The expression of the microsomal/peroxidase autoantigen in human thyroid cells is thyrotrophin-dependent.
In the present report the mechanisms responsible for the expression of the thyroid microsomal autoantigen (M-Ag) were studied in primary cultures of human thyroid cells prepared from Graves' or non-toxic goitres. The indirect immunofluorescence (IFL) technique using human sera positive for anti-microsomal antibody (anti-MAb) was employed to detect M-Ag. Studies were performed to ascertain whether M-Ag recognized by anti-MAb could be identified with thyroid peroxidase (TPO). Preabsorption experiments showed that, similarly to solubilized thyroid microsomes, purified human TPO abolished the binding of anti-MAb to thyrocytes, while no inhibition was obtained with control human tissues. The identity of M-Ag and TPO was also demonstrated using a double layer IFL technique which allowed a simultaneous staining of the antigen(s) recognized by anti-MAb and by a monoclonal anti-TPO antibody. After 5-15 days of TSH withdrawal from the culture medium the M/TPO-Ag disappeared from the surface and the cytoplasm of human thyroid cells. Readdition of TSH (0.1-100 mU/ml) to cells lacking M/TPO-Ag elicited its reappearance within 48-72 h. This effect of TSH was prevented by 10 microM cycloheximide but not by methimazole (0.1-2 mM). Two stimulators of the adenylate cyclase-cAMP system, cholera toxin and forskolin, and 8-bromo-cAMP mimicked TSH in inducing M/TPO-Ag. Thyroid stimulating antibody (TSAb) of Graves' disease also reproduced the effect of TSH on M/TPO-Ag reexpression in human thyroid cells. By contrast, epidermal growth factor, oestradiol or NaI were ineffective in inducing M/TPO-Ag. The present data indicate that: (i) the expression of M/TPO-AG in human thyroid cells is dependent on TSH stimulation, through pathways which involve cAMP production and protein synthesis, (ii) TSAb reproduces this effect of TSH; (iii) oestradiol and NaI have no direct influence on the expression of M/TPO-Ag.