Interferon consensus sequence-binding protein is constitutively expressed and differentially regulated in the ocular lens

IRF8 Regulates Intrinsic Ferroptosis through Repressing p53 Expression to Maintain Tumor Cell Sensitivity to Cytotoxic T Lymphocytes

Ferroptosis has emerged as a cytotoxic T lymphocyte (CTL)-induced tumor cell death pathway. The regulation of tumor cell sensitivity to ferroptosis is incompletely understood. Here, we report that interferon regulatory factor 8 (IRF8) functions as a regulator of tumor cell intrinsic ferroptosis. Genome-wide gene expression profiling identified the ferroptosis pathway as an IRF8-regulated pathway in tumor cells. IRF8.KO tumor cells acquire resistance to intrinsic ferroptosis induction and IRF8-deficient tumor cells also exhibit decreased ferroptosis in response to tumor-specific CTLs. Irf8 deletion increased p53 expression in tumor cells and knocking out p53 in IRF8.KO tumor cells restored tumor cell sensitivity to intrinsic ferroptosis induction. Furthermore, IRF8.KO tumor cells grew significantly faster than WT tumor cells in immune-competent mice. To restore IRF8 expression in tumor cells, we designed and synthesized codon usage-optimized IRF8-encoding DNA to generate IRF8-encoding plasmid NTC9385R-mIRF8. Restoring IRF8 expression via a lipid nanoparticle-encapsulated NTC9385R-mIRF8 plasmid therapy suppressed established tumor growth in vivo. In human cancer patients, nivolumab responders have a significantly higher IRF8 expression level in their tumor cells as compared to the non-responders. Our data determine that IRF8 represses p53 expression to maintain tumor cell sensitivity to intrinsic ferroptosis.

IRF8: Mechanism of Action and Health Implications

Interferon regulatory factor 8 (IRF8) is a transcription factor of the IRF protein family. IRF8 was originally identified as an essentialfactor for myeloid cell lineage commitment and differentiation. Deletion of Irf8 leads to massive accumulation of CD11b⁺Gr1⁺ immature myeloid cells (IMCs), particularly the CD11b⁺Ly6C^hi/+Ly6G⁻ polymorphonuclear myeloid-derived suppressor cell-like cells (PMN-MDSCs). Under pathological conditions such as cancer, Irf8 is silenced by its promoter DNA hypermethylation, resulting in accumulation of PMN-MDSCs and CD11b⁺ Ly6G⁺Ly6C^lo monocytic MDSCs (M-MDSCs) in mice. IRF8 is often silenced in MDSCs in human cancer patients. MDSCs are heterogeneous populations of immune suppressive cells that suppress T and NK cell activity to promote tumor immune evasion and produce growth factors to exert direct tumor-promoting activity. Emerging experimental data reveals that IRF8 is also expressed in non-hematopoietic cells. Epithelial cell-expressed IRF8 regulates apoptosis and represses Osteopontin (OPN). Human tumor cells may use the IRF8 promoter DNA methylation as a mechanism to repress IRF8 expression to advance cancer through acquiring apoptosis resistance and OPN up-regulation. Elevated OPN engages CD44 to suppress T cell activation and promote tumor cell stemness to advance cancer. IRF8 thus is a transcription factor that regulates both the immune and non-immune components in human health and diseases.

An osteopontin/CD44 immune checkpoint controls CD8+ T cell activation and tumor immune evasion

