Interleukin 7 receptor-deficient mice lack gammadelta T cells

T-Cell-Specific Deletion of Map3k1 Reveals the Critical Role for Mekk1 and Jnks in Cdkn1b-Dependent Proliferative Expansion

MAPK signaling is important for T lymphocyte development, homeostasis, and effector responses. To better understand the role of Mekk1 (encoded by Map3k1) in T cells, we conditionally deleted Map3k1 in Lck^Cre/+Map3k1^f/f mice, and these display larger iNKT cell populations within the liver, spleen, and bone marrow. Mekk1 signaling controls splenic and liver iNKT cell expansion in response to glycolipid antigen. Lck^Cre/+Map3k1^f/f mice have enhanced liver damage in response to glycolipid antigen. Mekk1 regulates Jnk activation in iNKT cells and binds and transfers Lys63-linked poly-ubiquitin onto Carma1. Map3k1 is critical for the regulation of p27^Kip1 (encoded by Cdkn1b).

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iNKT cell expansion is aberrant in Lck^Cre/+Map3k1^f/f mice

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Lck^Cre/+Map3k1^f/f mice have enhanced liver damage in response to glycolipids

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Mekk1 regulates TCR-dependent Jnk activation

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Mekk1 regulates p27^Kip1 expression to regulate proliferation

Differential Responsiveness of Innate-like IL-17- and IFN-γ-Producing γδ T Cells to Homeostatic Cytokines

γδ T cells respond to molecules upregulated following infection or cellular stress using both TCR and non-TCR molecules. The importance of innate signals versus TCR ligation varies greatly. Both innate-like IL-17-producing γδ T (γδT-17) and IFN-γ-producing γδ T (γδT-IFNγ) subsets tune the sensitivity of their TCR following thymic development, allowing robust responses to inflammatory cytokines in the periphery. The remaining conventional γδ T cells retain high TCR responsiveness. We determined homeostatic mechanisms that govern these various subsets in the peripheral lymphoid tissues. We found that, although innate-like γδT-17 and γδT-IFNγ cells share elements of thymic development, they diverge when it comes to homeostasis. Both exhibit acute sensitivity to cytokines compared with conventional γδ T cells, but they do not monopolize the same cytokine. γδT-17 cells rely exclusively on IL-7 for turnover and survival, aligning them with NKT17 cells; IL-7 ligation triggers proliferation, as well as promotes survival, upregulating Bcl-2 and Bcl-xL. γδT-IFNγ cells instead depend heavily on IL-15. They display traits analogous to memory CD8(+) T cells and upregulate Bcl-xL and Mcl-1 upon cytokine stimulation. The conventional γδ T cells display low sensitivity to cytokine-alone stimulation and favor IL-7 for their turnover, characteristics reminiscent of naive αβ T cells, suggesting that they may also require tonic TCR signaling for population maintenance. These survival constraints suggest that γδ T cell subsets do not directly compete with each other for cytokines, but instead fall into resource niches with other functionally similar lymphocytes.

STAT5 Orchestrates Local Epigenetic Changes for Chromatin Accessibility and Rearrangements by Direct Binding to the TCRγ Locus

The transcription factor STAT5, which is activated by IL-7R, controls chromatin accessibility and rearrangements of the TCRγ locus. Although STAT-binding motifs are conserved in Jγ promoters and Eγ enhancers, little is known about their precise roles in rearrangements of the TCRγ locus in vivo. To address this question, we established two lines of Jγ1 promoter mutant mice: one harboring a deletion in the Jγ1 promoter, including three STAT motifs (Jγ1P(Δ/Δ)), and the other carrying point mutations in the three STAT motifs in that promoter (Jγ1P(mS/mS)). Both Jγ1P(Δ/Δ) and Jγ1P(mS/mS) mice showed impaired recruitment of STAT5 and chromatin remodeling factor BRG1 at the Jγ1 gene segment. This resulted in severe and specific reduction in germline transcription, histone H3 acetylation, and histone H4 lysine 4 methylation of the Jγ1 gene segment in adult thymus. Rearrangement and DNA cleavage of the segment were severely diminished, and Jγ1 promoter mutant mice showed profoundly decreased numbers of γδ T cells of γ1 cluster origin. Finally, compared with controls, both mutant mice showed a severe reduction in rearrangements of the Jγ1 gene segment, perturbed development of γδ T cells of γ1 cluster origin in fetal thymus, and fewer Vγ3(+) dendritic epidermal T cells. Furthermore, interaction with the Jγ1 promoter and Eγ1, a TCRγ enhancer, was dependent on STAT motifs in the Jγ1 promoter. Overall, this study strongly suggests that direct binding of STAT5 to STAT motifs in the Jγ promoter is essential for local chromatin accessibility and Jγ/Eγ chromatin interaction, triggering rearrangements of the TCRγ locus.

IL-18 acts in synergy with IL-7 to promote ex vivo expansion of T lymphoid progenitor cells

Although IL-18 has not previously been shown to promote T lymphopoiesis, results obtained via a novel data mining algorithm (global microarray meta-analysis) led us to explore a predicted role for this cytokine in T cell development. IL-18 is a member of the IL-1 cytokine family that has been extensively characterized as a mediator of inflammatory immune responses. To assess a potential role for IL-18 in T cell development, we sort-purified mouse bone marrow-derived common lymphoid progenitor cells, early thymic progenitors (ETPs), and double-negative 2 thymocytes and cultured these populations on OP9-Delta-like 4 stromal layers in the presence or absence of IL-18 and/or IL-7. After 1 wk of culture, IL-18 promoted proliferation and accelerated differentiation of ETPs to the double-negative 3 stage, similar in efficiency to IL-7. IL-18 showed synergy with IL-7 and enhanced proliferation of both the thymus-derived progenitor cells and the bone marrow-derived common lymphoid progenitor cells. The synergistic effect on the ETP population was further characterized and found to correlate with increased surface expression of c-Kit and IL-7 receptors on the IL-18-treated cells. In summary, we successfully validated the global microarray meta-analysis prediction that IL-18 affects T lymphopoiesis and demonstrated that IL-18 can positively impact bone marrow lymphopoiesis and T cell development, presumably via interaction with the c-Kit and IL-7 signaling axis.

Role of the intestinal cytokine microenvironment in shaping the intraepithelial lymphocyte repertoire

Local resident IELs are composed of distinct subsets of T cells with potent cytolytic and immunoregulatory capacities. As IELs are located within this unique interface between the core of the body and the outside environment, the specific development and function of intestinal IELs must be tightly regulated. To accomplish this, the cytokine microenvironment of the intestine has evolved sophisticated mechanisms that modulate the phenotype, ontogeny, and function of these cells. In this review, we summarize the evidence demonstrating the origin of certain intestinal cytokines, including IL-7, IL-15, IL-2, TGF-β, and SCF and discuss what influence such cytokines may have on IELs. Moreover, we review data suggesting that the abnormal expression of cytokines that leads to the heightened activation of IELs may also contribute to immunopathological responses or exacerbate inflammatory diseases, such as IBD and celiac disease, or promote cancer development and progression.

