Functional Role for IκBNS in T Cell Cytokine Regulation As Revealed by Targeted Gene Disruption

IκBNS-deficiency protects mice from fatal Listeria monocytogenes infection by blunting pro-inflammatory signature in Ly6Chigh monocytes and preventing exaggerated innate immune responses

IκB proteins regulate the inhibition and activation of NF-κB transcription factor complexes. While classical IκB proteins keep NF-κB complexes inactive in the cytoplasm, atypical IκB proteins act on activated NF-κB complexes located in the nucleus. Most of the knowledge regarding the function of IκB proteins has been collected in vitro, while far less is known regarding their impact on activation and regulation of immune responses during in vivo infections. Combining in vivo Listeria monocytogenes (Lm) infection with comparative ex vivo transcriptional profiling of the hepatic response to the pathogen we observed that in contrast to wild type mice that mounted a robust inflammatory response, IκB_NS-deficiency was generally associated with a transcriptional repression of innate immune responses. Whole tissue transcriptomics revealed a pronounced IκB_NS-dependent reduction of myeloid cell-associated transcripts in the liver together with an exceptionally high Nfkbid promoter activity uncovered in Ly6C^high inflammatory monocytes prompted us to further characterize the specific contribution of IκB_NS in the inflammatory response of monocytes to the infectious agent. Indeed, Ly6C^high monocytes primed during Lm infection in the absence of IκB_NS displayed a blunted response compared to wild type-derived Ly6C^high monocytes as evidenced by the reduced early expression of hallmark transcripts of monocyte-driven inflammation such as Il6, Nos2 and Il1β. Strikingly, altered monocyte activation in IκB_NS-deficient mice was associated with an exceptional resistance against Lm infection and protection was associated with a strong reduction in immunopathology in Lm target organs. Of note, mice lacking IκB_NS exclusively in myeloid cells failed to resist Lm infection, indicating that the observed effect was not monocyte intrinsic but monocyte extrinsic. While serum cytokine-profiling did not discover obvious differences between wild type and IκB_NS ^-/- mice for most of the analyzed mediators, IL-10 was virtually undetectable in IκB_NS-deficient mice, both in the steady state and following Lm infection. Together, we show here a crucial role for IκB_NS during Lm infection with IκB_NS-deficient mice showing an overall blunted pro-inflammatory immune response attributed to a reduced pro-inflammatory signature in Ly6C^high monocytes. Reduced immunopathology and complete protection of mice against an otherwise fatal Lm infection identified IκB_NS as molecular driver of inflammation in listeriosis.

IκBNS expression in B cells is dispensable for IgG responses to T cell-dependent antigens

Mice lacking the atypical inhibitory kappa B (IκB) protein, IκBNS, a regulator of the NF-κB pathway encoded by the nfkbid gene, display impaired antibody responses to both T cell-independent (TI) and T cell-dependent (TD) antigens. To better understand the basis of these defects, we crossed mice carrying floxed nfkbid alleles with mice expressing Cre under the transcriptional control of the Cd79a gene to create mice that lacked IκBNS expression only in B cells. Analyses of these conditional knock-out mice revealed intact CD4+ and CD8+ T cell populations, including preserved frequencies of FoxP3+ regulatory T cells, which are known to be reduced in IκBNS knock-out mice. Like IκBNS knock-out mice, mice with conditional IκBNS ablation in B cells displayed defective IgM responses to TI antigens and a severe reduction in peritoneal B-1a cells. However, in contrast to mice lacking IκBNS altogether, the conditional IκBNS knock-out mice responded well to TD antigens compared to the control mice, with potent IgG responses following immunization with the viral antigen, rSFV-βGal or the widely used hapten-protein model antigen, NP-CGG. Furthermore, B cell intrinsic IκBNS expression was dispensable for germinal center (GC) formation and T follicular helper cell responses to NP-CGG immunization. The results presented here suggest that the defect in antibody responses to TD antigens observed in IκBNS knock-out mice results from a B cell extrinsic defect.

Nfkbid Overexpression in Nonobese Diabetic Mice Elicits Complete Type 1 Diabetes Resistance in Part Associated with Enhanced Thymic Deletion of Pathogenic CD8 T Cells and Increased Numbers and Activity of Regulatory T Cells

Type 1 diabetes (T1D) in both humans and NOD mice is caused by T cell-mediated autoimmune destruction of pancreatic β cells. Increased frequency or activity of autoreactive T cells and failures of regulatory T cells (Tregs) to control these pathogenic effectors have both been implicated in T1D etiology. Due to the expression of MHC class I molecules on β cells, CD8 T cells represent the ultimate effector population mediating T1D. Developing autoreactive CD8 T cells normally undergo extensive thymic negative selection, but this process is impaired in NOD mice and also likely T1D patients. Previous studies identified an allelic variant of Nfkbid, a NF-κB signal modulator, as a gene strongly contributing to defective thymic deletion of autoreactive CD8 T cells in NOD mice. These previous studies found ablation of Nfkbid in NOD mice using the clustered regularly interspaced short palindromic repeats system resulted in greater thymic deletion of pathogenic CD8 AI4 and NY8.3 TCR transgenic T cells but an unexpected acceleration of T1D onset. This acceleration was associated with reductions in the frequency of peripheral Tregs. In this article, we report transgenic overexpression of Nfkbid in NOD mice also paradoxically results in enhanced thymic deletion of autoreactive CD8 AI4 T cells. However, transgenic elevation of Nfkbid expression also increased the frequency and functional capacity of peripheral Tregs, in part contributing to the induction of complete T1D resistance. Thus, future identification of a pharmaceutical means to enhance Nfkbid expression might ultimately provide an effective T1D intervention approach.