Despite breakthroughs in immune checkpoint inhibitor (ICI) immunotherapy, not all human cancers respond to ICI immunotherapy and a large fraction of patients with the responsive types of cancers do not respond to current ICI immunotherapy. This clinical conundrum suggests that additional immune checkpoints exist. We report here that interferon regulatory factor 8 (IRF8) deficiency led to impairment of cytotoxic T lymphocyte (CTL) activation and allograft tumor tolerance. However, analysis of chimera mice with competitive reconstitution of WT and IRF8-KO bone marrow cells as well as mice with IRF8 deficiency only in T cells indicated that IRF8 plays no intrinsic role in CTL activation. Instead, IRF8 functioned as a repressor of osteopontin (OPN), the physiological ligand for CD44 on T cells, in CD11b+Ly6CloLy6G+ myeloid cells and OPN acted as a potent T cell suppressor. IRF8 bound to the Spp1 promoter to repress OPN expression in colon epithelial cells, and colon carcinoma exhibited decreased IRF8 and increased OPN expression. The elevated expression of OPN in human colon carcinoma was correlated with decreased patient survival. Our data indicate that myeloid and tumor cell-expressed OPN acts as an immune checkpoint to suppress T cell activation and confer host tumor immune tolerance.

Production of IL-35 by Bregs is mediated through binding of BATF-IRF-4-IRF-8 complex to il12a and ebi3 promoter elements

IL-10 and IL-35 suppress excessive immune responses and therapeutic strategies are being developed to increase their levels in autoimmune diseases. In this study, we sought to identify major cell types that produce both cytokines in-vivo and to characterize mechanisms that regulate their production. Experimental autoimmune uveitis (EAU) is a CNS autoimmune disease that serves as model of human uveitis. We induced EAU in C57BL/6J mice and investigated whether T cells, B lymphocytes, or myeloid cells are the major producers of IL-10 or IL-35 in blood, lymph nodes (LNs), spleen, and at the site of ocular inflammation, the neuroretina. Analysis of these tissues identified B cells as the major producers of IL-10 and IL-35 in-vivo. Compared to regulatory T cells (Tregs), IL-10- or IL-35-producing regulatory B cells (Bregs) are substantially expanded in blood, LNs, spleen, and retina of mice with EAU. We performed EMSA and chromatin immunoprecipitation (ChIP) assays on activated B cells stimulated with IL-35 or TLR agonists. We found that BATF, IFN regulatory factor (IRF)-4, and IRF-8 transcription factors were recruited and bound to AP1-IRF-composite elements (AICEs) of il12a, ebi3, and/or il10 loci, suggesting their involvement in regulating IL-10 and IL-35 transcriptional programs of B cells. Showing that B cells are major source of IL-10 and IL-35 in-vivo and identifying transcription factors that contribute to IL-10 and IL-35 expression in the activated B-cell, suggest that the BATF/IRF-4/IRF-8 axis can be exploited therapeutically to regulate physiological levels of IL-10/IL-35-Bregs and that adoptive transfer of autologous Bregs might be an effective therapy for autoimmune and neurodegenerative diseases.

Interleukin 27 induces the expression of complement factor H (CFH) in the retina

Complement factor H (CFH) is a central regulator of the complement system and has been implicated in the etiology of age-related macular degeneration (AMD), a leading cause of blindness in the elderly. In view of previous studies showing that reduced expression of CFH in the retina is a risk factor for developing AMD, there is significant interest in understanding how CFH expression is regulated in the retina. In this study, we have shown that the anti-inflammatory cytokine, IL-27, induced CFH expression in mouse retinal cells and human retinal pigmented epithelial cells (RPE) through STAT1-mediated up-regulation of Interferon Regulatory Factor-1 (IRF-1) and IRF-8. We further show that cells in the ganglion and inner-nuclear layers of the retina constitutively express IRF-1 and IRF-8 and enhanced CFH expression in the retina during ocular inflammation correlated with significant increase in the expression of IRF-1, IRF-8 and IL-27 (IL-27p28 and Ebi3). Our data thus reveal a novel role of IL-27 in regulating complement activation through up-regulation of CFH and suggest that defects in IL-27 signaling or expression may contribute to the reduction of CFH expression in the retina of patients with AMD.

