Blockage of alpha beta T-cell development by TCR gamma delta transgenes

N6 - Methyladenosine defines a new checkpoint in γδ T cell development

T cells, which are derived from hematopoietic stem cells (HSCs), are the most important components of adaptive immune system. Based on the expression of αβ and γδ receptors, T cells are mainly divided into αβ and γδ T cells. In the thymus, they share common progenitor cells, while undergoing a series of well-characterized and different developmental processes. N⁶ -Methyladenosine (m⁶ A), one of the most abundant modifications in mRNAs, plays critical roles in cell development and maintenance of function. Recently, we have demonstrated that the depletion of m⁶ A demethylase ALKBH5 in lymphocytes specifically induces an expansion of γδ T cells through the regulation of Jag1/Notch2 signaling, but not αβ T cells, indicating a checkpoint role of ALKBH5 and m⁶ A modification in the early development of γδ T cells. Based on previous studies, many key pathway molecules, which exert dominant roles in γδ T cell fate determination, have been identified as the targets regulated by m⁶ A modification. In this review, we mainly summarize the potential regulation between m⁶ A modification and these key signaling molecules in the γδ T cell lineage commitment, to provide new perspectives in the checkpoint of γδ T cell development.

Resolving the mystery-How TCR transgenic mouse models shed light on the elusive case of gamma delta T cells

Cutting-edge questions in αβ T cell biology were addressed by investigating a range of different genetically modified mouse models. In comparison, the γδ T cell field lacks behind on the availability of such models. Nevertheless, transgenic mouse models proved useful for the investigation of γδ T cell biology and their stepwise development in the thymus. In general, animal models and especially mouse models give access to a wide range of opportunities of modulating γδ T cells, which is unachievable in human beings. Because of their complex biology and specific tissue tropism, it is especially challenging to investigate γδ T cells in in vitro experiments since they might not reliably reflect their behavior and phenotype under physiologic conditions. This review aims to provide a comprehensive historical overview about how different transgenic mouse models contributed in regards of the understanding of γδ T cell biology, whereby a special focus is set on studies including the elusive role of the γδTCR. Furthermore, evolutionary and translational remarks are discussed under the aspect of future implications for the field. The ultimate full understanding of γδ T cells will pave the way for their usage as a powerful new tool in immunotherapy.

Integration of T-cell receptor, Notch and cytokine signals programs mouse γδ T-cell effector differentiation

γδ T‐cells perform a wide range of tissue‐ and disease‐specific functions that are dependent on the effector cytokines produced by these cells. However, the aggregate signals required for the development of interferon‐γ (IFNγ) and interleukin‐17 (IL‐17) producing γδ T‐cells remain unknown. Here, we define the cues involved in the functional programming of γδ T‐cells, by examining the roles of T‐cell receptor (TCR), Notch, and cytokine‐receptor signaling. KN6 γδTCR‐transduced Rag2^−/− T‐cell progenitors were cultured on stromal cells variably expressing TCR and Notch ligands, supplemented with different cytokines. We found that distinct combinations of these signals are required to program IFNγ versus IL‐17 producing γδ T‐cell subsets, with Notch and weak TCR ligands optimally enabling development of γδ17 cells in the presence of IL‐1β, IL‐21 and IL‐23. Notably, these cytokines were also shown to be required for the intrathymic development of γδ17 cells. Together, this work provides a framework of how signals downstream of TCR, Notch and cytokine receptors integrate to program the effector function of IFNγ and IL‐17 producing γδ T‐cell subsets.

Gamma delta T-cell differentiation and effector function programming, TCR signal strength, when and how much?

γδ T-cells boast an impressive functional repertoire that can paint them as either champions or villains depending on the environmental and immunological cues. Understanding the function of the various effector γδ subsets necessitates tracing the developmental program of these subsets, including the point of lineage bifurcation from αβ T-cells. Here, we review the importance of signals from the T-cell receptor (TCR) in determining αβ versus γδ lineage fate, and further discuss how the molecular components of this pathway may influence the developmental programming of γδ T-cells functional subsets. Additionally, we discuss the role of temporal windows in restricting the development of IL-17 producing γδ T-cell subtypes, and explore whether fetal and adult hematopoietic progenitors maintain the same potential for giving rise to this important subset.

