The BTB-zinc finger transcriptional regulator PLZF controls the development of invariant natural killer T cell effector functions. Nat Immunol. 2008;9:1055-1064. [PMID: 18660811 DOI: 10.1038/ni.1641]

The sequential activity of Gata3 and Thpok is required for the differentiation of CD1d-restricted CD4+ NKT cells

While most CD4(+) T cells are MHC class II-restricted, a small subset, including the CD1d-restricted 'invariant' NKT (iNKT) cells, are selected on non-classical MHC-I or MHC-I-like molecules. We previously showed that the sequential activity of two zinc finger transcription factors, Gata3 and Thpok, promotes the differentiation of conventional, MHC II-restricted thymocytes into CD4(+) T cells. In the current study, we show that a Gata3-Thpok cascade is required for the differentiation of CD4(+) iNKT cells. Gata3 is required for iNKT cells to express Thpok, whereas Thpok is needed for proper NKT cell differentiation, and notably for NKT cells to maintain CD4 and terminate CD8 expression. These findings identify the sequential activity of Gata3 and Thpok as a hallmark of CD4(+) T-cell differentiation, regardless of MHC restriction.

Directing traffic on the NKT-cell highway: a key role for ThPOK

CD1d-restricted NKT cells include CD4(+) and DN subsets, with an additional CD8(+) subset that is present in humans but not in mice. The molecular regulation of CD4/CD8 expression by NKT cells, and differentiation of these NKT-cell subsets, is poorly understood. The transcription factors GATA3 and ThPOK regulate lineage commitment of conventional MHC class II-restricted CD4(+) T cells; however, their role in CD4/CD8 expression by CD1d-restricted NKT cells is less clear. A new study in this issue of the European Journal of Immunology demonstrates a key role for ThPOK in differentiation of NKT-cell subsets. This study reveals that GATA3 and ThPOK are necessary for the development of CD4(+) NKT cells. Furthermore, ThPOK-deficient mice generate an unusual population of CD8(+) NKT cells, which are absent in control mice. This study sheds new light on the underlying molecular events leading to the emergence of distinct NKT-cell subsets.

Interleukin-2-inducible T cell kinase (Itk) network edge dependence for the maturation of iNKT cell

Invariant natural killer T (iNKT) cells are a unique subset of innate T lymphocytes that are selected by CD1d. They have diverse immune regulatory functions via the rapid production of interferon-γ (IFN-γ) and interleukin-4 (IL-4). In the absence of signaling nodes Itk and Txk, Tec family non-receptor tyrosine kinases, mice exhibit a significant block in iNKT cell development. We now show here that although the Itk node is required for iNKT cell maturation, the kinase domain edge of Itk is not required for continued maturation iNKT cells in the thymus compared with Itk-null mice. This rescue is dependent on the expression of the Txk node. Furthermore, this kinase domain independent edge rescue correlates with the increased expression of the transcription factors T-bet, the IL-2/IL-15 receptor β chain CD122, and suppression of eomesodermin expression. By contrast, α-galactosyl ceramide induced cytokine secretion is dependent on the kinase domain edge of Itk. These findings indicate that the Itk node uses a kinase domain independent edge, a scaffolding function, in the signaling pathway leading to the maturation of iNKT cells. Furthermore, the findings indicate that phosphorylation of substrates by the Itk node is only partially required for maturation of iNKT cells, while functional activation of iNKT cells is dependent on the kinase domain/activity edge of Itk.

Type I natural killer T cells: naturally born for fighting

Type capital I, Ukrainian natural killer T cells (NKT cells), a subset of CD1d-restricted T cells with invariant Valphabeta TCR, are characterized by prompt production of large amounts of Th1 and/or Th2 cytokines upon primary stimulation through the TCR complex. The rapid release of cytokines implies that type capital I, Ukrainian NKT cells may play a critical role in modulating the upcoming immune responses, such as anti-tumor response, protection against infection, and autoimmunity. As a bridge between innate and adaptive immunity, type capital I, Ukrainian NKT cells differentiate and mature upon stimulations to achieve and maintain a homeostasis. Orchestrating with other arms of adaptive immunity, type capital I, Ukrainian NKT cells show strong cytotoxic effects in response to various tumors in a direct and/or indirect manner(s). This review will focus primarily on type capital I, Ukrainian NKT cell development, homeostasis, and effector functions, especially in anti-tumor immunity, and followed by their potential applications in treatment of cancers.

Invariant natural killer T cells: bridging innate and adaptive immunity

Cells of the innate immune system interact with pathogens via conserved pattern-recognition receptors, whereas cells of the adaptive immune system recognize pathogens through diverse, antigen-specific receptors that are generated by somatic DNA rearrangement. Invariant natural killer T (iNKT) cells are a subset of lymphocytes that bridge the innate and adaptive immune systems. Although iNKT cells express T cell receptors that are generated by somatic DNA rearrangement, these receptors are semi-invariant and interact with a limited set of lipid and glycolipid antigens, thus resembling the pattern-recognition receptors of the innate immune system. Functionally, iNKT cells most closely resemble cells of the innate immune system, as they rapidly elicit their effector functions following activation, and fail to develop immunological memory. iNKT cells can become activated in response to a variety of stimuli and participate in the regulation of various immune responses. Activated iNKT cells produce several cytokines with the capacity to jump-start and modulate an adaptive immune response. A variety of glycolipid antigens that can differentially elicit distinct effector functions in iNKT cells have been identified. These reagents have been employed to test the hypothesis that iNKT cells can be harnessed for therapeutic purposes in human diseases. Here, we review the innate-like properties and functions of iNKT cells and discuss their interactions with other cell types of the immune system.

