Interferon consensus sequence-binding protein, a member of the interferon regulatory factor family, suppresses interferon-induced gene transcription

Modulation of the tumor immune microenvironment by Interferon Regulatory Factor 8 enhances immunotherapy in lung adenocarcinoma

Interferon regulatory factors (IRFs) are integral in governing the expression of Type I interferon (IFN) genes. However, the precise role of IRFs in lung adenocarcinoma remains elusive. Our objective is to elucidate the prognostic implications of IRFs and their potential influence on the immunotherapeutic response in patients with lung adenocarcinoma (LUAD). The association between IRFs expression and clinical as well as prognostic features was evaluated utilizing the TCGA database. Prognostic determinants for LUAD were pinpointed via univariate and multivariate analyses. Nomogram to evaluate prognosis predicated on IRF expression levels. Gene enrichments were conducted to elucidate the mechanisms of action. The degree of immune infiltration was using bioinformatics methods and was validated through a single-cell dataset. We compiled our unique cohort of LUAD patients who underwent anti-PD-1 therapy for subsequent immunohistochemistry and multicolor immunofluorescence staining to gauge the conclusion above. Our findings revealed that IRF8 serves as an independent risk factor for overall survival (OS) in patients with LUAD. An analysis of patients undergoing immunotherapy revealed a positive association between the expression of IRF8 and the response to the treatment. In our specific cohort treated with anti-PD-1, high IRF8 expression was observed to enhance immunotherapy response and prolong OS by modulating immune cell infiltration. Our retrospective analysis suggests that elevated IRF8 expression correlates with improved prognosis in LUAD, with higher IRF8 expression being predictive of a more robust immunotherapy response. Mechanistically, IRF8 expression is associated with a modulated tumor immune microenvironment and improved immunotherapeutic response.

The multiple roles of interferon regulatory factor family in health and disease

Interferon Regulatory Factors (IRFs), a family of transcription factors, profoundly influence the immune system, impacting both physiological and pathological processes. This review explores the diverse functions of nine mammalian IRF members, each featuring conserved domains essential for interactions with other transcription factors and cofactors. These interactions allow IRFs to modulate a broad spectrum of physiological processes, encompassing host defense, immune response, and cell development. Conversely, their pivotal role in immune regulation implicates them in the pathophysiology of various diseases, such as infectious diseases, autoimmune disorders, metabolic diseases, and cancers. In this context, IRFs display a dichotomous nature, functioning as both tumor suppressors and promoters, contingent upon the specific disease milieu. Post-translational modifications of IRFs, including phosphorylation and ubiquitination, play a crucial role in modulating their function, stability, and activation. As prospective biomarkers and therapeutic targets, IRFs present promising opportunities for disease intervention. Further research is needed to elucidate the precise mechanisms governing IRF regulation, potentially pioneering innovative therapeutic strategies, particularly in cancer treatment, where the equilibrium of IRF activities is of paramount importance.

The HIV latency reversing agent HODHBt inhibits the phosphatases PTPN1 and PTPN2

Nonreceptor tyrosine phosphatases (NTPs) play an important role in regulating protein phosphorylation and have been proposed as attractive therapeutic targets for cancer and metabolic diseases. We have previously identified that 3-Hydroxy-1,2,3-benzotriazin-4(3H)-one (HODHBt) enhanced STAT activation upon cytokine stimulation, leading to increased reactivation of latent HIV and effector functions of NK and CD8 T cells. Here, we demonstrate that HODHBt interacted with and inhibited the NTPs PTPN1 and PTPN2 through a mixed inhibition mechanism. We also confirm that PTPN1 and PTPN2 specifically controlled the phosphorylation of different STATs. The small molecule ABBV-CLS-484 (AC-484) is an active site inhibitor of PTPN1 and PTPN2 currently in clinical trials for advanced solid tumors. We compared AC-484 and HODHBt and found similar effects on STAT5 and immune activation, albeit with different mechanisms of action leading to varying effects on latency reversal. Our studies provide the first specific evidence to our knowledge that enhancing STAT phosphorylation via inhibition of PTPN1 and PTPN2 is an effective tool against HIV.

XAF1 prevents hyperproduction of type I interferon upon viral infection by targeting IRF7

Interferon regulatory factor (IRF) 3 and IRF7 are master regulators of type I interferon (IFN-I)-dependent antiviral innate immunity. Upon viral infection, a positive feedback loop is formed, wherein IRF7 promotes further induction of IFN-I in the later stage. Thus, it is critical to maintain a suitably low level of IRF7 to avoid the hyperproduction of IFN-I. In this study, we find that early expression of IFN-I-dependent STAT1 promotes the expression of XAF1 and that XAF1 is associated specifically with IRF7 and inhibits the activity of XIAP. XAF1-knockout and XIAP-transgenic mice display resistance to viral infection, and this resistance is accompanied by increases in IFN-I production and IRF7 stability. Mechanistically, we find that the XAF1-XIAP axis controls the activity of KLHL22, an adaptor of the BTB-CUL3-RBX1 E3 ligase complex through a ubiquitin-dependent pathway. CUL3-KLHL22 directly targets IRF7 and catalyzes its K48-linked ubiquitination and proteasomal degradation. These findings reveal unexpected functions of the XAF1-XIAP axis and KLHL22 in the regulation of IRF7 stability and highlight an important target for antiviral innate immunity.

Hepatic interferon regulatory factor 8 expression suppresses hepatocellular carcinoma progression and enhances the response to anti-programmed cell death protein-1 therapy

BACKGROUND AND AIMS

Therapeutic blockade of the programmed cell death protein-1 (PD-1) immune checkpoint pathways has resulted in significant reactivation of T cell-mediated antitumor immunity and is a promising clinical anticancer treatment modality in several tumor types, but the durable response rate remains relatively low (15%-20%) in most patients with HCC for unknown reasons. Evidence reveals that the interferon signaling pathway plays a critical role in modulating the efficacy and sensitivity of anti-PD-1 therapy against multiple tumor types, but the mechanisms are unclear.

