Major histocompatibility complex (MHC) class II-associated invariant chain (Ii) gene and the MHC class II molecules are physically and functionally associated within class II-expressing cell types. These genes are generally co-expressed in various cell types and coordinately induced by cytokines such as interferon-gamma (IFN-gamma). Human Ii gene shares a regulatory mechanism with MHC class II genes via the X and Y cis-acting elements. Ii X and Y are required for constitutive expression in B lymphoid cell lines and for induction by IFN-gamma in a glioblastoma cell line. The Servenius element (S) contributes 5-fold transcriptional activity to class II gene HLA-DRA, and an S homologue has been implied in Ii gene regulation. Now we report that along with X and Y, the Ii S box functions in a positive manner to regulate Ii transcription in B cell lines but not in T cell lines. In addition, S,X, and Y are all necessary for induced expression of Ii in an IFN-gamma-regulated glioblastoma cell line and in primary untransformed glial cells. Transcriptional activity of Ii X and Y elements correlates with the presence of in vivo protein/DNA interactions in Ii-expressing cells. Most interestingly, in vivo interactions are induced upon IFN-gamma induction in a time-dependent fashion. Thus, the Ii promoter contains elements that coordinately regulate expression with the MHC class II genes, and these functional sites are contacted in vivo emphasizing their importance in Ii gene transcription.
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