Enforcement of γδ-lineage commitment by the pre-T-cell receptor in precursors with weak γδ-TCR signals

Developing thymocytes bifurcate from a bipotent precursor into αβ- or γδ-lineage T cells. Considering this common origin and the fact that the T-cell receptor (TCR) β-, γ-, and δ-chains simultaneously rearrange at the double negative (DN) stage of development, the possibility exists that a given DN cell can express and transmit signals through both the pre-TCR and γδ-TCR. Here, we tested this scenario by defining the differentiation outcomes and criteria for lineage choice when both TCR-β and γδ-TCR are simultaneously expressed in Rag2(-/-) DN cells via retroviral transduction. Our results showed that Rag2(-/-) DN cells expressing both TCRs developed along the γδ-lineage, down-regulated CD24 expression, and up-regulated CD73 expression, showed a γδ-biased gene-expression profile, and produced IFN-γ in response to stimulation. However, in the absence of Inhibitor of DNA-binding 3 expression and strong γδ-TCR ligand, γδ-expressing cells showed a lower propensity to differentiate along the γδ-lineage. Importantly, differentiation along the γδ-lineage was restored by pre-TCR coexpression, which induced greater down-regulation of CD24, higher levels of CD73, Nr4a2, and Rgs1, and recovery of functional competence to produce IFN-γ. These results confirm a requirement for a strong γδ-TCR ligand engagement to promote maturation along the γδ T-cell lineage, whereas additional signals from the pre-TCR can serve to enforce a γδ-lineage choice in the case of weaker γδ-TCR signals. Taken together, these findings further cement the view that the cumulative signal strength sensed by developing DN cells serves to dictate its lineage choice.

Visualization and quantification of monoallelic TCRα gene rearrangement in αβ T cells

T-cell receptor α (TCRα) chain rearrangement is not constrained by allelic exclusion and thus αβ T cells frequently have rearranged both alleles of this locus. Thereby, stepwise secondary rearrangements of both TCRα loci further increase the odds for generation of an α-chain that can be positively selected in combination with a pre-existing TCRβ chain. Previous studies estimated that approximately 2-12% of murine and human αβ T cells still carry one TCRα locus in germline configuration, which must comprise a partially or even fully rearranged TCRδ locus. However, these estimates are based on a relatively small amount of individual αβ T-cell clones and αβ T-cell hybridomas analyzed to date. To address this issue more accurately, we made use of a mouse model, in which a fluorescent reporter protein is introduced into the constant region of the TCRδ locus. In this TcrdH2BeGFP system, fluorescence emanating from retained TCRδ loci enabled us to quantify monoallelically rearranged αβ T cells on a single-cell basis. Via fluorescence-activated cell sorting analysis, we determined the frequency of monoallelic TCRα rearrangements to be 1.7% in both peripheral CD4(+) and CD8(+) αβ T cells. Furthermore, we found a skewed 5' Jα gene utilization of the rearranged TCRα allele in T cells with monoallelic TCRα rearrangements. This is in line with previous descriptions of a tight interallelic positional coincidence of Jα gene segments used on both TCRα alleles. Finally, analysis of T cells from transgenic mice harboring only one functional TCRα locus implied the existence of very rare unusual translocation or episomal reintegration events of formerly excised TCRδ loci.

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.

gammadelta and alphabeta T cell lineage choice: resolution by a stronger sense of being

A common bipotent thymocyte precursor gives rise to both lineages of T cells, alphabeta and gammadelta. However, the cell intrinsic and extrinsic factors that influence alphabeta- versus gammadelta-lineage bifurcation remain controversial. gammadelta T cells play a unique and vital role in host defense, from maintaining integrity at epithelial and mucosal barriers to their newly defined role as an important innate source of interleukin-17. Although a T cell receptor (TCR)-independent fate choice may take place, emerging data supports a model in which the differential signaling capacity of alphabeta and gammadeltaTCRs play an instructional role in specifying lineage fate, with strength of signal measured by the amount of ERK/MAPK pathway activation. Here we discuss how the interplay between intrinsic TCR signals and cell extrinsic signals provided by Notch and TCR ligands help to assign and support a final lineage fate decision.

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.

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.

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.

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.

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.

