An excess of dietary iodine accelerates the development of a thyroid-associated lymphoid tissue in autoimmune prone BB rats

Abstract Previous studies have shown that dietary iodine enhances the severity and incidence of focal thyroiditis in autoimmune BB rats and OS chickens. However, which lymphoid cells are involved in the development of the iodine-induced focal thyroiditis and what the consequences are for the anti-colloid antibody production have not been studied in detail. We therefore performed a study in which 3-week-old female BB rats were kept on either an enriched iodine diet (EID; iodine intake, 100 μg iodine/day) or a normal iodine diet (NID; iodine intake, 7 μg iodine/day) for a period of 18 weeks. The development of the focal thyroiditis was immunohistologically studied. Immunohistological data were compared to the thyroid hormone status and anti-colloid antibody production. Our data confirm that a high dietary iodine intake results in an accelerated development of the focal lymphoid cell infiltrates in the thyroid of the BB rat. After 12-18 weeks of an EID 50% of the BB rats developed these infiltrates. Our data additionally show that: (a) the process starts with increases in the number of infiltrating MHC class II-positive dendritic cells and a clustering of these cells with T cells, B cells, and some macrophages and (b) the focal infiltrates are highly organized and consist of central B cell follicle-like structures surrounded by rims and areas of T cells. The architecture of the focal thyroiditis is hence very similar to mucosa-associated lymphoid tissue and secondary lymphoid organs (spleen and lymph node). Only minor signs of thyrocyte destruction were observed. We therefore consider the term "thyroiditis" as inappropriate and prefer the term "thyroid-associated lymphoid tissue." Since the thyroiditis component was small, it is also not surprising that the BB rats on the EID remained euthyroid. The presence of the thyroid-associated lymphoid tissue in the BB rats was positively correlated to the presence of anti-colloid antibody in the serum of the BB rats. We speculate that the dietary iodine might have direct effects on cells of the immune system or on cells forming the microenvironment of lymphoid tissue (reticulum cells). A role for highly iodinated thyroglobulin in the accelerated development of thyroid-associated lymphoid tissue is also possible.