APPROACH AND RESULTS

Using Kaplan-Meier survival analysis based on HCC databases, we found that deceased expression of interferon regulatory factor (IRF) 8 in HCC, among all the nine IRF members that regulate interferon signals, was associated with poor prognosis of patients with HCC. Moreover, gene set enrichment analysis identified the interferon-gamma and PD-1 signaling signatures as the top suppressed pathways in patients with IRF8-low HCC. Contrarily, overexpression of IRF8 in HCC cells significantly enhanced antitumor effects in immune-competent mice, modulating infiltration of tumor-associated macrophages (TAMs) and T cell exhaustion in tumor microenvironment. We further demonstrated that IRF8 regulated recruitment of TAMs by inhibiting the expression of chemokine (C-C motif) ligand 20 (CCL20). Mechanically, IRF8-mediated repression of c-fos transcription resulted in decreased expression of CCL20, rather than directly bound to CCL20 promoter region. Importantly, adeno-associated virus 8-mediated hepatic IRF8 rescue significantly suppressed HCC progression and enhanced the response to anti-PD-1 therapy.

CONCLUSIONS

This work identified IRF8 as an important prognostic biomarker in patients with HCC that predicted the response and sensitivity to anti-PD-1 therapy and uncovered it as a therapeutic target for enhancing the efficacy of immune therapy.

IFN Regulatory Factor 3 in Health and Disease

Immunity to viruses requires an array of critical cellular proteins that include IFN regulatory factor 3 (IRF3). Consequently, most viruses that infect vertebrates encode proteins that interfere with IRF3 activation. This review describes the cellular pathways linked to IRF3 activation and where those pathways are targeted by human viral pathogens. Moreover, key regulatory pathways that control IRF3 are discussed. Besides viral infections, IRF3 is also involved in resistance to some bacterial infections, in anticancer immunity, and in anticancer therapies involving DNA damage agents. A recent finding shows that IRF3 is needed for T cell effector functions that are involved in anticancer immunity and also in T cell autoimmune diseases. In contrast, unregulated IRF3 activity is clearly not beneficial, considering it is implicated in certain interferonopathies, in which heightened IRF3 activity leads to IFN-β-induced disease. Therefore, IRF3 is involved largely in maintaining health but sometimes contributing to disease.

Transcriptome analysis based on RNA-seq of common innate immune responses of flounder cells to IHNV, VHSV, and HIRRV

Viral hemorrhagic septicemia virus (VHSV) and hirame rhabdovirus (HIRRV) belong to the genus Novirhabdovirus and are the causative agents of a serious disease in cultured flounder. However, infectious hematopoietic necrosis virus (IHNV), a prototype of the genus Novirhabdovirus, does not cause disease in flounder. To determine whether IHNV growth is restricted in flounder cells, we compared the growth of IHNV with that of VHSV and HIRRV in hirame natural embryo (HINAE) cells infected with novirhabdoviruses at 1 multiplicity of infection. Unexpectedly, we found that IHNV grew as well as VHSV and HIRRV. For successful growth in host cells, viruses modulate innate immune responses exerted by virus-infected cells. Our results suggest that IHNV, like VHSV and HIRRV, has evolved the ability to overcome the innate immune response of flounder cells. To determine the innate immune response genes of virus-infected HINAE cells which are commonly modulated by the three novirhabdoviruses, we infected HINAE cells with novirhabdoviruses at multiplicity of infection (MOI) 1 and performed an RNA sequencing-based transcriptome analysis at 24 h post-infection. We discovered ~12,500 unigenes altered by novirhabdovirus infection and found that many of these were involved in multiple cellular pathways. After novirhabdovirus infection, 170 genes involved in the innate immune response were differentially expressed compared to uninfected cells. Among them, 9 genes changed expression by more than 2-fold and were commonly modulated by all three novirhabdoviruses. Interferon regulatory factor 8 (IRF8), C-X-C motif chemokine receptor 1 (CXCR1), Toll/interleukin-1 receptor domain-containing adapter protein (TIRAP), cholesterol 25-hydroxylase (CH25H), C-X-C motif chemokine ligand 11, duplicate 5 (CXCL11.5), and Toll-like receptor 2 (TLR2) were up-regulated, whereas C-C motif chemokine receptor 6a (CCR6a), interleukin-12a (IL12a), and Toll-like receptor 1 (TLR1) were down-regulated. These genes have been reported to be involved in antiviral responses and, thus, their modulation may be critical for the growth of novirhabdovirus in flounder cells. This is the first report to identify innate immune response genes in flounder that are commonly modulated by IHNV, VHSV, and HIRRV. These data will provide new insights into how novirhabdoviruses survive the innate immune response of flounder cells.

Intrinsic activation of the vitamin D antimicrobial pathway by M. leprae infection is inhibited by type I IFN

Following infection, virulent mycobacteria persist and grow within the macrophage, suggesting that the intrinsic activation of an innate antimicrobial response is subverted by the intracellular pathogen. For Mycobacterium leprae, the intracellular bacterium that causes leprosy, the addition of exogenous innate or adaptive immune ligands to the infected monocytes/macrophages was required to detect a vitamin D-dependent antimicrobial activity. We investigated whether there is an intrinsic immune response to M. leprae in macrophages that is inhibited by the pathogen. Upon infection of monocytes with M. leprae, there was no upregulation of CYP27B1 nor its enzymatic activity converting the inactive prohormone form of vitamin D (25-hydroxyvitamin D) to the bioactive form (1,25α-dihydroxyvitamin D). Given that M. leprae-induced type I interferon (IFN) inhibited monocyte activation, we blocked the type I IFN receptor (IFNAR), revealing the intrinsic capacity of monocytes to recognize M. leprae and upregulate CYP27B1. Consistent with these in vitro studies, an inverse relationship between expression of CYP27B1 vs. type I IFN downstream gene OAS1 was detected in leprosy patient lesions, leading us to study cytokine-derived macrophages (MΦ) to model cellular responses at the site of disease. Infection of IL-15-derived MΦ, similar to MΦ in lesions from the self-limited form of leprosy, with M. leprae did not inhibit induction of the vitamin D antimicrobial pathway. In contrast, infection of IL-10-derived MΦ, similar to MΦ in lesions from patients with the progressive form of leprosy, resulted in induction of type I IFN and suppression of the vitamin D directed pathway. Importantly, blockade of the type I IFN response in infected IL-10 MΦ decreased M. leprae viability. These results indicate that M. leprae evades the intrinsic capacity of human monocytes/MΦ to activate the vitamin D-mediated antimicrobial pathway via the induction of type I IFN.