Decision checkpoints in the thymus

The development of T cells in the thymus involves several differentiation and proliferation events, during which hematopoietic precursors give rise to T cells ready to respond to antigen stimulation and undergo effector differentiation. This review addresses signaling and transcriptional checkpoints that control the intrathymic journey of T cell precursors. We focus on the divergence of alphabeta and gammadelta lineage cells and the elaboration of the alphabeta T cell repertoire, with special emphasis on the emergence of transcriptional programs that direct lineage decisions.

gammadeltaTCR ligands and lineage commitment

Two major T lymphocyte lineages--alphabeta and gammadelta T cells--develop in the thymus from common precursors. Differentiation of both lineages requires signals coming from TCRs. Development of alphabeta T cells is driven at early stages by signaling from the pre-TCR, most likely in a ligand-independent fashion, and later--by signals delivered by alphabetaTCRs binding to their ligands--classical or non-classical MHC molecules. gammadelta lineage cells likewise require TCR signaling for their differentiation. Recent work from several groups suggests that TCR signaling not only ensures the developmental progression towards alphabeta and gammadelta lineages but that signal strength instructs lineage fate: weaker TCR signal results in alphabeta and stronger--in gammadelta lineage commitment. However, as most gammadeltaTCRs remain orphan receptors, it is still debated whether strong signals from gammadeltaTCRs in development are generated in a ligand-dependent manner (as in the case of alphabetaTCRs), ligand-independent manner (as for pre-TCR) or both. Here we summarize evidence supporting a possible role for ligands in gammadelta T cell lineage commitment and the generation of gammadelta sublineages.

SAP protein-dependent natural killer T-like cells regulate the development of CD8(+) T cells with innate lymphocyte characteristics

CD8(+) T cells are selected via low-affinity interaction with MHC class I molecules on thymic epithelial cells (TECs). However, compromised T cell receptor signaling was proposed to force CD8(+) T cell selection on hematopoietic cells through a SLAM-associated protein (SAP)-dependent mechanism similar to NKT cells. The outcome is an unconventional CD8(+) T cell with phenotypic and functional characteristics of innate lymphocytes. Here we showed that Id3(-/-) CD8(+) T cells had an innate-like phenotype and required SAP for their development. However, like conventional CD8(+) T cells, Id3(-/-) CD8(+) thymocytes were selected on TECs. The requirement for SAP and the innate-like phenotype was not intrinsic to Id3(-/-) CD8(+) thymocytes. Rather, an expanded population of NKT-like cells induced the innate phenotype on CD8(+) T cells through production of interleukin-4. Our findings reveal that accumulation of NKT-like cells promotes conventional CD8(+) thymocytes to acquire innate lymphocyte characteristics.

Murine induced pluripotent stem cells can be derived from and differentiate into natural killer T cells

NKT cells demonstrate antitumor activity when activated to produce Th1 cytokines by DCs loaded with alpha-galactosylceramide, the prototypic NKT cell-activating glycolipid antigen. However, most patients do not have sufficient numbers of NKT cells to induce an effective immune response in this context, indicating a need for a source of NKT cells that could be used to supplement the endogenous cell population. Induced pluripotent stem cells (iPSCs) hold tremendous potential for cell-replacement therapy, but whether it is possible to generate functionally competent NKT cells from iPSCs has not been rigorously assessed. In this study, we successfully derived iPSCs both from embryonic fibroblasts from mice harboring functional NKT cell-specific rearranged T cell receptor loci in the germline and from splenic NKT cells from WT adult mice. These iPSCs could be differentiated into NKT cells in vitro and secreted large amounts of the Th1 cytokine IFN-gamma. Importantly, iPSC-derived NKT cells recapitulated the known adjuvant effects of natural NKT cells and suppressed tumor growth in vivo. These studies demonstrate the feasibility of expanding functionally competent NKT cells via an iPSC phase, an approach that may be adapted for NKT cell-targeted therapy in humans.

T cells expressing the transcription factor PLZF regulate the development of memory-like CD8+ T cells

Several gene deficiency models promote the development of “innate CD8⁺ T cells” that have diverse TCRs, but display a memory phenotype and rapidly produce cytokines. We here demonstrate that similar cells develop in Kruppel-Like Factor 2 (KLF2) deficient mice. However, this is not due to intrinsic deficiency in KLF2, but rather to interleukin 4 (IL-4) produced by an expanded population of T cells expressing the PLZF transcription factor. The development of innate CD8⁺ T cells in ITK and CBP transcription factor deficient mice is also attributable to this IL-4-dependent mechanism. Finally, we show that the same mechanism drives innate CD8⁺ T cell differentiation in BALB/c mice. These findings reveal a novel mechanism of regulation of CD8⁺ T cells via PLZF⁺ T cell production of IL-4.

Gammadelta T cell effector functions: a blend of innate programming and acquired plasticity

Gammadelta T cells have several innate cell-like features that allow their early activation following recognition of conserved stress-induced ligands. Here we review recent observations revealing the ability of gammadelta T cells to rapidly produce cytokines that regulate pathogen clearance, inflammation and tissue homeostasis in response to tissue stress. These studies provide insights into how they acquire these properties, through both developmental programming in the thymus and functional polarization in the periphery. Innate features of gammadelta T cells underlie their non-redundant role in several physiopathological contexts and are therefore being exploited in the design of new immunotherapeutic approaches.

PLZF induces the spontaneous acquisition of memory/effector functions in T cells independently of NKT cell-related signals

The broad complex, tramtrack, bric-a-brac-zinc finger (BTB-ZF) transcription factor promyelocytic leukemia zinc finger (PLZF) is required for development of the characteristic innate/effector functions of NKT cells. In this study, we report the characterization and functional analysis of transgenic mouse T cells with forced expression of PLZF. PLZF expression was sufficient to provide some memory/effector functions to T cells without the need for Ag stimulation or proliferation. The acquisition of this phenotype did not require the proliferation typically associated with T cell activation. Furthermore, PLZF transgenic cells maintained a diverse TCR repertoire, indicating that there was no preferential expansion of specific clones. Functionally, PLZF transgenic CD4 and CD8 lymphocytes were similar to wild type memory cells, in that they had similar requirements for costimulation and exhibited a similar pattern of cytokine secretion, with the notable exception that transgenic T cells produced significantly increased levels of IL-17. Whereas transgene-mediated PLZF expression was not sufficient to rescue NKT cell development in Fyn- or signaling lymphocytic activation-associated protein (SAP)-deficient mice, the acquisition of memory/effector functions induced by PLZF in conventional T cells was independent of Fyn and SAP. These data show that PLZF is sufficient to promote T cell effector functions and that PLZF acts independently of SAP- and Fyn-mediated signaling pathways.

