T cell-mediated mechanisms in autoimmune thyroiditis

Experimental rat models for Hashimoto's thyroiditis

PURPOSE

Hashimoto's thyroiditis (HT) is an autoimmune thyroid disease characterized by T lymphocyte-mediated destruction of thyroid follicles. To study the pathogenesis of HT and the efficacy of new substances for its treatment, an easily obtained and adequate to the human disease experimental model is needed. The aim of our study was to find out whether it is possible to induce experimental autoimmune thyroiditis (EAT) similar to Hashimoto's thyroiditis by injecting with thyroglobulin (Tg) without using agents that enhance its thyroiditogenicity and without taking into account the genetic sensitivity of animals.

METHODS

Wistar rats were immunized with freshly isolated rat Tg or porcine Tg. In 8 weeks, histological studies of the thyroid and parathyroid glands were performed. Thyroid function and total serum calcium level were also evaluated.

RESULTS

Immunization with both rat and porcine freshly isolated Tg caused T lymphocytic infiltration of the thyroid gland, thyroid follicle atrophy and degradation in Wistar rats. EAT caused by porcine Tg was characterized by greater severity than EAT induced with rat Tg. In 55% of rats with porcine Tg-induced EAT, oxyphilic metaplasia was detected in the parathyroid glands. In addition, low total serum calcium was observed in these rats.

CONCLUSION

Two rat models of autoimmune thyroiditis were obtained. EAT caused in Wistar rats by immunization with rat Tg is similar to Hashimoto's thyroiditis. EAT induced with porcine Tg was accompanied by oxyphil cell metaplasia in the parathyroids and hypocalcemia.

Immunogenetic aspects of human thyroglobulin-reactive T cell lines and hybridomas

The in vitro proliferative response to T cells primed with human thyroglobulin (Tg) was compared in 11 independent haplotypes on B10 background. B10.K and B10.S mice were the most responsive, whereas, with the exception of B10.PL (H-2u), all other B10 congenics were intermediate responders. The two best responders to in vitro challenge with human Tg, of the k and s haplotype, are the same as those showing H-2-linked susceptibility to induction of experimental autoimmune thyroiditis (EAT) with mouse Tg. Since shared epitopes on human and mouse Tgs have been shown to be thyroiditogenic by adoptive transfer studies in CBA (H2k) mice, the findings indicate that shared epitopes may be studied in appropriate (i.e. EAT-susceptible) strains of mice. Therefore, we proceeded to develop methods to produce T-cell lines and hybridomas to human Tg in B10.K and B10.S mice, test their cross-reactivity to heterologous Tgs and their Ia restriction patterns. By using antigen-presenting cells from recombinant strains, we identified restriction elements encoded by the I-A subregion alone and a combinatorial molecule from the I-A/I-E subregions.

The effect of methimazole on the immune system is unlikely to operate directly on T lymphocytes

It has been shown that the antithyroid drug methimazole (MMI) may affect B cells and possibly accessory cell function. In the present study we investigated in detail the effects of MMI on T cell in vitro proliferation. The following variables were evaluated: T cell proliferation following stimulation with phytohemagglutinin (PHA), and anti-CD3 or anti-CD2 monoclonal antibodies; interleukin-2 (IL-2) and gamma-interferon (gamma-IFN) production by PHA-stimulated T cells in bulk culture and by T cell clones; PHA-induced IL-2 receptor expression; LPS-induced interleukin-1 production by accessory cells. The results obtained failed to demonstrate any effect of MMI on T cells in vitro proliferation, whatever the activation pathway considered. In addition, IL-2 and gamma-IFN productions were substantially unaffected by the drug, as well as IL-1 production by accessory cells. However, a slight reduction of PHA-induced IL-2 receptor expression was observed. Although the hypothesis of an effect of MMI on some specialized T cell functions cannot be ruled out, it is likely that the supposed "immunosuppressive" effect of the drug does not concern primarily the T lymphocyte.

Importance of cytolytic activity and dietary iodine in the pathogenesis of postpartum thyroiditis

The incidence of lymphocytic thyroiditis (LT) has increased since the introduction of iodine prophylaxis, and LT is claimed to represent an adverse reaction to iodine. This theory is supported by animal studies as well as experimental studies in humans. In pregnancy iodine turnover is increased. During the immunological rebound period occurring postpartum the thyroid autoantibodies increase, and may lead to postpartum thyroiditis (PPT). The iodine content within the gland may be important for this reaction. In vitro it has been shown that iodine inhibits the cAMP-dependent events of the thyroid cell function, but iodine may also stimulate the intrathyroidal immuno-competent cells leading to cytokine synthesis. Cytokines have been shown to interfer with the thyroid cell function. Iodine may thus trigger the autoreactive attack and enhance the local autoantibody production, which plays a primary pathogenic role for development of PPT.