Thyrocyte HLA class II expression and regulation in relation to thyroid autoimmunity

The occurrence of HLA Class II expression by thyroid (and other endocrine) epithelia in autoimmune diseases suggests that these cells may facilitate their own destruction by immunogenically presenting autoantigens. This is supported by the findings that Class II+ thyrocytes can specifically stimulate virus-specific and autoreactive T cell clones, and that Class II expression by thyrocytes correlates with the occurrence of thyroid autoantibodies. A variety of factors may contribute to the regulation of Class II expression by thyrocytes: this is induced by interferon (IFN-gamma), and is enhanced by thyroid stimulating hormone (TSH) and by tumour necrosis factor (TNF). Conversely, epidermal growth factor (EGF) suppresses the induction of Class II in thyrocytes. This complex regulation is reflected in differences in HLA-D subregion expression between patients (DR greater than DP greater than DQ). The immune-based mechanisms of thyrocyte Class II regulation are clearly applicable to the on-going disease in an infiltrated thyroid, but the possibility of nonimmune Class II induction deserves attention, particularly in identifying factors which might contribute to the initial autoimmune attack. The possible involvement of such mechanisms in autoimmunity is supported by findings in Type I diabetes in which Class II+ islet beta cells can be found in the absence of infiltration. Further evidence is provided by the observation that a proportion of thyrocytes transformed with SV40 DNA constitutively express Class II molecules. Finally, the 'activated' state of capillary endothelial cells in organs subject to autoimmune attack suggests that they may play an important role in facilitating the autoreactive infiltration of the tissues.