Regulation of LMP2 and TAP1 genes by IRF-1 explains the paucity of CD8+ T cells in IRF-1-/- mice

HIV Tat-mediated transcriptional regulation of proteasome protein cleavage specificity

The major antigen-adapted immune response protecting a vertebrate against virus infection is that mediated by CTLs (cytotoxic T-lymphocytes). CTLs destroy virus-infected cells, thereby containing the infection. They are activated by recognition of peptide antigens or epitopes, presented to them in the context of MHC I proteins. These epitopes are derived from proteolytic degradation of endogenously synthesized proteins, which is mediated by the proteasome. Augmentation of epitope presentation by MHC I is thought to be effected by the immunoproteasome, induced in response to IFN-gamma (interferon-gamma) in some cells, and constitutively expressed in others. In this issue of the Biochemical Journal, Remoli and colleagues describe the manipulation of the immunoproteasome by the Tat (transcriptional activation) protein of HIV. The authors show that Tat deregulates the balance of the three proteins, LMP2 (low-molecular-mass polypeptide 2), LMP7 and MECL1 (multicatalytic endopeptidase complex-like 1), which distinguish the immunoproteasome from the proteasome, and they provide a molecular explanation. Intracellular Tat sequesters IRF-1 (interferon-regulatory factor-1) from its cognate promoter element, where normally it associates with STAT1 (signal transducer and activator of transcription 1) to activate basal transcription of the LMP2 gene. LMP2 expression is inhibited as a consequence, skewing the stoichiometry of the immunoproteasome and changing its enzymatic activity. These findings provide a molecular account of an immunomodulatory activity of HIV: changing the peptide antigen profile of cells expressing or exposed to Tat. They may also provide an avenue for manipulating vaccine efficacy and specificity with Tat-based adjuvants.

The mutation in the ATP-binding region of JAK1, identified in human uterine leiomyosarcomas, results in defective interferon-gamma inducibility of TAP1 and LMP2

The presentation of human leukocyte antigens (HLA) class I requires the coordinated expression of numerous components involved in antigen presentation. Tumor cells may alter the antigen presentation by HLA class I, allowing them to evade antitumor immunity. In many cases, the lack of antigen presentation can be attributed to the downregulation of genes needed for antigen processing, such as the transporters associated with antigen processing (TAP) 1, and the proteasomal component, low molecular weight proteins (LMP) 2. The TAP1 and LMP2 genes are transcribed from a shared bidirectional promoter containing an interferon (IFN)-gamma-response factor element; thus, the IFN-gamma-signal strongly induces both TAP1 and LMP2 expression. Low molecular weight proteins2-deficient mice exhibited the development of uterine leiomyosarcomas. Here, the differential responsiveness to IFN-gamma of the SKN human uterine leiomyosarcomas cell line was investigated. We now identify the G871E mutation in the ATP-binding region of Janus kinases 1, suggesting that the loss of TAP1 and LMP2 induction is a defect in the earliest steps of the IFN-gamma-signal pathway, resulting in the inability of SKN cells to upregulate the antigen-processing pathway. Understanding the mechanisms by which these tumors circumvent cytokine signalling, thereby evading antitumor-specific immunity, would greatly aid the efficacy of immunotherapy for treating uterine leiomyosarcomas.

Expression and DNA binding activity of the recombinant interferon regulatory factor-1 (IRF-1) of mouse

Interferon regulatory factor-1 (IRF-1) is a positive transcription factor for genes involved in immune response, cell growth regulation and apoptosis in mammalian cells. Many agents like virus, interferon (IFN), double-stranded RNA (dsRNA), proinflammatory cytokines, prolactin etc. induce IRF-1 at transcriptional level. IRF-1 transcriptionally activates many IRF-1-regulated genes during normal physiological and pathological conditions. We have expressed recombinant mouse IRF-1 (329 amino acids) as a GST(glutathione-S-transferase)-IRF-1 fusion protein from a 1029 bp IRF-1 cDNA in pGEX-2TK expression vector in Escherichia coli XL-1 blue cells. Recombinant GST-IRF-1 was highly expressed as a approximately 66 kDa soluble protein by IPTG-induction, and was biologically active in terms of its DNA binding activity with a 24 bp specific oligonucleotide, i.e. 32P(GAAAGT)4 but not with a similar but nonspecific oligonucleotide i.e. 32P(GAAA)6. GST-alone expressed from the vector did not bind 32P(GAAAGT)4. We observed multiple (1-4) GST-IRF-1-(GAAAGT)4 protein-DNA complexes which were competed out by 25x- to 100x-fold molar excess of (GAAAGT)4 showing that the complexes 1-4 were specific for IRF-1. Such GAAANN (N = any nucleotide) hexanucleotides occur in the promoters of many virus- and interferon-inducible mammalian genes. Multimeric GAAAGT/C sequences are inducible by virus, IFN, dsRNA and IRF-1. Specificity of DNA binding by IRF-1 lies in the 5th and 6th nucleotides in the GAAAGT sequence. Multiple IRF-1-DNA complexes should stimulate transcription by IRF-1.

Expression of the MHC class II transactivator (CIITA) type IV promoter in B lymphocytes and regulation by IFN-gamma

The MHC class II transactivator (CIITA), the master regulator of MHC class II (MHC II) expression, is a co-activator that controls MHC II transcription. Human B lymphocytes express MHC II constitutively due to persistent activity of CIITA promoter III (pIII), one of the four potential promoters (pI-pIV) of this gene. Although increases in MHC II expression in B cells in response to cytokines have been observed and induction of MHC II and CIITA by IFN-gamma has been studied in a number of different cell types, the specific effects of IFN-gamma on CIITA expression in B cells have not been studied. To investigate the regulation of CIITA expression by IFN-gamma in B cells, RT-PCR, in vivo and in vitro protein/DNA binding studies, and functional promoter analyses were performed. Both MHC II and CIITA type IV-specific RNAs increased in human B lymphocytes in response to IFN-gamma treatment. CIITA promoter analysis confirmed that pIV is IFN-gamma inducible in B cells and that the GAS and IRF-E sites are necessary for full induction. DNA binding of IRF-1 and IRF-2, members of the IFN regulatory factor family, was up-regulated in B cells in response to IFN-gamma and increased the activity of CIITA pIV. In vivo genomic footprint analysis demonstrated proteins binding at the GAS, IRF-E and E box sites of CIITA pIV. Although CIITA pIII is considered to be the hematopoietic-specific promoter of CIITA, these findings demonstrate that pIV is active in B lymphocytes and potentially contributes to the expression of CIITA and MHC II in these cells.

IFN regulatory factor-2 cooperates with STAT1 to regulate transporter associated with antigen processing-1 promoter activity

Class I MHC complexes (MHC(I)) are essential in mediating immune response. The transport of antigenic peptides (TAP) to MHC(I) and the stable expression of MHC(I) on the cell surface require the presence of a dedicated TAP. In this study we report that IFN-gamma and thrombopoietin (TPO) strongly increase TAP1 protein expression in megakaryocytes, followed by an enhanced expression of MHC(I) on the cell surface. This expression parallels the enhanced TAP1 promoter activity and TAP1 mRNA expression, which are independent of protein synthesis. We also show that this cytokine-dependent expression of TAP1 transcripts depends on STAT1 and IFN regulatory factor-2 (IRF-2), but not on IRF-1, and provide evidence that IRF-2 constitutively binds to the TAP1 gene promoter and enhances TAP1 promoter activity. We show that IRF-2 forms a complex with STAT1 and the cytokine-responsive region of the TAP1 promoter in any TPO or IFN-gamma target cells tested. Interaction of IRF-2 and STAT1 on the promoter depends on the DNA-binding domain of IRF-2. Overall, our data indicate that TPO and IFN-gamma activate the expression of TAP1 via a new mechanism that involves functional cooperation between STAT1 and IRF-2 on the TAP1 promoter.

