Evidence that productive rearrangements of TCR gamma genes influence the commitment of progenitor cells to differentiate into alpha beta or gamma delta T cells

Direct regulation of TCR rearrangement and expression by E proteins during early T cell development

γδ T cells are widely distributed throughout mucosal and epithelial cell-rich tissues and are an important early source of IL-17 in response to several pathogens. Like αβ T cells, γδ T cells undergo a stepwise process of development in the thymus that requires recombination of genome-encoded segments to assemble mature T cell receptor (TCR) genes. This process is tightly controlled on multiple levels to enable TCR segment assembly while preventing the genomic instability inherent in the double-stranded DNA breaks that occur during this process. Each TCR locus has unique aspects in its structure and requirements, with different types of regulation before and after the αβ/γδ T cell fate choice. It has been known that Runx and Myb are critical transcriptional regulators of TCRγ and TCRδ expression, but the roles of E proteins in TCRγ and TCRδ regulation have been less well explored. Multiple lines of evidence show that E proteins are involved in TCR expression at many different levels, including the regulation of Rag recombinase gene expression and protein stability, induction of germline V segment expression, chromatin remodeling, and restriction of the fetal and adult γδTCR repertoires. Importantly, E proteins interact directly with the cis-regulatory elements of the TCRγ and TCRδ loci, controlling the predisposition of a cell to become an αβ T cell or a γδ T cell, even before the lineage-dictating TCR signaling events. This article is categorized under: Immune System Diseases > Stem Cells and Development Immune System Diseases > Genetics/Genomics/Epigenetics.

Interaction between γδTCR signaling and the E protein-Id axis in γδ T cell development

γδ T cells acquire their functional properties in the thymus, enabling them to exert rapid innate-like responses. To understand how distinct γδ T cell subsets are generated, we have developed a Two-Stage model for γδ T cell development. This model is predicated on the finding that γδTCR signal strength impacts E protein activity through graded upregulation of Id3. Our model proposes that cells enter Stage 1 in response to a γδTCR signaling event in the cortex that activates a γδ T cell-specific gene network. Part of this program includes the upregulation of chemokine receptors that guide them to the medulla. In the medulla, Stage 1 cells receive distinct combinations of γδTCR, cytokine, and/co-stimulatory signals that induce their transit into Stage 2, either toward the γδT1 or the γδT17 lineage. The intersection between γδTCR and cytokine signals can tune Id3 expression, leading to different outcomes even in the presence of strong γδTCR signals. The thymic signaling niches required for γδT17 development are segregated in time and space, providing transient windows of opportunity during ontogeny. Understanding the regulatory context in which E proteins operate at different stages will be key in defining how their activity levels impose functional outcomes.

Interleukin-17-Producing γδ T Cells Originate from SOX13+ Progenitors that Are Independent of γδTCR Signaling

Lineage-committed αβ and γδ T cells are thought to originate from common intrathymic multipotent progenitors following instructive T cell receptor (TCR) signals. A subset of lymph node and mucosal Vγ2⁺ γδ T cells is programmed intrathymically to produce IL-17 (Tγδ17 cells), however the role of the γδTCR in development of these cells remains controversial. Here we generated reporter mice for the Tγδ17 lineage-defining transcription factor SOX13 and identified fetal-origin, intrathymic Sox13⁺ progenitors. In organ culture developmental assays, Tγδ17 cells derived primarily from Sox13⁺ progenitors, and not from other known lymphoid progenitors. Single cell transcriptome assays of the progenitors found in TCR-deficient mice demonstrated that Tγδ17 lineage programming was independent of γδTCR. Instead, generation of the lineage committed progenitors and Tγδ17 cells was controlled by TCF1 and SOX13. Thus, T lymphocyte lineage fate can be prewired cell-intrinsically and is not necessarily specified by clonal antigen receptor signals.

Evidence for the divergence of innate and adaptive T-cell precursors before commitment to the αβ and γδ lineages

In addition to adaptive T cells, the thymus supports the development of unconventional T cells such as natural killer T (NKT) and CD8αα intraepithelial lymphocytes (IELs), which have innate functional properties, particular antigenic specificities, and tissue localization. Both conventional and innate T cells are believed to develop from common precursors undergoing instructive, TCR-mediated lineage fate decisions, but innate T cells are proposed to undergo positive instead of negative selection in response to agonistic TCR signals. In the present study, we show that, in contrast to conventional αβT cells, innate αβT cells are not selected against functional TCRγ rearrangements and express TCRγ mRNA. Likewise, in contrast to the majority of γδT cells, thymic innate γδT cells are not efficiently selected against functional TCRβ chains. In precursors of conventional T cells, autonomous TCR signals emanating from the pre-TCR or γδTCR in the absence of ligand mediate selection against the TCR of the opposite isotype and αβ/γδ lineage commitment. Our data suggest that developing innate T cells ignore such signals and rely solely on agonistic TCR interactions. Consistently, most innate T cells reacted strongly against autologous thymocytes. These results suggest that innate and adaptive T-cell lineages do not develop from the same pool of precursors and potentially diverge before αβ/γδ lineage commitment.

