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

A global network of transcription factors, involving E2A, EBF1 and Foxo1, that orchestrates B cell fate

It is now established that the transcription factors E2A, EBF1 and Foxo1 play critical roles in B cell development. Here we show that E2A and EBF1 bound regulatory elements present in the Foxo1 locus. E2A and EBF1 as well as E2A and Foxo1, in turn, were wired together by a vast spectrum of cis-regulatory codes. These associations were dynamic during developmental progression. Occupancy by the E2A isoform, E47, directly elevated the abundance as well as the pattern of histone H3K4 monomethylation across putative enhancer regions. Finally, the pro-B cell epigenome was divided into clusters of loci that show E2A, EBF and Foxo1 occupancy. From this analysis a global network consisting of transcriptional regulators, signaling and survival factors, was constructed that we propose orchestrates the B cell fate.

FOXO1, T-cell trafficking and immune responses

Efficient T-cell adaptive immune response require a faultless coordination between migration of naive T-cells into secondary lymphoid organs and critical biological outcomes driven by antigen such as cell division and cell differentiation into effector and memory cells. Recent works have shown that the phosphoinositide 3-kinase (PI3K) pathway could govern several of these processes. In this control, transcriptional factors of the Forkhead box O (FoxO) family, in particular FOXO1, a downstream effector of PI3K, appears to play a major role by coordinating both cellular proliferation of T-cells after antigen recognition and expression of homing molecules essential for their trafficking in the body.

Foxo proteins cooperatively control the differentiation of Foxp3+ regulatory T cells

CD4(+) regulatory T cells (T(reg) cells) characterized by expression of the transcription factor Foxp3 have a pivotal role in maintaining immunological tolerance. Here we show that mice with T cell-specific deletion of both the Foxo1 and Foxo3 transcription factors (collectively called 'Foxo proteins' here) developed a fatal multifocal inflammatory disorder due in part to T(reg) cell defects. Foxo proteins functioned in a T(reg) cell-intrinsic manner to regulate thymic and transforming growth factor-beta (TGF-beta)-induced Foxp3 expression, in line with the ability of Foxo proteins to bind to Foxp3 locus and control Foxp3 promoter activity. Transcriptome analyses showed that Foxo proteins regulated the expression of additional T(reg) cell-associated genes and were essential for inhibiting the acquisition of effector T cell characteristics by T(reg) cells. Thus, Foxo proteins have crucial roles in specifying the T(reg) cell lineage.

A robust and highly efficient immune cell reprogramming system

Here we describe a lineage reprogramming system consisting of a B cell line with an estradiol-inducible form of C/EBPalpha where cells can be converted into macrophage-like cells at 100% efficiency within 2 to 3 days. The reprogrammed cells are larger, contain altered organelle and cytoskeletal structures, are phagocytic, and exhibit an inflammatory response. Time-lapse experiments showed that the cells acquire a macrophage morphology and increased migratory activity as early as 10 hr. During induction, thousands of genes become up- or downregulated, including several dozen transcription and chromatin-remodeling factors. Time-limited exposure of cells to the inducer showed that the reprogrammed cells become transgene independent within 1 to 2 days. The reprogramming can be inhibited, at least partially, by perturbation experiments with B cell and macrophage transcription factors. The tightness, robustness, and speed of the system described make it a versatile tool to study biochemical and biological aspects of lineage reprogramming.

Foxp1 is an essential transcriptional regulator for the generation of quiescent naive T cells during thymocyte development

Proper thymocyte development is required to establish T-cell central tolerance and to generate naive T cells, both of which are essential for T-cell homeostasis and a functional immune system. Here we demonstrate that the loss of transcription factor Foxp1 results in the abnormal development of T cells. Instead of generating naive T cells, Foxp1-deficient single-positive thymocytes acquire an activated phenotype prematurely in the thymus and lead to the generation of peripheral CD4(+) T and CD8(+) T cells that exhibit an activated phenotype and increased apoptosis and readily produce cytokines upon T-cell receptor engagement. These results identify Foxp1 as an essential transcriptional regulator for thymocyte development and the generation of quiescent naive T cells.