IRF-1 mediates upregulation of LMP7 by IFN-gamma and concerted expression of immunosubunits of the proteasome

An immunoproteasome subunit low molecular weight protein 7 (LMP7) plays critical roles in major histocompatibility complex class I antigen processing; however, the mechanism for its expression has remained unclear. We demonstrate that interferon (IFN) regulatory factor-1 (IRF-1) has a pivotal role in IFN-gamma-dependent LMP7 expression, as was shown for the other two immunosubunits. A tetracycline-inducible system for IRF-1 revealed its function in the LMP7 expression, and a genomic region functionally interacting with IRF-1 was also determined. Furthermore, the role of IRF-1 in IFN-gamma-inducible LMP7 transcription was confirmed by employing small interfering RNA experiments and IRF-1-/- mice. These results suggest that IRF-1 acts as a master regulator for the concerted expression of immunoproteasome components.

Chicken TAP genes differ from their human orthologues in locus organisation, size, sequence features and polymorphism

We have previously shown that in the chicken major histocompatibility complex, the two transporters associated with antigen processing genes (TAP1 and TAP2) are located head to head between two classical class I genes. Here we show that the region between these two TAP genes has transcription factor-binding sites in common with class I gene promoters. The TAP genes are also up-regulated by interferon-gamma in a similar way to mammalian TAP genes and in a way that suggests they are both transcribed from a bi-directional promoter. The gene structures of TAP1 and TAP2 differ from that of human TAPs in that TAP1 has a truncated exon 1 and TAP2 has fused exons, resulting in a much smaller gene size. The truncation of TAP1 results in the loss of approximately 150 amino acids, which are thought to be involved in endoplasmic reticulum retention, heterodimer formation and tapasin binding, compared to human TAP1. Most of the protein sequence features involved in binding ATP are conserved, with two exceptions: chicken TAP1 has a glycine in the switch region where other TAPs have glutamine or histidine, and both chicken TAP genes have serines in the C motif where mammalian TAP2 has an alanine. Lastly, the chicken TAP genes are highly polymorphic, with at least as many TAP alleles as there are class I alleles, as seen by investigating nine inbred lines of chicken. The close proximity of the TAP genes to the class I genes and the high level of polymorphism may allow co-evolution of the genes, allowing TAP molecules to transport peptides specifically for the class I molecules of that haplotype.

Chemoprevention by Hippophae rhamnoides: Effects on Tumorigenesis, Phase II and Antioxidant Enzymes, and IRF-1 Transcription Factor

Fruits or berries of Hippophae rhamnoides (sea buckthorn), a rich source of vitamins A, C, and E, carotenes, flavonoids, and microelements such as sulfur, selenium, zinc, and copper, are edible and have been shown to protect from atopic dermatitis, hepatic injury, cardiac disease, ulcer, and atherosclerosis. However, its mechanism of action is not clear. We show that Hippophae inhibits benzo(a)pyrene-induced forestomach and DMBA-induced skin papillomagenesis in mouse. This decrease in carcinogenesis may be attributed to the concomitant induction of phase II enzymes such as glutathione S-transferase and DT-diaphorase and antioxidant enzymes such as superoxide dismutase, catalase, glutathione peroxidase, and glutathione reductase in the mouse liver. This was accompanied by a remarkable induction of the transcription factor interferon regulatory factor-1 in the Hippophae-treated liver. Our results strongly suggest that Hippophae fruit is able to decrease carcinogen-induced forestomach and skin tumorigenesis, which might involve up-regulation of phase II and antioxidant enzymes as well as DNA-binding activity of IRF-1, a known antioncogenic transcription factor causing growth suppression and apoptosis induction for its anticancer effect.

Regulation of murine Tap1 and Lmp2 genes in macrophages by interferon gamma is mediated by STAT1 and IRF-1

The genes of the transporter associated with antigen processing (Tap)-1, and the low molecular weight peptide (Lmp)-2, are crucial for class I major histocompatibility complex function and share a common bidirectional promoter. In murine bone marrow-derived macrophages, interferon gamma (IFN-gamma) induced Tap-1 and upregulated Lmp-2, which is constitutively expressed at low levels. The IFN-gamma-induction was independent of early gene synthesis. The mRNA induced by IFN-gamma was very stable. In macrophages from STAT1 knockout mice, IFN-gamma did not induce the expression of Tap-1 or Lmp-2. Several areas in the promoter can be controlled by IFN-gamma, such as proximal and distal GAS boxes in the direction of the Tap-1 gene, NFgammaB and IRF-1 boxes. By making deletions of the promoter, we found that only the proximal GAS and IRF-1 boxes are required for IFN-gamma induction of Tap-1 and Lmp-2. Experiments using nuclear extracts from macrophages treated for 30 min with IFN-gamma and gel shift analysis indicated that STAT1 binds to the GAS box. The nuclear extracts from macrophages treated for at least 2 h with IFN-gamma bound to the IRF-1 box. These results indicate that both STAT1 and IRF-1 are required for the IFN-gamma induction of Tap-1 and Lmp-2 genes.

STAT1 regulates lipopolysaccharide- and TNF-alpha-dependent expression of transporter associated with antigen processing 1 and low molecular mass polypeptide 2 genes in macrophages by distinct mechanisms

Transporter associated with Ag processing 1 and low molecular mass polypeptide 2 (LMP2) are essential for class I MHC function and share a common bidirectional promoter. In murine bone marrow-derived macrophages, LPS and TNF-alpha induced Tap1 and up-regulated Lmp2, which is constitutively expressed at low levels. These two genes are induced by LPS and TNF-alpha with distinct kinetics, at 6 and 12-24 h, respectively. Using macrophages derived from the TNF-alpha receptors of knockout mice, we found that induction by LPS is not due to the autocrine production of TNF-alpha. In macrophages from STAT-1 knockout mice, neither LPS nor TNF-alpha induced the expression of Tap1 or Lmp2. The shared promoter contains several areas that can be controlled by STAT-1, such as the proximal and distal IFN-gamma activation site (GAS) boxes in the direction of the Tap1 gene. By making deletions of the promoter, we determined that only the proximal GAS box is required for LPS induction of Tap1 and Lmp2. In contrast, TNF-alpha induction of these two genes is dependent on the IFN regulatory factor-1 and NF-kappaB boxes, and not on the GAS box. Our experiments using gel shift analysis and Abs indicated that STAT1 binds to the GAS box in nuclear extracts from LPS-treated macrophages. The nuclear extracts obtained from macrophages treated with TNF-alpha bound to the IFN regulatory factor-1 and NF-kappaB boxes. These results show that LPS and TNF-alpha regulate the induction of Tap1 and Lmp2 through STAT1, but use distinct areas of the promoter.

Alternate interferon signaling pathways

More than a half a century ago, interferons (IFN) were identified as antiviral cytokines. Since that discovery, IFN have been in the forefront of basic and clinical cytokine research. The pleiotropic nature of these cytokines continues to engage a large number of investigators to define their actions further. IFN paved the way for discovery of Janus tyrosine kinase (JAK)-signal transducing activators of transcription (STAT) pathways. A number of important tumor suppressive pathways are controlled by IFN. Several infectious pathogens counteract IFN-induced signaling pathways. Recent studies indicate that IFN activate several new protein kinases, including the MAP kinase family, and downstream transcription factors. This review not only details the established IFN signaling paradigms but also provides insights into emerging alternate signaling pathways and mechanisms of pathogen-induced signaling interference.