Mixing Signals: Molecular Turn Ons and Turn Offs for Innate γδ T-Cells

Lymphocytes of the gamma delta (γδ) T-cell lineage are evolutionary conserved and although they express rearranged antigen-specific receptors, a large proportion respond as innate effectors. γδ T-cells are poised to combat infection by responding rapidly to cytokine stimuli similar to innate lymphoid cells. This potential to initiate strong inflammatory responses necessitates that inhibitory signals are balanced with activation signals. Here, we discuss some of the key mechanisms that regulate the development, activation, and inhibition of innate γδ T-cells in light of recent evidence that the inhibitory immunoglobulin-superfamily member B and T lymphocyte attenuator restricts their differentiation and effector function.

CD8αα⁺ innate-type lymphocytes in the intestinal epithelium mediate mucosal immunity

Innate immune responses are critical for mucosal immunity. Here we describe an innate lymphocyte population, iCD8α cells, characterized by expression of CD8α homodimers. iCD8α cells exhibit innate functional characteristics such as the capacity to engulf and kill bacteria. Development of iCD8α cells depends on expression of interleukin-2 receptor γ chain (IL-2Rγc), IL-15, and the major histocompatibility complex (MHC) class Ib protein H2-T3, also known as the thymus leukemia antigen or TL. While lineage tracking experiments indicated that iCD8α cells have a lymphoid origin, their development was independent of the transcriptional suppressor Id2, suggesting that these cells do not belong to the family of innate lymphoid cells. Finally, we identified cells with a similar phenotype in humans, which were profoundly depleted in newborns with necrotizing enterocolitis. These findings suggest a critical role of iCD8α cells in immune responses associated with the intestinal epithelium.

The combined loss of Gads and CD127 reveals a novel function of Gads prior to TCRβ expression

The Gads adaptor protein is an essential component of the T cell signaling complex critical for T cell receptor-mediated calcium mobilization. After expression of TCRβ in T cell precursors, Gads is required for optimal Bcl-2 expression and cell survival. Similarly, the IL-7 receptor chain CD127 is also necessary for optimal Bcl-2 expression and cell survival in TCRβ-expressing thymocytes. Based on these observations, we tested whether Gads and CD127 might regulate convergent or linear signaling pathways by crossing Gads(-/-) mice with CD127(-/-) mice. Thymi from Gads(-/-)CD127(-/-) mice were barely detectable and many of the thymocytes were within the DN1 population. By contrast, B cell development in the Gads(-/-)CD127(-/-) mice was comparable to that of CD127(-/-) mice, indicating that the combined loss of Gads and CD127 did not lead to a global deficit in hematopoiesis. Analysis of Lin(-)Sca-1(+)c-kit(+) bone marrow cells and bone marrow chimera experiments indicated that Gads(-/-)CD127(-/-) T cell precursors either failed to migrate into the thymus or survive in the thymus. These data demonstrate that Gads functions at a stage of T cell development that had not been previously described.

Complexity of cytokine network regulation of innate lymphoid cells in protective immunity

The body's surface provides a critical barrier shielding us from various mechanical and pathogenic insults by virtue of the physical protection it provides and the presence of specialized populations of innate lymphoid cells (ILCs) that sense inflammatory signals induced by pathogens. This response plays a central role in the development and activation of early immune responses. While ILCs depend on common γ-chain cytokine signaling for their development, an essential component of the armory of these cells is their capacity to produce defensive cytokines when activated by viruses, microbes and other parasites. In this review, we describe the multiple intrinsic and extrinsic pathways that comprise the cytokine circuitry regulating the development and function of ILC necessary for protective immunity.

The inhibitory receptor BTLA controls γδ T cell homeostasis and inflammatory responses

γδ T cells rapidly secrete inflammatory cytokines at barrier sites that aid in protection from pathogens, but mechanisms limiting inflammatory damage remain unclear. We found that retinoid-related orphan receptor gamma-t (RORγt) and interleukin-7 (IL-7) influence γδ T cell homeostasis and function by regulating expression of the inhibitory receptor, B and T lymphocyte attenuator (BTLA). The transcription factor RORγt, via its activating function-2 domain, repressed Btla transcription, whereas IL-7 increased BTLA levels on the cell surface. BTLA expression limited γδ T cell numbers and sustained normal γδ T cell subset frequencies by restricting IL-7 responsiveness and expansion of the CD27(-)RORγt(+) population. BTLA also negatively regulated IL-17 and TNF production in CD27(-) γδ T cells. Consequently, BTLA-deficient mice exhibit enhanced disease in a γδ T cell-dependent model of dermatitis, whereas BTLA agonism reduced inflammation. Therefore, by coordinating expression of BTLA, RORγt and IL-7 balance suppressive and activation stimuli to regulate γδ T cell homeostasis and inflammatory responses.

Origin and evolution of adaptive immunity

The evolutionary emergence of vertebrates was accompanied by major morphological and functional innovations, including the development of an adaptive immune system. Vertebrate adaptive immunity is based on the clonal expression of somatically diversifying antigen receptors on lymphocytes. This is a common feature of both the jawless and jawed vertebrates , although these two groups of extant vertebrates employ structurally different types of antigen receptors and principal mechanisms for their somatic diversification . These observations suggest that the common vertebrate ancestor must have already possessed a complex immune system, including B- and T-like lymphocyte lineages and primary lymphoid organs, such as the thymus, but possibly lacked the facilities for somatic diversification of antigen receptors. Interestingly, memory formation, previously considered to be a defining feature of adaptive immunity, also occurs in the context of innate immune responses and can even be observed in unicellular organisms, attesting to the convergent evolutionary history of distinct aspects of adaptive immunity.

Clinical strategies to enhance thymic recovery after allogeneic hematopoietic stem cell transplantation

The thymus is particularly sensitive to injury caused by cytoreductive chemo- or radiation therapy, shock, infection and graft versus host disease. Insufficient thymic recovery has been directly correlated with increased risk of opportunistic infections and poor clinical outcomes in recipients of allogeneic hematopoietic stem cell transplantation (allo-HSCT). Prolonged immune deficiency is particularly pronounced in older patients whose thymi are already significantly impaired due to age-related thymic involution. Preclinical and clinical studies have revealed several strategies that can enhance thymic function and immune reconstitution after transplant, including sex steroid ablation, growth factors (growth hormone, keratinocyte growth factor, insulin-like growth factor 1, interleukin-7) and ex vivo generated precursor T cells. In addition, recent studies have shown that other approaches, such as interleukein-22 and nutritional changes, may represent additional candidates to enhance thymic regeneration. In this review we provide updates on these strategies and comment on their potential to be translated into clinical therapies.