The pre-TCR signal induces transcriptional silencing of the TCRγ locus by reducing the recruitment of STAT5 and Runx to transcriptional enhancers

The mouse TCRγ locus is positively regulated by the transcription factors STAT5 and Runx. While the locus undergoes frequent rearrangements in T lymphocytes, TCRγ transcription is repressed in αβ T cells. This phenomenon, known as TCRγ silencing, depends on pre-TCR-induced thymocyte proliferation. The molecular basis for TCRγ silencing, however, is largely unknown. Here, we show that pre-TCR signaling reduces transcription and histone acetylation of the TCRγ locus irrespective of V-J rearrangements. We also demonstrate that Runx is recruited to Eγ and HsA enhancer elements of the TCRγ locus, primarily at the CD4(-)CD8(-) double-negative stage and that Runx binding to these elements decreases at later stages of thymocyte development. Importantly, anti-CD3 antibody treatment decreased IL-7R expression levels, STAT5 phosphorylation and recruitment of STAT5 and Runx to Eγ and HsA elements in RAG2-deficient thymocytes, suggesting that pre-TCR signaling triggers reduced binding of STAT5 and Runx to the enhancer elements. Furthermore, we observed that misexpression of STAT5 or Runx in the CD4(+)CD8(+) double-positive cell line DPK induces TCRγ gene transcription. Finally, we showed that TCRγ transcription is induced in αβ T cells from Runx3 transgenic mice, suggesting that Runx3 counteracts TCRγ silencing in αβ T cells in vivo. Our results suggest that pre-TCR signaling indirectly inactivates TCRγ enhancers by reducing recruitment of STAT5 and Runx and imply that this effect is an important step for TCRγ silencing in αβ T cells.

Towards a molecular understanding of the differential signals regulating alphabeta/gammadelta T lineage choice

While insights into the molecular processes that specify adoption of the alphabeta and gammadelta fates are beginning to emerge, the basis for control of specification remains highly controversial. This review highlights the current models attempting to explain T lineage commitment. Recent observations support the hypothesis that the T cell receptor (TCR) provides instructive cues through differences in TCR signaling intensity and/or longevity. Accordingly, we review evidence addressing the importance of differences in signal strength/longevity, how signals differing in intensity/longevity may be generated, and finally how such signals modulate the activity of downstream effectors to promote the opposing developmental fates.

Forced expression of Id2 in fetal thymic T cell progenitors allows some of their progeny to adopt NK cell fate

The E proteins are indispensable for early T cell development. On the other hand, we previously demonstrated that their inhibitor Id2 is essential for NK lineage commitment from bipotent progenitors generating both T and NK cells (p-T/NK). To shed more light on the role of E proteins and Id2 in the development of early intrathymic progenitors, we performed a clonal analysis: individual fetal thymic CD4(-)CD8(-)CD44(+)CD25(-)CD122(-) (DN1CD122(-)) cells were retrovirally transduced with an Id2-internal ribosomal entry site (IRES)-green fluorescent protein (GFP) (Id2-GFP) gene or a control IRES-GFP (GFP) gene, and cultured in a modified fetal thymus organ culture able to support T and NK cell development. After the culture, both T and NK cells, T cells and no NK cells, NK cells and no T cells, or completely no cells were generated from single cells in each lobe. Hence, the seeded cells were regarded as p-T/NK, unipotent progenitors generating T cells (p-T), unipotent NK progenitors, or cells without progenitor activity, respectively. With Id2-GFP transduction, p-T disappeared and more p-T/NK emerged than with GFP transduction. This increase corresponded to the number of p-T that was counted when the vector-transduced-DN1CD122(-) cells of the same number were examined. Additionally, a fraction of GFP(-) NK cells obtained after Id2-GFP transduction underwent TCRbeta D-J rearrangement. Our data strongly suggest that forced expression of Id2 allows some progeny of p-T to adopt an NK cell fate, and that p-T retain a program for NK lineage development that can be implemented by inhibiting the function of E proteins.

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.

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.

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.

Mechanisms controlling termination of V-J recombination at the TCRgamma locus: implications for allelic and isotypic exclusion of TCRgamma chains

Analyses of Vgamma-Jgamma rearrangements producing the most commonly expressed TCRgamma chains in over 200 gammadelta TCR(+) thymocytes showed that assembly of TCRgamma V-region genes display properties of allelic exclusion. Moreover, introduction of functionally rearranged TCRgamma and delta transgenes results in a profound inhibition of endogenous TCRgamma rearrangements in progenitor cells. The extent of TCRgamma rearrangements in these cells is best explained by a model in which initiation of TCRgamma rearrangements at both alleles is asymmetric, occurs at different frequencies depending on the V or J segments involved, and is terminated upon production of a functional gammadelta TCR. Approximately 10% of the cells studied contained two functional TCRgamma chains involving different V and Jgamma gene segments, thus defining a certain degree of isotypic inclusion. However, these cells are isotypically excluded at the level of cell surface expression possibly due to pairing restrictions between different TCRgamma and delta chains.