Characteristic expression profiles induced by genotoxic carcinogens in rat liver

When applied in toxicological studies, the recently developed gene expression profiling techniques using microarrays, which brought forth the new field of toxicogenomics, facilitate the interpretation of a toxic compound's mechanism of action. In this study, we investigated whether genotoxic carcinogens at doses known to induce liver tumors in the 2-year rat bioassay deregulate a common set of genes in a short-term in vivo study and, if so, whether these deregulated genes represent defined biological pathways. Rats were dosed with the four genotoxic hepatocarcinogens dimethylnitrosamine (4 mg/kg/day), 2-nitrofluorene (44 mg/kg/day), aflatoxin B1 (0.24 mg/kg/day), and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK, 20 mg/kg/day). After treatment for up to 14 days, the expression profiles of the livers were analyzed on Affymetrix RG_U34A microarrays. Among the significantly upregulated genes were a set of target genes of the tumor suppressor protein p53, indicating a DNA damage response. Such a response was expected and, therefore, confirmed the validity of our approach. In addition, the gene expression changes suggest a specific detoxification response, the activation of proliferative and survival signaling pathways, and some cell structural changes. These responses were strong throughout the 14 day time course for 2-nitrofluorene and aflatoxin B1; in the case of dimethylnitrosamine and NNK, the effects were weakly detectable at day 1 and then increased with time. For dimethylnitrosamine and aflatoxin B1, which caused observable inflammation in vivo, we found a corresponding upregulation of inflammatory genes at the same time points. Thus, by the toxicogenomic analysis of short-term in vivo studies, we identified genes and pathways commonly deregulated by genotoxic carcinogens, which may be indicative for the early events in tumorigenesis and, thus, predictive of later tumor development.

Pregnancy and interferon tau regulate major histocompatibility complex class I and beta2-microglobulin expression in the ovine uterus

Major histocompatibility complex (MHC) class I molecules, consisting of an alpha chain and beta2-microglobulin (beta2MG), play an important role in immune rejection responses by discriminating self and nonself and are increased by type I interferons during antiviral responses. Interferon tau (IFNtau), the pregnancy-recognition signal in ruminants, is a type I interferon produced by the ovine conceptus between Days 11 and 21 of gestation. In study 1, expression of MHC class I alpha chain and beta2MG mRNA and protein was detected primarily in endometrial luminal epithelium (LE) and glandular epithelium (GE) on Days 10 and 12 of the estrous cycle and pregnancy. On Days 14-20 of pregnancy, MHC class I and beta2MG expression increased only in endometrial stroma and GE and, concurrently, was absent in LE and superficial ductal GE (sGE). Although neither MHC class I nor beta2MG proteins were detected in Day 20 trophectoderm, beta2MG mRNA was detected in conceptus trophectoderm. In study 2, cyclic ewes were ovariectomized on Day 5, treated daily with progesterone to Day 16, received intrauterine infusions between Days 11 and 16 of either control serum proteins or recombinant ovine IFNtau, and were hysterectomized on Day 17. The IFNtau increased MHC class I and beta2MG expression only in endometrial stroma and GE. During pregnancy, MHC class I and beta2MG gene expression is inhibited in endometrial LE and sGE but, paradoxically, is stimulated by IFNtau in the stroma and GE. The silencing of MHC class I alpha chain and beta2MG genes in the endometrial LE and sGE during pregnancy recognition and establishment may be a critical mechanism preventing immune rejection of the conceptus allograft.

Innate and acquired immunity intersect in a global view of the acute-phase response

Recognition of stereotypic chemical patterns by sentinel cells of the innate immune system provokes a transient deviation from homeostasis, the acute-phase response (APR). Although APR effectors have been identified individually, the complexity of the response suggested that emergent properties would be uncovered by a more comprehensive examination. Our global assessment revealed that approximately 7% of genes in the mouse are mobilized in the hepatic APR to endotoxin. Extensive metabolic adjustments include suppression of pathways for cholesterol, fatty acid, and phospholipid synthesis. Increased expression of genes for innate defense was accompanied by coordinate induction of the MHC class I antigen presentation machinery, illustrating an intersection between innate and adaptive immunity.

The IRF-2 DNA binding domain facilitates the activation of the class II transactivator (CIITA) type IV promoter by IRF-1

IFN-gamma induced transcription of class II transactivator (CIITA), a major regulator of MHC class II gene expression, is directed by the CIITA type IV promoter. The IFN-gamma activation of the CIITA type IV promoter is mediated by STAT1 and IRF-1, which bind to the GAS and IRF-E of the promoter, respectively. We and others have determined that IRF-2, another member of the IRF family, also activates the CIITA type IV promoter, by binding to the IRF-E. Also, IRF-2 cooperates with IRF-1 to activate the promoter. DNA binding analyses determined that IRF-1 and IRF-2 can co-occupy the IRF-E of the CIITA type IV promoter. To further understand the mechanism of IRF-1 and IRF-2 cooperativity in the activation of CIITA type IV promoter, we characterized the binding of IRF-1 and IRF-2 to the CIITA IRF-E and mapped the domains of IRF-2 required for the cooperative transactivation. Off-rate experiments revealed that the IRF-2/IRF-E complex was more stable than the IRF-1/IRF-E complex and that the affinity of IRF-1 for the IRF-E was increased when IRF-1 co-occupied the IRF-E with IRF-2. Deletion analysis of functional domains of IRF-2 revealed that a previously described latent activation domain of IRF-2 was essential for IRF-2 transactivation and participated in cooperative activation of the CIITA promoter by IRF-1 and IRF-2. However, the DNA binding domain of IRF-2 was sufficient for cooperativity with IRF-1 in the activation of the CIITA type IV promoter. DNA binding assay demonstrated that, like the full-length IRF-2, the IRF-2 DNA binding domain could co-occupy the CIITA IRF-E with IRF-1.

Transcriptional regulation of CCSP by interferon-gamma in vitro and in vivo

Interferon gamma (IFN-gamma), a potent cytokine inducing a wide range of immunologic activities, is increased in the airway secondary to viral infection or during an inflammatory response. This increase in IFN-gamma concentration may alter the expression of specific airway epithelial cell genes that regulate adaptation of airway inflammatory responses. One protein induced by IFN-gamma is Clara cell secretory protein (CCSP), which may contribute to the attenuation of airway inflammation. This study was done to investigate the molecular mechanism by which IFN-gamma stimulates the expression of the CCSP gene in mouse transformed Clara cells and transgenic mice. Deletion mapping and linker-scanning mutations demonstrated that IFN-gamma-induced expression of CCSP was regulated, in part, at the level of transcription. In vitro and in vivo studies verified that the minimal IFN-gamma-responsive segment was localized to the proximal 166 bp of the 5'-flanking region. Additionally, IFN-gamma-induced expression of CCSP was mediated indirectly through an interferon regulatory factor-1-mediated increase in hepatocyte nuclear factor-3beta.

The role of interferon regulatory factors in the cardiac response to viral infection

Reovirus-induced murine myocarditis provides an excellent model for the human disease. Cardiac tissue damage varies between reovirus strains, and is caused by a direct viral cytopathogenic effect. One determinant of virus-induced cardiac tissue damage is the cardiac interferon-beta (IFN-beta) response to viral infection. Nonmyocarditic reoviruses induce more IFN-beta and/or are more sensitive to the antiviral effects of IFN-beta in cardiac cells than myocarditis reoviruses. The roles of interferon regulatory factors (IRFs) in the cardiac response to viral infection are reviewed, and results suggest possible cardiac-specific variations in IRF-3 and IRF-1 function. In addition, data are presented indicating that the role of IRF-2 in regulation of IFN-beta expression is cell type-specific and differs between skeletal and cardiac muscle cells. Together, results suggest that the heart may provide a unique environment for IRF function, critical for protection against virus-induced cardiac damage.

