ICSBP/IRF-8 transactivation: a tale of protein-protein interaction

IRF8-mutant B cell lymphoma evades immunity through a CD74-dependent deregulation of antigen processing and presentation in MHCII complexes

The mechanism by which interferon regulatory factor 8 (IRF8) mutation contributes to lymphomagenesis is unknown. We modeled IRF8 variants in B cell lymphomas and found that they affected the expression of regulators of antigen presentation. Expression of IRF8 mutants in murine B cell lymphomas suppressed CD4, but not CD8, activation elicited by antigen presentation and downmodulated CD74 and human leukocyte antigen (HLA) DM, intracellular regulators of antigen peptide processing/loading in the major histocompatibility complex (MHC) II. Concordantly, mutant IRF8 bound less efficiently to the promoters of these genes. Mice harboring IRF8 mutant lymphomas displayed higher tumor burden and remodeling of the tumor microenvironment, typified by depletion of CD4, CD8, and natural killer cells, increase in regulatory T cells and T follicular helper cells. Deconvolution of bulk RNA sequencing data from IRF8-mutant human diffuse large B cell lymphoma (DLBCL) recapitulated part of the immune remodeling detected in mice. We concluded that IRF8 mutations contribute to DLBCL biology by facilitating immune escape.

IRF8-mutant B cell lymphoma evades immunity through a CD74-dependent deregulation of antigen processing and presentation in MHC CII complexes

In diffuse large B-cell lymphoma (DLBCL), the transcription factor IRF8 is the target of a series of potentially oncogenic events, including, chromosomal translocation, focal amplification, and super-enhancer perturbations. IRF8 is also frequently mutant in DLBCL, but how these variants contribute to lymphomagenesis is unknown. We modeled IRF8 mutations in DLBCL and found that they did not meaningfully impact cell fitness. Instead, IRF8 mutants, mapping either to the DNA-binding domain (DBD) or c-terminal tail, displayed diminished transcription activity towards CIITA, a direct IRF8 target. In primary DLBCL, IRF8 mutations were mutually exclusive with mutations in genes involved in antigen presentation. Concordantly, expression of IRF8 mutants in murine B cell lymphomas uniformly suppressed CD4, but not CD8, activation elicited by antigen presentation. Unexpectedly, IRF8 mutation did not modify MHC CII expression on the cell surface, rather it downmodulated CD74 and HLA- DM, intracellular regulators of antigen peptide processing/loading in the MHC CII complex. These changes were functionally relevant as, in comparison to IRF8 WT, mice harboring IRF8 mutant lymphomas displayed a significantly higher tumor burden, in association with a substantial remodeling of the tumor microenvironment (TME), typified by depletion of CD4, CD8, Th1 and NK cells, and increase in T-regs and Tfh cells. Importantly, the clinical and immune phenotypes of IRF8-mutant lymphomas were rescued in vivo by ectopic expression of CD74. Deconvolution of bulk RNAseq data from primary human DLBCL recapitulated part of the immune remodeling detected in mice and pointed to depletion of dendritic cells as another feature of IRF8 mutant TME. We concluded that IRF8 mutations contribute to DLBCL biology by facilitating immune escape.

Mechanism of transforming growth factor-1 induce renal fibrosis based on transcriptome sequencing analysis

OBJECTIVES

To explore the mechanism of transforming growth factor-β1 (TGF-β1) induce renal fibrosis.

METHODS

Renal fibroblast NRK-49F cells treated with and without TGF-β1 were subjected to RNA-seq analysis. DESeq2 was used for analysis. Differentially expressed genes were screened with the criteria of false discovery rate<0.05 and l o g 2 F C >1. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analyses were performed for differentially expressed genes. Genes encoding transcription factors were further screened for differential expression genes. Then, the expression of these genes during renal fibrosis was verified using unilateral ureteral obstruction (UUO)-induced mouse renal fibrosis model and a public gene expression dataset (GSE104954).

RESULTS

After TGF-β1 treatment for 6, 12 and 24 h, 552, 1209 and 1028 differentially expressed genes were identified, respectively. GO analysis indicated that these genes were significantly enriched in development, cell death, and cell migration. KEGG pathway analysis showed that in the early stage of TGF-β1 induction (TGF-β1 treatment for 6 h), the changes in Hippo, TGF-β and Wnt signaling pathways were observed, while in the late stage of TGF-β1 induction (TGF-β1 treatment for 24 h), the changes of extracellular matrix-receptor interaction, focal adhesion and adherens junction were mainly enriched. Among the 291 up-regulated differentially expressed genes treated with TGF-β1 for 6 h, 13 genes (Snai1, Irf8, Bhlhe40, Junb, Arid5a, Vdr, Lef1, Ahr, Foxo1, Myc, Tcf7, Foxc2, Glis1) encoded transcription factors. Validation in a cell model showed that TGF-β1 induced expression of 9 transcription factors (encoded by Snai1, Irf8, Bhlhe40, Junb, Arid5a, Vdr, Lef1, Myc, Tcf7), while the expression levels of the other 4 genes did not significantly change after TGF-β1 treatment. Validation results in UUO-induced mouse renal fibrosis model showed that Snai1, Irf8, Bhlhe40, Junb, Arid5a, Myc and Tcf7 were up-regulated after UUO, Vdr was down-regulated and there was no significant change in Lef1. Validation based on the GSE104954 dataset showed that IRF8 was significantly overexpressed in the renal tubulointerstitium of patients with diabetic nephropathy or IgA nephropathy, MYC was highly expressed in diabetic nephropathy, and the expressions of the other 7 genes were not significantly different compared with the control group.

CONCLUSIONS

TGF-β1 induces differentially expressed genes in renal fibroblasts, among which Irf8 and Myc were identified as potential targets of chronic kidney disease and renal fibrosis.

Polyphenols extracted from Shanxi-aged vinegar exert hypolipidemic effects on OA-induced HepG2 cells via the PPARα-LXRα-ABCA1 pathway

Hyperlipidemia is one of the key risk factors causing many chronic diseases, and lowering blood lipid levels can prevent many diseases. In this paper, a hyperlipidemic cell model of oleic acid (OA) induced hepatocellular carcinoma cells (HepG2) was established using polyphenols extracted from Shanxi-aged vinegar (SAVEP). The effects of SAVEP on nuclear damage, mitochondrial membrane potential, apoptosis, cellular lipid deposition, and lipid metabolism protein expression in HepG2 hyperlipidemic cells were examined to investigate the lipid-lowering mechanism of SAVEP at the cellular level. The results showed that SAVEP could reduce the content of TC/TG index, repair the nuclear damage, reduce lipid accumulation and finally decrease the rate of apoptosis by up-regulating the expression of key proteins such as PPARα, LXRα, and ABCA1 in the process of lipid metabolism. PRACTICAL APPLICATIONS: In this thesis, the hypolipidemic activity of polyphenol extracts from Shanxi-aged vinegar was analyzed on the level of HepG2 cells. The hypolipidemic mechanism of oxidative stress, lipid metabolism and inflammatory stress was also elucidated. It provided a theoretical basis for the in-depth understanding of the hypolipidemic health effects of Shanxi-aged vinegar.

Loss of a Negative Feedback Loop between IRF8 and AR Promotes Prostate Cancer Growth and Enzalutamide Resistance

In incurable castration-resistant prostate cancer (CRPC), resistance to the novel androgen receptor (AR) antagonist enzalutamide is driven mainly by AR overexpression. Here we report that the expression of interferon regulatory factor 8 (IRF8) is increased in primary prostate cancer but decreased in CRPC compared with normal prostate tissue. Decreased expression of IRF8 positively associated with CRPC progression and enzalutamide resistance. IRF8 interacted with AR and promoted its degradation via activation of the ubiquitin/proteasome systems. Epigenetic knockdown of IRF8 promoted AR-mediated prostate cancer progression and enzalutamide resistance in vitro and in vivo. Furthermore, IFNα increased expression of IRF8 and improved the efficacy of enzalutamide in CRPC by targeting the IRF8-AR axis. We also provide preliminary evidence for the efficacy of IFNα with hormonotherapy in a clinical study. Collectively, this study identifies IRF8 both as a tumor suppressor in prostate cancer pathogenesis and a potential alternative therapeutic option to overcome enzalutamide resistance. SIGNIFICANCE: These findings identify IRF8-mediated AR degradation as a mechanism of resistance to AR-targeted therapy, highlighting the therapeutic potential of IFNα in targeting IRF8-AR axis in CRPC. GRAPHICAL ABSTRACT: http://cancerres.aacrjournals.org/content/canres/80/13/2927/F1.large.jpg.