Our macrophages are equipped with the ability to detect and kill invading pathogens, and yet, these cells of the innate immune system are still subject to infection by intracellular bacterium. In particular, mycobacterium, the type of intracellular bacteria responsible for diseases such as tuberculosis and leprosy, are very successful at establishing infection within macrophages. By studying Mycobacterium leprae, the etiological agent of leprosy, we describe an immune evasion mechanism whereby this bacterial pathogen utilizes our own antiviral immune response against the macrophage. Type I interferons (IFN) are a major part of our immune response to viral infections; however, this response will also suppress our ability to fight opportunistic bacterial infection. During infection of our macrophages, M. leprae induces an aberrant type I IFN response that subsequently suppresses our macrophage’s ability to activate the vitamin D-mediated antimicrobial pathway, a critical antimicrobial response for containment of mycobacterium. Thus, understanding how these pathogens can evade our immune response will be important for the development of new therapies against these chronic infections.

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.

Field Synopsis and Re-analysis of Systematic Meta-analyses of Genetic Association Studies in Multiple Sclerosis: a Bayesian Approach

To provide an up-to-date summary of multiple sclerosis-susceptible gene variants and assess the noteworthiness in hopes of finding true associations, we investigated the results of 44 meta-analyses on gene variants and multiple sclerosis published through December 2016. Out of 70 statistically significant genotype associations, roughly a fifth (21%) of the comparisons showed noteworthy false-positive rate probability (FPRP) at a statistical power to detect an OR of 1.5 and at a prior probability of 10^-6 assumed for a random single nucleotide polymorphism. These associations (IRF8/rs17445836, STAT3/rs744166, HLA/rs4959093, HLA/rs2647046, HLA/rs7382297, HLA/rs17421624, HLA/rs2517646, HLA/rs9261491, HLA/rs2857439, HLA/rs16896944, HLA/rs3132671, HLA/rs2857435, HLA/rs9261471, HLA/rs2523393, HLA-DRB1/rs3135388, RGS1/rs2760524, PTGER4/rs9292777) also showed a noteworthy Bayesian false discovery probability (BFDP) and one additional association (CD24 rs8734/rs52812045) was also noteworthy via BFDP computation. Herein, we have identified several noteworthy biomarkers of multiple sclerosis susceptibility. We hope these data are used to study multiple sclerosis genetics and inform future screening programs.

Deciphering of the Human Interferon-Regulated Proteome by Mass Spectrometry-Based Quantitative Analysis Reveals Extent and Dynamics of Protein Induction and Repression

Interferons (IFNs) are pleotropic cytokines secreted upon encounter of pathogens and tumors. Applying their antipathogenic, antiproliferative, and immune stimulatory capacities, recombinant IFNs are frequently prescribed as drugs to treat different diseases. IFNs act by changing the gene expression profile of cells. Due to characteristics such as rapid gene induction and signaling, IFNs also represent prototypical model systems for various aspects of biomedical research (e.g., signal transduction). In regard to the signaling and activated promoters, IFNs can be subdivided into two groups. Here, alterations of the cellular proteome of human cells treated with IFNα and IFNγ were elucidated in a time-resolved manner by quantitative proteome analysis. The majority of protein regulations were strongly IFN type and time dependent. In addition to the expected upregulation of IFN-responsive proteins, an astonishing number of proteins became profoundly repressed especially by IFNγ. Thus, our comprehensive analysis revealed important insights into the human IFN-regulated proteome and its dynamics of protein induction and repression. Interestingly, the new class of IFN-repressed genes comprises known host factors for highly relevant pathogens such as HIV, dengue virus, and hepatitis C virus.

Mysm1 is required for interferon regulatory factor expression in maintaining HSC quiescence and thymocyte development

Mysm1(-/-) mice have severely decreased cellularity in hematopoietic organs. We previously revealed that Mysm1 knockout impairs self-renewal and lineage reconstitution of HSCs by abolishing the recruitment of key transcriptional factors to the Gfi-1 locus, an intrinsic regulator of HSC function. The present study further defines a large LSKs in >8-week-old Mysm1(-/-) mice that exhibit increased proliferation and reduced cell lineage differentiation compared with those of WT LSKs. We found that IRF2 and IRF8, which are important for HSC homeostasis and commitment as transcription repressors, were expressed at lower levels in Mysm1(-/-) HSCs, and Mysm1 enhanced function of the IRF2 and IRF8 promoters, suggesting that Mysm1 governs the IRFs for HSC homeostasis. We further found that the lower expressions of IRF2 and IRF8 led to an enhanced transcription of p53 in Mysm1(-/-) HSCs, which was recently defined to have an important role in mediating Mysm1(-/-)-associated defects. The study also revealed that Mysm1(-/-) thymocytes exhibited lower IRF2 expression, but had higher Sca1 expression, which has a role in mediating thymocyte death. Furthermore, we found that the thymocytes from B16 melanoma-bearing mice, which display severe thymus atrophy at late tumor stages, exhibited reduced Mysm1 and IRF2 expression but enhanced Sca1 expression, suggesting that tumors may downregulate Mysm1 and IRF2 for thymic T-cell elimination.

Increased expression of interferon signaling genes in the bone marrow microenvironment of myelodysplastic syndromes

Introduction

The bone marrow (BM) microenvironment plays an important role in the pathogenesis of myelodysplastic syndromes (MDS) through a reciprocal interaction with resident BM hematopoietic cells. We investigated the differences between BM mesenchymal stromal cells (MSCs) in MDS and normal individuals and identified genes involved in such differences.

Materials and Methods

BM-derived MSCs from 7 MDS patients (3 RCMD, 3 RAEB-1, and 1 RAEB-2) and 7 controls were cultured. Global gene expression was analyzed using a microarray.

Result

We found 314 differentially expressed genes (DEGs) in RCMD vs. control, 68 in RAEB vs. control, and 51 in RAEB vs. RCMD. All comparisons were clearly separated from one another by hierarchical clustering. The overall similarity between differential expression signatures from the RCMD vs. control comparison and the RAEB vs. control comparison was highly significant (p = 0), which indicates a common transcriptomic response in these two MDS subtypes. RCMD and RAEB simultaneously showed an up-regulation of interferon alpha/beta signaling and the ISG15 antiviral mechanism, and a significant fraction of the RAEB vs. control DEGs were also putative targets of transcription factors IRF and ICSBP. Pathways that involved RNA polymerases I and III and mitochondrial transcription were down-regulated in RAEB compared to RCMD.