Stochastic Modeling of T cell receptor gene rearrangement

The mechanisms controlling the recombination process of the gamma genes that encode the gamma chain of the antigen receptor of the gammadelta T lymphocytes are unclear. Based on experimental data on the recombination status of the two major TCR gamma genes expressed in V(gamma)4+ and V(gamma)1+ thymocytes, we tested the plausibility of three possible rearrangement mechanisms: (1) a time window mechanism according to which the two chromosomes are accessible to the recombination machinery during a defined period of time; (2) a feedback mechanism in which recombination stops shortly after the first in-frame rearrangement event anywhere in both chromosomes; and (3) a feedback mechanism with asynchronous chromosome accessibility, in which there is a first period when only one chromosome is accessible for recombination, followed by a second period when both chromosomes are accessible; shortly after the first in-frame rearrangement event, during any of these two periods, recombination will definitely stop. We model the time window mechanism using a pure probabilistic approach and the two feedback mechanisms using a continuous-time Markov chain formalism. We used maximum likelihood methodology to infer the goodness-of-fit of the models showing evidence for the last model, which best fits the data. Further analysis of this model suggests an evolutionary tradeoff between allelic and isotypic exclusion and the probability that a precursor differentiates into a mature gammadelta T lymphocyte.

Rates of recombination and chain pair biases greatly influence the primary gammadelta TCR repertoire in the thymus of adult mice

Analyses of the rearrangement status of the TCRgamma and TCRdelta chain loci in progenies of individual gammadelta thymocytes showed a hierarchy of the different Vgamma and Vdelta gene segments to participate in a recombination reaction. Moreover, individual TCRgamma chains only pair efficiently with a variable number of TCRdelta chains. Interestingly, these two parameters are inversely correlated such that the TCRgamma and TCRdelta chains that rearrange more often show a higher level of restriction in their pairing capabilities. Our data suggest that these mechanisms, together with a natural variation affecting the expected frequencies at which rearrangement of different Vgamma gene segments give raise to functional TCRgamma chains, have coevolved to maximize the diversity of the gammadelta TCR repertoire minimizing the risk that a gammadelta T cell will express more than one TCR specificity at the cell surface, despite the fact that multiple TCRgamma rearrangements take place in the same progenitor cell.

Early Expression of a Functional TCRβ Chain Inhibits TCRγ Gene Rearrangements without Altering the Frequency of TCRγδ Lineage Cells

To investigate the consequences of the simultaneous expression in progenitor cells of a TCRgammadelta and a pre-TCR on alphabeta/gammadelta lineage commitment, we have forced expression of functionally rearranged TCRbeta, TCRgamma, and TCRdelta chains by means of transgenes. Mice transgenic for the three TCR chains contain numbers of gammadelta thymocytes comparable to those of mice transgenic for both TCRgamma and TCRdelta chains, and numbers of alphabeta thymocytes similar to those found in mice solely transgenic for a rearranged TCRbeta chain gene. gammadelta T cells from the triple transgenic mice express the transgenic TCRbeta chain, but do not express a TCRalpha chain, and, by a number of phenotypic and molecular parameters, appear to be bona fide gammadelta thymocytes. Our results reveal a remarkable degree of independence in the generation of alphabeta and gammadelta lineage cells from progenitor cells that, in theory, could simultaneously express a TCRgammadelta and a pre-TCR.

Thymic selection revisited: how essential is it?

Intrathymic T cell development represents one of the best studied paradigms of mammalian development. Lymphoid committed precursors enter the thymus and the Notch1 receptor plays an essential role in committing them to the T cell lineages. The pre-T cell receptor (TCR), as an autonomous cell signaling receptor, commits cells to the alphabeta lineage while its rival, the gammadeltaTCR, is involved in generating the gammadelta lineage of T cells. Positive and negative selection of immature alphabetaTCR-expressing cells are essential mechanisms for generating mature T cells, committing them to the CD4 and CD8 lineages and avoiding autoimmunity. Additional lineages of alphabetaT cells, such as the natural killer T cell lineage and the CD25+ regulatory T cell lineage, are formed when the alphabetaTCR encounters specific ligands in suitable microenvironments. Thus, positive selection and receptor-instructed lineage commitment represent a hallmark of the thymus. Ectopically expressed organ-specific antigens contribute to thymic self-nonself discrimination, which represents an essential feature for the evolutionary fitness of mammalian species.

Evidence that gammadelta versus alphabeta T cell fate determination is initiated independently of T cell receptor signaling

Two types of T cells, alphabeta and gammadelta, develop in vertebrates. How these two T cell lineages arise from a common thymic T progenitor is poorly understood. Differentiation of alphabeta lineage T cells requires the surrogate alpha chain (pTalpha), which associates with the T cell receptor (TCR) beta chain to form the pre-TCR. gammadelta lineage development does not appear to involve an obligatory surrogate chain, but instead requires productive rearrangement and expression of both TCR gamma and delta genes. It has been proposed that the quality of signals transmitted by the pre-TCR and gammadelta TCR are distinct and that these "instructive" signals determine the lineage fate of an uncommitted progenitor cell. Here we show that the thymic T progenitor cells (CD25(+)CD44(+)c-kit(+)CD3(-)CD4(-)CD8(-) thymocytes, termed pro-T cells) from young adult mice that have yet to express TCRs can be subdivided based on interleukin 7 receptor (IL-7R) expression. These subsets exhibit differential potential to develop into gammadelta versus alphabeta lineage (CD4+CD8+ cells) in the thymus. Upon intrathymic injection, IL-7R(neg-lo) pro-T cells generated a 13-fold higher ratio of alphabeta lineage to gammadelta lineage cells than did IL-7R(+) pro-T cells. Much of this difference was due to a fivefold greater potential of IL-7R(+) pro-T cells to develop into TCR-gammadelta T cells. Evidence indicates that this biased developmental potential is not a result of enhanced TCR-gamma gene rearrangement/expression in IL-7R(+) pro-T cells. These results indicate that the pro-T cells are heterogeneous in developmental potential before TCR gene rearrangement and suggest that in some precursor cells the initial lineage commitment is independent of TCR-mediated signals.