Chromosomal excision of TCRδ chain genes is dispensable for αβ T cell lineage commitment

TCRbeta, delta and gamma chain genes are assembled and expressed in double-negative thymocytes prior to alphabeta or gammadelta T cell lineage commitment. Thus, cells committed to the alphabeta T cell lineage can possess completely assembled TCRdelta and/or TCRgamma chain genes. However, these genes are not expressed. TCRgamma chain gene expression may be silenced through the activity of a cis-acting silencer element. In the TCRalpha/delta locus, the TCRdelta genes lie between the Valpha and Jalpha gene segments, which rearrange by deletion. Moreover, Valpha to Jalpha rearrangements occur on both alleles in essentially all developing alphabeta T cells. Consequently, both TCRdelta chain genes are excised from the chromosome and placed on extrachromosomal circles in mature alphabeta T cells. It has been proposed that this excision process is important for silencing TCRdelta gene expression and permitting alphabeta T cell lineage commitment. A gene-targeting Cre-loxP strategy was used to invert a 75-kb region of the TCRalpha/delta locus encompassing all the Jalpha gene segments, generating the TCRalpha/delta(I) allele. Initial Valpha to Jalpha rearrangements on the TCRalpha/delta(I) allele occur by inversion, resulting in chromosomal retention of TCRdelta chain genes. These TCRdelta chain genes can be productively rearranged and are expressed at levels similar to TCRdelta chain genes in gammadelta T cells. However, alphabeta T cell development appears unperturbed in TCRalpha/delta(I/I) mice. Thus, excision of TCRdelta genes from the chromosome per se is not required for commitment of developing lymphocytes to the alphabeta T cell lineage.

Access roads for RAG-ged terrains: control of T cell receptor gene rearrangement at multiple levels

Antigen-specific immune response requires the generation of a diverse antigen (Ag)-receptor repertoire. The primary repertoire is generated through somatic gene rearrangement and molded by subsequent cellular selection. Constraints during gene recombination influence the ultimate shape of the repertoire. One major control mechanism of gene rearrangement, investigated for many years, is exerted through regulated chromosomal accessibility of the recombinase to the antigen receptor loci. More recent studies began to explore the role of interactions between the recombinase and its cognate recognition DNA sequences. The emerging results suggest that formation of the primary repertoire is controlled by two, partially independent factors: chromosomal accessibility and direct recombinase-DNA interactions.

Genomic organization of the family of CNR cadherin genes in mice and humans

The cadherin-related neuronal receptor (CNR) family is localized to the synaptic junction, and their cytoplasmic domains interact with Fyn-tyrosine kinase. Here, we describe the chromosomal locations and the orthologous genomic structures of CNR family members in mice and humans. In the genomic organization, distinct exons, each of which encodes the N-terminus of a different CNR ("variable region"), are clustered in a tandem array, and these exons are spliced to a common region composed of three exons ("constant region"). We also discovered three alternative versions of the transcripts; a single variable exon connects with three different C-terminal tails, comparable to class-switching in the immunoglobulin heavy chain. Thus the CNR family in the central nervous system has similarities to the immunoglobulin and T-cell receptor genes in the immune system.

Unexpectedly late expression of intracellular CD3epsilon and TCR gammadelta proteins during adult thymus development

During adult thymus development immature CD4(-)CD8(-) [double-negative (DN)] precursor cells pass through four phenotypically distinct stages defined by expression of CD44 and CD25: CD44(hi)CD25(-) (DN1), CD44(hi)CD25(+) (DN2), CD44(lo)CD25(+) (DN3) and CD44(lo)CD25(-) (DN4). Although it is well established that the TCR beta, gamma and delta genes are rearranged and expressed in association with the CD3 components in DN thymocytes, the precise timing of expression of the TCR and CD3 proteins has not been determined. In this report we have utilized a sensitive intracellular (ic) staining technique to analyze the expression of ic CD3epsilon, TCR beta and TCR gammadelta proteins in immature DN subsets. As expected from previous studies of TCR beta rearrangement and mRNA expression, icTCR beta(+) cells were first detected in the DN3 subset and their proportion increased thereafter. Surprisingly, however, both icCD3epsilon(+) and icTCR gammadelta(+) cells were detected at later stages of development than was predicted by molecular studies. In particular icCD3epsilon protein expression coincided with the transition from the DN2 to DN3 stage of development, whereas icTCR gammadelta protein expression was only detected in a minor subset of DN4 cells. The implications of these findings for alphabeta lineage divergence will be discussed.

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.

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.

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.