Differential expression of IRF8 in subsets of macrophages and dendritic cells and effects of IRF8 deficiency on splenic B cell and macrophage compartments

IRF8, a transcription factor restricted primarily to hematopoietic cells, is known to influence the differentiation and function of dendritic cells (DC), macrophages, granulocytes and B cells. In human tonsil, IRF8 is expressed at high levels by intrafollicular macrophages and DC, but at much lower levels by tingible body macrophages in germinal centers (GCs) and little, if at all, by follicular DC. Spleens of IRF8-deficient mice had reduced numbers of white pulp follicles and GCs that were irregular in shape. The frequency of follicular B cells was significantly reduced while the population of marginal zone (MZ) B cells was increased. In addition, MZ macrophages were reduced in number and abnormally distributed, while metallophilic macrophages were normal. These findings demonstrate differential requirements for IRF8 among distinct subsets of B cells, DC, and macrophages.

SOCS1 regulates CCR7 expression and migration of CD4+ T cells into peripheral tissues

Suppressors of cytokine signaling (SOCS) proteins control many aspects of lymphocyte function through regulation of STAT pathways. SOCS1-deficient mice develop severe skin and eye diseases that result from massive infiltration of inflammatory cells into these tissues. In this study, we have used SOCS1-, STAT1-, or STAT6-deficient mice, as well as, T cells with stable overexpression or deletion of SOCS1, to examine whether SOCS1 is involved in regulating lymphocyte trafficking to peripheral tissues. We show that SOCS1-deficient mice have increased numbers of T cells with characteristics of effector memory cells and expression of CCR7, a protein that promotes retention of T cells in lymphoid tissues, is markedly reduced in these cells. The decrease in CCR7 expression correlates with hyperactivation of STAT6, suggesting that aberrant recruitment of T cells into SOCS1-deficient mouse skin or eye results from abrogation of negative feedback regulation of STAT6 activation and CCR7 expression. Consistent with in vivo regulation of CCR7 expression and lymphocyte migration by SOCS1, forced overexpression of SOCS1 in T cells up-regulates CCR7 expression and enhances chemotaxis toward CCL19 or CCL21. CCR6 and CXCR3 are also up-regulated on SOCS1-deficient T cells and in situ analysis of the cornea or retina further reveal that these cells may mediate the chronic skin and eye inflammation through recruitment of Th1 and Th17 cells into these tissues. Collectively, these results suggest that SOCS1 regulates steady-state levels of chemokine receptors through its inhibitory effects on STAT pathways and this may underscore its role in regulating recruitment and retention of effector cells into nonlymphoid tissues.

Interferon-γ induces regression of epithelial cell carcinoma: critical roles of IRF-1 and ICSBP transcription factors

We have developed an epithelial cell carcinoma model for studying efficacy of IFNgamma gene therapy and have identified components of IFNgamma-signaling pathway responsible for its direct anti-tumor actions. The tumor results from ectopic expression of SV40 Large T-Antigen (SV40 T-Ag) oncogene in lens of transgenic mouse (alphaT3) and complete regression of the tumor is induced by targeting expression of IFNgamma into malignant lens cells. Inflammatory cells are absent in lens of alphaT3 or DT (co-expressing IFNgamma and SV40-T-Antigen) mice and the transformed lens cells are non-immunogenic, suggesting non-involvement of immunologic cells. We show that IFNgamma has direct growth-inhibitory effects on tumor cells, induces death of tumor cells by apoptosis and that these effects are mediated by two transcription factors, IRF-1 (interferon-regulatory factor-1) and ICSBP (interferon-consensus sequence-binding protein) induced by IFNgamma. Furthermore, stable transfection with ICSBP or IRF-1 construct inhibits lens carcinoma cell growth by upregulating Caspase-1, p21(WAF1) and p27 expression. In contrast, tumor progression in alphaT3 lens correlates with inhibition of IRF-1 and ICSBP expression. Our results suggest that IFNgamma gene therapy maybe effective in malignant diseases for which DNA tumor viruses are etiologic agents and that antitumor actions of IRF-1/ICSBP can be exploited therapeutically to circumvent adverse clinical effects associated with IFN therapy.

