Mutant human cells defective in induction of major histocompatibility complex class II genes by interferon gamma

TRF1 averts chromatin remodelling, recombination and replication dependent-break induced replication at mouse telomeres

Telomeres are a significant challenge to DNA replication and are prone to replication stress and telomere fragility. The shelterin component TRF1 facilitates telomere replication but the molecular mechanism remains uncertain. By interrogating the proteomic composition of telomeres, we show that mouse telomeres lacking TRF1 undergo protein composition reorganisation associated with the recruitment of DNA damage response and chromatin remodellers. Surprisingly, mTRF1 suppresses the accumulation of promyelocytic leukemia (PML) protein, BRCA1 and the SMC5/6 complex at telomeres, which is associated with increased Homologous Recombination (HR) and TERRA transcription. We uncovered a previously unappreciated role for mTRF1 in the suppression of telomere recombination, dependent on SMC5 and also POLD3 dependent Break Induced Replication at telomeres. We propose that TRF1 facilitates S-phase telomeric DNA synthesis to prevent illegitimate mitotic DNA recombination and chromatin rearrangement.

Tyk2 and Stat3 regulate brown adipose tissue differentiation and obesity

Mice lacking the Jak tyrosine kinase member Tyk2 become progressively obese due to aberrant development of Myf5+ brown adipose tissue (BAT). Tyk2 RNA levels in BAT and skeletal muscle, which shares a common progenitor with BAT, are dramatically decreased in mice placed on a high-fat diet and in obese humans. Expression of Tyk2 or the constitutively active form of the transcription factor Stat3 (CAStat3) restores differentiation in Tyk2(-/-) brown preadipocytes. Furthermore, Tyk2(-/-) mice expressing CAStat3 transgene in BAT also show improved BAT development, normal levels of insulin, and significantly lower body weights. Stat3 binds to PRDM16, a master regulator of BAT differentiation, and enhances the stability of PRDM16 protein. These results define Tyk2 and Stat3 as critical determinants of brown fat lineage and suggest that altered levels of Tyk2 are associated with obesity in both rodents and humans.

Hepatitis C virus inhibits cell surface expression of HLA-DR, prevents dendritic cell maturation, and induces interleukin-10 production

Hepatitis C virus (HCV) chronic infection is characterized by low-level or undetectable cellular immune responses against HCV antigens. HCV proteins have been shown to affect various intracellular events and modulate immune responses, although the precise mechanisms used to mediate these effects are not fully understood. In this study, we have examined the effect of HCV proteins on the modulation of major histocompatibility complex (MHC) class II expression and other functions important for antigen presentation in humans. Expression of an HCV(1-2962) genomic clone (HCV-FL) in human fibrosarcoma cells (HT1080) inhibited gamma interferon (IFN-gamma)-induced upregulation of human leukocyte antigen-DR (HLA-DR) cell surface expression. Furthermore, inhibition of promoter activities of MHC class II transactivator (CIITA), IFN-gamma-activated site (GAS), and HLA-DR was observed in IFN-gamma-inducible HT1080 cells expressing HCV-FL by in vitro reporter assays. Exposure of human monocyte-derived dendritic cells (DCs) to cell culture-grown HCV (HCVcc) genotype 1a (clone H77) or 2a (clone JFH1) significantly inhibited DC maturation and was associated with the production of IL-10. Furthermore, DCs exposed to HCVcc were impaired in their functional ability to stimulate antigen-specific CD4-positive (CD4(+)) and CD8(+) T-cell responses. Taken together, our results indicated that HCV can have direct and/or indirect inhibitory effects on antigen-presenting cells, resulting in reduction of antigen-specific T-cell activation. These effects may account for or contribute to the low overall level of immunogenicity of HCV observed in chronically infected patients.