Interleukin 7 induces TCR gene rearrangement in adult marrow-resident murine precursor T cells

Rearrangement of the T cell antigen receptor genes is a complex, highly regulated process. To gain a better understanding of the extracellular factors involved in the regulation of TCR beta and gamma gene rearrangement in adult murine bone marrow-resident precursor T cells, several cytokines were tested for their ability to induce gene recombination. A selected population of C58/J bone marrow cells (Thy 1(low), CD3, CD8, B220) that is enriched for pre-T cell activity was propagated in vitro in medium supplemented with IL-3 and mast cell growth factor (MGF, also referred to as stem cell factor, Steele factor and c-kit ligand). These cytokines were required for the maintenance of pre-T cell activity in culture, but had no effect on TCR gene expression. Several additional cytokines were added to the culture medium. Of all those tested, only IL-7 induced complete rearrangement of the TCR gamma locus. Complete rearrangement of the TCR beta locus was not induced under any of the culture conditions analysed here. The bone marrow cells cultured in IL-3, MGF and IL-7 did not begin to express mature T cell proteins and maintained their in vivo progenitor potential. Furthermore, IL-7 cultured bone marrow cells were capable of differentiation in vivo into all phenotypic subpopulations of T cells, without an apparent bias toward the gammadelta lineage. The data presented here suggest that TCR gamma gene rearrangement in adult pre-T cells is regulated by IL-7, but that the TCR beta locus requires additional or alternative signals for the induction of complete rearrangement.

Gamma delta T lymphocytosis associated with common variable immunodeficiency

We present the case of a 28-year-old Caucasian female with common variable immunodeficiency (CVID) since age 5 who had a long history of hospitalizations for unexplained fevers and pulmonary infiltrates. The patient developed mild lymphocytosis 7 months prior to our evaluation. Flow cytometry of peripheral blood revealed an expansion of gamma delta T lymphocytes, mild CD4 T lymphocytopenia, and a reduced CD4/CD8 ratio (0.2). Two subpopulations of gamma delta T lymphocytes were found (CD3+/CD4-/CD8+, 47%; CD3+/CD4-/CD8-, 53%), the vast majority of which expressed V-delta 1. An infectious cause for the patient's gamma delta T lymphocytosis could not be found. The sputum was chronically colonized with Staphylococcus aureus, and the organism produced TSST-1 in vitro. A bronchoalveolar lavage (BAL) revealed marked lymphocytosis, but gamma delta T lymphocytes were not overrepresented in the BAL. Lymphocyte functional studies revealed poor proliferative responses to mitogens and staphylococcal superantigens and diminished cytokine production. V-delta 1 T lymphocytes from the patient's blood were not expanded in vitro in response to staphylococcal superantigens. TCR gene rearrangement studies confirmed the presence of J gamma and J beta 1 clonal rearrangements accounting for only a small subpopulation of the gamma delta T lymphocytes. These studies were repeated 5 months later and were unchanged. A bone marrow biopsy was negative for leukemia. Hence, the cause of the patient's gamma delta T lymphocytosis could not be determined despite evaluation for underlying malignancy, occult infection, or superantigen-driven stimulation. The patient ultimately died of progressive respiratory insufficiency. The state of current knowledge regarding gamma delta T lymphocytosis, decreased production of alpha beta T lymphocytes, and a low CD4/ CD8 ratio in association with CVID is discussed.

V-D-J rearrangements at the T cell receptor delta locus in mouse thymocytes of the alpha beta lineage

The T cell receptor (TCR) delta locus lies within the TCR alpha locus and is excised from the chromosome by V alpha-J alpha rearrangement. We show here that delta sequences persist in a large fraction of the DNA from mature CD4+CD8- alpha beta+ mouse thymocytes. Virtually all delta loci in these cells are rearranged and present in extrachromosomal DNA. In immature alpha beta lineage thymocytes (CD3-/loCD4+CD8+) and in CD4+CD8- alpha beta+ thymocytes expressing a transgene-encoded alpha beta receptor, rearranged delta genes are present both in chromosomal and extrachromosomal DNA. Thus, contrary to earlier proposals, commitment to the alpha beta lineage does not require recombinational silencing of the delta locus or its deletion by a site-specific mechanism prior to V alpha-J alpha rearrangement.

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

Two models have been considered to account for the differentiation of gamma delta and alpha beta T cells from a common hematopoietic progenitor cell. In one model, progenitor cells commit to a lineage before T cell receptor (TCR) rearrangement occurs. In the other model, progenitor cells first undergo rearrangement of TCR gamma, delta, or both genes, and cells that succeed in generating a functional receptor commit to the gamma delta lineage, while those that do not proceed to attempt complete beta and subsequently alpha gene rearrangements. A prediction of the latter model is that TCR gamma rearrangements present in alpha beta T cells will be nonproductive. We tested this hypothesis by examining V gamma 2-J gamma 1C gamma 1 rearrangements, which are commonly found in alpha beta T cells. The results indicate that V gamma 2-J gamma 1C gamma 1 rearrangements in purified alpha beta T cell populations are almost all nonproductive. The low frequency of productive rearrangements of V gamma 2 in alpha beta T cells is apparently not due to a property of the rearrangement machinery, because a transgenic rearrangement substrate, in which the V gamma 2 gene harbored a frame-shift mutation that prevents expression at the protein level, was often rearranged in a productive configuration in alpha beta T cells. The results suggest that progenitor cells which undergo productive rearrangement of their endogenous V gamma 2 gene are selectively excluded from the alpha beta T cell lineage.