The role of ThPOK in control of CD4/CD8 lineage commitment

During alphabeta T cell development, cells diverge into alternate CD4 helper and CD8(+) cytotoxic T cell lineages. The precise correlation between a T cell's CD8 and CD4 choice and its TCR specificity to class I or class II MHC was noted more than 20 years ago, and establishing the underlying mechanism has remained a focus of intense study since then. This review deals with three formerly discrete topics that are gradually becoming interconnected: the role of TCR signaling in lineage commitment, the regulation of expression of the CD4 and CD8 genes, and transcriptional regulation of lineage commitment. It is widely accepted that TCR signaling exerts a decisive influence on lineage choice, although the underlying mechanism remains intensely debated. Current evidence suggests that both duration and intensity of TCR signaling may control lineage choice, as proposed by the kinetic signaling and quantitative instructive models, respectively. Alternate expression of the CD4 and CD8 genes is the most visible manifestation of lineage choice, and much progress has been made in defining the responsible cis elements and transcription factors. Finally, important clues to the molecular basis of lineage commitment have been provided by the recent identification of the transcription factor ThPOK as a key regulator of lineage choice. ThPOK is selectively expressed in class II-restricted cells at the CD4(+)8(lo) stage and is necessary and sufficient for development to the CD4 lineage. Given the central role of ThPOK in lineage commitment, understanding its upstream regulation and downstream gene targets is expected to reveal further important aspects of the molecular machinery underlying lineage commitment.

Disorderly conduct in gammadelta versus alphabeta T cell lineage commitment

The mechanism of T cell precursor commitment to the gammadelta or alphabeta T cell lineage remains unclear. While TCR signal strength has emerged as a key factor in lineage commitment based on TCR transgenic models, the entire TCR repertoire may not possess the same discriminatory power. A counterbalance to the TCR as the lineage determinant is the pre-existing heterogeneity in gene expression among precursors, which suggests that single precursors are unlikely to respond homogeneously to a given instructive signal.

Co-receptor choice by V alpha14i NKT cells is driven by Th-POK expression rather than avoidance of CD8-mediated negative selection

Mouse natural killer T (NKT) cells with an invariant Vα14-Jα18 rearrangement (Vα14 invariant [Vα14i] NKT cells) are either CD4⁺CD8⁻ or CD4⁻CD8⁻. Because transgenic mice with forced CD8 expression in all T cells exhibited a profound NKT cell deficit, the absence of CD8 has been attributed to negative selection. We now present evidence that CD8 does not serve as a coreceptor for CD1d recognition and that the defect in development in CD8 transgene homozygous mice is the result of a reduction in secondary T cell receptor α rearrangements. Thymocytes from mice hemizygous for the CD8 transgene have a less severe rearrangement defect and have functional CD8⁺ Vα14i NKT cells. Furthermore, we demonstrate that the transcription factor Th, Poxviruses and Zinc finger, and Krüppel family (Th-POK) is expressed by Vα14i NKT cells throughout their differentiation and is necessary both to silence CD8 expression and for the functional maturity of Vα14i NKT cells. We therefore suggest that Th-POK expression is required for the normal development of Vα14i NKT cells and that the absence of CD8 expression by these cells is a by-product of such expression, as opposed to the result of negative selection of CD8-expressing Vα14i NKT cells.

The transcription factor c-Myb primes CD4+CD8+ immature thymocytes for selection into the iNKT lineage

Type I invariant NKT cells (iNKT cells) are a subset of alphabeta T cells characterized by the expression of an invariant alpha-chain variable region 14-alpha-chain joining region 18 (V(alpha)14J(alpha)18) T cell antigen receptor (TCR) alpha-chain. The iNKT cells derive from CD4(+)CD8(+) double-positive (DP) thymocytes, and their generation requires a long half-life of DP thymocytes to allow V(alpha)14-J(alpha)18 rearrangements, expression of glycolipid-loaded CD1d on DP thymocytes, and signaling through the signaling-activation molecule SLAM-adaptor SAP pathway. Here we show that the transcription factor c-Myb has a central role in priming DP thymocytes to enter the iNKT lineage by simultaneously regulating CD1d expression, the half-life of DP cells and expression of SLAMF1, SLAMF6 and SAP.

Inhibitor of DNA binding 3 limits development of murine slam-associated adaptor protein-dependent "innate" gammadelta T cells

BACKGROUND

Id3 is a dominant antagonist of E protein transcription factor activity that is induced by signals emanating from the alphabeta and gammadelta T cell receptor (TCR). Mice lacking Id3 were previously shown to have subtle defects in positive and negative selection of TCRalphabeta+ T lymphocytes. More recently, Id3(-/-) mice on a C57BL/6 background were shown to have a dramatic expansion of gammadelta T cells.

METHODOLOGY/PRINCIPAL FINDINGS

Here we report that mice lacking Id3 have reduced thymocyte numbers but increased production of gammadelta T cells that express a Vgamma1.1+Vdelta6.3+ receptor with restricted junctional diversity. These Vgamma1.1+Vdelta6.3+ T cells have multiple characteristics associated with "innate" lymphocytes such as natural killer T (NKT) cells including an activated phenotype, expression of the transcription factor PLZF, and rapid production of IFNg and interleukin-4. Moreover, like other "innate" lymphocyte populations, development of Id3(-/-) Vgamma1.1+Vdelta6.3+ T cells requires the signaling adapter protein SAP.

