Autoimmune thyroiditis: a model uniquely suited to probe regulatory T cell function

Abstract

Murine experimental autoimmune thyroiditis (EAT) is a model for Hashimoto's thyroiditis that has served as a prototype of T cell-mediated autoimmunity for more than three decades. Key roles for MHC restriction and autoantigen influence on susceptibility to autoimmunity have been demonstrated in EAT. Moreover, it has served a unique role in investigations of self tolerance. In the early 1980s, self tolerance and resistance to EAT induction could be enhanced by increasing circulating levels of the autoantigen, thyroglobulin (Tg), by exogenous addition as well as endogenous release. This observation, directly linking circulating self antigen to self tolerance, led to subsequent investigations of the role of regulatory T cells (Tregs) in self tolerance. These studies revealed that protection against autoimmunity, in both naive and tolerized mice, was mediated by thymically-derived CD4(+)CD25(+)Foxp3(+) Tregs. Moreover, these naturally-existing Tregs required proper costimulation, in context with autoantigen presentation, to maintain and enhance self tolerance. In particular was the selected use of MHC- and heterologous Tg-restricted models from both conventional and transgenic mice. These models helped to elucidate the complex interplay between autoantigen presentation and MHC class II-mediated T cell selection in the development of Treg and autoreactive T cell repertoires determining susceptibility to autoimmunity. Here we describe these investigations in further detail, providing a context for how EAT has helped shape our understanding of self tolerance and autoimmunity.

Fig. 1

In vivo depletion of CD4⁺CD25⁺Foxp3⁺ T cells abrogates tolerance induced by exogenous or endogenous mTg. Circulatory mTg level was raised either by 100 μg deaggregated mTg i.v. on days −24, −17 (A), or by TSH-containing osmotic pump i.p. on days −26 or −22 (B). All mice were then injected i.v. with two 1-mg doses of CD25 mAb and challenged with 40 μg mTg + 20 μg LPS on days 0 and 7. Thyroiditis was assessed in individual mice on day 28. Adapted from Morris et al. (A) [21] and Morris et al. (B) [24].

Fig. 2

Immunization with hTg induces Tg-specific cytotoxic T cells. (A) E⁺B10.Ab⁰ (A⁻E⁺) mice were immunized with 100 μg hTg and 20 μg LPS on days 0 and 7 and their splenocytes were cultured with 10 μg/ml hTg410 or hTg2344 for 5 days. (B) A⁻E⁺ mice were immunized with 150 μg peptide hTg410 or hTg2344 in CFA and their lymph node cells were cultured for 5 days with the priming peptide. The cultured cells were then used as effectors in a ⁵¹Cr-release assay; EL-4 lymphoma cells (10⁴/well) loaded with hTg or peptide served as target cells; effector:target ratio = 100:1.

Fig. 3

Exacerbation of hTg- or mTg-induced thyroiditis by depletion of CD4⁺CD25⁺ T cells is more pronounced in less susceptible DQ8⁺ mice. DR3⁺ Ab⁰/NOD (A) or DQ8⁺ Ab⁰/NOD mice (B) were injected with 1-mg doses of CD25 mAb on days −14 and −10 prior to EAT induction with 100 μg hTg or 40 μg mTg + 20 μg LPS on days 0 and 7. Thyroiditis was assessed in individual mice on day 28.

Fig. 4

Competition by Eα52–68 peptide interferes with clonal deletion of mTg-specific autoreactive T (Tauto) cells and selection of Tregs in H2A^b mice. (A) In the absence of H2E class II molecules, Tg-derived peptides are efficiently presented by H2A class II molecules for clonal deletion and selection of Tregs. (B) In the presence of H2E class II molecules, Eα-derived peptides, such as Eα52–68, compete with Tg-derived peptides for presentation by H2A class II molecules. Reduced presentation of Tg-derived peptides allows Tg-specific Tauto cells to escape clonal deletion, and decreases the selection of Tg-specific Tregs.