T cells require Foxo1 to populate the peripheral lymphoid organs

Targeting senescence and GATA4 in age-related cardiovascular disease: a comprehensive approach

The growing prevalence of age-related cardiovascular diseases (CVDs) poses significant health challenges, necessitating the formulation of novel treatment approaches. GATA4, a vital transcription factor identified for modulating cardiovascular biology and cellular senescence, is recognized for its critical involvement in CVD pathogenesis. This review collected relevant studies from PubMed, Google Scholar, and Science Direct using search terms like 'GATA4,' 'cellular senescence,' 'coronary artery diseases,' 'hypertension,' 'heart failure,' 'arrhythmias,' 'congenital heart diseases,' 'cardiomyopathy,' and 'cardiovascular disease.' Additionally, studies investigating the molecular mechanisms underlying GATA4-mediated regulation of GATA4 and senescence in CVDs were analyzed to provide comprehensive insights into this critical aspect of potential treatment targeting. Dysregulation of GATA4 is involved in a variety of CVDs, as demonstrated by both experimental and clinical research, comprising CAD, hypertension, congenital heart diseases, cardiomyopathy, arrhythmias, and cardiac insufficiency. Furthermore, cellular senescence enhances the advancement of age-related CVDs. These observations suggested that therapies targeting GATA4, senescence pathways, or both as necessary may be an effective intervention in CVD progression and prognosis. Addressing age-related CVDs by targeting GATA4 and senescence is a broad mechanism approach. It implies further investigation of the molecular nature of these processes and elaboration of an effective therapeutic strategy. This review highlights the importance of GATA4 and senescence in CVD pathogenesis, emphasizing their potential as therapeutic targets for age-related CVDs.

Regulation of intestinal tissue‑resident memory T cells: a potential target for inflammatory bowel disease

Tissue-resident memory T (TRM) cells are populations which settle down in non-lymphoid tissues instead of returning to secondary lymph organs after the antigen presentation. These cells can provide rapid on-site immune protection as well as long-term tissue damage. It is reported that TRM cells from small intestine and colon exhibited distinctive patterns of cytokine and granzyme expression along with substantial transcriptional and functional heterogeneity. In this review, we focus on the reason why they lodge in intestinal tract, their developmental plasticity of going back to to circulation, as well as their regulators associated with retention, maintenance, exhaustion and metabolism. We also elaborate their role in the inflammatory bowel disease (IBD) and discuss the potential therapeutic strategies targeting TRM cells.

Induction of Thymus Atrophy and Disruption of Thymocyte Development by Fipronil through Dysregulation of IL-7-Associated Genes

The susceptibility of the immune system to immunotoxic chemicals is evident, particularly in the thymus, a vital primary immune organ prone to atrophy due to exposure to toxicants. Fipronil (FPN), a widely used insecticide, is of concern due to its potential neurotoxicity, hepatotoxicity, and immunotoxicity. Our previous study showed that FPN disturbed the antigen-specific T-cell functionality in vivo. As T-cell lineage commitment and thymopoiesis are closely interconnected with the normal function of the T-cell-mediated immune responses, this study aims to further examine the toxic effects of FPN on thymocyte development. In this study, 4-week-old BALB/c mice received seven doses of FPN (1, 5, 10 mg/kg) by gavage. Thymus size, medulla/cortex ratio, total thymocyte counts, double-positive thymocyte population, and IL-7-positive cells decreased dose-dependently. IL-7 aids the differentiation of early T-cell precursors into mature T cells, and several essential genes contribute to the maturation of T cells in the thymus. Foxn1 ensures that the thymic microenvironment is suitable for the maturation of T-cell precursors. Lyl1 is involved in specifying lymphoid cells and maintaining T-cell development in the thymus. The c-Kit/SCF collaboration fosters a supportive thymic milieu to promote the formation of functional T cells. The expression of IL-7, IL-7R, c-Kit, SCF, Foxn1, and Lyl1 genes in the thymus was significantly diminished in FPN-treated groups with the concordance with the reduction of IL-7 signaling proteins (IL-7, IL-7R, c-KIT, SCF, LYL1, FOXO3A, and GABPA), suggesting that the dysregulation of T-cell lineage-related genes may contribute to the thymic atrophy induced by FPN. In addition, FPN disturbed the functionality of thymocytes with an increase of IL-4 and IFN-γ production and a decrease of IL-2 secretion after T-cell mitogen stimulation ex vivo. Collectively, FPN significantly deregulated genes related to T-cell progenitor differentiation, survival, and expansion, potentially leading to impaired thymopoiesis.

KMT2D regulates activation, localization, and integrin expression by T-cells

Individuals with Kabuki syndrome present with immunodeficiency; however, how pathogenic variants in the gene encoding the histone-modifying enzyme lysine methyltransferase 2D (KMT2D) lead to immune alterations remain poorly understood. Following up on our prior report of KMT2D-altered integrin expression in B-cells, we performed targeted analyses of KMT2D's influence on integrin expression in T-cells throughout development (thymocytes through peripheral T-cells) in murine cells with constitutive- and conditional-targeted Kmt2d deletion. Using high-throughput RNA-sequencing and flow cytometry, we reveal decreased expression (both at the transcriptional and translational levels) of a cluster of leukocyte-specific integrins, which perturb aspects of T-cell activation, maturation, adhesion/localization, and effector function. H3K4me3 ChIP-PCR suggests that these evolutionary similar integrins are under direct control of KMT2D. KMT2D loss also alters multiple downstream programming/signaling pathways, including integrin-based localization, which can influence T-cell populations. We further demonstrated that KMT2D deficiency is associated with the accumulation of murine CD8+ single-positive (SP) thymocytes and shifts in both human and murine peripheral T-cell populations, including the reduction of the CD4+ recent thymic emigrant (RTE) population. Together, these data show that the targeted loss of Kmt2d in the T-cell lineage recapitulates several distinct features of Kabuki syndrome-associated immune deficiency and implicates epigenetic mechanisms in the regulation of integrin signaling.