NF-κB in control of regulatory T cell development, identity, and function

Regulatory T cells (Treg cells) act as a major rheostat regulating the strength of immune responses, enabling tolerance of harmless foreign antigens, and preventing the development of pathogenic immune responses in various disease settings such as cancer and autoimmunity. Treg cells are present in all lymphoid and non-lymphoid tissues, and the latter often fulfill important tasks required for the physiology of their host organ. The activation of NF-κB transcription factors is a central pathway for the reprogramming of gene expression in response to inflammatory but also homeostatic cues. Genetic mouse models have revealed essential functions for NF-κB transcription factors in modulating Treg development and function, with some of these mechanistic insights confirmed by recent studies analyzing Treg cells from patients harboring point mutations in the genes encoding NF-κB proteins. Molecular insights into the NF-κB pathway in Treg cells hold substantial promise for novel therapeutic strategies to manipulate dysfunctional or inadequate cell numbers of immunosuppressive Treg cells in autoimmunity or cancer. Here, we provide an overview of the manifold roles that NF-κB factors exert in Treg cells.

Genetic mapping reveals Nfkbid as a central regulator of humoral immunity to Toxoplasma gondii

Protective immunity to parasitic infections has been difficult to elicit by vaccines. Among parasites that evade vaccine-induced immunity is Toxoplasma gondii, which causes lethal secondary infections in chronically infected mice. Here we report that unlike susceptible C57BL/6J mice, A/J mice were highly resistant to secondary infection. To identify correlates of immunity, we utilized forward genetics to identify Nfkbid, a nuclear regulator of NF-κB that is required for B cell activation and B-1 cell development. Nfkbid-null mice (“bumble”) did not generate parasite-specific IgM and lacked robust parasite-specific IgG, which correlated with defects in B-2 cell maturation and class-switch recombination. Though high-affinity antibodies were B-2 derived, transfer of B-1 cells partially rescued the immunity defects observed in bumble mice and were required for 100% vaccine efficacy in bone marrow chimeric mice. Immunity in resistant mice correlated with robust isotype class-switching in both B cell lineages, which can be fine-tuned by Nfkbid gene expression. We propose a model whereby humoral immunity to T. gondii is regulated by Nfkbid and requires B-1 and B-2 cells for full protection.

Eukaryotic parasitic diseases account for approximately one fifth of all childhood deaths, yet no highly protective vaccine exists for any human parasite. More research must be done to discover how to elicit protective vaccine-induced immunity to parasitic pathogens. We used an unbiased genetic screen to find key genes responsible for immunity to the eukaryotic parasite Toxoplasma gondii. Our screen found Nfkbid, a transcription factor regulator, which controls B cell activation and innate-like B-1 cell development. Mice without Nfkbid were not protected against T. gondii and were deficient at making antibodies against the parasite. Our survival studies of vaccinated mice with and without B-1 compartments found that B-1 cells improved survival, suggesting that B-1 cells act in conjunction with B-2 cells to provide vaccine-induced immunity. Nfkbid and other loci identified in our unbiased screen represent potential targets for vaccines to elicit protective immune responses against parasitic pathogens.

NF-κB and Its Regulators During Pregnancy

The transcriptional factor NF-κB is a nuclear factor involved in both physiological and pathological processes. This factor can control the transcription of more than 400 genes, including cytokines, chemokines, and their modulators, immune and non-immune receptors, proteins involved in antigen presentation and cell adhesion, acute phase and stress response proteins, regulators of apoptosis, growth factors, other transcription factors and their regulators, as well as different enzymes; all these molecules control several biological processes. NF-κB is a tightly regulated molecule that has also been related to apoptosis, cell proliferation, inflammation, and the control of innate and adaptive immune responses during onset of labor, in which it has a crucial role; thus, early activation of this factor may have an adverse effect, by inducing premature termination of pregnancy, with bad outcomes for the mother and the fetus, including product loss. Reviews compiling the different activities of NF-κB have been reported. However, an update regarding NF-κB regulation during pregnancy is lacking. In this work, we aimed to describe the state of the art around NF-κB activity, its regulatory role in pregnancy, and the effect of its dysregulation due to invasion by pathogens like Trichomonas vaginalis and Toxoplasma gondii as examples.

IκBNS and IL-6 expression is differentially established in the uterus of pregnant healthy and infected mice

During pregnancy, NF-κB plays an important role for embryo implantation and the onset of labor. Regulated IL-6 production, under transcriptional control of NF-κB, is essential for a successful pregnancy outcome and the atypical regulator IκBNS is involved in this process. Previously, we showed that IκBNS negatively regulates IL-6 in uterine tissues during mouse estrous cycle. In this work, we analyzed if IκBNS and IL-6 expression in pregnant mice under physiological or L. monocytogenes-infected conditions would remain as observed in estrous cycle. In the healthy pregnancy IL-6 was highly expressed during implantation/placentation and labor stages but decreased during fetal development and post-partum stages. In contrast, in mice infected before pregnancy, IL-6 expression was not increased in the implantation stage, and its regulator IκBNS increased more in the infected condition rather than in the healthy pregnancy. IκBNS expression was reduced in post-implantation infection, allowing for IL-6 overexpression. The IκBNS-unrelated cytokine IL-36γ, used as inflammatory cytokine marker, was severely increased in the infected uterine tissues. When we analyzed the effect of infection over the fetuses, we found that pre-implantation infection caused the resorption (rejection) of some products, while the post-implantation infection restricted the intrauterine growth of fetuses. The results suggest that in the uterine tissue of pregnant mice the regulatory effect of IκBNS over IL-6 is more evident in an infection status rather than in a healthy condition.

