Roles of the NF-kappaB pathway in lymphocyte development and function

30 Years of NF-κB: A Blossoming of Relevance to Human Pathobiology

NF-κB was discovered 30 years ago as a rapidly inducible transcription factor. Since that time, it has been found to have a broad role in gene induction in diverse cellular responses, particularly throughout the immune system. Here, we summarize elaborate regulatory pathways involving this transcription factor and use recent discoveries in human genetic diseases to place specific proteins within their relevant medical and biological contexts.

A20 Curtails Primary but Augments Secondary CD8+ T Cell Responses in Intracellular Bacterial Infection

The ubiquitin-modifying enzyme A20, an important negative feedback regulator of NF-κB, impairs the expansion of tumor-specific CD8⁺ T cells but augments the proliferation of autoimmune CD4⁺ T cells. To study the T cell-specific function of A20 in bacterial infection, we infected T cell-specific A20 knockout (CD4-Cre A20^fl/fl) and control mice with Listeria monocytogenes. A20-deficient pathogen-specific CD8⁺ T cells expanded stronger resulting in improved pathogen control at day 7 p.i. Imaging flow cytometry revealed that A20-deficient Listeria-specific CD8⁺ T cells underwent increased apoptosis and necroptosis resulting in reduced numbers of memory CD8⁺ T cells. In contrast, the primary CD4⁺ T cell response was A20-independent. Upon secondary infection, the increase and function of pathogen-specific CD8⁺ T cells, as well as pathogen control were significantly impaired in CD4-Cre A20^fl/fl mice. In vitro, apoptosis and necroptosis of Listeria-specific A20-deficient CD8⁺ T cells were strongly induced as demonstrated by increased caspase-3/7 activity, RIPK1/RIPK3 complex formation and more morphologically apoptotic and necroptotic CD8⁺ T cells. In vitro, A20 limited CD95L and TNF-induced caspase3/7 activation. In conclusion, T cell-specific A20 limited the expansion but reduced apoptosis and necroptosis of Listeria-specific CD8⁺ T cells, resulting in an impaired pathogen control in primary but improved clearance in secondary infection.

TAK1 maintains the survival of immunoglobulin λ-chain-positive B cells

TAK1 (MAP3K7) mediation of the IκB kinase (IKK) complex-nuclear factor-κB (NF-κB) pathway is crucial for the activation of immune response and to perpetuate inflammation. Although progress has been made to understand TAK1 function in the B-cell receptor (BCR) signaling, the physiological roles of TAK1 in B-cell development, particularly in the bone marrow (BM), remain elusive. Previous studies suggested that the IKK complex is required for the development of immunoglobulin light chain λ-positive B cells, but not for receptor editing. In contrast, NF-κB activity is suggested to be involved in the regulation of receptor editing. Thus, NF-κB signaling in early B-cell development is yet to be fully characterized. Therefore, we addressed the role of TAK1 in early B-cell development. TAK1-deficient mice showed significant reduction of BM Igλ-positive B-cell numbers without any alteration in the BCR editing. Furthermore, the expression of survival factor Bcl-2 was reduced in TAK1-deficient BM B cells as assessed by microarray and quantitative PCR analyses. Ex vivo over-expression of exogenous Bcl-2 enhanced the survival of TAK1-deficient Igλ-positive B cells. TAK1-IKK-NF-κB signaling contributes to the survival of λ-chain-positive B cells through NF-κB-dependent anti-apoptotic Bcl-2 expression.

Differential requirements for the canonical NF-κB transcription factors c-REL and RELA during the generation and activation of mature B cells

Signaling through the canonical NF-κB pathway is critical for the generation and maintenance of mature B-cells and for antigen-dependent B-cell activation. c-REL (rel) and RELA (rela) are the downstream transcriptional activators of the canonical NF-κB pathway. Studies of B-cells derived from constitutional rel knockout mice and chimeric mice repopulated with rela^−/− fetal liver cells provided evidence that the subunits can have distinct roles during B-cell development. However, the B-cell-intrinsic functions of c-REL and RELA during B-cell generation and antigen-dependent B-cell activation have not been determined in vivo. To clarify this issue, we crossed mice with conditional rel and rela alleles individually or in combination to mice that express Cre-recombinase in B-cells. We here report that, whereas single deletion of rel or rela did not impair mature B-cell generation and maintenance, their simultaneous deletion led to a dramatic reduction of follicular and marginal zone B-cells. Upon T-cell-dependent immunization, B-cell-specific deletion of the c-REL subunit alone abrogated the formation of germinal centers (GC), whereas rela deletion did not affect GC formation. T-independent responses were strongly impaired in mice with B-cell-specific deletion of rel, and only modestly in mice with RELA-deficient B-cells. Our findings identify differential requirements for the canonical NF-κB subunits c-REL and RELA at distinct stages of mature B-cell development. The subunits are jointly required for the generation of mature B-cells. During antigen-dependent B-cell activation, c-REL is the critical subunit required for the initiation of the GC-reaction and for optimal T-independent antibody responses, with RELA being largely dispensable at this stage.

Endogenous n-3 Polyunsaturated Fatty Acids Attenuate T Cell-Mediated Hepatitis via Autophagy Activation

Omega-3 polyunsaturated fatty acids (n-3 PUFAs) exert anti-inflammatory effects in several liver disorders, including cirrhosis, acute liver failure, and fatty liver disease. To date, little is known about their role in immune-mediated liver diseases. In this study, we used fat-1 transgenic mice rich in endogenous n-3 PUFAs to examine the role of n-3 PUFAs in immune-mediated liver injury. Concanavalin A (Con A) was administered intravenously to wild-type (WT) and fat-1 transgenic mice to induce T cell-mediated hepatitis. Reduced liver damage was shown in Con A-administrated fat-1 transgenic mice, as evidenced by decreased mortality, attenuated hepatic necrosis, lessened serum alanine aminotransferase activity, and inhibited production of pro-inflammatory cytokines (e.g., TNF-α, IL-6, IL-17A, and IFN-γ). In vivo and in vitro studies demonstrated that n-3 PUFAs significantly inhibited the activation of hepatic T cells and the differentiation of Th1 cells after Con A challenge. Further studies showed that n-3 PUFAs markedly increased autophagy level in Con A-treated fat-1 T cells compared with the WT counterparts. Blocking hepatic autophagy activity with chloroquine diminished the differences in T cell activation and liver injury between Con A-injected WT and fat-1 transgenic mice. We conclude that n-3 PUFAs limit Con A-induced hepatitis via an autophagy-dependent mechanism and could be exploited as a new therapeutic approach for autoimmune hepatitis.

DNA methylome analysis of acute lymphoblastic leukemia cells reveals stochastic de novo DNA methylation in CpG islands

AIM

To identify regions of aberrant DNA methylation in acute lymphoblastic leukemia (ALL) cells of different subtypes on a genome-wide scale.

MATERIALS & METHODS

Whole-genome bisulfite sequencing (WGBS) was used to determine the DNA methylation levels in cells from four pediatric ALL patients of different subtypes. The findings were confirmed by 450k DNA methylation arrays in a large patient set.

RESULTS

Compared with mature B or T cells WGBS detected on average 82,000 differentially methylated regions per patient. Differentially methylated regions are enriched to CpG poor regions, active enhancers and transcriptional start sites. We also identified approximately 8000 CpG islands with variable intermediate DNA methylation that seems to occur as a result of stochastic de novo methylation.