In-frame TCR delta gene rearrangements play a critical role in the alpha beta/gamma delta T cell lineage decision

Using a quantitative multiprobe Southern blot analysis, we demonstrate the surprising result that a significant proportion of alpha beta T cells and thymocytes retain T cell receptor delta locus sequences. A substantial portion of the retained delta locus is in a fully V-to-D-to-J rearranged configuration and 20% of these delta rearrangements are functional, significantly less than the 33% predicted for random gene rearrangements. Our observations are in conflict with the idea that alpha beta and gamma delta T cells derive from distinct precursors and suggest that commitment of a common precursor to the gamma delta lineage depends upon expression of a gamma delta T cell receptor. We propose that the intrathymic T cell lineage decision is determined by a competition between the production of functional gamma delta and beta-pre-T cell receptor complexes.

Alpha beta and gamma delta T cells can share a late common precursor

BACKGROUND

The subdivision of T cells into alpha beta and gamma delta subtypes is conserved throughout vertebrate development. The respective alpha beta and gamma delta T-cell receptors (TCRs) are encoded by somatically rearranged genes. There has been broad speculation as to whether an individual thymocyte can become either a gamma delta T cell or an alpha beta T cell as a result of stochastic gene rearrangement processes, or whether the two types of T cell are derived from separate lineages. Many of the experimental findings are apparently conflicting, however, and the issue--a basic one in immunology and development--remains unresolved.

RESULTS

To address this issue, we have used the recently developed polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) technique, which allows us to examine quantitatively the status of TCR gamma and delta genes in postnatal alpha beta T cells and their progenitors. Interestingly, such cells are depleted of productively rearranged delta and gamma genes, which can encode delta and gamma TCR polypeptide chains. However, in mice that can rearrange TCR delta gene segments, but in which the TCR delta gene is non-functional in other respects, no such depletion of productive rearrangements is seen.

CONCLUSION

The quantitative data that we have obtained fulfill the predictions of the stochastic hypothesis: that is, a progenitor T cell first attempts to become a gamma delta T cell and, if unsuccessful, then attempts to become an alpha beta T cell. Thus, alpha beta and gamma delta T cells can derive from a common precursor thymocyte. In the simplest case, therefore, lineage-determining factors are the successful rearrangement of both gamma and delta genes before TCR alpha gene rearrangements occur, which lead to deletion of the TCR delta locus and thereby preclude further gamma delta T-cell differentiation. In contrast, successful rearrangement of the TCR beta locus remains compatible with cells becoming either gamma delta or alpha beta T cells.

gamma delta lineage-specific transcription of human T cell receptor gamma genes by a combination of a non-lineage-specific enhancer and silencers

The expression of the T cell receptor (TcR) gamma genes is restricted to TcR gamma delta + T lymphocytes. Transgenic and somatic cell hybrid experiments had suggested that the expression of a functionally rearranged TcR gamma gene was extinguished in TcR alpha beta + T cells, possibly by putative cis-acting transcriptional silencers. We have identified such negative cis-acting sequences in the 3' non-coding region of the human TcR gamma (TRG) locus, upstream of an enhancer located at 6.5 kb of the TcR C gamma 2 gene (TRGC2). These silencers were capable of repressing the transcription from a minimal heterologous promoter in a position- and orientation-independent fashion. When analyzed individually, the silencers and the enhancer were equally active in the TcR alpha beta + and TcR gamma delta + T cell lines studied. In contrast, the association of the enhancer with either silencer was shown to restrict transcription to the TcR gamma delta + T cell lines.