Genome reading by the NF-κB transcription factors

The NF-κB family of dimeric transcription factors regulates transcription by selectively binding to DNA response elements present within promoters or enhancers of target genes. The DNA response elements, collectively known as κB sites or κB DNA, share the consensus 5'-GGGRNNNYCC-3' (where R, Y and N are purine, pyrimidine and any nucleotide base, respectively). In addition, several DNA sequences that deviate significantly from the consensus have been shown to accommodate binding by NF-κB dimers. X-ray crystal structures of NF-κB in complex with diverse κB DNA have helped elucidate the chemical principles that underlie target selection in vitro. However, NF-κB dimers encounter additional impediments to selective DNA binding in vivo. Work carried out during the past decades has identified some of the barriers to sequence selective DNA target binding within the context of chromatin and suggests possible mechanisms by which NF-κB might overcome these obstacles. In this review, we first highlight structural features of NF-κB:DNA complexes and how distinctive features of NF-κB proteins and DNA sequences contribute to specific complex formation. We then discuss how native NF-κB dimers identify DNA binding targets in the nucleus with support from additional factors and how post-translational modifications enable NF-κB to selectively bind κB sites in vivo.

B-1a cells acquire their unique characteristics by bypassing the pre-BCR selection stage

B-1a cells are long-lived, self-renewing innate-like B cells that predominantly inhabit the peritoneal and pleural cavities. In contrast to conventional B-2 cells, B-1a cells have a receptor repertoire that is biased towards bacterial and self-antigens, promoting a rapid response to infection and clearing of apoptotic cells. Although B-1a cells are known to primarily originate from fetal tissues, the mechanisms by which they arise has been a topic of debate for many years. Here we show that in the fetal liver versus bone marrow environment, reduced IL-7R/STAT5 levels promote immunoglobulin kappa gene recombination at the early pro-B cell stage. As a result, differentiating B cells can directly generate a mature B cell receptor (BCR) and bypass the requirement for a pre-BCR and pairing with surrogate light chain. This 'alternate pathway' of development enables the production of B cells with self-reactive, skewed specificity receptors that are peculiar to the B-1a compartment. Together our findings connect seemingly opposing lineage and selection models of B-1a cell development and explain how these cells acquire their unique properties.

Generation of Foxp3+CD25- Regulatory T-Cell Precursors Requires c-Rel and IκBNS

Next to the classical developmental route, in which first CD25 and subsequently Foxp3 are induced to generate thymic regulatory T (Treg) cells, an alternative route has been described. This alternative route is characterized by reciprocal induction of Foxp3 and CD25, with CD25 induction being required to rescue developing Treg cells from Foxp3-induced apoptosis. NF-κB has been demonstrated to be crucial for the development of thymic Treg cells via the classical route. However, its impact on the alternative route is poorly characterized. Using single and double deficient mice for key regulators of the classical route, c-Rel and IκB_NS, we here demonstrate that NF-κB is essential for the generation of alternative CD25⁻Foxp3⁺ precursors, as well. Thus, c-Rel and IκB_NS govern both routes of thymic Treg cell development.

Essential role of IκBNS for in vivo CD4+ T-cell activation, proliferation, and Th1-cell differentiation during Listeria monocytogenes infection in mice

Acquisition of effector functions in T cells is guided by transcription factors, including NF‐κB, that itself is tightly controlled by inhibitory proteins. The atypical NF‐κB inhibitor, IκB_NS, is involved in the development of Th1, Th17, and regulatory T (Treg) cells. However, it remained unclear to which extend IκB_NS contributed to the acquisition of effector function in T cells specifically responding to a pathogen during in vivo infection. Tracking of adoptively transferred T cells in Listeria monocytogenes infected mice antigen‐specific activation of CD4⁺ T cells following in vivo pathogen encounter to strongly rely on IκB_NS. While IκB_NS was largely dispensable for the acquisition of cytotoxic effector function in CD8⁺ T cells, IκB_NS‐deficient Th1 effector cells exhibited significantly reduced proliferation, marked changes in the pattern of activation marker expression, and reduced production of the Th1‐cell cytokines IFN‐γ, IL‐2, and TNF‐α. Complementary in vitro analyses using cells from novel reporter and inducible knockout mice revealed that IκB_NS predominantly affects the early phase of Th1‐cell differentiation while its function in terminally differentiated cells appears to be negligible. Our data suggest IκB_NS as a potential target to modulate specifically CD4⁺ T‐cell responses.

Defective respiration and one-carbon metabolism contribute to impaired naïve T cell activation in aged mice

T cell-mediated immune responses are compromised in aged individuals, leading to increased morbidity and reduced response to vaccination. Finding new ways to boost T cell immunity in the elderly is key for enhancing their immune competence. In this work, we performed a systematic analysis of proteins and metabolites in young versus aged T cells. Metabolic rewiring occurs in young T cells following stimulation but is dampened in aged T cells. Moreover, we show that aged T cell functions can be enhanced by metabolite addition.

T cell-mediated immune responses are compromised in aged individuals, leading to increased morbidity and reduced response to vaccination. While cellular metabolism tightly regulates T cell activation and function, metabolic reprogramming in aged T cells has not been thoroughly studied. Here, we report a systematic analysis of metabolism during young versus aged naïve T cell activation. We observed a decrease in the number and activation of naïve T cells isolated from aged mice. While young T cells demonstrated robust mitochondrial biogenesis and respiration upon activation, aged T cells generated smaller mitochondria with lower respiratory capacity. Using quantitative proteomics, we defined the aged T cell proteome and discovered a specific deficit in the induction of enzymes of one-carbon metabolism. The activation of aged naïve T cells was enhanced by addition of products of one-carbon metabolism (formate and glycine). These studies define mechanisms of skewed metabolic remodeling in aged T cells and provide evidence that modulation of metabolism has the potential to promote immune function in aged individuals.

