Thyroid hormone analogues potentiate the antiviral action of interferon-γ by two mechanisms

L-thyroxine (L-T₄) potentiates the antiviral activity of human interferon-γ (IFN-γ) in HeLa cells. We have added thyroid hormone and analogues to cells either 1) for 24 h pretreatment prior to 24 h of IFN-γ (1.0 IU/ml), 2) for 24 h cotreatment with IFN-γ, 3) for 4 h, after 20 h cell incubation with IFN-γ, alone, or 4) for 24 h pretreatment and 24 h cotreatment with IFN-γ. The antiviral effect of IFN-γ was then assayed. L-T₄ potentiated the antiviral action of IFN-γ by a reduction in virus yield of more than two logs, the equivalent of a more than 100-fold potentiation of the IFN's antiviral effect. 3,3′,5-L-triiodothyronine (L-T₃) was as effective as L-T₄ when coincubated for 24 h with IFN-γ but was less effective than L-T₄ when coincubated for only 4 h. D-T₄, D-T₃, 3,3′,5-triiodothyroacetic acid (triac), tetraiodothyroacetic acid (tetrac), and 3,5-diiodothyronine (T₂) were inactive. When preincubated with L-T₄ for 24 h prior to IFN-γ treatment, tetrac blocked L-T₄ potentiation, but, when coincubated with L-T₄ for 4 h after 20 h IFN-γ, tetrac did not inhibit the L-T₄ effect. 3,3′,5′-L-triiodothyronine (rT₃) also potentiated the antiviral action of IFN-γ, but only in the preincubation model. Furthermore, the effects of rT₃ preincubation and L-T₃ coincubation were additive, resulting in 100-fold potentiation of the IFN-γ effect. When L-T₄, L-T₃, or rT₃, plus cycloheximide (5 μg/ml), was added to cells for 24 h and then removed prior to 24 h IFN-γ exposure, the potentiating effect of the three iodothyronines was completely inhibited. In contrast, IFN-γ potentiation by 4 h of L-T₄ or L-T₃ coincubation was not inhibited by cycloheximide (25 μg/ml). These studies demonstrate two mechanisms by which thyroid hormone can potentiate IFN-γ's effect: 1) a protein synthesis-dependent mechanism evidenced by enhancement of IFN-γ's antiviral action by L-T₄, L-T₃, or rT₃ preincubation, and inhibition of enhancement by tetrac and cycloheximide, and 2) a protein synthesis-independent (posttranslational) mechanism, not inhibited by tetrac or cycloheximide, demonstrated by 4 h coincubation of L-T₄ or L-T₃, but not rT₃, with IFN-γ. The protein synthesis-dependent pathway is responsive to rT₃, a thyroid hormone analogue generally thought to have little effect on protein synthesis. A posttranslational mechanism by which the antiviral action of IFN-γ can be regulated has not previously been described. © 1996 Wiley-Liss, Inc.