IKKα negatively regulates IRF-5 function in a MyD88–TRAF6 pathway

Defactinib inhibits PYK2 phosphorylation of IRF5 and reduces intestinal inflammation

Interferon regulating factor 5 (IRF5) is a multifunctional regulator of immune responses, and has a key pathogenic function in gut inflammation, but how IRF5 is modulated is still unclear. Having performed a kinase inhibitor library screening in macrophages, here we identify protein-tyrosine kinase 2-beta (PTK2B/PYK2) as a putative IRF5 kinase. PYK2-deficient macrophages display impaired endogenous IRF5 activation, leading to reduction of inflammatory gene expression. Meanwhile, a PYK2 inhibitor, defactinib, has a similar effect on IRF5 activation in vitro, and induces a transcriptomic signature in macrophages similar to that caused by IRF5 deficiency. Finally, defactinib reduces pro-inflammatory cytokines in human colon biopsies from patients with ulcerative colitis, as well as in a mouse colitis model. Our results thus implicate a function of PYK2 in regulating the inflammatory response in the gut via the IRF5 innate sensing pathway, thereby opening opportunities for related therapeutic interventions for inflammatory bowel diseases and other inflammatory conditions.

Involvement of Interleukin-1 Receptor-Associated Kinase 4 and Interferon Regulatory Factor 5 in the Immunopathogenesis of SARS-CoV-2 Infection: Implications for the Treatment of COVID-19

Interleukin-1 receptor-associated kinase 4 (IRAK4) and interferon regulatory factor 5 (IRF5) lie sequentially on a signaling pathway activated by ligands of the IL-1 receptor and/or multiple TLRs located either on plasma or endosomal membranes. Activated IRF5, in conjunction with other synergistic transcription factors, notably NF-κB, is crucially required for the production of proinflammatory cytokines in the innate immune response to microbial infection. The IRAK4-IRF5 axis could therefore have a major role in the induction of the signature cytokines and chemokines of the hyperinflammatory state associated with severe morbidity and mortality in COVID-19. Here a case is made for considering IRAK4 or IRF5 inhibitors as potential therapies for the "cytokine storm" of COVID-19.

DJ-1 Regulates Microglial Polarization Through P62-Mediated TRAF6/IRF5 Signaling in Cerebral Ischemia-Reperfusion

The polarization of microglia/macrophage, the resident immune cells in the brain, plays an important role in the injury and repair associated with ischemia-reperfusion (I/R). Previous studies have shown that DJ-1 has a protective effect in cerebral I/R. We found that DJ-1 regulates the polarization of microglial cells/macrophages after cerebral I/R and explored the mechanism by which DJ-1 mediates microglial/macrophage polarization in cerebral I/R. Middle cerebral artery occlusion/reperfusion (MCAO/R) and oxygen and glucose deprivation/reoxygenation (OGD/R) models were used to simulate cerebral I/R in vivo and in vitro, respectively. DJ-1 siRNA and the DJ-1-based polypeptide ND13 were used to produce an effect on DJ-1, and the P62-specific inhibitor XRK3F2 was used to block the effect of P62. Enhancing the expression of DJ-1 induced anti-inflammatory (M2) polarization of microglia/macrophage, and the expression of the anti-inflammatory factors IL-10 and IL-4 increased. Interference with DJ-1 expression induced pro-inflammatory (M1) polarization of microglia/macrophage, and the expression of the proinflammatory factors TNF-α and IL-1β increased. DJ-1 inhibited the expression of P62, impeded the interaction between P62 and TRAF6, and blocked nuclear entry of IRF5. In subsequent experiments, XRK3F2 synergistically promoted the effect of DJ-1 on microglial/macrophage polarization, further attenuating the interaction between P62 and TRAF6.

Fish IKKα from Japanese eel (Anguilla japonica) can activate NF-κB, AP1, and type I IFN signaling pathways

