Attenuation of gammadeltaTCR signaling efficiently diverts thymocytes to the alphabeta lineage

NKT cells: from totipotency to regenerative medicine

The recent discovery that natural killer T (NKT) cell nuclei are totipotent opens a novel avenue for further understanding NKT cell function in normal and diseased states. The progeny of a cloned mouse harboring the in-frame rearranged Valpha14-Jalpha18 T cell receptor in one allele showed a significant increase in NKT cell number compared with wild-type or littermate control mice that possessed a different TCR. Importantly, NKT cells from such progeny produced both interferon-gamma and interleukin-4, a hallmark of NKT cells. In these progeny, NKT cell development appeared to be instructively, rather than permissively, determined. Using embryonic stem cells prepared via the somatic cell nuclear transfer of NKT nuclei, relatively mature NKT cells were induced under conditions permissible for T cell induction. Furthermore, these NKT cells matured autonomously upon injection into mice, resulting in an antigen-specific adjuvant effect.

TCR-beta chains derived from peripheral gammadelta T cells can take part in alphabeta T-cell development

Between 10 and 20% of the peripheral gammadelta T cells express cytoplasmic TCR-beta proteins, but whether such TCR-beta chains can partake in alphabeta T-cell development has never been systematically investigated. Therefore, we reconstituted the T-cell compartment of CD3epsilon-deficient mice with Pax5-TCR-beta deficient proB cells expressing, via a retroviral vector, TCR-beta chains from either peripheral gammadelta or alphabeta T cells. Recipient thymi reconstituted with proB cells containing empty vector were small (<15x10(6) cells), contained few gammadelta T but no alphabeta T cells. In contrast, thymi from mice receiving proB cells containing gammadelta or alphabeta T-cell-derived TCR-beta chains contained 80-130x10(6) cells, and showed a normal CD4, CD8 and alphabeta TCR expression pattern. However, regardless of the source of TCR-beta chain, reconstituted mice rapidly showed signs of autoimmunity dying 5-15 wk following reconstitution. Autoimmune disease induction could be prevented by co-transfer of Treg cells thereby allowing the functionality of the generated T cells to be assessed. Results obtained show that TCR-beta chains from gammadelta T cells can efficiently take part in alphabeta T-cell development. The implications of these findings for gammadelta T-cell development will be discussed.

An early decrease in Notch activation is required for human TCR-alphabeta lineage differentiation at the expense of TCR-gammadelta T cells

Although well characterized in the mouse, the role of Notch signaling in the human T-cell receptor alphabeta (TCR-alphabeta) versus TCR-gammadelta lineage decision is still unclear. Although it is clear in the mouse that TCR-gammadelta development is less Notch dependent compared with TCR-alphabeta differentiation, retroviral overexpression studies in human have suggested an opposing role for Notch during human T-cell development. Using the OP9-coculture system, we demonstrate that changes in Notch activation are differentially required during human T-cell development. High Notch activation promotes the generation of T-lineage precursors and gammadelta T cells but inhibits differentiation toward the alphabeta lineage. Reducing the amount of Notch activation rescues alphabeta-lineage differentiation, also at the single-cell level. Gene expression analysis suggests that this is mediated by differential sensitivities of Notch target genes in response to changes in Notch activation. High Notch activity increases DTX1, NRARP, and RUNX3 expression, genes that are down-regulated during alphabeta-lineage differentiation. Furthermore, increased interleukin-7 levels cannot compensate for the Notch dependent TCR-gammadelta development. Our results reveal stage-dependent molecular changes in Notch signaling that are critical for normal human T-cell development and reveal fundamental molecular differences between mouse and human.

Molecular mechanisms that control mouse and human TCR-alphabeta and TCR-gammadelta T cell development

Following specification of hematopoietic precursor cells into the T cell lineage, several developmental options remain available to the immature thymocytes. The paradigm is that the outcome of the T cell receptor rearrangements and the corresponding T cell receptor signaling events will be predominant to determine the first of these choices: the alphabeta versus gammadelta T cell pathways. Here, we review the thymus-derived environmental signals, the transcriptional mediators, and other molecular mechanisms that are also involved in this decision in both the mouse and human. We discuss the differences in cellular events between the alphabeta and gammadelta developmental pathways and try to correlate these with a corresponding complexity of the molecular mechanisms that support them.