High starch diets attenuate the immune function of Micropterus salmoides immune organs by modulating Keap1/Nrf2 and MAPK signaling pathways

Based on their good physiological functions and physical properties, carbohydrates are widely used in fish feed. However, excessive use of carbohydrates such as starch in fish feed may reduce the immunity of the fish and cause a series of health problems. In order to more clearly clarify the effects of different starch levels in feed on the immune organs of Micropterus salmoides, this study took the immune organs as the entry point and explored it from several perspectives, including differences in enzyme activity in plasma, changes in gene expression in immune organs, and resistance to pathogenic bacteria. The results showed that (1) high starch feed activates inflammatory responses in the spleen and head kidney through the MAPK signaling pathway. This leads to a decrease in the number of lymphocytes and weakens the resistance to pathogens; (2) high starch diet affects the antioxidant capacity of the trunk kidney by regulating the Keap1/Nrf2 pathway; (3) There was a strong correlation between gene expression patterns in the head kidney and lysozyme content in plasma. This implies that the high starch diet may regulate lysozyme production by affecting gene expression in the head kidney and further affect immune function. This study helps to reveal the interaction between starch and the immune system and provide scientific basis for the development of reasonable dietary recommendations and disease prevention.

Get me out of here: Sphingosine 1-phosphate signaling and T cell exit from tissues during an immune response

During an immune response, the duration of T cell residence in lymphoid and non-lymphoid tissues likely affects T cell activation, differentiation, and memory development. The factors that govern T cell transit through inflamed tissues remain incompletely understood, but one important determinant of T cell exit from tissues is sphingosine 1-phosphate (S1P) signaling. In homeostasis, S1P levels are high in blood and lymph compared to lymphoid organs, and lymphocytes follow S1P gradients out of tissues into circulation using varying combinations of five G-protein coupled S1P receptors. During an immune response, both the shape of S1P gradients and the expression of S1P receptors are dynamically regulated. Here we review what is known, and key questions that remain unanswered, about how S1P signaling is regulated in inflammation and in turn how S1P shapes immune responses.

PP2A and Its Inhibitors in Helper T-Cell Differentiation and Autoimmunity

Protein phosphatase 2A (PP2A) is a highly complex heterotrimeric Ser/Thr phosphatase that regulates many cellular processes. The role of PP2A as a tumor suppressor has been extensively studied and reviewed. However, emerging evidence suggests PP2A constrains inflammatory responses and is important in autoimmune and neuroinflammatory diseases. Here, we reviewed the existing literature on the role of PP2A in T-cell differentiation and autoimmunity. We have also discussed the modulation of PP2A activity by endogenous inhibitors and its small-molecule activators as potential therapeutic approaches against autoimmunity.

IL-6/STAT3/Foxo1 Axis as a Target of Lycopene Ameliorates Atrazine-Induced Thymic Mitophagy and Pyroptosis Cross-talk

The intensive adoption of atrazine (ATZ) has been a persistently widespread pollutant in daily life. However, ATZ is still used as an essential herbicide in numerous countries because its toxic...

Zeb1 represses TCR signaling, promotes the proliferation of T cell progenitors and is essential for NK1.1+ T cell development

T cell development proceeds under the influence of a network of transcription factors (TFs). The precise role of Zeb1, a member of this network, remains unclear. Here, we report that Zeb1 expression is induced early during T cell development in CD4-CD8- double-negative (DN) stage 2 (DN2). Zeb1 expression was further increased in the CD4+CD8+ double-positive (DP) stage before decreasing in more mature T cell subsets. We performed an exhaustive characterization of T cells in Cellophane mice that bear Zeb1 hypomorphic mutations. The Zeb1 mutation profoundly affected all thymic subsets, especially DN2 and DP cells. Zeb1 promoted the survival and proliferation of both cell populations in a cell-intrinsic manner. In the periphery of Cellophane mice, the number of conventional T cells was near normal, but invariant NKT cells, NK1.1+ γδ T cells and Ly49+ CD8 T cells were virtually absent. This suggested that Zeb1 regulates the development of unconventional T cell types from DP progenitors. A transcriptomic analysis of WT and Cellophane DP cells revealed that Zeb1 regulated the expression of multiple genes involved in the cell cycle and TCR signaling, which possibly occurred in cooperation with Tcf1 and Heb. Indeed, Cellophane DP cells displayed stronger signaling than WT DP cells upon TCR engagement in terms of the calcium response, phosphorylation events, and expression of early genes. Thus, Zeb1 is a key regulator of the cell cycle and TCR signaling during thymic T cell development. We propose that thymocyte selection is perturbed in Zeb1-mutated mice in a way that does not allow the survival of unconventional T cell subsets.

Phosphoinositide 3-Kinase p110 Delta Differentially Restrains and Directs Naïve Versus Effector CD8+ T Cell Transcriptional Programs

Phosphoinositide 3-kinase p110 delta (PI3K p110δ) is pivotal for CD8+ T cell immune responses. The current study explores PI3K p110δ induction and repression of antigen receptor and cytokine regulated programs to inform how PI3K p110δ directs CD8+ T cell fate. The studies force a revision of the concept that PI3K p110δ controls metabolic pathways in T cells and reveal major differences in PI3K p110δ regulated transcriptional programs between naïve and effector cytotoxic T cells (CTL). These differences include differential control of the expression of cytolytic effector molecules and costimulatory receptors. Key insights from the work include that PI3K p110δ signalling pathways repress expression of the critical inhibitory receptors CTLA4 and SLAMF6 in CTL. Moreover, in both naïve and effector T cells the dominant role for PI3K p110δ is to restrain the production of the chemokines that orchestrate communication between adaptive and innate immune cells. The study provides a comprehensive resource for understanding how PI3K p110δ uses multiple processes mediated by Protein Kinase B/AKT, FOXO1 dependent and independent mechanisms and mitogen-activated protein kinases (MAPK) to direct CD8+ T cell fate.

Pro-Survival Lipid Sphingosine-1-Phosphate Metabolically Programs T Cells to Limit Anti-tumor Activity

Sphingosine 1-phosphate (S1P), a bioactive lysophospholipid generated by sphingosine kinase 1 (SphK1), regulates lymphocyte egress into circulation via S1P receptor 1 (S1PR1) signaling, and it controls the differentiation of regulatory T cells (Tregs) and T helper-17 cells. However, the mechanisms by which receptor-independent SphK1-mediated intracellular S1P levels modulate T cell functionality remains unknown. We show here that SphK1-deficient T cells maintain central memory phenotype and exhibit higher mitochondrial respiration and reduced differentiation to Tregs. Mechanistically, we discovered a direct correlation between SphK1-generated S1P and lipid transcription factor PPARγ (peroxisome proliferator-activated receptor gamma) activity, which in turn regulates lipolysis in T cells. Genetic and pharmacologic inhibition of SphK1 improved metabolic fitness and anti-tumor activity of T cells against murine melanoma. Further, inhibition of SphK1 and PD1 together led to improved control of melanoma. Overall, these data highlight the clinical potential of limiting SphK1/S1P signaling for enhancing anti-tumor-adoptive T cell therapy.