Inhibitor of nuclear factor kappa-B kinase subunit alpha (IKKα) plays a pivotal role in the activation of nuclear factor kappa-B (NF-κB) pathway in response to pathogens infections in mammals, but the information about IKKα in the regulation of immune responses is still limited in teleost fishes. In the present study, the full-length cDNA of an IKKα homologue, AjIKKα, was cloned by 5' and 3' SMART RACE from Japanese eel, and its characteristics of expression in response to various PAMPs and A. hydrophila infection were investigated both in vivo and in vitro using quantitative real-time polymerase chain reaction (qRT-PCR). In addition, the subcellular localization of AjIKKα GFP fusion protein and the induction of AjIKKα in the activation of NF-κB, type I IFN and AP1 performed using Dual-Glo luciferase assay system were also detected. Sequence comparison analysis revealed that AjIKKα has typical conserved domains, including an N-terminal kinase domain, an ubiquitin-like domain, a scaffold dimerization domain, and a C-terminal NEMO-binding domain. The predicted three-dimensional structure of AjIKKα is similar to that of human IKKα. Quantitative real-time polymerase chain reaction (qRT-PCR) analysis revealed a broad expression for AjIKKα in a wide range of tissues, with the highest expression in the liver, followed by the intestine, gills, and spleen, and with a lower expression in the muscle and heart. The AjIKKα expressions in the liver and kidney were significantly induced following injection with the viral mimic poly I:C and Aeromonas hydrophila infection, whereas the bacterial mimic LPS down-regulated the expression of AjIKKα in the spleen. In vitro, the AjIKKα transcripts of Japanese eel liver cells were significantly enhanced by the treatment of LPS, poly I:C, CpG-DNA, and PGN or the stimulation of different concentration of Aeromonas hydrophila (1 × 10⁶ cfu/mL, 1 × 10⁷ cfu/mL, and 1 × 10⁸ cfu/mL). Luciferase assays demonstrated that AjIKKα expression could significantly induce NF-κB, AP-1 and type I IFN promoter activation in a dose-dependent manner. Additionally, subcellular localization studies showed that AjIKKα was evenly distributed in the cytoplasm in the natural state, but AjIKKα was found to aggregate into spots in the cytoplasm after the stimulation of LPS and poly I:C. These results collectively indicated that AjIKKα plays an important role in innate immunity of host against antibacterial and antiviral infection likely via the activation of NF-κB, AP1and type I IFN signaling pathway.

IRF5 Signaling in Phagocytes Is Detrimental to Neonatal Hypoxic Ischemic Encephalopathy

Immune responses to neonatal hypoxic ischemic encephalopathy (HIE) exacerbate brain injury. Phagocytes, including microglia, play a central role in the immune response, but how the activation of phagocytes is regulated remains elusive. Previously, we have reported that interferon regulatory factor 5 (IRF5) signaling is closely correlated with a pro-inflammatory microglial phenotype in adult mice after stroke. The present study investigated IRF5's regulatory role in post-HIE inflammation. Male IRF5 conditional knockout (CKO) and IRF5^fl/fl postnatal day 10 (P10) pups were subjected to the Rice-Vannucci model (RVM) to induce HIE. Outcomes including morphological and neurobehavioral changes were evaluated at day 7 after HIE. Microglia/macrophage phenotypes and inflammatory responses were evaluated by flow cytometry (FC), RT-PCR, and multiplex cytokine assays. Lenti-IRF5 virus was administered in microglia-neuron co-cultures to evaluate the effects of microglial IRF5 upregulation in ischemic neurons exposed to oxygen-glucose deprivation (OGD). Deletion of phagocytic IRF5 resulted in significantly decreased IRF5 expression, attenuated pro-inflammatory and enhanced anti-inflammatory responses to HIE, and improved outcomes compared with IRF5^fl/fl control pups. In vitro lentivirus transfection experiments revealed that overexpression of IRF5 in microglia amplified pro-inflammatory signals and exacerbated OGD-induced neuronal apoptosis and neurite fragmentation. IRF5 signaling mediates microglial pro-inflammatory activation and also affects anti-inflammatory responses. Phagocytic IRF5 signaling is detrimental in HIE and is a potential therapeutic target for post-ischemic inflammation.

Grass carp (Ctenopharyngodon idellus) TRAF6 up-regulates IFN1 expression by activating IRF5

In mammals, interferon regulatory factor 5 (IRF5) can be activated by tumor necrosis factor receptor-associated factor 6 (TRAF6). Upon activation, IRF5 translocates into the nucleus, where it binds to IFN promoter and up-regulates IFN expression. However, there are few reports on the molecular mechanism by which TRAF6 up-regulates IFN expression in fish. In this study, we explored how Grass carp (Ctenopharyngodon idellus) TRAF6 initiated innate immunity by activating IRF5. We found that CiTRAF6, CiIRF5 and CiIFN1 were all significantly up-regulated in LPS-stimulated CIK cells and the expression of CiTRAF6 was earlier than the expressions of CiIRF5 and CiIFN1. These findings suggested that CiIFN1 expression might be induced by CiTRAF6 in fish. CiIFN1 expression, CiIFN1 promoter activity and CO cells viability were all significantly up-regulated in the overexpression experiments, but they were significantly down-regulated in the gene silencing experiments. This indicated that CiTRAF6, along with CiIRF5, regulated CiIFN1 expression. The localization analysis found that both CiTRAF6 and CiIRF5 located in the cytoplasm. Following LPS stimulation, CiIRF5 was observed to translocate to the nucleus. GST-pull down and co-IP experiments revealed that CiTRAF6 interacted with CiIRF5. The colocalization analysis also showed that CiTRAF6 bound with CiIRF5 in the cytoplasm. Overexpression of CiTRAF6 increased the endogenous CiIRF5, promoted its ubiquitination and nuclear translocation. In conclusion, CiTRAF6 bound to CiIRF5 in the cytoplasm, and then activated CiIRF5, resulting in up-regulating the expression of CiIFN1.