MHC class II gene expression is not induced in HPIV3-infected respiratory epithelial cells

The major histocompatibility complex (MHC) class II transactivator (CIITA) typically is required for both constitutive and inducible expression of MHC class II genes. However, transcription of class II MHC genes has been observed in specific cell types (e.g., thymic epithelial cells) in CIITA-deficient mice as well as in specific situations (e.g., following viral infections or in natural killer [NK]/target cell interaction). These observations have been interpreted by some to indicate that a CIITA-independent pathway of class II gene expression might be germane to processes such as the acquisition of tolerance during thymic selection or in the evasion of immune surveillance by a subset of viruses. One of the most striking examples of CIITA-independent, inducible class II gene expression has involved the de novo expression of class II MHC molecules on respiratory epithelial cells following infection by human parainfluenza virus type 3 (HPIV3). We report here that despite careful analysis using multiple techniques, we have been unable to detect HPIV3-dependent, CIITA-independent (or CIITA-dependent) induction of class II MHC genes. Thus, whereas there may still be an intriguing role for CIITA-independent gene expression in facets of the immune response, this is unlikely to manifest in the analysis of HPIV3 infection of respiratory epithelial cells.

Small ISGs coming forward

Interferons (IFNs) were first characterized as antiviral proteins. Since then, IFNs have proved to be involved in malignant, angiogenic, inflammatory, immune, and fibrous diseases and, thus, possess a broad spectrum of pathophysiologic properties. IFNs activate a cascade of intracellular signaling pathways leading to upregulation of more than 1000 IFN-stimulated genes (ISGs) within the cell. The function of some of the IFN-induced proteins is well described, whereas that of many others remain poorly characterized. This review focuses on three families of small intracellular and intrinsically nonsecreted proteins (10-20 kDa) separated into groups according to their amino acid sequence similarity: the ISG12 group (6-16, ISG12, and ISG12-S), the 1-8 group (9-27/Leu13, 1-8U, and 1-8D), and the ISG15 group (ISG15/UCRP). These IFN-induced genes are abundantly and widely expressed and mainly induced by type I IFN. ISG15 is very well described and is a member of the ubiquitin-like group of proteins. 9-27/Leu-13 associates with CD81/TAPA-1 and plays a role in B cell development. The functions of 1-8U, 1-8D, 6-16, ISG12, and ISG12-S proteins are unknown at present.

A 36-amino-acid region of CIITA is an effective inhibitor of CBP: novel mechanism of gamma interferon-mediated suppression of collagen alpha(2)(I) and other promoters

The class II transactivator (CIITA) is induced by gamma interferon (IFN-gamma) and activates major histocompatibility complex class II; however, this report shows it suppresses other genes. An N-terminal 36 amino acids of CIITA mediates suppression of the collagen alpha(2)(I) promoter via binding to CREB-binding protein (CBP). Reconstitution of cells with CBP reverts this suppression. IFN-gamma is known to inhibit collagen gene expression; to test if CIITA mediates this gene suppression, a mutant cell line defective in CIITA induction but not in the activation of STAT1/JAK/IRF-1 is studied. IFN-gamma suppression of the collagen promoter and the endogenous gene is observed in the wild-type control but not in the mutant line. Suppression is restored when CIITA is introduced. Other targets of CIITA-mediated promoter suppression include interleukin 4, thymidine kinase, and cyclin D1.

Identification of novel Mycoplasma hyorhinis gene fragments by differential display analysis of co-cultures

Mycoplasmas are a diverse group of wall-less prokaryotes that have evolved an unusually small genome by adopting a parasitic mode of life. Recently, intense efforts have been made to sequence mycoplasma genomes and to define a minimal genome using mycoplasma as a model. Due to their parasitic nature, mycoplasma species are often difficult to cultivate, making it challenging to identify and sequence mycoplasma genes. In this report, we describe a method for identifying mycoplasma gene fragments from co-cultures using differential display analysis. Using this technique, we have identified fragments of seven putative genes from Mycoplasma hyorhinis. Sequence similarities suggest that four of these genes are members of the proposed minimal mycoplasma genome. The application of differential display analysis to co-cultures should be useful in the identification of genes from a variety of pathogenic organisms that are difficult to cultivate without a host.