PI3Ks in lymphocyte signaling and development

Lymphocyte development and function are regulated by tyrosine kinase and G-protein coupled receptors. Each of these classes of receptors activates phosphoinositide 3-kinase (PI3K). In this chapter, we summarize current understanding of how PI3K contributes to key aspects of the adaptive immune system.

c-Myb is required for pro-B cell differentiation

The c-Myb transcription factor is required for normal adult hematopoiesis. However, the embryonic lethality of Myb-null mutations has been an impediment to identifying roles for c-Myb during lymphocyte development. We have used tissue-specific inactivation of the Myb locus in early progenitor cells to demonstrate that c-Myb is absolutely required for the differentiation of CD19(+) B-lineage cells and B cell differentiation is profoundly blocked beyond the pre-pro-B cell stage in Myb(f/f) Mb1-cre mice. We demonstrate that c-Myb is required for the intrinsic survival of CD19(+) pro-B cells as well as the proper expression of the alpha-chain of the IL-7 receptor (CD127) and Ebf1. However, survival of c-Myb-deficient CD19(+) pro-B cells cannot be rescued by transduction with CD127-producing retrovirus, suggesting that c-Myb controls a survival pathway independent of CD127. Furthermore, c-Myb-deficient progenitor cells inefficiently generate CD19(+) B-lineage cells during stromal cell culture but this process can be partially rescued with exogenous Ebf1. Thus, c-Myb does not appear to be required for commitment to B cell differentiation but is crucial for B cell differentiation to the CD19(+) pro-B cell stage as well as survival of CD19(+) pro-B cells. Surprisingly, forced c-Myb expression in lymphoid-primed multipotent progenitors favors differentiation toward the myeloid lineage, suggesting that proper c-Myb expression is crucial for B-lineage development.

PI3 kinase signals BCR-dependent mature B cell survival

Previous work has shown that mature B cells depend upon survival signals delivered to the cells by their antigen receptor (BCR). To identify the molecular nature of this survival signal, we have developed a genetic approach in which ablation of the BCR is combined with the activation of specific, BCR dependent signaling cascades in mature B cells in vivo. Using this system, we provide evidence that the survival of BCR deficient mature B cells can be rescued by a single signaling pathway downstream of the BCR, namely PI3K signaling, with the FOXO1 transcription factor playing a central role.

Domain requirements and sequence specificity of DNA binding for the forkhead transcription factor FOXP3

The forkhead, winged-helix transcription factor FOXP3 is preferentially expressed in T regulatory (Treg) cells and is critical for their immunosuppressive function. Mutations that abolish FOXP3 function lead to systemic autoimmunity in mice and humans. However, the manner by which FOXP3 recognizes cognate DNA elements is unclear. Here we identify an in vitro optimized DNA sequence to assess FOXP3 DNA binding by electrophoretic mobility shift assay (EMSA). The optimized sequence contains two tandem copies of a core DNA element resembling, but not identical to, the canonical forkhead (FKH) binding element. The tandem nature of this optimized FOXP3-binding oligonucleotide suggests a requirement for multimerization, and EMSA experiments confirm that both the DNA-binding FKH domain and an intact leucine-zipper domain, which mediates homo-multimerization of FOXP3, are required for DNA binding. These results establish a practical framework for understanding the molecular basis by which FOXP3 regulates gene transcription and programs Treg suppressive function.

Suppression of phosphatidylinositol 3,4,5-trisphosphate production is a key determinant of B cell anergy

Anergy is a critical physiologic mechanism to sensor self-reactive B cells. However, a biochemical understanding of how anergy is achieved and maintained is lacking. Herein, we investigated the role of the phosphoinositide 3-kinase (PI3K) lipid product PI(3,4,5)P(3) in B cell anergy. We found reduced generation of PI(3,4,5)P(3) in anergic B cells, which was attributable to reduced phosphorylation of the PI3K membrane adaptor CD19, as well as increased expression of the inositol phosphatase PTEN. Sustained production of PI(3,4,5)P(3) in B cells, achieved through conditional deletion of Pten, resulted in failed tolerance induction and abundant autoantibody production. In contrast to wild-type immature B cells, B cell receptor engagement of PTEN-deficient immature B cells resulted in activation and proliferation, indicating a central defect in early B cell responsiveness. These findings establish repression of the PI3K signaling pathway as a necessary condition to avert the generation, activation, and persistence of self-reactive B cells.