Interferon regulatory factor 4/5 signaling impacts on microglial activation after ischemic stroke in mice

Microglial activation is a key element in initiating and perpetuating inflammatory responses to stroke. Interferon regulatory factor 5 (IRF5) and IRF4 signaling have been found critical in mediating macrophage pro‐inflammatory (M1) and anti‐inflammatory (M2) phenotypes, respectively, in peripheral inflammation. We hypothesize that the IRF5/4 regulatory axis also mediates microglial activation after stroke. C57BL6 mice of 8–12 weeks were subject to a 90‐min middle cerebral artery occlusion, and the brains evaluated at 24 h, 3, 10 and 30 days after reperfusion. Flow cytometry was utilized to examine microglial activation and cytokine expression. RT‐PCR was performed for mRNA levels of IRF5/4 in sorted microglia. Microglial expression of IRF5/4 was examined by immunohistochemistry, and brain cytokine levels were determined by ELISA. Our results revealed that the IRF5 mRNA level in sorted microglia increased at 3 days of stroke; whereas IRF4 mRNA level exhibited biphasic increases, with a transient rise at 24 h and a peak at 10 days. The same pattern was seen in IRF5/4 protein colocalization with Iba‐1⁺ cells by IHC. Intracellular levels of TNF‐α and IL‐1β in microglia peaked at 3 days of stroke, and IL‐4⁺IL‐10⁺ double‐positive microglia significantly increased at day 10. Brain levels of these cytokines were consistent with microglial cytokine changes. Worse behavior test results were seen at 3 days vs. 10 days of stroke. We conclude that microglia phenotypes are dynamic to ischemic stroke, and IRF5/4 signaling may regulate microglial M1/M2 activation and impact on stroke outcomes.

Regulation and role of the transcription factor IRF5 in innate immune responses and systemic lupus erythematosus

The transcription factor interferon regulatory factor-5 (IRF5) plays an important role in innate immune responses via the TLR-MyD88 (Toll-like receptor - myeloid differentiation primary response 88) pathway. IRF5 is also involved in the pathogenesis of the autoimmune disease systemic lupus erythematosus (SLE). Recent studies have identified new regulators, both positive and negative, which act on IRF5 activation events in the TLR-MyD88 pathway such as post-translational modifications, dimerization and nuclear translocation. A model of the causal relationship between IRF5 activation and SLE pathogenesis proposes that a loss of the negative regulation of IRF5 causes its hyperactivation, resulting in hyperproduction of type I interferons and other cytokines, and ultimately in the development of SLE. Importantly, to our knowledge, all murine models of SLE studied thus far have shown that IRF5 is required for the pathogenesis of SLE-like diseases. During the development of SLE-like diseases, IRF5 plays key roles in various cell types, including dendritic cells and B cells. It is noteworthy that the onset of SLE-like diseases can be inhibited by reducing the activity or amount of IRF5 by half. Therefore, IRF5 is an important therapeutic target of SLE, and selective suppression of its activity and expression may potentially lead to the development of new therapies.

Therapeutic Targeting of IRFs: Pathway-Dependence or Structure-Based?

The interferon regulatory factors (IRFs) are a family of master transcription factors that regulate pathogen-induced innate and acquired immune responses. Aberration(s) in IRF signaling pathways due to infection, genetic predisposition and/or mutation, which can lead to increased expression of type I interferon (IFN) genes, IFN-stimulated genes (ISGs), and other pro-inflammatory cytokines/chemokines, has been linked to the development of numerous diseases, including (but not limited to) autoimmune and cancer. What is currently lacking in the field is an understanding of how best to therapeutically target these transcription factors. Many IRFs are regulated by post-translational modifications downstream of pattern recognition receptors (PRRs) and some of these modifications lead to activation or inhibition. We and others have been able to utilize structural features of the IRFs in order to generate dominant negative mutants that inhibit function. Here, we will review potential therapeutic strategies for targeting all IRFs by using IRF5 as a candidate targeting molecule.