SCART scavenger receptors identify a novel subset of adult gammadelta T cells

Although there has been great progress in the characterization of alphabeta T cell differentiation, selection, and function, gammadelta T cells have remained poorly understood. One of the main reasons for this is the lack of gammadelta T cell-specific surface markers other than the TCR chains themselves. In this study we describe two novel surface receptors, SCART1 and SCART2. SCARTs are related to CD5, CD6, and CD163 scavenger receptors but, unlike them, are found primarily on developing and mature gammadelta T cells. Characterization of SCART2 positive immature and peripheral gammadelta T cells suggests that they undergo lineage specification in the thymus and belong to a new IL-17-producing subset with distinct homing capabilities.

Thymic selection determines gammadelta T cell effector fate: antigen-naive cells make interleukin-17 and antigen-experienced cells make interferon gamma

gammadelta T cells uniquely contribute to host immune defense, but how this is accomplished remains unclear. Here, we analyzed the nonclassical major histocompatibility complex class I T10 and T22-specific gammadelta T cells in mice and found that encountering antigen in the thymus was neither required nor inhibitory for their development. But when triggered through the T cell receptor, ligand-naive lymphoid-gammadelta T cells produced IL-17, whereas ligand-experienced cells made IFN-gamma. Immediately after immunization, a large fraction of IL-17(+) gammadelta T cells were found in the draining lymph nodes days before the appearance of antigen-specific IL-17(+) *beta T cells. Thus, thymic selection determines the effector fate of gammadelta T cells rather than constrains their antigen specificities. The swift IL-17 response mounted by antigen-naive gammadelta T cells suggests a critical role for these cells at the onset of an acute inflammatory response to novel antigens.

The thymus chapter in the life of gut-specific intra epithelial lymphocytes

The intestinal intraepithelial lymphocytes (IEL) represent multi-lineage T cell populations. In addition to a major gammadeltaTCR(+) T cell subset, many IEL express alphabetaTCRs and they can be separated into alphabeta sublineages. Some TCRalphabeta(+)IEL have characteristics in common with conventional TCRalphabeta(+)T cells whereas others share an unconventional phenotype with their TCRgammadelta(+) counterparts. Because the latter are enriched for autoreactive TCRs and can be generated in the absence of a thymus, it has long been postulated that some IEL subsets develop locally in the intestine. Several new data however, indicate that under physiological conditions, IEL require a thymic education that directs lineage commitment and functional differentiation. This review will discuss the contributions of the thymus in shaping the various intestinal IEL sublineages.

T cell receptor-instructed alphabeta versus gammadelta lineage commitment revealed by single-cell analysis

alphabeta and gammadelta T cell lineages develop in the thymus from a common precursor. It is unclear at which stage of development commitment to these lineages takes place and in which way T cell receptor signaling contributes to the process. Recently, it was demonstrated that strong TCR signals favor gammadelta lineage development, whereas weaker TCR signals promote alphabeta lineage fate. Two models have been proposed to explain these results. The first model suggests that commitment occurs after TCR expression and TCR signaling directly instructs lymphocytes to adopt one or the other lineage fate. The second model suggests that commitment occurs before TCR expression and that TCR signaling merely confirms the lineage choice. By tracing the fate of single T cell precursors, this study shows that there is no commitment to either the alphabeta or gammadelta lineage before TCR expression and that modulation of TCR signaling in progeny of a single TCR-expressing cell changes lineage commitment.

MHC-restricted T cell receptor signaling is required for alpha beta TCR replacement of the pre T cell receptor

A developmental block is imposed on CD25(+)CD44(-) thymocytes at the beta-selection checkpoint in the absence of the pre T cell receptor (preTCR) alpha-chain, pTalpha. Early surface expression of a transgenic alphabeta TCR has been shown to partially circumvent this block, such that thymocytes progress to the CD4(+)CD8(+) double-positive stage. We wanted to analyze whether a restricting MHC element is required for alphabeta TCR-expressing double-negative (DN) thymocytes to overcome the developmental block in pTalpha-deficient animals. We used the HY-I knock-in model that endows thymocytes with alphabeta TCR expression in the DN compartment but has the advantage of physiological expression levels, in contrast to conventional TCR transgenes. On a pTalpha-deficient background, this HY-I TCR transgene 'rescued' CD25(+)CD44(-) thymocytes from apoptosis and enabled progression to later differentiation stages. On a non-selecting MHC background, however, pTalpha-deficient HY-I mice presented a pronounced reduction in numbers of splenocytes and thymocytes when compared to animals of selecting MHC genotype, showing that MHC restriction is necessary to drive HY-TCR-mediated rescue of pTalpha-deficient thymocytes.

