Distinct functions for the transcription factor Foxo1 at various stages of B cell differentiation

Functional regulation of FoxO1 in neural stem cell differentiation

Forkhead transcription factor family O (FoxO) maintains adult stem cell reserves by supporting their long-term proliferative potential. MicroRNAs (miRs) regulate neuronal stem/progenitor cell (NSPC) proliferation and differentiation during neural development by controlling the expression of a specific set of target genes. In the neurogenic subventricular zone, FoxO1 is specifically expressed in NSPCs and is no longer detected during the transition to neuroblast stage, forming an inverse correlation with miR-9 expression. The 3'-untranslated region of FoxO1 contains a conserved target sequence of miR-9 and FoxO1 expression is coordinated in concert with miR-9 during neuronal differentiation. Our study demonstrates that FoxO1 contributes to NSPC fate decision through its cooperation with the Notch signaling pathway.

An Enhancer of the IL-7 Receptor α-Chain Locus Controls IL-7 Receptor Expression and Maintenance of Peripheral T Cells

The IL-7R plays critical roles in lymphocyte development and homeostasis. Although IL-7R expression is strictly regulated during lymphocyte differentiation and the immune response, little is known regarding its in vivo regulation. To address this issue, we established a mouse line with targeted deletion of the conserved non-coding sequence 1 (CNS1) element found 3.6 kb upstream of the IL-7Rα promoter. We report that IL-7Rα is expressed normally on T and B cells in thymus and bone marrow of CNS1(-/-) mice except for in regulatory T cells. In contrast, these mice show reduced IL-7Rα expression in conventional CD4 and CD8 T cells as well as regulatory T, NKT, and γδ T cells in the periphery. CD4 T cells of CNS1(-/-) mice showed IL-7Rα upregulation in the absence of growth factors and IL-7Rα downregulation by IL-7 or TCR stimulation, although the expression levels were lower than those in control mice. Naive CD4 and CD8 T cells of CNS1(-/-) mice show attenuated survival by culture with IL-7 and reduced homeostatic proliferation after transfer into lymphopenic hosts. CNS1(-/-) mice exhibit impaired maintenance of Ag-stimulated T cells. Furthermore, IL-7Rα upregulation by glucocorticoids and TNF-α was abrogated in CNS1(-/-) mice. This work demonstrates that the CNS1 element controls IL-7Rα expression and maintenance of peripheral T cells, suggesting differential regulation of IL-7Rα expression between central and peripheral lymphoid organs.

The pre-B-cell receptor checkpoint in acute lymphoblastic leukaemia

The B-cell receptor (BCR) and its immature form, the precursor-BCR (pre-BCR), have a central role in the control of B-cell development, which is dependent on a sequence of cell-fate decisions at specific antigen-independent checkpoints. Pre-BCR expression provides the first checkpoint, which controls differentiation of pre-B to immature B-cells in normal haemopoiesis. Pre-BCR signalling regulates and co-ordinates diverse processes within the pre-B cell, including clonal selection, proliferation and subsequent maturation. In B-cell precursor acute lymphoblastic leukaemia (BCP-ALL), B-cell development is arrested at this checkpoint. Moreover, malignant blasts avoid clonal extinction by hijacking pre-BCR signalling in favour of the development of BCP-ALL. Here, we discuss three mechanisms that occur in different subtypes of BCP-ALL: (i) blocking pre-BCR expression; (ii) activating pre-BCR-mediated pro-survival and pro-proliferative signalling, while inhibiting cell cycle arrest and maturation; and (iii) bypassing the pre-BCR checkpoint and activating pro-survival signalling through pre-BCR independent alternative mechanisms. A complete understanding of the BCP-ALL-specific signalling networks will highlight their application in BCP-ALL therapy.

Processing of CD74 by the Intramembrane Protease SPPL2a Is Critical for B Cell Receptor Signaling in Transitional B Cells

The invariant chain (CD74), a chaperone in MHC class II-mediated Ag presentation, is sequentially processed by different endosomal proteases. We reported recently that clearance of the final membrane-bound N-terminal fragment (NTF) of CD74 is mediated by the intramembrane protease signal peptide peptidase-like (SPPL)2a, a process critical for B cell development. In mice, SPPL2a deficiency provokes the accumulation of this NTF in endocytic vesicles, which leads to a B cell maturation arrest at the transitional 1 stage. To define the underlying mechanism, we analyzed the impact of SPPL2a deficiency on signaling pathways involved in B cell homeostasis. We demonstrate that tonic as well as BCR-induced activation of the PI3K/Akt pathway is massively compromised in SPPL2a(-/-) B cells and identify this as major cause of the B cell maturation defect in these mice. Altered BCR trafficking induces a reduction of surface IgM in SPPL2a-deficient B cells, leading to a diminished signal transmission via the BCR and the tyrosine kinase Syk. We provide evidence that in SPPL2a(-/-) mice impaired BCR signaling is to a great extent provoked by the accumulating CD74 NTF, which can interact with the BCR and Syk, and that impaired PI3K/Akt signaling and reduced surface IgM are not directly linked processes. In line with disturbances in PI3K/Akt signaling, SPPL2a(-/-) B cells show a dysregulation of the transcription factor FOXO1, causing elevated transcription of proapoptotic genes. We conclude that SPPL2a-mediated processing of CD74 NTF is indispensable to maintain appropriate levels of tonic BCR signaling to promote B cell maturation.

Transcription factor networks in B-cell differentiation link development to acute lymphoid leukemia

B-lymphocyte development in the bone marrow is controlled by the coordinated action of transcription factors creating regulatory networks ensuring activation of the B-lymphoid program and silencing of alternative cell fates. This process is tightly connected to malignant transformation because B-lineage acute lymphoblastic leukemia cells display a pronounced block in differentiation resulting in the expansion of immature progenitor cells. Over the last few years, high-resolution analysis of genetic changes in leukemia has revealed that several key regulators of normal B-cell development, including IKZF1, TCF3, EBF1, and PAX5, are genetically altered in a large portion of the human B-lineage acute leukemias. This opens the possibility of directly linking the disrupted development as well as aberrant gene expression patterns in leukemic cells to molecular functions of defined transcription factors in normal cell differentiation. This review article focuses on the roles of transcription factors in early B-cell development and their involvement in the formation of human leukemia.