CONCLUSION

WGBS provides an unbiased view and novel insights into the DNA methylome of ALL cells.

Genetic drivers of NF-κB deregulation in diffuse large B-cell lymphoma

Diffuse large B cell lymphoma (DLBCL) is the most common form of B cell non-Hodgkin lymphoma worldwide and comprises a heterogeneous group of malignancies that originate from the malignant transformation of germinal center (GC) B cells. Over the past decade, significant improvement has been achieved in our understanding of the molecular pathogenesis underlying this disease, thanks in part to the implementation of powerful genomic technologies allowing genome-wide structural and functional analyses. These studies revealed the presence of multiple oncogenic alterations dysregulating signal transduction pathways that are normally required for the normal biology of the cells from which these tumors are derived. Among the pathways identified as recurrent targets of genetic lesions in DLBCL, NF-κB has emerged as a central player in the development and maintenance of this disease, particularly in the less curable, activated B cell (ABC)- like subtype. These lesions reveal vulnerabilities of the lymphoma cells that can be exploited for the design of more rationale therapeutic approaches. The purpose of this review is to summarize recent progresses in understanding the role of NF-κB deregulation in the pathogenesis of DLBCL, with emphasis on the genetic basis underlying its aberrant activation, in relationship to the normal biology of B lymphocytes, and the modelling of these lesions in the mouse.

Systematic identification of protein combinations mediating chromatin looping

Chromatin looping plays a pivotal role in gene expression and other biological processes through bringing distal regulatory elements into spatial proximity. The formation of chromatin loops is mainly mediated by DNA-binding proteins (DBPs) that bind to the interacting sites and form complexes in three-dimensional (3D) space. Previously, identification of DBP cooperation has been limited to those binding to neighbouring regions in the proximal linear genome (1D cooperation). Here we present the first study that integrates protein ChIP-seq and Hi-C data to systematically identify both the 1D- and 3D-cooperation between DBPs. We develop a new network model that allows identification of cooperation between multiple DBPs and reveals cell-type-specific and -independent regulations. Using this framework, we retrieve many known and previously unknown 3D-cooperations between DBPs in chromosomal loops that may be a key factor in influencing the 3D organization of chromatin.

Transcription factors of the alternative NF-κB pathway are required for germinal center B-cell development

The NF-κB signaling cascade relays external signals essential for B-cell growth and survival. This cascade is frequently hijacked by cancers that arise from the malignant transformation of germinal center (GC) B cells, underscoring the importance of deciphering the function of NF-κB in these cells. The NF-κB signaling cascade is comprised of two branches, the canonical and alternative NF-κB pathways, mediated by distinct transcription factors. The expression and function of the transcription factors of the alternative pathway, RELB and NF-κB2, in late B-cell development is incompletely understood. Using conditional deletion of relb and nfkb2 in GC B cells, we here report that ablation of both RELB and NF-κB2, but not of the single transcription factors, resulted in the collapse of established GCs. RELB/NF-κB2 deficiency in GC B cells was associated with impaired cell-cycle entry and reduced expression of the cell-surface receptor inducible T-cell costimulator ligand that promotes optimal interactions between B and T cells. Analysis of human tonsillar tissue revealed that plasma cells and their precursors in the GC expressed high levels of NF-κB2 relative to surrounding lymphocytes. Accordingly, deletion of nfkb2 in murine GC B cells resulted in a dramatic reduction of antigen-specific antibody-secreting cells, whereas deletion of relb had no effect. These results demonstrate that the transcription factors of the alternative NF-κB pathway control distinct stages of late B-cell development, which may have implications for B-cell malignancies that aberrantly activate this pathway.

Disturbed canonical nuclear factor of κ light chain signaling in B cells of patients with common variable immunodeficiency

BACKGROUND

Most patients with common variable immunodeficiency (CVID) present with severely reduced switched memory B-cell counts, and some display an increase of CD21^low B-cell counts (CVID 21low), whereas others do not (CVID 21norm). Altered B-cell receptor (BCR) signaling might contribute to the defective memory formation observed in patients with CVID.

OBJECTIVE

We sought to investigate canonical nuclear factor of κ light chain (NF-κB) signaling in B cells from patients with CVID as a central pathway in B-cell differentiation.

METHODS

Degradation of inhibitor of κBα (IκBα) and p65 phosphorylation, nuclear translocation of p65, and regulation of target genes and cell function were investigated after different modes of B-cell stimulation.

RESULTS

BCR-mediated canonical NF-κB signaling was impaired in all mature naive CVID-derived B cells. This impairment was more profound in naive B cells from CVID 21low patients than CVID 21norm patients and most pronounced in CD21^low B cells. The signaling defect translated into reduced induction of Bcl-xL and IκBα, 2 bona fide target genes of the canonical NF-κB pathway. CD40 ligand- and Toll-like receptor 9-mediated signaling were less strongly altered. Signaling in CD21^low B cells but not CD21⁺ B cells of patients with HIV was similarly affected.

CONCLUSION

Combined with the previous description of disturbed Ca²⁺ signaling, the discovery of NF-κB signaling defects, especially in CVID 21low patients, suggests a broad underlying signaling defect affecting especially BCR-derived signals. Given the immune phenotype of monogenic defects affecting Ca²⁺ and NF-κB signaling, the latter is more likely to contribute to the humoral deficiency. The strongly disturbed BCR signaling of CD21^low B cells is characteristic for this cell type and independent of the underlying disease.

Genetic risk variants for autoimmune diseases that influence gene expression in thymus

Genome-wide association studies (GWAS) have boosted our knowledge of genetic risk variants in autoimmune diseases (AIDs). Most risk variants are located within or near genes with immunological functions, and the majority is found to be non-coding, pointing towards a regulatory role. In this study, we performed a cis expression quantitative trait locus (eQTL) screen restricted to 353 AID associated risk variants selected from the GWAS catalog to investigate whether these single nucleotide polymorphisms (SNPs) influence gene expression in thymus. Genotypes were obtained by Immunochip (Ichip) and tested against expression of surrounding genes (±1 Mb) in human thymic tissue (n = 42). We identified eight significant eQTLs located within seven genetic regions (FCRL3, RNASET2, C2orf74, NPIPB8, SIRPG, SYS1 and AJ006998.2) where the expression was associated with AID risk SNPs at a study-wide level of significance (P < 2.7 × 10^-5). In NPIPB8 and AJ006998.2, the eQTL signals appeared to be thymus-specific. Furthermore, many AID risk SNPs from GWAS have been subsequently fine-mapped in recent Ichip projects, and fine-mapped AID SNPs overlapped with the thymic eQTLs within RNASET2 and SIRPG Finally, in all the eQTL regions, except C2orf74, SNPs underlying the thymic eQTLs were predicted to interfere with transcription factors important in T cell development. Our study therefore reveals autoimmune risk variants that act as eQTLs in thymus, and suggest that thymic gene regulation may play a functional role at some AID risk loci.