A Hypermorphic Nfkbid Allele Contributes to Impaired Thymic Deletion of Autoreactive Diabetogenic CD8+ T Cells in NOD Mice

In both NOD mice and humans, the development of type 1 diabetes (T1D) is dependent in part on autoreactive CD8+ T cells recognizing pancreatic β cell peptides presented by often quite common MHC class I variants. Studies in NOD mice previously revealed that the common H2-Kd and/or H2-Db class I molecules expressed by this strain aberrantly lose the ability to mediate the thymic deletion of pathogenic CD8+ T cell responses through interactions with T1D susceptibility genes outside the MHC. A gene(s) mapping to proximal chromosome 7 was previously shown to be an important contributor to the failure of the common class I molecules expressed by NOD mice to mediate the normal thymic negative selection of diabetogenic CD8+ T cells. Using an inducible model of thymic negative selection and mRNA transcript analyses, we initially identified an elevated Nfkbid expression variant as a likely NOD-proximal chromosome 7 region gene contributing to impaired thymic deletion of diabetogenic CD8+ T cells. CRISPR/Cas9-mediated genetic attenuation of Nfkbid expression in NOD mice resulted in improved negative selection of autoreactive diabetogenic AI4 and NY8.3 CD8+ T cells. These results indicated that allelic variants of Nfkbid contribute to the efficiency of intrathymic deletion of diabetogenic CD8+ T cells. However, although enhancing thymic deletion of pathogenic CD8+ T cells, ablating Nfkbid expression surprisingly accelerated T1D onset that was associated with numeric decreases in both regulatory T and B lymphocytes in NOD mice.

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.

IκBNS induces Muc5ac expression in epithelial cells and causes airway hyper-responsiveness in murine asthma models

BACKGROUND

In allergic asthma, environmental allergens including house dust mite (HDM) trigger pattern recognition receptors and activate downstream signaling pathways including NF-κB pathways not only in immune cells but also in airway epithelial cells. Recent studies have shown that NF-κB activation is regulated positively or negatively depending on the cellular context by IκBNS (encoded by the gene Nfkbid), one of atypical IκB proteins, in the nucleus. Therefore, we hypothesized that IκBNS expressed in immune cells or epithelial cells is involved in the regulation of asthmatic responses.

AIM

To determine the roles of IκBNS in HDM-induced asthmatic responses.

METHODS

Roles of IκBNS in HDM-induced airway inflammation and airway hyper-responsiveness (AHR) were examined by using IκBNS-deficient (Nfkbid^-/- ) mice. Roles of IκBNS expressed in hematopoietic cells and nonhematopoietic cells were separately evaluated by bone marrow chimeric mice. Roles of IκBNS expressed in murine tracheal epithelial cells (mTECs) were examined by air-liquid interface culture.

RESULTS

House dust mite-induced airway inflammation and AHR were exacerbated in mice lacking IκBNS in hematopoietic cells. In contrast, HDM-induced airway inflammation was exacerbated, but AHR was attenuated in mice lacking IκBNS in nonhematopoietic cells. The induction of Muc5ac, a representative mucin in asthmatic airways, was reduced in Nfkbid^-/- mTEC, whereas the induction of Spdef, a master regulator of goblet cell metaplasia, was not impaired in Nfkbid^-/- mTEC. Moreover, IκBNS bound to and activated the MUC5AC distal promoter in epithelial cells.

CONCLUSION

IκBNS is involved in inducing Muc5ac expression in lung epithelial cells and causing AHR in HDM-induced asthma models.

c-REL and IκBNS Govern Common and Independent Steps of Regulatory T Cell Development from Novel CD122-Expressing Pre-Precursors

Foxp3-expressing regulatory T cells (Tregs) are essential regulators of immune homeostasis and, thus, are prime targets for therapeutic interventions of diseases such as cancer and autoimmunity. c-REL and IκB_NS are important regulators of Foxp3 induction in Treg precursors upon γ-chain cytokine stimulation. In c-REL/IκB_NS double-deficient mice, Treg numbers were dramatically reduced, indicating that together, c-REL and IκB_NS are pivotal for Treg development. However, despite the highly reduced Treg compartment, double-deficient mice did not develop autoimmunity even when aged to more than 1 y, suggesting that c-REL and IκB_NS are required for T cell effector function as well. Analyzing Treg development in more detail, we identified a CD122⁺ subset within the CD25^-Foxp3^- precursor population, which gave rise to classical CD25⁺Foxp3^- Treg precursors. Importantly, c-REL, but not IκB_NS, controlled the generation of classical CD25⁺Foxp3^- precursors via direct binding to the Cd25 locus. Thus, we propose that CD4⁺GITR⁺CD122⁺CD25^-Foxp3^- cells represent a Treg pre-precursor population, whose transition into Treg precursors is mediated via c-REL.