Expression, subcellular localization, and potential antiviral function of three interferon regulatory factors in the big-belly seahorse (Hippocampus abdominalis)

Interferon regulatory factors (IRFs) are among the most important transcription mediators and have multiple biological functions, such as antiviral and antimicrobial defense, cell differentiation, immune modulation, and apoptosis. Three IRF family members (HaIRF4-like, HaIRF6, and HaIRF8) of the big belly seahorse (Hippocampus abdominalis) were molecularly and functionally characterized at the sequence and transcriptional level. The coding sequences of HaIRF4-like, HaIRF6, and HaIRF8 were 1214, 1485, and 1266 bp in length, encoding proteins of size 46.21, 55.32, and 47.56 kDa, respectively. Potential viral transcription and replication was detected against VHSV infection using qPCR in HaIRFs-transfected FHM cells. IRFs significantly reduced viral gene expression at 24 h and 48 h post infection and the expression of interferon-stimulated genes (ISGs) was modulated at transcriptional level upon HaIRF overexpression in FHM cells. Subcellular HaIRF localization was observed using GFP-tagged expression vectors in FHM cells. HaIRF4-like and HaIRF8 were localized to the nucleus, whereas HaIRF6 was observed in the cytoplasm. All three IRFs were ubiquitously expressed in all analyzed tissues of the big belly seahorse. The mRNA expression of IRF4-like, IRF6, and IRF8 increased significantly post injection in the blood and gills following LPS, poly (I:C), and Streptococcus iniae challenge. These findings demonstrate that seahorse IRFs are involved in host defense mechanisms against immune stimulants and HaIRFs induce interferon and ISGs which trigger antiviral activity against viral infections in the host.

Functional characterization of IRF8 regulation of type II IFN in golden pompano (Trachinotus ovatus)

Interferon regulatory factor 8 (IRF8) increases type I IFN transcription levels by binding to IFN promoters, thereby playing a role in innate immunity. Nevertheless, the detailed mechanism through which IRF8 regulates type II IFN in fish remains ambiguous. In the present study, two genes from the golden pompano (Trachinotus ovatus), IRF8 (ToIRF8) and IFN gamma (ToIFNγ), were identified in the IFN/IRF-based signalling pathway. The full-length ToIRF8 cDNA was composed of 2,141 bp and encoded a 421 amino acid polypeptide; the genomic DNA was 2,917 bp in length and consisted of 8 exons and 7 introns. The putative protein showed the highest sequence identity (90-92%) with fish IRF8 and possessed a DNA-binding domain (DBD), an IRF-association domain (IAD) and a nuclear localization signal (NLS) motif consistent with those of IRF8 in other vertebrates. Furthermore, the ToIRF8 transcripts were expressed in all examined tissues of healthy fish, with higher levels observed in the central nervous and immune relevant tissues. They were upregulated by polyinosinic acid: polycytidylic acid [poly (I: C)], lipopolysaccharide (LPS) and flagellin treatments in the blood, liver, intestine and kidney. The results from assays of subcellular localization showed that ToIRF8 was localized to the cytoplasm. Moreover, to investigate whether ToIRF8 was a regulator of ToIFNγ, a promoter analysis was performed using progressive deletion mutations of ToIFNγ. The results indicated that the region from -601 bp to -468 bp includes the core promoter. Mutation analyses indicated that the activity of the ToIFNγ promoter significantly decreased after the targeted mutation of the M1-M3 binding sites. Additionally, overexpressed ToIRF8 in vitro notably increased the expression of several IFN/IRF-based signalling pathway genes. These results suggest that IRF8 is vital in the defence of T. ovatus against bacterial infection and contributes to a better understanding of the transcriptional mechanisms of ToIRF8 on type II IFN in fish.

Interferonregulatoryfactor-8(IRF-8) regulates the expression of matrix metalloproteinase-13 (MMP-13) in chondrocytes

Low levels of inflammation-induced expression of matrix metalloproteinase (MMP) play a crucial role in articular cartilage matrix destruction in osteoarthritis (OA) patients. Interferon regulatory factor-8 (IRF-8), an important member in the IRF family, plays a key role in regulating the inflammation-related signaling pathway. The aim of this study is to investigate the physiological roles of IRF-8 in the pathological progression of OA. We found that IRF-8 was expressed in human primary chondrocytes. Interestingly, the expression of IRF-8 was upregulated in OA chondrocytes. In addition, IRF-8 was increased in response to interleukin-1β (IL-1β) treatment, mediated by the Janus kinase 2 (JAK2) pathway. Overexpression of IRF-8 in human chondrocytes by transduction with lentiviral-IRF-8 exacerbated IL-1β-induced expression of matrix metalloproteinase-13 (MMP-13) in human chondrocytes. In contrast, knockdown of IRF-8 inhibited IL-1β-induced expression of MMP-13. Importantly, IRF-8 could bind to the promoter of MMP-13 and stimulate its activity. Additionally, overexpression of IRF-8 exacerbated IL-1β-induced degradation of type II collagen. However, silencing IRF-8 abrogated the degradation of type II collagen. Taken together, our findings identified a novel function of IRF-8 in regulating articular cartilage matrix destruction by promoting the expression of MMP-13.

Relevance of Interferon Regulatory Factor-8 Expression in Myeloid-Tumor Interactions

Perturbations in myelopoiesis are a common feature in solid tumor biology, reflecting the central premise that cancer is not only a localized affliction but also a systemic disease. Because the myeloid compartment is essential for the induction of adaptive immunity, these alterations in myeloid development contribute to the failure of the host to effectively manage tumor progression. These "dysfunctional" myeloid cells have been coined myeloid-derived suppressor cells (MDSCs). Interestingly, such cells not only arise in neoplasia but also are associated with many other inflammatory or pathologic conditions. MDSCs affect disease outcome through multiple mechanisms, including their ability to mediate generalized or antigen-specific immune suppression. Consequently, MDSCs pose a significant barrier to effective immunotherapy in multiple disease settings. Although much interest has been devoted to unraveling mechanisms by which MDSCs mediate immune suppression, a large gap has remained in our understanding of the mechanisms that drive their development in the first place. Investigations into this question have identified an unrecognized role of interferon regulatory factor-8 (IRF-8), a member of the IRF family of transcription factors, in tumor-induced myeloid dysfunction. Ordinarily, IRF-8 is involved in diverse stages of myelopoiesis, namely differentiation and lineage commitment toward monocytes, dendritic cells, and granulocytes. Several recent studies now support the hypothesis that IRF-8 functions as a "master" negative regulator of MDSC formation in vivo. This review focuses on IRF-8 as a potential target suppressed by tumors to cripple normal myelopoiesis, redirecting myeloid differentiation toward the emergence of MDSCs. Understanding the bases by which neoplasia drives MDSC accumulation has the potential to improve the efficacy of therapies that require a competent myeloid compartment.

Transcriptional regulation of lipopolysaccharide (LPS)-induced Toll-like receptor (TLR) expression in murine macrophages: role of interferon regulatory factors 1 (IRF-1) and 2 (IRF-2)

Activation of TLRs is most closely associated with induction of pro-inflammatory gene expression; however, expression of many other genes, including the TLR genes themselves, has also been shown to be modulated following TLR engagement. A large family of nuclear transcription factors, the interferon regulatory factors (IRFs), have been implicated in TLR signaling leading to pro-inflammatory gene expression. Given that IRF-1 and IRF-2 counter-regulate the transcriptional activity of many genes, we hypothesized that IRF-1 and IRF-2 might also regulate TLR gene expression following LPS stimulation of murine macrophages. mRNA derived from medium- or LPS-treated primary peritoneal macrophages was analyzed for TLR gene expression using quantitative real-time PCR. In wild-type macrophages, LPS up-regulated expression of TLRs 1—3 and 6—9 steady-state mRNA, while TLR4 mRNA was modestly downregulated. IRF-2—/ — macrophages responded to LPS with dysregulated expression of TLR3, TLR4, and TLR5 mRNA, whereas IRF-1 deficiency dampened LPS-induced mRNA expression for TLR3, TLR6, and TLR9. Functional studies revealed aberrant TLR3 signaling in IRF-2—/ — macrophages. Collectively, these findings reveal an additional level of complexity associated with TLR transcriptional regulation and suggest that the trans-acting factors, IRF-1 and IRF-2, contribute to the innate immune response to infections by regulating TLR gene expression.

IRF5 and IRF8 modulate the CAL-1 human plasmacytoid dendritic cell line response following TLR9 ligation

Synthetic oligonucleotides (ODNs) containing CpG motifs stimulate human plasmacytoid dendritic cells (pDCs) to produce type-1 interferons (IFNs) and proinflammatory cytokines. Previous studies demonstrated that interferon regulatory factors (IRFs) play a central role in mediating CpG-induced pDC activation. This work explores the inverse effects of IRF5 and IRF8 (also known as IFN consensus sequence-binding protein) on CpG-dependent gene expression in the human CAL-1 pDC cell line. This cell line shares many of the phenotypic and functional properties of freshly isolated human pDCs. Results from RNA interference and microarray studies indicate that IRF5 upregulates TLR9-driven gene expression whereas IRF8 downregulates the same genes. Several findings support the conclusion that IRF8 inhibits TLR9-dependent gene expression by directly blocking the activity of IRF5. First, the inhibitory activity of IRF8 is only observed when IRF5 is present. Second, proximity ligation analysis shows that IRF8 and IRF5 colocalize within the cytoplasm of resting human pDCs and cotranslocate to the nucleus after CpG stimulation. Taken together, these findings suggest that IRF5 and IRF8, two transcription factors with opposing functions, control TLR9 signaling in human pDCs.

