B cell homeostasis and plasma cell homing controlled by Krüppel-like factor 2

RNA-seq analysis of the kidneys of broiler chickens fed diets containing different concentrations of calcium

Calcium (Ca) is required for normal growth and is involved in cellular physiology, signal transduction, and bone mineralization. In humans, inadequate Ca intake causes hypocalcaemia, and excessive Ca intake causes hypercalcemia. In chicken, Ca is also required for body weight gain and eggshell formation. However, transcriptomic responses to low/high Ca intake, and mechanisms affecting body weight have not been explored. In this study, we performed comparative RNA sequencing (RNA-seq) using the kidney of broiler chickens fed diets containing 0.8, 1.0, and 1.2% Ca. Annotation of RNA-seq data revealed a significant number of differentially expressed genes (DEGs) in the kidney via pairwise comparison using Cufflinks and edgeR. Using edgeR, we identified 12 DEGs; seven overlapped with those found by cufflinks. Seven DEGs were validated by real-time quantitative-PCR (qRT-PCR) in Ca-supplemented kidneys, and the results correlated with the RNA-seq data. DEGs identified by cufflinks/edgeR were subjected to pathway enrichment, protein/protein interaction, and co-occurrence analyses to determine their involvement in disease. The National Research Council (NRC) recommended Ca intake for 21-day post-hatch broilers is about 1.0%. Our findings suggest that higher-than-recommended Ca intake (1.2%) could reduce body weight gain in broilers, and that affected DEGs are related to stress-induced diseases, such as hypertension.

Maintenance of the marginal-zone B cell compartment specifically requires the RNA-binding protein ZFP36L1

RNA-binding proteins of the ZFP36 family are best known for inhibiting the expression of cytokines through binding to AU-rich elements in the 3' untranslated region and promoting mRNA decay. Here we identified an indispensable role for ZFP36L1 as the regulator of a post-transcriptional hub that determined the identity of marginal-zone B cells by promoting their proper localization and survival. ZFP36L1 controlled a gene-expression program related to signaling, cell adhesion and locomotion; it achieved this in part by limiting expression of the transcription factors KLF2 and IRF8, which are known to enforce the follicular B cell phenotype. These mechanisms emphasize the importance of integrating transcriptional and post-transcriptional processes by RNA-binding proteins for maintaining cellular identity among closely related cell types.

Effector and regulatory B cells in Multiple Sclerosis

The role of B cells in the pathogenesis of Multiple Sclerosis (MS), an autoimmune neurodegenerative disease, is becoming eminent in recent years, but the specific contribution of the distinct B cell subsets remains to be elucidated. Several B cell subsets have shown regulatory, anti-inflammatory capacities in response to stimuli in vitro, as well as in the animal model of MS: Experimental Autoimmune Encephalomyelitis (EAE). However, the functional role of the B regulatory cells (Bregs) in vivo and specifically in the human disease is yet to be clarified. In the present review, we have summarized the updated information on the roles of effector and regulatory B cells in MS and the immune-modulatory effects of MS therapeutic agents on their phenotype and function.

Molecular pathogenesis of splenic and nodal marginal zone lymphoma

Genomic studies have improved our understanding of the biological basis of splenic (SMZL) and nodal (NMZL) marginal zone lymphoma by providing a comprehensive and unbiased view of the genes/pathways that are deregulated in these diseases. Consistent with the physiological involvement of NOTCH, NF-κB, B-cell receptor and toll-like receptor signaling in mature B-cells differentiation into the marginal zone B-cells, many oncogenic mutations of genes involved in these pathways have been identified in SMZL and NMZL. Beside genetic lesions, also epigenetic and post-transcriptional modifications contribute to the deregulation of marginal zone B-cell differentiation pathways in SMZL and NMZL. This review describes the progress in understanding the molecular mechanism underlying SMZL and NMZL, including molecular and post-transcriptional modifications, and discusses how information gained from these efforts has provided new insights on potential targets of diagnostic, prognostic and therapeutic relevance in SMZL and NMZL.

NF-κB deregulation in splenic marginal zone lymphoma

Splenic marginal zone lymphoma is a rare mature B-cell malignancy involving the spleen, bone marrow and blood. Over the past years, the rapid expansion of sequencing technologies allowing the genome-wide assessment of genomic, epigenetic and transcriptional changes has revolutionized our understanding of the biological basis of splenic marginal zone lymphoma by providing a comprehensive and unbiased view of the genes/pathways that are deregulated in this disease. NF-κB is a family of transcription factors that plays critical roles in development, survival, and activation of B lymphocytes. Consistent with the physiological involvement of NF-κB signalling in proliferation and commitment of mature B-cells to the marginal zone of the spleen, many oncogenic mutations involved in constitutive activation of the NF-κB pathway were recently identified in splenic marginal zone lymphoma. This review describes the progress in understanding the mechanism of NF-κB activation in splenic marginal zone lymphoma, including molecular, epigenetic and post-transcriptional modifications of NF-κB genes and of upstream pathways, and discusses how information gained from these efforts has provided new insights on potential targets of diagnostic, prognostic and therapeutic relevance for splenic marginal zone lymphoma.