Human parainfluenza virus type 3 inhibits gamma interferon-induced major histocompatibility complex class II expression directly and by inducing alpha/beta interferon

Human parainfluenza virus type 3 (HPIV3) is one of the major causes of bronchiolitis, pneumonia, and croup in newborns and infants. Cellular immunity involving major histocompatibility complex (MHC) class I and class II molecules plays an important role in controlling virus infection. Several viruses have been shown to down-regulate gamma interferon (IFN-gamma)-mediated MHC class II expression. In this communication, we show that HPIV3 strongly inhibits the IFN-gamma-induced MHC class II expression in HT1080 human fibrosarcoma cells. The culture supernatant of HPIV3-infected cells also inhibited IFN-gamma-induced MHC class II expression, a phenomenon that was found to be due, in large part, to alpha/beta interferon (IFN-alpha/beta). Expression of MHC class I and intercellular adhesion molecule 1 occurred efficiently in cells simultaneously infected with HPIV3 and treated with IFN-gamma, indicating that the inhibitory effect of HPIV3 was specific to MHC class II. STAT1 activation was not affected by HPIV3 at early postinfection times but was partially inhibited at later times. These data suggested that the potent inhibition of MHC class II expression was, in major part, due to a defect downstream of STAT1 activation in the IFN-gamma-induced MHC class II expression pathway. Class II transactivator (CIITA) is the unique mediator of IFN-gamma-induced transcription from the MHC class II promoter. By RNase protection analysis, CIITA expression was found to be strongly inhibited in HPIV3-infected cells. The culture supernatant containing IFN-alpha/beta, on the other hand, inhibited MHC class II expression without affecting STAT1 and CIITA expression. These data indicate that HPIV3 inhibits IFN-gamma-induced MHC class II expression primarily by the viral gene products targeting CIITA and additionally by inducing IFN-alpha/beta to target one or more steps further downstream.

Identification of class II transcriptional activator-induced genes by representational difference analysis: discoordinate regulation of the DN alpha/DO beta heterodimer

Class II transcriptional activator (CIITA) is a master regulator of MHC class II genes, including DR, DP, and DQ, and MHC class II-associated genes DM and invariant chain. To determine the repertoire of genes that is regulated by CIITA and to identify uncharacterized CIITA-inducible genes, we used representational difference analysis. Representational difference analysis screens for differentially expressed transcripts. All CIITA-induced genes were MHC class II related. We have identified the alpha subunit, DN alpha, of the class II processing factor DO as an additional CIITA-inducible gene. Northern analysis confirmed that DN alpha is induced by IFN-gamma in 2fTGH fibrosarcoma cells, and CIITA is necessary for high-level expression in B cells. The beta subunit, DO beta, is not inducible in fibrosarcoma cells by IFN-gamma or exogenous CIITA expression. Moreover, in contrast to other class II genes, DO beta expression remains high in the absence of CIITA in B cells. The promoters for DN alpha and DO beta contain the highly conserved WXY motifs, and, like other class II genes, expression of both DN alpha and DO beta requires RFX. These findings demonstrate that both DN alpha and DO beta are regulated by RFX. However, DN alpha is defined for the first time as a CIITA-inducible gene, and DO beta as a MHC class II gene whose expression is independent of CIITA.

Ting J. Analysis of the Defect in IFN-γ Induction of MHC Class II Genes in G1B Cells: Identification of a Novel and Functionally Critical Leucine-Rich Motif (62-LYLYLQL-68) in the Regulatory Factor X 5 Transcription Factor