Structural Studies of IRF4 Reveal a Flexible Autoinhibitory Region and a Compact Linker Domain

IRF4 is a unique member of the interferon regulatory factor (IRF) family playing critical regulatory roles in immune cell development, regulation of obesity-induced inflammation, and control of thermogenic gene expression. The ability of IRF4 to control diverse transcriptional programs arises from its proficiency to interact with numerous transcriptional partners. In this study, we present the structural characterization of full-length IRF4. Using a combination of x-ray and small angle x-ray scattering studies, we reveal unique features of the interferon activation domain, including a set of β-sheets and loops that serve as the binding site for PU.1, and also show that unlike other IRF members, IRF4 has a flexible autoinhibitory region. In addition, we have determined the small angle x-ray scattering solution structure of full-length IRF4, which, together with circular dichroism studies, suggests that the linker region is not extended but folds into a domain structure.

Structural and expression studies of interferon regulatory factor 8 in Japanese flounder, Paralichthys olivaceus

Interferon regulatory factor 8 (IRF8) plays a role in both innate and adaptive systems in mammals. In this study, the gene and promoter sequences of Japanese flounder, Paralichthys olivaceus, (Po) IRF8 were cloned, and its expression in response to polyinosinic:polycytidylic acid (poly I:C) and lymphocystis disease virus (LCDV) challenges was studied in vivo. The PoIRF8 gene spans over 3.3 kb with a structure of 9 exon-8 intron and encodes 420 amino acids. The putative protein shows the highest sequence identity (69.5-89.0%) to fish IRF8 and possesses a DNA-binding domain (DBD), an IRF-association domain (IAD) and a nuclear localization signal (NLS) of vertebrate IRF8. Phylogenetic analysis classified PoIRF8 into the cluster of fish IRF8 within vertebrate IRF8 group of IRF4 subfamily. A number of transcription factor binding sites were identified in the 2348-bp 5' flanking region of PoIRF8 gene, including those of transcription factors for type Ⅰ and type Ⅱ interferon (IFN) inducible genes and genes regulating the development and function of lymphomyeloid cells in mammals. The PoIRF8 transcripts were expressed in all examined tissues of healthy flounders, with higher levels observed in the immune relevant tissues. They were up-regulated by both poly I:C and LCDV treatments in the spleen, head kidney, gills and muscle in an early phase of immune responses, with initiation and peak time points of induction prior to type Ⅰ IFN and Mx. Relative to LCDV, the induction by poly I:C was quicker in all four tissues. These results indicate an involvement of PoIRF8 in the host's antiviral responses and a functional conservation of IRF8 between fish and mammals.

IRF-5 and NF-κB p50 co-regulate IFN-β and IL-6 expression in TLR9-stimulated human plasmacytoid dendritic cells

Synthetic oligonucleotides (ODN) expressing CpG motifs mimic the ability of bacterial DNA to trigger the innate immune system via TLR9. Plasmacytoid dendritic cells (pDCs) make a critical contribution to the ensuing immune response. This work examines the induction of antiviral (IFN-β) and pro-inflammatory (IL-6) cytokines by CpG-stimulated human pDCs and the human CAL-1 pDC cell line. Results show that interferon regulatory factor-5 (IRF-5) and NF-κB p50 are key co-regulators of IFN-β and IL-6 expression following TLR9-mediated activation of human pDCs. The nuclear accumulation of IRF-1 was also observed, but this was a late event that was dependant on type 1 IFN and unrelated to the initiation of gene expression. IRF-8 was identified as a novel negative regulator of gene activation in CpG-stimulated pDCs. As variants of IRF-5 and IRF-8 were recently found to correlate with susceptibility to certain autoimmune diseases, these findings are relevant to our understanding of the pharmacologic effects of "K" ODN and the role of TLR9 ligation under physiologic, pathologic, and therapeutic conditions.

IFN regulatory factor 8 is a key constitutive determinant of the morphological and molecular properties of microglia in the CNS

IFN regulatory factor (IRF) 8 is a transcription factor that has a key role in the cellular response to IFN-γ and is pivotal in myeloid cell differentiation. Whether IRF8 plays a role in the development and function of microglia, the tissue-resident myeloid cells of the brain, is unknown. Here, we show IRF8 is a constitutively produced nuclear factor in microglia, which suggested that IRF8 might also be a key homeostatic transcriptional determinant of the microglial cell phenotype. In support of this, in mice with a targeted disruption of the IRF8 gene, microglia were increased in number and showed gross alterations in morphology and surface area. In situ analysis of some key myeloid markers revealed that IRF8-deficient microglia had significantly reduced levels of Iba1, but increased levels of CD206 (mannose receptor) and F4/80 as well as increased tomato lectin binding. Analysis of microglia ex vivo revealed IRF8-deficient microglia had significantly increased levels of CD45, CD11b and F4/80, but significantly decreased levels of the chemokine receptors CCR2, CCR5 and CX3CR1. The known involvement of some of these molecular markers in membrane dynamics and phagocytosis led us to examine the phagocytic capacity of cultured IRF8-deficient microglia, however, this was found to be similar to wild type microglia. We conclude IRF8 is a constitutively produced nuclear factor in resident microglia of the CNS being a crucial transcriptional determinant of the phenotype of these cells in the healthy brain.

Genetics of myasthenia gravis: a case-control association study in the Hellenic population

Myasthenia gravis (MG) is an heterogeneous autoimmune disease characterized by the production of autoantibodies against proteins of the postsynaptic membrane, in the neuromuscular junction. The contribution of genetic factors to MG susceptibility has been evaluated through family and twin studies however, the precise genetic background of the disease remains elusive. We conducted a case-control association study in 101 unrelated MG patients of Hellenic origin and 101 healthy volunteers in order to assess the involvement of common genetic variants in susceptibility to MG. We focused on three candidate genes which have been clearly associated with several autoimmune diseases, aiming to investigate their potential implication in MG pathogenesis. These are interferon regulatory factor 5 (IRF-5), TNFα-induced protein 3 (TNFAIP3), also known as A20, and interleukin-10 (IL-10), key molecules in the regulation of immune function. A statistical trend of association (P = 0.068) between IL-10 promoter single nucleotide polymorphisms (SNPs) and the subgroups of early and late-onset MG patients was revealed. No statistically significant differences were observed in the rest of the variants examined. As far as we are aware, this is the first worldwide attempt to address the possible association between IRF-5 and TNFAIP3 common genetic variants and the genetic basis of MG.

IRF8 regulates acid ceramidase expression to mediate apoptosis and suppresses myelogeneous leukemia

IFN regulatory factor 8 (IRF8) is a key transcription factor for myeloid cell differentiation and its expression is frequently lost in hematopoietic cells of human myeloid leukemia patients. IRF8-deficient mice exhibit uncontrolled clonal expansion of undifferentiated myeloid cells that can progress to a fatal blast crisis, thereby resembling human chronic myelogeneous leukemia (CML). Therefore, IRF8 is a myeloid leukemia suppressor. Whereas the understanding of IRF8 function in CML has recently improved, the molecular mechanisms underlying IRF8 function in CML are still largely unknown. In this study, we identified acid ceramidase (A-CDase) as a general transcription target of IRF8. We demonstrated that IRF8 expression is regulated by IRF8 promoter DNA methylation in myeloid leukemia cells. Restoration of IRF8 expression repressed A-CDase expression, resulting in C16 ceramide accumulation and increased sensitivity of CML cells to FasL-induced apoptosis. In myeloid cells derived from IRF8-deficient mice, A-CDase protein level was dramatically increased. Furthermore, we demonstrated that IRF8 directly binds to the A-CDase promoter. At the functional level, inhibition of A-CDase activity, silencing A-CDase expression, or application of exogenous C16 ceramide sensitized CML cells to FasL-induced apoptosis, whereas overexpression of A-CDase decreased CML cells' sensitivity to FasL-induced apoptosis. Consequently, restoration of IRF8 expression suppressed CML development in vivo at least partially through a Fas-dependent mechanism. In summary, our findings determine the mechanism of IRF8 downregulation in CML cells and they determine a primary pathway of resistance to Fas-mediated apoptosis and disease progression.