Integrated analysis of gene expression and microRNA regulation in three leukemia-related lymphoblastic cell lines

RNA-sequencing technology is progressively being applied in various fields since high-throughput data analysis provides deeper mining on both the genomic and transcriptomic level. Lymphoblastic leukemogenesis is a complex process caused by abnormalities occurring during lymphocyte differentiation, and can be initiated by various triggers. Each leukemia subtype has distinct characteristics that can be identified in the corresponding cell lines; the detection of the exclusive genetic signatures of these varying cell lines is critical. Our analysis revealed that approximately 8000 human genes were differentially expressed between samples. Signaling pathways such as the NOD-like signaling pathway, cell surface receptor signaling pathways, and leukemia-related pathways were significantly regulated, as determined by KEGG annotation. Furthermore, several oncogenes and differentiation-related genes were differentially expressed between leukemia cell lines and lymphocyte types, respectively. Our miRNA analysis demonstrated that the expression of approximately one-third of all expressed miRNAs appeared to be significantly different between the leukemia cell lines studied. We also analyzed the mRNA-miRNA regulatory networks of both lymphocyte differentiation and leukemogenesis to determine key regulators of interest. We combined the results of the mRNA and miRNA analyses in order to investigate the regulatory relationship between them. This study not only identifies differences in the pathways and networks of acute lymphocytic leukemia (ALL) relative to normal lymphocytes, but also identifies unique functional characteristics of lymphoid cells and distinct gene expression patterns during lymphoid development. The discovery of leukemia-related miRNAs may provide meaningful insights into the biology of the disease.

Nfix expression critically modulates early B lymphopoiesis and myelopoiesis

The commitment of stem and progenitor cells toward specific hematopoietic lineages is tightly controlled by a number of transcription factors that regulate differentiation programs via the expression of lineage restricting genes. Nuclear factor one (NFI) transcription factors are important in regulating hematopoiesis and here we report an important physiological role of NFIX in B- and myeloid lineage commitment and differentiation. We demonstrate that NFIX acts as a regulator of lineage specification in the haematopoietic system and the expression of Nfix was transcriptionally downregulated as B cells commit and differentiate, whilst maintained in myeloid progenitor cells. Ectopic Nfix expression in vivo blocked early B cell development stage, coincident with the stage of its downregulation. Furthermore, loss of Nfix resulted in the perturbation of myeloid and lymphoid cell differentiation, and a skewing of gene expression involved in lineage fate determination. Nfix was able to promote myeloid differentiation of total bone marrow cells under B cell specific culture conditions but not when expressed in the hematopoietic stem cell (HSPC), consistent with its role in HSPC survival. The lineage choice determined by Nfix correlated with transcriptional changes in a number of genes, such as E2A, C/EBP, and Id genes. These data highlight a novel and critical role for NFIX transcription factor in hematopoiesis and in lineage specification.

The transcriptional repressor HDAC7 promotes apoptosis and c-Myc downregulation in particular types of leukemia and lymphoma

The generation of B cells is a complex process requiring several cellular transitions, including cell commitment and differentiation. Proper transcriptional control to establish the genetic programs characteristic of each cellular stage is essential for the correct development of B lymphocytes. Deregulation of these particular transcriptional programs may result in a block in B-cell maturation, contributing to the development of hematological malignancies such as leukemia and lymphoma. However, very little is currently known about the role of transcriptional repressors in normal and aberrant B lymphopoiesis. Here we report that histone deacetylase 7 (HDAC7) is underexpressed in pro-B acute lymphoblastic leukemia (pro-B-ALL) and Burkitt lymphoma. Ectopic expression of HDAC7 induces apoptosis, leads to the downregulation of c-Myc and inhibits the oncogenic potential of cells in vivo, in a xenograft model. Most significantly, we have observed low levels of HDAC7 expression in B-ALL patient samples, which is correlated with the increased levels of c-Myc. From a mechanistic angle, we show that ectopically expressed HDAC7 localizes to the nucleus and interacts with the transcription factor myocyte enhancer factor C (MEF2C) and the corepressors HDAC3 and SMRT. Accordingly, both the HDAC7–MEF2C interaction domain as well as its catalytic domain are involved in the reduced cell viability induced by HDAC7. We conclude that HDAC7 has a potent anti-oncogenic effect on specific B-cell malignancies, indicating that its deregulation may contribute to the pathogenesis of the disease.

Assembly and Function of the Precursor B-Cell Receptor

During early stages of development, precursor B lymphocytes express a characteristic type of antigen receptor known as the pre-B-cell receptor (pre-BCR). This receptor differs from conventional BCRs in that it possesses a germ line-encoded surrogate light chain (SLC), which is associated with the signal transduction machinery via heavy chain (HC) proteins that have been generated by productive rearrangement of the immunoglobulin HC genes. The pre-BCR marks a key step of B-cell commitment, as it activates the B-cell-specific signaling cascade and mediates the selection, expansion, and differentiation of cells expressing a productively rearranged HC protein. Another difference between the pre-BCR and conventional BCR might be the initial event that triggers receptor activation, as the pre-BCR is activated in the absence of external ligands, while conventional BCRs require antigen for activation. Nonetheless, the pre-BCR downstream signaling cascade is largely similar to that of the BCR suggesting that the characteristic LC of the pre-BCR mediates important receptor interactions thereby providing distinctive, germ line-encoded features to the pre-BCR. In fact, the SLC enables the pre-BCR to act as a surrogate autoreactive receptor. Here, we outline the structure and function of the pre-BCR and how the autonomous signaling capacity might be a direct consequence of pre-BCR assembly. In addition to its role in early B-cell development, we discuss how the ordered activation of downstream signaling cascades enables the pre-BCR to activate seemingly opposing cellular programs such as proliferation and differentiation.

PI3K signalling in inflammation

PI3Ks regulate several key events in the inflammatory response to damage and infection. There are four Class I PI3K isoforms (PI3Kα,β,γ,δ), three Class II PI3K isoforms (PI3KC2α, C2β, C2γ) and a single Class III PI3K. The four Class I isoforms synthesise the phospholipid 'PIP3'. PIP3 is a 'second messenger' used by many different cell surface receptors to control cell movement, growth, survival and differentiation. These four isoforms have overlapping functions but each is adapted to receive efficient stimulation by particular receptor sub-types. PI3Kγ is highly expressed in leukocytes and plays a particularly important role in chemokine-mediated recruitment and activation of innate immune cells at sites of inflammation. PI3Kδ is also highly expressed in leukocytes and plays a key role in antigen receptor and cytokine-mediated B and T cell development, differentiation and function. Class III PI3K synthesises the phospholipid PI3P, which regulates endosome-lysosome trafficking and the induction of autophagy, pathways involved in pathogen killing, antigen processing and immune cell survival. Much less is known about the function of Class II PI3Ks, but emerging evidence indicates they can synthesise PI3P and PI34P2 and are involved in the regulation of endocytosis. The creation of genetically-modified mice with altered PI3K signalling, together with the development of isoform-selective, small-molecule PI3K inhibitors, has allowed the evaluation of the individual roles of Class I PI3K isoforms in several mouse models of chronic inflammation. Selective inhibition of PI3Kδ, γ or β has each been shown to reduce the severity of inflammation in one or more models of autoimmune disease, respiratory disease or allergic inflammation, with dual γ/δ or β/δ inhibition generally proving more effective. The inhibition of Class I PI3Ks may therefore offer a therapeutic opportunity to treat non-resolving inflammatory pathologies in humans. This article is part of a Special Issue entitled Phosphoinositides.