Gene rearrangement patterning and DNase-I hypersensitive sites within the T-cell receptor J alpha locus

For several years, the relationship between alpha beta and gamma delta T-cell progenitors has been a topic of debate. Some argue that a subset of T-cell progenitors is "pre-committed" to the alpha beta lineage and is thus programmed to rearrange alpha, but not delta genes. It is further argued that the deletion of the delta locus by a unique rearrangement, delta rec-psi J alpha, may be the critical forerunner to V-J alpha joins in alpha beta committed cells and that a hypersensitive site (HS) termed 5'TEA might regulate such rearrangement. Here we present an alternative hypothesis. We first emphasize that directed J alpha gene rearrangements do not exclusively target the psi J alpha gene, but that clustered gene rearrangements occur throughout the J alpha locus during T-cell development. We describe the existence of not one, but at least two HS sites distributed along the J alpha locus which might serve as regulators for the gene rearrangement event. Finally, we suggest that progenitor T-cells are not committed to a particular delta or alpha gene rearrangement, but that a flexible progenitor responds to complex interactions between environmental signals and multiple regulatory elements interspersed among delta/alpha genes.

Identification of a T cell-specific transcriptional enhancer 3' of the human T cell receptor gamma locus

Positive and negative transcriptional regulatory mechanisms are thought to play a major role in the expression of T cell antigen receptor (TCR) genes. Since the alpha beta and gamma delta T cell receptor heterodimers are expressed in a mutually exclusive fashion and since TCR genes are sequentially activated during T cell ontogeny, transcriptional activation and repression must at least in part determine T lineage-specific and developmental-specific expression of these genes. We have identified a transcriptional enhancer located 6.5 kb downstream from the human T cell receptor gamma (TRG) locus. The nucleotide sequence of the enhancer core element shows strong sequence homology to the recently identified murine C gamma 1 enhancer. The enhancer demonstrates T cell-specific activity, but not gamma delta sublineage-specificity in combination with either a heterologous or gene-specific promoter. Thus, additional regulatory elements may be required to repress the expression of rearranged TRG genes in non-gamma delta T cells.

Fetal thymocyte potential for T cell receptor V gamma 3-J gamma 1 junctional modification

Junctional modifications of T cell receptor (TcR) and immunoglobulin (Ig) gene joining regions provide great diversity to respective protein repertoires. The addition of non-germ-line-encoded nucleotides (N-regions) in the V-J gamma junction is one such modification which is developmentally regulated, rarely evident in the fetal animal, but common in the adult. A question has recently arisen as to whether developmentally patterned N-region additions in V-J gamma joins are a reflection of T cell progenitors which are committed to particular types of rearrangement prior to the event, or of changing environmental influences on uncommitted cell populations. To address this question with regard to the V gamma 3-J gamma 1 join, T cells were examined in the fetal thymic organ culture (FTOC), a system with which the environment of early progenitor cells could be deliberately altered. At various times following FTOC initiation, cells were isolated for examination by the polymerase chain reaction, cloning and sequencing. V gamma 3-J gamma 1 sequences within genomic DNA as well as cDNA were evaluated. Data from these studies revealed frequent N-region additions within V-J gamma joins among day 14 fetal thymocyte populations, a situation dissimilar from that in vivo. Also dissimilar from the in vivo situation was the degree of exonuclease activity evident in FTOC. The canonical V gamma 3-J gamma 1 join (a frequent junction lacking N-region addition) was recognized in all experiments, but was least common among DNA versus cDNA sequences. Results illustrate that early progenitor cell populations are not programmed to exclude junctional modifications from V gamma 3-J gamma 1 joins.

Accessory cell-derived signals required for T cell activation

Various populations of accessory cells differ in their abilities to function as effective antigen-presenting cells (APC) and stimulate CD4+ T cells to produce interleukin-2. Three important factors directly related to APC potency are the expression of class II major histocompatibility complex molecules and the ability to present peptide antigens to the T cell antigen receptor, the expression of costimulatory ligands which deliver important activation signals independent of T cell receptor occupancy and the expression of adhesion molecules which promote conjugate formation so that these activation signals can be effectively delivered to the T cells. The relative importance of these accessory cell functions in T cell activation will be discussed, with an emphasis on costimulation and the CD28/B7 receptor/ligand pair. The consequence of inadequate costimulation by an otherwise effective APC in inducing T cell anergy will also be discussed.

T cell receptor delta gene mutant mice: independent generation of alpha beta T cells and programmed rearrangements of gamma delta TCR genes

T cells bearing T cell receptor (TCR) gamma and delta chain heterodimers are first generated early in ontogeny. They form distinct subsets that differ in their TCR repertoires and tissue distribution. Disruption of the mouse TCR C delta gene segment by a gene targeting method caused the complete loss of T cells bearing TCR gamma delta chains, but had little or no effect on the development of T cells bearing TCR alpha beta chains. The analyses of TCR gamma and delta genes in the mutant mice suggest that intracellular mechanisms acting at the level of DNA rearrangement play key roles in the differential gamma and delta gene rearrangements and in the generation of the highly restricted junctional sequences during fetal thymic development.