The Thymus as an Inductive Site for T Lymphopoiesis

Like all hematopoietic cells, T lymphocytes are derived from bone-marrow-resident stem cells. However, whereas most blood lineages are generated within the marrow, the majority of T cell development occurs in a specialized organ, the thymus. This distinction underscores the unique capacity of the thymic microenvironment to support T lineage restriction and differentiation. Although the identity of many of the contributing thymus-derived signals is well established and rooted in highly conserved pathways involving Notch, morphogenetic, and protein tyrosine kinase signals, the manner in which the ensuing cascades are integrated to orchestrate the underlying processes of T cell development remains under investigation. This review focuses on the current definition of the early stages of T cell lymphopoiesis, with an emphasis on the nature of thymus-derived signals delivered to T cell progenitors that support the commitment and differentiation of T cells toward the alphabeta and gammadelta T cell lineages.

T cells developing in fetal thymus of T-cell receptor alpha-chain transgenic mice colonize gammadelta T-cell-specific epithelial niches but lack long-term reconstituting potential

The gammadelta T cells generated during mouse fetal development are absolutely dependent on their invariant T-cell receptors (TCRs) for their function. However, there is little information on whether the epithelial homing properties of fetal T cells might also be developmentally induced by factors unrelated to TCR specificity. We have previously described TCR alpha-chain transgenic (2B4 TCR-alpha TG) mice, in which the transgenic TCR alpha-chain is expressed early, already at embryonic day 14 (E14). These mice have a large population of 'gammadelta T-cell-like' CD4- CD8- (double-negative; DN) alphabeta T cells, some of which develop during E14-E18 contemporarily to intraepithelial lymphocytes (IELs) expressing invariant TCR-gammadelta. Using the 2B4 TCR-alpha TG mouse model we have been able to more precisely study the impact of a variant TCR expression on IEL development and homing. In this study we show that TCR-alpha TG and TCR-alpha TG crossed to TCR-delta-deficient mice (TCR-alpha TG x TCR-delta-/-) carry TG TCR-alpha+ dendritic epidermal T cells (DETCs) and TCR-alpha TG+ IELs in the small intestine. The TG+ DETCs develop and seed the epidermis with similar kinetics as Vgamma5+ DETCs of normal mice, in contrast to the TCR-alphabeta+ DETCs found in TCR-delta-/- mice. However, whereas the intestinal TCR-alpha TG+ IELs persist in old mice (> 20 months), the TCR-alpha TG+ DETCs do not. The data in this study indicate that the timing of TCR expression and thereby development during ontogeny regulates the specific homing potential for fetal T cells but not their subsequent functions and properties.

Regulation of alphabeta/gammadelta T cell development by the activator protein 1 transcription factor c-Jun

c-Jun is a member of the AP-1 family of transcription factors, the activity of which is strongly augmented by TCR signaling. To elucidate the functions of c-Jun in mouse thymic lymphopoiesis, we conditionally inactivated c-Jun specifically during early T cell development. The loss of c-Jun resulted in enhanced generation of gammadelta T cells, whereas alphabeta T cell development was partially arrested at the double-negative 3 stage. The increased generation of gammadelta T cells by loss of c-Jun was cell autonomous, because in a competitive reconstitution experiment the knockout-derived cells produced more gammadelta T cells than did the control cells. C-jun-deficient immature T cells failed to efficiently repress transcription of IL-7Ralpha, resulting in augmented IL-7Ralpha mRNA and surface levels. Chromatin immunoprecipitation assays revealed binding of c-Jun to AP-1 binding sites present in the IL-7Ralpha promoter, indicating direct transcriptional regulation. Thus, c-Jun controls the transcription of IL-7Ralpha and is a novel regulator of the alphabeta/gammadelta T cell development.

