IDENTIFICATION OF DISTINCT REGIONS OF 5'-FLANKING DNA THAT MEDIATE CONSTITUTIVE, IFN-GAMMA, STAT1, AND TCF-BETA-REGULATED EXPRESSION OF THECLASS-II TRANSACTIVATOR GENE

Class II transactivator (CIITA) is a master regulator required for con stitutive and IFN-gamma-inducible expression of class II MHC genes. Al though the role of CIITA is greatly appreciated, the mechanisms underl ying constitutive and IFN-gamma-induced expression of CIITA are not un derstood. The study of CIITA induction is extremely important, but has been fraught with difficulty. This study describes for the first time a large (7-kb) fragment of 5' flanking sequences that mediates the B cell-specific, IFN-gamma-induced, and TGF-beta-suppressed expression o f CIITA. This pattern of expression matches the authentic expression o f the endogenous gene. Within the 7-kb fragment, sequences that lie be tween nucleotides -545 and -113 relative to the transcriptional start site are critical for constitutive promoter expression in B cells. In contrast, inducible activation of CIITA by IFN-gamma requires sequence s contained in an additional 4 kb of upstream DNA. This region mediate s an IFN-gamma response when linked to either the endogenous CIITA pro moter or a heterologous promoter. A role for STAT1 in regulation of th e CIITA promoter is shown by the rescue of IFN-gamma induction by expr ession of STAT1 in STAT1-defective U3A cells. TGF-beta significantly i nhibits IFN-gamma-mediated induction of the CIITA promoter in 2ftGH fi broblasts, which indicates that the promoter is a target for TGF-beta. This inhibition is achieved by suppression of the basal promoter. Thi s study provides a focal point for understanding the mechanism of B ce ll-specific, IFN-gamma-induced, and TGF-beta-suppressed expression of CIITA.