Splenic marginal zone lymphoma: from genetics to management

Splenic marginal zone lymphoma (SMZL) is a rare B-cell malignancy involving the spleen, bone marrow, and frequently the blood. SMZL lymphomagenesis involves antigen and/or superantigen stimulation and molecular deregulation of genes (NOTCH2 and KLF2) involved in the physiological differentiation of spleen marginal zone B cells. Diagnosis requires either spleen histology or, alternatively, the documentation of a typical cell morphology and immunophenotype on blood cells coupled with the detection of intrasinusoidal infiltration by CD20+ cells in the bone marrow. Among B-cell tumors, deletion of 7q and NOTCH2 mutations are almost specific lesions of SMZL, thus representing promising diagnostic biomarkers of this lymphoma. Although the majority of SMZLs show an indolent course with a median survival of approximately 10 years, nearly 30% of patients experience a poor outcome. No randomized trials are reported for SMZL, and few prospective trials are available. A watch-and-wait approach is advisable for asymptomatic patients. Treatment options for symptomatic patients ranges from splenectomy to rituximab alone or combined with chemotherapy. In some geographic areas, a subset of patients with SMZL associates with hepatitis C virus infection, prompting virus eradication as an effective lymphoma treatment. It would be worthwhile to explore deregulated cellular programs of SMZL as therapeutic targets in the future; improved clinical and biological prognostication will be essential for identifying patients who may benefit from novel approaches.

Transcription factor KLF2 regulates homeostatic NK cell proliferation and survival

Natural killer (NK) cells are innate lymphocytes that recognize and lyse virally infected or transformed cells. This latter property is being pursued in clinics to treat leukemia with the hope that further breakthroughs in NK cell biology can extend treatments to other cancers. At issue is the ability to expand transferred NK cells and prolong their functionality within the context of a tumor. In terms of NK cell expansion and survival, we now report that Kruppel-like factor 2 (KLF2) is a key transcription factor that underpins both of these events. Excision of Klf2 using gene-targeted mouse models promotes spontaneous proliferation of immature NK cells in peripheral tissues, a phenotype that is replicated under ex vivo conditions. Moreover, KLF2 imprints a homeostatic migration pattern on mature NK cells that allows these cells to access IL-15-rich microenvironments. KLF2 accomplishes this feat within the mature NK cell lineage via regulation of a subset of homing receptors that respond to homeostatic ligands while leaving constitutively expressed receptors that recognize inflammatory cytokines unperturbed. Under steady-state conditions, KLF2-deficient NK cells alter their expression of homeostatic homing receptors and subsequently undergo apoptosis due to IL-15 starvation. This novel mechanism has implications regarding NK cell contraction following the termination of immune responses including the possibility that retention of an IL-15 transpresenting support system is key to extending NK cell activity in a tumor environment.

Multifunctional role of the transcription factor Blimp-1 in coordinating plasma cell differentiation

The transcription factor Blimp-1 is necessary for the generation of plasma cells. Here we studied its functions in plasmablast differentiation by identifying regulated Blimp-1 target genes. Blimp-1 promoted the migration and adhesion of plasmablasts. It directly repressed genes encoding several transcription factors and Aicda (which encodes the cytidine deaminase AID) and thus silenced B cell-specific gene expression, antigen presentation and class-switch recombination in plasmablasts. It directly activated genes, which led to increased expression of the plasma cell regulator IRF4 and proteins involved in immunoglobulin secretion. Blimp-1 induced the transcription of immunoglobulin genes by controlling the 3' enhancers of the loci encoding the immunoglobulin heavy chain (Igh) and κ-light chain (Igk) and, furthermore, regulated the post-transcriptional expression switch from the membrane-bound form of the immunoglobulin heavy chain to its secreted form by activating Ell2 (which encodes the transcription-elongation factor ELL2). Notably, Blimp-1 recruited chromatin-remodeling and histone-modifying complexes to regulate its target genes. Hence, many essential functions of plasma cells are under the control of Blimp-1.