Zoned out: functional mapping of stromal signaling microenvironments in the thymus

All hematopoietic cells, including T lymphocytes, originate from stem cells that reside in the bone marrow. Most hematopoietic lineages also mature in the bone marrow, but in this respect, T lymphocytes differ. Under normal circumstances, most T lymphocytes are produced in the thymus from marrow-derived progenitors that circulate in the blood. Cells that home to the thymus from the marrow possess the potential to generate multiple T and non-T lineages. However, there is little evidence to suggest that, once inside the thymus, they give rise to anything other than T cells. Thus, signals unique to the thymic microenvironment compel multipotent progenitors to commit to the T lineage, at the expense of other potential lineages. Summarizing what is known about the signals the thymus delivers to uncommitted progenitors, or to immature T-committed progenitors, to produce functional T cells is the focus of this review.

Ablation of ribosomal protein L22 selectively impairs alphabeta T cell development by activation of a p53-dependent checkpoint

The alphabeta and gammadelta T lineages are thought to arise from a common precursor; however, the regulation of separation and development of these lineages is not fully understood. We report here that development of alphabeta and gammadelta precursors was differentially affected by elimination of ribosomal protein L22 (Rpl22), which is ubiquitously expressed but not essential for translation. Rpl22 deficiency selectively arrested development of alphabeta-lineage T cells at the beta-selection checkpoint by inducing their death. The death was caused by induction of p53 expression, because p53 deficiency blocked death and restored development of Rpl22-deficient thymocytes. Importantly, Rpl22 deficiency led to selective upregulation of p53 in alphabeta-lineage thymocytes, at least in part by increasing p53 synthesis. Taken together, these data indicate that Rpl22 deficiency activated a p53-dependent checkpoint that produced a remarkably selective block in alphabeta T cell development but spared gammadelta-lineage cells, suggesting that some ribosomal proteins may perform cell-type-specific or stage-specific functions.

Functional genomics analyses of differential macaque peripheral blood mononuclear cell infections by human immunodeficiency virus-1 and simian immunodeficiency virus

The pathogenicity of the primate lentiviruses, human, and simian immunodeficiency viruses, is host-specific. Previous studies indicated that the highly pathogenic human lentivirus HIV-1 has markedly reduced pathogenicity compared to the pathogenic simian lentivirus SIV in pigtail macaques (Macaca nemestrina). We therefore hypothesized that the pigtail macaque peripheral blood mononuclear cells (mPBMCs) would respond differently to infections of HIV-1 and pathogenic SIV. To elucidate the cellular responses to the infections of HIV-1 and SIV, we infected mPBMC with these two viruses. Like infections in vivo, HIV-1 and SIV demonstrated distinct replication kinetics in mPBMCs, with HIV-1 replicating at significantly lower levels. Similarly, gene expression profiling facilitated by macaque-specific oligonucleotide microarrays also revealed distinct expression patterns of genes between the HIV-1- and SIV-infected mPBMCs; in particular, genes associated with the antigen presentation, T cell receptor, ERK/MAPK signaling, Wnt/beta-catenin signaling, and natural killer cell signaling pathways were differentially regulated between these two viruses. Most interestingly, despite the lower levels of replication, HIV-1 triggered a more robust regulation of immune response genes early after infection; the converse was true in SIV-infected mPBMCs. Our results therefore suggest that macaques may be controlling the infection of HIV-1 at an early stage through coordinated regulation of host defense pathways.

Key factors in the organized chaos of early T cell development

A fundamental issue in T cell development is what controls whether a thymocyte differentiates into a gammadelta T cell or an alphabeta T cell, each defined by their distinct T cell receptor. Most likely, lessons learned in studying that issue will also provide insight into how the thymus produces T cell subsets with distinct functional and regulatory potentials. Here we review recent experiments, focusing on three factors that regulate thymocyte differentiation up to and including the expression of the first products of antigen receptor gene rearrangements. Those factors are the archetypal developmental regulator Notch, intrinsic signals emanating from antigen-receptor complexes, and trans conditioning, which reflects communication between different subsets of thymocytes. We also review new findings on the positive selection of gammadelta T cells and on extrathymic T cell development.