PML is a key component for the differentiation of myeloid progenitor cells to macrophages

IFN regulatory factor-8 (IRF-8, previously known as ICSBP) is a key transcription factor driving the differentiation of granulocyte\monocyte progenitor (GMP) cells toward monocyte\macrophage lineage. The promyelocytic leukemia (PML) gene is an immediate target gene regulated by IRF-8 in response to IFN-γ activation. PML is a multifunctional protein that has many isoforms serving as the scaffold components for nuclear bodies (NBs) engaged in numerous proteins interactions. The role of PML in the retinoic acid pathway that drives GMPs to granulopoiesis is documented in the literature. Here, we show that PML is also involved in monopoiesis by mediating some of the IRF-8 activities during the differentiation of murine-derived bone marrow macrophages (BMMs). PML silencing resulted in altered expression level of key transcription factors essential for monopoiesis that was accompanied by silencing of typical myeloid-specific genes. Interestingly, this altered expression resembled that of the GMPs and that of BMMs derived from IRF-8(-/-) mice altogether supporting the role of PML in monopoiesis. Further, PML silencing led to reduced colony-forming capacity of bone marrow cells highlighting the dual function of PML in myelopoiesis. Last, PML overexpression only partially rescued the phenotype of IRF-8(-/-) BMMs. Together, our data show that PML is an important factor for monopoiesis and not solely for granulopoiesis. This suggests that PML-NBs respond to an incoming signal that affects the fate of GMP driving cell differentiation to granulocytes or monocytes.

Exon 6 variants carried on systemic lupus erythematosus (SLE) risk haplotypes modulate IRF5 function

Interferon regulatory factor 5 (IRF5) regulates innate immune responses to viral infection. IRF5 genetic variants have been shown to be strongly associated with risk for systemic lupus erythematosus (SLE). Functional roles of IRF5 exon 6 structural variants that occur as part of a SLE risk-associated haplotype, including a 30-bp in/del (in/del-10) and a 48-bp splice-site variant (SV-16), have not been established. In this study, we used IRF5-deficient cells overexpressing human IRF5 (hIRF5) variants to investigate the roles of exon 6 in/del-10 and SV-16 in regulation of the apoptosis response, nuclear translocation, and ability to transactivate IRF5 responsive cytokines. We found that expression of IRF5 isoforms including either SV-16 or in/del-10 confers ability of IRF5 to impair the apoptotic response and correlates with reduced capacity for IRF5 nuclear translocation in MEFs after a DNA-damaging stimulus treatment. Interestingly, the presence or absence of both SV-16 and in/del-10 results in abrogation of both the anti-apoptotic and enhanced nuclear translocation effects of IRF5 expression. Only cells expressing IRF5 bearing SV-16 show increased IL-6 production upon lipopolysaccharide stimulation. MEFs expressing hIRF5 variants containing in/del-10 showed no significant difference from the control; however, cells carrying hIRF5 lacking both SV-16 and in/del-10 showed reduced IL-6 production. Our overall findings suggest that exon 6 SV-16 is more potent than in/del-10 for IRF5-driven resistance to apoptosis and promotion of cytokine production; however, in/del-10 co-expression can neutralize these effects of SV-16.

Activation of IL-27 p28 gene transcription by interferon regulatory factor 8 in cooperation with interferon regulatory factor 1

Interferon regulatory factor (IRF) family members, especially interferon regulatory factor-1 (IRF-1) and interferon regulatory factor-8 (IRF-8 or ICSBP), play important roles in interferon signaling in a wide range of host responses to infection and tumor growth. Interleukin-27 (IL-27), as a member of the IL-12 cytokine family, not only acts as a proinflammatory cytokine that regulates the differentiation of naive T helper cells but also possesses anti-inflammatory properties. IL-27 consists of EBI3 (Epstein-Barr virus-induced gene 3) and p28 subunits. Our previous work has shown that IRF-1 regulates IL-27 p28 gene transcription by specifically binding to the IRF-1 response element in the p28 promoter. In this study, we found that IRF-8-deficient macrophages were highly defective in the production of IL-27 p28 at both mRNA and protein levels. Circulating IL-27 p28 in serum was also decreased in IRF-8(-/-) mice in a septic shock model. Lipopolysaccharide, as a potent inducer of IL-27 p28 expression, could activate IRF-8 expression in a MyD88-dependent pathway, which in turn induced p28 gene transcription through NF-kappaB and/or IRF-8. Transcriptional analyses revealed that IRF-8 activated p28 gene transcription through binding to a site located at -57 to -48 in the p28 promoter overlapping the IRF-1 binding site. Consistent with this observation, overexpression of both IRF-8 and IRF-1 additively activated IL-27 p28 promoter. This study provides further mechanistic information regarding how signals initiated during innate and adaptive immune responses synergize to yield greater IL-27 production and sustained cellular immunity.

Disseminated and rapidly fatal tuberculosis in mice bearing a defective allele at IFN regulatory factor 8

The interferon regulatory factor (IRF) family member IRF-8 participates in IFN-gamma-dependent transcriptional activation of genes containing in their promoter regions IFN-stimulated response element or IFN-gamma activation site elements. To test the role of IRF-8 in host defenses against tuberculosis, BXH-2 mice, which bear a defective IRF-8(R294C) allele, were challenged with low doses of virulent Mycobacterium tuberculosis via the i.v. and aerosol routes. BXH-2 mice were found to be extremely susceptible to M. tuberculosis, as demonstrated by rapid and uncontrolled microbial replication in spleen, liver, and lungs leading to very early death. The BXH-2 defect was expressed very early (10 days postinfection) as uncontrolled intracellular pathogen replication in NOS2-expressing lung macrophages, impaired granuloma formation, rapid dissemination of the infection to distant sites, and rapid necrosis of infected tissues. There was complete absence of IL-12p40 induction, severely reduced IFN-gamma production, and impaired T cell priming in the lungs of infected BXH-2, highlighting the critical role of IRF-8 in this process. Collectively, these results identify IRF-8 as a critical regulator of host defenses against tuberculosis.

Interferon regulatory factor 6 (IRF6) is expressed in the ovine uterus and functions as a transcriptional activator

Interferon tau (IFNT), the maternal recognition of pregnancy signal in sheep and other ruminants, is secreted by the conceptus and regulates the expression of a number of genes in a cell-specific manner within the uterus. The response of different endometrial cell types to IFNT appears to be specified by IFN regulatory factors (IRFs). IRF2, a potent repressor of gene transcription, is expressed only by luminal (LE) and superficial glandular epithelia (sGE), whereas IRF1 and IRF9, activators of gene transcription, are expressed only in GE and stromal cells of the uterus during early pregnancy. In the present study, IRF6 was found to be expressed in LE/sGE and middle GE of the ovine uterine endometrium as well as conceptus trophectoderm. IRF family members can regulate transcription via IFN-stimulated response elements (ISREs). Transient transfection analyses found that IRF6 enhanced basal activity of ISRE-containing promoters, but did not enhance IFNT stimulation of ISRE-containing promoters in variety of different cell types. Further, IRF6 did not cooperate with IRF1 or reduce IRF2 repression of ISRE-containing promoter activity. These results establish that IRF6 is a transcriptional activator that is preferentially expressed in the endometrial epithelia and conceptus trophectoderm. IRF6 is hypothesized to play critical roles in endometrial gene expression as well as in conceptus trophectoderm growth and differentiation.

IFN regulatory factor 8 sensitizes soft tissue sarcoma cells to death receptor-initiated apoptosis via repression of FLICE-like protein expression

IFN regulatory factor 8 (IRF8) has been shown to suppress tumor development at least partly through regulating apoptosis of tumor cells; however, the molecular mechanisms underlying IRF8 regulation of apoptosis are still not fully understood. Here, we showed that disrupting IRF8 function resulted in inhibition of cytochrome c release, caspase-9 and caspase-3 activation, and poly(ADP-ribose) polymerase cleavage in soft tissue sarcoma (STS) cells. Inhibition of the mitochondrion-dependent apoptosis signaling cascade is apparently due to blockage of caspase-8 and Bid activation. Analysis of signaling events upstream of caspase-8 revealed that disrupting IRF8 function dramatically increases FLIP mRNA stability, resulting in increased IRF8 protein level. Furthermore, primary myeloid cells isolated from IRF8-null mice also exhibited increased FLIP protein level, suggesting that IRF8 might be a general repressor of FLIP. Nuclear IRF8 protein was absent in 92% (55 of 60) of human STS specimens, and 99% (59 of 60) of human STS specimens exhibited FLIP expression, suggesting that the nuclear IRF8 protein level is inversely correlated with FLIP level in vivo. Silencing FLIP expression significantly increased human sarcoma cells to both FasL-induced and tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-induced apoptosis, and ectopic expression of IRF8 also significantly increased the sensitivity of these human sarcoma cells to FasL- and TRAIL-induced apoptosis. Taken together, our data suggest that IRF8 mediates FLIP expression level to regulate apoptosis and targeting IRF8 expression is a potentially effective therapeutic strategy to sensitize apoptosis-resistant human STS to apoptosis, thereby possibly overcoming chemoresistance of STS, currently a major obstacle in human STS therapy.

Interferon regulatory factor 4 and 8 in B-cell development

Interferon regulatory factor 4 (IRF4) and 8 are members of the interferon regulatory factor family of transcription factors and have been shown to be essential for the development and function of T cells, macrophages and dendritic cells. A series of recent studies have further demonstrated critical functions for IRF4 and 8 at several stages of B-cell development including pre-B-cell development, receptor editing, germinal center reaction and plasma cell generation. Collectively, these new studies provide molecular insights into the function of IRF4 and 8 and underscore a requirement for IRF4 and 8 throughout B-cell development. This review focuses on the recent advances on the roles of IRF4 and 8 in B-cell development.