CONCLUSIONS

Our data provide novel insight into the requirements for development of Vgamma1.1+Vdelta6.3+ T cells and indicate a role for Id3 in repressing the response of "innate" gammadelta T cells to SAP-mediated expansion or survival.

Transcriptional regulation of NKT cell development and homeostasis

NKT cells comprise a distinct T cell subset that acquires effector function during development and prior to antigen exposure. NKT cells are of limited antigen specificity but possess the ability to be recruited into an immune response without the need for further differentiation or proliferation and thus may be considered to function as memory cells or as part of the innate immune system. Although the development and maturation of NKT cells share some similarities with conventional T cell populations, many transcriptional regulators and signaling molecules are known to be uniquely required for NKT cell development. Recently, new transcription factors that specify NKT lineage and effector function and novel roles for previously identified transcriptional regulators in the differentiation of the NKT cell population have been discovered.

An essential role for the transcription factor HEB in thymocyte survival, Tcra rearrangement and the development of natural killer T cells

E proteins are basic helix-loop-helix transcription factors that regulate many key aspects of lymphocyte development. Thymocytes express multiple E proteins that are thought to provide cooperative and compensatory functions crucial for T cell differentiation. Contrary to that, we report here that the E protein HEB was uniquely required at the CD4(+)CD8(+) double-positive (DP) stage of T cell development. Thymocytes lacking HEB showed impaired survival, failed to make rearrangements of variable-alpha (V(alpha)) segments to distal joining-alpha (J(alpha)) segments in the gene encoding the T cell antigen receptor alpha-chain (Tcra) and had a profound, intrinsic block in the development of invariant natural killer T cells (iNKT cells) at their earliest progenitor stage. Thus, our results show that HEB is a specific and essential factor in T cell development and in the generation of the iNKT cell lineage, defining a unique role for HEB in the regulation of lymphocyte maturation.

Raising the NKT cell family

Natural killer T cells (NKT cells) are CD1d-restricted, lipid antigen-reactive, immunoregulatory T lymphocytes that can promote cell-mediated immunity to tumors and infectious organisms, including bacteria and viruses, yet paradoxically they can also suppress the cell-mediated immunity associated with autoimmune disease and allograft rejection. Furthermore, in some diseases, such as atherosclerosis and allergy, NKT cell activity can be deleterious to the host. Although the precise means by which these cells carry out such contrasting functions is unclear, recent studies have highlighted the existence of many functionally distinct NKT cell subsets. Because their frequency and number vary widely between individuals, it is important to understand the mechanisms that regulate the development and maintenance of NKT cells and subsets thereof, which is the subject of this review.

Alphabeta versus gammadelta lineage choice at the first TCR-controlled checkpoint

Alphabeta and gammadelta T cells develop in the thymus from a common precursor. Although lineages initially were defined by the type of TCR they express, it soon became clear that the TCR type per se does not play a deterministic role in the lineage decision, since in various transgenic and knockout models, as well as in a small fraction of cells in wt mice, the TCRgammadelta can drive the differentiation of alphabeta lineage cells and the TCRalphabeta can drive differentiation of gammadelta lineage cells. Thus until recently it was unclear what determines lineage choice and at which stage the two lineages diverge. Recent observations suggest that TCR signal strength determines lineage fate and that lineage choice is made at or shortly after the first TCR-controlled checkpoint. While it is clear that the decision between alphabeta and gammadelta lineages is made at the first TCR-controlled checkpoint and the alphabeta sublineages split off later, it is less clear whether gammadelta sublineages divert already at the first TCR-controlled checkpoint or later. Recent experiments support the former view.

Essential role of the p110beta subunit of phosphoinositide 3-OH kinase in male fertility

Phosphoinositide 3-kinases (PI3K) are key molecular players in male fertility. However, the specific roles of different p110 PI3K catalytic subunits within the spermatogenic lineage have not been characterized so far. Herein, we report that male mice expressing a catalytically inactive p110beta develop testicular hypotrophy and impaired spermatogenesis, leading to a phenotype of oligo-azoospermia and defective fertility. The examination of testes from p110beta-defective tubules demonstrates a widespread loss in spermatogenic cells, due to defective proliferation and survival of pre- and postmeiotic cells. In particular, p110beta is crucially needed in c-Kit-mediated spermatogonial expansion, as c-Kit-positive cells are lost in the adult testis and activation of Akt by SCF is blocked by a p110beta inhibitor. These data establish that activation of the p110beta PI3K isoform by c-Kit is required during spermatogenesis, thus opening the way to new treatments for c-Kit positive testicular cancers.

Generation of PLZF+ CD4+ T cells via MHC class II-dependent thymocyte-thymocyte interaction is a physiological process in humans

Human thymocytes, unlike mouse thymocytes, express major histocompatibility complex (MHC) class II molecules on their surface, especially during the fetal and perinatal stages. Based on this observation, we previously identified a novel developmental pathway for the generation of CD4⁺ T cells via interactions between MHC class II–expressing thymocytes (thymocyte–thymocyte [T–T] interactions) with a transgenic mouse system. However, the developmental dissection of this T–T interaction in humans has not been possible because of the lack of known cellular molecules specific for T–T CD4⁺ T cells. We show that promyelocytic leukemia zinc finger protein (PLZF) is a useful marker for the identification of T–T CD4⁺ T cells. With this analysis, we determined that a substantial number of fetal thymocytes and splenocytes express PLZF and acquire innate characteristics during their development in humans. Although these characteristics are quite similar to invariant NKT (iNKT) cells, they clearly differ from iNKT cells in that they have a diverse T cell receptor repertoire and are restricted by MHC class II molecules. These findings define a novel human CD4⁺ T cell subset that develops via an MHC class II–dependent T–T interaction.