Development and selection of gammadelta T cells

Two main lineages of T cells develop in the thymus: those that express the alphabeta T-cell receptor (TCR) and those that express the gammadelta TCR. Whereas the development, selection, and peripheral localization of newly differentiated alphabeta T cells are understood in some detail, these processes are less well characterized in gammadelta T cells. This review describes research carried out in this laboratory and others, which addresses several key aspects of gammadelta T-cell development, including the decision of precursor cells to differentiate into the gammadelta versus alphabeta lineage, the ordered differentiation over the course of ontogeny of functional gammadelta T-cell subsets expressing distinct TCR structures, programming of ordered Vgamma gene rearrangement in the thymus, including a molecular switch that ensures appropriate Vgamma rearrangements at the appropriate stage of development, positive selection in the thymus of gammadelta T cells destined for the epidermis, and the acquisition by developing gammadelta T cells of cues that determine their correct localization in the periphery. This research suggests a coordination of molecularly programmed events and cellular selection, which enables specialization of the thymus for production of distinct T-cell subsets at different stages of development.

gammadelta T-cell receptors: functional correlations

The gammadelta T-cell receptors (TCRs) are limited in their diversity, suggesting that their natural ligands may be few in number. Ligands for gammadeltaTCRs that have thus far been determined are predominantly of host rather than foreign origin. Correlations have been noted between the Vgamma and/or Vdelta genes a gammadelta T cell expresses and its functional role. The reason for these correlations is not yet known, but several different mechanisms are conceivable. One possibility is that interactions between particular TCR-V domains and ligands determine function or functional development. However, a recent study showed that at least for one ligand, receptor specificity is determined by the complementarity-determining region 3 (CDR3) component of the TCR-delta chain, regardless of the Vgamma and/or Vdelta. To determine what is required in the TCR for other specificities and to test whether recognition of certain ligands is connected to cell function, more gammadeltaTCR ligands must be defined. The use of recombinant soluble versions of gammadeltaTCRs appears to be a promising approach to finding new ligands, and recent results using this method are reviewed.

TCR and Notch synergize in αβ versus γδ lineage choice

At two checkpoints, T cell development is controlled by T cell receptor (TCR) signaling, which determines survival and lineage commitment. At the first of these checkpoints, signaling by the pre-TCR, the gammadeltaTCR or the alphabetaTCR has a major but nonexclusive impact on whether cells will become CD4-CD8- gammadelta or CD4+CD8+ alphabeta lineage cells. Pre-TCR signals synergize with moderate Notch signals to generate alphabeta lineage cells. Relatively strong signals by the gammadeltaTCR (or early expressed alphabetaTCR) in the absence of Notch signaling are sufficient to yield gammadelta lineage cells. However, relatively weak signals of the latter two receptors combined with strong Notch signaling result in the formation of alphabeta lineage cells that generate a diverse alphabetaTCR repertoire in pre-TCR-deficient mice. It remains to be determined whether TCR and/or Notch signals instruct or confirm predetermined lineage fate.

Molecular events that regulate alphabeta versus gammadelta T cell lineage commitment: old suspects, new players and different game plans

The divergence of alphabeta and gammadelta T cells from a common precursor in the thymus is regulated by multiple cell-intrinsic and cell-extrinsic factors, most of which are not well defined. Recent studies have provided crucial data regarding the precise timing of lineage commitment and some clarification on the extent of the involvement of Notch and T-cell receptor signaling in this process. Combined with new insights into the differential regulation of molecular pathways active in alphabeta and gammadelta precursors, these data have led to the generation of a revised model of lineage commitment.

TCRgamma silencing during alphabeta T cell development depends upon pre-TCR-induced proliferation

During thymus development, immature T cells become committed to two distinct lineages based upon expression of alphabeta or gammadelta TCR. In the alphabeta lineage, developing thymocytes progressively extinguish transcription of the TCRgamma genes by a poorly understood process known as gamma silencing. We show that alphabeta lineage thymocytes in mice lacking a functional pre-TCR undergo limited proliferation and fail to silence TCRgamma genes during development. Stimulation of pre-TCR-deficient immature thymocytes with anti-CD3 Abs does not directly down-regulate TCRgamma transcription but restores TCRgamma silencing following proliferation. Collectively our data reveal an important role for pre-TCR induced proliferation in activating the TCRgamma silencer in alphabeta lineage thymocytes, a process that may reinforce alphabeta or gammadelta lineage commitment.