mTOR signaling at the crossroads of environmental signals and T-cell fate decisions

The evolutionarily conserved serine/threonine kinase mTOR (mechanistic target of rapamycin) forms the distinct protein complexes mTORC1 and mTORC2 and integrates signals from the environment to coordinate downstream signaling events and various cellular processes. T cells rely on mTOR activity for their development and to establish their homeostasis and functional fitness. Here, we review recent progress in our understanding of the upstream signaling and downstream targets of mTOR. We also provide an updated overview of the roles of mTOR in T-cell development, homeostasis, activation, and effector-cell fate decisions, as well as its important impacts on the suppressive activity of regulatory T cells. Moreover, we summarize the emerging roles of mTOR in T-cell exhaustion and transdifferentiation. A better understanding of the contribution of mTOR to T-cell fate decisions will ultimately aid in the therapeutic targeting of mTOR in human disease.

PU.1 regulates Ccr7 gene expression by binding to its promoter in naïve CD4+ T cells

C‐C chemokine receptor type 7 (CCR7) is expressed on naïve T cells, B cells, and activated dendritic cells (DCs). We previously demonstrated that the transcription factor PU.1/Spi1 positively regulates the expression of CCR7 in DCs. In the present study, we investigated the role of PU.1 in CCR7 expression in T cells. To confirm whether PU.1 is involved in the expression of CCR7, we conducted a ChIP assay in various T cells purified from splenocytes and thymocytes and found that PU.1 binds to the Ccr7 promoter‐proximal region in spleen naïve CD4⁺ T cells, but not in thymocytes. Small interfering RNA‐mediated PU.1 knockdown resulted in decreased CCR7 expression in spleen naïve CD4⁺ T cells. Compared to naïve CD4⁺ T cells, Spi1 and Ccr7 mRNA levels decreased in Th1 and Th2 cells, in which PU.1 did not bind to the Ccr7 promoter, suggesting that CCR7 expression decreases due to the dissociation of PU.1 from the Ccr7 promoter during the development of effector T cells from naïve T cells. Collectively, we concluded that CCR7 expression level correlates with the binding level of PU.1 to the Ccr7 promoter and PU.1 acts as a transcriptional activator of the Ccr7 gene in naïve CD4⁺ T cells.

A key control point in the T cell response to chronic infection and neoplasia: FOXO1

T cells able to control neoplasia or chronic infections display a signature gene expression profile similar or identical to that of central memory T cells. These cells have qualities of self-renewal and a plasticity that allow them to repeatedly undergo activation (growth, proliferation, and differentiation), followed by quiescence. It is these qualities that define the ability of T cells to establish an equilibrium with chronic infectious agents, and also preserve the ability of T cells to be re-activated (by checkpoint therapy) in response to malignant cancers. Here we describe distinctions between the forms of inhibition mediated by tumors and persistent viruses, we review the properties of T cells associated with long-term immunity, and we identify the transcription factor, FOXO1, as the control point for a program of gene expression that allows CD8⁺ T cells to undergo serial reactivation and self-renewal.

Survival of Naïve T Cells Requires the Expression of Let-7 miRNAs

Maintaining the diversity and constant numbers of naïve T cells throughout the organism's lifetime is necessary for efficient immune responses. Naïve T cell homeostasis, which consists of prolonged survival, occasional proliferation and enforcement of quiescence, is tightly regulated by multiple signaling pathways which are in turn controlled by various transcription factors. However, full understanding of the molecular mechanisms underlying the maintenance of the peripheral T cell pool has not been achieved. In the present study, we demonstrate that T cell-specific deficiency in let-7 miRNAs results in peripheral T cell lymphopenia resembling that of Dicer1 knockout mice. Deletion of let-7 leads to profound T cell apoptosis while overexpression prevents it. We further show that in the absence of let-7, T cells cannot sustain optimal levels of the pro-survival factor Bcl2 in spite of the intact IL-7 signaling, and re-expression of Bcl2 in let-7 deficient T cells completely rescues the survival defect. Thus, we have uncovered a novel let-7-dependent mechanism of post-transcriptional regulation of naïve T cell survival in vivo.

Metabolism in Immune Cell Differentiation and Function

The immune system is a central determinant of organismal health. Functional immune responses require quiescent immune cells to rapidly grow, proliferate, and acquire effector functions when they sense infectious agents or other insults. Specialized metabolic programs are critical regulators of immune responses, and alterations in immune metabolism can cause immunological disorders. There has thus been growing interest in understanding how metabolic processes control immune cell functions under normal and pathophysiological conditions. In this chapter, we summarize how metabolic programs are tuned and what the physiological consequences of metabolic reprogramming are as they relate to immune cell homeostasis, differentiation, and function.

Transcription factors FOXO in the regulation of homeostatic hematopoiesis

PURPOSE OF REVIEW

Work in the past decade has revealed key functions of the evolutionary conserved transcription factors Forkhead box O (FOXO) in the maintenance of homeostatic hematopoiesis. Here the diverse array of FOXO functions in normal and diseased hematopoietic stem and progenitor cells is reviewed and the main findings in the past decade are highlighted. Future work should reveal FOXO-regulated networks whose alterations contribute to hematological disorders.

RECENT FINDINGS

Recent studies have identified unanticipated FOXO functions in hematopoiesis including in hematopoietic stem and progenitor cells (HSPC), erythroid cells, and immune cells. These findings suggest FOXO3 is critical for the regulation of mitochondrial and metabolic processes in hematopoietic stem cells, the balanced lineage determination, the T and B homeostasis, and terminal erythroblast maturation and red blood cell production. In aggregate these findings highlight the context-dependent function of FOXO in hematopoietic cells. Recent findings also question the nature of FOXO's contribution to heme malignancies as well as the mechanisms underlying FOXO's regulation in HSPC.

SUMMARY

FOXO are safeguards of homeostatic hematopoiesis. FOXO networks and their regulators and coactivators in HSPC are greatly complex and less well described. Identifications and characterizations of these FOXO networks in disease are likely to uncover disease-promoting mechanisms.

