Increased binding of IFN regulating factor 1 mediates the synergistic induction of CIITA by IFN-γ and tumor necrosis factor-α in human thyroid carcinoma cells

Expression of MHC class II molecules is restricted to professional antigen-presenting immune cells, but it can be induced by IFN-gamma in other cell types. Thyroid cells have been shown to induce class II expression (mainly HLA-DR) following stimulation with IFN-gamma and addition of tumor necrosis factor (TNF)-alpha synergistically enhanced this expression. Class II transactivator (CIITA) has been implicated as the master regulator of MHC class II molecules and its transcription has been shown to be regulated from four different promoters, one of which is responsible for its induction by IFN-gamma. The aim of this study was to find whether CIITA is synergistically induced by IFN-gamma and TNF-alpha in the human thyroid MRO-87-1 cell line, and to investigate the molecular mechanisms responsible for this synergism. We have demonstrated that IFN-gamma and TNF-alpha synergistically induce HLA-DR alpha and CIITA mRNAs, but prolonged incubation resulted in the inhibition of CIITA mRNA accumulation. Several potential mechanisms that could explain the synergistic effect were explored. NF-kappaB did not bind the CIITA inducible promoter and addition of SN50, which inhibits NF-kappaB translocation to the nucleus, did not change the synergistic effect. Furthermore, IFN-gamma did not induce I kappaB alpha degradation. Synergistic activation of signal transducer and activator of transcription (STAT)-1 or IFN regulating factor (IRF)-1 was not observed, and STAT-1 did not bind the CIITA inducible promoter. IRF-1, although not synergistically induced or activated, bound synergistically to its specific cis element on the CIITA type IV promoter. Thus we propose that IRF-1 binding mediates the synergistic induction of HLA-DRa and CIITA in thyroid cells.