Cooperative binding between factors RFX and X2bp to the X and X2 boxes of MHC class II promoters

Regulation of transcription of MHC class II genes

Genetic and biochemical analyses have identified multiple DNA-binding and non-DNA-binding proteins that functionally regulate MHC class II genes. These include RFX, X2BP, NF-Y, CIITA, OCT-2 and Bob1. One of the essential non-DNA-binding proteins, CIITA, appears to function as a limiting molecular switch that is responsible for the control of class II expression and the regulation of expression by interferon-gamma.

Regulation of MHC class II genes: lessons from a disease

Precise regulation of major histocompatibility complex class II (MHC-II) gene expression plays a crucial role in the control of the immune response. A major breakthrough in the elucidation of the molecular mechanisms involved in MHC-II regulation has recently come from the study of patients that suffer from a primary immunodeficiency resulting from regulatory defects in MHC-II expression. A genetic complementation cloning approach has led to the isolation of CIITA and RFX5, two essential MHC-II gene transactivators. CIITA and RFX5 are mutated in these patients, and the wild-type genes are capable of correcting their defect in MHC-II expression. The identification of these regulatory factors has furthered our understanding of the molecular mechanisms that regulate MHC-II genes. CIITA was found to be a non-DNA binding transactivator that functions as a molecular switch controlling both constitutive and inducible MHC-II expression. The finding that RFX5 is a subunit of the nuclear RFX-complex has confirmed that a deficiency in the binding of this complex is indeed the molecular basis for MHC-II deficiency in the majority of patients. Furthermore, the study of RFX has demonstrated that MHC-II promoter activity is dependent on the binding of higher-order complexes that are formed by highly specific cooperative binding interactions between certain MHC-II promoter-binding proteins. Two of these proteins belong to families of which the other members, although capable of binding to the same DNA motifs, are probably not directly involved in the control of MHC-II expression. Finally, the facts that CIITA and RFX5 are both essential and highly specific for MHC-II genes make possible novel strategies designed to achieve immunomodulation via transcriptional intervention.

Molecular defects in the bare lymphocyte syndrome and regulation of MHC class II genes

The complex pattern of expression of major histocompatibility complex (MHC) class II molecules plays an essential role in the control of the immune response. Our understanding of the molecular mechanisms controlling this expression has benefited greatly from the identification of the regulatory factors defective in two forms of a hereditary disease of MHC class II regulation: bare lymphocyte syndrome. This has also provided new tools for the experimental modulation of MHC class II expression.

Repression of I-A beta gene expression by the transcription factor PU.1

The PU.1 protein is an ets-related transcription factor that is expressed in macrophages and B lymphocytes. We present evidence that PU.1 binds to the promoter of the I-A beta gene, i.e. a PU box located next to the Y box. Transfection of PU.1 in B lymphocytes or in interferon-gamma-treated macrophages represses I-A beta gene expression. The inhibitory effect of PU.1 was obtained with the DNA binding domain of the protein, but not with the activation domain. Using the gel shift retardation assay we found that in vitro transcribed/translated NF-YA and NF-YB bind to the Y box of the I-A beta promoter. When PU.1 was added to the assay, a supershifted DNA band was found, indicating that PU.1 and NFY proteins bind to the same DNA molecule. We conclude that I-A beta gene expression is repressed by PU.1 binding to the PU box domain.

Complementation cloning of mammalian transcriptional regulators: the example of MHC class II gene regulators

Cloning by complementation of mutant cell lines is a powerful way in which to identify and isolate important regulatory genes on the basis of functional assays. The recent cloning of two essential regulators of major histocompatibility complex (MHC) class II gene expression has not only advanced our understanding of the complex mechanisms controlling these genes, but also helps to illustrate the feasibility of this approach for the study of mammalian gene regulation.

Transcriptional regulation of MHC class II genes

MHC class II molecules play a fundamental role in the homeostasis of the immune response, functioning as receptors for antigenic peptides to be presented to regulatory T cells. Both quantitative and qualitative alterations in the expression of these molecules on the cell surface dramatically affect the onset of the immune response, and may be the basis of a wide variety of disease states, such as autoimmunity, immunodeficiencies, and cancer. Most regulation of MHC class II molecule expression is under the control of transcription mechanisms which are both cell type and development specific. In the last few years classical genetics together with molecular biology have greatly contributed to the widening of our knowledge on the regulatory mechanisms operating in the control of class II gene expression. This review deals with the latest developments in this fundamental area of immunology.

A novel DNA-binding regulatory factor is mutated in primary MHC class II deficiency (bare lymphocyte syndrome)

Regulation of MHC class II gene expression is an essential aspect of the control of the immune response. Primary MHC class II deficiency is a genetically heterogeneous disease of gene regulation that offers the unique opportunity of a genetic approach for the identification of the functionally relevant regulatory genes and factors. Most patients exhibit a characteristic defect in the binding of a nuclear complex, RFX, to the X box motif of MHC class II promoters. Genetic complementation of a B-lymphocyte cell line from such a patient with a cDNA expression library has allowed us to isolate RFX5, the regulatory gene responsible for the MHC class II deficiency. This gene encodes a novel DNA-binding protein that is indeed a subunit of the RFX complex. Mutations in the RFX5 gene have been characterized in two patients. Transfection of the patient's cells with the RFX5 cDNA repairs the binding defect and fully restores expression of all the endogenous MHC class II genes in vivo.

Activation of class II MHC genes requires both the X box region and the class II transactivator (CIITA)

CIITA, a gene that can complement a transcriptional mutation of the major histocompatibility complex (MHC) class II genes, was tested for its ability to function as a coactivator, CIITA cDNA clones isolated showed alternative RNA splicing, but only one splice site combination was able to restore class II MHC gene expression. DNA-mediated transfection experiments showed that CIITA directs its activity through the X box element; the presence of CIITA leads to the formation of a higher order complex at the X box region; and CIITA contains a potent activation domain. These findings support the hypothesis that CIITA directly interacts with the MHC class II-specific transcription factors and is required for expression.