The MHC class II transactivator (CIITA) activates the expression of multiple genes involved in Ag presentation, but inhibits Th2-type cytokine production, including IL-4, during Th1 cell differentiation. Th1 cells derived from CIITA-deficient mice produce both Th1- and Th2-type cytokines, and the introduction of CIITA to Th2 cells down-regulates Th2-type cytokine gene transcription. Here we show that the IL-4 promoter is regulated by multiple protein-protein interactions among CIITA, NF-AT, and coactivator CBP/p300. The introduction of CBP/p300 and NF-AT enhances the IL-4 promoter activity, and this activation was repressed by CIITA. Furthermore, our data show that CIITA competes with NF-AT to bind CBP/p300 and that this competition dramatically influences transcriptional activation of the IL-4 promoter. We identified two domains of CIITA that interact with two distinct domains of CBP/p300 that are also recognized by NF-AT. CIITA mutants that retain the ability to interact with CBP/p300 are sufficient to inhibit NF-AT-mediated IL-4 gene expression.

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The MHC class II transactivator (CIITA) plays a central role in adaptive immune responses by controlling the expression of MHC class II genes. CIITA binds DNA-binding proteins and co-activator proteins to form an enhanceosome complex necessary for MHC class II gene expression. Here we demonstrate that CIITA interactions depend upon the phosphorylation status of CIITA. Hyper-phosphorylated CIITA interacts with co-activator p300, RFX5 and CIITA itself, which in turn results in induction of MHC class II promoter activity. Moreover, the C-terminal region of CIITA containing leucine-rich repeats (LRR) is a regulatory domain for CIITA self-association and LRR binding to CIITA is negatively regulated by phosphorylation. cAMP-dependent protein kinase (PKA) phosphorylates CIITA, and serine residues residing in a region between the proline/serine/threonine-rich domain and the GTP-binding domain are phosphorylated by PKA in vitro. The maximum transactivation potential of CIITA requires PKA phosphorylation as demonstrated by reduced transactivation activities of the mutant bearing substitutions of serine residues at the PKA site.

The major histocompatibility complex (MHC) class II transactivator (CIITA) regulates the expression of genes involved in the immune response, including MHC class II genes and the interleukin-4 gene. Interactions between CIITA and sequence-specific, DNA-binding proteins are required for CIITA to function as an activator of MHC class II genes. CIITA also interacts with the coactivators CBP (also called p300), and this interaction leads to synergistic activation of MHC class II promoters. Here, we report that CIITA forms complexes with itself and that a central region, including the GTP-binding domain is sufficient for self-association. Additionally, this central region interacts with the C-terminal leucine-rich repeat as well as the N-terminal acidic domain. LXXLL motifs residing in the GTP-binding domain are essential for self-association. Finally, distinct differences exist among various CIITA mutant proteins with regard to activation function, subcellular localization, and association with wild-type protein and dominant-negative potential.Major histocompatibility complex (MHC) class II molecules present exogenously derived antigenic peptides to CD4 ϩ T cells. The recognition of alien peptide by these T cells allows a host to immunologically respond to foreign pathogens. MHC class II molecules are constitutively expressed on B cells and dendritic cells and inducible upon other cells, such as macrophages, all of which are capable of the uptake and processing of foreign invaders. In the absence of MHC class II molecules, individuals are unable to mount a T-cell-mediated immune response and overwhelming infection ensues. A group of immunodeficient patients which lack MHC class II molecules have been identified, and this disease has been coined bare lymphocyte syndrome (BLS) (8,15). One class of these BLS patients (group A) lack MHC class II molecules on their cellular surfaces due to a defect in the MHC class II transactivator, CIITA (38).The regulation of MHC class II gene expression is primarily at the transcriptional level. The promoters of MHC class II genes contain at least four conserved sequences: the S, X, X2, and Y boxes (reviewed in reference 28). These cis-acting elements are occupied by sequence-specific transcription factors; the heterotrimeric NF-Y complex binds to the Y box (25), the multimeric RFX proteins bind to the S and X boxes (10, 21), and the cyclic AMP response element binding protein (CREB) binds to the X2 box (31). Protein-protein interactions stabilize the binding of these proteins to MHC class II promoter DNA as illustrated by interactions between the RFX complex and 40,41). and the enhancement of RFX complex binding facilitated by CREB (27, 41). However, the binding of all of these transcription factors to their respective cis-acting elements is insufficient to lead to MHC class II promoter activity. CIITA is required for both the constitutive and the gamma interferon-inducible expression of MHC class II genes (4, 38, 39). CIITA does not bind directly to DNA. The exact mechanism of CIIT...

The class II transactivator (CIITA) is a potent and critical transcriptional regulator. It activates genes necessary for antigen presentation function. It also regulates cytokine gene expression in CD4 T cells. We recently found that CIITA prevents cell death by inhibiting Fas ligand (FasL) gene expression. Thus, CIITA regulates multiple immune responses. The activation and the repression function of CIITA are mediated by its interaction with other transcription factors. To activate the target gene, CIITA interacts with DNA binding proteins and recruits the coactivator CBP/p300 to the promoter forming an enhanceosome necessary for transcription. In addition, CIITA interacts with self. Inter- and intramolecular interactions of CIITA are essential for transactivation function. Each domain of CIITA has a distinct role and all domains are required for CIITA activity. However, the regulatory mechanisms of CIITA interaction with self and other proteins are poorly understood and remain to be investigated.