Thyroid autoantigens and their relevance in the pathogenesis of thyroid autoimmunity

Abstract Humoral and cellular immune responses are both involved in autoimmune disorders of the thyroid gland. In the last five years, new substantial data have been obtained on the nature and the expression of thyroid cell surface autoantigens and on the demonstration of the functional heterogeneity of autoantibodies to the thyroid stimulating hormone (TSH) receptor. In the present report, attention will be mainly focused on recent studies carried out in our laboratory. The main autoantigens so far identified include the ‘microsomal’ antigen, thyroglobulin and the TSH receptor. For many years the ‘microsomal’ antigen (M) was considered a poorly characterized constituent of the cytoplasm of the thyroid cell. In the last five years, several lines of evidence were provided indicating that M is also well represented on the surface of the follicular cell and is identical to thyroid peroxidase (TPO). The use of anti-TPO monoclonal antibodies, presently available, have confirmed this antigenic identity. Microsomal (anti-TPO) antibodies are very useful markers of autoimmune thyroid disorders and are generally present in Hashimoto's thyroiditis, idiopathic myxedema and Graves' disease. TSH receptor antibodies (TRAb) are present in the sera of patients with Graves' disease. TRAb are able to stimulate thyroid adenylate cyclase and also to mimic TSH in its thyroid growth stimulation. Thus, these antibodies may have a pathogenetic role in goiter formation and in thyroid hyperfunction in Graves' disease. TRAb were also shown to inhibit both TSH binding to its receptor and TSH-stimulated adenylate cyclase activity. Recently TRAb, which inhibited TSH-stimulated adenylate cyclase activity, were found in idiopathic myxedema patients and may be responsible for impairment of thyroid function. In conclusion, it appears that the majority of thyroid autoantibodies may interfere in some peculiar thyroid function activities.