Epidermal growth factor and transforming growth factor-alpha suppress HLA class II induction in human thyroid epithelial cells.

Inappropriate expression of HLA class II by human thyroid epithelial cells (thyrocytes) occurs in autoimmune thyroid diseases where it may contribute to the pathogenesis. Several substances have been found to induce or up-regulate thyrocyte HLA class II expression in vitro. The present investigations show that the induction of HLA class II in human thyrocytes cultured with interferon (IFN)-gamma can be partially suppressed by exposure of the thyrocytes to epidermal growth factor (EGF): this occurs when the thyrocytes are treated with the two reagents simultaneously and also when the exposure to EGF is before or after that with IFN-gamma. Concentrations of EGF at least as low as 0.1 ng/ml show this inhibitory effect, which can be over-ridden by very high concentrations of IFN-gamma. Thyrocyte HLA class II expression stimulated by thyroid-stimulating hormone (TSH) (in the presence or absence of IFN-gamma) is also suppressed by EGF. Transforming growth factor-alpha (TGF alpha), which is structurally related to EGF and interacts with the same cell surface receptor, has a similar inhibitory activity on the induction of thyrocyte HLA class II expression. The existence of substances which can down-regulate, as well as those which can up-regulate, thyrocyte HLA class II expression raises the possibility that the occurrence of such expression in vivo may be determined by the balance between factors with opposing modulatory effects.