Detection and regulation of rat thyroid MHC class II (RT1.D) transcripts.

We have reexamined 1B-6 cells, a cloned line of FRTL-5 rat thyroid cells in order to determine whether such cells demonstrate constitutive MHC class II gene expression. A new homologous cDNA probe for the RT1.D alpha-chain region was developed and proved significantly more sensitive at detecting specific mRNA levels in 1B-6 cells than a human class II alpha-chain specific probe used in previous studies. Recombinant rat gamma-interferon induced MHC class II (RT1.D) antigen at concentrations as low as 1 U/ml (in the presence of bovine TSH), as detected by both alpha-chain specific mRNA transcripts and surface antigen expression. There was no evidence for constitutive expression of MHC class II transcripts in proliferating 1B-6 cells in the absence of gamma-interferon, even when bovine TSH was present. These studies add further evidence to our hypothesis that MHC class II antigen expression in thyroid cells is secondary to external stimulating factors.