Premature expression of T cell receptor (TCR)alphabeta suppresses TCRgammadelta gene rearrangement but permits development of gammadelta lineage T cells

The T cell receptor (TCR)gammadelta and the pre-TCR promote survival and maturation of early thymocyte precursors. Whether these receptors also influence gammadelta versus alphabeta lineage determination is less clear. We show here that TCRgammadelta gene rearrangements are suppressed in TCRalphabeta transgenic mice when the TCRalphabeta is expressed early in T cell development. This situation offers the opportunity to examine the outcome of gammadelta versus alphabeta T lineage commitment when only the TCRalphabeta is expressed. We find that precursor thymocytes expressing TCRalphabeta not only mature in the alphabeta pathway as expected, but also as CD4(-)CD8(-) T cells with properties of gammadelta lineage cells. In TCRalphabeta transgenic mice, in which the transgenic receptor is expressed relatively late, TCRgammadelta rearrangements occur normally such that TCRalphabeta(+)CD4(-)CD8(-) cells co-express TCRgammadelta. The results support the notion that TCRalphabeta can substitute for TCRgammadelta to permit a gammadelta lineage choice and maturation in the gammadelta lineage. The findings could fit a model in which lineage commitment is determined before or independent of TCR gene rearrangement. However, these results could be compatible with a model in which distinct signals bias lineage choice and these signaling differences are not absolute or intrinsic to the specific TCR structure.

T cell development in TCR beta enhancer-deleted mice: implications for alpha beta T cell lineage commitment and differentiation

T cell differentiation in the mouse thymus is an intricate, highly coordinated process that requires the assembly of TCR complexes from individual components, including those produced by the precisely timed V(D)J recombination of TCR genes. Mice carrying a homozygous deletion of the TCR beta transcriptional enhancer (E beta) demonstrate an inhibition of V(D)J recombination at the targeted TCR beta locus and a block in alpha beta T cell differentiation. In this study, we have characterized the T cell developmental defects resulting from the E beta-/- mutation, in light of previously reported results of the analyses of TCR beta-deficient (TCR beta-/-) mice. Similar to the latter mice, production of TCR beta-chains is abolished in the E beta-/- animals, and under these conditions differentiation into cell-surface TCR-, CD4+CD8+ double positive (DP) thymocytes depends essentially on the cell-autonomous expression of TCR delta-chains and, most likely, TCR gamma-chains. However, contrary to previous reports using TCR beta-/- mice, a minor population of TCR gamma delta+ DP thymocytes was found within the E beta-/- thymi, which differ in terms of T cell-specific gene expression and V(D)J recombinase activity, from the majority of TCR-, alpha beta lineage-committed DP thymocytes. We discuss these data with respect to the functional role of E beta in driving alpha beta T cell differentiation and the mechanism of alpha beta T lineage commitment.

Defective development of gamma/delta T cells in interleukin 7 receptor-deficient mice is due to impaired expression of T cell receptor gamma genes

Mice lacking the interleukin 7 receptor (IL-7R) generate alpha/beta T cells at a detectable but greatly reduced rate, but gamma/delta T cells are completely absent. The special role of IL-7R signaling in gamma/delta T cell development has remained unclear. IL-7Ralpha(-/-) mice exhibit a paucity of gamma gene rearrangements. This striking observation can be explained by a defect in T cell receptor (TCR)-gamma gene rearrangement, a defect in TCR-gamma gene transcription leading to death of gamma/delta lineage cells, and/or a requirement for IL-7R in commitment of cells to the gamma/delta lineage. To determine the role of IL-7R signaling in gamma/delta T cell development, we examined transcription of a prerearranged TCR-gamma transgene in IL-7Ralpha(-/-) mice, as well as the effects of IL-7 on transcription of endogenous, rearranged TCR-gamma genes in alpha/beta lineage cells. The results demonstrate that IL-7R-mediated signals are necessary for the normal expression of rearranged TCR-gamma genes. Equally significant, the results show that the poor expression of TCR-gamma genes in IL-7Ralpha(-/-) mice is responsible for the selective deficit in gamma/delta cells in these mice, since a high copy TCR-gamma transgene exhibited sufficient residual expression in IL-7Ralpha(-/-) mice to drive gamma/delta cell development. The results indicate that the absence of gamma/delta T cells in IL-7Ralpha(-/-) mice is due to insufficient TCR-gamma gene expression.

A developmental switch from TCR delta enhancer to TCR alpha enhancer function during thymocyte maturation

V(D)J recombination and transcription within the TCR alpha/delta locus are regulated by three characterized cis-acting elements: the TCR delta enhancer (Edelta), TCR alpha enhancer (Ealpha), and T early alpha (TEA) promoter. Analysis of enhancer and promoter occupancy and function in developing thymocytes in vivo indicates Edelta and Ealpha to be developmental-stage-specific enhancers, with Edelta "on" and Ealpha "off" in double-negative III thymocytes and Edelta "off" and Ealpha "on" in double-positive thymocytes. Edelta downregulation reflects a loss of occupancy. Surprisingly, Ealpha and TEA are extensively occupied even prior to activation. TCR delta downregulation in double-positive thymocytes depends on two events, Edelta inactivation and removal of TCR delta from the influence of Ealpha by chromosomal excision.