Conclusion

Gene expression in the MDS BM microenvironment was different from that in normal BM and exhibited altered expression according to disease progression. The present study provides genetic evidence that inflammation and immune dysregulation responses that involve the interferon signaling pathway in the BM microenvironment are associated with MDS pathogenesis, which suggests BM MSCs as a possible therapeutic target in MDS.

Interferon regulatory factor 5 in the pathogenesis of systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is an autoimmune disease characterized by multiple genetic risk factors, high levels of interferon alpha (IFN-α), and the production of autoantibodies against components of the cell nucleus. Interferon regulatory factor 5 (IRF5) is a transcription factor which induces the transcription of IFN-α and other cytokines, and genetic variants of IRF5 have been strongly linked to SLE pathogenesis. IRF5 functions downstream of Toll-like receptors and other microbial pattern-recognition receptors, and immune complexes made up of SLE-associated autoantibodies seem to function as a chronic endogenous stimulus to this pathway. In this paper, we discuss the physiologic role of IRF5 in immune defense and the ways in which IRF5 variants may contribute to the pathogenesis of human SLE. Recent data regarding the role of IRF5 in both serologic autoimmunity and the overproduction of IFN-α in human SLE are summarized. These data support a model in which SLE-risk variants of IRF5 participate in a “feed-forward” mechanism, predisposing to SLE-associated autoantibody formation, and subsequently facilitating IFN-α production downstream of Toll-like receptors stimulated by immune complexes composed of these autoantibodies.

Interferon regulatory factor 8/interferon consensus sequence binding protein is a critical transcription factor for the physiological phenotype of microglia

Background

Recent fate-mapping studies establish that microglia, the resident mononuclear phagocytes of the CNS, are distinct in origin from the bone marrow-derived myeloid lineage. Interferon regulatory factor 8 (IRF8, also known as interferon consensus sequence binding protein) plays essential roles in development and function of the bone marrow-derived myeloid lineage. However, little is known about its roles in microglia.

Methods

The CNS tissues of IRF8-deficient mice were immunohistochemically analyzed. Pure microglia isolated from wild-type and IRF8-deficient mice were studied in vitro by proliferation, immunocytochemical and phagocytosis assays. Microglial response in vivo was compared between wild-type and IRF8-deficient mice in the cuprizon-induced demyelination model.

Results

Our analysis of IRF8-deficient mice revealed that, in contrast to compromised development of IRF8-deficient bone marrow myeloid lineage cells, development and colonization of microglia are not obviously affected by loss of IRF8. However, IRF8-deficient microglia demonstrate several defective phenotypes. In vivo, IRF8-deficient microglia have fewer elaborated processes with reduced expression of IBA1/AIF1 compared with wild-type microglia, suggesting a defective phenotype. IRF8-deficient microglia are significantly less proliferative in mixed glial cultures than wild-type microglia. Unlike IRF8-deficient bone marrow myeloid progenitors, exogenous macrophage colony stimulating factor (colony stimulating factor 1) (M-CSF (CSF1)) restores their proliferation in mixed glial cultures. In addition, IRF8-deficient microglia exhibit an exaggerated growth response to exogenous granulocyte-macrophage colony stimulating factor (colony stimulating factor 2) (GM-CSF (CSF2)) in the presence of other glial cells. IRF8-deficient microglia also demonstrate altered cytokine expressions in response to interferon-gamma and lipopolysaccharide in vitro. Moreover, the maximum phagocytic capacity of IRF8-deficient microglia is reduced, although their engulfment of zymosan particles is not overtly impaired. Defective scavenging activity of IRF8-deficient microglia was further confirmed in vivo in the cuprizone-induced demyelination model in mice.

Conclusions

This study is the first to demonstrate the essential contribution of IRF8-mediated transcription to a broad range of microglial phenotype. Microglia are distinct from the bone marrow myeloid lineage with respect to their dependence on IRF8-mediated transcription.

The International Multiple Sclerosis Genetics Consortium. The genetic association of variants in CD6, TNFRSF1A and IRF8 to multiple sclerosis: a multicenter case-control study

Background

In the recently published meta-analysis of multiple sclerosis genome-wide association studies De Jager et al. identified three single nucleotide polymorphisms associated to MS: rs17824933 (CD6), rs1800693 (TNFRSF1A) and rs17445836 (61.5 kb from IRF8). To refine our understanding of these associations we sought to replicate these findings in a large more extensive independent sample set of 11 populations of European origin.

Principal Findings

We calculated individual and combined associations using a meta-analysis method by Kazeem and Farral (2005). We confirmed the association of rs1800693 in TNFRSF1A (p 4.19×10−7, OR 1.12, 7,665 cases, 8,051 controls) and rs17445836 near IRF8 (p 5.35×10−10, OR 0.84, 6,895 cases, 7,580 controls and 596 case-parent trios) The SNP rs17824933 in CD6 also showed nominally significant evidence for association (p 2.19×10−5, OR 1.11, 8,047 cases, 9,174 controls, 604 case-parent trios).

Conclusions

Variants in TNFRSF1A and in the vicinity of IRF8 were confirmed to be associated in these independent cohorts, which supports the role of these loci in etiology of multiple sclerosis. The variant in CD6 reached genome-wide significance after combining the data with the original meta-analysis. Fine mapping is required to identify the predisposing variants in the loci and future functional studies will refine their molecular role in MS pathogenesis.

What is the role of alternate splicing in antigen presentation by major histocompatibility complex class I molecules?

The expression of major histocompatibility complex (MHC) class I molecules on the cell surface is critical for recognition by cytotoxic T lymphocytes (CTL). This recognition event leads to destruction of cells displaying MHC class I-viral peptide complexes or cells displaying MHC class I-mutant peptide complexes. Before they can be transported to the cell surface, MHC class I molecules must associate with their peptide ligand in the endoplasmic reticulum (ER) of the cell. Within the ER, numerous proteins assist in the appropriate assembly and folding of MHC class I molecules. These include the heterodimeric transporter associated with antigen processing (TAP1 and TAP2), the heterodimeric chaperone-oxidoreductase complex of tapasin and ERp57 and the general ER chaperones calreticulin and calnexin. Each of these accessory proteins has a well-defined role in antigen presentation by MHC class I molecules. However, alternate splice forms of MHC class I heavy chains, TAP and tapasin, have been reported suggesting additional complexity to the picture of antigen presentation. Here, we review the importance of these different accessory proteins and the progress in our understanding of alternate splicing in antigen presentation.