The KDM3A-KLF2-IRF4 axis maintains myeloma cell survival

KDM3A is implicated in tumorigenesis; however, its biological role in multiple myeloma (MM) has not been elucidated. Here we identify KDM3A–KLF2–IRF4 axis dependence in MM. Knockdown of KDM3A is toxic to MM cells in vitro and in vivo. KDM3A maintains expression of KLF2 and IRF4 through H3K9 demethylation, and knockdown of KLF2 triggers apoptosis. Moreover, KLF2 directly activates IRF4 and IRF4 reciprocally upregulates KLF2, forming a positive autoregulatory circuit. The interaction of MM cells with bone marrow milieu mediates survival of MM cells. Importantly, silencing of KDM3A, KLF2 or IRF4 both decreases MM cell adhesion to bone marrow stromal cells and reduces MM cell homing to the bone marrow, in association with decreased ITGB7 expression in MAF-translocated MM cell lines. Our results indicate that the KDM3A–KLF2–IRF4 pathway plays an essential role in MM cell survival and homing to the bone marrow, and therefore represents a therapeutic target.

Several histone modifiers have been implicated in the survival of multiple myeloma cells. Here, the authors reveal a role for the histone demethylase KDM3A in the survival of this haematologic cancer, and show that mechanistically KDM3A removes H3K9 methylation from the promoters of KLF2 and IRF4, genes essential for myeloma cell survival.

Essential control of early B-cell development by Mef2 transcription factors

The sequential activation of distinct developmental gene networks governs the ultimate identity of a cell, but the mechanisms involved in initiating downstream programs are incompletely understood. The pre-B-cell receptor (pre-BCR) is an important checkpoint of B-cell development and is essential for a pre-B cell to traverse into an immature B cell. Here, we show that activation of myocyte enhancer factor 2 (Mef2) transcription factors (TFs) by the pre-BCR is necessary for initiating the subsequent genetic network. We demonstrate that B-cell development is blocked at the pre-B-cell stage in mice deficient for Mef2c and Mef2d TFs and that pre-BCR signaling enhances the transcriptional activity of Mef2c/d through phosphorylation by the Erk5 mitogen-activating kinase. This activation is instrumental in inducing Krüppel-like factor 2 and several immediate early genes of the AP1 and Egr family. Finally, we show that Mef2 proteins cooperate with the products of their target genes (Irf4 and Egr2) to induce secondary waves of transcriptional regulation. Our findings uncover a novel role for Mef2c/d in coordinating the transcriptional network that promotes early B-cell development.

The generation of antibody-secreting plasma cells

The regulation of antibody production is linked to the generation and maintenance of plasmablasts and plasma cells from their B cell precursors. Plasmablasts are the rapidly produced and short-lived effector cells of the early antibody response, whereas plasma cells are the long-lived mediators of lasting humoral immunity. An extraordinary number of control mechanisms, at both the cellular and molecular levels, underlie the regulation of this essential arm of the immune response. Despite this complexity, the terminal differentiation of B cells can be described as a simple probabilistic process that is governed by a central gene-regulatory network and modified by environmental stimuli.

A novel role for KLF14 in T regulatory cell differentiation

BACKGROUND AND AIMS

KLF proteins function as epigenetic reprogramming factors during cell differentiation in many cell populations and in engineered iPS cells. In this study, we determined KLF14 function in the regulation of FOXP3, a transcription factor critical for Treg cell differentiation.

METHODS

We studied the effects of KLF14 on FOXP3 expression at the level of the protein and mRNA. We evaluated the functional relevance of KLF14 to FOXP3⁺ Treg cells in vitro and in vivo through suppression assays and two colitis models. Finally, we analyzed the effect of KLF14 on the epigenetic landscape of the FOXP3 promoter locus through chromatin immuno-precipitation.

RESULTS

KLF14, induced upon activation of naïve CD4⁺ T cells, segregates to the FOXP3^- population and is inversely associated with FOXP3 expression and Treg function. KLF14 KO CD4⁺ cells differentiated into adaptive Tregs more readily in vitro and in vivo. KLF14 KO cells demonstrated enhanced Treg suppressor function in vitro and in vivo. KLF14 repressed FOXP3 at the level of the mRNA and protein, and by ChIP assay KLF14 was found to bind to the TSDR enhancer region of FOXP3. Furthermore, loss of KLF14 reduced the levels of H3K9me3, HP1 and Suv39H1at the TSDR.

CONCLUSIONS

These results outline a novel mechanism by which KLF14 regulates Treg cell differentiation via chromatin remodeling at the FOXP3 TSDR. To our knowledge, this is the first evidence supporting a role for KLF14 in maintaining the differentiated state of Treg cells and outlines a potential mechanism to modify the expression of immune genes, such as FOXP3, which are critical to T cell fate.