Disruption of alpha beta but not of gamma delta T cell development by overexpression of the helix-loop-helix protein Id3 in committed T cell progenitors

Enforced expression of Id3, which has the capacity to inhibit many basic helix-loop-helix (bHLH) transcription factors, in human CD34(+) hematopoietic progenitor cells that have not undergone T cell receptor (TCR) gene rearrangements inhibits development of the transduced cells into TCRalpha beta and gamma delta cells in a fetal thymic organ culture (FTOC). Here we document that overexpression of Id3, in progenitors that have initiated TCR gene rearrangements (pre-T cells), inhibits development into TCRalpha beta but not into TCRgamma delta T cells. Furthermore, Id3 impedes expression of recombination activating genes and downregulates pre-Talpha mRNA. These observations suggest possible mechanisms by which Id3 overexpression can differentially affect development of pre-T cells into TCRalpha beta and gamma delta cells. We also observed that cell surface CD4(-)CD8(-)CD3(-) cells with rearranged TCR genes developed from Id3-transduced but not from control-transduced pre-T cells in an FTOC. These cells had properties of both natural killer (NK) and pre-T cells. These findings suggest that bHLH factors are required to control T cell development after the T/NK developmental checkpoint.

Developmental regulation of TCR delta locus accessibility and expression by the TCR delta enhancer

We have used gene-targeted mutation to assess the role of the T cell receptor delta (TCR delta) enhancer (E delta) in alphabeta and gammadelta T cell development. Mice lacking E delta exhibited no defects in alphabeta T cell development but had a severe reduction in thymic and peripheral gammadelta T cells and decreased VDJ delta rearrangements. Simultaneous deletion of both E delta and the TCR alpha enhancer (E alpha) demonstrated that residual TCR delta rearrangements were not driven by E alpha, implicating additional elements in TCR delta locus accessibility. Surprisingly, while deletion of E delta severely impaired germline TCR delta expression in double-negative thymocytes, absence of E delta did not affect expression of mature delta transcripts in gammadelta T cells. We conclude that E delta has an important role in TCR delta locus regulation at early, but not late, stages of gammadelta T cell development.

Characterization of TCR Gene Rearrangements During Adult Murine T Cell Development

Development of the αβ and γδ T cell lineages is dependent upon the rearrangement and expression of the TCRα and β or γ and δ genes, respectively. Although the timing and sequence of rearrangements of the TCRα and TCRβ loci in adult murine thymic precursors has been characterized, no similar information is available for the TCRγ and TCRδ loci. In this report, we show that approximately half of the total TCRδ alleles initiate rearrangements at the CD44highCD25+ stage, whereas the TCRβ locus is mainly in germline configuration. In the subsequent CD44lowCD25+ stage, most TCRδ alleles are fully recombined, whereas TCRβ rearrangements are only complete on 10–30% of alleles. These results indicate that rearrangement at the TCRδ locus can precede that of TCRβ locus recombination by one developmental stage. In addition, we find a bias toward productive rearrangements of both TCRδ and TCRγ genes among CD44highCD25+ thymocytes, suggesting that functional γδ TCR complexes can be formed before the rearrangement of TCRβ. These data support a model of lineage commitment in which sequential TCR gene rearrangements may influence αβ/γδ lineage decisions. Further, because TCR gene rearrangements are generally limited to T lineage cells, these analyses provide molecular evidence that irreversible commitment to the T lineage can occur as early as the CD44highCD25+ stage of development.

Cloning thymic precursor cells: demonstration that individual pro-T1 cells have dual T-NK potential and individual pro-T2 cells have dual alphabeta-gammadelta T cell potential

Thymic progenitors have the capacity to generate alphabeta T cells, gammadelta T cells, and NK cells. To determine whether these three lineages derive from a single precursor cell or from different precursors, a procedure was developed for cloning precursor cells from mouse embryonic thymus. The progeny of each pro-T cell clone were then tested for the potential to generate alphabeta, gammadelta, and NK cells. Of these precursor clones, about half displayed dual potential, developing into either T cells or NK cells, demonstrating the existence of a common T/NK precursor cell in the thymus. The other half of the clones were restricted to T cell development. No precursor clones were restricted to NK development. The common T/NK precursors were shown to be of the pro-T1 (CD25(-)) stage whereas the T-restricted precursors were shown to be of the later pro-T2 (CD25(+)) stage. Both alphabeta and gammadelta T cells were generated from all clones derived from either pro-T1 or -T2 precursors. This shows that commitment of a cell to the alphabeta versus gammadelta lineages does not precede rearrangement of the TCR genes (which occurs immediately after the pro-T2 stage).

Distinct Effects of Jak3 Signaling on αβ and γδ Thymocyte Development

Janus kinase 3 (Jak3) plays a central role in the transduction of signals mediated by the IL-2 family of cytokine receptors. Targeted deletion of the murine Jak3 gene results in severe reduction of αβ and complete elimination of γδ lineage thymocytes and NK cells. The developmental blockade appears to be imposed on early thymocyte differentiation and/or expansion. In this study, we show that bcl-2 expression and in vivo survival of immature thymocytes are greatly compromised in Jak3−/− mice. There is no gross deficiency in rearrangements of the TCRδ and certain γ loci in pre-T cells, and a functional γδ TCR transgene cannot rescue γδ lineage differentiation in Jak3−/− mice. In contrast, a TCRβ transgene is partially able to restore αβ thymocyte development. These data suggest that the signals mediated by Jak3 are critical for survival of all thymocyte precursors particularly during TCRβ-chain gene rearrangement, and are continuously required in the γδ lineage. The results also emphasize the fundamentally different requirements for differentiation of the αβ and γδ T cell lineages.