FOXO1 repression contributes to block of plasma cell differentiation in classical Hodgkin lymphoma

FOXO1 directly activates PRDM1α, the master regulator of PC differentiation, and it enriches a PC signature in cHL cell lines. PRDM1α is a tumor suppressor in cHL.

mTOR kinase inhibitors promote antibody class switching via mTORC2 inhibition

The mammalian target of rapamycin (mTOR) is a kinase that functions in two distinct complexes, mTORC1 and mTORC2. In peripheral B cells, complete deletion of mTOR suppresses germinal center B-cell responses, including class switching and somatic hypermutation. The allosteric mTORC1 inhibitor rapamycin blocks proliferation and differentiation, but lower doses can promote protective IgM responses. To elucidate the complexity of mTOR signaling in B cells further, we used ATP-competitive mTOR kinase inhibitors (TOR-KIs), which inhibit both mTORC1 and mTORC2. Although TOR-KIs are in clinical development for cancer, their effects on mature lymphocytes are largely unknown. We show that high concentrations of TOR-KIs suppress B-cell proliferation and differentiation, yet lower concentrations that preserve proliferation increase the fraction of B cells undergoing class switching in vitro. Transient treatment of mice with the TOR-KI compound AZD8055 increased titers of class-switched high-affinity antibodies to a hapten-protein conjugate. Mechanistic investigation identified opposing roles for mTORC1 and mTORC2 in B-cell differentiation and showed that TOR-KIs enhance class switching in a manner dependent on forkhead box, subgroup O (FoxO) transcription factors. These observations emphasize the distinct actions of TOR-KIs compared with rapamycin and suggest that TOR-KIs might be useful to enhance production of class-switched antibodies following vaccination.

The regulatory network of B-cell differentiation: a focused view of early B-cell factor 1 function

During the last decades, many studies have investigated the transcriptional and epigenetic regulation of lineage decision in the hematopoietic system. These efforts led to a model in which extrinsic signals and intrinsic cues establish a permissive chromatin context upon which a regulatory network of transcription factors and epigenetic modifiers act to guide the differentiation of hematopoietic lineages. These networks include lineage-specific factors that further modify the epigenetic landscape and promote the generation of specific cell types. The process of B lymphopoiesis requires a set of transcription factors, including Ikaros, PU.1, E2A, and FoxO1 to 'prime' cis-regulatory regions for subsequent activation by the B-lineage-specific transcription factors EBF1 and Pax-5. The expression of EBF1 is initiated by the combined action of E2A and FoxO1, and it is further enhanced and maintained by several positive feedback loops that include Pax-5 and IL-7 signaling. EBF1 acts in concert with Ikaros, PU.1, Runx1, E2A, FoxO1, and Pax-5 to establish the B cell-specific transcription profile. EBF1 and Pax-5 also collaborate to repress alternative cell fates and lock cells into the B-lineage fate. In addition to the functions of EBF1 in establishing and maintaining B-cell identity, EBF1 is required to coordinate differentiation with cell proliferation and survival.

Rictor is required for early B cell development in bone marrow

The development of early B cells, which are generated from hematopoietic stem cells (HSCs) in a series of well-characterized stages in bone marrow (BM), represents a paradigm for terminal differentiation processes. Akt is primarily regulated by phosphorylation at Thr308 by PDK1 and at Ser473 by mTORC2, and Akt signaling plays a key role in hematopoiesis. However, the role of mTORC2 in the development of early B cells remains poorly understood. In this study, we investigated the functional role of mTORC2 by specifically deleting an integral component, Rictor, in a hematopoietic system. We demonstrated that the deletion of Rictor induced an aberrant increase in the FoxO1 and Rag-1 proteins in BM B cells and that this increase was accompanied by a significant decrease in the abundance of B cells in the peripheral blood (PB) and the spleen, suggesting impaired development of early B cells in adult mouse BM. A BM transplantation assay revealed that the B cell differentiation defect induced by Rictor deletion was not affected by the BM microenvironment, thus indicating a cell-intrinsic mechanism. Furthermore, the knockdown of FoxO1 in Rictor-deleted HSCs and hematopoietic progenitor cells (HPCs) promoted the maturation of B cells in the BM of recipient mice. In addition, we revealed that treatment with rapamycin (an mTORC1 inhibitor) aggravated the deficiency in B cell development in the PB and BM. Taken together, our results provide further evidence that Rictor regulates the development of early B cells in a cell-intrinsic manner by modifying the expression of FoxO1 and Rag-1.

NEDD4 ubiquitinates TRAF3 to promote CD40-mediated AKT activation

CD40, a member of tumour necrosis factor receptor (TNFR) superfamily, has a pivotal role in B-cell-mediated immunity through various effector pathways including AKT kinase, but the signal transduction of CD40-meidated AKT activation is poorly understood. Here we report that the neural precursor cell expressed developmentally downregulated protein 4 (NEDD4), homologous to E6-AP Carboxyl Terminus family E3 ubiquitin ligase, is a novel component of the CD40 signalling complex. It has a key role in CD40-mediated AKT activation and is involved in modulating immunoglobulin class switch through regulating the expression of activation-induced cytidine deaminase. NEDD4 constitutively interacts with CD40 and mediates K63-linked ubiquitination of TNFR-associated factor3 (TRAF3). The ubiquitination of TRAF3 by NEDD4 is critical for CD40-mediated AKT activation. Thus, NEDD4 is a previously unknown component of the CD40 signalling complex necessary for AKT activation.

B cell biology: an overview

In this review we summarize recent insights into the development of human B cells primarily by studying immunodeficiencies. Development and differentiation of B cells can be considered as a paradigm for many other developmental processes in cell biology. However, it differs from the development of many other cell types by phases of extremely rapid cell division and by defined series of somatic recombination and mutation events required to assemble and refine the B cell antigen receptors. Both somatic DNA alteration and proliferation phases take place in defined sites but in different organs. Thus, cell migration and timely arrival at defined sites are additional features of B cell development. By comparing experimental mouse models with insights gained from studying defined genetic defects leading to primary immunodeficiencies and hypogammaglobulinemia, we address important features that are characteristic for human B cells. We also summarize recent advances made by developing improved in vitro and in vivo systems allowing the development of human B cells from hematopoietic stem cells. Combined with genetic and functional studies of immunodeficiencies, these models will contribute not only to a better understanding of disease affecting the B lymphocyte compartment, but also to designing better and safer novel B cell-targeted therapies in autoimmunity and allergy.