Transcription elongation factor ELL2 drives Ig secretory-specific mRNA production and the unfolded protein response

Differentiation of B cells into Ab-secreting cells induces changes in gene transcription, IgH RNA processing, the unfolded protein response (UPR), and cell architecture. The transcription elongation factor eleven nineteen lysine-rich leukemia gene (ELL2) stimulates the processing of the secreted form of the IgH mRNA from the H chain gene. Mice (mus musculus) with the ELL2 gene floxed in either exon 1 or exon 3 were constructed and crossed to CD19-driven cre/CD19(+). The B cell-specific ELL2 conditional knockouts (cKOs; ell2(loxp/loxp) CD19(cre/+)) exhibit curtailed humoral responses both in 4-hydroxy-3-nitrophenyl acetyl-Ficoll and in 4-hydroxy-3-nitrophenyl acetyl-keyhole limpet hemocyanin immunized animals; recall responses were also diminished. The number of immature and recirculating B cells in the bone marrow is increased in the cKOs, whereas plasma cells in spleen are reduced relative to control animals. There are fewer IgG1 Ab-producing cells in the bone marrow of cKOs. LPS ex vivo-stimulated B220(lo)CD138(+) cells from ELL2-deficient mouse spleens are 4-fold less abundant than from control splenic B cells; have a paucity of secreted IgH; and have distended, abnormal-appearing endoplasmic reticulum. IRE1α is efficiently phosphorylated, but the amounts of Ig κ, ATF6, BiP, Cyclin B2, OcaB (BOB1, Pou2af1), and XBP1 mRNAs, unspliced and spliced, are severely reduced in ELL2-deficient cells. ELL2 enhances the expression of BCMA (also known as Tnfrsf17), which is important for long-term survival. Transcription yields from the cyclin B2 and the canonical UPR promoter elements are upregulated by ELL2 cDNA. Thus, ELL2 is important for many aspects of Ab secretion, XBP1 expression, and the UPR.

Gene expression profiling of the response to interferon beta in Epstein-Barr-transformed and primary B cells of patients with multiple sclerosis

The effects of interferon-beta (IFN-β), one of the key immunotherapies used in multiple sclerosis (MS), on peripheral blood leukocytes and T cells have been extensively studied. B cells are a less abundant leukocyte type, and accordingly less is known about the B cell-specific response to IFN-β. To identify gene expression changes and pathways induced by IFN-β in B cells, we studied the in vitro response of human Epstein Barr-transformed B cells (lymphoblast cell lines-LCLs), and validated our results in primary B cells. LCLs were derived from an MS patient repository. Whole genome expression analysis identified 115 genes that were more than two-fold differentially up-regulated following IFN-β exposure, with over 50 previously unrecognized as IFN-β response genes. Pathways analysis demonstrated that IFN-β affected LCLs in a similar manner to other cell types by activating known IFN-β canonical pathways. Additionally, IFN-β increased the expression of innate immune response genes, while down-regulating many B cell receptor pathway genes and genes involved in adaptive immune responses. Novel response genes identified herein, NEXN, DDX60L, IGFBP4, and HAPLN3, B cell receptor pathway genes, CD79B and SYK, and lymphocyte activation genes, LAG3 and IL27RA, were validated as IFN-β response genes in primary B cells. In this study new IFN-β response genes were identified in B cells, with possible implications to B cell-specific functions. The study's results emphasize the applicability of LCLs for studies of human B cell drug response. The usage of LCLs from patient-based repositories may facilitate future studies of drug response in MS and other immune-mediated disorders with a B cell component.

Leukocyte β7 integrin targeted by Krüppel-like factors

Constitutive expression of Krüppel-like factor 3 (KLF3, BKLF) increases marginal zone (MZ) B cell numbers, a phenotype shared with mice lacking KLF2. Ablation of KLF3, known to interact with serum response factor (SRF), or SRF itself, results in fewer MZ B cells. It is unknown how these functional equivalences result. In this study, it is shown that KLF3 acts as transcriptional repressor for the leukocyte-specific integrin β7 (Itgb7, Ly69) by binding to the β7 promoter, as revealed by chromatin immunoprecipitation. KLF2 overexpression antagonizes this repression and also binds the β7 promoter, indicating that these factors may compete for target sequence(s). Whereas β7 is identified as direct KLF target, its repression by KLF3 is not connected to the MZ B cell increase because β7-deficient mice have a normal complement of these and the KLF3-driven increase still occurs when β7 is deleted. Despite this, KLF3 overexpression abolishes lymphocyte homing to Peyer's patches, much like β7 deficiency does. Furthermore, KLF3 expression alone overcomes the MZ B cell deficiency when SRF is absent. SRF is also dispensable for the KLF3-mediated repression of β7. Thus, despite the shared phenotype of KLF3 and SRF-deficient mice, cooperation of these factors appears neither relevant for the formation of MZ B cells nor for the regulation of β7. Finally, a potent negative regulatory feedback loop limiting KLF3 expression is shown in this study, mediated by KLF3 directly repressing its own gene promoter. In summary, KLFs use regulatory circuits to steer lymphocyte maturation and homing and directly control leukocyte integrin expression.