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.

SOCS5 mRNA levels in peripheral blood mononuclear cells (PBMC): a potential bio-marker for monitoring response of uveitis patients to Daclizumab therapy

Uveitis is an intraocular inflammatory disease mediated by Th1 lymphocytes. Therapy for severe uveitis is frequently long-term immunosuppression using systemic corticosteroids and cytotoxic agents, but side effects make long-term therapy difficult. Long-term humanized anti-interleukin-2 (IL-2) receptor alpha (Daclizumab) therapy has few side effects and is as effective as standard immunosuppression for treating severe uveitis. However, it is necessary to carefully monitor levels of activated T cells in the eye to allow prompt re-institution of standard immunosuppressive therapy to non-responders to Daclizumab therapy. Suppressors of cytokine signaling (SOCS) are feedback regulators of Th1/Th2 cytokines. SOCS5 and SOCS3 are preferentially expressed in Th1 and Th2 cells, respectively, and are thought to be lineage markers for T-helper cells. In this study, we have investigated whether SOCS3 or SOCS5 expression can serve as surrogate markers of T-cell levels in the eye. Compared to healthy volunteers, SOCS5 mRNA is significantly elevated in PBMC of uveitis patients while SOCS3 is decreased. However, after Daclizumab therapy SOCS5 mRNA level is significantly decreased, suggesting that SOCS5 mRNA can be used as diagnostic tool to monitor therapeutic response of uveitis patients. Our data also suggest that SOCS5 may serve as a new therapeutic target for uveitis and other autoimmune diseases.

Cell proliferation and STAT6 pathways are negatively regulated in T cells by STAT1 and suppressors of cytokine signaling

Suppressor of cytokine signaling (SOCS) proteins have emerged as important regulators of cytokine signals in lymphocytes. In this study, we have investigated regulation of SOCS expression and their role in Th cell growth and differentiation. We show that SOCS genes are constitutively expressed in naive Th cells, albeit at low levels, and are differentially induced by Ag and Th-polarizing cytokines. Whereas cytokines up-regulate expression of SOCS1, SOCS2, SOCS3, and cytokine-induced Src homology 2 protein, Ags induce down-regulation of SOCS3 within 48 h of Th cell activation and concomitantly up-regulate SOCS1, SOCS2, and cytokine-induced Src homology 2 protein expression. We further show that STAT1 signals play major roles in inducing SOCS expression in Th cells and that induction of SOCS expression by IL-4, IL-12, or IFN-gamma is compromised in STAT1-deficient primary Th cells. Surprisingly, IL-4 is a potent inducer of STAT1 activation in Th2 but not Th1 cells, and SOCS1 or SOCS3 expression is dramatically reduced in STAT1(-/-) Th2 cells. To our knowledge, this is the first report of IL-4-induced STAT1 activation in Th cells, and suggests that its induction of SOCS, may in part, regulate IL-4 functions in Th2 cells. In fact, overexpression of SOCS1 in Th2 cells represses STAT6 activation and profoundly inhibits IL-4-induced proliferation, while depletion of SOCS1 by an anti-sense SOCS1 cDNA construct enhances cell proliferation and induces constitutive activation of STAT6 in Th2 cells. These results are consistent with a model where IL-4 has dual effects on differentiating T cells: it simulates proliferation/differentiation through STAT6 and autoregulates its effects on Th2 growth and effector functions via STAT1-dependent up-regulation of SOCS proteins.