MicroRNA-31 Reduces the Motility of Proinflammatory T Helper 1 Lymphocytes

Proinflammatory type 1 T helper (Th1) cells are enriched in inflamed tissues and contribute to the maintenance of chronic inflammation in rheumatic diseases. Here we show that the microRNA- (miR-) 31 is upregulated in murine Th1 cells with a history of repeated reactivation and in memory Th cells isolated from the synovial fluid of patients with rheumatic joint disease. Knock-down of miR-31 resulted in the upregulation of genes associated with cytoskeletal rearrangement and motility and induced the expression of target genes involved in T cell activation, chemokine receptor- and integrin-signaling. Accordingly, inhibition of miR-31 resulted in increased migratory activity of repeatedly activated Th1 cells. The transcription factors T-bet and FOXO1 act as positive and negative regulators of T cell receptor (TCR)-mediated miR-31 expression, respectively. Taken together, our data show that a gene regulatory network involving miR-31, T-bet, and FOXO1 controls the migratory behavior of proinflammatory Th1 cells.

CCR2 Signal Facilitates Thymic Egress by Priming Thymocyte Responses to Sphingosine-1-Phosphate

The signal mediated by sphingosine-1-phosphate receptor 1 (S1P1) is essential but seemingly insufficient for thymic export of newly generated T cells. Here, we reported the identification of CCR2 as an additional regulator of this process. CCR2 showed a markedly increased expression in the most mature subset of single-positive (SP) thymocytes. Its deficiency led to a reduction of recent thymic emigrants in the periphery and a simultaneous accumulation of mature SP cells in the thymus. The CCR2 signaling promoted thymic emigration primarily through modulating the chemotactic responses to S1P1 engagement. On the one hand, the chemokinesis induced by CCR2 activation endowed thymocytes with enhanced capacity to respond to S1P-induced migration. On the other hand, CCR2 signaling through Stat3 augmented forkhead box O1 activity, leading to increased expression of S1P1. Taken together, the present study highlights a unique and novel function of CCR2 signaling in the regulation of thymic egress.

Foxo transcription factors in T cell biology and tumor immunity

The evolutionally conserved forkhead box O (Foxo) family of transcription factors is pivotal in the control of nutrient sensing and stress responses. Recent studies have revealed that the Foxo proteins have been rewired to regulate highly specialized T cell activities. Here, we review the latest advances in the understanding of how Foxo transcription factors control T cell biology, including T cell trafficking, naive T cell homeostasis, effector and memory responses, as well as the differentiation and function of regulatory T cells. We also discuss the emerging evidence on Foxo-mediated regulation in antitumor immunity. Future work will further explore how the Foxo-dependent programs in T cells can be exploited for cancer immunotherapy.

CD38-NAD+Axis Regulates Immunotherapeutic Anti-Tumor T Cell Response

Heightened effector function and prolonged persistence, the key attributes of Th1 and Th17 cells, respectively, are key features of potent anti-tumor T cells. Here, we established ex vivo culture conditions to generate hybrid Th1/17 cells, which persisted long-term in vivo while maintaining their effector function. Using transcriptomics and metabolic profiling approaches, we showed that the enhanced anti-tumor property of Th1/17 cells was dependent on the increased NAD⁺-dependent activity of the histone deacetylase Sirt1. Pharmacological or genetic inhibition of Sirt1 activity impaired the anti-tumor potential of Th1/17 cells. Importantly, T cells with reduced surface expression of the NADase CD38 exhibited intrinsically higher NAD⁺, enhanced oxidative phosphorylation, higher glutaminolysis, and altered mitochondrial dynamics that vastly improved tumor control. Lastly, blocking CD38 expression improved tumor control even when using Th0 anti-tumor T cells. Thus, strategies targeting the CD38-NAD⁺ axis could increase the efficacy of anti-tumor adoptive T cell therapy.

FOXOs in cancer immunity: Knowns and unknowns

In the tumor microenvironment (TME), cancer cells, stromal cells, and immune cells, along with their extracellular factors, have profound effects on either promoting or repressing anti-cancer immunity. Accumulating evidence has shown the paradoxical intrinsic role of the Forkhead box O (FOXO) family of transcription factors in cancer, which can act as a tumor repressor while also maintaining cancer stem cells. FOXOs also regulate cancer immunity. FOXOs promote antitumor activity through negatively regulating the expression of immunosuppressive proteins, such as programmed death 1 ligand 1 (PD-L1), and vascular endothelial growth factor (VEGF) in tumor cells or stromal cells, which can shape an immunotolerant state in the TME. FOXOs also intrinsically control the anti-tumor immune response as well as the homeostasis and development of immune cells, including T cells, B cells, natural killer (NK) cells, macrophages, and dendritic cells. As a cancer repressor, reviving the activity of Foxo1 forces tumor-infiltrating activated regulatory T (T_reg) cells to egress from tumor tissues. As a promoter of cancer development, Foxo3 and Foxo1 negatively regulate cytotoxicity of both CD8⁺ T cells and NK cells against tumor cells. In this review, we focus on the complex role of FOXOs in regulating cancer immunity due to the various roles that they play in cancer cells, stromal cells, and immune cells. We also speculate on some possible additional roles of FOXOs in cancer immunity based on findings regarding FOXOs in non-cancer settings, such as infectious disease.

FOXO1 Regulates Bacteria-Induced Neutrophil Activity

Neutrophils play an essential role in the innate immune response to microbial infection and are particularly important in clearing bacterial infection. We investigated the role of the transcription factor FOXO1 in the response of neutrophils to bacterial challenge with Porphyromonas gingivalis in vivo and in vitro. In these experiments, the effect of lineage-specific FOXO1 deletion in LyzM.Cre⁺FOXO1^L/L mice was compared with matched littermate controls. FOXO1 deletion negatively affected several critical aspects of neutrophil function in vivo including mobilization of neutrophils from the bone marrow (BM) to the vasculature, recruitment of neutrophils to sites of bacterial inoculation, and clearance of bacteria. In vitro FOXO1 regulated neutrophil chemotaxis and bacterial killing. Moreover, bacteria-induced expression of CXCR2 and CD11b, which are essential for several aspects of neutrophil function, was dependent on FOXO1 in vivo and in vitro. Furthermore, FOXO1 directly interacted with the promoter regions of CXCR2 and CD11b. Bacteria-induced nuclear localization of FOXO1 was dependent upon toll-like receptor (TLR) 2 and/or TLR4 and was significantly reduced by inhibitors of reactive oxygen species (ROS and nitric oxide synthase) and deacetylases (Sirt1 and histone deacetylases). These studies show for the first time that FOXO1 activation by bacterial challenge is needed to mobilize neutrophils to transit from the BM to peripheral tissues in response to infection as well as for bacterial clearance in vivo. Moreover, FOXO1 regulates neutrophil function that facilitates chemotaxis, phagocytosis, and bacterial killing.