Development of promyelocytic zinc finger and ThPOK-expressing innate gamma delta T cells is controlled by strength of TCR signaling and Id3

The broad-complex tramtrack and bric a brac-zinc finger transcriptional regulator (BTB-ZF), promyelocytic leukemia zinc finger (PLZF), was recently shown to control the development of the characteristic innate T cell phenotype and effector functions of NK T cells. Interestingly, the ectopic expression of PLZF was shown to push conventional T cells into an activated state that seems to be proinflammatory. The factors that control the normal expression of PLZF in lymphocytes are unknown. In this study, we show that PLZF expression is not restricted to NK T cells but is also expressed by a subset of gammadelta T cells, functionally defining distinct subsets of this innate T cell population. A second BTB-ZF gene, ThPOK, is important for the phenotype of the PLZF-expressing gammadelta T cells. Most importantly, TCR signal strength and expression of inhibitor of differentiation gene 3 control the frequency of PLZF-expressing gammadelta T cells. This study defines the factors that control the propensity of the immune system to produce potentially disease-causing T cell subsets.

Enhanced Th2 cell differentiation and allergen-induced airway inflammation in Zfp35-deficient mice

Studies of human asthma and of animal models of allergic airway inflammation revealed a crucial role for Th2 cells in the pathogenesis of allergic asthma. Kruppel-type zinc finger proteins are the largest family of a regulatory transcription factor for cellular development and function. Zinc finger protein (Zfp) 35 is an 18-zinc finger motif-containing Kruppel-type zinc finger protein, while its function remains largely unknown. The aim of this study was to clarify the role of Zfp35 in the pathogenesis of Th2-dependent allergic inflammation, such as allergic asthma. We examined airway eosinophilic inflammation and hyperresponsiveness in two mouse models, which use our newly generated Zfp35-deficient (Zfp35(-/-)) mice and adoptive transfer of cells. In Zfp35(-/-) mice, Th2 cell differentiation, Th2 cytokine production, eosinophilic inflammation, and airway hyperresponsiveness were substantially enhanced. Furthermore, adoptive transfer of Ag-sensitized Zfp35(-/-) CD4 T cells into the asthmatic mice resulted in enhanced airway inflammation and airway hyperresponsiveness. These results indicate that Zfp35 controls Th2 cell differentiation, allergic airway inflammation, and airway hyperresponsiveness in a negative manner. Thus, Zfp35 may control Th2-dependent diseases, such as allergic asthma.

PLZF is a regulator of homeostatic and cytokine-induced myeloid development

A major question in hematopoiesis is how the system maintains long-term homeostasis whereby the generation of large numbers of differentiated cells is balanced with the requirement for maintenance of progenitor pools, while remaining sufficiently flexible to respond to periods of perturbed cellular output during infection or stress. We focused on the development of the myeloid lineage and present evidence that promyelocytic leukemia zinc finger (PLZF) provides a novel function that is critical for both normal and stress-induced myelopoiesis. During homeostasis, PLZF restricts proliferation and differentiation of human cord blood-derived myeloid progenitors to maintain a balance between the progenitor and mature cell compartments. Analysis of PLZF promoter-binding sites revealed that it represses transcription factors involved in normal myeloid differentiation, including GFI-1, C/EBPalpha, and LEF-1, and induces negative regulators DUSP6 and ID2. Loss of ID2 relieves PLZF-mediated repression of differentiation identifying it as a functional target of PLZF in myelopoiesis. Furthermore, induction of ERK1/2 by myeloid cytokines, reflective of a stress response, leads to nuclear export and inactivation of PLZF, which augments mature cell production. Thus, negative regulators of differentiation can serve to maintain developmental systems in a primed state, so that their inactivation by extrinsic signals can induce proliferation and differentiation to rapidly satisfy increased demand for mature cells.

NKT cells in mucosal immunity

The gastrointestinal tract allows the residence of an almost enumerable number of bacteria. To maintain homeostasis, the mucosal immune system must remain tolerant to the commensal microbiota and eradicate pathogenic bacteria. Aberrant interactions between the mucosal immune cells and the microbiota have been implicated in the pathogenesis of inflammatory disorders, such as inflammatory bowel disease (IBD). In this review, we discuss the role of natural killer T cells (NKT cells) in intestinal immunology. NKT cells are a subset of non-conventional T cells recognizing endogenous and/or exogenous glycolipid antigens when presented by the major histocompatibility complex (MHC) class I-like antigen-presenting molecules CD1d and MR1. Upon T-cell receptor (TCR) engagement, NKT cells can rapidly produce various cytokines that have important roles in mucosal immunity. Our understanding of NKT-cell-mediated pathways including the identification of specific antigens is expanding. This knowledge will facilitate the development of NKT cell-based interventions and immune therapies for human intestinal diseases.

Dicer-dependent microRNA pathway controls invariant NKT cell development

Invariant NK T (iNKT) cells are a separate lineage of T lymphocytes with innate effector functions. They express an invariant TCR specific for lipids presented by CD1d and their development and effector differentiation rely on a unique gene expression program. We asked whether this program includes microRNAs, small noncoding RNAs that regulate gene expression posttranscriptionally and play a key role in the control of cellular differentiation programs. To this aim, we investigated iNKT cell development in mice in which Dicer, the RNase III enzyme that generates functional microRNAs, is deleted in cortical thymocytes. We find that Dicer deletion results in a substantial reduction of iNKT cells in thymus and their disappearance from the periphery, unlike mainstream T cells. Without Dicer, iNKT cells do not complete their innate effector differentiation and display a defective homeostasis due to increased cell death. Differentiation and homeostasis of iNKT cells require Dicer in a cell-autonomous fashion. Furthermore, we identify a miRNA profile specific for iNKT cells, which exhibits features of activated/effector T lymphocytes, consistent with the idea that iNKT cells undergo agonist thymic selection. Together, these results define a critical role of the Dicer-dependent miRNA pathway in the physiology of iNKT cells.

