The locus control region of the MHC class II promoter acts as a repressor element, the activity of which is inhibited by CIITA

The closest region of the promoter of MHC II genes and particularly three conserved boxes (X, Y and S) are fundamental for the transcriptional regulation. A second set of conserved sequences is present approximately 1200–1500 bp upstream in opposite orientation. In transient transfection experiments in IFN-γ-treated macrophages and in B lymphocytes, we determined the expression of a fragment of 2035 bp of the I-Aβ gene, which contains the upstream boxes. Mutation of the distal boxes increased induction, thereby suggesting a repressive effect on transcription. In vitro, the proximal and distal ends of I-Aβ promoter were ligated in the presence of nuclear extracts from untreated macrophages but not when the extracts were obtained from IFN-γ-stimulated cells. The mutation of distal or proximal boxes resulted in a decrease in the ligation assay. The addition of recombinant CIITA to untreated nuclear extracts decreased the capacity of the promoter to be ligated. Finally, we observed increased capacity to ligate the promoter in extracts from B cells lacking CIITA, but not from B cells lacking RFXANK. These results allow us to postulate a model where the proteins in the proximal and distal conserved sequences interact. When CIITA is induced, these proteins make an enhanceosome, allowing chromatin to open and initiate transcription.