In vitro induction of regulatory CD4+CD8α+ T cells by TGF-β, IL-7 and IFN-γ

In vitro CD4(+) T cell differentiation systems have made important contributions to understanding the mechanisms underlying the differentiation of naive CD4(+) T cells into effector cells with distinct biological functions. Mature CD4(+) T cells expressing CD8αα homodimers are primarily found in the intestinal mucosa of men and mice, and to a lesser extent in other tissues such as peripheral blood. Although CD4(+)CD8α(+) T cells are easily identified, very little is known about their development and immunological functions. It has been reported, however, that CD4(+)CD8α(+) T cells possess regulatory properties. In this report, we present a novel in vitro differentiation system where CD4(+) T cells are stimulated to become CD4(+)CD8α(+) T cells in the presence of TGF-β, IL-7 and IFN-γ, resulting in cells with very similar features as CD4(+)CD8α(+) intraepithelial lymphocytes. This novel in vitro differentiation culture should provide a powerful and tractable tool for dissecting the differentiation and biological functions of CD4(+)CD8α(+) T cells.

Progress in multiple sclerosis genetics

A genetic component in the susceptibility to multiple sclerosis (MS) has long been known, and the first and major genetic risk factor, the HLA region, was identified in the 1970’s. However, only with the advent of genome-wide association studies in the past five years did the list of risk factors for MS grow from 1 to over 50. In this review, we summarize the search for MS risk genes and the latest results. Comparison with data from other autoimmune and neurological diseases and from animal models indicates parallels and differences between diseases. We discuss how these translate into an improved understanding of disease mechanisms, and address current challenges such as genotype-phenotype correlations, functional mechanisms of risk variants and the missing heritability.

IL-7 produced by thymic epithelial cells plays a major role in the development of thymocytes and TCRγδ+ intraepithelial lymphocytes

IL-7 is a cytokine essential for T cell development and survival. However, the local function of IL-7 produced by thymic epithelial cells (TECs) is poorly understood. To address this question, we generated IL-7-floxed mice and crossed them with FoxN1 promoter-driven Cre (FoxN1-Cre) mice to establish knockout mice conditionally deficient for the expression of IL-7 by TECs. We found that αβ and γδ T cells were significantly reduced in the thymus of IL-7(f/f) FoxN1-Cre mice. Proportion of mature single-positive thymocytes was increased. In lymph nodes and the spleen, the numbers of T cells were partially restored in IL-7(f/f) FoxN1-Cre mice. In addition, γδ T cells were absent from the fetal thymus and epidermis of IL-7(f/f) FoxN1-Cre mice. Furthermore, TCRγδ(+) intraepithelial lymphocytes (IELs) were significantly decreased in the small intestines of IL-7(f/f) FoxN1-Cre mice. To evaluate the function of IL-7 produced in the intestine, we crossed the IL-7(f/f) mice with villin promoter-driven Cre (Vil-Cre) mice to obtain the mice deficient in IL-7 production from intestinal epithelial cells. We observed that αβ and γδ IELs of IL-7(f/f) Vil-Cre mice were comparable to control mice. Collectively, our results suggest that TEC-derived IL-7 plays a major role in proliferation, survival, and maturation of thymocytes and is indispensable for γδ T cell development. This study also demonstrates that IL-7 produced in the thymus is essential for the development of γδ IELs and indicates the thymic origin of γδ IELs.

Interleukin-7 supports survival of T-cell receptor-β-expressing CD4(-) CD8(-) double-negative thymocytes

Among the milestones that occur during T-cell development in the thymus is the expression of T-cell receptor-β (TCR-β) and the formation of the pre-TCR complex. Signals emanating from the pre-TCR trigger survival, proliferation and differentiation of T-cell precursors. Although the pre-TCR is essential for these cell outcomes, other receptors, such as Notch and CXCR4, also contribute. Whether interleukin-7 (IL-7) participates in promoting the survival or proliferation of pre-TCR-expressing cells is controversial. We used in vitro and in vivo models of T-cell development to examine the function of IL-7 in TCR-β-expressing thymocytes. Culturing TCR-β-expressing CD4(-) CD8(-) double-negative thymocytes in an in vitro model of T-cell development revealed that IL-7 reduced the frequency of CD4(+) CD8(+) double-positive thymocytes at the time of harvest. The mechanism for this change in the percentage of double-positive cells was that IL-7 promoted the survival of thymocytes that had not yet differentiated. By preserving the double-negative population, IL-7 reduced the frequency of double-positive thymocytes. Interleukin-7 was not required for proliferation in the in vitro system. To follow this observation, we examined mice lacking CD127 (IL-7Rα). In addition to the known effect of CD127 deficiency on T-cell development before TCR-β expression, CD127 deficiency also impaired the development of TCR-β-expressing double-negative thymocytes. Specifically, we found that Bcl-2 expression and cell cycle progression were reduced in TCR-β-expressing double-negative thymocytes in mice lacking CD127. We conclude that IL-7 continues to function after TCR-β is expressed by promoting the survival of TCR-β-expressing double-negative thymocytes.

Glutamine modulates CD8αα(+) TCRαβ(+) intestinal intraepithelial lymphocyte expression in mice with polymicrobial sepsis

OBJECTIVES

CD8αα(+) T-cell receptor (TCR) αβ(+) intestinal intraepithelial lymphocytes (IELs) were found to have a regulatory function in the mucosal immune system. Glutamine (GLN) is an amino acid with immunomodulatory effects. The aim of this study was to investigate the influences of GLN on the proportion of CD8αα(+) TCRαβ(+) IELs and associated inflammatory mediator gene expression in polymicrobial sepsis.

METHODS

Mice were randomly assigned to a normal (NC) group, a sepsis with saline (SS) group, or a sepsis with GLN (SG) group. The NC group was fed a chow diet. Sepsis was induced by cecal ligation and puncture (CLP). The SS group was administered saline, and the SG group was given 0.75 g GLN/kg body weight via a tail vein after CLP. Mice were sacrificed 12 h after CLP, and CD8αα(+) TCRαβ(+) IELs were isolated for further analysis.

RESULTS

Sepsis resulted in a lower percentage of CD8αα(+) TCRαβ(+) IELs, and higher messenger (m)RNA expression of complement 5a receptor, interleukin (IL)-2 receptor β, IL-15 receptor α, and interferon-γ by CD8αα(+) TCRαβ(+) IELs. These immunomodulatory mediator genes decreased, whereas IL-7 receptor and transforming growth factor-β expressions increased in CD8αα(+) TCRαβ(+) IELs in septic mice with GLN administration. Annexin V⁄7-AAD staining revealed significantly lower apoptotic rates of CD8αα(+) TCRαβ(+) IELs in the SG group.