Forkhead transcription factors in chronic inflammation

Forkhead (Fox) transcription factors have been increasingly recognized to play key roles in immune homeostasis, especially Foxp3 for its role in the development and function of regulatory T cells, and Foxo family members for their regulatory role in T and B lymphocytes as well as other leukocytes. Although these transcription factors positively regulate the expression of multiple target genes, a unique functional attribute of these genes is the maintenance of leukocyte homeostasis, such as the preservation of the naïve or quiescent T cell state and prevention of autoimmunity. As a result, many chronic inflammatory processes appear to reflect a relative loss of activity of one of these transcription factors, raising the possibility that therapeutic approaches which confer gain-of-function Fox activity may be beneficial. On the other hand, however, some of the Fox family members also appear to promote and/or maintain chronic inflammation by preserving inflammatory leukocyte survival and/or otherwise promoting the expression of inflammatory target genes, at least in some cell types such as neutrophils. Therefore, although the role of Fox in inflammatory disorders remains complex and incompletely understood, the continued study of these factors provides new insight into the initiation, maintenance, and propagation of inflammation.

B cell signaling and fate decision

Antigen receptors on the surface of B lymphocytes trigger adaptive immune responses after encountering their cognate antigens but also control a series of antigen-independent checkpoints during B cell development. These physiological processes are regulated by the expression and function of cell surface receptors, intracellular signaling molecules, and transcription factors. The function of these proteins can be altered by a dynamic array of post-translational modifications, using two interconnected mechanisms. These modifications can directly induce an altered conformational state in the protein target of the modification itself. In addition, they can create new binding sites for other protein partners, thereby contributing to where and when such multiple protein assemblies are activated within cells. As a new type of post-transcriptional regulator, microRNAs have emerged to influence the development and function of B cells by affecting the expression of target mRNAs.

FoxO4 inhibits NF-kappaB and protects mice against colonic injury and inflammation

BACKGROUND & AIMS

FoxO4 is a member of the forkhead box transcription factor O (FoxO) subfamily. FoxO proteins are involved in diverse biological processes. In this study, we examine the role of FoxO4 in intestinal mucosal immunity and inflammatory bowel disease (IBD).

METHODS

Foxo4-null mice were subjected to trinitrobenzene sulfonic acid (TNBS) treatment. Microarray analysis and quantitative reverse transcription polymerase chain reaction were used to identify the cytokine transcripts that were altered by Foxo4 deletion. The effects of Foxo4 deficiency on the intestinal epithelial permeability and levels of tight junction proteins were examined by permeable fluorescent dye and Western blot. The molecular and cellular mechanisms by which FoxO4 regulates the mucosal immunity were explored through immunologic and biochemical analyses. The expression level of FoxO4 in intestinal epithelial cells of patients with IBD was examined with immunohistochemistry.

RESULTS

Foxo4-null mice were more susceptible to TNBS injury-induced colitis. The chemokine CCL5 is significantly up-regulated in the colonic epithelial cells of Foxo4-null mice, with increased recruitment of CD4(+) intraepithelial T cells and up-regulation of cytokines interferon-gamma and tumor necrosis factor-alpha in the colon. Foxo4 deficiency also resulted in an increase in intestinal epithelial permeability and down-regulation of the tight junction proteins ZO-1 and claudin-1. Mechanistically, FoxO4 inhibited the transcriptional activity of nuclear factor-kappaB (NF-kappaB), and Foxo4 deficiency is associated with increased NF-kappaB activity in vivo. FoxO4 transcription is transiently repressed in response to TNBS treatment and in patients with IBD.

CONCLUSION

These results indicate that FoxO4 is an endogenous inhibitor of NF-kappaB and identify a novel function of FoxO4 in the regulation of NF-kappaB-mediated mucosal immunity.

The cunning little vixen: Foxo and the cycle of life and death

A screen for increased longevity in Caenorhabditis elegans has identified a transcription factor that programs cells for resistance to oxidative stress, DNA repair and cell cycle control. The mammalian orthologs of this factor are referred to as 'Foxo' for 'Forkhead box', with the second 'o' in the name denoting a subfamily of four members related by sequence. This family of factors is regulated by growth factors, oxidative stress or nutrient deprivation. Thus, it might readily control the inflammatory conflagration associated with infection-driven lymphocyte proliferation. Surprisingly, the first insights into Foxo-mediated immune regulation have instead revealed direct control of highly specialized genes of the adaptive immune system.

