CIITA is a transcriptional coactivator that is recruited to MHC class II promoters by multiple synergistic interactions with an enhanceosome complex

By virtue of its control over major histocompatibility complex class II (MH C-II) gene expression, CIITA represents a key molecule in the regulation of adaptive immune responses. It was first identified as a factor that is def ective in MHC-II deficiency, a hereditary disease characterized by the abse nce of MHC-II expression. CIITA is a highly regulated transactivator that g overns all spatial, temporal, and quantitative aspects of MHC-II expression . It has been proposed to act as a non-DNA-binding transcriptional coactiva tor, but evidence that it actually functions at the level of MHC-II promote rs was lacking. By means of chromatin immunoprecipitation assays, we show h ere for the first time that CIITA is physically associated with MHC-II, as well as HLA-DM, Ii, MHC-I, and beta(2)m promoters in vivo. To dissect the m echanism by which CIITA is recruited to the promoter, we have developed a D NA-dependent coimmunoprecipitation assay and a pull-down assay using immobi lized promoter templates. We demonstrate that CIITA recruitment depends on multiple, synergistic protein-protein interactions with DNA-bound factors c onstituting the MHC-II enhanceosome. CIITA therefore represents a paradigm for a novel type of regulatory and gene-specific transcriptional cofactor.