The Interferon (IFN) Class of Cytokines and the IFN Regulatory Factor (IRF) Transcription Factor Family

Interferons (IFNs) are a broad class of cytokines elicited on challenge to the host defense and are essential for mobilizing immune responses to pathogens. Divided into three classes, type I, type II, and type III, all IFNs share in common the ability to evoke antiviral activities initiated by the interaction with their cognate receptors. The nine-member IFN regulatory factor (IRF) family, first discovered in the context of transcriptional regulation of type I IFN genes following viral infection, are pivotal for the regulation of the IFN responses. In this review, we briefly describe cardinal features of the three types of IFNs and then focus on the role of the IRF family members in the regulation of each IFN system.

Differential and Overlapping Immune Programs Regulated by IRF3 and IRF5 in Plasmacytoid Dendritic Cells

We examined the signaling pathways and cell type-specific responses of IFN regulatory factor (IRF) 5, an immune-regulatory transcription factor. We show that the protein kinases IKKα, IKKβ, IKKε, and TANK-binding kinase 1 each confer IRF5 phosphorylation/dimerization, thus extending the family of IRF5 activator kinases. Among primary human immune cell subsets, we found that IRF5 is most abundant in plasmacytoid dendritic cells (pDCs). Flow cytometric cell imaging revealed that IRF5 is specifically activated by endosomal TLR signaling. Comparative analyses revealed that IRF3 is activated in pDCs uniquely through RIG-I-like receptor (RLR) signaling. Transcriptomic analyses of pDCs show that the partitioning of TLR7/IRF5 and RLR/IRF3 pathways confers differential gene expression and immune cytokine production in pDCs, linking IRF5 with immune regulatory and proinflammatory gene expression. Thus, TLR7/IRF5 and RLR-IRF3 partitioning serves to polarize pDC response outcome. Strategies to differentially engage IRF signaling pathways should be considered in the design of immunotherapeutic approaches to modulate or polarize the immune response for specific outcome.

Advances and challenges in targeting IRF5, a key regulator of inflammation

Interferon regulatory factor 5 (IRF5) belongs to a family of transcription factors, originally implicated in antiviral responses and interferon production. However, studies conducted in different laboratories over the last decade have placed IRF5 as a central regulator of the inflammatory response. It has become clear that IRF5 contributes to the pathogenesis of many inflammatory and autoimmune diseases, such as rheumatoid arthritis, inflammatory bowel disease and systemic lupus erythematosus. Given the role of IRF5 in physiology and disease, IRF5 represents a potential therapeutic target. However, despite a significant interest from the pharmaceutical industry, inhibitors that interfere with the IRF5 pathway remain elusive. Here, we review the advances made by various studies in targeting multiple steps of signalling leading to IRF5 activation with their therapeutic potential, and the possible complications of such strategies are discussed.

The NEDD8 E3 ligase DCNL5 is phosphorylated by IKK alpha during Toll-like receptor activation

The activity of Cullin-RING ubiquitin E3 ligases (CRL) is regulated by NEDD8 modification. DCN-like proteins promote Cullin neddylation as scaffold-like E3s. One DCNL, DCNL5, is highly expressed in immune tissue. Here, we provide evidence that DCNL5 may be involved in innate immunity, as it is a direct substrate of the kinase IKKα during immune signalling. We find that upon activation of Toll-like receptors, DCNL5 gets rapidly and transiently phosphorylated on a specific N-terminal serine residue (S41). This phosphorylation event is specifically mediated by IKKα and not IKKβ. Our data for the first time provides evidence that DCNL proteins are post-translationally modified in an inducible manner. Our findings also provide the first example of a DCNL member as a kinase substrate in a signalling pathway, indicating that the activity of at least some DCNLs may be regulated.

Src Family Kinases Regulate Interferon Regulatory Factor 1 K63 Ubiquitination following Activation by TLR7/8 Vaccine Adjuvant in Human Monocytes and B Cells