NF-κB Pathways in the Pathogenesis of Multiple Sclerosis and the Therapeutic Implications

Nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) signaling pathways are involved in cell immune responses, apoptosis and infections. In multiple sclerosis (MS), NF-κB pathways are changed, leading to increased levels of NF-κB activation in cells. This may indicate a key role for NF-κB in MS pathogenesis. NF-κB signaling is complex, with many elements involved in its activation and regulation. Interestingly, current MS treatments are found to be directly or indirectly linked to NF-κB pathways and act to adjust the innate and adaptive immune system in patients. In this review, we will first focus on the intricacies of NF-κB signaling, including the activating pathways and regulatory elements. Next, we will theorize about the role of NF-κB in MS pathogenesis, based on current research findings, and discuss some of the associated therapeutic implications. Lastly, we will review four new MS treatments which interrupt NF-κB pathways—fingolimod, teriflunomide, dimethyl fumarate (DMF) and laquinimod (LAQ)—and explain their mechanisms, and the possible strategy for MS treatments in the future.

The Nuclear Protein IκBζ Forms a Transcriptionally Active Complex with Nuclear Factor-κB (NF-κB) p50 and the Lcn2 Promoter via the N- and C-terminal Ankyrin Repeat Motifs

The nuclear protein IκBζ, comprising the N-terminal trans-activation domain and the C-terminal ankyrin repeat (ANK) domain composed of seven ANK motifs, activates transcription of a subset of nuclear factor-κB (NF-κB)-dependent innate immune genes such as Lcn2 encoding the antibacterial protein lipocalin-2. Lcn2 activation requires formation of a complex containing IκBζ and NF-κB p50, a transcription factor that harbors the DNA-binding Rel homology region but lacks a trans-activation domain, on the promoter with the canonical NF-κB-binding site (κB site) and its downstream cytosine-rich element. Here we show that IκBζ productively interacts with p50 via Asp-451 in the N terminus of ANK1, a residue that is evolutionarily conserved among IκBζ and the related nuclear IκB proteins Bcl-3 and IκBNS Threonine substitution for Asp-451 abrogates direct association with the κB-site-binding protein p50, complex formation with the Lcn2 promoter DNA, and activation of Lcn2 transcription. The basic residues Lys-717 and Lys-719 in the C-terminal region of ANK7 contribute to IκBζ binding to the Lcn2 promoter, probably via interaction with the cytosine-rich element required for Lcn2 activation; glutamate substitution for both lysines results in a loss of transcriptionally active complex formation without affecting direct contact of IκBζ with p50. Both termini of the ANK domain in Bcl-3 and IκBNS function in a manner similar to that of IκBζ to interact with promoter DNA, indicating a common mechanism in which the nuclear IκBs form a regulatory complex with NF-κB and promoter DNA via the invariant aspartate in ANK1 and the conserved basic residues in ANK7.

The atypical IκB protein IκB(NS) is important for Toll-like receptor-induced interleukin-10 production in B cells

Although a major function of B cells is to mediate humoral immunity by producing antigen-specific antibodies, a specific subset of B cells is important for immune suppression, which is mainly mediated by the secretion of the anti-inflammatory cytokine interleukin-10 (IL-10). However, the mechanism by which IL-10 is induced in B cells has not been fully elucidated. Here, we report that IκBNS , an inducible nuclear IκB protein, is important for Toll-like receptor (TLR)-mediated IL-10 production in B cells. Studies using IκB(NS) knockout mice revealed that the number of IL-10-producing B cells is reduced in IκB(NS)(-/-) spleens and that the TLR-mediated induction of cytoplasmic IL-10-positive cells and IL-10 secretion in B cells are impaired in the absence of IκB(NS). The impairment of IL-10 production by a lack of IκB(NS) was not observed in TLR-triggered macrophages or T-cell-receptor-stimulated CD4(+) CD25(+) T cells. In addition, IκB(NS)-deficient B cells showed reduced expression of Prdm1 and Irf4 and failed to generate IL-10(+) CD138(+) plasmablasts. These results suggest that IκB(NS) is selectively required for IL-10 production in B cells responding to TLR signals, so defining an additional role for IκB(NS) in the control of the B-cell-mediated immune responses.

The nuclear IκB family protein IκBNS influences the susceptibility to experimental autoimmune encephalomyelitis in a murine model

The nuclear IκB family protein IκB_NS is expressed in T cells and plays an important role in Interferon (IFN)-γ and Interleukin (IL)-2 production. IκB-ζ, the most similar homolog of IκB_NS, plays an important role in the generation of T helper (Th)17 cells in cooperation with RORγt, a master regulator of Th17 cells. Thus, IκB-ζ deficient mice are resistant to Th17-dependent experimental autoimmune encephalomyelitis (EAE). However, IκB-ζ deficient mice develop the autoimmune-like Sjögren syndrome with aging. Here we found that IκB_NS-deficient (Nfkbid^−/−) mice show resistance against developing Th17-dependent EAE. We found that Nfkbid^−/− T cells have decreased expression of IL-17-related genes and RORγt in response to Transforming Growth Factor (TGF)-β1 and IL-6 stimulation. Thus, IκB_NS plays a pivotal role in the generation of Th17 cells and in the control of Th17-dependent EAE.

Regulation of mIκBNS stability through PEST-mediated degradation by proteasome

Negative regulatory proteins in a cytokine signaling play a critical role in restricting unwanted excess activation of the signaling pathway. At the same time, negative regulatory proteins need to be removed rapidly from cells to respond properly to the next incoming signal. A nuclear IκB protein called IκBNS is known to inhibit a subset of NF-κB target genes upon its expression by NF-κB activation. Here, we show a mechanism to control the stability of mIκBNS which might be important for cells to prepare the next round signaling. We found that mIκBNS is a short-lived protein of which the stability is controlled by proteasome, independent of ubiquitylation process. We identified that the N-terminal PEST sequence in mIκBNS was critical for the regulation of stability.