Regulation of the expression of mouse TAP-associated glycoprotein (tapasin) by cytokines

The expression of antigen presenting MHC class I molecules can be enhanced through cytokines, e.g. upon infection with bacteria or viruses, either directly by enhancing class I gene transcription or by increasing the amounts of accessory proteins of the loading complex. Tapasin plays a significant role in the peptide loading of class I molecules. Here, we describe recognition motifs of cytokine inducible transcription factors in the promoter region of the mouse tapasin gene, most of them clustered within the 140 base pairs upstream of the start codon. Tapasin mRNA was strongly induced in vivo after infection with the facultatively intracellular bacterium Listeria monocytogenes in an IFN-gamma-dependent fashion. Accordingly, both tapasin mRNA and protein were strongly induced in a time and dose dependent manner in embryonic fibroblasts treated with the cytokines IFN-gamma and IFN-beta, and weakly induced after treatment with TNF-alpha. Co-stimulation of tapasin by TNF-alpha and IFN-gamma resulted in a weak synergistic effect. Using fibroblasts either lacking IRF-1 or inhibited in protein synthesis we show that secondary transcription factors are necessary for a maximal stimulation of tapasin expression upon IFN-gamma stimulation. The sequential induction of TAP1, LMP2, and tapasin before the stimulated expression of class I heavy chain is discussed.

Regulation of immunoproteasome subunit expression in vivo following pathogenic fungal infection

The proteasome catalytic beta subunits LMP2, LMP7, and MECL-1 and two proteasome activator proteins, PA28 alpha and beta, are induced following exposure to IFN-gamma in vitro. Induction of these immunosubunits and the PA28 alpha/beta hetero-oligomer alters proteasome catalytic functions and specificity and enhances production of certain MHC class I epitopes. We sought to determine whether and to what extent proteasome subunit composition is regulated in vivo and to elucidate the mechanisms of such regulation. We analyzed basal expression levels of these inducible genes in normal, IFN-gamma-deficient, and Stat-1-deficient mice. Mice of all three genotypes display constitutive expression of the immunosubunits and PA28, demonstrating that basal expression in vivo is independent of endogenous IFN-gamma production. However, basal expression levels are reduced in Stat-1(-/-) mice, demonstrating a role for Stat-1 independent of IFN-gamma signaling. To demonstrate that IFN-gamma can induce these genes in vivo, mice were infected with Histoplasma capsulatum. Elevated expression of these genes followed the same time course as IFN-gamma expression in infected mice. IFN-gamma-deficient mice did not display elevated protein expression following infection, suggesting that other inflammatory cytokines produced in infected mice are unable to influence proteasome expression. Cytokines other than IFN-gamma also failed to influence proteasome gene expression in vitro in cell lines that had no basal expression of LMP2, LMP7, or MECL-1. Thus, both in vitro and in vivo data demonstrate that IFN-gamma is essential for up-regulation, but not constitutive expression, of immunoproteasome subunits in mice.

Type I interferons and autoimmunity: lessons from the clinic and from IRF-2-deficient mice

Type I interferons (IFN-alpha/beta) are produced upon viral and bacterial infections and play essential roles in host defense. However, since IFN-alpha/beta have multiple regulatory functions on innate and adoptive immunity, dysregulation of the IFN-alpha/beta system both in uninfected hosts and during immune responses against infection can result in immunopathologies. In fact, IFN-alpha/beta therapy often accompanies autoimmune-like symptoms. In this regard, we have recently found that mice lacking IFN regulatory factor (IRF)-2, a negative regulator of IFN-alpha/beta signaling, develop spontaneous, CD8(+) T cell-dependent skin inflammation. This unique animal model, together with other animal models, highlights the importance of the mechanism maintaining the homeostasis in the IFN-alpha/beta system even in the absence of infection.

Interferon regulatory factor-1, interferon-beta, and reovirus-induced myocarditis

Viral myocarditis is an important human disease, and reovirus-induced myocarditis in mice provides an excellent model to study direct viral damage to the heart. Previously, we showed that reovirus induction of and sensitivity to interferon-beta (IFN-beta) is an important determinant of viral pathogenicity in the heart and that the transcription factor interferon regulatory factor-3 (IRF-3) is required for reovirus induction of IFN-beta in primary cardiac myocyte cultures. Given several lines of evidence suggesting a possible distinctive environment for IRFs in the heart, we have now focused on IRF-1. Previous studies demonstrated that viruses, double-stranded-RNA (dsRNA), and IFN-alpha/beta can each induce IRF-1 and that IRF-1 plays a role in dsRNA, but perhaps not viral, induction of IFN-alpha/beta. Importantly, none of these studies used a virus with a dsRNA genome (such as reovirus), none of them used a highly differentiated nonlymphoid cell type, and none of them addressed whether viral induction of IRF-1 is direct or is mediated through viral induction of IFN-beta. Indeed, as recently as this year it has been assumed that viral induction of IRF-1 is direct. Here, we found that reovirus induced IRF-1 in primary cardiac myocyte cultures, but that IRF-1 was not required for reovirus induction of IFN-beta. Surprisingly, we found that reovirus failed to induce IRF-1 in the absence of the IFN-alpha/beta response. This provides the first evidence that viruses may not induce IRF-1 directly. Finally, nonmyocarditic reovirus strains induced more cardiac lesions in mice deficient for IRF-1 than they did in wildtype mice, directly demonstrating a protective role for IRF-1. Together, the results indicate that while IRF-1 is downstream of the IFN-beta response, it plays an important protective role against viral myocarditis.

Isolation and characterization of a human STAT1 gene regulatory element. Inducibility by interferon (IFN) types I and II and role of IFN regulatory factor-1

The transcription factor STAT1 plays a pivotal role in signal transduction of type I and II interferons (IFNs). STAT1 activation leads to changes in expression of key regulatory genes encoding caspases and cell cycle inhibitors. Deficient STAT1 expression in human cancer cells and virally mediated inhibition of STAT1 function have been associated with cellular resistance to IFNs and mycobacterial infection in humans. Thus, given the relative importance of STAT1, we isolated and characterized a human STAT1 intronic enhancer region displaying IFN-regulated activity. Functional analyses by transient expression identified a repressor region and type I and II IFN-inducible elements within the STAT1 enhancer sequence. A candidate IRF-E/GAS/IRF-E (IGI) sequence containing GAAANN nucleotide repeats was shown by gel shift assay to bind to IFN regulatory factor-1 (IRF-1), but not to IFN-stimulated gene factor-3 (ISGF-3) or STAT1-3. An additional larger IGI-binding complex containing IRF-1 was identified. Mutation of the GAAANN repeats within the IGI DNA element eliminated IRF-1 binding and the IFN-regulated activity of the STAT1 intronic enhancer region. Transfection of the IFN-resistant MM96 cell line to express increased levels of IRF-1 protein also elevated STAT1, STAT2, and p48/IRF-9 expression and enhanced cellular responsiveness to IFN-beta. Reciprocating regulation between IRF-1 and STAT1 genes and encoded proteins indicates that an intracellular amplifier circuit exists controlling cellular responsiveness to the IFNs.

IFN regulatory factor-1 regulates IFN-gamma-dependent cathepsin S expression

Cathepsin S is a cysteine protease with potent endoproteolytic activity and a broad pH profile. Cathepsin S activity is essential for complete processing of the MHC class II-associated invariant chain within B cells and dendritic cells, and may also be important in extracellular matrix degradation in atherosclerosis and emphysema. Unique among cysteine proteases, cathepsin S activity is up-regulated by IFN-gamma. Given its importance, we sought to elucidate the pathway by which IFN-gamma increases cathepsin S expression. Our data demonstrate that the cathepsin S promoter contains an IFN-stimulated response element (ISRE) that is critical for IFN-gamma-induced gene transcription in a cell line derived from type II alveolar epithelial (A549) cells. IFN response factor (IRF)-2 derived from A549 nuclear extracts associates with the ISRE oligonucleotide in gel shift assays, but is quickly replaced by IRF-1 following stimulation with IFN-gamma. The time course of IRF-1/ISRE complex formation correlates with increased levels of IRF-1 protein and cathepsin S mRNA. Overexpression of IRF-1, but not IRF-2, markedly augments cathepsin S promoter activity in A549 cells. Furthermore, overexpression of IRF-1 increases endogenous cathepsin S mRNA levels in 293T epithelial cells. Finally, freshly isolated bone marrow cells from IRF-1(-/-) mice fail to up-regulate cathepsin S activity in response to IFN-gamma. Thus, IRF-1 is the critical transcriptional mediator of IFN-gamma-dependent cathepsin S activation. These data elucidate a new pathway by which IRF-1 may affect MHC class II processing and presentation.