PKB Rescues Calcineurin/NFAT-Induced Arrest of Rag Expression and Pre-T Cell Differentiation

Protein kinase B (PKB), an Ag receptor activated serine-threonine kinase, controls various cellular processes including proliferation and survival. However, PKB function in thymocyte development is still unclear. We report PKB as an important negative regulator of the calcineurin (CN)-regulated transcription factor NFAT in early T cell differentiation. Expression of a hyperactive version of CN induces a profound block at the CD25+CD44- double-negative (DN) 3 stage of T cell development. We correlate this arrest with up-regulation of Bcl-2, CD2, CD5, and CD27 proteins and constitutive activation of NFAT but a severe impairment of Rag1, Rag2, and intracellular TCR-beta as well as intracellular TCR-gammadelta protein expression. Intriguingly, simultaneous expression of active myristoylated PKB inhibits nuclear NFAT activity, restores Rag activity, and enables DN3 cells to undergo normal differentiation and expansion. A correlation between the loss of NFAT activity and Rag1 and Rag2 expression is also found in myristoylated PKB-induced CD4+ lymphoma cells. Furthermore, ectopic expression of NFAT inhibits Rag2 promoter activity in EL4 cells, and in vivo binding of NFATc1 to the Rag1 and Rag2 promoter and cis-acting transcription regulatory elements is verified by chromatin immunoprecipitation analysis. The regulation of CN/NFAT signaling by PKB may thus control receptor regulated changes in Rag expression and constitute a signaling pathway important for differentiation processes in the thymus and periphery.

Recent insights into the signals that control alphabeta/gammadelta-lineage fate

During thymopoiesis, two major types of mature T cells are generated that can be distinguished by the clonotypic subunits contained within their T-cell receptor (TCR) complexes: alphabeta T cells and gammadelta T cells. Although there is no consensus as to the exact developmental stage where alphabeta and gammadelta T-cell lineages diverge, gammadelta T cells and precursors to the alphabeta T-cell lineage (bearing the pre-TCR) are thought to be derived from a common CD4- CD8- double-negative precursor. The role of the TCR in alphabeta/gammadelta lineage commitment has been controversial, in particular whether different TCR isotypes intrinsically favor adoption of the corresponding lineage. Recent evidence supports a signal strength model of lineage commitment, whereby stronger signals promote gammadelta development and weaker signals promote adoption of the alphabeta fate, irrespective of the TCR isotype from which the signals originate. Moreover, differences in the amplitude of activation of the extracellular signal-regulated kinase- mitogen-activated protein kinase-early growth response pathway appear to play a critical role. These findings will be placed in context of previous analyses in an effort to more precisely define the signals that control T-lineage fate during thymocyte development.

Progression of regulatory gene expression states in fetal and adult pro-T-cell development

Precursors entering the T-cell developmental pathway traverse a progression of states characterized by distinctive patterns of gene expression. Of particular interest are regulatory genes, which ultimately control the dwell time of cells in each state and establish the mechanisms that propel them forward to subsequent states. Under particular genetic and developmental circumstances, the transitions between these states occur with different timing, and environmental feedbacks may shift the steady-state accumulations of cells in each state. The fetal transit through pro-T-cell stages is faster than in the adult and subject to somewhat different genetic requirements. To explore causes of such variation, this review presents previously unpublished data on differentiation gene activation in pro-T cells of pre-T-cell receptor-deficient mutant mice and a quantitative comparison of the profiles of transcription factor gene expression in pro-T-cell subsets of fetal and adult wildtype mice. Against a background of consistent gene expression, several regulatory genes show marked differences between fetal and adult expression profiles, including those encoding two basic helix-loop-helix antagonist Id factors, the Ets family factor SpiB and the Notch target gene Deltex1. The results also reveal global differences in regulatory alterations triggered by the first T-cell receptor-dependent selection events in fetal and adult thymopoiesis.

Strength of signal: a fundamental mechanism for cell fate specification

How equipotent cells develop into complex tissues containing many diverse cell types is still a mystery. However, evidence is accumulating from different tissue systems in multiple organisms that many of the specific receptor families known to regulate cell fate decisions target conserved signaling pathways. A mechanism for preserving specificity in the cellular response that has emerged from these studies is one in which quantitative differences in receptor signaling regulate the cell fate decision. A signal strength model has recently gained support as a means to explain alphabeta/gammadelta lineage commitment. In this review, we compare the alphabeta/gammadelta fate decision with other cell fate decisions that occur outside of the lymphoid system to attain a better picture of the quantitative signaling mechanism for cell fate specification.