Immunoproteasome expression in a nonimmune tissue, the ocular lens

Interferon gamma (IFN gamma) induces the expression of three catalytic subunits of the 20S proteasome that can replace their constitutive homologues to form the "immunoproteasome," named to reflect its antigen presentation function. However, immunoproteasome levels and their modulation in nonimmune tissues remain unknown. A disrupted lens differentiation program observed in transgenic mice that constitutively express IFN gamma in the immune-privileged lens tissue suggests a role for this cytokine in differentiation. We have developed a competitive RT-PCR assay that demonstrates substantially increased levels of immuno subunits and unchanged levels of constitutive subunits in transgenic compared to wild-type lenses. Similar results were observed with IFN gamma treated alpha TN4-1 lens epithelial cells. A comparison of these subunits in different immune and nonimmune mouse tissues revealed unique expression patterns. The presence of immuno subunits in nonimmune tissues such as lens suggests that the immunoproteasome may also have nonimmune functions, such as that in lens differentiation.

Suppressors of cytokine signaling proteins are differentially expressed in Th1 and Th2 cells: implications for Th cell lineage commitment and maintenance

Positive regulatory factors induced by IL-12/STAT4 and IL-4/STAT6 signaling during T cell development contribute to polarized patterns of cytokine expression manifested by differentiated Th cells. These two critical and antagonistic signaling pathways are under negative feedback regulation by a multimember family of intracellular proteins called suppressor of cytokine signaling (SOCS). However, it is not known whether these negative regulatory factors also modulate Th1/Th2 lineage commitment and maintenance. We show here that CD4(+) naive T cells constitutively express low levels of SOCS1, SOCS2, and SOCS3 mRNAs. These mRNAs and their proteins increase significantly in nonpolarized Th cells after activation by TCR signaling. We further show that differentiation into Th1 or Th2 phenotype is accompanied by preferential expression of distinct SOCS mRNA transcripts and proteins. SOCS1 expression is 5-fold higher in Th1 than in Th2 cells, whereas Th2 cells contain 23-fold higher levels of SOCS3. We also demonstrate that IL-12-induced STAT4 activation is inhibited in Th2 cells that express high levels of SOCS3 whereas IL-4/STAT6 signaling is constitutively activated in Th2 cells, but not Th1 cells, with high SOCS1 expression. These results suggest that mutually exclusive use of STAT4 and STAT6 signaling pathways by differentiated Th cells may derive in part, from SOCS3- or SOCS1-mediated repression of IL-12/STAT4- or IL-4/STAT6 signaling in Th2 and Th1 cells, respectively. Given the strong correlation between distinct patterns of SOCS expression and differentiation into the Th1 or Th2 phenotype, SOCS1 and SOCS3 proteins are therefore Th lineage markers that can serve as therapeutic targets for immune modulation therapy.

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.

PU.1 and Multiple IFN Regulatory Factor Proteins Synergize to Mediate Transcriptional Activation of the Human IL-1β Gene

Both lymphoid and myeloid cells express two related members of the IFN regulatory factor (IRF) family of transcription factors, specifically IRF-4 and IFN consensus binding protein (ICSBP or IRF-8). We previously reported that macrophages express IRF-4 and in combination with the ETS-like protein PU.1 can synergistically activate a human IL-1beta reporter gene. Here we report that this synergy is mediated by a composite PU.1/IRF element located within an upstream enhancer known to confer cytokine- and LPS-inducible expression. In macrophages, synergistic activation of IL-1beta reporter gene expression was preferentially mediated by IRF-4, whereas IRF-4 and ICSBP were equally capable of synergizing with PU.1 when coexpressed in fibroblasts. Furthermore, coexpression of IRF-1 and IRF-2 dramatically increased the capacity of both PU.1/IRF-4 and PU.1/ICSBP to induce IL-1beta reporter gene expression in fibroblasts. The additional synergy observed with IRF-1 and IRF-2 coexpression is mediated by a region of DNA distinct from either the IL-1beta enhancer or promoter. We also assessed the capacity of these transcription factors to activate endogenous IL-1beta gene when overexpressed in human embryonic kidney 293 cells. Although ectopic expression of PU.1 alone was sufficient to activate modest levels of IL-1beta transcripts, endogenous IL-1beta expression was markedly increased following coexpression of additional IRF proteins. Thus, maximal expression of both a human IL-1beta reporter gene and the endogenous IL-1beta gene was observed in cells that coexpressed PU.1, IRF-4 (or ICSBP), IRF1, and IRF2. Together, our observations suggest that these factors may function together as an enhanceosome.

