Iodine in autoimmune thyroiditis.

Abstract

The aim of this chapter was to present a theory or concept of autoimmune disease that, in a sense, deemphasized the importance of immunoregulatory defects but rather concentrated on defects and/or changes in a target organ that might stimulate immune responses. Defects in immune regulation were not discussed because there is little evidence that generalized defects in immunoregulation occur in patients with autoimmune thyroid disease (38). There is some evidence of decreased numbers of thyroid-antigen-specific T-suppressor cells in patients with autoimmune thyroid disease; however, this has been detected only after the appearance of frank disease (38). Thus a reduction of T-suppressor cells specific for the thyroid microsomal antigen at a time when the immune system is responding vigorously to that antigen is to be expected and cannot a priori be assumed to be the reason for the immune response. One other reason for questioning the role of defective antigen-specific T-suppressor cells as an initiating event is that individual patients with autoimmune thyroid disease produce antibodies to a variety of unrelated thyroid antigens (Tg, microsomal antigen, which is now known to be thyroid peroxidase, TSH receptor, and others) (39). One would have to assume that the majority of thyroid patients spontaneously lose several unrelated clones of specific suppressor T cells. An alternative scenario of events in the pathogenesis of autoimmune disease is as follows: An environmental agent, whether it be iodide, alone, or in combination with high TSH, or a virus causes damage to the thyroid gland. The iodide-induced damage, perhaps mediated by hydroxyl radicals (40), is more severe and/or prolonged if the gland has a defect in iodide organification or perhaps, as seen in some susceptible chicken strains, partially autonomous thyroid function. As a result of the damage, leukocytes migrate into the gland. Once leukocytes arrive, a number of interesting phenomena occur. First, monocytes may secrete IL-1, which is directly cytotoxic for endocrine cells (41) and provides an accessory signal to T-helper cells. Second, T and B cells migrate into the damaged gland and into the follicles, where at least two of the important thyroid antigens are located, thyroglobulin and thyroid peroxidase. These two proteins are highly immunogenic: the thyroglobulin due to its increased iodine content and the thyroid peroxidase because it is a membrane-bound antigen sequestered in thyroid follicles. Third, the T cells, once activated, provide help to B cells and secrete gamma interferon, which induces the expression of class II MHC antigens on the thyroid epithelial cells (42).(ABSTRACT TRUNCATED AT 400 WORDS)