CONCLUSION

A single dose of GLN administered after the initiation of sepsis increased the percentage of CD8αα(+) TCRαβ(+) IELs, prevented apoptosis of CD8αα(+) TCRαβ(+) IELs, and downregulated CD8αα(+) TCRαβ(+) IEL-expressed inflammatory mediators. These results suggest that GLN influenced the distribution and cytokine secretion of the CD8αα(+) TCRαβ(+) IEL subset, which may ameliorate sepsis-induced inflammatory reactions and thus mitigate the severity of intestinal epithelial injury.

Type 1 diabetes therapy beyond T cell targeting: monocytes, B cells, and innate lymphocytes

Recent clinical trials, investigating type 1 diabetes (T1D), have focused mainly on newly diagnosed individuals who have developed diabetes. We need to continue our efforts to understand disease processes and to rationally design interventions that will be safe and specific for disease, but at the same time not induce undesirable immunosuppression. T cells are clearly involved in the pathogenesis of T1D, and have been a major focus for both antigen-specific and non-antigen-specific therapy, but thus far no single strategy has emerged as superior. As T1D is a multifactorial disease, in which multiple cell types are involved, some of these pathogenic and regulatory cell pathways may be important to consider. In this review, we examine evidence for whether monocytes, B cells, and innate lymphocytes, including natural killer cells, may be suitable targets for intervention.

Glutamine modulates sepsis-induced changes to intestinal intraepithelial γδT lymphocyte expression in mice

This study investigated the effect of glutamine (GLN) on intestinal intraepithelial lymphocyte (IEL) γδT-cell cytokines and immune regulatory factor gene expressions in a mouse model of polymicrobial sepsis. Mice were randomly assigned to a normal group, a sepsis with saline (SS) group, or a sepsis with GLN (SG) group. All mice were fed a chow diet. Sepsis was induced by cecal ligation and puncture (CLP). The SS group was injected with saline, and the SG group was given 0.75 g GLN/kg body weight once via a tail vein 1 h after CLP. Septic mice were killed 12 h after CLP, and IEL γδT cells of the animals were isolated for further analysis. Results showed that compared with normal mice, sepsis resulted in lower IEL γδT-cell percentage and higher messenger RNA expressions of interferon γ, tumor necrosis factor α, interleukin 4 (IL-4), IL-13, IL-17, retinoid acid receptor-related orphan receptor γt, and complement 5a receptor by IEL γδT cells. These immunomodulatory mediator genes exhibited decreases, whereas IL-7 receptor expression increased in IEL γδT cells in septic mice with GLN administration. Annexin V/7-amino-actinomycin D stain revealed significantly lower rates of apoptosis, and IEL γδT-cell percentage was higher in the SG group. The histological findings also showed that damage to intestinal epithelial cells was less severe in the SG group. These results indicated that a single dose of GLN administered as treatment after the initiation of sepsis prevented apoptosis of IEL γδT cells and downregulated γδT cell-expressed inflammatory mediators that may consequently ameliorate the severity of sepsis-induced intestinal epithelial injury.

Perspectives on interleukin-7 therapy in HIV infection

PURPOSE OF REVIEW

HIV infection is characterized by depletion of CD4 T cells and altered immune function, leading to severe immune deficiency. Mechanisms leading to this T-cell depletion are not completely understood. Potent antiretroviral therapy restores T-cell counts and improves prognosis. Apart from antiviral therapy for the infection, immunotherapies such as interleukin-7 that influence T-cell homeostatic mechanisms are undergoing clinical evaluation. Because of its pleiotropic effects on developing and mature T cells, interleukin-7 may help to restore immune function during HIV infection.

RECENT FINDINGS

Recent studies explored the therapeutic use of interleukin-7 in simian immunodeficiency virus models and in HIV-infected patients. Interleukin-7 can help to restore CD4 T-cell number and function.

SUMMARY

Numerous recent findings highlight the importance of interleukin-7 pathway impairment in the pathogenesis of HIV infection. Notably, interleukin-7 levels increased with advancing CD4 T-cell lymphopenia, whereas interleukin-7 receptor expression is downregulated mainly on CD8 T cells. Therapeutic trials conducted in monkeys and in humans (phase I) have provided evidence on the role of interleukin-7 in thymopoiesis and in restoration of T-cell functions. Interleukin-7 appeared to be well tolerated and to have no deleterious effects on viral load. These results should be confirmed in larger phase I/II studies.

Role of hepatocyte-derived IL-7 in maintenance of intrahepatic NKT cells and T cells and development of B cells in fetal liver

The liver contains a variety of resident immune cells, such as NK cells, NKT cells, T cells, macrophages, and dendritic cells. However, little is known about how IL-7, which is produced by hepatocytes, functions locally in development and maintenance of liver immune cells. To address this question, we established IL-7-floxed mice and crossed them with albumin promoter-driven Cre (Alb-Cre) transgenic mice to establish conditional knockout of IL-7 in hepatocytes. The levels of IL-7 transcripts were reduced 10-fold in hepatocyte fraction. We found that the absolute numbers of NKT and T cells were significantly decreased in adult liver of IL-7(f/f) Alb-Cre mice compared with IL-7(f/f) control mice. In contrast, NK cells, dendritic cells, and B cells were unchanged in the IL-7(f/f) Alb-Cre liver. The number of Vα14(+) invariant NKT cells was significantly reduced in liver, but not in thymus and spleen, of IL-7(f/f) Alb-Cre mice. Furthermore, B cell development was impaired in perinatal liver of IL-7(f/f) Alb-Cre mice. This study demonstrates that hepatocyte-derived IL-7 plays an indispensable role in maintenance of NKT and T cells in adult liver and development of B cells in fetal liver, and suggests that hepatocytes provide a unique IL-7 niche for intrahepatic lymphocytes.

IL-7Rα and L-selectin, but not CD103 or CD34, are required for murine peanut-induced anaphylaxis

Background

Allergy to peanuts results in severe anaphylactic responses in affected individuals, and has dramatic effects on society and public policy. Despite the health impacts of peanut-induced anaphylaxis (PIA), relatively little is known about immune mechanisms underlying the disease. Using a mouse model of PIA, we evaluated mice with deletions in four distinct immune molecules (IL7Rα, L-selectin, CD34, CD103), for perturbed responses.

Methods

PIA was induced by intragastric sensitization with peanut antigen and cholera toxin adjuvant, followed by intraperitoneal challenge with crude peanut extract (CPE). Disease outcome was assessed by monitoring body temperature, clinical symptoms, and serum histamine levels. Resistant mice were evaluated for total and antigen specific serum IgE, as well as susceptibility to passive systemic anaphylaxis.

