The Smad3 protein is involved in TGF-beta inhibition of class II transactivator and class II MHC expression

TGF-beta is a immunoregulatory cytokine that inhibits class ii MHC expressi on in a variety of cell types. Previous studies have shown that the class I I MHC transactivator (CIITA), a master regulator that controls class II MHC expression, is targeted by TGF-beta for repression of IFN-gamma -induced c lass II MHC expression in astrocytes. The mechanism(s) underlying the TGF-b eta inhibitory effect is not understood. In this study, we demonstrate that TGF-beta inhibition of CIITA expression occurs at the transcriptional leve l, and that both constitutive and IFN-gamma -induced human CIITA type IV pr omoter activity is inhibited by TGF-beta. TGF-beta does not affect the sign aling events that mediate IFN-gamma activation of CIITA expression; i.e, TG F-beta does not inhibit IFN-gamma -induced STAT-la phosphorylation and/or D NA binding ability, nor is IFN-gamma induction of IFN regulatory factor aff ected. The inhibitory effect of TGF-beta on the type IV CIITA promoter is m ediated through a promoter region within 80 bp from the transcription start site. Elimination of TGF-beta inhibition of class II MHC and CIITA express ion 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 expres sion.