MHC class II deficiency found in bare lymphocyte syndrome patients results from the absence or dysfunction of MHC class II transcriptional regulators, such as regulatory factor X (RFX) and class II transactivator (CIITA). Understanding the roles of these factors has been greatly facilitated by the study of genetic defects in cell lines of bare lymphocyte syndrome patients, as well as in cell lines that have been generated by chemical mutagenesis in vitro. The latter group includes MHC class II-deficient lines that are no longer responsive to induction by IFN-γ. Here, we show that the defect in G1B, one such cell line, is attributed to the lack of functional RFX5, the largest subunit of RFX. The RFX5 gene isolated from G1B cells contains two separate single-base pair mutations. One alteration does not exhibit a phenotype, whereas a leucine-to-histidine mutation eliminates DNA-binding and transactivating functions. This mutation lies outside of previously defined functional domains of RFX5 but within an unusual, leucine-rich region (62-LYLYLQL-68). To further investigate the significance of the leucine-rich region, we targeted all neighboring leucine residues for mutagenesis. These mutants were also unable to transactivate a MHC class II reporter gene, confirming that these leucine residues play an essential role in RFX activity and characterize a novel leucine-rich motif.

Somatic cell mutants resistant to retrovirus replication: intracellular blocks during the early stages of infection

To identify cellular functions involved in the early phase of the retroviral life cycle, somatic cell mutants were isolated after selection for resistance to infection. Rat2 fibroblasts were treated with chemical mutagens, and individual virus-resistant clones were recovered after selection for resistance to infection. Two clones were characterized in detail. Both mutant lines were resistant to infection by both ecotropic and amphotropic murine viruses, as well as by human immunodeficiency virus type 1 pseudotypes. One clone showed a strong block to reverse transcription of the retroviral RNA, including formation of the earliest DNA products. The second clone showed normal levels of viral DNA synthesis but did not allow formation of the circular DNAs normally found in the nucleus. Cell fractionation showed that the viral preintegration complex was present in a form that could not be extracted under conditions that readily extracted the complex from wild-type cells. The results suggest that the DNA was trapped in a nonproductive state and excluded from the nucleus of the infected cell. The properties of these two mutant lines suggest that host gene products play important roles both before and after reverse transcription.

Human parainfluenza virus type 3 up-regulates major histocompatibility complex class I and II expression on respiratory epithelial cells: involvement of a STAT1- and CIITA-independent pathway

Human parainfluenza virus type 3 (HPIV3) infection causes severe damage to the lung epithelium, leading to bronchiolitis, pneumonia, and croup in newborns and infants. Cellular immunity that plays a vital role in normal antiviral action appears to be involved, possibly because of inappropriate activation, in the infection-related damage to the lung epithelium. In this study, we investigated the expression of major histocompatibility complex (MHC) class I and II molecules on human lung epithelial (A549) and epithelium-like (HT1080) cells following HPIV3 infection. MHC class I was induced by HPIV3 in these cells at levels similar to those observed with natural inducers such as beta and gamma interferon (IFN-beta and -gamma). MHC class II was also efficiently induced by HPIV3 in these cells. UV-irradiated culture supernatants from infected cells were able to induce MHC class I but not MHC class II, suggesting involvement of released factors for the induction of MHC class I. Quantitation of IFN types I and II in the culture supernatant showed the presence of IFN-beta as the major cytokine, while IFN-gamma was undetectable. Anti-IFN-beta, however, blocked the HPIV3-mediated induction of MHC class I only partially, indicating that viral antigens, besides IFN-beta, are directly involved in the induction process. The induction of MHC class I and class II directed by the viral antigens was confirmed by using cells lacking STAT1, an essential intermediate of the IFN signaling pathways. HPIV3 induced both MHC class I and class II molecules in STAT1-null cells. Furthermore, MHC class II was also induced by HPIV3 in cells defective in class II transactivator, an important intermediate of the IFN-gamma-mediated MHC class II induction pathway. Together, these data indicate that the HPIV3 gene product(s) is directly involved in the induction of MHC class I and II molecules. The induction of MHC class I and II expression by HPIV3 suggests that it plays a role in the infection-related immunity and pathogenesis.

Two distinct gamma interferon-inducible promoters of the major histocompatibility complex class II transactivator gene are differentially regulated by STAT1, interferon regulatory factor 1, and transforming growth factor beta

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.