Results

PIA responses were dramatically reduced in IL7Rα^−/− and L-selectin^−/− mice, despite normal peanut-specific IgE production and susceptibility to passive systemic anaphylaxis. In contrast, CD34^−/− and CD103^−/− mice exhibited robust PIA responses, indistinguishable from wild type controls.

Conclusions

Loss of L-selectin or IL7Rα function is sufficient to impair PIA, while CD34 or CD103 ablation has no effect on disease severity. More broadly, our findings suggest that future food allergy interventions should focus on disrupting sensitization to food allergens and limiting antigen-specific late-phase responses. Conversely, therapies targeting immune cell migration following antigen challenge are unlikely to have significant benefits, particularly considering the rapid kinetics of PIA.

Regulation of in vitro human T cell development through interleukin-7 deprivation and anti-CD3 stimulation

BACKGROUND

The role of IL-7 and pre-TCR signaling during T cell development has been well characterized in murine but not in human system. We and others have reported that human BM hematopoietic progenitor cells (HPCs) display poor proliferation, inefficient double negative (DN) to double positive (DP) transition and no functional maturation in the in vitro OP9-Delta-like 1 (DL1) culture system.

RESULTS

In this study, we investigated the importance of optimal IL-7 and pre-TCR signaling during adult human T cell development. Using a modified OP9-DL1 culture ectopically expressing IL-7 and Fms-like tyrosine kinase 3 ligand (Flt3L), we demonstrated enhanced T cell precursor expansion. IL-7 removal at various time points during T cell development promoted a slight increase of DP cells; however, these cells did not differentiate further and underwent cell death. As pre-TCR signaling rescues DN cells from programmed cell death, we treated the culture with anti-CD3 antibody. Upon pre-TCR stimulation, the IL-7 deprived T precursors differentiated into CD3+TCRαβ+DP cells and further matured into functional CD4 T cells, albeit displayed a skewed TCR Vβ repertoire.

CONCLUSIONS

Our study establishes for the first time a critical control for differentiation and maturation of adult human T cells from HPCs by concomitant regulation of IL-7 and pre-TCR signaling.

Close link between development and function of gamma-delta T cells

Murine γδ T cells develop as the first T-cell lineage within the fetal thymus and disproportionately localize in mucosal tissues such as lung, skin, uterus, and intestine of adult mice. These unique developmental features and distribution patterns of γδ T cells enable rapid functioning against various insults from pathogens. γδ T cells are also able to respond to local inflammation and consequently regulate the pathogenesis of autoimmune disorders and development of tumors in mice and humans. Hence, it is clinically important to understand the mechanisms that regulate γδ T cell functions. Recent evidence has shown that generations of effector γδ T cell subsets producing IFN-γ, IL-4, and IL-17 are programmed in the murine thymus before their migration to peripheral tissues. This review outlines our current understanding of the development and function of γδ T cells as they influence both innate and acquired immunity.

Synergistic effects of interleukin-7 and pre-T cell receptor signaling in human T cell development

The role of IL-7 in pre-T cell receptor (TCR) signaling during human T cell development is poorly understood. To study this, we engineered Molt3, a T cell progenitor T-acute lymphoblastic leukemia cell line, using lentiviral IL-7 receptor α (IL-7Rα) to serve as a model system. IL-7 promoted pre-TCR activation in IL-7Rα(hi) Molt3 as illustrated by CD25 up-regulation after anti-CD3 stimulation. Anti-CD3 treatment activated Akt and Erk1/2 signaling pathways as proven using specific inhibitors, and IL-7 further enhanced both signaling pathways. The close association of IL-7Rα with CD3ζ in the pre-TCR complex was illustrated through live imaging confocal fluorescence microscopy. These results demonstrate a direct and cooperative role of IL-7 in pre-TCR signaling.

Osteoblast-specific overexpression of human interleukin-7 rescues the bone mass phenotype of interleukin-7-deficient female mice

Interleukin-7 is a critical cytokine for lymphoid development and a direct inhibitor of in vitro osteoclastogenesis in murine bone marrow cultures. To explore the role of IL-7 in bone, we generated transgenic mouse lines bearing the 2.3-kb rat collagen 1α1 promoter driving the expression of human IL-7 specifically in osteoblasts. In addition, we crossed these mice with IL-7-deficient mice to determine if the alterations in lymphopoiesis, bone mass, and osteoclast formation observed in the IL-7 knockout (KO) mice could be rescued by osteoblast-specific overexpression of IL-7. Here, we show that mice overexpressing human IL-7 in the osteoblast lineage showed increased trabecular bone volume in vivo by µCT and decreased osteoclast formation in vitro. Furthermore, targeted overexpression of IL-7 in osteoblasts rescued the osteopenic bone phenotype and B-cell development of IL-7 KO mice but did not have an effect on T lymphopoiesis, which occurs in the periphery. The bone phenotypes in IL-7 KO mice and targeted IL-7-overexpressing mouse models were observed only in females. These results likely reflect both direct inhibitory effects of IL-7 on osteoclastogenesis in vivo and sex-specific differences in responses to IL-7.

Multifaceted roles of interleukin-7 signaling for the development and function of innate lymphoid cells

Recently, additional innate lymphocyte subsets have been identified that express germline encoded immunoreceptors and respond to cytokine cues. Among these, innate lymphoid cells (ILC) at mucosal surfaces are of significant interest because they were found to play important roles for lymphoid organogenesis, tissue homeostasis and repair, for immunity to various infections but also have been involved as disease-promoting cells in models of chronic inflammatory diseases and of autoimmunity. Their functional and transcriptional programs strikingly resemble that of the various T helper cell subsets suggesting that these programs are already pre-formed in the innate immune system and that these may be more conserved than previously appreciated. Interestingly, all ILC subsets express the interleukin 7 receptor α chain and IL-7 signaling has been involved in various aspects of their developmental and functional programs. Here, we will review the role of IL-7 signaling for the differentiation, maintenance and function of two important ILC subsets, lymphoid tissue inducer cells (i.e., RORγt(+) ILC) and natural helper cells (i.e., type 2 ILC). We will also put emphasis on the recently discovered role of IL-7 in controlling plasticity of RORγt(+) ILC.

Expression and function of interleukin-7 in secondary and tertiary lymphoid organs

Interleukin-7 (IL-7) is known since many years as stromal-cell derived cytokine that plays a key role for the adaptive immune system. It promotes lymphocyte development in the bone marrow and thymus as well as naive and memory T cell homeostasis in the periphery. More recently, IL-7 reporter mice and other approaches have led to the further characterization of the various stromal cell sources of IL-7 in secondary lymphoid organs (SLO) and other tissues. We will review these advances along with a discussion of the regulation of IL-7 and its receptor, and compare the biological effects IL-7 has on adaptive as well as innate immune cells in SLO. Finally, we will review the role of IL-7 in development of SLO and tertiary lymphoid tissues that frequently are associated with sites of chronic inflammation.