Foxo3-/- mice demonstrate reduced numbers of pre-B and recirculating B cells but normal splenic B cell sub-population distribution

B cell antigen receptor (BCR) cross-linking promotes proliferation and survival of mature B cells. Phosphoinositide-3-kinase-mediated down-regulation of pro-apoptotic and anti-mitogenic genes such as the Foxo family of transcription factors is an important component of this process. Previously, we demonstrated that BCR signaling decreases expression of transcripts for Foxo1, Foxo3 and Foxo4. We now show that BCR-induced down-regulation of Foxo3 and Foxo4 mRNA expression occurs via distinct mechanisms from those established for Foxo1. While Foxo1, Foxo3 and Foxo4 bind the same DNA sequence, the differential control of their expression upon B cell activation suggests that they may have unique functions in the B lineage. To begin to address this issue, we evaluated B cell development and function in Foxo3-/- mice. No effect of Foxo3 deficiency was observed with respect to the following parameters in the splenic B cell compartment: sub-population distribution, proliferation, in vitro differentiation and expression of the Foxo target genes cyclin G2 and B cell translocation gene 1. However, Foxo3-/- mice demonstrated increased basal levels of IgG2a, IgG3 and IgA. A significant reduction in pre-B cell numbers was also observed in Foxo3-/- bone marrow. Finally, recirculating B cells in the bone marrow and peripheral blood were decreased in Foxo3-/- mice, perhaps due to lower than normal expression of receptor for sphingosine-1 phosphate, which mediates egress from lymphoid organs. Thus, Foxo3 makes a unique contribution to B cell development, B cell localization and control of Ig levels.

Regulation of B-cell proliferation and differentiation by pre-B-cell receptor signalling

The pre-B-cell receptor (pre-BCR) is expressed following the productive recombination of the immunoglobulin heavy chain gene. Signals through the pre-BCR are required for initiating diverse processes in pre-B cells, including proliferation and recombination of the light chain gene, which eventually lead to the differentiation of pre-B cells to immature B cells. However, the molecular mechanisms by which the pre-BCR promotes these processes remain largely unresolved. Recent findings suggest that forkhead box O (FOXO) transcription factors connect pre-BCR signalling to the activation of the recombination machinery. In this Review, we discuss how FOXO transcription factors are regulated by the pre-BCR to allow the progression of the cell cycle and the recombination of the light chain gene.

Mechanisms controlling expression of the RAG locus during lymphocyte development

Recombination activating genes (RAG)1 and RAG2 are expressed in developing B and T lymphocytes and are required for the rearrangement of antigen receptor genes. In turn, RAG expression is regulated by the products of these assembled immunoglobulin (Ig) and T cell receptor (TCR) genes. Upon successful assembly of Ig genes, the antigen receptor is expressed on the immature B cell surface and tested for autoreactivity leading to either maintenance or inactivation of RAG expression. Successful assembly of TCR genes is followed by surface TCR expression and testing for its ability to interact with self-MHC, which if appropriate leads to the inactivation of RAG expression. Recent studies in B and T lymphocytes demonstrate that the reduction in RAG expression at the immature B and double-positive (DP) T cell stages is mediated through tonic (foreign antigen independent) receptor signaling. In B cells, tonic signaling activates PI(3)K and Akt kinases, which phosphorylate and lead to the cytoplasmic sequestration of FoxO proteins, the key transcriptional activators of RAG expression. In T cells, tonic signaling activates Abl and Erk kinases, leading to the transcriptional inactivation of RAGs.

Foxo1 links homing and survival of naive T cells by regulating L-selectin, CCR7 and interleukin 7 receptor

Foxo transcription factors have a conserved role in the adaptation of cells and organisms to nutrient and growth factor availability. Here we show that Foxo1 has a crucial, nonredundant role in T cells. In naive T cells, Foxo1 controlled the expression of the adhesion molecule L-selectin, the chemokine receptor CCR7 and the transcription factor Klf2, and its deletion was sufficient to alter lymphocyte trafficking. Furthermore, Foxo1 deficiency resulted in a severe defect in interleukin 7 receptor alpha-chain (IL-7Ralpha) expression associated with its ability to bind an Il7r enhancer. Finally, growth factor withdrawal induced a Foxo1-dependent increase in Sell, Klf2 and Il7r expression. These data suggest that Foxo1 regulates the homeostasis and life span of naive T cells by sensing growth factor availability and regulating homing and survival signals.