LRRK1 is critical in the regulation of B-cell responses and CARMA1-dependent NF-κB activation

B-cell receptor (BCR) signaling plays a critical role in B-cell activation and humoral immunity. In this study, we discovered a critical function of leucine-rich repeat kinase 1 (LRRK1) in BCR-mediated immune responses. Lrrk1^−/− mice exhibited altered B1a-cell development and basal immunoglobulin production. In addition, these mice failed to produce IgG3 antibody in response to T cell–independent type 2 antigen due to defects in IgG3 class-switch recombination. Concomitantly, B cells lacking LRRK1 exhibited a profound defect in proliferation and survival upon BCR stimulation, which correlated with impaired BCR-mediated NF-κB activation and reduced expression of NF-κB target genes including Bcl-x_L, cyclin D2, and NFATc1/αA. Furthermore, LRRK1 physically interacted and potently synergized with CARMA1 to enhance NF-κB activation. Our results reveal a critical role of LRRK1 in NF-κB signaling in B cells and the humoral immune response.

Post-Translational Modifications of RelB NF-κB Subunit and Associated Functions

The family of NF-κB transcription factors plays a key role in diverse biological processes, such as inflammatory and immune responses, cell survival and tumor development. Beyond the classical NF-κB activation pathway, a second NF-κB pathway has more recently been uncovered, the so-called alternative NF-κB activation pathway. It has been shown that this pathway mainly controls the activity of RelB, a member of the NF-κB family. Post-translational modifications, such as phosphorylation, acetylation, methylation, ubiquitination and SUMOylation, have recently emerged as a strategy for the fine-tuned regulation of NF-κB. Our review discusses recent progress in the understanding of RelB regulation by post-translational modifications and the associated functions in normal and pathological conditions.

Negative role of TAK1 in marginal zone B-cell development incidental to NF-κB noncanonical pathway activation

The transcription factor nuclear factor-κB (NF-κB) signaling pathway is crucial in B-cell physiology. One key molecule regulating this pathway is the serine/threonine kinase TAK1 (MAP3K7). TAK1 is responsible for positive feedback mechanisms in B-cell receptor signaling that serve as an NF-κB activation threshold. This study aimed to better understand the correlation between TAK1-mediated signaling and B-cell development and humoral immune responses. Here we showed that a B-cell conditional deletion of TAK1 using mb1-cre resulted in a dramatic elimination of the humoral immune response, consistent with the absence of the B-1 B-cell subset. When monitoring the self-reactive B-cell system (the immunoglobulin hen egg lysozyme/soluble hen egg lysozyme double-transgenic mouse model), we found that TAK1-deficient B cells exhibited an enhanced susceptibility to cell death that might explain the disappearance of the B1 subset. In contrast, these mice gained numerous marginal zone (MZ) B cells. We consequently examined the basal and B-cell receptor-induced activity of NF-κB2 that is reported to regulate MZ B-cell development, and demonstrated that the activity of NF-κB2 increased in TAK1-deficient B cells. Thus, our results present a novel in vivo function, the negative role of TAK1 in MZ B-cell development that is likely associated with NF-κB2 activation.

Canonical NF-κB signaling is uniquely required for the long-term persistence of functional mature B cells

Although canonical NF-κB signaling is crucial to generate a normal mature B-cell compartment, its role in the persistence of resting mature B cells is controversial. To resolve this conflict, we ablated NF-κB essential modulator (NEMO) and IκB kinase 2 (IKK2), two essential mediators of the canonical pathway, either early on in B-cell development or specifically in mature B cells. Early ablation severely inhibited the generation of all mature B-cell subsets, but follicular B-cell numbers could be largely rescued by ectopic expression of B-cell lymphoma 2 (Bcl2), despite a persisting block at the transitional stage. Marginal zone (MZ) B and B1 cells were not rescued, indicating a possible role of canonical NF-κB signals beyond the control of cell survival in these subsets. When canonical NF-κB signaling was ablated specifically in mature B cells, the differentiation and/or persistence of MZ B cells was still abrogated, but follicular B-cell numbers were only mildly affected. However, the mutant cells exhibited increased turnover as well as functional deficiencies upon activation, suggesting that canonical NF-κB signals contribute to their long-term persistence and functional fitness.

The NF-κB transcription factor RelA is required for the tolerogenic function of Foxp3(+) regulatory T cells

The properties of CD4(+) regulatory T cell (Treg) subsets are dictated by distinct patterns of gene expression determined by FOXP3 and different combinations of various transcription factors. Here we show the NF-κB transcription factor RelA is constitutively active in naïve and effector Tregs. The conditional inactivation of Rela in murine FOXP3(+) cells induces a rapid onset, multi-focal autoimmune disease that depends on RelA being expressed in conventional T cells. In addition to promoting Treg lineage stability, RelA determines the size of the effector Treg population, a function influenced by the presence or absence of RelA in conventional T cells. These findings showing that RelA controls Treg stability and promotes the competitive fitness of effector Tregs highlight the importance of RelA activity in peripheral Treg induced tolerance.

NFκB1 is essential to prevent the development of multiorgan autoimmunity by limiting IL-6 production in follicular B cells

de Valle et al. show that, with age, NFκB1-deficient B cells spontaneously secrete IL-6 and cause a multiorgan autoimmune disease.

We examined the role of NFκB1 in the homeostasis and function of peripheral follicular (Fo) B cells. Aging mice lacking NFκB1 (Nfκb1^−/−) develop lymphoproliferative and multiorgan autoimmune disease attributed in large part to the deregulated activity of Nfκb1^−/− Fo B cells that produce excessive levels of the proinflammatory cytokine interleukin 6 (IL-6). Despite enhanced germinal center (GC) B cell differentiation, the formation of GC structures was severely disrupted in the Nfκb1^−/− mice. Bone marrow chimeric mice revealed that the Fo B cell–intrinsic loss of NFκB1 led to the spontaneous generation of GC B cells. This was primarily the result of an increase in IL-6 levels, which promotes the differentiation of Fo helper CD4⁺ T cells and acts in an autocrine manner to reduce antigen receptor and toll-like receptor activation thresholds in a population of proliferating IgM⁺Nfκb1^−/− Fo B cells. We demonstrate that p50-NFκB1 represses Il-6 transcription in Fo B cells, with the loss of NFκB1 also resulting in the uncontrolled RELA-driven transcription of Il-6. Collectively, our findings identify a previously unrecognized role for NFκB1 in preventing multiorgan autoimmunity through its negative regulation of Il-6 gene expression in Fo B cells.

Disruption of Tumor Necrosis Factor Receptor-Associated Factor 5 Exacerbates Murine Experimental Colitis via Regulating T Helper Cell-Mediated Inflammation

Tumor necrosis factor (TNF) receptor-associated factor 5 (TRAF5) is a key mediator of TNF receptor superfamily members and is important in both T helper (Th) cell immunity and the regulation of multiple signaling pathways. To clarify TRAF5's influence on inflammatory bowel diseases (IBDs), we investigated TRAF5 deficiency's effect on dextran sulfate sodium- (DSS-) induced colitis. Colitis was induced in TRAF5 knockout (KO) mice and their wild-type (WT) littermates by administering 3% DSS orally for 7 days. The mice were then sacrificed, and their colons were removed. Our data suggested that KO mice were more susceptible to DSS-induced colitis. TRAF5 deficiency significantly enhanced IFN-γ, IL-4, and IL-17a mRNA and protein levels in the colons of DSS-fed mice, and the mRNA expression of T-bet and GATA-3 was also markedly elevated. However, ROR-α and ROR-γt mRNA levels did not differ between DSS-induced KO and WT mice. Flow cytometry showed increased frequencies of Th2 and IFN-γ/IL-17a-coproducing CD4⁺ T cells in the colons of DSS-induced KO mice. Additionally, TRAF5 deficiency significantly enhanced the activation of NF-κB in CD4⁺ T cells after DSS administration. These results indicated that TRAF5 deficiency significantly aggravated DSS-induced colitis, most likely by regulating Th cell-mediated inflammation.