Interferon regulatory factor subcellular localization is determined by a bipartite nuclear localization signal in the DNA-binding domain and interaction with cytoplasmic retention factors

The transduction of type I interferon signals to the nucleus relies on activation of a protein complex, ISGF3, involving two signal transducers and activators of transcription (STAT) proteins, STAT1 and STAT2, and the interferon (IFN) regulatory factor (IRF) protein, p48/ISGF3gamma. The STAT subunits are cytoplasmically localized in unstimulated cells and rapidly translocate to the nucleus of IFN-stimulated cells, but the p48/ISGF3gamma protein is found in both the nucleus and the cytoplasm, regardless of IFN stimulation. Here, we demonstrate that p48 is efficiently and constitutively targeted to the nucleus. Analysis of the subcellular distribution of green fluorescent protein-p48 fragments indicates that p48 contains a bipartite nuclear retention signal within its amino-terminal DNA-binding domain. This signal is preserved in two other IRF proteins involved in immune responses, ICSBP and IRF4. Mutations to clustered basic residues within amino acids 50-100 of p48 or IRF4 disrupt their nuclear accumulation, and DNA-binding ability is not required for nuclear targeting. This is the only example of a nuclear localization signal for any ISGF3 component and assigns a second function to the IRF DNA-binding domain. We also demonstrate that the nuclear distribution of p48 is dramatically altered by coexpression of the STAT2 protein, indicating that STAT2 forms a cytoplasmic complex with p48, overriding the intrinsic p48 nuclear targeting. Retention by STAT2 may serve to regulate the activity of free p48 and/or guarantee that cytoplasmic pools of preassociated STAT2:p48 are available for rapid activation of the IFN response. These findings suggest that analogous mechanisms may exist for regulating the distribution of other IRF proteins.

Differential Expression and Distinct Functions of IFN Regulatory Factor 4 and IFN Consensus Sequence Binding Protein in Macrophages

IFN regulatory factor 4 (IRF4) and IFN consensus sequence binding protein (ICSBP) are highly homologous members of the growing family of IRF proteins. ICSBP expression is restricted to lymphoid and myeloid cells, whereas IRF4 expression has been reported to be lymphoid-restricted. We present evidence that primary murine and human macrophages express IRF4, thereby extending its range of expression to myeloid cells. Here, we provide a comparative analysis of IRF4 and ICSBP expression and function in distinct cell types. These IRF proteins can form specific complexes with the Ets-like protein PU.1, and can activate transcription via binding to PU.1/IRF composite sequences. EMSA analysis revealed that murine macrophages contained both IRF4/PU.1 and ICSBP/PU.1 complexes, analogous to B cells. Over-expression of ICSBP in these macrophages activated transcription of a PU.1/IRF-dependent promoter, whereas over-expression of IRF4 had no effect on this promoter. In contrast, over-expression of either IRF4 or ICSBP in both macrophages and NIH-3T3 fibroblasts suppressed transcription of the PU.1-independent H-2Ld MHC class I promoter. In NIH-3T3 fibroblasts, IRF4 and ICSBP also synergized with exogenous PU.1 to activate transcription of a PU.1/IRF-dependent promoter. Furthermore, both IRF4 and ICSBP activated transcription of the IL-1β promoter in both cell types. While this promoter is PU.1-dependent, it lacks any known PU.1/IRF composite binding sites. Synergistic activation of the IL-1β promoter by these IRF proteins and PU.1 was found to require PU.1 serine 148. Together, these data demonstrate that IRF4 and ICSBP are dichotomous regulators of transcription in macrophages.