Integrated genetic approaches identify the molecular mechanisms of Sox4 in early B-cell development: intricate roles for RAG1/2 and CK1ε

Commitment of hematopoietic stem cells to B lineage precursors and subsequent development of B lineage precursors into mature B cells is stringently controlled by stage-specific transcription factors. In this study, we used integrated genetic approaches and systematically determined the role of Sry-related high mobility group box (Sox) 4 and the underlying molecular mechanisms in early B-cell development. We found that Sox4 coordinates multilevel controls in the differentiation of early stage B cells. At the molecular level, Sox4 orchestrates a unique gene regulatory program, and its function was predominantly mediated through a conventional Sox4-binding motif as well as an unconventional GA-binding protein α chain binding motif. Our integrated gene network and functional analysis indicated that Sox4 functions as a bimodular transcription factor and ensures B lineage precursor differentiation through 2 distinct mechanisms. It positively induces gene rearrangements at immunoglobulin heavy chain gene loci by transcriptionally activating the Rag1 and Rag2 genes and negatively regulates Wnt signaling, which is critical for self-renewal, by inducing the expression of casein kinase 1 ε. Our findings illustrate that Sox4 mediates critical fine-tuning of the 2 opposing forces in early B-cell development and also set forth a model for characterization of critical genes whose deficiency, like Sox4 deficiency, is detrimental to this process.

The Interplay between Chromatin and Transcription Factor Networks during B Cell Development: Who Pulls the Trigger First?

All mature blood cells derive from hematopoietic stem cells through gradual restriction of their cell fate potential and acquisition of specialized functions. Lineage specification and cell commitment require the establishment of specific transcriptional programs involving the activation of lineage-specific genes and the repression of lineage-inappropriate genes. This process requires the concerted action of transcription factors (TFs) and epigenetic modifying enzymes. Within the hematopoietic system, B lymphopoiesis is one of the most-studied differentiation programs. Loss of function studies allowed the identification of many TFs and epigenetic modifiers required for B cell development. The usage of systematic analytical techniques such as transcriptome determination, genome-wide mapping of TF binding and epigenetic modifications, and mass spectrometry analyses, allowed to gain a systemic description of the intricate networks that guide B cell development. However, the precise mechanisms governing the interaction between TFs and chromatin are still unclear. Generally, chromatin structure can be remodeled by some TFs but in turn can also regulate (i.e., prevent or promote) the binding of other TFs. This conundrum leads to the crucial questions of who is on first, when, and how. We review here the current knowledge about TF networks and epigenetic regulation during hematopoiesis, with an emphasis on B cell development, and discuss in particular the current models about the interplay between chromatin and TFs.

FOXO transcription factors: their clinical significance and regulation

Members of the class O of forkhead box transcription factors (FOXO) have important roles in metabolism, cellular proliferation, stress resistance, and apoptosis. The activity of FOXOs is tightly regulated by posttranslational modification, including phosphorylation, acetylation, and ubiquitylation. Activation of cell survival pathways such as phosphoinositide-3-kinase/AKT/IKK or RAS/mitogen-activated protein kinase phosphorylates FOXOs at different sites which regulate FOXOs nuclear localization or degradation. FOXO transcription factors are upregulated in a number of cell types including hepatocytes, fibroblasts, osteoblasts, keratinocytes, endothelial cells, pericytes, and cardiac myocytes. They are involved in a number of pathologic and physiologic processes that include proliferation, apoptosis, autophagy, metabolism, inflammation, cytokine expression, immunity, differentiation, and resistance to oxidative stress. These processes impact a number of clinical conditions such as carcinogenesis, diabetes, diabetic complications, cardiovascular disease, host response, and wound healing. In this paper, we focus on the potential role of FOXOs in different disease models and the regulation of FOXOs by various stimuli.

CDK4 deficiency promotes genomic instability and enhances Myc-driven lymphomagenesis

The G1 kinase CDK4 is amplified or overexpressed in some human tumors and promotes tumorigenesis by inhibiting known tumor suppressors. Here, we report that CDK4 deficiency markedly accelerated lymphoma development in the Eμ-Myc transgenic mouse model of B lymphoma and that silencing or loss of CDK4 augmented the tumorigenic potential of Myc-driven mouse and human B cell lymphoma in transplant models. Accelerated disease in CDK4-deficient Eμ-Myc transgenic mice was associated with rampant genomic instability that was provoked by dysregulation of a FOXO1/RAG1/RAG2 pathway. Specifically, CDK4 phosphorylated and inactivated FOXO1, which prevented FOXO1-dependent induction of Rag1 and Rag2 transcription. CDK4-deficient Eμ-Myc B cells had high levels of the active form of FOXO1 and elevated RAG1 and RAG2. Furthermore, overexpression of RAG1 and RAG2 accelerated lymphoma development in a transplant model, with RAG1/2-expressing tumors exhibiting hallmarks of genomic instability. Evaluation of human tumor samples revealed that CDK4 expression was markedly suppressed, while FOXO1 expression was elevated, in several subtypes of human non-Hodgkin B cell lymphoma. Collectively, these findings establish a context-specific tumor suppressor function for CDK4 that prevents genomic instability, which contributes to B cell lymphoma. Furthermore, our data suggest that targeting CDK4 may increase the risk for the development and/or progression of lymphoma.

BACH2-BCL6 balance regulates selection at the pre-B cell receptor checkpoint

At the pre-B cell receptor (BCR) checkpoint, developing pre-B cells are selected for successful rearrangement of V(H)-DJ(H) gene segments and expression of a pre-BCR. Reduced stringency at this checkpoint may obstruct the B cell repertoire with nonfunctional B cell clones. Earlier studies have described that activation of B cell lymphoma/leukemia (BCL)6 by a functional pre-BCR mediates positive selection of pre-B cells that have passed the checkpoint. This concept is now further elaborated by the recent finding that the BTB and CNC homology 1 basic leucine zipper transcription factor 2 (BACH2) induces negative selection and opposes BCL6 function prior to the pre-BCR checkpoint. Here, we discuss the antagonism between BCL6 and BACH2 during early B cell development, as well as its implications in both repertoire selection and counter-selection of premalignant clones for leukemia suppression.