Lack of Ikaros cripples expression of Foxo1 and its targets in naive T cells

Ikaros is a transcription factor that regulates lymphocyte development from the level of the haematopoietic stem cell. Lack of Ikaros reduces the ability of progenitor cells to commit to the T-cell lineage, resulting in reduced numbers of early thymic T-cell progenitors and mature T cells. Mature CD4 T cells that lack Ikaros have defects in proliferation, T helper cell differentiation, cytokine expression and the ability to become anergic. A role for Ikaros in the naive T cell has not yet been identified. The receptors interleukin-7 receptor α (IL-7Rα) and l-selectin are important for ensuring survival and proper homing of naive T cells, respectively. Here we show that lack of Ikaros leads to reduced expression of these receptors in naive T cells, which impacts their ability to home and survive in response to IL-7. We define the mechanism underlying this phenotype as a requirement for Ikaros in maintenance of expression of Foxo1, a transcriptional regulator that is required for their expression. We also demonstrate that CD4 T cells lacking Ikaros are significantly crippled in their ability to become induced regulatory T cells, a phenotype also linked to reduced Foxo1 expression. Finally, we show that restoring Ikaros function to Ikaros-deficient CD4 T cells increases levels of Foxo1 message. Together, these studies define, for the first time, a role for Ikaros in naive T cells and establish it as the first transcriptional regulator required for maintaining levels of Foxo1 gene expression in these cells.

A type 2 cytokine axis for thymus emigration

In the thymus, stromal microenvironments support a developmental program that generates mature T cells ready for thymic exit. The cellular and molecular specialization within thymic stromal cells that enables their regulation of specific stages of thymocyte development is poorly understood. Here, we show the thymic microenvironment expresses the type 2 IL-4R complex and is functionally responsive to its known ligands, IL-4 and IL-13. Absence of IL-4Rα limits thymocyte emigration, leading to an intrathymic accumulation of mature thymocytes within medullary perivascular spaces and reduced numbers of recent thymic emigrants. Thymus transplantation shows this requirement maps to IL-4Rα expression by stromal cells, and we provide evidence that it regulates thymic exit via a process distinct from S1P-mediated migration. Finally, we reveal a cellular mechanism by which IL-4⁺IL-13⁺ invariant NKT cells are necessary for IL-4Rα signaling that regulates thymic exit. Collectively, we define a new axis for thymic emigration involving stimulation of the thymic microenvironment via type 2 cytokines from innate T cells.

Regulators of Tfh Cell Differentiation

The follicular helper T (Tfh) cells help is critical for activation of B cells, antibody class switching, and germinal center (GC) formation. The Tfh cells are characterized by the expression of CXC chemokine receptor 5 (CXCR5), ICOS, programed death 1 (PD-1), B cell lymphoma 6 (BCL-6), and IL-21. They are involved in clearing infections and are adversely linked with autoimmune diseases and also have a role in viral replication as well as clearance. On the one hand, Tfh cells are generated from naive CD4⁺ T cells with sequential steps involving cytokine signaling (IL-21, IL-6, IL-12, activin A), migration, and positioning in the GC by CXCR5, surface receptors (ICOS/ICOSL, signaling lymphocyte activation molecule-associated protein/signaling lymphocyte activation molecule) as well as transcription factor (BCL-6, c-Maf, and signal transducer and activator of transcription 3) signaling and repressor miR155. On the other hand, Tfh generation is negatively regulated at specific steps of Tfh generation by specific cytokine (IL-2, IL-7), surface receptor (PD-1, CTLA-4), transcription factors B lymphocyte maturation protein 1, signal transducer and activator of transcription 5, T-bet, KLF-2 signaling, and repressor miR 146a. Interestingly, miR-17-92 and FOXO1 act as a positive as well as a negative regulator of Tfh differentiation depending on the time of expression and disease specificity. Tfh cells are also generated from the conversion of other effector T cells as exemplified by Th1 cells converting into Tfh during viral infection. The mechanistic details of effector T cells conversion into Tfh are yet to be clear. To manipulate Tfh cells for therapeutic implication and or for effective vaccination strategies, it is important to know positive and negative regulators of Tfh generation. Hence, in this review, we have highlighted and interlinked molecular signaling from cytokines, surface receptors, transcription factors, ubiquitin ligase, and microRNA as positive and negative regulators for Tfh differentiation.

T Cell Adolescence: Maturation Events Beyond Positive Selection

Single-positive thymocytes that successfully complete positive and negative selection must still undergo one final step, generally termed T cell maturation, before they gain functional competency and enter the long-lived T cell pool. Maturation initiates after positive selection in single-positive thymocytes and continues in the periphery in recent thymic emigrants, before these newly produced T cells gain functional competency and are ready to participate in the immune response as peripheral naive T cells. Recent work using genetically altered mice demonstrates that T cell maturation is not a single process, but a series of steps that occur independently and sequentially after positive selection. This review focuses on the changes that occur during T cell maturation, as well as the molecules and pathways that are critical at each step.

An Lck-cre transgene accelerates autoantibody production and lupus development in (NZB × NZW)F1 mice

Lupus is an autoimmune disease characterized by the development of antinuclear autoantibodies and immune complex-mediated tissue damage. T cells in lupus patients appear to undergo apoptosis at an increased rate, and this enhanced T cell apoptosis has been postulated to contribute to lupus pathogenesis by increasing autoantigen load. However, there is no direct evidence to support this hypothesis. In this study, we show that an Lck-cre transgene, which increases T cell apoptosis as a result of T cell-specific expression of cre recombinase, accelerates the development of autoantibodies and nephritis in lupus-prone (NZB × NZW)F1 mice. Although the enhanced T cell apoptosis in Lck-cre transgenic mice resulted in an overall decrease in the relative abundance of splenic CD4(+) and CD8(+) T cells, the proportion of activated CD4(+) T cells was increased and no significant change was observed in the relative abundance of suppressive T cells. We postulate that the Lck-cre transgene promoted lupus by enhancing T cell apoptosis, which, in conjunction with the impaired clearance of apoptotic cells in lupus-prone mice, increased the nuclear antigen load and accelerated the development of anti-nuclear autoantibodies. Furthermore, our results also underscore the importance of including cre-only controls in studies using the cre-lox system.