Cutting Edge: Class II Transactivator-Independent Endothelial Cell MHC Class II Gene Activation Induced by Lymphocyte Adhesion

NK cells induce MHC class II molecules on the surface of allogeneic endothelial cells in an adhesion-dependent, IFN-γ-independent manner. Here, we demonstrate that NK cells induce HLA-DR on the surface of a mutant cell line that is defective in IFN-γ-induced MHC class II expression. RNA analysis in these cells and in a cell line that is defective in class II transactivator (CIITA) demonstrates that NK cell-induced HLA-DRα mRNA expression is also CIITA-independent. The Janus kinase-1-deficient cell line U4A expresses HLA-DRα mRNA in response to NK cell activation, and HLA-DRα promoter constructs transfected into these cells are induced by NK cells but not IFN-γ. These data indicate that the IFN-γ-independent component of the target cell HLA-DR expression induced by lymphocyte adhesion uses a signaling pathway that is distinct from the IFN-γ-dependent mechanism and also suggest that CIITA is not required.

CIITA stimulation of transcription factor binding to major histocompatibility complex class II and associated promoters in vivo

CIITA is a master transactivator of the major histocompatibility complex class II genes, which are involved in antigen presentation. Defects in CIITA result in fatal immunodeficiencies. CIITA activation is also the control point for the induction of major histocompatibility complex class II and associated genes by interferon-gamma, but CIITA does not bind directly to DNA. Expression of CIITA in G3A cells, which lack endogenous CIITA, followed by in vivo genomic footprinting, now reveals that CIITA is required for the assembly of transcription factor complexes on the promoters of this gene family, including DRA, Ii, and DMB. CIITA-dependent promoter assembly occurs in interferon-gamma-inducible cell types, but not in B lymphocytes. Dissection of the CIITA protein indicates that transactivation and promoter loading are inseparable and reveal a requirement for a GTP binding motif. These findings suggest that CIITA may be a new class of transactivator.

Expression and prognostic significance of immunoregulatory molecules in esophageal cancer

Major histocompatibility complex molecules (HLA), the co-stimulatory molecule B7 and the intercellular adhesion molecule 1 (ICAM-1) are key molecules involved in T cell-mediated immune surveillance. We aimed at assessing the expression pattern of these immunoregulatory molecules on primary esophageal carcinomas and evaluating their prognostic significance. Representative samples of primary tumors were obtained from 53 patients who had undergone radical en bloc esophagectomy without residual tumor. Cryostat sections of these tumors were stained with monoclonal antibodies (MAbs) directed against either HLA class I, HLA class II, B7 or ICAM-1. The median follow-up was 19 months (range, 6-43). We found that HLA class I expression was deficient on 27 tumors, while a significant neo-expression of HLA class II, B7 and ICAM-1 (> or =25% positive tumor cells) was observed on 17, 29 and 25, tumors, respectively. Kaplan-Meier analyses revealed a significant beneficial influence on relapse-free survival for patients with tumors expressing HLA class I, HLA class II and B7. Cox's regression analyses demonstrated that co-expression of HLA class I and ICAM-1 was a significant and independent predictor of a reduced risk of developing tumor recurrence, whereas expression of ICAM-1 on HLA class I negative tumors was correlated with an increased risk of tumor relapse.

CCAAT-binding factor NF-Y and RFX are required for in vivo assembly of a nucleoprotein complex that spans 250 base pairs: the invariant chain promoter as a model

The events that lead to promoter accessibility within chromatin are not completely understood. The invariant chain (Ii) promoter was used as a model to determine the contribution of different DNA-binding factors in establishing occupancy of a complex promoter. Gamma interferon induction of the Ii promoter requires the cooperation of multiple cis elements including distal S, X, and Y/CCAAT elements along with proximal GC and Y/CCAAT elements. The heteromeric transcription factor NF-Y binds to both Y/CCAAT elements. Genomic footprinting was used to analyze in vivo protein-DNA contacts for integrated Ii promoters bearing mutations in each element. The results reveal a hierarchy of transcription factor loading with NF-Y binding to the distal Y/CCAAT element being required for establishing protein-DNA interactions over the entire 250 bp analyzed. Mutation of the X box disrupts binding primarily at the adjacent Y/CCAAT element along with a lesser effect on GC box binding. Importantly, this finding is verified with a cell line which lacks a functional X-box-binding factor, RFX, providing physiological validity for the strategy described here. Mutation of both the S element and the GC box results in either no or little effect on transcription factor binding. However, mutation of the proximal Y/CCAAT element disrupts binding to the adjacent GC box and partially reduces binding in the distal S/X/Y domain. The crucial role for NF-Y in establishing promoter occupancy may be related to its histone fold motif, the essential component for assembling nucleosome-like structures.