FOXO1 transcription factor: a critical effector of the PI3K-AKT axis in B-cell development

B-cell development and differentiation are controlled at multiple levels by the complex interplay of specific receptors and a variety of transcription factors. Several receptors involved in regulating this process, such as IL-7R, pre-B cell receptor (pre-BCR), and BCR, share the ability to trigger the signaling via the phosphoinositide 3-kinase (PI3K)-AKT pathway. FOXO1 transcription factor, a major PI3K-AKT downstream effector, regulates the expression of genes critical for progress through consecutive steps of B-cell differentiation. FOXO1 directs or fine-tunes multiple biological functions that are crucial for differentiating cells, including the cell cycle, apoptosis, oxidative stress response or DNA damage repair. Recent studies have highlighted the key role that FOXO1 plays in the maintenance of the hematopoietic stem cell pool, regulation of progenitor commitment, development of early B-cell precursors, induction of B-cell tolerance, peripheral B-cell homeostasis, and terminal differentiation. FOXO1 deficiency impairs B-cell development, due to decreased expression of its critical target genes, that include early B-cell factor (EBF1), IL-7 receptor, recombination activating genes (RAG1 and 2), activation-induced cytidine deaminase (AID), L-selectin, and BLNK. Taken together, FOXO1 is an important node in a dynamic network of transcription factors that orchestrate B-cell differentiation and specialization. Herein, we review molecular mechanisms of the PI3K-AKT-dependent signal transduction and their impact on early B-cell development, peripheral B-cell homeostasis, and terminal differentiation.

Ursodeoxycholic acid influences the expression of p27kip1 but not FoxO1 in patients with non-cirrhotic primary biliary cirrhosis

Background. Enhanced expression of cell cycle inhibitor p27^kip1 suppresses cell proliferation. Ursodeoxycholic acid (UDCA) delays progression of primary biliary cirrhosis (PBC) but its effect on p27^kip1 expression is uncertain. Aims. To analyze the expression of p27^kip1 and its transcription modulator FoxO1 in patients with PBC, and to assess the impact of UDCA on this pathway. Materials and Methods. The examined human tissue included explanted livers from patients with cirrhotic PBC (n = 23), primary sclerosing cholangitis (PSC; n = 9), alcoholic liver disease (ALD; n = 9), and routine liver biopsies from patients with non-cirrhotic PBC (n = 26). Healthy liver samples served as controls (n = 19). Livers of FoxO-deficient mice were also studied. mRNA and protein expressions were analyzed by real-time PCR and Western blot. Results. p27^kip1 expression was increased in cirrhotic and non-cirrhotic PBC. FoxO1 mRNA levels were increased in PBC (8.5-fold increase versus controls). FoxO1 protein expression in PBC was comparable to controls, but it was decreased in patients with PSC and ALD (63% and 70% reduction, respectively; both P < 0.05 versus control). UDCA-treated non-cirrhotic patients with PBC showed decreased expression of p27^kip1 mRNA. Conclusion. PBC progression is characterized by a FoxO1-independent increase of p27^kip1 expression. In early PBC, UDCA may enhance liver regeneration via p27^kip1-dependent mechanism.

Transcriptional control of early T and B cell developmental choices

T and B cells share a common somatic gene rearrangement mechanism for assembling the genes that code for their antigen receptors; they also have developmental pathways with many parallels. Shared usage of basic helix-loop-helix E proteins as transcriptional drivers underlies these common features. However, the transcription factor networks in which these E proteins are embedded are different both in membership and in architecture for T and B cell gene regulatory programs. These differences permit lineage commitment decisions to be made in different hierarchical orders. Furthermore, in contrast to B cell gene networks, the T cell gene network architecture for effector differentiation is sufficiently modular so that E protein inputs can be removed. Complete T cell-like effector differentiation can proceed without T cell receptor rearrangement or selection when E proteins are neutralized, yielding natural killer and other innate lymphoid cells.

Transcriptional control of pre-B cell development and leukemia prevention

The differentiation of early B cell progenitors is controlled by multiple transcriptional regulators and growth-factor receptors. The triad of DNA-binding proteins, E2A, EBF1, and PAX5 is critical for both the early specification and commitment of B cell progenitors, while a larger number of secondary determinants, such as members of the Ikaros, ETS, Runx, and IRF families have more direct roles in promoting stage-specific pre-B gene-expression program. Importantly, it is now apparent that mutations in many of these transcription factors are associated with the progression to acute lymphoblastic leukemia. In this review, we focus on recent studies that have shed light on the transcriptional hierarchy that controls efficient B cell commitment and differentiation as well as focus on the oncogenic consequences of the loss of many of the same factors.

Orchestrating B cell lymphopoiesis through interplay of IL-7 receptor and pre-B cell receptor signalling

The development of B cells is dependent on the sequential DNA rearrangement of immunoglobulin loci that encode subunits of the B cell receptor. The pathway navigates a crucial checkpoint that ensures expression of a signalling-competent immunoglobulin heavy chain before commitment to rearrangement and expression of an immunoglobulin light chain. The checkpoint segregates proliferation of pre-B cells from immunoglobulin light chain recombination and their differentiation into B cells. Recent advances have revealed the molecular circuitry that controls two rival signalling systems, namely the interleukin-7 (IL-7) receptor and the pre-B cell receptor, to ensure that proliferation and immunoglobulin recombination are mutually exclusive, thereby maintaining genomic integrity during B cell development.

Context-specific BAFF-R signaling by the NF-κB and PI3K pathways

BAFF is a soluble factor required for B cell maturation and survival. BAFF-R signals via the noncanonical NF-κB pathway regulated by the TRAF3/NIK/IKK1 axis. We show that deletion of Ikk1 during early B cell development causes a partial impairment in B cell maturation and BAFF-dependent survival, but inactivation of Ikk1 in mature B cells does not affect survival. We further show that BAFF-R employs CD19 to promote survival via phosphatidylinositol 3-kinase (PI3K), and that coinactivation of Cd19 and Ikk1 causes a profound block in B cell maturation at the transitional stage. Consistent with a role for PI3K in BAFF-R function, inactivation of PTEN mediates a partial rescue of B cell maturation and function in Baff(-/-) animals. Elevated PI3K signaling also circumvents BAFF-dependent survival in a spontaneous B cell lymphoma model. These findings indicate that the combined activities of PI3K and IKK1 drive peripheral B cell differentiation and survival in a context-dependent manner.

Ebf1 and c-Myb repress rag transcription downstream of Stat5 during early B cell development

The temporal control of RAG (Rag) expression in developing lymphocytes prevents DNA breaks during periods of proliferation that could threaten genomic integrity. In developing B cells, the IL-7R and precursor B cell Ag receptor (pre-BCR) synergize to induce proliferation and the repression of Rag at the protein and mRNA levels for a brief period following successful Ig H chain gene rearrangement. Whereas the mechanism of RAG2 protein downregulation is well defined, little is known about the pathways and transcription factors that mediate transcriptional repression of Rag. Using Abelson murine leukemia virus-transformed B cells to model this stage of development, we identified early B cell factor 1 (Ebf1) as a strong repressor of Rag transcription. Short hairpin RNA-mediated knockdown of either Ebf1 or its downstream target c-Myb was sufficient to induce Rag transcription in these highly proliferative cells. Ebf1 and c-Myb antagonize Rag transcription by negatively regulating the binding of Foxo1 to the Rag locus. Ebf1 accomplishes this through both direct negative regulation of Foxo1 expression and direct positive regulation of Gfi1b expression. Ebf1 expression is driven by the IL-7R downstream effector Stat5, providing a link between the negative regulation of Rag transcription by IL-7 and a novel repressive pathway involving Ebf1 and c-Myb.