Expression of the interferon regulatory factor 8/ICSBP-1 in human reactive lymphoid tissues and B-cell lymphomas: a novel germinal center marker

To assess the role of interferon regulatory factor (IRF) 8 in B-cell development and lymphomagenesis, we studied its expression in reactive lymphoid tissues, its relationship to other B-cell transcription factors, and its expression in a series of 232 B-cell tumors and 30 cell lines representing a variety of B-cell developmental stages. We found that although IRF8 was detectable in most reactive B-cells, its expression levels differed with developmental stage. Germinal center B cells contained the highest levels of IRF8, with lower levels seen in mantle and marginal zone B cells and none in plasma cells. IRF8 was coexpressed with PAX-5, Pu.1, and B-cell lymphoma (BCL)-6, and similar to BCL-6, was absent from the small population of IRF4-positive germinal center B cells thought to be committed to postgerminal center developmental programs. Similarly, IRF8 was most strongly expressed in lymphomas of germinal center origin with lower levels present in mantle cell lymphomas, chronic lymphocytic leukemia, and marginal zone lymphomas, and no expression observed in plasmacytic/plasmablastic neoplasms. The reciprocal expression pattern with IRF4 in reactive tissues was generally maintained in lymphomas with some exceptions. These results suggest an important role for IRF8 during germinal center B-cell development and in related lymphomas, and provide a new diagnostic marker helpful in distinguishing B-cell non-Hodgkin lymphoma subtypes.

The IRF family transcription factors in immunity and oncogenesis

The interferon regulatory factor (IRF) family, consisting of nine members in mammals, was identified in the late 1980s in the context of research into the type I interferon system. Subsequent studies over the past two decades have revealed the versatile and critical functions performed by this transcription factor family. Indeed, many IRF members play central roles in the cellular differentiation of hematopoietic cells and in the regulation of gene expression in response to pathogen-derived danger signals. In particular, the advances made in understanding the immunobiology of Toll-like and other pattern-recognition receptors have recently generated new momentum for the study of IRFs. Moreover, the role of several IRF family members in the regulation of the cell cycle and apoptosis has important implications for understanding susceptibility to and progression of several cancers.

Leishmania major infection activates NF-kappaB and interferon regulatory factors 1 and 8 in human dendritic cells

The salient feature of dendritic cells (DC) is the initiation of appropriate adaptive immune responses by discriminating between pathogens. Using a prototypic model of intracellular infection, we previously showed that Leishmania major parasites prime human DC for efficient interleukin-12 (IL-12) secretion. L. major infection is associated with self-limiting cutaneous disease and powerful immunity. In stark contrast, the causative agent of visceral leishmaniasis, Leishmania donovani, does not prime human DC for IL-12 production. Here, we report that DC priming by L. major infection results in the early activation of NF-kappaB transcription factors and the up-regulation and nuclear translocation of interferon regulatory factor 1 (IRF-1) and IRF-8. The inhibition of NF-kappaB activation by the pretreatment of DC with caffeic acid phenethyl ester blocks L. major-induced IRF-1 and IRF-8 activation and IL-12 expression. We further demonstrate that IRF-1 and IRF-8 obtained from L. major-infected human DC specifically bind to their consensus binding sites on the IL-12p35 promoter, indicating that L. major infection either directly stimulates a signaling cascade or induces an autocrine pathway that activates IRF-1 and IRF-8, ultimately resulting in IL-12 transcription.

The BXH2 mutation in IRF8 differentially impairs dendritic cell subset development in the mouse

Among dendritic cell (DC) subsets, CD8alpha(+) DCs and plasmacytoid DCs (pDCs) produce high levels of IL12 and type I interferons (IFNs), respectively, and confer early innate immunity. Development of CD8alpha(+) DCs and pDCs requires the interferon regulatory factor 8 (IRF8). Recently, a spontaneous point mutation was identified in the Irf8/Icsbp gene in the BXH2 mouse, which exhibits an immunodeficient phenotype similar to the IRF8 knockout (KO) mouse. We show that this mutation, designated IRF8(R294C), abolishes the development of CD8alpha(+) DCs without impairing pDC development, and eliminates production of IL12p40, while retaining that of type I IFNs. Electrophoretic mobility shift and chromatin immunoprecipitation assays indicated that IRF8(R294C) failed to interact with partner transcription factors and did not bind certain promoters that require partner interactions. Together, this work indicates that IRF8-partner interactions play different roles in CD8alpha(+) DCs and pDCs, revealing a mechanistic separation that underlies development of these DC subsets.

Innate immunity to intraphagosomal pathogens is mediated by interferon regulatory factor 8 (IRF-8) that stimulates the expression of macrophage-specific Nramp1 through antagonizing repression by c-Myc

Macrophages are a central arm of innate immune defense against intracellular pathogens. They internalize microbes into phagosomes where the invaders are being killed by oxygen and nitrogen reactive species. Despite this battery of antimicrobial molecules, some are able to thrive within the phagosome thus termed intraphagosomal pathogens among which are Salmonella, Leishmania, and Mycobacteria. In mice, a single dominant gene termed Nramp1/Slc11a1 controls innate resistance to such pathogens. This gene is expressed exclusively in myeloid cells. Previously, we have shown that the restricted expression of Nramp1 is regulated by a myeloid cell-specific transcription factor termed IRF-8/ICSBP. It is demonstrated here that the induction of Nramp1 expression in activated macrophages is accompanied by a promoter shift from a repression state elicited by c-Myc to an activation state elicited by the induction of IRF-8 in activated macrophages. This transition from repression to activation is facilitated by a competitive protein-protein interaction with the transcription factor Miz-1. To show that IRF-8 is directly involved in the elimination of intraphagosomal pathogens through the regulation of Nramp1 gene expression, we bred wild type as well as IRF-8 and Nramp1 null mouse strains and examined macrophages derived from bone marrow and peritoneum. Our results clearly show that the absence of IRF-8 and Nramp1 leads to the same phenotype; defective killing of intraphagosomal Salmonella enterica serovar typhimurium and Mycobacterium bovis. Thus, interplay between repression and activation state of the Nramp1 promoter mediated by IRF-8 provides the molecular basis by which macrophages resist intraphagosomal pathogens at early stage after infection.

IFN regulatory factor 8 mediates apoptosis in nonhemopoietic tumor cells via regulation of Fas expression

IFN regulatory factor 8 (IRF8) is a transcription factor that was originally identified in myeloid cells and has been shown to be essential for differentiation and function of hemopoietic cells. Mice with a null mutation of IRF8 exhibit uncontrolled expansion of the granulocytic and monocytic lineages that progress into a phenotype resembling human chronic myelogenous leukemia. In human patients with chronic myelogenous leukemia, IRF8 transcript levels are frequently diminished. Therefore, IRF8 is a key regulator of myeloid tumor development. In this study, we report that IRF8 is a critical regulator of apoptosis in nonhemopoietic tumor cells. Disruption of IRF8 function with IRF8 dominant-negative mutants diminished Fas-mediated apoptosis in sarcoma tumor cells. Both constitutively expressed and IFN-gamma-activated IRF8 were involved in regulation of apoptosis. Furthermore, it was found that constitutively expressed IRF8 is associated with the Fas promoter to activate Fas transcription. In addition, disruption of constitutively expressed IRF8 function diminished JAK1 expression and thereby inhibited IFN-gamma-initiated induction of STAT1 phosphorylation, which in turn, blocked IFN-gamma-induced Fas up-regulation. Interestingly, the constitutively expressed IRF8 was also essential for TNF-alpha sensitization of Fas-mediated apoptosis because disruption of IRF8 function also inhibited TNF-alpha-sensitized and Fas-mediated apoptosis. Taken together, our data suggest that IRF8 is an essential mediator of Fas-mediated apoptosis and that IRF8 mediates apoptosis through regulation of Fas expression in nonhemopoietic tumor cells.

Icsbp1/IRF-8 is required for innate and adaptive immune responses against intracellular pathogens

The chronic myeloid leukemia syndrome of the BXH-2 mouse strain (Mus musculus) is caused by a recessive mutation (R294C) in the transcriptional regulator Icsbp1/IRF-8. In trans activation assays using an IL-12p40 gene reporter construct introduced in RAW 264.7 mouse macrophages, we show that the Icsbp1(C294) isoform behaves as a partial loss-of-function. The Icsbp1(C294) hypomorph allele appears to have a threshold effect on IL-12 production, with pleiotropic consequences on resistance to different types of infections in vivo. Despite the presence of a resistance Nramp1(G169) allele, BXH-2 mice (Icsbp1(C294)) show impaired control of Mycobacterium bovis (bacille Calmette-Guérin) multiplication both early and late during infection, with uncontrolled replication linked to inability to form granulomas in infected liver and spleen. Studies in informative (BXH-2 x BALB/cJ)F(2) mice show that homozygosity for Icsbp1(C294) causes susceptibility to Salmonella enterica serovar Typhimurium to a level comparable to that seen for mice lacking functional Nramp1 or TLR4. Finally, impaired Icsbp1(C294) function is associated with the following: 1) increased replication of the Plasmodium chabaudi AS malarial parasite during the first burst of blood parasitemia, and 2) recurring waves of high blood parasitemia late during infection. These results show that Icsbp1 is required for orchestrating early innate responses and also long-term immune protection against unrelated intracellular pathogens.