PLZF outreach: a finger in interferon's pie

In this issue of Immunity, Xu et al. (2009) find that the transcription factor PLZF activates interferon-stimulated genes and facilitates natural killer cell functions. Interferon-induced PLZF phosphorylation and histone deacetylase 1 recruitment probably mediates the repressor-to-activator conversion.

TCR-inducible PLZF transcription factor required for innate phenotype of a subset of gammadelta T cells with restricted TCR diversity

Some gammadelta and alphabeta T lymphocytes exhibit an "innate" phenotype associated with rapid cytokine responses. The PLZF transcription factor is essential for the innate phenotype of NKT cells. This report shows that PLZF is likewise responsible for the innate, NKT-like phenotype of Vgamma1+Vdelta6.3/Vdelta6.4+ cells. TCR cross-linking induced PLZF expression in all polyclonal immature gammadelta thymocytes, suggesting that agonist selection might be required for PLZF induction. Transgenic expression of Vgamma1Vdelta6.4 TCR was sufficient to support the development of large numbers of PLZF+ T cells, further supporting the importance of the TCR for PLZF induction. Interestingly, expression of this TCR transgene led to the development of spontaneous dermatitis.

Promyelocytic leukemia zinc finger protein regulates interferon-mediated innate immunity

Interferons (IFNs) direct innate and acquired immune responses and, accordingly, are used therapeutically to treat a number of diseases, yet the diverse effects they elicit are not fully understood. Here, we identified the promyelocytic leukemia zinc finger (PLZF) protein as a previously unrecognized component of the IFN response. IFN stimulated an association of PLZF with promyelocytic leukemia protein (PML) and histone deacetylase 1 (HDAC1) to induce a decisive subset of IFN-stimulated genes (ISGs). Consequently, PLZF-deficient mice had a specific ISG expression defect and as a result were more susceptible to viral infection. This susceptibility correlated with a marked decrease in the expression of the key antiviral mediators and an impaired IFN-mediated induction of natural killer cell function. These results provide new insights into the regulatory mechanisms of IFN signaling and the induction of innate antiviral immunity.

Where do MAIT cells fit in the family of unconventional T cells?

Mucosal-associated invariant T cells are newly identified subpopulation of T cells. A new study highlights their developmental pathway and functional features that allow these cells to assume a unique position in the family of unconventional T cells.

Stepwise development of MAIT cells in mouse and human

Mucosal-associated invariant T (MAIT) cells display two evolutionarily conserved features: an invariant T cell receptor (TCR)α (iTCRα) chain and restriction by the nonpolymorphic class Ib major histocompatibility complex (MHC) molecule, MHC-related molecule 1 (MR1). MR1 expression on thymus epithelial cells is not necessary for MAIT cell development but their accumulation in the gut requires MR1 expressing B cells and commensal flora. MAIT cell development is poorly known, as these cells have not been found in the thymus so far. Herein, complementary human and mouse experiments using an anti-humanVα7.2 antibody and MAIT cell-specific iTCRα and TCRβ transgenic mice in different genetic backgrounds show that MAIT cell development is a stepwise process, with an intra-thymic selection followed by peripheral expansion. Mouse MAIT cells are selected in an MR1-dependent manner both in fetal thymic organ culture and in double iTCRα and TCRβ transgenic RAG knockout mice. In the latter mice, MAIT cells do not expand in the periphery unless B cells are added back by adoptive transfer, showing that B cells are not required for the initial thymic selection step but for the peripheral accumulation. In humans, contrary to natural killer T (NKT) cells, MAIT cells display a naïve phenotype in the thymus as well as in cord blood where they are in low numbers. After birth, MAIT cells acquire a memory phenotype and expand dramatically, up to 1%–4% of blood T cells. Finally, in contrast with NKT cells, human MAIT cell development is independent of the molecular adaptor SAP. Interestingly, mouse MAIT cells display a naïve phenotype and do not express the ZBTB16 transcription factor, which, in contrast, is expressed by NKT cells and the memory human MAIT cells found in the periphery after birth. In conclusion, MAIT cells are selected by MR1 in the thymus on a non-B non-T hematopoietic cell, and acquire a memory phenotype and expand in the periphery in a process dependent both upon B cells and the bacterial flora. Thus, their development follows a unique pattern at the crossroad of NKT and γδ T cells.

White blood cells, or lymphocytes, play an important role in defending the body from infection and disease. T lymphocytes come in many varieties with diverse functions. Mucosal-associated invariant T (MAIT) cells constitute a subset of unconventional T lymphocytes, characterized by their invariant T cell receptor (TCR)α chain and their requirement for the nonpolymorphic class Ib (MHC) molecule, MR1. MAIT cells are extremely abundant in human blood and mucosae. Contrary to mainstream T cells, their development requires B cells and commensal microbial flora. To shed light on the little-understood MAIT cells, we used new tools, including an antibody that we recently developed to detect human MAIT cells, and we were able to show that MAIT cell development is a stepwise process, with an intra-thymic selection followed by peripheral expansion. We show that thymic selection is MR1 dependent but requires neither B cells nor the commensal flora, which are both necessary for the expansion in the periphery. In contrast with the other evolutionarily conserved invariant subset, the natural killer T (NKT) cells, we found that MAIT cells exit the thymus as “naïve” cells before becoming antigen-experienced memory cells and expanding in number to represent a significant 1%–4% of peripheral T cells in human blood. In mice, we found that MAIT cells remain naïve and do not expand substantially. We conclude that MAIT cell development follows a unique scheme, where, unlike NKT cells, MAIT cell selection and expansion are uncoupled events that are mediated by distinct cell types in different compartments.

Mucosal-associated invariant T cells, the most abundant invariant T cell subset in humans, arise via a distinct developmental pathway that represents a hybrid of that seen for NKT and γδ T cells, two other unconventional T cell subsets.

