Precise regulation of MHC class II gene expression is crucial for development and function of the immune system. Class II trans-activator (CIITA) has been shown to be required for constitutive and IFN-gamma-induced MHC class II transcription. TNF-alpha is commonly coexpressed with IFN-gamma during immune-mediated inflammatory responses and modulates IFN-gamma-stimulated MHC class II expression. The effect of TNF-alpha on MHC class II expression depends on cell type and cellular differentiation state. We show here that TNF-alpha suppresses IFN-gamma-induced CIITA mRNA accumulation, resulting in decreased MHC class II expression in human fibrosarcoma HT1080 cells. TNF-alpha also inhibits CIITA mRNA accumulation and protein expression in a tetracycline-regulated system without affecting promoter activity. CIITA mRNA, regulated by either IFN-gamma or tetracycline, was destabilized in the presence of TNF-alpha, suggesting that TNF-alpha utilizes a distinct mechanism to suppress MHC class II expression in HT1080 cells. Consistent with this interpretation, TNF-alpha blocked IFN-gamma-induced CIITA and MHC class II expression in mutant cells that are unresponsive to TGF-beta or IFN-beta. This is the first instance in which MHC class II expression is inhibited by destabilizing CIITA mRNA.