To investigate the stability of FRTL-5 cells we cloned stock cells by limiting dilution in a 6-hormone medium with bovine TSH (bTSH) (10 mU/ml) (6H medium). One third of the wells with irradiated fibroblast feeder cells developed FRTL-5 colonies. One hundred and twenty clones were obtained, of which 29 (8%) developed sufficiently for functional analysis. Clones were tested for growth responses to bTSH stimulation as evidenced by 72-h [3H]thymidine uptake and TSH receptor activation by cAMP generation. Clonal growth responses to bTSH were of three types: dose-related growth increase, absence of growth stimulation, and stimulation by bTSH in concentrations up to 100 microU/ml and inhibition of growth above 100 microU/ml. All clones tested showed evidence of extracellular cAMP accumulation in response to bTSH. However, sensitivity to bTSH varied from 1 to 100 microU/ml and maximum cAMP secretion with 1 mU/ml bTSH varied from 2 to 13 pmol/ml. Whereas certain clones showed high sensitivity to bTSH with respect to both growth and cAMP responses (e.g. 1B-6), there were clones which showed disparity in this relationship, as evidenced by poor growth dependency but high cAMP responses or by growth stimulation yet insensitivity with respect to cAMP secretion. These data demonstrate that the FRTL-5 line contains cells with variable responsiveness to bTSH. Whereas the FRTL-5 line is heterogeneous and subject to developmental variation, cloning by limiting dilution allows the derivation of highly bTSH-sensitive cells, such as 1B-6, from the stock cultures.

We have investigated the influence of antithyroid drugs (methimazole and propylthiouracil) and sodium iodide on the expression of major histocompatibility (MHC) class II antigen expression in human and rat thyroid cells. While methimazole and propylthiouracil significantly inhibited lectin-induced MHC class II (HLA-DR) antigen in crude human thyroid monolayer preparations these drugs had no influence on gamma-interferon induction of class II antigens in similar cell preparations. Hence, antithyroid drugs probably had a direct effect on thyroid monolayer T cells which are known to effect the lectin induction of MHC class II antigen via T-cell secretion of the cytokine, gamma-interferon (IF). Sodium iodide, similarly, had no direct influence on human thyroid cell MHC class II antigen expression induced by gamma-IF. However, iodide significantly inhibited the gamma-IF-induced expression of MHC class II antigens in a proliferating rat thyroid cell clone derived from FRTL-5 cells (clone 1B-6). In the rat thyroid cell clone this antagonistic action appeared to be exerted via inhibition of TSH-induced proliferation. These data add further support to the multifactorial nature of the profound immunosuppressive influence of antithyroid drugs in autoimmune thyroid disease and experimental thyroiditis. In contrast, iodides may have a major, and often overlooked, influence on thyroid cell proliferation and antigen expression.