KLF2--a negative regulator of pre-B cell clonal expansion and B cell activation

Maturation as well as antigen-dependent activation of B cells is accompanied by alternating phases of proliferation and quiescence. We and others have previously shown that Krüppel-like factor 2 (KLF2), a regulator of T cell quiescence and migration, is upregulated in small resting precursor (pre)-B cells after assembly of the immature pre-B cell receptor (pre-BCR) and is downregulated upon antigen-induced proliferation of mature B cells. These findings suggest that KLF2, besides its function in maintaining follicular B cell identity, peripheral B cell homeostasis and homing of antigen-specific plasma cells to the bone marrow, also controls clonal expansion phases in the B cell lineage. Here, we demonstrate that enforced expression of KLF2 in primary pre-B cells results in a severe block of pre-BCR-induced proliferation, upregulation of the cell cycle inhibitors p21 and p27 and downregulation of c-myc. Furthermore, retroviral KLF2 transduction of primary B cells impairs LPS-induced activation, favors apoptosis and results in reduced abundance of factors, such as AID, IRF4 and BLIMP1, that control the antigen-dependent phase of B cell activation and plasma cell differentiation. Hence, we conclude that KLF2 is not only a key player in terminating pre-B cell clonal expansion but also a potent suppressor of B cell activation.

Plasma cell formation, secretion, and persistence: the short and the long of it

B cells can be activated by cognate antigen, anti-B-cell receptor antibody, complement receptors, or polyclonal stimulators like lipopolysaccharide; the overall result is a large shift in RNA processing to the secretory-specific form of immunoglobulin (Ig) heavy chain mRNA and an upregulation of Igh mRNA amounts. Associated with this shift is the large-scale induction of Ig protein synthesis and the unfolded protein response to accommodate the massive quantity of secretory Ig that results. Stimulation to secretion also produces major structural accommodations and stress, with extensive generation of endoplasmic reticulum and Golgi as part of the cellular architecture. Reactive oxygen species can lead to either activation or apoptosis based on context and the high or low oxygen tension surrounding the cells. Transcription elongation factor ELL2 plays an important role in the induction of Ig secretory mRNA production, the unfolded protein response, and gene expression during hypoxia. After antigen stimulation, activated B cells from either the marginal zones or follicles can produce short-lived antibody secreting cells; it is not clear whether cells from both locations can become long-lived plasma cells. Autophagy is necessary for plasma cell long-term survival through the elimination of some of the accumulated damage to the ER from producing so much protein. Survival signals from the bone marrow stromal cells also contribute to plasma cell longevity, with BCMA serving a potentially unique survival role. Integrating the various information pathways converging on the plasma cell is crucial to the development of their long-lived, productive immune response.

Immune regulation by atypical chemokine receptors

Chemokines have fundamental roles in regulating immune and inflammatory responses, primarily through their control of leukocyte migration and localization. The biological functions of chemokines are typically mediated by signalling through G protein-coupled chemokine receptors, but chemokines are also bound by a small family of atypical chemokine receptors (ACKRs), the members of which are unified by their inability to initiate classical signalling pathways after ligand binding. These ACKRs are emerging as crucial regulatory components of chemokine networks in a wide range of developmental, physiological and pathological contexts. In this Review, we discuss the biochemical and immunological properties of ACKRs and the potential unifying themes in this family, and we highlight recent studies that identify novel roles for these molecules in development , homeostasis, inflammatory disease, infection and cancer.

The impact of KLF2 modulation on the transcriptional program and function of CD8 T cells

Krüppel-like factor 2 (KLF2) is a transcription factor that is highly expressed in quiescent T lymphocytes and downregulated in effector T cells. We now show that antigen receptor engagement downregulates KLF2 expression in a graded response determined by the affinity of T cell antigen receptor (TCR) ligand and the integrated activation of protein kinase B and the MAP kinases ERK1/2. The present study explores the importance of KLF2 downregulation and reveals that the loss of KLF2 controls a select portion of the CD8 effector T cell transcriptional program. In particular, KLF2 loss is required for CD8 T cells to express the inflammatory chemokine receptor CXCR3 and for maximum clonal expansion of T cells. KLF2 thus negatively controls the ability of CD8 T cells to respond to the CXCR3 ligand CXCL10. Strikingly, the KLF2 threshold for restraining expression of CXCR3 is very low and quite distinct to the KLF2 threshold for restraining T cell proliferation. KLF2 is thus an analogue (tunable) not a digital (on/off) cellular switch where the magnitude of KLF2 expression differentially modifies the T cell responses.