Usp9X Is Required for Lymphocyte Activation and Homeostasis through Its Control of ZAP70 Ubiquitination and PKCβ Kinase Activity

To achieve a durable adaptive immune response, lymphocytes must undergo clonal expansion and induce a survival program that enables the persistence of Ag-experienced cells and the development of memory. During the priming phase of this response, CD4(+)T lymphocytes either remain tolerized or undergo clonal expansion. In this article, we show that Usp9X functions as a positive regulatory switch during T lymphocyte priming through removal of inhibitory monoubiquitination from ZAP70. In the absence of Usp9X, an increased amount of ZAP70 localized to early endosomes consistent with the role of monoubiquitin in endocytic sorting. Usp9X becomes competent to deubiquitinate ZAP70 through TCR-dependent phosphorylation and enhancement of its catalytic activity and association with the LAT signalosome. In B lymphocytes, Usp9X is required for the induction of PKCβ kinase activity after BCR-dependent activation. Accordingly, inUsp9Xknockout B cells, there was a significant reduction in phospho-CARMA1 levels that resulted in reduced CARMA1/Bcl-10/MALT-1 complex formation and NF-κB-dependent cell survival. The pleiotropic effect of Usp9X during Ag-receptor signaling highlights its importance for the development of an effective and durable adaptive immune response.

Heterozygous Mutation in IκBNS Leads to Reduced Levels of Natural IgM Antibodies and Impaired Responses to T-Independent Type 2 Antigens

Mice deficient in central components of classical NF-κB signaling have low levels of circulating natural IgM antibodies and fail to respond to immunization with T-independent type 2 (TI-2) antigens. A plausible explanation for these defects is the severely reduced numbers of B-1 and marginal zone B (MZB) cells in such mice. By using an ethyl-N-nitrosourea mutagenesis screen, we identified a role for the atypical IκB protein IκBNS in humoral immunity. IκBNS-deficient mice lack B-1 cells and have severely reduced numbers of MZB cells, and thus resemble several other strains with defects in classical NF-κB signaling. We analyzed mice heterozygous for the identified IκBNS mutation and demonstrate that these mice have an intermediary phenotype in terms of levels of circulating IgM antibodies and responses to TI-2 antigens. However, in contrast to mice that are homozygous for the IκBNS mutation, the heterozygous mice had normal frequencies of B-1 and MZB cells. These results suggest that there is a requirement for IκBNS expression from two functional alleles for maintaining normal levels of circulating natural IgM antibodies and responses to TI-2 antigens.

Unexpected functions of nuclear factor-κB during germinal center B-cell development: implications for lymphomagenesis

PURPOSE OF REVIEW

B-cell tumors originating from the transformation of germinal center B cells frequently harbor genetic mutations, leading to constitutive activation of the nuclear factor-κB (NF-κB) signaling pathway. The present review highlights recent insights into the roles of separate NF-κB transcription factors in germinal center B-cell development and discusses implications of the results for germinal center lymphomagenesis.

RECENT FINDINGS

Understanding how aberrant NF-κB activation promotes tumorigenesis requires the understanding of the role of NF-κB in the tumor-precursor cells. Despite extensive knowledge on NF-κB biology, the function of this complex signaling pathway in the differentiation of germinal center B cells is largely unknown. The present review will discuss recent findings that revealed distinct roles of separate NF-κB transcription factors during the germinal center reaction in the context of germinal center lymphomagenesis. Most notably, a single NF-κB subunit, c-REL, was found to be required for the maintenance of the germinal center reaction and was associated with the activation of a metabolic program that promotes cell growth.

SUMMARY

Identifying the biological roles of the separate NF-κB transcription factor subunits in germinal center biology will help to better understand the pathogenic consequences of their constitutive activation in B-cell tumors. This knowledge may be exploited for the development of targeted antitumor therapies aimed at inhibiting selectively those components of aberrant NF-κB activity which contribute to pathogenesis.

Impairment of Mature B Cell Maintenance upon Combined Deletion of the Alternative NF-κB Transcription Factors RELB and NF-κB2 in B Cells

BAFF is critical for the survival and maturation of mature B cells. BAFF, via BAFFR, activates multiple signaling pathways in B cells, including the alternative NF-κB pathway. The transcription factors RELB and NF-κB2 (p100/p52) are the downstream mediators of the alternative pathway; however, the B cell-intrinsic functions of these NF-κB subunits have not been studied in vivo using conditional alleles, either individually or in combination. We in this study report that B cell-specific deletion of relb led to only a slight decrease in the fraction of mature splenic B cells, whereas deletion of nfkb2 caused a marked reduction. This phenotype was further exacerbated upon combined deletion of relb and nfkb2 and most dramatically affected the maintenance of marginal zone B cells. BAFF stimulation, in contrast to CD40 activation, was unable to rescue relb/nfkb2-deleted B cells in vitro. RNA-sequencing analysis of BAFF-stimulated nfkb2-deleted versus normal B cells suggests that the alternative NF-κB pathway, in addition to its critical role in BAFF-mediated cell survival, may control the expression of genes involved in the positioning of B cells within the lymphoid microenvironment and in the establishment of T cell-B cell interactions. Thus, by ablating the downstream transcription factors of the alternative NF-κB pathway specifically in B cells, we identify in this study a critical role for the combined activity of the RELB and NF-κB2 subunits in B cell homeostasis that cannot be compensated for by the canonical NF-κB pathway under physiological conditions.

B-cell survival and development controlled by the coordination of NF-κB family members RelB and cRel

Targeted deletion of BAFF causes severe deficiency of splenic B cells. BAFF-R is commonly thought to signal to nuclear factor κ-light-chain-enhancer of activated B cells (NF-κB)-inducing kinase dependent noncanonical NF-κB RelB. However, RelB-deficient mice have normal B-cell numbers. Recent studies showed that BAFF also signals to the canonical NF-κB pathway, and we found that both RelB and cRel are persistently activated, suggesting BAFF signaling coordinates both pathways to ensure robust B-cell development. Indeed, we report now that combined loss of these 2 NF-κB family members leads to impaired BAFF-mediated survival and development in vitro. Although single deletion of RelB and cRel was dispensable for normal B-cell development, double knockout mice displayed an early B-cell developmental blockade and decreased mature B cells. Despite disorganized splenic architecture in Relb(-/-)cRel(-/-) mice, generation of mixed-mouse chimeras established the developmental phenotype to be B-cell intrinsic. Together, our results indicate that BAFF signals coordinate both RelB and cRel activities to ensure survival during peripheral B-cell maturation.