Characterization of T cells expressing the gamma/delta antigen receptor in human renal allografts

To investigate the role of gamma/delta+ T cells in allograft rejection, we have studied the TCR phenotype and function of lymphocytes infiltrating rejecting, rejected, and nonrejecting human renal allografts. Two-color immunohistologic staining showed that 19% of rejecting biopsies and 40% of rejected nephrectomies had significant infiltration (> 10% of the total T-cell population) with gamma/delta+ T cells. No biopsies from nonrejecting kidneys showed > 10% gamma/delta+ T cells. Flow-cytometry analysis of T-cell populations expanded from rejecting and rejected allografts demonstrated that 33% of biopsy- and 40% of nephrectomy-derived populations had significant percentages (> 10%) of gamma/delta+ T cells. Six cell lines with increased numbers of gamma/delta+ T cells were tested for cytolytic activity against the NK target cell line K562 and compared with cytotoxic activity of exclusively alpha/beta T-cell populations. Lysis was noted by all gamma/delta+, but no gamma/delta-, populations. To confirm that the cytotoxicity of these gamma/delta+ T-cell populations was not MHC directed, one nephrectomy-derived population with 69% gamma/delta+ T cells by cytometry and > 50% by immunohistology was studied extensively. High levels of killing were seen against the NK targets K562 and Daudi as well as other malignant, benign, and third-party renal cell lines, but relevant alloantigen-expressing targets were not killed. Sterile cell sorting was used to isolate the gamma/delta+ T cells. The gamma/delta+ cells displayed enhanced killing of K562 while the gamma/delta- cells showed no cytolytic activity. Cytotoxicity mediated by gamma/delta+ T cells was also demonstrated against donor-derived, untransformed renal cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Co-expression of T-cell receptor beta and delta mRNA detected at high frequency in hybridomas derived from adult thymus

Two different hybridoma collections from adult C3H/HeJ thymus were generated in order to analyse T-cell receptor (TcR) rearrangements, surface expression of T-cell receptors and differentiation markers as well as lymphokine production. Large, low density thymocytes were either directly fused to the thymoma BW 5147 alpha-beta- variant, or fused after stimulation with Concanavalin A in the presence of interleukin-2 for 48 h. The hybrids obtained from Concanavalin A-stimulated cells represented rather mature thymocytes, with regard to TcR rearrangements and surface T-cell receptor expression. The collection of hybrids derived from freshly isolated large thymocytes contained cells in various stages of T-cell development. An unexpectedly large number of hybrids (46 out of 84) from this group expressed full-length C beta together with full-length, or shorter, C delta mRNA. This finding suggests that a considerable proportion of alpha beta T cells proceeds through a stage in development where delta genes are being rearranged and transcribed.

Lymphoid development in mice congenitally lacking T cell receptor alpha beta-expressing cells

Vertebrate T cells express either an alpha beta or gamma delta T cell receptor (TCR). The developmental relatedness of the two cell types is unresolved. alpha beta + T cells respond to specific pathogens by collaborating with immunoglobulin-producing B cells in distinct lymphoid organs such as the spleen and Peyer's patches. The precise influence of alpha beta + T cells on B cell development is poorly understood. To investigate the developmental effects of alpha beta + T cells on B cells and gamma delta + T cells, mice homozygous for a disrupted TCR alpha gene were generated. The homozygotes showed elimination of alpha beta + T cells and the loss of thymic medullae. Despite this, gamma delta + T cells developed in normal numbers, and there was an increase in splenic B cells.

Development and selection of gamma delta T cells

The gamma delta T-cell population, a subpopulation of T cells formed through cell lineages that are independent of the alpha beta T-cell lineage, consists of multiple subsets with distinct receptor repertoires and homing properties. While the cell sublineage is a critical factor in the determination of homing specificity, both cell sublineage and receptor-dependent selection are instrumental in the determination of the functional repertoire.