Regulation of PKR and IRF-1 during hepatitis C virus RNA replication

The virus-host interactions that influence hepatitis C virus (HCV) replication are largely unknown but are thought to involve those that disrupt components of the innate intracellular antiviral response. Here we examined cellular antiviral pathways that are triggered during HCV RNA replication. We report that (i) RNA replication of HCV subgenomic replicons stimulated double-stranded RNA (dsRNA) signaling pathways within cultured human hepatoma cells, and (ii) viral RNA replication efficiency corresponded with an ability to block a key cellular antiviral effector pathway that is triggered by dsRNA and includes IFN regulatory factor-1 (IRF-1) and protein kinase R (PKR). The block to dsRNA signaling was mapped to the viral nonstructural 5A (NS5A) protein, which colocalized with PKR and suppressed the dsRNA activation of PKR during HCV RNA replication. NS5A alone was sufficient to block both the activation of IRF-1 and the induction of an IRF-1-dependent cellular promoter by dsRNA. Mutations that clustered in or adjacent to the PKR-binding domain of NS5A relieved the blockade to this IRF-1 regulatory pathway, resulting in induction of IRF-1-dependent antiviral effector genes and the concomitant reduction in HCV RNA replication efficiency. Our results provide further evidence to support a role for PKR in dsRNA signaling processes that activate IRF-1 during virus infection and suggest that NS5A may influence HCV persistence by blocking IRF-1 activation and disrupting a host antiviral pathway that plays a role in suppressing virus replication.

Cis elements for transporter associated with antigen-processing-2 transcription: two new promoters and an essential role of the IFN response factor binding element in IFN-gamma-mediated activation of the transcription initiator

Expression of cell surface MHC class I:peptide complex requires coordinated expression of multiple genes such as MHC class I heavy chain, beta(2)-microglobulin (beta(2)m), transporters associated with antigen-processing (TAP)-1 and TAP-2, and proteosomal components low-molecular weight polypeptide (LMP)-2 and LMP-7. All of these genes are expressed at defined and distinct levels in normal tissues, and are inducible by IFN-gamma. While the cis elements involved in transcription of the MHC class I heavy chain, beta(2)m, TAP-1 and LMP-2 have been analyzed extensively, those for TAP-2 and LMP-7 have not been well studied. Here we systematically analyzed the cis elements for TAP-2 transcription. We found at least two independent elements that are sufficient to activate transcription of a reporter gene. One (hereby called TAP-2 P1) is located 5' to the TAP-2 exon 1, while the other (hereby called TAP-2 P2) is a transcription initiator residing in intron 1. Analysis of the 5' sequence of TAP-2 mRNA indicates that both promoters are active. Moreover, while the TAP-2 promoter region contains cis elements that can mediate TAP-2 induction by IFN-gamma, such as gamma-activation site and IFN response factor binding element (IRFE), only the IRFE is required for IFN-gamma induction of TAP-2 promoter in vitro. The IRFE appears to work as an enhancer for the initiator (P2). Together with another promoter recently identified by others, TAP-2 therefore has three independent promoters that can be differentially regulated.

IRF and tuberculosis

Tuberculosis is the most prevalent infectious disease and causes more deaths than any other, yet only 5%-10% of people infected by the causative agent, Mycobacterium tuberculosis, will develop the disease. Thus, natural resistance among humans is the norm. Fundamental immune responses to M. tuberculosis are being elucidated, including induction of interferon regulatory factor-1 (IRF-1). Moreover, IRF-1 has been found necessary for normal resistance to infection by mycobacteria in mice. Roles for IRF-1 in a plethora of immune system functions have been described. This review considers molecular responses to infection by M. tuberculosis that might account for induction of IRF-1 and highlights putative connections between immunomodulatory functions of IRF-1 and immune responses relevant to infection by M. tuberculosis. However, the complexity inherent in pleiotropy and redundancy limits the ability to draw firm conclusions. In many cases, it remains to be demonstrated that a particular function of IRF-1 is the basis for a known response to infection. For example, although IRF-1 is required for a Th1 cell-mediated, adaptive immune response in some circumstances, it is not known if the Th1 response to infection by M. tuberculosis requires IRF-1. Conversely, some known contributions by IRF-1 to fundamental aspects of the immune system are not yet proven relevant in the host response to infection. For example, it is not known if control of T cell subset development by IRF-1 is significant for host defense against M. tuberculosis. Functions of other IRF that overlap with or are distinct from the functions of IRF-1 also could be important for the immune response to M. tuberculosis.

IRF regulation of HIV-1 long terminal repeat activity

Interferon (IFN) regulatory factors (IRF) constitute a family of transcriptional activators and repressors implicated in multiple biologic processes, including regulation of immune responses and host defense, cytokine signalling, cell growth regulation, and hematopoietic development. All members are characterized by well-conserved DNA binding domains at the N-terminal region that recognize similar DNA sequences termed IRF-binding element/IFN-stimulated response element (IRF-E/ISRE) present on the promoter of the IFN-alpha/beta genes and of some IFN-stimulated genes (ISG). Recently, a sequence homologous to the ISRE has been identified downstream of the 5' human immunodeficiency virus type 1 (HIV-1) long terminal repeat (LTR). This sequence is a binding site for IRF-1 and IRF-2. Deletion of the LTR-ISRE results in impaired LTR promoter activity and decreased synthesis of viral RNA and proteins. Here, we briefly summarize characteristics of IRF-1 and IRF-2 binding to the HIV-1 LTR-ISRE and the data obtained to date on the functionality of this cis-element and on the role of IRF in the regulation of HIV-1 LTR transcriptional activity.

Activities of IRF-1

Interferon (IFN) regulatory factor-1 (IRF-1) was isolated by virtue of its affinity to specific DNA sequences in the IFN-beta promoter that mediate virus responsiveness. IRF-1 was the first factor identified of the IRF family and was most extensively characterized at the molecular level. Also, its physiologic role in host defense against pathogens, tumor prevention, and development of the immune system was investigated in detail. Even though some of the functions first associated with IRF-1 were later found to be mediated in part or predominantly by other activators of the IRF family of transcription factors, IRF-1 has remained a central paradigm in the transcriptional regulation of the IFN response.

Interferon regulatory factor-1 is required for interferon-gamma-induced MHC class I genes in astrocytes

Recent studies have shown that the role of the transcription factor interferon regulatory factor-1 (IRF-1) in the expression of major histocompatibility complex (MHC) class I molecules is tissue-specific. Our previous studies indicated a role for IRF-1 in expression of MHC class I genes in cultured astrocytes in response to interferon-gamma (IFN-gamma). However, the requirement for IRF-1 in MHC class I expression has not been directly analyzed in neural tissue. To further ascertain the importance of IRF-1 in the induction of MHC class I genes in astrocytes in response to IFN-gamma, we analyzed astrocytes from mice with a targeted disruption of the IRF-1 gene (IRF-1(-/-) mice). As expected, astrocytes from wild type (IRF-1(+/+)) mice showed a coordinate increase in both IRF-1 and MHC class I gene expression in response to IFN-gamma. To the contrary, astrocytes from IRF-1(-/-) mice had greatly reduced MHC class I mRNA expression. MHC class I gene promoter activity in astrocytes was controlled entirely through a single enhancer, the MHC-IRF-E, to which IRF-1 bound in response to IFN-gamma in wild type but not in IRF-1(-/-) mouse astrocytes. In vivo, astrocytes in brains of wild type mice readily responded to IFN-gamma to express MHC class I molecules. This correlated with increased MHC class I mRNA in the brain. In contrast, brains of IRF-1(-/-) mice showed no MHC class I gene induction following exposure to IFN-gamma indicating that all cells in the central nervous system (CNS) including astrocytes with the potential to express MHC class I molecules were dependent on IRF-1. These studies conclusively demonstrate a major role for IRF-1/MHC-IRF-E interactions in controlling MHC class I gene expression in astrocytes in response to IFN-gamma.