Transcription factors in the control of dendritic cell life cycle

Dendritic cells (DCs) are potent antigen-presenting cells that guard all parts of the body. They have the unique ability to prime T cells and generate primary immune responses. Their journey beginning with the development from precursor cells and ending with their death is controlled by a group of transcription factors. Some of the transcription factors like PU.1 are involved in more than one stage of DC life. Other transcription factors including Ikaros and JAK3 are involved in the development of more than one cell type. For a long time, the cellular and molecular mechanisms underlying the development, differentiation, maturation, and other stages of DC life were not well understood. However, in recent years novel information has been published by many researchers to better understand the molecular mechanisms of the development and function of DCs in immunological diseases such as asthma, cancer, autoimmunity, and transplantation. This review will discuss the various transcription factors and signaling pathways involved in each stage of the life cycle of DCs.

The role of the interferon regulatory factor (IRF) family in dendritic cell development and function

Dendritic cells (DCs) are powerful sensors of foreign pathogens as well as cancer cells and provide the first line of defence against infection. They also serve as a major link between innate and adaptive immunity. Immature DCs respond to incoming danger signals and undergo maturation to produce high levels of proinflammatory cytokines including type I interferons (IFNs) to establish innate immunity. They then present antigens to T lymphocytes to stimulate lasting specific immune responses. Recent studies point to the importance of DCs in the induction of peripheral tolerance. Transcription factors of the IRF family have emerged as crucial controllers of many aspects of DC activity, playing an essential role in the establishment of early innate immunity. Furthermore, eight of the nine members of the IRF family have been shown to control either the differentiation and/or the functional activities of DCs. In this review, we focus on three aspects of DC properties that are under the control of IRFs: (1) the development and differentiation, (2) maturation in response to toll-like receptor (TLR) signalling and the production of anti-microbial cytokines, and (3) activation and expansion of lymphocytes to generate protective or tolerogenic immune responses.

The feedback phase of type I interferon induction in dendritic cells requires interferon regulatory factor 8

Dendritic cells (DCs) produce type I interferons (IFNs) in greater amounts than other cells, but the mechanisms remain elusive. Here we studied the role of a transcription factor, IRF8, in DC induction of type I IFNs. Upon newcastle disease virus (NDV) infection, bone marrow-derived plasmacytoid and conventional DCs induced IFN transcripts, exhibiting two-phase kinetics. The second, amplifying phase represented an IFN feedback response that accounted for much of IFN protein production. Induction of second phase transcription required IRF8. Mouse cytomegalovirus (MCMV) and Toll-like receptor-mediated IFN induction in DCs also required IRF8. Chromatin immunoprecipitation analysis showed that IRF7, IRF8, and RNA polymerase II were recruited to the IFN promoters upon stimulation. Moreover, sustained RNA polymerase II recruitment to the promoters critically depended on IRF8. Together, these data indicate that IRF8 magnifies the second phase of IFN transcription in DCs by prolonging binding of basic transcription machinery to the IFN promoters, thereby playing a role in innate immunity.

A compass that points to lupus: genetic studies on type I interferon pathway

It was more than 20 years ago that patients with systemic lupus erythematosus (SLE) were first reported to display elevated serum levels of type I interferon (IFN). Since then, extensive studies revealed a crucial role for type I IFN in SLE pathogenesis. The current model proposes that small increase of type I IFN production by plasmacytoid dendritic cells (pDCs) is sufficient to induce unabated activation of immature peripheral DCs. IFN-matured DCs select and activate autoreactive T cells and B cells, rather than deleting them, resulting in peripheral tolerance breakdown, a characteristic feature of SLE. Furthermore, immune complexes provide an amplification loop to pDCs for further IFN production. In the past 5 years, high-throughput technologies such as expression profiling and single-nucleotide polymorphism (SNP) typing established the role of altered type I IFN system in SLE, and a detailed picture of its molecular mechanisms is beginning to emerge. In this review, we discuss two major lines of genetics studies on type I IFN pathway related to human SLE: (1) expression profiling of IFN-responsive genes and (2) disease-associated SNPs of IFN-related genes, especially IRF5 (IFN-regulatory factor 5). Lastly, we discuss how such genetic alterations in type I IFN pathway fit in the current model of SLE pathogenesis.

The interferon regulatory factor family in host defense: mechanism of action

Transcription factors of the interferon regulatory factor (IRF) family commands the entire type I interferon (IFN) system from induction of IFNs to diverse IFN responses, thereby providing a principal basis for host resistance against pathogens. However, the family has various additional roles. Regulating the development of the immune system, IRFs shape the establishment and execution of innate and adaptive immunity. IRFs also regulate growth and differentiation of many cell types, thus playing a role in leukemia and other cancers. In addition, evidence indicates that IRFs confer antiviral mechanisms not directly ascribed to the IFN system. This review deals with the diverse roles of IRFs in host defense and discusses the molecular mechanisms by which they regulate target gene transcription.

Three functional variants of IFN regulatory factor 5 (IRF5) define risk and protective haplotypes for human lupus

Systematic genome-wide studies to map genomic regions associated with human diseases are becoming more practical. Increasingly, efforts will be focused on the identification of the specific functional variants responsible for the disease. The challenges of identifying causal variants include the need for complete ascertainment of genetic variants and the need to consider the possibility of multiple causal alleles. We recently reported that risk of systemic lupus erythematosus (SLE) is strongly associated with a common SNP in IFN regulatory factor 5 (IRF5), and that this variant altered spicing in a way that might provide a functional explanation for the reproducible association to SLE risk. Here, by resequencing and genotyping in patients with SLE, we find evidence for three functional alleles of IRF5: the previously described exon 1B splice site variant, a 30-bp in-frame insertion/deletion variant of exon 6 that alters a proline-, glutamic acid-, serine- and threonine-rich domain region, and a variant in a conserved polyA+ signal sequence that alters the length of the 3' UTR and stability of IRF5 mRNAs. Haplotypes of these three variants define at least three distinct levels of risk to SLE. Understanding how combinations of variants influence IRF5 function may offer etiological and therapeutic insights in SLE; more generally, IRF5 and SLE illustrates how multiple common variants of the same gene can together influence risk of common disease.

Phosphorylation of IRF8 in a pre-associated complex with Spi-1/PU.1 and non-phosphorylated Stat1 is critical for LPS induction of the IL1B gene

Rapid induction of transcription is known to be mediated by factors which bind DNA following post-translational modification. We report here that non-tyrosine phosphorylated (NTP)-Stat1 is involved in a cooperative interaction with Spi-1/PU.1 and IRF8 to form a pre-associated, poised complex for IL1B gene induction. A double point mutation at a putative STAT binding site, which overlaps this composite Spi-1 x IRF8 site located in the LPS and IL-1 response element (LILRE), inhibited human IL1B LPS-dependent reporter activity to about 10 percent of the control wild type vector. Chromatin immunoprecipitation revealed stimulation-independent constitutive binding of IRF8, Spi-1 and NTP-Stat1 at the LILRE, while binding of C/EBP beta was activated at an adjacent C/EBP beta site after LPS stimulation. In contrast to Stat1, IRF8 was tyrosine phosphorylated following LPS treatment. Supporting the involvement of NTP-Stat1, LPS-induced IL1B reporter activity in monocytes was enhanced by ectopic expression of NTP-Stat1 Y701F. In contrast, co-expression of a Y211F IRF8 mutein functioned as a dominant-negative inhibitor of LPS-induced IL1B reporter activity. In vitro DNA binding using extracts from LPS-treated monocytes confirmed that the LILRE enhancer constitutively binds a trimolecular complex containing IRF8, Spi-1 and NTP-Stat1. Binding studies using in vitro-expressed proteins revealed that NTP-Stat1 enhanced the binding of Spi-1 and IRF8 to the LILRE. Co-expression of TRAF6, an LPS surrogate, with Spi-1 and IRF8 enhanced IL1B reporter activity in HEK293R cells, which was dramatically reduced when Y211F IRF8 was co-expressed. These results suggest that the rapid transcriptional induction of an important inflammatory gene is dependent upon constitutive cooperative binding of a Spi-1 x IRF8 x NTP-Stat1 complex to the LILRE, which primes the gene for immediate induction following IRF8 phosphorylation. Phosphorylation of chromatin pre-associated factors like IRF8 may be an important strategy for the rapid transcriptional activation of genes involved in innate immunity.