Intrathymic proliferation wave essential for Valpha14+ natural killer T cell development depends on c-Myc

The molecular requirements for invariant Valpha14-bearing natural killer T cells (iNKT) in the thymus are poorly understood. A minute population of approximately 500 newly selected CD69(+)CD24(+) stage 0 (ST0) iNKT cells gives rise to approximately 100 times more CD44(neg/lo)CD24(-) stage 1 (ST1) cells, which then generate similar frequencies of CD44(hi)CD24(-) stage 2 (ST2) and mature iNKT cells. Although the increased number of ST1 compared with ST0 cells indicates the initiation of a proliferation wave in the very early stages of iNKT cell development, details about the controlling mechanism are currently lacking. Here, we show that the transcription factor c-Myc is required for iNKT cell development. Conditional ablation of c-Myc in double-positive thymocytes specifically impacted iNKT but not conventional T cell development. Within the iNKT population, a progressive reduction of iNKT cells was observed starting at ST1 (approximately 50-fold) and ST2 (approximately 350-fold), with a complete lack of mature cells in thymus, spleen, and liver. ST0/ST1 c-Myc-deficient iNKT cells showed reduced proliferation. In contrast, annexin V staining did not reveal increased apoptosis, and transgenic overexpression of BCL-2 did not rescue iNKT cell development in c-Myc-deficient mice. Moreover, expression of known iNKT differentiation factors such as Plzf and Gata3 was not dramatically altered. These, findings provide compelling evidence that c-Myc mediates an intrathymic proliferation wave immediately after agonist selection of iNKT cells and illustrate the importance of this expansion for the generation of mature iNKT cells in vivo.

Tec kinase Itk in gammadeltaT cells is pivotal for controlling IgE production in vivo

In conventional alphabeta T cells, the Tec family tyrosine kinase Itk is required for signaling downstream of the T cell receptor (TCR). Itk also regulates alphabeta T cell development, lineage commitment, and effector function. A well established feature of Itk(-/-) mice is their inability to generate T helper type 2 (Th2) responses that produce IL-4, IL-5, and IL-13; yet these mice have spontaneously elevated levels of serum IgE and increased numbers of germinal center B cells. Here we show that the source of this phenotype is gammadelta T cells, as normal IgE levels are observed in Itk(-/-)Tcrd(-/-) mice. When stimulated through the gammadelta TCR, Itk(-/-) gammadelta T cells produce high levels of Th2 cytokines, but diminished IFNgamma. In addition, activated Itk(-/-) gammadelta T cells up-regulate costimulatory molecules important for B cell help, suggesting that they may directly promote B cell activation and Ig class switching. Furthermore, we find that gammadelta T cells numbers are increased in Itk(-/-) mice, most notably the Vgamma1.1(+)Vdelta6.3(+) subset that represents the dominant population of gammadelta NKT cells. Itk(-/-) gammadelta NKT cells also have increased expression of PLZF, a transcription factor required for alphabeta NKT cells, indicating a common molecular program between alphabeta and gammadelta NKT cell lineages. Together, these data indicate that Itk signaling regulates gammadelta T cell lineage development and effector function and is required to control IgE production in vivo.

NK and NKT cells in aging and longevity: role of zinc and metallothioneins

INTRODUCTION

During aging, dysregulated immune functions occur contributing to increased susceptibility to morbidity and mortality. However, these dysregulations are normally counterbalanced by continuous adaptation of the body to the deteriorative changes occurring over time. These adaptive changes well occur in healthy centenarians.

DISCUSSION

Both innate (natural) and adaptive (acquired) immune responses decline with advancing age. Natural killer (NK) and natural killer T (NKT) cell cytotoxicity, representing one of best models of innate immune response, decreases in aging as well as interferon-gamma (IFN-gamma) production by both activated types of cells. Both NK and NKT cell cytotoxicity and IFN-gamma production increase in very old age with respect to normal aging, especially by NKT cells bearing TCRgammadelta. The role played by zinc and metallothioneins (MT) is crucial because this affects NK and NKT cell development, maturation, and functions. In particular, some MT polymorphisms are involved in maintaining innate immune response and intracellular zinc ion availability in aging with thus a role of MT genetic background to escape some age-related diseases with subsequent healthy aging and longevity.

Selective requirement for c-Myc at an early stage of V(alpha)14i NKT cell development

Valpha14 invariant (Valpha14i) NKT cells are a subset of regulatory T cells that utilize a semi-invariant TCR to recognize glycolipids associated with monomorphic CD1d molecules. During development in the thymus, CD4(+)CD8(+) Valpha14i NKT precursors recognizing endogenous CD1d-associated glycolipids on other CD4(+)CD8(+) thymocytes are selected to undergo a maturation program involving sequential expression of CD44 and NK-related markers such as NK1.1. The molecular requirements for Valpha14i NKT cell maturation, particularly at early developmental stages, remain poorly understood. In this study, we show that CD4-Cre-mediated T cell-specific inactivation of c-Myc, a broadly expressed transcription factor with a wide range of biological activities, selectively impairs Valpha14i NKT cell development without perturbing the development of conventional T cells. In the absence of c-Myc, Valpha14i NKT cell precursors are blocked at an immature CD44(low)NK1.1(-) stage in a cell autonomous fashion. Residual c-Myc-deficient immature Valpha14i NKT cells appear to proliferate normally, cannot be rescued by transgenic expression of BCL-2, and exhibit characteristic features of immature Valpha14i NKT cells such as high levels of preformed IL-4 mRNA and the transcription factor promyelocytic leukemia zinc finger. Collectively our data identify c-Myc as a critical transcription factor that selectively acts early in Valpha14i NKT cell development to promote progression beyond the CD44(low)NK1.1(-) precursor stage.

