunopositivity in the HT group. Plasma Th17-associated cytokines’ levels were positively correlated with the severity of hyperthyroidism, independent of autoantibody levels, thus suggesting their possible role in GD pathogenesis. The changes in molecules of thyrocyte junctional complexes highlighting impairment of the integrity of thyroid follicle in HT were observed, but no significant association with IL-17 was found. Although no difference in selenium levels was observed between the AITD patients and controls, the results of the given research suggest the selenium status of the Latvian patients with newly diagnosed GD or HT is at a suboptimal level. Plasma selenium levels were negatively correlated with anti-thyroperoxidase (TPO) autoantibody titres in the HT patients, thus supporting the immunomodulatory role of selenium in AITD. Moreover, HT patients with higher anti-TPO autoantibody levels had lower levels of selenium, suggesting that these patients might benefit from selenium supplementation. Essential information deepening our knowledge about thyroid autoimmunity was obtained conducting this research, however, further experimental studies exploring the role and regulatory effects of Th17-related cytokines in the pathogenesis of AITD are required. More data from clinical studies are needed for a better understanding of the relationship between selenium supplementation and immune response