Toll-like receptors (TLRs) play a key role in the activation of innate immune cells, in which their engagement leads to production of cytokines and co-stimulatory molecules. TLRs signaling requires recruitment of toll/IL-1R (TIR) domain-containing adaptors, such as MyD88 and/or TRIF, and leads to activation of several transcription factors, such as NF-κB, the AP1 complex, and various members of the interferon regulatory factor (IRF) family, which in turn results in triggering of several cellular functions associated with these receptors. A role for Src family kinases (SFKs) in this signaling pathway has also been established. Our work and that of others have shown that this type of kinases is activated following engagement of several TLRs, and that this event is essential for the initiation of specific downstream cellular response. In particular, we have previously demonstrated that activation of SFKs is required for balanced production of pro-inflammatory cytokines by monocyte-derived dendritic cells after stimulation with R848, an agonist of human TLRs 7/8. We also showed that TLR7/8 triggering leads to an increase in interferon regulatory factor 1 (IRF-1) protein levels and that this effect is abolished by inhibition of SFKs, suggesting a critical role of these kinases in IRF-1 regulation. In this study, we first confirmed the key role of SFKs in TLR7/8 signaling for cytokine production and accumulation of IRF-1 protein in monocytes and in B lymphocytes, two other type of antigen-presenting cells. Then, we demonstrate that TLR7 triggering leads to an increase of K63-linked ubiquitination of IRF-1, which is prevented by SFKs inhibition, suggesting a key role of these kinases in posttranslational regulation of IRF-1 in the immune cells. In order to understand the mechanism that links SFKs activation to IRF-1 K63-linked ubiquitination, we examined SFKs and IRF-1 possible interactors and proved that activation of SFKs is necessary for their interaction with TNFR-associated factor 6 (TRAF6) and promotes the recruitment of both cIAP2 and IRF-1 by TRAF6. Collectively, our data demonstrate that TLR7/8 engagement leads to the formation of a complex that allows the interaction of cIAP2 and IRF-1 resulting in IRF-1 K63-linked ubiquitination, and that active SFKs are required for this process.

NF-κB and the Transcriptional Control of Inflammation

The NF-κB transcription factor was discovered 30 years ago and has since emerged as the master regulator of inflammation and immune homeostasis. It achieves this status by means of the large number of important pro- and antiinflammatory factors under its transcriptional control. NF-κB has a central role in inflammatory diseases such as rheumatoid arthritis, inflammatory bowel disease, and autoimmunity, as well as diseases comprising a significant inflammatory component such as cancer and atherosclerosis. Here, we provide an overview of the studies that form the basis of our understanding of the role of NF-κB subunits and their regulators in controlling inflammation. We also describe the emerging importance of posttranslational modifications of NF-κB in the regulation of inflammation, and highlight the future challenges faced by researchers who aim to target NF-κB transcriptional activity for therapeutic benefit in treating chronic inflammatory diseases.

The impact of interferon-regulatory factors to macrophage differentiation and polarization into M1 and M2

The mononuclear phagocytes control the body homeostasis through the involvement in resolving tissue injury and further wound healing. Indeed, local tissue microenvironmental changes can significantly influence the functional behavior of monocytes and macrophages. Such microenvironmental changes for example occur in an atherosclerotic plaque during all progression stages. In response to exogenous stimuli, macrophages show a great phenotypic plasticity and heterogeneity. Exposure of monocytes to inflammatory or anti-inflammatory conditions also induces predominant differentiation to proinflammatory (M1) or anti-inflammatory (M2) macrophage subsets and phenotype switch between macrophage subsets. The phenotype transition is accompanied with great changes in the macrophage transcriptome and regulatory networks. Interferon-regulatory factors (IRFs) play a key role in hematopoietic development of monocytes, their differentiation to macrophages, and regulating macrophage maturation, phenotypic polarization, phenotypic switch, and function. Of 9 IRFs, at least 3 (IRF-1, IRF-5, and IRF-8) are involved in the commitment of proinflammatory M1 whereas IRF-3 and IRF-4 control M2 polarization. The role of IRF-2 is context-dependent. The IRF impact on macrophage phenotype plasticity and heterogeneity is complex and involves activating and repressive function in triggering transcription of target genes.

Characterization of Amphioxus IFN Regulatory Factor Family Reveals an Archaic Signaling Framework for Innate Immune Response

The IFN regulatory factor (IRF) family encodes transcription factors that play important roles in immune defense, stress response, reproduction, development, and carcinogenesis. Although the origin of the IRF family has been dated back to multicellular organisms, invertebrate IRFs differ from vertebrate IRFs in genomic structure and gene synteny, and little is known about their functions. Through comparison of multiple amphioxus genomes, in this study we suggested that amphioxus contains nine IRF members, whose orthologs are supposed to be shared among three amphioxus species. As the orthologs to the vertebrate IRF1 and IRF4 subgroups, Branchiostoma belcheri tsingtauense (bbt)IRF1 and bbtIRF8 bind the IFN-stimulated response element (ISRE) and were upregulated when amphioxus intestinal cells were stimulated with poly(I:C). As amphioxus-specific IRFs, both bbtIRF3 and bbtIRF7 bind ISRE. When activated, they can be phosphorylated by bbtTBK1 and then translocate into nucleus for target gene transcription. As transcriptional repressors, bbtIRF2 and bbtIRF4 can inhibit the transcriptional activities of bbtIRF1, 3, 7, and 8 by competing for the binding of ISRE. Interestingly, amphioxus IRF2, IRF8, and Rel were identified as target genes of bbtIRF1, bbtIRF7, and bbtIRF3, respectively, suggesting a dynamic feedback regulation among amphioxus IRF and NF-κB. Collectively, to our knowledge we present for the first time an archaic IRF signaling framework in a basal chordate, shedding new insights into the origin and evolution of vertebrate IFN-based antiviral networks.