Depletion and recovery of lymphoid subsets following morphine administration

BACKGROUND AND PURPOSE

Opioid use and abuse has been linked to significant immunosuppression, which has been attributed, in part, to drug-induced depletion of lymphocytes. We sought to define the mechanisms by which lymphocyte populations are depleted and recover following morphine treatment in mice.

EXPERIMENTAL APPROACH

Mice were implanted with morphine pellets and B- and T-cell subsets in the bone marrow, thymus, spleen and lymph nodes were analysed at various time points. We also examined the effects of morphine on T-cell development using an ex vivo assay.

KEY RESULTS

The lymphocyte populations most susceptible to morphine-induced depletion were the precursor cells undergoing selection. As the lymphocytes recovered, more lymphocyte precursors proliferated than in control mice. In addition, peripheral T-cells displayed evidence that they had undergone homeostatic proliferation during the recovery phase of the experiments.

CONCLUSIONS AND IMPLICATIONS

The recovery of lymphocytes following morphine-induced depletion occurred in the presence of morphine and via increased proliferation of lymphoid precursors and homeostatic proliferation of T-cells.

Nucleotides enhance the secretion of interleukin 7 from primary-cultured murine intestinal epithelial cells

Our previous studies showed that dietary nucleotides fed to mice enhanced the secretion of interleukin 7 (IL-7) and transforming growth factor beta (TGF-beta) from intestinal epithelial cells (IECs). To explore whether nucleotides influence IECs directly to enhance the secretion of the cytokines or not, the effects of nucleotides added in vitro on the cytokine secretion from primary-cultured murine IECs were examined. When the mixture of nucleotide 5'-monophosphates (CMP, GMP, IMP, and UMP) or individual nucleotide 5'-monophosphates were added to the primary culture of IECs derived from BALB/c mice, the secretion of IL-7, but not that of TGF-beta, was increased significantly. Addition of nucleotides to the culture did not alter the number of the IECs. Secretion of IL-6 and granulocyte-macrophage colony-stimulating factor, which are known to be secreted from IECs, was not enhanced by the addition of nucleotides. These results demonstrate that nucleotides can affect IECs directly to enhance the secretion of IL-7, and suggest that the increased secretion of TGF-beta from IECs by dietary nucleotides was due to indirect effects of the nucleotides, which may affect intestinal microflora or cells other than IECs that in turn influence the cytokine secretion of IECs.

Dietary nucleotides increase the mucosal IgA response and the secretion of transforming growth factor beta from intestinal epithelial cells in mice

We have investigated the influence of dietary nucleotides on the intestinal immune system in ovalbumin (OVA)-specific T-cell receptor (TCR) transgenic mice (OVA-TCR Tg mice). When mice were supplied with water supplemented with 2% OVA ad libitum, the faecal OVA-specific immunoglobulin A (IgA) level significantly increased in those fed a nucleotide-supplemented diet (NT(+) diet) compared with those fed a nucleotide-free control diet (NT(-) diet). In the NT(+) diet-fed mice, secretion of transforming growth factor beta (TGF-beta), which is an isotype-specific switch factor for IgA, from intestinal epithelial cells (IECs) was significantly increased. Furthermore, an increased proportion of intestinal intraepithelial lymphocytes (IELs) bearing gammadelta TCR (TCRgammadelta(+) IELs) and increased secretion from IECs of interleukin 7 (IL-7), which is essential for the development of TCRgammadelta(+) IELs, were also observed in OVA-TCR-Tg mice fed the NT(+) diet, as we previously demonstrated using BALB/c mice (Nagafuchi et al., Biosci. Biotechnol. Biochem. 64: 1459-65 (2000)). Considering that TCRgammadelta(+) T cells and TGF-beta are important for an induction of the mucosal IgA response, our results suggest that dietary nucleotides augment the mucosal OVA-specific IgA response by increasing the secretion of TGF-beta from IECs and the proportion of TCRgammadelta(+) IELs.

Dynamics of the interaction of γδ T cells with their neighbors in vivo

γδ T cells are a diverse component of the immune system in humans and mice with presumably important but still largely unknown functions. Understanding the dynamic interaction of γδ T cells with their neighbors should help to understand their physiological role. This review addresses recent advances and strategies to visualize the dynamic interactions of γδ T cells with their neighbors in vivo. Current knowledge regarding the dynamic contacts of tissue resident γδ T cells and epithelial cells, but also of the communication between circulating γδ T cells and DCs, monocytes and FoxP3(+) regulatory T cells is revisited with emphasis on the role of γδ T cell motility.

Upregulated IL-7 receptor α expression on colitogenic memory CD4+ T cells may participate in the development and persistence of chronic colitis

We have previously demonstrated that IL-7 is essential for the persistence of colitis as a survival factor of colitogenic IL-7Rα-expressing memory CD4(+) T cells. Because IL-7Rα is broadly expressed on various immune cells, it is possible that the persistence of colitogenic CD4(+) T cells is affected by other IL-7Rα-expressing non-T cells. To test this hypothesis, we conducted two adoptive transfer colitis experiments using IL-7Rα(-/-) CD4(+)CD25(-) donor cells and IL-7Rα(-/-) × RAG-2(-/-) recipient mice, respectively. First, IL-7Rα expression on colitic lamina propria (LP) CD4(+) T cells was significantly higher than on normal LP CD4(+) T cells, whereas expression on other colitic LP immune cells, (e.g., NK cells, macrophages, myeloid dendritic cells) was conversely lower than that of paired LP cells in normal mice, resulting in predominantly higher expression of IL-7Rα on colitogenic LP CD4(+) cells, which allows them to exclusively use IL-7. Furthermore, RAG-2(-/-) mice transferred with IL-7Rα(-/-) CD4(+)CD25(-) T cells did not develop colitis, although LP CD4(+) T cells from mice transferred with IL-7Rα(-/-) CD4(+)CD25(-) T cells were differentiated to CD4(+)CD44(high)CD62L(-) effector-memory T cells. Finally, IL-7Rα(-/-) × RAG-2(-/-) mice transferred with CD4(+)CD25(-) T cells developed colitis similar to RAG-2(-/-) mice transferred with CD4(+)CD25(-) T cells. These results suggest that IL-7Rα expression on colitogenic CD4(+) T cells, but not on other cells, is essential for the development of chronic colitis. Therefore, therapeutic approaches targeting the IL-7/IL-7R signaling pathway in colitogenic CD4(+) T cells may be feasible for the treatment of inflammatory bowel diseases.