Virus-associated activation of innate immunity induces rapid disruption of Peyer's patches in mice

Early in the course of infection, detection of pathogen-associated molecular patterns by innate immune receptors can shape the subsequent adaptive immune response. Here we investigate the influence of virus-associated innate immune activation on lymphocyte distribution in secondary lymphoid organs. We show for the first time that virus infection of mice induces rapid disruption of the Peyer's patches but not of other secondary lymphoid organs. The observed effect was not dependent on an active infectious process, but due to innate immune activation and could be mimicked by virus-associated molecular patterns such as the synthetic double-stranded RNA poly(I:C). Profound histomorphologic changes in Peyer's patches were associated with depletion of organ cellularity, most prominent among the B-cell subset. We demonstrate that the disruption is entirely dependent on type I interferon (IFN). At the cellular level, we show that virus-associated immune activation by IFN-α blocks B-cell trafficking to the Peyer's patches by downregulating expression of the homing molecule α4β7-integrin. In summary, our data identify a mechanism that results in type I IFN-dependent rapid but reversible disruption of intestinal lymphoid organs during systemic viral immune activation. We propose that such rerouted lymphocyte trafficking may impact the development of B-cell immunity to systemic viral pathogens.

Expression of CXCL12 receptors in B cells from Mexican Mestizos patients with systemic Lupus erythematosus

BACKGROUND

Systemic Lupus Erythematosus (SLE) is a chronic autoimmune disease characterized by B-cell hyper-reactivity and the production of pathogenic anti-nuclear-directed auto-antibodies (Abs). B-cell ontogeny is partly dependent on the CXCL12/CXCR4 axis for which the contribution to SLE pathogenesis remains unclear. CXCR7, the novel receptor for CXCL12, is differentially expressed among memory B-cell subsets. However, its biological role in SLE remains to be explored.

METHODS

Relative CXCR4 and CXCR7 expression levels were compared by quantitative PCR in leukocytes from blood samples of 41 Mexican Mestizos patients with SLE and 45 ethnicity-matched healthy subjects. Intracellular and membrane expression of both receptors was analyzed by flow cytometry in naive and Ab-secreting B cells. B-cell responsiveness to CXCL12 was investigated using Transwell-based chemotaxis assays. Data were analyzed using the Kruskal-Wallis test for comparisons of values amongst healthy controls and patients with inactive or active SLE, and non-parametrically using the Mann-Whitney U-test for multiple comparisons and unpaired samples. Correlations were determined by Spearman's ranking.

RESULT

SLE leukocytes displayed reduced levels of CXCR4 and CXCR7 transcripts. In SLE patients, a significant defect in CXCR4 expression was detected at the surface of naive and Ab-secreting B cells, associated with an abnormal intracellular localization of the receptor. CXCR7 predominantly localized in cytosolic compartments of B cells from healthy and SLE individuals. Disease activity did not impact on these expression patterns. Altered receptor compartmentalization correlated with an impaired CXCL12-promoted migration of SLE B cells.

CONCLUSIONS

Our data highlight a down-regulation of CXCL12 receptors on circulating B cells from SLE patients that likely influences their migratory behavior and distribution.

Kruppel-like factor 2 modulates CCR5 expression and susceptibility to HIV-1 infection

CCR5, a cell surface molecule critical for the transmission and spread of HIV-1, is dynamically regulated during T cell activation and differentiation. The molecular mechanism linking T cell activation to modulation of CCR5 expression remains undefined. Kruppel-like factor 2 (KLF2) is a transcription factor that promotes quiescence, survival, and in part by modulating chemokine receptor levels, induces homing to secondary lymphoid organs. Given the relationship between T cell activation and chemokine receptor expression, we tested whether the abundance of KLF2 after T cell activation regulates CCR5 expression and, thus, susceptibility of a T cell to CCR5-dependent HIV-1 strains (R5). We observed a strong correlation between T cell activation, expression of KLF2 and CCR5, and susceptibility to infection. To directly measure how KLF2 affects CCR5 regulation, we introduced small interfering RNA targeting KLF2 expression and demonstrated that reduced KLF2 expression also resulted in less CCR5. Chromatin immunoprecipitation assays identified KLF2 bound to the CCR5 promoter in resting but not CD3/28 activated T cells, suggesting that KLF2 directly regulates CCR5 expression. Introduction of KLF2 under control of a heterologous promoter could restore CCR5 expression and R5 susceptibility to CD3/28 costimulated T cells and some transformed cell lines. Thus, KLF2 is a host factor that modulates CCR5 expression in CD4 T cells and influences susceptibility to R5 infection.