T-cell development, doing it in a dish

The thymus provides a unique environment for the development of T lymphocytes from bone marrow-derived progenitor cells. Several environmental factors have been identified that influence the development of T cells in the thymus. In particular, the Notch pathway has emerged as critical for the induction of T-lineage commitment and differentiation. Until recently, however, the precise nature of the thymus-derived signals that drive T-cell development were unclear, and the only reliable in vitro culture system that supported T-cell differentiation required the use of thymus organ cultures. Here, we discuss recent advances in the identification of critical Notch receptor ligands that have facilitated the development of a simple in vitro model for the differentiation of T cells 'in a dish', providing an alternate approach for studying T lymphopoiesis.

Stage-specific and differential notch dependency at the alphabeta and gammadelta T lineage bifurcation

Signals transduced by Notch receptors are indispensable for T cell specification and differentiation of alphabeta T lineage cells. However, the role of Notch signals during alphabeta versus gammadelta T lineage decision remains controversial. Here, we addressed this question by employing a clonal analysis of CD4(-)CD8(-) (DN) progenitor potential to position the divergence of alphabeta and gammadelta T cell lineages to the late DN2 to DN3 developmental stages. Accordingly, alphabeta and gammadelta precursor frequencies within these T cell progenitor subsets were determined, both in the presence and absence of Notch signaling through Delta-like 1. Notch signals were found to be critical for the DN to CD4(+)CD8(+) (DP) transition, irrespective of the identity (pTalphabeta or gammadelta) of the inducing T cell receptor complex, whereas gammadelta T cells developed from gammadeltaTCR-expressing T cell progenitors in the absence of further Notch ligand interaction. Collectively, our findings demonstrate a differential, stage-specific requirement for Notch receptor-ligand interactions in the differentiation of alphabeta and gammadelta T cells from T cell progenitors.

Differential synergy of Notch and T cell receptor signaling determines alphabeta versus gammadelta lineage fate

Thymic precursors expressing the pre–T cell receptor (TCR), the γδTCR, or the αβTCR can all enter the CD4⁺8⁺ αβ lineage, albeit with different efficacy. Here it is shown that proliferation and differentiation of precursors with the different TCRs into αβ lineage cells require Notch signaling at the DN3 stage of thymic development. At the DN4 stage, Notch signaling still significantly contributes to the generation of αβ T cells. In particular, in αβ lineage commitment, the pre-TCR synergizes more efficiently with Notch signals than the other two TCRs, whereas γδTCR-expressing cells can survive and expand in the absence of Notch signals, even though Notch signaling enhances their proliferation. These observations suggest a new model of αβ versus γδ lineage choice in which lineage fate is determined by the extent of synergy between TCR and Notch signaling and in which the evolutionarily recent advent of the cell-autonomously signaling pre-TCR increased the efficacy of αβ T cell generation.

Human alpha beta and gamma delta thymocyte development: TCR gene rearrangements, intracellular TCR beta expression, and gamma delta developmental potential--differences between men and mice

To evaluate the role of the TCR in the alphabeta/gammadelta lineage choice during human thymocyte development, molecular analyses of the TCRbeta locus in gammadelta cells and the TCRgamma and delta loci in alphabeta cells were undertaken. TCRbeta variable gene segments remained largely in germline configuration in gammadelta cells, indicating that commitment to the gammadelta lineage occurred before complete TCRbeta rearrangements in most cases. The few TCRbeta rearrangements detected were primarily out-of-frame, suggesting that productive TCRbeta rearrangements diverted cells away from the gammadelta lineage. In contrast, in alphabeta cells, the TCRgamma locus was almost completely rearranged with a random productivity profile; the TCRdelta locus contained primarily nonproductive rearrangements. Productive gamma rearrangements were, however, depleted compared with preselected cells. Productive TCRgamma and delta rearrangements rarely occurred in the same cell, suggesting that alphabeta cells developed from cells unable to produce a functional gammadelta TCR. Intracellular TCRbeta expression correlated with the up-regulation of CD4 and concomitant down-regulation of CD34, and plateaued at the early double positive stage. Surprisingly, however, some early double positive thymocytes retained gammadelta potential in culture. We present a model for human thymopoiesis which includes gammadelta development as a default pathway, an instructional role for the TCR in the alphabeta/gammadelta lineage choice, and a prolonged developmental window for beta selection and gammadelta lineage commitment. Aspects that differ from the mouse are the status of TCR gene rearrangements at the nonexpressed loci, the timing of beta selection, and maintenance of gammadelta potential through the early double positive stage of development.