Analysis of microRNA expression profile by small RNA sequencing in Down syndrome fetuses

Down syndrome (DS) is caused by trisomy of human chromosome 21 (Hsa21) and is associated with numerous deleterious phenotypes, including cognitive impairment, childhood leukemia and immune defects. Five Hsa21‑derived microRNAs (i.e., hsa-miR-99a, let-7c, miR-125b-2, miR-155 and miR-802) are involved in variable phenotypes of DS. However, the changes involved in the genome-wide microRNA expression of DS fetuses under the influence of trisomy 21 have yet to be determined. To investigate the expression characteristic of microRNAs during the development of DS fetuses and identify whether another microRNA gene resides in the Hsa21, Illumina high-throughput sequencing technology was employed to comprehensively characterize the microRNA expression profiles of the DS and normal fetal cord blood mononuclear cells (CBMCs). In total, 149 of 395 identified microRNAs were significantly differentially expressed (fold change >2.0 and P<0.001) and 2 of 181 candidate novel microRNAs were identified as residing within the ̔DS critical region̓ of human chromosome 21 (chr21q22.2‑22.3). Additionally, 7 of 14 Hsa21-derived microRNAs were detected, although not all seven were overexpressed in DS CBMCs compared with the control. Gene ontology enrichment analyses revealed that a set of abnormally expressed microRNAs were involved in the regulation of transcription, gene expression, cellular biosynthetic process and nucleic acid metabolic process. Significantly, most of the mRNA targets in these categories were associated with immune modulation (i.e., SOD1, MXD4, PBX1, BCLAF1 and FOXO1). Findings of the present study provided a considerable insight into understanding the expression characteristic of microRNAs in the DS fetal CBMCs. To the best of our knowledge, this is the first study to examine genome-wide microRNA expression profiles in the DS fetus. Differentially expressed microRNAs may be involved in hemopoietic abnormalities and the immune defects of DS fetuses and newborns.

Complementary regulation of early B-lymphoid differentiation by genetic and epigenetic mechanisms

Although B lymphopoiesis is one of the best-defined paradigms in cell differentiation, our knowledge of the regulatory mechanisms underlying its earliest processes, in which hematopoietic stem cells (HSCs) enter the B lineage, is limited. However, recent methodological advances in sorting progenitor cells and monitoring their epigenetic features have increased our understanding of HSC activities. It is now known that even the highly enriched HSC fraction is heterogeneous in terms of lymphopoietic potential. While surface markers and reporter proteins provide information on the sequential differentiation of B-lineage progenitors, complex interactions between transcription factors have also been shown to play a major role in this process. Epigenetic regulation of histones, nucleosomes, and chromatin appears to play a crucial background role in this elaborate transcription network. In this review, we summarize recent findings on the physiological processes of early B-lineage differentiation, which provides a new paradigm for understanding the harmonious action of genetic and epigenetic mechanisms.

Nck-mediated recruitment of BCAP to the BCR regulates the PI(3)K-Akt pathway in B cells

The adaptor Nck links receptor signaling to cytoskeleton regulation. Here we found that Nck also controlled the phosphatidylinositol-3-OH kinase (PI(3)K)-kinase Akt pathway by recruiting the adaptor BCAP after activation of B cells. Nck bound directly to the B cell antigen receptor (BCR) via the non-immunoreceptor tyrosine-based activation motif (ITAM) phosphorylated tyrosine residue at position 204 in the tail of the immunoglobulin-α component. Genetic ablation of Nck resulted in defective BCR signaling, which led to hampered survival and proliferation of B cells in vivo. Indeed, antibody responses in Nck-deficient mice were also considerably impaired. Thus, we demonstrate a previously unknown adaptor function for Nck in recruiting BCAP to sites of BCR signaling and thereby modulating the PI(3)K-Akt pathway in B cells.

Requirement for Rictor in homeostasis and function of mature B lymphoid cells

The mammalian target of rapamycin (mTOR), an essential serine/threonine kinase, functions in biochemically distinct multiprotein complexes, but little is known about roles of the complexes in B cells. The acutely rapamycin-sensitive mTOR complex 1 (mTORC1) is defined by a core subunit Raptor, whereas mTORC2 lacks Raptor and, instead, has Rictor and SIN1 as distinct essential components. We now show that homeostasis and function of B cells require Rictor. Conditional deletion of Rictor before lymphoid specification impaired generation of mature follicular, marginal zone, and B1a B lymphocytes. Induced inactivation in adult mice caused cell-autonomous defects in B lymphoid homeostasis and antibody responses in vivo, along with affecting plasma cells in bone marrow. Survival of B lymphocytes depended on Rictor, which was vital for normal induction of prosurvival genes, suppression of proapoptotic genes, nuclear factor κB induction after B-cell receptor stimulation, and B-cell activating factor-induced nuclear factor κB2/p52 generation. Collectively, the findings provide evidence that mTOR signaling affects survival and proliferation of mature B lymphocytes, and establish Rictor as an important signal relay in B-cell homeostasis, fate, and functions.

MK5 activates Rag transcription via Foxo1 in developing B cells

The kinase MK5 phosphorylates and activates Foxo1 at serine 215, and this modification is required for Foxo1 to induce Rag transcription.

Foxo1 is a critical, direct regulator of Rag (recombination activating gene) transcription during B cell development and is thus essential for the generation of a diverse repertoire of antigen receptors. Although Foxo1 regulation has been widely studied in many cell types, pathways regulating Foxo1 in B cells have not been fully elucidated. By screening a panel of Foxo1 mutants, we identified serine 215 on Foxo1 as a novel phosphorylation site that is essential for the activation of Rag transcription. Mutation of S215 strongly attenuated transactivation of Rag but did not affect most other Foxo1 target genes. We show that MK5, a MAPK-activated protein kinase, is a previously unidentified upstream regulator of Foxo1. MK5 was necessary and sufficient to activate Rag transcription in transformed and primary pro–B cells. Together, our experiments show that MK5 positively regulates Rag transcription via phosphorylation of Foxo1 in developing B cells.