IFN-gamma affects homing of diabetogenic T cells

IFN-gamma is a cytokine with pleiotropic functions that participates in immune and autoimmune responses. The lack of IFN-gamma is known to delay the development of autoimmune diabetes in nonobese diabetic (NOD) mice. Splenocytes from diabetic NOD and IFN-gamma knockout (KO) NOD mice transfer diabetes into NOD recipients equally well. However, adoptive transfer of diabetogenic T cells from NOD mice into NOD.IFN-gamma-KO or NOD mice lacking beta-chain of IFN-gamma receptor (NOD.IFN-gammaRbeta-KO) appeared to be much less efficient. We found that IFN-gamma influences the ability of diabetogenic cells to penetrate pancreatic islets. Tracing in vivo of insulin-specific CD8+ T cells has shown that homing of these cells to the islets of Langerhans was affected by the lack of IFN-gamma. While adhesion of insulin-specific CD8+ cells to microvasculature was normal, the diapedesis was significantly impaired. This effect was reversible by treatment of the animals with rIFN-gamma. Thus, IFN-gamma may, among other effects, influence immune and autoimmune responses by supporting the homing of activated T cells.

Selected contribution: Association of gender-related LMP2 inactivation with autoimmune pathogenesis

Recent results in an animal model of autoimmune diabetes, the nonobese diabetic (NOD) mouse, suggest a hypothesis to explain the role of major histocompatibility complex (MHC) in autoimmunity. The genome MHC region contains immune response genes that are important for T cell education and antigen presentation by MHC molecules. Two such genes encoding the LMP2 and LMP7 proteasome subunits are located in this high-risk MHC genomic region. Proteasome containing the LMP2 subunit is essential for T cell education and proteolytically activates transcription factor nuclear factor-kappaB. Splenocytes of NOD mouse with marked female specificity for disease expression are defective in LMP2 expression. The spontaneous defective LMP2 expression in NOD mice, which is gender biased toward female cohorts, is restricted to select lymphoid and myeloid cells and is developmentally controlled with lowered LMP2 protein and heightened tumor necrosis factor-alpha-induced apoptosis. These defects are apparent only after approximately 7 wk of age. These data suggest a proteasome role in autoimmune progression, and a gender developmental and lineage restriction of LMP2 expression may contribute to the diverse autoimmune characteristics preferentially observed in female NOD mice.

Knockout mice: a paradigm shift in modern immunology

In the past decade, advances in genetic engineering and mouse knockout technology have transformed our understanding of the immune system. In particular, new perspectives on T-cell development, co-stimulation and activation have emerged from the study of single and multiple gene-knockout animals, as well as from conditional knockout and 'knock-in' mutants. Analysis of these animals has clarified important intracellular signalling pathways and has shed light on the regulatory mechanisms that govern normal immune responses and autoimmunity.

Experimental allergic myositis in SJL/J mouse. Reappraisal of immune reaction based on changes after single immunization

SJL/J mice have been subjected to immunization with wide varieties of antigens to produce models of autoimmune disorders including experimental myositis. They also have a defect in dysferlin gene and spontaneously develop muscle fiber degeneration, a condition akin to limb-girdle type muscular dystrophy and Miyoshi myopathy. To know whether muscle inflammation of SJL mice after immunization with muscle fractions really represents immune-mediated myositis or no more than an epiphenomenon of muscle degeneration due to dysferlin defect, we studied immunological parameters after immunization with rabbit myosin B fraction. Initial infiltration of macrophages and CD4+ lymphocytes on day 11 was followed by increase in number of CD8+ cells. Such increase was not observed in the nontreated and adjuvant controls. Some infiltrating cells were interferon gamma (IFN-gamma) positive. Furthermore, increased expression of the signal transducers and activator of transcription 1 (STAT-1) and interferon regulatory factor 1 (IRF-1) mRNA was shown in the first 2 weeks. These results indicate Th1 system activity in the muscle, rather than simple dysferlin deficiency, particularly 1-3 weeks after immunization. Thus it is concluded that an immune-mediated myositis is taking place at this stage. This model can be helpful in understanding pathomechanisms involved in the early stage of human myositides. It has also important implications concerning immune reactions associated with transplantation or gene therapy for muscular dystrophies.

Complete suppression of insulitis and diabetes in NOD mice lacking interferon regulatory factor-1

Interferon regulatory factor-1 (IRF-1), a transcriptional factor, regulates type I interferon and interferon-induced genes. It was reported that IRF-1 regulates important molecules required for inflammation and immune reactions. To investigate the role of IRF-1 in the development of autoimmune diabetes, we established IRF-1 deficient (IRF-1(-/-)) non-obese diabetic (NOD) mice. IRF-1-deficient C57BL/6J mice were out-crossed to NOD mice, and F1 were backcrossed to NOD mice. At the N8 generation, the heterozygote for IRF-1 mutation was intercrossed and N8F1 was obtained. Out of three NOD genotypes, IRF-1(+/+) and IRF-1(+/-) developed spontaneous diabetes with an incidence of 47% (9/19) and 50% (10/20) by 30 weeks of age, respectively; whereas IRF-1(-/-) did not develop diabetes (0/18, P< 0.01 vs. (+/+) and (+/-)). Histologically, IRF-1(+/+) and IRF-1(+/-) had various degrees of insulitis, but IRF-1(-/-) had no insulitis. In comparison with IRF-1(+/+), the percentage of CD4(+) and Mac-1(+) splenic cells significantly increased, whereas CD3(+), CD8(+) and B220(+) cells decreased in IRF-1(-/-). Furthermore, spleen cell proliferation in response to Con A or murine GAD65 peptide, a major autoantigen of the pancreatic beta-cell, significantly increased, and the IFN-gamma/IL-10 ratio in the culture supernatant significantly decreased in IRF-1(-/-), suggesting Th2 deviation in cytokine balance. These results indicate that IRF-1 plays a key role in developing insulitis and diabetes in NOD mice.

IRF family of transcription factors as regulators of host defense

Interferon regulatory factors (IRFs) constitute a family of transcription factors that commonly possess a novel helix-turn-helix DNA-binding motif. Following the initial identification of two structurally related members, IRF-1 and IRF-2, seven additional members have now been reported. In addition, virally encoded IRFs, which may interfere with cellular IRFs, have also been identified. Thus far, intensive functional analyses have been done on IRF-1, revealing a remarkable functional diversity of this transcription factor in the regulation of cellular response in host defense. Indeed, IRF-1 selectively modulates different sets of genes, depending on the cell type and/or the nature of cellular stimuli, in order to evoke appropriate responses in each. More recently, much attention has also been focused on other IRF family members. Their functional roles, through interactions with their own or other members of the family of transcription factors, are becoming clearer in the regulation of host defense, such as innate and adaptive immune responses and oncogenesis.

TAP1 down-regulation in primary melanoma lesions: an independent marker of poor prognosis

Melanoma tumor thickness is a major prognostic factor. Thin lesions, however, may metastasize, and sometimes thick tumors may not. To investigate the role of HLA class I-mediated antigen presentation, we correlated the expression of components of the antigen-processing machinery in primary melanoma lesions with their thickness and with the development of metastases. Seventeen formalin-fixed, paraffin-embedded primary melanomas thinner than 0.76 mm and 21 thicker than 1.50 mm were stained with anti-LMP2, -LMP7, -TAP1, -TAP2, -HLA class I and -beta2-microglobulin monoclonal antibodies. Twenty patients remained tumor-free in the follow-up period (10.5 +/- 1.8 years). Eighteen patients relapsed within a median period of 15.0 months following tumor excision. Expression of all markers in the tested lesions was down-regulated, the frequency ranging from about 40% for LMP and TAP subunits to about 70% for HLA class I antigens. Expression of all markers was not correlated with tumor thickness. Only TAP1 and TAP2 down-regulation was significantly (p = 0.026 and 0.042, respectively) correlated with the development of metastases. This correlation was independent of tumor thickness for TAP1. We suggest that TAP1 and probably TAP2 expression in primary lesions represents an independent prognostic marker in melanoma. Abnormalities in antigen presentation may account for the lack of absolute correlation between tumor thickness and prognosis.