Antagonistic interplay between ThPOK and Runx in lineage choice of thymocytes

Differentiation of CD4(+)CD8(+) double-positive (DP) thymocytes into either CD4(+)-helper or CD8(+)-cytotoxic lineages involves several phases. It has been suggested that, following initial specification to one of the lineages by a set of lineage-specific genes during positive selection, stable cell identity is established during the commitment process by eliminating differentiation potential toward the other lineage. While the Runx3 transcription factor fixes the Cd4 gene into a silenced state during cytotoxic-lineage cell differentiation, the ThPOK transcription factor is both necessary and sufficient to generate a CD4(+)CD8(-) phenotype in post-selection thymocytes, regardless of the MHC specificity of the TCRs. Recent studies have revealed that a reciprocal antagonistic interplay between Runx3 and ThPOK is a central component in the transcription factor network governing the helper versus cytotoxic-lineage decision.

Tec kinases regulate T-lymphocyte development and function: new insights into the roles of Itk and Rlk/Txk

The Tec (tyrosine kinase expressed in hepatocellular carcinoma) family of non-receptor tyrosine kinases consists of five members: Tec, Bruton's tyrosine kinase (Btk), inducible T-cell kinase (Itk), resting lymphocyte kinase (Rlk/Txk), and bone marrow-expressed kinase (Bmx/Etk). Although their functions are probably best understood in antigen receptor signaling, where they participate in the phosphorylation and regulation of phospholipase C-gamma (PLC-gamma), it is now appreciated that these kinases contribute to signaling from many receptors and that they participate in multiple downstream pathways, including regulation of the actin cytoskeleton. In T cells, three Tec kinases are expressed, Itk, Rlk/Txk, and Tec. Itk is expressed at highest amounts and plays the major role in regulating signaling from the T-cell receptor. Recent studies provide evidence that these kinases contribute to multiple aspects of T-cell biology and have unique roles in T-cell development that have revealed new insight into the regulation of conventional and innate T-cell development. We review new findings on the Tec kinases with a focus on their roles in T-cell development and mature T-cell differentiation.

The Tec kinases Itk and Rlk regulate conventional versus innate T-cell development

Tec family kinases are important components of antigen receptor signaling pathways in B cells, T cells, and mast cells. In T cells, three members of this family, inducible T-cell kinase (Itk), resting lymphocyte kinase (Rlk), and Tec, are expressed. In the absence of Itk and Rlk, T-cell receptor signaling is impaired, with defects in mitogen-activated protein kinase activation, Ca(2+) mobilization, and actin polymerization. During T-cell development in the thymus, no role has been found for these kinases in the CD4(+) versus CD8(+) T-cell lineage decision; however, several studies indicate that Itk and Rlk contribute to the signaling leading to positive and negative selection. In addition, we and others have recently described an important role for Itk and Rlk in the development of conventional as opposed to innate CD4(+) and CD8(+) T cells. Natural killer T and gammadelta T-cell populations are also altered in Itk- and Rlk/Itk-deficient mice. These findings strongly suggest that the strength of T-cell receptor signaling during development determines whether T cells mature into conventional versus innate lymphocyte lineages. This lineage decision is also influenced by signaling via signaling lymphocytic activation molecule (SLAM) family receptors. Here we discuss these two signaling pathways that each contribute to conventional versus innate T-cell lineage commitment.

MHC class II-dependent T-T interactions create a diverse, functional and immunoregulatory reaction circle

Unlike conventional T cells, innate-like T cells such as natural killer (NK) T cells are selected by homotypic T-cell interactions. Recently, a few reports have shown that T-T CD4(+) T cells can be generated in a similar manner to that for NKT cells. These two types of cells share common functional properties such as rapid response to antigenic encounters and the potential for a panoply of cytokine secretion. However, T-T CD4(+) T cells differ from NKT cells in that they are restricted by highly polymorphic major histocompatibility complex (MHC) II molecules and have a diverse T-cell receptor repertoire. Additional example of T-T interactions was recently reported in which peripheral T cells re-circulate to the thymus and participate in the thymocyte selection process. In this review, we dissect the cellular mechanisms underlying the production of T-T CD4(+) and NKT cells, with particular emphasis on the differences between these two T-cell prototypes. Finally, we propose that T-T CD4(+) T cells serve two major functions: one as an acute-phase reactant against viral infection and the other is the generation of anti-ergotypic CD4(+) T cells for regulatory purposes. All of these features make it possible to create a diverse set of functional cells through MHC class II-restricted T-T interactions.

The transcriptional regulator PLZF induces the development of CD44 high memory phenotype T cells

Transcriptional pathways controlling the development of CD44(hi) memory phenotype (MP) T cells with "innate-like" functions are not well understood. Here we show that the BTB (bric-a-brac, tramtrack, broad complex) domain-containing protein promyelocytic leukemia zinc finger (PLZF) is expressed in CD44(hi), but not in CD44(lo), CD4(+) T cells. Transgenic expression of PLZF during T cell development and in CD4(+) and CD8(+) T cells induced a T cell intrinsic program leading to an increase in peripheral CD44(hi) MP CD4(+) and CD8(+) T cells and a corresponding decrease of naïve CD44(lo) T cells. The MP CD4(+) and CD8(+) T cells produced IFNgamma upon PMA/ionomycin stimulation, thus showing innate-like function. Changes in the naïve versus memory-like subset distribution were already evident in single-positive thymocytes, indicating PLZF-induced T cell developmental alterations. In addition, CD1d-restricted natural killer T cells in PLZF transgenic mice showed impaired development and were severely reduced in the periphery. Finally, after anti-CD3/CD28 stimulation, CD4(+) transgenic T cells showed reduced IL-2 and IFNgamma production but increased IL-4 secretion as a result of enhanced IL-4 production of the CD44(hi)CD62L(+) subset. Our data indicate that PLZF is a novel regulator of the development of CD44(hi) MP T cells with a characteristic partial innate-like phenotype.