B cell-specific deficiencies in mTOR limit humoral immune responses

Generation of high-affinity Abs in response to Ags/infectious agents is essential for developing long-lasting immune responses. B cell maturation and Ab responses to Ag stimulation require Ig somatic hypermutation (SHM) and class-switch recombination (CSR) for high-affinity responses. Upon immunization with either the model Ag 4-hydroxy-3-nitrophenylacetyl hapten (NP) conjugated to chicken γ globulin lysine (NP-CGG) or heat-killed Streptococcus pneumoniae capsular type 14 protein (Pn14), knock-in (KI) mice hypomorphic for mTOR function had a decreased ability to form germinal centers, develop high-affinity anti-NP-specific or anti-Pn14-specific Abs, and perform SHM/CSR. Hypomorphic mTOR mice also had a high mortality (40%) compared with wild-type (WT) (0%) littermates and had lower pneumococcal surface protein A-specific Ab titers when immunized and challenged with live S. pneumoniae infection. Mice with mTOR deleted in their B cell lineage (knockout [KO]) also produced fewer splenic germinal centers and decreased high-affinity Ab responses to NP-CGG than did their WT littermates. CSR rates were lower in mTOR KI and KO mice, and pharmacologic inhibition of mTOR in WT B cells resulted in decreased rates of ex vivo CSR. RNA and protein levels of activation-induced cytidine deaminase (AID), a protein essential for SHM and CSR, were lower in B cells from both KI and B cell-specific KO mice, concomitant with increases in phosphorylated AKT and FOXO1. Rescue experiments increasing AID expression in KI B cells restored CSR levels to those in WT B cells. Thus, mTOR plays an important immunoregulatory role in the germinal center, at least partially through AID signaling, in generating high-affinity Abs.

Transcription factor EBF1 is essential for the maintenance of B cell identity and prevention of alternative fates in committed cells

The transcription factors EBF1 and Pax5 have been linked to activation of the B cell lineage program and irreversible loss of alternative lineage potential (commitment), respectively. Here we conditionally deleted Ebf1 in committed pro-B cells after transfer into alymphoid mice. We found that those cells converted into innate lymphoid cells (ILCs) and T cells with variable-diversity-joining (VDJ) rearrangements of loci encoding both B cell and T cell antigen receptors. As intermediates in lineage conversion, Ebf1-deficient CD19(+) cells expressing Pax5 and transcriptional regulators of the ILC and T cell fates were detectable. In particular, genes encoding the transcription factors Id2 and TCF-1 were bound and repressed by EBF1. Thus, both EBF1 and Pax5 are required for B lineage commitment by repressing distinct and common determinants of alternative cell fates.

HDAC7 is a repressor of myeloid genes whose downregulation is required for transdifferentiation of pre-B cells into macrophages

B lymphopoiesis is the result of several cell-commitment, lineage-choice, and differentiation processes. Every differentiation step is characterized by the activation of a new, lineage-specific, genetic program and the extinction of the previous one. To date, the central role of specific transcription factors in positively regulating these distinct differentiation processes to acquire a B cell–specific genetic program is well established. However, the existence of specific transcriptional repressors responsible for the silencing of lineage inappropriate genes remains elusive. Here we addressed the molecular mechanism behind repression of non-lymphoid genes in B cells. We report that the histone deacetylase HDAC7 was highly expressed in pre-B cells but dramatically down-regulated during cellular lineage conversion to macrophages. Microarray analysis demonstrated that HDAC7 re-expression interfered with the acquisition of the gene transcriptional program characteristic of macrophages during cell transdifferentiation; the presence of HDAC7 blocked the induction of key genes for macrophage function, such as immune, inflammatory, and defense response, cellular response to infections, positive regulation of cytokines production, and phagocytosis. Moreover, re-introduction of HDAC7 suppressed crucial functions of macrophages, such as the ability to phagocytose bacteria and to respond to endotoxin by expressing major pro-inflammatory cytokines. To gain insight into the molecular mechanisms mediating HDAC7 repression in pre-B cells, we undertook co-immunoprecipitation and chromatin immunoprecipitation experimental approaches. We found that HDAC7 specifically interacted with the transcription factor MEF2C in pre-B cells and was recruited to MEF2 binding sites located at the promoters of genes critical for macrophage function. Thus, in B cells HDAC7 is a transcriptional repressor of undesirable genes. Our findings uncover a novel role for HDAC7 in maintaining the identity of a particular cell type by silencing lineage-inappropriate genes.

Through the hematopoietic system, all the distinct mature blood cell types are generated, thereby constituting one of the best-studied paradigms for cell lineage commitment and differentiation in biology. B lymphocytes are generated through several cell-commitment, lineage-choice, and differentiation processes. To date, the central role of lineage-specific transcription factors in positively regulating these distinct developmental steps is well established. However, in the absence of proper transcriptional repression, an “adolescent cell” will never be able to reach its “adulthood identity,” having a potential impact in the development of hematological malignancies. In this article, we examined the molecular mechanism responsible for the gene silencing of lineage undesirable genes in B cell precursors and uncovered the role played in this process by the histone deacetylase HDAC7. We show that HDAC7 is expressed in B cell precursors where it interacts with the transcription factor MEF2C and is recruited to the promoters of non-B cell genes. While HDAC7 is down-regulated during the lineage conversion of pre-B cells into macrophages, re-expression of HDAC7 interferes with both the acquisition of the myeloid gene transcriptional program and macrophage-specific cell functions. We therefore have identified a novel lineage-specific transcriptional repressor in the hematopoietic system.

The epigenetic landscape of B lymphocyte tolerance to self

Despite frequent exposures to a variety of potential triggers, including antigens produced by pathogens or commensal microbiota, B-lymphocytes are able to mount highly protective responses to a variety of threats, while remaining tolerant to self-components. A number of cytokines, signaling pathways and transcription factors have been characterized to elucidate the mechanisms underlying B cell tolerance to self. It is, however, unclear how the signals received by B-lymphocytes are converted into complex and sustained patterns of gene expression that can allow production of protective antibodies and maintain immune tolerance to self-components. Mounting evidence now suggests an important role for epigenetic mechanisms in modulating and transmitting signals for B lymphocyte tolerization to self-antigens. It is likely that a better insight into epigenetic regulation of B cell tolerance will lead to development of gene-specific therapeutic approaches that optimize host defense mechanisms to exogenous threats, while preventing development and/or progression of autoimmune inflammatory diseases.

The microRNA miR-181 is a critical cellular metabolic rheostat essential for NKT cell ontogenesis and lymphocyte development and homeostasis

Regulation of metabolic pathways in the immune system provides a mechanism to actively control cellular function, growth, proliferation, and survival. Here, we report that miR-181 is a nonredundant determinant of cellular metabolism and is essential for supporting the biosynthetic demands of early NKT cell development. As a result, miR-181-deficient mice showed a complete absence of mature NKT cells in the thymus and periphery. Mechanistically, miR-181 modulated expression of the phosphatase PTEN to control PI3K signaling, which was a primary stimulus for anabolic metabolism in immune cells. Thus miR-181-deficient mice also showed severe defects in lymphoid development and T cell homeostasis associated with impaired PI3K signaling. These results uncover miR-181 as essential for NKT cell development and establish this family of miRNAs as central regulators of PI3K signaling and global metabolic fitness during development and homeostasis.