Enhancer repertoires are reshaped independently of early priming and heterochromatin dynamics during B cell differentiation

A widely accepted model posits that activation of enhancers during differentiation goes through a priming step prior to lineage commitment. To investigate the chronology of enhancer repertoire establishment during hematopoiesis, we monitored epigenome dynamics during three developmental stages representing hematopoietic stem cells, B-cell progenitors and mature B-cells. We find that only a minority of enhancers primed in stem cells or progenitors become active at later stages. Furthermore, most enhancers active in differentiated cells were not primed in earlier stages. Thus, the enhancer repertoire is reshaped dynamically during B-cell differentiation and enhancer priming in early stages does not appear to be an obligate step for enhancer activation. Furthermore, our data reveal that heterochromatin and Polycomb-mediated silencing have only a minor contribution in shaping enhancer repertoires during cell differentiation. Together, our data revisit the prevalent model about epigenetic reprogramming during hematopoiesis and give insights into the formation of gene regulatory networks.

Enhancers in differentiated haematopoietic cells are generally believed to be primed prior to lineage commitment. Here, the authors show that early priming and Polycomb group mediated silencing have minor roles in shaping the enhancer repertoire in differentiated B cells and that most active enhancers are generated de novo.

The AhR and NF-κB/Rel Proteins Mediate the Inhibitory Effect of 2,3,7,8-Tetrachlorodibenzo-p-Dioxin on the 3' Immunoglobulin Heavy Chain Regulatory Region

Transcriptional regulation of the murine immunoglobulin (Ig) heavy chain gene (Igh) involves several regulatory elements including the 3'Igh regulatory region (3'IghRR), which is composed of at least 4 enhancers (hs3A, hs1.2, hs3B, and hs4). The hs1.2 and hs4 enhancers exhibit the greatest transcriptional activity and contain binding sites for several transcription factors including nuclear factor kappaB/Rel (NF-κB/Rel) proteins and the aryl hydrocarbon receptor (AhR). Interestingly, the environmental immunosuppressant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), which potently inhibits antibody secretion, also profoundly inhibits 3'IghRR and hs1.2 enhancer activation induced by the B-lymphocyte activator lipopolysaccharide (LPS), but enhances LPS-induced activation of the hs4 enhancer. Within the hs1.2 and hs4 enhancers, the AhR binding site is in close proximity or overlaps an NF-κB/Rel binding site suggesting a potential reciprocal modulation of the 3'IghRR by AhR and NF-κB/Rel. The objective of the current study was to evaluate the role of NF-κB/Rel and the AhR on the 3'IghRR and its enhancers using the AhR ligand TCDD, the AhR antagonist CH223191, and toll-like receptor agonists LPS, Resiquimod (R848), or cytosine-phosphate-guanine-oligodeoxynucleotides (CpG). Utilizing the CH12.LX B-lymphocyte cell line and variants expressing either a 3'IghRR-regulated transgene reporter or an inducible IκBα (inhibitor kappa B-alpha protein) superrepressor (IκBαAA), we demonstrate an AhR- and NF-κB/Rel-dependent modulation of 3'IghRR and hs4 activity. Additionally, in mouse splenocytes or CH12.LX cells, binding within the hs1.2 and hs4 enhancer of the AhR and the NF-κB/Rel proteins RelA and RelB was differentially altered by the cotreatment of LPS and TCDD. These results suggest that the AhR and NF-κB/Rel protein binding profile within the 3'IghRR mediates the inhibitory effects of TCDD on Ig expression and therefore antibody levels.

Bryant V, Frede N, Slade C, Woon ST, Lehnert K, Winzer S, Bulashevska A, Scerri T, Leung E, Jordan A, Keller B, de Vries E, Cao H, Yang F, Schäffer A, Warnatz K, Browett P, Douglass J, Ameratunga R, van der Meer J, Grimbacher B. Haploinsufficiency of the NF-κB1 Subunit p50 in Common Variable Immunodeficiency

Common variable immunodeficiency (CVID), characterized by recurrent infections, is the most prevalent symptomatic antibody deficiency. In ∼90% of CVID-affected individuals, no genetic cause of the disease has been identified. In a Dutch-Australian CVID-affected family, we identified a NFKB1 heterozygous splice-donor-site mutation (c.730+4A>G), causing in-frame skipping of exon 8. NFKB1 encodes the transcription-factor precursor p105, which is processed to p50 (canonical NF-κB pathway). The altered protein bearing an internal deletion (p.Asp191_Lys244delinsGlu; p105ΔEx8) is degraded, but is not processed to p50ΔEx8. Altered NF-κB1 proteins were also undetectable in a German CVID-affected family with a heterozygous in-frame exon 9 skipping mutation (c.835+2T>G) and in a CVID-affected family from New Zealand with a heterozygous frameshift mutation (c.465dupA) in exon 7. Given that residual p105 and p50—translated from the non-mutated alleles—were normal, and altered p50 proteins were absent, we conclude that the CVID phenotype in these families is caused by NF-κB1 p50 haploinsufficiency.

Dynamic aberrant NF-κB spurs tumorigenesis: a new model encompassing the microenvironment

Recently it was discovered that a transient activation of transcription factor NF-κB can give cells properties essential for invasiveness and cancer initiating potential. In contrast, most oncogenes to date were characterized on the basis of mutations or by their constitutive overexpression. Study of NF-κB actually leads to a far more dynamic perspective on cancer: tumors caused by diverse oncogenes apparently evolve into cancer after loss of feedback regulation for NF-κB. This event alters the cellular phenotype and the expression of hormonal mediators, modifying signals between diverse cell types in a tissue. The result is a disruption of stem cell hierarchy in the tissue, and pervasive changes in the microenvironment and immune response to the malignant cells.

Hypoxia modulates innate immune factors: A review

Hypoxia is an important factor for transcriptional regulation of cell metabolism and the adaptation to cellular stress. It modulates the function of phagocytic cells by stimulating surface receptors such as scavenger receptors, toll like receptors and their downstream signaling cascades. In response to hypoxia, innate immune modifiers are upregulated through pathways involving the key immune response master regulator nuclear factor-κB leading to the modulation of inflammatory cytokines. In this review, we highlighted the effects of hypoxia on different innate immune factors and consequences thereof.

NF-κB signaling in B-1 cell development

NF-κB transcription factors play essential roles in hematopoiesis. In this review, we summarize the requirements of different components of the NF-κB pathway for B-1 cell development and maintenance. The B-1 cell developmental steps are also reviewed, with particular emphasis on stages where NF-κB signaling may be critical.

Signaling in lymphocyte activation

The fate of T and B lymphocytes, the key cells that direct the adaptive immune response, is regulated by a diverse network of signal transduction pathways. The T- and B-cell antigen receptors are coupled to intracellular tyrosine kinases and adaptor molecules to control the metabolism of inositol phospholipids and calcium release. The production of inositol polyphosphates and lipid second messengers directs the activity of downstream guanine-nucleotide-binding proteins and protein and lipid kinases/phosphatases that control lymphocyte transcriptional and metabolic programs. Lymphocyte activation is modulated by costimulatory molecules and cytokines that elicit intracellular signaling that is integrated with the antigen-receptor-controlled pathways.