A systems model of phosphorylation for inflammatory signaling events

Phosphorylation is a fundamental biochemical reaction that modulates protein activity in cells. While a single phosphorylation event is relatively easy to understand, multisite phosphorylation requires systems approaches for deeper elucidation of the underlying molecular mechanisms. In this paper we develop a mechanistic model for single- and multi-site phosphorylation. The proposed model is compared with previously reported studies. We compare the predictions of our model with experiments published in the literature in the context of inflammatory signaling events in order to provide a mechanistic description of the multisite phosphorylation-mediated regulation of Signal Transducer and Activator of Transcription 3 (STAT3) and Interferon Regulatory Factor 5 (IRF-5) proteins. The presented model makes crucial predictions for transcription factor phosphorylation events in the immune system. The model proposes potential mechanisms for T cell phenotype switching and production of cytokines. This study also provides a generic framework for the better understanding of a large number of multisite phosphorylation-regulated biochemical circuits.

The IκB kinase complex in NF-κB regulation and beyond

The IκB kinase (IKK) complex is the signal integration hub for NF-κB activation. Composed of two serine-threonine kinases (IKKα and IKKβ) and the regulatory subunit NEMO (also known as IKKγ), the IKK complex integrates signals from all NF-κB activating stimuli to catalyze the phosphorylation of various IκB and NF-κB proteins, as well as of other substrates. Since the discovery of the IKK complex components about 15 years ago, tremendous progress has been made in the understanding of the IKK architecture and its integration into signaling networks. In addition to the control of NF-κB, IKK subunits mediate the crosstalk with other pathways, thereby extending the complexity of their biological function. This review summarizes recent advances in IKK biology and focuses on emerging aspects of IKK structure, regulation and function.

IRF5 is a specific marker of inflammatory macrophages in vivo

Macrophages are an integral part of the innate immune system and key players in pathogen clearance and tissue remodelling. Both functions are accomplished by a pivotal network of different macrophage subtypes, including proinflammatory M1 and anti-inflammatory M2 macrophages. Previously, our laboratory identified the transcription factor interferon regulatory factor 5 (IRF5) as the master regulator of the M1 macrophage polarisation. IRF5 was found to be highly expressed in human M1 compared to M2 macrophages. Furthermore, IRF5 dictates the expression of proinflammatory genes such as IL12b and IL23a whilst repressing anti-inflammatory genes like IL10. Here we show that murine bone marrow derived macrophages differentiated in vitro with GM-CSF are also characterised by high levels of IRF5 mRNA and protein and express proinflammatory cytokines upon LPS stimulation. These macrophages display characteristic expression of M1-marker MHC II but lack the M2-marker CD206. Significantly, we develop intracellular staining of IRF5- expressing macrophages and utilise it to recapitulate the in vitro results in an in vivo model of antigen-induced arthritis, emphasising their physiological relevance. Thus, we establish the species-invariant role of IRF5 in controlling the inflammatory macrophage phenotype both in vitro and in in vivo.

Molecular effects of autoimmune-risk promoter polymorphisms on expression, exon choice, and translational efficiency of interferon regulatory factor 5

The rs2004640 single nucleotide polymorphism and the CGGGG copy-number variant (rs77571059) are promoter polymorphisms within interferon regulatory factor 5 (IRF5). They have been implicated as susceptibility factors for several autoimmune diseases. IRF5 uses alternative promoter splicing, where any of 4 first exons begin the mRNA. The CGGGG indel is in exon 1A's promoter; the rs2004640 allele creates a splicing recognition site, enabling usage of exon 1B. This study aimed at characterizing alterations in IRF5 mRNA due to these polymorphisms. Cells with risk polymorphisms exhibited ~2-fold higher levels of IRF5 mRNA and protein, but demonstrated no change in mRNA stability. Quantitative PCR demonstrated decreased usage of exons 1C and 1D in cell lines with the risk polymorphisms. RNA folding analysis revealed a hairpin in exon 1B; mutational analysis showed that the hairpin shape decreased translation 5-fold. Although translation of mRNA that uses exon 1B is low due to a hairpin, increased IRF5 mRNA levels in individuals with the rs2004640 risk allele lead to higher overall protein expression. In addition, several new splice variants of IRF5 were sequenced. IRF5's promoter polymorphisms alter first exon usage and increase transcription levels. High levels of IRF5 may bias the immune system toward autoimmunity.