Cutting edge: Krűppel-like factor 2 is required for phenotypic maintenance but not development of B1 B cells

Several recent studies reported that Krüppel-like factor (KLF)2 controls trafficking, development, and function of B cells. Conditional B cell KLF2 knockout mice have increased numbers of marginal zone B cells and decreased numbers of B1 phenoytpe cells. However, it was unclear whether KLF2 is required for B1 B cell development, survival, or phenotypic maintenance. We show that B1 phenotype B cells are present in neonatal mice with B cell-specific KLF2 deficiency, suggesting that B1 differentiation can occur even in the absence of KLF2. Furthermore, by use of an inducible knockout strategy, we show that deletion of KLF2 in mature B1 cells causes loss of phenotypic markers associated with B1 cell identity, but it has a minimal effect on short-term cell survival. Taken together, our findings suggest that KLF2 is necessary for the maintenance of B1 cell identity rather than differentiation or survival of the population.

Krüppel-like factors in lymphocyte biology

The Krüppel-like factor family of transcription factors plays an important role in differentiation, function, and homeostasis of many cell types. While their role in lymphocytes is still being determined, it is clear that these factors influence processes as varied as lymphocyte quiescence, trafficking, differentiation, and function. This review will present an overview of how these factors operate and coordinate with each other in lymphocyte regulation.

MEK/Erk-based negative feedback mechanism involved in control of Steel Factor-triggered production of Krüppel-like factor 2 in mast cells

The receptor tyrosine kinase, c-kit (Steel Factor (SF) receptor) controls survival, proliferation, chemotaxis, and secretion of proinflammatory cytokines in mast cells (MCs). Activation of c-kit results, amongst others, in induction of the PI3K and MEK/Erk pathways. Comparison of two MEK inhibitors, the specific, widely used U0126 and the more selective PD0325901, in different MC models revealed severe differences on SF-induced expression of proinflammatory cytokines IL-6 and TNF-α as well as the transcription factor Krüppel-like factor 2 (KLF2). Expression of the latter in MCs was not investigated so far. Whereas SF-induced expression of IL-6, TNF-α, and KLF2 was unaltered by U0126, it was significantly augmented by PD0325901. The effect of PD0325901 was corroborated by a second selective MEK inhibitor, PD184352 (Cl-1040), indicating the presence of MEK/Erk-based negative feedback mechanism(s) downstream of c-kit activation. Further analysis of KLF2 production revealed a positive function of PI3K. Depending on additional stimuli (e.g. antigen, IGF-1, LPS, thapsigargin), SF-triggered KLF2 expression was differentially modified, most likely controlled by the respective ratio between MEK/Erk and PI3K pathway activation. Moreover, the statin, simvastatin, was demonstrated to upregulate expression of KLF2 in MCs. In conclusion, data obtained by solely using the MEK inhibitor U0126 have to be carefully corroborated by using more selective inhibitors, such as PD0325901 or PD184352. SF-induced expression of the transcription factor KLF2 and its regulation by the MEK/Erk and PI3K pathways could impact on physiological as well as pathophysiological MC functions.

Sphingosine-1-phosphate and lymphocyte egress from lymphoid organs

Much has been learned about how cells enter lymphoid tissues. But how do they leave? Sphingosine-1-phosphate (S1P) has emerged over the past decade as a central mediator of lymphocyte egress. In this review, we summarize the current understanding of how S1P promotes exit from the secondary lymphoid organs and thymus. We review what is known about additional requirements for emigration and summarize the mostly distinct requirements for exit from the bone marrow. Egress from lymphoid organs is limited during immune responses, and we examine how this regulation works. There is accumulating evidence for roles of S1P in directing immune cell behavior within lymphoid tissues. How such actions can fit together with the egress-promoting role of S1P is discussed. Finally, we examine current understanding of how FTY720, a drug that targets S1P receptors and is approved for the treatment of multiple sclerosis, causes immune suppression.