Sp2 regulates interferon-gamma-mediated socs1 gene expression

Suppressor of cytokine signalling (SOCS) proteins are inducible feedback inhibitors of Janus kinase (JAK) and signal transducers and activators of transcription signalling (STAT) pathways. Interferon (IFN)-gamma induces the expression of the socs1 gene in several cell types through several cis elements present in its promoter and their binding proteins. Socs1 expression is induced in the human keratinocytes HaCaT cell line through sequential activation of STAT1 and IRF-1. Comparison of the 5'-upstream sequences of the mouse and human socs1 genes identified conserved binding sites for IRF-1 regulatory elements. Although this response element is able to bind IRF-1 in human cells, no IFN-gamma responsiveness was observed with human socs1 promoter reporter constructs containing this element. In contrast the mouse socs1 promoter was fully responsive. The mouse promoter contains two cis-acting elements which modulate its expression and are recognized by IRF-1 and Sp2. Despite the absence of Sp2 in the 5'-upstream sequence of the human promoter, silencing of Sp2 by RNA interference clearly demonstrated that Sp2 is required for IFN-gamma-induced regulation of socs1 mRNA both in human and mouse.

Interferon regulatory factor family of transcription factors and regulation of oncogenesis

A family of transcription factors, the interferon regulatory factors (IRF), was identified originally in the context of the regulation of the type I interferon (IFN)-alpha/beta system. The IRF family has now expanded to nine members, and gene-disruption studies have revealed the critical involvement of these members in multiple facets of host defense systems, such as innate and adaptive immune responses and tumor suppression. In the present review article, we aim at summarizing our current knowledge of the roles of IRF in host defense, with special emphasis on their involvement in the regulation of oncogenesis.

The interferon consensus sequence-binding protein (ICSBP/IRF8) represses PTPN13 gene transcription in differentiating myeloid cells

The interferon consensus sequence-binding protein (ICSBP/IRF8) is an interferon regulatory factor that is expressed in myeloid and B-cells. ICSBP-deficient mice develop a myeloproliferative disorder characterized by cytokine hypersensitivity and apoptosis resistance. To identify ICSBP target genes involved in these effects, we screened a CpG island microarray with chromatin that co-immunoprecipitated with ICSBP from myeloid cells. Using this technique, we identified PTPN13 as an ICSBP target gene. PTPN13 encodes Fas-associated phosphatase 1 (Fap-1), a ubiquitously expressed protein-tyrosine phosphatase. This was of interest because interaction of Fap-1 with Fas results in Fas dephosphorylation and inhibition of Fas-induced apoptosis. In this study, we found that ICSBP influenced Fas-induced apoptosis in a Fap-1-dependent manner. We also found that ICSBP interacted with a cis element in the proximal PTPN13 promoter and repressed transcription. This interaction increased during myeloid differentiation and was regulated by phosphorylation of conserved tyrosine residues in the interferon regulatory factor domain of ICSBP. ICSBP deficiency was present in human myeloid malignancies, including chronic myeloid leukemia. Therefore, these studies identified a mechanism for increased survival of mature myeloid cells in the ICSBP-deficient murine model and in human myeloid malignancies with decreased ICSBP expression.

Constitutive activation of SHP2 protein tyrosine phosphatase inhibits ICSBP-induced transcription of the gene encoding gp91PHOX during myeloid differentiation

The IFN consensus sequence-binding protein (ICSBP; also referred to as IFN regulatory factor 8) is a transcription factor which is expressed in myeloid and B cells. In previous studies, we found that ICSBP activated transcription of the gene encoding gp91(PHOX) (the CYBB gene), a rate-limiting component of the phagocyte respiratory burst oxidase expressed exclusively after the promyelocyte stage of myelopoiesis. Previously, we found that CYBB transcription was dependent on phosphorylation of specific ICSBP tyrosine residues. Since ICSBP is tyrosine-phosphorylated during myelopoiesis, this provided a mechanism of differentiation stage-specific CYBB transcription. In the current studies, we found that ICSBP was a substrate for Src homology-containing tyrosine phosphatase 2 (SHP2-PTP) in immature myeloid cells but not during myelopoiesis. Therefore, SHP2-PTP inhibited CYBB transcription and respiratory burst activity in myeloid progenitor cells by dephosphorylating ICSBP. In contrast, we found that ICSBP was a substrate for a leukemia-associated, constitutively active mutant form of SHP2, described previously, throughout differentiation. Consistent with this, constitutive SHP2 activation blocked ICSBP-induced CYBB transcription and respiratory burst activity in differentiating myeloid cells. ICSBP-deficiency and constitutive SHP2 activation have been described in human myelodysplastic syndromes. As these two abnormalities may coexist, our results identified a potential molecular mechanism for impaired phagocyte function in this malignant myeloid disease.

PU.1, Interferon Regulatory Factor (IRF) 2, and the Interferon Consensus Sequence-binding Protein (ICSBP/IRF8) Cooperate to Activate NF1 Transcription in Differentiating Myeloid Cells

Nf1 (neurofibromin 1) is a Ras-GAP protein that regulates cytokine-induced proliferation of myeloid cells. In previous studies, we found that the interferon consensus sequence-binding protein (ICSBP; also referred to as interferon regulatory factor 8) activates transcription of the gene encoding Nf1 (the NF1 gene) in differentiating myeloid cells. We also found that NF1 activation requires cytokine-stimulated phosphorylation of a conserved tyrosine residue in the interferon regulatory factor (IRF) domain of ICSBP/IRF8. In this study, we found that ICSBP/IRF8 cooperates with PU.1 and interferon regulatory factor 2 to activate a composite ets/IRF-cis element in the NF1 promoter. We found that PU.1 binds directly to the NF1-cis element, and DNA-bound PU.1 interacts with IRF2, recruiting IRF2 to the cis element. This interaction requires cytokine-induced phosphorylation of specific serine residues in the PU.1 PEST domain and of a conserved tyrosine residue in the IRF domain of IRF2. We found that ICSBP/IRF8 interaction with the NF1-cis element requires pre-binding of PU.1 and IRF2. The conserved IRF domain tyrosine in ICSBP/IRF8 is required for interaction with the DNA-bound PU.1-IRF2 heterodimer. NF1 deficiency in myeloid progenitor cells results in cytokine hypersensitivity and myeloproliferation. Therefore, these studies identify a target gene for the previously observed tumor-suppressor effect of PU.1. Additionally, these studies identify a tumor-suppressor function for the "oncogenic" transcription factor, IRF2.

