Major histocompatibility (MHC) class II antigen expression by thyroid epithelial cells has been widely implicated in the pathogenesis of autoimmune thyroid disease. We have examined rat MHC (RTl) class II antigen gene regulation in 1B-6 cells, cloned in our laboratory from the Fisher rat thyroid line FRT-L5. 1B-6 cells are TSH dependent for growth and proliferation, and are responsive to more than 5 µU/ml bovine TSH (bTSH) in terms of extracellular cAMP accumulation. Recombinant rat γ interferon (γIF; 1–1000 U/ml) treatment for 5 days together with bTSH (103 µU/ml) was able to induce class II antigen expression in up to 90% of 1B-6 cells, as detected by a murine monoclonal antibody to rat MHC class II antigen (FITC-OX-6). Total cellular RNA was examined in Northern blot analyses using an HLA-DR α-chain-specific RNA probe, which shares 78% sequence homology with the rat RTl.D α-chain gene. A 1.4-kilobase mRNA transcript was detected in γlF-stimulated cells, but not in untreated cells. Dose-response studies of MHC class II gene expression, using a cytoplasmic RNA slot blot technique for α-chain mRNA levels and FITCOX 6 monoclonal antibody for detection of MHC class II antigen expression, indicated 1B-6 sensitivity to γIF in the range of 10–100 U/ml (in the presence of 103 µU/ml bTSH) and to bTSH in the range of 10–100 µU/ml (in the presence of 102 U/ml γIF). These data demonstrate dual control of MHC class II antigen gene expression by γIF and bTSH in a differentiated rat thyroid cell. Clone 1B-6 represents a powerful means of analyzing reproducibly and in long term studies the regulation of thyroid cell MHC class II gene expression.
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