Inhibition of Oncogenic Transcription Factor REL by the Natural Product Derivative Calafianin Monomer 101 Induces Proliferation Arrest and Apoptosis in Human B-Lymphoma Cell Lines

Increased activity of transcription factor NF-κB has been implicated in many B-cell lymphomas. We investigated effects of synthetic compound calafianin monomer (CM101) on biochemical and biological properties of NF-κB. In human 293 cells, CM101 selectively inhibited DNA binding by overexpressed NF-κB subunits REL (human c-Rel) and p65 as compared to NF-κB p50, and inhibition of REL and p65 DNA binding by CM101 required a conserved cysteine residue. CM101 also inhibited DNA binding by REL in human B-lymphoma cell lines, and the sensitivity of several B-lymphoma cell lines to CM101-induced proliferation arrest and apoptosis correlated with levels of cellular and nuclear REL. CM101 treatment induced both phosphorylation and decreased expression of anti-apoptotic protein Bcl-XL, a REL target gene product, in sensitive B-lymphoma cell lines. Ectopic expression of Bcl-XL protected SUDHL-2 B-lymphoma cells against CM101-induced apoptosis, and overexpression of a transforming mutant of REL decreased the sensitivity of BJAB B-lymphoma cells to CM101-induced apoptosis. Lipopolysaccharide-induced activation of NF-κB signaling upstream components occurred in RAW264.7 macrophages at CM101 concentrations that blocked NF-κB DNA binding. Direct inhibitors of REL may be useful for treating B-cell lymphomas in which REL is active, and may inhibit B-lymphoma cell growth at doses that do not affect some immune-related responses in normal cells.

Dynamics of B cells in germinal centres

Humoral immunity depends on the germinal centre (GC) reaction during which somatically mutated high-affinity memory B cells and plasma cells are generated. Recent studies have uncovered crucial cues that are required for the formation and the maintenance of GCs and for the selection of high-affinity antibody mutants. In addition, it is now clear that these events are promoted by the dynamic movements of cells within and between GCs. These findings have resolved the complexities of the GC reaction in greater detail than ever before. This Review focuses on these recent advances and discusses their implications for the establishment of humoral immunity.

A multi-scale approach reveals that NF-κB cRel enforces a B-cell decision to divide

Understanding the functions of multi-cellular organs in terms of the molecular networks within each cell is an important step in the quest to predict phenotype from genotype. B-lymphocyte population dynamics, which are predictive of immune response and vaccine effectiveness, are determined by individual cells undergoing division or death seemingly stochastically. Based on tracking single-cell time-lapse trajectories of hundreds of B cells, single-cell transcriptome, and immunofluorescence analyses, we constructed an agent-based multi-modular computational model to simulate lymphocyte population dynamics in terms of the molecular networks that control NF-κB signaling, the cell cycle, and apoptosis. Combining modeling and experimentation, we found that NF-κB cRel enforces the execution of a cellular decision between mutually exclusive fates by promoting survival in growing cells. But as cRel deficiency causes growing B cells to die at similar rates to non-growing cells, our analysis reveals that the phenomenological decision model of wild-type cells is rooted in a biased race of cell fates. We show that a multi-scale modeling approach allows for the prediction of dynamic organ-level physiology in terms of intra-cellular molecular networks.

CXCR4 expression on pathogenic T cells facilitates their bone marrow infiltration in a mouse model of aplastic anemia

Aplastic anemia (AA) is a disease characterized by T-cell-mediated destruction of bone marrow (BM) hematopoietic stem and progenitor cells. Physiologically, T cells migrate to the BM in response to chemokines, such as SDF-1α, the ligand for CXCR4. However, how T cells traffic to the BM in AA is poorly understood. CXCR4 is aberrantly expressed in immune-mediated diseases and its regulation by nuclear factor-κB (NF-κB) in cancer models is well documented. In this study, we show that CXCR4 is highly expressed on BM-infiltrating CD4(+) and CD8(+) T cells in a mouse model of AA. Inhibiting CXCR4 in AA mice, using CXCR4(-/-) splenocytes or AMD3100, significantly reduced BM infiltration of T cells. We also report that NF-κB occupancy at the CXCR4 promoter is enhanced in BM-infiltrating CD8(+) T cells of AA mice. Moreover, inhibiting NF-κB signaling in AA mice using Bay11 or dehydroxymethylepoxyquinomicin, or transferring p50(-/-) splenocytes, decreased CXCR4 expression on CD8(+) T cells, significantly reduced BM infiltration of T cells, and strongly attenuated disease symptoms. Remarkably, therapeutic administration of Bay11 significantly extended survival of AA mice. Overall, we demonstrate that CXCR4 mediates migration of pathogenic T cells to the BM in AA mice, and inhibiting NF-κB signaling may represent a novel therapeutic approach to treating AA.

Non-canonical NF-κB signaling in rheumatoid arthritis: Dr Jekyll and Mr Hyde?

The nuclear factor-κB (NF-κB) family of transcription factors is essential for the expression of pro-inflammatory cytokines, but can also induce regulatory pathways. NF-κB can be activated via two distinct pathways: the classical or canonical pathway, and the alternative or non-canonical pathway. It is well established that the canonical NF-κB pathway is essential both in acute inflammatory responses and in chronic inflammatory diseases, including rheumatoid arthritis (RA). Although less extensively studied, the non-canonical NF-κB pathway is not only central in lymphoid organ development and adaptive immune responses, but is also thought to play an important role in the pathogenesis of RA. Importantly, this pathway appears to have cell type-specific functions and, since many different cell types are involved in the pathogenesis of RA, it is difficult to predict the net overall contribution of the non-canonical NF-κB pathway to synovial inflammation. In this review, we describe the current understanding of non-canonical NF-κB signaling in various important cell types in the context of RA and consider the relevance to the pathogenesis of the disease. In addition, we discuss current drugs targeting this pathway, as well as future therapeutic prospects.

Roles of the NF-κB Pathway in B-Lymphocyte Biology

NF-κB was originally identified as a family of transcription factors that bind the enhancer of the immunoglobulin κ light-chain gene. Although its function in the regulation of immunoglobulin κ light-chain gene remains unclear, NF-κB plays critical roles in development, survival, and activation of B lymphocytes. In B cells, many receptors, including B-cell antigen receptor (BCR), activate NF-κB pathway, and the molecular mechanism of receptor-mediated activation of IκB kinase (IKK) complex has been partially revealed. In addition to normal B lymphocytes, NF-κB is also involved in the growth of some types of B-cell lymphomas, and many oncogenic mutations involved in constitutive activation of the NF-κB pathway were recently identified in such cancers. In this review, we first summarize the function of NF-κB in B-cell development and activation, and then describe recent progress in understanding the molecular mechanism of receptor-mediated activation of the IKK complex, focusing on the roles of the ubiquitin system. In the last section, we describe oncogenic mutations that induce NF-κB activation in B-cell lymphoma.

Transcriptional Control of NK Cells

Natural killer (NK) cells are innate lymphocytes that survey the environment and protect the host from infected and cancerous cells. As their name implies, NK cells represent an early line of defense during pathogen invasion by directly killing infected cells and secreting inflammatory cytokines. Although the function of NK cells was first described more than four decades ago, the development of this cytotoxic lineage is not well understood. In recent years, we have begun to identify specific transcription factors that control each stage of development and maturation, from ontogeny of the NK cell progenitor to the effector functions of activated NK cells in peripheral organs. This chapter highlights the transcription factors that are unique to NK cells, or shared between NK cells and other hematopoietic cell lineages, but govern the biology of this cytolytic lymphocyte.