Inverse regulation of inducible nitric oxide synthase (iNOS) and arginase I by the protein tyrosine phosphatase SHP-1 in CNS glia

We have previously shown that the SH2 domain-containing protein tyrosine phosphatase SHP-1 plays a critical role in controlling virus infection in CNS glia in vivo and in vitro. The present study addressed whether increased virus replication in SHP-1-deficient glia in vitro may be a result of altered expression of inducible nitric oxide synthase (iNOS/NOS2). First, we observed a profound reduction in iNOS protein expression and production of nitric oxide (NO) in response to the viral mimic double-stranded RNA (dsRNA), despite the induction of high levels of iNOS mRNA, in SHP-1-deficient motheaten mouse compared to wild type littermate mouse glia. Because both iNOS expression and NO production are suppressed by multiple pathways involving arginase I activity, it was important that we observed abnormally high constitutive expression of arginase I in cultured glia of SHP-1-deficient compared to wild type mice. Further, both constitutive and IL-4/IL-10-induced expression of arginase I correlated with elevated STAT6 nuclear binding activity, decreased NO production, and increased virus replication in motheaten compared to wild type astrocytes. These findings provide the first evidence of an inverse relationship between NO and arginase I activity regulated by SHP-1 in CNS glia that is relevant to modulation of innate anti-viral responses. Thus, we propose that SHP-1 is a critical regulator of innate immunity to virus infections in CNS cells.

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.

Genomic response to interferon-alpha in chimpanzees: implications of rapid downregulation for hepatitis C kinetics

The mechanism of the interferon-alpha (IFN-alpha)-induced antiviral response during hepatitis C virus (HCV) therapy is n o t completely understood. In this study,we examined the transcriptional response to IFN-alpha in uninfected chimpanzees after single doses of chimpanzee, human, or human-pegylated IFN-alpha. Liver and peripheral blood mononuclear cell (PBMC) samples were used for total genome microarray analysis. Most induced genes achieved maximal response within 4 hours, began to decline by 8 hours, and were at baseline levels by 24 hours postinoculation, a time when high levels of circulating pegylated IFN-alpha were still present. The rapid downregulation of the IFN-alpha response may be involved in the transition between the observed phase I and phase II viral kinetics during IFN-alpha therapy in HCV-infected patients. The response to all three forms of IFN-alpha was similar; thus, the reasons for previous failures in antiviral treatment of chimpanzees with human IFN-alpha were not due to species specificity of IFN-alpha. The response to IFN-alpha was partially tissue-specific. A total of 1778 genes were altered in expression by twofold or more by IFN-alpha, with 538 and 950 being unique to the liver or PBMC, respectively. Analysis of the IFN-alpha and IFN-gamma responses in primary chimpanzee and human hepatocytes were compared as well. IFN-alpha and IFN-gamma induced partially overlapping sets of genes in hepatocytes. In conclusion, the response to IFN-alpha is largely tissue-specific, and the response is rapidly downregulated in vivo, which may have a significant influence on the kinetics of antiviral response.

A locus for eosinophilia in the MES rat is on Chromosome 19

Matsumoto Eosinophilia Shinshu (MES) is a rat strain that spontaneously develops eosinophilia and eosinophil-related inflammatory lesions in many organs. We performed chromosomal mapping of the gene for eosinophilia by breeding backcross progeny. The onset of eosinophilia appeared to be delayed in the progeny compared with that in MES, with the prevalence of eosinophilia in the backcross progeny at 12 weeks of age being 22.5%. Genetic linkage analysis with marker loci indicated the major locus for eosinophilia was located at the end of the q arm region of Chromosome 19 (between D19Rat8 and telomere). The locus was denoted eosinophilia 1 (eos1). These data will form the basis for identification of the eos1 gene using a reverse genetic approach, which will hopefully lead to elucidation of the mechanisms involved in eosinophilia and eosinophilopoiesis.

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.

Ubiquitin-dependent degradation of interferon regulatory factor-8 mediated by Cbl down-regulates interleukin-12 expression

Interferon regulatory factor (IRF)-8/interferon consensus sequence-binding protein is regulated by both transcription and degradation. IRF-8 induced in peritoneal macrophages by interferon-gamma and lipopolysaccharide was degraded rapidly, and degradation of IRF-8 was blocked by MG132, the proteasome inhibitor, but inhibitors of calpain and lysosomal enzymes had no effect. The ubiquitination of IRF-8 was shown by co-immunoprecipitation from RAW264.7 macrophages retrovirally transduced with IRF-8 and hemagglutinin-ubiquitin. The dominant negative ubiquitin mutants K48R and K29R inhibited IRF-8 degradation in 293T cells, confirming the relationship between ubiquitination of IRF-8 and its degradation. IRF-8 carboxyl-terminal truncation mutants were not ubiquitinated and were consequently stable, indicating that the carboxyl-terminal domain of IRF-8 controls ubiquitination. The ubiquitin-protein isopeptide ligase (E3) that ubiquitinated IRF-8 was likely to be Cbl, which formed a complex with IRF-8, demonstrable by both immunoprecipitation and gel filtration. Furthermore, IRF-8 stability was increased by dominant negative Cbl, and IRF-8 ubiquitination was significantly attenuated in Cbl-/- cells. Reflecting increased stability and expression, the IRF-8 carboxyl-terminal deletion mutant induced interleukin (IL)-12 p40 promoter activity much more strongly than IRF-8 did. Furthermore, IRF-8-induced IL-12 p40 synthesis in RAW264.7 cells was enhanced by dominant negative Cbl, and peritoneal macrophages from Cbl-/- mice showed increased IL-12 p40 protein production. Taken together, these results suggest that the proteasomal degradation of IRF-8 mediated by the ubiquitin E3 ligase Cbl down-regulates IL-12 expression.

Roles of interferon-regulatory factors in T-helper-cell differentiation

Members of the interferon-regulatory factor family of transcription factors have long been known to be intracellular mediators of the effects of interferons. In recent years, interferon-regulatory factors have also been shown to have an essential role in the differentiation of T helper cells, both by modulating the functions of antigen-presenting cells and by having direct effects on the T helper cells themselves. Depending on the interferon-regulatory factor involved, the differentiation of T helper cells to either T helper 1 cells or T helper 2 cells can be influenced. In this article, we provide an overview of this relatively new and still underappreciated role of interferon-regulatory factors.

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.