The development of functionally responsive T cells

The work reviewed in this article separates T cell development into four phases. First is an expansion phase prior to TCR rearrangement, which appears to be correlated with programming of at least some response genes for inducibility. This phase can occur to some extent outside of the thymus. However, the profound T cell deficit of nude mice indicates that the thymus is by far the most potent site for inducing the expansion per se, even if other sites can induce some response acquisition. Second is a controlled phase of TCR gene rearrangement. The details of the regulatory mechanism that selects particular loci for rearrangement are still not known. It seems that the rearrangement of the TCR gamma loci in the gamma delta lineage may not always take place at a developmental stage strictly equivalent to the rearrangement of TCR beta in the alpha beta lineage, and it is not clear just how early the two lineages diverge. In the TCR alpha beta lineage, however, the final gene rearrangement events are accompanied by rapid proliferation and an interruption in cellular response gene inducibility. The loss of conventional responsiveness is probably caused by alterations at the level of signaling, and may be a manifestation of the physiological state that is a precondition for selection. Third is the complex process of selection. Whereas peripheral T cells can undergo forms of positive selection (by antigen-driven clonal expansion) and negative selection (by abortive stimulation leading to anergy or death), neither is exactly the same phenomenon that occurs in the thymic cortex. Negative selection in the cortex appears to be a suicidal inversion of antigen responsiveness: instead of turning on IL-2 expression, the activated cell destroys its own chromatin. The genes that need to be induced for this response are not yet identified, but it is unquestionably a form of activation. It is interesting that in humans and rats, cortical thymocytes undergoing negative selection can still induce IL-2R alpha expression and even be rescued in vitro, if exogenous IL-2 is provided. Perhaps murine thymocytes are denied this form of rescue because they shut off IL-2R beta chain expression at an earlier stage or because they may be uncommonly Bcl-2 deficient (cf. Sentman et al., 1991; Strasser et al., 1991). Even so, medullary thymocytes remain at least partially susceptible to negative selection even as they continue to mature.(ABSTRACT TRUNCATED AT 400 WORDS)

Induction of CD8 molecules on thymic gamma/delta T cells in vitro is dependent upon alpha/beta T cells

Thymocytes differentiate upon interactions with microenvironmental components, but the precise role of different stromal cells, or other T cells, in early differentiative events remains unclear. Here we have analyzed the in vitro differentiation of double-negative (DN) thymocytes from young adult mice. We demonstrate that a substantial proportion of DN thymocytes differentiate into CD8+ gamma/delta T cells upon stimulation with concanavalin A and recombinant interleukin 2. However, if alpha/beta T cells are excluded from the initial population of DN thymocytes, the CD8+ gamma/delta T cells do not appear in the cultures. These results suggest a role for T-T cell interactions in thymic differentiative events, and provide evidence for physiological interactions between the alpha/beta and gamma/delta T cell compartments within the thymus.

A pgk::hprt fusion as a selectable marker for targeting of genes in mouse embryonic stem cells: disruption of the T-cell receptor delta-chain-encoding gene

We have constructed a hypoxanthine phosphoribosyl transferase-selectable marker (hprt) under the control of the phosphoglycerate kinase (pgk) promoter. This construct permits cell growth in hypoxanthine/aminopterin/thymidine media and confers 6-thioguanine sensitivity upon mouse Hprt- embryonic stem cells, allowing either positive or negative selection in gene-targeting experiments. We have successfully targeted the gene encoding the T-cell receptor delta-chain using the pgk::hprt fusion for counterselection.

Developmental T cell receptor gene rearrangements: relatedness of the alpha/beta and gamma/delta T cell precursor

To examine the relationships between T cell populations at various stages of development, T cell receptor (TcR) gene rearrangements were compared between the four murine populations of (a) early thymocytes, (b) early splenocytes, (c) adult thymocytes and (d) adult splenocytes. TcR alpha gene rearrangements were shown to progress from 5' to 3' regions of the J alpha locus and from 3' to 5' regions of the V alpha locus during the development of T cells in both the thymus and spleen. Thus, the gene rearrangement potentials of proximal genes varied with age, yielding a biased repertoire in the young vs. adult animal. As evidence that gamma/delta and alpha/beta gene rearrangements appeared concomitantly in individual precursors, it was found that: (a) multiple adult thymocytes bore alpha gene rearrangements on one chromosome and delta gene rearrangements on the homologous chromosome, and (b) V gamma 3-J gamma 1 rearrangements, prominent joins in the early gamma/delta T cell population, were also prominent in the early alpha/beta T cell subset. These data illustrate the non-random nature of the developmental TcR gene rearrangement and suggest that alpha/beta and gamma/delta T cell populations derive from related, if not identical, T cell precursor populations.