Bcl11b prevents the intrathymic development of innate CD8 T cells in a cell intrinsic manner

If Bcl11b activity is compromised, CD4(+)CD8(+) double-positive (DP) thymocytes produce a greatly increased fraction of innate CD8(+) single-positive (SP) cells highly producing IFN-γ, which are also increased in mice deficient of genes such as Itk, Id3 and NF-κB1 that affect TCR signaling. Of interest, the increase in the former two is due to the bystander effect of IL-4 that is secreted by promyelocytic leukemia zinc finger-expressing NKT and γδT cells whereas the increase in the latter is cell intrinsic. Bcl11b zinc-finger proteins play key roles in T cell development and T cell-mediated immune response likely through TCR signaling. We examined thymocytes at and after the DP stage in Bcl11b (F/S826G) CD4cre, Bcl11b (F/+) CD4cre and Bcl11b (+/S826G) mice, carrying the allele that substituted serine for glycine at the position of 826. Here we show that Bcl11b impairment leads to an increase in the population of TCRαβ(high)CD44(high)CD122(high) innate CD8SP thymocytes, together with two different developmental abnormalities: impaired positive and negative selection accompanying a reduction in the number of CD8SP cells, and developmental arrest of NKT cells at multiple steps. The innate CD8SP thymocytes express Eomes and secrete IFN-γ after stimulation with PMA and ionomycin, and in this case their increase is not due to a bystander effect of IL-4 but cell intrinsic. Those results indicate that Bcl11b regulates development of different thymocyte subsets at multiple stages and prevents an excess of innate CD8SP thymocytes.

NIAM-deficient mice are predisposed to the development of proliferative lesions including B-cell lymphomas

Nuclear Interactor of ARF and Mdm2 (NIAM, gene designation Tbrg1) is a largely unstudied inhibitor of cell proliferation that helps maintain chromosomal stability. It is a novel activator of the ARF-Mdm2-Tip60-p53 tumor suppressor pathway as well as other undefined pathways important for genome maintenance. To examine its predicted role as a tumor suppressor, we generated NIAM mutant (NIAM^m/m) mice homozygous for a β-galactosidase expressing gene-trap cassette in the endogenous gene. The mutant mice expressed significantly lower levels of NIAM protein in tissues compared to wild-type animals. Fifty percent of aged NIAM deficient mice (14 to 21 months) developed proliferative lesions, including a uterine hemangioma, pulmonary papillary adenoma, and a Harderian gland adenoma. No age-matched wild-type or NIAM^+/m heterozygous animals developed lesions. In the spleen, NIAM^m/m mice had prominent white pulp expansion which correlated with enhanced increased reactive lymphoid hyperplasia and evidence of systemic inflammation. Notably, 17% of NIAM mutant mice had splenic white pulp features indicating early B-cell lymphoma. This correlated with selective expansion of marginal zone B cells in the spleens of younger, tumor-free NIAM-deficient mice. Unexpectedly, basal p53 expression and activity was largely unaffected by NIAM loss in isolated splenic B cells. In sum, NIAM down-regulation in vivo results in a significant predisposition to developing benign tumors or early stage cancers. These mice represent an outstanding platform for dissecting NIAM's role in tumorigenesis and various anti-cancer pathways, including p53 signaling.

Nuclear factor κB inhibition reduces lung vascular lumen obliteration in severe pulmonary hypertension in rats

NF-κB and IL-6, a NF-κB downstream mediator, play a central role in the inflammatory response of tissues. We aimed to determine the role of the classical NF-κB pathway in severe pulmonary arterial hypertension (PAH) induced by SU5416 and chronic hypoxia (SuHx) in rats. Tissue samples from patients with idiopathic PAH (iPAH) and control subjects were investigated. SuHx rats were treated from Days 1 to 3, 1 to 21, and 29 to 42 with the NF-κB inhibitor pyrrolidine dithiocarbamate (PDTC) and/or from Days 1 to 21 with anti-IL-6 antibody. Nuclear staining for NF-κB, an indicator of the activation of the classical NF-κB pathway, was detected in pulmonary arterial lesions of patients with iPAH and SuHx rats. NF-κB inhibition with PDTC prevented and reduced pulmonary arterial obliteration without reducing muscularization. However, the elevated lung levels of IL-6 were not reduced in PDTC-treated SuHx animals. PDTC treatment prevented or reduced apoptosis of pulmonary artery wall cells and pulmonary arterial obliteration. IL-6 inhibition had only a partial effect on apoptosis and obliteration. Pulmonary arterial media wall thickness was not affected by any of these treatments. Preventive and therapeutic PDTC treatment promoted immune regulation by increasing the number of perivascular CD4(+) T cells, in particular regulatory T cells (early treatment), and by reducing the number of perivascular CD8(+) T lymphocytes and CD45RA(+) B lymphocytes. Therapeutic PDTC treatment further preserved right ventricular function in SuHx animals. Inhibition of NF-κB may represent a therapeutic option for pulmonary arterial obliteration via reduced vessel wall cell apoptosis and improved regulation of the immune system.

Outer membrane protein A (OmpA) of Shigella flexneri 2a induces TLR2-mediated activation of B cells: involvement of protein tyrosine kinase, ERK and NF-κB

B cells are critically important in combating bacterial infections and their differentiation into plasma cells and memory cells aids bacterial clearance and long-lasting immunity conferred by essentially all vaccines. Outer membrane protein A (OmpA) of Shigella flexneri 2a has been demonstrated to induce the production of IgG and IgA in vivo following immunization of mice through intranasal route, but the direct involvement of B cells in OmpA-mediated immune regulation was not determined. Consequently, we investigated whether OmpA can modulate B cell functions and identified the molecular events involved in OmpA-induced B cell immune response in vitro. We show that OmpA of S. flexneri 2a activates B cells to produce protective cytokines, IL-6 and IL-10 as well as facilitates their differentiation into antibody secreting cells (ASCs). The immunostimulatory properties of OmpA are attributed to the increased surface expression of MHCII and CD86 on B cells. We also report here that B cell activation by OmpA is mediated strictly through recognition by TLR2, resulting in initiation of cascades of signal transduction events, involving increased phosphorylation of protein tyrosine kinases (PTKs), ERK and IκBα, leading to nuclear translocation of NF-κB. Importantly, a TLR2 antibody diminishes OmpA-induced upregulation of MHCII and CD86 on B cell surface as well as significantly inhibits B cell differentiation and cytokine secretion. Furthermore, we illustrate that B cell differentiation into ASCs and induction of cytokine secretion by OmpA are dependent on PTKs activity. Moreover, we identify that OmpA-induced B cell differentiation is entirely dependent on ERK pathway, whereas both NF-κB and ERK are essential for cytokine secretion by B cells. Overall, our data demonstrate that OmpA of S. flexneri 2a amplifies TLR signaling in B cells and triggers B cell immune response, which is critical for the development of an effective adaptive immunity to an optimal vaccine antigen.