On the role of the pre-T cell receptor in alphabeta versus gammadelta T lineage commitment

The role of the pre-T cell receptor (TCR) in lineage commitment to the gammadelta versus alphabeta lineage of T cells was addressed by analyzing TCRbeta chain rearrangements in gammadelta T cells from wild-type and pre-TCR-deficient mice by single cell polymerase chain reaction. Results show that the pre-TCR selects against gammadelta T cells containing rearranged Vbeta genes and that gammadelta T cell precursors but not gammadelta T cells express the pre-TCRalpha protein. Furthermore, pre-TCR-induced proliferation could not be detected in gammadelta T cells. We propose that the pre-TCR commits developing T cells to the alphabeta lineage by an instructive mechanism that has largely replaced an evolutionary more ancient stochastic mechanism of lineage commitment.

Kinetics of T cell receptor beta, gamma, and delta rearrangements during adult thymic development: T cell receptor rearrangements are present in CD44(+)CD25(+) Pro-T thymocytes

We performed a comprehensive analysis of T cell receptor (TCR) gamma rearrangements in T cell precursors of the mouse adult thymus. Using a sensitive quantitative PCR method, we show that TCRgamma rearrangements are present in CD44(+)CD25(+) Pro-T thymocytes much earlier than expected. TCRgamma rearrangements increase significantly from the Pro-T to the CD44(-)CD25(+) Pre-T cell transition, and follow different patterns depending on each Vgamma gene segment, suggesting that ordered waves of TCRgamma rearrangement exist in the adult mouse thymus as has been described in the fetal mouse thymus. Recombinations of TCRgamma genes occur concurrently with TCRdelta and D-Jbeta rearrangements, but before Vbeta gene assembly. Productive TCRgamma rearrangements do not increase significantly before the Pre-T cell stage and are depleted in CD4(+)CD8(+) double-positive cells from normal mice. In contrast, double-positive thymocytes from TCRdelta-/- mice display random proportions of TCRgamma rearranged alleles, supporting a role for functional TCRgamma/delta rearrangements in the gammadelta divergence process.

The role of the T-cell receptor (TCR) in alpha beta/gamma delta lineage commitment: clues from intracellular TCR staining

T cells belong to two mutually exclusive lineages expressing either alpha beta or gamma delta T-cell receptors (TCR). Although alpha beta and gamma delta cells are known to share a common precursor the role of TCR rearrangement and specificity in the lineage commitment process is controversial. Instructive lineage commitment models endow the alpha beta or gamma delta TCR with a deterministic role in lineage choice, whereas separate lineage models invoke TCR-independent lineage commitment followed by TCR-dependent selection and maturation of alpha beta and gamma delta cells. Here we review the published data pertaining to the role of the TCR in alpha beta/gamma delta lineage commitment and provide some additional information obtained from recent intracellular TCR staining studies. We conclude that a variant of the separate lineage model is best able to accommodate all of the available experimental results.

Developmental regulation of V(D)J recombination at the TCR alpha/delta locus

The T-cell receptor (TCR) alpha/delta locus includes a large number of V, D, J and C gene segments that are used to produce functional TCR delta and TCR alpha chains expressed by distinct subsets of T lymphocytes. V(D)J recombination events within the locus are regulated as a function of developmental stage and cell lineage during T-lymphocyte differentiation in the thymus. The process of V(D)J recombination is regulated by cis-acting elements that modulate the accessibility of chromosomal substrates to the recombinase. Here we evaluate how the assembly of transcription factor complexes onto enhancers, promoters and other regulatory elements within the TCR alpha/delta locus imparts developmental control to VDJ delta and VJ alpha rearrangement events. Furthermore, we develop the notion that within a complex locus such as the TCR alpha/delta locus, highly localized and region-specific control is likely to require an interplay between positive regulatory elements and blocking or boundary elements that restrict the influence of the positive elements to defined regions of the locus.

Developmentally programmed rearrangement of T cell receptor Vgamma genes is controlled by sequences immediately upstream of the Vgamma genes

Distinct subsets of gammadelta T cells expressing different Vgamma and Vdelta chains arise in ordered waves during thymic development. In the murine Jgamma1-Cgamma1 cluster, the Vgamma3 gene segment is utilized earliest in fetal thymic development, in progenitors of dendritic epidermal T cells (DECs). The Vgamma2 gene segment predominates in the late fetal stages and beyond, in cells destined for the secondary lymphoid organs. Using transgenic TCRgamma recombination substrates, we demonstrate that this restricted Vgamma gene usage is determined by developmentally targeted gene rearrangement. We show that sequences immediately upstream of the Vgamma2 and Vgamma3 genes direct the rearrangement pattern in adult thymocytes. Thus, the choice of Vgamma gene for recombination is coordinated with distinct differentiation programs in gammadelta subsets.

