Background
Autoimmune (Hashimoto’s) thyroiditis is characterized by lymphocytic infiltration of the parenchyma, causing a dense accumulation of lymphocytes, plasma cells, and macrophages with germinal center formation and thyroid enlargement. Although the pathogenesis of autoimmune thyroiditis is not entirely clear, studies have demonstrated that increasing the local production of inflammatory cytokines in the thyroid microenvironment plays a critical role in facilitating apoptosis in thyrocytes, leading to autoimmune thyroiditis [
1‐
3]. Among various inflammatory cytokines, IFN-γ, TNF-α and IL-1β appear to be critical since the combination of two such cytokines can significantly facilitate the development of experimental autoimmune thyroiditis (EAT) in mice [
1,
3,
4], a well-recognized animal model of autoimmune thyroiditis.
BH3 interacting-domain death agonist (BID) is a pro-apoptotic Bcl-2 family member that functions as a bridge molecule between two classic apoptotic pathways to augment apoptotic signaling. Our previous studies showed that the expression of BID in primary normal thyroid cells was significantly increased by inflammatory cytokines in vitro [
3]. The pretreatment of those inflammatory cytokines can also sensitize thyroid epithelia cells to death-receptor mediated apoptosis [
2,
3]. This finding suggests a potential role for BID in the pathogenesis of autoimmune thyroiditis. A number of recent reports have also indicated the involvement of BID in the development of other autoimmune diseases [
5‐
7]. We therefore hypothesized that the overexpression of BID plays a positive role in the development of autoimmune thyroiditis. To test this hypothesis we first established a transgenic mouse line that expresses human BID specifically in the thyroid, and then tested whether the overexpression of BID alone is sufficient for the development of autoimmune thyroiditis.
It has also been shown that there is connection between iodine ingestion and autoimmune thyroiditis. Indeed, iodine administration has been implicated in increasing incidence and/or severity of spontaneous thyroiditis in BB/W rats, CS chickens, hamsters and NOD-H-2 h4 mice [
8]. CBA/J (H-2 k) mice, however, are genetically susceptible to EAT induced by Tg but are resistant to EAT induced by iodine intake alone [
8‐
10]. In this study, we employed CBA/J (H-2 k) mice to examine whether the overexpression of BID can overcome this resistance, and sensitize these mice to iodine-induced autoimmune thyroiditis.
Discussion
Our previous work demonstrated that the Fas pathway in thyrocytes can be activated by Fas ligand in the presence of combinations of pro-inflammatory cytokines such as IL-1β, IFNγ and TNFα [
1,
2]. Further experiments revealed that the induction of apoptosis in thyrocytes is correlated with an increase of BID and Bak levels but a decrease of p44/42 mitogen-activated protein kinase activity [
2,
14,
15]. These findings suggest that the Fas apoptotic pathway activity may be further amplified by the bridge pro-apoptotic molecule BID, leading to the release of Bak from the mitochondria and activation of caspases. Thus, apoptosis of thyrocytes in EAT or autoimmune thyroiditis is dependent on both cell death receptors and mitochondrial elements, in which BID plays a critical role [
16,
17]. In contrast to most apoptotic activity, apoptotic cell death in EAT or autoimmune thyroiditis is not anti-inflammatory [
18]. Instead, apoptotic thyroid cells and secondary necrotic cells induce a pro-inflammatory environment, which may provide sufficient levels of self-antigens to intensify a deregulated immune response [
18,
19]. The occurrence of inflammation is now known to be closely associated with a BID-related pathway that is either dependent or independent of apoptosis [
20‐
22].
To test whether the genetic changes in BID play a role in the development of autoimmune thyroiditis, we successfully developed a transgenic mouse line in which BID is specifically over-expressed in the thyroid. We observed Tg-BID transgenic mice for 15 months, and during this period these mice remained healthy. These mice showed no obvious difference in activity and appearance compared with the control mice. In addition, no difference in thyroid histology and morphology were observed. We found that regardless of the level of BID in the thyroid of these transgenic animals, no differences were observed in the growth of mice, their activity, or serum T4 levels. Importantly, the classical features of thyroiditis, such as thyroid auto-antibodies, did not appear in any of the transgenic Tg-BID mice. These findings indicate that the overexpression of BID alone is not sufficient to cause the pathologic phenotype characteristic of spontaneous thyroiditis in mice.
The observation that Tg-BID transgenic mice do not develop spontaneous autoimmune thyroiditis is not totally unexpected. Autoimmune thyroiditis is a disorder with multiple causative factors including genetic, environmental, and immunological elements [
18,
23]. Among these elements, iodine is able to exert pleiotropic effects on either metabolic or immunological processes of thyroid. While iodine is an indispensable constituent of the two major thyroid hormones T3 and T4 [
24], it contributes to the development of autoimmune thyroiditis by enhancing the antigenicity of thyroglobulin and reducing regulatory T cells [
13,
25]. Though iodine is known to contribute to the development of autoimmune thyroiditis [
8‐
10], it may not be able to do so without other co-factors. For example, iodine is able to induce thyroiditis in nearly 100% of NOD.H-2 h4 mice that express I-Ak on the NOD genetic background, but cause none in mice such as CBA/J (H-2 k) (H-2 k) and NOD.SWR(H-2(q)) that do not carry I-Ak [
8]. It is unknown whether mice with BID overexpression would sensitize thyroiditis induced by iodine. To this end, we employed iodine-resistant CBA/J (H-2 k) mice in this study. We show that upon iodine administration, Tg-BID transgenic CBA/J (H-2 k) mice do indeed develop autoimmune thyroiditis in about 8 weeks, which is evident by a significant increase in serum anti-Tg autoantibody and 30% of Tg-BID transgenic CBA/J (H-2 k) mice having mononuclear cell infiltration into the thyroid glands. The timing of iodine-induced autoimmune thyroiditis in Tg-BID transgenic CBA/J (H-2 k) mice appears to be quite similar to that in NOD-H-2 h4 mice [
8].
Competing interest
All authors have no conflict of interest.
Authors’ contributions
SHW and JRB designed the study, and analyzed results. SHW and YF carried out experiments and drafted the manuscript. All authors have read and approved the final manuscript.