Lack of association between FOXO1 polymorphisms and bacteremia

Increasing evidence suggests that FOXO1, one critical gene related to the human immune system, probable is closely to the human infection. In the present study we aimed to investigate genetic association of FOXO1 with bacteremia in Han Chinese. 188 patients with bacteremia diagnosed with blood culture and 250 healthy blood donors without signs of infection were studied, two tagging SNPs of FOXO1 (rs9532571, rs3751436) were selected and genotyped using predesigned TaqMan allelic discrimination assays. The results showed that the allele frequency of rs9532571 and rs3751436 in FOXO1 was not associated with an increased risk of bacteremia (P=0.762, OR=1.05, 95% CI 0.77-1.43; P=0.059, OR=1.34, 95% CI 0.99-1.81 respectively), the genotype distribution of these two SNPs was also not significantly different between bacteremia patients and control groups (P=0.9; P=0.16). Haplotypes in this block were not significantly associated with bacteremia risk.

CONCLUSION

the association between FOXO1 genetic polymorphism and bacteremia has not been observed in the study, maybe a larger sample population and more SNPs in the FOXO1 need to reveal the role in bacteremia in the future.

The ubiquitin-specific protease USP8 is critical for the development and homeostasis of T cells

The modification of proteins by ubiquitin has a major role in cells of the immune system and is counteracted by various deubiquitinating enzymes (DUBs) with poorly defined functions. Here we identified the ubiquitin-specific protease USP8 as a regulatory component of the T cell antigen receptor (TCR) signalosome that interacted with the adaptor Gads and the regulatory molecule 14-3-3β. Caspase-dependent processing of USP8 occurred after stimulation of the TCR. T cell-specific deletion of USP8 in mice revealed that USP8 was essential for thymocyte maturation and upregulation of the gene encoding the cytokine receptor IL-7Rα mediated by the transcription factor Foxo1. Mice with T cell-specific USP8 deficiency developed colitis that was promoted by disturbed T cell homeostasis, a predominance of CD8(+) γδ T cells in the intestine and impaired regulatory T cell function. Collectively, our data reveal an unexpected role for USP8 as an immunomodulatory DUB in T cells.

FOXO1 regulates dendritic cell activity through ICAM-1 and CCR7

The transcription factor FOXO1 regulates cell function and is expressed in dendritic cells (DCs). We investigated the role of FOXO1 in activating DCs to stimulate a lymphocyte response to bacteria. We show that bacteria induce FOXO1 nuclear localization through the MAPK pathway and demonstrate that FOXO1 is needed for DC activation of lymphocytes in vivo. This occurs through FOXO1 regulation of DC phagocytosis, chemotaxis, and DC-lymphocyte binding. FOXO1 induces DC activity by regulating ICAM-1 and CCR7. FOXO1 binds to the CCR7 and ICAM-1 promoters, stimulates CCR7 and ICAM-1 transcriptional activity, and regulates their expression. This is functionally important because transfection of DCs from FOXO1-deleted CD11c.Cre(+)FOXO1(L/L) mice with an ICAM-1-expressing plasmid rescues the negative effect of FOXO1 deletion on DC bacterial phagocytosis and chemotaxis. Rescue with both CCR7 and ICAM-1 reverses impaired DC homing to lymph nodes in vivo when FOXO1 is deleted. Moreover, Ab production following injection of bacteria is significantly reduced with lineage-specific FOXO1 ablation. Thus, FOXO1 coordinates upregulation of DC activity through key downstream target genes that are needed for DCs to stimulate T and B lymphocytes and generate an Ab defense to bacteria.

CXCR4 and a cell-extrinsic mechanism control immature B lymphocyte egress from bone marrow

Joao Pereira and colleagues at Yale University show that B cell egress from bone marrow is a passive process, similar to that of red blood cells. Immature B cells that approached bone marrow sinusoids decreased their expression of CXCR4 and rounded up, allowing them to be passively swept away.

Leukocyte residence in lymphoid organs is controlled by a balance between retention and egress-promoting chemoattractants sensed by pertussis toxin (PTX)–sensitive Gαi protein–coupled receptors (GPCRs). Here, we use two-photon intravital microscopy to show that immature B cell retention within bone marrow (BM) was strictly dependent on amoeboid motility mediated by CXCR4 and CXCL12 and by α4β1 integrin–mediated adhesion to VCAM-1. However, B lineage cell egress from BM is independent of PTX-sensitive GPCR signaling. B lineage cells expressing PTX rapidly exited BM even though their motility within BM parenchyma was significantly reduced. Our experiments reveal that when immature B cells are near BM sinusoids their motility is reduced, their morphology is predominantly rounded, and cells reverse transmigrate across sinusoidal endothelium in a largely nonamoeboid manner. Immature B cell egress from BM was dependent on a twofold CXCR4 down-regulation that was antagonized by antigen-induced BCR signaling. This passive mode of cell egress from BM also contributes significantly to the export of other hematopoietic cells, including granulocytes, monocytes, and NK cells, and is reminiscent of erythrocyte egress.

Reduced FOXO1 expression accelerates skin wound healing and attenuates scarring

The forkhead box O (FOXO) family has been extensively investigated in aging and metabolism, but its role in tissue-repair processes remains largely unknown. Herein, we clarify the molecular aspect of the FOXO family in skin wound healing. We demonstrated that Foxo1 and Foxo3a were both up-regulated during murine skin wound healing. Partial knockout of Foxo1 in Foxo1(+/-) mice throughout the body led to accelerated skin wound healing with enhanced keratinocyte migration, reduced granulation tissue formation, and decreased collagen density, accompanied by an attenuated inflammatory response, but we observed no wound phenotype in Foxo3a(-/-) mice. Fibroblast growth factor 2, adiponectin, and notch1 genes were significantly increased at wound sites in Foxo1(+/-) mice, along with markedly altered extracellular signal-regulated kinase 1/2 and AKT phosphorylation. Similarly, transient knockdown of Foxo1 at the wound site by local delivery of antisense oligodeoxynucleotides enhanced skin wound healing. The link between FOXO1 and scarring extends to patients, in particular keloid scars, where we see FOXO1 expression markedly increased in fibroblasts and inflammatory cells within the otherwise normal dermis. This occurs in the immediate vicinity of the keloid by comparison to the center of the mature keloid, indicating that FOXO1 is associated with the overgrowth of this fibrotic response into adjacent normal skin. Overall, our data indicate that molecular targeting of FOXO1 may improve the quality of healing and reduce pathological scarring.