Impaired innate host defense causes susceptibility to respiratory virus infections in cystic fibrosis

Viral infection is the primary cause of respiratory morbidity in cystic fibrosis (CF) infants. Here, we identify that host factors allow increased virus replication and cytokine production, providing a mechanism for understanding the severity of virus disease in CF. Increased virus is due to lack of nitric oxide synthase 2 (NOS2) and 2', 5' oligoadenylate synthetase (OAS) 1 induction in response to virus or IFNgamma. This can be attributed to impairment of activation of signal transducer and activator of transcription (STAT)1, a fundamental component to antiviral defense. NO donor or NOS2 overexpression provides protection from virus infection in CF, suggesting that NO is sufficient for antiviral host defense in the human airway and is one strategy for antiviral therapy in CF children.

Complex formation of the interferon (IFN) consensus sequence-binding protein with IRF-1 is essential for murine macrophage IFN-gamma-induced iNOS gene expression

This study describes the role of the interferon (IFN) consensus sequence-binding protein (ICSBP or IRF-8) in iNOS gene expression by murine macrophages. An ICSBP binding site in the iNOS promoter region (-923 to -913) was identified using an electrophoretic mobility shift assay and chromatin co-immunoprecipitation. Overexpression of ICSBP greatly enhanced IFN-gamma-induced iNOS promoter activation in RAW264.7 cells, and IFN-gamma-induced iNOS promoter activation was abolished in ICSBP-/- macrophages. Furthermore, transduction of retrovirus-ICSBP in ICSBP-/- macrophages rescued IFN-gamma-induced iNOS gene expression. However, transduction of retrovirus-ICSBP in the absence of IFN-gamma activation did not induce iNOS expression in either RAW264.7 cells or ICSBP-/- macrophages. Interestingly, ICSBP alone transduced into ICSBP-/- macrophages did not bind to IFN-stimulated response element site (-923 to -913) of the iNOS promoter region, although following activation with IFN-gamma, a DNA.protein complex was formed that contains ICSBP and IRF-1. Co-transduction of ICSBP with IRF-1 clearly induces nitric oxide production. In addition, interleukin-4 inhibits IFN-gamma-induced iNOS gene expression by attenuating the physical interaction of ICSBP with IRF-1. Complex formation of ICSBP with IRF-1 is essential for iNOS expression, and interleukin-4 attenuates the physical interaction of ICSBP with IRF-1 resulting in the inhibition of INOS gene expression.

IRF-8/ICSBP and IRF-1 cooperatively stimulate mouse IL-12 promoter activity in macrophages

IRF-8/ICSBP and IRF-1 are IRF family members whose expression is induced in response to IFN-gamma in macrophages. IL-12 is a cytokine produced in macrophages that plays a critical role in host defense. IFN-gamma and bacterial lipopolysaccharide (LPS) induce IL-12p40 transcription, which is necessary for the production of IL-12. We have previously shown that IL-12p40 expression is impaired in ICSBP-deficient mice and that transfection of ICSBP together with IRF-1 can activate IL-12p40 expression in mouse macrophage cells. To further study the role of ICSBP and IRF-1, we investigated murine IL-12p40 promoter activity in the macrophage cell line RAW 264.7. We show here that co-transfection of ICSBP and IRF-1 synergistically stimulates IL-12 promoter activity to a level comparable to that induced by IFN-gamma/LPS. Mutation of the Ets or NFkappaB site previously shown to be important for IL-12p40 transcription did not abolish the activation by ICSBP and IRF-1. However, mutation of the ISRE-like site found downstream from the NFkappaB and C/EBP sites abrogated the activation by ICSBP and IRF-1. Together, these results indicate that ICSBP and IRF-1 cooperatively stimulate murine IL-12 transcription through a novel regulatory element in the murine promoter.

A novel transactivating factor that regulates interferon-gamma-dependent gene expression

We have previously identified a novel interferon (IFN)-stimulated cis-acting enhancer element, gamma-IFN-activated transcriptional element (GATE). GATE differs from the known IFN-stimulated elements in its primary sequence. Preliminary analysis has indicated that the GATE-dependent transcriptional response requires the binding of novel transacting factors. A cDNA expression library derived from an IFN-gamma-stimulated murine macrophage cell line was screened with a (32)P-labeled GATE probe to identify the potential GATE-binding factors. A cDNA coding for a novel transcription-activating factor was identified. Based on its discovery, we named it as GATE-binding factor-1 (GBF-1). GBF-1 homologs are present in mouse, human, monkey, and Drosophila. It activates transcription from reporter genes carrying GATE. It possesses a strong transactivating activity but has a weak DNA binding property. GBF-1 is expressed in most tissues with relatively higher steady-state levels in heart, liver, kidney, and brain. Its expression is induced by IFN-gamma treatment. GBF-1 is present in both cytosolic and nuclear compartments. These studies thus identify a novel transactivating factor in IFN signaling pathways.

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.

Preferential binding sites for interferon regulatory factors 3 and 7 involved in interferon-A gene transcription

Transcription of the murine interferon-A4 (IFN-A4) gene is mediated by a virus responsive element (VRE-A4) located in the promoter proximal [-120 to -43] region. VRE-A4 contains four DNA modules (A to D) which cooperate for maximal IFN-A4 activation following virus infection. The differential expression between the highly expressed IFN-A4 and the weakly inducible IFN-A11 gene promoters is essentially due to point mutations within the C and D modules of the virus-responsive element VRE-A11. We now demonstrate that in murine L929 and human 293 cells, transcription factors IRF-3 and IRF-7, which are potent activators of virus-induced type I IFN transcription, differentially affect IFN-A4 and IFN-A11 promoter activities. Using electrophoretic mobility shift assays and DNase I footprinting data, our studies demonstrate that the AB modules correspond to a preferential site for IRF-7, whereas the C module is preferentially recognized by IRF-3. Furthermore, transfection of reporter constructs driven by four copies of different GAAANN hexameric motifs found within VRE-A4 indicates that the NN residues of these hexameric sequences define the preferential binding sites for IRF-3 or IRF-7. Together, these experiments clarify the molecular basis for differential expression of IFN-A genes following virus infection by delineating the sequence requirements for IRF association with the virus responsive elements of the IFN-A genes.