T cell receptor gamma gene regulatory sequences prevent the function of a novel TCRgamma/pTalpha pre-T cell receptor

Expression of a TCRgamma transgene in RAG-1-/- mice resulted in the development of a limited number of CD4+CD8+ (DP) thymocytes. In vivo treatments with anti-TCRgamma antibody enhanced the number of DP thymocytes, demonstrating that TCRgamma chains were expressed on the cell surface in the absence of delta, alpha, or beta chains. Mutations in pTalpha or CD3epsilon genes abolished transgene-induced DP cell development, indicating that TCRgamma can associate with pTalpha and CD3 to form a novel pre-TCR. With a transgene containing additional regulatory sequences, TCRgamma expression was down-regulated in DP cells, and little DP cell development occurred. Thus, the function of the endogenous TCRgamma/pTalpha is limited by the transcriptional down-regulation of TCRgamma genes that normally accompanies DP cell development.

Alternative splicing of rearranged T cell receptor delta sequences to the constant region of the alpha locus

The T cell receptor (TCR) alpha/delta locus is composed of a common, shared set of variable (V) and distinct diversity (D), joining (J), and constant (C) genes. It has been recognized for several years that transcripts of the rearranged VDJdelta or VJalpha genes are spliced to the Cdelta or Calpha genes, respectively, encoding distinct TCR delta and alpha proteins. Herein, we describe the discovery of a splicing variation that allows the assembled VDJdelta genes to be fused with the Calpha gene. This variation is prominent in TCRdelta gene-deficient mice but is also detectable in wild-type mice. Furthermore, we show that several in-frame VDJdelta rearrangements in TCRdelta gene-deficient mice are strikingly underrepresented, suggesting that the alternative transcripts, with protein coding capacity, influence the development of alphabeta thymocytes. In-frame TCRgamma gene rearrangements do not appear underrepresented, indicating that the effect is not mediated by the gamma chain. Instead, indirect evidence supports the hypothesis that the delta/alpha chimeric protein acts in conjunction with the TCRbeta chain. These results have implications for the transcriptional control of the TCRalpha/delta locus and provide a novel insight into the distinct functional capacities of the TCR alpha and delta proteins during thymocyte development.

The developmental fate of T cells is critically influenced by TCRgammadelta expression

Differentiation of gammadelta and alphabeta T cells from a common precursor cell depends on productive rearrangement and expression of TCRgammadelta or TCRbeta genes, but whether it is an instructive or a stochastic mechanism that is responsible for this process is unclear. We report that expression of the productively rearranged TCRgamma transgene competitively inhibits alphabeta thymocyte development under conditions where TCRbeta gene rearrangement is limiting. The status of TCRdelta gene rearrangements in the remaining alphabeta-lineage cells indicates that the effect is mediated by the intact gammadelta receptor. Paradoxically, in TCRbeta-/- mice, gammadelta receptor expression can also drive differentiation of some alphabeta-lineage cells. To resolve this paradox, we provide evidence for a minor population of gammadelta-dependent alphabeta-lineage cells in normal mice. The results indicate that the T cell lineage commitment process is either error-prone or stochastic.

The alpha beta versus gamma delta T-cell lineage choice

During thymic development, immature T cells rearrange and express the genes encoding the T-cell antigen receptor and mature as either alpha beta or gamma delta lineage T cells. In the past year, advances have been made in understanding the role of individual components of the T-cell antigen receptor complex in the development of alpha beta and gamma delta lineage T cells. In addition, the transmembrane receptor Notch has recently been implicated as a new player in alpha beta versus gamma delta lineage determination.

A TCR alpha chain transgene induces maturation of CD4- CD8- alpha beta+ T cells from gamma delta T cell precursors

The proportion of CD4- CD8- double-negative (DN) alpha beta T cells is increased both in the thymus and in peripheral lymphoid organs of TCR alpha chain-transgenic mice. In this report we have characterized this T cell population to elucidate its relationship to alpha beta and gamma delta T cells. We show that the transgenic DN cells are phenotypically similar to gamma delta T cells but distinct from DN NK T cells. The precursors of DN cells have neither rearranged endogenous TCR alpha genes nor been negatively selected by the MIsa antigen, suggesting that they originate from a differentiation stage before the onset of TCR alpha chain rearrangements and CD4/CD8 gene expression. Neither in-frame V delta D delta J delta nor V gamma J gamma rearrangements are over-represented in this population. However, since peripheral gamma delta T cells with functional TCR beta gene rearrangements have been depleted in the transgenics, we propose that the transgenic DN population, at least partially, originates from the precursors of those cells. The present data lend support to the view that maturation signals to gamma delta lineage-committed precursors can be delivered via TCR alpha beta heterodimers.

Alpha beta lineage-committed thymocytes can be rescued by the gamma delta T cell receptor (TCR) in the absence of TCR beta chain

Commitment of the alpha beta and gamma delta T cell lineages within the thymus has been studied in T cell receptor (TCR)-transgenic and TCR mutant murine strains. TCR gamma delta-transgenic or TCR beta knockout mice, both of which are unable to generate TCR alpha beta-positive T cells, develop phenotypically alpha beta-like thymocytes in significant proportions. We provide evidence that in the absence of functional TCR beta protein, the gamma delta TCR can promote the development of alpha beta-like thymocytes, which, however, do not expand significantly and do not mature into gamma delta T cells. These results show that commitment to the alpha beta lineage can be determined independently of the isotype of the TCR, and suggest that alpha beta versus gamma delta T cell lineage commitment is principally regulated by mechanisms distinct from TCR-mediated selection. To accommodate our data and those reported previously on the effect of TCR gamma and delta gene rearrangements on alpha beta T cell development, we propose a model in which lineage commitment occurs independently of TCR gene rearrangement.