Maturation and emigration of single-positive thymocytes

T lymphopoiesis in the thymus was thought to be completed once it reaches the single positive (SP) stage, a stage when T cells are “fully mature” and waiting to be exported at random or follow a “first-in-first-out” manner. Recent evidence, however, has revealed that the newly generated SP thymocytes undergo a multistage maturation program in the thymic medulla. Such maturation is followed by a tightly regulated emigration process and a further postthymic maturation of recent thymic emigrants (RTEs). This review summarizes recent progress in the late stage T cell development. The regulation of this developmental process is discussed.

The transcription factor Foxo1 controls central-memory CD8+ T cell responses to infection

Memory T cells protect hosts from pathogen reinfection, but how these cells emerge from a pool of antigen-experienced T cells is unclear. Here, we show that mice lacking the transcription factor Foxo1 in activated CD8+ T cells have defective secondary, but not primary, responses to Listeria monocytogenes infection. Compared to short-lived effector T cells, memory-precursor T cells expressed higher amounts of Foxo1, which promoted their generation and maintenance. Chromatin immunoprecipitation sequencing revealed the transcription factor Tcf7 and the chemokine receptor Ccr7 as Foxo1-bound target genes, which have critical functions in central-memory T cell differentiation and trafficking. These findings demonstrate that Foxo1 is selectively incorporated into the genetic program that regulates memory CD8+ T cell responses to infection.

The thymic medulla is required for Foxp3+ regulatory but not conventional CD4+ thymocyte development

A key role of the thymic medulla is to negatively select autoreactive CD4(+) and CD8(+) thymocytes, a process important for T cell tolerance induction. However, the involvement of the thymic medulla in other aspects of αβ T cell development, including the generation of Foxp3(+) natural regulatory T cells (nTreg cells) and the continued maturation of positively selected conventional αβ T cells, is unclear. We show that newly generated conventional CD69(+)Qa2(-) CD4 single-positive thymocytes mature to the late CD69(-)Qa2(+) stage in the absence of RelB-dependent medullary thymic epithelial cells (mTECs). Furthermore, an increasing ability to continue maturation extrathymically is observed within the CD69(+)CCR7(-/lo)CCR9(+) subset of conventional SP4 thymocytes, providing evidence for an independence from medullary support by the earliest stages after positive selection. In contrast, Foxp3(+) nTreg cell development is medullary dependent, with mTECs fostering the generation of Foxp3(-)CD25(+) nTreg cell precursors at the CD69(+)CCR7(+)CCR9(-) stage. Our results demonstrate a differential requirement for the thymic medulla in relation to CD4 conventional and Foxp3(+) thymocyte lineages, in which an intact mTEC compartment is a prerequisite for Foxp3(+) nTreg cell development through the generation of Foxp3(-)CD25(+) nTreg cell precursors.

FOXO transcription factors throughout T cell biology

The outcome of an infection with any given pathogen varies according to the dosage and route of infection, but, in addition, the physiological state of the host can determine the efficacy of clearance, the severity of infection and the extent of immunopathology. Here we propose that the forkhead box O (FOXO) transcription factor family--which is central to the integration of growth factor signalling, oxidative stress and inflammation--provides connections between physical well-being and the form and magnitude of an immune response. We present a case that FOXO transcription factors guide T cell differentiation and function in a context-driven manner, and might provide a link between metabolism and immunity.

FOXO1 Up-Regulates Human L-selectin Expression Through Binding to a Consensus FOXO1 Motif

L-selectin plays important roles in lymphocyte homing and leukocyte rolling. Mounting evidence shows that it is involved in many disease entities including diabetes, ischemia/reperfusion injuries, inflammatory diseases, and tumor metastasis. Regulation of L-selectin at protein level has been well characterized. However, the regulation of human L-selectin transcription remains largely unknown. To address transcriptional regulation of L-selectin, we cloned 1088 bp 5' of the start codon ATG. Luciferase analysis of the serial 5' deletion mutants located the core promoter region at -288/-1. A major transcription initiation site was mapped at -115 by 5'RACE. Transcription factors Sp1, Ets1, Mzf1, Klf2, and Irf1 bind to and transactivate the L-selectin promoter. Significantly, FOXO1 binds to a FOXO1 motif, CCCTTTGG, at -87/-80, and transactivates the L-selectin promoter in a dose-dependent manner. Over-expression of a constitutive-active FOXO1 increased the endogenous L-selectin expression in Jurkat cells. We conclude that FOXO1 regulates L-selectin expression through targeting its promoter.

FoxO-dependent regulation of diacylglycerol kinase α gene expression

Diacylglycerol kinase α (DGKα) regulates diacylglycerol levels, catalyzing its conversion into phosphatidic acid. The α isoform is central to immune response regulation; it downmodulates Ras-dependent pathways and is necessary for establishment of the unresponsive state termed anergy. DGKα functions are regulated in part at the transcriptional level although the mechanisms involved remain poorly understood. Here, we analyzed the 5' end structure of the mouse DGKα gene and detected three binding sites for forkhead box O (FoxO) transcription factors, whose function was confirmed using luciferase reporter constructs. FoxO1 and FoxO3 bound to the 5' regulatory region of DGKα in quiescent T cells, as well as after interleukin-2 (IL-2) withdrawal in activated T cells. FoxO binding to this region was lost after complete T cell activation or IL-2 addition, events that correlated with FoxO phosphorylation and a sustained decrease in DGKα gene expression. These data strongly support a role for FoxO proteins in promoting high DGKα levels and indicate a mechanism by which DGKα function is downregulated during productive T cell responses. Our study establishes a basis for a causal relationship between DGKα downregulation, IL-2, and anergy avoidance.

IL-7 signaling and CD127 receptor regulation in the control of T cell homeostasis

After their development in the thymus, mature T cells are maintained in the periphery by two sets of survival signals, namely TCR signals from contact with self-peptide/MHC ligands and the cytokine receptor signals from binding IL-7 and IL-15. These signals cooperate to maximize the utility of finite resources to support a diverse pool of mature T cells. It is becoming increasingly clear that multiple mechanisms exist to regulate expression of IL-7R at the transcriptional and post-translational levels. The interplay between TCR signals and IL-7R signals are also important in regulation of IL-7R expression. This review will focus on regulation of T cell homeostasis by IL-7R signaling, with an emphasis on the cross talk between signals from TCR and IL-7R.