Apoptosis and cell growth inhibition as antitumor effector functions of interferons

Since their introduction to the clinic some 30 yr ago, interferons (IFNs) have become standard therapy for a range of disorders, including malignant and benign tumors as well as various viral diseases. Although IFNs will induce remissions in some patients with cancer, they are of no benefit or, at best, lead only to minor improvements in the great majority of patients with malignant disease. One of the great challenges of IFN research is to understand the multiple ways by which IFNs influence the behavior of tumor cells and to identify the factors that underlie the resistance of some tumors to IFNs. This review is written with a focus on two anticellular effects of IFN, inhibition of proliferation and induction of apoptosis, possible mechanisms underlying the antitumor action of IFN. In addition, possible reasons for IFN tumor cell resistance are also discussed.

The role of IRF-4 in B and T cell activation and differentiation

Appropriate activation and differentiation of lymphocytes are critical for effective immune responses. These processes are normally guided by exposure of lymphocytes to different stimuli, which need to be appropriately integrated in order for lymphocytes to proceed along their activation and differentiation pathways. Although the early steps in lymphocyte activation have been studied extensively, the downstream effectors of these activation pathways and the basic mechanisms employed by lymphocytes to integrate the information provided by different activation stimuli are not fully characterized. Interferon (IFN) regulatory factor-4 (IRF-4) is a recently described member of the IRF family of transcription factors whose expression is largely restricted to lymphocytes. Genetic studies have indicated that IRF-4 is critical for the function of mature T and B cells. Here we review the role of IRF-4 as a downstream effector and potentially an integrator of lymphocyte responses.

ICSBP/IRF-8: its regulatory roles in the development of myeloid cells

Interferon (IFN) consensus sequence binding protein (ICSBP)/IFN regulatory factor (IRF)-8 is an IFNgamma-inducible transcription factor of the IRF family and regulates transcription through multiple target DNA elements, such as IFN-stimulated response element (ISRE), Ets/IRF composite element, and IFN-gamma activation site (GAS). ICSBP(-/-) mice are immunodeficient and susceptible to various pathogens. They have defects in the macrophage function, including the ability to induce interleukin-12 (IL-12) p40 and some IFN-gamma-responsible genes. In addition, ICSBP(-/-) mice develop a chronic myelogenous leukemia (CML)-like syndrome, where a systemic expansion of granulocytes is followed by a fatal blast crisis. ICSBP(-/-) mice harbor an increased number of myeloid progenitor cells, and the -/- progenitors preferentially give rise to granulocytes, although they cannot efficiently generate another descendant of the myeloid lineage, macrophages. Studies with myeloid progenitor cells have shown that ICSBP drives their differentiation toward macrophage, whereas it inhibits granulocyte differentiation. Furthermore, myeloid cells from ICSBP(-/-) mice are resistant to apoptosis. These results illustrate the mechanism by which the loss of ICSBP leads to immunodeficiency and CML-like syndrome and suggest ICSBP's critical role in the development of myeloid cells.

The role of IRF-4 in transcriptional regulation

Gene expression is a tightly regulated process involving multiple levels of control spanning histone acetylation to protein turnover. One of the first events in this cascade is transcription, which itself is a multistep process involving protein-protein interaction and macromolecular assembly. Here we review the role of the interferon (IFN) regulatory factor (IRF) transcription factor family member IRF-4 in transcriptional regulation. IRF-4 was initially characterized in lymphocytes and was shown to function as both a transcriptional repressor and activator. More recently, IRF-4 expression and function have been reported in macrophages. The ability of IRF-4 to serve as both a transcriptional activator and repressor is determined, in part, by binding to distinct DNA-binding motifs and through interaction with various additional transcription factors, most notably with the Ets family member PU.1. The details governing these functional differences are the focus of this review. Importantly, the role of posttranslational modification and nuclear translocation of IRF-4 in transcriptional regulation are addressed. Several possible paradigms of transcriptional regulation by IRF-4 are proposed, where these paradigms may describe regulatory mechanisms common to many distinct transcription factor families.

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

Interferon (IFN) consensus sequence binding protein (ICSBP) is a member of a family of transcription factors termed IFN regulatory factors (IRF) and is also called IRF-8. Its expression is restricted mainly to cells of the immune system, and it plays a key role in the maturation of macrophages. ICSBP exerts its activity through the formation of different DNA-binding heterocomplexes. The interacting partner dictates a specific DNA recognition sequence, thus rendering ICSBP dual transcriptional activity, that is, repression or activation. Accordingly, such DNA elements were identified at the promoter regions of target genes that manifest macrophage action. A specific module (IRF association domain [IAD]) within ICSBP and a PEST domain located on the interacting partners mediate this association. Thus, ICSBP serves as an excellent prototype, demonstrating how a small subset of transcription factors can regulate gene expression in a spatial, temporal, and delicate tuning through combinatorial protein-protein interactions on different enhanceasomes.

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.

Specific deficiency in nuclear factor-kappaB activation in neurons of the central nervous system

The expression and activation of nuclear factor-kappaB (NF-kappaB) in neurons and glia of the central nervous system (CNS) has been intensely investigated because of its potential importance in understanding how this multifunctional transcription factor controls developmental and pathological processes. In particular, there has been interest in how NF-kappaB may be differentially regulated in these two major functional subgroups of cells in the CNS to provide for specific responses to various stimuli. Of special interest are responses to both proinflammatory cytokines and microbial products that signal from specific cell receptors to activate NF-kappaB. In the present studies, both neurons and glia (astrocytes) in vivo expressed latent cytoplasmic NF-kappaB analyzed by immunofluorescence microscopy and electrophoretic mobility shift analysis. In vitro, neurons and astrocytes expressed comparable levels of latent NF-kappaB molecules, but NF-kappaB nuclear localization stimulated by proinflammatory cytokines or microbial products was markedly deficient in neurons. In accord with this finding, the rapid degradation of inhibitor of NF-kappaB alpha (IkappaBalpha) that is seen in astrocytes did not occur in neurons in response to these agents. However, long-term exposure to translational inhibitors resulted in IkappaBalpha decay and activation of latent NF-kappaB in neurons, indicating potential NF-kappaB activity in these cells. Analysis of NF-kappaB-responsive interferon regulatory factor-1 gene expression indicated that increased nuclear NF-kappaB in neurons had transcriptional potential. We conclude that mechanisms responsible for inducible targeting of IkappaBalpha are uniquely regulated in neurons and account for the hypo-responsiveness of these cells to signals generated during microbial infections in the CNS. Thus, modulation of signals that target IkappaBalpha degradation may be unique and a key component of specific NF-kappaB regulation in neurons.

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.