Atypical IκB proteins - nuclear modulators of NF-κB signaling

Nuclear factor κB (NF-κB) controls a multitude of physiological processes such as cell differentiation, cytokine expression, survival and proliferation. Since NF-κB governs embryogenesis, tissue homeostasis and the functions of innate and adaptive immune cells it represents one of the most important and versatile signaling networks known. Its activity is regulated via the inhibitors of NF-κB signaling, the IκB proteins. Classical IκBs, like the prototypical protein IκBα, sequester NF-κB transcription factors in the cytoplasm by masking of their nuclear localization signals (NLS). Thus, binding of NF-κB to the DNA is inhibited. The accessibility of the NLS is controlled via the degradation of IκBα. Phosphorylation of the conserved serine residues 32 and 36 leads to polyubiquitination and subsequent proteasomal degradation. This process marks the central event of canonical NF-κB activation. Once their NLS is accessible, NF-κB transcription factors translocate into the nucleus, bind to the DNA and regulate the transcription of their respective target genes. Several studies described a distinct group of atypical IκB proteins, referred to as the BCL-3 subfamily. Those atypical IκBs show entirely different sub-cellular localizations, activation kinetics and an unexpected functional diversity. First of all, their interaction with NF-κB transcription factors takes place in the nucleus in contrast to classical IκBs, whose binding to NF-κB predominantly occurs in the cytoplasm. Secondly, atypical IκBs are strongly induced after NF-κB activation, for example by LPS and IL-1β stimulation or triggering of B cell and T cell antigen receptors, but are not degraded in the first place like their conventional relatives. Finally, the interaction of atypical IκBs with DNA-associated NF-κB transcription factors can further enhance or diminish their transcriptional activity. Thus, they do not exclusively act as inhibitors of NF-κB activity. The capacity to modulate NF-κB transcription either positively or negatively, represents their most important and unique mechanistic difference to classical IκBs. Several reports revealed the importance of atypical IκB proteins for immune homeostasis and the severe consequences following their loss of function. This review summarizes insights into the physiological processes regulated by this protein class and the relevance of atypical IκB functioning.

IκB(NS) protein mediates regulatory T cell development via induction of the Foxp3 transcription factor

Forkhead box P3 positive (Foxp3(+)) regulatory T (Treg) cells suppress immune responses and regulate peripheral tolerance. Here we show that the atypical inhibitor of NFκB (IκB) IκB(NS) drives Foxp3 expression via association with the promoter and the conserved noncoding sequence 3 (CNS3) of the Foxp3 locus. Consequently, IκB(NS) deficiency leads to a substantial reduction of Foxp3(+) Treg cells in vivo and impaired Foxp3 induction upon transforming growth factor-β (TGF-β) treatment in vitro. Moreover, fewer Foxp3(+) Treg cells developed from IκB(NS)-deficient CD25(-)CD4(+) T cells adoptively transferred into immunodeficient recipients. Importantly, IκB(NS) was required for the transition of immature GITR(+)CD25(+)Foxp3(-) thymic Treg cell precursors into Foxp3(+) cells. In contrast to mice lacking c-Rel or Carma1, IκB(NS)-deficient mice do not show reduced Treg precursor cells. Our results demonstrate that IκB(NS) critically regulates Treg cell development in the thymus and during gut inflammation, indicating that strategies targeting IκB(NS) could modulate the Treg cell compartment.

It takes two to tango: IκBs, the multifunctional partners of NF-κB

The inhibitory IκB proteins have been discovered as fundamental regulators of the inducible transcription factor nuclear factor-κB (NF-κB). As a generally excepted model, stimulus-dependent destruction of inhibitory IκBs and processing of precursor molecules, both promoted by components of the signal integrating IκB kinase complex, are the key events for the release of various NF-κB/Rel dimers and subsequent transcriptional activation. Intense research of more than 20 years provides evidence that the extending family of IκBs act not simply as reversible inhibitors of NF-κB activation but rather as a complex regulatory module, which assures feedback regulation of the NF-κB system and either can inhibit or promote transcriptional activity in a stimulus-dependent manner. Thus, IκB and NF-κB/Rel family proteins establish a complex interrelationship that allows modulated NF-κB-dependent transcription, tailored to the physiological environment.

A forward genetic screen reveals roles for Nfkbid, Zeb1, and Ruvbl2 in humoral immunity

Using chemical germ-line mutagenesis, we screened mice for defects in the humoral immune response to a type II T-independent immunogen and an experimental alphavirus vector. A total of 26 mutations that impair humoral immunity were recovered, and 19 of these mutations have been positionally cloned. Among the phenovariants were bumble, cellophane, and Worker ascribed to mutations in Nfkbid, Zeb1, and Ruvbl2, respectively. We show that IκBNS, the nuclear IκB-like protein encoded by Nfkbid, is required for the development of marginal zone and peritoneal B-1 B cells and additionally required for extrafollicular antibody responses to T-independent and -dependent immunogens. Zeb1 is also required for marginal zone and peritoneal B-1 B-cell development as well as T-cell development, germinal center formation, and memory B-cell responses. Finally, Ruvbl2 is required for T-cell development and maximal T-dependent antibody responses. Collectively, the mutations that we identified give us insight into the points at which disruption of an antibody response can occur. All of the mutations identified to date directly affect lymphocyte development or function; none have an exclusive effect on cells of the innate immune system.