Impaired B cell development in the absence of Krüppel-like factor 3

Krüppel-like factor 3 (Klf3) is a member of the Klf family of transcription factors. Klfs are widely expressed and have diverse roles in development and differentiation. In this study, we examine the function of Klf3 in B cell development by studying B lymphopoiesis in a Klf3 knockout mouse model. We show that B cell differentiation is significantly impaired in the bone marrow, spleen, and peritoneal cavity of Klf3 null mice and confirm that the defects are cell autonomous. In the bone marrow, there is a reduction in immature B cells, whereas recirculating mature cells are noticeably increased. Immunohistology of the spleen reveals a poorly structured marginal zone (MZ) that may in part be caused by deregulation of adhesion molecules on MZ B cells. In the peritoneal cavity, there are significant defects in B1 B cell development. We also report that the loss of Klf3 in MZ B cells is associated with reduced BCR signaling strength and an impaired ability to respond to LPS stimulation. Finally, we show increased expression of a number of Klf genes in Klf3 null B cells, suggesting that a Klf regulatory network may exist in B cells.

A blood flow–dependent klf2a-NO signaling cascade is required for stabilization of hematopoietic stem cell programming in zebrafish embryos

Blood flow has long been thought to be important for vessel development and function, but its role in HSC development is not yet fully understood. Here, we take advantage of zebrafish embryos with circulation defects that retain relatively normal early development to illustrate the combinatorial roles of genetic and hemodynamic forces in HSC development. We show that blood flow is not required for initiation of HSC gene expression, but instead is indispensable for its maintenance. Knockdown of klf2a mimics the silent heart (sih/tnnt2a) phenotype while overexpression of klf2a in tnnt2a morphant embryos can rescue HSC defects, suggesting that klf2a is a downstream mediator of blood flow. Furthermore, the expression of NO synthase (nos) was reduced in klf2a knockdown embryos, and ChIP analysis showed that endogenous Klf2a is bound to the promoters of nos genes in vivo, indicating direct gene regulation. Finally, administration of the NO agonist S-nitroso N-acetylpenicillamine (SNAP) can restore HSC development in tnnt2a and klf2a morphants, suggesting that NO signaling is downstream of Klf2a which is induced by hemodynamic forces. Taken together, we have demonstrated that blood flow is essential for HSC development and is mediated by a klf2a-NO signaling cascade in zebrafish.

The transcription factor Bright plays a role in marginal zone B lymphocyte development and autoantibody production

Previous data suggested that constitutive expression of the transcription factor Bright (B cell regulator of immunoglobulin heavy chain transcription), normally tightly regulated during B cell differentiation, was associated with autoantibody production. Here we show that constitutive Bright expression results in skewing of mature B lineage subpopulations toward marginal zone cells at the expense of the follicular subpopulation. C57Bl/6 transgenic mice constitutively expressing Bright in B lineage cells generated autoantibodies that were not the result of global increases in immunoglobulin or of breaches in key tolerance checkpoints typically defective in other autoimmune mouse models. Rather, autoimmunity correlated with increased numbers of marginal zone B cells and alterations in the phenotype and gene expression profiles of lymphocytes within the follicular B cell compartment. These data suggest a novel role for Bright in the normal development of mature B cell subsets and in autoantibody production.

The outs and the ins of sphingosine-1-phosphate in immunity

The potent lipid mediator sphingosine-1-phosphate (S1P) is produced inside cells by two closely related kinases, sphingosine kinase 1 (SPHK1) and SPHK2, and has emerged as a crucial regulator of immunity. Many of the actions of S1P in innate and adaptive immunity are mediated by its binding to five G protein-coupled receptors, designated S1PR1-5, but recent findings have also identified important roles for S1P as a second messenger during inflammation. In this Review, we discuss recent advances in our understanding of the roles of S1P receptors and describe the newly identified intracellular targets of S1P that are crucial for immune responses. Finally, we discuss the therapeutic potential of new drugs that target S1P signalling and functions.

Krüppel-like factor 2 (KLF2) regulates B-cell reactivity, subset differentiation, and trafficking molecule expression

The transcription factor Krüppel-like factor 2 (KLF2) is critical for normal trafficking of T lymphocytes, but its role in B cells is unclear. We report that B cell-specific KLF2 deficiency leads to decreased expression of the trafficking molecules CD62L and β7-integrin, yet expression of sphingosine-1 phosphate receptor 1 (which is a critical target of KLF2 in T cells) was, unexpectedly, minimally altered. Unexpectedly, Klf2 deletion led to a drastic reduction in the B1 B-cell pool and a substantial increase in transitional and marginal zone B-cell numbers. In addition, we observed that KLF2-deficient B cells showed increased apoptosis and impaired proliferation after B-cell receptor cross-linking. Gene expression analysis indicated that KLF2-deficient follicular B cells display numerous characteristics shared by normal marginal zone B cells, including reduced expression of several signaling molecules that may contribute to defective activation of these cells. Hence, our data indicate that KLF2 plays a critical role in dictating normal subset differentiation and functional reactivity of mature B cells.