Thyroid hormone potentiates the antiviral action of interferon-γ in cultured human cells

Human interferon-γ (hIFN γ) exhibits a number of biological effects, including antiviral activity in homologous cells. The antiviral activity of recombinant (r) hIFN γ, estimated by inhibition of vesicular stomatitis virus yield, was potentiated up to 120-fold in human fibroblast (BG-9) cells exposed to free L-T₄ (0.5 × 10^-10 mol/L). Thyroid hormone alone did not induce the antiviral state in BG-9 cells. L-T₃ also potentiated the antiviral action of rhIFN γ, but D-T4 and D-T₃ were ineffective. The antiviral effect of hIFN α in BG-9 cells was not influenced by thyroid hormone. Exposure of rhIFN γ-treated BG-9 cells to L-T₄ for only 3 h was sufficient to potentiate hIFN γ-mediated antiviral activity. Similar potentiation by L-T₄ of the antiviral effect of rhIFN γ in HeLa cell cultures was also observed. Although the mechanism of potentiation of rhIFN γ action by thyroid hormone is incompletely understood, the absence of antiviral activity of thyroid hormone alone indicates that the iodothyronine effect does not depend upon hormonal action on genes able to be stimulated by IFN γ.