Hypomethylation enhances Ia antigen expression by the FRTL5 rat thyroid cell line

The effect of DNA methylation on Ia antigen expression by FRTL5 rat thyroid cells was assessed using 5-azacytidine. Hypomethylation with this agent did not alone induce Ia antigen expression but did increase the expression of Ia antigen induced by suboptimal concentrations of rat gamma-interferon, present in a T cell conditioned medium or in recombinant form. These results suggest that DNA methylation does not account for the constitutive lack of Ia expression by thyroid cells, but methylation of gamma-interferon response regions may confer partial resistance to the Ia-inducing effect of this lymphokine.