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Biomedical subjects

B Mach

Publications and source records attributed to B Mach.

At least 73 records · Page 4Linked to original sources
PubMed1995

The two novel MHC class II transactivators RFX5 and CIITA both control expression of HLA-DM genes.

MHC-encoded HLA-DMA and -DMB molecules are atypical MHC chains that play an essential role in antigen presentation by MHC class II molecules. They resemble both MHC class I and II molecules but are not expressed at the cell surface. From the study of MHC class II regulatory mutants, it was found recently that two novel transactivators, CIITA and RFX5, are essential for the control of MHC class II gene expression. We report here that CIITA and RFX5, although operating at different levels of transcriptional control, are also both essential regulators of HLA-DMA and -DMB genes. This is true for both the constitutive and the inducible mode of DM gene expression. Indeed, both CIITA and RFX5 cDNA can correct the HLA-DMA and -DMB gene expression defect in the respective regulatory mutants. The involvement of these two transcription factors accounts for the coordinate expression of MHC class II and HLA-DM, two sets of molecules that perform quite different functions in the overall process of antigen presentation.

Cell Line↗
PubMed1994

Developmental extinction of major histocompatibility complex class II gene expression in plasmocytes is mediated by silencing of the transactivator gene CIITA.

Constitutive major histocompatibility complex (MHC) class II gene expression is tightly restricted to antigen presenting cells and is under developmental control. Cells of the B cell lineage acquire the capacity to express MHC class II genes early during ontogeny and lose this property during terminal differentiation into plasma cells. Cell fusion experiments have suggested that the extinction of MHC class II expression in plasma cells is due to a dominant repression, but the underlying mechanisms are not understood. CIITA was recently identified as an MHC class II transactivator that is essential for MHC class II expression in B lymphocytes. We show here that inactivation of MHC class II genes in plasmocytes is associated with silencing of the CIITA gene. Moreover, experimentally induced expression of CIITA in plasmocytes leads to reexpression of MHC class II molecules to the same level as that observed on B lymphocytes. We therefore conclude that the loss of MHC class II expression observed upon terminal differentiation of B lymphocytes into plasmocytes results from silencing of the transactivator gene CIITA.

B-Lymphocytes↗
PubMed1994

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

Transcription of major histocompatibility complex (MHC) class II genes is controlled primarily by the promoter, which contains several conserved cis-acting elements, including the X, X2, and Y boxes. We show here that RFX, the X box-binding protein that is deficient in certain MHC class II regulatory mutants, binds cooperatively with an X2 box-binding protein (X2bp) to form an RFX.X2bp.DNA complex in which the interaction of the two factors with their target sites is strongly stabilized. A functional role of this RFX.X2bp complex is consistent with mutational analysis of the X and X2 boxes of the DRA and DRB1 class II promoters. Together with previous results demonstrating cooperative binding between RFX and the Y box-binding protein NF-Y, our results indicate that RFX plays a central role in promoting cooperative binding interactions required for stable occupation of the MHC class II promoter. This may explain why the highly specific defect in binding of RFX observed in certain MHC class II regulatory mutants is associated in vivo with a bare promoter in which all of the cis-acting elements, including the X, X2, and Y boxes, are unoccupied.

Base Sequence↗
PubMed1994

Regulation of MHC class II expression by interferon-gamma mediated by the transactivator gene CIITA.

Major histocompatibility complex (MHC) class II genes are expressed constitutively in only a few cell types, but they can be induced in the majority of them, in particular by interferon-gamma (IFN-gamma). The MHC class II transactivator gene CIITA is defective in a form of primary MHC class II deficiency. Here it is shown that CIITA expression is controlled and induced by IFN-gamma. A functional CIITA gene is necessary for class II induction, and transfection of CIITA is sufficient to activate expression of MHC class II genes in class II-negative cells in the absence of IFN-gamma. CIITA is therefore a general regulator of both inducible and constitutive MHC class II expression.

Cell Line↗
PubMed1994

Function of major histocompatibility complex class II promoters requires cooperative binding between factors RFX and NF-Y.