Interleukin-1beta selectively expands and sustains interleukin-22+ immature human natural killer cells in secondary lymphoid tissue

Among human natural killer (NK) cell intermediates in secondary lymphoid tissue (SLT), stage 3 CD34(-)CD117(+)CD161(+)CD94(-) immature NK (iNK) cells uniquely express aryl hydrocarbon receptor (AHR) and interleukin-22 (IL-22), supporting a role in mucosal immunity. The mechanisms controlling proliferation and differentiation of these cells are unknown. Here we demonstrate that the IL-1 receptor IL-1R1 was selectively expressed by a subpopulation of iNK cells that localized proximal to IL-1beta-producing conventional dendritic cells (cDCs) within SLT. IL-1R1(hi) iNK cells required continuous exposure to IL-1beta to retain AHR and IL-22 expression, and they proliferate in direct response to cDC-derived IL-15 and IL-1beta. In the absence of IL-1beta, a substantially greater fraction of IL-1R1(hi) iNK cells differentiated to stage 4 NK cells and acquired the ability to kill and secrete IFN-gamma. Thus, cDC-derived IL-1beta preserves and expands IL-1R1(hi)IL-22(+)AHR(+) iNK cells, potentially influencing human mucosal innate immunity during infection.

An overview of IL-7 biology and its use in immunotherapy

Interleukin (IL)-7 is required for T-cell development as well as for the survival and homeostasis of mature T-cells. In the thymus, the double negative (DN) CD4(-) CD8(-) thymocyte progenitor transition into double positive CD4+ CD8+ cells requires Notch and IL-7 signaling. Importantly, IL-7 seems to have a dose effect on T-cell development and, at high doses, DN progression is blocked. Naïve T-cells in the thymus, and after their exit to the periphery, are dependent on IL-7 and TCR signaling for survival. Upon antigen exposure, they proliferate and differentiate into memory T-cells. Because IL-7 intervenes at all stages of T-cell development and maintenance, it has been introduced recently into clinical trials as an immunotherapeutic agent for cancer patients (of particular note, those who had undergone T-cell depleting therapy) in an attempt to increase their population sizes of CD4+ and CD8+ cells overall, and specifically of CD8+ (CD45RA+)CCR7+ and/or CD27+), CD4+ (CD45RA+CD31+), and CD4+ central memory T-cells (CD45RA(-)CCR7+). Interestingly, IL-7 in humans induced a preferential expansion of naïve T-cells, resulting in a broader T-cell repertoire than before the treatment; this effect was independent of age. This suggests that IL-7 therapy could enhance immune responses in patients with limited naïve T-cell numbers as in aged patients or after disease-induced or iatrogenic T-cell depletion. This overview highlights the role of IL-7 on T-cells in mice and humans.

Activation of the mouse TCRgamma enhancers by STAT5

The IL-7R controls local accessibility of joining (J) gamma gene segments in the mouse TCRgamma locus by recruiting signal transducers and activators of transcription (STAT) 5 and transcriptional coactivators to the Jgamma germ line promoters and inducing histone acetylation and germ line transcription. Because STAT consensus motifs are conserved not only in the Jgamma promoters but also in the TCRgamma 3' enhancer (Egamma) elements, it is possible that STAT5 interacts with and activates Egamma. To address this question, we first showed that the lysine 4 residue of histone H3 is substantially methylated at Egamma1 and Egamma4 elements in wild-type early thymocytes and that the levels of the methylation are reduced in IL-7R alpha chain-deficient mice. We also showed that STAT5 has potential to elevate histone acetylation of the Egamma elements in a cytokine-dependent cell line by cytokine stimulation. Next, we demonstrated that STAT5 is recruited to the STAT consensus motifs in the Egamma elements after cytokine stimulation and that transcription factors Runt-related (Runx) and c-Myb are constitutively recruited to Egamma. Furthermore, we showed that STAT5 augments basal Egamma activity controlled by Runx and c-Myb. These results suggest that STAT5 is recruited to the consensus motifs in the Egamma elements by cytokine stimulation and augments basal Egamma activity independent of Runx and c-Myb. Therefore, this study implies that the Egamma elements might be activated in two successive steps, first by Runx and c-Myb and next by STAT5.

Thymic involution and immune reconstitution

Chronic thymus involution associated with aging results in less efficient T-cell development and decreased emigration of naïve T cells to the periphery. Thymic decline in the aged is linked to increased morbidity and mortality in a wide range of clinical settings. Negative consequences of these effects on global health make it of paramount importance to understand the mechanisms driving thymic involution and homeostatic processes across the lifespan. There is growing evidence that thymus tissue is plastic and that the involution process might be therapeutically halted or reversed. We present here progress on the exploitation of thymosuppressive and thymostimulatory pathways using factors such as keratinocyte growth factor, interleukin 7 or sex steroid ablation for therapeutic thymus restoration and peripheral immune reconstitution in adults.

Differentiation, expansion, and homeostasis of autoreactive T cells in type 1 diabetes mellitus

Autoreactive T cells play a major role in the pathogenesis of type 1 diabetes mellitus (T1DM) and are considered a major target of immunomodulatory strategies aimed at preventing or delaying the disease onset. However, the T-cell response against insulin-producing beta cells is still poorly understood. T cells potentially able to recognize and destroy beta cells are present in most individuals, but only in a few do they differentiate into pathogenic effectors. Recent and novel findings in T-cell biology on the dynamics of T-cell activation and memory maintenance are shedding new light on the general mechanisms of the T-cell response. In this article, we discuss how new discoveries about T-cell differentiation, expansion, and homeostasis could help to clarify mechanisms of autoimmunity that lead to T1DM.

Distinct and overlapping patterns of cytokine regulation of thymic and bone marrow-derived NK cell development

Although bone marrow (BM) represents the main site for postnatal NK cell development, recently a distinct thymic-dependent NK cell pathway was identified. These studies were designed to investigate the role of cytokines in regulation of thymic NK cells and to compare with established regulatory pathways of BM-dependent NK cell compartment. The common cytokine receptor gamma-chain (Il2rg) essential for IL-15-induced signaling, and FMS-like tyrosine kinase 3 (FLT3) receptor ligand (Flt3l) were previously identified as important regulatory pathways of the BM NK cell compartment based on lack of function studies in mice, however their complementary action remains unknown. By investigating mice double-deficient in Il2rg and Flt3l (Flt3l(-/-) Il2rg(-/-)), we demonstrate that FLT3L is important for IL2Rg-independent maintenance of both immature BM as well as peripheral NK cells. In contrast to IL-7, which is dispensable for BM but important for thymic NK cells, IL-15 has a direct and important role in both thymic and BM NK cell compartments. Although thymic NK cells were not affected in Flt3l(-/-) mice, Flt3l(-/-)Il2rg(-/-) mice lacked detectable thymic NK cells, suggesting that FLT3L is also important for IL-2Rg-independent maintenance of thymic NK cells. Thus, IL-2Rg cytokines and FLT3L play complementary roles and are indispensable for homeostasis of both BM and thymic dependent NK cell development, suggesting that the cytokine pathways crucial for these two distinct NK cell pathways are largely overlapping.