Induction of MHC class I expression by the MHC class II transactivator CIITA

Major histocompatibility complex (MHC) class I-deficient cell lines were used to demonstrate that the MHC class II transactivator (CIITA) can induce surface expression of MHC class I molecules. CIITA induces the promoter of MHC class I heavy chain genes. The site alpha DNA element is the target for CIITA-induced transactivation of class I. In addition, interferon-gamma (IFNgamma)-induced MHC class I expression also requires an intact site alpha. The G3A cell line, which is defective in CIITA induction, does not induce MHC class I antigen and promoter in response to IFNgamma. Trans-dominant-negative forms of CIITA reduce class I MHC promoter function and surface antigen expression. Collectively, these data argue that CIITA has a role in class I MHC gene induction.

Importance of acidic, proline/serine/threonine-rich, and GTP-binding regions in the major histocompatibility complex class II transactivator: generation of transdominant-negative mutants

The class II transactivator (CIITA) is a master transcription regulator of gene products involved in the exogenous antigen presentation pathway, including major histocompatibility complex (MHC) class II, invariant chain, and DM. An extensive analysis of the putative functional domains of CIITA is undertaken here to explore the action of CIITA. Antibodies to CIITA protein were produced to verify that these mutant proteins are expressed. Both acidic and proline/serine/threonine-rich domains are essential for class II MHC promoter activation. In addition, three guanine nucleotide-binding motifs are essential for CIITA activity. Of these mutants, two exhibited strong transdominant-negative functions. These two mutants provide a plausible approach to manipulate MHC class II expression and immune responses.

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.

Interferon (IFN) beta acts downstream of IFN-gamma-induced class II transactivator messenger RNA accumulation to block major histocompatibility complex class II gene expression and requires the 48-kD DNA-binding protein, ISGF3-gamma

Interferon (IFN) gamma, a cardinal proinflammatory cytokine, induces expression of the gene products of the class II locus of the major histocompatibility complex (MHC), whereas IFN-alpha or -beta suppresses MHC class II expression. The mechanism of IFN-beta-mediated MHC class II inhibition has been unclear. Recently, a novel factor termed class II transactivator (CIITA) has been identified as essential for IFN-gamma-induced MHC class II transcription. We studied the status of IFN-gamma-induced CIITA messenger RNA (mRNA) accumulation and CIITA-driven transactivation in IFN-beta-treated cells and used cell lines that had defined defects in the type I IFN response pathway to address the roles of IFN signaling components in the inhibition of MHC class II induction. IFN-beta treatment did not suppress IFN-gamma-induced accumulation of CIITA mRNA. After cells were stably transfected with CIITA, endogenous MHC class II genes were constitutively expressed, and MHC class II promoters, delivered by transfection, were actively transcribed in CIITA-expressing cells. Expression of these promoters was significantly impaired by pretreatment with IFN-beta. These results suggest that IFN-beta acts downstream of CIITA mRNA accumulation, and acts in part by reducing the functional competence of CIITA for transactivating MHC class II promoters. IFN stimulated gene factor 3 (ISGF3) gamma was essential for IFN-beta to mediate inhibition of MHC class II induction, regardless of whether MHC class II transcription was stimulated by IFN-gamma or directly by CIITA expression. Results of these experiments suggest that inhibition of MHC class II in IFN-beta-treated cells requires expression of gene(s) directed by the ISGF3-IFN-stimulated response element pathway, and that these gene product(s) may act by blocking CIITA-driven transcription of MHC class II promoters.