Interferon regulatory factor-8 is indispensable for the expression of promyelocytic leukemia and the formation of nuclear bodies in myeloid cells

Interferon (IFN) regulatory factor-8 (IRF-8), previously known as ICSBP, is a myeloid cell essential transcription factor. Mice with null mutation in IRF-8 are defective in the ability of myeloid progenitor cells to mature toward macrophage lineage. Accordingly, these mice develop chronic myelogenous leukemia (CML). We demonstrate here that IRF-8 is an obligatory regulator of the promyelocytic leukemia (PML) gene in activated macrophages, leading to the expression of the PML-I isoform. This regulation is most effective together with two other transcription factors, IRF-1 and PU.1. PML is a tumor suppressor gene that serves as a scaffold protein for nuclear bodies. IRF-8 is not only essential for the IFN-gamma-induced expression of PML in activated macrophages but also for the formation of nuclear bodies. Reduced IRF-8 transcript levels were reported in CML patients, and a recovery to normal levels was observed in patients in remission following treatment with IFN-alpha. We demonstrate a significant correlation between the levels of IRF-8 and PML in these CML patients. Together, our results indicate that some of the myeloleukemia suppressor activities of IRF-8 are mediated through the regulation of PML. When IRF-8 levels are compromised, the reduced PML expression may lead to genome instability and eventually to the leukemic phenotype.

IRF family proteins and type I interferon induction in dendritic cells

Dendritic cells (DC), although a minor population in hematopoietic cells, produce type I interferons (IFN) and other cytokines and are essential for innate immunity. They are also potent antigen presenters and regulate adaptive immunity. Among DC subtypes plasmacytoid DC (pDC) produce the highest amounts of type I IFN. In addition, pro- and anti-inflammatory cytokines such as IL-12 and IL-10 are induced in DC in response to Toll like receptor (TLR) signaling and upon viral infection. Proteins in the IRF family control many aspects of DC activity. IRF-8 and IRF-4 are essential for DC development. They differentially control the development of four DC subsets. IRF-8-/- mice are largely devoid of pDC and CD8alpha+ DC, while IRF-4-/- mice lack CD4+DC. IRF-8-/-, IRF4-/-, double knock-out mice have only few CD8á-CD4-DC that lack MHC II. IRF proteins also control type I IFN induction in DC. IRF-7, activated upon TLR signaling is required for IFN induction not only in pDC, but also in conventional DC (cDC) and non-DC cell types. IRF-3, although contributes to IFN induction in fibroblasts, is dispensable in IFN induction in DC. Our recent evidence reveals that type I IFN induction in DC is critically dependent on IRF-8, which acts in the feedback phase of IFN gene induction in DC. Type I IFN induction in pDC is mediated by MyD88 dependent signaling pathway, and differs from pathways employed in other cells, which mostly rely on TLR3 and RIG-I family proteins. Other pro-inflammatory cytokines are produced in an IRF-5 dependent manner. However, IRF-5 is not required for IFN induction, suggesting the presence of separate mechanisms for induction of type I IFN and other pro-inflammatory cytokines. IFN and other cytokines produced by activated DC in turn advance DC maturation and change the phenotype and function of DC. These processes are also likely to be governed by IRF family proteins.

Transcriptional regulation of lipopolysaccharide (LPS)-induced Toll-like receptor (TLR) expression in murine macrophages: role of interferon regulatory factors 1 (IRF-1) and 2 (IRF-2)

Activation of TLRs is most closely associated with induction of pro-inflammatory gene expression; however, expression of many other genes, including the TLR genes themselves, has also been shown to be modulated following TLR engagement. A large family of nuclear transcription factors, the interferon regulatory factors (IRFs), have been implicated in TLR signaling leading to pro-inflammatory gene expression. Given that IRF-1 and IRF-2 counter-regulate the transcriptional activity of many genes, we hypothesized that IRF-1 and IRF-2 might also regulate TLR gene expression following LPS stimulation of murine macrophages. mRNA derived from medium- or LPS-treated primary peritoneal macrophages was analyzed for TLR gene expression using quantitative real-time PCR. In wild-type macrophages, LPS up-regulated expression of TLRs 1-3 and 6-9 steady-state mRNA, while TLR4 mRNA was modestly down-regulated. IRF-2(-/-) macrophages responded to LPS with dysregulated expression of TLR3, TLR4, and TLR5 mRNA, whereas IRF-1 deficiency dampened LPS-induced mRNA expression for TLR3, TLR6, and TLR9. Functional studies revealed aberrant TLR3 signaling in IRF-2(-/-) macrophages. Collectively, these findings reveal an additional level of complexity associated with TLR transcriptional regulation and suggest that the trans-acting factors, IRF-1 and IRF-2, contribute to the innate immune response to infections by regulating TLR gene expression.

Retroviral integration site analysis identifies ICSBP as a collaborating tumor suppressor gene in NUP98-TOP1-induced leukemia

INTRODUCTION

The NUP98-TOP1 fusion gene is one of 18 distinct translocations identified in acute myeloid leukemia involving the N-terminal portion of the nucleoporin NUP98. We previously reported that expression of NUP98-TOP in murine bone marrow induces a lethal, transplantable leukemia. However, the long latency suggests the in vivo acquisition of additional mutations and/or time required for clonal outgrowth of rare transformed cells arising from the collaboration of NUP98-TOP1 and a cooperating event. The aim of this study was to test whether retroviral insertional mutagenesis contributes to disease onset and whether integration site analysis can identify collaborating genes.

METHODS

The genomic sites of retroviral integration in NUP98-TOP1-induced leukemic mice were analyzed. This screen identified a proviral integration that disrupts expression of the Interferon consensus sequence binding protein (ICSBP) tumor suppressor gene. Intriguingly, an ICSBP deficiency induces a chronic myeloid leukemia-like disease in mice and its reduced expression has been observed in several human leukemias. To ascertain whether an ISCBP deficiency collaborates with NUP98-TOP1 in leukemogenesis, we expressed NUP98-TOP1 in ICSBP(-/-) bone marrow.

RESULTS

The in vivo myeloproliferation induced by NUP98-TOP1 was markedly exaggerated with the ICSBP(-/-) deficiency. Moreover, NUP98-TOP1/ICSBP(-/-) mice had a reduced survival compared with NUP98-TOP1/ICSBP(+/+) mice.

CONCLUSION

These results reveal the novel finding of collaboration between the ICSBP tumor suppressor gene and NUP98-TOP1 in leukemogenesis. Moreover they further illustrate the power of retroviral integration site analysis for identifying novel cooperating tumor suppressor genes.

The Human Papillomavirus Type 16 E7 Protein Binds Human Interferon Regulatory Factor-9 via a Novel PEST Domain Required for Transformation

It is critical that viruses are able to avoid the antiviral activities of interferon (IFN). We have shown previously that the human papillomavirus (HPV) is able to avoid IFN-alpha via interaction of the HPV-16 E7 protein with IFN regulatory factor-9 (IRF-9). Here, we investigated the details of the interaction using HPV-16 E7 peptide mapping to show that IRF-9 binds HPV-16 E7 in a domain encompassing amino acids 25-36. A closer examination of this region indicates this is a novel proline, glutamate, serine, and threonine-rich (PEST) domain, with a PEST score of 8.74. We have also mapped the region of interaction within IRF-9 and found that amino acids 354-393 play an important role in binding to HPV-16 E7. This region of IRF-9 encompasses the IRF association domain (IAD), a region important for protein-protein interaction central to IRF function. Finally, we used alanine-scanning mutagenesis to determine if E7-IRF-9 interaction was important for E7-mediated cellular transformation and found that the HPV-16 E7 mutants Y25A, E26A, S31A, S32A, and E35A, but not L28A and N29A, caused loss of transformation ability. Preliminary data suggest loss of IRF-9 interaction with E7 mutants correlated with transformation. Our work suggests E7-IRF-9 interaction is important for the transforming ability of HPV-16 E7 and that HPV-16 E7 may interact with other IRF proteins that have IAD domains.

The Icsbp locus is a common proviral insertion site in mature B-cell lymphomas/plasmacytomas induced by exogenous murine leukemia virus

ICSBP (interferon consensus sequence binding protein)/IRF8 (interferon regulatory factor 8) is an interferon gamma-inducible transcription factor expressed predominantly in hematopoietic cells, and down-regulation of this factor has been observed in chronic myelogenous leukemia and acute myeloid leukemia in man. By screening about 1200 murine leukemia virus (MLV)-induced lymphomas, we found proviral insertions at the Icsbp locus in 14 tumors, 13 of which were mature B-cell lymphomas or plasmacytomas. Only one was a T-cell lymphoma, although such tumors constituted about half of the samples screened. This indicates that the Icsbp locus can play a specific role in the development of mature B-lineage malignancies. Two proviral insertions in the last Icsbp exon were found to act by a poly(A)-insertion mechanism. The remaining insertions were found within or outside Icsbp. Since our results showed expression of Icsbp RNA and protein in all end-stage tumor samples, a simple tumor suppressor function of ICSBP is not likely. Interestingly, proviral insertions at Icsbp have not been reported from previous extensive screenings of mature B-cell lymphomas induced by endogenous MLVs. We propose that ICSBP might be involved in an early modulation of an immune response to exogenous MLVs that might also play a role in proliferation of the mature B-cell lymphomas.