Expression of IL-5Ralpha on B-1 cell progenitors in mouse fetal liver and involvement of Bruton's tyrosine kinase in their development

B-1 cells are a subset of B cells responsible for the production of natural antibodies. Although the amount of natural antibody is tightly regulated, how this regulation occurs remains unknown. We examined the expression of IL-5 receptor, a cytokine receptor critical for homeostatic proliferation of B-1 cells, on B-1 cell progenitors in the fetal liver. We identified B-1 progenitors expressing low levels of IL-5 receptor alpha chain (IL-5Ralpha) and eosinophil progenitors expressing higher levels of IL-5Ralpha in the fetal liver. Moreover, the number of these B-1 progenitors were significantly reduced in the fetuses of mice deficient in Bruton's tyrosine kinase (Btk), even though IL-5 and thymic stroma lymphopoietin signaling are intact in early B lineage cells in Btk-deficient mice. These data suggest that IL-5 is possibly involved in B-1 cell development and an uncharacterized, Btk-dependent regulatory signaling pathway is involved in unexpectedly early stages of B-1 cell differentiation.

Intestinal specific overexpression of interleukin-7 attenuates the alternation of intestinal intraepithelial lymphocytes after total parenteral nutrition administration

OBJECTIVE

Total parenteral nutrition (TPN), with the complete removal of enteral nutrition, results in marked changes in intestinal intraepithelial lymphocyte (IEL) function and phenotype. Previous work shows that TPN results in a loss of intestinal epithelial cell-derived interleukin-7 (IL-7), and this loss may play an important role in development of such TPN-associated IEL changes.

METHODS

To further understand this relation, we generated a transgenic mouse (IL-7), which overexpresses IL-7 specifically in intestinal epithelial cells. We hypothesized that this localized overexpression would attenuate many of the observed TPN-associated IEL changes.

RESULTS

Our study showed that TPN administration led to significant changes in IEL phenotype, including a marked decline in the CD8alphabeta+, CD4+, and alphabeta-TCR+ populations. IEL basal proliferation decreased 1.7-fold compared with wild-type TPN mice. TPN administration in wild-type mice resulted in several changes in IEL-derived cytokine expression. IL-7 mice given TPN, however, maintained IEL proliferation, and sustained normal IEL numbers and phenotype.

CONCLUSIONS

We conclude that specific intestinal IL-7 overexpression significantly attenuated many IEL changes in phenotype and function after TPN administration. These findings suggest a mechanism by which TPN results in observed IEL changes.

FLT3 ligand and not TSLP is the key regulator of IL-7-independent B-1 and B-2 B lymphopoiesis

Phenotypically and functionally distinct progenitors and developmental pathways have been proposed to exist for fetally derived B-1 and conventional B-2 cells. Although IL-7 appears to be the primary cytokine regulator of fetal and adult B lymphopoiesis in mice, considerable fetal B lymphopoiesis and postnatal B cells are sustained in the absence of IL-7; in humans, B-cell generation is suggested to be largely IL-7-independent, as severe combined immune-deficient patients with IL-7 deficiency appear to have normal B-cell numbers. However, the role of other cytokines in IL-7-independent B lymphopoiesis remains to be established. Although thymic stromal lymphopoietin (TSLP) has been proposed to be the main factor driving IL-7-independent B lymphopoiesis and to distinguish fetal from adult B-cell progenitor development in mice, recent studies failed to support a primary role of TSLP in IL-7-independent fetal B-cell development. However, the role of TSLP in IL-7-independent adult B lymphopoiesis and in particular in regulation of B-1 cells remains to be established. Here we demonstrate that, rather than TSLP, IL-7 and FLT3 ligand are combined responsible for all B-cell generation in mice, including recently identified B-1-specified cell progenitors. Thus, the same IL-7- and FLT3 ligand-mediated signal-ing regulates alternative pathways of fetal and adult B-1 and B-2 lymphopoiesis.

Lineage development of hematopoietic stem and progenitor cells

Hematopoietic stem cells have the potential to develop into multipotent and different lineage-restricted progenitor cells that subsequently generate all mature blood cell types. The classical model of hematopoietic lineage commitment proposes a first restriction point at which all multipotent hematopoietic progenitor cells become committed either to the lymphoid or to the myeloid development, respectively. Recently, this model has been challenged by the identification of murine as well as human hematopoietic progenitor cells with lymphoid differentiation capabilities that give rise to a restricted subset of the myeloid lineages. As the classical model does not include cells with such capacities, these findings suggest the existence of alternative developmental pathways that demand the existence of additional branches in the classical hematopoietic tree. Together with some phenotypic criteria that characterize different subsets of multipotent and lineage-restricted progenitor cells, we summarize these recent findings here.

MEK1/2 induces STAT5-mediated germline transcription of the TCRgamma locus in response to IL-7R signaling

The IL-7R plays an essential role in gammadelta T cell development by inducing V-J recombination of the TCRgamma locus through STAT5. Although tyrosine residues in the intracellular domain of the mouse IL-7R alpha-chain (IL-7Ralpha) have been implicated in STAT5 activation, it is still unknown whether they are essential for gammadelta T cell development. In this study, we showed that those IL-7Ralpha tyrosine residues are not essential for gammadelta T cell development, because phenylalanine replacement of four intracellular tyrosine residues (IL-7R-FFFF) partially rescued gammadelta T cell development of IL-7Ralpha-/- progenitors. To examine signaling pathways activated by IL-7R-FFFF, we introduced a chimeric receptor consisting of the human IL-4R alpha-chain and mouse IL-7R-FFFF (4R/7R-FFFF) into an IL-7-dependent pre-B cell line and found that 4R/7R-FFFF induced TCRgamma germline transcription and STAT5 activation. Treatment of cells with MEK1/2 inhibitors significantly decreased levels of TCRgamma germline transcription and STAT5 tyrosine phosphorylation mediated by 4R/7R-FFFF, suggesting that MEK1/2 plays an alternative role in STAT5 activation by IL-7R. MEK1/2 associated with STAT5 and induced STAT5 tyrosine phosphorylation and DNA binding activity. Furthermore, MEK1 directly phosphorylated a STAT5 tyrosine residue in vitro. Finally, active MEK1 partially rescued TCRgamma germline transcription by IL-7R in a pre-T cell line. These results demonstrate that MEK1/2 induces TCRgamma germline transcription by phosphorylating STAT5 through IL-7R-FFFF and suggest a potential role for MAPK in IL-7R tyrosine-independent activation of STAT5.