Diabetes during pregnancy influences Hofbauer cells, a subtype of placental macrophages, to acquire a pro-inflammatory phenotype

Growing evidence indicates that maternal pathophysiological conditions, such as diabetes, influence fetal growth and could program metabolic disease in adulthood. Placental cells, particularly Hofbauer cells (HBCs), which are placental macrophages characterized by an anti-inflammatory profile (M2), can sense the modified maternal environment. The goal of this study was to investigate the direct effect of hyperglycemia on HBCs. We studied, at mRNA and protein levels, some markers of M2 and M1 (pro-inflammatory) macrophages in placentae from control and diabetic patients to assess the balance between pro- and anti-inflammatory macrophages: an imbalance of M2 to M1 macrophages has been observed in humans. We used pregnant rats, receiving a single injection of streptozotocin (STZ), as a model of maternal diabetes. We noticed a M2-to-M1 macrophage unbalance as we observed in human. An in vitro model of isolated rat HBCs was used to identify the direct effects of high glucose. We found that high glucose stimulation activated genes belonging to TLR (Toll-Like Receptor)-dependent inflammatory pathways. Moreover, the HBCs stimulated by high glucose switched their M2 profile towards M1, with increased expression of pro-inflammatory cytokines and markers. We also noticed that the oxidative-stress pathway was activated in response to high glucose driven by Hif-1α. In this study, we demonstrated that diabetes/hyperglycemia affect the anti-inflammatory profile of HBCs, by stimulating these cells to acquire an inflammatory profile leading to adverse consequences for the fetal-placental-maternal axis.

Human parainfluenza virus type 2 V protein inhibits TRAF6-mediated ubiquitination of IRF7 to prevent TLR7- and TLR9-dependent interferon induction

Paramyxovirus V proteins block Toll-like receptor 7 (TLR7)- and TLR9-dependent signaling leading to alpha interferon production. Our recent study has provided evidence that interaction of the V proteins with IRF7 is important for the blockade. However, the detailed mechanisms still remain unclear. Here we reexamined the interaction of the human parainfluenza virus type 2 (HPIV2) V protein with signaling molecules involved in TLR7/9-dependent signaling. Immunoprecipitation experiments in HEK293T cells transfected with V protein and one of the signaling molecules revealed that the V protein interacted with not only IRF7 but also TRAF6, IKKα, and MyD88. Whereas overexpression of TRAF6 markedly enhanced the level of V protein associating with IRF7, IKKα, and MyD88 in HEK293T cells, the level of V protein associating with TRAF6 was little affected by overexpression of IRF7, IKKα, and MyD88. Moreover, knockdown or knockout of endogenous TRAF6 in HEK293T or mouse embryonic fibroblast cells resulted in dissociation of the V protein from IRF7, IKKα, and MyD88. These results demonstrate that binding of the V protein to IRF7, IKKα, and MyD88 is largely indirect and mediated by endogenous TRAF6. It was found that the V protein inhibited TRAF6-mediated lysine 63 (K63)-linked polyubiquitination of IRF7, which is prerequisite for IRF7 activation. Disruption of the tryptophan-rich motif of the V protein significantly affected its TRAF6-binding efficiency, which correlated well with the magnitude of inhibition of K63-linked polyubiquitination and the resultant activation of IRF7. Taken together, these results suggest that the HPIV2 V protein prevents TLR7/9-dependent interferon induction by inhibiting TRAF6-mediated K63-linked polyubiquitination of IRF7.