NF-κB, the first quarter-century: remarkable progress and outstanding questions

The ability to sense and adjust to the environment is crucial to life. For multicellular organisms, the ability to respond to external changes is essential not only for survival but also for normal development and physiology. Although signaling events can directly modify cellular function, typically signaling acts to alter transcriptional responses to generate both transient and sustained changes. Rapid, but transient, changes in gene expression are mediated by inducible transcription factors such as NF-κB. For the past 25 years, NF-κB has served as a paradigm for inducible transcription factors and has provided numerous insights into how signaling events influence gene expression and physiology. Since its discovery as a regulator of expression of the κ light chain gene in B cells, research on NF-κB continues to yield new insights into fundamental cellular processes. Advances in understanding the mechanisms that regulate NF-κB have been accompanied by progress in elucidating the biological significance of this transcription factor in various physiological processes. NF-κB likely plays the most prominent role in the development and function of the immune system and, not surprisingly, when dysregulated, contributes to the pathophysiology of inflammatory disease. As our appreciation of the fundamental role of inflammation in disease pathogenesis has increased, so too has the importance of NF-κB as a key regulatory molecule gained progressively greater significance. However, despite the tremendous progress that has been made in understanding the regulation of NF-κB, there is much that remains to be understood. In this review, we highlight both the progress that has been made and the fundamental questions that remain unanswered after 25 years of study.

NF-κB regulation: lessons from structures

The signaling module that specifies nuclear factor-κΒ (NF-κB) activation is a three-component system: NF-κB, inhibitor of NF-κΒ (IκΒ), and IκΒ kinase complex (IKK). IKK receives upstream signals from the surface or inside the cell and converts itself into a catalytically active form, leading to the destruction of IκB in the inhibited IκB:NF-κB complex, leaving active NF-κB free to regulate target genes. Hidden within this simple module are family members that all can undergo various modifications resulting in expansion of functional spectrum. Three-dimensional structures representing all three components are now available. These structures have allowed us to interpret cellular observations in molecular terms and at the same time helped us to bring forward new concepts focused towards understanding the specificity in the NF-κB activation pathway.

Impaired B cell development and function in the absence of IkappaBNS

IκBNS has been identified as a member of the IκB family of NF-κB inhibitors, which undergoes induction upon TCR signaling. Mice carrying a targeted gene disruption of IκBNS demonstrate dysregulation of cytokines in T cells, macrophages, and dendritic cells. IκBNS mediates both positive and negative gene regulation, depending on individual cell type and/or cytokine. In this study, we demonstrate an additional role for IκBNS in the B cell lineage. B cells from IκBNS knockout (KO) mice were impaired in proliferative responses to LPS and anti-CD40. IgM and IgG3 Igs were drastically reduced in the serum of IκBNS KO mice, although IκBNS KO B cells exhibited a higher level of surface IgM than that found in wild-type mice. Switching to IgG3 was significantly reduced in IκBNS KO B cells. The in vitro induction of plasma cell development demonstrated that progression to Ab-secreting cells was impaired in IκBNS KO B cells. In agreement with this finding, the number of Ab-secreting cells in the spleens of IκBNS KO mice was reduced and production of Ag-specific Igs was lower in IκBNS KO mice after influenza infection as compared with wild-type mice. Additionally, IκBNS KO mice lacked B1 B cells and exhibited a reduction in marginal zone B cells. Thus, IκBNS significantly impacts the development and functions of B cells and plasma cells.

NFKBIL1 confers resistance to experimental autoimmune arthritis through the regulation of dendritic cell functions

We and others have reported that human NF-κB inhibitor-like-1 (NFKBIL1) was a putative susceptible gene for autoimmune diseases such as rheumatoid arthritis (RA). However, its precise role in the pathogenesis of RA is still largely unknown. In this study, we generated transgenic mice expressing human NFKBIL1 (NFKBIL1-Tg) and examined whether NFKBIL1 plays some role(s) in the development of autoimmune arthritis. In both a collagen-induced arthritis model and a collagen antibody-induced arthritis model, NFKBIL1-Tg mice showed resistance to arthritis compared to control mice, indicating that the gene product of NFKBIL1 was involved in the control of thusly induced arthritis. Total spleen cells of NFKBIL1-Tg mouse showed decreased proliferation to mitogenic stimuli, consistent with its resistance to arthritis. Unexpectedly, purified T cells of NFKBIL1-Tg mouse showed increased proliferation and cytokine production. This apparent discrepancy was accounted for by the impaired functions of antigen-presenting cells of NFKBIL1-Tg mouse; both T/B cell-depleted spleen cells and bone marrow-derived dendritic cells of the Tg mouse induced less prominent proliferation and IL-2 production of T cells. Furthermore, dendritic cells (DCs) derived from NFKBIL1-Tg mouse showed lower expression of co-stimulatory molecules and decreased production of inflammatory cytokines when they were activated by lipopolysaccharide. Taken together, these results indicated that NFKBIL1 affected the pathogenesis of RA at least in part through the regulation of DC functions.

Structure-function relationship of cytoplasmic and nuclear IκB proteins: an in silico analysis

Cytoplasmic IκB proteins are primary regulators that interact with NF-κB subunits in the cytoplasm of unstimulated cells. Upon stimulation, these IκB proteins are rapidly degraded, thus allowing NF-κB to translocate into the nucleus and activate the transcription of genes encoding various immune mediators. Subsequent to translocation, nuclear IκB proteins play an important role in the regulation of NF-κB transcriptional activity by acting either as activators or inhibitors. To date, molecular basis for the binding of IκBα, IκBβ and IκBζ along with their partners is known; however, the activation and inhibition mechanism of the remaining IκB (IκBNS, IκBε and Bcl-3) proteins remains elusive. Moreover, even though IκB proteins are structurally similar, it is difficult to determine the exact specificities of IκB proteins towards their respective binding partners. The three-dimensional structures of IκBNS, IκBζ and IκBε were modeled. Subsequently, we used an explicit solvent method to perform detailed molecular dynamic simulations of these proteins along with their known crystal structures (IκBα, IκBβ and Bcl-3) in order to investigate the flexibility of the ankyrin repeat domains (ARDs). Furthermore, the refined models of IκBNS, IκBε and Bcl-3 were used for multiple protein-protein docking studies for the identification of IκBNS-p50/p50, IκBε-p50/p65 and Bcl-3-p50/p50 complexes in order to study the structural basis of their activation and inhibition. The docking experiments revealed that IκBε masked the nuclear localization signal (NLS) of the p50/p65 subunits, thereby preventing its translocation into the nucleus. For the Bcl-3- and IκBNS-p50/p50 complexes, the results show that Bcl-3 mediated transcription through its transactivation domain (TAD) while IκBNS inhibited transcription due to its lack of a TAD, which is consistent with biochemical studies. Additionally, the numbers of identified flexible residues were equal in number among all IκB proteins, although they were not conserved. This could be the primary reason for their binding partner specificities.

