The Smad3 Protein Is Involved in TGF-β Inhibition of Class II Transactivator and Class II MHC Expression

TGF-beta is a immunoregulatory cytokine that inhibits class II MHC expression in a variety of cell types. Previous studies have shown that the class II MHC transactivator (CIITA), a master regulator that controls class II MHC expression, is targeted by TGF-beta for repression of IFN-gamma-induced class II MHC expression in astrocytes. The mechanism(s) underlying the TGF-beta inhibitory effect is not understood. In this study, we demonstrate that TGF-beta inhibition of CIITA expression occurs at the transcriptional level, and that both constitutive and IFN-gamma-induced human CIITA type IV promoter activity is inhibited by TGF-beta. TGF-beta does not affect the signaling events that mediate IFN-gamma activation of CIITA expression; i.e, TGF-beta does not inhibit IFN-gamma-induced STAT-1alpha phosphorylation and/or DNA binding ability, nor is IFN-gamma induction of IFN regulatory factor affected. The inhibitory effect of TGF-beta on the type IV CIITA promoter is mediated through a promoter region within 80 bp from the transcription start site. Elimination of TGF-beta inhibition of class II MHC and CIITA expression in Smad3-deficient astrocytes, as well as restoration of the inhibitory effect by overexpression of the Smad3 protein, demonstrates that Smad3 is essential in mediating TGF-beta inhibition of CIITA and class II MHC expression.