4F decreases IRF5 expression and activation in hearts of tight skin mice

The apoAI mimetic 4F was designed to inhibit atherosclerosis by improving HDL. We reported that treating tight skin (Tsk^−/+) mice, a model of systemic sclerosis (SSc), with 4F decreases inflammation and restores angiogenic potential in Tsk^−/+ hearts. Interferon regulating factor 5 (IRF5) is important in autoimmunity and apoptosis in immune cells. However, no studies were performed investigating IRF5 in myocardium. We hypothesize that 4F differentially modulates IRF5 expression and activation in Tsk^−/+ hearts. Posterior wall thickness was significantly increased in Tsk^−/+ compared to C57Bl/6J (control) and Tsk^−/+ mice with 4F treatment assessed by echoradiography highlighting reduction of fibrosis in 4F treated Tsk^−/+ mice. IRF5 in heart lysates from control and Tsk/+ with and without 4F treatment (sc, 1 mg/kg/d, 6–8 weeks) was determined. Phosphoserine, ubiquitin, ubiquitin K⁶³ on IRF5 were determined on immunoprecipitates of IRF5. Immunofluorescence and TUNEL assays in heart sections were used to determine positive nuclei for IRF5 and apoptosis, respectively. Fluorescence-labeled streptavidin (SA) was used to determine endothelial cell uptake of biotinylated 4F. SA-agarose pulldown and immunoblotting for IRF5 were used to determine 4F binding IRF5 in endothelial cell cytosolic fractions and to confirm biolayer interferometry studies. IRF5 levels in Tsk^−/+ hearts were similar to control. 4F treatments decrease IRF5 in Tsk^−/+ hearts and decrease phosphoserine and ubiquitin K⁶³ but increase total ubiquitin on IRF5 in Tsk^−/+ compared with levels on IRF5 in control hearts. 4F binds IRF5 by mechanisms favoring association over dissociation strong enough to pull down IRF5 from a mixture of endothelial cell cytosolic proteins. IRF5 positive nuclei and apoptotic cells in Tsk^−/+ hearts were increased compared with controls. 4F treatments decreased both measurements in Tsk^−/+ hearts. IRF5 activation in Tsk^−/+ hearts is increased. 4F treatments decrease IRF5 expression and activation in Tsk^−/+ hearts by a mechanism related to 4F’s ability to bind IRF5.

Characteristics of the interferon regulatory factor 5 (IRF5) and its expression in response to LCDV and poly I:C challenges in Japanese flounder, Paralichthys olivaceus

Interferon regulatory factor 5 (IRF5) has been identified as a key transcriptional mediator regulating expression of both type I interferons (IFNs) and proinflammatory cytokines. In this study, the cDNA and genomic sequences of IRF5 were isolated from Japanese flounder, Paralichthys olivaceus. The gene of Japanese flounder (Jf)IRF5 is 7326 bp long, contains 9 exons and 8 introns and encodes a putative protein of 472 amino acids. The predicted protein sequence shares 61.1-81.9% identity to fish IRF5 and possesses a DNA-binding domain (DBD), a middle region (MR), an IRF association domain (IAD), a virus activated domain (VAD) and two nuclear localization signals (NLSs) conserved in all known IRF5s. Phylogenetic analysis clustered it into the teleost IRF5 subgroup within vertebrate IRF5 group. JfIRF5 mRNA was constitutively expressed in all tissues examined, with higher levels observed in the gills and head kidney. Gene expression of JfIRF5 was analyzed over a 7-day time course in the gills, head kidney, spleen and muscle of Japanese flounders challenged with lymphocystis disease virus (LCDV) and polyinosinic:polycytidylic acid (poly I:C). The data showed that JfIRF5 expression was slightly up-regulated by LCDV, but its induction time was clearly moved up; in contrast, the induction upon poly I:C challenge started not earlier than day 2 post-injection and was stronger and more persistent with a later peak time in all four organs. The late and long-lasting inductive expression of JfIRF5 following poly I:C challenge suggests that it might be an interferon stimulated gene (ISG), the induction of which is driven by poly I:C-induced type I IFNs.

IKK biology

The inhibitor of nuclear factor-κB (IκB) kinase (IKK) complex is the master regulator of the NF-κB signaling pathway. The activation of the IKK complex is a tightly regulated, highly stimulus-specific, and target-specific event that is essential for the plethora of functions attributed to NF-κB. More recently, NF-κB-independent roles of IKK members have brought increased complexity to its biological function. This review highlights some of the major advances in the studies of the process of IKK activation and the biological roles of IKK family members, with a focus on NF-κB-independent functions. Understanding these complex processes is essential for targeting IKK for therapeutics.

Crosstalk in NF-κB signaling pathways

NF-κB transcription factors are critical regulators of immunity, stress responses, apoptosis and differentiation. A variety of stimuli coalesce on NF-κB activation, which can in turn mediate varied transcriptional programs. Consequently, NF-κB-dependent transcription is not only tightly controlled by positive and negative regulatory mechanisms but also closely coordinated with other signaling pathways. This intricate crosstalk is crucial to shaping the diverse biological functions of NF-κB into cell type- and context-specific responses.