Understanding the logic of IκB:NF-κB regulation in structural terms

NF-κB is an inducible transcription factor that controls expression of diverse stress response genes. The entire mammalian NF-κB family is generated from a small cadre of five gene products that assemble with one another in various combinations to form active homo- and heterodimers. The ability of NF-κB to alter target gene expression is regulated at many levels. Chief among these regulatory mechanisms is the noncovalent association in the cell cytoplasm of NF-κB dimers with IκB inhibitor proteins. Removal of IκB leads to accumulation of active NF-κB within the cell nucleus where it binds to specific DNA sequences contained within the promoter regions of target genes and initiates recruitment of general transcription factors and assembly of the basal transcription machinery. Here we provide a detailed description of these fundamental NF-κB regulatory events using as a basis macromolecular structures and experimental data derived from structure-based biochemistry.

New regulators of NF-kappaB in inflammation

Research on the biological function of nuclear factor-kappaB (NF-kappaB), a key mediator of inducible transcription in the immune system, has traditionally focused on its role in the initiation of innate and adaptive immune responses. These studies have largely concentrated on the mechanisms of signalling that lead to NF-kappaB activation and on the positive role of NF-kappaB in both physiological immunity and pathological inflammation. More recently, there has been growing interest in the mechanisms that directly regulate the NF-kappaB transcriptional programmes. As a result, several new NF-kappaB regulatory components have been identified and some of the known components have been assigned new roles. In this Review, we discuss these new insights into the regulation of NF-kappaB.

Role of nuclear IkappaB proteins in the regulation of host immune responses

A variety of microbial components activate a transcription factor called nuclear factor-kappaB (NF-kappaB) that plays an essential role in the optimal activation of host immune systems. The transcriptional activity of NF-kappaB is tightly regulated at multiple steps in immune signaling pathways, because excessive activation is detrimental to the host. One mechanism to prevent NF-kappaB activation is mediated by cytoplasmic IkappaB family proteins. Although cytoplasmic IkappaBs interact with NF-kappaB subunits in the cytoplasm of unstimulated cells, IkappaBs are rapidly degraded on stimulation, allowing free NF-kappaB to translocate into the nucleus and activate the transcription of genes encoding various immune mediators. After the translocation of NF-kappaB from the cytoplasm to the nucleus, nuclear proteins that are structurally similar to cytoplasmic IkappaBs take part in the regulation of NF-kappaB transcriptional activity, as activators or inhibitors, by associating with NF-kappaB subunits. Therefore, the regulatory IkappaB-like nuclear molecules are described as "nuclear IkappaB proteins." In this review, the in vivo function of the nuclear IkappaB proteins, Bcl-3, IkappaBzeta, and IkappaBNS in the context of host immune responses and diseases will be discussed.

The nuclear I kappaB protein I kappaB zeta specifically binds NF-kappaB p50 homodimers and forms a ternary complex on kappaB DNA

Although they share sequence homology to classical cytoplasmic I kappaB inhibitors of transcription factor NF-kappaB, the proteins I kappaB zeta, Bcl-3, and I kappa BNS function in the nucleus as factors that influence NF-kappaB-dependent gene expression profiles. Through the use of purified recombinant proteins and by comparison with the classical I kappaB protein I kappaB alpha, we have discovered mechanistic details of the interaction between I kappaB zeta and functional NF-kappaB dimers. Whereas I kappaB alpha and other classical I kappaB proteins bind tightly to NF-kappaB dimers that possess the p65 subunit, I kappaB zeta binds preferentially to NF-kappaB p50 homodimers. This altered specificity is particularly interesting in light of the fact that both NF-kappaB subunits exhibit high sequence and structural homology, while the I kappaB alpha and I kappaB zeta proteins are also conserved in primary amino acid sequence. We further show that I kappaB zeta is capable of forming a stable ternary complex with the NF-kappaB p50 homodimer and kappaB DNA. Again, this is a stark contrast from I kappaB alpha, which inhibits NF-kappaB p65 homodimer binding to NF-kappaB target DNA sequences. Removal of the DNA sequences flanking the NF-kappaB binding site does not directly affect the interaction of p50 and I kappaB zeta. Rather, we have discovered that the carboxy-terminal glycine-rich region of the NF-kappaB p50 homodimer is involved in mediating high-affinity binding of I kappaB zeta and NF-kappaB p50. We conclude that the NF-kappaB p50 homodimer functions as a legitimate activator of gene expression through formation of a ternary complex between itself, I kappaB zeta, and DNA. The requirement for formation of this complex could explain why the nuclear I kappaB protein I kappaB zeta is absolutely required for expression of the pluripotent pro-inflammatory cytokine interleukin-6 in peritoneal macrophages.