Specificity of signaling by STAT1 depends on SH2 and C-terminal domains that regulate Ser727 phosphorylation, differentially affecting specific target gene expression

Complete activation of signal transducer and activator of transcription 1 (STAT1) requires phosphorylation at both Y701 and a conserved PMS(727)P sequence. S727 phosphorylation of STAT1 in interferon-gamma (IFN-gamma)-treated mouse fibroblasts occurred without a need for p38 mitogen-activated protein kinase (MAPK), extracellular signal-regulated kinases 1 and 2 or c-Jun kinases, and required both an intact SH2 domain and phosphorylation of Y701. In contrast, UV irradiation-induced STAT1 phosphorylation on S727 required p38MAPK, but no SH2 domain- phosphotyrosine interactions. Mutation of S727 differentially affected IFN-gamma target genes, at the level of both basal and induced expression. Particularly strong effects were noted for the GBP1 and TAP1 genes. The PMS(727)P motif of STAT3 was phosphorylated by stimuli and signaling pathways different from those for STAT1 S727. Transfer of the STAT3 C-terminus to STAT1 changed the stimulus and pathway specificity of STAT1 S727 phosphorylation to that of STAT3. Our data suggest that STAT C-termini contribute to the specificity of cellular responses by linking individual STATs to different serine kinase pathways and through an intrinsically different requirement for serine phosphorylation at different target gene promoters.

Interferon regulatory factor 7 mediates activation of Tap-2 by Epstein-Barr virus latent membrane protein 1

Transporter associated with antigen processing 2 (Tap-2) is responsible for ATP-dependent transport of peptides from the cytosol to the endoplasmic reticulum, where peptides bind to newly synthesized human leukocyte antigen (HLA) class I molecules, which are essential for cellular immune responses. Epstein-Barr virus (EBV) latent membrane protein 1 (LMP-1) has been shown to induce the expression of Tap-2. In this study, the induction of endogenous Tap-2 by LMP-1 is shown to be associated with and requires the expression of interferon regulatory factor 7 (IRF-7). In DG75 Burkitt's lymphoma (BL) cells, in which LMP-1 induces the expression of IRF-7, LMP-1 induced endogenous Tap-2, and ectopic expression of IRF-7 could enhance the induction. In Akata BL cells, in which LMP-1 could not induce IRF-7, LMP-1 could not induce Tap-2. Addition of IRF-7, which complements the defect in Akata cells, could stimulate the expression of Tap-2. Furthermore, LMP-1 and IRF-7A but not other IRF-7 splicing variants could activate endogenous Tap-2. A Tap-2 promoter reporter construct could be activated by the overexpression of IRF-7A. The activation could be specifically enhanced by LMP-1 and was dependent on an intact interferon-stimulated response element (ISRE) present in the Tap-2 promoter. Also, IRF-7 can bind to the Tap-2 promoter under physiological conditions in vivo, as shown by formaldehyde cross-linking, as well as to the Tap-2 ISRE in vitro, as shown by gel mobility shift assays. Furthermore, LMP-1 facilitates the phosphorylation and nuclear translocation of IRF-7. These data point to the role of IRF-7 as a secondary mediator of LMP-1-activated signal transduction for Tap-2 as follows: LMP-1 stimulates the expression of IRF-7 and facilitates its phosphorylation and nuclear translocation, and then the activated IRF-7 mediates the activation of the cellular Tap-2 gene. The induction of Tap-2 by IRF-7 and LMP-1 may have an important implication for the immune response to EBV and its persistence in vivo.

Molecular biology of NK T cell specificity and development

NK T cells recognize a glycosphingolipid, alpha -galactosyl ceramide, presented by CD1d. CD1d is capable of binding a variety of lipids, however, and the hydrophilic groups of the antigen contribute relatively little to CD1d binding. Amino acids in the CD1d groove and the top of the alpha helices are involved in lipid antigen presentation, suggesting a conventional mode of presentation and antigen recognition. NK T cells also have unique requirements for their differentiation, as suggested by the analysis of a number of mouse germline mutations. For example, the development of NK T cells, unlike conventional T lymphocytes, is highly lymphotoxin dependent.

Identification of a novel cytokine response element in the human IFN regulatory factor-1 gene promoter

The present study investigates the regulatory mechanisms involved in the cooperation between IFN-gamma and TNF-alpha to promote transcription from IFN regulatory factor-1 (IRF-1). A transient transfection analysis revealed that the region between -218 and -144, where +1 is the transcription start site, as well as previously reported downstream elements, ppkappaB and IFN-gamma activation site/kappaB, were required for the optimal response to the two cytokines. A subsequent DNase I footprint analysis showed that the region between -171 and -144 was inducibly protected with stimulation by TNF-alpha, and this protection was significantly enhanced with the combination of IFN-gamma and TNF-alpha. In an EMSA with the protected region as a probe, a TNF-alpha-inducible complex (C1) and an IFN-gamma-inducible complex (C2), but no synergy-specific DNA-protein complexes, were recognized. The C1 complex consisted of a pre-existing factor (p65/p50), whereas the C2 complex consisted of a newly synthesized IRF-1-related factor. A methylation interference assay revealed the critical G residues (from -167 to -151) for the DNA-protein complex formation specific to the cytokine response, and within this region the novel kappaB sequence, the promoter distal kappaB (pdkappaB) element (5'-GGGGAAG TAC-3'), was identified. Because the base substitutions over the pdkappaB region (from -171 to -144) affected not only the TNF-alpha-response but also that of IFN-gamma, this region might contribute to the cooperative action of the NF-kappaB subunits with the IRF-1-related factor. Finally, we demonstrated that none of the cis-acting elements, ppkappaB, pdkappaB, or IFN-gamma activation site/kappaB, is dispensable for the optimal synergism in response to IFN-gamma and TNF-alpha.

Identification of genes specifically expressed in human activated and mature dendritic cells through serial analysis of gene expression

Dendritic cells (DCs) are professional antigen-presenting cells in the immune system and can be generated in vitro from hematopoietic progenitor cells, DC precursors, and monocytes in peripheral blood. Serial analysis of gene expression (SAGE) was conducted in lipopolysaccharide (LPS)-stimulated mature and activated DCs (MADCs) derived from human blood monocytes. A total of 31 837 tag sequences from an MADC cDNA library represented 10 962 different genes, and these data were compared with SAGE data for monocyte-derived immature DCs (IMDCs). Many of the genes, such as germinal center kinase–related protein kinase, cystatin F, interferon (IFN)-α–inducible protein p27, EBI3, HEM45, actin-bundling protein, ELC, DC-LAMP, serine/threonine kinase 4, and several genes in expressed sequence tags, were differentially expressed in MADCs, and those encode proteins related to cell structure, antigen-processing enzymes, chemokines, and IFN-inducible proteins. The profile of MADCs was also compared with that of LPS-stimulated monocytes. The Epstein-Barr virus–induced gene 3 and IFN-α–inducible protein p27 are newly identified to be specifically and highly expressed in MADCs, but not in LPS-stimulated monocytes. The comprehensive identification of specific genes expressed in human IMDCs and MADCs should provide candidate genes to define heterogeneous subsets as well as the function and maturation stage of DCs.