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.

Deficient antigen-presenting cell function in multiple genetic complementation groups of type II bare lymphocyte syndrome

The absence of HLA class II gene expression in type II bare lymphocyte syndrome (BLS) results from defective transcriptional activation of class II histocompatibility genes. Genetic studies have revealed that distinct defects in multiple trans-acting factors result in the immunodeficient BLS phenotype. We studied antigen-presenting cell (APC) function in DR-transfected BLS cells derived from multiple complementation groups. Each BLS cell line displayed the same defective APC phenotype: an inability to mediate class II-restricted presentation of exogenous protein antigens, and structurally altered class II alpha beta dimers. Expression of the HLA class II-like genes DMA and DMB, previously implicated in antigen presentation, was reduced or absent in the BLS cells. Fusion of BLS cells with cell line 721.174, which has a genomic deletion of HLA class II genes, coordinately restores class II structural gene and DM gene expression and a wild-type APC phenotype. Thus each of the molecular defects that silences class II structural gene transcription also results in a defective APC phenotype, providing strong evidence for coregulation of these two functionally linked pathways.

Establishment of new interferon-gamma-resistant mutant cells with dominant phenotypes

We established interferon-gamma-resistant (IGR) cells from a human colon adenocarcinoma cell line, LoVo. Their resistance was extremely high, and the ED50 values of IFN-gamma were > 10(5) IU/ml. Interestingly, although IGR-5 cells were still sensitive to the antiproliferative effect of IFN-alpha, the cells lost responsiveness to the antiviral effects of both IFN-alpha and gamma. Another clone, IGR-53, was unresponsive to both the antiproliferative and antiviral effects of either IFN-alpha or gamma. Furthermore, the IFN-gamma-resistant phenotypes of IGR cells were apparently dominant to the parental LoVo cells based on complementation tests. Although IGR-53 cells lack IFN-gamma receptors, IGR-5 cells seemed to have functional IFN-gamma receptors and processing mechanisms of IFN-gamma bound to the receptors. Northern analysis showed that IGR-5 cells responded to IFN-gamma and alpha, but the enhancement of IRF-1 expression by IFN-gamma was markedly suppressed.

Jak-STAT pathways and transcriptional activation in response to IFNs and other extracellular signaling proteins

Through the study of transcriptional activation in response to interferon alpha (IFN-alpha) and interferon gamma (IFN-gamma), a previously unrecognized direct signal transduction pathway to the nucleus has been uncovered: IFN-receptor interaction at the cell surface leads to the activation of kinases of the Jak family that then phosphorylate substrate proteins called STATs (signal transducers and activators of transcription). The phosphorylated STAT proteins move to the nucleus, bind specific DNA elements, and direct transcription. Recognition of the molecules involved in the IFN-alpha and IFN-gamma pathway has led to discoveries that a number of STAT family members exist and that other polypeptide ligands also use the Jak-STAT molecules in signal transduction.

YB-1 DNA-binding protein represses interferon gamma activation of class II major histocompatibility complex genes

Interferon gamma (IFN-gamma) is the most potent inducer of class II major histocompatibility complex (MHC) genes. This induction is uniquely mediated by three DNA elements in the promoter region of class II MHC genes. One of these DNA elements, Y, contains an inverted CCAAT box. Previously, we have screened a lambda gt11 library for Y-binding proteins and identified the YB-1 gene. Here we provide evidence that YB-1 can repress the IFN-gamma induction of class II MHC promoter as well as the Invariant chain (Ii) gene which also contains a Y element in its promoter. This was demonstrated by cotransfecting a YB-1 expression vector with promoter-reporter gene constructs. As an alternate approach, an efficient transient transfection system was developed which resulted in a > 70% transfection efficiency. Transfection of YB-1 by this procedure resulted in the near abrogation of IFN-gamma induced HLA-DR antigen and mRNA expression. These findings show the functional suppression of class II MHC gene induction by the YB-1 protein.