Identification of IRF-8 and IRF-1 target genes in activated macrophages

Interferon regulatory factor 1 (IRF-1) and IRF-8, also known as interferon consensus sequence binding protein (ICSBP), are important regulators of macrophage differentiation and function. These factors exert their activities through the formation of heterocomplexes. As such, they are coactivators of various interferon-inducible genes in macrophages. To gain better insights into the involvement of these two transcription factors in the onset of the innate immune response and to identify their regulatory network in activated macrophages, DNA microarray was employed. Changes in the expression profile were analyzed in peritoneal macrophages from wild type mice and compared to IRF-1 and IRF-8 null mice, before and following 4 h exposure to IFN-gamma and LPS. The expression pattern of 265 genes was significantly changed (up/down) in peritoneal macrophages extracted from wild type mice following treatment with IFN-gamma and LPS, while no changes in the expression levels of these genes were observed in samples of the same cell-type from both IRF-1 and IRF-8 null mice. Among these putative target genes, numerous genes are involved in macrophage activity during inflammation. The expression profile of 10 of them was further examined by quantitative RT-PCR. In addition, the promoter regions of three of the identified genes were analyzed by reporter gene assay for the ability to respond to IRF-1 and IRF-8. Together, our results suggest that both IRF-1 and IRF-8 are involved in the transcriptional regulation of these genes. We therefore suggest a broader role for IRF-1 and IRF-8 in macrophages differentiation and maturation, being important inflammatory mediators.

Immune cell-specific amplification of interferon signaling by the IRF-4/8-PU.1 complex

Both type I interferon (IFN-alpha/beta) and type II IFN (IFN-gamma) exert many functions that are restricted to immune cells. Thus, they play critical roles in innate and adaptive immunity. IFN regulatory factor-4 (IRF-4) and IRF-8 (formerly PU.1 interaction partner [Pip] and IFN consensus sequence binding domain [ICSBP], respectively) are immune cell-specific members of the IRF family that regulate the development of myeloid, lymphoid, and dendritic cells. They form a heterodimeric complex with another immune cell-specific transcription factor PU.1-Spi-1 and regulate transcription of genes in the immune system. This review describes the role of the IRF-8-PU.1 complex in modulating IFN signaling in an immune cell-specific manner. Our studies revealed that some but not all IFN-gamma-inducible genes carry an IFN-gamma activation site (GAS) element that contains a binding site for the IRF- 8-PU.1 complex. The IRF-8-PU.1 complex can take part in GAS-mediated transcription and amplify expression of IFN-gamma-responsive genes initiated by Stat1 in macrophages. Similarly, some but not all IFN-alpha/beta-responsive genes are shown to carry an IFN-stimulated response element (ISRE) that contains an IRF-8-PU.1 binding site. The participation of IRF-8-PU.1 in ISRE-mediated transcription results in the augmentation of IFN-stimulated gene factor 3 (ISGF3)-induced transcription in macrophages. Thus, GAS and ISRE elements, classically defined as universal IFN-alpha/beta and IFN-gamma response sequences, are not the same, and some harbor an embedded motif for IRF- 8-PU.1 binding that functions only in immune cells. Accordingly, the IRF-8-PU.1complex provides secondary IFN signaling pathways unique to the immune system. Collectively, the contribution of IRF-8 and PU.1 to IFN-regulated gene expression may in part account for immune cell-specific functions of IFNs.

Regulation of the germinal center gene program by interferon (IFN) regulatory factor 8/IFN consensus sequence-binding protein

Interferon (IFN) consensus sequence-binding protein/IFN regulatory factor 8 (IRF8) is a transcription factor that regulates the differentiation and function of macrophages, granulocytes, and dendritic cells through activation or repression of target genes. Although IRF8 is also expressed in lymphocytes, its roles in B cell and T cell maturation or function are ill defined, and few transcriptional targets are known. Gene expression profiling of human tonsillar B cells and mouse B cell lymphomas showed that IRF8 transcripts were expressed at highest levels in centroblasts, either from secondary lymphoid tissue or transformed cells. In addition, staining for IRF8 was most intense in tonsillar germinal center (GC) dark-zone centroblasts. To discover B cell genes regulated by IRF8, we transfected purified primary tonsillar B cells with enhanced green fluorescent protein-tagged IRF8, generated small interfering RNA knockdowns of IRF8 expression in a mouse B cell lymphoma cell line, and examined the effects of a null mutation of IRF8 on B cells. Each approach identified activation-induced cytidine deaminase (AICDA) and BCL6 as targets of transcriptional activation. Chromatin immunoprecipitation studies demonstrated in vivo occupancy of 5' sequences of both genes by IRF8 protein. These results suggest previously unappreciated roles for IRF8 in the transcriptional regulation of B cell GC reactions that include direct regulation of AICDA and BCL6.

Towards an understanding of the transcription factor network of dendritic cell development

Dendritic cells (DCs) are antigen-presenting cells of the immune system and develop from hematopoietic stem cells through successive steps of lineage commitment and differentiation. The three major DC populations are epidermal Langerhans cells, tissue/interstitial/dermal DCs and plasmacytoid DCs. We review how gene-targeted mutations in mice have contributed to our understanding of how the various DC subpopulations develop. These studies have revealed both overlapping and distinct pathways of DC differentiation and show that there is no obvious correlation between transcription factor knockout phenotypes and a lymphoid or myeloid origin of DCs.

Characterization of two candidate genes, NCoA3 and IRF8, potentially involved in the control of HIV-1 latency

BACKGROUND

The persistence of latent HIV-1 reservoirs is the principal barrier preventing the eradication of HIV-1 infection in patients by current antiretroviral therapy. It is thus crucial to understand the molecular mechanisms involved in the establishment, maintenance and reactivation of HIV-1 latency. Since chromatin remodeling has been implicated in the transcriptional reactivation of the HIV-1 promoter, we assessed the role of the histone deacetylase inhibitor sodium butyrate (NaB) on two HIV-1 latently infected cell lines (U1 and ACH-2) gene expression.

RESULTS

Analysis of microarrays data led us to select two candidate genes: NCoA3 (Nuclear Receptor Coactivator 3), a nuclear receptor coactivator and IRF8 (Interferon Regulatory Factor 8), an interferon regulatory factor. NCoA3 gene expression is upregulated following NaB treatment of latently infected cells whereas IRF8 gene expression is strongly downregulated in the promonocytic cell line following NaB treatment. Their differential expressions were confirmed at the transcriptional and translational levels. Moreover, NCoA3 gene expression was also upregulated after treatment of U1 and ACH-2 cells with phorbol myristyl acetate (PMA) but not trichostatin A (TSA) and after treatment with NaB of two others HIV-1 latently infected cell lines (OM10.1 and J1.1). IRF8 gene is only expressed in U1 cells and was also downregulated after treatment with PMA or TSA. Functional analyses confirmed that NCoA3 synergizes with Tat to enhance HIV-1 promoter transcription and that IRF8 represses the IRF1-mediated activation through the HIV-1 promoter Interferon-stimulated response element (ISRE).

CONCLUSION

These results led us to postulate that NCoA3 could be involved in the transcriptional reactivation of the HIV-1 promoter from latency and that IRF8 may contribute to the maintenance of the latent state in the promonocytic cell line. Implication of these factors in the maintenance or reactivation of the viral latency may provide potential new targets to control HIV-1 replication in latent viral reservoirs.

Interferon regulatory factor 5 represses expression of the Epstein-Barr virus oncoprotein LMP1: braking of the IRF7/LMP1 regulatory circuit

We have reported evidence for a positive regulatory circuit between interferon regulatory factor 7 (IRF7) and the Epstein-Barr virus (EBV) oncoprotein 1 (LMP1) (S. Ning, A. M. Hahn, and J. S. Pagano, J. Virol. 77:9359-9368, 2003). To explore a possible braking mechanism for this circuit, several type II EBV-infected cell lines that express different levels of LMP1 and IRF7 proteins and therefore are convenient for studying modulation of expression of LMP1 were analyzed. Endogenous levels of IRF7 and LMP1 were directly correlated. Transient expression of an IRF7 dominant-negative mutant decreased LMP1 levels. Endogenous IRF5 and IRF7 proteins were shown to physically associate in EBV-positive cells. Transient expression of IRF5 decreased activation of the LMP1 promoter by IRF7 in a dose-dependent manner. Finally, transfection of either an IRF5 dominant-negative construct or IRF5 small interfering RNA in these cells resulted in increases in endogenous levels of LMP1. These results indicate that IRF5 can downregulate IRF7's induction of expression of LMP1 most likely by interacting with IRF7 and provide a means of modulating a regulatory circuit between IRF7 and LMP1.