Identification of genes specifically expressed in human activated and mature dendritic cells through serial analysis of gene expression

Dendritic cells (DCs) are professional antigen-presenting cells in the immune system and can be generated in vitro from hematopoietic progenitor cells, DC precursors, and monocytes in peripheral blood. Serial analysis of gene expression (SAGE) was conducted in lipopolysaccharide (LPS)-stimulated mature and activated DCs (MADCs) derived from human blood monocytes. A total of 31 837 tag sequences from an MADC cDNA library represented 10 962 different genes, and these data were compared with SAGE data for monocyte-derived immature DCs (IMDCs). Many of the genes, such as germinal center kinase–related protein kinase, cystatin F, interferon (IFN)-α–inducible protein p27, EBI3, HEM45, actin-bundling protein, ELC, DC-LAMP, serine/threonine kinase 4, and several genes in expressed sequence tags, were differentially expressed in MADCs, and those encode proteins related to cell structure, antigen-processing enzymes, chemokines, and IFN-inducible proteins. The profile of MADCs was also compared with that of LPS-stimulated monocytes. The Epstein-Barr virus–induced gene 3 and IFN-α–inducible protein p27 are newly identified to be specifically and highly expressed in MADCs, but not in LPS-stimulated monocytes. The comprehensive identification of specific genes expressed in human IMDCs and MADCs should provide candidate genes to define heterogeneous subsets as well as the function and maturation stage of DCs.

Synergistic induction of the Tap-1 gene by IFN-gamma and lipopolysaccharide in macrophages is regulated by STAT1

Proper regulation of the Tap-1 gene is critical for the initiation and continuation of a cellular immune response. Analysis of the Tap-1/low molecular mass polypeptide 2 bidirectional promoter showed that the IFN-gamma activation site element is critical for the rapid induction of the promoter by IFN-gamma following transfection into the human macrophage cell line THP-1. Furthermore, activation of STAT1 binding to this site was important for the synergistic response seen following the stimulation with both IFN-gamma and LPS. Mutation of an IFN-stimulated regulatory element that binds IFN regulatory factor 1 appeared to enhance the response to IFN-gamma and LPS. These data show that STAT1 is necessary for the activation of Tap-1 gene expression in APCs and initiation of cellular immune responses. Furthermore, our data suggest that bacterial products such as LPS may enhance cellular immune responses through augmenting the ability of STAT1 to regulate IFN-gamma-inducible genes.

How Stat1 mediates constitutive gene expression: a complex of unphosphorylated Stat1 and IRF1 supports transcription of the LMP2 gene

Analysis of mRNA levels in cells that express or lack signal transducers and activators of transcription 1 (Stat1) reveals that Stat1 mediates the constitutive transcription of many genes. Expression of the low molecular mass polypeptide 2 (LMP2), which requires Stat1, has been studied in detail. The overlapping interferon consensus sequence 2/gamma-interferon-activated sequence (ICS-2/GAS) elements in the LMP2 promoter bind to interferon regulatory factor 1 (IRF1) and Stat1 and are occupied constitutively in vivo. The point mutant of Stat1, Y701F, which does not form dimers involving SH2-phosphotyrosine interactions, binds to the GAS element and supports LMP2 expression. Unphosphorylated Stat1 binds to IRF1 directly and we conclude that this complex uses the ICS-2/GAS element to mediate constitutive LMP2 transcription in vivo. The promoter of the IRF1 gene, which also contains a GAS site but not an adjacent ICS-2 site, is not activated by Stat1 Y701F. The promoters of other genes whose constitutive expression requires Stat1 may also utilize complexes of unphosphorylated Stat1 with IRF1 or other transcription factors.

Adenovirus E1A down-regulates LMP2 transcription by interfering with the binding of stat1 to IRF1

The LMP2 gene, which encodes a protein required for efficient presentation of viral antigens, requires both unphosphorylated Stat1 and IRF1 for basal expression. LMP2 expression is down-regulated by the adenovirus protein E1A, which binds to Stat1 and CBP/p300, and by the mutant E1A protein RG2, which binds to Stat1 but not to CBP/p300, but not by the mutant protein Delta2-36, which does not bind to either Stat1 or CBP/p300. Stat1 and IRF1 associate in untreated cells and bind as a complex to the overlapping ICS-2/GAS element of the LMP2 promoter. E1A interferes with the formation of this complex by occupying domains of Stat1 that bind to IRF1. These results reveal how adenovirus infection attenuates LMP2 expression, thereby interfering with the presentation of viral antigens.

IFN regulatory factor-1-mediated transcriptional activation of mouse STAT-induced STAT inhibitor-1 gene promoter by IFN-gamma

STAT-induced STAT inhibitor-1 (SSI-1), also referred to as suppressor of cytokine signaling-1 and JAK-binding protein, is a member of a new family, the members of which are negative regulators of cytokine signals. SSI-1 is induced by various cytokines; however, the transcriptional mechanism of the SSI-1 gene is not fully understood. Here, we showed that transcription of the mouse SSI-1 gene was initiated from six adjoining sites accompanying three GC boxes and a single GC box-like element near them, but not from the TATA box or an initiator sequence. We also showed that IFN-gamma induced SSI-1 mRNA more strongly than IL-6 in NIH-3T3 fibroblasts and that this IFN-gamma effect was mediated by Stat1. To determine the signal pathway downstream of Stat1, transcriptional activities of several mutant promoters were examined. The region mediating stimulatory effect of IFN-gamma to the gene transcription was localized to the -88/-60 region containing three tandem GAAA units, named variant IFN-gamma-responsive element (VIRE), while four IFN-gamma activation site (GAS)-like elements located far upstream were not related to the IFN-gamma response. Gel-shift assays revealed that IFN-gamma induced IFN regulatory factor-1 (IRF-1) binding to VIRE, but not that of IRF-2 or three components of ISGF3. Furthermore, forced expression of IRF-1 mimicked and that of IRF-2 inhibited the stimulatory effect of IFN-gamma on SSI-1 gene transcription. Finally, mouse embryonal fibroblasts lacking IRF-1 showed impaired SSI-1 mRNA induction by IFN-gamma. These results demonstrated that IRF-1, which is induced by activation of Stat1, mediated transcriptional activation of the SSI-1 gene by IFN-gamma via VIRE.

Complex roles of Stat1 in regulating gene expression

Stat1 is a fascinating and complex protein with multiple, yet contrasting transcriptional functions. Upon activation, it drives the expression of many genes but also suppresses the transcription of others. These opposing characteristics also apply to its role in facilitating crosstalk between signal transduction pathways, as it participates in both synergistic activation and inhibition of gene expression. Stat1 is a functional transcription factor even in the absence of inducer-mediated activation, participating in the constitutive expression of some genes. This review summarizes the well studied involvement of Stat1 in IFN-dependent and growth factor-dependent signaling and then describes the roles of Stat1 in positive, negative and constitutive regulation of gene expression as well as its participation in crosstalk between signal transduction pathways. Oncogene (2000).

Immune-inflammatory mechanisms in IFNgamma-mediated anti-tumor activity

IFNgamma is a functionally pleiotropic cytokine which shows considerable potency in promoting anti-tumor functions in vivo. Despite limited efficacy when delivered systemically either to experimental animals or patients, IFNgamma appears to play an important and perhaps critical role in directing the development of immune-mediated tumor destruction when expressed within the tumor bed. This has been demonstrated both by use of tumor cells transduced to express IFNgamma and by the use of IL-12 which is able, at least is murine models, to promote an IFNgamma-dependent, T cell mediated anti-tumor response. Recent studies indicate that the therapeutic efficacy of IFNgamma in tumor models depends critically upon the ability of the tumor cells themselves to respond to IFNgamma. Though IFNgamma is able to induce anti-viral activity and has direct anti-proliferative effects on some tumor cell lines, immunomodulatory function also appears to be an important component of its anti-tumor action. This is mediated through the action of several different classes of IFNgamma-inducible gene expression which control antigen processing and presentation, leukocyte trafficking, and indirect tumor cytotoxicity.