Transcription of major histocompatibility complex (MHC) class II genes is controlled largely by the conserved promoter elements called the X and Y boxes. We show here that RFX, the X box-binding protein deficient in certain MHC class II-deficient immunodeficiency patients (CID), and the Y box-binding protein NF-Y bind cooperatively. Functional relevance of this protein-protein interaction is suggested by the fact that promoter activity correlates with cooperative binding of RFX and NF-Y rather than with binding of RFX or NF-Y alone. Stability of the RFX/NF-Y complex is affected by alterations in X-Y box spacing. These results are consistent with the fact that MHC class II promoter function is dependent on correct stereospecific alignment of the X and Y boxes. Cooperative binding involving RFX, NF-Y, and perhaps other MHC class II promoter-binding proteins may explain why the highly specific defect in binding of RFX observed in CID cells is associated in vivo with a bare promoter in which all of the cis-acting elements, including the X and Y boxes, are unoccupied.

Base Sequence↗
PubMed1994

An HLA-DRB alpha-helix motif shared by DR11 and DR8 alleles is implicated in the pluriallelic restriction of peptide-specific T-cell lines.

The T-cell recognition of HLA-DR-peptide complexes is generally restricted by the polymorphism of the DRB molecules but pluriallelic restriction has been described. The molecular basis of restriction and promiscuity of such peptide-specific responses is poorly understood. We isolated a panel of T-cell lines specific for the tetanus toxin peptide p2 (TT830-843) exhibiting pluriallelic restriction by DR11 and DR8 alleles. Fine restriction specificity of the T-cell lines was examined in functional assays against DR oligotyped APCs expressing different variants of DR11 and DR8 alleles. Our results show that (a) polymorphisms between serologically related alleles are relevant in terms of restriction of the peptide-specific T-cell response; in some instances, a single amino acid substitution can determine the restriction of a T-cell line; (b) different patterns of restriction are not the result of specific differences in DR-p2 binding as p2 peptide binds to all DR11 and DR8 alleles tested (DRB1* 1101, -1102, -1103, -1104, 110X, -0801, -0802, -0803, and -0806); and (c) pluriallelic restriction of the peptide-specific T-cell response correlates with the presence of a DRB1 alpha-helix motif (67-71-86) shared by some DR11 and DR8 alleles. Possible implications of pluriallelic restriction of peptide-specific T-cell response in autoimmune disorders associated with DR11 and DR8 are discussed.

Alleles↗
PubMed1994

Functional complementation of major histocompatibility complex class II regulatory mutants by the purified X-box-binding protein RFX.

Major histocompatibility complex (MHC) class II deficiency, or bare lymphocyte syndrome (BLS), is a disease of gene regulation. Patients with BLS have been classified into at least three complementation groups (A, B, and C) believed to correspond to three distinct MHC class II regulatory genes. The elucidation of the molecular basis for this disease will thus clarify the mechanisms controlling the complex regulation of MHC class II genes. Complementation groups B and C are characterized by a lack of binding of RFX, a nuclear protein that normally binds specifically to the X box cis-acting element present in the promoters of all MHC class II genes. We have now purified RFX to near homogeneity by affinity chromatography. Using an in vitro transcription system based on the HLA-DRA promoter, we show here that extracts from RFX-deficient cells from patients with BLS (BLS cells) in groups B and C, which are transcriptionally inactive in this assay, can be complemented to full transcriptional activity by the purified RFX. As expected, purified RFX also restores a completely normal pattern of X box-binding complexes in these mutant extracts. This provides the first direct functional evidence that RFX is an activator of MHC class II gene transcription and that its absence is indeed responsible for the regulatory defect in MHC class II gene expression in patients with BLS.

Base Sequence↗
PubMed1994

RFX1, a transactivator of hepatitis B virus enhancer I, belongs to a novel family of homodimeric and heterodimeric DNA-binding proteins.

RFX1 is a transactivator of human hepatitis B virus enhancer I. We show here that RFX1 belongs to a previously unidentified family of DNA-binding proteins of which we have cloned three members, RFX1, RFX2, and RFX3, from humans and mice. Members of the RFX family constitute the nuclear complexes that have been referred to previously as enhancer factor C, EP, methylation-dependent DNA-binding protein, or rpL30 alpha. RFX proteins share five strongly conserved regions which include the two domains required for DNA binding and dimerization. They have very similar DNA-binding specificities and heterodimerize both in vitro and in vivo. mRNA levels for all three genes, particularly RFX2, are elevated in testis. In other cell lines and tissues, RFX mRNA levels are variable, particularly for RFX2 and RFX3. RFX proteins share several novel features, including new DNA-binding and dimerization motifs and a peculiar dependence on methylated CpG dinucleotides at certain sites.

Alternative Splicing↗