The influence of iodine on the intensity of the intrathyroidal autoimmune process in Graves' disease

Autoreactivity to lipoate and a conjugated form of lipoate in primary biliary cirrhosis

BACKGROUND & AIMS

Although considerable effort has been directed toward the mapping of peptide epitopes by autoantibodies, the role of nonprotein molecules has been less well studied. The immunodominant autoantigen in primary biliary cirrhosis (PBC), E2 components of pyruvate dehydrogenase complexes (PDC-E2), has a lipoate molecule bonded to the domain to which autoantibodies are directed.

METHODS

We examined sera from patients with PBC (n = 105), primary sclerosing cholangitis (n = 70), and rheumatoid arthritis (n = 28) as well as healthy volunteers (n = 43) for reactivity against lipoic acid. The lipoic acid hapten specificity of the reactive antibodies in PBC sera was determined following incubation of aliquots of the sera with human serum albumin (HSA), lipoylated HSA (HSA-LA), PDC-E2, lipoylated PDC-E2, polyethylene glycol (PEG), lipoylated PEG, free lipoic acid, and synthetic molecular mimics of lipoic acid.

RESULTS

Anti-lipoic acid specific antibodies were detected in 81% (79 of 97) of antimitochondrial antibody (AMA)-positive patients with PBC but not in controls. Two previously unreported specificities in AMA-positive sera that recognize free lipoic acid and a carrier-conjugated form of lipoic acid were also identified.

CONCLUSIONS

We hypothesize that conjugated form(s) of native or xenobiotic lipoic acid mimics contribute to the initiation and perpetuation of autoimmunity by at first breaking self-tolerance and participating in subsequent determinant spreading. The variability in the immunoreactive carrier/lipoate conjugates provides an experimental framework on which potential mechanisms for the breakdown of self-tolerance following exposure to xenobiotics can be investigated. The data have implications for patients taking lipoic acid as a dietary supplement.

Kinetics of mononuclear cell infiltration and cytokine expression in iodine-induced thyroiditis in the NOD-H2h4 mouse

Mononuclear cell infiltration of the thyroid gland is a common histologic feature of chronic lymphocytic thyroiditis. Although the infiltrating mononuclear cells have been implicated in the destruction of the thyroid, information concerning the progression of infiltration into the thyroid is limited. In this report, we examine the composition and kinetics of mononuclear cell infiltration in the thyroid and the expression of major histocompatibility complex class II (I-Ak), IL-12, and IFN-gamma in the thyroid of the NOD-H2h4 mouse, a model of spontaneous autoimmune thyroiditis accelerated by the administration of excess dietary iodine. Mice were given a low dose of 0.015% NaI in their drinking water for 2, 4, 6, 8, and 16 weeks, and thyroids were removed, serially sectioned, and stained in an avidin-biotin peroxidase assay. The thyroid infiltrate included CD4+ and CD8+ T cells, F4/80+ macrophages, and B220+ B cells. After 2 weeks of iodine treatment, CD4+ T cells were the first seen in the thyroid, followed by CD8+ T cells and F4/80+ macrophages. B220+ B cells entered the thyroid after 4 weeks of iodine treatment. IL-12 and IFN-gamma positive cells were located in the thyroid early in disease and were up-regulated in the focal accumulations of infiltrating cells. Thyrocytes clearly expressed I-Ak after 4 weeks of iodine treatment near the location of mononuclear cell infiltration.

Iodine and thyroid autoimmune disease in animal models

Thyroid autoimmune diseases are complex, polygenic afflictions the penetrance of which is heavily dependent on various environmental influences. In their pathogenesis, an afferent stage (enhanced autoantigen presentation), a central stage (excessive expansion and maturation of autoreactive T and B cells), and an efferent stage (effects of autoreactive T cells and B cells on their targets) can be discerned. At each stage, a plethora of inborn, endogenous or exogenous factors is able to elicit the abnormalities characteristic of that stage, thus opening the gateway to thyroid autoimmunity. Iodine is an important exogenous modulating factor of the process. In general, iodine deficiency attenuates, while iodine excess accelerates autoimmune thyroiditis in autoimmune prone individuals. In nonautoimmune prone individuals, the effects of iodine are different. Here iodine deficiency precipitates a mild (physiological) form of thyroid autoimmune reactivity. Iodine excess stimulates thymus development. Iodine probably exerts these effects via interference in the various stages of the autoimmune process. In the afferent and efferent stage, iodine-induced alterations in thyrocyte metabolism and even necrosis most likely play a role. By contrast, in the central phase, iodine has direct effects on thymus development, the development and function of various immune cells (T cells, B cells macrophages and dendritic cells) and the antigenicity of thyroglobulin.

[Iodine and thyroid function]

Iodine is a raw material for the thyroid production of hormone which is on the major external control of TSH. The thyroid adaptation to iodine deficiency consists in an increasing iodine concentration from the circulation, an enhancing iodination of the tyrosyl residues in thyroglobulin, a decreasing iodine storage associated to a better recycling of non hormonal iodine and thyroid hyperplasia to provide more synthetic possibilities. Genetic variation and environmental factors explain the wide variation of individuals response to iodine deficiency resulting in a high prevalence of goiter, a mild TSH level increase or overt hypothyroidism. At long term iodine deficiency may have severe pathological consequences and induce neuropsychological deficits in school-children. A policy of iodine supplementation mainly by iodized salt must be undertaken in many areas in the world. Effects of an iodine excess on thyroid function are variable depending upon the underlying thyroid disorder and ambient iodine intake. The most subjects remain euthyroid by mechanisms of autoregulation based on an inhibition of thyroid hormone synthesis and a decrease in the thyroid iodide trap. Euthyroid individuals from high iodine intake areas or those with a history of lymphocytic thyroiditis, treated Graves' disease or subtotal thyroidectomy develop hypothyroidism. On the other hand iodine induced hyperthyroidism is more common in areas of iodine deficiency and in older patients with nodular goiter.