NF-κB factors control the induction of NFATc1 in B lymphocytes

In peripheral lymphocytes, the transcription factors (TFs) NF-κB, NFAT, and AP-1 are the prime targets of signals that emerge from immune receptors. Upon activation, these TFs induce gene networks that orchestrate the growth, expansion, and effector function of peripheral lymphocytes. NFAT and NF-κB factors share several properties, such as a similar mode of induction and architecture in their DNA-binding domain, and there is a subgroup of κB-like DNA promoter motifs that are bound by both types of TFs. However, unlike NFAT and AP-1 factors that interact and collaborate in binding to DNA, NFAT, and NF-κB seem neither to interact nor to collaborate. We show here that NF-κB1/p50 and c-Rel, the most prominent NF-κB proteins in BCR-induced splenic B cells, control the induction of NFATc1/αA, a prominent short NFATc1 isoform. In part, this is mediated through two composite κB/NFAT-binding sites in the inducible Nfatc1 P1 promoter that directs the induction of NFATc1/αA by BCR signals. In concert with coreceptor signals that induce NF-κB factors, BCR signaling induces a persistent generation of NFATc1/αA. These data suggest a tight connection between NFATc1 and NF-κB induction in B lymphocytes contributing to the effector function of peripheral B cells.

IKK-induced NF-κB1 p105 proteolysis is critical for B cell antibody responses to T cell-dependent antigen

Jacque et al. investigate the functions of NF-κB1 p105 and its associated NF-κB–binding partners in B cells, using a mutant mouse strain that carries a form of the NF-κB1 precursor that is resistant to IKK-induced proteolysis. They identify a critical B cell–intrinsic role for this IKK signaling pathway in the antigen-induced survival and differentiation of follicular mature B cells.

The importance of IκB kinase (IKK)–induced proteolysis of NF-κB1 p105 in B cells was investigated using Nfkb1^SSAA/SSAA mice, in which this NF-κB signaling pathway is blocked. Nfkb1^SSAA mutation had no effect on the development and homeostasis of follicular mature (FM) B cells. However, analysis of mixed bone marrow chimeras revealed that Nfkb1^SSAA/SSAA FM B cells were completely unable to mediate T cell–dependent antibody responses. Nfkb1^SSAA mutation decreased B cell antigen receptor (BCR) activation of NF-κB in FM B cells, which selectively blocked BCR stimulation of cell survival and antigen-induced differentiation into plasmablasts and germinal center B cells due to reduced expression of Bcl-2 family proteins and IRF4, respectively. In contrast, the antigen-presenting function of FM B cells and their BCR-induced migration to the follicle T cell zone border, as well as their growth and proliferation after BCR stimulation, were not affected. All of the inhibitory effects of Nfkb1^SSAA mutation on B cell functions were rescued by normalizing NF-κB activation genetically. Our study identifies critical B cell-intrinsic functions for IKK-induced NF-κB1 p105 proteolysis in the antigen-induced survival and differentiation of FM B cells, which are essential for T-dependent antibody responses.

Germinal center B cell maintenance and differentiation are controlled by distinct NF-κB transcription factor subunits

Germinal centers (GCs) are the sites where memory B cells and plasma cells producing high-affinity antibodies are generated during T cell-dependent immune responses. The molecular control of GC B cell maintenance and differentiation remains incompletely understood. Activation of the NF-κB signaling pathway has been implicated; however, the distinct roles of the individual NF-κB transcription factor subunits are unknown. We report that GC B cell-specific deletion of the NF-κB subunits c-REL or RELA, which are both activated by the canonical NF-κB pathway, abolished the generation of high-affinity B cells via different mechanisms acting at distinct stages during the GC reaction. c-REL deficiency led to the collapse of established GCs immediately after the formation of dark and light zones at day 7 of the GC reaction and was associated with the failure to activate a metabolic program that promotes cell growth. Conversely, RELA was dispensable for GC maintenance but essential for the development of GC-derived plasma cells due to impaired up-regulation of BLIMP1. These results indicate that activation of the canonical NF-κB pathway in GC B cells controls GC maintenance and differentiation through distinct transcription factor subunits. Our findings have implications for the role of NF-κB in GC lymphomagenesis.

Curcumin ameliorates autoimmune diabetes. Evidence in accelerated murine models of type 1 diabetes

Type 1 diabetes (T1DM) is a T cell-mediated autoimmune disease that selectively destroys pancreatic β cells. The only possible cure for T1DM is to control autoimmunity against β cell-specific antigens. We explored whether the natural compound curcumin, with anti-oxidant and anti-inflammatory activities, might down-regulate the T cell response that destroys pancreatic β cells to improve disease outcome in autoimmune diabetes. We employed two accelerated autoimmune diabetes models: (i) cyclophosphamide (CYP) administration to non-obese diabetic (NOD) mice and (ii) adoptive transfer of diabetogenic splenocytes into NODscid mice. Curcumin treatment led to significant delay of disease onset, and in some instances prevented autoimmune diabetes by inhibiting pancreatic leucocyte infiltration and preserving insulin-expressing cells. To investigate the mechanisms of protection we studied the effect of curcumin on key immune cell populations involved in the pathogenesis of the disease. Curcumin modulates the T lymphocyte response impairing proliferation and interferon (IFN)-γ production through modulation of T-box expressed in T cells (T-bet), a key transcription factor for proinflammatory T helper type 1 (Th1) lymphocyte differentiation, both at the transcriptional and translational levels. Also, curcumin reduces nuclear factor (NF)-κB activation in T cell receptor (TCR)-stimulated NOD lymphocytes. In addition, curcumin impairs the T cell stimulatory function of dendritic cells with reduced secretion of proinflammatory cytokines and nitric oxide (NO) and low surface expression of co-stimulatory molecules, leading to an overall diminished antigen-presenting cell activity. These in-vitro effects correlated with ex-vivo analysis of cells obtained from curcumin-treated mice during the course of autoimmune diabetes. These findings reveal an effective therapeutic effect of curcumin in autoimmune diabetes by its actions on key immune cells responsible for β cell death.

Olive (Olea europaea) leaf extract induces apoptosis and monocyte/macrophage differentiation in human chronic myelogenous leukemia K562 cells: insight into the underlying mechanism

Differentiation therapy is an attractive approach aiming at reversing malignancy and reactivating endogenous differentiation programs in cancer cells. Olive leaf extract, known for its antioxidant activity, has been demonstrated to induce apoptosis in several cancer cells. However, its differentiation inducing properties and the mechanisms involved are still poorly understood. In this study, we investigated the effect of Chemlali Olive Leaf Extract (COLE) for its potential differentiation inducing effect on multipotent leukemia K562 cells. Results showed that COLE inhibits K562 cells proliferation and arrests the cell cycle at G0/G1, and then at G2/M phase over treatment time. Further analysis revealed that COLE induces apoptosis and differentiation of K562 cells toward the monocyte lineage. Microarray analysis was conducted to investigate the underlying mechanism of COLE differentiation inducing effect. The differentially expressed genes such as IFI16, EGR1, NFYA, FOXP1, CXCL2, CXCL3, and CXCL8 confirmed the commitment of K562 cells to the monocyte/macrophage lineage. Thus our results provide evidence that, in addition to apoptosis, induction of differentiation is one of the possible therapeutic effects of olive leaf in cancer cells.

NF-κB control of T cell development

The NF-κB signal transduction pathway is best known as a major regulator of innate and adaptive immune responses, yet there is a growing appreciation of its importance in immune cell development, particularly of T lineage cells. In this Review, we discuss how the temporal regulation of NF-κB controls the stepwise differentiation and antigen-dependent selection of conventional and specialized subsets of T cells in response to T cell receptor and costimulatory, cytokine and growth factor signals.