Separately expressed T cell receptor alpha and beta chain transgenes exert opposite effects on T cell differentiation and neoplastic transformation

Two aspects of T cell differentiation in T cell receptor (TCR)-transgenic mice, the generation of an unusual population of CD4-CD8-TCR+ thymocytes and the absence of gamma delta cells, have been the focus of extensive investigation. To examine the basis for these phenomena, we investigated the effects of separate expression of a transgenic TCR alpha chain and a transgenic TCR beta chain on thymocyte differentiation. Our data indicate that expression of a transgenic TCR alpha chain causes thymocytes to differentiate into a CD4-CD8-TCR+ lineage at an early developmental stage, depleting the number of thymocytes that differentiate into the alpha beta lineage. Surprisingly, expression of the TCR alpha chain transgene is also associated with the development of T cell lymphosarcoma. In contrast, expression of the transgenic TCR beta chain causes immature T cells to accelerate differentiation into the alpha beta lineage and thus inhibits the generation of gamma delta cells. Our observations provide a model for understanding T cell differentiation in TCR-transgenic mice.

T-call development: is notch a key player in lineage decisions?

At two consecutive 'checkpoints' in their development, T cells have to be rescued from programmed cell death and to choose between distinct lineage fates; recent results show that the Notch transmembrane receptor can significantly influence T-cell development at both of these points.

Notch activity influences the alphabeta versus gammadelta T cell lineage decision

The choice between the alphabeta or gammadelta T cell fates is influenced by the production of functional, in-frame rearrangements of the TCR genes, but the mechanism that controls the lineage choice is not known. Here, we show that T cells that are heterozygous for a mutation of the Notch1 gene are more likely to develop as gammadelta T cells than as alphabeta T cells, implying that reduced Notch activity favors the gammadelta T cell fate over the alphabeta T cell fate. A constitutively activated form of Notch produces a reciprocal phenotype and induces thymocytes that have functional gammadeltaTCR gene rearrangements to adopt the alphabeta T cell fate. Our data indicate that Notch acts together with the newly formed T cell antigen receptor to direct the alphabeta versus gammadelta T cell lineage decision.

Induction of TCR gene rearrangements in uncommitted stem cells by a subset of IL-7 producing, MHC class-II-expressing thymic stromal cells

The embryonic thymic microenvironment provides the necessary elements for T cell lineage commitment, but the precise role of individual stromal cell components remains to be determined. Here we address the question of which stromal cell types are required for initiation of V-DJ rearrangements of the TCR-beta and TCR-delta locus in CD117+CD45+ uncommitted fetal liver progenitors. We show that fetal thymic stroma alone is necessary and sufficient for induction of TCR-beta and TCR-delta rearrangements. Furthermore, the ability to induce this T cell commitment step is confined to a subset of MHC class II-positive epithelial cells. Thymic stroma derived from mice with a targeted deletion in the IL-7 gene, however, lacks this ability. These findings set the stage for a further definition of the nature of the thymic stromal cell support in the regulation of T cell commitment.

T cell receptor alpha gene rearrangement and transcription in adult thymic gamma delta cells

T cells belong to two separate lineages based on surface expression of alpha beta or gamma delta T cell receptors (TCR). Since during thymus development TCR beta, gamma, and delta genes rearrange before alpha genes, and gamma delta cells appear earlier than alpha beta cells, it has been assumed that gamma delta cells are devoid of TCR alpha rearrangements. We show here that this is not the case, since mature adult, but not fetal, thymic gamma delta cells undergo VJ alpha rearrangements more frequently than immature alpha beta lineage thymic precursors. Sequence analysis shows VJ alpha rearrangements in gamma delta cells to be mostly (70%) nonproductive. Furthermore, VJ alpha rearrangements in gamma delta cells are transcribed normally and, as shown by analysis of TCR beta-/- mice, occur independently of productive VDJ beta rearrangements. These data are interpreted in the context of a model in which precursors of alpha beta and gamma delta cells differ in their ability to express a functional pre-TCR complex.

The alpha beta T cell receptor can replace the gamma delta receptor in the development of gamma delta lineage cells

In peripheral lymphoid tissues of TCR transgenic mice that express the nominal antigen (HY peptide plus H-2Db MHC) recognized by the transgenic TCR, there exist unusual CD4-CD8- and CD4-CD8low cells bearing the transgenic TCR. Here we show that, unlike TCR alpha beta T cells that are generated in the absence of nominal antigen, these unusual cells do not express endogenous TCR alpha genes, have maintained the TCR delta locus on both chromosomes, and can coexpress TCR alpha beta and TCR gamma delta chains on the cell surface. The latter is also true for CD4-CD8-, HSA+ TCR alpha beta + thymocytes in male and female TCR transgenic mice. The number of TCR alpha beta and TCR gamma delta coexpressing cells is increased in pre-TCR-deficient mice. The data indicate that the TCR alpha beta can replace the TCR gamma delta in the development of gamma delta lineage cells and that the pre-TCR interferes with the generation of gamma delta-expressing cells.