Modulation of major histocompatibility complex (MHC) class I genes in adenovirus 12 transformed cells: interferon-gamma increases class I expression by a mechanism that circumvents E1A induced-repression and tumor necrosis factor enhances the effect of interferon-gamma.

The protein products of the E1A gene of adenovirus type-12 (Ad12) block transcription of major histocompatibility (MHC) class I genes in both rodent and human transformed cells and interferon-gamma (IFN-gamma) is able to override this repression. Although IFN-gamma is known to stimulate class I transcription, we investigated whether its dominance over E1A repression could alternatively result from the ability of this cytokine to induce antiviral mechanisms. We show that this is not so, since the accumulation of Ad12 E1A mRNA and protein are unabated in the presence of IFN-gamma. Also, tumor necrosis factor (TNF) was shown to act synergistically with IFN-gamma to enhance class I antigen levels, although it had little effect alone. These results suggest that the normal pathway by which IFN-gamma acts to enhance the level of class I mRNAs, circumvents the block by which E1A represses class I transcription.