Thymic microenvironments for T cell differentiation and selection

The adult thymus provides a variety of specialized microenvironments that support and direct T cell differentiation and selection. In this review, we summarize recent advances in the understanding of the function of microenvironments in shaping a diverse T cell repertoire. In particular, we focus on how thymocytes move in and out of these specialized thymic compartments in response to homing signals, differential chemokine gradients and other factors that regulate T cell migration. In addition, we discuss the diverse developmental signals provided by these microenvironments that contribute to the generation of divergent T cell lineages.

Marking emerging beta- and gammadelta-selected T cells

How does signaling through the pre-TCR and gammadelta TCR generate distinct cell lineages? A new study shows that a cell-surface marker, CD27, identifies the first point of demarcation of these lineages and allows further probing into the critical regulatory factors that control beta and gammadelta selection.

Developmental and molecular characterization of emerging beta- and gammadelta-selected pre-T cells in the adult mouse thymus

The first checkpoint in T cell development, beta selection, has remained incompletely characterized for lack of specific surface markers. We show that CD27 is upregulated in DN3 thymocytes initiating beta selection, concomitant with intracellular TCR-beta expression. Clonal analysis determined that CD27high DN3 cells generate CD4+CD8+ progeny with more than 90% efficiency, faster and more efficiently than the CD27low majority. CD27 upregulation also occurs in gammadelta-selected DN3 thymocytes in TCR-beta-/- mice and in IL2-GFP transgenic reporter mice where GFP marks the earliest emerging TCR-gammadelta cells from DN3 thymocytes. With CD27 to distinguish pre- and postselection DN3 cells, a detailed gene expression analysis defined regulatory changes associated with checkpoint arrest, with beta selection, and with gammadelta selection. gammadelta selection induces higher CD5, Egr, and Runx3 expression as compared to beta selection, but it triggers less proliferation. Our results also reveal differences in Notch/Delta dependence at the earliest stages of divergence between developing alphabeta and gammadelta T-lineage cells.

Regulation of T-cell progenitor survival and cell-cycle entry by the pre-T-cell receptor

Pre-T-cell receptor (pre-TCR) functions and the study of early thymocyte development continue to fascinate immunologists more than 10 years after the first description and cloning of the receptor. Although multiple reports have addressed several aspects of pre-TCR signaling and function, its ability to regulate diverse functions, including proliferation, survival, and allelic exclusion of the TCR-beta locus, remains an open question. What fascinates us is its central role in the fine balance between physiological differentiation and thymocyte transformation that leads to T-cell leukemia and lymphomas. In this review, we integrate pre-TCR signaling pathways and study their effects on the regulation of T-cell progenitor cell-cycle entry and cell survival. We also connect aberrant pre-TCR signaling to deregulated proliferation and apoptotic balances and thymocyte transformation.

Inhibition of Immune Responses by ITAM-Bearing Receptors

Cells of the immune system possess many multisubunit receptors that are composed of a ligand-binding subunit associated with distinct signaling adaptors containing one or more immunoreceptor tyrosine-based activation motifs (ITAMs). These receptors include the T cell receptor, the B cell receptor, and many Fc receptors, as well as families of activating receptors on myeloid and natural killer cells. Receptors that associate with ITAM-containing adaptors classically have been viewed as transducing activating signals involving phosphorylation of the tyrosines within the ITAM and recruitment of Syk family tyrosine kinases. Receptors associated with ITAM-containing adaptors in myeloid cells have also been implicated in inhibition of cellular activation. Here, we discuss these new negative roles for signaling by receptors that associate with ITAM-bearing adaptors in myeloid and other cell types within the immune system.