The major histocompatibility complex (MHC) class II transactivator (CIITA) is the master regulatory factor required for appropriate expression of class II MHC genes. Understanding the expression of CIITA is key to understanding the regulation of class II MHC genes. This report describes the independent regulation of two distinct CIITA promoters by cytokines with opposing functions, gamma interferon (IFN-gamma) and transforming growth factor beta (TGF-beta). A functional analysis of deletion mutations of the upstream promoter (promoter III) identified an IFN-gamma-responsive region located approximately 5 kb from the transcriptional start site. An in vivo DNase I hypersensitivity analysis detected a hypersensitive site in this area which supports the relevance of this region. When the downstream promoter (promoter IV) was studied by in vivo genomic footprinting, IFN-gamma-induced changes at putative binding sites for STAT1, interferon regulatory factor 1 (IRF-1), and E-box proteins were seen. Gel shift and supershift analyses for IRF-1 confirmed the in vivo footprint results. The role of the IFN-gamma-inducible transcription factor STAT1 was examined functionally. Although both promoters were controlled by STAT1, promoter-specific regulation was exhibited. The IFN-gamma response of promoter III was completely dependent on STAT1 and not IRF-1, while promoter IV was partially activated by IRF-1 in the total absence of STAT1 expression. While both promoters were affected by TGF-beta, activation of promoter III by IFN-gamma was more severely diminished by TGF-beta treatment. The differential control of CIITA promoters by TGF-beta, IRF-1, and STAT1 may be important in refining regulation of class II MHC genes in different cell types and under different stimulatory conditions.
Class II transactivator (CIITA), a coactivator required for class II major histocompatibility complex (MHC) transcription, is expressed in B cells but extinguished in plasma cells. This report identifies B lymphocyte-induced maturation protein I (BLIMP-I), a transcriptional repressor that is capable of triggering plasma cell differentiation, as a developmentally regulated repressor of CIITA transcription. BLIMP-I represses the B cell-specific promoter of the human gene that encodes CIITA (MHC2TA) in a binding site-dependent manner. Decreased CIITA correlates with increased BLIMP-I during plasma cell differentiation in cultured cells. Ectopic expression of BLIMP-I represses endogenous mRNA for CIITA and the CIITA targets, class II MHC, invariant chain and H2-DM (the murine equivalent of HLA-DM) in primary splenic B cells as well as 18-81 pre-B cells. Thus, the BLIMP-I program of B cell differentiation includes loss of antigen presentation via extinction of CIITA expression.
Major histocompatibility complex (MHC) class II molecules play a central role in immune responses, and transcription of this family of genes requires the MHC class II transactivator (CIITA). CIITA has four promoters, which are transcribed in a tissue-specific manner. CIITA promoter III is constitutively active in mature B-lymphocytes. This report now describes the minimal 319-base pair promoter region necessary for maximal transcriptional activity in B-lymphocytes. Ultraviolet light and dimethylsulfate in vivo genomic footprinting analyses reveal five occupied DNA sequence elements present in intact B-lymphocytes. Functional analysis of these elements using promoter deletions and site-specific mutations demonstrates that at least two of the sites occupied in vivo are critical for transcriptional activity. In vitro protein/DNA analysis suggests that one of the sites is a TEF-2-like element and the other is occupied by a novel transcription activator. In addition, nuclear factor-1 associates with the promoter both in vivo and in vitro. In myeloma cell lines, loss of CIITA transcription correlates with a completely unoccupied CIITA promoter III. These findings suggest that CIITA transcription in B-lymphocytes is activated through at least two strong promoter elements, while loss of expression in myeloma cells is mediated through changes in promoter assembly.Major histocompatibility complex (MHC) 1 class II molecules play a fundamental role in presenting exogenous antigenic peptides to CD4 ϩ helper T lymphocytes (1). These activated CD4 ϩ T cells release stimulatory cytokines that augment the response of CD8 ϩ cytotoxic T lymphocytes (2). Constitutive expression of MHC class II molecules is restricted to professional antigen-presenting cells, such as B lymphocytes and dendritic cells, and can be induced by interferon-␥ (IFN-␥) in macrophages, endothelial cells, and fibroblasts (3, 4). The MHC class II family of genes are coordinately regulated at the level of transcription through a conserved trimeric promoter motif (3, 5). A major component of this transcription complex was cloned by genetic complementation of an in vitro mutagenized MHC class II negative B cell line, RJ2.2.5. This component was named the MHC class II transactivator (CIITA) and restored MHC class II cell surface expression in one subgroup of bare lymphocyte syndrome patient cells (6). CIITA has since been shown to be a key regulatory molecule in the expression of all the known genes involved in the MHC class II antigen-presenting pathway (6 -9). CIITA is required for constitutive expression of MHC class II on B-lymphocytes (10) and with a few exceptions, such as in respiratory epithelial cells and dendritic cells (11,12), is required for cytokine-induced expression in other cell types. Mice in which functional CIITA protein was ablated clearly demonstrated an absolute requirement for CI-ITA in B cell expression of MHC class II (13). It has been shown that the CIITA gene has four distinct promoters each transcribing a unique first exon, which are lo...
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