PDK1 regulates VDJ recombination, cell-cycle exit and survival during B-cell development

Phosphoinositide-dependent kinase-1 (PDK1) controls the activation of a subset of AGC kinases. Using a conditional knockout of PDK1 in haematopoietic cells, we demonstrate that PDK1 is essential for B cell development. B-cell progenitors lacking PDK1 arrested at the transition of pro-B to pre-B cells, due to a cell autonomous defect. Loss of PDK1 decreased the expression of the IgH chain in pro-B cells due to impaired recombination of the IgH distal variable segments, a process coordinated by the transcription factor Pax5. The expression of Pax5 in pre-B cells was decreased in PDK1 knockouts, which correlated with reduced expression of the Pax5 target genes IRF4, IRF8 and Aiolos. As a result, Ccnd3 is upregulated in PDK1 knockout pre-B cells and they have an impaired ability to undergo cell-cycle arrest, a necessary event for Ig light chain rearrangement. Instead, these cells underwent apoptosis that correlated with diminished expression of the pro-survival gene Bcl2A1. Reintroduction of both Pax5 and Bcl2A1 together into PDK1 knockout pro-B cells restored their ability to differentiate in vitro into mature B cells.

Analysis of FOXO1 mutations in diffuse large B-cell lymphoma

Diffuse large B-cell lymphoma (DLBCL) accounts for 30% to 40% of newly diagnosed lymphomas and has an overall cure rate of approximately 60%. Previously, we observed FOXO1 mutations in non-Hodgkin lymphoma patient samples. To explore the effects of FOXO1 mutations, we assessed FOXO1 status in 279 DLBCL patient samples and 22 DLBCL-derived cell lines. FOXO1 mutations were found in 8.6% (24/279) of DLBCL cases: 92.3% (24/26) of mutations were in the first exon, 46.2% (12/26) were recurrent mutations affecting the N-terminal region, and another 38.5% (10/26) affected the Forkhead DNA binding domain. Recurrent mutations in the N-terminal region resulted in diminished T24 phosphorylation, loss of interaction with 14-3-3, and nuclear retention. FOXO1 mutation was associated with decreased overall survival in patients treated with rituximab, cyclophosphamide, doxorubicin, vincristine, and prednisone (P = .037), independent of cell of origin (COO) and the revised International Prognostic Index (R-IPI). This association was particularly evident (P = .003) in patients in the low-risk R-IPI categories. The independent relationship of mutations in FOXO1 to survival, transcending the prognostic influence of the R-IPI and COO, indicates that FOXO1 mutation is a novel prognostic factor that plays an important role in DLBCL pathogenesis.

The BCL11A transcription factor directly activates RAG gene expression and V(D)J recombination

Recombination-activating gene 1 protein (RAG1) and RAG2 are critical enzymes for initiating variable-diversity-joining (VDJ) segment recombination, an essential process for antigen receptor expression and lymphocyte development. The transcription factor BCL11A is required for B cell development, but its molecular function(s) in B cell fate specification and commitment is unknown. We show here that the major B cell isoform, BCL11A-XL, binds the RAG1 promoter and Erag enhancer to activate RAG1 and RAG2 transcription in pre-B cells. We employed BCL11A overexpression with recombination substrates in a cultured pre-B cell line as well as Cre recombinase-mediated Bcl11a(lox/lox) deletion in explanted murine pre-B cells to demonstrate direct consequences of BCL11A/RAG modulation on V(D)J recombination. We conclude that BCL11A is a critical component of a transcriptional network that regulates B cell fate by controlling V(D)J recombination.

Constitutive Akt1 signals attenuate B-cell receptor signaling and proliferation, but enhance B-cell migration and effector function

Ligation of the BCR induces a complex signaling network that involves activation of Akt, a family of serine/threonine protein kinases associated with B-cell development, proliferation, and tumor formation. Here, we analyzed the effect of enhanced Akt1 signals on B-cell maturation and function. Unexpectedly, we found that peripheral B cells overexpressing Akt1 were less responsive to BCR stimuli. This correlated with a decrease in Ca(2+) -mobilization and proliferation, in an impaired activation of Erk1/2 and mammalian target of rapamycin (mTOR) kinases and poor mobilization of NFATc1 and NF-κB/p65 factors. In contrast, activation of STAT5 and migration of B cells toward the chemokine SDF1α was found to be enhanced. Akt1 Tg mice showed an altered maturation of peritoneal and splenic B1 B cells and an enhanced production of IgG1 and IgG3 upon immunization with the T-cell independent Ag TNP-Ficoll. Furthermore, mice homo-zygous for Tg Akt1 showed a severe block in the maturation of B-cell precursors in BM and a strong enrichment of plasma cells in spleen. Altogether, our data reveal that enhanced Akt1 signals modify BCR signaling strength and, thereby, B-cell development and effector function.

FOXOs: signalling integrators for homeostasis maintenance

Forkhead box O (FOXO) transcription factors are involved in the regulation of the cell cycle, apoptosis and metabolism. In model organisms, FOXO activity also affects stem cell maintenance and lifespan as well as age-related diseases, such as cancer and diabetes. Multiple upstream pathways regulate FOXO activity through post-translational modifications and nuclear-cytoplasmic shuttling of both FOXO and its regulators. The diversity of this upstream regulation and the downstream effects of FOXOs suggest that they function as homeostasis regulators to maintain tissue homeostasis over time and coordinate a response to environmental changes, including growth factor deprivation, metabolic stress (starvation) and oxidative stress.

Signaling by the phosphoinositide 3-kinase family in immune cells

Phosphoinositide 3-kinases (PI3Ks) control many important aspects of immune cell development, differentiation, and function. Mammals have eight PI3K catalytic subunits that are divided into three classes based on similarities in structure and function. Specific roles for the class I PI3Ks have been broadly investigated and are relatively well understood, as is the function of their corresponding phosphatases. More recently, specific roles for the class II and class III PI3Ks have emerged. Through vertebrate evolution and in parallel with the evolution of adaptive immunity, there has been a dramatic increase not only in the genes for PI3K subunits but also in genes for phosphatases that act on 3-phosphoinositides and in 3-phosphoinositide-binding proteins. Our understanding of the PI3Ks in immunity is guided by fundamental discoveries made in simpler model organisms as well as by appreciating new adaptations of this signaling module in mammals in general and in immune cells in particular.