Transcription factor Foxo1 represses T-bet-mediated effector functions and promotes memory CD8(+) T cell differentiation

The evolutionary conserved Foxo transcription factors are important regulators of quiescence and longevity. Although, Foxo1 is known to be important in regulating CD8(+) T cell trafficking and homeostasis, its role in functional differentiation of antigen-stimulated CD8(+) T cells is unclear. Herein, we demonstrate that inactivation of Foxo1 was essential for instructing T-bet transcription factor-mediated effector differentiation of CD8(+) T cells. The Foxo1 inactivation was dependent on mTORC1 kinase, given that blockade of mTORC1 abrogated mTORC2-mediated Akt (Ser473) kinase phosphorylation, resulting in Foxo1-dependent switch from T-bet to Eomesodermin transcription factor activation and increase in memory precursors. Silencing Foxo1 ablated interleukin-12- and rapamycin-enhanced CD8(+) T cell memory responses and restored T-bet-mediated effector functions. These results demonstrate an essential role of Foxo1 in actively repressing effector or terminal differentiation processes to promote memory CD8(+) T cell development and identify the functionally diverse mechanisms utilized by Foxo1 to promote quiescence and longevity.

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.

CD8 T cell quiescence revisited

Naïve T cells are typically considered to be in a default state of quiescence, whereas memory T cells undergo basal proliferation and quickly exhibit effector responses when stimulated. Over the past few years, however, a more complex picture has emerged, with evidence that naïve T cell quiescence is actively enforced, and that heterogeneity among naïve T cells influences their capacity to escape quiescence in response to homeostatic cues. Furthermore, the active state of memory T cells may also be instructed, requiring contact with dendritic cells to avoid reversion to quiescence. Here, we discuss these new findings and propose that there is much more flexibility in the quiescent state of naïve and memory T cells than previously thought.

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.

The molecular signature underlying the thymic migration and maturation of TCRαβ+ CD4+ CD8 thymocytes

BACKGROUND

After positive selection, the newly generated single positive (SP) thymocytes migrate to the thymic medulla, where they undergo negative selection to eliminate autoreactive T cells and functional maturation to acquire immune competence and egress capability.

METHODOLOGY/PRINCIPAL FINDINGS

To elucidate the genetic program underlying this process, we analyzed changes in gene expression in four subsets of mouse TCRαβ(+)CD4(+)CD8(-) thymocytes (SP1 to SP4) representative of sequential stages in a previously defined differentiation program. A genetic signature of the migration of thymocytes was thus revealed. CCR7 and PlexinD1 are believed to be important for the medullary positioning of SP thymocytes. Intriguingly, their expression remains at low levels in the newly generated thymocytes, suggesting that the cortex-medulla migration may not occur until the SP2 stage. SP2 and SP3 cells gradually up-regulate transcripts involved in T cell functions and the Foxo1-KLF2-S1P(1) axis, but a number of immune function-associated genes are not highly expressed until cells reach the SP4 stage. Consistent with their critical role in thymic emigration, the expression of S1P(1) and CD62L are much enhanced in SP4 cells.

CONCLUSIONS

These results support at the molecular level that single positive thymocytes undergo a differentiation program and further demonstrate that SP4 is the stage at which thymocytes acquire the immunocompetence and the capability of emigration from the thymus.

Normal T cell homeostasis: the conversion of naive cells into memory-phenotype cells

Weak T cell antigen receptor (TCR) signals from contact with self ligands act in synergy with antiapoptotic signals induced by interleukin 7 (IL-7) to promote the survival of naive T cells in a resting state. The amount of background TCR signaling in naive T cells is set by post-thymic TCR tuning and operates at an intensity just below that required to induce entry into the cell cycle. Costimulation from higher concentrations of IL-7 and other common γ-chain cytokines can induce T cells to undergo homeostatic proliferation and conversion into cells with a memory phenotype; many of these memory phenotype cells may be the progeny of cells responding to self antigens. The molecular mechanisms that control the conversion of naive resting T cells into memory-phenotype cells TCR-dependent in normal animals are beginning to be understood.

Signal integration and crosstalk during thymocyte migration and emigration

The thymus produces self-tolerant functionally competent T cells. This process involves the import of multipotent haematopoietic progenitors that are then signalled to adopt the T cell fate. Expression of T cell-specific genes, including those encoding the T cell receptor (TCR), is followed by positive and negative selection and the eventual export of mature T cells. Significant progress has been made in elucidating the signals that direct progenitor cell trafficking to, within and out of the thymus. These advances are the subject of this Review, with a particular focus on the role of reciprocal cooperative and regulatory interactions between TCR- and chemokine receptor-mediated signalling.

Foxo transcription factors control regulatory T cell development and function

Foxo transcription factors integrate extrinsic signals to regulate cell division, differentiation and survival, and specific functions of lymphoid and myeloid cells. Here, we showed the absence of Foxo1 severely curtailed the development of Foxp3(+) regulatory T (Treg) cells and those that developed were nonfunctional in vivo. The loss of function included diminished CTLA-4 receptor expression as the Ctla4 gene was a direct target of Foxo1. T cell-specific loss of Foxo1 resulted in exocrine pancreatitis, hind limb paralysis, multiorgan lymphocyte infiltration, anti-nuclear antibodies and expanded germinal centers. Foxo-mediated control over Treg cell specification was further revealed by the inability of TGF-β cytokine to suppress T-bet transcription factor in the absence of Foxo1, resulting in IFN-γ secretion. In addition, the absence of Foxo3 exacerbated the effects of the loss of Foxo1. Thus, Foxo transcription factors guide the contingencies of T cell differentiation and the specific functions of effector cell populations.

Highly specialized role of Forkhead box O transcription factors in the immune system

Recent studies have highlighted a fundamental role for Forkhead box O (Foxo) transcription factors in immune system homeostasis. Initial reports designed to dissect function of individual Foxo isoforms in the immune system were based on in vitro overexpression systems, and these experiments suggested that Foxo1 and Foxo3 are important for growth factor withdrawal-induced cell death. Moreover, Foxo factors importantly regulate basic cell cycle progression, and so the implication was that these factors may control lymphocyte homeostasis, including a critical function in the termination and resolution of an immune response. Most recently, cell-type-specific loss mutants for the different Foxo isoforms have revealed unexpected and highly specialized functions in the control of multiple cell types in the immune system, but they have yet to reveal a role in cell death or proliferation. This review will focus on the recent advances made in the understanding of the many ways that Foxo factors regulate the immune system, including a discussion of how the specialized versus redundant functions of Foxo transcription factors impact immune system homeostasis.